Theoretical Investigations on the Photoinduced Phase Transition Mechanism of Tetrathiafulvalene-p-chloranil

The photoinduced phase transition (PIPT) mechanism of tetrathiafulvalene-p-chloranil (TTF-CA) molecular crystal was theoretically investigated using the long-range corrected time-dependent density functional theory (LC-TDDFT) combined with a local response dispersion (LRD) method, which enables us to quantitatively reproduce charge transfer (CT) excitations of van der Waals clusters. By calculating the excitation spectrum and potential energy surface, we found that the PIPT of TTF-CA crystal may proceed through the angle change of the molecular planes. We also found that the CT excitation of one TTF-CA pair helps other neighboring TTF-CA pairs to become excited. Consequently, we theoretically proposed the initial structural change in the neutral-to-ionic PIPT of TTF-CA crystal, which is consistent with experiments.