Synthesis and photoluminescence analysis of europium(III) complexes with pyrazole acid and nitrogen containing auxiliary ligands

SPECTROSCOPY LETTERS

https://doi.org/10.1080/00387010.2020.1817093

Synthesis and photoluminescence analysis of europium(III) complexes with

pyrazole acid and nitrogen containing auxiliary ligands

a

b

a

Jyoti Khanagwal , S. P. Khatkar , Priyanka Dhankhar , Manju Bala , Rajesh Kumar , Priti Boora and

V. B. Taxak

a

b

Department of Chemistry, Maharshi Dayanand University, Rohtak, India; University Institute of Engineering and Technology (UIET),

Maharshi Dayanand University, Rohtak, India

ABSTRACT

ARTICLE HISTORY

Four red photoluminescent europium(III) complexes have been synthesized by using fluori-

nated carboxylate ligand, 1-(4-methoxyphenyl)-5-(trifluoromethyl)-1H-pyrazole-4-carboxylic

acid as main ligand and bathophenanthroline, 1,10-phenanthroline and 2,2-bipyridyl as aux-

iliary ligands, through solution precipitation approach. The complexes possess an optimum

thermal stability, which is sufficient for organic light emitting diodes fabrication. Under ultra-

violet excitation, the complexes display an intense emission peak of europium ion, which

makes them a promising red color emitter in display devices. Internal quantum efficiency,

color coordinates, Judd–Ofelt parameters, and energy transfer mechanism have been

explored. The investigated antimicrobial and antioxidant activity suggest that the synthe-

sized complexes are potent antimicrobial and antioxidant agents.

Received 21 August 2020

Accepted 22 August 2020

KEYWORDS

Antioxidant; luminescence;

Judd–Ofelt parameters;

optical; spectroscopic

RESEARCH HIGHLIGHTS

ꢀ

A new ligand L has been synthesized by an ecofriendly method.

Four Eu(III) complexes have been synthesized through a facile solution precipitation

method by using ligand L and auxiliary ligands.

ꢀ

CHN, EDAX, IR, NMR, TG-DTG, DR, UV–vis, and PL spectroscopic techniques and antioxi-

dant analysis are employed.

Potential candidate for display devices, laser devices, and antimicrobial agents.

Introduction

near-infrared region. As a consequence of parity

rule, 4f–4f electronic transitions are laporte for-

bidden transitions, which are responsible for low

molar absorptivity and less possibility of direct

excitation of lanthanide ion. This inconvenience

can be controlled by coordinating the central lan-

thanide ion to the light harvesting organic chro-

mophores, which sensitize the lanthanide ion by

antenna effect. On the other hand, the lumines-

cence efficiency is also determined by the energy

transfer process from the excited triplet energy

levels of organic ligand to the emissive level of

lanthanide ion and better compatibility between

both energy levels directly improves the lumines-

Up to now, a surge of investigations by material

scientists is accelerating on lanthanide lumines-

cent materials day by day owing to their interest-

ing optoelectronic properties such as high

luminescence efficiency, static luminescent loca-

tion, long luminescence decay time, and good

[

1]

color purity,

which endorsed their potential

applications in display systems, solid state light-

[

2]

ing systems mainly white organic light emitting

[

3]

[4]

diodes (W-OLEDs), security inks and tags,

biomedical sensors, optical cell imaging,

[

5]

and

[

6]

biological systems. Therefore, the synthesis and

the development of lanthanide chelates with high

luminescence efficiency have become a hot

research topic at present time. The luminescence

of lanthanide ion is associated with 4f–4f

electronic transitions in ultra-visible and

[

7]

cence efficiency of lanthanide ion. Currently,

among different types of organic ligands, one of

the most promising light harvesting chromo-

phores, fluorinated aromatic carboxylic acid

CONTACT V. B. Taxak

v_taxak@yahoo.com

Department of Chemistry, Maharshi Dayanand University, Rohtak, India.

ß 2020 Taylor & Francis Group, LLC

2

J. KHANAGWAL ET AL.

ligand containing extended p-conjugation, more

binding modes, high thermal, and light stability,

attracts much attention for complexation with

The energy transfer mechanism explicates the

luminescence efficiency of the complexes and

biological investigations suggest that the synthe-

sized complexes are potent antimicrobial and

[

1,8]

lanthanide ion.

The organic ligands possessing

C–F oscillators diminish the energy loss in non-

antioxidant

agents.

The

synthesized

radiative decay process and facilitate the energy

europium(III) complexes are considered as poten-

tial candidates in laser and solar technology,

advanced luminescent devices, new electronic

technology, and fabrication of tricolor based

white light emitting diodes. The pyrazole-based

complexes play a great role for inhibition of

growth of microbes, therefore, the complexes

exhibit promising applications in the biological

and pharmacological fields.

[

9]

transfer mechanism.

It can be acknowledged

that the high-energy O–H, N–H, and C–H oscil-

lators of solvent and binding ligands in coordin-

ation sphere quench the metal excited states

through non-radiative decay process leading to

shorter lifetime of excited states resulting into

lower photoluminescence efficiency of lanthanide

[

10]

complexes.

But the solvent molecules get easily

substituted by highly conjugated N-containing

auxiliary ligands, which control non-radiative

[

11]

Experimental

decay process via synergistic effect.

Also, aux-

iliary ligands provide high rigidity, chemical sta-

bility, and satisfy the coordination environment

Materials and methods

A series of luminescent europium(III) complexes

were synthesized through a facile solution pre-

cipitation method by utilizing highly pure

[

12]

around lanthanide ion.

Among lanthanide

ions, the europium(III) ion gains burgeoning

attention due to environment independent line

like emission at 615 nm and long luminescent

decay time.

(

99.9%) europium(III) nitrate pentahydrate, 2,2-

bipyridyl, bathophenanthroline, and 1,10-phenan-

throline. The chemicals and solvents used in the

synthesis of complexes and ligand were of high

purity and analytical reagent grade. The chemi-

cals were purchased from easily available com-

mercial source, Merck (Mumbai, India) and used

as such without further purification. 2,2-

Diphenyl-1-picrylhydrazyl hydrate (DPPH) and

ascorbic acid consumed in the estimation of anti-

oxidant property of complexes were purchased

from Central drug house (CDH), New Delhi,

India. The double-distilled water was used for the

synthesis of complexes.

Keeping the above facts in mind, the main flu-

orinated aromatic carboxylic acid, 1-(4-methoxy-

phenyl)-5-(trifluoromethyl)-1H-pyrazole-4-car-

boxylic acid (L) and its corresponding

europium(III) complexes, Eu(L) .2H O (C1),

3

2

Eu(L) .batho (C2), Eu(L) .phen (C3), and

3

Eu(L) .bipy (C4) have been synthesized by using

3

bathophenanthroline (batho), 1,10-phenanthro-

line (phen), and 2,2-bipyridyl (bipy) as auxiliary

ligands, through solution precipitation approach.

The synthesized ligand (L) and europium(III)

complexes are characterized by various techni-

ques, energy dispersive X-ray analysis (EDAX),

elemental analysis, proton nuclear magnetic res-

Carbon, hydrogen and nitrogen (CHN) ana-

lysis were performed on Perkin Elmer 2400 CHN

elemental analyzer (PerkinElmer, Waltham, MA)

and EDAX analysis was done on Ametek EDAX.

The complexometric titration with ethylenediami-

netetraacetic acid (EDTA) was used to detect the

europium(III) ion content in the complexes. The

FT-IR spectra of europium(III) complexes and

ligand (L) were recorded with the help of Perkin

Elmer Spectrum 400 spectrometer (PerkinElmer,

Waltham, MA) in wavenumber range of

1

onance ( H-NMR), carbon nuclear magnetic res-

13

onance ( C-NMR), Fourier transform infrared

FT-IR), ultraviolet–visible absorption (UV–vis),

(

thermogravimetric (TG) and differential thermog-

ravimetric (DTG) analyses, diffuse reflectance

(

DR), scanning electron microscopy (SEM), and

photoluminescence spectroscopy (PL). The lumi-

nescence parameters such as radiative lifetime

(

s), non-radiative rate (A_n_r_a_d), radiative rate

A ), Judd–Ofelt parameters (X ), and internal

ꢁ1

4000–400 cm , by dispersing the solid sample

rad

k

1

13

quantum efficiency (g) are explained in detail.

on dry KBr disks. H-NMR and C-NMR of

SPECTROSCOPY LETTERS

3

complexes were recorded on Bruker Avance II

00 NMR spectrometer (Bruker, Fremont, CA) in

carboxylate:

4-methoxyphenyl

hydrazine

4

(0.15 mL) and ethyl 2-((N-trifluoromethyl-N-

methylamino)methylene)-3-oxobutanoate (0.978

mmol) were dissolved in ethanol (5 mL) and

refluxed for about 3 hr, then evaporated under

reduced pressure. The obtained residue was dis-

solved in ethyl acetate, washed three times with

saturated aqueous solution of sodium bicarbon-

ate, and subsequently extracted with ethyl acetate.

The extracts were dried with the help of anhyd-

6

deutrated dimethylsulfoxide (d -DMSO) solvent

with respect to reference tetramethylsilane (TMS)

13

while C-NMR of ligand (L) was recorded in

CDCl (deuterated chloroform). UV–vis absorp-

3

tion and solid diffuse reflectance spectral analysis

of ligand (L) and complexes were performed on

Shimadzu UV-3600 plus UV–visible spectropho-

tometer (Shimadzu, Kyoto, Japan) in wavelength

range of 200–800 nm. Thermogravimetric investi-

gation was done on SDT Q600 thermal analyzer

rous magnesium sulfate (MgSO ) and then

4

evaporated under reduced pressure. The oily

product of ethyl 1-(4-methoxyphenyl)-5-(trifluor-

omethyl)-1H-pyrazole-4-carboxylate is obtained

under inert atmosphere of N gas with the heat-

2

ꢂ

ing rate of 20 C/min. The morphological features

were assessed by using Jeol JSM-6510 SEM (JEOL

Ltd., Tokyo, Japan). Furthermore, the phosphor-

escence and luminescence spectral data was col-

after

chromatography.

purification

of

extracts

through

Step 3: Synthesis of 1-(4-methoxyphenyl)-5-

trifluoromethyl)-1H-pyrazole-4-carboxylic acid

L): Ethanolic (2 mL) solution of potassium

lected

on

Hitachi

F-7000

fluorescence

(

spectrophotometer (Hitachi, Shiga, Japan)

employed with xenon lamp, which acted as a

source of radiations. The decay curves of euro-

pium complexes were determined by using soft-

ware (FL solution of F-7000) of the

spectrophotometer and color coordinates were

calculated with the help of MATLAB software.

The biological properties such as antimicrobial

and antioxidant were investigated through tube

dilution and DPPH methods, respectively.

hydroxide (4.07 mmol) and ethyl 1-(4-methoxy-

phenyl)-5-(trifluoromethyl)-1H-pyrazole-4-carb-

oxylate (0.604 mmol) was refluxed for about 6 hr.

The resulting mixture was allowed to evaporate

under reduced pressure. The aqueous solution of

hydrochloric acid (6 N) was added to obtained

residue to adjust the pH ꢃ1 resulting into white

solid was separated which was extracted with

ethyl acetate and dried over anhydrous MgSO₄.

The solid product was purified by chromatog-

raphy to obtain the pure white solid 1-(4-

methoxyphenyl)-5-(trifluoromethyl)-1H-pyrazole-

Synthesis of ligand 1-(4-methoxyphenyl)-5-

(trifluoromethyl)-1H-pyrazole-4-carboxylic acid (L)

The ligand L was synthesized in three step as

4

-carboxylic acid (L) and progress of reaction

[

13]

shown below (Fig. 1a)-

:

was checked by thin layer chromatography. 78%

yield of product (L) was received with 166 C

melting point. IR (KBr): cm

Step 1: Synthesis of ethyl 2-((N-trifluoro-

methyl-N-methylamino)methylene)-3-oxobuta-

noate: ethylacetoacetate (0.2 mL) was stirred

along with continuous dropwise addition of N-

ꢂ

ꢁ1

3443 (b), 2967

(

w), 2850 (w), 2764 (w), 2363 (w), 1709 (s), 1604

s), 1564 (s), 1462 (s), 1423 (s), 1374 (m), 1305

s), 1255 (s), 1183 (s), 1144 (s), 1074 (s), 1025

s), 962 (s), 922 (s), 838 (s), 777 (s), 744 (m), 667

m), 603 (m), 546 (s), 508 (w) 446 (w); H-NMR

DMSO, 400 MHz): d 3.85 (s, 3H, -OCH₃),

(

dimethoxymethyl)-1,1,1-trifluoro-N-methylme-

thanamine (0.3 mL) at room temperature. The

reaction mixture was left for about 16 hr with

constant stirring at room temperature and then

evaporated under reduced pressure. The resulting

product was purified by chromatography, which

produced the oily substance of ethyl 2-((N-tri-

fluoromethyl-N-methylamino)methylene)-3-

oxobutanoate.

