Lea Gimeno

Brian T Phelan

Emily A Sprague-Klein

Thierry Roisnel

Errol Blart

Christophe Gourlaouen
email: gourlaouen@unistra.fr

Lin X Chen
email: lchen@anl.gov

Yann Pellegrin
email: yann.pellegrin@univ-nantes.fr

A bulky and stable

des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Copper(I) complexes are drawing increased attention as photosensitizers, efficiently replacing heavy metal complexes in lighting devices, 1 dye sensitized solar cells, 2 solar fuel production [START_REF] Khnayzer | Robust Cuprous Phenanthroline Sensitizer for Solar Hydrogen Photocatalysis[END_REF] and organic photochemistry. [START_REF] Baralle | Aryl Radical Formation by Copper(I) Photocatalyzed Reduction of Diaryliodonium Salts: NMR Evidence for a CuII/CuI Mechanism[END_REF] Homoleptic copper(I) complexes of the general formula [CuL2] + where L is a diimine ligand are particularly interesting. They absorb over a large wavelength range (400-550 nm or farther in the red depending on L) [START_REF] Meyer | π-π Stacking-Induced Cooperativity in Copper(I) Complexes with Phenanthroline Ligands[END_REF][START_REF] Kern | Photoassisted C-C coupling via electron transfer to benzylic halides by a bis(di-imine) copper(I) complex[END_REF] through a metal to ligand charge transfer transition (MLCT) and emit in the red to near infrared, with respectable luminescence quantum yields Φ (up to 0.063) [START_REF] Mccusker | Design of a Long-Lifetime, Earth-Abundant, Aqueous Compatible Cu(I) Photosensitizer Using Cooperative Steric Effects[END_REF] and lifetime τ up to a few microseconds. [START_REF] Green | Photophysical Characteristics and Reactivity of Bis(2,9-ditert-butyl-1,10-phenanthroline)copper(I)[END_REF] Importantly, it is necessary to tether sterically challenging groups in positions 2 and 9 of 1,10-phenanthroline (phen) to promote advantageous excited-state properties. [START_REF] Mcmillin | A light-induced redox reaction of bis(2,9-dimethyl-1,10phenanthroline)copper(I)[END_REF]10 The rationale is that photoexcitation of the MLCT transition entails electron transfer from copper-centered d orbitals towards π* ligand-centered orbitals. Thus, [Cu I L2] + transiently turns into [Cu II (L •-)L] + , leading to a drastic geometry change from pseudo-tetrahedral D2d towards square planar D2. In other words, the coordination sphere flattens to make the transient Cu II center in [Cu II (L •-)L] + more exposed to the solvent molecules, enabling fast quenching of the excited state assuming the solvent-solute interactions lower the energy gap between the MLCT triplet and the ground states.

Grafting bulky groups in α-positions of the nitrogen atoms on phen hinders the flattening of the copper(I) coordination sphere in the MLCT state and shields the Cu II center from solvent quenching, thus enhancing the excited-state lifetime as well as other photochemical properties.

In addition, increasing the steric bulkiness around copper(I) elevates the energy of the excited state, [START_REF] Miller | Effects of Sterics and Electronic Delocalization on the Photophysical, Structural, and Electrochemical Properties of 2,9-Disubstituted 1,10-Phenanthroline Copper(I) Complexes[END_REF][START_REF] Gimeno | Non-Symmetrical Sterically Challenged Phenanthroline Ligands and Their Homoleptic Copper(I) Complexes with Improved Excited-State Properties[END_REF] improving its reactivity from a thermodynamic point of view. Thus, larger steric strain around copper(I) generally leads to larger fluorescence yield Φ and longer excited-state lifetime τ. [START_REF] Mccusker | Design of a Long-Lifetime, Earth-Abundant, Aqueous Compatible Cu(I) Photosensitizer Using Cooperative Steric Effects[END_REF][START_REF] Green | Photophysical Characteristics and Reactivity of Bis(2,9-ditert-butyl-1,10-phenanthroline)copper(I)[END_REF][START_REF] Garakyaraghi | Cuprous Phenanthroline MLCT Chromophore Featuring Synthetically Tailored Photophysics[END_REF][START_REF] Eggleston | Steric Effects in the Ground and Excited States of Cu(NN)2+ Systems[END_REF] In dichloromethane (DCM), for example, complexes like [Cu(dipp)2] +, [START_REF] Garakyaraghi | Cuprous Phenanthroline MLCT Chromophore Featuring Synthetically Tailored Photophysics[END_REF] (where dipp stands for 2,9-diisopropyl-1,10-phenanthroline) or [Cu(dsbtmp)2] +,7 (where dsbtmp stands for 2,9-disec-butyl-3,4,7,8-tetramethyl-1,10-phenanthroline) exhibit Φ of 0.4 and 6.3 %, and τ of 0.37 and 2.8 µs, respectively. [Cu(dmp)2] + (where dmp stands for 2,9-dimethyl-1,10phenanthroline) features more modest Φ (0.02%) and τ (ca. 55 ns), 10 while [Cu(phen)2] + is non luminescent at all. On the other hand, increasing the steric bulk around copper(I) too much can make it difficult to prepare the homoleptic complex. [START_REF] Gandhi | Facile Oxidation-Based Synthesis of Sterically Encumbered Four-Coordinate Bis(2,9-di-tert-butyl-1,10-phenanthroline)copper(I) and Related Three-Coordinate Copper(I) Complexes[END_REF][START_REF] Miller | A Highly Emissive Heteroleptic Copper(I) Bis(phenanthroline) Complex: [Cu(dbp)(dmp)[END_REF] For example, the synthesis of [Cu(dtbp)2] + (dtbp stands for 2,9-di-tert-butyl-1,10-phenanthroline) is very challenging, [START_REF] Gandhi | Facile Oxidation-Based Synthesis of Sterically Encumbered Four-Coordinate Bis(2,9-di-tert-butyl-1,10-phenanthroline)copper(I) and Related Three-Coordinate Copper(I) Complexes[END_REF] and the resulting complex is highly unstable. [START_REF] Green | Photophysical Characteristics and Reactivity of Bis(2,9-ditert-butyl-1,10-phenanthroline)copper(I)[END_REF] Nevertheless, even if [Cu(dtbp)2] + is an unstable complex, it exhibits fascinating properties, notably the longest luminescence lifetime recorded for a homoleptic copper(I) complex in solution (ca. 3.3 µs in DCM). 8 Structurally, stable [Cu(dipp)2] + differs from unstable [Cu(dtbp)2] + by only two methyl groups on the substituents of each phenanthroline ligand. With this in mind, we were curious if removing only one methyl group from dtbp, yielding the non-symmetrical phenanthroline ligand 2-tertbutyl-9-isopropyl-1,10-phenanthroline (L1), would be sufficient to afford a stable copper(I) complex [Cu(L1)2] + in the usual mild conditions used for the synthesis of these coordination scaffolds (Figure 1). In addition, we were interested to study the optical and electrochemical properties of the corresponding complex in both the ground and excited states. In this contribution, we report the synthesis of ligand L1 and associated complex [Cu(L1)2] + (named Cu1) as well as a detailed optical and electrochemical characterization. The stability and luminescence properties of Cu1 demonstrate that steric strain is not the sole factor governing the excited-state properties of copper(I) complexes. [START_REF] Gimeno | Non-Symmetrical Sterically Challenged Phenanthroline Ligands and Their Homoleptic Copper(I) Complexes with Improved Excited-State Properties[END_REF][START_REF] Brown-Xu | Intriguing Effects of Halogen Substitution on the Photophysical Properties of 2,9-(Bis)halo-Substituted Phenanthrolinecopper(I) Complexes[END_REF]

