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]

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,*,

a Department of Chemistry, Kuvempu University, P G Centre, Kadur 577548, India

b Department of Chemistry, University College of Science, Tumkur University, Tumakuru 572 103, India

c Prof C.N.R Rao Centre for Advanced Materials Research, Tumkur University, Tumkur 572 103, India

d Forensic Science Laboratory, Madivala, Bengaluru 560068, India

e Department 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 4 December 2016

Received in revised form

20 February 2017

Accepted 26 February 2017

Available online 3 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

2468-2179/© 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

Journal of Science: Advanced Materials and Devices 2 (2017) 156e164

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 cm
1using 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

1 mol of cyclohexane-1,2-diamine (1), 2 mol of

salicylaldehydes/2-hydroxy-1-naphthaldehyde (2a-c) and 1 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 1 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 1 and aldehydic group of 2a-c

Fig 2 FTIR spectrum of the complex bis(salicylidene)cyclohexyl-1,2-diamino zinc(II) metal complex (3a) (i) Wave numbers from 2500 to 4000 cm
1(ii) Wave numbers

1

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 1 H 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)

d6.0e8.5 ppm.

M Srinivas et al / Journal of Science: Advanced Materials and Devices 2 (2017) 156e164 158

1H NMR, Scanning Electron Microscope (SEM) and EDAX analysis.

The reaction scheme for the synthesis of 3a-c complexes is given

inFig 1

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)ncm
1:

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)ncm
1: 2928(CeH), 1603(C]N),1246(CeO)

1H NMR (400 MHz, DMSO-d6) 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)ncm
1: 2932(CeH), 1619(C]N), 1238(CeO)

1H NMR (400 MHz, DMSO-d6) 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 cm
1), carbon-oxygen stretching frequency (between 1238 and 1246 cm
1) and carbon-hydrogen stretching frequency (at 2932 cm
1) 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

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

Fig 6 Emission colours of 3a-c in ethanol as photographed under a long wave length (z366 nm) UV light.

M Srinivas et al / Journal of Science: Advanced Materials and Devices 2 (2017) 156e164 160

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)

Fig 7 (i)Diffuse reflectance spectra and (ii) Energy band gaps from the Plot of [F(R )hn] 1/2 versus photon energy (hn) of 3(aec).

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

Fig 8 Finger marks developed with the prepared complexes 3a-c and standard black and white powder on various surfaces The photos were captured under UV light (~366 nm) (i)

M Srinivas et al / Journal of Science: Advanced Materials and Devices 2 (2017) 156e164 162

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

following equations:

FðR∞Þ ¼ð1
R∞Þ2

hn¼1240

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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