Solid-State Thermochromism and Phase Transitions of Charge Transfer 1,3-Diamino-4,6-dinitrobenzene Dyes

The lower 1,3-bis(hydroxyalkylamino) homologues of the strong intramolecular X-type charge transfer (CT) system 1,3-diamino-4,6-dinitrobenzene (DADNB) exhibit reversible color change in the solid state from yellow at room temperature (RT) to orange and red at high temperature (HT). To investigate the structural prerequisites for occurrence of this phenomenon, we prepared 10 new derivatives of DADNB where the hydroxyalkyl arms at the amino groups were replaced with substituents having different electronic and steric profiles. Two of the new materials exhibit sharp and reversible thermochromic change in the solid state: when heated, the bis(aminoethyl) derivative (DADNB-1) undergoes color change from orange-red to brown, while one of the three polymorphs of the bisphenyl product (DADNB-2) changes its color from red to yellow. The physicochemical analysis and the crystal structures of seven of these compounds, one of which is trimorphic, confirmed that both phenomena are due to solid–solid phase transitions. The brown high-temperature phase of DADNB-1 presents the first example where the absorption is shifted beyond the red region. Form C of DADNB-2 is the first material of this group that exhibits “negative” thermochromism, where the high-temperature phase absorbs at lower wavelength than the low-temperature one. The results demonstrate the potentials of these simple and easily accessible organic molecular materials for thermal switching of the optical properties by utility of intermolecular interactions to modulate the intramolecular CT.强分子内X型电荷转移（charge transfer, CT）体系1,3-二氨基-4,6-二硝基苯（1,3-diamino-4,6-dinitrobenzene, DADNB）的1,3-双（羟烷基氨基）同系物，在固态下表现出可逆变色行为：室温（RT）下呈黄色，高温（HT）下变为橙色至红色。为探究该现象出现的结构先决条件，我们制备了10种DADNB的新型衍生物，将氨基上的羟烷基侧臂替换为具有不同电子与空间位阻特性的取代基。其中两种新型材料在固态下表现出敏锐且可逆的热致变色变化：双（氨乙基）衍生物（DADNB-1）受热时由橙红色变为棕褐色；而双苯基产物（DADNB-2）的三种多晶型之一，受热时由红色转变为黄色。对其中7种化合物（其中一种为三晶型）的理化分析与晶体结构表征证实，上述两种变色现象均源于固-固相变。DADNB-1的高温棕褐色相是首个吸收峰红移至红光区域之外的此类体系。DADNB-2的晶型C是该系列中首个表现出“负热致变色（negative thermochromism）”的材料——其高温相的吸收波长低于低温相。本研究结果表明，这类简单易得的有机分子材料，可通过利用分子间相互作用调控分子内电荷转移，实现光学性质的热开关应用。