# Encumbering the Copper(I)-Diimine Coordination Sphere with a Benzyl Group: Impact on the Stability and Photoredox Properties

Lea Gimeno, Emily A Sprague-Klein, Brian T Phelan, Tyler N Haddock, Nicolas Oppé, Maryline Beyler, Charlotte David, Errol Blart, Eric Levillain, Olivier Alévêque, et al.

► To cite this version:

Lea Gimeno, Emily A Sprague-Klein, Brian T Phelan, Tyler N Haddock, Nicolas Oppé, et al. "Encumbering the Copper(I)-Diimine Coordination Sphere with a Benzyl Group: Impact on the Stability and Photoredox Properties." Journal of Physical Chemistry A, 2026, 10.1021/acs.jpca.5c08355. hal-05463684

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Distributed under a Creative Commons CC BY 4.0 - Attribution - International License
Encumbering the Copper(I)-Diimine Coordination Sphere with a Benzyl Group: Impact
on the Stability and Photoredox Properties.

Lea Gimeno,<fnref n="1" /> Emily A. Sprague-Klein,<fnref n="2" />,<fnref n="3" /> Brian T. Phelan,<fnref n="2" />,<fnref n="4" /> Tyler N. Haddock,<fnref n="2" /> Nicolas
Oppé,<fnref n="1" /> Maryline Beyler,<fnref n="5" /> Charlotte David,<fnref n="1" /> Errol Blart,<fnref n="1" /> Eric Levillain,<fnref n="6" /> Olivier Aleveque,<fnref n="6" />
Christophe Gourlaouen,<fnref n="7" />* Lin X. Chen,<fnref n="2" />,<fnref n="4" />* Yann Pellegrin<fnref n="1" />*

gourlaouen@unistra.fr

lchen@anl.gov

yann.pellegrin@univ-nantes.fr

**Abstract**

Cu(I)-bisdiimine complexes, [Cu(NN)₂]⁺ (where NN represents a diimine such as phenanthroline or bipyridine derivatives), are considered promising photosensitizers for various photochemical applications. However, their effectiveness is subject to several key challenges. In particular, controlling steric strain around the copper(I) center by introducing appropriate substituents (R) at the α-position of the nitrogen atoms is crucial for optimizing the excited-state properties of the complex. In brief, increasing the size of R leads to longer emission lifetimes and higher quantum yields. Additionally, the energy of the singlet excited state rises with increasing steric bulk, enhancing photoinduced reactivity. However, excessive steric strain from bulky substituents can significantly destabilize the coordination sphere. To balance complex stability with increased steric bulk around the metal center, we have developed two novel non-symmetrical ligands featuring branched alkyl chains and benzyl groups at the α-position of the nitrogen atoms. Our findings demonstrate that intramolecular π-stacking interactions between the benzyl group and the opposing phenanthroline ligand

<footnote n="1">¹Nantes Université, CNRS, CEISAM, UMR 6230, F-44000 Nantes, France</footnote>

<footnote n="2">²Chemical Science and Engineering Division, Argonne National Laboratory, Lemont, Illinois
60439, United States</footnote>

<footnote n="3">³Brown University, Geochemistry Building, Providence, RI 02912, United States</footnote>

<footnote n="4">⁴Department of Chemistry, Northwestern University, Evanston, Illinois, United States</footnote>

<footnote n="5">⁵Univ Brest, UMR-CNRS 6521 CEMCA, 6 av. Victor le Gorgeu, 29200 Brest, France</footnote>

<footnote n="6">⁶MOLTECH Anjou, Université d'Angers, rue Lavoisier, 49000, Angers, France</footnote>

<footnote n="7">⁷Laboratoire de Chimie Quantique Institut de Chimie UMR 7177 CNRS-Université de
Strasbourg, 4, Rue Blaise Pascal CS 90032, F-67081 Strasbourg Cedex, France</footnote>
contribute to stabilizing the coordination sphere. Furthermore, the flexibility of the benzyl group reinforces the tetrahedral geometry around copper(I), resulting in an increased singlet excited-state energy compared to benchmark complexes. Notably, we show that this enhancement in excited-state energy translates into greater excited-state reactivity.

**Keywords:** Copper(I), homoleptic complexes, ligand design, phenanthroline, oxidative and reductive quenching, photochemistry

**Introduction**

In the family of copper (I)-based photosensitizers, homoleptic Cu(I) complexes of the general formula $[Cu(L)₂]⁺$ have proven to be very efficient photosensitizers in a large number of photo-induced processes.<sup>1,2</sup> They are indeed colourful complexes, and some of them are featuring long-lived excited-state lifetimes and broad emission bands often situated in the near infrared region of the electromagnetic spectrum. Overall, the ground- and MLCT excited-state optical properties strongly depend on the steric hindrance around the Cu(I) ion.<sup>3–6</sup> Indeed, the tetrahedral copper(I) complexes are known to undergo a photo-induced flattening of their structure, allowing solvent attack of the transient Cu(II) centre in the MLCT state, lowering the ground and MLCT state energy gap and leading to a premature extinction of the excited state within a few picoseconds.<sup>7,8</sup> Considering the case where Cu(I) is coordinated by a phenanthroline ligand (phen), the presence of two bulky substituents in positions 2 and 9 promotes the tetrahedral geometry and hinders the Jahn-Teller distortion (and thus solvent attack of the transient Cu(II) centre) resulting in long-lived excited states. A corollary to this fact is that the bulkier the groups, the less likely the deformation and solvent attack are to occur, and hence the excited state lifetimes are prolonged.<sup>3,5,6</sup> In addition, the energy of the excited state E<sup>00</sup> usually rises with strong steric hindrance around the Cu centre, improving the redox reactivity upon illumination.<sup>9</sup> Indeed, the higher E<sup>00</sup>, the stronger the photo-oxidizing and photo-reducing powers, as shown by Rehm and Weller<sup>10</sup> (Figure S1). However, overly bulky groups tend to destabilize the coordination scaffold, which is exacerbated by the lability of Cu-L bonds.<sup>5</sup> When aromatic rings are used as bulky groups, inter-ligand intramolecular π-stacking interactions occur, which stabilize the coordination scaffold<sup>11–14</sup> but significantly decrease E<sup>00</sup>, as observed for Cu(dpp)₂⁺ (where dpp stands for 2,9-diphenyl-1,10-phenanthroline).<sup>9,15</sup>

Aiming at fostering stable complexes with a high E<sup>00</sup>, we designed in a previous contribution non-symmetrical phen ligands, sterically strained by a branched alkyl chain on one position and by a phenyl group on the other, in order to gather the advantages of branched bulky
aliphatic groups such as in Cu(dipp)$_2^+$ (dipp = 2,9-diisopropyl-1,10-phenanthroline) exhibiting
high quantum yields, long lifetimes, with the stabilizing effect of π-stacking promoters such as
phenyl moieties (complexes C1 and C2, Figure 1).⁹ However in the case of C1, the branched
group in position 2 (isopropyl) was not bulky enough and the stacking of the phenyl group in
position 9 with the vicinal phen entailed a distortion of the structure in the ground state and a
mild rise of E⁰⁰. In the case of C2, the branched group (tert-butyl) was too large and limited the
stacking interaction. As a result, a high E⁰⁰ was measured but C2 was less stable than C1.⁹

In order to improve our design, we decided to prepare phenyl-bearing phen ligands, where the
phenyl groups would be tethered to the rigid phen *via* a flexible linker. This flexibility was
hypothesized to allow the phenyl groups to arrange and stack more freely, impacting to a lesser
extent the geometry of the ground state.

Copper(I) complexes bearing phen ligands substituted at positions 2 and 9 by aromatic groups
connected by a flexible linker are available in the literature. For example, Mayor et al. have
reported the synthesis of a CuL<sub>2</sub><sup>+</sup> copper complex where L is a phen ligand functionalized in
positions 2 and 9 by an O-Ph phenoxy group (2,9-diphenoxy-1,10-phenanthroline).<sup>16</sup> The
oxygen atom acts as a spacer between phen and the Ph group. However, the presence of
intramolecular π-stacking interactions is unlikely according to calculations carried out on this
molecule. In contrast, Karnahl et al. presented a series of hetero- and homoleptic complexes
in which benzene rings were attached to phen *via* a flexible 2-chain spacer containing a
heteroatom (phen-CH<sub>2</sub>-O-Ph and phen-CH<sub>2</sub>-S-Ph).<sup>17</sup> The X-ray structures clearly demonstrate
the existence of π-stacking between the Ph groups and the opposite phen. Nevertheless, the
complexes are not luminescent, probably due to a rapid luminescence quenching process by
exciplex formation. Indeed, the excited state of copper complexes is effectively quenched in
the presence of nucleophiles.<sup>18</sup> This is all the more true if the nucleophiles (e.g. oxygen or
sulfur atoms) are localized close to the copper(I) cation, namely on the substituents attached
in α of the nitrogen atoms.<sup>19</sup>

As demonstrated by Nierengarten, when phenyl is attached to phen *via* a CH<sub>2</sub>-CH<sub>2</sub> spacer, no
intramolecular exciplex quenching occurs and intramolecular edge-to-face π-stacking
interactions are observed.<sup>20</sup> Those interactions are nevertheless less stabilizing than face-to-
face stacking.

In the light of these results, we propose to graft a benzyl group at position 2 of phen, and bulky
isopropyl and *tert*-butyl groups at position 9. The absence of a heteroatom eliminates the risk
of exciplex quenching, and the shorter spacer size (CH<sub>2</sub> vs. CH<sub>2</sub>-CH<sub>2</sub>) could promote the
development of face-to-face π-stacking interactions. Finally, the presence of branched
aliphatic groups should increase the energy of the excited state.
<figure><img src="image_2.png" /><figcaption>**Figure 1.** Design of new Cu(I) complexes for the implementation of intramolecular π-stacking without deformation of the tetrahedral coordination cavity for the promotion of excited-state properties.</figcaption></figure>

Our main findings are that, although the occurrence of intramolecular π-stacking is not clearly observed, the coordination spheres of the new complexes are more stable than the previous ones. Importantly the new complexes described herein exhibit a larger excited-state energy, leading to enhanced photo-oxidative strength and improved reactivity in the presence of sacrificial donors.

## Methods

All methods and experimental details are given in the SI.

