Abstract
The relaxation dynamics of excited electronic states of [(1,1′-biphenyl)-4,4′-diyldi-2,1-ethenediyl]bis(dim-ethylsilane) 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 ∼100 fs, and two decay time constants, 7-65 ps and ∼1 ns. We attribute the former decay to upper excited states to the S <sub>1</sub> state, and the latter decay to geometric relaxation and the lifetime of the S <sub>1</sub> 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 ∼34° for trans and ∼38° 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 <sub>1</sub> state. Enhanced vibrational relaxation of S <sub>1</sub> 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 <sup>-1</sup> ). © 2009 American Chemical Society.