Photoinduced Neutral-to-Ionic Phase Transition in Tetrathiafulvalene-p-chloranil Studied by Time-Resolved Vibrational Spectroscopy

We studied the photoinduced neutral-to-ionic phase transition in tetrathiafulvalene- p -chloranil (TTF-CA) using time-resolved infrared (IR) vibrational spectroscopy with a broadband femtosecond IR laser pulse and a linear IR detector array. After photoexcitation, the strength of the TTF a g ν 3 band increased, indicating that dimerization is activated within ∼1 ps. In contrast, the strength of the CA b 1 u ν 10 band, whose frequency is proportional to the degree of charge transfer (ρ), decreased, and a broad weak band appeared in the lower-frequency region. This implies that large fluctuations in charge transfer continued until about 20 ps after the photoexcitation. These results indicate a large difference between the electronic structures of the ionic phases generated by thermalexcitation and photoexcitation, although a dimerized structure was observed in both cases.