Katie Eastham

Paul A Scattergood
email: p.scattergood@hud.ac.uk

Danny Chu

Rayhaan Z Boota

Adrien Soupart

Fabienne Alary

Isabelle M Dixon

Craig R Rice

Samantha J O Hardman

Paul I P Elliott
email: p.i.elliott@hud.ac.uk

3MCcis States in the Photochemical N∧N Ligand Release from [Ru(bpy)2(N∧N)]2+ Complexes

Introduction

The photochemistry of ruthenium(II) complexes is a current subject of some prominence in the literature due to their potential application in photoactivated chemotherapy (PACT). [START_REF] Havrylyuk | Ru(II) photocages enable precise control over enzyme activity with red light[END_REF][START_REF] Bonnet | Why develop photoactivated chemotherapy?[END_REF][START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF][START_REF] Toupin | Photosensitive Ru(II) Complexes as Inhibitors of the Major Human Drug Metabolizing Enzyme CYP3A4[END_REF] Here, a non-toxic ruthenium(II) complex that is photochemically labile undergoes photorelease of a ligand, [START_REF] Havrylyuk | Toward Optimal Ru(II) Photocages: Balancing Photochemistry, Stability, and Biocompatibility Through Fine Tuning of Steric, Electronic, and Physiochemical Features[END_REF][START_REF] Li | Ru(ii) polypyridyl complexes as photocages for bioactive compounds containing nitriles and aromatic heterocycles[END_REF] yielding cytotoxic metal-containing and/or ligand fragments [START_REF] Cuello-Garibo | To cage or to be caged? The cytotoxic species in rutheniumbased photoactivated chemotherapy is not always the metal[END_REF][START_REF] Azar | Phototoxicity of strained Ru(ii) complexes: is it the metal complex or the dissociating ligand?[END_REF] and enables excellent spatial and temporal control of drug release and anticancer activity. Further, PACT does not rely on the presence of molecular oxygen required for photodynamic therapy (PDT) [START_REF] Mcfarland | Metal-based photosensitizers for photodynamic therapy: the future of multimodal oncology?[END_REF][START_REF] Monro | Transition Metal Complexes and Photodynamic Therapy from a Tumor-Centered Approach: Challenges, Opportunities, and Highlights from the Development of TLD1433[END_REF] and therefore has advantages under the hypoxic conditions found in tumours. [START_REF] Hompland | Tumor Hypoxia as a Barrier in Cancer Therapy: Why Levels Matter[END_REF] Key to achieving efficient photochemical reactivity in complexes of this class is the accessibility of photoreactive triplet metal centred ( 3 MC) states (associated with population of Ru-N antibonding ds* orbitals) via thermal population from triplet metal-to-ligand charge transfer ( 3 MLCT) states, [START_REF] Van Houten | Temperature dependence of the photophysical and photochemical properties of the tris(2,2'-bipyridyl)ruthenium(II) ion in aqueous solution[END_REF][START_REF] Damrauer | Femtosecond Dynamics of Excited-State Evolution in [Ru(bpy)3]2+[END_REF] themselves resulting from intersystem crossing from photoexcited singlet MLCT states (Figure 1a). [START_REF] Bhasikuttan | Ultrafast Fluorescence Detection in Tris(2,2'bipyridine)ruthenium(II) Complex in Solution: Relaxation Dynamics Involving Higher Excited States[END_REF][START_REF] Cannizzo | Broadband Femtosecond Fluorescence Spectroscopy of [Ru(bpy)3]2+[END_REF] Due to the population of antibonding orbitals [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MC states experience distortions and metal-ligand bond elongations with potential energy surface (PES) minima that are significantly displaced relative to minima for the ground and MLCT states (Figure 1b). [START_REF] Alary | Is the 3MLCT the Only Photoreactive State of Polypyridyl Complexes?[END_REF][START_REF] Wagenknecht | Metal centered ligand field excited states: Their roles in the design and performance of transition metal based photochemical molecular devices[END_REF][START_REF] Alary | Theoretical Characterization of the Lowest Triplet Excited States of the Tris-(1,4,5,8-tetraazaphenanthrene) Ruthenium Dication Complex[END_REF] This can therefore result in rapid nonradiative decay to the ground state [START_REF] Soupart | DFT rationalization of the room-temperature luminescence properties of Ru(bpy)32+and Ru(tpy)22+: 3MLCT-3MC minimum energy path from NEB calculations and emission spectra from VRES calculations[END_REF][START_REF] Sun | On the role of ligand-field states for the photophysical properties of ruthenium(II) polypyridyl complexes[END_REF] but also ligand dissociation and the formation of photoproducts. [START_REF] Juris | Ru(II) polypyridine complexes: photophysics, photochemistry, eletrochemistry, and chemiluminescence[END_REF] Figure 1. Qualitative Jablonski (a) and potential energy surface (b) diagrams depicting photophysical and photochemical processes for a [Ru(bpy)3] 2+ -like complex.

The archetypal complex [Ru(bpy)3] 2+ (bpy = 2,2'-bipyridyl) [START_REF] Adamson | New photosensitizer. Tris(2,2'-bipyridine)ruthenium(II) chloride[END_REF] exhibits efficient luminescence from the [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MLCT state [START_REF] Juris | Ru(II) polypyridine complexes: photophysics, photochemistry, eletrochemistry, and chemiluminescence[END_REF] and is well-known to undergo photoanation with release of a bpy ligand [START_REF] Durham | Photochemistry of tris(2,2'-bipyridine)ruthenium(2+) ion[END_REF][START_REF] Durham | Synthetic applications of photosubstitution reactions of poly(pyridyl) complexes of ruthenium(II)[END_REF][START_REF] Gleria | Photochemistry of tris(2,2′-bipyridine)ruthenium(II) in chlorinated solvents[END_REF][START_REF] Hoggard | Photoanation of the tris(2,2'-bipyridine)ruthenium(II) cation by thiocyanate[END_REF][START_REF] Van Houten | Photochemistry of tris(2,2'-bipyridyl)ruthenium(II) in aqueous solutions[END_REF] with realisation of selective photochemical ligand release possible in heteroleptic complexes. [START_REF] Hughes | Photophysical and Photochemical Properties of Dinuclear Ruthenium(II) Complexes Containing 2,2′-Bipyridine and 1,10-Phenanthroline Moieties[END_REF][START_REF] Wang | pH Control of Photoreactivity of Ru(II) Pyridyltriazole Complexes: Photoinduced Linkage Isomerism and Photoanation[END_REF] The efficiency of selective photochemical ligand release can be significantly enhanced in sterically encumbered derivatives featuring weakened Ru-N bonds. [START_REF] Laemmel | Efficient and Selective Photochemical Labilization of a Given Bidentate Ligand in Mixed Ruthenium(II) Complexes of the Ru(phen)2L2+ and Ru(bipy)2L2+ Family (L = Sterically Hindering Chelate)[END_REF] This stabilises the [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MC state relative to the [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MLCT state which promotes efficient photochemical ligand release. For example, the complex [Ru(bpy)2(dmbpy)] 2+ (dmbpy = 6,6'-dimethyl-2,2'-bipyridyl) is not luminescent in room temperature solution and readily releases the dmbpy ligand upon irradiation in donor solvents. [START_REF] Howerton | Strained Ruthenium Complexes Are Potent Light-Activated Anticancer Agents[END_REF] The historically accepted mechanism for photoinitiated ligand release from trischelate complexes (Scheme 1) [START_REF] Juris | Ru(II) polypyridine complexes: photophysics, photochemistry, eletrochemistry, and chemiluminescence[END_REF][START_REF] Durham | Photochemistry of tris(2,2'-bipyridine)ruthenium(2+) ion[END_REF][START_REF] Hoggard | Photoanation of the tris(2,2'-bipyridine)ruthenium(II) cation by thiocyanate[END_REF][START_REF] Van Houten | Photochemistry of tris(2,2'-bipyridyl)ruthenium(II) in aqueous solutions[END_REF][START_REF] Tachiyashiki | Mechanisms of the photosubstitution of ruthenium(II) polypyridine complexes: formation of an intermediate with a monodentate polypyridine ligand and its reactions[END_REF] involves population of thermally accessible [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MC states from the [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MLCT state which can go on to facilitate either ground state recovery or ligand dechelation. Subsequent solvent trapping of the coordinatively unsaturated species yields a ligand-loss intermediate with a monodentate N^N ligand. [START_REF] Durham | Photochemistry of tris(2,2'-bipyridine)ruthenium(2+) ion[END_REF][START_REF] Durham | Synthetic applications of photosubstitution reactions of poly(pyridyl) complexes of ruthenium(II)[END_REF][START_REF] Gleria | Photochemistry of tris(2,2′-bipyridine)ruthenium(II) in chlorinated solvents[END_REF] A secondary photochemical or thermally driven process then results in formal loss of the monodentate N^N ligand and formation of the final bis-solvento product complex.

However, commonly-invoked ligand loss intermediates from tris(diimine) complexes are rarely observed. [START_REF] Buchanan | The first report of the isolation and characterisation of an intermediate formed upon photolysis of a ruthenium(II) diimine compound, in which one of the chelating ligands is bound in a monodentate fashion[END_REF][START_REF] Arakawa | Detection of Reaction Intermediates: Photosubstitution of (Polypyridine)ruthenium(II) Complexes Using Online Electrospray Mass Spectrometry[END_REF][START_REF] Tachiyashiki | Identification of an Intermediate of the Photosubstitution of a Ruthenium(II) Diimine Complex with a Monodentate Chelating Ligand: 1H NMR and HPLC Evidence[END_REF][START_REF] Welby | Photochemistry of RuII 4,4′Bi-1,2,3-triazolyl (btz) complexes: Crystallographic characterization of the photoreactive ligand-loss intermediate trans-[Ru(bpy)(κ2-btz)(κ1-btz) (NCMe)]2+[END_REF][START_REF] Welby | Unambiguous characterization of a photoreactive ligand-loss intermediate[END_REF] Further, the exact nature of the processes occurring on the triplet excited state potential energy surface have not been well understood, in particular, the exact geometric and electronic character of the [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MC states in question. [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MC states are generally spectroscopically dark and therefore intrinsically difficult to study. [START_REF] Cadranel | Spectroscopic signatures of ligand field states in {RuII(imine)} complexes[END_REF][START_REF] Sun | Experimental Evidence of Ultrafast Quenching of the 3MLCT Luminescence in Ruthenium(II) Tris-bipyridyl Complexes via a 3dd State[END_REF] Scheme 1. Historically accepted mechanism for photochemical ligand release from [Ru(bpy)3] 2+ -type complexes (sol = coordinating solvent ligand).

