Semiempirical Molecular-Orbital Calculations. VI. Dimers of Benzene

Abstract

Semiempirical charge self-consistent MO calculations have been carried out for a large number of benzene dimers of different geometric conformations (e.g., D6h, C2h, S2, C6, S12, C2υ, etc.). It has been found that the most stable excimer conformation is not the most symmetric (i.e., D6h), but rather one in which the rings are rotated relatively about the D6h axis and/or tilted, one with respect to the other. The lifetime of excimer fluorescence can be satisfactorily explained in two ways: (1) The transition is made vibronically allowed in much the same way as the B2u1→1A1gtransition of benzene; or (2) the excimer possesses C1 or C2υ symmetry, in which case a very small relative tilting of the benzene planes can induce transition moments of proper magnitude without invoking vibronic effects at all. A number of suggestions are made concerning solvent effects, and the presence of underlying continua in discrete absorption spectra, etc. In particular, a concept of contact excimer absorption is broached and rationalized. The results of computation are in excellent agreement with the general run of available experimental data.