Transition from charge-transfer to largely locally excited exciplexes, from structureless to vibrationally structured...

Exciplexes of 9,10-dicyanoanthracene (DCA) with alkylbenzene donors in cyclohexane show structureless emission spectra, typical of exciplexes with predominantly charge-transfer (CT) character, when the donor has a relatively low oxidation potential (Eox ), e.g. hexamethylbenzene (HMB). With increasing Eox and stronger mixing with a locally excited (LE) state, vibrational structure begins to appear with 1,2,3,5-tetramethylbenzene and becomes prominent with p-xylene (p-Xy). A simple theoretical model reproduces the spectra and the radiative rate constants, and it reveals several surprises: Even in this nonpolar solvent, the fractional CT character of a highly mixed exciplex varies widely in response to fluctuations in the microscopic environment. Environments that favor the LE (or CT) state contribute more to the blue (or red) side of the overall spectrum. It is known that sparsely substituted benzene radical cations, e.g., p-Xy(•+) , are stabilized more in acetonitrile than the heavily substituted HMB(•+) . Remarkably, ion pairing with DCA(•-) in cyclohexane leads to even larger differences in the stabilization of these radical cations. The spectra of the low-Eox donors are almost identical except for displacements that approximately equal the differences in Eox , even though the exciplexes have varying degrees of CT character. These similarities result from compensation among several nonobvious, but quantified factors.