Prof Heinze Katja

Prof Oliver Wenger

Adrien Soupart

Fabienne Alary

Jean-Louis Heully

Paul I P Elliott

Isabelle M Dixon

Light-Controlled Reactivity of Metal Complexes

A complete mechanistic picture for the photochemical release of bpy from the archetypal complex [Ru(bpy) 3 ] 2+ is presented for the first time following the description of the ground and lowest triplet potential energy surfaces, as well as their key crossing points, involved in successive elementary steps along pathways towards cis-and trans-[Ru(bpy) 2 (NCMe) 2 ] 2+ . This work accounts for two main pathways that are identified involving a) two successive photochemical reactions for photodechelation followed by photorelease of a monodentate bpy ligand, and b) a novel onephoton mechanism in which initial photoexcitation is followed by dechelation, solvent coordination and bpy release processes, all of which occur sequentially within the triplet excited state manifold before final relaxation to the singlet state and formation of final photoproducts. For the reaction between photoexcited [Ru(bpy) 3 ] 2+ and acetonitrile, which is taken as a model reaction, pathways towards cis and trans photoproducts are uphill processes, in line with the comparative inertness of the complex in this solvent. Factors involving the nature of the departing ligand and retained 'spectator' ligands are considered, and their role in selecting mechanistic pathways involving overall two sequential photon absorptions versus one photon absorption for the formation of both cis or trans photoproducts are discussed in relation to notable examples from the literature. This study ultimately provides a generalised road map of accessible photoproductive pathways for light-induced reactivity mechanisms of photolabile [Ru(N^N)(N^N')(N^N")] 2+ -type complexes.

