Synthesis, Photophysical and Computational Study of Novel Coumarin-based Organic Dyes

Article abstract of DOI:10.1111/php.12852

A series of novel coumarin pyrazoline moieties combined with tetrazoles, 3-(1-phenyl-4-(1H-tetrazol-5-yl)-1H-pyrazol-3-yl)-2H-chromen-2-one, 6-chloro-3-(1-phenyl-4-(1H-tetrazol-5-yl)-1H-pyrazol-3-yl)-2H-chromen-2-one, 6-bromo-3-(1-phenyl-4-(1H-tetrazol-5-

Full text of DOI:10.1111/php.12852

Article type
: Research Article
Synthesis, Photophysical and Computational Study of Novel
Coumarin Based Organic Dyes
1
2
1
Mahadev N. Kumbar , Madivalagouda S. Sannaikar , Saba Kauser J. Shaikh , Atulkumar
1
2
2
3
4
A. Kamble , Manjunath N. Wari , Sanjeev R. Inamdar , Qiquan Qiao , Bhavya N. R ,
4
5
*1
Mahendra M , Jagadeesh Prasad D and Ravindra R. Kamble
1
Department of Chemistry, Karnatak University, Dharwad-580003, Karnataka, India.
2
Laser Spectroscopy Lab and UGC-CPEPA Programme, Department of Physics, Karnatak
University, Dharwad -580003, Karnataka, India.
3
Centre for Advanced Photovoltaics, Department of Electrical Engineering and Computer
Science, South Dakota State University, Brookings, SD,57007, USA.
4
Department of Studies in Physics, Manasagangotri, University of Mysore, Mysore - 570006
Karnataka, India.
5
Department of Chemistry, Mangalore University, Mangalagangothri-574199, Karnataka, India.
*
Corresponding author email: kamchem9@gmail.com (Ravindra R. Kamble)
ABSTRACT
A series of novel coumarin pyrazoline moieties combined with tetrazoles, 3-(1-phenyl-4-(1H-
tetrazol-5-yl)-1H-pyrazol-3-yl)-2H-chromen-2-one, 6-chloro-3-(1-phenyl-4-(1H-tetrazol-5-yl)-
This article has been accepted for publication and undergone full peer review but has not been
through the copyediting, typesetting, pagination and proofreading process, which may lead to
differences between this version and the Version of Record. Please cite this article as doi:
1
0.1111/php.12852
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1
H-pyrazol-3-yl)-2H-chromen-2-one, 6-bromo-3-(1-phenyl-4-(1H-tetrazol-5-yl)-1H-pyrazol-3-
yl)-2H-chromen-2-one and 6-bromo-3-(1-(4-bromophenyl)-4-(1H-tetrazol-5-yl)-1H pyrazol-3-
yl)-2H-chromen-2-one 7(a-d) were designed and synthesized. Single crystal X-ray diffraction
and their interactions were studied by Hirshfeld Surface Analysis. Thermal stabilities and
electrochemical properties of these compounds were examined from Differential Scanning
Calorimetry (DSC), Thermo gravimetric (TGA) and Cyclic Voltametric (CV) studies. Their
spectroscopic properties were analysed in various alcohols and general solvents by UV-Vis
absorption, fluorescence and time resolved spectroscopy. In addition, the ground and excited
state electronic properties were investigated using density functional theory (DFT). The
calculated highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital
g
(LUMO) and energy band gap (E ) values have revealed the effect of substitution of halogens.
The substitution has equally affected the ground and excited states of 7(a-d) compounds. The
solvatochromism on absorption, fluorescence spectra and fluorescence lifetimes of these
compounds were investigated. All these results showed the chromen-2-one of pyrazoline
tetrazole derivatives could play an important role in photonic and electronic devices.
INTRODUCTION
Fluorescent heterocyclic compounds are of huge interest as useful materials and have received
increasing attention in recent years due to their wide spread practical applications such as
fluorescence sensors and biological imaging, molecular probes used for biochemical research,
organic light-emitting diodes (OLEDs) and emitters of electroluminescence devices (1). In the
present scenario, OLEDs tend to revolutionize the display technologies in the field of
optoelectronics (2,3). Also there are many reports on using fluorescent compounds in
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biochemical and medical research (4,5). Since fluorescent dyes exhibit attractive properties,
these dyes are carefully integrated with designed functional groups to induce an intended
reaction for the generation of the highly fluorescent device (6). However, fluorescent molecules
still have unresolved complications such as operational stability of the devices and colour shift
after the operation. These problems can be resolved by introducing different substituent to the
structure backbone and hence the steric functionalization of the core organic moiety is necessary
(7).
Synthetic strategies and the fluorescent nature of coumarin derivatives have been
thoroughly reviewed and a worth of coumarin analogs have been studied for absorbance,
emissive properties and applications in optoelectronic devices like well known coumarin based
dyes; C151, C120, C343, NKX-2311, NKX-2586, NKX-2753 and NKX-2593 (8,9). Considering
the high photostability and strong fluorescence of coumarin derivatives, π-extended coumarins
are expected to provide several advantageous features over the corresponding coumarins (10,11).
Pyrazole and coumarin containing derivatives are used extensively as fluorescence probes for
their excellent photophysical properties of high fluorescence quantum yield and efficient
membrane permeability. Therefore, we designed a chemosensor due to tetrazole with an active
proton, which were combined via intramolecular hydrogen bond upon interaction with various
alcohols and general solvents, the intramolecular hydrogen bond was broken and tetrazole anion
were formed, which induced its optical changes from weak fluorescence to strong fluorescence.
Some of the representative derivatives are presented in Fig. 1.
<
Fig. 1 >
Therefore herein, the synthesis of four compounds was accomplished with electron
th
th
withdrawing substituent (halogens), especially on 6 and 4 position of coumarin and phenyl
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rings, respectively. Due to the importance of these dyes, we have initiated to study the
photophysical properties. These tetrazole derivatives of coumarins yield enhanced
photoluminescence (PL) and exhibited fluorescence in the blue region ( 450 nm). This
highlights the importance of substituent selection in determining the photophysical properties
(
12-14), high quantum yield (QY) and also electron transporting materials in OLEDs. However,
the photochemical and photophysical properties of intramolecular, electron donor-acceptor
systems depend on the excited state processes that occur after absorption of a photon by
interacting with surrounding various alcoholic and general solvent molecules (15-17). In such
case, rapid electron transfer from donor to acceptor takes place by n-π* conjugated systems (18-
2
0).
Therefore, we systematically investigated photophysical, electrochemical and thermal
properties of novel synthesised compounds by CV, DSC, TGA, time-resolved fluorescence
decay, QY, HOMO-LUMO and energy band gap (E ) calculations with a view to examine their
g
suitability in developing an electroluminescence device. Further, we report the effects of solvents
polarity (solvatochromism) on absorption and fluorescence spectra to explore strong
intramolecular charge transfer (ICT) properties of synthesized dyes. All these results showed
tetrazole derivatives of coumarins exhibit good electronic, photophysical and optoelectronic
properties.
MATERIALS AND METHODS
Materials, methods and instruments. All the reagents and solvents used were of analytical grade
purchased from Sigma-Aldrich and E-Merck and used without further purification. IR spectra were
recorded in KBr and determined on a disc matrix using an IMPACT-410 Nicolet (USA) Fourier
transform infrared spectroscopy (FTIR). Progress of the reaction was checked by Aluchrosep Silica
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Gel 60/UV 254 TLC on silica gel coated plates in ethyl acetate and hexane (3:7, 30%) as eluent. The
1
13
H and C-NMR spectra were recorded on Bruker Avance 300-MHz FT NMR spectrometers with
(
DMSO-d ) as solvent and tetramethylsilane (TMS) as internal standard. The mass spectra were
6
recorded on Shimadzu GC-MS instrument operating at 70 eV spectrometer and elemental analyses
were carried out using Heraus CHN rapid analyzer.
Steady State and Time Resolved Measurements. The steady state absorption and fluorescence
spectral measurements of samples at room temperature were performed by UV-Vis
Spectrophotometer (Hitachi-2800) and Spectrofluorometer (Jobin Yvon Horiba, Fluoromax-4),
respectively. And time resolved measurements were conducted by Time Core related Single Photon
Counting (TCSPC) instrument (ISS ChronosBH) and data were analysed with Vinci Analysis fitting
software.
Computational details. The ground and first excited state molecular geometry of the synthesized
dyes were optimized by DFT (B3LYP) calculations and configuration interaction singles (CIS) with
the 6-31G basis set as implemented in the Gaussian 09 (version D.01) program package. The TD-
