Organic semiconducting oligomers containing three to nine heterocyclic units and based on 1,4-dimethoxybenzene, thiophene and thiazole have been studied by DFT and TDDFT at the B3LYP/6-31G(d,p) level. The energy of each molecule, those of the frontier orbitals HOMO and LUMO,the width of the band gaps and the optical properties have been calculated. The inter-ring distances, dihedral angles and torsional angles have been elucidated. These molecules are planar π-conjugated systems. The band gaps of the longer oligomers studied range from 2.53 to 2.72 eV and their excitation energies calculated by TDDFT vary from 2.22 to 2.36 eV. Wavelengths of absorption bands of 524 to 556 nm are also obtained by TDDFT. The results of this theoretical study show that these organic molecules have interesting properties and can potentially be used as components in solar cells. In this work, a theoretical investigation on the geometries and optoelectronic properties of oligomers based on 1,4-dimethoxybenzene, thiophene and thiazole has been done. Molecules that contain up to nine rings have been studied. For all these oligomers, the elongation of the molecular chain led to a gradual decrease of the energy gap between the frontier orbitals. This energy splitting varied from 2.5 to 3.7 eV. This indicates that these compounds may exhibit semi-conducting properties. The increase in the molecular chain length also induced a decrease in the excitation energy determined by TDDFT. These oligomers present many intra and inter-unit interactions which contribute to their rigidity and to a better delocalization of the π-electrons.
Laboratoire de Chimie Moléculaire et des Matériaux, Université de Ouagadougou, UFR/SEA 03 BP 7021, Burkina Faso.
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