Torsional Profiles of Thiophene and Furan Oligomers: Probing the Effects of Heterogeneity and Chain Length.

A systematic analysis of the torsional profiles of 55 unique oligomers composed of two to four thiophene and/or furan rings (n = 2 to 4) has been conducted using three density functional theory (DFT) methods along with MP2 and three different coupled-cluster methods. Two planar or quasi-planar minima were identified for each n = 2 oligomer system. In every case, the torsional angle (τ) between the heteroatoms about the carbon-carbon bond connecting the two rings is at or near 180° for the global minimum and 0° for the local minimum, referred to as anti and syn conformations, respectively. These oligomers have rotational barrier heights ranging from ca. 2 kcal mol-1 for 2,2'-bithiophene to 4 kcal mol-1 for 2,2'-bifuran, based on electronic energies computed near the CCSD(T) complete basis set (CBS) limit. The corresponding rotational barrier for the heterogeneous 2-(2-thienyl)furan counterpart falls approximately halfway between those values. The energy differences between the minima are approximately 2 and 0.4 kcal mol-1 for the homogeneous 2,2'-bifuran and 2,2'-bithiophene, respectively, whereas the energy difference between the planar local and global minima (at τ = 0 and 180°, respectively) is only 0.3 kcal mol-1 for 2-(2-thienyl)furan. Extending these three oligomers by adding one or two additional thiophene and/or furan rings resulted in only minor changes to the torsional profiles when rotating around the same carbon-carbon bond as the two-ring profiles. Relative energy differences between the syn and anti conformations were changed by no more than 0.4 kcal mol-1 for the corresponding n = 3 and 4 oligomers, while the rotational barrier height increased by no more than 0.8 kcal mol-1.