Computational studies have provided highly illuminating results in this regard which have shed light on the nature of these important excited states. Numerous reports have detailed the pivotal role of [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MC states in mediating photochromic [START_REF] Ding | Excited-State Proton Transfer Controls Irreversibility of Photoisomerization in Mononuclear Ruthenium(II) Monoaquo Complexes: A DFT Study[END_REF][START_REF] Göttle | Adiabatic Versus Nonadiabatic Photoisomerization in Photochromic Ruthenium Sulfoxide Complexes: A Mechanistic Picture from Density Functional Theory Calculations[END_REF][START_REF] Göttle | Unravelling the S → O Linkage Photoisomerization Mechanisms in cis-and trans-[Ru(bpy)2(DMSO)2]2+ Using Density Functional Theory[END_REF] and photoracemisation [START_REF] Feng | A Key Factor Dominating the Competition between Photolysis and Photoracemization of [Ru(bipy)3]2+ and [Ru(phen)3]2+ Complexes[END_REF][START_REF] Feng | Triplet Ground-State-Bridged Photochemical Process: Understanding the Photoinduced Chiral Inversion at the Metal Center of [Ru(phen)2(l-ser)]+ and Its Bipy Analogues[END_REF] reactions of metal complexes and photochemical ligand substitution reactivity. [START_REF] Salassa | Mechanism of Ligand Photodissociation in Photoactivable [Ru(bpy)2L2]2+ Complexes: A Density Functional Theory Study[END_REF][START_REF] Salassa | Ligand-Selective Photodissociation from [Ru(bpy)(4AP)4]2+: a Spectroscopic and Computational Study[END_REF][START_REF] Camilo | Photosolvolysis of cis-[Ru(α-diimine)2(4aminopyridine)2]2+ Complexes: Photophysical, Spectroscopic, and Density Functional Theory Analysis[END_REF][START_REF] Greenough | Ultrafast photo-induced ligand solvolysis of cis-[Ru(bipyridine)2(nicotinamide)2]2+: experimental and theoretical insight into its photoactivation mechanism[END_REF][START_REF] Tu | Selective Photodissociation of Acetonitrile Ligands in Ruthenium Polypyridyl Complexes Studied by Density Functional Theory[END_REF][START_REF] Nisbett | DFT Investigation of Ligand Photodissociation in [RuII(tpy)(bpy)(py)]2+ and [RuII(tpy)(Me2bpy)(py)]2+ Complexes[END_REF] These studies reveal that metal complexes can access a number of [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MC states of differing geometric and electronic character. [START_REF] Soupart | Recent progress in ligand photorelease reaction mechanisms: Theoretical insights focusing on Ru(II) 3MC states[END_REF] Further, investigations have suggested that structural modification of complexes can modulate the relative accessibility of different [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MC states, states which may have preferential roles in either promoting ground state recovery or ligand substitution, thus modulating photochemical ligand substitution efficiency. [START_REF] Göttle | Pivotal Role of a Pentacoordinate 3MC State on the Photocleavage Efficiency of a Thioether Ligand in Ruthenium(II) Complexes: A Theoretical Mechanistic Study[END_REF] In our studies of the coordination chemistry of 1,2,3-triazole-based ligands [START_REF] Scattergood | An unexpected journey from highly tunable phosphorescence to novel photochemistry of 1,2,3-triazole-based complexes[END_REF][START_REF] Scattergood | Photophysics and photochemistry of 1,2,3-triazole-based complexes[END_REF] and their photophysics and photoreactivity of their resultant complexes [START_REF] Welby | Photochemistry of RuII 4,4′Bi-1,2,3-triazolyl (btz) complexes: Crystallographic characterization of the photoreactive ligand-loss intermediate trans-[Ru(bpy)(κ2-btz)(κ1-btz) (NCMe)]2+[END_REF][START_REF] Welby | Unambiguous characterization of a photoreactive ligand-loss intermediate[END_REF][START_REF] Scattergood | Photochemistry of [Ru(pytz)(btz)2]2+ and Characterization of a κ1-btz Ligand-Loss Intermediate[END_REF][START_REF] Scattergood | Unravelling the Mechanism of Excited-State Interligand Energy Transfer and the Engineering of Dual Emission in [Ir(C∧N)2(N∧N)]+ Complexes[END_REF][START_REF] Scattergood | Labilizing the Photoinert: Extraordinarily Facile Photochemical Ligand Ejection in an [Os(N^N)3]2+ Complex[END_REF][START_REF] Welby | Synthesis, characterisation and theoretical study of ruthenium 4,4′-bi-1,2,3-triazolyl complexes: fundamental switching of the nature of S1 and T1 states from MLCT to MC[END_REF] we were able to identify differing structural classes of hexacoordinate 3 MC states, [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MCtrans and [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MCcis, where trans and cis denote the relative regiochemistry of the elongated Ru-N bonds. [START_REF] Soupart | Recent progress in ligand photorelease reaction mechanisms: Theoretical insights focusing on Ru(II) 3MC states[END_REF][START_REF] Dixon | Theoretical illumination of highly original photoreactive 3MC states and the mechanism of the photochemistry of Ru(ii) tris(bidentate) complexes[END_REF][START_REF] Soupart | Exploration of Uncharted 3PES Territory for [Ru(bpy)3]2+: A New 3MC Minimum Prone to Ligand Loss Photochemistry[END_REF] These latter [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MCcis states were shown to be crucial in the observed photochemistry of Ru(II) bis(bitriazolyl) complexes. [START_REF] Dixon | Theoretical illumination of highly original photoreactive 3MC states and the mechanism of the photochemistry of Ru(ii) tris(bidentate) complexes[END_REF] From these studies, what seems clear is that the lowest triplet state potential energy surface of complexes of this type is a highly complex landscape comprising local minima for many [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MC states of different geometric character (e.g. relative stereochemistry of elongated metal-ligand bonds) and denticity (hexacoordinate versus pentacoordinate, etc). It is likely that many possible routes to photochemical ligand release exist through this excited state potential energy landscape which then represent different photochemical mechanistic regimes. Structure-property relationships then determine which [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MC states are preferentially accessible in this landscape and so which route, or routes will dominate for a particular complex and governing the efficiencies of ligand photorelease and ground state recovery.

In this contribution we explore the relative importance of 3 MC state-stabilising steric effects and 3 MLCT statedestabilising electronic effects on the ordering and energy gap between these states, and on promoting photochemical reactivity of complexes of the form [Ru(bpy)2(N^N)] 2+ (where N^N is a bipyridyl-, pyridyltriazole-or bitriazolyl-based ligand). We show that both inclusion of a sterically encumbering methyl group or a triazole donor in the N^N ligand results in rapid deactivation of the 3 MLCT state. However, incorporation of a non-sterically demanding triazole donor surprisingly leads to 10-fold higher photochemical quantum yield for N^N ligand release. Our results suggest that structure-property relationships exist, governing the type of 3 MC states that are accessible from the [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MLCT state. The data reported here, combined with data from other recent studies, provides a compelling case for a pivotal role that [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MCcis states [START_REF] Soupart | Exploration of Uncharted 3PES Territory for [Ru(bpy)3]2+: A New 3MC Minimum Prone to Ligand Loss Photochemistry[END_REF] play in mediating photochemical ligand release in [Ru(N^N)3] 2+ type complexes, offering significant insight into the photoreactive mechanistic regimes in which photochemistry for these complexes operate. In particular, the possible involvement of [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MCcis states in mediating photochemical ligand release and solvent coordination in a single step without need for a [Ru(bpy)2(k 1 -N^N)(solvent)] 2+ -type intermediate is discussed.

Results & Discussion

The complexes included in this study are depicted in Figure 2. The tris-bpy-based complexes 2 and 3 feature increasing steric congestion adjacent to the coordinating N-atoms of one bpy-based ligand in order to stabilise their [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MC states relative to their [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MLCT states when compared to the unencumbered complex 1. Complexes 4 and 6 feature a pyridyltriazole and btz ligand respectively, included in order to destabilise the [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MLCT state relative to their [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MC states when compared to 1. [START_REF] Dixon | Theoretical illumination of highly original photoreactive 3MC states and the mechanism of the photochemistry of Ru(ii) tris(bidentate) complexes[END_REF] 1,2,3-Triazoles have been shown by Sarkar and co-workers to be slightly weaker overall donors relative to pyridine based on infrared spectra of Re(I) carbonyl complexes though weaker p-acceptors based on electrochemical reduction potentials. [START_REF] Suntrup | Gauging Donor/Acceptor Properties and Redox Stability of Chelating Click-Derived Triazoles and Triazolylidenes: A Case Study with Rhenium(I) Complexes[END_REF] Other studies have indicated monodentate triazole and pyridine ligands can be broadly comparable as donors. [START_REF] Uppal | Synthesis and characterisation of luminescent rhenium tricarbonyl complexes with axially coordinated 1,2,3-triazole ligands[END_REF][START_REF] Suijkerbuijk | Click" 1,2,3-triazoles as tunable ligands for late transition metal complexes[END_REF] The smaller ring size for a triazole donor and the absence of a C-H proton adjacent to the coordinating N-atom, which is present for pyridine, makes the triazole donor less sterically demanding.

