Resolution of Three Fluorescence Components in the Spectra of all-trans-1,6-Diphenyl-1,3,5-hexatriene under Isopolarizability Conditions

Abstract

all-trans-1,6-diphenyl-1,3,5-hexatriene (DPH) fluorescence in solution consists of emissions from the S1 (2(1)A(g)) and S2 (1(1)B(u)) 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 longer wavelengths, and both minor emissions, s-c-DPH and 1(1)B(u) s-t-DPH fluorescence, contribute more at higher temperatures (Ts). Resolution of a spectrothermal matrix of DPH fluorescence spectra by principal component analysis with self-modeling (PCA-SM) is hampered by T-dependent changes in the spectra of the individual components. We avoided differential polarizability-dependent spectral shifts by measuring the spectra in n-alkanes (Cn, C8 to C16 with n even) at T values selected to keep the index of refraction constant, hence under isopolarizability conditions. Compensation of the spectra for T-induced broadening allowed resolution of the spectral matrix into its three components. The optimum van't Hoff plot gives Delta H = 2.83 kcal/mol for s-c-DPH/s-t-DPH equilibration, somewhat smaller than the 3.4 kcal/mol calculated value, and the optimum Boltzmann distribution law plot gives Delta E(ab) = 4.09 kcal/mol for 1(1)B(u)/2(1)A(g) equilibration. The 1(1)B(u) fluorescence spectrum 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 1(1)B(u) and 2(1)A(g) fluorescence spectra. The results give V(ab) = 198 +/- 12 cm(-1) for the vibronic matrix coupling element between the 2(1)A(g) and 1(1)B(u) states. Fluorescence quantum yields and lifetimes under isopolarizability conditions reveal an increase in the effective radiative rate constant of s-t-DPH with increasing T.