DFT calculations for the compounds were performed by using B3LYP/6-31G and using triple zeta
*
functions B3LYP/6-31G . And also the results were checked against the functional dependence by
using CAM-B3LYP and B3PWM91 functional.
X-ray crystal analysis. The X-ray diffraction structure measurements were carried out using Bruker
SMART CCD area-detector with monochromatic Mo Kα radiation at room temperature. The raw
data frameworks were unified with the SAINT (21) program by using a narrow-frame algorithm.
2
The structures were solved by direct methods and refined by full-matrix least-squares methods on F
using SHELXL (22) in anisotropic approximation for all non-hydrogen atoms.
Hirshfeld Surface Analysis. The application of Hirshfeld surface has been increasing in the field of
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crystallography. This approach is a convenient tool for the investigation of intermolecular
interactions, because the crystal structure provides the most definite understanding of the
intermolecular contacts and crystal packing feature (compound 7b and 7c). Hirshfeld surface based
tools also appear to be particularly suitable for visualizing variations in the intermolecular
interactions of crystals. Hirshfeld surfaces and their associated fingerprint plots were generated
using Crystal Explorer 3.1 software (23) from the crystal structure coordinates supplied in the format
of Crystallographic Information File (CIF). Tonto, a program feature of Crystal Explorer was
implemented to generate a map of electrostatic potential on the Hirshfeld surface by using ab initio
Hartree Fock (HF) method with STO-3G basis set (24).
Thermal properties of synthesized compounds. Thermal stabilities of synthesized compounds were
examined by DSC then data were obtained using standard aluminium pans with heating rates of
-1
1
0°C min from 40 to 400°C, unless otherwise stated. The TGA experiments were recorded with
Perkin-Elmer Thermal Analysis at heat rate from 40 to 700°C at 10°C/min in a platinum pan.
Electrochemical Analysis. The cyclic voltametry is one of the standard techniques used for
electrochemical investigation. The CV experiments were performed by using ACM instrument gill
AC 660 potentiostat equipped with a conventional three electrodes (Ag/AgCl couple and platinum
plate were reference electrode and auxiliary electrode) at a scan rate of 50 mV/s at room temperature
2
under N atm. The setup is computer controlled consisting of a potentiostat VoltaLab 40.
Tetrabutylammonium perchlorate in anhydrous acetonitrile (CH
solution (0.1M).
3
CN) was used as electrolyte
Synthesis. 3-(2-Oxo-2H-chromen-3-yl)-1-phenyl-1H-pyrazole-4-carbaldehydes 4(a-d) were
prepared according to the literature method (25).
Procedure for the conversion of carbaldehyde 4(a-d) into oximes 5(a-d). A mixture of compound
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4
(a-d) (0.50 gm, 0.0018 m mol) in ethanol (12 ml) was taken in RB flask (25 ml). To this,
hydroxylamine hydrochloride (1 gm, 0.0035 m mol) and anhydrous sodium acetate (1.50 gm, 0.054
m mol) were added. Then reaction mixture refluxed for 1h. After completion of the reaction (TLC),
reaction mixture was cooled to room temperature and poured into ice cold water. The solid thus
obtained was filtered, washed with water and recrystallized from ethanol to get pure crystals of 5(a-
d).
Procedure for the conversion of oximes 5(a-d) into nitriles 6(a-d). Finely powdered Na
gm, 1m mol) was then mixed with thionyl chloride (2.20 ml, 1m mol) in a RB flask (25 ml) at 0-5C.
To the resulting powder, a solution of oxime (5a-d, 1m mol) in dry CH Cl (15 ml) was slowly
2 3
CO (0.16
2
2
added and stirred at 0°C for 1 h. The progress of the reaction was monitored by TLC. After
completion of the reaction, the mixture was shaken with DCM (5 ml) and filtered. The residue was
further washed with DCM and solvent was evaporated under reduced pressure to afford the final
product 6(a-d).
Synthesis of 3-(1-aryl-4-(1H-tetrazol-5-yl)-1H-pyrazol-3-yl)-2H-chromen-2-one 7(a-d). A mixture of
3
-(2-oxo-2H-chromen-3-yl)-1-aryl-1H-pyrazole-4-carbonitrile, 6(a-d) (0.50 gm, 0.003 m mol),
NaN (0.90 gm, 0.0134 m mol) and tri-ethylamine hydrochloride (1.90 gm, 0.0134 m mol) in toluene
10.00 ml) was heated to 105-110°C for 48 h with constant stirring. The reaction mixture was cooled
3
(
to room temperature and stirred for 30 minutes at RT. The product was extracted with water (15ml)
and NaOH solution (5%, 2ml). Dil. HCL was added drop-wise, the solid thus obtained was filtered
to get a pure tetrazole derivative 7(a-d).
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H
N
X
O
N
H
X
N
N
DMF/ POCl3
N
NH2 EtOH / AcOH
RT Stirring
R
5
0 - 60 oC
Reflux, 4 h
O
O
O
O
O
R
X
R
O
OHC
1
a-d
2 a-b
3
a-d
4 a-d
Reflux,
for 1 h
H2NOH.HCl
anhyd. NaOAc
X
X
N
N
NaN3,
TEA.HCl
anhyd. Na CO3
X
2
R
R
SOCl , DCM
N
N
2
O
N
N
R
O
O
O
Toluene
0
-5 °C
N
NH
O
O
OH
N
N
NC
N
7
a-d
6 a-d
5 a-d
Where, 7a; R = -H, X = -H, 7b; R = -6Cl, X = -H, 7c; R = -6Br, X = -H, 7d; R = -6Br, X = -4Br
Scheme 1 Synthetic approach for target compounds 7(a-d).
The synthetic route of the target compounds is outlined in Scheme 1. Initial cyclization of
aryl hydrazones 3(a-b) of 3-acetylcoumarin 1(a-d) by Vilsmeier–Haack reaction afforded a 3-(2-
oxo-2H-chromen-3-yl)-1-phenyl-1H-pyrazole-4-carbaldehydes 4(a-d). These on reaction with
hydroxylamine hydrochloride in presence of anhydrous sodium acetate (NaOAc) under reflux
gave oximes 5(a-d). Further, these oximes 5(a-d) were converted into nitriles by thionyl chloride
2 2 3 2 2
(SOCl ) and anhydrous Na CO in DCM (CH Cl ) under cold condition to afford coumarin
pyrazolo nitriles 6(a-d). 1,3-Dipolar cyclization of these nitriles 6(a-d) with sodium azide in the
presence of TEA.HCl (phase transfer catalyst and base) at 105-110°C in toluene gave the title
compounds 3-(1-aryl-4-(1H-tetrazol-5-yl)-1H-pyrazol-3-yl)-2H-chromen-2-ones 7(a-d). Such
1
13
newly synthesized compounds were characterized by IR, H and C NMR, Mass spectral and
elemental analyses (see Supporting Information).
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Spectral characterizations.
E)-2-(6-Bromo-2H-chromen-2-one)-1-(1-(4-bromophenyl)ethylidene)hydrazine (3d)
(
-1
Yield: 82%, colour: pale yellow. MP = 272-274°C (dec.). IR (KBr, νmax, cm ): 1707 (coumarin,
1
C=O), 3481 (N-H). H NMR (400 MHz, DMSO-d
6
): δ 8.78 (s, 1H, coumarin C
4
-H), 8.42 (s, 1H,
Ar-H), 8.42 (s, 1H, N-H), 8.24-7.72 (m, 4H, Ar-H), 7.49 (d, 1H, Ar-H), 7.39 (d, 1H, Ar-H), 1.22
1
3
(
s, 3H, CH
3
) ppm. C NMR (100 MHz, DMSO-d
44.22, 143.14, 136.45, 131.99, 131.43, 128.55, 122.40, 120.29, 120.21, 118.65, 118.26, 118.12,
12 6 2
4.10 (CH ) ppm. MS-EI, (m/z): = 437 [M+4], 435 [M=2], 433.92 [M]; Anal. For C19H N O :
6
): δ 156.63 (coumarin C=O), 152.27, 145.31,
1
3
3
calcd. C, 46.82; H, 2.77; N, 6.42; Found: C 46.85, H 2.80, N 6.46.
-(7-Bromo-2-oxo-2H-chromen-3-yl)-1-(4-bromophenyl)-1H-pyrazole-4-carbaldehyde (4d)
3
-1
Yield: 87%, colour: Pale brown. MP = 256-258°C (dec.). IR (KBr, νmax, cm ): 1736 (coumarin,
1
C=O), 1678 (pyrazole, C=O). H NMR (400 MHz, DMSO-d
6
): δ 9.28 (s, 1H, pyrazole, C-H),
8
7
1
1
4
.45 (s, 1H, coumarin C -H), 8.42 (d, 2H, Ar-H), 7.97-7.69 (m, 4H, Ar-H), 7.51 (d, 1H, Ar-H),
1
3
6
.44 (d, 1H, Ar-H) ppm. C NMR (100 MHz, DMSO-d ): δ 162.41 (coumarin C=O), 158.43,
55.76, 148.74, 145.12, 138.82, 135.97, 133.64, 131.52, 128.45, 126.48, 125.36, 124.20, 122.61,
20.26, 119.86, 119.54, 118.63, 118.83 ppm. MS-EI, (m/z): = 476 [M+4], 474 [M=2], 472 [M];
Anal. For C19
2 2 3
H10Br N O : calcd. C, 48.13; H, 2.13; N, 5.91; Found: C 48.15, H 2.16, N 5.96.
1
-(4-Chlorophenyl)-3-(2-oxo-2H-chromen-3-yl)-1H-pyrazole-4-oxime (5b)
-1
Yield: 78%, colour: brown. MP = 268-270 °C (dec.). IR (KBr, νmax, cm ): 1728 (coumarin,
1
C=O), 3285 (-OH). H NMR (400 MHz, DMSO-d
6
): δ 9.09 (s, 1H, pyrazole, C-H), 8.80 (s, 1H,
coumarin C
Ar-H), 7.38 (d, 1H, Ar-H) ppm. C NMR (100 MHz, DMSO-d
46.86, 145.50, 143.75, 142.72, 140.91, 138.91, 137.39, 132.37, 132.21, 131.08, 129.61, 128.92,
4
-H), 8.32 (d, 1H, Ar-H), 8.06 (s, 1H, Ar-H), 7.93-7.51 (m, 6H, Ar-H), 7.43 (d, 1H,
1
3
6
): δ 158.98 (coumarin C=O),
1
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1
28.81, 127.21, 127.08, 126.91, 124.68, 120.98 ppm. MS-EI, (m/z): = 367 [M=2], 365 [M];
Anal. For C19 : calcd. C, 62.39; H, 3.31; N, 11.49; Found: C 62.41, H 3.35, N 11.53.
-(6-Bromo-2-oxo-2H-chromen-3-yl)-1-(4-bromophenyl)-1H-pyrazole-4-oxime (5d)
3 3
H12ClN O
3
-1
Yield: 78%, colour: Light brown. MP = 277-279 °C (dec.). IR (KBr, νmax, cm ): 1737
1
(
coumarin, C=O), 3445 (-OH). H NMR (400 MHz, DMSO-d
6
): δ 9.09 (s, 1H, pyrazole, C-H),
8
7
d
1
.80 (s, 1H, coumarin C
.55 (d, 1H, Ar-H), 7.40 (d, 1H, Ar-H), 7.36 (d, 1H, Ar-H) ppm. C NMR (100 MHz, DMSO-
): δ 159.37 (coumarin C=O), 153.42, 146.88, 145.52, 143.75, 142.72, 140.91, 138.91, 137.39,
32.37, 132.21, 131.08, 129.61, 128.92, 128.82, 127.21, 127.08, 126.90, 124.68, 120.96 ppm.
: calcd. C, 46.66; H,
4
-H), 8.36 (d, 1H, Ar-H), 8.06 (s, 1H, Ar-H), 7.93-7.64 (m, 4H, Ar-H),
