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

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.

Sensitive thermal transitions of nanoscale polymer samples using the bimetallic effect: Application to ultra-thin polythiophene

“…These results are fully consistent with the rotational profile predicted by quantum mechanical methods for the inter-ring dihedral angle, [45][46][47][48][49] which indicated that the syn conformation is severely destabilized when adjacent substituted thiophene rings show a head-head linkage but energetically accessible for head-tail and tail-tail linkages. From an electronic point of view, the distortion of the interring dihedral angle from anti to syn destroys the extent of the π -conjugation along the polymer chain.…”

La modelización molecular como herramienta para el diseño de nuevos polímeros conductores

“…La geometría y el perfil rotacional de estos compuestos fueron analizados mediante cálculos ab initio de mecánica c u á n t i c a [12] , obteniendo resultados que concuerdan perfectamente con los datos experimentales disponibles. Más recientemente, extendimos el estudio a oligómeros de tiofeno de mayor tamaño [13,14] . Como ejemplo de este tipo de estudios, en la Figura 3 se representa el perfil rotacional teórico del 2,2'-bitiofeno, obtenido a nivel de cálculo B3PW91/6-31+G(d,p) [15] , donde se puede apreciar la presencia de dos conformaciones de energía mínima [16] .…”

“…Desde el punto de vista teórico, se puede modelizar separadamente la cadena polimérica neutra y la eléctricamente cargada, para después analizar las diferencias estructurales entre ambas. De esta manera, pudimos comprobar que la estructura electrónica del 2,2'-bitiofeno y la del 2,2',5',2"-tertiofeno son aromáticas, mientras que sus respectivos dicationes presentan una distribución de electrones con carácter quinoideo [13,20] . Posteriormente, con la intención de acercarnos más a las características reales del politiofeno, utilizamos modelos de mayor tamaño que contenían hasta 13 anillos de tiofeno [21] .…”

Aplicaciones de los métodos computacionales al estudio de la estructura y propiedades de polímeros

“…Por ejemplo, se puede determinar los modos normales de vibración (frecuencia e intensidad) característicos de los espectroscopía i.r. [73][74][75][76] , las constantes de acoplamiento de E.P.R [77,78] , etc……”