Results and discussion

Synthesis

The synthesis of ligand L1 is performed in two steps (Figure S1). First, nucleophilic attack of anhydrous phen by tert-butyllithium followed by re-aromatization with MnO2 afforded monosubstituted 2-tert-butyl-1,10-phenanthroline. The organolithium solution was carefully added (over 20 min) to limit the formation of the bis-substituted ligand. 2-tert-butyl-1,10phenanthroline was then subjected to the same series of reactions with isopropyllithium as the organolithium reagent, affording L1 in 77% yield over all steps. The homoleptic copper(I) complex [Cu(L1)2] + was finally assembled by mixing two equivalents of the ligand with one equivalent of copper(I) tetrakis acetonitrile hexafluorophosphate in degassed DCM at room temperature.

Structural properties

After solvent evaporation under reduced pressure, the obtained red powder was recrystallized from hot toluene. We obtained monocrystals which were suitable for a structural determination by X-ray scattering (Figure 2 and Figure S2). Relevant angles and distances are reported in Table 1 and nomenclature for the different atoms is given in figure S3. The structure reveals that the copper(I) ion is in a nearly tetrahedral environment. Cu-N distances are significantly influenced by the nature of the substituent in α of the corresponding nitrogen atom: tert-butyl groups impose a longer Cu-N distance (ca. 2.16 Å on average) whereas isopropyl allow for a shorter Cu-N bond length (2.04 Å on average) and likely a tighter coordination bond.

Interestingly, the phenanthroline plane is slightly farther away from the bulkier tert-butyl group than from the less challenging isopropyl one (Figure S4), translating in a small waggling distortion (see figure S5 for the definition of all distortions in homoleptic copper(I) complexes).

Nevertheless, the structures of Cu1 and [Cu(dtbp)2] + are very close as evidenced by the calculation of the rocking, waggling and flattening angles (respectively θx, θy and θz) which are close to 90° in all cases (Figures S6 and S7 τ4 [START_REF] Yang | Structural variation in copper(i) complexes with pyridylmethylamide ligands: structural analysis with a new four-coordinate geometry index[END_REF] 0.66 0.79 0.76 We computed the structure of the different possible conformers for Cu1 (Figure 3). The first one (Cu1sym1) is symmetric and corresponds to the case where both of the tert-Butyl groups on each ligand are in the staggered conformation. The second one (Cu1noSym) is not symmetric, corresponding to the X-ray experimental structure displayed in Figure 2, namely one tert-Butyl is in a staggered conformation and the other one in an eclipsed conformation. Finally, the last calculated conformation (Cu1sym2) corresponds to a symmetric structure where both tert-Butyl groups are in an eclipsed conformation. All relevant parameters (bond length, angles for each structure) are given in Table ST1.

Absorption properties of complex Cu1

Cu1 was dissolved in DCM, yielding orange solutions which remained stable over several days when stored in the dark (Figure S10). The UV-Vis absorption spectrum of Cu1 (Figure 4a) is The theoretical spectra (Figure 5) of Cu1nosym reproduces the features of the experimental one.

dominated
The spectra of the two symmetric rotamers Cu1Sym1 and Cu1Sym2 are very similar (Figure S11).

The nature of the electronic transitions was determined by mean of TheoDORE analysis (Figure 6). In this approach, the Cu1nosym complex is divided into three fragments: the metallic cation and the two ligands. The nature of each transition is determined by the contribution of each fragment to either the emptied orbital or the filled one. In the computed spectra, several weak absorption bands are present between 350 and 550 nm corresponding to the broad and weak experimental bands observed in the same domain. The spectrum is dominated by an intense band at 270 nm (exp. 275 nm). The detailed nature of the transitions can be found in Table ST2. The low energy domain (316 to 526 nm) is composed only of MLCT states (S1 to S23, Figure 6). combination of numerous electronic transitions (see comments below Table ST2). The nature of the lowest triplet states is identical to that of the corresponding singlet states.

UV-Vis monitoring of the stability of Cu1 in presence of acetonitrile

As mentioned above, Cu1 remained stable in aerated solution despite its crowded coordination sphere. In order to appreciate and compare the stabilities of Cu1 and [Cu(dtbp)2] + , we studied the evolution of the UV-Vis spectrum of Cu1 in DCM when aliquots of acetonitrile were added in the medium (figure S12). Indeed, acetonitrile (ACN) is a weak ligand which nevertheless proved capable to destroy [Cu(dtbp)2] + efficiently following the equation:

[Cu(dtbp)2] + + ACN → [Cu(dtbp)(ACN)] + + dtbp. 8
The MLCT transition of Cu1 collapses upon addition of ACN whereas model [Cu(dipp)2] + is unaffected (figure S12) showing that the coordination sphere of Cu1 is bulky enough to favor ligand exchange with weakly coordinating acetonitrile for steric relief. On the other hand, Cu1

is much more stable than [Cu(dtbp)2] + since ca. 5x10 4 equivalents of acetonitrile were necessary to destroy half the quantity of Cu1 whereas only one equivalent was sufficient to quantitatively disrupt the coordination scaffold of [Cu(dtbp)2] + . [START_REF] Green | Photophysical Characteristics and Reactivity of Bis(2,9-ditert-butyl-1,10-phenanthroline)copper(I)[END_REF][START_REF] Gandhi | Facile Oxidation-Based Synthesis of Sterically Encumbered Four-Coordinate Bis(2,9-di-tert-butyl-1,10-phenanthroline)copper(I) and Related Three-Coordinate Copper(I) Complexes[END_REF] Conclusively, the removal of one mere methyl group per phenanthroline from the structure of [Cu(dtbp)2] + is enough to significantly improve the stability of the complex, although the coordination sphere is weakened because of the strong steric strain imposed by the remaining tert-butyl group, as previously observed. 12

Study of the photoluminescence properties of complex Cu1

The strong steric bulk surrounding the copper(I) ion is expected to be a positive boon for the promotion of the photo-luminescence properties. Indeed, solutions of Cu1 in DCM are brightly luminescent upon excitation of the MLCT transition. The usual broad emission band can be monitored at 678 nm (Figure 4b, red line). The corresponding excitation spectrum (emission at 678 nm) is given in Figure S13 and reproduces the absorption features of the MLCT, confirming that the luminescence originates indeed from the light induced population of MLCT excited states. We retrieved E 00 = 2.14 eV from the emission spectrum (using the tangent method, figure S14). This value is very close to E 00 ([Cu(dipp)2] + ) despite Cu1 be more sterically constrained than [Cu(dipp)2] + . In addition, the emission quantum yield and lifetime are more than twice lower than those of [Cu(dipp)2] + (Table 2 andFigure S15): these results are in contradiction with the expected trend, and show that steric strain is not the sole parameter to influence the excited-state properties of copper(I) complexes. In previous works, it has been noticed that copper(I) complexes built with phenanthroline ligands which are substituted by two different bulky groups in positions 2 and 9 show decreased luminescence performances due to a high non-radiative rate constant knr. [START_REF] Meyer | π-π Stacking-Induced Cooperativity in Copper(I) Complexes with Phenanthroline Ligands[END_REF][START_REF] Gimeno | Non-Symmetrical Sterically Challenged Phenanthroline Ligands and Their Homoleptic Copper(I) Complexes with Improved Excited-State Properties[END_REF][START_REF] Felder | Highly luminescent Cu(I)phenanthroline complexes in rigid matrix and temperature dependence of the photophysical properties[END_REF] Likewise, a large knr of 7.86x10 6 s -1 was calculated in the case of Cu1 (Table 2). Nevertheless, Cu1 is the most luminescent complex reported in the series of homoleptic copper(I) complexes bearing non symmetrical phenanthroline ligands;

it is as well the most sterically challenged, underlining that steric strain remains an important factor to take into consideration to increase Φ, τ and E 00 , all things otherwise being equal.