1

7

.09–7.12 (d, 2H, Ar-H), 7.45–7.47 (d, 2H, Ar-

H), 8.21 (s, 1H, pyrazoleꢁCꢁH), 13.31 (s, 1H,

1

3

ꢁ

COOH) ppm; C-NMR (100 MHz, CDCl ): d

3

5.6 (OꢁCH ), 114.3 (ArꢁCꢁC ¼ O), 127.1

5

3

Step 2: Synthesis of ethyl 1-(4-methoxy-

phenyl)-5-(trifluoromethyl)-1H-pyrazole-4-

(ArꢁCH),

132.1

(ArꢁCN),

143.03

(pyrazoleꢁCH), 160.6 (ꢁCOOH) ppm. Anal. Cal.

4

J. KHANAGWAL ET AL.

Figure 1. Synthetic methods and chemical structures of [13] ligand 1-(4-methoxyphenyl)-5-(trifluoromethyl)-1H-pyrazole-4-carbox-

ylic acid (a) and europium(III) complexes C –C (b). The ligand gets deprotonated on addition of alkali solution and deprotonated

1

5

ligand is coordinated to europium(III) ion at ꢃ7 pH value.

SPECTROSCOPY LETTERS

5

for C H N O F : C, 50.36; H, 3.17; N, 9.79;

found: C, 50.32; H, 3.14; N, 9.78.

2362 (w), 2338 (w), 1640 (m), 1614 (s), 1581 (m),

1567 (s), 1552 (m), 1518 (s), 1492 (w), 1461 (m),

12

9 2 3 3

1

9

439 (s), 1402 (s), 1326 (s), 1292 (s), 1255 (s),

228 (m), 1182 (s), 1140 (s), 1078 (m), 1039 (s),

70 (s), 834 (s), 800 (s), 764 (m), 748 (w), 701

Synthesis of europium(III) complexes

Ethanolic solution of ligand (3 mmol) was added

to aqueous solution of europium nitrate (1 mmol)

with constant stirring on magnetic stirrer at

room temperature. The pH of resulting solution

was adjusted in the range of 6–7 with the add-

ition of dilute NaOH solution (0.05 M) dropwise.

Furthermore, the reaction mixture was stirred for

about 4 hr on the magnetic stirrer. The obtained

white crude product was filtered on suction

pump and subsequently washed with water, then

with ethanol three times to remove unreacted lig-

and from the reaction mixture. The filtered pre-

cipitates were dried in oven and finally in

vacuum desiccator to obtain the white powder of

C1 complex (Fig. 1b).

(

w), 644 (w), 628 (w), 604 (m), 572 (w), 545 (m),

1

5

4

6

05 (w), 478 (w), 464 (w); H-NMR (DMSO,

00 MHz): d 3.76 (s, 9H, -OCH ), 6.94–7.07 (d,

3

H, Ar-H), 7.75 (s, 14H, Ar-H), 8.04 (s, 3H,

pyrazoleꢁCꢁH), 8.32–8.54 (d, 6H,₁Ar-H),

3

9

.02–9.09 (s, 2H, Ar-H) ppm;

C-NMR

(

1

100 MHz, DMSO): d 55.4 (OꢁCH ), 113.9

3

ArꢁCꢁC ¼ O), 124.2 (ꢁCF ), 127.1 (ArꢁCH),

3

31.7

(ArꢁCN),

137.6

(pyrazoleꢁCH),

147.2–156.4 (ArꢁC¼Cꢁ), ppm. Anal. Cal. for

EuC H N O F : C, 53.78; H, 3.01; N, 8.36; Eu,

6

0

40 8 9 9

11.34; found: C, 53.76; H, 3.02; N, 8.34;

Eu, 11.32.

Eu(L) .phen (C3): White solid, yield 75%; IR

3

ꢁ1

(KBr): cm 3005 (w), 2961 (w), 2936 (w), 2839

(w), 2360 (w), 2340 (w), 1639 (m), 1620 (s), 1590

(s), 1565 (s), 1553 (m), 1519 (s), 1462 (m), 1430

(s), 1401 (w), 1324 (s), 1313 (s), 1293 (s), 1254

(s), 1230 (m), 1179 (s), 1134 (s), 1080 (m), 1038

(s), 971 (s), 863 (w), 841 (s), 800 (s), 768 (w),

749 (w), 701 (w), 668 (w), 630 (w), 603 (w), 572

Eu(L) .(H O) (C1): White solid, yield 70%; IR

3

2

ꢁ

1

(

KBr): cm 3409 (b), 3005 (w), 2936 (w), 2838

w), 2359 (w), 2341 (w), 1640 (w), 1582 (m),

1

8

564 (s), 1519 (s), 1463 (w), 1432 (s), 1404 (w),

327 (s), 1301 (w), 1292 (m), 1257 (s), 1228 (w),

180 (s), 1137 (s) , 1080 (m), 1040 (s), 971 (s),

1

36 (s), 799 (s), 749 (w), 683 (w), 630 (w), 604

(w), 547 (m), 507 (w), 477 (w), 465 (w); H-

NMR (DMSO, 400 MHz): d 3.77 (s, 9H,

1

(

w), 546 (m), 507 (w), 478 (w), 464 (w); H-

NMR (400 MHz, DMSO): d 3.78 (s, 9H,

OCH ), 6.99 (s, 3H, pyrazole –CꢁH), 7.13–7.14

ꢁOCH ), 6.96–7.11 (d, 6H, Ar-H), 7.47 (s, 8H,

3

ꢁ

Ar-H), 7.96 (s, 3H, pyrazoleꢁCꢁH), 8.40–8.41(d,

3

1

3

13

(

d, 12H, Ar-H) ppm;

C-NMR (100 MHz,

4H, Ar-H), 8.70 (s, 2H, Ar-H) ppm; C-NMR

DMSO): d 55.6 (O-CH ), 114.7 (>C ¼ O), 124.7

(100 MHz, DMSO): d 55.4 (OꢁCH ), 113.9

3

(

CF ), 131.8 (Ar -CN), 138.8 (pyrazoleꢁCH)

(ArꢁCꢁC ¼ O), 124.2 (ꢁCF ), 127.1–127.6

3

ppm. Anal. Cal. for EuC H N O F : C, 41.43;

(ArꢁCH), 131.7 (ArꢁCN), 137.3 (pyrazoleꢁCH),

142.1–159.6 (ArꢁC¼Cꢁ) ppm. Anal. Cal. for

EuC H N O F : C, 48.54; H, 2.72; N, 9.43; Eu,

3

6

28 6 11 9

H, 2.70; N, 8.05; Eu, 14.56; found: C, 41.41; H,

2.67; N, 8.08; Eu, 14.58.

4

8

32 8 9 9

The similar approach was adopted for the syn-

12.79; found: C, 48.55; H, 2.71; N, 9.41;

Eu, 12.79.

thesis of C2–C4 complexes along with the add-

ition of ethanolic solution of auxiliary ligands,

bathophenanthroline (1 mmol), 1,10-phenanthro-

line (1 mmol) and 2,2-bipyridyl (1 mmol),

respectively, in the resulting solution of aqueous

europium nitrate (1 mmol) and ethanolic solution

of ligand (3 mmol). To illustrate the triplet energy

level of ligand, the gadolinium complex of ligand

L [Gd(L) .2H O] (G ) was also synthesized by

Eu(L) .bipy (C4): White solid, yield 76%; IR

3

ꢁ1

(KBr): cm 3004 (w), 2959 (w), 2936 (w), 2837

(w), 2360 (w), 2341 (w), 1640 (w), 1619 (s), 1563

(s), 1518 (s), 1494 (w), 1461 (m), 1431 (s), 1401

(w), 1323 (s), 1292 (m), 1252 (s), 1231 (m), 1175

(s), 1137 (s), 1079 (s), 1040 (s), 970 (s), 833 (s),

800 (s), 766 (s), 741 (s), 703 (s), 664 (w), 628

(m), 603 (w), 574 (w), 546 (m), 505 (w), 478 (w),

3

2

1

adopting similar strategy as used for C1 complex.

464 (w); H-NMR (400 MHz, DMSO): d 3.76 (s,

Eu(L) .batho (C2): White powder, yield 78%;

IR (KBr): cm 3008 (w), 2942 (w), 2839 (w),

9H, ꢁOCH ), 6.95–7.07 (d, 6H, Ar-H), 7.76 (s,

3

ꢁ1

10H, Ar-H), 8.03 (s, 3H, pyrazoleꢁCꢁH),

6

J. KHANAGWAL ET AL.

1

3

8

.32–8.54 (d, 4H, Ar-H) ppm;

C-NMR

minimum quantity of complexes was sufficient to

prevent the growth of bacteria and fungi, thereby,

can be employed as commercial bactericidal and

(

100 MHz, DMSO): d 55.4 (OꢁCH ), 113.5

3

ArꢁCꢁC ¼ O), 124.3 (ꢁCF ), 127.3–127.7

3

[

14]

ArꢁCH), 131.5 (ArꢁCN), 137.6 (pyrazoleꢁCH),

fungicides.

The complexes can be used as

1

41.4–152.3 (ArꢁC¼Cꢁ) ppm. Anal. Cal. for

excellent bactericidal and fungicides.

EuC H N O F : C, 47.48; H, 2.77; N, 9.63; Eu,

46

32 8 9 9

1

3.06; found: C, 47.50; H, 2.76; N, 9.62;

Eu, 13.05.

Gd(L) .(H O) (G1): White solid, yield 60%;

In vitro antioxidant activity

3

2

1

2,2-Diphenyl-1-picrylhydrazyl hydrate (DPPH)

nitrogen centered stable free radical method was

employed to investigate the antioxidant activity

of ligand and its corresponding europium com-

plexes. The antioxidant activity of complexes was

studied by reaction of DPPH with antioxidant

samples in which the purple color of DPPH solu-

tion convert to yellow color, which produced a

significant absorbance at 517 nm with the help of

UV–vis spectrophotometer. The stock solutions

of tested samples and standard sample (ascorbic

acid) were prepared with different concentrations

such as 25 mg/mL, 50 mg/mL, 75 mg/mL, and

ꢁ

IR (KBr): cm

3412 (b), 3009 (w), 2936 (w),

2

838 (w), 2359 (w), 2343 (w), 1642 (w), 1582

(

m), 1564 (s), 1552 (m), 1520 (s), 1463 (w), 1431

s), 1404 (w), 1327 (s), 1302 (w), 1292 (m), 1257

s), 1228 (w), 1182 (s), 1137 (s) , 1080 (m), 1040

s), 971 (s), 836 (s), 799 (s), 749 (w), 683 (w),

630 (w), 604 (w), 546 (m), 507 (w), 478 (w), 464

(

w). Anal. Cal. for GdC₃₆H N O F : C, 41.22;

28

6 11 9

H, 2.69; N, 8.01; Gd, 14.99; found: C, 41.19; H,

.66; N, 7.99; Gd, 15.01.

2

Biological properties

1

00 mg/mL in ethanol. Then, 1 mL solution of

In vitro antimicrobial properties

each concentration of different samples and

standard sample was mixed with 1 mL DPPH

solution (3 mg/mL) in their respective flasks and

DPPH solution was considered as control. After

that, the mixtures were thoroughly stirred and

kept for incubation in dark for 30 min. After the

completion of incubation period, the absorbance

of different mixtures was recorded at 517 nm

against blank then it converted into percentage

scavenging activity by using Eq. (1). These inves-

tigations were carried out in triplicate containing

The antibacterial activity of the complexes and

ligand (L) was assessed against Gram-positive

and Gram-negative bacterial pathogens named as

Staphylococcus aureus (S. aureus), Escherichia coli

(E. coli), and Pseudomonas aeruginosa (P. aerugi-

nosa), respectively, whereas the antifungal activity

was evaluated against fungal pathogens such as

Aspergillus niger (A. niger) and Candida albicans

(C. albicans). The ciprofloxacin and fluconazole

were used as the standard drugs for estimation of

antibacterial and antifungal activity, respectively.

