Semiconducting Oligomers of 1,4-dimethoxybenzene, Thiophene and Thiazole: A Theoretical Study

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.

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