Synthesis, crystal structure and excellent photoluminescence properties of copper (II) and cobalt (II) complexes with Bis (1[(4-butylphenyl)imino]methyl naphthalen-2-ol) Schiff base

Encouraged by the photoluminescence properties of metal complexes derived from Schiff bases, an attempt has been made to synthesize and study the photoluminescent properties of Copper (I[r]

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Original Article

V.B Nagavenia,b, K.M Mahadevana, G.R Vijayakumarc,*, H Nagabhushanad, S Naveene,

N.K Lokanathf

aDepartment of Chemistry, Kuvempu University, P G Centre, Kadur, 577548, India bDepartment of Chemistry, Government Science College, Chitradurga, 577501, India

cDepartment of Chemistry, University College of Science, Tumkur University, Tumakuru, 572 103, India dProf C.N.R Rao Centre for Advanced Materials Research, Tumkur University, Tumakuru, 572 103, India eDepartment of Basic Sciences, School of Engineering& Technology, Jain University, Bangalore 562 112, India fDepartment of Studies in Physics, Manasagangotri, University of Mysore, Mysuru, 570006, India

a r t i c l e i n f o

Article history:

Received 30 September 2017 Received in revised form 31 December 2017 Accepted 16 January 2018 Available online 31 January 2018 Keywords:

1[(4-butylphenyl)imino]methylnaphthalen-2-ol

Schiff base

Cu (II) and Co (II)complex Photoluminescence Single crystal XRD OLED

a b s t r a c t

Copper (II) and Cobalt (II) metal complexes (4a- and 4b-complexes) using Schiff base ligand 1-[(4-butylphenyl)imino]methyl naphthalen-2-ol (3) have been synthesized The single crystals of Copper (II) and Cobalt (II) complex phosphors were grown and characterized by Fourier-Transform Infrared (FT-IR), single crystal X-ray diffraction (XRD), SEM (Scanning Electron Microscope) and EDS (Energy Dispersive X-ray spectroscopy) Photoluminescence study of the phosphors revealed the presence of excitation peaks at 333 nm and 360 nm for 4a-complex (lemi¼ 495 nm) and excitation peaks at 300 nm

and 360 nm for 4b-complex (lemi¼ 496 nm) The calculated CCT values of the complexes pointed out

that these materials can be used to obtain cold white light from the light emitting devices Diffuse reﬂectance spectra (DRS) showed the measured band gap energies of 1.78 eV and 1.44 eV for Cu (II) and Co (II) complexes, respectively It is concluded that the 4a- and 4b-complexes become white and blue green light emitting diodes respectively and will be useful in the development of strong electrolumi-nescent materials

© 2018 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/)

1 Introduction

New metal-organic ligand materials with efficient light emitting properties have attracted many researchers in recent years[1e3] due to their prospective applications as organic light-emitting di-odes (OLEDs)[4e6], light emitting electro-chemical cells (LEECs) [7], lumophores for cell imaging [8,9], conductors and semi-conductors [10e13] Some organic transition metal complexes, which exhibit strong luminescence at low driving voltages, are of much interest for designing a flat panel display system [14,15] Additionally, transition metal complexes of Schiff bases have received significant importance in material chemistry owing to

their wide range of applications[16e18] Metal complexes of Zn (II), Pt (II), Co (III), Cu (II) and Ag (I) with Schiff bases were described as luminescence organic materials that are used in organic optoelec-tronics [17,18e21] In particular, cobalt complex [Co(PLAGeỵ2H)(NH3)3]NO3 was reported as a higher

photo-luminescence material [22] and the methanolic solution of {[Cu(2,5-pdc)(H2O)4]$H2O} complex exhibitsﬂuorescence at room

temperature [23] Further, the novel Cu (I) complex of [Cu(ABPQ)(DPEphos)]BF4 was shown to be a good

photo-luminescence material[24e27] Recently, copper (II) complex with (1R, 2R)-cyclohexanediamine derived Schiff base has been pre-pared and used as a dye for light emitting devices due to its excellent luminescence properties[28]

Encouraged by the photoluminescence properties of metal complexes derived from Schiff bases, an attempt has been made to synthesize and study the photoluminescent properties of Copper (II) and Cobalt (II) complex phosphors (4a- and 4b-complexes) of * Corresponding author

E-mail address:vijaykumargr18@gmail.com(G.R Vijayakumar) Peer review under responsibility of Vietnam National University, Hanoi