## Results and discussion

### Synthesis

The syntheses of 2-benzyl-9-isopropyl-1,10-phenanthroline (L3) and 2-benzyl-9-(tert-butyl)-1,10-phenanthroline (L4) are based on the stepwise alkylation of anhydrous phen by the adequate organolithium reagents. The branched alkyl chains were first tethered to phen by reacting R¹-Li (isopropyllithium and tert-butyllithium) with anhydrous phen, followed by oxidation in presence of MnO₂ and yielding R¹-phen in good yields (scheme 1a). Benzyllithium R²Li is then conveniently prepared in toluene in the presence of *n*-BuLi and N,N,N',N'-tetramethylethylenediamine (TMEDA), whose purpose is to increase the reactivity of the organolithium reagents.²¹ R²Li is immediately reacted with the previously obtained mono-alkylated ligands; after re-aromatization in the presence of MnO₂, batches of ligands L3 and
**L4** were obtained with 26 and 32% yield, respectively. The low yields are due to several parasitic reactions (grafting of two benzyl units on R¹-phen and addition of a butyl chain rather than the desired benzyl because of the presence of unreacted *n*-BuLi). Pure batches of compounds could nevertheless be obtained from the crude mixtures by column chromatography (see SI for experimental details).

Cu(I) complexes were then prepared by mixing two equivalents of ligands **L3** or **L4** with one equivalent of tetrakis(acetonitrile)copper hexafluorophosphate in degassed dichloromethane. All ligands and complexes were characterized by ¹H, ¹³C NMR and HR mass spectrometry (see experimental part) and the data confirmed their structures. Interestingly, the ¹H NMR signals associated to the aromatic ring of the benzyl group in **C3** and **C4** are substantially shielded compared to ligands **L3** and **L4**, respectively (Figures S2 and S3). This is in favour of the localization of this aromatic ring in the shielding zone of the vicinal phen. Although this is not evidencing intramolecular π-stacking, this nevertheless points toward the localization of the benzyl ring above the phen core.

<figure><img src="image_3.png" /><figcaption>**Scheme 1.** (a) Synthesis of the new ligands and complexes described in this article. (b) Synthesis of benzyllithium reagent.</figcaption></figure>

In spite of our repeated efforts, we were unable to produce single crystals of **C3** and **C4** that would be suitable for an X-ray diffraction structure resolution. We thus turned to computational chemistry to shed more light on the possible conformational arrangements of the complexes. DFT calculations (ADF, B3LYP/GD3/TZP) were performed to calculate their geometry. This approach was already successfully employed in previous study on related systems.⁹,²² The flexibility of the structures of **C3** and **C4** in the ground state was explored in solution in dichloromethane. The non-symmetrical nature of the ligands and the flexibility of the methylene spacer allow a large variety of conformations (Figure 2 and 3 for the two most stable of **C3** and **C4**), which are presented in supporting information (files C3.xyz and C4.xyz in the SI). In the case of **C3**, six representative structures can describe the molecule in its ground state. The
calculated geometrical parameters are all given in Table S1. The position of the phenyl rings has a significant impact on the structure stability, however four of the computed geometries are very close in energy (C3_g3, C3_g4, C3_g5 and C3_g6). The energy between the most stable (C3_g6) and the least stable (C3_g5) of these four is 2.1 kcal.mol⁻¹. The two other structures are more than 5 kcal.mol⁻¹ above C3_g6. This is the sign that, in solution, C3 does not have a well-defined structure and likely interchanges between different conformations following a Boltzmann distribution. The same investigations were performed on C4, leading to similar results. C4 presents many virtually degenerate conformers, four of them ranging by less than 1 kcal.mol⁻¹. Conformers in which the aryl ring points away are the least stable by far (more than 6 kcal.mol⁻¹ energy difference). Structures in which the aryl rings remain in the vicinity of the phen core are favoured. Interestingly, the lowest energy structures C3_g6 and C4_g8 do not present π-stacking interactions between the aryl ring and the phen core, although they are present in slightly less stable ones (e.g. C3_g4 and C4_g6, respectively).

In summary, the theoretical results are consistent with the experimental ones: the structures in which the phenyl ring are away of the phen cores are much less stable than those in which they are close as shown experimentally. However, the high degree of flexibility makes the π-stacking interaction quite elusive.

<figure><img src="image_4.png" /><figcaption>**Figure 2.** Structure C3_g3 (left) and C3_g6 (right) in their ground state (top) and lowest triplet state (bottom).</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>**Figure 3.** Structure C4_g5 (left) and C4_g8 (right) in their ground state (top) and lowest triplet state (bottom).</figcaption></figure>

## UV-Vis absorption

Electronic absorption spectra of complexes **C3** and **C4** were recorded in dichloromethane and show the typical absorption features of homoleptic Cu(I) – bis(diimine) complexes (Figure 4a, Table 1), in particular a broad metal-to-ligand charge-transfer (MLCT) band spreading between 400 and 550 nm. The extinction coefficients of the latter are in the range expected for these kinds of complexes. The MLCT of **C4** is less intense than **C3**, probably because the steric bulk imposed by the *tert*-butyl group entails a weaker Cu-N bond and a reduced interaction between **L4** and Cu(I). The same observation was made with **C1** and **C2**.⁹ The bluest MLCT band of the four is shown in **C4**, again due to the large steric bulk of the *tert*-butyl groups, which favours the tetrahedral geometry and stabilizes the HOMO.³

The shape of the spectrum for the MLCT transition gives important information about the geometry of the coordination sphere in [CuL₂]⁺ complexes. Indeed, the MLCT is a mixture of several S₀→Sₙ transitions (n ≥ 1, see calculations below) and it is well known that the intensity of the S₀ to S₁ transition (I₅₀→₅₁) increases as the geometry of the coordination sphere (in the ground state) moves from a perfect tetrahedron (I₅₀→₅₁ = 0) to a flattened tetrahedron (I₅₀→₅₁ ≠ 0).
0).²³⁻²⁵ In our previous work, the spectrum of complex C1 clearly exhibited a S₀→S₁ transition, evidencing that the environment around Cu(I) was distorted, descending from D₂d to D₂ symmetry.⁹ This was assigned to intramolecular π-stacking which imposed the geometry around Cu(I). This transition is almost invisible in the case of C3 and C4. To better appreciate the intensity of the S₀→S₁ transition, the UV-Vis spectra of C1-4 and model [Cu(dipp)₂]⁺ were normalized in intensity (Figure S4) and C3 and C4 exhibit the lowest intensity S₀→S₁ transitions in the series. The weak S₀→S₁ transition does not mean that there is no intramolecular π-stacking within the structures of C3 and C4, but rather it is less impactful on the ground state structure of the complex than in the case of C1 (or C2 to a lesser extent) thanks to the additional flexibility brought by the methylene spacer.

<figure><img src="image_6.png" /><figcaption>**Figure 4.** (a) Experimental UV-Vis absorption spectra of C3 (blue line) and C4 (red line) and (b) computed absorption spectra in dichloromethane on the most stable conformer of C3 (C3_g6) and C4 (C4_g8). (c) Emission spectra of C3 (blue line) and C4 (red line) in dichloromethane. Emission: λ<sub>excitation</sub> set at the wavelength of maximum absorbance. The solvent used was degassed CH₂Cl₂.</figcaption></figure>

<table><caption>**Table 1.** Optical and electrochemical figures of merit for complexes C3, C4, and model Cu(dipp)₂⁺.</caption><thead><tr><th></th><th>λ<sub>abs</sub> / nm<br>(ε / M<sup>-1</sup>·cm<sup>-1</sup>)</th><th>λ<sub>em</sub> / nm<br>(Φ<sub>em</sub>)</th><th>τ / ns</th><th>k<sub>r</sub> / s<sup>-1</sup></th><th>k<sub>nr</sub> / s<sup>-1</sup></th><th>E<sup>00</sup> / eV</th><th>E<sub>ox</sub> / V<sup>a,b</sup></th><th>E<sub>red</sub> / V<sup>a,c</sup></th><th>E<sub>ox</sub><sup>*</sup> / V<sup>a</sup></th><th>E<sub>red</sub><sup>*</sup> / V<sup>a</sup></th></tr></thead><tbody><tr><td>C3</td><td>456<br>(6500)</td><td>708<br>8x10<sup>-4</sup></td><td>118</td><td>6.78x10<sup>3</sup></td><td>8.47x10<sup>6</sup></td><td>2.07</td><td>0.52</td><td>-2.06</td><td>-1.55</td><td>0.01</td></tr><tr><td>C4</td><td>440<br>(4700)</td><td>685<br>6x10<sup>-4</sup></td><td>78</td><td>7.69x10<sup>3</sup></td><td>1.28x10<sup>7</sup></td><td>2.10</td><td>0.62</td><td>-2.07</td><td>-1.48</td><td>0.03</td></tr><tr><td>[Cu(dipp)₂]<sup>+</sup></td><td>453<br>(7260)</td><td>679<br>4x10<sup>-3</sup></td><td>365</td><td>1.10x10<sup>4</sup></td><td>2.73x10<sup>6</sup></td><td>2.16</td><td>0.65</td><td>-2.10</td><td>-1.51</td><td>0.06</td></tr></tbody></table>

λ<sub>abs</sub>: wavelength at maximum absorption intensity. λ<sub>em</sub>: wavelength at maximum emission intensity (excitation at λ<sub>abs</sub> in each case). τ: emission lifetime recorded by TC-SPC. k<sub>r</sub>: radiative constant. k<sub>nr</sub>: non-radiative constant. E<sup>00</sup>: singlet excited state energy. All optical measurements were recorded in dichloromethane. E<sub>ox</sub>: oxidation half-wave potential. E<sub>red</sub>: reduction half-wave potential. E<sub>ox</sub><sup>*</sup>: excited state oxidation potential E(Cn⁺/Cn⁺). E<sub>red</sub><sup>*</sup>: excited state reduction potential E(Cn⁺/Cn⁺). n = 3 or 4.

