Density functional theory studies of new bipolar carbazole–benzothiazole: Electronic and vibrational properties

An intramolecular donor–acceptor compound containing carbazole as a donor group and two benzothiazole rings as electron acceptors, has been theoretically studied using the density functional theory (DFT) with the hybrid B3LYP exchange correlation function and the split valence 6-31G* basis set. The ground-state geometries’ optimization was carried out, showing a reduction in the band gap when going from the bicarbazole ((Cz)2) to the carbazole attached to benzothiazole rings in both sides (Cz–(Bzt)2). Structural parameters, electronic and vibrational properties have been analyzed and discussed in terms of conjugative pathway between the electron-donating and electron-accepting moieties. It is predicted that benzothiazole systems exhibit large optical responses based on intramolecular charge transfer (CT) process. Optical properties may be altered by changing the compound’s molecular structure by a series of π-bridges, such as ethylene (eth) and phenylene-vinylene (PV) units. These modifications lead to obtain a resulting compound having better optoelectronic properties, through the redistribution of its HOMO and LUMO energy levels. On the other hand and in order to check the real properties of the resulting compound, calculations have also been performed for all compounds in solvated media (chloroform). The obtained results reveal a significant influence of solvation on optical properties, due to the change of the spectral response caused by the increase of ground state CT process.