1
3
6
2 3 3
MS-EI, (m/z): = 490 [M+4], 488 [M+2], 486 [M]; Anal. For C19H11Br N O
2
.27; N, 8.59; Found: C 46.69, H 2.30, N 8.61.
-(2-Oxo-2H-chromen-3-yl)-1-phenyl-1H-pyrazole-4-carbonitrile (6a)
3
-1
Yield: 89%, colour: pale yellow. MP = 212-214 °C. IR (KBr, νmax, cm ): 1726 (coumarin, C=O),
1
1
602 (pyrazole, C=N), 2224 (Nitrile). H NMR (400 MHz, DMSO-d
6
): δ 9.44 (s, 1H, pyrazole,
C-H), 8.56 (s, 1H, coumarin C
.73 (t, 1H, Ar-H), 7.61 (t, 1H, Ar-H), 7.51- 7.41 (m, 4H, Ar-H) ppm. MS-EI, (m/z): = 313
M+1]; Anal. For C19 : calcd. C 72.84, H 3.54, N 13.41; Found: C 72.90, H 3.61, N
4
-H), 7.94 (d, 1H, Ar-H), 7.90 (d, 1H, Ar-H), 7.88 (d, 1H, Ar-H),
7
[
11 3 2
H N O
1
1
3.49.
-(4-Chlorophenyl)-3-(2-oxo-2H-chromen-3-yl)-1H-pyrazole-4-carbonitrile (6b)
-1
Yield: 72%, colour: White. MP = 198-200 °C. IR (KBr, νmax, cm ): 1721 (coumarin, C=O), 1601
1
(
pyrazole, C=N), 2218 (Nitrile). H NMR (400 MHz, DMSO-d
6
): δ 8.73 (s, 1H, pyrazole, C-H),
8
7
.70 (s, 1H, coumarin C
4
-H), 8.05 (d, 1H, Ar-H), 7.79 (d, 1H, Ar-H), 7.76 (d, 1H, Ar-H), 7.72-
1
3
.14 (m, 4H, Ar-H), 7.08 (t, 1H, Ar-H), ppm. C NMR (100 MHz, DMSO-d
6
): δ 161.59
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(
coumarin C=O), 156.46, 151.83, 151.67, 151.61, 144.91, 144.12, 139.20, 134.49, 133.87,
32.62, 13198, 129.01, 128.80, 128.39, 122.48, 120.58 ppm. MS-EI, (m/z): = 349 [M+2], 347
M]; Anal. For C H ClN O : calcd. C, 65.62; H, 2.90; N, 12.08; Found: C 65.66, H 2.94, N
1
[
1
9
10
3
2
1
1
2.12.
-(4-Bromophenyl)-3-(2-oxo-2H-chromen-3-yl)-1H-pyrazole-4-carbonitrile (6c)
-1
Yield: 78%, colour: Light brown. MP = 220-222 °C. IR (KBr, νmax, cm ): 1731 (coumarin,
1
C=O), 1601 (pyrazole, C=N), 2223 (Nitrile). H NMR (400 MHz, DMSO-d
6
): δ 9.44 (s, 1H,
pyrazole, C-H), 8.57 (s, 1H, coumarin C
H, Ar-H), 7.71-7.49 (m, 4H, Ar-H), 7.43 (t, 1H, Ar-H), ppm. C NMR (100 MHz, DMSO-d
δ 158.21 (coumarin C=O), 153.50, 148.20, 143.55, 138.20, 135.92, 133.01, 129.72, 129.30,
4
-H), 7.97 (d, 1H, Ar-H), 7.88 (d, 1H, Ar-H), 7.73 (d,
1
3
1
6
):
1
3
28.13, 124.95, 119.45, 118.53, 117.81, 116.19, 112.95, 93.95 ppm. MS-EI, (m/z): = 393 [M+2],
91 [M]; Anal. For C19 : calcd. C, 58.18; H, 2.57; N, 10.71; Found: C 58.21, H 2.62,
3 2
H10BrN O
N 10.74.
-(6-Bromo-2-oxo-2H-chromen-3-yl)-1-(4-bromophenyl)-1H-pyrazole-4-carbonitrile (6d)
3
-1
Yield: 75%, colour: Brown. MP = 187-189 °C. IR (KBr, νmax, cm ): 1731 (coumarin, C=O),
1
1
600 (pyrazole, C=N), 2224 (Nitrile). H NMR (400 MHz, DMSO-d
6
): δ 9.91 (s, 1H, pyrazole,
4
C-H), 9.26 (s, 1H, coumarin C -H), 8.39 (d, 1H, Ar-H), 8.12 (d, 1H, Ar-H), 7.98 (d, 1H, Ar-H),
1
3
7
1
1
6
.96-7.42 (m, 4H, Ar-H), ppm. C NMR (100 MHz, DMSO-d ): δ 162.50 (coumarin C=O),
59.20, 151.66, 150.73, 144.91, 144.12, 142.68, 139.28, 134.89, 132.73, 130.00, 129.73, 127.87,
22.48, 120.58, 119.67, 118.49 ppm. MS-EI, (m/z): = 393 [M+2], 391 [M]; Anal. For
C
19
H
9
Br
2 3 2
N O : calcd. C, 48.44; H, 1.93; N, 8.92; Found: C 48.47, H 1.96, N 8.97.
3
-(1-Phenyl-4-(1H-tetrazol-5-yl)-1H-pyrazol-3-yl)-2H-chromen-2-one (7a)
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-1
Yield: 84%, colour: yellow. MP = 284-285 °C (dec.). IR (KBr, νmax, cm ): 1726 (coumarin,
1
C=O), 1606 (pyrazole, C=N), 3428 (Tetrazole, N-H). H NMR (400 MHz, DMSO-d
6
): δ 9.18 (s,
1
H, tetrazole, N-H), 8.47 (s, 1H, pyrazole, C-H), 7.99 (s, 1H, coumarin C -H), 7.89 (d, 2H, Ar-
4
H), 7.86 (d, 1H, Ar-H), 7.70 (t, 1H, Ar-H), 7.58 (t, 1H, Ar-H), 7.48 (d, 1H, Ar-H), 7.45 (t, 1H,
1
3
Ar-H), 7.43 (t, 1H, Ar-H), 7.41 (t, 1H, Ar-H) ppm. C NMR (100 MHz, DMSO-d
6
): δ 164.24
), 158.73 (coumarin C-O), 153.53, 145.57, 143.24, 138.71,
32.50, 129.74, 128.97, 127.50, 124.80, 119.00, 116.15, 115.97, 113.21, 110.11 (pyrazole C
(coumarin C=O), 162.00 (tetrazole C
4
1
3
)
12 6 2
ppm. MS-EI, (m/z): = 356 [M+1]; Anal. For C19H N O : calcd. C 64.04, H 3.39, N 23.58;
Found: C 64.09, H 3.46, N 23.67.
6
-Chloro-3-(1-phenyl-4-(1H-tetrazol-5-yl)-1H-pyrazol-3-yl)-2H-chromen-2-one (7b)
-1
Yield: 82%, colour: yellow. MP = 280-282 °C (dec.). IR (KBr, νmax, cm ): 1720 (coumarin,
1
C=O), 1606 (pyrazole, C=N), 3224 (tetrazole, N-H). H NMR (400 MHz, DMSO-d
6
): δ 9.22 (s,
1
1
H, tetrazole, N-H), 8.43 (s, 1H, pyrazole, C-H), 8.00 (d, 1H, Ar-H), 7.97 (s, 1H, Ar-H), 7.95 (s,
1
3
H, Ar-H), 7.74 (d, 1H, Ar-H), 7.61-7.53 (m, 4H, Ar-H), 7.43 (t, 1H, Ar-H) ppm. C NMR (100
): δ 162.03 (coumarin C=O), 160.12 (tetrazole C ), 158.30 (coumarin C-O),
52.15, 145.24, 141.95, 138.66, 132.03, 129.78, 128.50, 127.94, 127.59, 120.32, 119.01, 118.20,
14.08, 110.15 (pyrazole C ) ppm. MS-EI, (m/z): = 392 [M+2], 390 [M]; Anal. For
ClO : calcd. C 58.40, H 2.84, N 21.51; Found: C 58.47, H 2.90, N 21.62.
-Bromo-3-(1-phenyl-4-(1H-tetrazol-5-yl)-1H-pyrazol-3-yl)-2H-chromen-2-one (7c)
MHz, DMSO-d
6
4
1
1
3
C
19
H
11
N
6
2
6
-1
Yield: 72%, colour: pale brown. MP = 272-274 °C (dec.). IR (KBr, νmax, cm ): 1726 (coumarin,
1
C=O), 1610 (pyrazole, C=N), 3123 (tetrazole, N-H). H NMR (400 MHz, DMSO-d
6
): δ 9.15 (s,
1
1
H, tetrazole, N-H), 8.45 (s, 1H, pyrazole, C-H), 7.95 (d, 1H, Ar-H), 7.84 (s, 1H, Ar-H), 7.65 (d,
1
3
6
H, Ar-H), 7.57 (d, 1H, Ar-H), 7.47-7.38 (m, 5H, Ar-H) ppm. C NMR (100 MHz, DMSO-d ):
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δ 161.61 (coumarin C=O), 159.37 (tetrazole C
4
), 155.08 (coumarin C-O), 146.15, 143.87,
1
39.24, 133.13, 130.35, 130.24, 129.57, 128.12, 126.01, 125.43, 120.73, 119.56, 116.75, 108.88
(
pyrazole C ) ppm. MS-EI, (m/z): = 436 [M+2], 434 [M]; Anal. For C H N BrO : calcd. C
3
19 11
6
2
5
6
2.43, H 2.55, N 19.31; Found: C 52.51, H 2.60, N 19.38.
-Bromo-3-(1-(4-bromophenyl)-4-(1H-tetrazol-5-yl)-1H-pyrazol-3-yl)-2H-chromen-2-one (7d)
-1
Yield: 70%, colour: brown. MP = 272-274 °C (dec.). IR (KBr, νmax, cm ): 1726 (coumarin,
1
C=O), 1606 (pyrazole, C=N), 3123 (tetrazole, N-H). H NMR (400 MHz, DMSO-d
6
): δ 9.18 (s,
1
2
1
1
H, tetrazole, N-H), 8.41 (s, 1H, pyrazole, C-H), 7.98 (d, 2H, Ar-H), 7.94 (s, 1H, Ar-H), 7.72 (d,
1
3
H, Ar-H), 7.50 (d, 2H, Ar-H), 7.40 (s, 1H, Ar-H) ppm. C NMR (100 MHz, DMSO-d
62.87 (coumarin C=O), 158.92 (tetrazole C ), 154.22 (coumarin C-O), 152.70, 145.79, 142.56,
39.18, 132.64, 130.38, 129.09, 128.51, 128.21, 121.90, 120.89, 119.56, 118.80, 105.06
pyrazole C ) ppm. MS-EI, (m/z): = 516 [M+4], 514 [M+2], 512 [M]; Anal. For C19 Br
6
): δ
4
(
3
H
10
N
6
2
O
2
:
calcd. C 44.39, H 1.96, N 16.35; Found: C 44.45, H 1.99, N 16.42.
RESULTS AND DISCUSSION
The conversion of coumarin pyrazolonitriles 6(a-d) to tetrazoles 7(a-d) was confirmed through
-1
IR spectra which showed disappearance of CN absorption band around 2224-2236 cm and
-1
appearance of new band in the range 3426-3450 cm due to N–H and also compounds have
shown strong adsorption band for carbonyl carbon of coumarin and tetrazole (N-H) at 1720-
-
1
1
1
9
727 and 3224-3428 cm , respectively. The H NMR spectrum of showed a singlet at δ 9.17-
.29, 7.74-7.99 and 8.40-8.51 ppm. due to N-H of tetrazole, C -H of coumarin and pyrazole ring
4
proton, respectively. Aromatic protons in all compounds appeared as multiplets in the range
1
3
7
.59-7.68 to 7.28-7.54 ppm. Further, C NMR spectral study indicated the number of signals in
consistent with number of magnetically non equivalent carbon atoms and also in mass spectra all
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the synthesised compounds have shown the molecular ion peaks at their respective m/z values
(
representative spectra are given in Supporting Information, Figs. S1-S51).
Subsequently, the X-ray diffraction structures of novel dyes 7b and 7c were obtained.
Both structures were obtained by using DMSO (HPLC grade) at room temperature and obtained
single crystal structure of the compound is characterized by a long range, well defined three
dimensional orders. The asymmetric unit of the enantiotopic structure of the compound is shown
1
in Fig. 2. These dyes crystallize in the monoclinic P2 space group. Both the compounds
crystallize in the same asymmetric space groups. In the refined structure, all bond lengths and
angles are within normal ranges (26) (Supporting Information, Table S52).
Crystal Data for Compound 7b:
C
19
H11ClN
6
O
2
, (M =390.79 g/mol): Monoclinic, space group P21 (no. 4), a = 6.2282(5) Å, b =
1
9.8260(14) Å, c = 15.5480(12) Å, β = 92.283(5)°, V = 1918.3(3) Å3, Z = 4, T = 296.15 K,
-1
μ(MoKα) = 0.226 mm , Dcalc = 1.353 g/cm3, 18814 reflections measured (3.33° ≤ 2Θ ≤
5
6.964°), 8338 unique (Rint = 0.0353, Rsigma = 0.0566) which were used in all calculations.
The final R1 was 0.0427 (I > 2σ(I)) and wR2 was 0.0907.
Crystal Data for Compound 7c:
C
19
H11BrN
6
O
2
, (M =435.25 g/mol): monoclinic, space group P21 (no. 4), a = 6.2436(4) Å, b =
9.8399(12) Å, c = 15.6428(10) Å, β = 92.017(3)°, V = 1936.5(2) Å3, Z = 4, T = 296.15 K,
μ(MoKα) = 2.150 mm-1, Dcalc = 1.493 g/cm3, 14713 reflections measured (3.316° ≤ 2Θ ≤
2.432°), 6778 unique (Rint = 0.0300, Rsigma = 0.0654) which were used in all calculations.
The final R1 was 0.0386 (I > 2σ(I)) and wR2 was 0.0885.
Fig. 2 
1
5

