**3. Results and discussions**

#### **3.1 Ground-state geometry optimizations**

The studied conjugated molecules are constructed based on CPDT units as donors with BT and DPP as acceptor units. Hence, these compounds are of ″pushpull″ type conjugated molecules [41, 42]. Both P-CPDTBT3 and SM-CPDTDPP were optimized in the ground state using DFT//B3LYP/6-311 g(d,p) method. This study aims to examine the effect of the conjugated molecular design on the optoelectronic and photovoltaic properties. Here, we have maintained the CPDT donor building block and we have tuned the acceptor moieties based on BT and DPP units. Besides, we are looking to reveal the difference of behavior between polymer and small molecule.

As it can be seen from **Figure 2**, both compounds exhibit a high planar optimized geometry. The dihedral angles are almost 0°, as observed from the side view of these molecules. These planar configurations are arising from the intramolecular non-covalent interactions of S---H, N---H and S---N types that take place within the conjugated framework [43]. These non-covalent bonds are found smaller than the sum of Van der Waals radii of the considered atoms. The planar backbone structure is one of the key factors to enhance the conjugation degree and accordingly increasing the π-staking for more charge transfer capability.

The bridge bonds are described as the bonds that link between the distinct building blocks such as electron donating units, electron acceptor units and π-spacer within the conjugated backbone. The interest of examining the bridge bond length is to get an idea about the interactions among the different building blocks. Where, the shorter bridge bond length leads to stronger intra-molecular interactions and higher charge transfer [44, 45]. For the studied compounds, the bridge bond defines the bond C-C between the CPDT donor and BT or DPP

**Figure 2.** *Ground state optimized structures of P-CPDTBT3 and SM-CPDTDPP at DFT/B3LYP/6-311 g(d,p) level of theory.*

acceptor units. The bridge bond lengths are found around 1.45 Å for P-CPDTBT3 and 1.43 Å for SM-CPDTDPP. The obtained values are higher than the regular C=C bond length (1.34 Å) and smaller than the regular C-C bond length (1.54 Å) which indicates that these bonds are still found to have double-bond character.

**69**

**Figure 4.**

**Table 1.**

*Designing Well-Organized Donor-Bridge-Acceptor Conjugated Systems Based…*

Based on the optimized ground state geometries, we deduce that important π-electron delocalization, within the conjugated frameworks; can induce intramolecular charge transfer (ICT) characteristics in push-pull donor materials. Further, Molecular Electrostatic Potential Surfaces (MEPs) were simulated to identify the electronic properties and molecular stability. The MEP is a helpful tool for specifying the reactive sites as it is related to the topology of molecular electron density [46]. The colors displayed in the MEP represent the different electrostatic potential values and charge distributions within the molecules. From **Figure 3**, the electron richregions (red color), usually have negative potentials, are mainly located over the dicyanomethylene bridge groups whereas the blue color depicts regions of more positive electrostatic potentials (electron-deficient) color are concentrated over H and S atoms. The MEP plots have shown the dominance of the zero potential which is presents green

color. This observation revel the high stability of the considered compounds.

The analysis of Frontier molecular orbitals (FMOs) gives a description of the electron delocalization as well as the electron transport capacities within the conjugated skeleton. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) strongly determine the optoelectronic properties of conjugated compounds, pointedly on the photovoltaic properties of donor materials. Largely, donor compound should tend to have a deep HOMO level to assure a high open circuit voltage VOC and a suitable LUMO energy level with

The FMOs of the considered materials are carried out based on DFT/B3LYP method at 6-311 g(d,p) and listed in **Table 1**. The FMOs contour plots are illustrated in **Figure4**.

**Compound** *EH*<sup>−</sup><sup>1</sup> **(eV)** *EH* **(eV)** *EL* **(eV)** *EL*<sup>+</sup>1 **(eV)** *Egap* **(eV) IP (eV) EA (eV)** P-CPDTBT3 −5.90 −5.49 −3.87 −7.74 1.62 6.10 3.20 SM-CPDTDPP −5.83 −5.20 −3.78 −3.72 1.42 5.90 3.09

*Electronic properties for studied materials obtained at DFT/B3LYP 6-311 g(d,p) level of theory.*

*FMOs contour plots at the optimized ground state of the considered materials.*

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

respect to that of the acceptor unit [47–49].

*DOI: http://dx.doi.org/10.5772/intechopen.94874*

**Figure 3.** *Molecular electrostatic potential (MEP) of the considered compounds.*

*Designing Well-Organized Donor-Bridge-Acceptor Conjugated Systems Based… DOI: http://dx.doi.org/10.5772/intechopen.94874*

Based on the optimized ground state geometries, we deduce that important π-electron delocalization, within the conjugated frameworks; can induce intramolecular charge transfer (ICT) characteristics in push-pull donor materials.

Further, Molecular Electrostatic Potential Surfaces (MEPs) were simulated to identify the electronic properties and molecular stability. The MEP is a helpful tool for specifying the reactive sites as it is related to the topology of molecular electron density [46]. The colors displayed in the MEP represent the different electrostatic potential values and charge distributions within the molecules. From **Figure 3**, the electron richregions (red color), usually have negative potentials, are mainly located over the dicyanomethylene bridge groups whereas the blue color depicts regions of more positive electrostatic potentials (electron-deficient) color are concentrated over H and S atoms. The MEP plots have shown the dominance of the zero potential which is presents green color. This observation revel the high stability of the considered compounds.
