Journal of Science: Advanced Materials and Devices (2016) 324e329 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Original Article Synthesis, crystal structure and photoluminescence study of green light emitting bis(1[(4-butylphenyl)imino]methyl naphthalen-2-ol) Ni(II) complex M Srinivas a, T.O Shrungesh Kumar a, K.M Mahadevan a, *, S Naveen b, G.R Vijayakumar c, H Nagabhushana d, M.N Kumara e, N.K Lokanath f a Department of Chemistry, Kuvempu University, P G Centre, Kadur 577548, India Institution of Excellence, Vijnana Bhavana, University of Mysore, Manasagangotri, Mysuru 570 006, India Department of Chemistry, University College of Science, Tumkur University, Tumkur 572 103, India d Prof C.N.R Rao Centre for Advanced Materials Research, Tumkur University, Tumkur 572 103, India e Department of Chemistry, Yuvarajas College, University of Mysore, Mysore 570005, India f Department of studies in Physics, Manasagangotri, University of Mysore, Mysore 570005, India b c a r t i c l e i n f o a b s t r a c t Article history: Received June 2016 Received in revised form July 2016 Accepted July 2016 Available online 11 July 2016 Synthetically feasible and cost effective Ni(II) complex phosphor (4) as green organic light emitting diode (OLED) was prepared by using Schiff base 1-[(4-butylphenyl)imino]methyl naphthalen-2-ol (3) The single crystals of Ni(II) complex were grown from chloroform and hexane (1:1 v/v) solution The green crystals of the complex were characterized by using single crystal XRD studies and were evaluated for their photophysical properties From the Diffused Reflectance Spectrum of the complex, the measured band gap energy was found to be 1.83 eV and the PL spectrum of the complex showed emission peak at 519 nm The excitation peaks at 519 nm were appeared at 394 nm and 465 nm The Commission Internationale De L'Eclairage (CIE) chromaticity diagram indicated that, the complex exhibit green color Hence, Ni(II) complex (4) could be a promising green OLED for developing strong electroluminescent materials for flat panel display applications © 2016 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) Keywords: 1[(4-butylphenyl)imino]methylnaphthalen2-ol Schiff base Ni(II) complex Photoluminescence Green OLED Introduction Many transition metal complexes were known to posses potential applications in developing energy-efficient, low-cost and full color flat panel OLED displays, which reveals their outstanding photo and electroluminescent (PL and EL) properties [1e3] These metal complexes were displaying an efficient electron transport and light emission, higher thermal stability and ease of sublimation [4,5] The aluminum complex with 8-hydroxy-quinoline and its derivatives (Alq3) were excellent metal-chelate complexes used widely as emitting materials and electron transporting materials in OLED applications [1,6] Thus in comparisons with Alq3, transition metal complexes of Schiff bases were being extensively reported to * Corresponding author E-mail address: mahadevan.kmm@gmail.com (K.M Mahadevan) Peer review under responsibility of Vietnam National University, Hanoi exhibit excellent luminescent properties and hence, they have gathered much attention [7e13] However, as far as their device fabrication is concerned, metal complexes need to possess high solubility in organic solvents Therefore most of the complexes could not be used for fabricating EL devices In this regard, there were some reports to improve the properties like solubility, stability and electron transporting capability by incorporating flexible alkyl chain in the molecules [14] Thus, the presence of alkyl groups was found to increase the polarity and solubility of the complexes in organic solvents Since three primary colors such as red, green and blue were used for full color displays for white light emission [15e20], we report in