**3.3 Optical properties**

### *3.3.1 Optical absorption properties*

The optical absorption properties of **M1** and **M2** have been investigated based on the simulated UV–Vis optical absorption spectra using TD-DFT method at 6-31 g(d) basis set. The obtained optical absorption curves with their Gaussian fitting peaks are given in **Figure 6**. The simulated corresponding maximum absorption wavelengths (λabs max), the electronic transition energy (Eex), the oscillator strength (f), the full-width at half maximum (FWHM) and the main electronic transitions are listed in **Table 3**.

As it can be seen from **Figure 6**, the studied molecules exhibit large and intense absorption bands in the visible zone ranging from 300 nm to 500 nm. These electronic transitions are defined as π! π\* transitions associated with the electron migration from the HOMOs mainly located over the DTP units to the LUMOs mainly concentrated over the anthracene units [38].

From **Table 3**, the maximum absorption wavelengths (λabs max) of **M1** and **M2** are found at 442 nm and 406 nm, respectively. The full-width at half maximum FWHM referring to the main absorption band for **M1** is found higher than that of **M2** (FWHM (**M1**) = 76.96 > FWHM (**M2**) = 58.70). This result shows the role played by the acceptor block in improving the optical absorption properties.

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

**Figure 6.** *Optical absorption spectra of M1 and M2 with fitting Gaussian peaks obtained at DFT//B3LYP/6-31 g(d) level.*

From the optical absorption analysis, it is obvious that the acceptor block configuration contributes significantly in enhancing the absorption properties of conjugated materials.

#### *3.3.2 Emission properties*

The emission properties of **M1** and **M2** are simulated on their optimized geometries at the first excited state by means of TD-DFT//B3LYP/6-31 g(d) method. The


**Table 3.**

*Maximum absorption wavelengths λabs max (nm), electronic transition energy, Eex (eV), oscillator strength f (a.u), full-width at half maximum FWHM (nm) and main electronic transitions calculated at B3LYP/6-31 g(d) level.*

emission spectra with their fitting Gaussian peaks are depicted in **Figure 7** and the corresponding emission characteristics are listed in **Table 4**.

As it can be seen from **Figure 7**, the studied materials exhibit large emission bands with maximum emission wavelengths of 478 nm and 554 nm for **M1** and **M2**, respectively. As mentioned in **Table 4**, the emission spectrum of **M2** exhibits larger FWHM comparing to **M2** which indicates the effect of acceptor moiety in enhancing the emission properties.

The photoluminescence chromaticity coordinates of **M1** and **M2** were carried out according to the CIE 1931 diagram. As illustrated in **Figure 8**, the CIE coordinates are found upon (x: 0.45, y: 0.45) and (x: 0.32, y: 0.43) for **M1** and **M2**, respectively. Hence, **M1** displays a pure green color while **M2** displays a pure yellow color.

From the emission investigation, it is revealed that the studied materials are promising materials for OLED applications. Indeed, OLEDs as they are promising organic electronic devices present the subject of intense research. The efficient OLED operation requires balanced charge injection, charge transport and charge recombination within the electronic device [39].

The considered materials exhibit appropriate optoelectronic characteristics allowing their use as emitting layers in two layers OLED device. Alq3 (Tris (8 hydroxyquinoline) aluminum) is a suitable material to act as an electron transport layer [40] with respect to the LUMO energy levels of **M1** and **M2** (See **Figure 8**).

Radiative lifetime (τ) introduces the average time of the stay of a molecule at the excited state before a photon-emission. The lower value of τ leads to a relevant emission of the conjugated material. The radiative lifetimeτcould be determined according to the following expression [41]:

$$\mathbf{\dot{\tau}} = \frac{\mathbf{C}^3}{2(\mathbf{E\_{flu}})^2 \mathbf{f}} \tag{1}$$

Where, c, Eflu and f represent the velocity of light, the fluorescent energy and the oscillator strength, respectively.

The radiative lifetime values are of 10.12 ns and 5.46 ns for **M1** and **M2**, respectively. The small values of τ denote the efficient light emission of these materials. The findings of the present report corroborate a slight difference in radiative lifetime values which are explained by the effect of DPA and DTA acceptor blocks within the conjugated structures.

According to these results and compared to some previous studies reported in Refs. [28, 42], it is revealed that **M1** and **M2** exhibit promising optoelectronic properties for high performance OLED devices.

