Chiral tetrathienylene: Synthesis and X-ray structure

Tetrakis(4-bromo-5-trimethylsilyl-2,3-thienylene) 1 with highly functionalized tetrathienylene ring may provide a building block for double helical oligothiophenes. Also, the tetrakis(2,3-thienylene) ring with relatively well-defined conformation provides an interesting probe for cross-conjugation and conjugation between thiophenes. Synthesis and X-ray structure of chiral tetrathienylene, tetrakis(4-bromo-5-trimethylsilyl-2,3-thienylene) 1, are reported. Treatment of 5,5′-bis(trimethylsilyl)-4,4′-dibromo-3, 3′-bithienyl with LDA, followed by oxidative coupling provided 1. The compound crystallizes with one chloroform molecule per formula unit in the chiral space group P21212 and is merohedrally twinned. The presence of four bromine atoms in the two bay areas of 1 leads to increased out-of-plane twisting (∼ 60°) of the chiral π-system along the β, β-linkages (CC bonds between the β-positions of thiophenes) within 3,3′- bithienylene units, compared to out-of-plane twisting (∼46°) in the parent tetrakis(2,3-thienylene). UV-vis spectra reveal a hypsochromic shift of about 50 nm for the onset of electronic absorption in 1, compared to tetrakis(5-trimethylsilyl-2,3-thienylene). This hypsochromic shift is assigned to the decreased cross-conjugation in 1 due to the increased out-of-plane twisting along the β, β-linkages. A large bathochromic shift of about 100 nm for the onset of electronic absorption in 1, compared to the corresponding acyclic 3,3′-bithienyl, i.e., 5,5′- bis(trimethylsilane)-4,4′-dibromo-3,3′-bithienyl, is assigned to the increased conjugation due to the presence of the α, α-linkages (CC bonds between α-positions of thiophenes) in 1.