Spectroscopic and Electrochemical Investigations of New 5-oxo-2- Phenyloxazol-4(5H)-ylidene Based Triazine Derivatives

Phenyloxazol-4(5H)-ylidene Based Triazine Derivatives Faheem Ahmad1, Abdullah1, M. Shaheer Akhtar2, Hafiz Ghulam Abbas3, Shabaz Alam1, Hyung-Shik Shin1,4*, Sadia Ameen5* 1Energy Materials & Surface Science Laboratory, Solar Energy Research Center, School of Chemical Engineering, Jeonbuk National University, Jeonju, 54896, Republic of Korea. 2New & Renewable Energy Material Development Center (NewREC), Jeonbuk National University, Jeonbuk, Republic of Korea. 3Department of Nano science and Nanotechnology, Research Institute of Physics and Chemistry, Jeonbuk National University, Jeonju, 54896, Republic of Korea. 4*Korea Basic Science Institute (KBSI), 169-148 Gwahak-ro, Yuseong-gu, Daejon, 34133, Republic of Korea. 5*Advanced Materials and Devices Laboratory, Department of Bio-Convergence Science, Jeongeub Campus, Jeonbuk National University, 56212, Republic of Korea. Current Research in Materials Chemistry Ahmad F, et al. Cur Res Mater Chem 2019, 1: 102 https://doi.org/10.33790/crmc1100102

Oxazolone derivatives exhibit interesting photophysical and electrochemical properties and therefore, utilized as the promising materials for the electronic devices, biosensors, non-linear optical materials, and wide varieties of dyes [21][22][23][24][25]. The photophysical property is one of the important parameters of organic molecules for the potential applications in photonics and electronics [26,27]. Several reports are available on the absorption and emission studies of differently substituted oxazolone derivatives like Murthy et al studied the photophysical behavior of five new oxazolone derivatives, which showed an intense absorption at 350-480 nm with the emission maxima at 390-535 nm [28]. In another work, Murthy and co-workers performed the photophysical studies of 4-(4′-N,Ndimethylbenzylidene)-2-phenyloxazol-5-(4H)-one and reported the absorption and emission maxima at ~465 and ~522 nm, respectively [23]. Ozturk et al reported the synthesis and the photophysical studies of four different oxazolone derivatives, substituted with N-phenylaza-15-crown-5 moiety which showed an intense visible absorption and an emission maxima in the range of 467-524 nm and 496-689 nm, respectively [29]. The versatile applicability of oxazolone molecule inspired us for assembling and decorating oxazolone molecule with diverse functional groups. In this work, a new oxazolone derivative, named as 4-((4,6-bis(4-((Z)-(5-oxo-2-phenyloxazol-4(5H)-ylidene) methyl)phenoxy)-1,3,5-triazin-2-yl) oxy)benzaldehyde (CBOZ (5), is synthesized containing the two oxazolone rings which are associated with a central triazine nucleus in their structural framework. To the best of our knowledge, for the first time, the synthesis of CBOZ (5) is reported and comprehensively characterized in terms of its structural, molecular, optical and electrochemical properties. To support our experimental studies, the theoretical calculations of optical and electrochemical properties are performed by DFT studies which are in good agreement with the experimental results.

Material and methods
All the reagents were purchased from TCI and Sigma-Aldrich as 'synthesis grade' and used without the further purifications. The progress of the reaction was monitored with the thin layer chromatography (TLC) plates of aluminum silica gel 60 F254 (Merck).

Synthesis of 4,4',4''-[1,3,5-triazine-2,4,6-triyltris(oxy)] trisbenzaldehyde (3)
The mixture of trichlorotriazine (1, 2 mmol), 4-hydroxybenzaldehyde (2, 6.17 mmol) and sodium carbonate in p-dioxane solvent was taken in a flask. The reaction mixture was stirred and refluxed under nitrogen atmosphere for 12 h. After completion of the reaction (as monitored by TLC), the reaction mixture was allowed to cool at the room temperature and poured into cold deionized (DI) water. The resulting precipitate was filtered off and washed with cold DI water to obtain the crude product. The crude product was subjected to column chromatography on silica gel with dichloromethane and hexane (1:1) to achieve the pure product (

