Pyrazole based solid state emissive NLOphores with TICT characteristics: Synthesis, DFT and TDDFT studies

Article abstract of DOI:10.1016/j.dyepig.2015.11.014

A series of pyrazole based D-π-A derivatives with large stokes shift have been synthesized from 1,3-dipheny-1H-pyrazole-3-carbaldehyde by reacting with a series of active methylenes and characterized by spectroscopic analysis. The synthesized dyes show blue to red solid state emissions in the range of 441-604 nm. All the pyrazole dyes, except dye 2.3.6.4-((1,3-dipheny-1H-pyrazole-4-yl)methylene)-3-methyl-1-phenyl-1H-pyrazol-5(4H)-one, display solid state emission characteristics. Experimental absorption and emission wavelengths measured for all the synthesized dyes are in good agreement with those predicted using the Time-Dependent Density Functional Theory. In this paper, the static first, second hyperpolarizability and its related properties estimated for pyrazole based "push-pull" dyes using B3LYP functional with 6-311G(d) basis sets. We have performed experimental calculation to determine the first hyperpolarizability using solvatochromic shift method. These dyes show high first hyperpolarizability in the range of 39.96-296.35 × 10^-30 esu by theoretical method and 35.8-302 × 10^-30 esu by solvatochromic shift method.

Full text of DOI:10.1016/j.dyepig.2015.11.014

Accepted Manuscript
Pyrazole based Solid State Emissive NLOphores with TICT Characteristics:
Synthesis, DFT and TDDFT studies
Sandip K. Lanke, Nagaiyan Sekar
PII:
S0143-7208(15)00450-7
10.1016/j.dyepig.2015.11.014
DYPI 5008
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 22 August 2015
Revised Date: 3 October 2015
Accepted Date: 11 November 2015
Please cite this article as: Lanke SK, Sekar N, Pyrazole based Solid State Emissive NLOphores with
TICT Characteristics: Synthesis, DFT and TDDFT studies, Dyes and Pigments (2015), doi: 10.1016/
j.dyepig.2015.11.014.
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ACCEPTED MANUSCRIPT
Pyrazole based Solid State Emissive NLOphores with TICT
Characteristics: Synthesis, DFT and TDDFT studies
Sandip K. Lanke, Nagaiyan Sekar*
Department of Dyestuff Technology, Institute of Chemical Technology, (Formerly UDCT)
Nathalal Parekh Marg, Matunga, Mumbai - 400 019. (India)
*Corresponding author. Tel.: +91 22 3361 1111/2222, 2707(direct),
Fax: +91 22 3361 1020. E-mail: n.sekar@ictmumbai.edu.in, nethi.sekar@gmail.com
Graphical Abstract:

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Pyrazole based Solid State Emissive NLOphores with TICT
Characteristics: Synthesis, DFT and TDDFT studies
Sandip K. Lanke, Nagaiyan Sekar*
Department of Dyestuff Technology, Institute of Chemical Technology, (Formerly UDCT)
Nathalal Parekh Marg, Matunga, Mumbai - 400 019. (India)
*Corresponding author. Tel.: +91 22 3361 1111/2222, 2707(direct),
Fax: +91 22 3361 1020. E-mail: n.sekar@ictmumbai.edu.in, nethi.sekar@gmail.com
Abstract:
A series of pyrazole based D-π-A derivatives with large stokes shift have been synthesized
from 1,3-dipheny-1H-pyrazole-3-carbaldehyde by reacting with a series of active methylenes
and characterized by spectroscopic analysis. The synthesized dyes show blue to red solid
state emissions in the range of 441-604 nm. All the pyrazole dyes, except dye 2.3.6.4-((1,3-
dipheny-1H-pyrazole-4-yl)methylene)-3-methyl-1-phenyl-1H-pyrazol-5(4H)-one,
display
solid state emission characteristics. Experimental absorption and emission wavelengths
measured for all the synthesized dyes are in good agreement with those predicted using the
Time-Dependent Density Functional Theory. In this paper, the static first, second
hyperpolarizability and its related properties estimated for pyrazole based “push-pull” dyes
using B3LYP functional with 6-311G(d) basis sets. We have performed experimental
calculation to determine the first hyperpolarizability using solvatochromic shift method.
These dyes show high first hyperpolarizability in the range of 39.96−296.35 × 10-30 esu by
theoretical method and 35.8−302 × 10-30 esu by solvatochromic shift method.
Keywords: Pyrazole, DFT, TDDFT, Solid state emission, NLO study.
