Claus Hierlinger

Amlan K Pal

Filippo Stella

Tomas Lebl

David B Cordes

Alexandra M Z Slawin

Denis Jacquemin
email: denis.jacquemin@univ-nantes.fr

Véronique Guerchais
email: veronique.guerchais@univ-rennes1.fr

Eli Zysman-Colman
email: eli.zysman-colman@st-andrews.ac.uk

Synthesis

Keywords:

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Introduction

One of the most promising and explored classes of luminescent transition metal complexes are those based on iridium(III) as they show excellent photo-and thermal stabilities.

They are typically highly efficient emitters with relatively short phosphorescent emission lifetimes, and their emission energy can be easily tuned as a function of ligand design. [1][START_REF] Zanoni | Ir(iii) complexes designed for light-emitting devices: beyond the luminescence color array[END_REF] The most widely studied class of iridium complexes are heteroleptic bis-cyclometalated complexes of the form [Ir(C^N)2(L^X)] n+ , where C^N is the cyclometalating ligand and L^X is either a monoanionic ligand such as acetylacetonate (acac, n = 0), or a neutral diimine ligand such as 2,2'-bipyridine (bpy, n = 1). The main strategies for tuning the emission energy of these complexes are the decoration of the ligands with either electron-withdrawing or electrondonating substituents, and the modification of the coordinating heterocycles. 1,[START_REF] Ladouceur | A Comprehensive Survey of Cationic Iridium(III) Complexes Bearing Nontraditional Ligand Chelation Motifs[END_REF][START_REF] Terki | Tailoring the Photoluminescence Properties of Ionic Iridium Complexes[END_REF][START_REF] Lu | Metal Complexes with Azolate-Functionalized Multidentate Ligands: Tactical Designs and Optoelectronic Applications[END_REF] The C^N ligands nearly always form five-membered chelates, typically based on 2-phenylpyridine (ppyH). Strategies involving interruption of the conjugation in these C^N ligands, such as going from a five-membered chelate to a six-membered chelate that incorporates a methylene spacer, are far less explored. Indeed, examples of photoactive iridium complexes incorporating sixmembered chelate C^N ligands remain rare and belong to two categories depending on the presence of conjugated [START_REF] Li | Synthesis, Characterization, and Photophysical Properties of Iridium Complexes with an 8-Phenylquinoline Framework. The First Six-Membered Chelated Iridium Complexes for Electroluminance[END_REF][START_REF] Zhu | Color Tuning Based on a Six-membered Chelated Iridium(III) Complex with Aza-aromatic Ligand[END_REF][START_REF] Thamilarasan | Greenemitting phosphorescent iridium(III) complex: Structural, photophysical and electrochemical properties[END_REF] or nonconjugated [START_REF] Song | Phosphorescent iridium(III) complexes with nonconjugated cyclometalated ligands[END_REF][START_REF] Chang | Highly Efficient Blue-Emitting Iridium(III) Carbene Complexes and Phosphorescent OLEDs[END_REF][START_REF] Zhang | Synthesis, characterization, and photophysical and electroluminescent properties of blue-emitting cationic iridium(III) complexes bearing nonconjugated ligands[END_REF][START_REF] Hierlinger | An Unprecedented Family of Luminescent Iridium(III) Complexes Bearing a Six-Membered Chelated Tridentate C^N^C Ligand[END_REF] bidentate chelating ligands.

To date, to the best of our knowledge, all iridium complexes employing nonconjugated six-membered chelate C^N ligands have included either a pyrazole [START_REF] Song | Phosphorescent iridium(III) complexes with nonconjugated cyclometalated ligands[END_REF][START_REF] Zhang | Synthesis, characterization, and photophysical and electroluminescent properties of blue-emitting cationic iridium(III) complexes bearing nonconjugated ligands[END_REF] or benzyl-derived Nheterocyclic carbene [START_REF] Chang | Highly Efficient Blue-Emitting Iridium(III) Carbene Complexes and Phosphorescent OLEDs[END_REF] coordinating moiety as part of the ligand. Surprisingly, the use of pyridine, such as with 2-benzylpyridinato (bnpy), has not yet been explored in the design of photoactive iridium complexes; this ligand has only been reported for iridium complexes used in catalysis. [START_REF] Li | C-H Activation of Phenyl Imines and 2-Phenylpyridines with [Cp*MCl2]2 (M = Ir, Rh): Regioselectivity, Kinetics, and Mechanism[END_REF][START_REF] Boutadla | The scope of ambiphilic acetateassisted cyclometallation with half-sandwich complexes of iridium, rhodium and ruthenium[END_REF] The two C^N-coordination moieties in bnpy are separated by a methylene spacer, which fully interrupts the π-conjugation within the C^N ligands. This allows the decoration of the pyridine without influencing the electronic properties of the phenyl ring and therefore the HOMO level of the complex; the HOMO being typically located on the phenyl rings of the C^N ligands and the iridium d-orbitals. [START_REF] Ladouceur | A Comprehensive Survey of Cationic Iridium(III) Complexes Bearing Nontraditional Ligand Chelation Motifs[END_REF] In the present study, we have targeted a series of four new cationic Ir(III) complexes (Chart 1) of the form [Ir(bnpy)2(N^N)](PF6), where the N^N ligand is a neutral diimine chelate, with the goal to evaluate whether breaking the conjugation within the C^N ligands can aid in pushing the emission to the blue, which has been an active research theme in the Zysman-Colman group. [START_REF] Ladouceur | Enhanced Luminescent Iridium(III) Complexes Bearing Aryltriazole Cyclometallated Ligands[END_REF][START_REF] Donato | Cationic iridium(iii) complexes bearing a bis(triazole) ancillary ligand[END_REF][START_REF] Fernandez-Hernandez | Blue light emitting electrochemical cells incorporating triazole-based luminophores[END_REF][START_REF] Ladouceur | Strongly Blue Luminescent Cationic Iridium(III) Complexes with an Electron-Rich Ancillary Ligand: Evaluation of Their Optoelectronic and Electrochemiluminescence Properties[END_REF][START_REF] Evariste | Fluorine-free blue-green emitters for light-emitting electrochemical cells[END_REF][START_REF] Henwood | Rigid biimidazole ancillary ligands as an avenue to bright deep blue cationic iridium(iii) complexes[END_REF][START_REF] Shavaleev | Green Phosphorescence and Electroluminescence of Sulfur Pentafluoride-Functionalized Cationic Iridium(III) Complexes[END_REF][START_REF] Henwood | Solubilised bright blue-emitting iridium complexes for solution processed OLEDs[END_REF][START_REF] Rota Martir | Enhancing the photoluminescence quantum yields of blue-emitting cationic iridium(iii) complexes bearing bisphosphine ligands[END_REF] Complexes 1 and 2 contain bipyridine-based ancillary ligands [1, N^N = bpy; 2 = 4,4'-di-tert-butyl-2,2'-bipyridine (dtBubpy)] while complexes 3 and 4 each contain two 5-membered heterocycles in the electron deficient 2,2'-bithiazole (bthz) and the electron-rich 1H,1'H-2,2'-biimidazole (biim) ligands, respectively. The motivation for the choice of N^N ligands was to have a series of ligands that (1) spanned a wide electronic range and (2) whose bite angles varied in order to assess what impact, if any, this has in conjunction with the C^N ligands. The optoelectronic properties of these complexes are explored and compared with benchmark complexes (R1-R4, see Chart 1)

where the bnpy ligands have been replaced with ppy; R1-R3 have been previously reported [START_REF] Ladouceur | Enhanced Luminescent Iridium(III) Complexes Bearing Aryltriazole Cyclometallated Ligands[END_REF][START_REF] Pal | Simple design to achieve red-to-near-infrared emissive cationic Ir(iii) emitters and their use in light emitting electrochemical cells[END_REF] while R4 is new. The physical and photophysical properties of these complexes are corroborated by density functional theory (DFT) and time-dependent DFT (TD-DFT) investigations.