Computational details
The geometry optimizations were performed using the Vienna ab-initio simulation package (VASP) [30,31]. The electron-ion interactions were described using the projector-augmented wave (PAW) method, which was primarily a frozen-core all-electron calculation [31]. The attractive Vander Waals interactions were included using Grimme's correction for PBE-D3 method [32]. For the structure optimization, atoms were relaxed in the direction of the Hellmann-Feynman force using the conjugate gradient method with an energy cut-off of ~400 eV until a stringent convergence criterion (of 0.02 eV/Å) was satisfied. The lattice constants were optimized using PBE-D3 exchange-correlation functional. The accurate electronic structure and optical calculations were performed using HSE06 functional [33]. By neglecting the local field effects, the imaginary part of the frequency-dependent dielectric matrix was determined from the equation:

Characterizations
Fourier transform infrared (FTIR) spectra were recorded in the 400-4000 cm -1 wave-number range by using FTIR-4100 (JASCO) spectrometer. 1 H NMR (400 MHz) and 13 C NMR (100 MHz) spectra were recorded through JEOL FT-NMR Spectrophotometer by using CDCl 3 as solvent. The chemical shifts (δ) were reported in ppm relative to TMS as an internal standard. The UV-Vis absorption and the photoluminescence spectrum were performed in chlorobenzene by using V-670 (JASCO) spectrophotometer and FP-6500 (JASCO) fluorometer, respectively. Cyclic voltammetry (CV) measurements were carried out by using an electrode system of WPG 100 Potentiostat/Galvanostat (WonA Tech). CV measurements were performed at a scan rate of 100 mV/s by using three-electrode cell system, containing a glassy carbon working electrode, a saturated calomel reference electrode (SCE) and a platinum wire counter electrode. Thermogravimetric analysis (TGA) was carried out under an inert atmosphere at a scan rate of ~10oC min-1 by using TA Q-50 Thermogravimetric Analyzer instrument. Differential scanning calorimetry (DSC) measurements were performed by DSC-2910 instrument at a heating and cooling rate of ~10 o C min -1 under nitrogen atmosphere.

Spectroscopic Studies
The synthesis of the target compound (5) was performed by the two step synthetic procedure, as depicted in Scheme 1. The intermediate (3) was synthesized by following the procedure as reported by Mikroyannidis et al [34]. The compound 5 was typically synthesized by cyclodehydration condensation reaction process which involved the intermediate (3) with hippuric acid in the presence of sodium acetate and acetic anhydride [20]. The structural descriptions of the synthesized CBOZ (5) are described on the basis of FTIR, 1 H NMR, 13 C NMR and Mass spectral studies which are found in good corroboration with the expected structural framework of the synthesized compound. The FTIR spectrum (ESI † ) displays the characteristic signals for (C=O) ald , lactone carbonyl, -(C=N), (C-O) and (C-N) at ~1780, ~1751, ~1654, ~1206 and ~1165 cm -1 , respectively [35]. However, the absorption band resonating at around ~1602 cm -1 is assigned to the exocyclic double bond attached to the oxazolone ring [35]. 1 H NMR spectral analysis, as shown in Figure. 1(a), exhibits the singlet appearing at δ ~9.91 and ~8.02 which are attributed to the aldehydic and olefinic protons, respectively. The phenyl ring attached to the oxazolone ring shows the doublet (δ ~8.22) and multiplet peak (δ ~7.48-7.60), representing the two ortho protons and three (2 para and 1 meta) protons, respectively. However, the double doublets at δ ~8.16, ~7.88, ~7.24 and ~7.16 are assigned to the aromatic protons of benzene rings, substituted to triazine nucleus. From 13 C NMR spectral study ( Figure. 1(b), the absorption band resonating at δ ~191.3 corresponds to carbonyl carbon of the aldehydic group, while the carbons of triaizne nucleus shows two different peaks at δ ~175.9 and ~169.4 due to the presence of two different substituents on the carbon atom of triazine. Similarly, the signals at δ ~167.9, ~164.0 and ~133.7 correspond to the carbonyl, C-10′ and C-12′ carbons of oxazolone ring, respectively [35]. The complete spectral assignment of CBOZ (5) is presented in Table S-1 Page 3 of 12 CBOZ (5) shows m/z value at ~727.65, as depicted in Figure. S-2, ESI † , is found in good conformity with the proposed structure. All the above spectral assignment regarding the structure elucidation confirms the

