New Type of Dual Solid-State Thermochromism: Modulation of Intramolecular Charge Transfer by Intermolecular π−π Interactions, Kinetic Trapping of the Aci-Nitro Group, and Reversible Molecular Locking

When heated above room temperature, some crystalline polymorphs of the 1,3-bis(hydroxyalkylamino)-4,6-dinitrobenzenes (BDBn, n = 2−5), bis(hydroxyalkyl) analogues of the intramolecular charge-transfer molecule 1,3-diamino-4,6-dinitrobenzene, exhibit “dual” thermochromism: gradual color change from yellow to orange at lower temperatures, and sharp color change from orange to red at higher temperatures. These two thermochromic changes are related to different solid-state processes. When allowed to cool to room temperature, the yellow color of the thermochromic molecules with different alkyl length (n) is recovered with unexpectedly different kinetics, the order of the respective rate constants ranging from 10−7−10−6 s−1 for BDB2 to about 0.1 s−1 in the case of BDB3. The thermochromic mechanism and the reasons behind the different kinetics were clarified on the basis of detailed crystallographic characterization, kinetic thermoanalysis, and spectroscopic study of eight crystalline forms (seven polymorphs and one solvate). It was found that the polymorphism is due to the possibility of “locking” and “unlocking” of the alkyl arms by formation of a strong intramolecular hydrogen bond between the hydroxyl groups at their hydroxyl termini. The locking of BDB2, with shortest alkyl arms, is reversible and it can be controlled thermally; either of the two conformations can be obtained in the solid state by proper thermal treatment. By use of high temperature in situ single crystal X-ray diffraction analysis of BDB3, direct evidence was obtained that the gradual thermochromic change is related to increased distance and weakened π−π interactions between the stacked benzene rings: the lattice expands preferably in the stacking direction, causing enhanced oscillator strength and red shift of the absorption edge of the intramolecular charge transfer transition. The second, sharp thermochromic change had been assigned previously to solid−solid phase transition triggered by intramolecular proton transfer of one amino proton to the nitro group, whereupon an aci-nitro form is thermally populated. Contrary to the numerous examples of solid thermochromic molecules based on either pericyclic reactions or keto−enol tautomerism, this system appears to be the first organic thermochromic family where the thermochromic change appears as an effect of intermolecular π−π interactions and thermal intramolecular proton transfer to aromatic nitro group.当加热至室温以上时，1,3-双(羟烷基氨基)-4,6-二硝基苯(BDBn, n=2−5)的部分结晶多晶型物——即分子内电荷转移分子1,3-二氨基-4,6-二硝基苯的双(羟烷基)类似物——表现出“双重”热致变色现象：在较低温度下呈现从黄色到橙色的缓慢颜色变化，而在较高温度下则发生从橙色到红色的突变色变。这两种热致变色变化对应不同的固态过程。当冷却至室温时，不同烷基链长(n)的热致变色分子的黄色会以差异显著的动力学过程恢复，其对应速率常数的数量级范围从BDB2的10⁻⁷~10⁻⁶ s⁻¹到BDB3的约0.1 s⁻¹。通过对8种晶型（7种多晶型物与1种溶剂化物）的详细晶体学表征、动力学热分析及光谱学研究，阐明了该体系的热致变色机理及动力学差异的根源。研究发现，该多晶型现象源于烷基臂可通过其末端羟基间形成的强分子内氢键实现“锁定”与“解锁”。烷基臂最短的BDB2的锁定过程是可逆的，且可通过热调控实现；通过恰当的热处理，可在固态下得到两种构象中的任意一种。通过对BDB3开展高温原位单晶X射线衍射分析，我们获得了直接证据：缓慢的热致变色变化与堆叠苯环间距离增大、π-π相互作用减弱相关——晶格沿堆叠方向优先膨胀，使得分子内电荷转移跃迁的吸收边振子强度增强并发生红移。此前研究将第二种突变型热致变色变化归因为固-固相变：该相变由一个氨基质子向硝基发生的分子内质子转移触发，从而热致生成酸式硝基异构体。与众多基于周环反应或酮-烯醇互变异构的固态热致变色分子体系不同，该体系是首个已知的有机热致变色家族，其热致变色变化源于分子间π-π相互作用以及热诱导的分子内质子向芳族硝基的转移过程。