PHOTOINDUCED ELECTRON-TRANSFER IN CHARGE-TRANSFER CRYSTALS BY DIFFUSE-REFLECTANCE (PICOSECOND) TIME-RESOLVED SPECTROSCOPY

Solid-state photoexcitation of crystalline charge-transfer complexes of arene donors (including various benzenes, naphthalenes, biphenyls, anthracenes, and phenanthrene) and either tetracyanobenzene (TCNB) or methyl viologen (MV) with a 25-ps laser pulse generates short-lived charge-transfer excitons, the transient diffuse-reflectance spectra of which strongly resemble those of the corresponding arene cation radicals. The first-order decays of the transients monitored on the picosecond and early nanosecond time scales are ascribed to back-electron transfer (BET) within the photogenerated ion pairs. Plots of the decay rate (In k(BET)) versus the charge-transfer energies (h nu(CT)) are linear with slopes similar to those obtained from the same charge-transfer complexes that are either dissolved in solution or encapsulated in porous glasses (beta = 2-3 eV(-1)). The data suggest that charge-transfer excitons generated by laser excitation of charge-transfer crystals can be considered as highly localized excited ion-pair states which behave similarly to contact ion pairs in solution.