This study focuses on the conformational energetics of three heterocyclic oligomers, 2,2’-bithiophene (TT), 2,2’-bifuran (FF), and 2,2’-thienylfuran (TF). Thiophene, and to a lesser extent furan, are common building blocks in organic semiconductors and electronics, such as organic light emitting diodes (OLEDs) or organic photovoltaics (OPVs) due to their synthetic accessibility (i.e., fabrication costs, synthetic versatility, and natural abundance). The effectiveness of π-conjugated molecules/oligomers for these purposes often depends on their degree of planarity, rigidity to interconversion, and solid-state packing efficiency. Full geometry optimizations and harmonic vibrational frequency computations are performed on TT, FF, and TF using the B3LYP, M06-2X, and ωB97XD density functionals and the MP2 ab initio method with two triple-ζ quality basis sets (6-311G(2df,2pd) and cc-pVTZ). Additionally, a series of relaxed torsional scans, where the heteroatom to heteroatom torsional angle is systematically varied from 0° to 180°, are performed at the same levels of theory. Results from these computations indicate that, although B3LYP, M06-2X, and ωB97XD provide reasonable results for the TT system, they all overestimate the barrier to interconversion in FF and TF. In fact, in the case of FF, the global minimum is incorrectly identified by all of the density functionals considered here. To validate these results, a set of CCSD(T)-F12/VTZ-F12 single energy-point computations are performed on the B3LYP and MP2 optimized geometries and the lowest energy conformations revealed by the torsional scans. According to MP2 and CCSD(T)-F12, both the syn-and the anti-conformations of TF are planar, and there is a reasonably significant energetic barrier to interconversion of nearly 4 kcal mol– 1. These results make TF a promising π-bridge candidate for building efficient organic semiconducting and electronic materials.

Conformational analysis of furan and thiophene oligomers

255th National Meeting (2018)

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