Contents lists available atScienceDirect

Journal of Science: Advanced Materials and Devices j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / j s a m d

https://doi.org/10.1016/j.jsamd.2018.01.001

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Schiff base ligand 1-[(4-butylphenyl)imino]methyl naphthalen-2-ol (3) As suggested by their structures, the prepared complexes include high electron mobility and are soluble in organic solvents The present work has been carried out based on our earlier in-vestigations on OLED materials, which yielded prominent results [29,30]

2 Experimental

2.1 Materials and methods

Chemicals and reagents used for the synthesis and analysis were procured from a commercial supplier, Sigma Aldrich, India Analytical grade solvents were purchased from SD Fine Pvt Ltd., Mumbai, India and used as such without puriﬁcation Melting points of the synthesized compounds were carried out by an open capillary method using electrical heating apparatus The wave number range from 650 cm1to 4000 cm1was used for recording Fourier-Transform Infrared (FT-IR) spectra of the samples using Agilent FT-IR Spectrometer Proton Nuclear Magnetic Resonance (NMR) spectrum of the ligand was recorded using JEOL 500 MHz NMR instrument X-ray diffraction data were collected at RT using Bruker Proteum2 CCD diffract meter Diffuse Reﬂectance Spectra (DRS) of the samples were recorded using l35 Perkin-Elmner

UVeVisible Spectrophotometer Photoluminescence (PL) spectra of the prepared samples were recorded on Jobin Yvon Spectro-ﬂourimeter Fluorolog-3 which used 450W Xenon lamp as an excitation source Surface morphology of the compounds was studied using Hitachi (Table top, Model TM 3000) Scanning Elec-tron Microscope (SEM)

2.2 Synthesis of ligand 1-[(4-butylphenyl)imino]methyl naphthalen-2-ol (3)

The solution of 4-butylaniline (1) (6.0 mmol) in dry ethanol (15 mL) was stirred on a magnetic stirrer at room temperature An ethanolic solution (15 mL) of 2-hydroxynapthalene-1-carbaldehyde (2) (6.0 mmol) was added slowly to the stirring so-lution of followed by the addition of catalytic amount of acetic acid The resulting reaction mixture was continued to be stirred for h at the same temperature, during which the color of the solution changes to yellow The TLC (mobile phase pet-ether:ethyl acetate 70:30 v/v) revealed the product formation, after which the solvent was removed under vacuum and triturated with petroleum ether (15 mLx2) to afford the ligand (3) as yellow solid The yellow solid was subjected to recrystallization using hot ethanol Yield: 90%, mp: 68e70 C IR (KBr), (n, cm1): 3054(¼CeH), 2928(CeH),

2854(CeH), 1619(C]N), 1214(CeO).1H NMR(400 MHz, DMSO-d 6): d(ppm) 0.92 (t, J¼ 6.00 Hz, 3H), 1.29e1.31 (m, 2H), 1.55e1.57 (m, 2H), 2.63 (t, J¼ 6.00 Hz, 2H), 6.99 (d, J ¼ 7.20 Hz, 1H), 7.31e7.35 (m, 3H), 7.52e7.56 (m, 3H), 7.78 (d, J ¼ 5.20 Hz, 1H), 7.91 (d, J ¼ 7.20 Hz, 1H), 8.48 (d, J¼ 6.40 Hz, 1H), 9.64 (s, 1H)

2.3 Synthesis of Cu (II) complex (4a-complex)

A mixture of ligand (5 mmol) in hot ethanol, CuCl2$2H2O

(2.5 mmol) and triethylamine (5 mmol) was refluxed for 8e10 h The Cu (II) complex (4a-complex) was precipitated out as brown solid after cooling the reaction mixture to room temperature The solid wasfiltered, washed with ethanol and dried in vacuum The product 4a-complex was further purified by recrystallization using a mixture of chloroform and hexane (1:1 v/v) which afford brown crystals of the complex Yield: 80%; mp: 180e184C; IR (KBr), (n,

cm1): 3021(CeH), 2926(¼CeH), 2854(CeH), 1614(C]N), 1215(CeO), 526(MO)

2.4 Synthesis of Co (II) complex (4b-complex)

A mixture of ligand (5 mmol) in hot ethanol, CoCl2$6H2O

(2.5 mmol) and triethylamine (5 mmol) was reﬂuxed for 8e10 h The Co (II)complex (4b-complex) was precipitated out as dark vi-olet colored solid after cooling the reaction mixture to room tem-perature The product 4b-complex was worked up and recrystallized as described in the earlier procedure (4a-complex) Yield: 80%; mp: 235e240 C; IR (KBr), (n, cm1): 3022(CeH),

