Fluorescence Thermochromism of Organic Crystals

物质的颜色随温度的变化而发生的可逆变化称为热致变色，长期以来一直受到化学家的广泛关注。在许多情况下，颜色随着温度的升高而加深，随着温度的降低而变淡。颜色的变化通常与可见光区吸收光谱的变化有关。因此，热致变色性被解释为组成物质的有色和无色分子之间的平衡随着温度的变化而变化。然而，当物质发光时，物质的颜色不仅受吸收的支配，而且受可见光的发射支配。如果发射光谱随着温度的变化而在可见光区发生变化，即使吸收光谱不变，也会发生热致变色。然而，这种被称为荧光热致变色的热致变色现象在有机化合物中鲜为人知。我们在这里证明了水杨亚基苯胺(1)及其同系物的热致变色是一类最著名的热致变色有机化合物，即荧光热致变色。据报道，热致变色水杨醛基苯胺的晶体在室温下通常是红色的，随着温度的降低而变成黄色。热致变色性归因于OH和NH之间的互变异构平衡随着温度的变化而移动[方程(L)]。基于对电子吸收光谱、荧光光谱和可感知颜色的重新考察，我们对热致变色性的起源提出了新的解释：水杨酰亚胺的热致变色性不是由OH和NH之间的互变异构平衡移动引起的，而主要是由于荧光强度随温度变化而引起的。

Reversible change in color of substances with variation of the temperature is known as thermochromism and has attracted much interest from chemists for a long time. In many cases the color deepens with raising of the temperature and pales with lowering of the temperature. The color change is usually associated with a change in absorption spectra in visible region. The thermochromism is, therefore, interpreted in terms of a temperature-dependent shift of an equilibrium between colored and colorless forms of molecules that compose the substances. The color of substances is, however, dominated not only by absorption but also by emission of visible light, when the substances are luminescent. If emission spectra change in visible region with variation of the temperature, a thermochromism would also occur even if absorption spectra do not change. Such a thermochromism, which is called fluorescence thermochromism, has been, however, hardly known for organic compounds.We demonstrate here that the thermochromism of salicylideneaniline (1) and its congeners, which belong to a class of the most well-known thermochromic organic compounds, is the fluorescence thermochromism. Crystals of thermochromic salicylideneanilines have been reported to be usually red at room temperature and become yellow with lowering of the temperature. The thermochromism was ascribed to the shift of the tautomeric equilibrium between the OH and NH forms with variation of the temperature [equation (l)]. Based on the reexamination of electronic absorption spectra, fluorescence spectra, and perceived colors, we present here a new interpretation for the origin of the thermochromism : The thermochromism of salicylideneanilines is not caused by the shift of the tautomeric equilibrium between the OH and NH forms but is caused mainly by the change of the fluorescence in intensity with variation of the temperature.