Cur Res Mater Chem
CRMC, an open access journal Volume 1. 2019. 102 successful synthesis of CBOZ (5). The optimized molecular structure of CBOZ (5) is performed by DFT calculations in XY plane, as shown in Figure.

Thermal analysis
The thermal characteristics of CBOZ (5) are evaluated by TGA and DSC in the temperature range of 25-700 ºC under N 2 atmosphere at the scan rate of 10 ºC/min, as shown in Figure. 3. The thermogram of CBOZ (5) shows a little weight loss of only ~5% at around ~258 ºC, indicating a good thermal stability [36]. The TGA curve shows two stages of the compound decomposition. loss of about ~64 %, and the second stage of the decomposition with ~14% weight loss occurs at the temperature of ~700 ºC. The DSC thermogram displays two melting transition (T m ) at ~178.31 and ~271.82 o C, indicating the liquid-crystalline (LC) property of CBOZ (5), which might be associated with the self-organization behavior due to LC-LC phase transition [37]. Also, the crystallization transition (T c ) at ~303.6 o C might represent the self-organization of the small molecule.

Photo-physical properties
The photo-physical properties of CBOZ (5) are investigated by UV-Vis absorption and photoluminescence (PL) emission studies. Initially, the absorption spectra of CBOZ (5) are investigated at the room temperature in 10 μM concentration of four different solvents such as chlorobenzene, chloroform, acetone and methanol. As depicted in Figure. 4(a), the absorption spectrum shows a single absorption band which corresponds to the electronic transition occurring from the π-molecular orbitals [38]. From Figure. 4(a), CBOZ (5) shows the maximum absorptivity and λmax (~370 nm) value in chlorobenzene solvent as compared to other solvents i.e. chloroform (λmax = ~367 nm), acetone (λ max = ~363 nm), and methanol (λ max = ~362 nm). Herein, the increase in the polarity of the solvent leads to the hypsochromic shift of the absorption band, indicating that CBOZ (5) is stabilized in ground state [39]. Figure. 4(b) shows the absorbance curves of CBOZ (5) in chlorobenzene of five different concentrations (such as 1, 3, 6, 8 and 10 μM). The absorbance of CBOZ (5) is performed at the room temperature which depicts an increase with the increasing concentrations, and the maximum peak position or the wavelength for all the concentration is found as ~370 nm. These results suggest that the concentration has substantially affected the optoelectronic properties of CBOZ (5). The optical band gap (E g ) of ~3.02 eV is estimated from the equation E g = 1240/λ onset [40].

Cur Res Mater Chem
CRMC, an open access journal Volume 1. 2019. 102 To get more insight into the optical properties of CBOZ (5), the PL properties are extendedly studied in 10 μM concentrations of four different solvents like chlorobenzene, chloroform, acetone and methanol at the excitation wavelength of ~380 nm. The two emission peaks at ~438 and ~553 nm are displayed, as shown in Figure. 5(a). The observed photoluminescence spectra of CBOZ (5) in the region of ~380-800 exhibits no mirror image relationship with the absorption spectra [39], which signifies the geometric rearrangement of CBOZ (5) in the excited state (S1) as compared to the initial ground state (S0). Figure. 5(a) also shows that the polarity of solvent does not affect the PL spectra of CBOZ (5), as it shows similar trends in each solvent. This might be attributed to the lack of intramolecular charge transfer (ICT) interaction within the conjugated backbone of CBOZ (5), which might lead to weak solvation effects between the solvents and the compound [41]. From Figure. 5(b), the intensity of the emission peak increases with the increase in the concentration of the compound, which evidences a strong mechanical property of CBOZ (5) [42]. Figure. 5(c) shows the emission spectra of CBOZ (5) at different excitation wavelengths ranging from 300-400 nm. The PL spectra exhibit an increase in an excitation wavelength intensity of the absorption peak, which might be related to the enlargement of π-π transitions. Moreover, with the increase of excitation wavelength, π-π excitation might be enhanced in CBOZ (5)