1. Introduction
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In recent years, organic fluorophores exhibiting high nano aggregation-induced emission and
solid state fluorescence efficiency have received increasing attention owing to their potential
applications in the optoelectronic devices, such as electroluminescence devices [1], organic
light emitting devices (OLED) [2–4], non-linear optical materials [5], active gain media for
optically pumped solid-state lasers [6,7] and fluorescent sensors for living cell bio-imagining
[8–10]. D- π -A systems are known to perturb the π-electron delocalization of the locally
excited state (LE) attained by the absorption which can promote intramolecular charge
transfer (ICT) and such phenomena are intensified in polar solvent environments [11–15].
Dyes having intramolecular charge transfer properties are one of the key feature to reduce the
energy gap between HOMO and LUMO orbital of the molecule for broadening the range of
absorption properties [16].
The electron-rich pyrazole unit has N atoms which attains conjugation by donating electron,
hence it acts as a donor-acceptor type of functional molecules because of its good electron-
donating and high hole transfer efficiency and some photoelectron characteristics [17].
Pyrazole derivatives have not only excellent hole-transfer performance but also excellent blue
emission [18,19]. Therefore, pyrazole derivatives have widely been used as hole-transport
materials in the electrophotography and electroluminescence fields. p-Aryl-substituted
pyrazole derivatives have also attracted considerable interest due to their wide range of
applications in DSSCs [20], organic light-emitting diodes (OLEDs) [19,21] as well as second
order nonlinear optical devices [22].
Organic materials showing nonlinear optical (NLO) properties have been the object of
extensive research from both experimental and computational points of view because of the
inherent disadvantages of the commonly used inorganic materials like lithium niobate in
optoelectronic devices [23,24]. There are huge efforts taken on the design of molecules to
improve their nonlinear optical (NLO) properties for various applications [25,26]. Conjugated
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organic molecules containing electron-donating and –accepting groups can in addition to
higher response time exhibit higher second-order nonlinearity, lower dielectric constants, and
higher optical thresholds than their inorganic counterparts. Their synthetic versatility in
custom tailoring the NLO properties makes them obvious candidates to the particular needs
of the specific application. Several donor-acceptor substituted π-electron conjugated organic
molecules viz, para-di substituted benzenes, stilbenes, porphyrins, phthalocyanines and styryl
dyes have been reported into potentially useful nonlinear optical (NLO) materials over the
past few years [27–35]. Organic based materials with large second-order hyperpolarizability
are used in the development of new optical data-storage systems [25,36], integrated optical
circuit for telecommunication [34,37], parametric oscillation (OPO) [38], electrooptic phase
modulation [38,39], laser devices [40] and chemical sensing [41]. The hyperpolarizability
value can be tuned by increase in conjugation, TICT characteristics, transition dipole
moment, λmax and solvent polarity [40,42,43]. The β value might be properly influenced by
adjusting the HOMO−LUMO energy gap of organic molecules [33].
In order to explore the relationship between chemical structures and various properties of
pyrazole based dyes, we have synthesized a new series of pyrazole substituted dyes by
incorporating cyano group, benzthiazole, and coumarin residues (electron acceptor) for
comparative study. We have found that structurally the pyrazole rings have richer π-electron
density than phenyl ring and increase the dimension of π-electron delocalization of the
system and hence these dyes show not only good photophysical properties but also strong
aggregation induced emissive properties. Photophysical studies of pyrazole based derivatives
were investigated using absorption and fluorescence spectroscopy, density functional theory
(DFT) calculations. These dyes show fluorescence in solution as well as in the solid state.
2. Experimental Section:
2.1. Computational Strategy
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DFT calculations were performed on a HP workstation XW 8600 with Xeon processor, 4 GB
RAM and Windows Vista as operating system. All the computations were performed with the
Gaussian 09 package [44]. DFT method was used for the ground state optimisation, while for
the excited state optimisation TD-DFT was employed. The hybrid functional namely B3LYP
(Becke3-Lee-Yang-Parr hybrid functional) [45–48] was used. The 6-311G(d) basis set was
used for all the atoms. The Polarizable Continuum Model (PCM) [49] was used to optimize
the ground and excited state geometries in the solvent environments. The excitation energies,
oscillator strengths and orbital contribution for the lowest 10 singlet-singlet transitions at the
optimized geometry in the ground state were obtained by TD-DFT calculations at the same
level of theory. The solvents used were toluene, tetrahydrofuran (THF), 1,4-dioxane
(dioxane), ethyl acetate (EtOAc), chloroform (CHCl3), dichloromethane (DCM), ethanol
(EtOH), methanol (MeOH), N,N-dimethyl formamide (DMF), dimethyl sulfoxide (DMSO).