Introduction

Ruthenium coordination compounds have a long and remarkable history in inorganic photochemistry. Since the early 1970s, fundamental knowledge has been gained on their photophysical properties, with particularly monumental contributions from the USA (e.g. from the groups of Adamson, 1,2 Crosby, [START_REF] Crosby | Spectroscopic Characterization of Complexes of Ruthenium(II) and Iridium(III) with 4,4'-Diphenyl-2,2'-Bipyridine and 4,7-Diphenyl-1,10-Phenanthroline[END_REF][START_REF] Demas | Quantum Efficiencies on Transition Metal Complexes. II. Charge-Transfer Luminescence[END_REF][START_REF] Harrigan | Evidence for Multiple-State Emission from Ruthenium(II) Complexes[END_REF][START_REF] Hager | Charge-Transfer Excited States of Ruthenium(II) Complexes. I. Quantum Yield and Decay Measurements[END_REF][START_REF] Crosby | Spectroscopic Investigations of Excited States of Transition-Metal Complexes[END_REF] Demas, 8,9,10 Endicott 11, [START_REF] Natarajan | Direct Observation of the Dibromide Radical Anion Oxidation of Tris(Bipyridyl)Ruthenium(II). Evidence for a Triplet-to-Triplet Energy Transfer Mechanism in the Photosensitized Redox Decomposition of Cobalt(III) Substrates[END_REF] Wrighton, [START_REF] Wrighton | Quenching of the Luminescent State of Tris(2,2'-Bipyridine)Ruthenium(II) by Electronic Energy Transfer[END_REF] Meyer, [START_REF] Bock | Electron Transfer Quenching of the Luminescent Excited State of Tris(2,2'-Bipyridine)Ruthenium(II). Flash Photolysis Relaxation Technique for Measuring the Rates of Very Rapid Electron Transfer Reactions[END_REF][START_REF] Young | Electron Transfer Quenching of Excited States of Metal Complexes[END_REF] Watts, [START_REF] Van Houten | Effect of Ligand and Solvent Deuteration on the Excited State Properties of the Tris(2,2'-Bipyridyl)Ruthenium(II) Ion in Aqueous Solution. Evidence for Electron Transfer to Solvent[END_REF] or Sutin, [START_REF] Navon | Mechanism of the Quenching of the Phosphorescence of Tris(2,2'-Bipyridine)Ruthenium(II) by Some Cobalt(III) and Ruthenium(III) Complexes[END_REF][START_REF] Lin | Mechanism of the Quenching of the Emission of Substituted Polypyridineruthenium(II) Complexes by Iron(III), Chromium(III), and Europium(III) Ions[END_REF][START_REF] Creutz | Electron-Transfer Reactions of Excited States. Reductive Quenching of the Tris(2,2'-Bipyridine)Ruthenium(II) Luminescence[END_REF][START_REF] Creutz | Electron-Transfer Reactions of Excited States: Direct Evidence for Reduction of the Charge-Transfer Excited State of Tris(2,2'-Bipyridine)Ruthenium(II)[END_REF] ) and Italy. [START_REF] Laurence | Reduction by the Triplet Charge-Transfer State of Tris(Bipyridyl)Ruthenium(II). Photochemical Reaction between Tris(Bipyridyl) Ruthenium(II) and Thallium(III)[END_REF][START_REF] Bolletta | Bimolecular Electron Transfer Reactions of Electronically Excited States of Coordination Compounds[END_REF][START_REF] Balzani | Quenching and Sensitization Processes of Coordination Compounds[END_REF][START_REF] Juris | Electron and Energy Transfer Mechanisms in the Quenching of the Tris(2,2'-Bipyridine)Ruthenium(II) Luminescence by Cyanide Complexes[END_REF] Since then, thousands of derivatives have found numerous applications including in water splitting, [START_REF] Meyer | Mechanisms of Molecular Water Oxidation in Solution and on Oxide Surfaces[END_REF][START_REF] Garrido-Barros | How to Make an Efficient and Robust Molecular Catalyst for Water Oxidation[END_REF][START_REF] Matheu | Seven Coordinated Molecular Ruthenium-Water Oxidation Catalysts: A Coordination Chemistry Journey[END_REF] dye-sensitised solar cells, [START_REF] Pashaei | Influence of Ancillary Ligands in Dye-Sensitized Solar Cells[END_REF][START_REF] Aghazada | Ruthenium Complexes as Sensitizers in Dye-Sensitized Solar Cells[END_REF] as sensitisers for photocatalysis, [START_REF] Prier | Visible Light Photoredox Catalysis with Transition Metal Complexes: Applications in Organic Synthesis[END_REF][START_REF] Buzzetti | Mechanistic Studies in Photocatalysis[END_REF] or as sensors or probes for biology. [START_REF] Shum | Luminescent Ruthenium(II) Polypyridine Complexes for a Wide Variety of Biomolecular and Cellular Applications[END_REF] What makes these compounds especially powerful is the tremendous tuning of their properties resulting from chemists' endless creativity. [START_REF] Balzani | Photophysical Properties of Ruthenium(II) Polypyridine Complexes[END_REF][START_REF] Juris | II) Polypyridine Complexes : Photophysics, Photochemistry, Electrochemistry, and Chemiluminescence[END_REF][START_REF] Lever | Electrochemical Parametrization of Metal Complex Redox Potentials, Using the Ruthenium(III)/Ruthenium(II) Couple to Generate a Ligand Electrochemical Series[END_REF][START_REF] Roundhill | Photochemistry and Photophysics of Metal Complexes[END_REF][START_REF] Campagna | Photochemistry and Photophysics of Coordination Compounds: Ruthenium[END_REF] Because photoinstability issues are deleterious to photophysical applications, [START_REF] Limburg | Rate and Stability of Photocatalytic Water Oxidation Using [Ru(Bpy) 3 ] 2+ as Photosensitizer[END_REF][START_REF] Chakraborty | Photochemical Hydrogen Evolution from Neutral Water with a Cobalt Metallopeptide Catalyst[END_REF] the same period has also witnessed thorough photoreactivity studies with a variety of starting compounds and incoming ligands (e.g. anions, coordinating solvents). [START_REF] Ford | Photochemical Reaction Pathways of Pentaammineruthenium(II) Complexes[END_REF][START_REF] Ford | Quantum Yields for the Photoaquation Reactions of [Ru(NH 3 ) 5 (Py)] 2+ and Related Complexes: An Unusual Dependence on Acid Concentration[END_REF][START_REF] Chaisson | Photochemical Reaction Pathways of Ruthenium(II) Complexes. III. Metal-to-Ligand Charge-Transfer Excitation of the Pentaamminepyridineruthenium(II) Cation and Related Species[END_REF][START_REF] Malouf | Photochemical Reaction Pathways of Ruthenium(II) Complexes. Evidence Regarding the Reactive Excited State(s) from Metal-to-Ligand Charge Transfer Excitation of Ru(NH 3 ) 5 2+ and Related Complexes[END_REF][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] Van Houten | Photochemistry of Tris(2,2'-Bipyridyl)Ruthenium(II) in Aqueous Solution[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] Synthetic applications of photosubstitution reactions followed shortly afterwards [START_REF] Wallace | A Simple Photochemical Synthesis of Some Bis(Bipyridyl)Ruthenium(II) Complexes[END_REF][START_REF] Durham | Synthetic Applications of Photosubstitution Reactions of Poly(Pyridyl) Complexes of Ruthenium(II)[END_REF][START_REF] Durham | Photochemistry of Ru(Bpy) 3 2+[END_REF][START_REF] Pinnick | Photosubstitution Reactions of Ru(Bpy) 2 XY n+ Complexes[END_REF][START_REF] Allen | Substitution Reactions of Ruthenium(II) Complexes Containing 2,2'-Bipyridine and 1,10-Phenanthroline[END_REF][START_REF] Suen | Photosubstitution Reactions of Terpyridine Complexes of Ruthenium(II)[END_REF][START_REF] Hecker | Evidence for Dissociative Photosubstitution Reactions of [Ru(Trpy)(Bpy)(NCCH 3 )] 2+ . Crystal and Molecular Structure of [Ru(Trpy)(Bpy)(Py)](PF 6 ) 2 .(CH 3 ) 2 CO[END_REF] and, more recently, applications in phototriggered molecular machines [START_REF] Bonnet | Ruthenium-Based Light-Driven Molecular Machine Prototypes: Synthesis and Properties[END_REF][START_REF] Colasson | Light-Driven Molecular Machines Based on Ruthenium(II) Polypyridine Complexes: Strategies and Recent Advances[END_REF] or the photorelease of biologically active molecules. [START_REF] Ford | Metal Complex Strategies for Photo-Uncaging the Small Molecule Bioregulators Nitric Oxide and Carbon Monoxide[END_REF][START_REF] White | An Overview of Photosubstitution Reactions of Ru(II) Imine Complexes and Their Application in Photobiology and Photodynamic Therapy. Inorganica[END_REF][START_REF] Li | II) Polypyridyl Complexes as Photocages for Bioactive Compounds Containing Nitriles and Aromatic Heterocycles[END_REF][START_REF] Li | II) Polypyridyl Complexes Derived from Tetradentate Ancillary Ligands for Effective Photocaging[END_REF][START_REF] Bonnet | Why Develop Photoactivated Chemotherapy? Dalton Trans[END_REF][START_REF] Shum | Luminescent Ruthenium(II) Polypyridine Complexes for a Wide Variety of Biomolecular and Cellular Applications[END_REF][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] Toupin | Dual-Action Ru(II) Complexes with Bulky π-Expansive Ligands: Phototoxicity without DNA Intercalation[END_REF][START_REF] Loftus | Photoinduced Ligand Dissociation Follows Reverse Energy Gap Law: Nitrile Photodissociation from Low Energy 3 MLCT Excited States[END_REF] As a continuation to Adamson's formulation of empirical rules regarding relative efficiency and selectivity of photoreactions, [START_REF] Adamson | Photochemistry of Complex Ions. IV. Role of Quartet Excited States in the Photochemistry of Chromium(III) Complexes[END_REF] several theoretical studies were published in the 1970s with the aim of rationalizing their outcome. Orbital and symmetry considerations have brought up enlightening mechanistic information, initially around Cr(III) and Co(III) photochemistry [START_REF] Zink | Model for Predicting the Photoreactions and Relative Quantum Yields of Transition Metal and Organometallic Complexes. I. Chromium(III)[END_REF][START_REF] Zink | Model for Predicting the Photoreactions and Relative Quantum Yields of Transition Metal and Organometallic Complexes. II. Cobalt(III) and Rhodium(III)[END_REF][START_REF] Zink | Relative Ligand Labilization in Transition Metal Photochemistry. Molecular Orbital Approach[END_REF] and which was then generalized. [START_REF] Wrighton | A Model for the Substitutional Reactivity of Ligand Field Excited States[END_REF][START_REF] Vanquickenborne | On the Photochemical Substitution Reactions of Hexacoordinated Transition Metal Complexes[END_REF][START_REF] Vanquickenborne | Photostereochemistry and Electronic Selection Rules in Strong-Field d 6 Transition-Metal Complexes[END_REF][START_REF] Vanquickenborne | Photolabilization and Bond Indices in Hexacoordinated Transition-Metal Complexes: The D 2h Case[END_REF] More recently, the parallel development of advanced quantum and ultrafast time-resolved spectroscopic methods has enabled an in-depth study of microscopic ligand photorelease mechanisms. [START_REF] Daniel | Chemical Bonding Alteration upon Electronic Excitation in Transition Metal Complexes[END_REF][START_REF] Chergui | Photoinduced Structural Dynamics of Molecular Systems Mapped by Time-Resolved X-Ray Methods[END_REF] Returning to ruthenium, the source of the fascinating properties of this family of complexes, especially its polypyridine derivatives, is found in their triplet excited states, particularly in the metal-to-ligand charge transfer ( 3 MLCT) state, and its rival brother, the ligand-field metalcentered ( 3 MC) state. Both excited states are antagonistic in the sense that the former is capable of photoluminescence and photoinduced electron or energy transfer, [START_REF] Thompson | Ru(Bpy) 3 ] 2+ * and Other Remarkable Metal-to-Ligand Charge Transfer (MLCT) Excited States[END_REF] whereas the latter undermines these properties and is involved in nonradiative deactivation and photoinstability issues, [START_REF] Chaisson | Photochemical Reaction Pathways of Ruthenium(II) Complexes. III. Metal-to-Ligand Charge-Transfer Excitation of the Pentaamminepyridineruthenium(II) Cation and Related Species[END_REF] as beautifully demonstrated for other metal complexes. [START_REF] Adamson | Photochemistry of Transition-Metal Coordination Compounds[END_REF][START_REF] Endicott | Structure and Reactivity of the Metal-Centered Transition Metal Excited States[END_REF] Tuning the relative energies of the [START_REF] Crosby | Spectroscopic Characterization of Complexes of Ruthenium(II) and Iridium(III) with 4,4'-Diphenyl-2,2'-Bipyridine and 4,7-Diphenyl-1,10-Phenanthroline[END_REF] MLCT and [START_REF] Crosby | Spectroscopic Characterization of Complexes of Ruthenium(II) and Iridium(III) with 4,4'-Diphenyl-2,2'-Bipyridine and 4,7-Diphenyl-1,10-Phenanthroline[END_REF] MC states is thus of utmost importance in driving the properties of complexes towards the one or other desired regime. [START_REF] Caspar | Photochemistry of MLCT Excited States. Effect of Nonchromophoric Ligand Variations on Photophysical Properties in the Series Cis-Ru(Bpy) 2 L 2 2+[END_REF] Following the first optimisation of a [START_REF] Crosby | Spectroscopic Characterization of Complexes of Ruthenium(II) and Iridium(III) with 4,4'-Diphenyl-2,2'-Bipyridine and 4,7-Diphenyl-1,10-Phenanthroline[END_REF] MC state in a ruthenium complex (a hexacoordinate [Ru(bpy) 3 ] 2+ excited state exhibiting two elongated and mutually trans Ru-N bonds that we term a 'classical' [START_REF] Crosby | Spectroscopic Characterization of Complexes of Ruthenium(II) and Iridium(III) with 4,4'-Diphenyl-2,2'-Bipyridine and 4,7-Diphenyl-1,10-Phenanthroline[END_REF] MC state), [START_REF] Alary | Is the 3 MLCT the Only Photoreactive State of Polypyridyl Complexes?[END_REF] several theoretical mechanistic studies focused on the photoinduced release of a monodentate ligand. [START_REF] Salassa | Mechanism of Ligand Photodissociation in Photoactivable [Ru(Bpy) 2 L 2 ] 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] Salassa | Photo-Induced Pyridine Substitution in Cis-[Ru(Bpy) 2 (Py) 2 ]Cl 2 : A Snapshot by Time-Resolved X-Ray Solution Scattering[END_REF][START_REF] Camilo | Photosolvolysis of Cis-[Ru(α-Diimine) 2 (4-Aminopyridine) 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 [Ru II (Tpy)(Bpy)(Py)] 2+ and [Ru II (Tpy)(Me 2 Bpy)(Py)] 2+ Complexes[END_REF] Our group initially became interested in photorelease mechanisms involving the loss of a thioether monodentate ligand in collaboration with Bonnet's group. [START_REF] Bahreman | Spontaneous Formation in the Dark, and Visible Light-Induced Cleavage, of a Ru-S Bond in Water: A Thermodynamic and Kinetic Study[END_REF] We hypothesised the involvement of [START_REF] Crosby | Spectroscopic Characterization of Complexes of Ruthenium(II) and Iridium(III) with 4,4'-Diphenyl-2,2'-Bipyridine and 4,7-Diphenyl-1,10-Phenanthroline[END_REF] MC states distinct from those we originally described: these new states display quasi pentacoordinated geometries and are local minima on the lowest triplet potential energy surface ( 3 PES). We proposed that triplet/singlet surface crossings in the region of the local [START_REF] Crosby | Spectroscopic Characterization of Complexes of Ruthenium(II) and Iridium(III) with 4,4'-Diphenyl-2,2'-Bipyridine and 4,7-Diphenyl-1,10-Phenanthroline[END_REF] MC minima were mechanistically determining, crossing with the reactant [START_REF] Adamson | A New Photosensitizer. Tris(2,2'-Bipyridine)Ruthenium(II) Chloride[END_REF] PES for the classical [START_REF] Crosby | Spectroscopic Characterization of Complexes of Ruthenium(II) and Iridium(III) with 4,4'-Diphenyl-2,2'-Bipyridine and 4,7-Diphenyl-1,10-Phenanthroline[END_REF] MC state vs crossing with the product [START_REF] Adamson | A New Photosensitizer. Tris(2,2'-Bipyridine)Ruthenium(II) Chloride[END_REF] PES for the quasi pentacoordinated [START_REF] Crosby | Spectroscopic Characterization of Complexes of Ruthenium(II) and Iridium(III) with 4,4'-Diphenyl-2,2'-Bipyridine and 4,7-Diphenyl-1,10-Phenanthroline[END_REF] MC state. [START_REF] Göttle | Pivotal Role of a Pentacoordinate 3 MC State on the Photocleavage Efficiency of a Thioether Ligand in Ruthenium(II) Complexes: A Theoretical Mechanistic Study[END_REF] From this viewpoint, the ease of population of this latter [START_REF] Crosby | Spectroscopic Characterization of Complexes of Ruthenium(II) and Iridium(III) with 4,4'-Diphenyl-2,2'-Bipyridine and 4,7-Diphenyl-1,10-Phenanthroline[END_REF] MC state can be correlated to the experimental photosubstitution quantum yield.

