Franck-Condon analysis of the absorption and fluorescence spectra of all trans α,ω-diphenylpolyenes with one to seven polyene double bonds

Fluorescence and absorption spectra have been measured for all-trans alpha,omega-diphenylpolyenes with one to seven polyene double bonds in room-temperature solution, along with the fluorescence spectra of those with one to six polyene double bonds in n-alkane matrices at 77 K. All the spectral data were fitted by sums of Gaussians to treat the Franck-Condon envelopes of the measured spectra quantitatively. The Franck-Condon analyses of the spectra in the harmonic limit revealed that the displacements of the C=C and C-C stretching vibrational modes in the 2 Ag state relative to those in the ground state, 1 1Ag(S0), increase, while those in the 1 1Bu state show a slight decrease with the increase of the polyene chain length. It is also shown that the bandwidths of the absorption and fluorescence spectra exhibit a monotonic decrease with the increase of the chain length.