the present work the synthesis of structurally very appropriate, low cost bis(1[(4-butylphenyl)imino]methyl naphthalen-2-ol) Ni(II) complex (4) and its use as green light emitting material The highly favorable and very much essential physical properties such as excellent solubility, stability and electron transporting capability http://dx.doi.org/10.1016/j.jsamd.2016.07.002 2468-2179/© 2016 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) M Srinivas et al / Journal of Science: Advanced Materials and Devices (2016) 324e329 325 were achieved The present report was based on our previous significant investigation on LED materials [21e24] MP: 68e70 C IR (KBr) n, cmÀ1 ¼ 3054 (]CeH), 2928 (CeH), 2854 (CeH), 1619 (C]N), 1214 (CeO) Experimental 2.3 Synthesis of Ni(II) complex (4) 2.1 Materials and methods A mixture of hot ethanolic solution of the ligand (3) (2.34 g; 0.72 mmol), NiCl2.6H2O (0.5 g; 0.36 mmol) and triethylamine (0.426 g; 0.72 mmol) was refluxed for h The resulting mixture was left overnight at room temperature The desired Ni(II) complex precipitated out as green colored solid was filtered, washed with ethanol and dried in vacuum This solid was then recrystallized from a mixture of chloroform and hexane (1:1 v/v) to get green crystals of the complex (4) Yield: 80%: MP: 218e220 C IR (KBr) n, cmÀ1 ¼ 3054 (]CeH), 2925.9 (CeH), 1598 (C]C), 1615 (C]N), 1213 (CeO) Commercially available chemicals & reagents obtained from Sigma Aldrich were used for synthesis of ligand and complex All solvents were reagent grade and used without further purification Melting points of the ligand and complex were determined by electrothermal apparatus in open capillaries and were uncorrected The FT-IR spectra were recorded by scan method in the range of 4000e500 cmÀ1 with an Agilent FT-IR Spectrometer The X-ray intensity data were collected at a temperature of 296 K on a Bruker Proteum CCD diffractometer equipped with an X-ray generator operating at 45 kV and 10 mA, using CuKa radiation of wavelength 1.54178 Å Diffused reflectance spectrum was recorded using l35 PerkineElmer UVeVisible Spectrometer The photoluminescence (PL) measurement was performed on a Jobin Yvon Spectroflourimeter Fluorolog-3 equipped with 450 W Xenon lamp as an excitation source Scanning electron microscopy (SEM) pictures were taken using Hitachi table top, Model TM 3000 Results and discussion 3.1 Synthesis Initially the schiff base 1-[(4-butylphenyl)imino]methyl-naphthalen-2-ol (3) was obtained by the reaction of equimolar quantity of 4-butylaniline (1) and 2-hydroxynapthalene-1-carbaldehyde (2) at room temperature stirring in dry ethanol in the presence of acetic acid as catalyst The obtained product ligand (3) was purified by recrystallization with ethanol and used for the preparation of Ni(II) complex(4) The complex was prepared by using ligand 1[(4-butylphenyl)imino]methyl-naphthalen-2-ol (3) and NiCl2.6H2O in ethanol in the presence of triethylamine as catalyst The solvent chloroform:hexane (1:1) mixture was found to be suitable solvent system for recrystallization of Ni(II) complex as green crystals The reaction sequence for the synthesis was as depicted in Fig The structure of green colored Ni(II) complex was established from single crystal X-ray diffraction studies (Fig 2a) 2.2 Synthesis of ligand 1-[(4-butylphenyl)imino]methyl naphthalen-2-ol (3) 4-Butylaniline (1) (1 g; 0.6 mmol) dissolved in 20 ml of dry ethanol was stirred for 30 at room temperature 2Hydroxynapthalene-1-carbaldehyde (2) (1.15 g; 0.6 mmol) dissolved in 20 ml of dry ethanol was added to the above solution drop wise with constant stirring in presence of catalytic amount of acetic acid The mixture was then stirred for 4e5 h at room temperature, during which the solution changes to yellow color The progress of the reaction was monitored by TLC using pet-ether and ethyl