2.2. Materials and equipments
All the chemicals were purchased from Sigma-Aldrich and S. D. Fine Chemicals Ltd,
Mumbai, India. The solid reagents were characterized by melting points and used without
further purification. The liquid reagents were distilled at their boiling points and used without
any further purification. Solvents were used after distillation at their boiling point and drying
according to standard processes. All the reactions were monitored on precoated silica gel
aluminium based plates kisel gel 60 F254 Merck, India. Purification of all the dyes was
achieved by recrystallization. Melting points were recorded on instrument from Sunder
Industrial Product Mumbai by using open capillary and are uncorrected. The absorption
spectra of the dyes were recorded on a Lambda 25 UV-visible spectrophotometer. The FT-IR
spectra were recorded on a Jasco 4100 Fourier Transform IR instrument (ATR accessories).
1H NMR spectra were recorded on a Varian Cary Eclipse Australia. The chemical shift values
are expressed in δ ppm using CDCl3 as a solvent and chemical shifts (δ) are reported relative
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to tetramethylsilane (Me4Si) (δ = 0.0) as an internal standard. High resolution mass spectra
were recorded on Agilent Technologies, USA, 6550 iFunnel Q-TOFs Mass spectrometer.
2.3. Synthesis
2.3.1. Synthesis of intermediates 1 and 2
The compound 1 was prepared by condensation of acetophenone and phenyl hydrazine in
ethanol at reflux temperature using the reported procedure [50]. The compound 1 was further
subjected to Vilsmeier-Haack reaction using DMF/POCl3 to afford 1,3-dipheny-1H-pyrazole-
3-carbaldehyde (2) in good yield according to the procedure that has been described by Kira
et. al. [51].
2.3.2. 2-[(1,3-Diphenyl-1H-pyrazol-4-yl)methylene]malononitrile (3a)
Under nitrogen atmosphere, 1,3-dipheny-1H-pyrazole-3-carbaldehyde (248 mg, 1 mmol, 1
equiv) and malononitrile (66 mg, 1 mmol, 1 equiv) were dissolved in dry methanol (15 mL).
0.1 mL of piperidine was added, and the solution was stirred at RT for 2 h. After completion
of the reaction, the off white colored solid was filtered, washed with methanol, and dried. The
dye obtained was purified by column chromatography using silica gel 100-200 mesh and
toluene as eluent system.
Color: Off -white solid, Yield: 240 mg, 81%, M. P.: 160–162 oC [52].
FT-IR (KBr, cm-1): 2224 (CN stretching).
1H NMR (400MHz, CDCl3, Me4Si) δ 7.455 (d, J = 7.6 Hz, 1 H, ArH), 7.524 ~7.564 (m, 7 H,
ArH), 7.803 (s, 1 H, N-CH), 7.811 ~ 7.834 (m, 2 H, ArH), 9.064 (s, 1 H, C=CH).
13C NMR (126 MHz, CDCl3, Me4Si) δ 78.5, 113.8, 113.9, 114.9, 120.1, 128.7, 129.2 (2),
129.3, 129.8 (2), 130.1, 138.5, 151.1, 156.4.
MS (m/z): 297.2 (M+H).
Anal. Calcd. for C19H12N4: C, 77.01; H, 4.08; N, 18.91. Found: C, 77.23; H, 4.01; N, 18.82.
HRMS m/z [M + H]+ Calcd for C19H12N4: 297.1140. Found: 297.1099
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2.3.3. Ethyl 2-cyano-3-(1,3-diphenyl-1H-pyrazol-4-yl)acrylate (3b)
Under nitrogen atmosphere, 1,3-dipheny-1H-pyrazole-3-carbaldehyde (248 mg, 1 mmol, 1
equiv) and ethyl-2-cyanoacetate (113 mg, 1 mmol, 1 equiv) were dissolved in dry methanol
(15 mL). 0.1 mL of piperidine was added, and the solution was stirred at RT for 2 h. After
completion of the reaction, the white solid was filtered, washed with methanol and dried. The
dye obtained was purified by column chromatography using silica gel 100-200 mesh and
toluene as eluent.
Color: White crystal, Yield: 268 mg, 78%, M. P.: 136–138 oC [52].
FT-IR (KBr, cm-1): 2218 (CN stretching), 1726 (ester C=O stretching), 1581 (Ar).
1H NMR (400 MHz, CDCl3) δ 1.382 (t, J = 7.2 Hz, 3 H, CH3), 4.327 ~ 4.380 (m, 2 H, CH2),
7.412 (t, J = 7.2 Hz, 1 H, ArH), 7.501 ~ 7.558 (m, 5 H, ArH), 7.621 (d, J = 6.4 Hz, 2 H, ArH),
7.837 (d, J =8.0 Hz, 2 H, ArH), 8.313 (s, 1 H, N-CH), 9.145 (s, 1 H,C=CH).