Hirshfeld surface determination for compound 7b and 7c
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Molecular Hirshfeld surfaces in the crystal structure are constructed basing on the electron
distribution calculated as the sum of spherical atom electron densities. For a given crystal
structure the Hirshfeld surface is unique and this property helps further gain additional insight
into the intermolecular interaction of molecular crystals. For each point on that isosurface, two
distances are defined: d
surface, and d is the distance to the nearest nucleus internal to the surface. The normalized
contact distance (dnorm) based on both d and d , and the Van-der-Waals (vdW) radii of the atom,
e
, the distance from the point to the nearest nucleus external to the
i
e
i
is given by the Eq. (1), which enables us to identify the regions of particular important
intermolecular interactions.
=
+
(1)
From Eq. (1),
represents the Van Der Waals (vdW) radius of the appropriate atom internal
or external to the surface.
positive for contacts greater than vdW separations, which are displayed using a red-white-blue
color scheme. The red regions correspond to closer contacts with negative value, the blue
regions correspond to longer contacts with positive value and the white regions
correspond to the value of zero (contacts around the vdW separation). Short contacts were
observed with larger and brighter red spots as inter nuclear separations ( and ) decreases.
Since is symmetric in and , any close intermolecular contacts are characterized by
two identical red regions. One act as a donor and another as an acceptor if or
respectively (27). The combination of d and d in the form of a 2D fingerprint plot provides
is negative where contacts shorter than vdW separations and
<
>
e
i
summary of intermolecular contacts in the crystal. In addition, crystal voids were generated by
plotting the isosurface at 0.002 au for the entire procrystal electron density.
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The Hirshfeld surfaces mapped with dnorm and Shape index are shown in Fig. 3. The 3-D dnorm
surfaces were mapped by using a fixed color scale of 0.223 (red) to 1.411 Å (blue) and Shape
index were mapped in the color range of ±1.000 Å. As we expected, the dnorm surfaces shown in
Fig. 3 a, b reveal the close contacts of hydrogen bond donors and acceptors, but other close
contacts are also evident here. In dnorm surfaces, for both compounds 7b and 7c, the large circular
depressions (deep red) are the indicators of hydrogen bonding contacts, which are due to N-
H….N close contacts, whereas other visible light red spots are due to O-H…O contacts. We have
identified six and eight intermolecular interactions for compounds 7b and 7c, respectively, which
are tabulated in Table S53. These interactions stabilize the crystal structure of the compounds.
The Shape index is a qualitative measure of shape. One important attribute of the Shape
index is that two shapes where the Shape index differs only by a sign representing
complementary "stamp" and "mould" pairs. These maps of Shape index on the Hirshfeld surfaces
were used to identify complementary hollows and bumps. The pattern of red and blue triangles
on the same region of the shape index surface (Fig. 3 c, d) is characteristic of
stacking
interactions. The pattern of red and blue triangles on this region of both sides of the molecule
shows how adjacent molecules in the crystal are related by symmetry. Blue triangles represent
convex regions due to ring carbon atoms of the molecule inside the surface, while red triangles
represent concave regions due to carbon atoms of the stacked molecule above it (28).