Bidentate ligand photorelease has been explored in recent years in view of biological applications. [START_REF] Howerton | Strained Ruthenium Complexes Are Potent Light-Activated Anticancer Agents[END_REF][START_REF] Wachter | Mechanistic Study on the Photochemical "Light Switch" Behavior of [Ru(Bpy) 2 Dmdppz] 2+[END_REF][START_REF] Havrylyuk | Photochemical Properties and Structure-Activity Relationships of Ru(II) Complexes with Pyridylbenzazole Ligands as Promising Anticancer Agents[END_REF][START_REF] Kohler | Photochemical and Photobiological Activity of Ru(II) Homoleptic and Heteroleptic Complexes Containing Methylated Bipyridyl-Type Ligands[END_REF][START_REF] Cuello-Garibo | To Cage or to Be Caged? The Cytotoxic Species in Ruthenium-Based Photoactivated Chemotherapy Is Not Always the Metal[END_REF][START_REF] Cuello-Garibo | Selective Preparation of a Heteroleptic Cyclometallated Ruthenium Complex Capable of Undergoing Photosubstitution of a Bidentate Ligand[END_REF][START_REF] Meijer | Diastereoselective Synthesis and Two-Step Photocleavage of Ruthenium Polypyridyl Complexes Bearing a Bis(Thioether) Ligand[END_REF][START_REF] Azar | Phototoxicity of Strained Ru(II) Complexes: Is It the Metal Complex or the Dissociating Ligand? Dalton Trans[END_REF][START_REF] Li | Visible Light Driven Bromide Oxidation and Ligand Substitution Photochemistry of a Ru Diimine Complex[END_REF][START_REF] Roque | Photoejecting Ru(II) Complexes That Are Cytotoxic Under Hypoxic Conditions[END_REF] A two-step mechanism was inferred from early spectroscopic observations invoking an intermediate containing a monocoordinated bidentate ligand (Scheme 1). [START_REF] Van Houten | Photochemistry of Tris(2,2'-Bipyridyl)Ruthenium(II) in Aqueous Solution[END_REF][START_REF] Durham | Synthetic Applications of Photosubstitution Reactions of Poly(Pyridyl) Complexes of Ruthenium(II)[END_REF][START_REF] Durham | Photochemistry of Ru(Bpy) 3 2+[END_REF][START_REF] Kazlauskas | Application of Rapid-Scan Fourier Transform Infrared Spectroscopy to Characterize the Monodentate Intermediate in the Photochemical Formation of Tetracarbonyl(4,4'-Dialkyl-2,2'-Bipyridine)Metal from Hexacarbonylmetal[END_REF]100,101,102,103,104 Structural characterisation of such κ 1 intermediate photoproducts has been provided by [START_REF] Adamson | A New Photosensitizer. Tris(2,2'-Bipyridine)Ruthenium(II) Chloride[END_REF] H-NMR spectroscopy 105,106,107,108,109 and X-ray diffraction studies. 110 Note that the reaction coordinate for stepwise bidentate ligand loss is inevitably more complex than the corresponding one for monodentate ligand loss. Building up on the mechanistic hypothesis for-mulated in 2016 (different roles played by distinct [START_REF] Crosby | Spectroscopic Characterization of Complexes of Ruthenium(II) and Iridium(III) with 4,4'-Diphenyl-2,2'-Bipyridine and 4,7-Diphenyl-1,10-Phenanthroline[END_REF] MC states), [START_REF] Göttle | Pivotal Role of a Pentacoordinate 3 MC State on the Photocleavage Efficiency of a Thioether Ligand in Ruthenium(II) Complexes: A Theoretical Mechanistic Study[END_REF] we embarked on the theoretical mechanistic study of photoinduced bidentate ligand loss in collaboration with Elliott's group. 111,105,110 Novel 3 MC states were characterised, displaying either hexacoordinate classical (trans Ru-N elongation) or flattened (involving elongation of two cis Ru-N bonds) hexa-and pentacoordinate geometries. 112 This study reinforced our conviction that surface crossings with reactant or product 1 PES were crucial in determining the outcome of a photochemical reaction.

As a side hypothesis, we also envisaged reaction in the triplet state, bypassing surface crossing to the 1 PES of the κ 1 intermediate photoproduct and directly populating the [START_REF] Crosby | Spectroscopic Characterization of Complexes of Ruthenium(II) and Iridium(III) with 4,4'-Diphenyl-2,2'-Bipyridine and 4,7-Diphenyl-1,10-Phenanthroline[END_REF] MLCT state of the κ 1bound species. The efficiency of this path is certainly moderate as it involves energy barriers around 20 kcal/mol. 113 Finally, the recent identification of a second, flattened [START_REF] Crosby | Spectroscopic Characterization of Complexes of Ruthenium(II) and Iridium(III) with 4,4'-Diphenyl-2,2'-Bipyridine and 4,7-Diphenyl-1,10-Phenanthroline[END_REF] MC state for [Ru(bpy) 3 ] 2+ , that we consider more prone to ligand loss than the classical 3 MC state of the type described above, 114 has prompted us to attempt to generalise the proposed mechanism for bidentate ligand loss. 115 In this work we thus sought to establish a general ligand photorelease mechanistic framework in agreement with all the available experimental data, i.e. stepwise mechanisms through a commonly invoked κ 1 intermediate photoproduct as well as concerted excited state mechanisms directly yielding final cis and trans photoproducts without involving κ 1 ground state intermediate. [START_REF] Hecker | Evidence for Dissociative Photosubstitution Reactions of [Ru(Trpy)(Bpy)(NCCH 3 )] 2+ . Crystal and Molecular Structure of [Ru(Trpy)(Bpy)(Py)](PF 6 ) 2 .(CH 3 ) 2 CO[END_REF] Note that photoisomerisation issues (Δ/Λ 116,117 and cis-trans isomerisations 118,119 ) are outside of the scope of this work. Scheme 1. General mechanistic picture for 2-step bidentate ligand loss and coordination of two solvent molecules (S) onto a metallic complex [M].

Methodology

The reaction investigated in this work involves [Ru(bpy) 3 ] 2+ as reactant despite other Ru(II) complexes being much more photoreactive with their photolysis products formed in much higher quantum yield. 120,121,122,123,124 Whilst [Ru(bpy) 3 ] 2+ is by comparison largely photoinert in acetoni- [START_REF] Durham | Synthetic Applications of Photosubstitution Reactions of Poly(Pyridyl) Complexes of Ruthenium(II)[END_REF] the photosolvolysis of this benchmark archetypal complex can be used as a highly informative model reaction that builds upon the results of our previous work. Concepts and understanding from this work may then be translated to other systems exhibiting greater photoreactive efficiency. Regarding the choice of coordinating solvent, acetonitrile indeed bears several advantages over the commonly encountered water, in that proton transfer, pH and H-bonding issues are avoided. Acetonitrile is a neutral ligand that allows charge conservation throughout the whole process (all complexes are dicationic and all free ligands are neutral). It bears a single coordinating lone pair giving rise to directional coordination, which facilitates the search for new structures. For all these reasons we considered that the approximation of successively reacting two 126,109 for example. The mechanism we are proposing is split into elementary steps, each of these involving well-defined singlet and triplet minima. The topology of the 1 PES and 3 PES is also described through the determination of energy barriers from minimum energy path calculations (NEB, nudged elastic band) complemented by the optimisation of singlet-triplet crossings (MECP, minimum energy crossing points), at the B3LYP 127,128 -D3BJ 129,130 /def2-ECP 131 /def2-TZVP(-f) 132 level of theory. Four successive regions, shown with different background colors in the schemes in this article, are defined as a function of the coordination sphere of the metal : (bpy) 3 ; (bpy) 2 (κ 1 -bpy); (bpy) 2 (κ 1 -bpy)(MeCN); (bpy) 2 (MeCN); (bpy) 2 (MeCN) 2 . The border zones between these regions (diffusion of the released ligand and coordination of the incoming ligand) are explored through NEB calculations, a method which we exploit here as powerful relaxed surface scans that do not require the (user-dependent) numerical definition of the reaction coordinate through geometrical parameters. The optimisation of minima and minimum energy paths has enabled us to construct singlet and triplet potential energy surfaces. Following the description of the key protagonists, we report full mechanisms for the formation of cis and trans-[Ru(bpy) 2 (NCMe) 2 ] 2+ and provide discussion around possible connections between these pathways. The results presented offer fundamental insights, not only for the photochemical properties and processes for [Ru(bpy) 3 ] 2+ under investigation but can be translated to the myriad photoactive complexes known in the literature.

[M] [M] [M] S +S +S - S [M] S trile,
(btz = 1,1'-dibenzyl-4,4'-bi- 1,2,3-triazolyl) 125 or [Ru(bpy) 2 (inv-pytri)] 2+ (inv-pytri = 2-(1H-1,2,3-triazol-1-yl)pyridine),

Results and discussion

In the nomenclature used herein (Table 1), the complexes (singlet ground states and triplet excited states) were named as a function of the ligands in the coordination sphere (A for acetonitrile, B for bipyridine) : B 3 when 3 bpy ligands are bound to the metal, κ 1 -B 3 when one bpy ligand is monodentate (pentacoordinate species), κ 1 -B 3 A 1 when one bpy ligand is monodentate and one NCMe molecule has been coordinated (primary photoproducts), B 2 A 1 following the loss of one bpy ligand (pentacoordinate species), and finally B 2 A 2 in the final photoproducts. In addition, (pro-)cis and (pro-)trans prefixes have been used to specify the relative positions of the κ 1bpy/vacancy or NCMe/vacancy (for pentacoordinate species), or the relative positions of the κ 1bpy/NCMe or NCMe/NCMe sites (for hexacoordinate species). Singlet and triplet states have been optimised, as well as singlet/triplet minimum energy crossing points (MECPs). All energies are given relative to the energy of [Ru(bpy) 3 ] 2+ + 2 MeCN isolated molecules computed in COSMO-SMD acetonitrile. The numbering scheme is shown on Figure 1. along the reaction path. This means that each singlet PES should also have two local minima of general formula κ 1 -B 3 and B 2 A 1 . In addition to B 3 -GS, minima have been located and geometries optimised for the two hexacoordinate species κ 1 -B 3 A 1 -GS and B 2 A 2 -GS and two pentacoordinate species κ 1 -B 3 -GS and B 2 A 1 -GS along the PES for the formation of both the cis and trans products. Finally, upon diffusion of the departing bpy ligand, one may envisage van der Waals adducts, which were also searched for and optimised (B 2 A 1 +B structures). If these structures exist, many isoenergetic conformers can be expected, only one of which is presented here. The van der Waals B 2 A 1 +B adducts are found about 10 kcal/mol higher in energy than their corresponding κ 1 -B 3 A 1 species. The geometries for all these closed shell ground state species are depicted in Fig-

ure 2 with key structural parameters presented in Table 2.