The new complex 5 featuring the mpytz ligand incorporates both a 3 MLCT state-destabilising triazole moiety as well as a 3 MC state-stabilising methyl substituent on the pyridine ring and thus might be expected to display the smallest [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MLCT-3 MC gap (or indeed inverted ordering relative to 1, 2, 4 & 6). The mpytz ligand was prepared in a two-step procedure through initial Sonogashira ethynylation of 2-bromo-6-methylpyridine followed by copper(I)-catalysed alkyne-azide cycloaddition with benzylazide. Subsequent reaction with [Ru(bpy)2Cl2] then resulted in formation and isolation of new complex 5 as its hexafluorophosphate salt. [START_REF] Hammarström | Structure and photophysical properties of novel ruthenium(II) complexes containing 6-substituted bipyridines[END_REF] ; dmbpy (3) [START_REF] Howerton | Strained Ruthenium Complexes Are Potent Light-Activated Anticancer Agents[END_REF] ; pytz (4) [START_REF] Lo | Comparison of Inverse and Regular 2-Pyridyl-1,2,3-triazole "Click" Complexes: Structures, Stability, Electrochemical, and Photophysical Properties[END_REF] ; mpytz (5); btz (6) [START_REF] Welby | Synthesis, characterisation and theoretical study of ruthenium 4,4′-bi-1,2,3-triazolyl complexes: fundamental switching of the nature of S1 and T1 states from MLCT to MC[END_REF] ).

Crystals of X-ray diffraction quality of 5 were grown and the molecular structure determined (Figure 3 with selected bond lengths and angle in Table 1). The Ru-N bond lengths are fairly typical for a ruthenium tris-diimine complex with values between 2.035 and 2.073 Å for the two bpy ligands. These distances are similar to those for 4 as reported by

Crowley and co-workers. [START_REF] Lo | Comparison of Inverse and Regular 2-Pyridyl-1,2,3-triazole "Click" Complexes: Structures, Stability, Electrochemical, and Photophysical Properties[END_REF] The Ru-N bonds to the mpytz ligand are elongated with respect to those reported for 4 (2.16

Å for the Ru-N bond to the methylpyridine donor compared to 2.08 Å for 4) demonstrating the steric demands imparted by the methyl group. This also results in a distortion in the neighbouring bpy ligand in 5 with an intercyclic torsion angle Table 1. Selected crystallographic bond lengths (Å) and angles (°) for [Ru(bpy)2(mpytz)](PF6)2.

based complexes 1 to 3 a total of three reduction processes are observed assigned to one-electron reduction processes for each of the three bpy-based ligands. The potentials of these processes are almost invariant for 1 to 3 indicating that methylation of one of the bpy ligands has little or no effect on the frontier orbital energies of the complexes. For the triazole-containing complexes 4 to 6 two reduction processes are observed, assigned to reduction of the two bpy ligands in each complex. The potentials for these reduction processes are slightly shifted to more negative potentials compared to the first and second reduction processes of complexes 1 to 3 indicating destabilisation of the LUMO in agreement with previous results. 58 In order to probe ligand effects on the 3 MLCT state energy, luminescence spectra were recorded (Figure 6). Since the complexes, with the exception of 1, are very weakly or non-emissive in room temperature fluid solutions, [START_REF] Welby | Synthesis, characterisation and theoretical study of ruthenium 4,4′-bi-1,2,3-triazolyl complexes: fundamental switching of the nature of S1 and T1 states from MLCT to MC[END_REF][START_REF] Hammarström | Structure and photophysical properties of novel ruthenium(II) complexes containing 6-substituted bipyridines[END_REF][START_REF] Lo | Comparison of Inverse and Regular 2-Pyridyl-1,2,3-triazole "Click" Complexes: Structures, Stability, Electrochemical, and Photophysical Properties[END_REF][START_REF] Yoshikawa | Emission property and DFT calculation for the 3MLCT luminescence of Ru(bpy)2(L)2+ complex[END_REF] spectra were recorded at 77 K in EtOH/MeOH glass matrices to enable direct comparison across the series. The spectra obtained for all complexes are structured, featuring clear vibronic progressions. 1 and 2 exhibit near-identically positioned emission bands (l em max 580 & 630 nm) with a very slight red-shift observed for the emission maxima of 3. In agreement with the electrochemical and electronic absorption data the emission maxima of 4 to 6 are all blue-shifted relative to those of 1, with 6 appearing at the highest energy (l em max 564 & 609 nm) in line with the complex having the most cathodic reduction potential. A small red-shift is observed for the progressions in the spectrum for the methyl-substituted complex 5 compared to 4. 4. Upon excitation all complexes display ground state bleach features between 400 and 500 nm which are coincident with the 1 MLCT absorption bands. Intense excited state absorption (ESA) bands are also observed between 350 and 400 nm along with a broad and less intense ESA feature beyond 500 nm which are assigned to the 3 MLCT state. [START_REF] Damrauer | Ultrafast Dynamics in the Metal-to-Ligand Charge Transfer Excited-State Evolution of [Ru(4,4'-diphenyl-2,2'-bipyridine)3]2+[END_REF] For all complexes, a very short rise time of <0.25 ps is observed, accompanied by a second, slower process (<30 ps) manifested as a secondary rise component for 1, 2 and 4 and as an initial decay for 3, 5 and 6 which are tentatively assigned to vibrational cooling, internal conversion, solvent reorganisation and energy redistribution processes. [START_REF] Damrauer | Ultrafast Dynamics in the Metal-to-Ligand Charge Transfer Excited-State Evolution of [Ru(4,4'-diphenyl-2,2'-bipyridine)3]2+[END_REF][START_REF] Henry | The Early Picosecond Photophysics of Ru(II) Polypyridyl Complexes: A Tale of Two Timescales[END_REF][START_REF] Wallin | Femtosecond Transient Absorption Anisotropy Study on [Ru(bpy)3]2+ and [Ru(bpy)(py)4]2+. Ultrafast Interligand Randomization of the MLCT State[END_REF] The archetypical complex 1 displays the longest lifetime, with both excited state transient and ground state bleach bands still evident at the end of the 3 ns time window of the experiment, as expected given its ns-µs [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MLCT lifetime. The inclusion of a single methyl group in 2 results in a significant shortening of the 3 MLCT state lifetime (t3 = 2.5 ns, in agreement with previously reported data indicated a lifetime of <10 ns [START_REF] Hammarström | Structure and photophysical properties of novel ruthenium(II) complexes containing 6-substituted bipyridines[END_REF][START_REF] Fujita | Photophysical properties of covalently attached tris(bipyridine)ruthenium(2+) and Mcyclam2+ (M = nickel, H2) complexes[END_REF] ) with the replacement of one pyridine ring with a triazole donor in 4 resulting in a comparably shortened [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MLCT state lifetime (t3 = 7.3 ns; this value agrees with data reported by Crowley and co-workers who determined that the lifetime must be <10 ns [START_REF] Lo | Comparison of Inverse and Regular 2-Pyridyl-1,2,3-triazole "Click" Complexes: Structures, Stability, Electrochemical, and Photophysical Properties[END_REF] ). Incorporation of the second methyl group in 3 results in a 10-fold reduction in the 3 MLCT lifetime compared to 2. For 6 the replacement of the pytz ligand present in 4 with the btz ligand results in a greater than 10-fold further reduction in the [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MLCT state lifetime (from 7.3 ns for 4 to 443 ps for 6). Delayed bleach recovery behaviour has previously been documented by Hauser and co-workers for sterically encumbered ruthenium(II) complexes and ascribed to the rapid depopulation of the 3 MLCT state to yield a metasTable 4MC state which then decays to the ground state. [START_REF] Sun | Experimental Evidence of Ultrafast Quenching of the 3MLCT Luminescence in Ruthenium(II) Tris-bipyridyl Complexes via a 3dd State[END_REF] We therefore have some confidence in assigning the shorter-lived spectral evolution observed for 5 (t2 = 14.8 ps) as arising from conversion of the 3 MLCT state to a 3 MC state (along with vibrational cooling, energy redistribution processes, etc), with the longer-lived process (t3 = 233 ps) representing decay of the 3 MC state to the ground state.

Collectively the transient absorption data show that the steric effects imparted by methylation have a slightly greater deactivating influence on the [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MLCT state lifetime (i.e. 1 compared to 2/3 versus 1 compared to 4/6) but that this is largely comparable to the electronic effect of replacing pyridine by triazole donors.

Photochemical reactivity. The photochemical ligand release reactivity of the complexes was investigated by UV-visible absorption spectroscopy in proteo-acetonitrile and also by [START_REF] Havrylyuk | Ru(II) photocages enable precise control over enzyme activity with red light[END_REF] H NMR spectroscopy in d3-acetonitrile for photoproduct identification. For NMR experiments, photolysis was conducted using irradiation with the mercury emission lines from a 23 W fluorescent light bulb, whilst for optical spectroscopic experiments a blue LED with an emission maximum at 446 nm was used. Under both these sets of conditions the photolysis of 1 is exceedingly slow relative to the photochemistry observed for the remainder of the complexes and so 1 is therefore considered "photoinert" by comparison (f < 0.0001).

When monitored by UV-visible absorption spectroscopy spectral features for the [START_REF] Havrylyuk | Ru(II) photocages enable precise control over enzyme activity with red light[END_REF] MLCT transitions between 400 to 500 nm for 2 to 6 are observed to evolve with clear isosbestic points indicating a one-step photolysis process (Figures 8 andS12). For complexes 4 to 6, this is accompanied by a bleaching of the bands at 300-350 nm consistent with loss of the triazole-containing ligand. The spectra for all complexes converge to a common band shape for the [START_REF] Havrylyuk | Ru(II) photocages enable precise control over enzyme activity with red light[END_REF] MLCT transitions consistent with the formation of [Ru(bpy)2(NCMe)2] 2+ . [START_REF] Welby | Photochemical ligand ejection from non-sterically promoted Ru(ii)bis(diimine) 4,4′-bi-1,2,3-triazolyl complexes[END_REF] The evolution of 1 H NMR spectra is much slower than that observed by UV-visible absorption spectroscopy consistent with the much higher concentration required, taking samples beyond the optically dilute regime. Spectral changes generally involve the loss of resonances for the starting complex and appearance of resonances for the photoproduct [Ru(bpy)2(NCMe)2] 2+ and those of the free ligand (mbpy, dmbpy, pytz, mpytz or btz) that has been released (Figures S13 to S17 and reference [START_REF] Welby | Photochemical ligand ejection from non-sterically promoted Ru(ii)bis(diimine) 4,4′-bi-1,2,3-triazolyl complexes[END_REF] ). In the case of 2, photolysis is extremely slow (the sample was monitored over more than 2 weeks of continual photolysis) and appears to proceed with competitive loss of both mbpy and bpy. Further, ligand scrambling processes are evident over the prolonged photolysis with observed formation of 1. However, UV-visible absorption spectra recorded during photolysis of 2 show only bleaching in the region for the 1 MLCT absorption maximum for 2 and also 1. Thus, due to these observations as well as the significantly prolonged timescales of photolysis, we suspect that the ligand scrambling is likely a secondary photochemical process and Photochemical quantum yields were determined from the evolution of UV-visible absorption spectra (Table 5) using the spectrometric approach reported by Slep and co-workers and modelled as a single-step photochemical reaction given the observed isosbestic points. [START_REF] Marcolongo | A chemometric approach for determining the reaction quantum yields in consecutive photochemical processes[END_REF] The data reveal that for 2 and 4, the inclusion of one methyl group or replacement of one pyridine for triazole in the departing N^N ligand leads to increased photochemical reactivity compared to 1.

