REQUIMTE, CQFB, Departamento de Química, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal.
Chemistry. 2011 Mar 21;17(13):3663-71. doi: 10.1002/chem.201002781. Epub 2011 Feb 21.
New trans-2-hydroxychalcones bearing a carboxylate group at position 2' (Ct(-)) were synthesized (compounds 2 and 3). These compounds lead to a network of chemical reactions depending on pH value, light, and solvent. In water, when the pH value is lowered, the ionized trans-chalcone is protonated and the flavylium cation AH(+) is formed at very acidic pH values through hemiketal B and cis-chalcone Cc, with global acidity constants of pK'(a) ≤ -1 and ≈ 0.1, respectively, for 2 and 3. The electron-acceptor character of the carboxylic substituent not only increases the observed acidity of the flavylium cation, but also decreases the rate of the ring-opening/-closing from a subsecond timescale to hours relative to model compound 1 (without carboxylate). The photochemistry of the network was studied in detail by means of continuous irradiation, monitored by UV/Vis absorption and (1)H and (13)C NMR spectroscopic analysis. Although compound 3 is only slightly photoactive, compound 2 (Ct(-)) reacts in aqueous solutions (λ(irr) = 313 nm) to form B(-) and Cc(-), with a global quantum yield of 0.15, and fully reverts back to Ct(-) with a rate constant of k = 6.7×10(-5) s(-1) . The flavylium cation is no longer formed in methanol, and irradiation of Ct(-) leads to the formation of B(-) and the new lactone-trapped chromene species La. The formation of La takes place through a sequence of three photochemical steps: photoisomerization of Ct(-), photo-ring-closing reaction of Cc(-), and photolactonization of B(-). Only the cis/trans isomerization and ring-closing reactions are thermally reversible on a timescale of seconds and hours, respectively. A photochromic system was achieved in rigid matrices of methanol (at 77 K) and 1-dodecanol (5 °C) by irradiating lactone La to give a red ortho-quinone allide through a photo-ring-opening reaction; the color disappears with a rate constant of k = 1.25×10(-2) s(-1) in 1-dodecanol at 5 °C.
新的 2'-位带有羧酸盐的反式-2-羟基查耳酮（Ct(-)）被合成（化合物 2 和 3）。这些化合物会导致一系列取决于 pH 值、光和溶剂的化学反应。在水中，当 pH 值降低时，离子化的反式查耳酮被质子化，并且在非常酸性的 pH 值下通过半缩酮 B 和顺式查耳酮 Cc 形成 flavylium 阳离子 AH(+)，对于 2 和 3，其全局酸度常数 pK'(a) 分别为≤-1 和≈0.1。羧酸取代基的电子接受特性不仅增加了 flavylium 阳离子的观察到的酸度，而且还将从亚秒级到小时级的开环/闭环速率相对于模型化合物 1（无羧酸盐）降低。通过连续照射详细研究了网络的光化学，通过 UV/Vis 吸收和 (1)H 和 (13)C NMR 光谱分析进行监测。尽管化合物 3 仅略有光活性，但化合物 2（Ct(-)）在水溶液中（λ(irr) = 313 nm）反应形成 B(-)和 Cc(-)，总量子产率为 0.15，并以速率常数 k = 6.7×10(-5) s(-1) 完全恢复到 Ct(-)。在甲醇中不再形成 flavylium 阳离子，并且 Ct(-)的辐照导致 B(-)和新的内酯捕获色烯物种 La 的形成。La 的形成经历三个光化学步骤的序列：Ct(-)的光异构化、Cc(-)的光环封闭反应和 B(-)的光内酯化。只有顺/反异构体化和环封闭反应在秒和小时的时间尺度上是热可逆的。在甲醇（在 77 K）和 1-十二醇（5°C）的刚性基质中通过辐照内酯 La 以通过光环打开反应形成红色邻醌全氮杂环丁烷来实现光致变色系统；在 5°C的 1-十二醇中，颜色以速率常数 k = 1.25×10(-2) s(-1) 消失。