M λabs / nm (ε / M -1 .cm -1 ) E(M + /M) a (V) E(M/M -) b (V) λem / nm (Φ/%) a E(M*/M -) c
τ / ns a kr d / s - The emission properties were computed and thus the lowest excited singlet and triplet states were optimized for the three rotamers Cu1sym1, Cu1sym2 and Cu1nosym (Table 3). The excited states (S1 or T1) are MLCT states and therefore can be seen as intramolecular charge transfer states where a hole is mostly located on copper and one electron is localized either on phenA or phenB. This results in two degenerate electronic states with C1 symmetry for Cu1Sym1 and Cu1sym2 and nearly degenerate electronic states with C1 symmetry for Cu1nosym, corresponding crudely to [Cu II (phenA -)(phenB)] + and [Cu II (phenA)(phenB -)] + . Shifting from one potential well to the other is possible through population of a transient state formally described as [Cu II (phenA (0.5-) )(phenB (0.5)-)] + (Figure S16). The latter has a C2 symmetry in the case of Cu1sym1 and Cu1sym2 (in the case of Cu1nosym, the symmetry of the intermediate state remains in C1 symmetry because of the different orientations of the tert-butyl groups).

In the C2 point group the electron density of the excited state is delocalized on both ligands phenA and phenB. Breaking the symmetry leads to its localization on one of the phen, which is associated to a breathing of the Cu-N distance without rotation of the tert-Butyl fragment. In the ground state, Cu1nosym was the lowest energy structure. In the excited states, the most stable geometries (in both S1 and T1) were generated by Cu1sym2. All the computed emission wavelengths em from the T1 states (Table 3) are inconsistent with the experimental maxima at 678 nm. This shows that like other similar copper complexes, the emission likely stems from the singlet S1 state, through thermally activated delayed fluorescence (TADF), where the luminescent S1 state is populated from the weakly luminescent T1 state by thermally activated reverse intersystem crossing. One should notice the strong influence of the tert-Butyl group orientations on the T1 state properties (Table 3). This means that a simple rotation of the tertbutyl group (an easy motion at room temperature given the low energy barriers between rotamers) leads to the existence of many different minima on the lowest excited singlet or triplet potential energy surface (PES), each of them with its own set of properties. For example, the spin orbit coupling (SOC) varies impressively from one rotamer to the other. This value is known to strongly impact intersystem crossing and therefore the probability that T1 either repopulates S1 (leading to TADF) or decays to S0 (either radiatively or non-radiatively).

We first computed the lowest excited S1 singlet state retaining the C2 symmetry for Cu1sym1

and Cu1sym2 and then we did the calculation without any symmetry (C1 point group) for the three rotamers. The em computed for the S1 states in C1 symmetry are too red-shifted (Table 4). However, the em of the S1 state of C2 symmetry of rotamers Cu1sym1 (663 nm) or Cu1sym2 (706 nm) agree well with the experimental values. In what follows, we will not consider rotamer Cu1sym1 because its energy is significantly higher than Cu1sym2, which means that this conformation will be minor in solution compared to the others. As mentioned above, the optimized S1 state of C1 symmetry of Cu1sym2 rotamer is the most stable structure, though that of Cu1nosym is close in energy. Importantly, the Cu1sym2 rotamer S1 state of C2 symmetry (transition state with the electron delocalized over the two phenanthroline ligands, Figure S16) is also quite low in energy, being only 0.118 eV above the structure without symmetry Cu1nosym.

From these data we can deduce the emission properties. After excitation, the complex relaxes in the lowest triplet PES where it hops between the different energetically accessible minima generated by two kinds of motion. The first one is due to the rotation of the tert-butyl groups and the second is the hopping of the exciton between the two ligands. The S1 PES is thermally repopulated, being accessible due to the small S1-T1 energy difference (Table 3) and significant SOC (as expected for copper(I) complexes with nearly tetrahedral geometries) whatever the geometry considered. [START_REF] Siddique | Structure-Dependent Photophysical Properties of Singlet and Triplet Metal-to-Ligand Charge Transfer States in Copper(I) Bis(diimine) Compounds[END_REF] From the S1 PES, the complex radiatively relaxes to the ground state, generating this broad emission band due to contributions from the various rotamers (Table 4).

These multiple transition structures entail that the barriers between the PES minima are low and that the complex evolves freely between them, populating each of these structures with almost equal probability. This tends to "dilute" the excited state over several states, enabling accelerated deactivation channels, as was observed previously. [START_REF] Gimeno | Non-Symmetrical Sterically Challenged Phenanthroline Ligands and Their Homoleptic Copper(I) Complexes with Improved Excited-State Properties[END_REF] In addition the SOC is significantly smaller for Cu1 than [Cu(dipp)2] + (table ST3). This means that the thermally activated reverse intersystem crossing is less efficient for Cu1 than

Optical Transient Absorption Spectroscopy

Optical transient absorption (TA) spectroscopy experiments were performed on solutions of Cu1 prepared in DCM at a concentration of about 0.5 mM (Figure 7A). Following excitation with 415 nm laser pulses (500 fs, 500 nJ), the difference spectra comprise negative features at wavelengths less than 480 nm and positive features at wavelengths greater than 480 nm, with excited-state absorption (ESA) maxima between 540-580 nm. At short time delays-0 to 2 ps-the ESA is broad and featureless, centered at 560 nm, and undergoes a subtle red-shift leading to a small decrease in the ESA intensity at 550 nm and a small increase in the ESA intensity at wavelengths greater than 560 nm. The broad ESA feature then becomes more structured over the time delay range of 2-50 ps, with strong maxima at 537 and 574 nm and weak maxima at 678 and 748 nm. The negative and positive features then decay concomitantly to zero over the time delay range of 50 ps to 1 µs. Importantly, the absorbance of the samples did not change during photolysis, proving that Cu1 is photo-stable during photolysis in degassed samples (Figure S17). The transient absorption data were analyzed using globalkinetic analysis to fit select kinetic traces with a first-order sequential kinetic model, A→B→C→GS (see Experimental Section and Figure S18 for details). The evolutionassociated spectra (EAS) were extracted from the raw data using the resultant time-dependent model populations (Figure 7B andS18D).