Chart 1. Synthesized and characterized complexes (1-4) and reference complexes (R1-R4) in this study.

Results and Discussion

Complexes 1-4 were obtained as their hexafluorophosphate salts from the chlorobridged dimer [Ir(bnpy)2Cl]2, upon reaction with the corresponding N^N ancillary ligand and subsequent anion exchange with NH4PF6 following standard reaction conditions; complex R4 was obtained analogously using [Ir(ppy)2Cl]2. [START_REF] Nonoyama | Benzo[h]quinolin-10-yl-N Iridium(III) Complexes[END_REF][START_REF] Henwood | In Iridium(III) in Optoelectronic and Photonics Applications[END_REF] In the first step, [Ir(bnpy)2Cl]2 was quantitatively obtained as a yellow solid by treatment of bnpyH with IrCl3.6H2O in a 3:1 mixture of 2-ethoxyethanol/H2O (125 °C, 24 h). This dimer was then cleaved with the appropriate N^N ligand in 1:1 mixture of CH2Cl2/MeOH (40 °C, 18 h) to afford the cationic Ir(III) complexes as their chloride salts. After column chromatography on silica (eluent: 5% MeOH in CH2Cl2) followed by an ion exchange with aqueous NH4PF6, complexes 1-4 were isolated in excellent yield (78% -91%) as their hexafluorophosphate salts. The ancillary ligands for 3 (bthz) [START_REF] Pal | Simple design to achieve red-to-near-infrared emissive cationic Ir(iii) emitters and their use in light emitting electrochemical cells[END_REF] and 4 (biim) [START_REF] Henwood | Rigid biimidazole ancillary ligands as an avenue to bright deep blue cationic iridium(iii) complexes[END_REF] were obtained according to previous reports. All complexes were characterized by 1 H, [START_REF] Li | C-H Activation of Phenyl Imines and 2-Phenylpyridines with [Cp*MCl2]2 (M = Ir, Rh): Regioselectivity, Kinetics, and Mechanism[END_REF] C and 31 P NMR spectroscopy, HR-ESI mass spectrometry, elemental analysis, and melting point determination (see Figures S1-S19 in the Supporting Information for NMR and HR-ESI-mass spectra). The structures of complexes 1-3 and the chloride salt of R4, R4.Cl, were determined by single crystal X-ray diffraction.

Crystal Structures

Single crystals of sufficient quality of 1-3 and R4.Cl were grown from vapor diffusion of a CH2Cl2 or acetone solution of the complex with diethyl ether acting as the anti-solvent.

The structures of 1-3 and R4.Cl were determined by single-crystal X-ray diffraction (Figure 1 the analogous bond lengths in R1-R3. [START_REF] Ladouceur | Enhanced Luminescent Iridium(III) Complexes Bearing Aryltriazole Cyclometallated Ligands[END_REF][START_REF] Pal | Simple design to achieve red-to-near-infrared emissive cationic Ir(iii) emitters and their use in light emitting electrochemical cells[END_REF][START_REF] Costa | Archetype Cationic Iridium Complexes and Their Use in Solid-State Light-Emitting Electrochemical Cells[END_REF][START_REF] Martir | Chiral Iridium(III) Complexes in Light-Emitting Electrochemical Cells: Exploring the Impact of Stereochemistry on the Photophysical Properties and Device Performances[END_REF] The bite angles of the N^N ligands in 1-3 vary from 75.24(15)-77.8(2)° [76.1(2)° for R4.Cl], which are again similar to the reference complexes R1-R3. The bite angles of the bnpy ligands vary between 87.97 (13) In the X-ray structures, the conformation of the bnpy ligands in complexes 1 and 2 differs from that in 3. Complexes 1 and 2 are found solely as one conformer where the pyridine rings of the C^N ligand are orientated towards the N^N ligand, whereas, in 3, the other conformer, where the methylene groups point towards the N^N ligand, is observed, despite this being the minor conformer observed by 1 H NMR (see below). Given the small differences between the conformer free energies computed in solution for 3 (see below), it is not surprising that packing effects can affect the equilibrium and that the most favored conformer differs in solution and in the solid-state. Complex 1 crystallizes in the polar space group P21, so individual crystals could be enantiopure, resulting from conglomerate crystallization; however the Flack parameter [0.305 (7)] indicates the likelihood of individually racemic crystals being present.

Complexes 2 and 3 crystallize in centrosymmetric space groups (P3 and P1 , respectively), so exist as racemates in the crystalline state.

Solution-State NMR studies

The room temperature 1 H NMR spectra in CDCl3 of 1-4 are depicted in Figure 2.

Complexes 1 and 2 each show one set of two doublets in the region of δ = 4.80 -3.30 ppm, corresponding to the diastereotopic methylene protons of the C^N ligand, the result of magnetic nonequivalence imparted by the proximal iridium stereocenter. By contrast, there are two sets of two doublets with an integration ratio of 1:1.2 in the same region of 1 H NMR spectrum of 3.