<sup>a</sup>Potentials given in volts vs. Fc⁺/Fc couple. <sup>b</sup>In DCM: sweep rate 100 mV·s<sup>-1</sup>. <sup>c</sup>In DMF: sweep rate 1 V·s<sup>-1</sup>.
The absorption spectra of all conformers were computed for both complexes (see SI for computational details, and figure S5 for a representative sample) and those of the most stable conformers (C3_g6 and C4_g8) are presented in Figure 4b. The agreement between theoretical and experimental spectra is good, with a computed main absorption band around 475 nm slightly red shifted compared to the experimental one by 20-30 nm. Contrary to the experiment, a well-defined weaker band is present around 530-540 nm in the computed spectra. In the particular case of C3 and C4, the conformation of the molecule has a small impact on the absorption spectra. There is a variation of the main absorption band position for both complexes ranging from 463 to 478 nm for C3 and 466 to 486 nm for C4. The impact of conformation on the low-energy shoulder is more important, especially on the intensity, which varies by one order of magnitude whereas its position is affected to the same amount as the main band (Figure S5). A similar observation can be made on the high-energy shoulder around 410 nm. If this mismatch is possibly due to the limitations of DFT, the changes in the aryl orientations affects three geometrical parameters (Cu-N distances, bending and flattening angles) that are significant for the electronic transitions by affecting the Cu-phenanthroline interactions. The presence of these shoulders on the theoretical spectra is due to the selection of one specific conformer which cannot reproduce the conformational flexibility in solution. The absorbing states are predominantly of MLCT character from the copper cation towards the phen core. The main absorption bands for C3_g6 and C4_g8 are due to the third absorbing singlet S<sub>3</sub> at 473 nm and 474 nm, respectively (Figure 5). LC states are only present at much higher energy with wavelength shorter than 315 nm. At the Franck-Condon geometry, the nature of the triplet states for the complexes is an exact reflection of the singlet states.

<figure><img src="image_7.png" /></figure>

**Figure 5.** Nature of the computed singlet state according to THEODore analysis. Horizontal axis: numbering of the singlet states S<sub>n</sub> for the corresponding S<sub>0</sub> → S<sub>n</sub> transition. Vertical axis: proportions (between 0 and 1) of metal centered states (MC, black), ligand centered π-π* 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 S<sub>0</sub> → S<sub>n</sub> transitions.
Sauvage et al.¹² showed that the estimation of the different metal-ligand association constants in Cu(I) complexes could give hints regarding the extent of intramolecular π-stacking. Two association constants are considered, associated to the following equilibria:

$$ \mathrm{Cu}^+ + \mathrm{L} \rightarrow \mathrm{CuL}^+ (K_1) $$

$$ \mathrm{CuL}^+ + \mathrm{L} \rightarrow \mathrm{CuL}_2^+ (K_2) $$

In presence of intramolecular π-stacking, the coordinated ligand in CuL⁺ assists the coordination of the second ligand through stabilizing interactions. This translates into a larger K₂ compared to cases where no such stabilizing interaction exists. The constants were estimated by titration of a Cu(I) solution by L3, L4 and neocuproine as benchmark, monitored by UV-Vis spectroscopy. The constants were retrieved using the REACTLAB software and are given in Table 2. First of all, K₁(CuL4⁺) is larger by almost one order of magnitude than K₁(CuL3⁺). This is probably due to the stronger σ donation of tert-butyl vs. isopropyl. Curiously, log K₁(Cu(neocuproine)⁺) is virtually identical to log K₁(CuL4⁺); this could stem from the larger steric hindrance of L4 (and L3), which might counteract the better σ-donating effects of the branched alkyl chains vs. methyl groups. The log K₂ value is weaker than log K₁ in all cases, but this does not disprove the existence of intramolecular π-stacking within C3 and C4. Examining the data closely, it can be noticed that log K₂ is weaker than log K₁ by two orders of magnitude for L = neocuproine. In this case, there cannot be any stabilizing intramolecular interactions. On the other hand, there is only ca. one order of magnitude drop between log K₁ and log K₂ for L = L3 and L4. This could originate in the competition between steric hindrance and implementation of intramolecular π-stacking. The coordinating cavity of L3 and L4 are very bulky, which could destabilize the coordination scaffold and lead to smaller log K₂ values despite intramolecular π-stacking.

<table><caption>**Table 2.** Association constants log K₁ and log K₂ measured by UV-Vis titration of a Cu(CH₃CN)₄⁺ dichloromethane solution by the appropriate ligand L.</caption><thead><tr><th>L</th><th>Log K<sub>1</sub></th><th>Log K<sub>2</sub></th></tr></thead><tbody><tr><td>L3</td><td>5.9</td><td>5.1</td></tr><tr><td>L4</td><td>6.7</td><td>5.7</td></tr><tr><td>Neocuproine</td><td>6.8</td><td>5.1</td></tr></tbody></table>

Qualitatively, complexes C3 and C4 are more stable than C1 and C2, respectively, in the same conditions. Indeed, while C1 is efficiently destroyed in presence of the weak competitor ligand CH₃CN, C3 seems virtually unaffected by the presence of acetonitrile (Figure S6). C2 and C4 both show instability because of the steric bulk imposed by the tert-butyl group. But ligand
exchange is less efficient in the case of **C4** than in the case of **C2**. The presence of a simple CH₂ spacer between the aryl ring and phen thus led to important changes in the stability of the coordination scaffold (**C3** and **C4** vs. **C1** and **C2**, respectively). This could be due to the existence of intramolecular π-stacking within **C3** and **C4**, but improved σ-donation from benzyl compared to phenyl groups cannot be excluded.

To summarize, the occurrence of intramolecular π-stacking within **C3** and **C4** is not obvious to observe because it does not lead to a distortion of the geometry of the molecules in the ground state (like in the case of Cu(dpp)₂⁺ or **C1**). Computational studies have shown that both complexes present numerous quasi degenerate conformers, some presenting these interactions and others not. The molecular flexibility will probably favor fast exchange between the different structures leading to thermic Boltzmann distribution. Computational studies show that if these interactions are present, the geometry is not fixed and may not be preponderant. This interpretation is corroborated by the measured association constants, and additional stability tests in presence of CH₃CN.

## Electrochemical properties

The electrochemical properties of both complexes were recorded in dichloromethane (anodic behaviour) or DMF (cathodic behaviour) in the presence of tetrabutylammonium hexafluorophosphate as the supporting electrolyte. All data are summarized in Table 1 (voltammograms are given in Figures S7 to S10). In both cases, reversible oxidation and reduction waves were observed and half-wave potentials are in line with previously published data on homoleptic Cu(I) complexes.<sup>3,9,22</sup> The assignment of redox events was confirmed through spectroelectrochemical analysis (Figure S11). Upon oxidation, spectral features characteristic of a Cu(II) species are observed, as expected, including ground-state bleaching at approximately 460 nm and the emergence of a weak d-d transition band at 610 nm. Similarly, one-electron reduction of **C3** and **C4** yields spectra displaying the characteristic signature of phen radicals, with broad, featureless absorption bands around 580 nm (**C3**) and 565 nm (**C4**), respectively.

E(**C4**⁺/**C4**) is 100 mV more positive than E(**C3**⁺/**C3**), likely due to steric effects. As previously observed, the bulkier *tert*-butyl group favours the tetrahedral geometry around the copper ion, thus stabilizing the HOMO. In addition, the oxidation half-wave potential of **C4** is the highest within the series of non-symmetrical complexes **C1-C4**. This trend cannot be attributed to electronic effects, as a benzyl group is less electron-withdrawing than a phenyl group. Instead, steric effects likely dominate, reinforcing a more tetrahedral environment around the copper centre in **C4**.
In contrast, the reduction waves of **C3**, **C4**, and [Cu(dipp)₂]⁺ are nearly identical, indicating that the nature of the bulky substituents at the α-position of the nitrogen atoms has little impact on the LUMO of these complexes.

## Excited-state properties

**C3** and **C4** are luminescent in degassed CH₂Cl₂ solutions at room temperature. Figure 4c shows representative spectra of the broad emission signal that is observed upon MLCT excitation. The emission maximum of **C4** is blue-shifted compared to that of **C3**, in line with the fact that **C4** is more sterically hindered. Overall, **C3** and **C4** are weak luminophores, with emission quantum yields below 0.1% and below those of the previously reported **C1** and **C2** complexes (Table S2). Table 2 shows the rate constants of emission decays recorded by time correlated single photon counting in degassed CH₂Cl₂ (Figure S12). The non-symmetric nature of the ligands is partly responsible for this behaviour as previously reported.⁹,²² Indeed, this broken symmetry entails the existence of many different and equally probable structures in dynamic equilibrium, multiplying non-radiative deactivation pathways. This is reflected in the relatively high non-radiative rate constant *k*<sub>nr</sub> for **C3** and **C4** (Table 1).

The fact that **C3** and **C4** feature weaker photophysical figures of merit than previously published **C1** and **C2** likely originates in the methylene spacer itself. The latter was included in the structures of **L3** and **L4** to promote intramolecular π-stacking within **C3** and **C4** by giving more freedom to the pending aromatic group to stack with the vicinal phen. In return, the ligands and complexes are less rigid and the new additional vibronic modes involving the CH₂ moiety could contribute to the enhanced deactivation of **C3** and **C4**'s luminescence. This assumption is supported by the *k*<sub>nr</sub> magnitudes for **C3** and **C4**, which are larger than those obtained for **C1** and **C2**.

The singlet excited-state energy E<sup>00</sup> could be retrieved from the steady-state spectra with the tangent method for complexes **C3** and **C4** (Figure S13). E<sup>00</sup>(**C4**) is the largest among our series of non-symmetrical complexes. This is likely due to a combination of the benzyl's better σ-donating ability and large steric bulk around the copper ion imposed by the *tert*-butyl group. Nevertheless, the E<sup>00</sup> values are in general close to each other among our series of complexes, larger than the value for Cu(dpp)₂⁺ (where intramolecular π-stacking severely distorts the coordination cage and leads to weak E<sup>00</sup>), but weaker than the singlet energy of benchmark Cu(dipp)₂⁺. As previously observed, these experimental results generally points to the fact that breaking the symmetry of the phen ligand is not favourable to endow the corresponding Cu(I) complexes with large E<sup>00</sup> or emission quantum yields and lifetimes, regardless the steric bulk around the metal cation.<sup>9,22</sup>
Emission properties were computed by optimizing the lowest excited singlet and triplet states of all structures for both complexes (Tables 3 and 4). These states retain their predominantly MLCT nature that was computed at Franck-Condon geometry, from the copper cation towards the phen core. The conformation of the complex has a strong impact on the emission properties as it influences several parameters, especially the singlet-triplet energy splitting (Δ*E*<sub>ST</sub>) and spin-orbit coupling values (SOC) as shown by Capano *et al.* on a series of Cu(I) bisphenathroline complexes.²⁶ This can be illustrated by the λ<sub>em</sub> of C4, which ranges from 796 nm (C4_g2) to 964 nm (C4_g4) in the case of T₁. In all cases, the computed emission wavelength for the T₁ states is localized in the infrared domain, far from the experimental values (Table 1), meaning that the emission is unlikely to stem from the sole triplet state. However, the computed singlet-triplet energy splitting (Δ*E*<sub>ST</sub>) and spin-orbit coupling values (SOC) are consistent with a TADF mechanism as already evidenced in C1 and C2.⁹ The computed emission wavelength for S1 states are closer to the experimental values, but still red-shifted by around 100 nm. This underlines the limitation of a static approach, which only considers energy minima and does not reproduce well the dynamical flexibility of the complex in solution. However, it should be noticed that whatever the conformation considered, the Δ*E*<sub>ST</sub> for C4 is significantly lower (0.167-0.269 eV, Table 4) than that of C3 (0.333-0.359 eV), which could be linked to the smaller lifetime of the C4 excited state. A smaller Δ*E*<sub>ST</sub> allows a more efficient thermal repopulation of the emitting S₁ state and a shorter lifetime.²⁷

