Conformational Analysis (ab Initio HF/3-21G*) and Optical Properties of Symmetrically Disubstituted Terthiophenes

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Abstract

We report a conformational analysis of several substituted terthiophenes using ab initio calculations performed at the HF/3-21G* level. Geometries of terthiophenes having methoxy substituents in 3,3‘‘ positions (DMOTT), methyl groups in the same positions (DMTT), and ethyl substituents in 3‘,4‘ positions (DETT) are compared with that of the unsubstituted molecule (TT). For all these symmetrical molecules, it is observed that the two dihedral angles are independent of each other. The most stable conformation of TT is found for dihedral angles θ = φ = 147.2°, whereas three maxima are located at 0°, 90°, and 180°. The insertion of methoxy groups in 3,3‘‘ positions favors a more planar conformation with a higher rotational barrier at 90°. This behavior is explained by the electron donor properties of the methoxy groups. By contrast, the addition of two methyl groups at the same positions induces a twisting in the molecule which is caused by the steric hindrance between the methyl substituents and the sulfur atom. The presence of two ethyl groups in 3‘,4‘ positions creates an even stronger steric effect, giving rise to a more twisted conformation for DETT compared to that of DMTT. Absorption and fluorescence spectra of each terthiophene derivative are also reported and are correlated with their respective potential energy surfaces. The more planar molecule (DMOTT) shows a red-shifted absorption band with a higher vibrational resolution and a smaller bandwidth. For more twisted molecules, the blue shift and the bandwidth of the absorption bands increase with twisting while the absorption coefficient decreases. The fluorescence bands, in all molecules, show a better vibrational resolution with a smaller bandwidth compared to their absorption counterparts, while their maximum wavelengths are practically the same, showing that in the first excited singlet state, all molecules relax to a more planar conformation.

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