It is well recognized that emission spectra from the reactions of excited-state electron acceptors (A*) with hexamethyl Dewar benzene (D) are typically dominated by fluorescence from exciplexes of its valence isomerization product hexamethylbenzene (B) (i.e., A(.-)B(.+)). We were able to obtain well-defined emission spectra of the A(.-)D(.+) exciplexes with several cyanonaphthalenes as the excited-state electron acceptors by subtraction of the dominant A(.-)B(.+) fluorescence from the total emission. Interestingly, a comparison of band shapes and maxima between the exciplexes of D and B reveals that the reorganization energy for return electron transfer in A(.-)D(.+) is much larger than in A(.-)B(.+) (similar to1.2 vs 0.57 eV). Furthermore, a comparison between exciplexes of D and of 1,2-dimethylcyclobutene as a model compound showed that a greater reorganization energy is associated with return electron transfer from the A(.-)D(.+) exciplexes. DFT calculations identified much of the "excess" reorganization energy in A(.-)D(.+) with a change in the dihedral angle (flap angle) between the two cyclobutene moieties, which is similar to12degrees smaller in D.+ than in D. Approximately one-fourth of the total reorganization energy of similar to1.2 eV for the D exciplexes is due to this angle deformation. Spectra of the exciplexes of B and of model olefins were analyzed by using a familiar two-mode model whereas those of D required a three-mode model to account for the intermediate frequency (491 cm(-1)) mode associated with the flap angle deformation. Remarkably, although the driving forces for return electron transfer are nearly identical for the exciplexes of D and B with the same acceptor, the rate constants for nonradiative return electron transfer are predicted to be 4 to 5 orders of magnitude greater for the A(.-)D(.+) exciplexes because of their larger reorganization energies.