Cyclic Tetrathiophenes Planarized by Silicon and Sulfur Bridges Bearing Antiaromatic Cyclooctatetraene Core: Syntheses, Structures, and Properties

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 H-1 NMR and UV-vis measurements of 1-3. In comparison of the H-1 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 CH2Cl2, 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(center dot+), radical anion 1(center dot-), and dianion 1(2-) were chemically generated and successfully characterized by means of UV-vis, ESR, and NMR spectroscopies.