However, the quantum yield of 4 (0.3 %) is an order of magnitude larger than that for 2 (0.02 %) indicating that the introduction of the triazole ring has a far greater effect in promoting photorelease despite 2 exhibiting a shorter [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MLCT state lifetime, one presumes through 3 MC state-mediated deactivation.

Interestingly, the incorporation of the additional methyl group in the mpytz ligand in 5 leads to the same, rather than increased quantum yield of N^N release compared to 4 as might have been expected if steric and electronic impacts on quantum yield combine in an additive fashion. Given that 5 displays the shortest [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MLCT lifetime in the series as determined by transient absorption spectroscopy, efficient depopulation of the 3 MLCT state indeed occurs, but not to a photoproductive 3 MC state. This piece of information will prove particularly important in the upcoming discussion and in the global mechanistic interpretation proposed in this work. Inclusion of sterically encumbering methyl groups on both donor rings for the departing dmbpy ligand in 3, or incorporation of the btz ligand in 6 both lead to a further significant increase in photochemical reactivity with quantum yields of 8.2 % and 2.0 % respectively.

The apparent contradictions between transient-absorption data and determined photochemical quantum yields means that interpretation of this data is not straightforward. However, deeper insight may be offered through computational calculations on the available excited state local minima for each complex.

Computational Calculations. To gain deeper insight into the photophysical and photochemical properties of 1 to 6 we carried out density functional theory calculations on the ground and excited states of the complexes (benzyl substituents on triazole rings being modelled as methyl groups to reduce computational expense and as they will have minimal impact on photophysical properties [START_REF] Scattergood | Photophysics and photochemistry of 1,2,3-triazole-based complexes[END_REF] ). In all cases the HOMO has predominantly ruthenium d-orbital character whilst the LUMO has primarily bpy p* character (Figure S19). Geometries are available in the Supporting Information and Ru-N bonds for all geometries are summarised in Table S1. For 1 the Ru-N bonds are 2.06 to 2.07 Å. [START_REF] Rillema | Comparison of the crystal structures of tris heterocyclic ligand complexes of ruthenium(II)[END_REF] For 2, five of the Ru-N bond distances are very similar to those of 1, however, the Ru-N bond to the methyl-substituted pyridine is significantly elongated at 2.15 Å. Similarly, the Ru-N bond length to the sterically encumbered pyridine ring of the mpytz ligand in 5 (2.17 Å) is significantly elongated compared to the equivalent Ru-N bond for 4 (2.11 Å). The Ru-N distance to the dmbpy ligand in 3 (2.13 Å) are 0.05 to 0.06 Å longer than the Ru-N bond for the unsubstituted bpy ligands. These ground state distortions therefore demonstrate the steric demands imparted by the methyl substituents.

In comparison, the Ru-N distances for the unstrained complexes 4 and 6 have much reduced deviation compared to 1.

Since 2 and 5 show only very little or no increase in photochemical quantum yield compared to those of 1 and 4

respectively, these ground state geometry elongations offer little insight into the relationships governing the efficiency of the observed photochemistry. Excited state optimisations are therefore required to derive additional arguments for the rationalisation of the varying behaviours observed. Relative energies of optimised [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MLCT and 3 MC states for 1 to 6 quoted relative to their respective optimised ground states (E = 0.0 eV) (bottom). For the [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MCcis states for 4 and 6 the ligand that is repelled is the pytz and btz ligand respectively.

The 3 MLCT state geometries for each complex were optimised, as well as geometries for possible 3 MC states using initial guess geometries based on structural parameters from our previous studies. [START_REF] Dixon | Theoretical illumination of highly original photoreactive 3MC states and the mechanism of the photochemistry of Ru(ii) tris(bidentate) complexes[END_REF][START_REF] Soupart | Exploration of Uncharted 3PES Territory for [Ru(bpy)3]2+: A New 3MC Minimum Prone to Ligand Loss Photochemistry[END_REF] The energies of the optimised [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MLCT and 3 MC states of 1 to 6, relative to the energies of their respective ground states, are depicted in Figure 9. The 3 MLCT states for all complexes exhibit one singly occupied natural orbital (SONO) of ruthenium d-orbital character and a second SONO (SONO+1) of bpy p* character (Figure S20). Mulliken spin densities of ~0.99 on the ruthenium atom confirm the charge transfer nature of these [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MLCT states. In agreement with expectations based on electrochemical and photophysical data, the [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MLCT states of the pyridyltriazole-based complexes 4 and 5 are higher in energy than that of 1 and the methylated bpy containing complexes 2 and 3 with the most destabilised [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MLCT state arising for 6. S1. For Ru(II) tris-bidentate complexes we have previously classified hexacoordinate [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MC states into two principal types.

Firstly, axial elongation to two Ru-N bonds situated trans to one another are characterised by population of a dz 2 -like ds* orbital [START_REF] Alary | Is the 3MLCT the Only Photoreactive State of Polypyridyl Complexes?[END_REF] and are thus termed [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MCtrans states (Figure 11). [START_REF] Soupart | Exploration of Uncharted 3PES Territory for [Ru(bpy)3]2+: A New 3MC Minimum Prone to Ligand Loss Photochemistry[END_REF] Secondly, population of a dx 2 -y 2 -like ds* orbital may result in elongation of two Ru-N bonds situated cis to one another which is accompanied by a widening of the N-Ru-N angle for the Ru-N bonds trans to those elongated (Figure 11). These states we term [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MCcis and have previously identified [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MCcis states for 1 and 6. [START_REF] Dixon | Theoretical illumination of highly original photoreactive 3MC states and the mechanism of the photochemistry of Ru(ii) tris(bidentate) complexes[END_REF][START_REF] Soupart | Exploration of Uncharted 3PES Territory for [Ru(bpy)3]2+: A New 3MC Minimum Prone to Ligand Loss Photochemistry[END_REF] Figure 10. Singly occupied natural orbitals (SONOs) for the lowest energy 3 MC states of 2 to 6 ( 3 MCtrans,A) and those for the [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MCcis states for 4 and 6. As the three N-Ru-N axes of 1 are equivalent, the three possible axial elongations give rise to equivalent 3 MCtrans states.

However, for the heteroleptic complexes 3 and 6 that incorporate a symmetrical dmbpy or btz ligand respectively, there are two unique types of N-Ru-N axis and thus two unique types of [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MCtrans state which were optimised in each case (elongation for N(bpy)-Ru-N(dmbpy/btz) and N(bpy)-Ru-N(bpy) axes, termed [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MCtrans,A and [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MCtrans,B respectively and as defined in Figure 9). For 2, 4 and 5 which incorporate an asymmetric third ligand, the three N-Ru-N axes are unique and thus three distinct [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MCtrans states are expected ( 3 MCtrans,A, [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MCtrans,B and 3 MCtrans,C, Figure 9) which were indeed located, in each case.

As one of the bpy ligands in 1 is replaced by pytz or btz in 4 and 6 respectively, [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MCtrans states are progressively stabilised with increasing triazole content. The 3 MCtrans,A states are the lowest in energy for 4 and 6 involving elongation of the Ru-N(triazole) bond. For 1 and 4 all the 3 MC states lie higher in energy than the [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MLCT state whereas for the other complexes the [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MLCT state energies sit within the range of energies for, and are thus straddled by, the 3 MC states. For each of the complexes 2, 3 and 5 incorporating (a) ligand methyl substituent(s), the [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MCtrans,A states involving elongation of the Ru-N bond to the methylated pyridine donor are significantly stabilised with respect to the other 3 MC states and lie 0.03, 0.13 & 0.14 eV below their respective [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MLCT states.

We attempted to locate [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MCcis states on the T1 PES for each complex in which the Ru-N bond elongations occur so as to repel the departing ligand. Whilst the geometry for a 3 MCcis state for 4 was indeed located, such states could not be found for 2, 3 and 5. This presumably stems from the steric encumbrance imparted by the departing ligand methyl substituents that would inhibit the widening of the angle between the two bpy ligands. For both 4 and 6 the 3 MCcis states sit within the range of energies for their respective [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MCtrans states and are highly accessible from the 3 MLCT state.

The trend for the relative energies of the 3 MLCT and lowest energy 3 MC states of the complexes is in good agreement with the reduced [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MLCT state lifetime resulting from either increasing the steric congestion through incorporation of ligand methyl-substituents, or through replacement of pyridine donors with triazole. Both act to make population of [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MC states from the [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MLCT state more favourable. However, it is noted that this does not translate to the trend in photochemical quantum yields for ligand release.

Rationalising 3 MLCT lifetimes versus photochemical reactivities.

