RADIATIVE ELECTRON-TRANSFER IN ARYL DERIVATIVES OF DIMETHYLANILINES

Radiative electron transfer (RET) has been studied in six members of the series of aryl derivatives of aromatic amines by means of the solvent and temperature effects on the position of CT fluorescence maxima, the half-widths, quantum yields (PHI(f)) and excited-state depopulation kinetics (tau(f)). The radiative rate constant (k(f) = PHI(f)/tau(f)) for all compounds decreases considerably with increasing solvent polarity and, contrary to our expectations, the fluorescence is not thermally activated. Moreover, k(f) values decrease markedly with the increase of the energy of the Franck-Condon (FC) ground state reached upon CT emission. The reduction of the half-width of the CT fluorescence spectra with lowering of temperature, as well as the corresponding solvatochromic effects, allow us to determine the outer (lambda(o)) and inner (lambda(i)) reorganization energies in terms of the Marcus theory in the inverted region. The results strongly suggest the nonorthogonal conformation of the CT fluorescent states of the compounds under study. The lowest excited state potential energy minimum of these compounds corresponds likely to the more similar values of the twist angle, THETA(F) almost-equal-to 55-degrees-70-degrees, between donor and acceptor moieties than those in the ground state. The transition dipole moment values of the radiative depopulation of the CT state are unexpectedly high as compared with those of the corresponding arenes. The mechanism of the radiative electron transfer (RET) is briefly discussed.