13C NMR (126 MHz, CDCl3, Me4Si) δ 14.2, 62.4, 99.8, 114.9, 116.7, 119.9, 128.5, 129.01,
129.2, 129.3, 129.4, 129.7, 130.8, 138.8, 146.3, 156.4, 162.7.
MS (m/z): 344.4 (M+H).
Anal. Calcd. for C21H17N3O2: C,73.45; H, 4.99; N, 12.24. Found: C, 73.49; H, 4.82; N, 12.28.
HRMS m/z [M + H]+ Calcd for C21H17N3O2: 344.1399. Found: 344.1340.
2.3.4. 2-(3-(1,3-Dipheny-1H-pyrazole-4-yl)-1-phenylallylidene)malononitrile (3c)
1,3-Dipheny-1H-pyrazole-3-carbaldehyde (248 mg,
1 mmol, 1 equiv) and 2-(1-
phenylallylidene)malononitrile (168 mg, 1mmol, 1 equiv) were dissolved in dry methanol (15
mL). 0.1 mL of piperidine were added, and the solution was reflux for 8h. After completion
of the reaction, the reaction mixture was cooled to room temperature and the yellow solid was
filtered, washed with methanol and dried. The dye obtained was purified by column
chromatography using silica gel 100-200 mesh and toluene as eluent system.
Color: Yellow solid, Yield: 302 mg, 76%, M. P.: 224–226 oC.
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FT-IR (KBr, cm-1): 2221 (CN), 1591 (Ar), 1236 (C-N).
1H NMR (500 MHz, CDCl3, Me4Si) δ 6.978 (d, J= 15.5 Hz, 1H),7.361 ~ 7.401 (m, 6 H, ArH),
7.425 (d, J= 15.5 Hz, 1H), 7.471 ~ 7.535 (m, 7 H, ArH), 7.811 (dd, J = 8 Hz & 2 Hz, 2 H,
ArH), 8.427 (s, 1 H, N-CH).
13C NMR (125 MHz, CDCl3, Me4Si) δ 80.3, 113.1, 113.6, 118.27, 119.5, 123.6, 126.9, 127.8,
128.5, 128.6, 128.7, 128.8(2), 128.9(2), 129.7, 131.1, 131.5, 133.0, 139.1, 140.3, 154.1, 171.6
MS (m/z): 399.6 (M+H).
Anal. Calcd. for C27H18N4: C, 81.39; H, 4.55; N, 14.06. Found: C, 81.37; H, 4.55; N, 14.10.
HRMS m/z [M + H]+ Calcd for C27H18N4: 399.1610. Found: 399.1549
2.3.5. 2-(Benzo[d]thiazol-2-yl)-3-(1,3-diphenyl-1H-pyrazol-4-yl)acrylonitrile (3d)
Under nitrogen atmosphere, 1, 3-dipheny-1H-pyrazole-3-carbaldehyde (248 mg, 1 mmol, 1
equiv) and 2-benzo[d]thiazol-2-yl)acetonitrile (174 mg, 1 mmol, 1 equiv) were dissolved in
dry methanol (15 mL). 0.1 mL of piperidine were added, and the solution was stirred at RT
for 2 h. After completion of the reaction, the greenish yellow solid was filtered, washed with
methanol and dried. The dye obtained was purified by column chromatography using silica
gel 100-200 mesh and toluene as eluent system.
Color: Greenish yellow solid, Yield: 350 mg, 84%, M. P.: 222–224 oC.
FT-IR (KBr, cm-1): 2224 (CN stretching)
1H NMR (500 MHz, CDCl3, Me4Si) δ 7.40 (t, J = 7Hz, 2H, ArH), 7.50 ~ 7.57 (m, 6H, ArH),
7.70 (d, J = 6.5 Hz, 2H, ArH), 7.86 (d, J = 8Hz, 3H, ArH), 8.05 (d, J = 8Hz, 1H, ArH), 8.19
(s, 1H, N-CH), 9.14 (s, 1H, C=CH).
13C NMR (126 MHz, CDCl3, Me4Si) δ 103.4, 115.6, 117.2, 119.9, 121.5, 123.5, 125.8, 126.8,
127.9, 128.3, 129.3, 129.2(2), 129.7, 131.3, 134.7, 138.1, 139.1, 153.7, 155.7, 162.5.
MS (m/z): 405.4 (M+H).
