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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The intramolecular electron-transfer reaction in crystal violet lactone in polar aprotic solvents is studied with femtosecond transient absorption spectroscopy. The initially excited charge transfer state (1)CT A is rapidly converted into a highly polar charge transfer state (1)CT B. This ultrafast electron transfer is seen as a solvent-dependent dual fluorescence in steady-state spectra. We find that the electron-transfer process can be followed by a change from a double-peaked transient absorption spectrum to a single-peak one in the low picosecond range. The transient absorption kinetic curves are multiexponential, and the fitted time constants are solvent dependent but do not reproduce the known solvation times. For 6-dimethylaminophthalide, the optically active constituent of crystal violet lactone, only a small temporal evolution of the spectra is found. To explain these findings, we present a model that invokes a time-dependent electron-transfer rate. The rate is determined by the instantaneous separation of the two charge-transfer states. Because of their differing dipole moments, they are dynamically lowered to a different extent by the solvation. When they temporarily become isoenergetic, equal forward and backward transfer rates are reached. The intrinsic electron-transfer ( (1)CT A --> (1)CT B) reaction is probably as fast as that in the structurally analogous malachite green lactone (on the 100 fs time scale). The key element for the dynamics is therefore its control by the solvent, which changes the relative energetics of the two states during the solvation process. With further stabilization of the more polar state, the final equilibrium in state population is reached.

The spectroscopy and photophysics of malachite green lactone (MGL), a lactonic form of the well-know malachite green dye, have been studied as functions of solvent polarity in aprotic and protic solvents at different temperatures in solution and in glass. It has been found that MGL photophysics substantially differs from that of other malachite green leucoderivatives (e.g. leuconitrile, leucohydroxide or halides). In aprotic solvents ultrafast intramolecular electron transfer (estimated t ET 130 fs) between MGL structural components (from initially excited insulated dimethylaniline, DMA, to phthalide, Pd) results in formation of a highly polar (25.0 D) intramolecular CT state which then emits fluorescence. The dynamics of the primary ET are determined mainly by intramolecular vibrational motions and not by the solvation process. The polar nature of the emitting state was verified with the solvatochromic method, and S n S 1 transient absorption spectra prove that charge separation results in formation of DMA radical cation in MGL. The separation of charges is maintained in the charge transfer triplet state, which lies below the local triplet levels of MGL structural components, making MGL a unique candidate for studying both CT triplet state and CT-singlet-triplet dynamics. The electronic structure of the 1 CT state stabilizes charge separation in moderately polar solvents (e.g. F fl and t fl in butyl acetate (BA) are 0.12 and 23.8 ns, respectively) and no evidence has been found for photodissociation of C-O bond in lactone ring in aprotic solvents, which is the dominant photoprocess in malachite green leuconitrile or halides. Further increase of solvent polarity results in dramatic enhancement of MGL nonradiative deactivation from the 1 CT state (increase in k nr by two orders of magnitude on going from BA to ACN) pointing to a solvent polarity-driven deactivation channel. The strong dependence of CT fluorescence quantum yield on CT transition energy found in supercooled and in glassy butyronitrile (BTN), where conformational motions are restricted, suggests that the nonradiative decay is (i) not related to conformational changes and (ii) consists in enhancement of nonradiative return electron transfer (direct radiationless charge recombination), which is proven by the good linear correlation between ln k nr and ñ n fl in more polar solvents (energy gap law). The large temperature-dependent blue shift of the fluorescence maximum in BTN below the melting point (rigidochromism) makes MGL a very sensitive probe for studying reorientation polarization in rigid and semi-rigid supercooled media.y Dedicated to Professor Dr Z. R. Grabowski and Professor Dr J. Wirz on the occasions of their 75th and 60th birthdays.

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Substitution of non-fluorescent phthalide (Pd) with amino group at meta (6) position in relation to the electron-accepting part of the lactone ring completely changes Pd photophysics: a new long-wavelength absorption band arises and the molecule becomes highly fluorescent. The experimental data and the analysis of vertical electronic transitions with TDDFT method indicate that the first absorption band in 6-aminophthalides (6-APds) comprises a single CT transition to the S1 state. Almost equal absorption and emission transition dipole moments indicate that S0 <--> S1 transition in all 6-APds is not affected by any mixing with other electronic states, the excited-state vibrational relaxation is not accompanied by significant conformational changes and the Stokes shifts reflect mainly solvation energetics of these molecules. Excited state dipole moments obtained from solvatochromic plots and from CASSCF calculations confirm large charge displacement from amino group towards the meta position of the benzene ring upon excitation of 6-APds to S1 state. Long fluorescence lifetimes and high fluorescence quantum yields demonstrate efficient and stable excited state charge separation in 6-APds. Taken together with sensitivity of 6-APds to polarity and proticity of the environment these properties make them good candidates for fluorescent probes of long-time scale molecular dynamics.

Crystal violet lactone (CVL) in solution displays unusually broad (FWHM > 9100 cm(-1)) dual fluorescence with the characteristics of white light. The emission combines a blue CT band from a local chromophore with an orange CT band from an intramolecular exciplex formed adiabatically at appropriate medium polarity. The fluorescence spectrum can be controlled by solvent polarity to yield tuneable emission colours in a broad range of coordinates in the CIE chromaticity diagram including the white region. We show that such dual emission is a general property of CVL-like D-A structures built on sp(3) carbon atoms. The dependence of excited state energetics on molecular structure allows the prediction of width, shape and other parameters of the dual fluorescence spectrum, and so enables the engineering and customised design of white fluorophores. The photophysics-structure relationship found for CVL and its analogues can be generalized into a novel concept of white light generation by small molecules. These D-A systems are studied as a template basis for design and development of white fluorophores.