Temperature dependence of aryl butatriene fluorescence: Barrier to twisting on S1 for 1,1,4,4-tetraphenylbutatriene

Article abstract of DOI:10.1016/S0022-2860(97)00363-3

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°.