Nonadiabatic Mixed Quantum−Classical Dynamic Simulation of π-Stacked Oligophenylenevinylenes

We present results from the first nonadiabatic (NA), nonequilibrium mixed quantum-classical molecular dynamics simulations of pi-stacked oligophenylvinylene (OPV) chains with a quantum electronic Hamiltonian (Pariser-Parr-Pople with excited states given by configuration interaction) that goes beyond the tight-binding approximation. The chains pack approximately 3.6 A apart in the ground state at 300 K, and we discuss how thermal motions, chiefly a relative sliding motion along the oligomer backbone, affect the electronic structure. We assign the electronic absorption spectrum primarily to the S(0) --> S(2) transition as transitions from the ground state to S(1) and S(3) are particularly weak. After photoexcitation, the system rapidly decays via NA transitions to S(1) in under 150 fs. On S(1), the system relaxes as a bound exciton, localized on one chain that may hop between chains with a characteristic time between 300 and 800 fs. We find that the system does not make a rapid transition to the ground state because both the NA and radiative couplings between S(1) and S(0) are weak.