Investigations on Tunable Blue Light Emitting P-Acetyl Biphenyl-DPQ Phosphor for OLED Applications

S.Y. Mullemwar¹, G.D. Zade², N. Thejo Kalyani³*, S.J. Dhoble¹ and Xiaoyong Huang⁵

• 1 D. D. Bhoyar College of Arts and Science Mouda, Nagpur - 441104, India
• 2 J. N. Arts, Commerce and Science College, Wadi, Nagpur - 440023, India
• 3 Department of Applied Physics, Laxminarayan Institute of Technology, Nagpur - 440033, India
• 4 Department of Physics, RTM, Nagpur University, Nagpur - 440033, India
• 5 College of Physics and Optoelectronics, Taiyuan University of Technology, Taiyuan 030024, PR. China

Introduction

The era of Organic Light-Emitting Diodes (OLEDs) and Polymeric Light Emitting Diodes (PLEDs) has evolved tremendously from the time when preliminary reports by (Tang and Van Slyke 1987; Burroughes et al. 1990) were accepted. Though researchers are determined to improve the quantum efficiency of photoluminescence (PL) and electroluminescence (EL) OLEDs, challenges still exist. (Zhu et al. 2003). Universally, the blends of three primary colors (red, green and blue) or complementary colours (blue and orange) give rise to white emission. Amongst all, the luminous efficiency of blue OLEDs needs improvement (Kato et al. 2015). Hence, it is imperative to come-up with novel blue light emitting materials, which can compete with their red and green light emissive counterpart materials with respect to luminous efficiency, life time so as to harvest stable white light emission from these three RGB materials. In this regard, organic phosphors based on quinoline comprise an imperative class of a heterocyclic group and hence create substantial awareness amongst researchers worldwide. Poly(quinoline)s were principally reported (Stille 1981) by using Friedlander 'Corresponding author: This email address is being protected from spam bots, you need Javascript enabled to view it condensation as a polymerization step, so as to stipulate thermally stable and instinctively strong polymers. They show evidence of adaptable properties such as elevated thermal stability with glass-transition temperatures, even greater than 200 °C and onset thermal decomposition temperatures >400 °C, elevated electrical conductivity, and exceptional film forming property (Pelter et al. 1990; Tunney et al. 1987). These distinctive features make them appealing for opto-electronic devices. Prior state of art reveals extensive research on distinguishing properties of poly(quinoline)s such as charge- transfer, photoluminescence (PL) and Electro Luminescence (EL) and electron transporting properties (Kim et al. 1992; Lee and Rhee 1995; Chen et al. 1996; Jen et al. 1998; Mullemwar et al. 2016; Ghate et al. 2017; Kim et al. 2000; Zhang et al. 1998; Kwon et al. 2004) for their potential applications in OLEDs devices (Zhang and Jenekhe 2000; Zhang et al. 1999; Jenekhe et al. 2001; Tonzola et al. 2004). These complexes can be integrated into numerous matrices including zeolites, semiconductors (Alam and Jenekhe 2001; Maas et al. 2002), porous materials (Xu et al. 2002; Minootar et al. 2002), polymer thin films (Wang et al. 2002) and a range of aqueous solutions (Kalyani et al. 2010) to avoid luminescent concentration quenching, elevate the processing ability and enhance thermal as well as mechanical stabilities. This chapter describes the synthesis and characterization of a novel phosphor (Acetyl biphenyl substituted diphenyl quinolines) 2-([l, l'-biphenyl]-4-yl)-4-phenylquinoline (P-Acetyl biphenyl-DPQ) in various organic solvents and polystyrene at different wt%.

Experimental

Reagent and Solvents

Starting materials used for the synthesis of 2-([l,l'-biphenyl]-4-yl)-4- phenylquinoline (C₂₇H_1QN) (P-Acetyl biphenyl-DPQ) complex are 4 Acetylbiphenyl (C₁₄H₁₂0) [Himedia] 2 Amino Benzophenone (C₁₃H_nNO) [Himedia] Diphenylphosphate (C„H₅0)₂P(0)0H) [Sigma Aldrich], mcresol [CH₃QH₄OH], Dichloromethane (CH₂C1₂) [Fisher scientific], Sodium hydroxide (NaOH) [Fisher scientific], Hexane (CH₃(CH,)₄CH₃) [Loba chemiecals] Chloroform (CHC1₃), [Fisher scientific], Polystyrene [CH₂CH (C₆H₅)]_n, [Sigma Aldrich] and double distilled water.