2926(¼CeH), 2851(CeH), 1613(C]N), 1253(CeO), 526(MO) Results and discussion

3.1 Formation of 4a- and 4b-complexes

The reaction between amino group of 4-butylaniline (1) and aldehyde group of 2-hydroxynapthalene-1-carbaldehyde (2) afforded the Schiff's base ligand 1-[(4-butylphenyl)imino]methyl-naphthalen-2-ol (3) [29] Reaction conditions and the use of catalyst were the key parameters in obtaining product with satisfactory yield In the preparation of 3, acetic acid was used as a catalyst The product was puriﬁed by recrystallization, charac-terized by FT-IR and NMR spectral analysis and utilized for further step to get Cu (II) and Co (II) complexes Metal salts CuCl2$2H2O

and CoCl2$6H2O were reacted with in the presence of

triethyl-amine as a mild base to afford the 4a-complex and 4b-complex, respectively The reaction scheme followed for the synthesis of Cu (II) complex (4a-complex) and Co (II) complex (4b-complex) is shown inFig During thefinal reaction one equivalent of metal reacted with two equivalents of ligand which produced a four coordinated complex (4a-complex and 4b-complex) The solvent mixture chloroform:hexane (1:1) was found to be suitable for the recrystallization of both 4a-complex and 4b-complex Structures of the complexes were established from single crystal X-ray diffraction studies (Fig 2) The single crystal X-ray diffraction confirms the trans orientation for 4a-complex and cis orientation for 4b-complex with respect to two N and two O donor atoms of the ligand Even though the ligand is same for the two complexes, the opposite orientation of the donor atoms in 4a-complex and 4b-complex may be due to unequal size and different interaction of metals with the ligand The square planar geometry with reference to metal and donor atoms in the 4a-complex has been confirmed from the bond angles of O1eCu1eN1 (~90), O1eCu1eO1 (180)

and N1eCu1eN1(180) But the planarity was almost lost in the

4b-complex, which resulted in distorted square pyramidal geom-etry having cobalt at the apical position and four donor atoms of the ligand occupy the basal equatorial positions with slight changes in the regular bond angles The L-M-L bond angles (º) of 4b-complex were found to be O2eCo1eO1 (88.27), O2eCo1eN1

(155.98), O1eCo1eN1 (91.54), O2eCo1eN2 (92.28), O1eCo1eN2

(151.66) and N1eCo1eN2(99.05)

3.2 Single crystal X-ray diffraction studies

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MERCURY [34] software programs were used for geometrical calculation and generation of molecular/packing diagrams respectively

3.3 UV-Visible spectrum

Diffuse reﬂectance (DR) study can be useful to know the conductance properties of the materials and hence the study was carried out for the synthesized materials DR spectra were measured in the range 200e1000 nm (Fig 3) The major peaks were identiﬁed in the range 300e400 nm, which may be accounted for the transition of electrons from the valence band to the conduction band The transitions involving surface traps or impurities resulted in the weak absorption in UV-Visible region of the spectra Band gaps of the complexes were determined from the respective DR spectra by using KubelkaeMunk theory The intercept of the tangents to the plots of [F(R∞)hn]1/2 versus photon energy hn was shown in Fig 3(b) The KubelkaeMunk function F(R∞) and photon energy (hn) were calculated using the following equations:[35]

FRị ẳ1 Rị

2R∞ (1)

hn¼1240

l (2)

where R∞ ¼ sample reﬂection coefﬁcient; l ¼ absorption

wavelength

The measured band gap energy for 4a-complex and 4b-complex were found to be 1.78 eV & 1.44 eV respectively It was clearly indicated from the band gap energy values that the allowed direct transitions were responsible for the inter band transitions Reaction conditions used for the preparation of materials were directly responsible for the Eg values These conditions may inhibit or favor the formation of defect, control the degree of structural order-disorder and consequently the number of intermediary energy levels within the band gap

3.4 Photoluminescence

PL Spectra of the 4a- and 4b-complexes recorded at room temperature are given in Fig The photoluminescence study revealed that the excitation peaks at 333 nm and 360 nm for 4a-complex atlemi¼ 495 nm, and the peaks at 300 nm and 360 nm for

4b-complex at lemi ¼ 496 nm were appeared The Commission

International De I-Eclairage (CIE) 1931 chromaticity co-ordinates [36,37] under different excitations were calculated for the

