Time-resolved dual fluorescence of 1-phenylpyrrole in acetonitrile: molecular dynamics simulations of solvent response to twisted intramolecular charge transfer

The real-time dynamics of solvation of 1-phenylpyrrole (PhPy) in acetonitrile (ACN) upon electronic excitation is investigated by means of non-equilibrium molecular dynamics simulations. The interaction is modeled by empirical intermolecular pair potentials using partial charges and intramolecular torsional potentials from high level ab initio calculations of ground and excited states of PhPy. The intramolecular torsional motion following sudden excitation from the twisted ground state to the 2 B-1 charge transfer state is strongly damped by the viscous ACN solvent, leading to a near-exponential approach of the perpendicular conformation on a timescale of about 5-10 ps. The intermolecular dynamics is characterized by rapid reorientation of the solvent molecules on a timescale of 100 fs followed by weak quasi-coherent librations. The solvatochromic red-shift of the charge transfer state with respect to the locally excited 1 B-1 state results in dual fluorescence, thus supporting the twisted intramolecular charge transfer (TICT) mechanism for PhPy in a polar solvent.