Synthesis of P-Acetyl Biphenyl-DPQ

Synthesis scheme of P-Acetyl biphenyl-DPQ is adopted from the literature (Zade et al. 2011) and successive reactions are shown in Fig. 6.1.

Preparation of Blended Films

The most popular polymer, namely polystyrene (PS) was chosen for blending the synthesized complex at known wt% such as 10 and 5 wt% and the blended films were made according to literature methods (Mullemwar et al. 2016) . Preparation of blended thin films is schematically represented in Fig. 6.2.

Fig. 6.1. Synthesis scheme of 2-( [1Д'-biphenyl] -4-yl)-4-phenylquinoline (P-Acetyl biphenyl-DPQ).

Fig. 6.2. Schematic representation: Preparation of blended thin films.

Results and Discussion

Physical, chemical and optical properties of the synthesized P-Acetyl biphenyl-DPQ phosphor were investigated as follows: 'H-NMR, ¹³C-NMR on Bruker Benchtop instrument, Fourier Transform Infrared (FTIR) spectra on Bruker-Alpha at room temperature, and Thermo gravimetric/ Differential Thermal Analysis (TGA/DTA) on Perkin Elmer diamond. Absorption and photo luminescence measurements were carried out on SPECORD 50 spectrophotometer and Humamatsu F-4500 spectrofluorometer, respectively. CIE coordinates were calculated by CIE1931 (Commission International d'Eclairage) system.

NMR Spectroscopy

‘H-NMR spectrum of P-Acetyl biphenyl-DPQ: ¹H NMR (400 MHz, chloroform-d(CDCl₃)) 5 (ppm) 8.35 (d, 3H), 7.97 (d, 1H), 7.92 (s, 1H), 7.81 (m, 2H), 7.74 (d, 2H), 7.62 (m, 4H), 7.53 (t, 4H), 7.43 (d, 2H). This spectrum confirms the presence of 19 H-atom in the synthesized compound. These peaks can be assigned to the aromatic protons (Fig. 6.3), which correlates with the structure as described in Fig. 6.1. Chemical shifts from ¹³C-NMR spectrum: ¹³C-NMR (100 MHz, CDC1₃) 5 (ppm) 156.43, 149.26, 148.90, 142.16, 140.63, 138.55, 130.21, 129.65, 128.90, 128.70, 128.47, 128.05, 127.61, 127.22, 126.40, 125.88, 125.69, 119.27 as shown in Fig. 6.4. Each carbon nucleus experiences different magnetic fields according to their electronic

Fig. 6.3. ‘H-NMR spectrum of P-Acetyl biphenyl-DPQ.

Fig. 6.4.¹³C-NMR spectrum of P-Acetyl biphenyl-DPQ.

environment. In the range of 115-160 ppm, P-Acetyl biphenyl-DPQ confirms 27 aromatic C-nuclei. Due to the symmetrically related nature of maximum C-nuclei, the ⁱ³C spectrum displays 18 signals for a total number of 27 nuclei. The signals between 70 and 80 ppm are due to the CDC1₃ solvent.

Fourier Transform Infrared (FTIR) Spectra

The molecular structure of P-Acetyl biphenyl-DPQ chromophore are confirmed by FT-IR spectra over the range 4000-400 cm¹ by averaging 500 scans at a maximum resolution of 20 cm'¹ as shown in Fig. 6.5. Peaks recorded

Fig. 6.5. FTIR spectrum of P-Acetyl biphenyl DPQ.

at 916.29, and 763.24 cm⁴ may be attributed to phenyl ring substituted bands, while the peaks at 1400 cm¹ confirms (C=N) group, revealing the presence of the quinoline ring. Peaks at 866.61, 763.24, 724.97 and 688.59 cm¹ can be assigned to the benzene ring. Conjugated C=0 stretching vibrations are clearly observed at 1664.97 cm¹, conjugated C=C stretching vibrations are portrayed at 1584.69 and 1534.74 cm¹, thereby declaring the complete structural formation of P-Acetyl biphenyl-DPQ phosphor.