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

**Figure 7.** *Emission spectra of M1 and M2 with fitting Gaussian peaks obtained at DFT//B3LYP/6-31 g(d) level.*

#### **3.4 Charge transfer properties**

Many factors are responsible for high performance organic optoelectronic devices such as hole/electron charge transfer balance. The reorganization energies of holes and electrons (λ<sup>h</sup> and λe) together with the ionization potential (IP) and the electron affinity (EA) have been calculated to evaluate the hole/electron transfer abilities. The reorganization energies were carried out from neutral, cationic and anionic geometries, as detailed in **Figure 9**.

The reorganization energies of holes and electrons can be calculated following the expression above [43]:


#### **Table 4.**

*Calculated maximum emission wavelengths* λem max *(nm), fluorescent energy* Eflu*(eV), oscillator strength f (a.u), full-width at half maximum FWHM (nm), main electronic transitions, stokes shift (nm) and radiative lifetime (*τ*).*

#### **Figure 8.**

*CIE color coordinates for the studied materials (right), schematic structure of proposed bilayer OLED based studied materials (left).*

$$
\lambda\_{\hbar}/\lambda\_{\mathbf{e}} = \left(\mathbf{E}\_{\mathbf{0}}^{\pm} - \mathbf{E}\_{\pm}^{\pm}\right) + \left(\mathbf{E}\_{\pm}^{\mathbf{0}} - \mathbf{E}\_{\mathbf{0}}^{\mathbf{0}}\right) \tag{2}
$$

Hence, E� <sup>0</sup> , E0 �, E� � and E0 <sup>0</sup> represent the cation/anion energy of cation/anion at neutral geometry, the energy of neutral structure in the cation/anion state, the energy of cation/anion in the cation/anion state and the energy of the neutral structure, respectively. Charge transfer parameters have been determined using DFT//B3LYP/6-31 g(d) method and the results are presented in **Table 5**. Based on the obtained results, it is found that **M1** exhibits higher hole and electron transfer abilities regarding the lower reorganization energies of hole and electron.

To get insights into the charge transport properties, the ionization potential (IP) and the electron affinity (AE) were carried out for better evaluating the electron extraction and attraction abilities, respectively [44]. In comparison with **M2**, **M1** exhibits relatively lower EA (0.94 eV versus 1.17 eV) which demonstrates that our materials exhibit low performance of grasping electrons. However, the low values of IP (6.02 eV for **M1** and 6.00 eV for **M2**) indicated the high ability of these materials to grasp hole (See **Table 5**). It is possible to better improve the mobility of charge carriers through the modification of the conjugate structure [45].

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

**Figure 9.** *Calculation details of reorganization energies from neutral cation and anion states.*


#### **Table 5.**

*Calculated charge transfer parameters (expressed in eV) of M1 and M2 at DFT//B3LYP/6-31 g(d) level of theory.*

Overall, the charge properties analysis of **M1** and **M2** shows that they are considered as promising materials for organic optoelectronic applications.

#### **3.5 Nonlinear optical (NLO) properties**

The principle of nonlinear optics represents the interaction between an incident electromagnetic field with a particular material leading to the generation of an electromagnetic field modified in wave number, phase or frequency [17]. NLO materials are increasingly applied in emerging technological fields such as telecommunications, optical memory, optical information processing, etc. [46].

NLO properties produced from the high delocalization of electrons within the molecule increase while increasing the molecular conjugation [47]. Further, the presence of electron donor blocks (D) and electron acceptor blocks (A) contributes to the improvement of the NLO properties [48–50]. Studies have shown that nonlinear organic optical materials possess higher optical nonlinearity compared to inorganic materials [51].