Furthermore, two sets of aromatic signals are also apparent, which implies the presence of two conformers (designed as a and b, see below) in solution. Two conformers, in a ratio of 1:0.2 for conformer a:conformer b, are also present in 4 as observed by 1 H NMR. This phenomenon was previously reported in related neutral Ir(III) complexes containing 1-(2,4-difluorobenzyl)-1Hpyrazole (dFbpzH) as the C^N ligands and was associated with restricted conformational flexibility of six membered C^N chelates. 9 This presence of two conformers prompted us to perform DFT calculations (see the Supporting Information for more details). For each of the complexes, we could optimize stable structures for both conformers, differing in orientation of the methylene bridge of the C^N ligand, as shown in Figure 3 for complex 3 (see Figure S21 the SI for 4). The conformer 3a has the same orientation of the methylene bridge as found in the solid state by X-ray crystallography, whereas the geometry of 3b is analogous to those found in the crystal structures of 1 and 2 (see above). For all compounds, both conformers belong to the C2 point group. Given that DFT calculations have been performed in MeCN solution, the DFT-determined bond lengths reasonably match their experimental counterparts, e.g., the Ir-CC^N bonds are 2.02 Å long by DFT and 2.022(5) -2.052(7) Å for 1-3 long in the X-ray, whereas the Ir-NC^N distances are 2.10 Å according to the calculation and 2.069(4) -2.098(11) Å for 1-3 in the solid-state structure. The chemical exchange cross-peaks (red highlighted) in the 2D ROESY spectrum indicate the existence of a dynamic equilibrium between conformers 3a and 3b in solution (Figure 4). Furthermore, the ROESY enables assignment of 1 H resonances to each of the conformers. The NOE cross-peak (blue highlighted) between doublets at 4.54 and 8.18 ppm is only possible for conformer 3a that has one proton of methylene bridge close (2.56 Å according to DFT) to the aromatic proton of the N^N ligand (Figure 3). By contrast, this NOE enhancement is not observed for conformer 3b due to a longer distance between the methylene bridge and the N^N ligand (5.12 Å according to DFT). DFT computed relative free energies and theoretical Boltzmann distribution can be found in Table 1. As can be seen, there is a remarkable agreement between the ratio of the two conformers determined by theory and experimentally from the relative integral intensities of methylene group 1 H resonances (Table 1). There seems to be a global trend that the steric stress, that decreases in the 1~2 > 3 > 4 series, makes conformer a more accessible in the latter compounds (3a and 4a) than in the former (1a and 2a). The DFT determined distances between the (closest) hydrogen atoms of the methylene and N^N ligand are: 2.15, 2.14, 2.23 and 2.28 Å, in 1a, 2a, 3a and 4a, respectively. For 3, we attempted to optimize by DFT a mixed conformer with one of the ligands in each conformation, but this induces a steric clash and the optimization process led back to one of the two conformers. This clearly suggests that the 3a 3b

transition from one conformation to the other should be a concerted process in which the two ligands simultaneously change their conformation. We therefore searched for the corresponding transition state, and we could locate it 66 kJ . mol -1 above the most stable 3b structure on the free energy scale. This indicates that while thermal equilibration between the two conformers is attainable at room temperature, the interconversion will not be rapid, which is consistent with the fact that two separate sets of signals could be detected on the NMR timescale. The corresponding imaginary frequency mode at 40.5 icm -1 is given in the SI (see video S1).

The interconversion of conformers 3a and 3b was also studied by variable temperature NMR spectroscopy from 280 to 315 K. The variable-temperature 1 H NMR studies showed considerable dependence of equilibrium constant, K, on the temperature (Figure S22). Analysis of the Van't Hoff plot yielded the following thermodynamic parameters ∆G o 298 = -0.63 ± 0.05 kJ . mol -1 , ∆H o = 7.3 ± 0.4 kJ . mol -1 and ∆S o = 26.6 ± 1.5 J . mol -1. K -1 , indicating that at 298 K 3b is moderately more stable than 3a. The rate constants of interconversion between the two conformers were derived from the intensity of the exchange cross peaks in the ROESY spectra (Figure S23). The corresponding activation parameters determined from an Eyring analysis are summarized in Table 2. The activation energy obtained experimentally for 3b to 3a (72.2 ± 3.2 kJ . mol -1 ) is in very good agreement with the DFT calculation (66.6 kJ . mol -1 ). Surprisingly both Eyring and Van't Hoff plot analysis suggest large positive entropy for the isomerization of 3a to 3b. These significant entropic changes were not reproduced by DFT calculations, but we note that the entropic term is the most approximated thermodynamic term in the traditional DFT calculations of total and reaction (free) energies.

Electrochemical properties

The electrochemical behavior for 1-4 and R4 was evaluated by cyclic voltammetry (CV) and differential pulse voltammetry (DPV) in deaerated MeCN solution containing n-Bu4NPF6

as the supporting electrolyte and ferrocene/ferrocenium (Fc/Fc + ) as the internal reference. The potentials are referenced with respect to SCE (Fc/Fc + = 0.38 V in MeCN) [START_REF] Pavlishchuk | Conversion constants for redox potentials measured versus different reference electrodes in acetonitrile solutions at 25°C[END_REF] at 298 K. The electrochemistry data, obtained at a scan rate of 100 mV s -1 , can be found in Table 3 (including data from R1-R4) and the voltammograms are shown in Figure 5 (R4 is shown in Figure S24 in the SI). All complexes show a quasi-reversible single electron oxidation peak, which is attributed to the Ir(III)/Ir(IV) redox couple with contributions from the phenyl rings of the bnpy.

Throughout the series, complexes 1-4 are easier to oxidize than their corresponding reference complexes R1-R4 with cathodic shifts ranging from 0.09 to 0.40 V as the result of the interruption of the π-conjugation. The absence of a conjugated coordinating pyridine results in a destabilized HOMO, compared to the analogous ppy reference complexes; for instance, according to DFT the HOMO is 0.24 eV lower in R1 than in 1 (see Figure S25). For instance, the oxidation potential of 1 at 1.12 V is significantly shifted to less positive potential than that of R1 ( / = 1.27 V). [START_REF] Ladouceur | Enhanced Luminescent Iridium(III) Complexes Bearing Aryltriazole Cyclometallated Ligands[END_REF] A more dramatic cathodic shift of 400 mV is observed for 2 ( / = 0.91 V) compared to R2 ( / = 1.31 V). [START_REF] Donato | Cationic iridium(iii) complexes bearing a bis(triazole) ancillary ligand[END_REF] The oxidation potential of 3 at 1.15 V is similar to 1 and, likewise is easier to oxidize than R3 ( / = 1.24 V). [START_REF] Pal | Simple design to achieve red-to-near-infrared emissive cationic Ir(iii) emitters and their use in light emitting electrochemical cells[END_REF] The oxidation potential of 4 at 1.05 is modestly cathodically shifted by 7 mV compared to 1 and by 10 mV compared to R4

( / = 1.15 V). The cathodic shifting of the complexes 2 and 4 compared to that of 1 reflect the more electron-rich nature of the ancillary ligand, which serves to modulate the electronics of the iridium center.

Complexes 1-3 show monoelectronic reversible first reduction waves, reflecting a reduction of the ancillary ligand in each case. No reduction wave was detected for 4, a feature also found for related biim-containing cationic Ir(III) complexes such as [(dFppy)2Ir(biim)](PF6) (where dFppy = 2-(2,4-difluorophenyl)pyridinato). [START_REF] Henwood | Rigid biimidazole ancillary ligands as an avenue to bright deep blue cationic iridium(iii) complexes[END_REF] The voltammograms for R4 however show two irreversible reduction waves at -1.95 V and -2.29

V. The reversible reduction potentials for 1-3 are in a similar range to those of R1-R3 and other related cationic Ir(III) complexes where N^N-based reduction is well-documented. [START_REF] Lowry | Accelerated Luminophore Discovery through Combinatorial Synthesis[END_REF] The reduction potential of 1 at -1.38 V is the same as that found for R1. [START_REF] Ladouceur | Enhanced Luminescent Iridium(III) Complexes Bearing Aryltriazole Cyclometallated Ligands[END_REF] Complex 2, bearing the electron-rich dtBubpy N^N ligand, is more difficult to reduce than 1, with / = -1.58 V. This reduction potential is significantly cathodically shifted however compared to R2 ( / = -1.40 V). In contrast the reduction potential for 3 is shifted anodically by 270 mV to -1.11 V compared to 1, which is similar to that of R3 ( / = -1.15 V). [START_REF] Pal | Simple design to achieve red-to-near-infrared emissive cationic Ir(iii) emitters and their use in light emitting electrochemical cells[END_REF] A second, irreversible reduction wave at -1.81 V is observed for 3, also present in the voltammogram of R3.