The dilutions of standard drugs, ligand and com-

plexes were done by dissolving in ethanol to pre-

pare 100 mg/mL concentration of solutions and

resulting solutions were left for incubation The

incubation periods for different pathogens were

9

9 blocks. IC₅₀(concentration implementing 50%

inhibition) values of samples were obtained from

the curve, which is plotted between percentage

scavenging activity (% SCA) and various concen-

trations of tested samples.

ꢂ

2

4

4 hr at 37 C (bacteria), 7 days at 25 C, and

The scavenging activity of DPPH was esti-

8 hr at 37 C for A. niger and C. albicans,

[15]

mated by given equation

:

respectively. The mediums such as double

strength nutrient broth I.P. (Indian

Scavenging activity ð%Þ

Pharmacopoeia) for antibacterial and Sabouraud

dextrose I.P. for antifungal activity were

employed for in vitro investigations. The anti-

microbial activity was determined in terms of

minimum inhibitory concentration (MIC) of

samples, which clearly pointed out that the

(1)

¼

½ðAbs

ꢁ Abs_s_a_m_p_l_eÞ=Abs_c_o_n_t_r_o_lꢄ ꢅ 100

control

where Abscontrol signifies the absorbance of

DPPH þ ethanol and Abssample represents the

absorbance of DPPH þ sample or standard.

SPECTROSCOPY LETTERS

7

Figure 2. Energy dispersive X-ray spectrum of C complex of europium(III) ion with main ligand 1-(4-methoxyphenyl)-5-(trifluoro-

4

methyl)-1H-pyrazole-4-carboxylic acid and auxiliary ligand 2,2-bipyridyl. The spectrum confirms the presence of carbon, nitrogen,

oxygen, fluorine and europium elements in C complex.

4

Table 1. Percentage analytical data (weight %) of free ligand 1-(4-methoxyphenyl)-5-(trifluoromethyl)-1H-pyr-

azole-4-carboxylic acid (main ligand), C1 complex of europium(III) ion with main ligand and complexes C2–C4

with main ligand and auxiliary ligands bathophenanthroline, 1,10-phenanthroline and 2,2-bipyridyl,

respectively.

Complexes

C (%) found (Cal.)

H (%) found (Cal.)

N (%) found (Cal.)

Eu (%) found (Cal.)

L

50.32 (50.36)

41.41 (41.43)

53.76 (53.78)

48.55 (48.54)

47.50 (47.48)

3.14 (3.17)

2.67 (2.70)

3.02 (3.01)

2.71 (2.72)

2.76 (2.77)

9.78 (9.79)

8.08 (8.05)

8.34 (8.36)

9.41 (9.43)

9.62 (9.63)

–

C1 [Eu(L)

C2 [Eu(L)

C3 [Eu(L)

C4 [Eu(L)

3

.2H

2

O]

14.58 (14.56)

11.32 (11.34)

12.79 (12.79)

13.05 (13.06)

.batho]

.phen]

.bipy]

L: 1-(4-methoxyphenyl)-5-(trifluoromethyl)-1H-pyrazole-4-carboxylic acid; batho: bathophenanthroline; phen: 1,10-phenanthroline;

bipy: 2,2-bipyridyl; C: carbon; H: hydrogen; N: nitrogen; Eu: europium; Cal: calculated. The calculated data are closed to the experi-

mentally recorded (found) data.

Results and discussion

Spectral analysis

Compositions and properties of complexes

The FT-IR spectra of ligand (L) (Fig. 3a) and

europium(III) complexes show a remarkable dif-

ferences. The spectra of all complexes C1–C4

resemble to each other indicating that the similar

mode of coordination in complexes, therefore, IR

spectrum of complex C4 is depicted in Fig. 3b as

representative of all complexes. The characteristic

absorption frequencies of ligand (L) and com-

plexes are displayed in Table 2. The absorption

bands of uncoordinated ligand (L) are situated at

The EDAX analysis authenticates the elemental

compositions (carbon, nitrogen, oxygen, fluorine,

and europium) possessed by the complexes,

which is illustrated by EDAX spectrum of C4

complex (Fig. 2). The CHN and complexometric

titration results (Table 1) are in good consonance

with each other, which clearly justify that the

molecular compositions of complexes are in

accordance with proposed formulae of the syn-

thesized complexes as given in Scheme 2. All the

europium complexes are soluble in DMSO, par-

tially soluble in chloroform, acetonitrile, and

ethanol while insoluble in diethyl ether.

ꢁ1

1

709, 1564, and 1183 cm

corresponding to

stretching vibrations of > C¼O, >C ¼ N, and

C–F groups, respectively. The IR spectrum of lig-

and (L) displays the absorption bands at 1305

ꢁ1

and 1144 cm , which are related to stretching

8

J. KHANAGWAL ET AL.

Figure 3. Infrared spectra of free ligand 1-(4-methoxyphenyl)-5-(trifluoromethyl)-1H-pyrazole-4-carboxylic acid (a) and europium(III)

complex C4 (b) with main ligand 1-(4-methoxyphenyl)-5-(trifluoromethyl)-1H-pyrazole-4-carboxylic acid and auxiliary ligand 2,2-

bipyridyl. The coordination of auxiliary ligand 2,2-bipyridyl and main ligand to europium(III) ion is confirmed by the spectral peaks

in C complex.

4

ꢁ

1

Table 2. The characteristics infrared spectral frequencies (cm ) of uncoordinated ligand 1-(4-methoxyphenyl)-5-(trifluoromethyl)-

H-pyrazole-4-carboxylic acid (main ligand) and its corresponding complexes, C1 complex of europium(III) ion with main ligand

1

and C2–C4 complexes with main ligand and auxiliary ligands bathophenanthroline, 1,10-phenanthroline and 2,2-bipyridyl,

respectively.

Complexes

ꢀ (O–H)

ꢀ (C¼O)

ꢀ

asym (OCO)

ꢀ

sym (OCO)

ꢀ (C¼N)

ꢀ (C–F)

ꢀ (Eu–N)

ꢀ (Eu–O)

L

3443 (b)

3409 (b)

1709 (s)

–

1564 (s)

1567 (s)

1565 (s)

1563 (s)

1183(s)

1180 (s)

1182 (s)

1179 (s)

1175 (s)

–

C1 [Eu(L)

C2 [Eu(L)

C3 [Eu(L)

C4 [Eu(L)

3

.2H

2

O]

–

1582 (m)

1614 (s)

1620 (s)

1619 (s)

1432 (s)

1439 (s)

1430 (s)

1431 (s)

464 (w)

465 (w)

464 (w)

.batho]

.phen]

.bipy]

–

572 (w)

574 (w)

L: 1-(4-methoxyphenyl)-5-(trifluoromethyl)-1H-pyrazole-4-carboxylic acid; batho-bathophenanthroline, phen-1,10-phenanthroline; bipy: 2,2-bipyridyl; ꢀ: fre-

quency of bands; b: broad; s: strong; m: medium; w: weak; sym: symmetric stretching; asym: asymmetric stretching. The changes in stretching frequencies

affirm that the auxiliary ligands and deprotonated ligand are coordinated to europium(III) ion.

ꢁ

1

ꢁ1 [16]

vibrations of C–N and N–N bonds, respectively.

of 150–190 cm (<225 cm ).

In IR spectra

ꢁ1

The broad band at 3443 cm corresponding to

stretching vibrations of –OH of carboxylic acid

group (–COOH) in IR spectrum of free ligand

of free ligand (L) and C1 complex, the band

ꢁ1

(1564 cm ) corresponding to >C¼N group of

ꢁ1

pyrazole ring is slightly shifted by 1–3 cm in IR

ꢁ1

(

L) while the broad band at 3409 cm

in the

spectra of C2–C4 complexes (1563–1567 cm ),

spectrum of C1 complex is ascertained to –OH

groups of coordinated water molecules in C1

complex. This broad band vanishes in C2–C4

complexes, showing successfully substitution of

water molecules by auxiliary ligands. The charac-

teristic absorption band of >C¼O group in com-

which may be due to overlapped bands of

>C¼N group of pyrazole ring and auxiliary

ligands. This overlapped band confirms the

coordination of auxiliary ligands to trivalent

3þ

europium ion (Eu ) in C2–C4 complexes.

Furthermore, the absorption bands present in the

ꢁ1

plexes is also disappeared while two extra

range of 572–574 cm

and 464–465 cm

in

ꢁ

asymmetric (ꢀ

COO ) and symmetric (ꢀ

complexes can be accredited to Eu-N and Eu-O,

asym,

sym,

ꢁ

3þ

COO ) absorption bands of carboxylate group

respectively, pointing out that Eu ion is coordi-

occur in the range of 1582–1614 and

nated through carboxylate group of main ligand

ꢁ1

[17,18]

1430–1439 cm , suggesting the coordination of

(L) and nitrogen atoms of auxiliary ligands.

1

13

carboxylate group in bidentate manner with the

The H and C-NMR spectra of ligand and

europium(III) complexes are recorded in deu-

3

þ

central Eu

ion as the difference of (ꢀ

asym,

ꢁ

6

COO ) and (ꢀ

COO ) is found in the range

trated dimethyl sulfoxide (d -DMSO) solvent at

sym,

SPECTROSCOPY LETTERS

9

1

room temperature. The H-NMR spectra of

complexes C1–C4 toward higher wavelength

region suggests the energy gap between HOMO

(highest occupied molecular orbital) and LUMO

(lowest unoccupied molecular orbital) decreases

upon coordination, which results into more

effective energy transfer from ligand to

Eu(III) complexes in comparison of spectra of

free ligand (L) show remarkable changes. The

characteristic absorption peaks of ligand at d

3.31, 8.21, 7.45–7.47, and 3.85 ppm are accred-

ited to –OH of carboxylic acid group, pyrazole

1

3þ

[19]

ring proton, aromatic protons and methoxy

Eu ion.

(

–OCH ) group, respectively Fig. 4a. The absorp-

3

tion peaks at d 8.21 and 3.85 ppm are shifted to

upfield in the range of d 6.99–8.04 and

Band-gap analysis

3.76–3.78 ppm, respectively, and most of peaks

The plot of absorption coefficient with photon

energy is obtained from the diffused reflectance

(DR) data by special calculation. The spectral

profile of europium(III) complexes (C1–C4) are

identical, hence, C4 complex acts as depictive of

all complexes C1–C4. The plots of absorption

coefficient against photon energy for ligand (L)

and its respective europium(III) complexes are

demonstrated in Fig. 6a, b, respectively, while

their respective diffuse reflectance spectra are dis-

played in insets of these figures. The optical

are broadened upon coordination in the com-

plexes suggesting the paramagnetic character of

3þ

Eu ion. The absorption peak at d 13.31 ppm of

ligand disappears in complexes clearly reveal that

3þ

Eu

ion coordinates with ligand (L) through

carboxylate group. The appearance of extra aro-

matic peaks ranging from d 7.75–9.09 ppm in

1

H-NMR spectra of C2–C4 complexes substanti-

ates the coordination of auxiliary ligands to

europium(III) ion in C2–C4 complexes. The

complex C3 acts as depictive of all the complexes

because the spectral profiles of all the complexes

energy band-gap (E ) values of complexes are

g

determined by transformation of diffused reflect-

ance spectra as shown by Kubelka and Munk and

it is narrated by employing the general equation

1

are alike, hence, H-NMR spectrum of C3 com-

1

3

plex is manifested in Fig. 4b. C-NMR peaks of

ligand (L) are positioned at d 143.0, 132.1, 127.1,

and 55.6 ppm corresponding to pyrazole –CH,

[

20]

as given below

:

n

½

FðR Þhꢀꢄ ¼ Cðhꢀ ꢁ E Þ

(2)

1

g

Ar–CN, Ar–CH, and –OCH groups, respectively,

3

In the above equation, E denotes the energy

which are shifted to higher field on complexation

as compared to free ligand (L) pointing out the

g

of band-gap, C represents the constant of propor-

tionality, h denotes Planck’ constant and ꢀ desig-

nates the frequency of incident radiation. The

values of exponent n for indirect allowed transi-

tion, direct allowed, indirect forbidden and direct

forbidden transition are 0.5, 2, 3, and 1.5,

3þ

paramagnetic chemical environment around Eu

ion. The peak at d 160.6 ppm due to –COOH

13

group in C-NMR spectrum of ligand (L) is not

visible in the complexes, which indicates that the

3þ

ligand L is coordinated to Eu

ion through

carboxylate group. Hence, the above analyses of

IR, H-NMR, and C-NMR spectroscopy are in

agreement with each other.

respectively. The symbol F(R

Kubelka–Munk function whose value is demar-

1

) signifies the

1

13

[

20]

cated as

:

2

ð1ꢁR Þ

K

1

2R1

FðR Þ ¼

¼

S

(3)

1

UV–visible absorption spectroscopy

where R ^s^y^m^b^o^lⁱ^z^e^s^t^h^e^r^a^tⁱ^o^o^f^Rnormal^/^Rstandard

while K and S presents the absorption and scat-

The UV–vis absorption spectra of ligand (L) and

Eu(III) complexes (C1–C4) are manifested in Fig.

