Excited-state dynamics of [(1,1'-biphenyl)-4,4-diyldi-2,1-ethenediyl]bis(dimethylsilane)

The relaxation dynamics of excited electronic states of [(1,1'-biphenyl)-4,4'-diyldi-2,1-ethenediyl]bis(dimethylsilane) dissolved in various solvents with varied polarity and viscosity have been investigated. Upon excitation at wavelength 266 nm, we measured the fluorescence curves that exhibit a rise time constant approximately 100 fs, and two decay time constants, 7-65 ps and approximately 1 ns. We attribute the former decay to upper excited states to the S(1) state, and the latter decay to geometric relaxation and the lifetime of the S(1) state. Only the tens of picosecond decay shows a dependence on the solvent viscosity, indicating that the torsional motion dominates the relaxation. Theoretical calculations were performed to obtain the optimized structures of the free [(1,1'-biphenyl)-4,4'-diyldi-2,1-ethenediyl]bis(dimethylsilane) molecule in its ground and first excited states with methods B3LYP/6-311G(d) and CIS/6-311G(d), respectively. The results of these calculations show that the dihedral angle between the two phenyl rings is approximately 34 degrees for trans and approximately 38 degrees for cis conformers in the ground state and that the first excited state has a planar structure, in agreement with the experimental results that indicate that the torsional motion of two phenyl groups elevates the relaxation of the S(1) state. Enhanced vibrational relaxation of S(1) in alcoholic solvents is observed. Rapid relaxation in methanol-OH compared with that in methanol-OD is explained by the excess energy dissipated efficiently through high-frequency vibrational mode (>500 cm(-1)).