In acetonitrile, the DFT-computed HOMO levels in acetonitrile of the major conformer of complexes 1-4 are -5.67 eV (1b), -5.63 eV (2b), -5.69 eV (3b), -5.74 eV (4a), respectively.

The HOMO levels inferred from the CV data are systematically slightly higher in energy compared to the DFT data at -5.54 eV (1), -5. a in degassed MeCN at a scan rate of 100 mV s -1 with Fc/Fc + as internal reference, and referenced with respect to SCE (Fc/Fc + = 0.38 V in MeCN); 29 b ∆Eredox is the difference (V) between first oxidation and first reduction potentials; c EHOMO/LUMO = -[E ox/red vs Fc/Fc + + 4.8] eV; 31 d irreversible; e not detectable in MeCN; f from ref [START_REF] Ladouceur | Enhanced Luminescent Iridium(III) Complexes Bearing Aryltriazole Cyclometallated Ligands[END_REF] ; g from ref [START_REF] Pal | Simple design to achieve red-to-near-infrared emissive cationic Ir(iii) emitters and their use in light emitting electrochemical cells[END_REF] .

Electrochemistry a / / V ∆ / mV / !" / V ∆ / mV ∆ !" b / V #$# c / eV %&$# c /

Photophysical properties

The UV-Visible absorption spectra of 1-4, recorded in MeCN at 298 K, are shown in Figure 6 and the data are summarized in Table S2; the spectrum of R4 is shown in Figure S26 in the SI. All complexes show intense high-energy (ε on the order of 2.0-3.5 × 10 4 M -1 cm -1 ) absorption bands below 280 nm that are attributed to 1 π-π* ligand-centered ( 1 LC) transitions localized on the ancillary ligand, analogous to the corresponding bands found for their reference complexes R1-R4. [START_REF] Ladouceur | Enhanced Luminescent Iridium(III) Complexes Bearing Aryltriazole Cyclometallated Ligands[END_REF][START_REF] Pal | Simple design to achieve red-to-near-infrared emissive cationic Ir(iii) emitters and their use in light emitting electrochemical cells[END_REF] The UV-vis spectrum of 3 is dominated by a moderately intense energy band at 322 nm (ε = 1.7 × 10 4 M -1 cm -1 ) and a second less intense band at 374 nm (ε = 0.5 × 10 4 M -1 cm -1 ). This absorption profile is similar to that for R3 [START_REF] Pal | Simple design to achieve red-to-near-infrared emissive cationic Ir(iii) emitters and their use in light emitting electrochemical cells[END_REF] and those observed in related systems containing thiazole-based ancillary ligands. [START_REF] Gärtner | Synthesis, Characterisation and Application of Iridium(III) Photosensitisers for Catalytic Water Reduction[END_REF][START_REF] Ertl | Highly Stable Red-Light-Emitting Electrochemical Cells[END_REF] Complex 4 shows moderately intense bands at 386 nm (ε = 0.4 × 10 4 M -1 cm -1 ), which are also present in both R4 and [Ir(dFppy)2(biim)] + . [START_REF] Henwood | Rigid biimidazole ancillary ligands as an avenue to bright deep blue cationic iridium(iii) complexes[END_REF] Similar to the bands observed for R1-R4, 1-4 show weak bands (ε on the order of 10 3 M -1 cm -1 ) in the region of 440-490 nm and tailing to 550 nm. To probe the nature of the low-lying transitions in these complexes, we have used TD-DFT. For 1, the TD-M06 calculations yield the four lowest singlet excited-states at 534 nm (f=0.003), 426 nm (f=0.002), 413 nm (f=0.010) and 399 nm (f=0.083). The former corresponds to a HOMO to LUMO transition (see Figure 7) and can therefore be ascribed to a CT from the phenyl rings of the C^N ligand and the metal atom to the bipyridine. The second involves a HOMO-2 to LUMO transition and is clearly MLCT. The third is a HOMO-1 to LUMO+1 transition and can therefore be characterized as mainly an ILCT, the electronic density moving from the phenyl (and in part the metal) to the pyridyl rings of the C^N ligands. The fourth transition is characterized by HOMO-1 to LUMO character and is also CT in nature, similar to the first band. For the R1 molecule, the two lowest TD-DFT singlets are located at 483 nm (f=0.0004) and 404 nm (f=0.064). In R1, the HOMO and LUMO have the same electronic distribution as in 1, but both HOMO-1 and LUMO+1 are more delocalized than in 1 (see Figure S25 in the SI), which is the consequence of breaking of the π-conjugation due to the methylene unit. For 2, the four lowest singlet excited-states are calculated at 500 nm (f=0.003), 415 nm (f=0.008), 406 nm (f=0.007) and 385 nm (f=0.098). There is therefore a moderate hypsochromic displacement of the transitions between the ground state and the first, second and fourth singlet excited states as a consequence of the presence of the more electron-rich dtBubpy N^N ligand in 2, and a smaller shift for the third state, which is itself consistent with the fact that this transition does not involve the bipyridine in 1 nor in 2. Indeed, the nature of the states and the shapes of the orbitals in 2 are essentially unchanged compared to 1. In 3, the four lowest-lying singlet excited-states are at 638 nm (f=0.002), 486 nm (f=0.002), 443 nm (f=0.096) and 413 nm (f=0.014), and they mainly correspond to HOMO to LUMO, HOMO-2 to LUMO, HOMO-1 to LUMO and HOMO to LUMO+1 transitions, respectively. In 3, the strongest low-lying transition is therefore significantly bathochromically shifted compared to 1, which fits experimental trends (Figure 6). The frontier MOs of 3b are displayed in Figure S27 in the SI and the same nature as in 1 is found but for the LUMO+2 that is centered on the pyridyl of the C^N ligand in 3 rather than ancillary ligand in 1. In 4, TD-DFT locates the lowest singlet states at higher energy than in 1-3: 392 nm (f=0.001, HOMO-LUMO), 380 nm (f=0.001, HOMO-LUMO+1), 359 nm (f=0.007, HOMO-LUMO+2) and 353 nm (f=0.165, HOMO-1-LUMO+1).

