Photoinduced intramolecular charge transfer (ICT) in a series of N-bonded donor−acceptor derivatives of 3,6-di-tert-butylcarbazole containing benzonitrile, nicotinonitrile, or various dicyanobenzenes as an electron acceptor has been studied in solutions. The latter group of compounds, contrary to benzonitrile and nicotinonitrile derivatives, shows a well-separated low-energy CT absorption band which undergoes a distinct blue shift with increasing solvent polarity. 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 of the compounds studied both in a polar and a nonpolar environment. 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 values of the fluorescence rate constants (k f) and corresponding transition dipole moments (M) and their solvent polarity dependence indicate that the electronic coupling between the emitting 1CT state and the ground state is a governing factor of the radiative transitions. The relatively large values of M indicate a nonorthogonal geometry of the donor and acceptor subunits in the fluorescent states. It is shown that Marcus theory can be applied for the quantitative description of the radiationless charge recombination processes in the cases when an intersystem crossing to the excited triplet states can be neglected.

Crystal violet lactone (CVL) emits in aprotic solvents at room temperature single (A band, low polar solvents) or dual (A and B bands, medium and highly polar solvents) fluorescence. Strong solvatochromic shifts of both fluorescence bands prove significant charge redistribution in both emitting states (1CTA and 1CTB). Comparison with model compounds mimicking structural subunits of CVL, 6-dimethylaminophthalide (6-DMAPd), and malachite green lactone (MGL) shows that the A band is displayed from a polar excited state localized within the 6-DMAPd subunit (1CTA, μ e ≅ 10.7 D), and the B band, from a highly polar excited state (1CTB, μ e ≅ 25.2 D) formed after electron transfer from one of the dimethylaniline groups to 6-DMAPd moiety. The 1CTB state becomes accessible and is populated during solvation by a sufficiently polar environment. CVL phosphorescence strictly matches that of 6-DMAPd, indicating ISC to a triplet state localized on 6-DMAPd moiety in low polar solvents. In polar solvents, transient absorption measurements indicate spin flip and relaxation to a charge-transfer triplet state, as evidenced by identical S1 → S n and T1 → T n spectra in acetonitrile indicating presence of dimethylaniline cation radical. In protic media, CVL undergoes fast photodissociation of the C−O bond in lactone ring and diabatic formation of a zwitterion stabilized by hydrogen bonding with solvent molecule.

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