Journal of Molecular Structure p. 107 - 113 (1998)
Update date:2022-08-11
Topics:
Connors, Robert E.
Chynwat, Veeradej
Clifton, Christine H.
Coffin, Tracy L.
The temperature dependence of aryl butatriene fluorescence is assigned to a competition between radiative decay and butatriene chain rotation against a barrier followed by radiationless decay from S1. Fluorescence quantum yields were measured as a function of temperature for 1,1,4,4-tetraphenylbutatriene (TPBT) in a series of n-alkane solvents. Application of the medium-enhanced barrier model and transition state theory allow the observed barrier to be expressed in terms of an intrinsic and solvent-induced contribution (ΔH(obsd)/(+) = ΔH(t)/(+) + ΔH(v)/(+)). It is found that there is little or no intrinsic barrier to rotation (ΔH(t)/(+) = 0.075 ± 0.155 kcal/mol) on S1 for TPBT. The solvent-induced component is related to E(η), the activation energy for viscous flow, by ΔH(v)/(+) = aE(η) with a = 0.86 ± 0.06 for TPBT. This is in contrast to twisting on S1 for trans-stilbene for which a significant intrinsic barrier has been reported. AM1 molecular orbital calculations for gas phase TPBT show a twist angle of ~O°on S0 with a barrier to rotation of 20.1 kcal/mol. Consistent with the experimental results, the calculations show that TPBT undergoes a barrierless relaxation on S1 from its Franck-Condon structure to a minimum with a twist angle of 85.6°.
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