The topology of the MOs of 4b are displayed in Figure S28 in the SI and it is notable that the LUMO and LUMO+1 are inverted compared to 3, but the nature of the most intense low-lying intense CT band is conserved. 4. The emission spectrum of R4 in degassed MeCN is shown in Figure S29 in the SI. Upon photoexcitation (λexc = 420 nm for 1-3 and 400 nm for 4) each of the emission profiles observed at room temperature is broad and unstructured, which is consistent with an emission from a mixed MLCT/LLCT state (Figure 7b), similar in character to that observed for R1-R3 [START_REF] Pal | Simple design to achieve red-to-near-infrared emissive cationic Ir(iii) emitters and their use in light emitting electrochemical cells[END_REF][START_REF] Ladouceur | Enhanced Luminescent Iridium(III) Complexes Bearing Aryltriazole Cyclometallated Ligands[END_REF] and R4. In solution, the emission maxima are ordered from 580, 585, 602 and 655 nm for 4, 2, 1 and 3, respectively. The emission energy of 1 is the same as that for R1 (λPL = 602 nm in MeCN). [START_REF] Ladouceur | Enhanced Luminescent Iridium(III) Complexes Bearing Aryltriazole Cyclometallated Ligands[END_REF] Complexes 2 and 3 are very slightly shifted (6 nm, 174 cm -1 , and 3 nm, 70 cm -1 ) compared to R2 and R3 (λem = 591 nm in MeCN [START_REF] Ladouceur | Enhanced Luminescent Iridium(III) Complexes Bearing Aryltriazole Cyclometallated Ligands[END_REF] and λem = 658 nm in MeCN). [START_REF] Pal | Simple design to achieve red-to-near-infrared emissive cationic Ir(iii) emitters and their use in light emitting electrochemical cells[END_REF] Comparing 4 to R4 (λPL = 489 nm) a much larger shift of 91 nm (3209 cm -1 ) is observed.

The photoluminescence quantum yields in MeCN (ΦPL, MeCN) for 1-4 are low (<10%)

and each complex shows a ΦPL that is reduced compared to the corresponding reference complexes, except complex 3. For instance, the ΦPL, MeCN for 1 and 2, at 3% and 8%, are lower than that of R1 and R2, at 9% and 27%, respectively. [START_REF] Ladouceur | Enhanced Luminescent Iridium(III) Complexes Bearing Aryltriazole Cyclometallated Ligands[END_REF] The ΦPL, MeCN for 3 is 3%, which is similar to that for R3 (2%). [START_REF] Pal | Simple design to achieve red-to-near-infrared emissive cationic Ir(iii) emitters and their use in light emitting electrochemical cells[END_REF] Complex 4 is very weakly emissive with a ΦPL, MeCN smaller than 1%, significantly lower than its R4 counterpart (ΦPL, MeCN = 66%), showing the deleterious effect of the bnpy ligand in this case. Note that 4 is the only complex in the series for which conformer a is the most stable in the ground electronic state at room temperature, and this specific arrangement has an influence on the resulting photophysical properties as shown by DFT studies (see below).

The increased conformational flexibility of the C^N ligands is the likely culprit for the reduced ΦPL values in 1-4 compared to R1-R4. To gain more insights, we performed DFT optimizations of the lowest triplet excited-states. We computed a 0-0 phosphorescence wavelength of 654 nm, 620 nm, 793 nm and 510 nm for 1, 2, 3 and 4, respectively. Despite the obvious fact that the absolute DFT values do not perfectly match the experimental values (ca.

0.22 eV average error compared to the λPL maxima in solution), we note that the theoretical 4 < 2< 1 < 3 ranking does match the experimental trend. More importantly, in the triplet state, DFT predicts that conformer b is always the most stable, even for 4. This suggests that, after intersystem crossing, which is generally very efficient in Ir-complexes, 4 might change conformation before phosphorescence takes place, which would obviously be detrimental for the photoluminescence quantum yield. In turn, this might explain why 4 is the poorest emitter of the series.

Complexes 1-4 all exhibit a three-component decay in the sub-microsecond regime (cf.

Table 4). The weighted average lifetimes, τPL, for 1-4 are 124, 275, 71 and 235 ns, respectively, and are shorter compared to their reference complexes (τPL = 275, 386, 81, 852 ns for R1-R3 [START_REF] Ladouceur | Enhanced Luminescent Iridium(III) Complexes Bearing Aryltriazole Cyclometallated Ligands[END_REF][START_REF] Pal | Simple design to achieve red-to-near-infrared emissive cationic Ir(iii) emitters and their use in light emitting electrochemical cells[END_REF] and R4, respectively). From these values, radiative and non-radiative rate constants in MeCN can be estimated. The radiative rate constants, kr, for 1 (2.41 × 10 5 s -1 ) and 2 (2.91 × 10 5 s -1 ) are similar and much larger than that for 4 with kr = 0.21 × 10 5 s -1 . These complexes (1, 2 and 4) all possess much smaller kr than their reference complexes (R1, R2 and R4). Complex 3 shows the highest radiative rate constant with kr = 4.2 × 10 5 s -1 , which is also higher than R3 (kr = 2.12 × 10 5 s -1 ). However, complex 3 also shows the highest non-radiative rate constant knr value (135.95 × 10 5 s -1 ), and which is slightly higher than the value of R3 (knr = 121.33 × 10 5 s -1 ).

Complex 1 (knr = 77.97 × 10 5 s -1 ) shows a higher knr than 2 (33.49 × 10 5 s -1 ), which can be explained in part by the red-shifted emission of 1 compared to that of 2. Compared to their reference complexes, both 1 and 2 show much higher knr values (knr = 33 × 10 5 s -1 for R1 and knr = 19 × 10 5 s -1 for R2). The poorly emissive complex 4 possesses a knr of 42.43 × 10 5 s -1 , which is much higher than the value for R4 (3.99 × 10 5 s -1 ).

The emission energies in PMMA-doped films (5 wt% of complex in PMMA) are not significantly changed compared from those in MeCN. The emission maxima in 1-3 remain essentially unchanged whereas complex 4 shows a modest blue shift of 15 nm (458 cm -1 ) in the film. The photoluminescence quantum yields of the films (ΦPL, PMMA) are expectedly significantly increased compared to the solution-state measurements and range from 1% (for 4)

to 7% (for 3) to 15% (1) to 21% (2). The increase in ΦPL in the doped films is attributed mainly to a reduction in knr due to the expected limitations of the conformational motion of the C^N ligands. Each of 1-4 in doped films exhibit a three-component emission decay in the submicrosecond regime. The weighted average lifetimes for 1-4 are 404, 509, 323 and 1196 ns, respectively, which are on average longer than in those measured in solution-state. The low-intensity emission spectra at 77 K in 2-MeTHF glasses of 1-4 are noisy. Figure 8b shows the smoothed emission profiles. A significant hypsochromic shift is observed compared to the measurements at 298 K, both in MeCN and as doped films. For instance, the profile for complex 1 shows an emission maximum at 544 nm and a shoulder at 572 nm resulting in a blue shift of 58 nm (900 cm -1 ) for the E0,0 peak compared to the spectrum in MeCN at 298 K with almost no shift compared to the emission of R1 in the 2-MeTHF glass (λPL = 542 nm).