Fig. 3 
The relative percentage contributions of the different interactions to the Hirshfeld surface were
calculated for both the compounds and are depicted in Fig. S54.
Fig. S55 (a-l) gives the percentage of contribution of intermolecular interactions to the
total Hirshfeld surface for compounds 7b and 7c. The fingerprint plots can be decomposed to
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highlight particular atoms pair close contacts (29,30). This decomposition enables separation of
contributions from different interaction types, which overlap in the full fingerprint. The
Fingerprint plots of compound 7b and 7c are almost similar for H···H, N···H, O···H and C···H
intercontacts. The intermolecular H···H contacts are predominant in the title compounds 7b & 7c
and their relative contributions being 25.6% and 25% respectively (Fig. S55 c, d). The
N···H/H···N interactions are identified by distinct long spikes (Fig. S55 e, f); the upper left spike
(donor) is due to H···N and lower right spike (acceptor) is due to N···H interactions and their
contribution to Hirshfeld surface are 8.8% and 10.7% for compound 7b and 9.3% and 10.5% for
compound 7c respectively. In Fig. S55 g, h, the characteristics wings for both the compounds 7b
&
7c are due to C-H···ᴨ interactions and their contributions are 15.6% and 12.4% to their
Hirshfeld surfaces, respectively. The O···H/H···O interactions are identified as a small spikes
Fig. S55 i, j); the upper left spike (donor) is due to H···O and lower right spike (acceptor) is due
(
to O···H interactions and their contribution to Hirshfeld surfaces are 6% and 6.3%, respectively.
In Fig. S55 k, l, another significant interaction in compound 7b is Cl…H/H…Cl and its
contribution to the Hirshfeld surface is 12.2%. Similarly in compound 7c the Br···H/H···Br
interaction contributes about 11.8% to the total Hirshfeld surface.
The electrostatic potential mapped onto Hirshfeld surface to rationalize intermolecular
interactions has been represented in Fig. 4. Here the red region is electronegative, while the blue
region is electropositive and both are complementing to each other (31). The electrostatic
complementarity can be viewed from Fig. 5. In Fig. 5a, electronegative and electropositive
patches are present near nitrogen atom and near hydrogen atom, respectively. Similarly in Fig.
5
b, electronegative and electropositive patches near carbon and hydrogen atoms are observed.
Fig. 4 >
<
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<
Fig. 5 >
Crystal voids
The anisotropic properties of the molecular solids are determined equally by the empty
spaces as well as the filled ones (32). Voids in the crystal can be defined as the region of empty
space between the molecules in the crystal and this empty space is the region which lies outside
the normal van der Waals surface area of molecules in the unit cell and within which no nuclei
exists. Voids in the crystalline material have been visualized by constructing the (0.002 au) -
isosurface of procrystal electron density. This 0.002 au electron density isosurface contains more
than 98% of the electronic charge of molecules and identifies the empty space in the crystal by
determining the shape and size of molecules. Fig. S56 displays the voids surface of the grown
crystals of compound 7b & 7c. For compound 7b & 7c, the volume of the void was computed to
3
3
be 224.92 Å and 214.62 Å , while from single crystal XRD analysis, the volume of the unit cell
3
3
comes out to be 1918.3 Å and 1936.5 Å . Thus the volume occupied by voids is around 11.72%
and 11.082% of the unit cell volume. Void volume, surface area and void volume as a percentage
of total unit cell volume of grown crystals computed at 0.002 au isosurface have been
summarized in Table S57.
Thermal and Electrochemical Properties:
From DSC studies, the phase transition properties of these compounds were analyzed. The DSC
curves of 7(a-d) compounds shows an exothermic shift related to the glass transition temperature
(
T
g
) at 94.41, 74.61, 69.19 and 95.32°C. The crystalline materials can be characterized by their
glass transition temperature (T ). If the material is stored below the T , the rate at which it can
g
g
recrystallize is significantly reduced relative to a material that is above the glass transition
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temperature. Crystallinity of these compounds found a sharp peak at 284.73, 282.02, 274.80°C
and 273.89°C. These values are significantly higher than those of other charge transport
materials such as the hole transport material N,N′-bis(3-methylphenyl)-N,N′-diphenylbenzidine
(
T
g
= 65 °C) (33). The glass transition temperature of compounds 7a, 7b, and 7c shows an
exothermic melting peak at around 74.61 to 95.32°C. However, compound 7d has shown broad
spectrum of recrystallization at 273.89°C with T equal to 69.19 °C which has been detected in
g
thermogram. The exothermic peaks of 7 (a-d) were observed at higher temperature resulting
from recrystallization and it revealed that these compounds are crystalline in nature with good
glass transition state stability. In TGA, high thermo-chemical stability of the compounds is
demonstrated as shown in Fig. 6 a, b. The thermal decomposition under nitrogen starts above
2
1
73-285°C and the temperature for 5% weight loss (T5d) were observed at a heating rate of
0°C/min. From all these results it is clear that the studied dyes exhibit good thermal stability
(
33).

