A strategy is presented to improve the excited state reactivity of homoleptic copper–bis(diimine) complexes CuL2+ by increasing the steric bulk around CuI whereas preserving their stability. Substituting the phenanthroline at the 2‐position by a phenyl group allows the implementation of stabilizing intramolecular π stacking within the copper complex, whereas tethering a branched alkyl chain at the 9‐position provides enough steric bulk to rise the excited state energy E00. Two novel complexes are studied and compared to symmetrical models. The impact of breaking the symmetry of phenanthroline ligands on the photophysical properties of the complexes is analyzed and rationalized thanks to a combined theoretical and experimental study. The importance of fine‐tuning the steric bulk of the N–N chelate in order to stabilize the coordination sphere is demonstrated. Importantly, the excited state reactivity of the newly developed complexes is improved as demonstrated in the frame of a reductive quenching step, evidencing the relevance of our strategy.

Three new copper(I) complexes [Cu(LX)2] + (PF6-) (where LX stands for 2,9-di-halo-1,10phenanthroline and X = Cl, Br and I) have been synthesized in order to study the impact of halogen substituents tethered in α position of the chelating nitrogen atoms on their physical properties. Photophysical properties of these new complexes (hereafter named Cu-X) were characterized both in their ground and excited states. Femtosecond ultrafast spectroscopy revealed that early photo-induced processes are faster for Cu-I than for Cu-Cl or Cu-Br, both showing similar behaviors. Their electronic absorption and electrochemical properties are comparable to benchmark [Cu(dmp)2] + (where dmp stands for 2,9-dimethyl-1,10phenanthroline); furthermore, their optical features were fully reproduced by TD-DFT and Ab Initio Molecular Dynamics (AIMD) calculations. All three complexes are luminescent at room temperature, showing that halogen atoms bound to positions 2 and 9 of phenanthroline are sufficiently bulky to prevent strong interactions between the excited Cu complexes and solvent molecules in the coordination sphere. Their behavior in the excited state, more specifically the extent of photoluminescence efficiency and its dependence on temperature, is however strongly dependent on the nature of the halogen. A combination of ultrafast transient absorption spectroscopy, temperature dependent steady state fluorescence spectroscopy and 2 computational chemistry allows to gain a deeper understanding of the behavior of all three complexes in their excited state.

In order to perform challenging reduction reactions with light, at low cost and low toxicity, we aim at using for the first time a reductive quenching cycle with a simple,...