Structural Properties and Degree of Intramolecular Charge Transfer of an<i>N</i>-Alkyl Indoline–Tricyanoquinodimethane System
作者:Tsuyoshi Murata、Kazukuni Nishimura、Yuichiro Enomoto、Genki Honda、Yasuhito Shimizu、Akihiro Otsuka、Gunzi Saito
DOI:10.1246/bcsj.81.869
日期:2008.7.15
A series of intramolecular charge-transfer compounds comprised of 1-alkyl-3,3-dimethyl-2-methyleneindoline (In, donor, n = alkyl chain length; 1–8, 10, 14, 16, 18, 20, and 22) and 7,8,8-tricyanoquinodimethane (=2-(4′-cyanomethylene-2′,5′-cyclohexadienylidene)malononitrile (3CNQ), acceptor) moieties linked through a π-bond, In–3CNQ were prepared, and their structures are discussed. The melting points of the In–3CNQ solids decreased with increasing alkyl chain length until reaching a minimum point (95 °C) at n=18, and then began to gradually increase, indicating self-aggregation of the alkyl chains. The In–3CNQ derivatives with long alkyl chains (n≥6) produced supercooled liquids displaying conspicuous color changes after melting. Diverse molecular packing patterns were observed in the crystal structures. Most In–3CNQ derivatives formed face-to-face and/or side-by-side dimeric motifs to cancel net dipole moments. I20–3CNQ formed a two-dimensional bilayer structure by π-stacking and self-assembly of the alkyl chains. Two types of molecular conformation, which significantly affect the electronic structures, were observed to be dependent on the alkyl chain length. A parameter to evaluate the degree of intramolecular charge transfer, namely bond length ratio, was proposed; it exhibited good agreement with solvatochromic shifts and molecular orbital calculations.
一系列分子内电荷转移化合物由 1-烷基-3,3-二甲基-2-亚甲基吲哚啉(In,供体,n = 烷基链长度;1-8、10、14、16、18、20 和 22)和 7,8,8-三氰基二甲烷(=2-(4′-氰基亚甲基-2′,5′-环己二烯亚基)丙二腈 (3CNQ),受体)分子通过 π 键连接而成的分子内电荷转移化合物,即 In-3CNQ,并讨论了它们的结构。In-3CNQ 固体的熔点随着烷基链长度的增加而降低,直到 n=18 时达到最低点(95 °C),然后开始逐渐升高,表明烷基链发生了自聚集。具有长烷基链(n≥6)的 In-3CNQ 衍生物产生过冷液体,熔化后显示出明显的颜色变化。在晶体结构中观察到了不同的分子堆积模式。大多数 In-3CNQ 衍生物形成面对面和/或并排的二聚体图案,以抵消净偶极矩。I20-3CNQ 通过烷基链的π堆积和自组装形成了二维双层结构。观察到两种对电子结构有重大影响的分子构象取决于烷基链的长度。提出了一个评估分子内电荷转移程度的参数,即键长比值;该参数与溶解色移和分子轨道计算结果十分吻合。