Excited-State Dynamics of Tetraphenylethylene:  Ultrafast Stokes Shift, Isomerization, and Charge Separation

Femtosecond pump−probe spectroscopy is used to explore the excited-state dynamics of TPE in polar and nonpolar solvents. Four excited states are shown to play an important role: the vertically excited S1 state, the Franck−Condon relaxed S1 state, a twisted charge-resonance state (in the literature often referred to as biradical), and a charge-separated state. The subpicosecond dynamics are dominated by a very large Stokes shift, which is primarily due to ultrafast elongation of the ethylenic C−C bond, and by wave packet motion of the phenyl-ring bending modes. On picosecond time scales, isomerization dynamics and charge separation by symmetry breaking occur. The latter process is made possible by an avoided crossing between the singly and doubly excited states of TPE, leading to a dramatic enhancement of the polarizability. The electron transfer across the C−C bond follows an adiabatic reaction path on the lower potential energy surface. In nonpolar solvents, an equilibrium is established with a symmetric charge resonance state, by thermally activated recrossing to the upper potential surface. In polar solvents this process is suppressed by solvent stabilization of the dipolar, zwitterionic form of TPE.