The blue shift is less pronounced for 2 (33 nm, 1022 cm -1 ), with peaks at 528, 552 and 536 (λmax = 552 nm). However, compared to R2 the highest intensity emission peak of complex 2 shows a significant red shift of 79 nm (3026 cm -1 ). For complex 3 the normalized profile is centered at 604 nm (blue shift of 51 nm, 1289 cm -1 compared to the spectrum in MeCN). Complex 4

(λexc = 380 nm) is also very poorly emissive and the profile shows two moderately intense bands at 428 nm and 484 nm (and a shoulder at 534 nm) and the λPL located at 458 nm shows significant blue shift compared to the spectrum in MeCN (122 nm, 4593 cm -1 ). The monoexponential emission lifetimes at 77 K in the microsecond regime are significantly longer compared to both sets of room-temperature measurements and range from 2088 ns (1), to 2120 ns (2), to 2363 ns (3) and 4423 ns (4). Compared to the reported emission lifetimes at 77 K of R1 (4770 ns) and R2 (4550 ns), [START_REF] Ladouceur | Enhanced Luminescent Iridium(III) Complexes Bearing Aryltriazole Cyclometallated Ligands[END_REF] 1 and 2 show significantly shorter lifetimes. [START_REF] Pal | Simple design to achieve red-to-near-infrared emissive cationic Ir(iii) emitters and their use in light emitting electrochemical cells[END_REF] .

Conclusions

Experimental Section

General Synthetic Procedures

Commercial chemicals were used as supplied. All reactions were performed using standard Schlenk techniques under inert (N2) atmosphere with reagent-grade solvents. Flash column chromatography was performed using silica gel (Silia-P from Silicycle, 60 Å, 40-63 µm).

Analytical thin layer chromatography (TLC) was performed with silica plates with aluminum backings (250 µm with indicator F-254). Compounds were visualized under UV light. 1 H, [START_REF] Li | C-H Activation of Phenyl Imines and 2-Phenylpyridines with [Cp*MCl2]2 (M = Ir, Rh): Regioselectivity, Kinetics, and Mechanism[END_REF] C and 31 P solution-phase NMR spectra were recorded on a Bruker Avance spectrometer operating at 11.7 T (Larmor frequencies of 500, 400, 126 and 162 MHz, respectively). The following abbreviations have been used for multiplicity assignments: "s" for singlet, "d" for doublet, "t" for triplet and "m" for multiplet. 1 H and [START_REF] Li | C-H Activation of Phenyl Imines and 2-Phenylpyridines with [Cp*MCl2]2 (M = Ir, Rh): Regioselectivity, Kinetics, and Mechanism[END_REF] C NMR spectra were referenced to the solvent peak. The IrCl3 . 6H2O (2.0 equiv.) and 2-benzylpyridine (5.0 equiv.) were suspended in a mixture of 2-ethoxyethanol/water (3:1). The mixture was heated and kept at 125 °C under stirring. After 24 h the mixture was allowed to cool to r.t. and distilled water was added. A precipitate was observed. It was washed with Et2O, H2O and dried under vacuum to give the intermediate bis(µ-Cl) dimer complex. A suspension of this dimer (1.0 equiv.), the corresponding ancillary ligand (2.2 equiv.) in a mixture of CH2Cl2/methanol (1:1) was degassed for 15 min and then heated and kept at reflux for 18 h under stirring. The solvent was then evaporated leaving a residue, which was purified over silica with dichloromethane and increasing percentages of methanol (0% -8%). The desired fractions were collected and reduced till dryness, giving a solid that was then dissolved in methanol. An aqueous NH4PF6 solution was added dropwise resulting in a precipitate. The suspension was stirred vigorously for 2 h and subsequently filtered. The solid was dissolved in CH2Cl2 and washed with water. The layers were separated, and the organic layer was reduced till dryness leaving a solid, which was recrystallized through vapor diffusion using CH2Cl2 as solvent and Et2O as precipitant. After filtration, the target complexes were obtained as solids.

,R4.

  Cl is shown in Figure S20 in the SI). All three (1-3) complexes possess a distorted octahedral geometry, with the two bnpy ligands in complexes 1-3 coordinated to iridium to form a six-membered chelate ring. The pyridine rings of the C^N ligand are disposed in a mutually trans arrangement while the cyclometalating carbon atoms are trans to the nitrogen atoms of the N^N ligand, presenting an analogous binding mode to the majority of cationic Ir(III) complexes such as R1-R4.Cl. In 1-3 the Ir-CC^N bonds are noticeably shorter [2.022(5) -2.052(7) Å for 1-3; 1.988(6) -2.016(7) for R4.Cl] than the Ir-NN^N bonds, which range from 2.148(5) to 2.203(4) Å [2.153(5) -2.184(5) Å for R4.Cl]. These bond lengths are all similar to

  and 87.98(13)° for 3, 88.1(3) and 88.3(6)° for 1, and 88.8(2) and 89.2(3)° for 2; reflecting the increasing steric demand of the different N^N ligands. These bite angles are in the same range as those observed in a related complex using a different nonconjugated C^N ligand with a methylene spacer, [Ir(dFbpz)2(pymbi)]PF6 [where dFbpzH is 1-(2,4-difluorobenzyl)-1H-pyrazole and pymbi 3methyl-1-(2-pyridyl)-benzimidazolin-2-ylidene-C,C 2 ], the bite-angle across the dFbpz ligand being 88.80°. 11 As expected, the bite angles of the bnpy ligands are significantly larger than those seen in the reference complexes, which contain five-membered ring C^N chelates [R1: 80.00°, R2: 80.42°, R3: 80.60° and R4.Cl: 80.1(3)-80.7(2)°].

Figure 1 . 2 vs 3 .

 123 Figure 1. Solid-state structures of complexes 1-3, with thermal ellipsoids drawn at the 50 % probability level. Hydrogen atoms, PF6 -counterions, solvent molecules and minor components of disorder are omitted for clarity. Note the different orientations of the bnpy ligand in 1 and 2 vs 3. Color codes are: C = grey, N = light purple, S = yellow and Ir = blue.

Figure 2 .

 2 Figure 2. Room-temperature 1 H-NMR spectra of complexes 1-4 in CDCl3.

Figure 3 .

 3 Figure 3. DFT-optimized geometry of conformers 3a (left) and 3b (right). Hydrogen atoms are omitted for clarity.

Figure 4 .

 4 Figure 4. 2D ROESY spectrum of 3 in CDCl3 at room temperature. The blue and red crosspeaks indicate magnetization transfer due to NOE and chemical exchange, respectively. The cross-peak between doublets at 4.54 and 8.18 ppm enables assignment of those resonances to conformer 3a.