According to the calculations, the 415 nm laser pulses were resonant with the S0→Sn (n = 3, 6-8, see calculations) MLCT transitions that are optically-allowed in the D2d structure adopted by copper(I) diimine complexes in the ground state. [START_REF] Everly | Reinvestigation of the absorbing and emitting charge-transfer excited states of [Cu(NN)2]+ systems[END_REF] Prior time-resolved spectroscopy experiments with shorter time resolution and theoretical studies demonstrated that the initial excited-state population rapidly relaxes from Sn to S1 via internal conversion (IC), with a portion of the population undergoing intersystem crossing (ISC), on the 100 fs timescale. [START_REF] Shaw | Ultrafast Structural Rearrangements in the MLCT Excited State for Copper(I) bis-Phenanthrolines in Solution[END_REF] The subtle red-shift of the broad ESA feature observed here occurs with τ = 1.2 ± 0.6 ps. This process is typically assigned to the pseudo Jahn-Teller flattening distortion commonly observed for photoexcited copper(I) diimine complexes bearing symmetrical phenanthroline ligands: as explained in the introduction, copper(I) is transiently oxidized into copper(II) upon excitation, thus moving from tetrahedral to square planar geometries, and leading to a flattening of the coordination scaffold. However, our calculations of the excited state of Cu1 and of copper(I) and copper(II) variants of Cu1 indicate that this excited-state structural distortion rather corresponds to the rocking motion of a phenanthroline ligand and leads to formation of the trigonal bipyramidal structure (calculated structures for the excited state of Cu1 and the oxidized state of Cu1 are available in the adf_struc.xyz and gauss_struc.xyz files, respectively, in SI. Also, see Table ST4 for relevant structural data on the excited states). This rocking motion is well illustrated in Figure 8, where a view of excited Cu1 is given. The flattening distortion was observed on the 200-600 fs timescale in [Cu(tbp)2] + (tbp stands for 2-tert-butyl-1,10-phenanthroline) and 700-800 fs timescale in [Cu(dipp)2] + , but was not observed in [Cu(dtbp)2] + due to the increased steric bulk of the tert-butyl substituents in the 2 and 9 positions of the phenanthroline ligands. [START_REF] Garakyaraghi | Cuprous Phenanthroline MLCT Chromophore Featuring Synthetically Tailored Photophysics[END_REF][START_REF] Gothard | Strong Steric Hindrance Effect on Excited State Structural Dynamics of Cu(I) Diimine Complexes[END_REF] The τ ~ 1 ps time constant for structural distortion in Cu1 is consistent with the intermediate steric bulk of L1 and our calculations, which predicted structural distortion upon oxidation of the copper. for copper(I) bisimine complexes decrease significantly with the degree of the flattening distortion, which explains the trend among the copper(I) complexes with symmetrically substituted phenanthrolines. [START_REF] Siddique | Structure-Dependent Photophysical Properties of Singlet and Triplet Metal-to-Ligand Charge Transfer States in Copper(I) Bis(diimine) Compounds[END_REF] However, our calculations reveal that Cu1 flattens only slightly (89.9° to 86.0°, see Table ST4), but distorts significantly via the rocking motion (160.4° to 148.6°, see Table ST4). Thus, distortions along nuclear coordinates other than flattening similarly decrease the rate of ISC and probably the magnitude of SOC in copper(I) bisphenanthroline complexes. This is corroborated by the lower calculated SOC value for Cu1 compared to [Cu(dipp)2] + (Table ST3).

While the picosecond dynamics of

Electrochemistry

The electrochemical behavior of Cu1 was investigated by cyclic voltammetry in DCM (anodic part, figure S20) and dimethylformamide (Figure S21). The voltammograms feature a reversible oxidation wave at 0.60 V and a reversible reduction wave at -2.10 V vs. Fc + /Fc, which is consistent with the values usually recorded for this kind of complex. E(Cu1 + /Cu1) is slightly below the oxidation potential of [Cu(dipp)2] + despite the fact that Cu1 is more sterically strained (which should imply the stabilization of the HOMO). Electronic factors could partly explain this result, since tert-butyl is more electron donating than isopropyl: L1 is thus slightly electron richer than dipp and could better stabilize Cu(II). In addition, the coordination scaffold is sterically ill-balanced, and oxidized Cu1 (i.e. [Cu II (L1)2] 2+ ) is certainly more warped than [Cu II (dipp)2] 2+ hence the observed trend of E(Cu II /Cu I ). This is confirmed by DFT calculations (Table ST4 in supporting information): upon oxidation, the phen-Cu-phen angle in [Cu II (dipp)2] 2+ shifts from 90.0° to 70.3° evidencing that this complex endures some flattening of about 20°. In addition, all four Cu-N distances shorten equivalently and the X-Cu-X angle stays at 180° (where X is the calculated centroid of the central C6 ring of phenanthroline, Table ST3). On the contrary, the distortions are stronger in oxidized Cu1. Indeed, due to the imbalance, there is a rocking of one of the phen plane with an X-Cu-X angle reduced to 148.6°.

The coordination tends towards a trigonal bipyramid (Figure 9) with the copper atom at the center of the structure and one methyl group of the tert-Butyl occupying an axial position. ST4.

Reductive quenching of the excited state

Knowing E 00 and E(Cu1/Cu1 -), one can estimate the photo-oxidizing power E(Cu1*/Cu1 -) at ca. 0.01 V vs. Fc + /Fc (table 1). This value is modest and compares well with the usual values estimated for homoleptic Cu(I) complexes. Increasing the steric bulk around the Cu(I) ion did not entail the expected rise of E 00 , and therefore E(Cu1*/Cu1) and E([