<table><caption>**Table 3.** Emission wavelength (λ<sub>em</sub> in nm), oscillator strength (f<sub>osc</sub>), Singlet-Triplet energy splitting (Δ*E*<sub>ST</sub> in eV), Spin-Orbit Coupling (SOC in cm<sup>-1</sup>) between S₁ and T₁ and relative stability of the different conformation (Δ*E* in kcal mol<sup>-1</sup>) at both S₁ and T₁ optimal geometries for C3 conformers. λ<sub>socem</sub> (in nm) stands for emission wavelength and f<sub>socosc</sub> stands for the oscillator strength of the three components of the spin-orbit perturbed T₁ state.</caption><thead><tr><th></th><th></th><th>C3_g1</th><th>C3_g2</th><th>C3_g3</th><th>C3_g4</th><th>C3_g5</th><th>C3_g6</th></tr></thead><tbody><tr><td rowspan="3">S<sub>1</sub></td><td>λ<sub>em</sub></td><td>785</td><td>748</td><td>759</td><td>783</td><td>857</td><td>793</td></tr><tr><td>f<sub>osc</sub></td><td>1.38 10<sup>-2</sup></td><td>1.58 10<sup>-2</sup></td><td>1.28 10<sup>-2</sup></td><td>4.04 10<sup>-3</sup></td><td>4.61 10<sup>-3</sup></td><td>1.47 10<sup>-2</sup></td></tr><tr><td>ΔE</td><td>5.8</td><td>11.4</td><td>3.3</td><td>0.3</td><td>0.0</td><td>0.8</td></tr><tr><td rowspan="5">T<sub>1</sub></td><td>λ<sub>em</sub></td><td>942</td><td>904</td><td>917</td><td>926</td><td>981</td><td>997</td></tr><tr><td>ΔE<sub>ST</sub></td><td>0.346</td><td>0.333</td><td>0.319</td><td>0.333</td><td>0.333</td><td>0.359</td></tr><tr><td>T<sub>1</sub>-S<sub>1</sub></td><td>53.5</td><td>4.8</td><td>61.4</td><td>61.9</td><td>63.4</td><td>59.2</td></tr><tr><td>SOC</td><td>3.5</td><td>41.2</td><td>6.5</td><td>5.3</td><td>2.2</td><td>2.2</td></tr><tr><td>ΔE</td><td>5.8</td><td>11.2</td><td>3.4</td><td>3.5</td><td>1.0</td><td>0.0</td></tr><tr><td rowspan="6">T<sub>1</sub> with SOC</td><td rowspan="3">λ<sub>socem</sub></td><td>946</td><td>908</td><td>921</td><td>930</td><td>986</td><td>1002</td></tr><tr><td>946</td><td>907</td><td>920</td><td>930</td><td>986</td><td>1002</td></tr><tr><td>945</td><td>907</td><td>920</td><td>930</td><td>986</td><td>1002</td></tr><tr><td rowspan="3">f<sub>socosc</sub></td><td>6.23 10<sup>-6</sup></td><td>4.94 10<sup>-6</sup></td><td>8.90 10<sup>-6</sup></td><td>6.06 10<sup>-6</sup></td><td>2.37 10<sup>-5</sup></td><td>3.82 10<sup>-6</sup></td></tr><tr><td>1.81 10<sup>-5</sup></td><td>1.63 10<sup>-5</sup></td><td>2.89 10<sup>-5</sup></td><td>2.87 10<sup>-5</sup></td><td>4.43 10<sup>-5</sup></td><td>1.94 10<sup>-5</sup></td></tr><tr><td>9.50 10<sup>-5</sup></td><td>1.17 10<sup>-4</sup></td><td>1.01 10<sup>-4</sup></td><td>9.87 10<sup>-5</sup></td><td>6.31 10<sup>-5</sup></td><td>9.63 10<sup>-5</sup></td></tr></tbody></table>
<table><caption>**Table 4.** Emission wavelength (λ<sub>em</sub> in nm), oscillator strength (f<sub>osc</sub>), Singlet-Triplet energy splitting (ΔE<sub>ST</sub> in eV), Spin-Orbit Coupling (SOC in cm<sup>-1</sup>) between S<sub>1</sub> and T<sub>1</sub> and relative stability of the different conformation (ΔE in kcal mol<sup>-1</sup>) at both S<sub>1</sub> and T<sub>1</sub> optimal geometries for C4 conformers. λ<sub>socem</sub> (in nm) stands for emission wavelength and f<sub>socosc</sub> stands for the oscillator strength of the three components of the spin-orbit perturbed T<sub>1</sub> state.</caption><thead><tr><th></th><th></th><th>C4_g1</th><th>C4_g2</th><th>C4_g3</th><th>C4_g4</th><th>C4_g5</th><th>C4_g6</th><th>C4_g7</th><th>C4_g8</th></tr></thead><tbody><tr><td rowspan="3">S<sub>1</sub></td><td>λ<sub>em</sub></td><td>734</td><td>723</td><td>705</td><td>707</td><td>764</td><td>816</td><td>871</td><td>745</td></tr><tr><td>f<sub>osc</sub></td><td>2.32<br>10<sup>-3</sup></td><td>1.60<br>10<sup>-3</sup></td><td>3.21<br>10<sup>-3</sup></td><td>2.94<br>10<sup>-3</sup></td><td>2.25<br>10<sup>-3</sup></td><td>4.74<br>10<sup>-3</sup></td><td>1.57<br>10<sup>-3</sup></td><td>3.49<br>10<sup>-3</sup></td></tr><tr><td>ΔE</td><td>4.8</td><td>7.3</td><td>2.1</td><td>2.2</td><td>0.9</td><td>1.7</td><td>0.0</td><td>0.8</td></tr><tr><td rowspan="4">T<sub>1</sub></td><td>λ<sub>em</sub></td><td>814</td><td>796</td><td>958</td><td>964</td><td>840</td><td>903</td><td>897</td><td>822</td></tr><tr><td>ΔE<sub>ST</sub></td><td>0.198</td><td>0.167</td><td>0.173</td><td>0.173</td><td>0.180</td><td>0.269</td><td>0.196</td><td>0.230</td></tr><tr><td rowspan="2">SOC</td><td>43.9<br>6.7</td><td>1.6<br>18.1</td><td>25.5<br>6.2</td><td>25.4<br>5.4</td><td>32.5<br>6.8</td><td>59.5<br>22.0</td><td>41.1<br>1.6</td><td>4.1<br>41.0</td></tr><tr><td>6.7</td><td>18.1</td><td>6.2</td><td>5.4</td><td>6.8</td><td>22.0</td><td>1.6</td><td>41.0</td></tr><tr><td>ΔE</td><td>5.3</td><td>8.1</td><td>0.0</td><td>0.0</td><td>1.5</td><td>1.5</td><td>1.1</td><td>1.1</td></tr><tr><td rowspan="6">T<sub>1</sub> with<br>SOC</td><td rowspan="3">λ<sub>socem</sub></td><td>818</td><td>800</td><td>963</td><td>969</td><td>845</td><td>908</td><td>902</td><td>827</td></tr><tr><td>817</td><td>800</td><td>963</td><td>969</td><td>844</td><td>908</td><td>901</td><td>826</td></tr><tr><td>817</td><td>800</td><td>963</td><td>968</td><td>844</td><td>908</td><td>901</td><td>825</td></tr><tr><td rowspan="3">f<sub>socosc</sub></td><td>1.80 10<sup>-5</sup></td><td>3.38 10<sup>-5</sup></td><td>1.60 10<sup>-5</sup></td><td>1.57 10<sup>-5</sup></td><td>1.54 10<sup>-5</sup></td><td>3.54 10<sup>-5</sup></td><td>1.37 10<sup>-5</sup></td><td>2.14 10<sup>-5</sup></td></tr><tr><td>7.21 10<sup>-5</sup></td><td>4.22 10<sup>-5</sup></td><td>3.49 10<sup>-5</sup></td><td>3.50 10<sup>-5</sup></td><td>3.53 10<sup>-5</sup></td><td>4.75 10<sup>-5</sup></td><td>5.35 10<sup>-5</sup></td><td>5.43 10<sup>-5</sup></td></tr><tr><td>5.92 10<sup>-5</sup></td><td>7.47 10<sup>-5</sup></td><td>2.63 10<sup>-5</sup></td><td>2.58 10<sup>-5</sup></td><td>4.66 10<sup>-5</sup></td><td>5.48 10<sup>-5</sup></td><td>8.43 10<sup>-5</sup></td><td>1.17 10<sup>-4</sup></td></tr></tbody></table>

Optical Transient Absorption Spectroscopy

Transient absorption spectroscopy was used to probe the photo-induced deformation of the coordination sphere of Cu(I) complexes because certain spectral features are strongly dependent on the geometry around the metal ion.<sup>4,28,29</sup> Optical transient absorption spectra were recorded for solutions of C3 and C4 dissolved in degassed dichloromethane at concentrations of ca. 0.5 mM after excitation with 415 nm laser pulses (Figure 6). The evolution-associated difference spectra (EADS) obtained following global kinetic analysis to a sequential first-order kinetic model for C3 and C4 are included in Figure 6, while the kinetic traces and fits and population traces can be found in the SI (Figures S14-S15).

