Near infrared absorbing benzobis(thiadiazole) derivatives: computational studies point to biradical nature of the ground states

Abstract

Main factors responsible for the red to near infrared (NIR) absorption of the benzobis(1,2,5‐thiadiazole) (BBT) derivatives have been investigated using high level computational studies. These molecules with NIR emission are of importance due to the recent reports of possible role in NIR organic light emitting diodes (OLED) development. The electronic structure has been determined using closed‐shell/open‐shell DFT methods (B3LYP and BHandHLYP). The wavefunction stabilities of these BBT derivatives have been tested. We notice that using the BHandHLYP functional, the wave function becomes instable though large HOMO–LUMO gaps (HLG) are obtained. On the other hand a stable wavefunction is obtained using the B3LYP functional but the HLG is small. The B3LYP HLG is in good agreement with the available experimental data. Nevertheless results from both functionals indicate a possible LUMO occupation/biradicaloid character (BRC). We calculated the BRC for all the molecules using different methods and observed that these molecules have a large BRC which correlates well with excitation energies. Larger the BRC smaller the excitation energy. Charge transfer based on Mulliken charges of both ground and excited state are obtained from high level SAC/SAC‐CI studies. We carried out the VB studies of the unsubstituted BBT to predict the relative weights of the individual Lewis structures to the resonance picture. It is concluded that a more general description which include the zwitterionic and biradical structures are necessary for these molecules and not just simple donor–acceptor–donor (D–A–D) structures as suggested in the literature. Copyright © 2011 John Wiley & Sons, Ltd.