  33 eV (2), -5.57 eV (3) and -5.47 eV(4). A notable destabilization of the experimentally determined HOMO of 1 and 2 is observed compared to their respective reference complexes R1 (-6.07 eV) and R2 (-6.11 eV); the same trend is found but is less pronounced when comparing the HOMO energies of 3 and 4 to R3 (-5.85 eV) and R4 (-5.57 eV). This destabilization is consistent with the interruption of the electronic communication between the phenyl ring and the electron-withdrawing pyridine in the C^N ligands.The DFT computed LUMO energies in acetonitrile of 1-4 are -2.42 eV (1b), -2.23 eV (2b) and -2.82 eV (3b); the calculated LUMO energy for 4a is -1.59 eV. The LUMO levels for 1-3 could be straightforwardly estimated from the CV data and the experimental results are in line with the computed trends. The experimentally inferred LUMO of 2 ( = -2.84 eV) is destabilized compared to 1 ( = -3.04 eV) as a result of the presence of the electrondonating t-butyl substituents on the N^N ligand. This effect is more pronounced than that observed between R1 and R2 ( = -3.42 eV and -3.40 eV, respectively). The presence of the strongly π-accepting bithiazole N^N ligand in 3 leads to a significant stabilization of the LUMO ( = -3.31 eV) compared to those of 1 and 2. Complex 3, however, exhibits a more stabilized LUMO level ( = -3.31 eV) than R3 ( = -2.76 eV). The HOMO-LUMO gap of complexes 1 and 2 (experiment: 2.50 and 2.49 eV, theory: 3.25 and 3.40 eV for 1 and 2, respectively) are larger than that of 3 (experiment 2.26 eV, theory: 2.87 eV) but are all smaller compared to their reference complexes R1-R3 (experiment 2.65, 2.71 and 2.39 eV, respectively).

Figure 5 .

 5 Figure5. Cyclic voltammograms (in solid lines) and differential pulse voltammetry (in dotted lines) for 1-4 carried out in degassed MeCN at a scan rate of 100 mV s -1 , with Fc/Fc + as the internal reference, referenced to SCE (0.38 V vs. SCE).[START_REF] Pavlishchuk | Conversion constants for redox potentials measured versus different reference electrodes in acetonitrile solutions at 25°C[END_REF]

Figure 6 .

 6 Figure 6. UV-vis absorption spectra in MeCN at 298 K of complexes 1-4.

Figure 7 .

 7 Figure 7. a) Representation of the Kohn-Sham molecular orbitals for 1. b) spin density difference of the lowest triplet state in its optimal geometry (two views).

Figure 8 .

 8 Figure 8. a) Photoluminescence spectra of complexes 1-4 recorded in MeCN (dotted lines) and PMMA doped films (5 wt% of complex in PMMA) in solid lines (λexc = 420 nm for 1-3 and λexc = 400 nm for 4). b) Photoluminescence spectra of complexes 1-4 recorded in 2-MeTHF at 77 K. λexc = 420 nm for complexes 1-3 and λexc = 380 nm for 4.

A 4 .

 4 new series of four cationic Ir(III) complexes bearing nonconjugated C^N ligands, using the bnpy ligand for the first time in luminescent Ir(III) complexes, have been synthesized and fully characterized. Comparative studies with the related series of ppy-based complexes show the impact of the methylene spacer present within the cyclometalated ligand on the electrochemical and photophysical properties. Analysis of the X-ray crystal structures for three of the complexes clearly illustrates the existence of two possible conformations. Depending on the nature of the ancillary ligand, phosphorescence ranging from yellow to deep red is observed in MeCN as well as in 5 wt% PMMA doped films. Variable-temperature 1 H and 2D ROESY-NMR studies show the presence of two conformers for complex 3 as a result of the fluxional behavior of the bridging methylene group of the cyclometalated bnpy ligand, which is consistent with DFT calculations yielding a small free energy difference but a rather large barrier to interconversion between the two conformers. DFT calculations suggest that the transition from one conformation to the other is a concerted process in which the two ligands simultaneously change their conformation. A similar behavior is observed 4, but for this complex the major conformer is the opposite to the one observed for 3. Under the same conditions, however, complexes 1 and 2 only show one conformer in the 1 H NMR at room temperature. All four complexes show quasi-reversible oxidation waves. Complexes 1-3 show quasi-reversible reduction waves while no reduction wave was detected within the solvent window for Compared to their reference complexes R1-R4, 1-4 emit at similar energies but show much lower photoluminescence quantum yields and shorter emission lifetimes. DFT calculations predict that conformer b is the most stable in the triplet state for 1-4. Breaking the conjugation of the C^N ligands did not change the triplet energy of the complexes despite reducing the electrochemical gap. This detailed joint theoretical and experimental study provides a better understanding of the role of the methylene spacer in the 2-benzylpyridinato cyclometalating ligands within this new series of Ir(III) complexes.

4 N

 4 Melting points(Mps) were recorded using open-ended capillaries on an electrothermal melting point apparatus and are uncorrected. High-resolution mass spectra were recorded at the EPSRC UK National Mass Spectrometry Facility at Swansea University on a quadrupole time-of-flight (ESI-Q-TOF), model ABSciex 5600 Triple TOF in positive electrospray ionization mode and spectra were recorded using sodium formate solution as the calibrant. Elemental analyses were performed by Mr. Stephen Boyer, London Metropolitan University.General procedure for the Synthesis of target Complexes 1-