1

5

. The absorption maxima of ligand (L) is

tering coefficient, respectively. The optical energy

ꢆ

obtained at 259 nm corresponding to p!p elec-

tronic transitions of ligand whereas the absorp-

tion maxima of complexes are located at 265 nm

band-gap (E

) of ligand and complexes are calcu-

g

lated by extrapolating a tangent against photon

2

energy (ht) with [F(R

)hꢀ] ¼ 0 in Tauc plots.

1

(

C1), 273 nm (C2–C3), and 282 nm (C4). This

The optical band-gap energy of C1–C4 complexes

(3.98–3.37 eV) are found to be less than that of

shift in absorption maxima (6–23 nm) of

1

0

J. KHANAGWAL ET AL.

Figure 4. Proton nuclear magnetic resonance spectra of free ligand 1-(4-methoxyphenyl)-5-(trifluoromethyl)-1H-pyrazole-4-carbox-

ylic acid (a) and C complex of europium(III) ion (b) with main ligand 1-(4-methoxyphenyl)-5-(trifluoromethyl)-1H-pyrazole-4-carbox-

3

ylic acid and auxiliary ligand 1,10-phenanthroline recorded in deuterated dimethyl sulfoxide solvent. The spectral profile of C₃

complex suggest that the main ligand is coordinated to europium(III) ion after removal of proton.

SPECTROSCOPY LETTERS

11

ligand (5.63 eV), which confirms the decrease of

energy difference between HOMO and LUMO.

This decrease results into more effective energy

transfer from ligand to excited levels of Eu(III)

ion. Further, the band-gap energy range

lasers, military radars and energy converting sys-

tems. Nowadays, according to the energy depart-

ment of US (United State), these europium(III)

complexes can be employed as efficient semicon-

ductors for developing the new electrical technol-

[

21]

(

3.98–3.37 eV) of complexes (C1–C4) is in

ogies and alternate energy devices.

accordance with the energy gap (2–4 eV) of wide

band-gap semiconductors (WBGs). Therefore, the

complexes are considered as worthy contender in

Thermal analysis

The thermal stability of europium(III) complexes

C1–C4) has been investigated by thermogravi-

metric (TG) and differential thermogravimetric

(

8

DTG) analyses from ambient temperature to

00 C in the presence of nitrogen gas flow. All

ꢂ

europium(III) complexes exhibit the similar order

of thermal decomposition, hence, the decompos-

ition pattern of C4 complex is considered as rep-

resentative of all complexes as shown in Fig. 7.

The TG curve of complex represents the loss of

moisture (3.47%) in first step. The complex starts

ꢂ

to decompose at 152˚C and ends up to 572 C,

which leads to removal of one auxiliary ligand

bipy and three main organic ligands (L) with

weight loss of 86.98%. The theoretical and calcu-

lated values are found to be close to the total

mass loss of complex. At last, europium oxide is

leftover as final residue. The above fact suggests

that these luminescent complexes have optimum

Figure 5. Absorption spectra of ligand 1-(4-methoxyphenyl)-5-

(trifluoromethyl)-1H-pyrazole-4-carboxylic acid (main ligand), C₁

complex of eupoium(III) ion with main ligand and C –C com-

2

4

plexes with main ligand and auxiliary ligands bathophenan-

throline, 1,10-phenanthroline and 2,2-bipyridyl, respectively,

recorded in dimethyl sulfoxide solvent. The absorption spectra

of complexes display the red shift as compared to ligand.

ꢂ

thermal stability up to 152 C, hence, the com-

plexes can be acted as emitting materials in

[

10]

OLEDs fabrication.

Figure 6. Tauc plots of main ligand 1-(4-methoxyphenyl)-5-(trifluoromethyl)-1H-pyrazole-4-carboxylic acid (a) and C complex of

4

europium(III) ion with main ligand and auxiliary ligand 2,2-bipyridyl (b) while their insets display the corresponding diffuse reflect-

ance spectra. The plots illustrate the decreasing of energy difference between highest occupied molecular orbital and lowest

unoccupied molecular orbital in C complex on the basis of band-gap energy.

4

1

2

J. KHANAGWAL ET AL.

Figure 7. Thermogravimetric and differential thermogravimet-

ric curves of complex C of europium(III) ion with main ligand

4

1

-(4-methoxyphenyl)-5-(trifluoromethyl)-1H-pyrazole-4-carbox-

Figure 8. Solid state excitation spectra of C₁complex of

europium(III) ion with main ligand 1-(4-methoxyphenyl)-5-(tri-

fluoromethyl)-1H-pyrazole-4-carboxylic acid and C –C com-

ylic acid and auxiliary ligand 2,2-bipyridyl documented under

inert atmosphere of dinitrogen gas. The curves indicate that

the complex C possess an optimum thermal stability (152 C).

2

4

ꢂ

4

plexes with main ligand and auxiliary ligands such as

bathophenanthroline, 1,10-phenanthroline and 2,2-bipyridyl,

respectively, recorded by fixing the wavelength of intense

emission peak. The wavelengths of excitation maxima are scru-

tinized as 320 nm (C ), 321 nm (C ), 344 nm (C ), and

Photoluminescence characterization

1

2

3

⁵⁶ⁿ^m⁽^C4⁾^.

To analyze the photoluminescence properties of

luminescent Eu(III) complexes, the excitation

spectra of complexes are recorded by fixing

5

7

[24]

D ! F transitions, respectively.

The weak

0

4

5

7

3þ

D ! F prominent emission transition of Eu

5

7

0

2

emission peaks corresponding to D ! F ,

1

0

ion, in 200–500 nm range with 400 PMT voltage

as displayed in Fig. 8. One can find that the exci-

tation spectra of the complexes consist of bands

in two regions, a remarkable broad band in

higher energy region (200–370 nm) with maxima

at different wavelengths for the corresponding

complexes such as 320 nm (C1), 321 nm (C2),

5

7

5

7

D ! F , D ! F transitions centered at 526,

1

2

5

36 and 554 (C1–C2, C4), 555 nm (C3) are

observed in the spectra of C1–C4 complexes,

which are presented in inset of Fig. 9 (C3–C4

complexes) for clear inspection of transitions. In

addition, a diminished peak at 468 nm assigned

5

7

to D ! F transition is also discernible in emis-

2

0

3

44 nm (C3), and 356 nm (C4), which can suc-

sion spectra of complexes. Among these emission

peaks, the most prominent peak located at

ꢆ

cessfully be accredited to p!p ligand centered

transition. The lower energy region (358–470 nm)

of excitation spectra of complexes contains the

less intense peaks at 361, 375, 382, 395, and

6

15 nm (C2–C4) and 614 nm (C1) emanates from

induced electric dipole allowed transition

5

7

5

D ! F , which corresponds to bright red emis-

0

2

4

65 nm, which are allocated to F ! D ,

0

4

3

þ

7

5

7

5

7

5

7

5

sion of Eu

ion. The magnetically allowed

F ! G , F ! G , F ! L , and F ! D char-

0

2

0

3

0

6

þ

0

[22,23]

2

5

7

3

dipole transition ( D ! F ) is insensitive to site

0

1

acteristics transitions of Eu ion.

symmetry whereas the emission signal corre-

Further, the emission spectra of europium(III)

complexes are scanned at room temperature

within the range of 400–700 nm on applying the

maximal excitation wavelength of corresponding

complexes C1–C4. The emission spectra of

Eu(III) complexes (Fig. 9) are composed of emis-

sion peaks at 580, 592, 614 (C1) and 615

5

7

sponding to electric dipole transition ( D ! F )

0

2

is extremely sensitive to local symmetry around

3þ

Eu ion in the coordination sphere. The transi-

5

7

5

7

tions D ! F and D ! F are parity forbidden

0

3

transitions according to selection rule while

5

7

D

! F

transition is electronically allowed tran-

4

0

(

C2–C4), 657 (C1–C2, C4), 661 (C3) and 698

sition and the peak corresponding to this transi-

(

C1–C3), 699 nm (C4), which are ascribed to

tion shows stark splitting, which supports the

5

7

5

7

5

7

5

7

3þ

D ! F , D ! F , D ! F , D ! F , and

asymmetric environment around Eu

ion. Of

0

1

0

2

0

3

SPECTROSCOPY LETTERS

13

Figure 9. Room temperature emission spectra of C complex of europium(III) ion with main ligand 1-(4-methoxyphenyl)-5-(trifluor-

1

omethyl)-1H-pyrazole-4-carboxylic acid and C –C complexes with main ligand and auxiliary ligands such as bathophenanthroline,

2

4

1,10-phenanthroline and 2,2-bipyridyl, respectively. The graph demonstrate the different transitions emanating from lower energy

excited level of europium(III) ion while insets display the less intense peaks corresponding to different transitions which are origi-

nated from lower energy and higher energy excited levels of europium(III) ion in C and C complexes.

3

4

note, the emission signal with respect to electric

the integrated area of peak corresponding to

5

7

5

7

dipole ( D ! F ) transition is about three fold

D ! F transition by total integrated area of all

0

2

0

2

stronger than the signal of magnetic dipole tran-

emission peaks. The branching ratios of the com-

3

þ

sition, which recommends that Eu

devoid of an inversion center.

ion is

plexes C1–C4 for electric dipole transition

[

25]

5

7

The C–F oscil-

( D ! F ) are laid in the range of 64.41–74.43%,

0

2

lators in main ligand (L) reduce energy loss non-

radiatively during vibration of organic ligand,

which improves the luminescence efficiency of

the complexes. It is worthy noted that the photo-

luminescence emission signal of europium(III)

complexes (C2–C4) can be further improved by

removing the vibronic quenching produced by

high energy O–H oscillators of water molecules

present in coordination sphere of C1 complex.

This can be removed by incorporation of rigid

and highly conjugated auxiliary ligands in coord-

ination sphere in place of water molecules, which

can be enhanced the emission signal strength of

C2–C4 complexes up to a great extent via syner-

gistic effect.

hence, the complexes act as promising candidates

in the laser diodes.

[

26]

The CIE (Commission Internationale de

I’Eclairage) color coordinates (x and y) of

europium(III) complexes are determined with the

help of emission spectral data by using MATLAB

software. The chromaticity coordinates of com-

plexes 0.4696, 0.2486 (C1), 0.5691, 0.2956 (C2),

0.6093, 0.3381 (C3), and 0.6403, 0.3462 (C4),

respectively, are tabularized in Table 3, which are

presented in the x–y plane. The chromaticity

coordinates of complexes C1–C4 are well located

in red region of chromaticity diagram as demon-

strated in Fig. 10. The CIE values of C4 complex

are well comparable to SMPTE (Society of

Motion Picture and Television, x ¼ 0.63, y ¼ 0.34)

and EBU (European Broadcasting Union,

x ¼ 0.64, y ¼ 0.33) due to high intense red color

of this complex, thus, proving to be an excellent

The percentage branching ratios of lumines-

cent europium(III) complexes are found to be

almost greater than 60% with respect to electric

5

7

dipole transition D ! F as listed in Table 3.

0

2

[

27]

The branching ratios are obtained on dividing

red component in OLEDs.

1

4

J. KHANAGWAL ET AL.

5

7

Table 3. Color coordinates, branching ratios with respect to D ! F transition of europium(III) ion and color purity of complex

0

2

C of europium(III) ion with 1-(4-methoxyphenyl)-5-(trifluoromethyl)-1H-pyrazole-4-carboxylic acid as main ligand and complexes

1

C –C with main ligand and auxiliary ligands bathophenanthroline, 1,10-phenanthroline and 2,2-bipyridyl, respectively.

