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•+ (∼1.2 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 ∼12° smaller in D•+ than in D. Approximately one-fourth of the total reorganization energy of ∼1.2 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.