NH2

H3C

H O

O H

Ethanol

Reflux, h AcOH

N

H3C

O H

MCl2.6H2O Ethanol, Et3N RT, Stirr, 4-5 h

1 2

3

+

4a-complex 4b-complex

M=Cu M=Co

CH3

N O C

H3

N O

M

CH3

N O C

H3

N O M

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Table

Crystal data and structure reﬁnement details for 4a- and 4b-complexes

4a-complex 4b-complex

CCDC number CCDC 1559055 CCDC 1559054

Empirical formula C42H40CuN2O2 C42H40CoN2O2

Formula weight 668.30 663.69

Temperature 296 (2) K 296 (2) K

Wavelength 1.54178 Å 1.54178 Å

Reﬂns for cell determination 2659 4976

qrange for above 5.63to 64.26 4.91to 64.75

Crystal system Triclinic Monoclinic

Space group P P 21/c

Cell dimensions a¼ 6.9703 (5) Å, b ¼ 8.5861 (7) Å, c ¼ 14.9618 (12) Å a¼ 88.264 (2),b¼ 87.177 (2),g¼ 66.336 (2)

a¼ 10.4699 (6) Å, b ¼ 26.1070 (15) Å, c ¼ 13.0709 (8) Å a¼ 90.00,b¼ 107.623 (2),g¼ 90.00

Volume 819.09 (11) Å3 3405.1 (3) Å3

Z

Density(calculated) 1.355 Mg m3 1.295 Mg m3

Absorption coefﬁcient 1.248 mm1 4.247 mm1

F000 351 1396

Crystal size 0.26 0.22 0.21 mm 0.28 0.25 0.22 mm

qrange for data collection 5.63to 64.26 4.91to 64.75

Index ranges 8 h

10 k 10 17 l 17

12 h 12 30 k 29 13 l 15

Reﬂections collected 11,026 32,941

Independent reﬂections 2684 [Rint¼ 0.0439] 5649 [Rint¼ 0.0602]

Absorption correction multi-scan multi-scan

Reﬁnement method Full matrix least-squares on F2 Full matrix least-squares on F2

Data/restraints/parameters 2684/0/215 5649/0/426

Goodness-of-ﬁt on F2 1.072 1.168

Final [I> 2s(I)] R1¼ 0.0349,uR2¼ 0.0899 R1¼ 0.0535,uR2¼ 0.1440

R indices (all data) R1¼ 0.0351,uR2¼ 0.0902 R1¼ 0.0647,uR2¼ 0.1662

Largest diff peak and hole 0.370 and0.564 e Å3 1.338 and0.335 e Å3

200 400 600 800 1000

0 5 10 15 20 25 30 35 40 45

R

eflectan

ce (%

)

Wavelength (nm) 4a-complex

(a)

2 3 4 5 6

0 100 200 300

400 (b)

[F

(R

)hv

]

1/

2

Energy (eV) 1.78 eV

4a-complex

200 400 600 800 1000

0 5 10 15 20 25 30 35 40

(a)

R

eflectan

ce (%

)

Wavelength (nm) 4b-complex

2 3 4 5 6

0 50 100 150 200 250 300 350

(b)

[F(R)hv

]

1/

2

Energy (eV) 1.44 eV

4b-complex

Fig (a) Diffuse reﬂectance spectra of Cu (II) complex (4a-complex) and Co (II) complex (4b-complex) (b) Plots of [F(R∞)hn]1/2against photon energy (hn) for these two samples,

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complexes The estimated CIE values were found to be X¼ 0.33474; Y¼ 0.34496 for 4a-complex and X ¼ 0.22326; Y ¼ 0.34695 for 4b-complex The location of the color coordinates were represented in the CIE chromaticity diagram by solid circle sign (star) indicates the color appearance of the sample powder From thisﬁgure, one can conclude that the 4a-complex and 4b-complex located in the white and blue green region so that these complexes become promising white and blue green light emitting diodes, respectively

The correlated color temperature (CCT) is a speciﬁcation of the color appearance of the light emitted by a light source, relating its color to the color of light with respect to a reference light source when heated up to a speciﬁc temperature, in Kelvin (K) The CCT rating for a lamp or a source is a general‘‘warmth’’ or ‘‘coolness’’

measure of its appearance However, opposite to the temperature scale, lamps with a CCT rating below 3200 K are usually considered ‘‘warm’’ sources, while those with a CCT above 4000 K are considered‘‘cool’’ in appearance.