We have previously postulated that [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MCtrans and [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MCcis states for [Ru(N^N)3] 2+ complexes, whilst both having the potential to deactivate [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MLCT states, have differing preferential roles with respect to facilitating ground state recovery versus promoting photochemical reactivity. [START_REF] Soupart | Recent progress in ligand photorelease reaction mechanisms: Theoretical insights focusing on Ru(II) 3MC states[END_REF] Whilst these states exhibit elongated Ru-N bonds they nevertheless remain hexacoordinate and in [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MCtrans states the metal centre remains significantly shielded from a potential incoming ligand. We have suggested that [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MCtrans states are therefore more prone to facilitating ground state recovery than in going on to form photoproducts. [START_REF] Soupart | Recent progress in ligand photorelease reaction mechanisms: Theoretical insights focusing on Ru(II) 3MC states[END_REF][START_REF] Soupart | Exploration of Uncharted 3PES Territory for [Ru(bpy)3]2+: A New 3MC Minimum Prone to Ligand Loss Photochemistry[END_REF] On the other hand, [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MCcis states, in which both Ru-N bonds for the departing ligand are elongated, also exhibit an open quadrant created by the widening of the angle between the 'spectator' ligands, thus potentially exposing the metal centre to incoming ligands. In our previous work we have proposed that [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MCcis states are therefore far more prone to result in photochemical reactivity. [START_REF] Soupart | Recent progress in ligand photorelease reaction mechanisms: Theoretical insights focusing on Ru(II) 3MC states[END_REF][START_REF] Soupart | Exploration of Uncharted 3PES Territory for [Ru(bpy)3]2+: A New 3MC Minimum Prone to Ligand Loss Photochemistry[END_REF] Indeed, an [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MCcis state was shown to be crucial in mediating the observed formation of the ligand loss intermediate and final photoproducts trans-[Ru(bpy)(k 2 -btz)(k 1 -btz)(NCMe)] 2+ and trans-[Ru(bpy)(btz)(NCMe)2] 2+ respectively, in acetonitrile solution in which the retained bidentate bpy and btz ligands are coplanar. [START_REF] Welby | Photochemistry of RuII 4,4′Bi-1,2,3-triazolyl (btz) complexes: Crystallographic characterization of the photoreactive ligand-loss intermediate trans-[Ru(bpy)(κ2-btz)(κ1-btz) (NCMe)]2+[END_REF][START_REF] Welby | Unambiguous characterization of a photoreactive ligand-loss intermediate[END_REF] For 1, the fact that the lowest [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MLCT state lies significantly below the energies of the 3 MC states agrees with the comparatively low photochemical reactivity of the complex. Incorporation of a single methyl group on one bpy ligand in 2 leads to a significant elongation in the ground state Ru-N bond for the pyridine ring bearing the methyl group. Thus, one would expect this steric encumbrance to result in a lowering in energy of [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MCtrans,A whose elongation is aligned with this axis. Indeed, calculations reveal a stabilisation of the 3 MC states for 2 compared to those of 1, particularly so for the [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MCtrans,A state, to the point that it is now lower in energy than the [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MLCT state. As is evident from the transient absorption data, the stabilisation of 3 MCtrans,A for 2 results in rapid deactivation of the 3 MLCT state (t3 = 2.5 ns). The still very low photochemical quantum yield for release of the mbpy ligand from 2 is, however, in agreement with [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MCtrans,A being prone to facilitating ground state recovery over photochemical reactivity.

For 4, the replacement of one pyridine for a triazole donor in the pytz ligand leads to a slight destabilisation of the [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MLCT state and a stabilisation of the 3 MC states compared to 1. Thus, the closer proximity of the 3 MLCT state to the closely spaced set of [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MC states (including 3 MCcis) is in agreement with the observed shortened lifetime of 4 compared to 1 (t3 = 7.3 ns). With the 3 MC states lying just above the [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MLCT state, this is also in agreement with a longer [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MLCT state lifetime compared to 2. However, despite the reduced accessibility of the 3 MC states for 4 compared to 2 it is noted that 4, for which a 3 MCcis state local minimum could be located, shows a 10-fold higher photochemical quantum yield for N^N loss.

For 5, the inclusion of a methyl group in the mpytz ligand leads to the shortest [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MLCT lifetime for all complexes in the series (t2 = 15 ps), significantly shorter than that of 4. In this case, the long-lived decay process instead stems from the return of a meta-sTable 4MC state to the ground state (t3 = 233 ps) as evidenced from the prompt decay of transient absorption bands for the [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MLCT state and delayed ground state bleach recovery. In agreement with 2, calculations reveal the lowest energy 3 MC state to involve elongation along the axis containing the methyl-substituted pyridine donor ( 3 MCtrans,A). However, the increased rate of [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MLCT deactivation compared to 4, with apparent promotion of the population of [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MCtrans,A, does not translate into increased photochemical reactivity, again highlighting its preferential role in promoting ground state recovery.

For the btz-containing complex 6, the inclusion of a second triazole ring leads, again, to a shortening of the 3 MLCT state lifetime (t3 = 443 ps) compared to 4. Calculations reveal that the 3 MCtrans,A state is slightly lower in energy than the [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MLCT state (which is itself further destabilised relative to that of 4) which is almost isoenergetic with the 3 MCcis state.

Thus, the 3 MLCT state is rapidly depopulated and a further 10-fold enhancement of photochemical quantum yield for ligand release is observed, but in the absence of any steric encumbrance.

The increased photochemical reactivity of 4 and 6 relative to 1 would therefore seem to correlate with the existence and accessibility on the T1 PES of [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MCcis states. These states could not be located for 2 and 5, seemingly accounting for their limited photoreactivity. As discussed above, [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MCcis states are important for the formation of trans photoproducts for [Ru(bpy)(btz)2] 2+ . As the angle between the retained bpy and btz ligand widens, the lack of steric impediment for the retained btz ligand enables it to become fully coplanar with the bpy ligand. On the other hand, whilst trans-[Ru(bpy)2(L)2] 2+/0 complexes are known, [START_REF] Cordes | Crystal and molecular structures of transbis(acetonitrile)bis(bipyridine)ruthenium(II) perchlorate and trans-diamminebis(bipyridine)ruthenium(II) perchlorate[END_REF][START_REF] Bonneson | Six-coordinate complexes with 1,10phenanthroline ligands in the trans configuration. Preparation of trans-bis(1,10-phenanthroline)ruthenium(II) complexes and crystal structure of trans-bis(1,10-phenanthroline)bis(pyridine)ruthenium(II) hexafluorophosphate[END_REF] coplanarisation of two bpy ligands is nonetheless inhibited by steric interactions between the H6 and H6' protons of the two bpy ligands and leads to severe distortions. Hence, cis-[Ru(bpy)2]-containing photoproducts predominate. However, as we outline below, and have shown previously, [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MCcis states may contribute to the formation of both cis and trans photoproduct formation. [START_REF] Soupart | Theoretical Study of the Full Photosolvolysis Mechanism of [Ru(bpy)3]2+: Providing a General Mechanistic Roadmap for the Photochemistry of [Ru(N^N)3]2+-Type Complexes toward Both Cis and Trans Photoproducts[END_REF] As the reader may note, we have yet to discuss complex 3 and will do so during the next section.

Broader mechanistic implications. Early work, for example by van Houten [START_REF] Van Houten | Photochemistry of tris(2,2'-bipyridyl)ruthenium(II) in aqueous solutions[END_REF][START_REF] Jones | Photochemistry of hetero-trischelated ruthenium(II) polypyridine complexes in dichloromethane[END_REF] and Meyer, [START_REF] Durham | Photochemistry of tris(2,2'-bipyridine)ruthenium(2+) ion[END_REF][START_REF] Allen | Synthetic control of excited-state properties. Tris-chelate complexes containing the ligands 2,2'-bipyrazine, 2,2'-bipyridine, and 2,2'-bipyrimidine[END_REF][START_REF] Caspar | Photochemistry of tris(2,2'-bipyridine)ruthenium(2+) ion (Ru(bpy)32+). Solvent effects[END_REF] provided compelling evidence for the photochemistry of [Ru(bpy)3] 2+ and related complexes as proceeding via a dissociative or interchangedissociative mechanism. For the ligand substitution of the thiocyanate salt, evidence by UV-visible absorption spectroscopy is observed for an unstable intermediate, presumably of the form [Ru(bpy)2(k 1 -bpy)(NCS)] + . [START_REF] Durham | Synthetic applications of photosubstitution reactions of poly(pyridyl) complexes of ruthenium(II)[END_REF] However, intermediates are not observed under other conditions with different incoming ligands. Meyer noted the dependence on the nature of the incoming ligand, attributing this to competition between coordination of the incoming ligand to the pentacoordinate [Ru(bpy)2(k 1 -bpy)] 2+ with favourable rechelation of the k 1 -bpy ligand. On the other hand, Glazer and co-workers reported evidence of associative photochemical ligand substitution for the complex [Ru(bpy)2(dmdppz)] 2+

(dmdppz = 3,6-dimethyldipyridylphenazine). [START_REF] Wachter | Mechanistic Study on the Photochemical "Light Switch" Behavior of [Ru(bpy)2dmdppz]2+[END_REF] This was attributed with the coordination of a solvent ligand to the electron deficient Ru(III) centre of the 3 MLCT state. However, in the same work the photochemical behaviour of 3 suggested dissociative character. In work on chelating bisthioether complexes of the form [Ru(bpy)2(S^S)] 2+ , Turro and Kodanko showed that the [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MLCT states can show significant dissociative character with large elongations of the Ru-S bonds. [START_REF] Garner | Effect of Electronic Structure on the Photoinduced Ligand Exchange of Ru(II) Polypyridine Complexes[END_REF] This and other prior work from a number of groups has therefore shown that the rate and efficiency of photochemical ligand substitution in Ru(II) trischelate complexes can show a dependence on the nature of the departing ligand, the incoming ligand, solvent and temperature. Further, this demonstrates that photochemical ligand substitution in these systems may operate by a variety of mechanisms.

