Kinetics of Isomerization via Photoinduced Electron Transfer. I. Spectral Analysis and Structural Reorganization of Hexamethyl Dewar Benzene Exciplexes

Abstract

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.