acetate (70:30 v/v) as mobile phase After completion, the reaction solution was concentrated by rotary evaporator which resulted ligand as yellow solid The yellow solid was washed with petroleum ether (10 ml  2) and then dried under vacuum Yield: 92%, 3.2 Crystal X-ray diffraction studies A yellow colored rectangle shaped single crystal of dimensions 0.28  0.25  0.22 mm of the title compound was chosen for an Xray diffraction study The X-ray intensity data were collected at a H3C NH2 AcOH H O HO H3C N Ethanol RT, Stirr, 4-5 h HO H3C N NiCl2 6H O Ethanol, Et3 N H3C Reflux, h N HO Ni O O N Green CH3 Fig Reaction scheme for the synthesis of bis(1[(4-butylphenyl)imino]methyl naphthalen-2-ol) Ni(II) complex (4) 326 M Srinivas et al / Journal of Science: Advanced Materials and Devices (2016) 324e329 Fig (a) ORTEP of the Ni(II) complex with thermal ellipsoids drawn at 50% probability (b) Packing of the Ni(II) metal complex exhibiting layered when viewed down along the ‘a’ axis The dotted line represents intramolecular hydrogen bonds temperature of 296 K on a Bruker Proteum2 CCD diffractometer equipped with an X-ray generator operating at 45 kV and 10 mA, using CuKa radiation of wavelength 1.54178 Å Data were collected for 24 frames per set with different settings of (0 and 90 ), keeping the scan width of 0.5 , exposure time of s, the sample to detector distance of 45.10 mm and 2q value at 46.6 A complete data set was processed using SAINT PLUS [25] The structure was solved by direct methods and refined by full-matrix least squares method on F2 using SHELXS and SHELXL programs [26] All the nonhydrogen atoms were revealed in the first difference Fourier map itself All the hydrogen atoms were positioned geometrically and refined using a riding model with Uiso(H) ¼ 1.2Ueq and 1.5Ueq (O) After ten cycles of refinement, the final difference Fourier map showed peaks of no chemical significance and the residuals saturated to 0.0371 The geometrical calculations were carried out using the program PLATON [27] The molecular and packing diagrams were generated using the software MERCURY [28] The details of the crystal data and structure refinement, bond lengths and bond angle values are given in Tables 1e3 The values were in good agreement with the standard values Fig 2a represents the ORTEP of the Ni(II) complex with thermal ellipsoids drawn at 50% probability The Ni(II) complex crystallizes in the triclinic space group P À with a single molecule in the asymmetric unit The average NieO and NieN bond lengths were 1.8237(12) Å and 1.9046(15) Å respectively The naphthalene ring was essentially planar with a maximum deviation of 0.013(2) Å for C12 The naphthalene ring system makes a dihedral angle of 45.58(7) with the plane of the phenyl ring The butyl group adopts an extended conformation and was twisted from the plane of the phenyl ring and adopts a ỵsynclinal conformation as indicated by the torsion angle value of 62.7(2) The structure exhibits an intermolecular hydrogen bond of the type CeH/O which helps in stabilizing the crystal structure The C20eH20/O14 hydrogen bond has a length of 3.238(2) Å and an angle of 141 with symmetry code ỵ x, y z The molecules appear to be stacked and this hydrogen bond when viewed along the axis links the molecules to form chains (Fig 2b) 3.3 UVevisible spectrum The diffuse reflectance (DR) spectrum of Ni(II) metal complex was measured in the range 200e1100 nm was shown in Fig 3a The spectrum exhibited major peaks in the range 300e400 nm due to transition between valence band and conduction band The weak M Srinivas et al / Journal of Science: Advanced Materials and Devices (2016) 324e329 Table Crystal data and structure refinement details CCDC number Empirical formula Formula weight Temperature Wavelength Reflns for cell determination q range for above Crystal system Space group Cell dimensions