Cu(dipp)2] + */[Cu(dipp)2] + )
are similar. Cu1 was put to the test in a reductive quenching experiment in the presence of decamethylferrocene dmFc as the electron donor. [START_REF] Cunningham | Reductive Quenching of Photoexcited Cu(dipp)2+ and Cu(tptap)2+ by Ferrocenes (dipp = 2,9-Diisopropyl-1,10-phenanthroline and tptap = 2,3,6,7-Tetraphenyl-1,4,5,8-tetraazaphenanthrene)[END_REF] The quenching of the emission of Cu1 was monitored for different concentrations of dmFc. Plotting τ0/τ (where τ0 and τ are the emission lifetimes in the absence and presence of dmFc, respectively) as a function of [dmFc] afforded a straight line (figure S22) whose slope gives the Stern-Volmer constant KSV. The latter is equal to kQ x τ0 where kQ is the bimolecular quenching rate. As stated in previous work, [START_REF] Gimeno | Non-Symmetrical Sterically Challenged Phenanthroline Ligands and Their Homoleptic Copper(I) Complexes with Improved Excited-State Properties[END_REF][START_REF] Cunningham | Competitive energy-transfer and reductive quenching of the CT excited states of copper(l) phenanthrolines[END_REF] the quenching of the excited state of a copper complex by dmFc may occur via two mechanisms: energy transfer ("EnT" from the MLCT excited state to low lying iron(II) based (d,d) states with kinetic rate constant kEnT = 2.1x10 8 s -1 ) [START_REF] Gimeno | Non-Symmetrical Sterically Challenged Phenanthroline Ligands and Their Homoleptic Copper(I) Complexes with Improved Excited-State Properties[END_REF] and electron transfer (from dmFc to excited copper complexes with kinetic rate constant kRQ). The quenching rate constants were anticipated to be rather large [START_REF] Cunningham | Reductive Quenching of Photoexcited Cu(dipp)2+ and Cu(tptap)2+ by Ferrocenes (dipp = 2,9-Diisopropyl-1,10-phenanthroline and tptap = 2,3,6,7-Tetraphenyl-1,4,5,8-tetraazaphenanthrene)[END_REF] and diffusion-limited processes (kinetic rate constant kd = 7x10 9 M -1 .s -1 for ferrocene in DCM) cannot be ignored. Thus, the equation [START_REF] Cunningham | Reductive Quenching of Photoexcited Cu(dipp)2+ and Cu(tptap)2+ by Ferrocenes (dipp = 2,9-Diisopropyl-1,10-phenanthroline and tptap = 2,3,6,7-Tetraphenyl-1,4,5,8-tetraazaphenanthrene)[END_REF] From the Stern-Volmer plots, we retrieved kRQ = 3.9x10 9 M -1 .s -1 . Interestingly, kRQ(Cu1) is higher than kRQ([Cu(dpp)2] + ) = 8x10 8 M -1 .s -1 (where dpp is 2,9-diphenyl-1,10phenanthroline): [START_REF] Cunningham | Competitive energy-transfer and reductive quenching of the CT excited states of copper(l) phenanthrolines[END_REF] this is consistent with E(Cu1*/Cu1 -) being significantly larger than E([Cu(dpp)2] + */[Cu(dpp)2] 0 ) (0.01 V vs. -0.07 V vs. Fc + /Fc respectively). 12,28 However, kRQ(Cu1) is smaller than kRQ([Cu(dipp)2] + ) (5.9x10 9 M -1 .s -1 ) recorded in the same conditions, despite their similar photo-oxidative powers. This evidences that thermodynamic factors are not sufficient to rationalize the reactivity of Cu1 and kinetic factors must be taken into account. [START_REF] Gushurst | Comparative studies of the photophysical properties of copper phenanthrolines: from Cu(dmp)2+ to the copper(I) catenates[END_REF] The nonsymmetrical nature of ligand L1 entails a large reorganization energy, as demonstrated by the distorted calculated structures of oxidized [Cu II (L1)2] 2+ (Figure 8) and S1 or T1 excited states (Tables 3 and4). Interestingly, Cu1sym2 exhibits the highest Edist in the series Cu1, [Cu(dipp)2] + and [Cu(dtbp)2] + (Tables 4 andST3). The reorganization energy offsets the thermodynamic driving force and decreases the electron-transfer kinetics, following Marcus theory.

1 𝑘 𝑄 = 1 𝑘 𝑑 + 1 𝑘 𝑅𝑄 +𝑘 𝐸𝑛𝑇 applies.

Conclusion

A sterically strained homoleptic copper(I) complex [Cu(L1)2] + or Cu1 has been designed, showing an improved stability compared to [Cu(dtbp)2] + in DCM solutions. The design is based on the synthesis of a non-symmetrical phenanthroline ligand where positions in α of the nitrogen atoms are occupied by two encumbering branched aliphatic chains, namely tert-butyl and isopropyl groups. The complex has been characterized in the ground and excited states.

Early after excitation, structural distortions are observed by ultrafast transient absorption spectroscopy, in contrast to previously studied [Cu(dtbp)2] + . Despite the increased steric strain imposed by the two substituents, the excited-state Cu1 exhibit a shorter lifetime and a weaker emission quantum yield than those of [Cu(dipp)2] + . The rationale behind this observation is two-fold. First, many energy minima exist on the potential energy surfaces (triplet and singlet) and the energy barriers between them is small. Statistically, several of these minima can be populated with almost equal probability and decay to the ground state, providing multiple deactivation channels. Second, because the vicinal isopropyl group is smaller than the tertbutyl group, the excited-state distortion corresponds to rocking motion of the ligand, which leads to a trigonal bipyramid structure of the excited state of Cu1 (as verified by calculations of the structures of the excited state and of the oxidized state of Cu1) opening new deactivation channels.

Experimental section

General: chemicals were purchased from Sigma-Aldrich or Fisher Scientific and used as received. 2,9-bis-isopropyl-1,10-phenanthroline (dipp), [START_REF] Pallenberg | Synthesis and Characterization of Some Copper(I) Phenanthroline Complexes[END_REF] [Cu(dipp)2] +13 and 2-tert-butyl-1,10phenanthroline [START_REF] Gimeno | Non-Symmetrical Sterically Challenged Phenanthroline Ligands and Their Homoleptic Copper(I) Complexes with Improved Excited-State Properties[END_REF] were synthesized as previously published. Thin-layer chromatography (TLC) was performed on aluminium sheets precoated with Merck 5735 Kieselgel 60F254. Column chromatography was carried out with Merck 5735 Kieselgel 60F (0.040-0.063 mm mesh). 1 H NMR spectra were recorded on an AVANCE 300 UltraShield BRUKER. Chemical shifts for 1 H NMR spectra are referenced relative to residual protium in the deuterated solvent (CDCl3 δ = 7.26 ppm). NMR spectra were recorded at room temperature, chemical shifts are written in ppm and coupling constants in Hz. Mass spectrometry was performed with a JEOL JMS-700 B/E spectrometer. Electrochemical measurements were made under an argon atmosphere in CH2Cl2 or in DMF with 0.1 M Bu4NPF6. Cyclic voltammetry experiments were performed by using an Autolab PGSTAT 302N potentiostat/galvanostat. A standard three-electrode electrochemical cell was used. Potentials were referenced vs. reversible ferrocenium/ferrocene couple. The working electrode was a glassy carbon disk and the auxiliary electrode was a Pt wire. UV-visible absorption spectra were recorded on an Analytik Jena spectrophotometer, using 1 cm path length cells. Emission spectra were recorded on a Fluoromax-3 Horiba spectrofluorimeter (1 cm quartz cells). E 00 values were calculated by the tangent method: a tangent to the emission spectrum was drawn on the blue side of the spectrum. The intersection with the baseline gives the wavelength λ 00 , used to calculate E 00 = 1240/λ 00 . Luminescence decays were recorded with a DELTAFLEX time correlated single photon counting system (HORIBA) on degassed DCM solutions.

A suitable crystal for X-ray diffraction single crystal experiment was selected and mounted with a cryoloop on the goniometer head of a D8 VENTURE Bruker AXS diffractometer equipped with a (CMOS) PHOTON 100 detector, using Mo-K radiation (λ = 0.71073 Å, multilayer monochromator) at T = 150 K. Crystal structure has been described in monoclinic symmetry and P 21/c space group. Cell parameters were found as follows: a = 16.0924 (13), b = 15.3035 (11), c = 15.6444(11) Å, β = 110.423(2)°, V = 3610.6(5) Å [START_REF] Khnayzer | Robust Cuprous Phenanthroline Sensitizer for Solar Hydrogen Photocatalysis[END_REF] . Number of formula unit Z is equal to 4 and calculated density d and absorption coefficient µ values are 1.408 g.cm -3 and 0.714 mm -1 respectively. The structure was solved by dual-space algorithm using the SHELXT program, [START_REF] Sheldrick | SHELXT -Integrated space-group and crystal-structure determination[END_REF] and then refined with full-matrix least-squares methods based on F 2 (SHELXL program). [START_REF] Sheldrick | Crystal structure refinement with SHELXL[END_REF] All non-hydrogen atoms were refined with anisotropic atomic displacement parameters. H atoms were finally included in their calculated positions and treated as riding on their parent atom with constrained thermal parameters. A final refinement on F 2 with 8267 unique intensities and 470 parameters converged at ωR(F 2 ) = 0.2218 (RF = 0.0869) for 7275 observed reflections with (I > 2σ).

