**1. Introduction**

Recently, several categories of conjugated materials have gained significant interest due to their relevant optoelectronic characteristics for the development of a wide range of organic electronic devices [1–3]. Remarkably, organic luminescent

materials are increasingly attractive in the manufacture of organic light-emitting diodes (OLEDs) and non-linear optical (NLO) devices [4–6].

OLEDs are desirable electronic devices due to their valuable advantages of full color emission, high brightness, flexibility and operation stability [7–11]. OLEDs were greatly developed in electronic fields and showed their successful applications in digital displays, mobile phones and flat panels in TV screens [12]. Various functional units, such as fluorene [13], carbazole [14] and anthracene [15] were utilized to develop efficient luminescent materials for their prominent photoluminescence properties and simple modifications. Among these functional units, anthracene and its derivatives were widely studied as potential building blocks in the development of active materials for OLED devices [16].

Apart from OLEDs, recent studies have demonstrated the potential application of conjugated materials in NLO devices [17, 18]. NLO materials have reached large interest of the scientific community for prosperous use in technological areas such as telecommunications, optical information processing and data storage [19–21]. NLO studies have shown the design of promising organic materials for nonlinear effect based on the introduction of highly delocalized electron fragments and additional electron donor and acceptor groups for enhancing the molecular conjugation [22].

Previous works have shown that D-A (Donor-Acceptor) systems dispose a successful architecture for non-linear optics and OLEDs. Where, D-A systems exhibit large charge transfer (CT) in which the electrons located in the electron rich donor unit undergo an intra-molecular charge transfer (ICT) to the electron deficient acceptor unit [23, 24]. The present CT phenomenon leads to excellent optoelectronic characteristics that encourage the use of D-A systems in NLO and OLED applications.

Dithieno [3,2-b: 2<sup>0</sup> ,30 -d] pyrrole (DTP) material has been recognized as one of the most efficient building blocks with high electron donation capacity [25, 26]. DTP building blocks have been widely incorporated into a variety of materials for the aim of reducing the band gap, improving the mobility of charge carriers, and reinforcing solution and solid state fluorescence.

**Figure 1.** *Molecular structures of the investigated compounds.*

*Computational Study on Optoelectronic Properties of Donor-Acceptor Type Small… DOI: http://dx.doi.org/10.5772/intechopen.98590*

Equally, anthracene has attracted significant attention to the construction of organic luminescent materials due to its unique features. Where, anthracene derivatives such as 9,10-diphenylanthracene (DPA) and 9,10-di(thiophen-2-yl)anthracene (DTA) have exhibited remarkable electronic and light-emitting properties to be applied in optical applications [27, 28].

In the present work, we have developed a theoretical investigation based on new D-A type small molecules for NLO and OLED applications. As mentioned in **Figure 1**, we have used DTP as an electron donor block and derivatives from DPA and DTA as an electron acceptor block. The addition of a strong electron withdrawing group as cyano group is for the reason of enhancing the polarization and improving the π-electrons delocalization [29].

A theoretical computational study using density functional theory (DFT) approach introduce excellent tools to predict the optoelectronic properties of molecular systems as well as the design of new materials for OLED and NLO devices [30]. Hence, the designed materials will be theoretically investigated and discussed to envisage the reliability for OLED and NLO applications.

### **2. Computational methods**

Theoretical calculations were performed using Density Functional Theory (DFT) approach implemented in Gaussian 09 software [31]. Previous studies have shown that DFT//B3LYP/6-31 g(d) method gives better accuracy in investigating the photo-physical properties of materials based on DTP and anthracene [32, 33]. Structural properties such as dihedral angles, torsion angles and bridge bond lengths were firstly investigated based on the geometry optimization of **M1** and **M2** in their ground states. Vibrational calculations were carried out to confirm the stable conformers with no imaginary frequencies. Frontier molecular orbitals (FMOs) and electron density difference (EDD) contour plots were carried out to examine the electron delocalization within the conjugated frameworks. The absorption and emission spectra were simulated using Time Dependent DFT (TD-DFT) at B3LYP/ 6-31 g(d) level. Photoluminescence color coordinates of **M1** and **M2** were determined using International Commission on Illumination (usually abbreviated CIE for its French name, Commission internationale de l'éclairage) process. Bilayer OLED devices are designed based on the optoelectronic properties of the studied molecules. Hole and electron charge transfer properties (λh, λe) were carried out on neutral, anionic and cationic states. The NLO properties involving the electric dipole moment (μ), the polarizability (α), the first order hyperpolarizability (β) and the second order hyperpolarizability (γ) were investigated.
