On the inversion of the 1 Bu and 2 Ag electronic states in α,ω-diphenylpolyenes

An alternative model to that of the inversion of the states 1 B-u and 2 A(g) is proposed for interpreting the photophysics of the alpha,omega-diphenylpolyenes. This model is based upon the existence of two chemical structures with B-u symmetry, which may be ascribed to the same excited electronic state 1 B-u. One of the two chemical structures corresponds to the Franck-Condon structure with conjugated single and double bonds for the polyene chain, and another consists of a nearly equivalent series of partial double bonds along the polyene chain. The latter relaxed structure is consistent with the observation of high torsional energy barriers and low photoisomerization quantum yields for diphenylhexatriene in the singlet excited state manifold. Interestingly, such a simple quantum model as that of the particle in a one-dimensional box provides quite an accurate description of the absorption spectroscopic properties of these major compounds. This is partly the result of the most stable structures for these compounds being of the all-trans type; such structures increase in length as additional ethylene units are added, which makes them very similar to a one-dimensional box becoming increasingly longer. (C) 2003 American Institute of Physics.