We have recently reported computational studies on the full photosolvolysis mechanism for 1, including routes involving and circumventing k 1 -bpy solvento intermediates, as a model system in acetonitrile from which we may draw parallels with other more photoreactive complexes. [START_REF] Soupart | Theoretical Study of the Full Photosolvolysis Mechanism of [Ru(bpy)3]2+: Providing a General Mechanistic Roadmap for the Photochemistry of [Ru(N^N)3]2+-Type Complexes toward Both Cis and Trans Photoproducts[END_REF] These studies provided three important insights: (1) the [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MCcis state was

shown to be able to contribute to product formation pathways for both cis and trans-[Ru(bpy)2(NCMe)2] 2+ ; (2) minimum energy path optimisations revealed that there are potential low energy pathways to direct solvent capture by [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MC states without contravention of Wigner's rules. [START_REF] Soupart | On the Possible Coordination on a 3MC State Itself? Mechanistic Investigation Using DFT-Based Methods[END_REF] Electrostatic repulsion between the lone pair of an approaching solvent molecule and the unpaired electron located in the ds* orbital can result in a switching to a triplet state of alternative character (e.g. another [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MC state with elongation of other Ru-N bonds, or a [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MLCT state) which enables solvent coordination; (3) in the triplet state solvent molecule capture for [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MCcis, the solvent can approach the Ru centre in the open quadrant between the two 'spectator' ligands, and on the opposing side of the metal centre to the departing bpy ligand whose Ru-N bonds are both elongated. During this process, rather than form an approximately, though distorted, coplanar arrangement, the 'spectator' bpy ligands rearrange their orientation whilst the Ru-N bonds to the departing bpy ligand formally rupture to yield a pentacoordinate 3 MC state of the form [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] [Ru(bpy)2(NCMe)] 2+ in which the departing bpy ligand remains associated in a van der Waal's adduct ({ 3 MCpenta + bpy} in Figure 12). Due to steric interactions between the two retained bpy ligands that prevents true coplanarity, one of the bpy ligands slides up over the other in forming this pentacoordinate [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MC state which then favours formation of the cis-[Ru(bpy)2(NCMe)2] 2+ photoproduct. The striking observation from these insights is that both solvent capture and rupture of both Ru-N bonds with formal release of departing ligand may occur as part of a single concerted photochemical process without the need for formation of the k 1 -N^N ligand-loss intermediate primary photoproduct. This would agree with the lack of intermediates observed for many complexes and the isosbestic behaviour when monitored by UV-visible absorption spectroscopy, and so this pathway may dominate in many cases.

On the other hand, a step-wise photochemical ligand release indeed proceeds with observation of a k 1 -N^N solvento intermediate in several other cases (Scheme 1). [START_REF] Lima | Bidentate Coordination of 2-Aminopyridine (2Apy) in cis-[Ru(phen)2(2Apy)]2+ Aiming at Photobiological Studies[END_REF] For example, for [Ru(bpy)2(S^S)] 2+ -type complexes where S^S is a bisthioether ligand with a flexible linker [START_REF] Meijer | Diastereoselective Synthesis and Two-Step Photocleavage of Ruthenium Polypyridyl Complexes Bearing a Bis(thioether) Ligand[END_REF] and for [Ru(bpy)2(3,3'-dimethyl-2,2'-bipyridyl)] 2+ where a steric clash between the methyl substituents results in a 'spring-loaded' photodechelation which favours solvento intermediate formation. [START_REF] Tachiyashiki | Mechanisms of the photosubstitution of ruthenium(II) polypyridine complexes: formation of an intermediate with a monodentate polypyridine ligand and its reactions[END_REF] Weak resonances for an intermediate are observed for 6 though isosbestic behaviour when monitored by UV-visible absorption spectroscopy suggests that this species never builds up to any appreciable concentration. Whilst one or other of these mechanistic pathways may dominate for a particular complex under a given set of conditions (solvent, identity of the incoming ligand, etc), it is possible that both may operate competitively for some systems. These possible pathways are illustrated in Scheme 2 for 6.

Scheme 2. Proposed excited state mechanisms for step-wise and concomitant solvent coordination and departing ligand release illustrated for 6 (a cis arrangement of the acetonitrile and k 1 -btz is depicted in the upper pathway on the basis that a trans arrangement would generate significant steric strain, [START_REF] Soupart | Theoretical Study of the Full Photosolvolysis Mechanism of [Ru(bpy)3]2+: Providing a General Mechanistic Roadmap for the Photochemistry of [Ru(N^N)3]2+-Type Complexes toward Both Cis and Trans Photoproducts[END_REF] however a trans isomer could also be envisaged).

We have yet to discuss complex 3, which of course displays the highest photochemical reactivity in the series. Bearing two methyl substituents in the dmbpy ligand, the complex is highly strained and exhibits significant distortion in the ground state. [START_REF] Yoshikawa | Emission property and DFT calculation for the 3MLCT luminescence of Ru(bpy)2(L)2+ complex[END_REF] For 2 and 5 bearing a single methyl substituent, population of the 3 MCtrans,A state relieves the steric strain present in the ground and [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MLCT states as the Ru-N bond to the methyl-substituted pyridine ring elongates. For 3, however, there will still be strain in the [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MCtrans,A state due to the second methyl substituent on the fully coordinated pyridine ring of the dmbpy ligand. As alluded to above, the formation of a 3 MCcis state will also be inhibited, with the methyl groups of dmbpy precluding the required angular opening between the two spectator bpy ligands. It is therefore likely that the 'brute force' approach of the two sterically encumbering methyl substituents pushes the complex over a tipping point beyond which photochemical reactivity is shunted into operating in a different mechanistic regime.

Rotation about the intercyclic C-C bond of the dmbpy ligand may enable access to photoproductive pentacoordinate trigonal bipyramidal [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MC states ( 3 MCpenta, which have not been calculated in this study. Scheme 2). 85 To underline this, Meijer has shown that the related 2,9-dimethylphenanthroline (dmphen) complex [Ru(bpy)2(dmphen)] 2+ , in which this bond rotational motion is not possible (and where [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MCcis state is presumably heavily disfavoured), shows substantially reduced photochemical reactivity compared to 3 (f £ 0.005. The complex in fact undergoes competitive photorelease of dmphen and bpy). [START_REF] Bonnet | [END_REF][START_REF] Meijer | Photo-activation of ruthenium-decorated upconverting nanoparticles[END_REF] Formation of a pentacoordinate 3 MC state for 3 might be expected to result in formation of a ligand-loss intermediate photoproduct of the form [Ru(bpy)2(k 1 -dmbpy)(NCMe)] 2+ , however, the occurrence of isosbestic points in UV-visible absorption spectra during photolysis and no detection of an intermediate is not consistent with this. The steric pressure imparted by the methyl substituent of the coordinated pyridine ring of the dmbpy ligand would result in the solvento intermediate being highly strained and thus the dmbpy may undergo rapid thermally-driven dissociation soon after formation, if it is formed at all. On the other hand, this same steric strain for a resultant 3 MLCT or hexacoordinate [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MC states could facilitate a concomitant solvent capture and ligand release process to yield 3 *[Ru(bpy)2(NCMe)] 2+ as a van der Waal's adduct with dmbpy in a related fashion to that proposed originally for 1 (Scheme 3). [START_REF] Soupart | Theoretical Study of the Full Photosolvolysis Mechanism of [Ru(bpy)3]2+: Providing a General Mechanistic Roadmap for the Photochemistry of [Ru(N^N)3]2+-Type Complexes toward Both Cis and Trans Photoproducts[END_REF][START_REF] Soupart | On the Possible Coordination on a 3MC State Itself? Mechanistic Investigation Using DFT-Based Methods[END_REF] Whilst steric strain in 2 and 5 would be largely relieved on forming the 3 MCtrans,A state, the lack of a 3 MCcis state may mean that photochemistry for these complexes may proceed by a similar [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MCpenta mechanism but with much reduced efficiency due to reduced propensity to become pentacoordinate. Interestingly, Kayanuma has very recently reported computational studies on the photoaquation of 3, detailing a similar 3 MC state solvent capture mechanism. This involved a calculated pathway in which a water ligand enters cis to the k 1 -dmbpy ligand at the 3 MCtrans,A state with subsequent dissociation of dmbpy to yield a 3 MC state of the form [ 3 Ru(bpy)2(OH2)] 2+ . 88 Scheme 3. Possible mechanism for the concerted solvent addition and dmbpy release for 3.

One also could venture that such bond rotations and pentacoordinate 3 MC states could come into play for 4 and 6 rather than photochemistry stemming from population of the hexacoordinate 3 MCcis state. Whilst this is possible, we have also recently reported the photochemistry of the tetraazaphenanthrene (TAP) complex [Ru(TAP)2(btz)] 2+ and also noted the known photochemical reactivity of the homoleptic complex [Ru(TAP)3] 2+ . [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MCcis states in which a TAP ligand is repelled (Figure 13). Thus, for [Ru(TAP)3] 2+ , the respectable quantum efficiency for TAP release (f = 0.02), the lack of a suitable C-C bond about which rotation may occur to favour formation of pentacoordinate species, and the optimisation of a 3 MCcis state are supportive of photochemistry occurring through this state and without the involvement of a ground state k 1 -TAP intermediate. The data discussed here, in conjunction with those reported previously, provide compelling support for a novel mechanistic regime for photochemical ligand release from ruthenium(II) tris-diimine type complexes. Whilst other competing mechanisms may also operate, photochemistry for these complexes is proposed to proceed through population of a 3 MCcis state which undergoes concomitant solvent capture and formal release of the departing ligand in a single excited state process that negates the need to invoke a sequential mechanism with intervening involvement of ground state ligand-loss intermediate complexes bearing a monodentate departing N^N ligand.

Conclusions

Photochemically reactive ruthenium(II) complexes are of significant interest for application in photoactivated chemotherapy (PACT). However, the excited state mechanistic details of photochemical ligand release from trisbidentate ruthenium(II) complexes have been the subject of considerable ambiguity. [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MC states which mediate these processes are either extremely short lived or are spectroscopically dark which makes their direct investigation challenging. We have reported here a systematic study through combined steady state & time-resolved spectroscopic and computational chemistry approaches to elucidate structure-property relationships which determine the photoreactivity for ruthenium(II) tris-diimine complexes. We have probed the contribution from steric strain, as well as the electronic effect of replacement of pyridine donors for triazole, on the deactivation of the 3 MLCT state and on the promotion of photochemical ligand release.

Introduction of a single methyl substituent adjacent to the coordinating N-atom of a bpy-based ligand results in steric strain which dramatically stabilises 3 MCtrans states. Whilst 3 MCtrans states in these complexes promotes rapid deactivation of their [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MLCT states, they counterintuitively do not promote significant enhancement of photochemical reactivity, but instead favour ground state recovery.