Electrochemical properties
In order to explore the electrochemical behavior of CBOZ (5), the electrochemical properties are studied through CV analysis by using thin film of CBOZ (5) in 0.1 M solution of tetrabutylammonium hexaflourophosphate [ n Bu 4 N] + [PF 6 ]in acetonitrile at a potential scan rate of ~100 mV/s. Figure. 6(a) shows CV of CBOZ (5), and the corresponding redox potentials are summarized in Table 1. CBOZ (5) exhibits the onset oxidation potential (E ox ) at ~1.47 V. From the oxidation potential, the HOMO energy level as calculated by the formula [HOMO = -e (E ox onset + 4.40) (eV)] is found as -5.87 eV. However, the LUMO energy level is estimated as -2.85 eV by using the formula [LUMO = HUMO + E g (eV)]. To investigate the stability of CBOZ (5), CV curve is further scanned for 25 cycles, as shown in Figure. 6(b). The multicycle CV curves displays a slightly distorted shape of CV and the peak position or the onset is marginally changed, which suggests the decent stability of CBOZ (5) in an oxidation process. Figure. 6 Cyclic Voltammetry plots of CBOZ (5) for (a) one cycle and (b) multi cycles.

DFT study
In order to acquire deeper insight about the electronic spectra of CBOZ (5), density functional study (DFT) is carried out at the HSE06 functional theory and the corresponding theoretical data are compared with the experimental values, as summarized in Table 1. From UV-vis spectra ( Figure. 4), the simulated UV-Vis spectrum shows a maximum absorption at wavelength of ~372 nm, which is very close to the experimental value of ~370 nm. The refractive index (n) is calculated as ~1.62. For CBOZ (5), the energy of the frontier molecular orbitals i.e. HOMO and LUMO energy levels are calculated by density functional study and are found as -5.77 and -2.67 eV, respectively, which matches well to the experimental electronic energy levels, as calculated by CV analysis. The excitation energy of HOMO→LUMO transition could be considered from the optical band gap, which is estimated as ~3.10 eV and matches well with the experimental optical band gap, as obtained from UV absorption spectrum (i.e. E g = ~3.02 eV). The optical band gap and energy of frontier molecular orbitals describe the chemical reactivity as well as kinetic stability of the molecules [30]. The high and a low stability of the molecule could be explained by a large and small gap, respectively. Moderate E g = ~3.02 eV value realizes that CBOZ (5) might exhibit good chemical reactivity and the kinetic properties. The spatial plots of HOMO and LUMO with energy eigen values and the optical band gaps are shown in Figure. 7. These spatial plots and low optical band gap explain the good charge transfer properties of CBOZ (5) upon excitation. In Figure. 7, the HOMO is delocalized fully over one side chain and LUMO is partially delocalized on the other side chain of triazine nucleus bearing oxazolone nucleus, which confirms the charge excitation mechanism. The charge transfer is noticed from one substituted chain to other upon HOMO→LUMO excitation.

Conclusions
In summary, a new and novel oxazolone molecule, CBOZ (5) with central triazine nucleus substituted with two oxazolone ring is successfully synthesized by two step reactions and characterized extensively by FTIR, 1 H NMR, 13 C NMR and MS spectroscopic analysis. CBOZ (5) exhibits relatively a good thermal stability, as observed by the decomposition temperature of ~258 ºC. The synthesized CBOZ (5) displays a reasonable good optical band gap with the HOMO of ~ -5.87 eV and LUMO of ~ -2.85 eV. The photoluminescence properties reveal that the significant mechanical strength of CBOZ (5) increases with the increase of the concentration and excitation wavelengths. Importantly, the optical and electrochemical studies of CBOZ (5) are comprehensibly consistent with the DFT calculated optical band gaps and HOMO and LUMO energy levels.