Complex 1 2 ( 4

 124 Red solid. Yield: 78%. Mp.: 244 -248 °C. 1 H NMR (500 MHz, CDCl3) δ (ppm): 9.01 (d, J = 8.2 Hz, 2H), 8.32 (d, J = 5.4 Hz, 2H), 8.25 (td, J = 7.9, 1.6 Hz, 2H), 7.64 (s, 4H), 7.38 (t, J = 6.6 Hz, 2H), 7.32 (s, 2H), 7.00 (s, 4H), 6.92 (t, J = 6.5 Hz, 2H), 6.87 (t, J = 6.6 Hz, 2H), 6.61 (d, J = 7.6 Hz, 2H), 3.93 (d, J = 15.6 Hz, 2H), 3.56 (d, J = 15.7 Hz, 2H).13 C NMR (126 MHz, CDCl3) δ (ppm): 163.39, 155.98, 150.88, 150.64, 139.77, 139.26, 138.32, 136.54, 135.87, 127.33, 127.07, 127.02, 126.90, 126.11, 123.88, 123.31, 49.18. 31 P NMR (162 MHz, CDCl3) δ (ppm): -144.34. HR-MS (FTMS + ): [M] + Calculated: (C34H28IrN4): 685.1939; Found: 685.1936. CHN: Calcd for (C34H28F6IrN4P): C, 49.21; H, 3.40; N, 6.75 Found: C, 49.06; H, 3.28; N, 6.68. Complex C30H24IrN4S2): 697.1067; Found: 697.1049. CHN: Calcd. for C30H24F6IrN4PS2: C, 42.80; H, 2.87; N, 6.66. Found: C, 42.43; H 2.82; N 6.55. The NMR data correspond to the mixture of3a and 3b at 298 K. Complex Beige solid. Yield: 80%. Mp.: °C (decomp.). 1 H NMR (400 MHz, CDCl3) δ (ppm): 8.13 (d, J = 6.9 Hz, 2H), 7.59 (td, J = 7.6, 1.5 Hz, 2H), 7.36 -7.32 (m, 3H), 7.24 (s, 2H), 7.17 (d, J = 7.7 Hz, 2H), 6.95 -6.84 (m, 3H), 6.80 -6.74 (m, 2H), 6.53 (t, J = 7.4 Hz, 2H), 5.43 (d, J = 8.7 Hz, 2H), 4.73 (d, J = 16.3 Hz, 2H), 4.04 (d, J = 16.0 Hz, 2H). 13 C NMR (126 MHz, CDCl3) δ (ppm): 162.82, 156.23, 141.86, 140.51, 138.47, 137.85, 137.74, 128.59, 126.13, 125.84, 124.40, 122.88, 122.50, 120.15, 48.54. 31 P NMR (162 MHz, CDCl3) δ (ppm): -143.81. HR-MS (FTMS + ): [M] + Calculated: (C30H26IrN6) 663.1844; Found: 663.1834. The NMR data correspond only to the major conformer 4a. Complex R4 Complex R4 was obtained according the general procedure using 2-phenylpyridine instead of 2-benzylpyridine. Beige solid. Yield: 80%. Mp.: 259 °C (decomp.). 1 H NMR (400 MHz, CDCl3) δ (ppm): 12.01 (s, 2H), 8.01 (d, J = 8.1 Hz, 2H), 7.84 -7.78 (m, 2H), 7.78 -7.68 (m, 4H), 7.25 (d, J = 1.5 Hz, 2H), 7.07 (ddd, J = 7.4, 5.8, 1.4 Hz, 2H), 6.99 -6.93 (m, 2H), 6.83 (td, J = 7.4, 1.4 Hz, 2H), 6.47 (d, J = 1.5 Hz, 2H), 6.33 (dd, J = 7.6, 1.2 Hz, 2H).13 C NMR (126 MHz, CDCl3) δ (ppm): 168.73, 150.44, 149.61, 145.52, 141.34, 138.78, 132.72, 130.45, 127.88, 125.18, 123.98, 122.59, 121.60, 120.00.HR-MS (FTMS + ): [M] + Calculated: (C30H26IrN6) 635.1531; Found: 653.1517. CHN: Calcd. for C28H22F6IrN6P: C, 43.13; H, 2.84; N, 10.78. Found: C, 43.16; H 2.91; N 10.83. The crystal structure obtained is of the chloride salt, denoted as R4.Cl.AcknowledgmentWe thank Umicore AG for the gift of materials. We thank the EPSRC UK National Mass Spectrometry Facility at Swansea University for analytical services. C.H. acknowledges the Région Bretagne, France for funding. D.J. acknowledges the European Research Council and the Région des Pays de la Loire for financial support in the framework of a Starting Grant (Marches -278845) and the LUMOMAT RFI project, respectively. This research used resources of 1) the GENCI-CINES/IDRIS, 2) the CCIPL (Centre de Calcul Intensif des Pays de Loire), 3) a local Troy cluster. E.Z.-C. acknowledges the University of St. Andrews and EPSRC (EP/M02105X/1) for financial support. Supporting information. CIF files for the X-ray structures of 1-3 and R4 (CCDC: 1584175-1584178). NMR and mass spectra for 1-4, R4, Supplementary crystallographic, electrochemical and photophysical data. Description of the computational protocol and supplementary computational results including a movie of the transition state connecting 3a and 3b. References

Table 1 .

 1 Comparison

	K, at 298 K and ratio of conformers derived from relative integral intensities of methylene
	group 1 H resonances.
		1	2	3	4
	∆G o / kJ . mol -1	-13.4	-9.6	-2.1	5.4
	Boltzmann ratio (a:b)	0.004:1	0.02:1	0.43:1	1:0.11
	K	223	48.2	2.33	0.113
	Ratio of conformers a:b	0:1	0:1	0.83:1	1:0.17

of relative DFT Gibbs energies of the conformers 1b -4b with respect to conformers 1a -4a, respectively (∆G o ), Boltzmann ratio, corresponding equilibrium constant,

Table 2 .

 2 Activation parameters for the interconversion between conformers 3a and 3b for complex 3 obtained by Eyring plot analysis of the rate constants for conformer interconversion at various temperatures.

	Conformer	‡	‡	‡
		/ kJ . mol -1	/ kJ . mol -1	/ J . mol -1. K -1
	3a	70.6 ± 4.9	80.0 ± 2.4	31.6 ± 8.1
	3b	72.2 ± 3.2	71.9 ± 1.6	-0.8 ± 5.5

Table 3 .

 3 Electrochemical

properties of 1-4 and R1-R4

Table 4 .

 4 Photophysical properties of 1-4 and R1-R4 In deaerated MeCN at 298 K; b at 298 K, spin-coated from a 2-methoxyethanol solution of 5 wt% of the complex in PMMA on a pristine quartz substrate; c in 2-MeTHF at 77 K, if not specified differently; d λexc = 420 nm, if not specified differently; e [Ru(bpy)3](PF6)2 in MeCN as reference (ΦPL = 1.8% in aerated MeCN at 298 K)[START_REF] Suzuki | Reevaluation of absolute luminescence quantum yields of standard solutions using a spectrometer with an integrating sphere and a back-thinned CCD detector[END_REF] ; f λexc = 378 nm, average weighted lifetimes were determined according to equation τPL,avg= ΣAiτi (Ai = pre-exponential factor of the ith lifetime τi); g Measured using an integrating sphere; h kr = ΦPL,MeCN / τPL; i knr = [(1 -ΦPL, MeCN)/ τPL]; j λexc = 400 nm; k λexc = 380 nm; l from ref[START_REF] Ladouceur | Enhanced Luminescent Iridium(III) Complexes Bearing Aryltriazole Cyclometallated Ligands[END_REF] ; m Measured in 1/1 MeOH/EtOH glass state at 77 K; n from ref

			MeCN a		Film b	Glass c
		λPL d	ΦPL e	τPL f	λPL d	ΦPL g	τPL f	λPL d	τPL f	kr h	knr i
		/ nm	/ %	/ ns	/ nm	/ %	/ ns	/ nm	/ ns	× 10 -5 / s -1	× 10 -5 / s -1
				26 (25%)			124 (11%)
				110 (60%)			316 (56%)
	1	602 3	346 (15%)	600 15	646 (33%)	544, 572 (sh)	2088 2.41	77.97
				Average			Average
				124			404
				52 (29%)			99 (4%)
				215 (58%)			318 (43%)	528,
	2	585 8	1038 (13%)	584 21	695 (53%)	536,	2120 2.91	33.49
				Average			Average	552
				275			509
				54 (79%)			93 (16%)
				95 (19%)			254 (57%)
	3	655 3	532 (2%)	655 7	605 (27%)	604	2363 4.20	135.95
				Average			Average
				71			323
				17 (3%)			49 (2%)
	4	580 j 0.5	131 (60%) 420 (37%)	565 j 1	361 (23%) 1482 (75%)	428, 458, 484,	4423 0.21	42.43
				Average			Average	534 (sh) k
				235			1196
	R1 l 602 9	275			-	542	4770 m 3.4	33
	R2 l 591 27	386			-	473	4550 m 7.0	19

a