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Anal. Calcd. for C25H16N4S: C, 74.23; H, 3.99; N, 13.85; S, 7.93. Found: C, 74.29; H, 3.93;
N, 13.82; S, 7.89.
HRMS m/z [M + H]+ Calcd for C25H16N4S: 405.1174. Found: 405.1124
2.3.6. 4-((1,3-Dipheny-1H-pyrazole-4-yl)methylene)-3-methyl-1-phenyl-1H-pyrazol-5(4H)-
one (3e)
1,3-Dipheny-1H-pyrazole-3-carbaldehyde (248 mg, 1 mmol, 1 equiv) and 3-methyl-1-phenyl-
1H-pyrazol-5(4H)-one (174 mg, 1 mmol, 1 equiv) were dissolved in dry methanol (15 mL).
0.1 mL of piperidine were added, and the solution was refluxed for 12h. After completion of
the reaction, the reaction mixture cooled to room temperature, the orange solid was filtered,
washed with methanol and dried. The dye obtained was purified by column chromatography
using silica gel 100-200 mesh and toluene as eluent system.
Color: Orange solid, Yield: 300 mg, 74%, M. P.: 239–240 oC.
FT-IR (KBr, cm-1): 1666 (amide stretching)
1H NMR (500 MHz, CDCl3, Me4Si) δ 2.26 (s, 3H, CH3), 7.20 (t, J = 6.9 Hz, 1H, ArH), 7.38
(t, J = 7.4 Hz, 1H, ArH), 7.41 ~ 7.47 (m, 2H, ArH), 7.48 (s, 1H, ArH), 7.49 ~ 7.60 (m, 5H,
ArH), 7.69 (d, J = 6.6 Hz, 2H, ArH), 7.93 (dd, J = 8.5, 0.9 Hz, 2H, ArH),7.97 (dd, J = 8.6, 1.0
Hz, 2H, ArH), 10.27 (s, 1H, N-CH).
13C NMR (126 MHz, CDCl3, Me4Si) δ 13.0, 115.9, 119.3, 119.9, 123.8, 124.9, 127.8, 128.8,
128.9, 129.1(2), 129.3, 129.4, 129.5, 131.4, 134.2, 135.4, 138.5, 139.1, 150.4, 157.2, 162.7.
MS (m/z): 405.5 (M+H).
Anal. Calcd. for C26H20N4O: C, 77.21; H, 4.98; N, 13.85. Found: C, 77.23; H, 4.99; N, 13.90.
HRMS m/z [M + H]+ Calcd for C26H20N4O: 405.1715. Found: 405.1653
2.3.7. 3-(3-(1, 3-Diphenyl-1-H-pyrazol-4-yl)-4-hydroxy-2H-chromen-2-one (3f)
Under nitrogen atmosphere, 1,3-dipheny-1H-pyrazole-3-carbaldehyde (248 mg, 1 mmol.1
equiv) and 3-acetyl-4-hydroxy-2H-chromen-2-one (204 mg, 1 mmol, 1 equiv) were dissolved
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in dry methanol (25 mL). 0.1 mL of piperidine was added and the solution was stirred at RT
for 6 h. After completion of the reaction, the yellow solid was filtered, washed with methanol
and dried. The dye obtained was purified by column chromatography using silica gel 100-200
mesh and toluene as eluent.
Color: Yellow solid, Yield: 320 mg, 73%, M. P.: 237–238 oC.
FT-IR (KBr, cm-1): 3654 (O-H stretching), 2224 (CN stretching), 1710 (lactone C=O
stretching),
1H NMR (500 MHz, CDCl3, Me4Si) δ 7.41~7.28 (m, 3H, ArH), 7.57 ~ 7.44 (m, 5H, ArH),
7.69 ~ 7.65 (t, 1H, ArH), 7.71 (d, J = 7.0 Hz, 2H, ArH),7.84 (d, J = 7.7 Hz, 2H, ArH), 8.09
(dd, J = 7.9, 1.4 Hz, 1H, ArH), 8.16 (d, J = 15.5 Hz, 1H, C=CH), 8.29 (d, J = 15.5 Hz, 1H,
HC=C), 8.54 (s, 1H, N-CH).
13C NMR (126 MHz, CDCl3, Me4Si) δ 100.4, 116.8, 116.9, 118.6, 119.6, 121.5, 124.3, 125.8,
127.5, 127.6, 128.8, 128.9(2), 129.9, 131.9, 135.8, 138.3, 139.3, 154.6(2), 181.6, 191.6.
MS (m/z): 435.4 (M+H).
Anal. Calcd. for C27H18N2O4: C, 74.64; H, 4.18; N, 6.45. Found: C, 74.61; H, 4.11; N, 6.49.
