Four bis-oxonols have been synthesized which possess different alkyl substituents appended to the thiobarbiturate subunit. The nature of the alkyl substituent affects the photophysical properties of the dye in solution since it modulates the rate of rotation of the thiobarbiturate subunit around one of the double bonds in the connecting trimethine bridge. Rates of light-induced (trans to cis) and thermal (cis to trans) isomerization processes have been measured for one of the dyes in protic (i.e. alkanols) and aprotic solvents at various temperatures. These rates, together with activation energies derived from Arrhenius plots, are discussed in terms of the hydrogen bonding and stereochemical properties of the solvent. The thermal step is very sensitive to the stereochemistry of the solvent while the light-induced process is controlled mostly by the size of the rotor and the solvent viscosity. The overall effects may be explained, at least in a qualitative sense, in terms of the medium-enhanced barrier model.
合成了四种双氧烯醇,它们具有不同的烷基取代基连接在
硫巴比妥酸亚基上。烷基取代基的性质影响
染料在溶液中的光物理性质,因为它调节了
硫巴比妥酸亚基围绕连接的三甲烯桥中的一个双键的旋转速率。针对其中一种
染料,在亲
水性(即
醇类)和非亲
水性溶剂中于不同温度下测量了光诱导(顺式转反式)和热诱导(反式转顺式)异构化过程的速率。这些速率以及通过阿伦尼乌斯图得出的活化能,结合氢键作用和溶剂的立体
化学特性进行了讨论。热过程对溶剂的立体
化学非常敏感,而光诱导过程则主要受旋转体的大小和溶剂粘度的控制。这些整体效应至少可以在定性意义上用介质增强的能障模型来解释。