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Abstract:
Intramolecular charge transfer (ICT) with crystal violet lactone (CVL) in the excited singlet state takes place in solvents more polar than n-hexane, such as ethyl acetate, tetrahydrofuran, and acetonitrile (MeCN). In these solvents, the fluorescence spectrum of CVL consists of two emission bands, from a locally excited (LE) and an ICT state. The dominant deactivation channel of the lowest excited singlet state is internal conversion, as the quantum yields of fluorescence (0.007) and intersystem crossing (0.015) in MeCN at 25 degrees C are very small. CVL is a weakly coupled electron donor/acceptor (D/A) molecule, similar to an exciplex (1)(A(-)D(+)). A solvatochromic treatment of the LE and ICT emission maxima results in the dipole moments mu(e)(LE) = 17 D and mu(e)(ICT) = 33 D, much larger than those previously reported. This discrepancy is attributed to different Onsager radii and spectral fluorimeter calibration. The LE and ICT fluorescence decays of CVL in MeCN are double exponential. As determined by global analysis, the LE and ICT decays at 25 degrees C have the times tau(2) = 9.2 ps and tau(1) = 1180 ps, with an amplitude ratio of 35.3 for LE. From these parameters, the rate constants k(a) = 106 x 10(9) s(-1) and k(d) = 3.0 x 10(9) s(-1) of the forward and backward reaction in the LE (sic) ICT equilibrium are calculated, resulting in a free enthalpy difference Delta G of -8.9 kJ/mol. The amplitude ratio of the ICT fluorescence decay equals -1.0, which signifies that the ICT state is not prepared by light absorption in the S-0 ground state, but originates exclusively from the directly excited LE precursor. From the temperature dependence of the fluorescence decays of CVL in MeCN (-45 to 75 degrees C), activation energies E-a = 3.9 kJ/mol (LE -> ICT) and E-d = 23.6 kJ/mol (ICT -> LE) are obtained, giving an enthalpy difference Delta H (= E-a - E-d) of -19.7 kJ/mol, and an entropy difference Delta S = -35.5 J mol(-1) K-1. These data show that the ICT reaction of CVL in MeCN is not barrierless. The ICT reaction time of 9.2 ps is much longer than the mean solvent relaxation time of MeCN (0.26 ps), indicating, in contrast with earlier reports in the literature, that the reaction is not solvent controlled. This conclusion is supported by the observation of double exponential LE and ICT fluorescence with the same decay times.