The electric dipole moment μ, the polarizabilities α, the first (β) and the secondorder hyperpolarizability (γÞ describe the nonlinear optical response of an isolated molecule within an electric field [52]. The total dipole moment μtot, the static polarizability α0, the static first hyperpolarizability β<sup>0</sup> and the static second order hyperpolarizability γ0are calculated using the expressions above [22, 53]:


#### **Table 6.**

*Calculated electric dipole moment <sup>μ</sup>tot (D), polarizability <sup>α</sup><sup>0</sup> (*�10�<sup>24</sup> *esu), first-order hyperpolarizability <sup>β</sup><sup>0</sup> (*�10�<sup>30</sup> *esu) and second-order hyperpolarizability* <sup>γ</sup><sup>0</sup> *(*�10�<sup>36</sup> *esu) of the studied materials at DFT//B3LYP/ 6-31 g(d) level.*

$$
\mu\_{\text{tot}} = \left(\mu\_{\text{x}}\,^2 + \mu\_{\text{y}}\,^2 + \mu\_{\text{z}}\,^2\right)^{\frac{1}{2}}\tag{3}
$$

$$\mathbf{a\_0} = \frac{1}{3} \left( \mathbf{a\_{xx}} + \mathbf{a\_{yy}} + \mathbf{a\_{zz}} \right) \tag{4}$$

$$\beta\_0 = \left[ \left( \mathfrak{F}\_{\rm{xxx}} + \mathfrak{f}\_{\rm{yy}} + \mathfrak{f}\_{\rm{xxx}} \right)^2 + \left( \mathfrak{f}\_{\rm{yy}} + \mathfrak{f}\_{\rm{yx}} + \mathfrak{f}\_{\rm{yxx}} \right)^2 + \left( \mathfrak{f}\_{\rm{xxx}} + \mathfrak{f}\_{\rm{xxx}} + \mathfrak{f}\_{\rm{xyy}} \right)^2 \right]^{\frac{1}{2}} \tag{5}$$

$$\gamma\_0 = (\mathbf{1}/\mathbf{5}) \left[ \gamma\_{\mathbf{xxxx}} + \gamma\_{\mathbf{yyyy}} + \gamma\_{\mathbf{zzzz}} + 2\gamma\_{\mathbf{xxyy}} + 2\gamma\_{\mathbf{yyzz}} + 2\gamma\_{\mathbf{zzxx}} \right] \tag{6}$$

DFT approach is used as a reliable method for the determination of NLO properties of organic materials [54]. To get insights into the NLO properties, theoretical calculations were performed on the ground state optimized geometries of **M1** and **M2** at DFT//B3LYP/6-31 g(d) level of theory and the results are listed in **Table 6**.

From **Table 6**, the first- and second-order hyperpolarizabilities of **M2** are found lower than those of **M1** explained by the distinct electron delocalization in the conjugated structures. Thus, as compared to **M2**, it is important to note that **M1** presents the best NLO properties.

Urea is a prototypical organic molecule used as a threshold comparison value in the study of the NLO properties of molecular materials [55]. The NLO parameters of urea calculated at DFT//B3LYP/6-31 g(d) level of theory are found of: μtot = 4.259 D, α<sup>0</sup> ¼3.749 � 10–24 esu and β<sup>0</sup> ¼0.557� 10–30 esu and γ<sup>0</sup> = 0.746� 10–36 esu.

Compared to urea, the high values of NLO parameters of **M1** and **M2** confirm the design of high performance nonlinear optical materials.

### **4. Conclusion**

In this study, we reported a DFT study based on structural, optoelectronic and nonlinear optical (NLO) properties of D-A small π-conjugated molecules based on *Computational Study on Optoelectronic Properties of Donor-Acceptor Type Small… DOI: http://dx.doi.org/10.5772/intechopen.98590*

DTP and anthracene. The optimized structures have shown the non-planarity of the investigated molecules **M1** and **M2** arising from the anthracene derivatives (DPA and DTA) conjugated configuration. FMOs analysis shows the appropriate HOMO/ LUMO energy levels with the low band gap energies. To support the FMOs analysis, EDD contour plots have been computed to identify the donor and acceptor moiety within **M1** and **M2** structures. The TD-DFT study demonstrated the role played by the acceptor block in improving the absorption properties of the studied materials. The emission properties revealed an intense emission in the pure green and pure yellow for **M1** and **M2**, respectively. These molecules have shown their promising abilities to be used in OLED devices. The charge transfer properties analysis attested the relevant hole/electron transport abilities of these materials. Computed static polarizability (α0), first-order hyperpolarizability (β0) and second-order hyperpolazabilty (ɣ0) indicated the excellent NLO properties of **M1** and **M2**. This NLO response suggested these compounds to be used as potential candidates for NLO applications. Overall, this study provided an insight into a promising D-A conjugated architecture with the role of the acceptor block on enhancing the optoelectronic performances of organic materials.