The shortest Ru-N bond is always found trans to the vacancy in pentacoordinate intermediate species (Ru-N3 in cis-κ 1 -B 3 -GS; Ru-N6 in trans-κ 1 -B 3 -GS; Ru-N2 in cis-B 2 A 1 -GS; Ru-NCMe in trans-B 2 A 1 -GS) or trans to the departing bpy in the van der Waals adducts (Ru-N2 in cis-B 2 A 1 -GS+B; Ru-NCMe in trans-B 2 A 1 -GS+B). In the κ 1 -B 3 species, a significant dihedral angle between the two rings of the κ 1 -bound bpy is found (>70°). In each pentacoordinate ground state species, we note the opening of a quadrant that allows the incoming ligand to approach : 165° in cis-κ 1 -B 3 -GS (Fig. S4) ; 153° in trans-κ 1 -B 3 -GS (Fig. S9) ; 175° in cis-B 2 A 1 -GS (Fig. S7) ; 164° in trans-B 2 A 1 -GS (Fig. S12). Structurally speaking, the removal of the κ 1 -bound bpy is accompanied by significant elongation of all Ru-pyridine distances in the van der Waals adducts (Fig. S6 and S11). Classical Ru-N distances are restored (Fig. S7 andS12) in the subsequent pentacoordinate B 2 A 1 -GS minima, which are located a further 25-30 kcal/mol higher in energy. It is noteworthy that some differences persist between the cis and trans GS structures, the trans isomer always displaying shorter Ru-acetonitrile and longer Ru-pyridine distances, in line with the strain caused by the two bipyridine ligands being approximately coplanar with one another.

The topologies of the cis and trans 1 PESs shown in Figure 2 were constructed by connecting optimised minima through minimum energy paths obtained from Nudged Elastic Band calculations (Supporting Information). Several experimental observations can lead us through the discussion.

First of all, [Ru(bpy) 3 ] 2+ is kinetically inert, in line with the fact that the computed energy barrier for thermal dechelation of bpy (B 3 -GS towards κ 1 -B 3 -GS, ca 50 kcal/mol) is too high to be overcome under normal reaction conditions. Formal loss of the monodentate bpy ligand from the intermediate species cis-and trans-κ 1 -B 3 A 1 -GS to form the pentacoordinate species cis-and trans-B 2 A 1 -GS also require surmounting significant energy differences (via their van der Waals adducts) of 35 and 38 kcal/mol respectively. Whilst loss of the bpy ligand clearly requires photochemical initiation, the results suggest that, if the singlet state solvent intermediates κ 1 -B 3 A 1 -GS are formed in the course of the ligand loss process, absorption of a second photon will be required at this mid-point. In agreement with the favoured formation of the cis-B 2 A 2 isomer, all minima on the cis 1 PES are found to be lower in energy than their trans counterparts by 6-12 kcal/mol.

(NCMe) 2 ] 2+ (Ru-N = 2.07-2.08 Å in B 3 -GS). cis-κ 1 -B 3 cis-κ 1 -B 3 A 1 cis-B 2 A 1 +B cis-B 2 A 1 cis-B 2 A 2 Ru-N1 2
- - - - 2.032 angle N5-C-C-N6 70° N2-Ru-N6 165° N5-C-C-N6 83° N5-C-C-N6 51° N3-Ru-N5 175° - trans-κ 1 -B 3 trans-κ 1 -B 3 A 1 trans-B 2 A 1 +B trans-B 2 A 1 trans- B 2 A 2 Ru-N1 2
- - - - 2.019 angle N5-C-C-N6 73° N2-Ru-N3 153° N5-C-C-N6 71° N5-C-C-N6 58° N1-Ru-N4 164° -

Bringing in the triplet PES : a tale of two sequential photochemical reactions

The energy barriers for both the initial bpy dechelation and subsequent formal dissociation are rather high and therefore preclude that either of these steps is thermally mediated. We therefore proceed to examine the photochemical route, bringing into consideration the evolution of the complex within the triplet manifold. In the following sections we examine the photochemical initiation, dechelation and solvation to form the ground state intermediate κ 1 -B 3 A 1 and subsequent photochemical bpy loss and second solvation to form B 2 A 2 for the two sequential photondriven mechanisms for both cis and trans product formation. The sequence of elementary steps connecting relevant excited and ground state species are depicted in Scheme 2 for the cis and trans product formation routes respectively. Table 3 summarises the main geometrical parameters for the triplet states involved along these paths.

Ruthenium polypyridine complexes display strong [START_REF] Adamson | A New Photosensitizer. Tris(2,2'-Bipyridine)Ruthenium(II) Chloride[END_REF] MLCT-based absorption bands in the visible region, a specificity that makes them amenable to application as dyes for solar cells, sensors and probes, or as photosensitisers. 29 , 133 , 134 , 32 Following light absorption that populates several [START_REF] Adamson | A New Photosensitizer. Tris(2,2'-Bipyridine)Ruthenium(II) Chloride[END_REF] MLCT states, ultrafast intersystem crossing occurs to populate 3 MLCT states. According to Kasha's rule, luminescence could occur from the lowest of these, but higher lying [START_REF] Crosby | Spectroscopic Characterization of Complexes of Ruthenium(II) and Iridium(III) with 4,4'-Diphenyl-2,2'-Bipyridine and 4,7-Diphenyl-1,10-Phenanthroline[END_REF] MLCT states could also be populated and could decay to various other triplet states by internal conversion processes. 135 We have previously reported two distinct [START_REF] Crosby | Spectroscopic Characterization of Complexes of Ruthenium(II) and Iridium(III) with 4,4'-Diphenyl-2,2'-Bipyridine and 4,7-Diphenyl-1,10-Phenanthroline[END_REF] MC states for [Ru(bpy) 3 ] 2+ having a different hole and particle pair, which we then termed 3 MC trans and 3 MC cis (cis and trans suffixes, in this context, refer to the positions of the elongated Ru-N bonds: two Ru-N bonds to distinct bpy ligands for 3 MC trans vs. both Ru-N bonds to the same bpy ligand in 3 MC cis ). 114 The singly occupied orbital hosting the particle is especially distinctive, being d z2 -like for [START_REF] Crosby | Spectroscopic Characterization of Complexes of Ruthenium(II) and Iridium(III) with 4,4'-Diphenyl-2,2'-Bipyridine and 4,7-Diphenyl-1,10-Phenanthroline[END_REF] MC trans and d x2-y2 -like for [START_REF] Crosby | Spectroscopic Characterization of Complexes of Ruthenium(II) and Iridium(III) with 4,4'-Diphenyl-2,2'-Bipyridine and 4,7-Diphenyl-1,10-Phenanthroline[END_REF] MC cis . Thus, we can expect the population of these states to occur efficiently from distinct [START_REF] Crosby | Spectroscopic Characterization of Complexes of Ruthenium(II) and Iridium(III) with 4,4'-Diphenyl-2,2'-Bipyridine and 4,7-Diphenyl-1,10-Phenanthroline[END_REF] MLCT states. In this respect, careful inspection of TD-DFT calculations can inform us on the localisation and energy of these states at the ground state geometry. Time dependent DFT at the B 3 -GS geometry (Table S1) shows that the three lowest [START_REF] Crosby | Spectroscopic Characterization of Complexes of Ruthenium(II) and Iridium(III) with 4,4'-Diphenyl-2,2'-Bipyridine and 4,7-Diphenyl-1,10-Phenanthroline[END_REF] MLCT states (T 1 -T 3 , 2.43-2.45 eV) share the same hole as B 3 -3 MC trans , whereas states T 4 -T 9 (2.49-2.71 eV) share the same hole as B 3 -3 MC cis . Given the energetic proximity of these states, one can suppose several of them be involved in the system's relaxation cascade, allowing the population of several 3 MC states from the [START_REF] Crosby | Spectroscopic Characterization of Complexes of Ruthenium(II) and Iridium(III) with 4,4'-Diphenyl-2,2'-Bipyridine and 4,7-Diphenyl-1,10-Phenanthroline[END_REF] MLCT manifold.

The first photochemical event : bpy dechelation

Identification and characterisation of 3 MC states undoubtedly represents the most demanding part of this kind of study. We have previously shown how crucial they are in photolabilisation mechanisms, 88 , 112 where they can be considered as pre-dissociated states. As such, they are worthy of devoting most of the human and computing resources to. Structural distortions due to the occupation of antibonding metal-ligand dσ* orbitals (bond elongations, angular distortions) are such in [START_REF] Crosby | Spectroscopic Characterization of Complexes of Ruthenium(II) and Iridium(III) with 4,4'-Diphenyl-2,2'-Bipyridine and 4,7-Diphenyl-1,10-Phenanthroline[END_REF] MC states that at these geometries the ground state is strongly destabilised, resulting in singlet/triplet crossing regions in the neighbourhood of [START_REF] Crosby | Spectroscopic Characterization of Complexes of Ruthenium(II) and Iridium(III) with 4,4'-Diphenyl-2,2'-Bipyridine and 4,7-Diphenyl-1,10-Phenanthroline[END_REF] MC minima. The optimisation of minimum energy crossing points (MECP) 136 , 137 is therefore systematically performed here and the orbital parentage with the parent [START_REF] Crosby | Spectroscopic Characterization of Complexes of Ruthenium(II) and Iridium(III) with 4,4'-Diphenyl-2,2'-Bipyridine and 4,7-Diphenyl-1,10-Phenanthroline[END_REF] MC minimum is verified.

