Synthesis, photoluminescence and forensic applications of blue light emitting azomethine-zinc (II) complexes of bis(salicylidene) cyclohexyl-1,2-diamino based organic ligands

Kang, Generation of blue light-emitting zinc complexes by band-gap control of the oxazolylphenolate ligand system: syntheses, char- acterizations, and organic light emitting device appli[r]

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Original Article

Synthesis, photoluminescence and forensic applications of blue light emitting azomethine-zinc (II) complexes of bis(salicylidene)

cyclohexyl-1,2-diamino based organic ligands

M Srinivasa,d, G.R Vijayakumarb, K.M Mahadevan a, H Nagabhushanac,*,

H.S Bhojya Naike

aDepartment of Chemistry, Kuvempu University, P G Centre, Kadur 577548, India

bDepartment of Chemistry, University College of Science, Tumkur University, Tumakuru 572 103, India cProf C.N.R Rao Centre for Advanced Materials Research, Tumkur University, Tumkur 572 103, India dForensic Science Laboratory, Madivala, Bengaluru 560068, India

eDepartment of Studies and Research in Industrial Chemistry, Kuvempu University, Jnana Sahyadri, Shankaraghatta 577 451, India

a r t i c l e i n f o

Article history:

Received December 2016 Received in revised form 20 February 2017 Accepted 26 February 2017 Available online March 2017

Keywords:

Azomethine-zinc (II) complexes Photoluminescence

OLED Salicylaldehyde

2-Hydroxy-1-naphthaldehyde Fingerprint

a b s t r a c t

Various azomethine-zinc(II) complexes (3a-c) of bis(salicylidene)cyclohexyl-1,2-diamino organic ligands were synthesized by one pot reaction of salicylaldehydes/2-hydroxy-1-naphthaldehyde (2 eq), cyclo-hexyl-1,2-diamine (1 eq) and zinc acetate (1 eq) in methanol solvent at reflux temperature The syn-thesized complexes were characterized by FTIR,1H NMR, and SEM Their photophysical properties such as Photoluminescence (PL) and Diffused Reflectance Spectra (DRS) were studied PL studies revealed that the emission peaks of the complexes in both solution and solid states appeared to occur at 395e600 nm and emitted blue light The band gap energies determined from DRS were 2.98 eV (3a), 2.91 eV (3b), and 2.73 eV (3c) Based on these results, we ascertain that these Zn(II) complexes can serve as a suitable non-dopant blue light emitting compound forflat panel display applications Latent fingerprint detection study indicated that the powder compounds show good adhesion and finger ridge details without background staining The demonstrated method can be applied to detectfingerprints on all types of smooth surfaces

© 2017 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

Metal complexes with organic ligands have been used in solid state lighting and ﬂat panel display applications due to their excellent electroluminescent properties [1,2] The organic light emitting diodes (OLED) based on small molecules exhibit a lot of advantages as they are thinner and lighter than liquid crystal display materials and they can work without a backlight Hence, these OLED materials could become potential replacements for LCD materials[3] Such metal complexes have been reported for use as strong electroluminescent materials, which display an efﬁcient electron transport ability, stronger light emission, higher thermal stability, and ease of sublimation [4,5] In particular, various azomethine-zinc complexes have been extensively investigated as

blue light emitting luminescent materials[6e9] Recently, some low-cost zinc complexes have been reported as novel materials for white organic light-emitting devices[10,11] Certain Zn(II) com-plexes of 2- (2-hydroxyphenyl)benzothiazolates ligands were used as blue-light emitting, electron transporter and also as host mate-rials in OLEDs [12e16] Thermally activated high performance green OLEDs of Zn(II) complexes have also been reported, and their high solubility in most organic solvents plays a major role as far as their device fabrications are concerned[17]

Hence, keeping in view of obtaining highly efﬁcient, thermally stable and highly soluble active emissive materials for OLEDs, we report herein the synthesis of novel bis(salicylidene)/2-hydroxy-1-napthlidene cyclohexyl-1,2-diamino Zn(II) metal complexes (3a-c) and their photo-luminescent properties The synthesized com-plexes have also been explored for forensic applications as ﬁnger-print developing dyes Apart from the excellent photoluminescent properties and forensic applications of the synthesized compounds, the scope of the work includes mild reaction conditions and better

* Corresponding author

E-mail address:bhushanvlc@gmail.com(H Nagabhushana)

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

http://dx.doi.org/10.1016/j.jsamd.2017.02.008

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yield with easy of one pot synthesis by using readily available sal-icylaldehydes/2-hydroxy-1-napthaldehyde, cylohexyl-1,2-diamine and zinc acetate

