On the UV spectrum of cross-conjugated 2,3-diphenyl-1,3-butadiene

The UV spectrum of the cross-conjugated compound 2,3-diphenyl-1,3-butadiene, which can occur in both an s-gauche and s-trans conformation, is reported. The spectrum is interpreted with the use of ZINDO/S calculations and compared to the spectrum of styrene. In both conformations the difference in electronic transition energy with styrene is negligible, despite a shift in HOMO and LUMO energy. This is the consequence of the decrease of Coulomb and exchange energies by which extension of the π-system is accompanied. As such, the UV spectrum of 2,3-diphenylbutadiene, as compared to that of styrene, does not reflect extension of the π-system.