**3. Chemical synthesis and experimental properties of the news organics active layers**

### **3.1 Brief description of the chemical synthesis and Photophysical properties of the PVK-MeT**

The chemical synthesis of the PVK-MeT starts with the cross-linking of the Poly(9-vinylcarbazole) (PVK) in presence of 3-(methyl thiophene) monomers in chloroform solution using the anhydrous ferric chloride (FeCl3) as an oxidant. The material was obtained in the doped state and the neutral state was fully obtained by chemical treatment described elsewhere in our previous paper [15]. The neutral and doped states were identified essentially by analyzing the obtained samples by infrared and Raman analysis as well as optical absorption tools to prove the successful dedoping of the sample [15].

#### **3.2 Photo-physical properties of the PVK-MeT**

Optical absorption spectra of the synthesized material PVK-MeT was first investigated by optical absorption and PLE spectroscopies in chloroform diluted solution. Obtained spectra are presented in **Figure 1**. The synthesized material absorbs essentially in the UV-visible part, with an intense narrow absorption centered at around 250 nm and a shoulder absorption band centered at 410 nm. The PLE spectrum registered for the investigated material in the solution state of 550 nm maximum emission, shows a maximum at around 410 nm, coinciding with the maximum absorption assigned to π-π\* optical transition. As mentioned above, poly (3-alkyl thiophene) and in particular Poly(3-methyl thiophene) synthesized by Fecl3 routes absorbs essentially in the visible part with an intense absorption located at around 500 nm in the condensed state [25].

In order to study the excited state of the synthesized organic material, the sample was excited under various excited lines going from UV to visible part using first steady-state photoluminescence spectroscopy. No optical signal is obtained under UV excitation and a high emission is obtained under visible excitation, confirming PLE results. On the second steep, PVK-MeT and PMeT powder were then excited under 400 nm laser pulsed excitation and the PL obtained spectra are presented in **Figure 2**.

From **Figure 2**, it is clearly seen that the PL intensity is improved upon using the Poly(N\_vinylcarbazole) (PVK) on the chemical synthesis of the Poly (3-methyl thiophene) (PMeT) by FeCl3 oxidative polymerization routes. In contrast with the PL

**Figure 1.** *Optical absorption (a) and PLE spectra (b) of the synthesized PVK-MeT.*

**Figure 2.** *Photoluminescence spectra of PVK-MeT and PMeT powder (excitation 400 nm).*

*Synthesis, Experimental and Theoretical Investigations on the Optical and Electronic Properties… DOI: http://dx.doi.org/10.5772/intechopen.103807*

spectrum of PMeT, PVK-MeT exhibits an intense emission (see **Figure 2**). Indeed, we calculate the ratio I1/I2, where, I1 and I2 present respectively the maximum intensity of the PL spectra of the PVK-MeT and that of the PMeT homopolymer. The ratio is at around 4. An emissive active layer based on thiophene derivative was successfully obtained. The improvement of the luminescence properties is accompanied by a blue shift of the maximum emission from 600 nm to 550 nm, showing a change in the spectral emission, which is shown by the CIE cartographic (See **Figure 6**). The enhancement of the luminescence properties is confirmed by the analysis of the PL decays analysis using transient photoluminescence analysis on the synthesized PVK-MeT and PMeT homopolymer powder, described in detail in our previous paper [23]. Transient photoluminescence spectroscopy confirms the change of the nature of the photogenerated species going from PMeT to PVK-MeT, which induce an increase of the PLQY from 1 to 2% for poly(3-alkylthiophene) [6, 16] to 13% for PVK-MeT in the condensed state [16].

Theoretical computations based essentially on DFT-B3LYP-6-31G(d) level of theory, was permitting to deduces first the chemical structure of the synthesized material and their electronic parameters such as HOMO, LUMO energies, and the energy gap

**Figure 3.**

*Optimized chemical structure of (SWCNTs) (5,5) and (SWCNTs) (6,4) and their contour plot HOMO and LUMO.*


