THE LOWEST EXCITED SINGLET-STATE OF ISOLATED 1-PHENYL-1,3-BUTADIENE AND 1-PHENYL-1,3,5-HEXATRIENE

We report vibrationally resolved S0 --> S1 excitation spectra and vibr onic level decay times for the phenyl-substituted polyenes 1-phenylbut adiene and 1-phenylhexatriene seeded in supersonic He expansions. This information was obtained using one- and two-color resonance-enhanced multiphoton ionization techniques. The shift in the excitation energy of the lowest excited singlet state upon deuteration of the phenyl rin g demonstrates that in 1-phenylbutadiene S1 is mainly an excitation of the benezene ring while in 1-phenylhexatriene S1 is mainly the 2 1A(g ) state of hexatriene. Analysis of the excitation spectrum of 1-phenyl hexatriene shows that the spectrum contains contributions from two spe cies, the trans,trans and the trans,cis isomers, whose 2 1A(g) state e xcitation energies differ by about 155 cm-1. The lifetimes of 2 1A(g) zero-point levels and, in particular, the activity of the C-C and C = C stretch vibrations are markedly different for the two isomers. From these results we obtain a picture of the influence that the phenyl gro up has on such aspects of the electronic structure of the 2 1A(g) stat e of hexatriene as the effective conjugation length, the symmetry prop erties, and the fluorescence quantum yield.