Azobenzene is a prototypical photochromic molecule existing in two isomeric forms, which has numerous photochemical applications that rely on a precise knowledge of the molar absorption coefficients (ε). Careful analysis revealed that the previously reported absorption spectra of the “pure” isomers were in fact mutually contaminated by small amounts of the other isomer. Therefore, the absorption spectra of both trans- and cis-azobenzene in methanol were re-determined at temperatures of 5–45 °C. The thermodynamically more stable trans-azobenzene was prepared by warming the solution in the dark. To obtain the spectrum of cis-azobenzene three methods were used, which gave consistent results within the limits of error. The method based on the subtraction of derivative spectra coupled with a global analysis of the spectra recorded during thermal cis–trans isomerization is shown to give slightly more reliable results than the method using isomeric ratios determined by ¹H-NMR. The described methods are readily generalizable to other azobenzene derivatives and to other photochromic systems. The practical implication of the re-determined ε values is demonstrated by a very high precision of spectrophotometric species analysis in azobenzene isomeric mixtures. The new ε values imply that the previously reported quantum yields must be revised.

中文翻译：

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顺式偶氮苯的吸收光谱

偶氮苯是一种典型的光致变色分子，以两种同分异构形式存在，其依赖于对摩尔吸收系数（ε）的精确了解，具有许多光化学应用。仔细的分析表明，先前报道的“纯”异构体的吸收光谱实际上被少量的其他异构体相互污染。因此，两者的吸收光谱反式-和顺-azobenzene在甲醇中在5-45℃的温度下重新确定。通过在黑暗中加热溶液来制备热力学更稳定的反式-偶氮苯。获得顺式光谱-偶氮苯使用了三种方法，在误差范围内给出了一致的结果。与基于¹ H-NMR测定的异构体比例的方法相比，该方法基于减去导数光谱并结合对热式顺式-反式异构化过程中记录的光谱进行整体分析，显示出可靠的结果。所描述的方法易于推广到其他偶氮苯衍生物和其他光致变色体系。重新测定的ε值的实际含义由偶氮苯异构体混合物中分光光度种类分析的极高精度所证实。新ε 值暗示必须修改先前报告的量子产率。