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 similar to 3.6 angstrom 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.