Table Bond angles ( ) CCDC 1494102 C42H40N2NiO2 663.47 296(2) K 1.54178 Å 2671 Volume Z Density (calculated) Absorption coefficient F000 Crystal size q range for data collection Index ranges Reflections collected Independent reflections Absorption correction Refinement method Data/restraints/parameters Goodness-of-fit on F2 Final [I > 2s(I)] R indices (all data) Largest diff peak and hole 5.68 e64.56 Triclinic PÀ1 a ¼ 6.9964(8) Å, b ¼ 8.4494(9) Å, c ¼ 15.0136(17) Å, a ¼ 89.521(4) , b ¼ 86.914(4) , g ¼ 67.266(4) 817.32(16) Å3 1.348 Mg mÀ3 1.164 mmÀ1 350 0.28  0.25  0.22 mm 5.68 to 64.56 À8 h 7, À9 k 9, À17 l 8455 2671 [Rint ¼ 0.0351] Multi-scan Full matrix least-squares on F2 2671/0/215 1.051 R1 ¼ 0.0371, wR2 ¼ 0.0996 R1 ¼ 0.0390, wR2 ¼ 0.1037 0.354 and À0.442eÅÀ3 17 absorption in the UVeVisible region is expected to arise due to transitions involving extrinsic states such as surface traps or defect states or impurities The KubelkaeMunk theory was used to determine the energy band gap of the synthesized Ni(II) metal complex from DR spectra The intercept of the tangents to the plots of [F(R∞)hn]1/2 versus photon energy hn was shown in Fig 3b The KubelkaeMunk function F(R∞) and photon energy (hn) was calculated by following equations [29]: FR ị ẳ hn ẳ 327 ð1 À R∞ Þ2 2R∞ (1) 1240 (2) l where R∞; reflection coefficient of the sample, l; the absorption wavelength Table Bond lengths (Å) Atoms Length Atoms Length Ni1eO14 Ni1eO14 Ni1eN2 Ni1eN2 O14eC13 N2eC3 N2eC15 C20eC19 C20eC15 C15eC16 C4eC13 C4eC3 C4eC5 C16eC17 C13eC12 1.8237(12) 1.8237(12) 1.9046(15) 1.9046(15) 1.302(2) 1.310(2) 1.438(2) 1.385(3) 1.392(2) 1.391(2) 1.402(3) 1.419(2) 1.452(2) 1.384(3) 1.432(3) C17eC18 C5eC6 C5eC10 C18eC19 C18eC21 C6eC7 C10eC9 C10eC11 C11eC12 C21eC22 C7eC8 C23eC22 C23eC24 C9eC8 1.394(3) 1.412(3) 1.420(3) 1.396(3) 1.508(2) 1.378(3) 1.413(3) 1.426(3) 1.356(3) 1.541(3) 1.400(3) 1.516(3) 1.524(3) 1.371(3) Atoms Angle Atoms Angle O14eNi1eO14 O14eNi1eN2 O14eNi1eN2 O14eNi1eN2 O14eNi1eN2 N2eNi1eN2 C13eO14eNi1 C3eN2eC15 C3eN2eNi1 C15eN2eNi1 C19eC20eC15 C16eC15eC20 C16eC15eN2 C20eC15eN2 C13eC4eC3 C13eC4eC5 C3eC4eC5 N2eC3eC4 C17eC16eC15 O14eC13eC4 O14eC13eC12 179.999(1) 91.84(6) 88.16(6) 88.17(6) 91.84(6) 179.999(1) 128.34(11) 115.12(14) 123.60(12) 121.20(11) 119.91(16) 119.47(16) 119.65(15) 120.88(15) 119.21(16) 119.81(16) 120.56(16) 126.80(16) 119.91(16) 124.16(16) 116.36(15) C4eC13eC12 C16eC17eC18 C6eC5eC10 C6eC5eC4 C10eC5eC4 C17eC18eC19 C17eC18eC21 C19eC18eC21 C20eC19eC18 C7eC6eC5 C9eC10eC5 C9eC10eC11 C5eC10eC11 C12eC11eC10 C11eC12eC13 C18eC21eC22 C6eC7eC8 C22eC23eC24 C8eC9eC10 C9eC8eC7 C23eC22eC21 119.45(16) 121.56(16) 117.15(16) 123.77(16) 119.07(16) 117.67(16) 121.85(16) 120.46(16) 121.46(16) 121.54(17) 120.15(17) 120.74(17) 119.11(17) 121.61(17) 120.93(17) 112.95(15) 120.91(18) 114.37(17) 121.19(18) 119.05(17) 114.60(15) The measured band gap energy for complex was found to be 1.83 eV This indicated that the allowed direct transition was responsible for the inter band transitions The Eg values were mainly depends on the preparation methods and experimental conditions which could favor or inhibit the formation of structural defects, which were able to control the degree of structural orderedisorder of the materials and consequently, the number of intermediary energy levels within the band gap 3.4 Photoluminescence Fig 4a and b shows the emission and excitation spectra of Ni(II) metal complex (4) phosphor which was recorded at room temperature In the excitation spectrum at 519 nm emission shows two major excitations at 394 and 465 nm along with several sharp lines at 450 nm, 468 nm and 481 nm, indicating that this phosphor can be effectively excited by UV LED chip (360e400 nm) Emission spectra exhibits sharp and broad peak at 519 nm (green) Further, it was noticed that there was no change in emission spectra for different