Carbocations as Electron Acceptors. Photoexcitation of the Charge-Transfer Complexes of Tropylium Salts and Aromatic Hydrocarbons

Article abstract of DOI:10.1021/ja00190a033

Tropylium cation (C7H7⁺) forms electron donor-acceptor or EDA complexes with various benzene, naphthalene, and anthracene donors (Ar), as evidenced by the series of brightly colored solutions showing absorption bands that vary with the arene ionization potentials according to Mulliken charge-transfer theory.The spectrophotometric measurement of the formation constants indicates the presence of uniformly weak (1:1) complexes +> with K<1M^-1 in acetonitrile.However, the successful isolation of single crystals of the unique cationic EDA complex allows X-ray crystallography to establish the charge-transfer (CT) transition as deriving from centrosymmetric layered structures of tropylium-naphthalene pairs at interannular separations of 3.3 Angstroem.The direct correspondence of such solid-state structures with the tropylium EDA complexes in solution is demonstrated by an excellent correlation of the relevant CT energies (hν_CT) obtained from the reflectance and absorption spectra, respectively.Time-resolved spectroscopy following the 532-nm irradiation of the tropylium EDA complex in acetonitrile with a 30-ps laser pulse leads to the resolved picosecond absorption spectra of arene cation radicals within the CT excited (ion-radical pair) state as .+, C7H7^.>.The temporal evolution of the transient Ar^.+ absorbance with first-order rate constants of k₁>=4 x 1E10s^-1 for various anthracenes relates to the back-electron-transfer from this ion-radical pair on its return to the original ground-state EDA complex.Such a facile annihilation of the solvent-caged ion-radical pair limits the photochemistry of tropylium EDA complexes to electron donors such as dianthracene and hexamethyl(Dewar benzene) in which the cation radical is highly labile.Thus, the brief lifetime of Ar^.+ with τ ca. 15 ps largely precludes second-order quenching processes.