Synthesis:

2-(tert-butyl)-9-isopropyl- ). The second harmonic was directed through a λ/2 waveplate and wiregrid polarizer to attenuate the pump pulse energy and set the relative polarization between the pump and probe to magic angle (54.7°). A mechanical chopper operating at 500 Hz was used to block every other pump pulse and the pump pulses were then focused to ~200-300 µm diameter at the sample with 500 nJ/pulse. The probe was generated by focusing about 3-4 µJ of the fundamental into a 3 mm sapphire crystal after the optical delay line. The probe pulses (450-800 nm) were set to horizontal polarization using a wiregrid polarizer and focused to about 200 µm at the sample.

After interacting with the sample, the probe and signal were coupled into a fiber optic cable connected to a CCD detector. The probe spectra and difference signal were recorded and calculated using the Helios software (Ultrafast Systems). The fsTA spectra were measured with 3 s total averaging at each time delay.

Nanosecond transient absorption (nsTA) spectroscopy experiments were performed using a previously described Nd:YAG laser system (PL2210 oscillator and amplifier, Ekspla) coupled to a supercontinuum laser (STM-1-UV, Leukos). [START_REF] Hayes | Excited state electron and energy relays in supramolecular dinuclear complexes revealed by ultrafast optical and Xray transient absorption spectroscopy[END_REF] The amplifier outputs a 0. The emission wavelength of lowest excited singlet and triplet states was computed on the basis of their optimized structures.

A second set of calculations was performed with GAUSSIAN09 at DFT level of theory (B3LYP functional) including Grimme's dispersion corrections. All atoms were described by the 6-31+G** basis set and the solvent (DCM) introduced through a PCM. All structures were fully optimized and the nature of the stationary point encountered determined by frequency calculations within the harmonic approximation. The minima were characterized by a full set of real frequencies.

Supporting information

ORTEP picture of the structure of Cu1, graphical illustrations, additional spectra and voltammograms, full data tables related to quantum calculations.

Table Of Content

Caption: "A stable copper(I) complex was prepared, whose steric hindrance is halfway between benchmark [Cu(dipp)2] + and [Cu(dtbp)2] + . The coordination sphere of the complex is not geometrically symmetric and this causes a geometric imbalance, which favors rotational and rocking motions of the ligands in the excited state, causing the premature quenching of the excited state."

Figure 1 .

 1 Figure 1. Complex Cu1 and models

  ). For example θx, θy and θz values are significantly different from 90° for [Cu(dipp)2] + (FigureS7) showing that the addition of one extra methyl on one isopropyl group of each phen ligand has substantially brought the structure closer to a tetrahedral geometry, as desired to promote the excited-state properties. This is confirmed bythe τ4 parameter (τ4 = 360°-(𝛼+𝛽) 141°,where α and β are the two largest N-Cu-N angles in the structure of Cu1, namely N1A-Cu-N2B and N2A-Cu-N2B, cf.

Figure 2 .

 2 Figure 2. Structure of complex Cu1. Hydrogen atoms and counter-anion have been removed for the sake of clarity.One last point concerns the orientation of the tert-Butyl groups of each ligand. As shown on Figure2, tert-butyl groups can be in a staggered or eclipsed conformation (figureS8). The two

  Cu1sym1 and Cu1sym2 are less stable than Cu1nosym (Figure S9), though the structure of Cu1sym2 is very close in energy. The associated transition states are very low in energy. The barrier from Cu1noSym to Cu1sym1 is 4.1 kcal mol -1 and that from Cu1nosym to Cu1sym2 is only 1.4 kcal mol -1 . These results show that there is free rotation of the tert-Butyl group at room temperature, as observed in the NMR spectra. The orientation of the tert-Butyl group impacts the structural parameters especially the Cu-N bond length. The difference between Cu-N2A or Cu-N2B (nitrogen atom close to tert-Butyl group) and Cu-N1A or Cu-N1B (nitrogen atom close to isopropyl groups) distances in the same ligand increases from Cu1sym1 (0.066 Å) to Cu1sym2 (0.175 Å), in accordance with the trend observed in the crystal structure.

Figure 3 :

 3 Figure 3: structures of the three most stable rotamers of Cu1. Only the hydrogen of the tBu groups pointing toward the copper are shown. The relative Gibbs Free energies from GAUSSIAN calculations are in kcal/mol.

Figure 4 .Figure 5 .

 45 Figure 4. a) UV-Vis (black line) and b) emission (red line) spectra (λexc = 438 nm) of complex Cu1 in DCM. Inset: zoom on the MLCT transition.

Figure 6 .

 6 Figure 6. Nature of the transitions generating the theoretical absorption spectra. Horizontal axis: numbering of the singlet states Sn for the corresponding S0→Sn transition. Vertical axis: proportions (between 0 and 1) of Metal Centered states (MC, black), ligand centered * transitions (on the same ligand) (LC, red), Metal-to-Ligand Charge transfer (MLCT, blue), Ligand-to-Metal Charge transfer (LMCT, yellow) and Ligand-to-Ligand Charge transfer (LLCT, green) to the overall calculated S0→Sn transitions.

[

  Cu(dipp)2] + . The TADF process is therefore less likely for Cu1, likely participating to the lower luminescence quantum yield and lifetime of the latter compared to model [Cu(dipp)2] + .One last point concerns the global shape of the T1 and S1 structures. The tetrahedral arrangement around the copper atom is almost lost in the excited state; the coordination sphere approached a trigonal bipyramid with the copper at the center of the equatorial plane and one methyl group of a tert-Butyl group occupying an axial position (see file adf_struc.xyz in the SI). Such distortion is particularly pronounced for complexes bearing tert-Butyl substituents in α of the nitrogen atoms. It is well known that extended distortions of the copper(I) complexes coordination sphere lead to high non-radiative rates knr.

Figure 7 .

 7 Figure 7. (A) Optical transient absorption spectra at select time delays following excitation by 415 nm laser pulses for a solution of Cu1 in DCM and (B) the evolution-associated spectra obtained via global-kinetic fitting of the raw data to a first-order sequential kinetic model, A→B→C→GS.

Figure 8 .

 8 Figure 8. Structure of Cu1 in the excited state (example of Cu1nosym in S1 state)

  Cu1 suggest intermediate effects of the structural distortions compared to those of the less sterically hindered [Cu(dipp)2] + and more sterically hindered [Cu(dtbp)2] + , the 3 MLCT lifetime of Cu1, τ = 127 ns, measured with TCSPC, is the shortest of this series of complexes (Table2). As discussed above, the duration of the 3 MLCT state typically decreases with increased flattening in the excited state for two reasons: the flattening distortion (1) enables quenching caused by exciplex-like solvent interactions with the transient copper(II) center and (2) decreases the energy gap between the T1 and S0 states. While Cu1 does not flatten significantly, the rocking distortion has the same two effects on the excited-state lifetime. Given that the picosecond dynamics and absorbance and emission properties correlate with the steric hindrance of the phenanthroline ligands, an additional factor beyond the steric hindrance and structural distortion likely contributes to the abbreviated[START_REF] Khnayzer | Robust Cuprous Phenanthroline Sensitizer for Solar Hydrogen Photocatalysis[END_REF] MLCT lifetime in copper(I) complexes bearing different substituents at the 2 and 9 positions.