Thermogravimetric and Differential Thermal Analysis (TGA/DTA)

Thermogram of P-Acetyl biphenyl-DPQ phosphor displays a horizontal plateau, till 300 °C as indicated in Fig. 6.6. With further increase in temperature, the thermogram takes a curved portion, indicating decomposition or weight

loss of the sample due to heating. Hence, the synthesized complex can be operated up till 300 °C without any degradation in its properties.

DTA curve of P-Acetyl biphenyl-DPQ displays two endothermic peaks, one centred at 25 °C and the other at 181 °C corresponding to the distortion of water and evaporation of residual moisture from the complex. Exothermic peaks roughly at 189 °C and 429 °C in the DTA curve can be accredited to the breakdown process of the residual organic molecules.

UV-visible Absorption Spectra

UV-visible absorption spectroscopy was carried out to investigate the optical absorption peaks and the role of polarity index of solvent on the absorption spectra of P-Acetyl biphenyl-DPQ phosphor in chloroform, dichloromethane, acetic acid and formic acid at 10³ mol/L. Two absorption peaks: one attributed to the я - я’ transition (at the lower wavelength with high energy) of the conjugated polymer main chains; another is attributed to the n - я transition (at a higher wavelength with low energy) of the conjugated side chains were observed. However, the peak position and optical absorption density were found to be different for different solvents as shown in Fig. 6.7. Solvated DPQ in formic acid displays a strong absorption peak at 366 run with a shoulder at 262 nm, while in acetic acid, it displays strong absorption peak at 364 nm with a weak shoulder at 258 nm.

In both the cases, the peak at the lower wavelength is due to their large molar extinction coefficients of the conjugated polymer main chains,

while the peak at the higher wavelength is attributed to conjugated side chains. Similarly in dichloromethane and chloroform absorption peaks were observed at 327 and 332 run with a weak shoulder at 234 and 245 run, respectively. Hypsochromic shift was monitored in the absorption spectra of P-Acetyl biphenyl-DPQ, when the solvent is changed from formic/acetic acid to chloroform and dichloromethane. These changes may be due to (i) protonation of the imine nitrogen of the quinoline ring to form the quinolium ion (Kalvani et al. 2012). Poor optical densities were observed in formic and acetic acid, indicating sparse decomposition of the ligand in acidic conditions.

Determination of Optical Band Gap

The optical band gap of P-Acetyl biphenyl-DPQ in various solvents was calculated by adopting the method given by Morita et al. (1995). These values were found to be 3.02,3.07,3.46 and 3.44 eV in formic acid, acetic acid, dichloromethane and chloroform, respectively as shown in Fig. 6.8.

Fig. 6.8. Energy band gap of P-Acetyl biphenyl-DPQ in various solvents.

Photoluminescence (PL) Spectra in Organic Solvents

Consecutively, to explore the spectral features of P-Acetyl biphenyl-DPQ in a solid state, various organic solvents and blended polystyrene films, PL spectra was carried out individually. The PL spectra of P-Acetyl biphenyl- DPQ in a solid state demonstrate intense blue light emission at 388 run, when excited at 362 run as shown in Fig. 6.9.

The PL spectra of solvated P-Acetyl biphenyl-DPQ in formic acid, acetic acid, chloroform and dichloromethane are shown iir Figs. 6.10 (a), (b), (c) and (d), respectively. Solvated P-Acetyl biphenyl-DPQ in formic acid peaks at 485 nm when excited at 412 run, while iir formic acid the emission peak shifted to 478 nm when excited at 409 run. Thus a hypsochromic shift of 7 run was

Fig. 6.9. PL spectra of P-Acetyl biphenyl-DPQ in powder form.

observed when the change in solvent from acetic to formic acid, while in dichloromethane emission peak was observed at 466 nm when excited at 369 nm, while in chloroform, the emission peaked at 381 nm when excited at 357 nm. Thus a considerable hypsochromic shift of 85 nm was observed when the solvent is changed from DCM to chloroform. However, all the solvated fluorophores emits intense blue light, which can be tuned with the change in solvent.