What seems evident is that for non-strained complexes with a small 3 MLCT- Where the complex is severely strained in dmbpy complexes, for example, where the departing ligand contains two sterically encumbering methyl substituents, the population of [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MCcis states is inhibited but photochemistry may proceed via alternative pathways involving coordinatively unsaturated pentacoordinate 3 MC states. What is clear is that a thorough understanding of the structure-property relationships that determine the nature of the 3 MC states that are accessible to a given complex is essential for the rational design of efficiently photoreactive complexes for PACT.

Improved understanding of these structure-property relationships may facilitate identification of potential departing ligands for the design of high potency and efficiently photoreactive PACT complexes without incorporating steric strain, and where the departing ligand itself is a pharmacologically active species.

The excited state landscape navigated by ruthenium(II) complexes during photochemical ligand substitution is clearly complex, with the route taken and thus the mechanistic pathway that predominates dependent on the nature of the departing, as well as incoming ligands. Further, whilst this work provides illuminating insights into the photochemistry of Ru(II) complexes, computational studies are based on static DFT calculations. Future studies will be required to refine our understanding of these systems which will require the dynamic quantum mechanical calculations to observe how the critical 3 MC states identified evolve. [START_REF] Dixon | Photoinduced Ligand Exchange Dynamics of a Polypyridyl Ruthenium Complex in Aqueous Solution[END_REF] We are currently in the planning stages for such work.

Experimental Section

General methods. Known complexes were prepared by literature methods. or an electronic fan (NMR samples). The determination of photochemical quantum yields was performed for MeCN solutions of known concentration (2.5 ml volume, 10 mm pathlength cuvette) under irradiation with the aforementioned blue LED excitation source, the photon flux density of which was determined to be 2.01 x 10 -5 einstein s -1 dm -3 through use of an K3Fe(C2O4)3.3H2O chemical actinometer. Photorelease quantum yield calculations were performed using GNU

Figure 2 .

 2 Figure 2. Structures of the complexes [Ru(bpy)2(N^N)](PF6)2 1 to 6 (N^N = bpy (1); mbpy (2)[START_REF] Hammarström | Structure and photophysical properties of novel ruthenium(II) complexes containing 6-substituted bipyridines[END_REF] ; dmbpy (3)[START_REF] Howerton | Strained Ruthenium Complexes Are Potent Light-Activated Anticancer Agents[END_REF] ; pytz (4)[START_REF] Lo | Comparison of Inverse and Regular 2-Pyridyl-1,2,3-triazole "Click" Complexes: Structures, Stability, Electrochemical, and Photophysical Properties[END_REF] ; mpytz (5); btz (6)[START_REF] Welby | Synthesis, characterisation and theoretical study of ruthenium 4,4′-bi-1,2,3-triazolyl complexes: fundamental switching of the nature of S1 and T1 states from MLCT to MC[END_REF] ).

N( 6 )

 6 -C-C-N(7) of 9.74 °. This compares to the much smaller values of 4.15 and 2.35 ° for the two crystallographically unique cations in the reported structure for 4.

Figure 3 .

 3 Figure 3. Structure of the cation for [Ru(bpy)2(mpytz)](PF6)2 (5, counterions, co-crystallised solvent molecules and hydrogen atoms removed for clarity. Thermal ellipsoids at 50 % probability. CCDC2162566).

Figure 4 .

 4 Figure 4. Cyclic voltammograms recorded for 1.5 mmol dm -3 acetonitrile solutions of 1 to 6 at 100 mV s -1 . Potentials are shown against the Fc + /Fc couple (E½ = +0.39 V vs SCE). The arrow indicates initial scan direction for all complexes.

Figure 5 .

 5 Figure 5. UV-Visible electronic absorption spectra recorded for acetonitrile solutions of 1 to 6. The inset shows detail of charge transfer absorption bands between 350 and 550 nm.

Figure 6 .

 6 Figure 6. Normalised photoluminescence spectra recorded for 1 to 6 at 77K in a 4:1 EtOH/MeOH glass.

Figure 7 .

 7 Figure 7. ps-Transient absorption spectra of 5 in acetonitrile solution overlaid with ground state steady state absorption spectrum (dashed line) (top) and kinetic traces at 370 and 443 nm (below).

  photolysis, we suspect that the ligand scrambling is likely a secondary photochemical process and not indicative of what

Figure 8 .

 8 Figure 8. UV-visible absorption spectra of 5 and 6 recorded during photolysis in acetonitrile solution at room temperature (l ex 446 nm).

Figure 9 .

 9 Figure 9. Definition of the inequivalent 3 MCtrans,A, 3 MCtrans,B and 3 MCtrans,C states for 2 to 5 based on colour coding of pyridine donors and corresponding Ru-N bonds along which respective axial elongation distortions occur (top).Relative energies of optimised[START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MLCT and 3 MC states for 1 to 6 quoted relative to their respective optimised ground states (E = 0.0 eV) (bottom). For the[START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MCcis states for 4 and 6 the ligand that is repelled is the pytz and btz ligand respectively.

For 3

 3 MC states relevant SONOs are plotted in Figures 10 & S21 and structural parameters collated in Table

Figure 11 . 2 -y 2 -

 1122 Figure 11. Classification of hexacoordinate 3 MC states as 3 MCtrans and 3 MCcis based on population of either dz 2 -like or dx 2 -y 2 -like ds* orbitals respectively.

Figure 12 .

 12 Figure 12. Concomitant solvent coordination and formal bpy release for 1 via a pentacoordinate 3 MC state van der Waal's adduct. Geometries of the cations depicted are DFT optimised minima from reference 76 .

Figure 13 .

 13 Figure 13. Structure of [Ru(TAP)3] 2+ and the optimised geometry of its 3 MCcis state (elongated bond lengths in Å and N-Ru-N angle in º).[START_REF] Boota | Photochemistry of Heteroleptic 1,4,5,8-Tetraazaphenanthrene-and Bi-1,2,3-triazolyl-Containing Ruthenium(II) Complexes[END_REF]

Electrochemistry.

  Photochemical ligand release photochemistry. Photolysis experiments were carried out by irradiating the appropriate

Table 2 .

 2 Summarised electrochemical data for complexes 1 to 6. All potentials are referenced against the ferrocene/ferrocenium couple in acetonitrile in the presence of n Bu4NPF6 as an electrolyte.

	Complex	Eox / V	Ered / V
	1	+0.89	-1.73, -1.89, -2.15
	2	+0.90	-1.71, -1.92, -2.16
	3	+0.91	-1.71, -1.92, -2.18
	4	+0.93	-1.74, -1.94
	5	+0.88	-1.79, -2.01
	6	+0.91	-1.80, -2.04
	UV-visible absorption spectra in acetonitrile solutions were recorded for each complex (Figure 5) and summarised
	photophysical data are presented in Table 3. All complexes display sharp and intense bands in the region around 290
	nm due to bpy-based p®p* ligand-centred transitions, with further weaker bands at lower energy assigned to singlet
	metal-to-ligand charge transfer ( 1 MLCT) transitions. In agreement with the electrochemical data, complexes 1 to 3

Table 3 .

 3 Summarised photophysical data for complexes 1 to 6. UV-visible absorption spectra were recorded in acetonitrile at room temperature whilst emission data were collected at 77 K in 4:1 EtOH/MeOH glass matrices.

Complex λ abs /nm (ε / mol -1 dm 3 cm -1 ) λ em / nm (77 K)

	1	288 (78,030)	580
		450 (13,290)	628
	2	288 (77,030)	580
		448 (13,370)	630
	3	289 (64,770)	583
		451 (11,170)	633
	4	285 (64,120)	569
		370 (7,450)	616
		443 (11,020)
	5	286 (58,050)	572
		368 (5,570)	620
		444 (8,390)
	6	285 (51,030)	564
		333 (8,090)	609
		440 (8,010)

Table 4 .

 4 Time constants for evolution of transient absorption spectra for complexes 1 to 6. Time constants assigned to decay of[START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MLCT states are highlighted in bold.

	Complex	t 1 (rise)	t 2	t 3

1 0.19 ± 0.02 ps 9.7 ± 1.3 ps >> 3 ns 2 0.13 ± 0.01 ps 27.9 ± 2.5 ps 2.5 ± 0.7 ns 3 0.16 ± 0.01 ps 7.3 ± 0.4 ps 243 ± 14 ps 4 0.24 ± 0.01 ps 15.7 ± 1.0 ps 7.3 ± 3.2 ns 5 0.12 ± 0.01 ps 14.8 ± 0.4 ps 233 ± 6 ps 6 0.21 ± 0.02 ps 18.4 ± 6.2 ps 443 ± 17 ps

Table 5 .

 5 Quantum yields for photochemical release of N^N from [Ru(bpy)2(N^N)] 2+ in acetonitrile.

	Complex	Ligand	Φ / %
	1	bpy	<0.01
	2	mbpy	0.02
	3	dmbpy	8.2
	4	pytz	0.3
	5	mpytz	0.3
	6	btz	2.0

  [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MC gap, the accessibility of[START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MCcis states leads to far greater propensity for photochemical reactivity. A photoreactive excited state mechanism in which solvent coordination and formal ligand release occurs at the 3 MCcis state at the same time negates the need for commonly invoked k 1 -N^N ligand-loss ground state intermediate complexes. Whilst more rarely observed they are nonetheless observed in some cases as discussed above and so one-step/no intermediate and two-step/with intermediate mechanisms may compete.