Starting from the previously described 3 MC trans and 3 MC cis states, 114 the 3 MC trans state becomes B 3 -3 MC trans in the current nomenclature and corresponds to the original 3 MC state characterised by population of a d z 2 -like dσ* orbital. [START_REF] Alary | Is the 3 MLCT the Only Photoreactive State of Polypyridyl Complexes?[END_REF] Given that the bond elongations in this state are towards two different bpy ligands, this state is considered not to be directly involved in the photoinduced ligand loss mechanism. On the other hand, [START_REF] Crosby | Spectroscopic Characterization of Complexes of Ruthenium(II) and Iridium(III) with 4,4'-Diphenyl-2,2'-Bipyridine and 4,7-Diphenyl-1,10-Phenanthroline[END_REF] MC cis , which we had estimated to be more predicated to ligand loss (as it repelled both coordinating atoms of a single bpy ligand) is here termed B 3 -3 MC cis . In this new study we have been able to refine its description further and have been able to optimise two conformers (identical electronic states according to their singly occupied molecular orbitals) which display an open quadrant 138 (which 3 MC trans does not have). One could view these two conformers (Figure 3) as predisposing the complex to coordination by solvent in a manner which could lead onwards either to the cis product (called pro-cis-B 3 -3 MC cis as the open quadrant is adjacent to the κ 1 -bpy ligand; N2-Ru-N6 = 128°) or to the trans product (called pro-trans-B 3 -3 MC cis as the open quadrant lies trans to the κ 1 -bpy ligand; N2-Ru-N3 = 121°). The energy barrier on the minimum energy path from the lowest [START_REF] Crosby | Spectroscopic Characterization of Complexes of Ruthenium(II) and Iridium(III) with 4,4'-Diphenyl-2,2'-Bipyridine and 4,7-Diphenyl-1,10-Phenanthroline[END_REF] MLCT state to either of these states is similar at 10-12 kcal/mol (Fig. S47 andS54). The optimised geometries of each of these conformers leads us to their MECPs (pro-cis-B 3 -MECP cis and pro-trans-B 3 -MECP cis ) that were also found separately from the corresponding κ 1 -B 3 -GS species (cis-κ 1 -B 3 -GS and trans-κ 1 -B 3 -GS). This is therefore a strong indication that the conformation of the B 3 -3 MC cis state is decisive in driving the system towards either the cis or trans-κ 1 -B 3 A 1 primary photoproduct. 139 , 140 We thus connect cis-κ 1 -B 3 -GS with pro-cis-MECP cis , itself connecting with pro-cis-B 3 -3 MC cis and similarly, trans-κ 1 -B 3 -GS with pro-trans-MECP cis and in turn pro-trans-B 3 -3 MC cis . The electrophilicity of the metal in the pentacoordinate singlet κ 1 -B 3 species is indeed seen in the presence of a low-lying vacant metallic orbital exhibiting a large lobe in the vacated coordination site (Fig. S4 andS9). Coordination of acetonitrile to cis/trans-κ 1 -B 3 -GS therefore readily yields the primary photoproducts cis/trans-κ 1 -B 3 A 1 -GS. Interestingly, the N6 atom that is the coordinating N-atom of the κ 1 -bpy ligand migrates from being situated cis to N2 in B 3 -MLCT to being trans to N2 in cis-κ 1 -B 3 -GS, which provides an unhindered coordination site for acetonitrile for the formation of cis-κ 1 -B 3 A 1 -GS.

To go forwards or backwards ? Evolution from the κ 1 -B 3 A 1 species

Having arrived at the primary photoproduct κ 1 -B 3 A 1 -GS, we have the choice to either release NCMe to reform B 3 -GS or move forward towards B 2 A 2 -GS via van der Waals adducts. Although the computed energy barriers are of the same order of magnitude with our model (30-40 kcal/mol), experiments 105 , 110 indicate that the thermal reverse reaction (computed barriers 33 and 31 kcal/mol for the cis and trans paths respectively, Fig. S38 andS43) is favoured over the forward one (computed barriers 37 and 39 kcal/mol for the cis and trans paths respectively, Fig. S40 and S45), which makes sense given the spatial proximity of the binding site provided by the chelating ligand. Ring closure of the metallacycle is then favoured over bpy loss. [START_REF] Durham | Photochemistry of Ru(Bpy) 3 2+[END_REF] However, just as the high energy barriers for bpy ligand dissociation would disfavour these processes occuring thermally, the energy barriers for going backwards toward B 3 -GS are similarly large. We therefore return to a photochemical pathway and evolution of the system on the triplet PES.

Experimental evidence for the existence of intermediate B 3 A 1 photoproducts implies that under continuous irradiation, the corresponding B 3 A 1 -3 MLCT states will become involved. We therefore optimised these states and found cis-κ 1 -B 3 A 1 -MLCT and trans-κ 1 -B 3 A 1 -MLCT to lie 51 and 48 kcal/mol higher in energy than the reference respectively. As with photoexcitation of B 3 -GS, light absorption by cis/trans-κ 1 -B 3 A 1 -GS will populate a number of MLCT states whose internal conversion and relaxation cascade may preferentially lead to population of particular 3 MC states.

We therefore carried out a similar TD-DFT analysis to that conducted for B 3 -GS, at the cis-κ 1 -B 3 A 1 -GS geometry (Table S2) and at the trans-κ 1 -B 3 A 1 -GS geometry (Table S3). It appears that the hole of the cis van der Waals adduct (cis-B 2 A 1 -3 MC+B) is the HOMO-2 of cis-κ 1 -B 3 A 1 -GS,

i.e. it is the same as in T 3 , T 5 or T 6 (2.59-2.78 eV) and as in S 5 , the most absorbing singlet NEB calculations were carried out to determine minimum energy paths for both MeCN loss (towards cis and trans-κ 1 -B 3 ) and bpy loss (towards these van der Waals adducts) from both cis-and trans-κ 1 -B 3 A 1 -MLCT states. Both forward and reverse energy barriers are small in our calculations (cis isomer : forward process 3 kcal/mol (Fig. S48) and backward process 4 kcal/mol (Fig. S50) ; trans isomer : forward process 2 kcal/mol (Fig. S55) and backward process 1.5 kcal/mol (Fig. S57)). The general trend of energy barriers being significantly reduced in the excited state is maintained.

Dissociation of bpy, second solvation and final photoproduct formation

Once the system has reached the van der Waals adducts cis-and trans-B 2 A 1 -3 MC+B, pulling away bpy to form pentacoordinate B 2 A 1 -3 MC states represents a moderate energy penalty of 13-14 kcal/mol (Fig. S49 and S56) (vs 25-30 kcal/mol in the corresponding singlet states, Fig. S39 bottom and S44 bottom). In the case of trans-B 2 A 1 -MC the relative arrangement of the remaining bpy ligands is retained with one bpy ligand significantly tilted up and away from the acetonitrile ligand. The open pro-trans quadrant is consequently much smaller in the resultant trans-B 2 A 1 -3 MC (120°, Fig. S33) than the pro-cis open quadrant for its cis counterpart (174°, Fig. S22) (Table 3). Importantly, the peculiar geometry of trans-B 2 A 1 -3 MC allows for a second open quadrant (141°), which we alluded to earlier for the corresponding van der Waals adduct. Cis and trans [START_REF] Crosby | Spectroscopic Characterization of Complexes of Ruthenium(II) and Iridium(III) with 4,4'-Diphenyl-2,2'-Bipyridine and 4,7-Diphenyl-1,10-Phenanthroline[END_REF] PESs from NEB calculations display similar energy profiles, although once again the two isomers can be expected to have different labilisation abilities, different solvation spheres and ion pairing capacities. 142 Once the cis/trans-B 2 A 1 -3 MC state is reached, intersystem crossing occurs to the singlet pentacoordinate cis/trans-B 2 A 1 -GS electrophilic species (Fig. S7 andS12), which are finally trapped with solvent to form the cis/trans-B 2 A 2 -GS final photoproducts (Fig. S35 and S40) lying 17 (cis) and 29 (trans) kcal/mol higher in energy than the reference.

Mixing between the cis and trans pathways

Up until this point we have, for reasons of simplicity, treated the cis and trans photoproduct formation pathways independently, but several interconnections can be envisaged. Very early in the process, in the B 3 -3 MC region, we note that the two conformers pro-cis-3 MC cis and pro-trans-3 MC cis are nearly degenerate. The whole B 3 -3 MC region is in fact a vast basin displaying a collection of local minima, between which interconversion is practically barrierless. The system can therefore certainly visit all these microstates. As soon as one bpy ligand is half decoordinated, the pentacoordinate κ 1 -B 3 ground state trans species could convert to the cis one for an energy barrier of 13 kcal/mol (Fig. S60). As these electrophilic species are expected to undergo rapid quenching by coordinating solvent, this trans-cis conversion is probably not competitive. Later in the process, the B 2 A 1 pentacoordinate ground state trans species could also convert to the cis one, this time for an energy barrier of 7 kcal/mol (Fig. S61), but again, this will be in competition with quenching by NCMe coordination. The B 2 A 1 -3 MC states, on the other hand, are almost degenerate and should be able to interconvert at almost no cost (barrierless process, calculated ΔE < 3 kcal/mol) (Fig. S62).

Direct solvent coordination to triplet states : possible one-photon processes?

In addition to mechanisms implying the sequential absorption of two photons and going through an intermediate photoproduct ('stepwise' viewpoint), one-photon processes with binding of NCMe in the triplet state and direct formation of final photoproduct can also be envisaged ('concerted' viewpoint). Ordinarily under Wigner's rules solvent coordination to a pentacoordinate [START_REF] Crosby | Spectroscopic Characterization of Complexes of Ruthenium(II) and Iridium(III) with 4,4'-Diphenyl-2,2'-Bipyridine and 4,7-Diphenyl-1,10-Phenanthroline[END_REF] MC state would be considered to be highly inefficient due to there being no metallic unoccupied acceptor orbital of the correct symmetry. However, we have recently postulated that electrostatic repulsion between an approaching ligand lone-pair and the unpaired metallic electron of a3 MC state could result in switching the ordering of metal based orbitals and relocalising the unpaired electron with generation of a triplet state of alternative character. 113 This would therefore vacate the orbital originally accommodating the unpaired electron, thus enabling the solvent ligand to coordinate. We have previously hinted at the solvent accessibility of MC states and now explicitly address it.