HRMS m/z [M + H]+ Calcd for C27H18N2O4: 435.1345. Found: 435.1285
2.3.8. 2-(3-(2(1,3-Dipheny-1H-pyrazole-4-yl)vinyl)5,5-dimethylcyclohex-2-en-1-
ylidene)malononitrile (3g)
1,3-Dipheny-1H-pyrazole-3-carbaldehyde (248 mg, 1 mmol, 1 equiv) and 2-(3,5,5-
trimethylcyclohex-2-enylidene)malononitrile (186 mg, 1 mmol, 1 equiv) were dissolved in
dry methanol (15 mL). 0.1 mL of piperidine were added and the solution was reflux for 8h.
After completion of the reaction, the reaction mixture was cooled to room temperature, the
red solid was filtered, washed with methanol and dried. The dye obtained was purified by
column chromatography using silica gel 100-200 mesh and toluene as eluent system.
Color: Red solid, Yield: 325 mg, 78%, M. P. 254–256 oC.
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FT-IR (KBr, cm-1): 2218 (CN stretching), 1519 (Ar).
1H NMR (500 MHz, CDCl3, Me4Si) δ 1.02 (s, 6H, 2 CH3), 2.35 (s, 2H, CH2), 2.54 (s, 2H,
CH2), 6.72 (s, 1H, = CH), 6.84 (d, J = 16 Hz, 1H, C=CH), 7.03 (d, J = 16 Hz, 1H, HC=C),
7.31 (t, J= 7.4 Hz, 1H, ArH), 7.45 (m, 5H, ArH), 7.63 (dd, J = 8.2, 1.2 Hz, 2H, ArH), 7.76
(dd, J = 8.6, 1.0 Hz, 2H, ArH), 8.29 (s, 1H, N-CH).
13C NMR (126 MHz, CDCl3, Me4Si) δ 27.9, 30.9, 31.9, 38.9, 42.9, 112.8, 113.6, 119.2, 122.6,
125.5, 127.2, 127.5, 128.6, 128.8, 128.9, 129.6, 132.5, 139.4, 152.9, 154.2, 169.2.
MS (m/z): 417.7 (M+H).
Anal. Calcd. for C28H24N4: C, 80.74; H, 5.81; N, 13.45%. Found: C, 80.68; H, 5.76; N,
13.40%.
HRMS m/z [M + H]+ Calcd for C28H24N4: 417.2079. Found: 417.2016
3. Results and Discussion:
3.1. Design and Synthesis.
To increase the scope of research in solid state emissive molecules we have designed and
synthesized a few pyrazole based donor acceptor molecules. We have synthesized this
molecule by multistep route. 1,3-Diphenyl-1H-pyrazole-4-carbaldehyde was synthesized by
using reported procedure [53,54]. Malononitrile, ethylcyanoacetate, isophorone, 4-hydroxy
coumarin, 2-aminothiophenol were available commercially. Active methylenes 1a-1e were
synthesized by the reported procedure [55–59]. All the synthesized intermediates and final
1
products were carefully purified and fully characterized by IR, H and 13C NMR, and mass
spectroscopic technique. All fluorophores are soluble in most of the organic solvents.
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Scheme 1. Synthesis of pyrazole dyes 3a-3g.
3.2. Photophysical properties
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3.2.1. Effect of solvent on photophysical properties of the dyes 3a-3g
In order to investigate their photophysical properties, the UV and PL spectra of 2.5 x 10-5 mol
L-1 solution of the dyes (3a-3g) in EtOH, MeOH, DMF, DCM, CHCl3, EtOAc, THF, dioxane,
toluene and solid state were recorded. The spectral data are summarized in Table 1. These
dyes are typical D-π-A consisting of an electron donating pyrazole unit and electron
withdrawing cyanovinyl and carboethoxy cyanovinyl unit. The results showed that these
pyrazole based dyes exhibited strong solvatofluoric and solid state fluorescence properties.
The pyrazole is well known as blue emitting chromophore but due to the introduction of
electron accepting groups in the positions induced intramolecular charge transfer [60–63] and
results into blue to red emission in the solid state. Depending upon the electron affinity of
electron accepting groups, the charge transfer band was red shifted. The electron accepting
ability of the different active methylenes directly influence their absorption and fluorescence
spectra. The observation suggests that both the absorption and the emission spectra are in
good agreement with the order of the extension of the π conjugation and the increase of
electron accepting ability of the acceptor: 3b < 3a < 3e < 3d < 3c < 3f < 3g. UV-vis
absorption spectra show similar absorption behaviour between the dyes (3a-3g) in different
solvents. The representative absorption and emission spectra of dyes 3a-3g in DMF solution
are shown in Fig. 1. Each of them reveals a common low-energy band at 340-435 nm
assigned to the π-π* transitions of the dyes. A change of solvent from DMF to THF resulted
in a little blue shift (7-18 nm) of the absorption maximum (Table 1). The detailed absorption
are shown in Figures S1 and S2 respectively. The detailed emission spectra are shown in
Figures S3 and S4.