Fig. 6 
Further, in order to understand the electro-chemical properties of dyes, we performed CV
measurements shown in Fig. 7, calculated HOMO-LUMO and energy band gap of these
compounds which are tabulated in Table 1. Band gap of these dyes were calculated from the
difference between HOMO and LUMO energy levels. The HOMO-LUMO energy levels were
computed using Eq. (2) and (3), respectively by measuring the redox potentials Ered and Eox and
ferrocene is used as the known reference (34).
onset
ox
HOMO = – [
E
+ 4.8] (eV)
(2)
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LUMO = [HOMO +
] (eV)
(3)
onset
ox
onset
are respectively, measured oxidation and reduction potentials relative to
red
Where,
E
and
E
+
Ag/Ag . From Table 1, it is clear that all 7(a-d) compounds are possessing almost same energy
band gap ranging from 3.95 to 4.03 eV. This is due to the presence of electron withdrawing
groups (halogens) on tetrazole linked to pyrazoline coumarin which simultaneously affects both
HOMO and LUMO levels and keeping the almost constant difference between HOMO-LUMO
(energy band gap) of corresponding dyes. Therefore, the substitution of tetrazole linked
pyrazoline derivatives of coumarin has affected both the ground and excited states. These results
revealed the oxidation behaviour and hence they can be applied as bipolar transport materials for
electroluminescence applications.

