Solvent dependence of the spectra and kinetics of excited-state charge transfer in three (alkylamino)benzonitriles.
Solvent dependence of the spectra and kinetics of excited-state charge transfer in three (alkylamino)benzonitriles.
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DOI：
10.1021/jp046605p
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发表时间：
2005-02
期刊：
影响因子：
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通讯作者：
K. Dahl;R. Biswas;N. Ito;M. Maroncelli
中科院分区：
文献类型：
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作者：
K. Dahl;R. Biswas;N. Ito;M. Maroncelli
报道了4-氨基苯甲腈(ABN)、4-(1-氮杂环丁基)苯甲腈(P4C)、4-(1-吡咯烷基)苯甲腈(P5C)和4-(1-哌啶基)苯甲腈(P6C)在24℃溶剂中的稳态吸收、发射光谱和发射衰减动力学。在中高极性的溶剂中，P4C、P5C和P6C表现出从初始制备的(LE)态到更极性的电荷转移(CT)态转变的双重荧光和发射衰减特征，而ABN不发生这种反应。所有这些溶质的稳态吸收和发射光谱的频率可以通过对溶剂的介电连续描述和只考虑反应表面上的极小值来合理化，这些极小值被假定涉及分子内(扭曲)和溶剂化坐标。由此分析得到的气相溶质的性质与电子结构计算符合得很好，表明它们光谱的不同主要反映了气相LE和CT态相对能量的不同。该模型不能很好地描述LE和CT发射的相对产额，并讨论了失败的原因。LE-->；CT反应的动力学随溶质和溶剂的不同而有很大差异。在许多溶剂中，P4C的发射衰减可以用一个简单的与时间无关的速率常数的两态动力学方案来描述。在P5C和P6C中，观察到了多指数衰减，这反映了组份光谱的随时间变化以及随时间变化的反应速率。对这些复杂动力学的简化分析提供了对广泛的溶质和溶剂组合的自由能变化Delta(R)G和(平均)LE-->；CT速率常数k(F)的估计值。反应驱动力(-Delta(R)G)遵循顺序P6C>；P5C>；P4C，并随溶剂极性的增加而增大。反应速率与Delta(R)G相关，并遵循相反的趋势。K(F)和Delta(R)G之间的关系表明，静态溶剂效应，即势垒高度的变化，是P4C中溶剂依赖性的主要决定因素。势垒校正速率和溶剂化时间之间的关联表明，动态溶剂效应对P5C，特别是P6C中的速率的溶剂依赖性有很大贡献。
Steady-state absorption and emission spectra and emission decay kinetics are reported for 4-aminobenzonitrile (ABN), 4-(1-azetidinyl)benzonitrile (P4C), 4-(1-pyrrolidinyl)benzonitrile (P5C), and 4-(1-piperidinyl)benzonitrile (P6C) in 24 room temperature solvents. In solvents of modest to high polarity, P4C, P5C, and P6C exhibit dual fluorescence and emission decays characteristic of the transformation from an initially prepared (LE) state to a more polar charge transfer (CT) state, whereas ABN does not undergo this reaction. The frequencies of the steady-state absorption and emission spectra of all of these solutes can be rationalized using a dielectric continuum description of the solvent and considering only the minima on the reactive surfaces, which are assumed to involve both an intramolecular (twisting) and a solvation coordinate. Characteristics of the gas-phase solutes deduced from this analysis are in good agreement with electronic structure calculations and indicate that differences in their spectra mainly reflect differences in the relative energies of the gas-phase LE and CT states. The relative yields of LE and CT emission are not described as satisfactorily by this model, and reasons for this failure are discussed. The kinetics of the LE --> CT reaction vary considerably with solute and solvent. In many solvents, the emission decays of P4C are reasonably described by a simple two-state kinetic scheme with time-independent rate constants. In P5C and P6C multiexponential decays are observed that reflect time-dependent shifts of the component spectra as well as time-dependent reaction rates. A simplified analysis of these complex dynamics provides estimates for both the free energy change Delta(r)G and (average) LE --> CT rate constant k(f) for a wide range of solute and solvent combinations. The driving force for reaction (-Delta(r)G) follows the order P6C > P5C > P4C and increases with increasing solvent polarity. The reaction rates are correlated to Delta(r)G and follow the opposite trend. The relationships observed between k(f) and Delta(r)G suggest that static solvent effects, i.e., barrier height changes, are the primary determinants of the solvent dependence in P4C. Correlations between barrier-corrected rates and solvation times suggest that dynamical solvent effects contribute substantially to the solvent dependence of the rates in P5C, and especially P6C.