科研之友
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摘要
The time-resolved fluorescence of p-terphenyl, 2,2 ''-dimethyl-p-terphenyl (dm-terphenyl), and tetracene dissolved in solvents was measured with fluorescence up-conversion and time-correlated single-photon counting. We characterized the relaxation dynamics of vibrational energy in these three molecules by examining their properties in solvents of varied physical properties. According to the measured curves of fluorescence decays obtained at excitation wavelength 266/256 nm, we propose that p-terphenyl and dm-terphenyl both undergo initially an ultrafast intramolecular redistribution of vibrational energy. A conformational relaxation of p-terphenyl in excited-state SI with time constant 6-13 Ps is proposed from its linear dependence on the viscosity of solvents. This process and vibrational energy relaxation are accelerated in a protic solvent involving efficient coupling to the high-energy bath modes of the solvent. Because the torsional conformational change of dm-terphenyl is effectively inhibited by the steric hindrance imposed by its methyl substituents, its vibrational relaxation is found to be independent of the viscosity of the solvent but remains rapid with time constant 5.2-8.8 ps. The time constant of vibrational relaxation in tetracene is observed to be 12-16 ps, with no evident dependence on solvent viscosity or thermal diffusivity. The fact that accepting modes with greater energy are more effective in transferring energy for a system with a large excess of vibrational energy is proposed to explain the ratio of the rates of vibrational relaxation for methanol MeOH/MeOD approximate to 2.
- 出版日期2011-11-17
- 单位清华大学
