THE EXCITED-STATE POTENTIAL-ENERGY SURFACE FOR THE PHOTOISOMERIZATION OF TETRAPHENYLETHYLENE - A FLUORESCENCE AND PICOSECOND OPTICAL CALORIMETRY INVESTIGATION

A rudimentary map of the photoisomerization potential energy surface for tetraphenylethylene in alkane solvents is experimentally determined. The line shapes of the vertical and relaxed excited states emissions at 294 K in methylcyclohexane are obtained from the steady state emission spectrum and the wavelength dependence of the time resolved fluorescence decays. Subsequently, the temperature dependences of the vertical and relaxed states emission quantum yields and of the time resolved fluorescence decays are determined. Analysis of these data in conjunction with the twisted excited state energy, determined using picosecond optical calorimetry, provides values for the energies of the vertical, conformationally relaxed, and twisted excited states on the photoisomerization surface, as well as the barriers to their interconversion. The energy difference between the latter two states is found to be 1.76 +/- 0.15 kcal/mol in methylcyclohexane. The information from this and prior investigations is discussed with the aim of constructing structure-energy and structure-kinetic correlations for the twisted excited states of tetraphenylethylenes and structurally related alkenes.