Properties of the intramolecular excited charge-transfer states of carlbazol-9-yl derivatives of aromatic ketones

Photoinduced intramolecular charge transfer (ICT) in a series of a newly sy nthesized N-bonded donor-acceptor derivatives of 3,6-di-tert-butylcarbazole containing benzophenone and acetophenone as an electron acceptor has been studied in solutions. In solvents more polar than butyl ether, excitation l eads to an emissive singlet state. Solvatochromic effects on the spectral p osition and profile of the stationary fluorescence spectra clearly indicate the CT character of the emitting singlet states of all the compounds studi ed. An analysis of the CT fluorescence and absorption band shapes leads to the quantities relevant for the electron transfer in the Marcus inverted re gion. The analysis of the fluorescence rate constants (k(f)) and correspond ing transition dipole moments (M) indicate that Marcus theory can be applie d for the quantitative description of the radiationless charge recombinatio n processes in such cases, when an intersystem crossing to the excited trip let states may be neglected (i.e., in the polar solvent). This last reactio n channel, however, seems to operate efficiently in nonpolar media in which charge-transfer fluorescence is totally quenched. The obtained results sup port the hypothesis that the photophysical behavior of a particular A-D com pound can be predicted from the properties of its donor and acceptor moieti es, taking also into account the manner of linking between the two subunits .