2 Experimental

2.1 Materials and instruments

Commercially available chemicals & reagents from Sigma Aldrich were used for the synthesis of 3a-c All the solvents were reagent grade and used without further purification Melting points of the complexes were determined by an electrothermal apparatus in open capillaries and were uncorrected The1H NMR was recorded at 400 MHz in DMSO-d6as solvent and TMS as an internal standard using Varian 400 NMR Autosampler (Varian, California, USA) The FT-IR spectrum was recorded by a scaning method in the range of 4000e500 cm1using Thermo Fisher iS 10 Nicolet FT-IR spectrometer (Thermo Fisher Scientific Inc Ger-many) Diffused Reflectance Spectra were recorded using l35 Perkin-Elmner UVeVisible Spectrometer (Perkin Elmer, Inc Wal-tham, USA) The photoluminescence (PL) measurement was per-formed on a Jobin Yvon Spectroflourimeter Fluorolog-3 (Jobin Yvon Inc 3880 Park Avenue, Edison, NJ 08820, USA) equipped with a 450 W Xenon lamp as an excitation source Scanning electron microscope (SEM) and energy dispersive X-rays analyzer (EDAX) measurements were performed on a VEGA3LMUVG13171475 of TESCAN, Brno, Kohoutovice, CZ

2.2 Synthesis of Zn(II) metal complexes (3a-c)

The metal complexes (3a-c) were prepared by the reaction of mol of cyclohexane-1,2-diamine (1), mol of salicylaldehydes/2-hydroxy-1-naphthaldehyde (2a-c) and mol of zinc acetate in methanol solvent at reﬂux temperature for 5e8 h The reaction was monitored by thin layer chromatography (TLC) using pet-ether and ethyl acetate (70:30 v/v) as mobile phase After completion of the reaction, the colour precipitate obtained was washed with meth-anol (10 mL x 2) and dried over vacuum to get pure Zn(II) metal complexes (3a-c) All zinc(II)metal complexes were sparingly sol-uble in methanol and ethanol, and they showed good solubility in DMSO and DMF solvents

3 Results&discussion 3.1 Synthesis

The reaction of two equivalents of salicylaldehydes/2-hydroxy-1-naphthaldehyde (2a-c) with one equivalents of cyclohexane-1,2-diamine (1) to afford Schiff base which in situ reacts with mol of zinc acetate in methanol solvent at reﬂux temperature to yield the ﬁnal bis(salicylidene)cyclohexyl-1,2-diamino based Zn(II) metal complexes (3a-c) [18] The salicyladehyde (2a), 5-chlorosalicyladehyde (2b) and 2-hydroxy-1-naphthaldehyde (2c) were employed for the synthesis of bis(salicylidene)cyclohexyl-1,2-diamino zinc(II)azomethine complex (3a), bis(5-chlorosalicylidene) cyclohexyl-1,2-diamino zinc(II) azomethine complex (3b) and bis(2-hydroxy-1-napthalidene)cyclohexyl-1,2-diamino zinc(II) azome-thine complex (3c) respectively The formation of metal complex and the disappearance of Schiff base ligand were monitored by using TLC The C]N groups[19,20]was formed by the condensation between amine groups of compound and aldehydic group of 2a-c

Fig Reaction scheme for the synthesis of zinc metal complexes 3a-c

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The resulted Schiff bases further react with zinc acetate to give metal complex 3a-c The two neutral coordinate covalent bonds and two anionic bonds were formed from ligand to zinc metal in the resulted tetrahedral tetra coordinated complex (Fig 1) Two neutral coordi-nate covalent bonds formed from two nitrogen atoms of the aza

methane group to zinc and two anionic covalent bonds formed by twoeOH group of the same ligand to zinc and acetic acid molecule was eliminated as by-product All the zinc complexes were sparingly soluble in methanol and showed good solubility in DMSO and DMF The resultingﬁnal complexes were characterized using FTIR,

Fig 3.1H NMR spectrum of the complex bis(salicylidene)cyclohexyl-1,2-diamino zinc(II)metal complex (3a) Chemical shift values are in the range (i)d0e10 ppm (ii)

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1H NMR, Scanning Electron Microscope (SEM) and EDAX analysis. The reaction scheme for the synthesis of 3a-c complexes is given inFig

3.2 Spectral characterization of 3a-c

Bis(salicylidene)cyclohexyl-1,2-diamino zinc (II) complex (3a): appearance: white solid; Yield: 90%; mp> 300C; IR (KBr)ncm1: 2932(CeH), 1630(C]N), 1370(N]O), 1264(CeO); 1H NMR (400 MHz, DMSO-d6)d(ppm): 1.392e1.372 (m, 4H, CH2of cyclo-hexyl), 1.886 (m, 2H, cyclohexyl CH2), 2.480e2.422 (m, 2H, cyclo-hexyl CH), 3.172 (m, 2H, cyclocyclo-hexyl CH2), 6.301e7.203 (m, 8H, Aromatic), 8.305 (s, 2H,eCH]N)