C3 and C4 feature similar transient absorption properties, in line with previously published data on homoleptic Cu(I) complexes.<sup>6,30,31</sup> Briefly, right after excitation (0-2 ps), the depletion of the ground state is clearly observed at ca. 450-470 nm, while a broad and featureless excited state absorption (ESA) is monitored at ca. 570 nm. Subtle variations of the general shape of the ESA are observed on a short timescale (τ<sub>1</sub>, Table 5). τ<sub>1</sub> is usually assigned to the Jahn-Teller distortion; namely, the kinetics of the photo-induced flattening of the CuL<sub>2</sub><sup>+</sup> coordination core. We note that the spectral signature of flattening for C3 was too weak to determine the τ<sub>1</sub> lifetime. Then, the ESA blue shifts, narrows slightly and becomes more structured within ca. 9 to 12 ps (τ<sub>2</sub>, Table 5), leading to a well-known “camel back” ESA typical of the lowest-lying
triplet state of Cu(I) complexes. The $τ_2$ value is thus assigned to intersystem crossing from the excited states of the singlet manifold towards the excited triplet state $T_1$. The shortest $τ_2$ is observed for **C4**, in line with the larger steric strain around the copper cation.

<figure><img src="image_8.png" /><figcaption>**Figure 6.** Femtosecond transient absorption spectra and resulting evolution-associated difference spectra of **C3** and **C4** recorded in degassed dichloromethane upon excitation at 415 nm (500 nJ/pulse). Spectrum A of the EADS accounts for cross-phase modulation signals that occur when the pump and probe are temporally overlapped.</figcaption></figure>

<table><caption>**Table 5.** Experimental time constants retrieved from femtosecond and nanosecond transient absorption spectroscopy</caption><thead><tr><th>Complex</th><th>τ<sub>1</sub> (ps)</th><th>τ<sub>2</sub> (ps)</th><th>τ<sub>3</sub> (ps)</th><th>τ<sub>4</sub> (ns)</th></tr></thead><tbody><tr><td>C1</td><td>0.9 ± 0.6</td><td>10.9 ± 0.6</td><td>-</td><td>193.1 ± 2.6</td></tr><tr><td>C2</td><td>1.0 ± 0.6</td><td>11.1 ± 0.6</td><td>360 ± 40</td><td>93.5 ± 0.8</td></tr><tr><td>C3</td><td>-</td><td>11.6 ± 0.5</td><td>-</td><td>101.5 ± 0.8</td></tr><tr><td>C4</td><td>1.2 ± 0.6</td><td>9.5 ± 0.6</td><td>64 ± 4</td><td>59.8 ± 0.6</td></tr></tbody></table>
The presence of the methylene spacer has no significant impact on the photophysical properties on the complexes during the first tens of picoseconds after photo-excitation. Indeed, the measured lifetimes, transient spectra and EADS are all very comparable in the series from **C1** to **C4** (see Figures S14 to S17). However, a faster ISC rate constant ($\tau_2$) is measured for **C4** vs. all other complexes. This is in line with a more rigid environment around the copper cation for this complex, imposed by the *tert*-butyl on one side, and the benzyl group on the other side of each phen ligand.

Finally, the ESA decays back to the ground state over several tens of ns ($\tau_4$, Table 5, Figure S18), consistent with the observed luminescence lifetime decays. Interestingly, an additional intermediate-timescale species is monitored in the case of **C2** and **C4** ($\tau_3$, Table 5), due to a slight blue shift of the transient signal compared to the final $T_1$ triplet state (see EADS in Figures S15 and S17). The nature of this transient species is unclear for now, but seems associated to the presence of *tert*-butyl groups. We tentatively propose that $\tau_3$ reflects “slow” conformational changes.

## Reductive and oxidative quenching of the excited states

As mentioned above, the increased $E^{\circ 00}$ could lead to enhanced photoredox properties, because of increase photo-oxidative and photo-reductive strengths (Figure S1, Rehm and Weller equation). We explored the latter in two different photochemical processes: oxidative quenching (OQ)- and reductive quenching (RQ)-initiated photochemical transformations.

RQ with homoleptic Cu(I) complexes is rarely observed because they are weak photo-oxidants. Implementing RQ-initiated photochemical reactions is nevertheless a worthwhile endeavour, as recently evidenced in photochemical organic transformations.<sup>32</sup> Here, dmFc was chosen as sacrificial donor because $E(\text{dmFc}^+/\text{dmFc})$ (-0.57 V vs. Fc<sup>+</sup>/Fc) is well suited for the low photo-oxidizing power of homoleptic Cu(I) complexes (Table 1). The occurrence of a photo-induced electron transfer from dmFc to photo-excited homoleptic Cu(I) complexes has been demonstrated before.<sup>9,15</sup>

We followed the RQ of **C3**\* and **C4**\* by dmFc by monitoring the emission decay of both complexes in presence of increasing quantities of dmFc, and retrieved the dynamic quenching constants $k_Q$ following the Stern-Volmer equation (Stern-Volmer plots are given in Figure S19). Importantly, the luminescence quenching can occur through both electron transfer ($k_{\text{ELT}}$) from dmFc to **C3**\*/**C4**\*, and an energy transfer ($k_{\text{ENT}}$) from **C3**\*/**C4**\* to dmFc ($k_Q = k_{\text{ELT}} + k_{\text{ENT}}$). The extent of the latter process has been previously estimated by studying the quenching of **C3** and **C4**'s excited states by ferrocene, a good model of dmFc which cannot be involved in
electron transfer for thermodynamic reasons. Average $k_{ENT}$ values of ca. $1.7 \times 10^8$ M$^{-1}$.s$^{-1}$, i.e. one order of magnitude lower than $k_{ELT}$ have been measured previously for related complexes,⁹,²² and similar values are expected for **C3** and **C4**. The values of $k_{ELT}$ for **C3**, **C4** and model $Cu(dipp)_{2}^{+}$ are given in Table 6. The data were fitted with equation (1), considering the bimolecular reaction would be diffusion-limited (the kinetic diffusion rate of dmFc in dichloromethane is $k_{diff} = 7 \times 10^9$ s$^{-1}$).¹⁵

$$ \frac{1}{k_Q} = \frac{1}{k_{diff}} + \frac{1}{k_{ELT} + k_{ENT}} \quad (\text{equation 1}) $$

Fast kinetics of photo-induced electron transfers from dmFc to **C3**\* and **C4**\* are measured. Calculated $\Delta G_{RQ}$ values (Table 6) are very similar for **C3**, **C4**, $Cu(dipp)_{2}^{+}$ and previously published **C1** and **C2**. However, $k_{ELT}$ for **C3** and **C4** surpass those of **C1**, **C2** and $Cu(dipp)_{2}^{+}$, demonstrating that the reactivity of these complexes in their excited state cannot be explained solely by thermodynamic considerations.

Similar observations can be made from our investigation of OQ-initiated photochemical processes. We monitored the OQ of all complexes, including previously published **C1** and **C2**, in their excited states using $CBr_4$, a well-known electron acceptor that reacts with photo-excited homoleptic $Cu(I)$ complexes (notably $Cu(dap)_{2}^{+}$, where dap stands for 2,9-dianisyl-1,10-phenanthroline).³³ As shown in Table 6, the estimated driving forces for the reaction are ranked as follows: **C1** > **C2** > **C3** > $Cu(dipp)_{2}^{+}$ > **C4**. However, the measured kinetic constants for the reaction follow a different order: **C2** > $Cu(dipp)_{2}^{+}$ > **C1** > **C4** > **C3**.

Both OQ and RQ results confirm that electron transfer rates cannot be directly correlated with the Gibbs free energy of the reaction for these $Cu(I)$ complexes. This highlights the importance of kinetic parameters, which are known to play a crucial role in electron transfer, according to Marcus theory. Moreover, our overall observations support previous reports where the reactivity of $Cu(I)$ complexes was shown to be difficult to predict.³⁴

Nevertheless, although the underlying rationale for the observed reactivities remains unclear, we note that $k_{ELT}(C4)$ and $k_{OQ}(C2)$ are the highest among the non-symmetrical complexes **C1** to **C4**, even exceeding that of model $Cu(dipp)_{2}^{+}$. This suggests that breaking the symmetry of the phenanthroline coordinating cavity positively influences the kinetics of photo-induced electron transfer, whether in the context of oxidative or reductive quenching of the copper complex's excited state. Nevertheless, the largest $K_{SV}$ constants are obtained for $Cu(dipp)_{2}^{+}$.
<table><caption>**Table 6.** Bimolecular quenching constants of the excited state of complexes **C3** and **C4** in presence of dmFc in dichloromethane. $\Delta G_{RQ}$ is the thermodynamic driving force for the reductive quenching step between $Cn^+$ (n = 1-4) and dmFc ($\Delta G_{RQ} = -E(Cn^*/Cn^-) + E(dmFc^+/dmFc)$, $E(dmFc^+/dmFc) = -0.57$ V vs. Fc$^+/Fc)$. $\Delta G_{OQ}$ is the thermodynamic driving force for the oxidative quenching step between $Cn^+$ (n = 1-4) and CBr$_4$ ($\Delta G_{OQ} = -E(Cn^*/Cn^-) + E(CBr$_4$/CBr$_4^–)$, $E(CBr$_4$/CBr$_4^–) = -0.88$ V vs Fc$^+/Fc)$.³³</caption><thead><tr><th rowspan="2"></th><th colspan="4">complex<sup>*</sup> + dmFc → complex<sup>–</sup> + dmFc<sup>+</sup></th><th colspan="2">complex<sup>*</sup> + CBr<sub>4</sub> → complex<sup>+</sup> + CBr<sub>3</sub><sup>–</sup> + Br<sup>–</sup></th></tr><tr><th>ΔG<sub>RQ</sub><br>(eV)</th><th>k<sub>Q</sub><br>(L.mol<sup>-1</sup>.s<sup>-1</sup>)</th><th>k<sub>ENT</sub><br>(L.mol<sup>-1</sup>.s<sup>-1</sup>)</th><th>k<sub>ELT</sub><br>(L.mol<sup>-1</sup>.s<sup>-1</sup>)</th><th>ΔG<sub>OQ</sub><br>(eV)</th><th>k<sub>Q</sub><br>(L.mol<sup>-1</sup>.s<sup>-1</sup>)</th></tr></thead><tbody><tr><td>C1</td><td>-0.60</td><td>2.0x10<sup>9</sup></td><td>1.9x10<sup>8</sup></td><td>1.8x10<sup>9</sup></td><td>-0.72</td><td>1.07x10<sup>6</sup></td></tr><tr><td>C2</td><td>-0.60</td><td>3.13x10<sup>9</sup></td><td>3.3x10<sup>8</sup></td><td>2.8x10<sup>9</sup></td><td>-0.68</td><td>1.32x10<sup>6</sup></td></tr><tr><td>C3</td><td>-0.58</td><td>2.89x10<sup>9</sup></td><td>1.25x10<sup>8</sup></td><td>4.8x10<sup>9</sup></td><td>-0.67</td><td>0.59x10<sup>6</sup></td></tr><tr><td>C4</td><td>-0.60</td><td>3.50x10<sup>9</sup></td><td>1.16x10<sup>8</sup></td><td>6.88x10<sup>9</sup></td><td>-0.60</td><td>0.73x10<sup>6</sup></td></tr><tr><td>Cu(dipp)<sub>2</sub><sup>+</sup></td><td>-0.63</td><td>3.38x10<sup>9</sup></td><td>2.76x10<sup>8</sup></td><td>3.10x10<sup>9</sup></td><td>-0.63</td><td>1.27x10<sup>6</sup></td></tr></tbody></table>

