Fluorescence Thermochromism of Organic Crystals

物质的颜色随温度的变化而发生可逆的变化，这种现象被称为热致变色现象，长期以来引起了化学家们的极大兴趣。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。颜色变化通常与可见光区吸收光谱的变化有关。因此，热致变色被解释为组成物质的有色和无色分子形式之间的平衡随温度变化。然而，当物质发光时，物质的颜色不仅由吸收决定，而且还由可见光的发射决定。如果发光光谱在可见光区随温度变化，即使吸收光谱不变，也会发生热致变色。这种热色性，即荧光热色性，在有机化合物中还不为人所知，本文证明了一类最著名的热色性有机化合物水杨醛苯胺（1）及其同系物的热色性，即荧光热色性。热致变色水杨醛苯胺类化合物的晶体在室温下通常为红色，随着温度的降低而变黄。热致变色性归因于OH和NH形式之间的互变异构平衡随温度的变化而移动[方程式（1）]。通过对电子吸收光谱、荧光光谱和颜色的重新研究，对水杨醛缩苯胺类化合物的热致变色机理提出了新的解释：水杨醛缩苯胺类化合物的热致变色不是由于OH和NH两种形式之间的互变异构平衡发生了变化，而是由于荧光强度随温度的变化而发生了变化。

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.