Solvent coordination to B 3 -MC and B 2 A 1 -MC states

From the B 3 -3 MC region, direct coordination of acetonitrile onto a B 3 -3 MC state could populate the κ 1 -B 3 A 1 -3 MLCT state (calculated energy barriers being 18 kcal/mol in the trans case, Fig S57 113 and 14 kcal/mol in the cis case, Fig S50). Alternatively, the κ 1 -B 3 A 1 -3 MLCT states could be circumvented entirely through direct and concerted solvent coordination and conversion from B 3 -MC states to the B 2 A 1 -3 MC+B van der Waals adducts. Indeed, significantly smaller energy barriers are encountered for these latter processes, being 8 kcal/mol for both isomers (Fig. S51 and S58). These barriers are moderate enough so that we can consider these one-photon processes to be feasible and may be operative, at least in competition with reverse intersystem crossing to the singlet manifold, and in line with the lack of systematic evidence for κ 1 -B 3 A 1 -GS primary photoproducts in some photolysis experiments. [START_REF] Howerton | Strained Ruthenium Complexes Are Potent Light-Activated Anticancer Agents[END_REF]124,125 Further along the mechanism, we note that the B 3 A 1 -3 MLCT and B 2 A 1 -3 MC states are almost isoenergetic, and their direct connection is not excluded (direct population of the B 2 A 1 -3 MC state from a higher-lying 3 MLCT state is not excluded either). In this view, the van der Waals B 2 A 1 +B-

The minimum energy path topologies of the cis and trans 3 PESs are shown in Figure 4. Note that two cis B 2 A 2 -3 MC states were identified as true minima, the one displaying elongated bonds towards the two bpy ligands (prefixed 'B+B', Fig. S26) being 5 kcal/mol higher in energy than the one displaying elongated bonds towards one acetonitrile and one bipyridine (prefixed 'A+B', Fig. S24). Barriers for their population from cis-B 2 A 1 -3 MC are 6 and 3 kcal/mol, respectively. In the trans series, one B 2 A 2 -3 MC minimum was optimised displaying elongations towards both bpy ligands (Fig. S35). The barrier for its population from trans-B 2 A 1 -3 MC is 7 kcal/mol. B 2 A 2 -

3 MLCT states were also optimised although they do not take part in the formation of the bis(solvento) products (Fig. S27 andS36). Table 4 summarises their main geometrical parameters and Figure 4 presents the triplet energy profiles. The overall energy profiles are much flatter than in the singlet state however they are also uphill processes, in line with the poor photoreactivity of [Ru(bpy) 3 ] 2+ in MeCN. As in the singlet state, the cis 3 PES is lower in energy than the trans 3 PES by a few kcal/mol. that it seems an acceptable model to consider bpy diffusion prior to NCMe approach and coordination.

Second solvent coordination to

Mixing the cis and trans pathways

Here we return to possible cis-trans conversions, which we had previously envisaged for earlier stages of the reaction. In the B 2 A 1 region, the peculiar geometry of the trans isomer displaying two open quadrants allows us to propose two preferential ways of approach for NCMe, depending on which, the two acetonitrile ligands will end in a cis or trans configuration. Nudged elastic band calculations were conducted between cis/trans-B 2 A 1 -3 MC and cis/trans-B 2 A 2 -3 MC minima (Fig. S52-S53, S59, S63-S64). Most interestingly, the energy barriers for the cis à cis and trans à cis conversions are very small (3-5 kcal/mol), and smaller than that for the trans à trans conversion, for which the energy barrier to populate the trans-B 2 A 2 -3 MC state is 7 kcal/mol (Figure 5). It is also worth noting that the population of the lowest of all B 2 A 2 -3 MC states involves, from the corresponding adducts in which the incoming NCMe is within van der Waals distance (minima in the grey-shaded regions in Figure 5), similarly small energy barriers of 5 kcal/mol. In the B 2 A 2 region the lowest triplet state is of cis configuration, which possibly also contributes to favouring the formation of the cis-B 2 A 2 final photoproduct from either the preceding cis or trans-B 2 A 1 -3 MC states.

Table 4 : Ru-N distances (Å) for B 2 A 2 triplet minima. A+B-elongated cis-B 2 A 2 -3 MC B+B-elongated cis-B 2 A 2 -3 MC cis-B 2 A 2 3 MLCT trans-B 2 A 2 3 MC trans-B 2 A 2 3 MLCT Ru-N1 2
In this section we have explored the possibility of a one-photon mechanism for photorelease of a bpy ligand from [Ru(bpy) 3 ] 2+ in which solvent ligand coordination and bpy ligand release may occur as sequential processes within the triplet manifold and circumvent ground state κ 1 -bpy solvento intermediates. To summarise this novel mechanism (Scheme 3), after initial photoexcitation to, and then depopulation of the resultant B 3 -MLCT state a solvent molecule may coordinate to the pro-cis-or pro-trans-B 3 -MC cis state to yield the van der Waals adducts cis-and trans-B 2 A 1 -MC+B respectively. Diffusion of the departing bpy ligand may then either be followed by intersystem crossing to the singlet state with a second solvent coordination, or solvent coordination to the B 2 A 1 -3 MC states and subsequent relaxation, to form the final B 2 A 2 -GS product. These final steps favour, both energetically and sterically, convergence of the preceding cis-and transpathways to formation the cis-B 2 A 2 photoproduct.

Translation to, and significance for other photochemically reactive systems

In carrying out our detailed survey of the singlet and triplet potential energy surfaces associated with photochemical chelate ligand release and solvation of the archetypal complex [Ru(bpy) 3 ] 2+

we have attempted to establish a generalised mechanistic road map for excited state evolution during photoproduct formation for the wider family of tris(bidentate) complexes to which it belongs. This road map must necessarily encompass all key ground and excited state species that may play roles in the formation of photoproducts. Further it must also detail routes that select the formation of either trans bis(solvento) photoproducts or their more commonly encountered cis counterparts. Therefore in our calculations we have sought to probe the potential energy surfaces for the formation of both cis-and trans-[Ru(bpy) 2 (NCMe) 2 ] 2+ through cis-and trans-[Ru(bpy) 2 (κ 1 -bpy)(NCMe)] 2+ intermediates. Clearly, comparable excited state geometries will be accessible and involved in the photochemical processes of other complexes of this class that display far more efficient photochemistry.

Chelate ligand release might be anticipated to occur via one of several mechanistic scenarios; i) thermal ligand dechelation followed by thermal ligand dissociation, ii) photochemical ligand dechelation followed by thermal ligand dissociation in the ground state, iii) photochemical ligand dechelation to yield a ground state intermediate with absorption of a second photon in a second photochemical ligand release step, and most intriguingly, iv) a single photon excitation process in which both dechelation and ligand release occur as sequential excited state events within the triplet manifold before finally forming closed shell ground state photoproducts. As is evident from the calculations presented here mechanism i) for the highly kinetically inert [Ru(bpy) 3 ] 2+ can be easily ruled out as activation barriers for the key processes would be prohibitively high. Similarly, mechanism ii) would be unlikely as thermal barriers for ligand dissociation from a

[Ru(bpy) 2 (κ 1 -bpy)(solvent)] 2+ type intermediate would be similarly prohibitive. However, we note that after photochemical initiation this may be feasible in cases with an extremely sterically encumbered ligand. We must therefore move on to mechanisms iii) and iv). In recent work reported by Bonnet, photolysis of bis(thiomethyl)propane (bthmp) containing complexes, e.g. [Ru(bpy) 2 (bthmp)] 2+ (Figure 6), reveals rapid photochemical conversion to ligand loss intermediates [Ru(bpy) 2 (κ 1 -bthmp)(solvent)] 2+ followed by a much slower photochemical release of the monodentate bthmp ligand. [START_REF] Meijer | Diastereoselective Synthesis and Two-Step Photocleavage of Ruthenium Polypyridyl Complexes Bearing a Bis(Thioether) Ligand[END_REF] The high degree of conformational flexibility of the bthmp ligand may favour formation of the ground state intermediate solvent complex. The much slower nature of the second step likely stems from photochemical solvent ligand exchange processes that are more favourable than photochemical bthmp ligand release in the [Ru(bpy) 2 (κ 1bthmp)(solvent)] 2+ intermediate. Similarly, the complex [Ru(bpy)(btz) 2 ] 2+ , which favours trans photoproduct formation, first rapidly and quantitatively forms the intermediate trans-

[Ru(bpy)(κ 2 -btz)(κ 1 -btz)(NCMe)] 2+ , with final photochemical loss of the κ 1 -btz ligand being slow. 110 Due to the trans arrangement of the monodentate btz and acetonitrile ligands, the 3 MC state responsible for elongation of the Ru-N bond to the former will also elongate the corresponding bond to the latter. It would be expected that loss of the acetonitrile would be favoured over loss of btz, thus again resulting in photochemical solvent exchange to be the dominant process thereby enabling the reported observation, and indeed isolation of this κ 1 -btz intermediate. 110 material may suggest that these intermediates may not in fact form at all in these systems. In our calculations we have shown that it is energetically feasible for acetonitrile to coordinate to B 3 -3 MC species to form B 2 A 1 -3 MC+B van der Waals adducts in which the departing bpy ligand can already be said to be "on its way". We have also shown that a second solvent ligand may also be coordinated to the subsequent B 2 A 1 - The nature of both the departing and retained spectator ligands present, as well as the nature of the incoming solvent (or coordinating anions) are therefore crucial for the selection of the preference for cis versus trans photoproduct formation but also the favourability of the formation of singlet ground state intermediate photoproducts and thus the operation of a two photon (iii) or single photon triplet state solvation (iv) mechanism. For some complexes it will also be possible that both mechanisms are simultaneously operative.

