Department of Chemistry, Graduate School of Science and Engineering, Tokyo Metropolitan University, Hachioji, Tokyo 192-0397, Japan.
J Am Chem Soc. 2010 Jan 27;132(3):1066-74. doi: 10.1021/ja908161r.
Cyclic tetrathiophenes 1, 2, and 3 planarized by dimethylsilyl, sulfur, and sulfone bridges bearing an antiaromatic cycloocatatetraene (COT) core were designed and synthesized to investigate the relationship among the bent angle, paratropicity, and HOMO-LUMO gap of the COT ring. The bent angles of the central COT rings of 1-3 were theoretically estimated and experimentally determined, and it was found that the planarity of the COT ring was finely adjusted in the order of 2 > 3 > 1 by using the small differences in the bond lengths between the bridging units and thiophene rings. From the comparisons of NICS values and calculated HOMO-LUMO gaps of cyclooctatetraene at various bent angles as well as the optimized structures of cyclic tetrathiophenes 1-3, similar enhancement of the paratropicity and narrowing of the HOMO-LUMO gap with decreasing bent angle of the COT rings were shown in both cyclooctatetraene and cyclic tetrathiophenes 1-3. Such predictions were experimentally proved for the first time by means of (1)H NMR and UV-vis measurements of 1-3. In comparison of the (1)H NMR chemical shifts of 1-3 with those of the corresponding precursors, upfield shifts due to a paratropic ring current in the COT ring were observed and the degree of shift increased with increasing planarity of the COT ring. Furthermore the colors of the solutions of 1 (lambda(max) = 483 nm), 2b (lambda(max) = 618 nm), and 3b (lambda(max) = 575 nm) were orange, purple, and red in CH(2)Cl(2), respectively, indicating that the HOMO-LUMO gaps of 1-3 become increasingly narrow with increasing planarity of the COT ring. Reflecting these electronic properties, CV measurements demonstrated the amphoteric redox properties of 1 and 2b, and the radical cation 1(+), radical anion 1(-), and dianion 1(2-) were chemically generated and successfully characterized by means of UV-vis, ESR, and NMR spectroscopies.
设计并合成了由二甲基硅、硫和砜桥连接的平面化环状四噻吩 1、2 和 3，这些噻吩带有反芳香性的环己四烯（COT）核心，以研究 COT 环的弯曲角度、para 性和 HOMO-LUMO 能隙之间的关系。通过理论估计和实验测定，计算出了 1-3 中环中心 COT 环的弯曲角度，发现通过桥接单元和噻吩环之间键长的微小差异，可以精细调节 COT 环的平面性，其顺序为 2 > 3 > 1。从比较不同弯曲角度下环辛四烯的 NICS 值和计算的 HOMO-LUMO 能隙以及环状四噻吩 1-3 的优化结构，可以看出 COT 环的弯曲角度越小，para 性增强，HOMO-LUMO 能隙变窄。这在环辛四烯和环状四噻吩 1-3 中都得到了证实。通过对 1-3 的 (1)H NMR 和 UV-vis 测量，首次从实验上证明了这一预测。与相应前体相比，1-3 的 (1)H NMR 化学位移发生了向上位移，这是由于 COT 环中的 para 环电流所致，并且随着 COT 环的平面化程度增加，位移程度也增加。此外，1（lambda(max) = 483 nm）、2b（lambda(max) = 618 nm）和 3b（lambda(max) = 575 nm）在 CH(2)Cl(2)中的溶液颜色分别为橙色、紫色和红色，这表明随着 COT 环的平面化程度增加，1-3 的 HOMO-LUMO 能隙变窄。反映这些电子性质，CV 测量证明了 1 和 2b 的两性氧化还原性质，自由基阳离子 1(+)、自由基阴离子 1(-)和二阴离子 1(2-)通过化学方法生成，并通过 UV-vis、ESR 和 NMR 光谱成功地进行了表征。
