Molecular and Electronic Structures of Heteroaromatic Oligomers: Model Compounds of Polymers with Quantum-Well Structures

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Abstract

Conformational preferences of 2,2‘-bithiophene, 2-(2-thienyl)pyrrole, and N-methyl-2-(2-thienyl)pyrrole have been investigated by means of computational methods. Calculations were performed at the ab initio HF/6-31G(d) and MP2/6-31G(d) levels and, additionally, with the density functional B3-LYP/6-31G(d). The results indicate that 2-(2-thienyl)pyrrole behaves similarly to the 2,2‘-bithiophene. Thus, two minimum energy conformations were found for each compound, which correspond to anti-gauche and syn-gauche. Such minima are separated by barriers of about 1.7 and 1.3 kcal/mol at the HF and MP2 levels. On the contrary, the preferences found for N-methyl-2-(2-thienyl)pyrrole were different, giving an almost negligible energy barrier between the two minimum energy conformations. Furthermore, at the MP2 level the anti-gauche and syn-gauche minimum energy conformations present an inter-ring dihedral of 135° and 68°, respectively, displaying deviations greater than those found in 2,2‘-bithiophene and 2-(2-thienyl)pyrrole. The conformational analysis was extended to the tricyclic compound N-methyl-2,5-di(2-thienyl)pyrrole. For this molecule, a contour map of the conformational energy as a function of the inter-ring dihedral angles was computed at the HF/6-31G(d) level. Minimum energy conformations were subsequently computed at the same level of theory. Results were in agreement with those obtained for the bicyclic compound N-methyl-2-(2-thienyl)pyrrole. Thus, the inter-ring dihedral angles of the minimum energy conformations present a large distortion with respect to the planarity, and such minimum energy conformations are separated by almost negligible energy barriers. Finally, N-hexyl-2,5-di(2-thienyl)pyrrole radical cation was observed by EPR spectroscopy. This is the first time that a mixed trimer of thiophene and pyrrole is oxidized to its radical cation and detected by means of EPR spectroscopy. The spectrum seems to be symmetric, which is accounted for by a fast interconversion between conformers. This supports the small energy barrier calculated between minima for the bicyclic and tricyclic compounds. Overall, the results presented in this work indicate that N-methylpyrrole rings are able to induce large rotational deffects in [...-(pyrrole)n-(thiophene)n-...] block copolymers.

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