Photoluminescence (PL) Spectra in Polystyrene

The PL spectra of molecularly doped P -Acetyl biphenyl-DPQ with polystyrene (PS) was investigated for different wt %. In case of P Acetyl biphenyl-DPQ+PS at 10 wt %, the PL spectrum displays emission at 384 nm, when excited at 360 nm and for 5 wt%, the polymeric chromophore emits intense blue emission at wavelength 381 nm, when excited at 357 nm as shown in Fig. 6.11. When these results are compared with the PL results of P-Acetyl biphenyl-DPQ in a solid state and in various solvents, the emission intensity was observed in the order of I_emi in blended films > I_emi in solvents > I_emiin a solid state. This infers that the synthesized phosphor form aggregates in PS matrix as well as in solvents and hence enhancement in intensity is observed in solvated P-Acetyl biphenyl-DPQ and its blended films (Kin et al. 2009; Raut et al. 2016).

Determination of Stokes Shift

The difference between the reciprocal of absorption to the emission wavelength (Stokes shift) was calculated by using the relation

Fig. 6.10. PL spectra of phosphor in (a) Formic acid, (b) Acetic acid, (c) Chloroform and (d) Dichloromethane.

Investigations on Tunable Blue Light Emitting P-Acetyl Biphemjl-DPQ...

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The synthesized phosphors display a Stokes shift of about 67, 65, 91, 38 nm in formic acid, acetic acid, dichloromethane and chloroform as shown in Figs. 6.12 (a), (b), (c) and (d), respectively.

Commission International d’Eclairage (CIE) Coordinates

CIE coordinates of P-Acetyl biphenyl-DPQ in various solvents were calculated on the CIE 1931 (Commission International d'Eclairage) system software (Dahule et al. 2015). They were found to be (0.154, 0.323) in formic

Fig. 6.12. Illustration of Stokes shift in P-Acetyl biphenyl-DPQ: (a) Formic acid, (b) Acetic acid, (c) Chloroform and (d) Dichloromethane.

acid, (0.143, 0.255) in acetic acid, (0.139, 0.160) in dichloromethane and (0.162, 0.063) in chloroform, respectively as shown in Fig. 6.13. A summary of optical and photometric properties of P-Acetyl biphenyl-DPQ in various environments is tabulated in Table 6.1.

Fig. 6.13. CIE1931 diagram of P-Acetyl biphenyl-DPQ in various solvents.

Conclusions

P-Acetyl biphenyl-DPQ was synthesized by Friedlander condensation reaction at 140 °C for 4 hours. ^XPI-NMR spectrum of P-Acetyl biphenyl-DPQ confirms the presence of the desired 19 Pi-atoms that can be assigned to the aromatic protons, which were found to be correlated with the structure. FTIR spectrum shows molecular confirmation of P-Acetyl biphenyl-DPQ chromophore. TGA/DTA result infers that the complex has thermal stability

till 300 °C. Two endothermic peaks, centred at 25 °C and 181 °C were observed in the DTA curve. UV-Vis absorption spectra reveal that the optical density of the complex in acetic acid and formic acid are much smaller than in chloroform and dichloromethane. The energy band gap of the synthesized complex was found to be 3.02, 3.07, 3.46 and 3.44 eV in formic acid, acetic acid, dichloromethane and chloroform, respectively. PL spectra of P-Acetyl biphenyl DPQ in solid state and blended films reveals tunable blue light emission within the range of 381 nm to 485 run with narrow Full Width at Half-Maximum (FWHM). The synthesized phosphors display a Stokes shift of about 67, 65, 91, 38 nm in formic acid, acetic acid, dichloromethane and chloroform, demonstrating its prospective as a tunable emissive material for OLED fabrication by solution processed techniques.

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