Table 1 
Fig. 7 

Photophysical studies:
To study the photophysical properties of 7(a-d) dyes, we have used optimised concentrations to
avoid aggregation and re-absorption of fluorescence by the dye molecules in steady state spectral
measurements. The concentrations were of the order of 1-2 µM.
Quantum yield calculation
The fluorescence quantum yields (Φ) of 7(a-d) are determined using coumarin 440 (C440) as
reference sample by using Eq. (4) (34) and results are summarized in Table 2.
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2
I
I
dye
ref
A
ref
n
n
dye
2


dye

ref
A
dye
ref
(
4)
where, Φdye and Φref are the fluorescence quantum yields of the dyes and reference (C440; Φref
=
0
.98 in ethanol) (35), respectively, Adye and Aref are the optical absorbances of the dye and
reference samples at the excitation wavelengths, Idye and Iref are the integrated emission
intensities of the dyes and reference samples, ndye and nref are the refractive indices of the
solvents used for the dyes and the reference, respectively. The Fig. 8 shows the typical overlay
of absorption and emission spectra of 7a, 7b, 7c, 7d and C440 samples. From Table 2, it is
observed that appending the tetrazole pyrazole to coumarin moiety resulted in drastically
enhanced PL and QY. However, among the studied pyrazole tetrazole coumarin derivatives, Br-
th
substituted (7c) at 6 position show comparatively high quantum yield (see Table. 2).

Table 2 
Fig. 8 

Optical band gap
The energy levels of the electronic states correspond to the energy carried by UV or Visible
radiation. At resonance, the molecules will absorb quantized energy transported by the
electromagnetic radiation and promote an electron from the low energy molecular orbital to
opt
higher energy molecular orbital (36). The optical band (E
g
) corresponds to the energy of the
long edge wavelength of the exciton absorption band (37). Conversely, the optical band gap
values of 7(a-d) dyes were approximated from the onset of the low energy side of their
absorption spectra (onset) to the baseline according to the Eq. (5) (38,39). The calculated optical
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E
g
values ranging from 3.96 to 4.03 eV as tabulated in Table 1. From this table we noticed that
th
the halide groups at 6 position of coumarin appended to tetrazole-pyrazole derivative has no
significant effect on E . But as compared to Dipyrazolopyridine derivatives (40), our tetrazole
g
pyrazole coumarin derivatives possessing slightly higher optical energy band gap. And good
agreement of their band gaps obtained by the absorption spectrum and CV measurements were
observed. And in both measurements, the constant E
g
values of 7(a-d) noticed and suggested the
less prominent effect of substituent (halogens) on E
g
. Also this fact is well supported by DFT
study which is discussed in the next section.
opt 1240
(
5)

E
g

onset
Substituent and Solvent Effect:
With the synthetic dyes in hand, it is most important to investigate effect of substituent on the
photophysical properties of dyes. Hence, absorption and fluorescence spectra of 7(a-d) dyes
were recorded in same solvent viz., ethyl acetate (EA) to study the effects of substituent on PL
properties and the results are summarized in Table 2 (see Supporting Information, Fig. S58 a, b).
From this table, we found that absorption wavelength of 7(b-d) dyes has been slightly blue
shifted relative to 7a dye. This is due to presence of conjugation in pyrazole linked tetrazole -NH
of the coumarin. Further, the corresponding fluorescence emission maxima of 7b, 7c and 7d dyes
also have been blue shifted. Therefore, both ground and excited states were affected equally due
th
to the halide functional group at 6 position of coumarin of all studied dyes. As a consequence,
there is no rapid variation in stokes shift and in turn absence in the variation of energy band gap
of corresponding dyes. This result is in consistent with the previously discussed CV and optical
band gap measurements.
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Now to understand the effects of solvents polarity on PL properties of tetrazole pyrazole
coumarin derivatives dyes containing different halogens as substituent were recorded in alcohols
(
methanol to decanol) and general solvents (DMSO, DMF, Acetonitrile, Chloroform and EA).
For instance, the overlay of absorption and emission spectra of dye 7a is shown in Fig. 9 a, b.
From this figure, it is observed that the fluorescence emission maximum of 7a has red shifted
(bathochromic shift) as a function of increasing solvent polarity. Further, the same results were
observed for 7(b-d) dyes (see Figs. S59-S61 and Figs. S62-S64). This is attributed to change in
the ground and excited state dipole moments of 7(a-d) dyes which implies the relative change in
ground and excited state energy distribution of these dyes. Comparable shifts observed in
absorption and emission spectra show the small difference between ground and excited state
dipole moment. These results are also confirmed by DFT studies given in the next section. The
bathochromic shift of these dyes is attributed to the interaction of solute molecule with the
surrounding solvent environment. Since the solute molecule finds itself in a cavity inside the
solvent that results in stabilization of the ground state. The Franck-Condon excited state is
formed in a solvent cage of already partly oriented solvent dipoles. The better stabilization of the
excited state relative to the ground state with increasing solvent polarity results in a
bathochromic shift (positive solvatochromism). It is also apparent from the slight increasing
values of dipole moments during the electron transition for dye molecules calculated from DFT
studies as given in Table 3 (  g,). Difference is not to a greater extent, the small change in
e
dipole moment value can be understood based on change in tendency of electron movement from
donor to acceptor group. This delocalization of electrons in aromatic ring is also affected by
change in the surrounding environment created by solvents of different polarity. Further,
observed Stokes shift in alcohols, as tabulated in Table 2 (for general solvents, see Supporting
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Information, Table S65), clearly suggested the presence of intramolecular charge transfer (ICT)
character in these dyes.

Table 3 
Fig. 9 

Computational studies
Further, to gain deep insight into the electronic structure and the HOMO–LUMO levels of 7(a-d)
dyes, DFT (B3LYP) calculations were performed. It has been reported that the B3LYP/6-31G
level of theory can give reliable geometry parameters of organic dyes. The ICT activities are
related to the orbital spatial distribution and the compositions of the frontier molecular orbital.
The HOMO-LUMO structures of 7(a-d) dyes are shown in Fig. 10. Computationally elaborated
electronic properties reveal the nature of HOMO and LUMO levels. The close observation of
this figure depicts for all dyes electron density in LUMO concentrated mainly on coumarin
moiety linked to different electron withdrawing groups while in HOMO level, electron density
distributed over tetrazole moiety. Therefore, all dyes possessed similar HOMO and LUMO
levels irrespective of substituent attached to coumarin moiety. Hence, the energy band gaps of all
molecules ranging from 3.96 to 4.03 eV.


Fig. 10 
Fig. 11 
The fluorescence decay lifetimes of dyes were measured using Time Correlated Single
Photon Counting (TCSPC) instrument. The instrument utilizes pico second light emitting diode
for the excitation generating pulses at 296 nm with instrument response function 851 ps. The
fluorescence decays were fitted to tri-exponential functions. The average lifetime, <τ> is
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calculated from Eq. (6) (41) in alcohols and are tabulated in Table 2 (for general solvents see
Supporting Information, Table S65).

 
i
i
(
6)




i
i i
where α and τ are the relative amplitude and lifetime components, respectively. The excitation
lifetime profiles in alcohols and general solvents are shown in Supporting Information, Figs.
S66-S67 and Figs. S68-S69. However, the fluorescence decay lifetimes profile of 7(a-d) in ethyl
acetate as depicted in Fig. 11 in order to understand the effects of substituent on ICT. From this
figure it is clear that the fluorescence decay time was slightly increased from 2.19 to 2.88 ns for
7
(a-d) dyes (see Table 2 for ethyl acetate). This is because the free –NH of tetrazole ring and
oxygen atoms in coumarin ring leads to facilitated ICT, which has slightly affected the HOMO-
LUMO levels and ground and excited state charge distribution. It is evident from electrostatic
potential map (ESP) that the electron cloud mainly localized on coumarin moiety of all
derivatives in which red and blue colours are respectively the indicators of high and low electron
density as shown in Fig. 12.

