Theoretical Investigation of New Organic Electroluminescent Materials Based on 4-Azaindole Groups and Oligopyrrole

Introduction The study of organic electroluminescence (EL) materials is now a rapidly developing field of material science due to promising practical applications [1]. In spite of the impressing achievement of the last decade, the problem of searching for new effective organic luminescent materials of different emission colours is still topical. Recently, many electronic and optical-electronic devices consisting of organic materials have been demonstrated; organic light emitting devices (OLEDs) require lower energy input, have a wider viewing angle with improved colour contrast, and can be made much thinner. The use of π-conjugated organic compounds as electroluminescent materials in organic light-emitting devices (OLEDs) was originally introduced by Van Slyke over two decades ago [2,3]. Since then, the development of new π-conjugated compounds especially small molecules with superior physical, optical, thermal, and electrochemical properties has become one of the most reviving research areas [4]. Although many fluorescent blue emitters have been reported, such as anthracene derivatives, phenylene derivatives, pyrene derivatives, fluorene derivatives, carbazolederivaties, triarylamine derivatives, and phosphorescent iridium complexes, there is still a clear need for further improvements in terms of efficiency and colour purity compared to red and green emitters [5].


Introduction
The study of organic electroluminescence (EL) materials is now a rapidly developing field of material science due to promising practical applications [1]. In spite of the impressing achievement of the last decade, the problem of searching for new effective organic luminescent materials of different emission colours is still topical. Recently, many electronic and optical-electronic devices consisting of organic materials have been demonstrated; organic light emitting devices (OLEDs) require lower energy input, have a wider viewing angle with improved colour contrast, and can be made much thinner. The use of π-conjugated organic compounds as electroluminescent materials in organic light-emitting devices (OLEDs) was originally introduced by Van Slyke over two decades ago [2,3]. Since then, the development of new π-conjugated compounds especially small molecules with superior physical, optical, thermal, and electrochemical properties has become one of the most reviving research areas [4]. Although many fluorescent blue emitters have been reported, such as anthracene derivatives, phenylene derivatives, pyrene derivatives, fluorene derivatives, carbazolederivaties, triarylamine derivatives, and phosphorescent iridium complexes, there is still a clear need for further improvements in terms of efficiency and colour purity compared to red and green emitters [5].
A new type of luminescent compounds based on Oligomers and azaindolyl groups were prepared by Hong et al. [5]. The presence of 4-azainole moieties at the end group of oligopyrrole greatly enhanced the photoluminescence by increasing the intrinsic stiffness of the polymer backbone; weaken intermolecular interaction [6][7][8]. The emission spectrum of a conjugated polymer depends basically on its π-π*band gap, which can be tailored using different structures. In this paper, we present a theoretical study of four new organic electroluminescent material (I-IV) based on (4-azaindolyl) oligopyrrole. We investigated theoretically, the effect of increasing pyrrole ring on 4-azainole moieties on the structural and optical-electronic properties of the compounds. Then, bridging effect was studied by bridging two central pyrrole rings with electron withdrawing 2-carboxylpropenoic acid group[C=C(CO 2 H) 2 ].
The chemical structures of the materials studied are shown in Figure 1. the π-conjugation length of the oligopyrrole units of the compounds (1-3) and these values are in perfect accordance with those estimated from the electronic transition data [10][11][12].
The distribution of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of studied compounds was also investigated at the DFT/B3LYP/ 6-31G (d) level for the geometry optimizations. The iso-density surface plot of HOMO and LUMO were exhibited above in Figure 2, it is seen from the figure that the electron density of HOMO is mainly localized on the pyrrolyl moieties while the electron density of LUMO is mainly on the 4-azaindole backbone except compound VI, where is localized on pyrrolyl groups with contribution from 2-carboxylpropenioc acid substituent group. The electron transition from the ground state to the excited state will be