## Conclusion

Aiming at implementing stabilizing intramolecular $\pi-\pi$ stacking within Cu(I) homoleptic complexes, two new non-symmetrically substituted phen ligands were synthesized and their homoleptic Cu(I) complexes were isolated. Branched alkyl chains (isopropyl and tert-butyl) on one side, and benzyl encumbering groups were thus tethered to phen in positions 2 and 9. The role of the benzyl was primarily to promote the implementation of stabilizing intramolecular $\pi$-stacking interactions within the corresponding Cu(I) complexes. More precisely, the $CH_2$ “rotula” inserted between the phenyl ring and the phen was imagined to allow the generation of such stacking interaction without leading to deformations of the coordination sphere, as previously observed.⁹,¹²,³⁵,³⁶ Experimental and computational measurements are in favour of the existence of such $\pi-\pi$ stacking interactions, although it is not clear whether the interactions are face-to-face or edge-to-face stacking. As previously observed, all the photophysical figures of merit associated to this new series of complexes are rather weak compared to benchmark $Cu(dipp)<sub>2</sub>^+$. This is grounded in the non-symmetrical nature of the bulky phen ligands, which favours non-radiative deactivation pathways. Nevertheless, the flexibility introduced in the new complexes presented herein allowed to improve their stability. Importantly, tert-butyl bearing complex **C4** features improved stability vs. the analogous more rigid complex **C2**, and improved reactivity in the frame of the reductive quenching of its excited state by decamethylferrocene. This is a direct consequence of the highly tetrahedral and rigid environment fostered by ligands **L4** around the copper cation. Nevertheless, this study clearly demonstrates that predicting the reactivity of chromophores in their excited states is inherently complex and cannot be adequately explained by thermodynamic considerations alone.
**Supporting information**

Description of the methods and experimental techniques, computational details, synthesis protocols and characterizations, NMR spectra, normalized experimental UV-Vis spectra and computed absorption spectra, stability assessment of the complexes by UV-Vis spectroscopy, cyclic voltammetry, UV-Vis spectra of oxidized and reduced species (spectroelectrochemistry), luminescence decays recorded by TC-SPC and normalized emission spectra, complementary transient absorption spectra and global kinetic analysis for **C3** and **C4**, decay traces to the ground state recorded by transient absorption spectroscopy, Stern-Volmer plots for **C3** and **C4**'s quenching by decamethylferrocene, computed geometrical parameters for **C3** and **C4** in the ground and excited states, summary of the figures of merit for complexes **C1**, **C2**, **C3** and **C4**.

**Acknowledgements**

The optical transient spectroscopic studies are partially supported by Chemical, Biological and Geological Science Division, Basic Energy Science, CBG Division, US Department of Energy through Argonne National Laboratory under Contract No. DE-AC02-06CH11357.

The authors wish to thank the ANR (PERCO program n°ANR-16-CE07-0012-01) for financial support, and 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.

**TOC Graphic**

<figure><img src="image_9.png" /></figure>
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Encumbering the Copper(I)-Diimine Coordination Sphere with a Benzyl Group: Impact
on the Stability and Photoredox Properties.

Lea Gimeno,¹ Emily A. Sprague-Klein,²,³ Brian T. Phelan,²,⁴ Tyler N. Haddock,² Nicolas
Oppé,¹ Maryline Beyler,⁵ Charlotte David,¹ Errol Blart,¹ Eric Levillain,⁶ Olivier Aleveque,⁶
Christophe Gourlaouen,⁷* Lin X. Chen,²,⁴* Yann Pellegrin¹*

¹Nantes Université, CNRS, CEISAM, UMR 6230, F-44000 Nantes, France

²Chemical Science and Engineering Division, Argonne National Laboratory, Lemont, Illinois
60439, United States

³Brown University, Geochemistry Building, Providence, RI 02912, United States

⁴Department of Chemistry, Northwestern University, Evanston, Illinois, United States

⁵Univ Brest, UMR-CNRS 6521 CEMCA, 6 av. Victor le Gorgeu, 29200 Brest, France

⁶MOLTECH Anjou, Université d'Angers, rue Lavoisier, 49000, Angers, France

⁷Laboratoire de Chimie Quantique Institut de Chimie UMR 7177 CNRS-Université de Strasbourg, 4, Rue Blaise Pascal CS 90032, F-67081 Strasbourg Cedex, France
# A. Methods and techniques

## General:

Chemicals were purchased from Sigma-Aldrich or Fisher Scientific and used as received. 2,9-bis-isopropyl-1,10-phenanthroline (dipp), and $[Cu(dipp)_2]^{+}$ were synthesized as previously published.<sup>1,2</sup> Thin-layer chromatography (TLC) was performed on aluminum sheets precoated with Merck 5735 Kieselgel 60F254. Column chromatography was carried out with Merck 5735 Kieselgel 60F (0.040-0.063 mm mesh). <sup>1</sup>H and <sup>13</sup>C NMR spectra were recorded on an AVANCE 300 UltraShield BRUKER. Chemical shifts for <sup>1</sup>H NMR spectra are referenced relative to residual protium in the deuterated solvent ($CDCl_3$ 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 $CH_2Cl_2$ or in DMF with 0.1 M $n-Bu_4NPF_6$. 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 dichloromethane (DCM) solutions.

## Computational details:

Calculations were performed with ADF 2019 at DFT level of theory (B3LYP functional). Weak interactions were included through Grimme3 corrections. 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). The emission wavelength of lowest excited singlet and triplet states was computed on the basis of their optimized structures. Spin-Orbit coupling was introduced perturbatively in the TD-DFT calculations.
**Association constants measurements:**

3 mL of a ligand L solution in DCM HPLC grade ([L] between 0.537 mM and 0.290 mM) was mixed with variable aliquots of a stock solution of Cu(CH₃CN)₄PF₆ in DCM HPLC grade ([Cul] between 7.27 and 5.08 mM). The solutions were allowed to equilibrate for at least 1h at 25°C. Absorption spectra were recorded on a Jasco V-760 spectrophotometer thermostated at 25°C by using 1 cm quartz cells. Absorption spectra were recorded from 350 to 650 nm for 25 solutions corresponding to VCu added = 0 to 250 µL. Data were analysed using the commercial REACTLAB software to calculated the association constants.

**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.³ 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 remaining 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°). 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).³ The amplifier outputs a 0.5 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 primary 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 primary 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 m[M]) 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 ($\tau_i = 1/k_i$), 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 sequential first-order rate matrix 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 difference spectra were then extracted from the raw data using the time-dependent populations for each state in the proposed kinetic model.

## Spectroelectrochemistry:

Time-resolved spectroelectrochemistry was performed using a custom-built spectroelectrochemical cell, as previously described.⁴ The distance between the electrode surface and the optical window was typically maintained between 25 and 50 µm. Electrochemical measurements were conducted using a platinum wire as the counter electrode and a silver wire as a quasi-reference electrode, with a Biologic SP-300 potentiostat controlled by EC-Lab software, which included ohmic drop compensation. Experiments were carried out
in anhydrous, HPLC-grade CH₂Cl₂ with TBAPF₆ (electrochemical grade, Fluka) as the supporting electrolyte. All solutions were prepared and transferred into the spectroelectrochemical cell inside a glovebox under an inert atmosphere of dry, oxygen-free (<1 ppm) argon at room temperature. Spectrophotometric measurements were performed using a custom-built setup consisting of various Princeton Instruments components, including light sources, optical fibers, monochromators, a spectroscopy camera, and associated software. To ensure simultaneous initiation of both experiments, the two setups were synchronized using TTL signals.

**Synthesis:**

2-isopropyl-1,10-phenanthroline and 2-tert-butyl-1,10-phenanthroline were prepared following previously published procedures.⁵

**2-benzyl-9-isopropyl-1,10-phenanthroline (L3):** under argon atmosphere, distilled TMEDA (tetramethylethylenediamine) (3.42 mmol, 0.50 mL) and dry toluene (10 mL) are stirred at room temperature. The *n*-BuLi (3.42 mmol, 1.40 mL) is added dropwise at 0°C. The solution is stirred at 0°C for 1 h, affording the benzyllithium solution. In another two-neck round bottom flask, 2-isopropyl-1,10-phenanthroline (0.86 mmol, 190 mg) is dissolved under argon in dry toluene (10 mL). The prepared benzyllithium solution is then added dropwise at 0°C. The solution is allowed to warm to room temperature and stirred overnight. The solution is then neutralized with water and extracted with DCM. Activated MnO₂ (21.4mmol, 1.90 g) is added to the organic phase and stirred. After 3h Na₂SO₄ is added to dry the organic phase, filtrated and evaporated. The resulting oil is purified through silica column with DCM (100%) to DCM /MeOH: (97/3%). The product was retrieved as an orange oil with 26% of yield.

HRMS (ESI⁺) for C₂₂H₂₁N₂ [M+H]⁺, m/z 313.1697 Da found, 313.1705 Da calc.

