Solvent Effect on the Photoinduced Structural Change of a Phosphorescent Molecular Butterfly

Photoinduced structural changes (PSC) is one of the fundamental excited-state dynamic processes, and yet often very challenging to distinguish from competing electronic excited-state relaxation channels having similar or even comparable timescales. Here, we report a detailed study on the PSC of a pyrazolate bridged platinum(II) binuclear complex, BFPtPZ (C^NPt(μ-pz')₂ PtC^N, C^N=2-(2,4-difluorophenyl)pyridine, pz'=pyrazolate), a molecular butterfly, using time-correlated single photon counting measurements at different wavelengths and sample temperatures. Analysis of the results obtained using dichloromethane (DCM) and ethylene carbonate (EC) as solvents enabled us to reveal an unexpected, strong solvent effect on the PSC processes. We show that a rapid PSC process with a characteristic timescale of 323 ps is observed in DCM, which leads to an excitation equilibrium between the ligand center/metal-to-ligand charge transfer (³ LC/MLCT) and metal-metal-to-ligand charge transfer (³ MMLCT) triplet states. The subsequent relaxation from these electronic states to the ground state takes place in several nanoseconds. In contrast, the corresponding PSC process in EC appears slow at all temperatures studied in our experiments and showed no sign of such excitation equilibrium. The observed solvent effect is found to arise from distinct solvent properties including their viscosities and polarities as well as the peculiar electronic excited-states of the butterfly-like molecules with charge transfer character.