Computational Method
Quantum chemical calculation of the ground state molecular structures of (I-IV) were performed using the Spartan'14 program package on HP 2000 inter (R) core (TM) i3-3110 (M) CPU @ 2.40 GHz processor machine (computer) having 6.00 GB installed memory (RAM) and 750 GB hard disc. The molecules were drawn with SPARTAN' 14 graphical user interface (GUI) and the energy minimization conducted using MMFFaq (Merck Molecular Force Field). MMFFaq, Molecular Mechanics Conformational Distribution gave rise to different conformers for each of the studied compounds with their corresponding energies. The most stable conformer (i.e. conformer with the lowest energy) obtained for each compounds were submitted into SPARTAN' 14 for energy optimization. The influence of increasing number of pyrrole rings on the opto-electronic properties of all the studied compounds were fully optimized at Density Functional Theory (Becke's Three Parameter Hybrid Functional using the Lee, Yang and Parr Correlation Functional-B3LYP) [8]. The basis sets: 6-31G (d) and EDF2 were used for all atoms. We have also examined HOMO and LUMO energies levels; the energy gap is evaluated as the difference between the HOMO and LUMO energies. The electronic transition properties, which include the maximum excitation wavelength (λ abs max ), excitation energy, relative intensities (oscillating strength), molecular orbitals character and coefficient of the compounds, were studied using Time Dependent Density Functional Theory (TD-DFT) [9].

Optical-electronic properties
The electronic properties of all the studied compounds were obtained by DFT calculations at B3LYP/6-31G (d) and EDF2/6-31G (d) levels. Table 1, Figures 2 and 3 show the analysis of HOMO-LUMO energy gap of the studied compounds.
Highest occupied molecular orbital (HOMO) represents the ability to donate an electron and negative values of HOMO energy is taken as the ionization potential. Using Polarized basis set of B3LYP/6-31G (d), it was found that ionization potential gradually decreased with increasing of the π-conjugation system. The HOMO energy levels of these materials were in the range of -5.75 to -4.83 eV, matching well with most work function of indium tin oxide (ITO) electrode and favouring the injection and transport of holes. Their LUMO energy levels were in the range of-0.91 to -0.81 eV.The HOMO-LUMO energy gaps were calculated to be decreased from 4.84 eV to 4.02 eV with an increase in

E-LUM0 (eV) E-HOMO (eV) E-gap (eV)
Compound I (n=1)   electron flowing from the pyrrolyl moieties to the azaindoly moieties. The electron density distribution of HOMO and LUMO suggests that the investigated compounds might possess beneficial electron injection and transport properties with the incorporation of electron withdrawing 2-carboxylpropenoic acid group [13].
The calculated band gap (E-gap) of the studied compounds decreases in the order as follows: I>II>III>IV. The significant reduction of E-gap of compound IV with 2.7 eV compared to 4.27 eV of compound II is due to the bridging effect of C=C(CO 2 H) 2 . This bridging effect causes a destabilization of both HOMO and LUMO levels; thus, producing the lowest value of the energy gap. The low HOMO energy level of compound IV with -5.16 eV suggests that the compound has high oxidation stability and potential application for charge transport material [14]. For organic polymeric chromophores, this energy gap ranges from 1.4 to 3.3 eV corresponding to light wavelength between 890-370 nm covering the visible region [15] (Table 2).

Absorption and emission properties
The absorption spectra of the studied compounds exhibited three absorption bands at 261-291 nm, 300-362 nm and 383-640 nm due to π-π* electronic transitions. The low-energy broad band at 383-640 nm is assigned to an intra-molecular charge transfer band from the pyrrole ring to the two 4-azaindole backbone. The lowest-energy electronic transition HOMO → LUMO consists mostly of the intramolecular charge transfer and an average absorption band of 471 nm. Other two absorption bands occur around 339 nm and 276 nm mainly consisting ofHOMO-2 → LUMO vertical transitions and HOMO-1 → LUMOvertical transitions respectively. The excitation energy, oscillator strength, coefficient and main configuration for most relevant absorption bands were listed in Table 2. The presence of the N-H group in the heterocyclic ring influences the excited electronic states. The first four vertical excitation energies are summarized. Only two electronic transitions from ground state (S 0 ) to third excited electronic state (S 3 ) have computed oscillator strength been higher than 0.5 while the other electronic transitions, with smaller oscillator strength have a negligible relevance in experimental works [16][17][18][19].

Conclusion
Conjugated oligomers and polymers have been studied in great details as electroluminescent materials for usage in organic light emitting devices (OLED). Azaindolyl oligomers-based materials have been investigated extensively because of the many attractive properties they possess. Compound IV is the least sterically hindered and may be promising candidate for hole transporting and bright blue to red emitting layer in organic light emitting device (OLED) fabrication.