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

Abstract

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.

Introduction

Recently, significant attention has been devoted to organic solar cells due to their flexibility, low cost and low weight. It has been reported that the majority of efficient solar cells make use of an interpenetrating network of electron donors and acceptors [1], [2]. Then, for such application, it is useful to develop bipolar molecules bearing both electron-donating (D) and electron accepting (A) moieties. These entities ensure charge carrier injection and facilitate acceptance of both holes and electrons [3], [4], [5], [6], [7], [8]. In fact, the incorporation of electron donating and electron accepting groups in π-conjugated organic molecules generally leads to a great enhancement of its photo-physical properties [9], [10]. Compared to the bi-layer structure, for which the separation process of the excitons’ entities occurs at the interface, this molecular engineering procedure leads to a local separation process. We are particularly interested in the investigation of a new carbazole/benzothiazole compound as a good example of such systems. As an excellent hole transporting material, the carbazole is widely used in such systems for two reasons. First, its charge transporting complexes can create free carriers. Secondly, its ring can be easily functionalized by covalently linking to other molecules [11], [12], [13]. On the other hand, the benzothiazole moiety is well-known as an excellent acceptor [9], [13], [14], [15]. It gives rise to a donor-π-acceptor type compound [13], [14], [16] due to its potential role in the modulation of the HOMO–LUMO gap. Yet, the interaction of the HOMO of the electron-donating moiety and the LUMO of the electron-accepting moiety causes the reduction of the band gap of the resulting compound [13], [16].

Due to their good electro-optical properties, these materials represent the most promising candidates for photovoltaïc applications.

In fact, systems containing carbazole and benzothiazole have been already synthesized and experimentally characterized, but theoretical studies of their properties are still limited. It has been reported that the theoretical studies of compounds permits, not only providing a structure–properties relationship, but also the prediction of specific applications [9], [17]. For this purpose, this paper relies on the investigation, by means of DFT calculations, of the compound’s properties resulting from carbazole and benzothiazole molecules as presented in Fig. 1. Our aim is to elucidate the effect of adding benzothiazole to the carbazole molecule in single and in both sides (Fig. 1b and c). Another subject of this work is to investigate the effect of spacers on the electro-optical properties of the acceptor–donor compound (Fig. 1d and e).

Then, in order to obtain satisfactory results of both electronic and optical properties compared to those obtained from experimental ones, the effect of solvation interactions [18], [19] is taken into account.