Ultrafast relaxation processes of excitons to the relaxed excited state in one-dimensional dimethylglyoxime platinum complexes driven by intramolecular vibration

Optical response due to the photo excited carrier or exciton dynamics in a one-dimensional (1D) dimethylglyoxime Pt complex $[Pt(dmg)2]$ has been investigated by femtosecond pump–probe spectroscopy. Measurements were made at several excitation energies between the exciton state and the free electron hole pair state. Induced absorption was observed in the low-energy side of the exciton band at any excitation energy. The spectral shape of the induced absorption changed with time between 0.1 and 0.3 ps after instantaneous rise up. These results can be explained by the formation and thermalization processes of the relaxed excited state on the adiabatic potential surface. The unthermalized relaxed excited state is formed in a very short time (<50 fs). From the Raman spectrum, the frequency of the intermolecular stretching mode along the chain was estimated to be 80 cm⁻¹ $(T∼400 fs).$ These facts suggest that, in contrast to other 1D systems such as polydiacetylenes (PDA) and halogen bridged mixed valence metal complexes (MX), the formation of the relaxed excited state is caused not by the stretching vibration along the 1D chain. We propose the possibility that the triggering of the primary relaxation is caused by a high frequency (>600 cm⁻¹) intramolecular mode which is not in the direction of the 1D chain.