Fig. 12 >
Solvatochromisim: Lippert-Mataga considerations
Ground state and excited state dipole moments for all four compounds are obtained by using
Lippert-Mataga equations. Further, difference in the excited state and ground state dipole
moments are experimentally calculated. The expression most commonly used in fluorescence
spectroscopy is, however, the somewhat simplified equation, first developed by Lippert (42) and
Mataga et al. (43). It is based on the Onsager’s reaction field theory, the so called Lippert–
Mataga equation is no longer applicable when, in addition to the non-specific interactions,
specificfluorophore/solvent interactions such as hydrogen bonding or electron-pair
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donor/electron-pair acceptor interactions also contribute significantly to the overall solute–
solvent interaction. Photophysical parameters from solvatochromism study are presented in
Table 4. Kawski and co-workers obtained a simple quantum mechanical second-order
a f
perturbation theory of absorption (ν ) and fluorescence (ν ) band shifts in different solvents of
varying permitivity (ε) and refractive index (n) relative to the band position of a solute molecule
based on which the following equations are obtained.
a  f  m f (, n)cons tan t
1
(
7)
8)
a  f  m [ f (, n) 2g(n)] cons tan t
2
(
Where,
n 1  1 n2 1
2




2
f (, n) 


2
2

 2
n  2 
n  2

(
(
(
(
9)
is the polarity of solvent and
3
2
n4 1






g(n) 
2
2

(n  2) 
10)
11)
12)
With
2
2
(   )
e
g
m1 
hca3
and
2
2
2
(   )
e
g
m 
2
hca3
h being Planck’s constant and c is the velocity of light in vacuum. The parameters m
be determined from absorption and fluorescence band shifts [Eqs. (7) and (8)], and the values of
and μ from Eqs. (13) and (14) can be given as,
1 2
and m can
μ
g
e
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1
3
2

m  m1 hca
2


g


2
2m


1 

(
13)
14)
1
3
2


m1  m2 hca
e



2
2m


1 

(
<
Table 4 >
The polarity functions graphs for 7a-7d are shown (Figs. S70-S77). These values are now
compared with corresponding µ values obtained from theoretical studies. The theoretically
evaluated HOMO/LUMO values were discussed and compared with experimentally calculated
values from CV studies.
To compute the UV-Vis transition of the compounds, the singlet excited state geometries
corresponding to the vertical excitations were optimized using the time dependent DFT (TD-
DFT) scheme starting with the ground state geometries optimized in gas phase (44,45). In order
to understand the nature of the underlying excited states involved in the experimental absorption
spectra, theoretical calculations are essential. The energy of each excited state is the vertical
excitation energy in electron volts (eV) from the ground state. The TD-DFT calculations for the
compounds were performed by using B3LYP/6-31G and using triple zeta functions B3LYP/6-
*
3
1G . And also the results were checked against the functional dependence by using CAM-
B3LYP and B3PWM91 functional. The calculated spin-allowed lowest-energy absorption bands
are obtained in the visible region for the studied four basis sets namely B3LYP/6-31G,
*
*
*
B3LYP/6-31G , CAM-B3LYP/6-31G and B3PWM91/ 6-31G . The results are tabulated in the
Table 5. For example, for B3LYP/6-31G basis set at 447 (2.77eV), 446 (2.78eV), 438 (2.76eV)
and 428nm (2.86eV), for 7a, 7b, 7c and 7d, respectively. These spin allowed transitions
corresponds to an HOMO/LUMO excitation. The corresponding oscillator strengths (f) for 7a,
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7
b, 7c and 7d are 0.177, 0.184, 0.186 and 0.221, respectively. These results reflect that upon
substitution of compound 7a by different halogens (-Cl, -Br and dibromo), the difference
between HOMO and LUMO levels are slightly shortening. These experimental results are in
good agreement with theoretically obtained data from DFT studies.
CONCLUSIONS
In summary, fluorescent dyes 3-(1-aryl-4-(2H-tetrazol-5-yl)-1H-pyrazol-3-yl)-2H-chromen-2-
ones 7(a-d) were designed and synthesized by simple synthetic protocol. Then successfully
established their spectroscopic, Hirshfeld Surface, thermal and electronic behaviours from steady
state, time resolved, DSC, TGA, CV, and DFT (TD-DFT) studies. Appending the tetrazole
pyrazole to coumarin moiety resulted in drastically enhanced PL and QY. Both ground and
th
excited states were affected equally due to the halide functional group at 6 position of coumarin
of all studied dyes. Hence, there is no rapid variation in stokes shift and energy band gap of
corresponding dyes. Therefore, E
all well matched. However, fluorescence emission maxima of 7(a-d) has been red shifted
bathochromic shift) as a function of increasing solvent polarity. All these results revealed that
g
of 7(a-d) dyes calculated from DFT, CV and onset method are
(
chromen-2-one of pyrazoline tetrazole derivatives could be a leading perception for future
optoelectronic applications.
ACKNOWLEDGEMENTS: The authors are thankful to University Grant Commission
(
UGC), New Delhi, India for providing financial assistance vide No. F540/2-DSA, 2013 (SAP-
I), UGC-UPE (F. No. 14-3/2012 (NS/PE)) and UGC-CPEPA programme (F. No. 8-2/2008
NS/PE)). Authors are also grateful to the university scientific instrumentation centre (USIC),
(
Karnatak University, Dharwad, for providing the facility to conduct the experiments.
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SUPPORTING INFORMATION
Additional Supporting Information may be found in the online version of this article:
1
13
Figure S1-S51: IR, H, C NMR and Mass spectra for synthesized compounds.
Table S52: Obtained main crystallographic parameters.
Table S53: Intermolecular interactions with distances for compounds 7b and 7c.
Figure S54: Relative contributions of various intermolecular contacts to the total Hirshfeld
surface for compounds 7b and 7c.
Figure S55: (a-l) Two dimensional fingerprint plots for the 7b (shown in a, c, e, g, i, k) and
compound 7c (shown in b, d, f, h, j, l).
Table S56: Different parameters of voids in the crystals.
Figure S57: Unit cell voids at (0.002 au) – isosurface.
Figure S58: (a) Normalized absorption (left). b) Emission spectra (right) of 7(a-d) in ethyl
acetate.
Figure S59-S61: Normalized absorption and emission of 7b, 7c and 7d in alcohols.
Figure S62-S64: Normalized absorption and emission of 7b, 7c and 7d in general solvents.
Table S65: Photophysical parameters in general solvents.
Figure S66-S67: Fluorescence lifetime profile of 7b, 7c and 7d in various alcohols.
Figure S68-S69: Fluorescence lifetime profile of 7b, 7c and 7d in various GS.
Figure S70-71: Polarity function versus Stokes shifts graphs for 7a.
Figure S72-73: Polarity function versus Stokes shifts graphs for 7b.
Figure S74-75: Polarity function versus Stokes shifts graphs for 7c.
Figure S76-77: Polarity function versus Stokes shifts graphs for 7d.
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