2

4

5

7

5

7

5

7

5

7

5

7

Complexes

Color coordinates (x, y)

D

0

! F

0

D

0

! F

1

D

0

! F

2

D

0

! F

3

D

0

! F

4

I

615/ I592

CP (%)

C1 [Eu(L)

C2 [Eu(L)

C3 [Eu(L)

C4 [Eu(L)

3

.2H

2

O]

(0.4696, .2486)

(0.5691, .2956)

(0.6093, .3381)

(0.6403, .3462)

2.84

2.28

2.21

2.15

25.66

22.89

22.15

22.06

64.41

73.72

73.58

74.43

3.25

0.92

0.56

0.45

3.84

0.19

1.49

0.91

2.51

3.22

3.32

3.37

56.40

61.44

84.64

93.83

.batho]

.phen]

.bipy]

L: 1-(4-methoxyphenyl)-5-(trifluoromethyl)-1H-pyrazole-4-carboxylic acid; batho: bathophenanthroline; phen: 1,10-phenanthroline; bipy: 2,2-bipyridy; x, y:

5

7

5

7

5

7

5

7

5

7

5

7

coordinates along x and y-axis; D

1

0

! F

0

, D

0

! F

1

, D

0

! F

2

, D

0

! F

3

and D

0

! F

4

: transitions from lowest excited level ( D

0

) to ground levels ( FJ ¼ 0,

, 2, 3, 4) of europium(III) ion, respectively; CP(%): percentage color purity; I615 and I592: integrated areas of emission peaks at wavelengths of 615 nm and

5

92 nm. The color purity and color coordinates confirm the light emitted by europium complexes.

sﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ

2

ðx ꢁx Þ þ ðy ꢁy Þ

s

i

s

i

CP ¼

ꢅ 100%

(4)

2

ðx ꢁ x Þ þ ðy ꢁ y Þ

d

i

d

i

The color purity (CP) of complexes (C1–C4) is

found in the range of 56.40–93.83%, which is

enlisted in Table 3. The dominant color purity of

C2–C4 complexes is due to presence of auxiliary

ligands which intensify the luminescence of these

complexes. Hence, the complexes can be consid-

ered as red light emitting component in fabrica-

tion of tricolor RGB (red, green, and blue)

based

diodes (WOLEDs).

To investigate the homogeneous environment

white

organic

light

emitting

[

29]

3þ

around the Eu

ion, the photoluminescence

decay curves of Eu(III) complexes are recorded

5

7

for D ! F transition and pulsed excitation of

0

2

corresponding complexes. The photolumines-

cence decay curves are satisfactory fitted into the

Figure 10. Color coordinates of complex C of europium(III)

1

ion with main ligand 1-(4-methoxyphenyl)-5-(trifluoromethyl)-

first order exponential equation as given

1

H-pyrazole-4-carboxylic acid and C –C complexes with main

2 4

[30]

below

:

ligand and auxiliary ligands bathophenanthroline, 1,10-phenan-

throline and 2,2-bipyridyl, respectively, in chromaticity diagram.

The chromaticity diagram reveals that the color coordinates of

complexes (C1: x ¼ 0.4696, y ¼ 0.2486, C2: x ¼ 0.5691, y ¼

I ¼ I exp ðꢁt=sÞ

(5)

8

where exp represents the exponential function, s

symbolizes the lifetime for radiative emission,

0

.2956, C3: x ¼ 0.6093, y ¼ 0.3381, C4: x ¼ 0.6403, y ¼

.3462) lie in red region of chromaticity diagram, which con-

while I and I are the integrated areas of photo-

0

firm the emitted color of complexes (color figure online).

5

7

emission peak corresponding to D ! F transi-

0

2

The color purity (CP) of the complexes is

determined with respect to illuminated coordi-

nates (x ¼ 0.33, y ¼ 0.33) in CIE color gamut

tion at time t and 0, respectively. The decay

curves are well fitted into first-order exponential

function, which prove the possibility of homo-

genous chemical environment around the emit-

i

space, indicates that how strongly a particular

complex may be acted as red color emitter. The

color purity is weighted average of dominant

emission color coordinates of that sample (x , y )

3þ

[7]

ting Eu ion as demonstrated in Fig. 11. The

decay time values of europium complexes

(C1–C4) are found in the range of

0.3893–1.5706 ms as reported in Table 4. The

shorter decay time of C1 complex is associated

with dominant non-radiative decay channels

induced by vibronic coupling of O–H oscillators

s

and dominant wavelength coordinates (x , y )

d

with respect to standard CIE 1931 illuminated

coordinates (x , y ) as shown by the following

i

[

28]

equation

:

SPECTROSCOPY LETTERS

15

of water molecules present in coordination sphere

of C1 complex. It is noteworthy pointing that the

presence of auxiliary ligands in place of water

molecules in C2–C4 complexes is diminished the

non-radiative decay channels. This also improves

the lifetime (s) of excited states through radiative

decay pathway resulting into an efficient energy

^Arad

g ¼

(6)

Arad þ Anrad

In aforementioned equation, A_n_r_a_dand A_r_a_drep-

resent the non-radiative and radiative decay rate

constants, respectively. The internal quantum effi-

ciency (g) helps to investigate the interaction

between the radiative and non-radiative decay

processes and lies in the range of 7.84–36.69% for

C1–C4 complexes (Table 4). It can be noticed that

the internal quantum efficiency (g) of C2–C4 com-

plexes is comparatively higher than C1 complex

due to the presence of auxiliary ligands which

populate the higher excited levels of Eu ion by

synergistic effect, thus, the complexes support the

advanced display devices fabrication.

Further, the radiative and non-radiative decay

processes are related to radiative lifetime (s)

according to given equation as expressed

3þ

[31]

transfer from ligands to Eu ion.

The luminescent properties of Eu(III) com-

plexes are further assessed by evaluating the

internal quantum efficiency (g) of the complexes.

5

The internal quantum efficiency (g) of D₀

3þ

excited level of Eu ion is calculated by employ-

ing the photoluminescence lifetime (s) and emis-

sion spectral data, which is demarcated by the

3þ

[

32]

following equation

:

[

33]

below

:

1

A_t_o_t_a_l¼ _s¼ A_r_a_dþ A_n_r_a_d

(7)

The certain photoluminescence parameters

(

A_t_o_t_a_l, A_n_r_a_d, A , and g) are tabulated in

rad

Table 4. The radiative decay rate (A ) is eval-

rad

uated by addition of radiative decay rates of each

5

7

distinctive transitions D ! F (J ¼ 0, 1, 2, 3, 4)

0

J

3þ

of Eu ion. The radiative decay rate, A is not

0

6

estimated due to absence of emission peak corre-

5

7

sponding to D ! F transition. Hence, this

0

6

radiative rate does not involve in the depopula-

tion of excited level of Eu ion. The radiative

3þ

decay rate (A ) of each transition can be calcu-

0

J

Figure 11. The photoluminescent decay curves of complex C₁

of europium(III) ion with main ligand 1-(4-methoxyphenyl)-5-

[34]

lated by using the following equation

:

ꢀ

ꢁ

ꢀ

ꢁ

(trifluoromethyl)-1H-pyrazole-4-carboxylic acid and complexes

^I0

J

^ꢀ01

C –C with main ligand and auxiliary ligands such as batho-

A ¼ A

0J

01

(8)

2

4

^I0

1

ꢀ₀_J

phenanthroline, 1,10-phenanthroline and 2,2-bipyridyl, respect-

ively, recorded with respect to wavelength of intense

emission peak.

where I₀_Jand ꢀ₀_Jrepresent the integrated areas

of emission peaks and energy barycenter for cor-

5

7

responding D ! F transitions (J ¼ 0, 1, 2, 3, 4),

0

J

Table 4. Experimentally calculated photoluminescent data of complex C of europium(III) ion with 1-(4-methoxyphenyl)-5-(tri-

1

fluoromethyl)-1H-pyrazole-4-carboxylic acid as main ligand and C –C complexes with main ligand and auxiliary ligands

2

4

bathophenanthroline, 1,10-phenanthroline and 2,2-bipyridyl, respectively.

ꢁ1

ꢁ20

2

ꢁ20

2

Complexes

s (ms)

A

total (s

)

Arad (s

)

A

nrad (s

)

X

2

(10 cm )

X

4

(10 cm )

g (%)

C1 [Eu(L)

C2 [Eu(L)

C3 [Eu(L)

C4 [Eu(L)

3

.2H

2

O]

0.3893

1.2651

1.5307

1.5706

2568.71

790.45

653.30

636.70

201.45

224.85

232.40

233.59

2367.26

565.6

420.9

2.88

3.71

3.82

3.88

0.20

0.01

0.09

0.05

7.84

28.45

35.57

36.69

.batho]

.phen]

.bipy]

403.11

s (ms): luminescent lifetime in millisecond; Arad: radiative rate constant; Anrad: non-radiative rate constant; Atotal: total radiative rate constant; g: internal

quantum efficiency; X and X : Judd–Ofelt parameters; L: 1-(4-methoxyphenyl)-5-(trifluoromethyl)-1H-pyrazole-4-carboxylic acid; batho: bathophenanthro-

line; phen: 1,10-phenanthroline; bipy: 2,2-bipyridyl.

2

4

1

6

J. KHANAGWAL ET AL.

respectively while A₀₁represents Einstein’s coeffi-

cient of spontaneous emission for parity-allowed

5

7

(

D ! F ) transition which is independent of lig-

0

1

and field in the coordination sphere, therefore, it

can be used as an internal reference and its value

comes out to be about 50 s in air.

ꢁ1

[35]

The Judd–Ofelt theory is put forward by Judd

and Ofelt scientists, which is very helpful in

investigation of asymmetric environment around

metal ion and also determine the f-f electronic

transitions by utilizing the Judd–Ofelt parameters

[

36]

X (k ¼ 2, 4).

X is primarily sensitive to

2

k

chemical field created by the surrounding ligands,

Figure 12. Scanning electron microscope image of C complex

4

while X mainly relies upon the metal–ligand dis-

tance which in turn related to rigidity of the sys-

of europium(III) ion with main ligand 1-(4-methoxyphenyl)-5-

4

(trifluoromethyl)-1H-pyrazole-4-carboxylic acid and auxiliary lig-

and 2,2-bipyridyl. The image indicates the crystalline nature of

particles which are possessed by complex C₄.

tem. Judd–Ofelt parameters (X₂and X ) are

4

calculated by using the following equation:

3

2

3

5

ðkÞ 7

2

X ¼ 3Éc A =4e x vh D jU j F i

(9)

Analysis of biological activity

k

0J

0

J

In the above equation, c denotes the velocity

Antimicrobial properties

of light, ¯¼h/2p, h corresponds to Planck’s con-

The antibacterial property of ligand (L) and its

respective complexes are assessed against Gram-

positive and Gram-negative bacterial pathogens

such as S. aureus and E. coli, P. aeruginosa,

respectively, by using ciprofloxacin as standard

drug. The fungal pathogens like A. niger and C.

albicans are used to determine the antifungal

activity of complexes against fluconazole as

standard drug. These antimicrobial activities are

investigated in terms of MIC values as reported

in Table 5 and represented by bar graph of C4

complex in Fig. 13. The recorded MIC values

suggest that europium(III) complexes are acted as

more potent than ligand (L) and reference drugs

used. For S. aureus, C2 and C3 complexes

2

stant, v ¼ n (n þ2) /9 represents the local field

Lorentz correction term, e symbolizes the elec-

ꢁ

10

tronic charge (4.8 ꢅ 10

e.s.u.), and n signify

5

(k)

the refractive index (n ¼ 1.5). The term h D jU j

0

7

2

F i denotes the squares of reduced matrix elem-

J

5

7

ent having values 0.0032 ( D ! F ) and 0.0023

0

2

5

7

[37]

(

D ! F ).

The calculated values of X and

2

0

4

X

for C1–C4 complexes are summarized in

4

Table 4. Higher value of X for C2–C4 complexes

2

reflects the presence of more covalence character

and asymmetric chemical environment around

the europium(III) ion whereas greater value of

X reflects the high rigidity of C2–C4 complexes

4

due to presence of auxiliary ligands in

(

6.25 mg/mL) are more active than C1 and C4

C2–C4 complexes.

complexes (12.5 mg/mL). Against E. coli, C2 com-

plex (3.13 mg/mL) shows significantly greater

activity and C3–C4 complexes (6.25 mg/mL) are

moderate active whilst C1 complex (12.5 mg/mL)

is inactive against this bacterial pathogen. In the

case of P. aeruginosa, C2–C3 complexes (3.13 mg/

mL) are more active than C1 and C4 complexes

(6.25 mg/mL). It is clearly revealed that the order

of antibacterial potential of the complexes is

found to be C2 > C3 > C4 > C1.

