LOW-LYING SINGLET-STATES OF ALPHA,OMEGA-DITHIENYLPOLYENES - ALPHA,OMEGA-DITHIENYLBUTADIENE, ALPHA,OMEGA-DITHIENYLHEXATRIENE, AND ALPHA,OMEGA-DITHIENYLOCTATETRAENE

Fluorescence and fluorescence excitation spectra for low temperature n-alkane solutions of alpha,omega-dithenylpolyenes with two, three, and four polyene double bonds have been measured. The fundamental frequencies in the vibronic development of the fully resolved spectra are either nearly identical to modes seen for unsubstituted polyenes or nearly identical to modes seen for polythiophene oligomers. The degree to which thiophene ring modes contribute to the spectra decreases with increasing polyene chain length. In the tetraene, the 2 1A(g) state is 2760 cm-1 lower in energy than the 1 1B(u) state, in the triene it is 1570 cm-1 lower, and in the diene these two states are nearly degenerate. The 2 1A(g) and 1 1B(u) excitation energies are well fit by a simple theoretical model which also gives a description of the pi-electron distributions in the 1 1A(g), 2 1A(g), and 1 1B(u) states.