Quantifying flattening distortion on the thermodynamics and kinetics of electron transfers of Cu(i) photoredox catalysts

Abstract

Cu(I) photoredox catalysts have become popular alternatives to traditional Ir and Ru photosensitizers. However, the flattening distortion during the Cu(I) to Cu(II) ³MLCT charge transfer is one of the main drawbacks preventing their broad-scale application. Avoiding this structural change using bulky ligands has been a strategy to increase the lifetime of the excited ³MLCT state. In this computational study, we discuss the effect of preventing the flattening distortion on the thermodynamics and kinetics of the electron transfer processes of the photoredox cycle. Theoretical square schemes were calculated to quantitatively separate the reduction processes into separate vertical electron transfer (ET) and structural relaxation (SR) steps. Computational results show that ET energies account for 89–99% of the reduction processes. Meanwhile, SR depends on the degree of structural changes during the Cu(II) → Cu(I) transition and becomes less important as the bulkiness of the ligand increases. The calculated internal reorganization energies for the electron-self exchange show that preventing flattening distortion in Cu(I) complexes may aid the electron transfer kinetics by reducing the reaction barriers. The calculated singlet–triplet energy gaps increase with the bulkiness of the ligand, which agrees with the increase of the excited state lifetimes observed in Cu(I) species by preventing the flattening distortion.