**2-benzyl-9-tert-butyl-1,10-phenanthroline (L4):** under argon atmosphere, distilled TMEDA (10.1 mmol, 1.50 mL) and dry toluene (24 mL) are stirred at room temperature. *n*-BuLi (10.1

<footnote n="1">¹H NMR spectrum (CDCl₃, 400 MHz) δ = 8.18 (d, JH,H = 8.4 Hz, 1H), 8.08 (d, JH,H = 8.3 Hz, 1 H), 7.70 (AB, JAB = 9.92 Hz, 2H), 7.59 (d, JH,H = 8.4 Hz, 1H), 7.38 (m, 5H), 7.26 (m, 1H), 4.63 (s, 2H), 3.62 (hept, JH,H = 6.9Hz, 1H), 1.49 (d, JH,H =6.9 Hz, 6H).</footnote>

<footnote n="13">¹³C NMR spectrum (CDCl₃, 75 MHz) δ = 168.21 (s), 161.68 (s), 136.52 (s), 136.41 (s), 129.75 (s), 128.60 (s), 126.42 (s), 125.79 (s), 125.45 (s), 122.62 (s), 120.12 (s), 45.69 (s), 37.47 (s), 23.05 (s).</footnote>
mmol, 4.0 mL) is added dropwise at 0°C. The solution is stirred at 0°C for 1 h, affording the benzyllithium solution. In another two-neck round bottom flask, 2-tert-butyl-1,10-phenanthroline (0.42 mmol, 100 mg) is dissolved under argon in dry toluene (10 mL). 1.54 mL of the prepared benzyllithium solution is then added dropwise at 0°C. The solution is allowed to warm to room temperature and stirred overnight. The solution is then neutralized with water and extracted with DCM. Activated $MnO_2$ (21.4mmol, 1.90 g) is added to the organic phase and stirred. After 3h $Na_2SO_4$ is added to dry the organic phase, filtrated and evaporated. The resulting oil is purified through silica column with DCM (100%) to DCM/MeOH: (97/3%). The product was retrieved as an orange oil with 32% of yield.

¹H NMR spectrum (CDCl₃, 400MHz) $\delta$ = 8.15 (d, JH,H = 8.5 Hz, 1H), 8.08 (d, JH,H = 8.2 Hz, 1H), 7.74 (d, JH,H = 8.5 Hz, 1H), 7.96 (AB, JAB = 4.6 Hz, 2H), 7.52 (m, 2H), 7.38 (d, JH,H = 8.2 Hz, 1H), 7.33 (m, 2H), 7.24 (m, 1H), 4.56 (s, 2H), 1.62 (s, 9H).

¹³C NMR spectrum (CDCl₃, 75 MHz) $\delta$ = 169.57 (s), 161.50 (s), 136.52 (s), 136.14 (s), 129.95 (s), 128.67 (s), 126.53 (s), 125.76 (s), 125.59 (s), 122.48 (s), 120.13 (s), 45.82 (s), 38.89 (s), 30.48 (s).

HRMS (ESI+) for C₂₃H₂₃N₂ [M+H]⁺, m/z 327.1867 Da found, 327.1861 Da calc.

[Cu(L3)₂]⁺(PF₆⁻) (C3): under argon atmosphere, Cu(CH₃CN)₄(PF₆) (26 mg, 0.07 mmol) is dissolved in degassed dichloromethane (4 mL). A solution of L3 (44 mg, 0.14 mmol) in degassed dichloromethane (4 mL) is then syringed into the previous clear solution, which turned from colorless to deep orange. The solution was stirred at room temperature overnight. Then the crude was precipitated in presence of hexane. The residue was purified on a silica plug (dichloromethane:methanol, 9:1). After removal of the solvent under reduced pressure, a dark orange powder was obtained with 75% yield (42 mg).

¹H NMR spectrum (CDCl₃, 300 MHz) $\delta$= 8.61 (d, JH,H = 8.3 Hz, 1H), 8.54 (d, JH,H = 8.3 Hz, 1H), 8.08 (s, 2H), 7.81 (d, JH,H = 8.5 Hz, 1H), 7.65 (d, JH,H = 8.2 Hz, 1H), 7.00 (t, JH,H = 7.6 Hz, z, 1H), 6.90 (t, JH,H = 7.0 Hz, 2H), 6.48 (d, JH,H = 7.3 Hz, 2H), 3.94 (AB, JAB = 46.7 Hz, 2H), 3.04 (hept, JH,H = 7.3 Hz, 1H), 1.01 (d, JH,H = 7.0 Hz, 3H), 0.95 (d, JH,H = 6.9 Hz, 3H).

¹³C NMR spectrum (CDCl₃, 75 MHz) $\delta$= 166.87 (s), 159.49 (s), 143.48 (s), 142.71 (s), 138.32 (s), 137.73 (s), 136.63 (s), 128.54 (s), 128.38 (s), 128.31 (s), 127.68 (s), 126.83 (s), 126.75 (s), 126.43 (s), 125.77 (s), 122.04 (s), 39.15 (s), 31.25 (s), 22.24 (s), 22.12 (s).

HRMS (ESI+) for C₄₄H₄₀N₄Cu [M]⁺, m/z 687.2559 Da found, 687.2549 Da calc.
[Cu(L4)2]⁺(PF6⁻) (C4): complex C4 was obtained following the same protocol than C3
(Cu(CH3CN)4(PF6): 19 mg, 0.05 mmol; L4: 33 mg, 0.1 mmol). Yield: 80% (34 mg).

¹H NMR spectrum (CDCl₃, 300 MHz) δ= 8.59 (d, JH,H = 8.6 Hz, 1H), 8.50 (d, JH,H = 8.3 Hz, 1H), 8.11 (d, JH,H = 8.6 Hz, 1H), 8.07 (s, 2H), 7.54 (d, JH,H = 8.4 Hz, 1H), 7.07 (t, JH,H = 7.1 Hz, 1H), 7.00 (t, JH,H = 7.4 Hz, 2H), 6.51 (d, JH,H = 7.2 Hz, 2H), 3.94 (AB, JAB = 73.0 Hz, 2H), 1.27 (s, 9H).

¹³C NMR spectrum (CDCl₃, 75 MHz) δ= 166.34 (s), 157.62 (s), 143.69 (s), 142.73 (s), 139.00 (s), 137.82 (s); 137.79 (s), 128.63 (s), 128.37 (s), 128.25 (s), 127.36 (s), 127.10 (s), 126.80 (s), 126.18 (s), 124.83 (s), 121.89 (s), 39.59 (s), 29.72 (s), 22.58 (s), 21.92 (s).

HRMS (ESI+) for C₄₆H₄₄N₄Cu [M]⁺, m/z 715.2853 Da found, 715.2862 Da calc.
B. Figures

$$E(\text{PS}^*/\text{PS}^-) = E(\text{PS}/\text{PS}^-) + E^{00}$$

$$E(\text{PS}^+/\text{PS}^-) = E(\text{PS}^+/\text{PS}) - E^{00}$$

**Figure S1.** Equations of Rehm and Weller allowing to estimate E(PS*/PS-) and E(PS+/PS*), quantitative parameters which describe respectively the photo-oxidative and photo-reductive powers of a photosensitizer PS. E(PS+/PS) is the half-wave potential of the oxidation wave of PS. E(PS/PS-) is the half-wave potential of the reduction wave of PS. E°° is the energy of the lowest excited state.
<figure><img src="image_10.png" /><figcaption>**Figure S2.** ¹H-NMR spectra of L3 and C3 in CDCl3. Residuals peaks at ca 1.5-1.6 ppm assigned to trace water. Residual peak at ca. 2.2 ppm assigned to trace acetone.</figcaption></figure>
<figure><img src="image_11.png" /></figure>

L4, from 9 to 3.8 ppm

<figure><img src="image_12.png" /></figure>

C4, from 9 to 3.8 ppm

<figure><img src="image_13.png" /></figure>

L4, from 3 to 0 ppm

<figure><img src="image_14.png" /></figure>

<figure><img src="image_15.png" /></figure>

C4, from 3 to 0 ppm

<figure><img src="image_16.png" /></figure>

<figure><img src="image_17.png" /></figure>

**Figure S3.** ¹H-NMR spectra of L4 and C4 in CDCl₃. Residual peaks at ca. 1.5 ppm assigned to trace water
<figure><img src="image_18.png" /></figure>

**Figure S4.** Normalized MLCT of complexes **C3** (red line), **C4** (blue line) and model [Cu(dipp)₂]⁺ (green line). The spectra of previously published⁵ **C1** (black) and **C2** (grey) are given for the reader's consideration.

<figure><img src="image_19.png" /></figure>

**Figure S5.** Computed absorption spectra in dichloromethane for (a) complex **C3** and (b) complex **C4** in their most stable conformations.
<figure><img src="image_20.png" /><figcaption>**Figure S6.** Evolution of the maximum intensity of the MLCT upon addition of acetonitrile to a dichloromethane solution of **C1** (black), **C2** (grey), **C3** (red) and **C4** (blue). Case of Cu(dipp)₂⁺ in green, in the same conditions. The concentration of each complex was ca. 0.04 mM; the volume of the solutions was 2.5 mL. Aliquots of CH₃CN (from 0 to 1000 µL) were added to the solution. The data shown in Figure S6 take the dilution factor into consideration.</figcaption></figure>

<figure><img src="image_21.png" /><figcaption>**Figure S7.** Cyclic voltammetry of complex **C3** in dichloromethane. Supporting electrolyte: tetrabutylammonium hexafluorophosphate 0.1 M. Working electrode: Pt disk. Counter electrode: platinum gauge. Reference electrode: Ag/AgCl electrode in LiCl saturated ethanol. Potentials reference vs. Fc⁺/Fc couple. Sweep rate: 100 mV·s⁻¹. The arrow indicates the direction of sweep. The additional wave at 0.2 V vs. Fc⁺/Fc on sweeping the potential back to open circuit potential is related to partial decoordination of the copper complex upon oxidation.⁶⁻⁸</figcaption></figure>
<figure><img src="image_22.png" /><figcaption>**Figure S8.** Cyclic voltammetry of complex C3 in dimethylformamide. Supporting electrolyte: tetrabutylammonium hexafluorophosphate 0.1 M. Working electrode: Pt disk. Counter electrode: platinum gauge. Reference electrode: Ag/AgCl electrode in LiCl saturated ethanol. Potentials reference vs. Fc+/Fc couple. Sweep rate: 1000 mV.s⁻¹ (to improve the reversibility of the reduction wave). The arrow indicates the direction of sweep.</figcaption></figure>

<figure><img src="image_23.png" /></figure>
**Figure S9.** Cyclic voltammetry of complex **C4** in dichloromethane. Supporting electrolyte: tetrabutylammonium hexafluorophosphate 0.1 M. Working electrode: Pt disk. Counter electrode: platinum gauge. Reference electrode: Ag/AgCl electrode in LiCl saturated ethanol. Potentials reference vs. Fc+/Fc couple. Sweep rate: 100 mV.s⁻¹. The arrow indicates the direction of sweep.