Correlated Color Temperature (CCT) can be estimated by Planckian locus, which is only a small portion of the (x, y) chro-maticity diagram and there exist many operating points outside the Planckian locus CCT values were determined as described in the earlier reports[25]and were found to be ~5400 K (atlexi¼ 333 nm)

and ~5755 K (atlexi¼ 300 nm) for 4a- and 4b-complexes

respec-tively Since the CCT values for both complexes were above 4000 K, these could be used as a cold light emitting source in commercial lightening applications

300 350 400 450

0.2 0.4 0.6 0.8 1.0 1.2 1.4

PL

Intensi

ty (a.

u.) ×10

6

Wavelength (nm)

λλemi = 495 nm

4a-complex 333 nm

360 nm

(a)

260 280 300 320 340 360 380 400

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

360 nm

300 nm

PL Intensity

(a

.u.)

×

10

6

Wavelength (nm)

λemi = 496 nm

4b-complex

(a)

400 450 500 550 600

0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6

PL Intensity (a.u.) ×10

6

Wavelength (nm)

λexi = 333 nm

4a-complex

457 nm

550 nm

(b)

375 400 425 450 475 500 525 550

1.5 1.6 1.7 1.8 1.9

λexi = 300 nm

496 nm

P

L

Intensity

(a

.u.)

×

10

6

Wavelength (nm) 411 nm

4b-complex

(b)

(c)

0.0 0.2 0.4 0.6 0.8

0.0 0.3 0.6 0.9

CIE Y

CIE X

λexi = 300 nm

4b-complex

(c)

0.0 0.2 0.4 0.6 0.8

0.0 0.3 0.6 0.9

CIE Y

CIE X

λexi= 333 nm

4a-complex

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3.5 SEM and EDS studies

The surface morphology of 4a- and 4b-complexes was studied by using scanning electron microscope images and composition of the complex was revealed by energy dispersive X-ray spectroscopy (EDS) analysis of the respective complexes The SEM and EDS im-ages of 4a- and 4b-complexes are shown inFig Broken pieces with smooth surface morphology for 4a-complex and soft surface stone like morphology for 4b-complex were observed in the respective SEM images Further these metal complexes were sub-jected to EDS analysis to conﬁrm the composition of the complexes and experimental values as shown in Table The EDS spectra (Fig 5) conﬁrm the elemental composition of copper, carbon and oxygen in 4a-complex and cobalt, carbon and oxygen in 4b-complex

4 Conclusion

In summary, the two transition metal complexes of Cu (II) and Co (II) containing Schiff base ligand 1-[(4-butylphenyl)imino] methyl naphthalen-2-ol (3) were synthesized using mild reaction conditions, and their physical properties were studied The single crystals of the complexes were grown and structures were identi-ﬁed by the single crystal XRD analysis The structures 4a-complex

and 4b-complex were further characterized by FT-IR, SEM and EDS studies

PL spectrum of 4a-complex showed the excitation peaks at 333 nm and 360 nm for the emission wavelength of 495 nm Similarly, 4b-complex yielded the excitation peaks at 300 nm and 360 nm for the emission wavelength of 496 nm The band gap energies were determined from the DRS study to be 1.78 eV (4a-complex) and 1.44 eV (4b-(4a-complex) The SEM images of the ma-terials revealed smooth broken piece surfaces for 4a-complex and the soft surface stone morphology for 4b-complex Both the com-plexes were highly soluble in THF, DMF and DMSO and found to be suitable for fabricating electroluminescent devices Due to their excellent photophysical properties, these complexes would be useful as the promising white and blue green light emitting diodes in fabricating strong electroluminescent materials for ﬂat panel display applications as an emissive layer

Fig (a) SEM and (b) EDS images of the 4a- and 4b-complexes Table

Elemental compositions of the 4a- and 4b-complexes obtained from EDS analysis

4a-complex 4b-complex

Element Weight % Atomic % Element Weight % Atomic %

C 81.49 90.88 C 80.69 88.12

O 8.32 6.97 O 12.87 10.55

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Acknowledgements

The authors are thankful to the DST-Purse, UGC-CPEPA and Institution of Excellence, Vijnana Bhavana, University of Mysore, Manasagangotri, Mysore, for providing the single-crystal X-ray diffraction data Authors acknowledges to DST, New Delhi SERB, for theﬁnancial support, Reference No: SB/EMEQ-351/2013 and DST-FIST No.SR/FST/ETT-378/2014

Supplementary information (SI)

Crystallographic data for the structural analysis were deposited with the CCDC numbers 1559055 (4a-complex) and 1559054 (4b-complex) Copy of this information will be obtained free of charge from‘The Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK (faxỵ44 1223 336033; e-mail:deposit@ccdc.cam.ac.uk orhttp:// www.ccdc.cam.ac.uk)

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