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 synthesized 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 leads to an emissive singlet state. Solvatochromic effects on the spectral position and profile of the stationary fluorescence spectra clearly indicate the CT character of the emitting singlet states of all the compounds studied. An analysis of the CT fluorescence and absorption band shapes leads to the quantities relevant for the electron transfer in the Marcus inverted region. The analysis of the fluorescence rate constants (k(f)) and corresponding transition dipole moments (M) indicate that Marcus theory can be applied for the quantitative description of the radiationless charge recombination processes in such cases, when an intersystem crossing to the excited triplet states may be neglected (i.e., in the polar solvent). This last reaction channel, however, seems to operate efficiently in nonpolar media in which charge-transfer fluorescence is totally quenched. The obtained results support the hypothesis that the photophysical behavior of a particular A-D compound can be predicted from the properties of its donor and acceptor moieties, taking also into account the manner of linking between the two subunits.