#### **Table 1.**

*Electronic parameters of the investigated organic active layer.*

HOMO-LUMO. The ionization potential (I.P) and electron affinity (E.A) are theoretically deduced using Koopman's theorem [26]. Obtained results are summarized in **Table 1**. Polythiophene derivatives and in particular Poly(3-methylthiophene) (PMeT) present a good absorption in the visible part and a red light emission is produced, when the PMeT homopolymer is excited by the visible line [27]. Else, in a condensed state (thin film) a low emission is produced with a low photoluminescence quantum yield (PLQY), which is at around 1–2% [6, 28]. The planarity of the chemical structure on the excited state of pol(3-alkylthioophenes) derivative, favors the formation of the nonemissive excimer species under the photo-excitation (excited state) [6, 28]. Further, the high π-conjugated system in polythiophenes favors the formation of the π-π stacking on the condensed state, which enhances the fast migration mechanism of the photo-generation species [6, 28, 29]. Consequently, non-radiative pathways are favored and a poor emission signal is obtained with fast multi-exponential decays are produced [29, 30].

In order to study the structure-properties relationship of the investigated materials, first, a simple optimization on the ground state of an oligomer of 6-thiophenes coupled at the 2–5 positions and of the deduced chemical structure of the PVK-MeT. Secondly, the fully optimized chemical structures on the first excited state are

#### **Figure 4.**

*Geometric parameters (dihedral angle ɸi) of the optimized chemical structure of the six-coupled non-regioregular 3-(methyl thiophene) and the PVK-MeT chemical structure on the ground (black line) (S0) and the first calculated excited state (S1) (red line).*

*Synthesis, Experimental and Theoretical Investigations on the Optical and Electronic Properties… DOI: http://dx.doi.org/10.5772/intechopen.103807*

successfully obtained by re-optimization of the optimized ground-state chemical structure with the RSCI/6-31G(d) level of theory. The optimized chemical structures on the ground and the first excited state are drawn with Gauss view software and presented in **Figure 4**.

Geometric parameters such as dihedral angle ɸi of 6-Oligothiophene optimized in the ground and the first excited-state state are presented in **Figure 4**. It is clearly seen from **Figure 4**, that the conformation of the non-regioregular coupled 3-methyl thiophene presents a non-planar conformation, and the distortion is higher on the TT coupling (ɸ3) in the ground state. Else, the first excited state of the non-regioregular 6-oligothiophene, obtained by the re-optimization of the optimized chemical structure by CIS/STO-3G present no distortion and a fully coplanar structure. Focused studies reveal that the luminescence in the conjugated system is only obtained when the main chain is deformed especially in the excited state [26]. Indeed, Bai et al. [6], suggest that upon excitation of the conjugated system in poly (3-alkyl thiophene), the planarity increases, which favors the formation of the nonemissive excimer species in the condensed state [6]. Consequently, a poor luminescence with low PLQY is obtained for polythiophenes derivatives in the condensed state [6]. Herein, the insertion of the cross-linked carbazole system (bicarbazole) grafted to the 6-oligothiophene system induces the increase of the distortion and the high deformation of the main chain in both grounds and excited state (**Figure 3**). These behaviors induced the decrease of the non-radiative photo-generated species and the improvement of the luminescence properties going from PMeT to PVK-MeT.

#### **3.3 Chemical elaboration of new small molecules for OLEDs application**

#### *3.3.1 Chemical synthesis of news small molecule: DOMCN and DBrCN*

To an equimolar stirred mixture of 1,4- phenylenediacetonitrile (1) (1 mmol) and aldehyde (2) (2 mmol) in 10 mL of tert-butyl alcohol (10 mL) were added a solution of tetrabutylammonium hydroxide (TBAH) (1 M solution in methanol). After 24 h of stirring at 50°C, the reaction mixture was cooled to room temperature and quenched with distilled water. Then, the obtained solution was extracted with chloroform. The organic layer was washed several times with distilled water and dried over anhydrous magnesium sulfate. The purification was carried out by precipitation in methanol. A yellow powder was filtered and dried in a vacuum for 24 h (**Figure 5**).

1 H NMR (CDCl 3) δ [ppm]: 7.89 (d, 4H, Ar-H), 7.71 (s, 4H, Ar-H), 7.49 (s, 2H, Vinyl-H), 6.96 (d, 4H, Ar-H), 4.04 (t, 6H, -OCH3). 13C NMR (CDCl3) δ [ppm]: 160.5, 142.4, 135.1, 131.5, 126.2, 126.2, 118.5, 115.1, 107.5, 68.1.

**Figure 5.** *The synthesis adopted procedure of the DOMCN and DBrCN.*
