Effect of Chalcogen Interaction on the Structure of Methine-Bridged Trichalcogenophenes

Abstract

Polythienylenemethylidenes (PTMs) are promising conjugated polymers for organic electronics owing to their narrow bandgaps and extended π-conjugation. However, their stereochemistry remains unexplored. In this study, methine-bridged trithiophene and trifuran analogs were synthesized to investigate stereochemistry and chalcogen bonding effects. The compounds were obtained as mixtures of ZZ, EZ(= ZE), and EE geometric isomers, established through detailed NMR analyses. At thermal equilibrium, the ZZ isomer predominated in trithiophene (ZZ:(EZ + ZE):EE = 58:35:6), whereas trifuran showed a near-statistical distribution. X-ray crystallography revealed intramolecular S···S chalcogen bonding in trithiophene with S···S distances (≈3.04 Å) shorter than van der Waals radii and C-S···S angles of 171°. Comprehensive conformer searches and DFT calculations not only validated the higher stability of the ZZ isomer in trithiophene but also provided calculated isomer distributions that closely matched the experimental values. Multi-faceted computational analysis (electron localization function (ELF), noncovalent interaction (NCI), quantum theory of atoms in molecules (QTAIM), and natural bond orbital (NBO)) confirmed the presence of these chalcogen-centered interactions and quantified their strength through lone pair (LP)(S)→σ*(S-C) donor-acceptor orbital interactions. Trithiophene exhibited a unique dual-chalcogen bonding mode in the ZZ configuration. These findings elucidate the role of chalcogen bonding in stabilizing ZZ-trithiophenes and contribute to designing PTMs with controlled stereochemistry for organic electronics applications.

Keywords: chalcogen bonding; conjugation; diastereoselectivity; noncovalent interactions; thiophene.

Figure 1

a) Structure of PTM. b) Structures of the three isomers of methine‐bridged trithiophene 1S and trifuran 1O investigated in this work.

Scheme 1

Synthesis of 1S and 1O.

Figure 2

¹H NMR (500 MHz, CDCl₃, r.t.) spectra of geometric isomer mixtures of a) 1S and b) 1O.

Figure 3

a) Top and b) side views of crystal structure of ZZ‐1S′ (Molecule B) (C = gray, H = white, S = orange, F = yellow, and Cl = green). Thermal ellipsoids are drawn at 50% probability level. One of the terminal thiophene rings shows disorder between syn‐ and anti‐conformations; the minor anti‐conformation is omitted for clarity.

Figure 4

Relative Gibbs free energy (referenced to the most stable conformer) of the conformer ensembles of 1S and 1O, optimized using the CENSO program. Red: ZZ‐configuration, green: EZ‐configuration, and blue: EE‐configuration. Temperature: 55 °C.

Figure 5

a) ELF isosurfaces (0.85 a.u.), b) NCI isosurfaces (0.5 a.u.), and c) the result of QTAIM of syn,syn‐ZZ‐1S.

Figure 6

Top and side views of the calculated NBOs and second‐order perturbation energy (E(2), kcal/mol) involved in the chalcogen bonding in syn,syn‐ZZ‐1S.