Zhang Shiguo, Zhang Qinghua, Ye Baoxi, Li Xueli, Zhang Xiaoping, Deng Youquan
Center for Green Chemistry and Catalysis, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China.
J Phys Chem B. 2009 Apr 30;113(17):6012-9. doi: 10.1021/jp9004218.
Photochromism of spiropyran and thermal reversion of MC in a series of imidazolium-based ionic liquids (ILs) were investigated via fluorescence and UV-vis absorption spectra. The fluorescence emission maxima (lambda(em)) of the colored merocyanine (MC) form is polarity dependent in all ILs and fluorescence intensity enhancement was first observed in ILs with respect to that observed in organic solvents (e.g., in [HOMIm][PF(6)], nearly 32-fold enhancement than ethanol). Moreover, the thermal decay of MC in all ILs is uniformly first order, and in comparison with polar solvents possessing comparable polarity (such as ethanol and acetonitrile), a significant retardation of the decay rate and increase of half-lifetime of MC form was observed, in particular at elevated temperature (e.g., at 313 K, MC decay with k of 5.19 x 10(-4) and 34.7 x 10(-4) s(-1) in [BMMIm][BF(4)] and ethanol, respectively). Abnormal thermodynamics of the thermal decay of MC in ILs were observed, activation energies in less polar ILs (88-97 kJ mol(-1)) have a larger value than in polar ILs (70-82 kJ mol(-1)), opposite that in organic solvents (70-80 kJ mol(-1) in nonpolar solvents and around 100 kJ mol(-1) in polar solvents), and all ILs exclusively exhibit minus activation entropies (-85.4 to -24.5 J K(-1) mol(-1)), similar to that of nonpolar solvents. Desolvation due to the conformational change during the thermal reversion between MC and the transition state may be the key to interpret this experimental result.
通过荧光光谱和紫外可见吸收光谱研究了一系列咪唑基离子液体（ILs）中螺吡喃的光致变色和MC的热逆转。在所有离子液体中，有色部花青（MC）形式的荧光发射最大值（λem）与极性有关，并且相对于在有机溶剂中观察到的情况（例如，在[HOMIm][PF6]中，比乙醇增强近32倍），在离子液体中首次观察到荧光强度增强。此外，所有离子液体中MC的热衰变均为均匀的一级反应，并且与具有可比极性的极性溶剂（如乙醇和乙腈）相比，观察到衰变速率显著减慢，MC形式的半衰期增加，特别是在升高的温度下（例如，在313 K时，[BMMIm][BF4]和乙醇中MC衰变的速率常数k分别为5.19×10-4和34.7×10-4 s-1）。观察到离子液体中MC热衰变的异常热力学，极性较小的离子液体中的活化能（88 - 97 kJ mol-1）比极性离子液体中的活化能（70 - 82 kJ mol-1）大，这与有机溶剂中的情况相反（非极性溶剂中为70 - 80 kJ mol-1，极性溶剂中约为100 kJ mol-1），并且所有离子液体都仅表现出负的活化熵（-85.4至-24.5 J K-1 mol-1），与非极性溶剂的情况类似。MC与过渡态之间热逆转过程中构象变化导致的去溶剂化可能是解释这一实验结果的关键。
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