Effect of the back skeleton ligand on the ultrafast excited-state dynamics of Cu(<scp>i</scp>) cyano substituted bipyridine complexes

Cu(I) complexes have attracted a lot of research interest as potential alternatives to functional noble metal complexes. In previous research studies of the ultrafast dynamics of Cu(I) complexes, most of the acceptor ligands used are symmetric and examples of only a limited number of asymmetric ligands were reported. To further understand the ultrafast excited state dynamics of Cu(I) complexes with an asymmetric cyano-substituted bipyridine electron acceptor ligand, Cu(I) complexes with 6-cyano-2,2'-bipyridine and 4,4'-dimethyl-6-cyano-2,2'-bipyridine ligands in dichloromethane and acetonitrile were investigated by applying femtosecond time-resolved transient absorption (TA) spectroscopy. From the TA spectra, it was found that two different metal-to-ligand charge transfer (MLCT) states with different nature could be populated after pseudo-Jahn-Teller distortion. Time-dependent density functional theory (TD-DFT) calculation results also support the hypothesis that, in one MLCT state, the electron density is donated from the Cu(I) center to the cyanobipyridine ligand with electron density delocalised on the whole bipyridine ligand and in the other MLCT state the electron density is donated from the Cu(I) center to the cyano-substituted pyridine fragment of the cyanobipyridine ligand. This result indicates that asymmetric electron acceptors may lead to the population of extra excited states compared with symmetric electron acceptors.