Max-Planck-Institut für biophysikalische Chemie, Spektroskopie und Photochemische Kinetik, 37070 Göttingen, Germany.
J Phys Chem A. 2013 Aug 22;117(33):7721-36. doi: 10.1021/jp405530j. Epub 2013 Jul 31.
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 °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 μe(LE) = 17 D and μe(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 °C have the times τ2 = 9.2 ps and τ1 = 1180 ps, with an amplitude ratio of 35.3 for LE. From these parameters, the rate constants ka = 106 × 10(9) s(-1) and kd = 3.0 × 10(9) s(-1) of the forward and backward reaction in the LE ⇄ ICT equilibrium are calculated, resulting in a free enthalpy difference ΔG 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 S0 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 °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 ΔH (= E(a) - E(d)) of -19.7 kJ/mol, and an entropy difference ΔS = -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.
分子内电荷转移（ICT）在激发单线态下的结晶紫内酯（CVL）在比正己烷更具极性的溶剂中发生，例如乙酸乙酯、四氢呋喃和乙腈（MeCN）。在这些溶剂中，CVL 的荧光光谱由两个发射带组成，来自局部激发（LE）和 ICT 态。最低激发单线态的主要失活通道是内转换，因为 MeCN 中荧光（0.007）和系间窜越（0.015）的量子产率在 25°C 时非常小。CVL 是一个弱耦合的电子供体/受体（D/A）分子，类似于激基复合物（1）（A(-)D(+))。LE 和 ICT 发射最大值的溶剂变色处理导致偶极矩 μe(LE) = 17 D 和 μe(ICT) = 33 D，远大于以前报道的值。这种差异归因于不同的 Onsager 半径和光谱荧光计校准。在 MeCN 中，CVL 的 LE 和 ICT 荧光衰减是双指数的。通过全局分析确定，LE 和 ICT 在 25°C 下的衰减时间τ2 = 9.2 ps 和τ1 = 1180 ps，LE 的幅度比为 35.3。从这些参数中，计算出 LE ⇄ ICT 平衡中正向和反向反应的速率常数 ka = 106×10(9) s(-1) 和 kd = 3.0×10(9) s(-1)，导致自由焓差 ΔG 为 -8.9 kJ/mol。ICT 荧光衰减的幅度比等于-1.0，这表明 ICT 态不是通过 S0 基态的光吸收来制备的，而是源自直接激发的 LE 前体。从 CVL 在 MeCN 中的荧光衰减的温度依赖性（-45 至 75°C），获得了 LE → ICT 的活化能 E(a) = 3.9 kJ/mol 和 ICT → LE 的活化能 E(d) = 23.6 kJ/mol，给出了焓差 ΔH（= E(a) - E(d))为 -19.7 kJ/mol，熵差 ΔS = -35.5 J mol(-1) K(-1)。这些数据表明，CVL 在 MeCN 中的 ICT 反应不是无势垒的。ICT 反应时间为 9.2 ps，远长于 MeCN 的平均溶剂弛豫时间（0.26 ps），这表明与文献中的早期报道相反，该反应不受溶剂控制。这一结论得到了 LE 和 ICT 荧光具有相同衰减时间的双指数衰减的观察结果的支持。
