Theoretical Study of the Properties of Isatin (1H-indole-2,3-dione) Based Alternating Donor-acceptor Type Conjugated Oligomers

The potential of Isatin (1H-indole-2,3-dione) as a donor moiety in an alternating donor-acceptor conjugated oligomer models was studied. Eight different electron acceptor molecules investigated are: thiazole (Z),thiadiazole (D), thienopyrazine (TP), thienothiadiazole (TD), benzothiadiazole (BT), thiadiazolothienopyrazine (TPD), benzobisthiadiazole (BDD) and thienopyridine (TPY). The geometry and electronic properties of the oligomers were investigated using density functional theory (DFT) method at B3LYP/ 6-31G (d) level. Properties such as torsional angle, bond length and intramolecular charge transfer were analysed. The lowest excitation energies (Eex.) and the maximal absorption wavelength (λabs.) were studied with time-dependent density functional theory (TD-DFT) method. The HOMO-LUMO level and band gap energies were calculated for all the model oligomers. The effect of addition of another isatin moiety to isatin-acceptor type oligomers give rise to D-A-D type structures and the band gap energies was investigated. It was discovered that (ISAT-TPD-ISAT) and (ISAT-BDD-ISAT) have lower band gaps (Eg) of 1.55 and 1.93 eV respectively. This result suggests that the two oligomers could be used as active layer materials in photovoltaic devices.