<figure><img src="image_24.png" /></figure>

**Figure S10.** Cyclic voltammetry of complex **C4** in dimethylformamide. Supporting electrolyte: tetrabutylammonium hexafluorophosphate 0.1 M. Working electrode: Pt disk. Counter electrode: platinum gauge. Reference electrode: Ag/AgCl electrode in LiCl saturated ethanol. Potentials reference vs. Fc+/Fc couple. Sweep rate: 1000 mV.s⁻¹ (to improve the reversibility of the reduction wave). The arrow indicates the direction of sweep. The additional irreversible wave at −1.9 V likely originates from side products formed in DMF, as **C4** shows instability in this solvent (see inset in Figure S10: monitoring of the MLCT of **C4** in 2.5 mL of dichloromethane upon the addition of 0 to 800 µL of DMF; the blue arrow indicates the collapse of the MLCT).
<figure><img src="image_25.png" /><figcaption>**Figure S11.** Spectro-electrochemistry on complexes **C3** and **C4** in dichloromethane (supporting electrolyte: tetrabutylammonium hexafluorophosphate 0.1 M). a) and b): complex **C3**. c) and d): complex **C4**. a) and c): spectral changes upon oxidation. b) and d): spectral changes upon reduction. The corresponding cyclic voltammograms of **C3** and **C4** are given in e) and f) respectively (5 mV/s sweep rate).</figcaption></figure>
<figure><img src="image_26.png" /><figcaption>**Figure S12.** Luminescence decay traces recorded for C3 (red) and C4 (blue) in degassed dichloromethane by time correlated single photon counting. The best fits of the experimental data are given as red lines.</figcaption></figure>

<figure><img src="image_27.png" /><figcaption>**Figure S13.** Normalized emission spectra used to retrieve E°° (singlet energy) for C3 (red), C4 (blue) and model Cu(dipp)₂⁺ (black)</figcaption></figure>
<figure><img src="image_28.png" /><figcaption>**Figure S14.** Global kinetic analysis results for the fsTA spectra measured for solutions of C3 dissolved in dichloromethane following excitation at 415 nm (500 nJ/pulse). Kinetic traces at select probe wavelengths overlaid with the optimized fits are on the left and the resultant populations for the sequential first order kinetic model are on the right.</figcaption></figure>

<figure><img src="image_29.png" /><figcaption>**Figure S15.** Global kinetic analysis results for the fsTA spectra measured for solutions of C4 dissolved in dichloromethane following excitation at 415 nm (500 nJ/pulse). Kinetic traces at select probe wavelengths overlaid with the optimized fits are on the left and the resultant populations for the sequential first order kinetic model are on the right.</figcaption></figure>
<figure><img src="image_30.png" /><figcaption>**Figure S16.** Femtosecond transient absorption spectra (upper left) and global kinetic analysis results for the fsTA spectra measured for solutions of C1 dissolved in dichloromethane following excitation at 415 nm (500 nJ/pulse). Kinetic traces at select probe wavelengths overlaid with the optimized fits are in the upper right along with the resultant evolution-associated difference spectra (lower left) and populations (lower right) for the sequential first order kinetic model.</figcaption></figure>
<figure><img src="image_31.png" /><figcaption>**Figure S17.** Femtosecond transient absorption spectra (upper left) and global kinetic analysis results for the fsTA spectra measured for solutions of C2 dissolved in dichloromethane following excitation at 415 nm (500 nJ/pulse). Kinetic traces at select probe wavelengths overlaid with the optimized fits are in the upper right along with the resultant evolution-associated difference spectra (lower left) and populations (lower right) for the sequential first order kinetic model.</figcaption></figure>

<figure><img src="image_32.png" /><figcaption>**Figure S18.** Decays of the transient absorption signals at chosen relevant wavelength for (a) C1, (b) C2, (c) C3 and (d) C4. Excitation at 415 nm in dichloromethane. Best fits as red lines on the experimental spectra.</figcaption></figure>
<figure><img src="image_33.png" /></figure>

<figure><img src="image_34.png" /></figure>

**Figure S19.** a) Stern-Volmer plots for C3 (red) and C4 (blue) in presence of dmFc (τ₀/τ as a function of [dmFc], where τ₀ and τ designate the emission lifetime in absence and in presence of dmFc, respectively). Data recorded in dichloromethane by single photon counting, excitation at 438 nm. b) Data recorded with ferrocene Fc to estimate the kinetic rate constant of energy transfer from the copper complexes' excited states towards the ferrocenic energy acceptors.
<figure><img src="image_35.png" /><figcaption>**Figure S20.** Stern-Volmer plots for **C1** (green), **C2** (purple), **C3** (red), **C4** (blue) and Cu(dipp)₂⁺ (black) in presence of CBr₄ (τ₀/τ as a function of [CBr₄], where τ₀ and τ designate the emission lifetime in absence and in presence of CBr₄, respectively). Data recorded in dichloromethane by single photon counting, excitation at 438 nm.</figcaption></figure>

<table><caption>**Table S1.** Ground-state geometry parameters for the six conformers computed for **C3**. Distances are in Angstroms and angle in degrees. The definition of atoms N1, N2, Cα and of position X is given in the above picture.</caption><thead><tr><th>Structure</th><th>C3_g1</th><th>C3_g2</th><th>C3_g3</th><th>C3_g4</th><th>C3_g5</th><th>C3_g6</th><th></th><th></th></tr></thead><tbody><tr><td>Flattening<br>X-Cα-Cα-X</td><td>87.5</td><td>-82.1</td><td>-83.0</td><td>79.5</td><td>99.9</td><td>-83.2</td><td></td><td></td></tr><tr><td>Bending<br>X-Cu-X</td><td>175.5</td><td>171.3</td><td>175.2</td><td>167.0</td><td>160.7</td><td>163.2</td><td></td><td></td></tr><tr><td>Cu-N1</td><td>2.064</td><td>2.078</td><td>2.078</td><td>2.064</td><td>2.062</td><td>2.114</td><td></td><td></td></tr><tr><td>Cu-N2</td><td>2.063</td><td>2.062</td><td>2.057</td><td>2.070</td><td>2.080</td><td>2.034</td><td></td><td></td></tr><tr><td>Cu-N1'</td><td>2.064</td><td>2.078</td><td>2.091</td><td>2.099</td><td>2.062</td><td>2.114</td><td></td><td></td></tr><tr><td>Cu-N2'</td><td>2.062</td><td>2.062</td><td>2.043</td><td>2.035</td><td>2.080</td><td>2.034</td><td></td><td></td></tr><tr><td>ΔE</td><td>6.8</td><td>7.8</td><td>0.2</td><td>0.4</td><td>2.1</td><td>0.0</td><td></td><td></td></tr><tr><td>Structure</td><td>C4_g1</td><td>C4_g2</td><td>C4_g3</td><td>C4_g4</td><td>C4_g5</td><td>C4_g6</td><td>C4_g7</td><td>C4_g8</td></tr><tr><td>Flattening<br>X-Cα-Cα-X</td><td>85.6</td><td>86.7</td><td>90.2</td><td>89.2</td><td>84.2</td><td>101.8</td><td>80.8</td><td>97.8</td></tr><tr><td>Bending<br>X-Cu-X</td><td>169.1</td><td>172.0</td><td>171.5</td><td>172.7</td><td>175.0</td><td>167.0</td><td>173.3</td><td>174.0</td></tr><tr><td>Cu-N1</td><td>2.127</td><td>2.214</td><td>2.252</td><td>2.186</td><td>2.167</td><td>2.267</td><td>2.276</td><td>2.030</td></tr><tr><td>Cu-N2</td><td>2.053</td><td>2.029</td><td>1.997</td><td>2.024</td><td>2.029</td><td>1.988</td><td>2.001</td><td>2.169</td></tr></tbody></table>
<table><tr><td>Cu-N1'</td><td>2.218</td><td>2.214</td><td>2.026</td><td>2.255</td><td>2.180</td><td>2.203</td><td>2.276</td><td>2.030</td></tr><tr><td>Cu-N2'</td><td>2.013</td><td>2.029</td><td>2.166</td><td>1.999</td><td>2.021</td><td>2.006</td><td>2.001</td><td>2.169</td></tr><tr><td>ΔE</td><td>7.3</td><td>6.0</td><td>0.4</td><td>0.6</td><td>0.2</td><td>2.4</td><td>2.7</td><td>0.0</td></tr></table>

<figure><img src="image_36.png" /><figcaption>**Table S2.** Optical and electrochemical figures of merit for complexes C1, C2, C3 and C4 and model Cu(dipp)₂⁺.</figcaption></figure>

<table><thead><tr><th></th><th>λ<sub>abs</sub> (nm)/<br>ε (M<sup>-1</sup>.cm<sup>-1</sup>)</th><th>λ<sub>em</sub><br>(nm)/<br>Φ<sub>em</sub></th><th>τ (ns)</th><th>k<sub>r</sub><br>(s<sup>-1</sup>)</th><th>k<sub>nr</sub><br>(s<sup>-1</sup>)</th><th>E<sup>00</sup><br>(eV)</th><th>E<sub>ox</sub><br>(V)<sup>a,b</sup></th><th>E<sub>red</sub><br>(V)<sup>a,c</sup></th><th>E<sub>red</sub><sup>*</sup><br>(V)<sup>a</sup></th></tr></thead><tbody><tr><td>C1</td><td>443, 540<br>(3990,<br>1000)</td><td>698<br>1x10<sup>-3</sup></td><td>216</td><td>4.63x10<sup>3</sup></td><td>4.63x10<sup>6</sup></td><td>2.09</td><td>0.49<br>(120)</td><td>-2.06<br>(130)</td><td>0.03</td></tr><tr><td>C2</td><td>450 (3850)</td><td>768<br>8x10<sup>-4</sup></td><td>92</td><td>8.70x10<sup>3</sup></td><td>1.09x10<sup>7</sup></td><td>2.11</td><td>0.55<br>(110)</td><td>-2.08<br>(90)</td><td>0.03</td></tr><tr><td>C3</td><td>456 (6500)</td><td>708<br>8x10<sup>-4</sup></td><td>118</td><td>6.78x10<sup>3</sup></td><td>8.47x10<sup>6</sup></td><td>2.07</td><td>0.52</td><td>-2.06</td><td>0.01</td></tr><tr><td>C4</td><td>440 (4700)</td><td>685<br>6x10<sup>-4</sup></td><td>78</td><td>7.69x10<sup>3</sup></td><td>1.28x10<sup>7</sup></td><td>2.10</td><td>0.62</td><td>-2.07</td><td>0.03</td></tr><tr><td>Cu(dipp)<sub>2</sub><sup>+</sup></td><td>453 (7260)</td><td>679<br>4x10<sup>-3</sup></td><td>365</td><td>1.10x10<sup>4</sup></td><td>2.73x10<sup>6</sup></td><td>2.16</td><td>0.65</td><td>-2.10</td><td>0.06</td></tr></tbody></table>

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