Bis(5-chlorosalicylidene)cyclohexyl-1,2-diamino zinc(II) metal complex (3b): appearance: Yellow colour solid; Yield: 80%; mp> 300C; IR (KBr)ncm1: 2928(CeH), 1603(C]N),1246(CeO). 1H NMR (400 MHz, DMSO-d

6) d (ppm): 1.382e1.337 (m, 4H, cyclohexyl CH2), 1.879 (m, 2H, cyclohexyl CH2), 2.480e2.402 (m, 2H, cyclohexyl CH), 3.182 (m, 2H, cyclohexyl CH2), 6.595

(d, J¼ 8.8HZ,2H, Aromatic), 7.082 (d, J¼ 10.40 HZ,2H, Aromatic), 7.300 (s, 2H, Aromatic), 8.468 (s, 2H,eCH]N)

Bis(2-hydroxy-1-napthalidene)cyclohexyl-1,2-diamino zinc(II) metal complex (3c): appearance: Yellow colour solid; Yield: 90%; mp> 300C; IR (KBr)ncm1: 2932(CeH), 1619(C]N), 1238(CeO). 1H NMR (400 MHz, DMSO-d

6) d (ppm): 1.486 (m, 4H, CH2 of cyclohexyl), 1.974 (m, 2H, cyclohexyl CH2), 2.656 (m, 2H, cyclohexyl CH), 3.310 (m, 2H, cyclohexyl CH2), 6.905 (d, J¼ 12.40 HZ, 2H, Aromatic), 7.109e7.672 (m, 8H, Aromatic), 8.053 (d, J ¼ 8.0 HZ, 2H, Aromatic), 9.209 (s, 2H,eCH]N)

FTIR analysis of the complexes shows the presence of carbon-nitrogen (eC]Ne) stretching frequency (between 1603 and 1630 cm1), carbon-oxygen stretching frequency (between 1238 and 1246 cm1) and carbon-hydrogen stretching frequency (at 2932 cm1) bands (Fig 2) The absence of eOH band of the salicylaldehydes further conﬁrmed the oxygen-metal bond for-mation in the metal complexes Since all metal complexes were able to record1H NMR spectra, a diamagnetic characteristic can be attributed to the tetrahedral structure of the complexes 3a-c

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Additionally, the absence of eOH group and the presence of oxygen-metal bond in the products were ascertained from the1H NMR spectra of the complexes (Fig 3)

3.3 SEM and EDAX studies

Surface morphology is one of the characteristics of the materials and from which the properties of the compounds present can be predicted Hence, surface morphology of the complexes 3a-c was analyzed by using SEM and composition of the complexes was studied using EDAX (Fig 4(ii)) The SEM images exhibit a cutting edge broken stone structure for 3a (size 10e20mm length), a broom brushwood structure for 3b (10e50mm length) and a brokenﬂat tiles morphology for the 3c (20e50mm length) complex SEM im-ages depicted a non-uniformly distributed structure for all the obtained complexes Our earlier report revealed the metal complex with a similar rod shape structure exhibited photoluminescence [21] The EDAX spectrum of 3a-c shows the elemental compositions of zinc, carbon, and oxygen atoms (Fig 4), which further conﬁrms the structures of these metal complexes

3.4 Photoluminescence study

The PL spectra can reﬂect some important information such as surface defects, oxygen vacancies, photo induced charge carrier separation and recombination processes in the prepared materials Excitation spectra, emission spectra and Commission International de I'Eclairage (CIE) spectra of the compounds were depicted in Fig 5(i), (ii), and (iii), respectively Excitation spectra of the samples

were obtained by monitoring the emission at the wavelength of 600 nm for 3a, and 513 nm for 3b and 3c The excitation spectrum consists of different emission peaks in the range of 300e500 nm PL emission spectra were recorded in the range of 450e750 nm under

Fig Photoluminescence spectra of the complexes 3a-c: (i) Excitation spectra; (ii) Emission spectra; (iii) CIE graphs

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UV excitation at 443 nm (3a), 394 nm (3b), and 378 nm (3c) wavelengths The correlated colour temperature (CCT) was one of the essential parameter to know the colour appearance of the light emitted by a light source with respect to a reference light source when heated up to a speciﬁc temperature, in Kelvin (K) CCT was estimated as described in our earlier report[22]and was found to be ~5239 K, ~9061 K, and ~4237 K for 3a, 3b, and 3c, respectively Since the CCT values are greater than 5000 K, the present com-pounds could be useful for artiﬁcial production of white light in illumination devices