Scanning electron microscopy analysis

The surface morphology of the particles is con-

firmed by SEM analysis which indicates that the

particles possessed by the complexes are crystal-

line in nature. The particles morphology of all

complexes is represented by SEM micrograph of

C4 complex as manifested in Fig. 12, which dem-

onstrates that the particles are crystalline in

nature and some particles are seen in conglomer-

ated stage.

Against C. albicans, C4 complex (3.13 mg/mL)

is more active than other complexes

SPECTROSCOPY LETTERS

17

Table 5. Minimum inhibitory concentration values of main lig-

and 1-(4-methoxyphenyl)-5-(trifluoromethyl)-1H-pyrazole-4-car-

this complex is considered as more powerful anti-

microbial agent. It is noteworthy pointed that

highly conjugated system of auxiliary ligand

batho with nitrogen donor atoms increase the

delocalization of p-electrons over the whole lig-

and, hence, it increases the lipid attracting ten-

boxylic acid, complex C of europium(III) ion with main ligand

1

and complexes C –C with main ligand and auxiliary ligands

2

4

bathophenanthroline, 1,10-phenanthroline and 2,2-bipyridyl,

respectively, determined against ciprofloxacin (bacteria) and

fluconazole (fungi) standard drugs.

Minimum inhibitory concentration (mg/mL)

3þ

dency of Eu

ion. Thus, Eu

ion can be

Complexes

L

MICsa

MICec

MICpa

MICca

MICan

penetrated into the cell wall of microorganisms

to a deep area, which consequently inhibits the

growth of microbes and enhance the antimicro-

12.5

6.25

12.5

3.13

6.25

12.25

6.25

3.13

6.25

25.0

6.25

12.5

3.13

12.5

3.13

6.25

C1 [Eu(L)

C2 [Eu(L)

C3 [Eu(L)

C4 [Eu(L)

3

2

.2H O]

.batho]

.phen]

.bipy]

bial potential of complexes as compared to ligand

a

b

SD 25.0

12.5

25.0

12.5

[38,39]

(

L) and reference drugs used.

MIC: minimum inhibitory concentration; L: 1-(4-methoxyphenyl)-5-(trifluor-

omethyl)-1H-pyrazole-4-carboxylic acid; batho: bathophenanthroline; phen:

Antioxidant activity

1

,10-phenanthroline; bipy: 2,2-bipyridyl; sa: Staphylococcus aureus; ec:

Escherichia coli; pa: Pseudomonas aeruginosa (bacterial pathogens) and

ca: Candida albicans; an: Aspergillus niger (fungal pathogens); a: ciprofloxa-

cin; b: fluconazole; SD: standard drugs; mg/mL: microgram per milliliter.

The bold values represent the lowest minimum inhibitory concentrations

and highest antimicrobial activities.

Antioxidant activity of complexes and ligand (L)

are determined by DPPH method. DPPH gets

diamagnetic character by accepting the hydrogen

radical from antioxidant complexes which results

into absorbance of the complexes gets decrease

and scavenging activity gets increased. A straight

line graph is obtained between percentage scav-

enging activity (% SCA) and concentrations of

samples, which is displayed in Fig. 14. Higher

percentage scavenging activity (% SCA) of com-

plexes increases the capability of complexes

toward antioxidant activity. IC₅₀and percentage

scavenging activity (% SCA) of the complexes are

indexed in Table 6. It is clear from Table 6 that

the free ligand L (IC₅₀¼ 61.92 mg/mL) is less

active than standard ascorbic acid (IC₅₀

¼

4

5

6.71 mg/mL) and complexes C1–C4 (IC₅₀

8.75–49.70 mg/mL). In the complexes, ligand

3þ

donates its electron density to Eu ion, hence,

hydrogen atom gets easily ionized, which quench

Figure 13. Antimicrobial activity of main ligand 1-(4-methoxy-

phenyl)-5-(trifluoromethyl)-1H-pyrazole-4-carboxylic acid and C₄

complex of europium(III) ion with main ligand and auxiliary lig-

and 2,2-bipyridyl investigated against the standard drugs,

ciprofloxacin (bacteria), and fluconazole (fungi). The bar graph

demonstrates that the ligand is less active agent than

C₄complex.

the DPPH activity and increase the antioxidant

[40]

activity of complexes.

The antioxidant activity

of C2–C4 complexes is enhanced through syner-

[

41]

gistic effect of auxiliary ligands.

The above

fact suggests that the complexes C2–C4 are acted

as an excellent antioxidant agent as compared to

C1 complex and ligand (L).

(

6.25–12.5 mg/mL), while ligand L (25.0 mg/mL) is

acted as inactive agent. Furthermore, the complex

C2 (3.13 mg/mL) is excellent and C3–C4 com-

plexes (6.25 mg/mL) are moderate antifungal

agents against A. niger whereas C1 complex

Energy transfer mechanism

The emission spectral data is very helpful to

interpret the energy transfer mechanism in the

complexes and also ascertain the strength of

photoemission signal of complexes. The mechan-

ism involves the energy transfer from excited

singlet level to triplet level of ligand (L) via

(

12.5 mg/mL) are totally inactive against this fun-

gal pathogen. It can be noted that the antimicro-

bial activity of C2 complex is comparatively

greater than other complexes (C1, C3–C4), hence,

1

8

J. KHANAGWAL ET AL.

Figure 14. Variation of percentage scavenging activity with concentration for main ligand 1-(4-methoxyphenyl)-5-(trifluoromethyl)-

H-pyrazole-4-carboxylic acid, C complex of europium(III) ion with main ligand and complexes C –C with main ligand and auxil-

1

2

4

iary ligands such as bathophenanthroline, 1,10-phenanthroline and 2,2-bipyridyl, respectively, determined against ascorbic acid

standard). The graph clearly reveals that the complex C is acted as an efficient antioxidant agent on the basis of percentage

(

4

scavenging activity.

Table 6. The recorded percentage scavenging activity and

their shortest edge wavelength of UV–vis absorp-

tion spectrum of ligand (L) (Fig. 5) and lowest

emission band of phosphorescence spectrum of

Gd(L) .2H O complex (G1) (Fig. 15), respect-

50% inhibitory concentration of free ligand 1-(4-methoxy-

phenyl)-5-(trifluoromethyl)-1H-pyrazole-4-carboxylic acid, C₁

complex of europium(III) ion with 1-(4-methoxyphenyl)-5-(tri-

fluoromethyl)-1H-pyrazole-4-carboxylic acid as main ligand

and C –C complexes with main ligand and auxiliary ligands

3

2

4

ively. The phosphorescence emission spectrum of

G1 complex is ligand centered emission because

bathophenanthroline, 1,10-phenanthroline and 2,2-bipyridyl,

respectively, against ascorbic acid (standard) at different

concentrations.

3

þ

the lowest emissive level of Gd

ion is posi-

ꢁ1

Concentration (lg/mL)

tioned at about 32,000 cm , which does not

allow the transfer of energy from excited triplet

level of ligand to emissive level of Gd ion.

Complexes

L

25

50

75

100

IC50

3þ

[18]

25.86

27.35

29.48

31.95

33.81

35.87

42.12

44.47

46.22

49.04

50.29

52.16

58.58

60.88

62.16

65.31

66.55

68.35

74.96

77.43

78.59

82.02

83.11

85.27

61.92

58.75

56.22

51.94

49.70

46.71

C1 [Eu(L)

C2 [Eu(L)

C3 [Eu(L)

C4 [Eu(L)

3

2

.2H O]

.batho]

.phen]

.bipy]

Similarly, the singlet and triplet energy levels of

ꢁ1

auxiliary ligands like bipy (S : 29,900 cm , T :

1

ꢁ1

Ascorbic acid

2

2,900 cm ), phen (S : 31,000 cm , T :

1 1

%

SCA: percentage scavenging activity; IC50: 50% inhibitory concentration;

ꢁ1

2,100 cm ), and batho (S : 29,000 cm , T :

1

L: 1-(4-methoxyphenyl)-5-(trifluoromethyl)-1H-pyrazole-4-carboxylic acid;

batho: bathophenanthroline, phen: 1,10-phenanthroline; bipy: 2,2-bipyr-

idyl; lg/mL: microgram per milliliter. The highest antioxidant activity of

complexes is confirmed by highest value of percentage scaveng-

ing activity.

ꢁ1

21,000 cm ) are determined, which are properly

matched with the values reported in literature.

[42]

The energy transfer rate depends upon two main

processes. In the first case, the energy transfer

from lowest excited triplet level of ligand to emis-

sive level of metal ion takes place according to

intersystem crossing, finally to excited emissive

5

3þ

levels ( D ) of Eu ion through excited triplet

J

5

[43]

level of ligand. The excited emissive level ( D )

gets relax to ground levels ( F ), which emit lumi-

Dexter’s electron exchange interaction theory.

0

7

This theory proposed that more efficient energy

transfer takes place, if more suitable energy gap

is present between triplet level of ligand and

emissive level of metal ion. The transfer

J

nescence in visible region. The excited singlet

ꢁ1

(

31,546 cm ) and triplet (24,752 cm ) energy

levels of ligand (L) are determined by referring to

SPECTROSCOPY LETTERS

19

ꢁ

1

Table 7. Energy (cm ) of excited levels of ligand 1-(4-

methoxyphenyl)-5-(trifluoromethyl)-1H-pyrazole-4-carboxylic

acid and auxiliary ligands bathophenanthroline, 1,10-phenan-

throline and 2,2-bipyridyl.

5

Ligands

(S

1

)

(T

1

)

DE

1

(S

–T

1 1

)

DE(T

1

– D

0

)

2 1 1

DE (T – D )

L

31,546

29,900

31,000

29,000

24,752

22,900

22,100

21,000

6794

7000

8900

8000

7511

5659

4859

3759

6095

4243

3443

2343

Bipy

Phen

Batho

S

1

and T

respectively; DE: energy difference;

europium(III) ion; L: 1-(4-methoxyphenyl)-5-(trifluoromethyl)-1H-pyrazole-

-carboxylic acid; batho: bathophenanthroline, phen: 1,10-phenanthroline;

bipy: 2,2-bipyridyl.

1

: lowest excited singlet and triplet energy levels of ligands,

5 5

0 1

D and D : excited energy levels of

4

5

7

D ! F (468 nm) transitions, which are observed

2

0

in emission spectra of complexes. The value of

energy gap (T -Eu ) for different emissive levels

3þ

1

Figure 15. Phosphorescence spectrum of Gadolinium complex

of ligand 1-(4-methoxyphenyl)-5-(trifluoromethyl)-1H-pyrazole-

3þ

of Eu ion and ligand (L) are found to be 7511

5

ꢁ1

4

-carboxylic acid in solid state. The energy of excited triplet

level of ligand is determined from the lowest band wavelength

404 nm).

( D ), 6095 ( D ), and 3384 ( D ) cm , which

0 1 2

5

indicates that the ligand (L) transfer energy to D

level of Eu ion as according to Latva empirical

2

(

3þ

ꢁ1

rule (T -Eu ¼ 2000–5000 cm ) and it sensitizes

1

3

þ

[45]

5

probability constant (P ) is expressed as

s

the Eu ion.

The D emissive level populates

2

ꢀ

ꢁ

ð

5

2

pZ²

the D

which in turn D g relax to ground levels ( F )

0 J

0

level via non-radiative decay process,

P_S¼

F ðEÞ:n ðEÞ:dE

(10)

5

7

S

s

h

radiatively and thus, emit luminescence in visible

region. The excited triplet level of ligand (L) is

located above the triplet level of auxiliary ligands,

hence, its triplet level transfer energy to excited

triplet level of auxiliary ligand, whereby the syner-

gistic effect of auxiliary ligands intensify the emis-

sion signal of C2–C4 complexes than C1 complex.