Figure

  FigureS19shows optical transient absorption spectra carried out on the nanosecond to microsecond timescales, with a measured[START_REF] Khnayzer | Robust Cuprous Phenanthroline Sensitizer for Solar Hydrogen Photocatalysis[END_REF] MLCT lifetime of 126 ns that is in agreement with the TCSPC results (t = 127 ns). The non-radiative decay rate constant of the 3 MLCT statecalculated from the emission decay rate and yield-is larger than that of [Cu(dipp)2] + and [Cu(dtbp)2] + , which could result from the decreased symmetry of Cu1 and may explain the relatively short excited-state lifetimes of the copper(I) complexes with non-symmetric substituents. This is in line with the distorted structures calculated for the S1 and T1 states. A secondary non-dominant decay component was also observed (t = 5 ± 1 ns), which may correspond to solvent interactions following the excited-state rocking distortion that increases the exposure of the transient Cu(II) center to solvent molecules in the excited state. This 5-ns component is reproducible across two separate measurements of the Cu1 compound in DCM (FigureS19c). Furthermore, global analysis of the ESA and ground-state bleach(GSB) showed that this secondary decay component (t = 5 ns) was present with similar relative amplitudes at both features. We assign this secondary decay component to the quenching of the[START_REF] Khnayzer | Robust Cuprous Phenanthroline Sensitizer for Solar Hydrogen Photocatalysis[END_REF] MLCT lifetime due to inner shell penetration of DCM and subsequent interaction with the Cu(II) center, drawing from our previous studies of the solute-solvent interactions of [Cu I (phen)2] + in both acetonitrile and DCM.[START_REF] Lockard | Influence of Ligand Substitution on Excited State Structural Dynamics in Cu(I) Bisphenanthroline Complexes[END_REF] In this earlier study, the MLCT triplet state lifetimes of the least sterically strained [Cu I (phen)2] + were significantly shortened in DCM to ~140 ps and acetonitrile to 16 ps compared to 100 ns and ~1 ns for [Cu I (dmp)2] + , where methyl groups are attached to the 2,9 positions of the phen ligands. Therefore, when the transient Cu(II) of the MLCT state is exposed to DCM, the lifetime will also be shortened even though the solvent is considered non-coordinating. Thus, we hypothesize that the observation of biexponential[START_REF] Khnayzer | Robust Cuprous Phenanthroline Sensitizer for Solar Hydrogen Photocatalysis[END_REF] MLCT quenching results from distinct populations of Cu1 with different orientations of the phenanthroline ligands/substituents that permit different degrees of solutesolvent (i.e. Cu(II)-DCM) interactions. The accelerated decay component (t = 5 ns) corresponds to the loss of a fraction of the 3 MLCT population through enhanced solute-solvent interactions. The relative amplitudes of the 5 ns and 126 ns decay components indicate about 1/5 to 1/4 of the Cu1 3 MLCT population adopted a structure that enabled closer approach of the DCM solvent molecules to the Cu(II) center. Because we observe the ~5 ns decay component in the GSB, we assume this process corresponds to Cu1 returning back to the ground state without any intervening processes. Thus, we conclude that although excited-state Cu1 was expected to be sterically hindered to solute-solvent interactions, it is still possible to observe large variations in excited-state quenching resulting from a distribution of coordination environments.

Figure 9 .

 9 Figure 9. Structure of [Cu II (L1)2] 2+ (oxidized Cu1) (calculated)

  5 kHz train of 355 nm, 25 ps, 0.3 mJ pulses which pumped an optical parametric generator (PG403, Ekspla) tuned to output 415 nm, 25 ps, 25 µJ pulses. The supercontinuum laser was triggered by the 1 kHz master trigger from the amplifier and output a 1 kHz train of broadband (400-1600 nm), 600 ps, 15 µJ pulses. The 415 nm pump was attenuated to 300 nJ/pulse and focused to about 200-300 µm at the sample. The broadband probe was focused to about 150 µm at the sample.After interacting with the sample, the probe and signal were detected using an SP-2150i Acton Series spectrograph (Princeton Instruments) coupled with a Spyder3 SG-14 (Teledyne DALSA) CCD camera. A laboratory-written application (LabVIEW 2014, National Instruments) provided the time delays to the supercontinuum laser via DG535 delay generators (Stanford Research Systems) externally triggered by the 1 kHz master trigger, recorded the probe spectra, and calculated the difference signal.The fsTA and nsTA data had the background signal (scattered pump photons and long-lived, >1 ms, transient signals) subtracted and were corrected for group delay dispersion in the probe pulses. The nsTA data were corrected for scan-to-scan drift in the pump-probe timing provided by the DG535 delay generators. Samples for the fsTA and nsTA spectroscopy experiments were prepared in DCM with an absorbance of about 0.5 (about 0.5 mM) at the peak of the MLCT transition. Three freeze-pump-thaw cycles were performed on the nsTA samples to degas the solutions and eliminate the effects of oxygen on the measured excited-state dynamics.Global-kinetic analysis of the transient absorption data was performed using MATLAB programs written by Matthew D. Krzyaniak in Michael R. Wasielewksi's group. Here, the program simultaneously fit raw kinetic traces at select probe wavelengths (450-700 nm at 10 nm increments) where the time constants (𝜏 𝑖 = 1 𝑘 𝑖 ⁄ ), time zero, and width of the instrument response function were shared and only the amplitudes varied between the kinetic traces. The program numerically solved the first-order kinetic equations for the corresponding rate matrix K (FigureS13) using matrix methods and convolved the solutions with a Gaussian instrument response function. The program employed least-squares fitting with the Levenberg-Marquardt method to find the parameters that best described the data, which were then fed back into the kinetic equations to solve for the time-dependent populations of each state in the kinetic model.The evolution-associated spectra were then extracted from the raw data using the timedependent populations for each state in the proposed kinetic model.ComputationalDetails. Two sets of calculation were performed. A first set were performed with ADF 2019 34 at DFT level of theory (B3LYP functional). All atoms were described by TZP basis set. Scalar relativistic effects were introduced through ZORA Hamiltonian and solvent (DCM) taken into account through a PCM. The UV-Visible absorption spectrum was computed by TD-DFT on the basis of ADF optimized structures. The transitions were analyzed by mean of TheoDORE tool (TheoDORE: Theoretical Density, Orbital Relaxation and Exciton analysis).

Table 1

 1

) which is 0.66, 0.76 and 0.79 for [Cu(dipp)2] + , Cu1 and [Cu(dtbp)2] + respectively, the expected value for a perfect tetrahedron being 0.93.

[START_REF] Yang | Structural variation in copper(i) complexes with pyridylmethylamide ligands: structural analysis with a new four-coordinate geometry index[END_REF]

Table 1 .

 1 Bond lengths and angles of interest for complex Cu1 and models [Cu(dipp)2] + and [Cu(dtbp)2] + obtained by X-ray scattering.