Commission International de I'Eclairage (CIE) 1931 x-y chro-maticity diagrams of the 3a-c samples were depicted inFig 5(iii), which indicated the excitation As shown in the inset of these Figures, the CIE chromaticity coordinates were located in the light green region for 3a and the blue region for 3b and 3c respectively To identify technical applicability of these blue green and blue emissions, the CCT was determined from CIE coordinates It was also noticed that the compounds displayed blue emission in a solution state when dissolved in ethanol, with images captured under UV light (Fig 6)

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3.5 DRS spectra

The diffuse reﬂectance (DR) spectra of Zn(II) metal complexes (3a-c) measured in the range 200e1100 nm were shown inFig 7(i) The spectra exhibited major peaks in the range 300e400 nm due to the transition between the valence and conduction band The weak

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 3a-c from the DRS spectra The intercept of the tangents to the plots of [F(R∞)hn]1/2versus photon energy hn was shown in Fig 7(ii) The KubelkaeMunk

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function F(R∞) and photon energy (hn) were calculated by the following equations:

FRị ẳ1 Rị

2R∞ (1)

hn¼1240

l (2)

where R∞is the reﬂection coefﬁcient of the sample, andlis the absorption wavelength

The measured band gap energy for these materials was found to be 2.98 eV (3a), 2.91 eV (3b), and 2.73 eV (3c) This indicated that the allowed direct transition was responsible for the inter band transitions The Egvalues were mainly dependent on the prepara-tion methods and experimental condiprepara-tions They could favor or inhibit the formation of structural defects, allowing for control of the degree of structural orderedisorder of the materials and hence the number of intermediate energy levels within the band gap 3.6 Fingerprint analysis

Manyfluorescent compounds have been used for developing weak prints under ultraviolet light [23,24], which can assist a forensic scientist for liftingfingerprints from the scene of crime and also a defence scientist for establishing the identity of deceased soldiers as well as of war prisoners[25] Since these metal com-plexes 3a-c exhibit fluorescence and emit deep blue light, a fingerprint analysis was carried out for all the complexes by selecting various substrates such as glass bulb, glass petri plate, steel spoon and steel cup at normal conditions Herein, we focused on how these compounds can be used to develop prints and visu-alized under ultraviolet light tofind out their feasibility to use in forensicfingerprint detection

Thefinger marks were collected from a single donor of 26 years old Before collection of thefingerprints, the hand of the donor was washed neatly with soap and was dried The right hand thumb was pressed on the various surfaces of different substrates and the sample in powder along with references (black and white powders) was applied on these surfaces with the help of a camel hair brush Since the compounds 3a-c emited blue light under UV light, the developedfinger marks were photographed using a digital camera seen under a long wave length UV light (366 nm) The photographs on various surfaces illustrated inFig 8indicate that the compounds 3a-c in powder showed good adhesion and finger ridge details without background staining Interestingly, the finger marks resulted from the use of the tested compounds 3a-c were found to be more pronounced than those of the standard black and white powders It was also demonstrated that the developed method could be efficiently applied to detect fingerprints on all type of smooth surfaces, having potentially important applications in latentfingerprint detection

4 Conclusion

In summary, we have developed the rapid one pot synthesis of salicylaldehydes/2-hydroxy-1-naphthaldehyde (2 eq), cyclohexyl-1,2-diamine(1 eq) and zinc acetate (1 eq) in methanol at reflux temperature Photoluminescence studies revealed that the emis-sion peaks of the complexes in both the solution and solid states appeared to occur at 395e600 nm and emitted blue light The band gap energies were determined from DRS to be 2.98 eV(3a), 2.91 eV (3b), and 2.73 eV (3c) These results indicate that the Zn(II)plexes can serve as suitable non-dopant blue light emitting com-pounds for use inflat panel displays Latent fingerprint detection

study of the title compounds indicates that theﬁnger marks using the (3a-c) compounds were observed to be more obvious than those of the currently using standard black and white powders Acknowledgements

The author Prof K M Mahadevan acknowledges DST, New Delhi SERB, for ﬁnancial support Reference No: SB/EMEQ-351/2013 Dated 29e10-2013 The authors thank Basavaraj R B and Darshan G P for their technical assistance in recording the spectra of the title compounds

Appendix A Supplementary data

Supplementary data related to this article can be found athttp:// dx.doi.org/10.1016/j.jsamd.2017.02.008

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