<< Please Insert: Fig. 1. Absorption and emission spectra of dyes 3a-3g in DMF solvent
(concentration 2.5×10-5 M). >>
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<< Please Insert: Table 1. Photophysical data of the dyes 3a-3g in selected solvents of
varying polarities.a >>
3.2.2. Solid State Emission and Aggregation induced emission enhancement properties
The day light and UV light photograph for the dyes 3a-3g are shown in Figures 2a and 2b.
<<Please Insert: Fig. 2a. Day light photograph of dyes 3a-3g. >>
<< Please Insert: Fig. 2b. UV light photograph of dyes 3a-3g. >>
The dyes 3a-3d and 3f-3g showed solid state fluorescence under long UV (365 nm). All the
parameters were kept constant such as source of light, slit width, sample holder. The solid
state emission spectra are shown in Fig 3. The dyes 3a-3d and 3f-3g show more
bathochromic shift in the emission spectra as compared to their emission in solution. While
the dye 3e does not show any solid state emission properties. The dyes 3a and 3b show blue
emission at 461 and 441 nm with maximum emission intensity while the dye 3g shows more
red shifted emission at 603 nm. The dyes 3c, 3d and 3f show emission at 531 nm, 499 nm and
523 nm which is also red shifted with good emission intensity. This solid emission
characteristics was due to the formation of aggregation.
<< Please Insert: Fig. 3. Solid state emission spectra of dyes 3a-3g. >>
The AIEE characteristics of the pyrazole dyes 3a-3g were investigated in a mixture of THF
and water (Figures 4, S5 and S6 ). In THF dyes 3a-3g were well-dispersed and displayed
weak fluorescence emissions in their solution state. To determine whether dyes 3a-3g have
AIEE characteristics, the fluorescence spectra of dyes were measured in a series of
THF/water mixtures with different volume fractions of water, since the dyes 3a-3g were
soluble in THF but not in water (Fig. 4) we added different amounts of water, a poor solvent
for the luminogens, to the pure THF solutions by defining the water fractions (fw) of 0-90%
and then monitored the change in the emission wavelength. The concentration was
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maintained at 2.5 × 10−5 mol L−1. Fig. 4 reveals that all pyrazole push-pull dyes showed
higher emission intensity when water fractions (fw) was 90% . However it is noteworthy that
with a gradual addition of water into the solution of dye 3a in THF, the emission intensity of
dye 3a was dramatically weakened and the emission wavelength was red shifted to 445 nm
when fw was 0 - 40 vol% but when fw > 40%, emission intensity increase suddenly with red
shifted emission. Similar observation are seen for the other dyes 3b, 3c, 3d, 3f and 3g
(Figures S5 and S6). The emission intensity of dye 3b was higher when water fractions (fw)
was 90%. It is notable that the emission wavelength decrease slowly with when water
fraction increases form 0 - 50 vol%, but increases sharply when fw was 60% again from 60%
to 90% the emission intensity increase slowly with red shift in emission. Similar
enhancement was observed in the behaviours of dyes 3c, 3d, 3f and dye 3g. The emission
intensity of dye 3g was significantly quenched at the initial stage from fw = 0 to 60 vol%.
Meanwhile, the emission wavelength of dye 3g in the solvent mixture with fw = 60 vol% was
red-shifted to 14 nm, compared with that in pure THF solution. It is noteworthy that
fluorescence intensity of the dye 3e dramatically weakened with increasing fw from 50 to
90%, At water fraction (fw) = 90 vol% the dye 3e showed almost no emission revealing the
aggregation caused quenching phenomenon. In conclusion, the dyes 3a-3g showed
aggregation induced emission enhancement properties while the dye 3e showed aggregation
caused quenching properties. The UV-light photograph for dyes 3a-3g in THF:H2O mixture
is shown in Fig. 5.