	Parameter	[Cu(dipp)2] + (BF4 -) 19	[Cu(dtbp)2] + (BF4 -) 15	Cu1(PF6)
		Distances (Angström)
	Cu-N1A	2.083	2.104(2)	2.057(4)
	Cu-N2A	2.017	2.111(2)	2.122(4)
	Cu-N1B	2.067	2.108(2)	2.031(4)
	Cu-N2B	2.030	2.105(2)	2.207(4)
		Angles (deg)
	N1A-Cu-N2A	82.25	84.53	82.09(14)
	N1A-Cu-N1B	107.35	123.26	120.32(15)
	N1A-Cu-N2B	118.71	122.92	126.36(14)
	N2A-Cu-N1B	128.97	121.37	126.10(15)
	N2A-Cu-N2B	138.38	124.93	126.94(13)
	N1B-Cu-N2B	81.52	84.82	81.06(16)
	θx	82.2	88.6	87.2
	θy	72.5	90.0	87.4
	θz	85.4	91.8	87.3

Table 2 .

 2 Ground

state and excited state parameters for molecules M = Cu1, [Cu(dipp)2] + and [Cu(dtbp)2] + . Measurements are performed in DCM unless otherwise mentioned.

Table 3 .

 3 Emission from the lowest T1 state (Eem), distortion energy Edist (defined as the difference between the energy of the ground state at its optimum and at the excited state geometry) and stabilization energy Estab (Estab = Edist + Eem) energies in eV, emission wavelength in nm (em), S1-T1 energy splitting (EST) in eV at T1 geometry and S1-T1 spin orbit coupling in cm -1 of the lowest triplet states T1 at T1 geometry for the three rotamers and group symmetry. a) The two phen ligand of Cu1 being inequivalent, the emission properties from both have been computed.

	Structure	Cu1sym1	Cu1nosym	Cu1sym2
	Symmetry	C2	C1	C1	C1 a	C2	C1
	Eem	1.668	1.507	1.460	1.449	1.544	1.405
	em	743	823	849	855	803	882
	Edist	0.281	0.371	0.365	0.387	0.333	0.404
	Estab	1.949	1.878	1.824	1.837	1.877	1.810
	EST	0.222	0.197	0.211	0.184	0.266	0.232
		0.1	19.0	33.6	15.0	0.0	25.8
	SOC	20.2	34.2	37.8	27.0	20.3	37.9
		20.2	34.2	37.8	27.0	20.3	37.9

Table 4 .

 4 Emission (Eem), distortion (Edist) and stabilization (Estab) energies in eV, emission wavelength in nm (em) and oscillator strength (fosc) of the lowest S1 states. a) The two phen ligands of Cu1 being inequivalent, the emission properties from each of them have been computed.

	Structure	Cu1sym1			Cu1nosym	Cu1sym2
	Symmetry	C2	C1	C1	C1 a	C2	C1

  .40 mmol). The red resulting solution was stirred overnight at room temperature. The mixture was precipitated in hexane to obtain a red powder.1 H NMR spectrum (CDCl3, 300MHz) δ= 8.58 (m,4H), 8.13 (d,[START_REF] Khnayzer | Robust Cuprous Phenanthroline Sensitizer for Solar Hydrogen Photocatalysis[END_REF] JH,H = 8.7Hz, 2H), 8.07 (s, 4H), 7.76 (d,[START_REF] Khnayzer | Robust Cuprous Phenanthroline Sensitizer for Solar Hydrogen Photocatalysis[END_REF] JH,H = 8.7Hz, 2H), 3.04 (sept,

	3 JH,H = 6.9Hz, 2H), 1.38 (s, 18H), 0.93 (d, 3 JH,H = 6.9Hz, 4H), 0.66 (d, 3 JH,H = 6.9Hz, 4H). 13 C
	NMR spectrum (CDCl3, 300MHz) δ= 138.55 (s), 138.41 (s), 128.66 (s), 127.03 (s), 126.82 (s),
	123.94 (s), 122.02 (s), 38.76 (s), 38.01 (s), 30.66 (s), 22.13 (s), 22.01 (s). HRMS (ESI + ) for
	C38H44CuN4 + m/z 619.2861 found, 619.2862 calc.
	Optical transient absorption spectroscopy: Femtosecond transient absorption (fsTA)
	spectroscopy experiments were performed on a previously described Ti:sapphire laser system
	(MaiTai oscillator / Spitfire Pro XP amplifier, Spectra-Physics) outputting a 1 kHz train of 830
	nm, 100 fs, 2.6 mJ laser pulses. 33 0.6 mJ of the fundamental were used to perform the fsTA
	experiments. A 10% reflective beamsplitter directed 60 µJ of the fundamental towards an
	optical delay line within the transient absorption spectrometer (Helios, Ultrafast Systems). The
	0.54 mJ of transmitted fundamental were telescoped to about 2 mm diameter and frequency-
	doubled using a 2 mm thick type I lithium triborate crystal (θ = 90°, φ = 28°
	1,10-phenanthroline :	2-(tert-butyl)-1,10-
	phenanthroline (226 mg, 0.96 mmol) is dissolved in dry toluene (15 mL). The solution is cooled
	to 0°C before dropwise addition of isopropylithium (0.7 M, 2.1 mL). The resulting solution is
	allowed to warm to room temperature and is stirred under argon for 18 h. Water (10 mL) is
	then added followed by a DCM extraction. Organic phases are stirred with activated
	manganese dioxide (2.15 g, 24 mmol) for 6h. The mixture is filtrated on celite, dried over
	magnesium sulfate and evaporated. The orange oil is purified by chromatography on silica
	(eluent: 0 to 5% of methanol in DCM). The title compound is obtained in quantitative yield (265
	mg). 1 H NMR spectrum (300MHz, CDCl3) δ= 8.14 (m, 2H), 7.73 (m, 3H), 7.54 (d, 3 JH,H = 8.4Hz,
	1H), 3.45 (sept, 3 JH,H = 6.9Hz , 1H), 1.60 (s, 9H), 1.55 (d, 3 JH,H = 6.9Hz, 6H). 13 C NMR spectrum
	(CDCl3, 75 MHz) δ= 169.29 (s), 167.36 (s), 136.15 (s), 135.96 (s), 127.20 (s), 126.84 (s),
	125.50 (s), 125.31 (s), 120.74 (s), 119.79 (s), 38.73 (s), 37.04 (s), 30.30 (s), 22.48 (s). HRMS
	(ESI + ) for C19H22N2 [M+H] + , m/z 279.1870 found, 279.1861 calc.

Complex Cu1: under argon atmosphere, [Cu(ACN)4]PF6 (74 mg, 0.20 mmol) was dissolved in 5 mL of degassed DCM. This solution was transferred into a vial that contains L1 (110 mg, 0

Acknowledgements

The optical transient spectroscopy part of the work is funded by Chemical, Biological and Geological Sciences Division, Basic Energy Science, Office of Science, the US Department of Energy under Contract No. DE-AC02-06CH11357.

The authors wish to thank the ANR (PERCO program n°ANR-16-CE07-0012-01) for financial support, the CEISAM (Nantes University) analysis platforms.

The calculations were carried out in part at the IDRIS computer centers through a grant of computer time from GENCI (A0050810629) and in part in the Strasbourg HPC. We would also like to thank Michael Eberhart for his assistance in the freeze-pump-thaw sample preparation procedure.