Effect of substituents at the 4,7-positions on the structural change dynamics of Cu(I) bis(phenanthroline) complexes in the MLCT excited state

Cu(I) bis(phenanthroline) complexes are known to show rapid structural changes in metal-to-ligand charge-transfer excited states, and the rates of the structural changes decrease with bulkier substituents at the 2,9-positions of phenanthroline. In this work, the structural change dynamics of Cu(I) bis(phenanthroline) complexes with substituents at the 4,7- positions of the ligands were investigated using femtosecond fluorescence upconversion measurements. The time constants of the structural changes for complexes with CH₃ and Ph substituents at the 4,7-positions were found to be the same as that of the unsubstituted Cu(I) complex (ca. 200 fs). This clearly indicates that solvent friction with the substituent moiety does not affect the structural change dynamics of the Cu(I) complexes. The structural change dynamics for the substituents at the 2,9- positions were determined to be because of steric effects among the ligands or substituents, rather than the friction of the solvent.