Here, F (E) represents the shape of emission

band of energy donor triplet level of ligand, n_s

E) shows the shape of absorption spectrum of

energy acceptor metal ion and 2pZ /h term is

constant. The second case deals with the thermal

de-excitation or inverse energy transfer from

metal ion to ligand, which can be represented as

DE

S

(

2

The energy values (DE ) for bipy, phen, and batho

_ðꢁ

2

Þ

KðTÞ ¼ Ae

RT

(11)

ꢁ1

are reckoned as 4243, 3443, and 2343 cm ,

Here, DE is energy difference between emissive

level of metal ion and lowest excited triplet level

of ligand and K(T) is thermal de-excitation con-

stant. Therefore, the value of transfer probability

respectively, which clearly reveal that the DE value

for bipy auxiliary ligand is highest among all auxil-

2

iary ligands. Hence, this energy value (DE ) for

2

bipy negates the chance of inverse energy transfer

from metal to ligand and exhibits the intense emis-

constant (P ) decide the efficiency of energy

S

transfer process from ligand to metal ion.

sion signal. On the contrary, the DE value of batho

2

On the other hand, the scientist Reinhoudt

proposed that the efficiency of intersystem cross-

is the lowest, which makes a possibility of inverse

energy transfer and gets least intense emission sig-

nal among the auxiliary ligands. The proposed

energy transfer mechanism of one of complex that

is C4 is displayed in Fig. 16.

ing controlled by energy gap DE (S –T ) between

1

excited singlet (S ) and triplet (T ) level of ligand

1

ꢁ

1 [44]

that should be at least 5000 cm .

The value

ꢁ1

of DE for ligand (L) is found to be 6794 cm as

1

demonstrated in Table 7, which permits the

effective intersystem crossing in the complexes.

Conclusions

3þ

The emissive levels of Eu ion are positioned at

In this reported research work, four red color

emitting europium(III) complexes (C1–C4) were

synthesized by using main organic ligand, 1-(4-

ꢁ

7

1

7,241, 18,657, and 21,368 cm corresponding to

5

7

5

D ! F (580 nm), D ! F (536 nm), and

0

1

2

0

J. KHANAGWAL ET AL.

Figure 16. The schematic energy transfer mechanism in C complex of europium(III) ion with main ligand 1-(4-methoxyphenyl)-5-

4

(trifluoromethyl)-1H-pyrazole-4-carboxylic acid and auxiliary ligand 2,2-bipyridyl. It demonstrates the energy transfer from excited

triplet level of main ligand and auxiliary ligand 2,2-bipyridyl to excited energy levels of europium(III) ion through different energy

5

transfer pathways. The energy difference between triplet level of ligand L and D of europium(III) ion is suitable for energy trans-

2

fer process and auxiliary ligand also help in sensitization of europium(III) ion.

methoxyphenyl)-5-(trifluoromethyl)-1H-pyrazole-

-carboxylic acid (L) and auxiliary ligands

devices. In addition, the energy transfer

occurred more efficiently in C2–C4 complexes

due to presence of auxiliary ligands which

reduced the quenching produced by water mol-

ecules in C1 complex and also provided stabil-

ity to the complexes, thereby, the luminescence

efficiency of C2–C4 complexes was enhanced.

The internal quantum efficiency (g) of

europium(III) complexes increased from 7.84%

to 36.69%, which was attributed to potential

advantages of the complexes in lighting and

display devices. Furthermore, the antimicrobial

and antioxidant activity of complexes were also

investigated via tube dilution and DPPH

approach, respectively. The biological properties

substantiated that the complexes were acted as

best antimicrobial and antioxidant agents. The

above characterization results proposed a prom-

ising role of the complexes in the fields of

advanced luminescent devices, fabrication of

white light emitting diodes, lasers, and solar

4

through a more facile and less time consuming

solution precipitation approach and character-

ized through various spectral techniques such

as elemental and EDAX analyses, IR, NMR,

DR, TG-DTG, and PL spectroscopy. The optical

band-gap values of complexes (C2–C4) were

found to be in the range of WBGs, which indi-

cated their behavior like wide-band gap semi-

conductors and thus, proved better luminescent

semiconductors than conventional semiconduc-

tors. TG-DTG study of complexes clearly

revealed that the complexes possessed thermal

ꢂ

stability up to 152 C, which supported the role

of them as emitting materials in OLEDs fabri-

cation. SEM analysis affirmed the crystalline

nature of particles, possessed by the complexes.

The photoluminescence features of the com-

plexes had been explored in detail, which sug-

gested that the complexes have remarkable

photoluminescence properties. The red emission

of complexes was obtained at 614 (C1) and 615

[

46–49]

technology.

5

7

(

C2–C4) nm corresponding to D ! F transi-

0

2

3þ

Ethics statement

tion of Eu ion whereas CIE color coordinates

were well situated in the red region reflected

that the complexes play a great role in display

ꢀ This is our original research work and it high-

lights new dimensions in the field of

SPECTROSCOPY LETTERS

21

luminescent materials and biological systems, by

utilizing various advanced spectroscopic techni-

ques. The work has not been published before; it

is not under consideration for publication any-

where else; and publication has been approved

by all coauthors and the responsible authorities

at the institute(s) where the work has been car-

ried out.

Chemistry 2015, 8, 1425–1434. DOI: 10.1002/ejic.

00401043 .

7] Raj, D. B. A.; Biju, S.; Reddy, M. L. P. One-, Two-,

2

[

3þ

and Three-Dimensional Arrays of Eu -4,4,5,5,5-

Pentafluoro-1-(Naphthalen-2-yl)Pentane-1,3-Dione

Complexes: Synthesis, Crystal Structure and

Photophysical Properties. Inorganic Chemistry 2008,

47, 8091–8100. DOI: 10.1021/ic8004757.

[

8] Yue, B.; Chen, Y. N.; Chu, H. B.; Qu, Y. R.; Wang,

A. L.; Zhao, Y. L. Synthesis, Crystal Structures and

Fluorescence Properties of Dinuclear Tb(III) and

Sm(III) Complexes with 2,4,6-Tri(2-Pyridyl)-1,3,5-

Triazine and Halogenated Benzoic Acid. Inorganica

Chimica Acta 2014, 414, 39–45. DOI: 10.1016/j.ica.

ꢀ

Moreover, the authors are ready to follow all

norms of the publications like copy right, etc.

Disclosure statement

2014.01.024.

No potential conflict of interest was reported by

the author(s).

[

9] Bala, M.; Kumar, S.; Taxak, V. B.; Boora, P.;

Khatkar, S. P. Synthesis, Photoluminescent Features

and Intramolecular Energy Transfer Mechanism of

Europium (III) Complexes with Fluorinate

b-Diketone Ligand and Auxiliary Ligands. Journal of

Fluorine Chemistry 2015, 178, 6–13. DOI: 10.1016/j.

jfluchem.2015.06.011.

Funding

Authors are thankful to the University Grants Commission

UGC) for granting funds under special Assistance program

No. F.540/17/DRS-I/2016 SAP-I] to Department

(

[

10] Devi, R.; Priyanka Chahar, S.; Khatkar, S. P.; Taxak,

V. B.; Boora, P. Judd Oflet Characterization and

Energy Transfer Mechanism of Highly Luminescent

Europium(III) Complexes with 1-(5-Chloro-2-

of Chemistry.

Hydroxyphenyl)-1,3-Butanedione.

Chimica Acta 2018, 471, 364–371. DOI: 10.1016/j.

Inorganica

References

[1] Huijuan, R.; Dehui, S.; Zhenfeng, C.; Mei, Y.;

ica.2017.11.006.

Guangyan, H. Synthesis and Characterization of

[11] Devi, R.; Chahar, S.; Khatkar, S. P.; Taxak, V. B.;

Boora, P. Relative Study of Luminescent Properties

with Judd-Ofelt Characterization in Trivalent

Terbium(III)-Pyromellitic

Acid

(H L)-1,10-

4

Phenanthroline (Phen) Luminescent Complex.

Journal Rare Earths 2010, 28, 47–50. DOI: 10.1016/

S1002-0721(10)60384-5 .

Europium

Complexes

Comprising

Ethyl-(4-

Fluorobenzoyl) Acetate. Journal of Fluorescence 2017,

27, 1349–1358. DOI: 10.1007/s10895-017-2069-3.

[12] Dhankhar, P.; Devi, R.; Devi, S.; Chahar, S.; Dalal,

M.; Taxak, V. B.; Khatkar, S. P.; Boora, P. Synthesis

and Photoluminescent Performance of Novel

Europium (III) Carboxylates with Heterocyclic

Ancillary Ligands. Rare Metals 2019. DOI: 10.1007/

s12598-019-01261-y.

[

2] Chen, W.; Li, M.; HuiBo, W.; ZhiWei, L.; ZuQiang,

B.; ChunHui, H. Advances in Luminescent

Lanthanide Complexes and Applications. Science

China-Technological Sciences 2018, 61, 1265–1285.

DOI: 10.1007/s11431-017-9212-7 .

3] Ilmi, R.; Khan, M. S.; Sun, W.; Zhou, L.; Wong,

W. Y.; Raithby, P. R. A Single Component White

Electroluminescent Device Fabricated from

a

[13] Qiu, X. Synthesis and Evaluation of New

Metallo-Organic Terbium Complex. Journal of

Materials Chemistry C 2019, 7, 13966–13975. DOI:

Antimicrobial

Fluorophores. PhD thesis, National University of

Ireland, Maynooth, 2013, 24–26.

Agents

and

Novel

Organic

10.1039/C9TC04653D .

[

4] Bunzli, J. C. G. Rising Stars in Science and

Technology: Luminescent Lanthanide Materials.

European Journal of Inorganic Chemistry 2017, 44,

[14] Onawumi, O. O. E.; Odunola, O. A.; Suresh, E.;

Paul, P. Synthesis, Structural Characterization and

Microbial Activities of Mixed Ligand Copper(II)

0

5058–5063. DOI: 10.1002/ejic.201701201 .

Complexes of 2,2 -Bipyridine and Acetylacetonate.

Inorganic Chemistry Communications 2011, 14,

1626–1631. DOI: 10.1016/j.inoche.2011.06.025 .

5] Piccinelli, F.; Bettinelli, M.; Melchior, A.; Grazioli,

C.; Tolazzi, M. Structural, Optical and Sensing

Properties of Novel Eu(III) Complexes with Furan-

[15] Kardong, D.; Upadhyaya, S.; Saikia, L. R. Screening

of Phytochemicals, Antioxidant and Antibacterial

Activity of Crude Extract of Pteridium Aquilinum

Kuhn. Journal of Pharmacy Research 2013, 6,

179–182. DOI: 10.1016/j.jopr.2012.11.037 .

ꢆ

and Pyridine- Based Ligands. Dalton Transactions

2

015, 44, 182–192. DOI: 10.1039/C4DT02326A.

[

6] Dossing, A. Luminescence from Lanthanide(3þ)

Ions in Solution. European Journal of Inorganic

2

J. KHANAGWAL ET AL.

[

16] Khullar, S.; Singh, S.; Das, P.; Mandal, S. K.

Complexes on the Basis of the Novel Ligand

b-Thujaplicin. Journal of Luminescence 2020, 218,

116852. DOI: 10.1016/j.jlumin.2019.116852.

ꢆ

Luminescent Lanthanide- Based Probes for the

Detection of Nitroaromatic Compounds in Water.

ACS Omega 2019, 4, 5283–5292. DOI: 10.1021/acso-

mega.9b00223.

[26] Gangan, T. V. U.; Sreenadh, S.; Reddy, M. L. P.

Visible-Light

Molecular Plastic Materials Derived from Eu

Excitable

Highly

Luminescent

3

þ

[

17] Gao, B.; Qiao, Z.; Chen, T. Structure and

Photoluminescence Property of Complexes of

-

Biphenyl Based b-Diketonate Ternary Complex and

Poly(Methylmethacrylate). Journal of Photochemistry

and Photobiology A 2016 , 328, 171–181. DOI: 10.

Aromatic

Carboxylic

Acid-Functionalized

Polysulfone with Eu(III) and Tb(III). Materials

Chemistry and Physics 2014 , 143 , 1119–1130. DOI:

1016/j.jphotochem.2016.06.005.

10.1016/j.matchemphys.2013.11.012.

[

27] Wang, X.; Sun, K.; Wang, L.; Tian, X.; Zhang, Q.;

Chen, B. Effect on the Fluorescence Branching Ratio

of Different Synergistic Ligands in Neodymium

Complex Doped PMMA. Journal of Non-Crystalline

Solids 2012 , 358, 1506–1510. DOI: 10.1016/j.jnoncry-

sol.2012.04.006.

28] Guo, F.; Karl, A.; Xue, Q. F.; Tam, K. C.; Forberich,

K.; Brabec, C. J. The Fabrication of Color-Tunable

Organic Light-Emitting Diode Displays via Solution

Processing. Light: Science & Applications 2017, 6,

e17094. DOI: 10.1038/lsa.2017.94.

