Chiral tetrathienylene: synthesis and X-ray structure

Abstract

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 P 2 1 2 1 2 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.

Synthesis and Characterization of Cyclooctatetrathiophenes with Different Connection Sequences

“…COTh s possess homologous cross-conjugated π-systems in which adjacent thiophene rings are alternatively connected, analogously to the linkages in tetraphenylenes . The out-of-plane twisting between the adjacent thiophene rings and the possible cross-conjugation may affect the delocalization in macrocyclic oligothiophenes. , From the absorbance spectra of COTh s (Figure ), we can see that all the four COTh s showed similar absorbance curves, and there were three absorption bands (bands I–III), which were located in the regions of 220–260, 260–320, and 330–400 nm, respectively. Similar maximum absorbance peaks (λ max ) are definitely observed as 284, 282, 287, and 295 nm for COTh-1 , COTh-2 , COTh-3 , and COTh-4 , respectively.…”