For [Ru(bpy) 3 ] 2+ the energy landscape associated with formation of acetonitrile-containing photoproducts involves an uphill climb through the triplet manifold and accounts for the low photochemical reactivity displayed by the complex. For the highly photochemically reactive [Ru(bpy) 2 (6,6'-dmbpy)] 2+ it is proposed that the steric congestion imparted by the 6'6'-dmbpy methyl group weakens the Ru-N bonds and thus stabilises the 3 MC states relative to the 3 MLCT state. This then makes the depopulation of the 3 MLCT states and initiation of photochemical reactivity far easier than for [Ru(bpy) 3 ] 2+ . However, it can also be said that the steric strain will also destabilise the ground and 3 MLCT states relative to [START_REF] Crosby | Spectroscopic Characterization of Complexes of Ruthenium(II) and Iridium(III) with 4,4'-Diphenyl-2,2'-Bipyridine and 4,7-Diphenyl-1,10-Phenanthroline[END_REF] MC states in which the strain is intrinsically relieved due to the occurrence of elongated metal-ligand bonds. As the energy profiles for the photoreaction mechanism are necessarily related to the reference of the energy of the ground state this should result in a flattening of the overall excited state minimum energy paths with respect to the starting 3 MLCT state. This would result in making states along this path inherently more accessible and therefore photochemical reactivity far more efficient. For [Ru(bpy) 2 (btz)] 2+ a similar result can be envisaged through a different cause : the destabilisation of the 3 MLCT state with respect to that of [Ru(bpy) 3 ] 2+ would again result in far more facile access to excited state PES photoproductive pathways. In order to verify these inferences we are currently using the generalised road map derived in this work to chart and explore the analogous ground and excited state potential energy landscapes for photoproduct formation in these and other photolabile systems.

Explicit solvation would also improve modelling by comparison to the current bimolecular reaction model, which in itself is a challenging prospect. Results from these on-going studies will be reported elsewhere in due course. that several mechanisms could be operative, as exemplified by the complex photochemistry of Fe(CO) 5. 146 , 147 , 148 , 141 , 149 Experimental conditions necessarily have a tremendous impact on the outcome of a photochemical reaction. 150 In this respect, ligand photoexchange experiments performed under pulsed excitation could bring a wealth of mechanistic insights. We hope this theoretical study will stimulate further experimental studies that would feed the discussion in this exciting field of research. We also hope that these results provide a transferable road map that can be translated to other systems where both departing and 'spectator' ligand influence the selection of whether stepwise/two-photon or concerted/one-photon mechanisms are operative and the selection of product stereochemistry. Further, we hope that this will offer insights that aid the development in the molecular design of efficient photoreactive complexes for PACT and as photomechanical molecular machines.

Conclusions

ASSOCIATED CONTENT

Supporting Information

Figure 1 .

 1 Figure 1. Numbering scheme used in this study for coordinating N-atoms in [Ru(bpy) 3 ] 2+ .

Figure 2 .

 2 Figure 2. Singlet potential energy profile towards cis-[Ru(bpy) 2 (NCMe) 2 ] 2+ (top) and trans-[Ru(bpy) 2 (NCMe) 2 ] 2+ (bottom).

Figure 3 .

 3 Figure 3. Structures of pro-cis-B 3 -3 MC cis (left) and pro-trans-B 3 -3 MC cis (right), highlighting their respective open quadrant that will facilitate the approach of incoming MeCN.

( 2 .

 2 97 eV). Regarding the trans van der Waals adduct (trans-B 2 A 1 -3 MC+B), the hole corresponds to the HOMO of trans-κ 1 -B 3 A 1 -GS, i.e. it is the same as in T 1 or T 3 . Now in the triplet state, the cis/trans-κ 1 -B 3 A 1 -MLCT states have the same choice and could either move forward towards bpy loss (productive pathway) or release NCMe and revert to the initial B 3 world (nonproductive pathway). The case of the trans-κ 1 -B 3 A 1 -3 MLCT state is particularly illustrative of this competition because population of a single dσ* orbital would repel both the halfbound bpy and the newly coordinated MeCN ligand, as they are located trans to one another. Following Zink's reasoning, 66 , 67 , 68 the Ru-NCMe overlap being larger than the Ru-pyridine overlap in the bonding dσ orbital, one can expect NCMe to be more repelled than pyridine when the corresponding dσ* orbital is populated, i.e. a return to the starting material would be particularly favoured in the trans case. This may be one of the reasons why trans-B 2 A 2 products are less fa-voured, in various photolysis experiments. For the forward reaction (bpy loss) along the cis and trans routes we find a minimum in each case represented by a pentacoordinate[START_REF] Crosby | Spectroscopic Characterization of Complexes of Ruthenium(II) and Iridium(III) with 4,4'-Diphenyl-2,2'-Bipyridine and 4,7-Diphenyl-1,10-Phenanthroline[END_REF] MC state with which the departing bpy ligand is still associated as a van der Waals adduct (cis-and trans-B 2 A 1 -3 MC+B, Fig.S20 and S31). These adducts are found to be lower in energy by 8 and 12 kcal/mol than their corresponding κ 1 -B 3 A 1 -MLCT state respectively. In cis-B 2 A 1 -MC+B the departing bpy ligand has a significantly elongated Ru-N distance to the N atom of the previously monodentate bpy ligand (Ru-N6 = 2.81 Å). The bpy ligand is significantly inclined with respect to this Ru-N distance, with the other pyridine ring (the decoordinated pyridine ring of the previously κ 1 -bpy) engaged in a π-stacking arrangement with one of the retained bpy ligands. In the case of trans-B 2 A 1 -MC+B, the retained bpy ligands undergo a rearrangement process upon conversion from the MLCT state in which one of the ligand tilts up to relieve steric strain (N1-Ru-N4 = 141°, Fig-ure S31). Here, the departing bpy engages in π-stacking with the retained bpy ligand that tilts up towards it. This arrangement interestingly presents a new open quadrant adjacent to the acetonitrile ligand (N4-Ru-NCMe = 129°), the possible significance of which we shall return to later.

  ) and S63 (top). The fact that initial barriers for bpy diffusion and final barriers for NCMe coordination are perfectly consistent with the barriers involving the model B 2 A 1 species indicates

Figure 4 :

 4 Figure 4: Triplet state minima and minimum energy paths along the cis (top) and trans (bottom) pathway.

Figure 6 )

 6 photolysis of the complex [Ru(bpy) 2 (3,3'-dmbpy)] 2+ (3,3'-dmbpy = 3,3'-dimethyl-2,2'-bipyridyl, Figure6) with monitoring by 1 H NMR spectroscopy, two weak singlet resonances are observed ascribed to the now inequivalent 3,3'-dmbpy methyl groups of the solvento intermediate [Ru(bpy) 2 (κ 1 -3,3'-dmbpy)(NCMe)] 2+ .102 Here, steric strain imparted by the 3-position methyl groups of the dmbpy ligand results in the complex being spring-loaded towards dechelation and thereby hindering rechelation, favouring formation of an observable monodentate intermediate.

Figure 5 :Scheme 3 .

 53 Figure 5: Minimum energy paths for cis-cis (purple), trans-trans (orange) and trans-cis (blue and red) connections for the coordination of the second acetonitrile ligand onto pentacoordinate B 2 A 1 3 MC states. The grey-shaded regions correspond to regions of van der Waals adducts with the incoming NCMe (energy plateau or slight decrease). Arrows near the trans-B 2 A 1 structure represent NCMe approaching in either open quadrant, leading to trans (orange arrow) or cis (blue or red arrows) products. Energies are given in kcal/mol with respect to the reference.

Figure 6 .

 6 Figure 6. Structures of selected photochemically reactive ruthenium(II) bis-bpy complexes.

  Ligand photorelease is a very active field of research in relation to photoactivated chemotherapy (PACT) or photorelease of biologically active molecules. From the point of view of fundamental inorganic photochemistry, the microscopic mechanisms underpinning multistep ligand photoex-change processes have been unclear and have still required thorough investigation. The theoretical study reported herein has allowed us to establish all the elementary steps of a dissociative ligand photoexchange mechanism on a model reaction involving [Ru(bpy) 3 ] 2+ as reactant and acetonitrile as the incoming ligand. This has enabled us to highlight two main mechanisms involving either two sequential photochemical processes or a single process after absorption of just one photon. The former involves stepwise bpy dissociation through the formation of κ 1 intermediate photoproducts incorporating one NCMe ligand and absorption of a second photon that opens the way to the photorelease of the κ 1 -bpy ligand, followed by the coordination of a second NCMe ligand. In the latter case, following absorption of just one photon the ligand dechelation, dissociation and solvent coordination may all happen as a set of sequential steps all occurring in the triplet excited state. Both cis and trans products could be formed in either mechanism, with energy barriers of similar magnitude. The trans pathway is generally slightly higher in energy than the cis pathway for the model [Ru(bpy) 3 ] 2+ system, except in a few regions where both are almost isoenergetic and may communicate at little cost. Associative mechanisms have been occasionally proposed in Ru(II),[START_REF] Wachter | Mechanistic Study on the Photochemical "Light Switch" Behavior of [Ru(Bpy) 2 Dmdppz] 2+[END_REF] Rh(I),144 and Re(I) 145 photochemistry and it is important to note

  individual MeCN molecules with [Ru(bpy) 3 ] 2+ in implicit MeCN (polarisable continuum model) is an acceptable model that should provide us with general mechanistic information in relation to bidentate ligand loss. As both cis-and trans-[Ru(bpy) 2 L 2 ] 2+ products are known, we envisaged forming cis-[Ru(bpy) 2 (NCMe) 2 ] 2+ and trans-[Ru(bpy) 2 (NCMe) 2 ] 2+ . cis-[Ru(bpy) 2 (NCMe) 2 ] 2+ is indeed the photosolvolysis product formed from Ru(bpy) 2 (btz) 2+

Table 1 .