  [START_REF] Howerton | Strained Ruthenium Complexes Are Potent Light-Activated Anticancer Agents[END_REF][START_REF] Welby | Synthesis, characterisation and theoretical study of ruthenium 4,4′-bi-1,2,3-triazolyl complexes: fundamental switching of the nature of S1 and T1 states from MLCT to MC[END_REF][START_REF] Hammarström | Structure and photophysical properties of novel ruthenium(II) complexes containing 6-substituted bipyridines[END_REF][START_REF] Lo | Comparison of Inverse and Regular 2-Pyridyl-1,2,3-triazole "Click" Complexes: Structures, Stability, Electrochemical, and Photophysical Properties[END_REF] NMR spectra were recorded on a Bruker Ascend 400 MHz spectrometer, with all chemical shifts being reported in ppm and referenced relative to the residual solvents signal (CHCl3, 1 H: δ 7.26, 13 C δ 77.16; MeCN 1 H: δ 1.94,[START_REF] Damrauer | Femtosecond Dynamics of Excited-State Evolution in [Ru(bpy)3]2+[END_REF] C δ 1.32, 118.26). Mass spectra were recorded at high resolution on an Agilent 6210 TOF instrument with a dual ESI source or on a Bruker Q-ToF mass spectrometer. UV-Visible absorption spectra were recorded on an Agilent Cary-60 spectrophotometer utilising quartz cuvettes of 10 mm pathlength. Photoluminescence spectra were recorded on a Horiba Fluoromax-4 spectrophotometer at 77 K in a 4:1 EtOH/MeOH glassing mixture.

Acknowledgements

The authors thank the University of Huddersfield for supporting this work as well as the 3M Buckley Innovation Centre, University of Huddersfield, for computational resources utilised in this work. Computational work performed in

Octave software (version 6.2.0), freely available at https://www.gnu.org/software/octave/, using the method of Slep and co-workers. [START_REF] Marcolongo | A chemometric approach for determining the reaction quantum yields in consecutive photochemical processes[END_REF] Transient absorption spectroscopy. Spectra were recorded using a broadband ultra-fast pump-probe transient absorption spectrometer 'Helios' (Ultrafast Systems LLC), collecting data over a 3 ns time window with a time resolution of approximately 250 fs. A Ti:Sapphire amplifier system (Newport Spectra Physics, Solstice Ace) producing 800 nm pulses at 1 kHz with 100 fs pulse duration was used to generate the probe beam and to also pump a TOPAS Prime OPA with associated NIR-UV-Vis unit to generate the excitation beam. The probe beam consisted of a white light continuum generated in a CaF2 crystal. Absorbance changes were monitored between 330-650 nm. Samples were excited with 0.5 µJ pulses at 285 nm, contained within a 0.2 cm pathlength quartz cuvette that was magnetically stirred during the measurements. Before data analysis, pre-excitation data was subtracted, and spectral chirp corrected for. Kinetics were analysed at the wavelengths of the highest intensity transient and bleach features. These traces were fitted with multiexponential functions with shared lifetime parameters.

Single crystal X-Ray diffraction. X-ray diffraction data for 5 were collected at 150 K on a Bruker D8 Venture diffractometer equipped with a graphite monochromated Mo(Kα) radiation source and a cold stream of N2 gas. Solutions were generated by conventional Patterson heavy atom or direct methods and refined by full-matrix least-squared on F 2 data, using SHELXS-97 and SHELXL software respectively. 92 Absorption corrections were applied based upon multiple and symmetry-equivalent measurements using SADABS. [START_REF] Sheldrick | SADABS: A Program for Absorption Correction with the Siemens SMART System[END_REF] One of the hexafluorophosphate counter ions displayed some rotational disorder and this was refined over two positions using the PART instruction in the l.s. refinement with the disordered fluorine atoms restrained using the SIMU and DELU instructions. Crystallographic data is available as supplementary information or can be downloaded from the Cambridge Crystallographic Datacentre. values were 0.1099 (I > 2σ(I)). The final R1 values were 0.0765 (all data). The final wR(F 2 ) = 0.1193 (all data). The goodness of fit on F 2 was 1.047. Largest peak and hole (e Å -3 ) 1.462 / -0.699.

Computational details.

The geometries of the ground states of complexes 1 to 6 were optimised using density functional theory using the B3LYP hybrid functional [START_REF] Becke | A new mixing of Hartree-Fock and local density-functional theories[END_REF][START_REF] Lee | Development of the Colle-Salvetti correlation-energy formula into a functional of the electron density[END_REF] as implemented in the Orca 4.2.1 software package. [START_REF] Neese | The ORCA program system[END_REF][START_REF] Neese | Software update: the ORCA program system, version 4.0[END_REF] Def2-ECP effective core potential and def2/j auxiliary basis set were used for ruthenium with def2-tzvp(-f) basis sets used for all other atoms. [START_REF] Weigend | Balanced basis sets of split valence, triple zeta valence and quadruple zeta valence quality for H to Rn: Design and assessment of accuracy[END_REF] All calculations were conducted using Grimme's D3-BJ dispersion correction [START_REF] Grimme | A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT-D) for the 94 elements H-Pu[END_REF][100] along with the SMD implicit solvation model (acetonitrile). 101 In these DFT calculations the resolution-of-identity (RI) approximation for hybrid functionals (as implemented in ORCA) was employed to calculate the Coulomb energy term using the Ahlrichs/Weigend Def2-TZV basis as the auxiliary basis set and the exchange term by the so-called 'chain-of-spheres exchange' (COSX) algorithm. For complexes 4 to 6 the benzyl substituents of the triazole rings were replaced by methyl groups as these will have little impact on the photophysical properties and also saves on computational expense. The 3 MLCT states of the complexes were optimised by unrestricted DFT starting from the ground state geometries whereas [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MCtrans and [START_REF] White | An overview of photosubstitution reactions of Ru(II) imine complexes and their application in photobiology and photodynamic therapy[END_REF] MCcis states were optimised from initial guess geometries whose key bond lengths and angles were informed by previous data on related complexes. [START_REF] Dixon | Theoretical illumination of highly original photoreactive 3MC states and the mechanism of the photochemistry of Ru(ii) tris(bidentate) complexes[END_REF][START_REF] Soupart | Exploration of Uncharted 3PES Territory for [Ru(bpy)3]2+: A New 3MC Minimum Prone to Ligand Loss Photochemistry[END_REF] Molecular orbitals were visualised using the Gabedit software package with isosurfaces set to 0.02.

Synthesis of 4-(2-methylpyrid-6-yl)-1,2,3-triazole (mpytz)

2-Methyl-6-(trimethylsilylethynyl)pyridine 102-103 (466 mg, 2.46 mmol), benzyl azide (362 mg, 2.72 mmol, 1.1 equiv.), sodium ascorbate (244 mg, 1.23 mmol, 0.5 equiv.), CuSO4.5H2O (154 mg, 0.62 mmol, 0.25 equiv.) and K2CO3 (467 mg, 3.38 mmol, 1.3 equiv.) were added to a solvent mixture consisting of H2O (25 ml), THF (25 ml), t BuOH (25 ml) and pyridine (5 ml). The reaction mixture was stirred at room temperature for 12 h. CH2Cl2 (75 ml) followed by conc. aq. NH3 (10 ml) were added and the mixture stirred vigorously at room temperature for a further 30 mins. The organic phase was separated and the remaining aqueous phase extracted with a further 100 ml portion of CH2Cl2. The combined organic layers were washed successively with dilute aq. NH3 (3 x 100 ml), H2O (100 ml) and sat. brine (2 x 200 ml).

The organic phase was dried over MgSO4, filtered and the solvent removed in vacuo. The crude product was purified by column chromatography (SiO2, 1% MeOH / CH2Cl2) affording the product as a white solid. Yield = 521 mg, 85 %

Synthesis of [Ru(bpy)2(mbpy)](PF6)2 (2).

[Ru(bpy)2Cl2] (320 mg 0.66 mmol) was dissolved in ethanol (30 ml) and combined with mbpy (110 mg 0.66 mmol, 1 equiv.). The solution was heated to 80 °C overnight under an N2 atmosphere in the dark. The solution was then cooled to room temperature and an excess of NH4PF6 (323 mg 1.98 mmol) along with ethanol (30 ml) was added. The resultant red precipitate was collected by filtration, recrystallised from MeCN/Et2O and purified further via column chromatography (SiO2, 10:1:1 (v/v/v) MeCN:H2O:KNO3 (aq.)). Subsequent counter-ion metathesis yielded the bright orange/red coloured product as its hexafluorophosphate salt. Yield = 0.185 g, 32%. [START_REF] Havrylyuk | Ru(II) photocages enable precise control over enzyme activity with red light[END_REF] [START_REF] Damrauer | Femtosecond Dynamics of Excited-State Evolution in [Ru(bpy)3]2+[END_REF] C NMR (101 MHz, CD3CN): δ 26. 40, 122.83, 125.21, 125.35, 125.39, 125.47, 125.60, 127.98, 128.37, 128.39, 128.58, 128.83, 129.44, 138.45, 138.58, 138.71, 138.74, 138.98, 139.16, 152.10, 152.26, 152.35, 152.61, 154.05, 157.61, 157.98, 158.11, 158.21, 158.26, 159.23, 166.23

Synthesis of [Ru(bpy)2(mpytz)](PF6)2 (5).

Ru(bpy)2Cl2 (204 mg, 0.42 mmol) and picTz (120 mg, 0.48 mmol, 1.1 equiv.) were added to ethylene glycol (6 ml) and heated to 150 o C for 17 h in the dark under an N2 atmosphere. The resulting red/orange coloured mixture was cooled to r.t. before the addition of an aqueous solution of NH4PF6 (703 mg, 4.31 mmol, 25 ml) which resulted in formation of a bright orange coloured precipitate. The mixture was stirred at room temperature for a further 10 mins and the solids collected by filtration, being washed with H2O (30 ml) followed by Et2O (30 ml). The solids were redissolved in the minimum volume of MeCN, filtered and the product reprecipitated through the addition of excess Et2O. The precipitate was collected by filtration, washed with Et2O and dried in vacuo, affording the title complex as a bright red/orange coloured powder. NMR data indicate the presence of the photolysis product of the complex as a very minor contaminant. Yield = 376 mg, 94 %. [START_REF] Havrylyuk | Ru(II) photocages enable precise control over enzyme activity with red light[END_REF]

Conflicts of Interest

There are no conflicts of interest to declare.

Supporting Information

NMR and UV-vis absorption spectra recorded during photolysis of complexes, transient absorption spectra and data from DFT calculations. Optimised geometries for ground, MLCT and MC states (xyz format).

Author Information

Paul Elliott -ORCID 0000-0003-1570-3289; p.i.elliott@hud.ac.uk Paul Scattergood -ORCID 0000-0001-9070-5933; P.Scattergood@hud.

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