**3. Results and discussion**

#### **3.1 Ground state optimized geometries**

The development of high performance luminescent materials is principally based on the enhancement of π-electrons delocalization within the conjugated architectures. In fact, the geometric structure gives an idea about the delocalization of the πelectrons as well as the charge transfer (CT) within the conjugated structure [34].

Hence, the ground state geometries of the studied materials were optimized using DFT//B3LYP/6-31 g(d) method in gaseous phase. The optimized geometries of **M1** and **M2** are illustrated in **Figure 2** and the geometric parameters involving bond lengths, torsion angles and dihedral angles are listed in **Table 1**.

**Figure 2.** *Ground state optimized geometries at B3LYP/6-31 g(d) level for the studied molecules.*


#### **Table 1.**

*Optimized ground state geometry parameters of the studied molecules.*

The considered materials exhibit non-coplanar structures (**Figure 2**). There are large torsion angles around 119° between anthracene and phenyl ring in **M1** and between anthracene and thiophene ring in **M2** (**Table 1**) which is a characteristic property of compounds based substituted anthracene [23, 35].

Furthermore, as mentioned in **Figure 1**, the characteristic bridge bonds between the donor and acceptor blocks (L1) and between the different groups of the acceptor units (L2 and L3) were calculated to have an idea about the charge transfer (CT) in the conjugated backbone. The calculated lengths of bond L1 were found equal to 1.46 and 1.44 Å for **M1** and **M2**, respectively. While, the bond lengths of L2 and L3 were found about 1.49 Å for **M1** and about 1.48 Å for **M2**. The calculated values are located in the interval between C-C single bond length (C-C = 1.54 Å) and C=C double-bond length (C=C = 1.33 Å) showing the high π-electron delocalization and interesting intra-molecular charge transfer (ICT) within the framework [36].

The structural analysis suggests that the studied compounds exhibit good conjugated structures with high π-electron delocalization, which is important for applications in organic electronic devices.

#### **3.2 Frontier molecular orbitals (FMOs)**

The electronic properties of the studied molecules are examined based on the frontier molecular orbital (FMOs) analysis. The highest occupied molecular orbitals

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

(HOMOs) and the lowest unoccupied molecular orbitals (LUMOs) contour plots were carried out using the DFT//B3LYP/6-31 g(d) method at the optimized ground state geometries. The energy values of HOMOs, LUMOs and band gap energies are listed in **Table 2**. The HOMOs and LUMOs distributions are illustrated in **Figure 3**.

As depicted in **Figure 3**, the HOMOs are mostly located over the DTP unit while the LUMOs are distributed over the DPA and DTA acceptor units that indicate the important electron charge transfer from the donor to acceptor moieties. Hence, the FMOs analysis has shown the considerable charge transfer taking place within the designed molecules.

The band gap energies are calculated from the difference between the HOMO and LUMO energy levels at DFT//B3LYP/6-31 g(d) method. The band gap energies are about 3.16 eV and 2.81 eV for **M1** and **M2**, respectively. These lower values of band gaps with the FMOs distributions demonstrate the presence of a significant intra-molecular charge transfer (ICT) that leads to enhance the electronic properties [37].

Density of states (DOS) is a helpful tool to examine the delocalization of πelectrons in the compound. The DOS plots were determined by DFT//B3LYP/6-31 g (d) method at the ground state geometry and illustrated in **Figure 4**. DOS plots of **M1** and **M2** show a large overlapping of electronic energy levels that indicates the high electron delocalization. In fact, the high electron delocalization within these materials could be explained by the mutual reactions of donor and acceptor constructive blocks.

To better understand the delocalization of π-electrons, electron density difference (EDD) was simulated between the ground state (S0) and the first excited state (S1). As seen in **Figure 5**, the EDD plots contain blue regions referring to electron density depletion and purple regions referring to electron density increment.

Hence, the regions of electron increment density are mainly located over the DPA and DTA blocks. While, the DTP block presents the region of depleted free carriers (depletion region). These observations decline the effective electron transfer from donor to acceptor units within the studied materials.


#### **Table 2.**

*DFT//B3LYP/6-31 g(d) calculated electronic properties of M1 and M2 molecules.*

**Figure 3.** *Frontier molecular orbitals in the optimized ground state for the studied molecules.*

**Figure 4.** *Density of states (DOS) plots of the studied materials.*

**Figure 5.** *Electron density difference (EDD) contour plots of the studied materials.*
