| Abstract
| - all-trans-1,6-Diphenyl-1,3,5-hexatriene (DPH) fluorescence in solution consists of emissions from the S1 (21Ag)and S2 (11Bu) states of the s-trans,s-trans conformer (s-t-DPH) and emission from the S1 state of the s-cis,s-trans conformer (s-c-DPH). The contribution of s-c-DPH fluorescence increases upon excitation at longerwavelengths, and both minor emissions, s-c-DPH and 11Bus-t-DPH fluorescence, contribute more at highertemperatures (Ts). Resolution of a spectrothermal matrix of DPH fluorescence spectra by principal componentanalysis with self-modeling (PCA-SM) is hampered by T-dependent changes in the spectra of the individualcomponents. We avoided differential polarizability-dependent spectral shifts by measuring the spectra inn-alkanes (Cn, C8 to C16 with n even) at T values selected to keep the index of refraction constant, henceunder isopolarizability conditions. Compensation of the spectra for T-induced broadening allowed resolutionof the spectral matrix into its three components. The optimum van't Hoff plot gives ΔH = 2.83 kcal/mol fors-c-DPH/s-t-DPH equilibration, somewhat smaller than the 3.4 kcal/mol calculated value, and the optimumBoltzmann distribution law plot gives ΔEab = 4.09 kcal/mol for 11Bu/21Ag equilibration. The 11Bu fluorescencespectrum bears mirror-image symmetry with the DPH absorption spectrum, and the energy gap, 1431 cm-1,is consistent with the 1615 cm-1 difference between the lowest energy bands in the 11Bu and 21Ag fluorescencespectra. The results give Vab = 198 ± 12 cm-1 for the vibronic matrix coupling element between the 21Agand 11Bu states. Fluorescence quantum yields and lifetimes under isopolarizability conditions reveal an increasein the effective radiative rate constant of s-t-DPH with increasing T.
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