excitations The Commission Internationale De L'Eclairage (CIE) 1931 chromaticity co-ordinates [30,31] for complex were calculated at excitation 394 nm The estimated CIE values for different excitations were tabulated in inset of Fig 4d The location of the color coordinates were represented in the CIE chromaticity diagram by solid circle sign (star) indicates the color of the complex From this figure, one could see that the color of a complex was located in the green region Further it was proved from the image (Fig 4c) of the complex dissolved in ethanol that the complex exhibited green color when it was placed in UV chamber at longer wavelength (z366 nm) Therefore, this complex could be a promising green component for possible applications in the field of OLEDs 3.5 SEM Surface morphology of the complex was studied by using Scanning Electron Microscope and images are shown in Fig SEM micrograph exhibits cutting edge rod shape with smooth surface morphology for the complex The width and length of the rods were observed to be 10e30 mm and 100e300 mm respectively Further from the SEM images it was confirmed that a nonuniformly distributed rods like structure was obtained for this 328 M Srinivas et al / Journal of Science: Advanced Materials and Devices (2016) 324e329 Fig (a) Diffuse reflectance spectrum of Ni(II) complex (b) Plot of [F(R∞)hn]1/2 versus photon energy (hn) complex It was evidenced by the earlier reports that organic materials having similar morphology with varied particle size showed photoluminescence properties [32,33] Conclusion In summery we have successfully tuned the Ni(II) complex structure (4) to get significant green light emission The final structure was characterized by single crystal XRD studies The CIE graph indicated that this phosphor might be very useful for green light emitting diodes and solid state lighting applications The complex was also found to be highly soluble in most of the common organic solvents find itself suitable for fabricating EL devices From the ease of synthesis it could be served as economically cheaper material for developing green component in white OLEDs, and also in many environment remedy applications Thus, based on Fig Photoluminescence spectra and CIE graph of the Ni(II) complex (a) Emission spectrum at lexi 394 nm (b) Excitation spectrum at lemi 519 nm (c) The image of the Ni(II) complex solution in ethanol at longer wavelength (z366 nm) (d) CIE graph of the complex (4) M Srinivas et al / Journal of Science: Advanced Materials and Devices (2016) 324e329 329 Fig SEM images of the Ni(II) complex displayed cutting edge rod shape with smooth surface morphology predicted excellent photophysical properties, it could be used as promising green light emitting diode in developing strong electroluminescent materials for flat panel displays applications as an emissive layer Acknowledgment The author Prof K M Mahadevan acknowledges to DST, New Delhi SERB, for the financial support Reference No: SB/EMEQ-351/ 2013 Dated 29-10-2013 The authors are grateful to the 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C23eC22eC 21 119 .45 (16 ) 12 1.56 (16 ) 11 7 .15 (16 ) 12 3.77 (16 ) 11 9.07 (16 ) 11 7.67 (16 ) 12 1.85 (16 ) 12 0 .46 (16 ) 12 1 .46 (16 ) 12 1. 54 (17 ) 12 0 .15 (17 ) 12 0. 74 (17 ) 11 9 .11 (17 ) 12 1. 61( 17) 12 0.93 (17 ) 11 2. 95 (15 ) 12 0. 91( 18)... 91. 84( 6) 17 9.999 (1) 12 8. 34 (11 ) 11 5 . 12 ( 14 ) 12 3.60 ( 12 ) 12 1 .20 (11 ) 11 9. 91( 16) 11 9 .47 (16 ) 11 9.65 (15 ) 12 0.88 (15 ) 11 9. 21 ( 16) 11 9. 81( 16) 12 0.56 (16 ) 12 6.80 (16 ) 11 9. 91( 16) 12 4 .16 (16 ) 11 6.36 (15 ) C4eC13eC 12 C16eC17eC18... Ni1 eO 14 Ni1 eO 14 Ni1 eN2 Ni1 eN2 O14eC13 N2eC3 N2eC15 C20eC19 C20eC15 C15eC16 C4eC13 C4eC3 C4eC5 C16eC17 C13eC 12 1. 823 7 ( 12 ) 1. 823 7 ( 12 ) 1. 9 046 (15 ) 1. 9 046 (15 ) 1. 3 02( 2) 1. 310 (2) 1. 43 8 (2) 1. 385(3) 1. 3 92( 2)