SOLVENT DEPENDENCE OF THE TWISTED EXCITED-STATE ENERGY OF TETRAPHENYLETHYLENE - EVIDENCE FOR A ZWITTERIONIC STATE FROM PICOSECOND OPTICAL CALORIMETRY

The energy and decay rate constant of the twisted excited singlet state of tetraphenylethylene are determined as a function of solvent by using picosecond optical calorimetry. The energy of the twisted excited state, relative to the planar ground state, in alkane solvents, diethyl ether, and tetrahydrofuran are 67.0 +/- 1.3, 66.2 +/- 1.4, and 65.3 +/- 1.7 kcal/mol, respectively. A dipole moment of 6 D for the twisted excited state is obtained from a dielectric continuum analysis of the solvent dependence of the twisted state energy. Using the activation energy for thermal cis-trans isomerization to obtain an estimate of the twisted ground state energy, a linear relationship is found between the logarithm of the decay rate constant and the energy gap at the twisted geometry. These results confirm a previous explanation for the solvent-dependent lifetimes of the twisted state and provide evidence for zwitterionic character in the lowest energy excited singlet state of tetraphenylethylene.