Substituent effect on the photoinduced structural change of Cu(I) complexes observed by femtosecond emission spectroscopy.

The Cu(I) complexes having phenanthroline derivatives as ligands are known to exhibit photo-induced 'flattening' structural change in the metal-to-ligand charge transfer (MLCT) excited state. Our recent ultrafast spectroscopic studies of Cu(dmphen)2 (dmphen = 2,9-dimethyl-1,10-phenanthroline) showed that the photo-induced structural change predominantly occurs in the S1 state on a subpicosecond time scale, with the appearance of the 'perpendicular' S1 state before the structural change. In this work, we carried out femto/picosecond time-resolved emission spectroscopy of Cu(phen)2 (phen = 1,10-phenanthroline) and Cu(dpphen)2 (dpphen = 2,9-diphenyl-1,10-phenanthroline) in dichloromethane with the S2 ← S0 photo-excitation to examine the substituent effect on the ultrafast structural change. The femtosecond time-resolved emission spectra of the two complexes exhibit ultrafast fluorescence changes that are attributed to the structural change in the S1 state after fast (50-100 fs) S2 → S1 internal conversion. By comparing with the dynamics of Cu(dmphen)2, it was found that the time constant of the structural change increases as the substituents at 2- and 9- positions of the ligand become bulkier, i.e., Cu(phen)2 (200 fs) < Cu(dmphen)2 (660 fs) < Cu(dpphen)2 (920 fs). This implies that the complex needs a longer time to flatten with the bulkier substituent. This demonstrates that the dynamics of the photo-induced structural change of Cu(I) complexes is substantially affected by the substituent of the ligand. The dynamics of the ultrafast structural change and the substituent effect are discussed with the multidimensional S1 potential energy surface of Cu(I) complexes.