[

18] Kharcheva, A. V.; Borisova, N. E.; Ivanov, A. V.;

Reshetova, M. D.; Kaminskaya, T. P.; Popov, V. V.;

Yuzhakov, V. I.; Patsaeva, S. V. Effect of Aliphatic

Chain Length in the Ligand on Photophysical

Properties and Thin Films Morphology of the

Europium Complexes. Russian Journal of Inorganic

Chemistry 2018 , 63, 219–228. DOI: 10.1134/

S0036023618020092.

[

19] Shen, L.; Yang, Z.; Tang, R. Synthesis, Luminescence

Properties of Eu(III) and Tb(III) Complexes with a

Novel Aromatic Carboxylic Acid and Their

[

29] Sehrawat, P.; Khatkar, A.; Boora, P.; Kumar, M.;

Malik, R. K.; Khatkar, S. P.; Taxak, V. B. Emanating

Interactions

with

Bovine

Serum

Albumin.

Spectrochimica Acta Part A: Molecular and

Biomolecular Spectroscopy 2012, 98, 170–177. DOI:

Cool White Light Emission from Novel Down-

3þ

Converted SrLaAlO :Dy

Nanophosphors for

4

10.1016/j.saa.2012.08.060.

Advanced Optoelectronic Applications. Ceramics

International 2020 , 46, 16274–16284. DOI: 10.1016/j.

ceramint.2020.03.184.

[

20] Sehrawat, P.; Khatkar, A.; Boora, P.; Kumar, M.;

Malik, R. K.; Khatkar, S. P.; Taxak, V. B.

Combustion derived color tunable Sm3þ activated

BaLaAlO4 nanocrystals for various innovative solid

stateilluminants. Chemical Physics Letters 2020.

DOI: 10.1016/j.cplett.2020.137937.

[

30] SeethaLekshmi, S.; Ramya, A. R.; Reddy, M. L. P.;

Varughese, S. Lanthanide Complex-Derived White-

Light Emitting Solids: A Survey on Design Strategies.

Journal of Photochemistry and Photobiology C 2017 ,

[

21] Sehrawat, P.; Khatkar, A.; Devi, S.; Hooda, A.; Singh,

S.; Malik, R. K.; Khatkar, S. P.; Taxak, V. B. An

Effective Emission of Characteristic Cool White

33 , 109–131. DOI: 10.1016/j.jphotochemrev.2017.11.

001.

3þ

[31] Sehrawat, P.; Khatkar, A.; Boora, P.; Hooda, A.;

Kumar, M.; Malik, R. K.; Khatkar, S. P.; Taxak, V. B.

A Novel Strategy for High Color Purity Virescent

Light from Dy Doped Perovskite Type SrLa Al O

2 7

2

Nanophosphors in Single-Phase Pc WLEDs.

Chemical Physics Letters 2019, 737, 136842. DOI: 10.

3

þ

Er -Doped SrLaAlO₄Nanocrystals for Solid-State

Lighting Applications. Journal of Materials Science:

Materials in Electronics 2020, 31, 6072–6083. DOI:

1016/j.cplett.2019.136842.

[

22] Yan, L.; Popescu, F.; Rao, M. R.; Meng, H.;

Perepichka, D. F. A Wide Band Gap Naphthalene

Semiconductor for Thin-Film Transistors. Advanced

Electronic Materials 2017, 3, 1600556–1600558. DOI:

10.1007/s10854-020-03160-w.

[

32] Francis, B.; Heering, C.; Freire, R. O.; Reddy,

M. L. P.; Janiak, C. Achieving Visible-Light

Excitation in Carbazole-Based Eu3þ–b-Diketonate

Complexes via Molecular Engineering. RSC Advances

10.1002/aelm.201600556.

23] Dalal, J.; Dalal, M.; Devi, S.; Dhankhar, P.; Hooda, A.;

Khatkar, A.; Taxak, V. B.; Khatkar, S. P. Structural

and Judd-Ofelt Intensity Parameters of a Down-

2

015, 5, 90720–90730. DOI: 10.1039/C5RA18819A.

3þ

[

33] Werts, M. H. V.; Jukes, R. T. F.; Verhoeven, J. W.

The Emission Spectrum and the Radiative Lifetime

Converting

Ba GdV O :Eu

Nanophosphors.

2

3 11

Materials Chemistry and Physics 2020 , 243, 122631.

DOI: 10.1016/j.matchemphys.2020.122631.

3

þ

of Eu

in Luminescent Lanthanide Complexes.

[

24] Dhankhar, P.; Bedi, M.; Khanagwal, J.; Taxak, V. B.;

Khatkar, S. P.; Doon, P. B. Photoluminescent Report

on Red Light Emitting Europium(III) Complexes

with Heterocyclic Acid. Spectroscopy Letters 2020,

Physical Chemistry Chemical Physics 2002, 4,

1542–1548. DOI: 10.1039/b107770h.

[34] Yan, B. M.; Guo, M. Photofunctional Eu /Tb

3

þ

3þ

Organic-Inorganic Polymeric Hybrid Microsphere

with Covalently Bonded Resin Hosts. Journal of

Photochemistry and Photobiology A 2013, 257, 34–43.

DOI: 10.1016/j.photochem.2013.02.013 .

5

3, 256–269. DOI: 10.1080/00387010.2020.1736099.

[

25] Liu, Y.; Liu, J.; Liu, Q.; He, W.; Kityk, I. V.

Photophysical Spectral Features of Fluorescent

SPECTROSCOPY LETTERS

23

[

35] Yan, B.; Lu, H. F. Lanthanide-Centered Inorganic/

Organic Hybrids from Functionalized 2-

and Biomolecular Spectroscopy 2014, 133, 785–793.

DOI: 10.1016/j.saa.2014.06.045.

Pyrrolidinone-5-Carboxylic Acid Bridge: Covalently

Bonded Assembly and Luminescence. The Journal of

Organometallic Chemistry 2009 , 694, 2597–2603.

DOI: 10.1016/j.jorganchem.2009.03.048.

[43] Wan, Y.; Lyu, H.; Du, H.; Wang, D.; Yin, G.

Synthesis and Photophysical Properties of Europium

Pentafluorinated b-Diketonate Complexes. Research

on Chemical Intermediates 2019, 45, 1669–1687.

DOI: 10.1007/s11164-018-3691-7.

[

36] de S ꢀa , G. F.; Malta, O. L.; de Mello Doneg ꢀa , C.;

Simas, A. M.; Longo, R. L.; Santa-Cruz, P. A.; da

Silva, E. F. Spectroscopic Properties and Design of

Highly Luminescent Lanthanide Coordination

Complexes. Coordination Chemistry Reviews 2000,

[

44] Bala, M.; Kumar, S.; Taxak, V. B.; Boora, P.;

Khatkar, S. P. Terbium(III) Complexes Sensitized

with b-Diketone and Ancillary Ligands: Synthesis,

Elucidation of Photoluminescence Properties and

Mechanism. Journal of Materials Science: Materials

in Electronics 2016, 27, 9306–9313. DOI: 10.1007/

s10854-016-4970-y.

1

96, 165–195. DOI: 10.1016/S0010-8545(99)00054-5 .

[

37] Hasegawa, Y.; Wada, Y.; Yanagida, S. Strategies for

the Design of Luminescent Lanthanide(III)

Complexes and Their Photonic Applications. Journal

of Photochemistry and Photobiology C 2004 , 5,

[45] Steemers, F. J.; Verboom, W.; Reinhoudt, D. N.; van

der Tol, E. B.; Verhoeven, J. W. New Sensitizer-

Modified

Calix[4]Arene

Enabling

near-UV

183–202. DOI: 10.1016/j.jphotochemrev.2004.10.003.

Excitation of Complexed Luminescent Lanthanide

Ions. The Journal of the American Chemical Society

1995, 117, 9408–9414. DOI: 10.1021/ja00142a004.

[

38] Ferreira, R. A.; S; Nobre, S. S.; Granadeiro, C. M.;

Nogueira, H. I. S.; Carlos, L. D.; Malta, O. L. A

5

7

Theoretical Interpretation of the Abnormal D ! F

0

4

3þ

[46] Latva, M.; Takalo, H.; Mukkala, V. M.; Matachescu,

C.; Rodr ꢀı guez-Ubis, J. C.; Kankare, J. Correlation

between the Lowest Triplet State Energy Level of the

Ligand and Lanthanide(III) Luminescence Quantum

Yield. Journal of Luminescence 1997, 75, 149–169.

DOI: 10.1016/S0022-2313(97)00113-0.

Intensity Based on the Eu Local Coordination in

the Na [EuW O ]ꢇ14H O Polyoxometalate. Journal

9

10 36

2

of Luminescence 2006, 121, 561–567. DOI: 10.1016/j.

jlumin.2005.12.044.

[

39] Ajlouni, A. M.; Taha, Z. A.; Momani, W. A.; Hijazi,

A. K.; Ebqa’ai, M. Synthesis, Characterization,

Biological Activities, and Luminescent Properties of

[

47] Sehrawat, P.; Khatkar, A.; Boora, P.; Khanagwal, J.;

Kumar, M.; Malik, R. K.; Khatkar, S. P.; Taxak, V. B.

Tailoring the Tunable Luminescence from Novel

0

Lanthanide Complexes with N, N -Bis (2-Hydroxy-1-

Naphthylidene)-1,

6-Hexadiimine.

Inorganica

3þ

Sm Doped SLAO Nanomaterials for NUV-Excited

WLEDs. Chemical Physics Letters 2020, 755, 137758.

DOI: 10.1016/j.cplett.2020.137758.

Chimica Acta 2012, 388, 120–126. DOI: 10.1016/j.

ica.2012.03.029.

[

40] Hossain, S.; Zakaria, C. M.; Kudrat-E-Zahan, M.

Metal Complexes as Potential Antimicrobial Agent:

48] Sehrawat, P.; Khatkar, A.; Boora, P.; Kumar, M.;

Singh, S.; Malik, R. K.; Khatkar, S. P.; Taxak, V. B.

A

Review. American Journal of Heterocyclic

Chemistry 2018, 4, 1–21. DOI: 10.11648/j.ajhc.

0180401.11.

3þ

Fabrication

of

Single-Phase

BaLaAlO :Dy

4

Nanophosphors by Combustion Synthesis. Materials

and Manufacturing Processes 2020 , 35, 1259–1267.

DOI: 10.1080/10426914.2020.1762206.

2

[

41] Ajlouni, A. M.; Salem, Q. A.; Taha, Z. A.; Hijazi,

A. K.; Momani, W. A. Synthesis, Characterization,

Biological Activities and Luminescent Properties of

Lanthanide Complexes with [2-Thiophenecarboxylic

Acid, 2-(2-Pyridinylmethylene)Hydrazide] Schiff

Bases Ligand. Journal of Rare Earths 2016, 34,

49] Sehrawat, P.; Khatkar, A.; Devi, S.; Kumar, R.; Malik,

R. K.; Khatkar, S. P.; Taxak, V. B. Crystal Structure

and Photophysical Features of Greenish Perovskite

3

þ

Type SrLa Al O :Er

Nanocrystals for down

2

2 7

Conversion White LEDs. Materials Research Express

2020, 6, 126213. DOI: 10.1088/2053-1591/ab6653.

986–993. DOI: 10.1016/S1002-0721(16)60125-4.

[

42] Ravichandran, J.; Gurumoorthy, P.; Musthafa,

M. A. I.; Rahiman, A. K. Antioxidant, DNA Binding

and Nuclease Activities of Heteroleptic Copper(II)

[50] Sehrawat, P.; Khatkar, A.; Hooda, A.; Kumar, M.;

Kumar, R.; Malik, R. K.; Khatkar, S. P.; Taxak, V. B.

3þ

An Energy-Efficient Novel Emerald Er

Doped

Complexes

Derived

from

2-((2-(Piperazin-1-

SrGdAlO Nanophosphor for PC WLEDs Excitable

4

yl)Ethylimino)methyl)-4-Substituted Phenols and

Diimines. Spectrochimica Acta. Part A, Molecular

by NUV Light. Ceramics International 2019 , 45,

24104–24114. DOI: 10.1016/j.ceramint.2019.08.118.

Article Doi

Synthesis and photoluminescence analysis of europium(III) complexes with pyrazole acid and nitrogen containing auxiliary ligands

DOI: 10.1080/00387010.2020.1817093

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Article abstract of DOI:10.1080/00387010.2020.1817093

Full text of DOI:10.1080/00387010.2020.1817093

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