 1 Abbreviations used throughout this work. All states can be derived as cis or trans isomers, with singlet or triplet spin multiplicity.

	abbreviation formula
	B 3	[Ru(bpy) 3 ] 2+
	κ 1 -B 3	[Ru(κ 1 -bpy)(bpy) 2 ] 2+
	κ 1 -B 3 A 1	[Ru(κ 1 -bpy)(bpy) 2 (NCMe)] 2+
	B 2 A 1 +B	[Ru(bpy) 2 (NCMe)] 2+ +bpy (adduct)
	B 2 A 1	[Ru(bpy) 2 (NCMe)] 2+
	B 2 A 2	[Ru(bpy) 2 (NCMe) 2 ] 2+
	1

. Topologies of the cis singlet PES and trans singlet PES

	Photochemical ligand release may involve the participation of a sequence of both singlet ground
	state and triplet excited state species. It is thus imperative to throughly characterise the singlet
	potential energy surface between the reactant and final photoproducts and capture these important
	intermediate species. Experimental studies conclude that cis-and trans-bis(solvento) adducts can
	be formed from prolonged photolysis of [Ru(bpy) 3 ] 2+ , the cis product being the major isomer.
	Similarly to what had been observed for osmium, 106 primary photoproducts bearing one mono-
	dentate bpy ligand and one bound solvent molecule could be formed as cis or trans isomers too.
	Thus each of the singlet PES for the formation of cis and trans-[Ru(bpy) 2 (NCMe) 2 ] 2+ should have
	three local minima of general formula B 3 , κ 1 -B 3 A 1 and B 2 A 2 . If one assumes a dissociative mech-
	anism this implies that photoreactivity proceeds through formally pentacoordinate intermediates

Table 2 :

 2 Ru-N distances (Å) and selected angles for closed-shell minima leading to cis-[Ru(bpy) 2 (NCMe) 2 ] 2+ and trans-[Ru(bpy) 2

Table 3 :

 3 Ru-N distances (Å) and selected angles for triplet minima leading to cis-and trans-[Ru(bpy) 2 (NCMe) 2 ] 2+ .

		pro-cis-B 3	3 MC cis		cis-κ 1 -B 3 A 1	3 MLCT	cis-B 2 A 1 +B 3 MC	cis-B 2 A 1	3 MC
	Ru-N1	2.106				2.107			2.133	2.116
	Ru-N2	2.300				2.072			2.344	2.260
	Ru-N3	2.215				2.014			2.060	2.068
	Ru-N4	2.088				2.042			2.063	2.063
	Ru-N5	2.547				3.899			3.825	-
	Ru-N6	2.162				2.156			2.815	-
	Ru-NCMe a	-				2.087			2.047	2.045
	angle	N2-Ru-N6 128°		N5-C-C-N6 71°	N5-C-C-N6 30°	N3-Ru-NCMe a 174°
		pro-trans-B 3	3 MC cis	a	trans-κ 1 -B 3 A 1	3 MLCT	trans-B 2 A 1 +B 3 MC	trans-B 2 A 1	3 MC
	Ru-N1	2.078				2.049			2.136	2.185
	Ru-N2	2.177				2.057			2.080	2.081
	Ru-N3	2.135				2.118			2.080	2.070
	Ru-N4	2.080				2.110			2.094	2.071
	Ru-N5	2.530				3.593			3.773	-
	Ru-N6	2.384				2.132			3.347	-
	Ru-NCMe a	-				2.055			2.196	2.086
	angle	N2-Ru-N3 121°		N5-C-C-N6 59°	N1-Ru-N4 129° N4-Ru-NCMe a 141° N5-C-C-N6 18°	N1-Ru-N4 120° N4-Ru-NCMe a 141°

a From ref 114 . Scheme 2. Proposed multistep productive mechanism towards cis-[Ru(bpy) 2 (NCMe) 2 ] 2+ (top) and trans-[Ru(bpy) 2 (NCMe) 2 ] 2+ (bottom) involving two sequential photochemical steps.

  of bpy is the prerequisite to the formation of a van der Waals adduct between a pentacoordinate metal centre and the incoming acetonitrile molecule. This process involves barriers between 11-13 kcal/mol, values that correspond well to the energy gaps found between the B 2 A 1 +B and corresponding B 2 A 1 species. Once the incoming NCMe is within van der Waals distance, a smaller barrier is encountered to populate the B 2 A 2 triplet state (3-4 kcal/mol). Again these values are in very good agreement with the values found for the model

3 MC state van der Waals adducts : bypassing the B 2 A 1 region One could argue that B 2 A 1 species are artificially brought in by our computational model and have little probability of existence in coordinating solvent. To probe this idea we have conducted NEB calculations bypassing the B 2 A 1 region, i.e. connecting directly the van der Waals adducts B 2 A 1 +B to the final B 2 A 2 region. In such calculations removal of bpy and approach of NCMe are concomitant. Representative examples are presented in Figures S52 (bottom) and S63 (bottom), showing that diffusion reactions approaching NCMe onto a pentacoordinate B 2 A 1 triplet state, presented in Figures S52 (top

  [START_REF] Crosby | Spectroscopic Characterization of Complexes of Ruthenium(II) and Iridium(III) with 4,4'-Diphenyl-2,2'-Bipyridine and 4,7-Diphenyl-1,10-Phenanthroline[END_REF] MC states and access the B 2 A 2 triplet region. Thus, through either intersystem crossing to the singlet state in the B 2 A 1 region followed by solvent coordination, or the sequential coordination of two solvent ligands to triplet states followed by intersystem crossing to the singlet ground state in the B 2 A 2 region, we show that it is entirely possible for As highlighted earlier, photochemical ligand release from Ru(bpy) 2 -based complexes tends to produce photoproducts with cis stereochemistry. Ru(bpy) 2 -based complexes with two monodentate ligands in a trans arrangement are known, however, and have been crystallographically characterised.143 In agreement with our calculations the two bpy ligands in these trans complexes deviate significantly from coplanarity due to a steric clash between the 6-position protons of the pyridine rings of the two ligands. This is therefore the likely reason for the relative dearth of ligand release Ru(bpy) 2 -based photoproducts exhibiting trans stereochemistry. Our calculations further reveal that the key van der Waals adduct trans-B 2 A 1 -3 MC+B and the pentacoordinate[START_REF] Crosby | Spectroscopic Characterization of Complexes of Ruthenium(II) and Iridium(III) with 4,4'-Diphenyl-2,2'-Bipyridine and 4,7-Diphenyl-1,10-Phenanthroline[END_REF] MC state trans-B 2 A 1 -3 MC in fact exhibit a hybrid structure partway between trans and cis stereochemistry, which relieves this steric strain. With access to the pro-trans quadrant of this 3 MC state being hindered by the departing bpy ligand this would therefore seem to favour solvent coordination at the second open quadrant, therefore driving the complex towards formation of the cis bis-acetonitrile product. Thus, whilst both the pro-trans and pro-cis excited state pathways are accessible through the B 3 , B 3 A 1 , B 2 A 1 +B and B 2 A 1 regions, these appear to converge to favour NCMe) 2 ] 2+ . One would therefore not expect the appearance of hybrid van der Waals adduct and pentacoordinate[START_REF] Crosby | Spectroscopic Characterization of Complexes of Ruthenium(II) and Iridium(III) with 4,4'-Diphenyl-2,2'-Bipyridine and 4,7-Diphenyl-1,10-Phenanthroline[END_REF] MC states comparable to trans-B 2 A 1 -MC+B (indeed, one would expect the bpy and btz to remain coplanar as in the cyrstallographically characterised κ 1btz intermediate),110 and thus little driving force to switch from trans to cis photoproduct formation pathways. Calculations on the cis and trans isomers of the final photoproducts showed the latter to be the most stable.105 Therefore it seems likely that the relative energy ordering of the comparable cis and trans pathways for [Ru(bpy) 2 (btz)] 2+ is inverted compared to [Ru(bpy) 3 ] 2+ .

	ruthenium(II) tris(diimine) complexes (for example, [Ru(bpy) 2 (6,6'-dmbpy)] 2+ and
	[Ru(bpy) 2 (btz)] 2+ ) to form the final photoproducts after absorption of a single photon and bypass
	formation of ground state κ 1 intermediate photoproducts entirely.

cis-B 2 A 2 , a configuration which, in addition, provides thermodynamically more stable final products. As such intermediate excited state species will be relevant to a large number of other photoreactive bis-bpy complexes this accounts for the predominance of photoproducts of cis stereochemistry.

Conversely, for the heteroleptic complex [Ru(bpy)(btz) 2 ] 2+ no such steric clash exists between the bpy and btz ligands when they become coplanar in trans-[Ru(bpy)(κ 2 -btz)(κ 1 -btz)(NCMe)] 2+ and trans-[Ru(bpy)(btz)(

MC states can be seen as traps to the desired reactivity through their neighbouring MECPs, which open a singlet state deactivation cascade to B 2 A 1 +B-GS and eventually κ 1 -B 3 A 1 -GS.

Computational details, structures, energies and selected orbitals of all minima and minimum energy crossing points, plots of minimum energy paths, and TDDFT calculations (pdf); Cartesian coordinates of all minima and MECPs (zip).

The Supporting Information is available free of charge on the ACS Publications website.

Acknowledgements

We thank the French Ministry for Higher Education and Research for a Ph. D. fellowship to AS. HPC resources from LCPQ and from CALMIP (p18013 project) are gratefully acknowledged. PE also thanks the University of Huddersfield for support and the 3M Business and Innovation Centre for additional HPC resources.

Funding Sources

No competing financial interests have been declared.

AUTHOR INFORMATION

Corresponding Authors * P.I.Elliott@hud.ac.uk * isabelle.dixon@irsamc.ups-tlse.fr

Author Contributions

The manuscript was written through contributions of all authors.

For Table of Contents Only

SYNOPSIS TOC

A full road map is provided for the multistep mechanism of photoinduced ligand loss from tris(diimine) Ru(II) complexes and formation of bis(solvento) cis and trans photoproducts. Two major pathways have been identified; one involves two sequential photonic excitations through the formation of an intermediate mono(solvento) product (red route). The other is a novel onephoton pathway (blue route) in which ligand dechelation, coordination of solvent and ligand loss all occur in the triplet excited state.

TOC GRAPHIC