Revealing the Photorelaxation Mechanism in a Molecular Solid Using Density-Functional Theory

Photorelaxation in a molecular crystal is investigated by a density-functional theory for the first time. A quasi-one-dimensional molecular compound, (EDO-TTF)(2)PF6, is known to exhibit a photoinduced phase transition, which is characterized as a transition from a (0110)-type charge-ordering insulator phase to a high-temperature metallic phase. First, we apply the method of embedding a cluster in a self-consistent environment and succeed in constructing a stable tetramer structure of EDO-TTF molecules. The reliance of this cluster is ensured by a vibrational analysis that well reproduces the IR and Raman frequencies particularly for C=C stretching modes including a relatively large degree of electron-'molecular vibration' coupling. Second, relaxations in the photoexcited states of this cluster are investigated by adiabatic potential-surface analyses and full structural optimization. A reaction coordinate is found to be quite unique for a relatively high-energy excitation, namely, the so-called CT2 excitation, which is interpreted as leading to the photoinduced phase transition. DOI: 10.1103/PhysRevLett.110.116401