<< Please Insert: Fig. 4. (a) Emission spectra of dye 3f in THF/H2O mixtures with different
water fractions (2.5× 10-5 M), λexc = 489 nm (slit width:5nm/5nm). (b) Plot of water fraction
versus ratiometric fluorescence intensity (red line) and water fraction versus emission
wavelength (green line) of dye 3f in THF/H2O mixtures with different water fractions.>>
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<< Please Insert: Fig. 5. UV-Light photograph of dyes 3a-3g in THF:H2O mixture (0 and 90
vol%). >>
3.3. DFT Studies
3.3.1. Optimized geometries of dyes 3a-3g
To gain insight into the geometrical and electronic structure of the dyes computational studies
were performed using density functional theory (DFT) at the B3LYP/6-311G(d) level.
Ground state geometry of the dyes were having planar arrangement with two of the phenyl
ring present on the pyrazole moiety showing twist in the dihedral angle. The resulting
optimized geometry of the dye 3g shows that it has a small twist in dihedral angle between
pyrazole moiety and N-phenyl ring, N14-N15-C3-C4 is 25.0o and dihedral angle between the
phenyl ring present on pyrazole and pyrazole core, C19-C17-C11-N14 is 39.5o (Fig. 6). The dye
3g did not possess a vertical plane of symmetry (CS) in the molecule. The optimized
geometry of the dye 3c is such that it has also a small twist in dihedral angle of 27.4o along
N14-N15-C3-C4 angle between the N-phenyl ring of pyrazole and pyrazole core. The dihedral
angle, C18-C17-C12-N14 also shows more twist of about 40.5o. However the phenyl ring present
across the π-bridge for the dye 3c showed the highest twist in dihedral angle, C39-C38-C30-C27
of 64.3o
.
Dye 3g was optimized in the excited state and major bond lengthening was observed between
the bonds N15-N14, C16-C13, C12-C13, C28-C29, C32-C33, C35-C41, C43-N45, C42-N44 by 0.011,
0.012, 0.028, 0.036, 0.041, 0.040, 0.005, 0.005 Å and bond length shortened for the bonds
C13-C28, C29-C32, C33-C35, C41-C42, C41-C43, with 0.028, 0.036, 0.028, 0.012, 0.011 Å (Fig. 6).
This is due to the partial double-bond character on the bridge bond between pyrazole and the
acceptor unit caused by π-bonding interaction, thereby strengthening and shortening the
bridge bond. Also, the phenyl ring present on pyrazole core becomes more planar in the
excited state as compared to the ground state and the dihedral angle N14-N15-C3-C4 gets
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reduced from 25.04o to 1.2o more twisting is being observed in the ground state as shown in
Fig. 6. Similarly the dihedral angle, C19-C17-C11-N14 reduced from 39.5o to 34.0o which
confirms that the intramolecular charge transfer is more favourable in the excited state as
compared to the ground state. The dye 3g shows more planar geometry in the excited state.
From the optimized geometries of the dyes 3a-3g The dihedral angle for the dyes 3b (C13-
C26-C27-C33), 3c (C13-C26-C27-C30), 3d (C2-C27-C29-C43) 3f (C2-C27-C29-C32) and 3g (C13-C28-
C29-C32) are 178.8o, 178.2o, 178.9o,179.7o and 177.1o respectively. From the dihedral angle it
is clear that the dyes 3c, 3d, 3f and 3g shows dihedral angle between 177.1o to 179.7 o which
indicated the trans geometries of the dyes. In case of dye 3e the dihedral angle, C13-C26-C27-
C31 is 1.7o which confirms cis geometry. The optimized geometries of the dyes 3a-3g are
shown in Figures S7, S8 and S9.
<< Please Insert: Fig. 6. Optimized geometry parameters of dye 3g in DMF solvent in the
ground state and excited state (bond length are in Å, dihedral angles are in degree).>>
3.3.2. Frontier molecular orbitals (FMO)
The absorption leading to the electronic excitation from the HOMO or HOMO-1 to the
LUMO orbital can constitute the charge transfer from the N-phenyl pyrazole donor unit to the
various acceptor units such as dicyanovinyl, carboethoxy cyanovinyl etc. In case of the dyes
3a, 3b, 3c, 3f and 3g the HOMO is mainly located on N-phenyl pyrazole core and LUMO is
located on the acceptor unit. However, for the dye 3d the HOMO is mainly located on
cyanovinyl benzthiazolyl unit and LUMO is spread all over the molecule, while for the dye
3e the HOMO is located on methyl pyrazole unit and the LUMO is located on N-phenyl
pyrazole unit and bridge bond. The HOMO and LUMO energy level diagram gives
quantitative idea of electronic structure and excitation properties. Fig. 7 reveals that the
calculated band gap of the dyes 3a-3g are found in the range of 2.97-3.92 eV in DMF solvent.
As shown in Fig. 7 the dye 3g shows lower energy gap as compared to the dye 3a-3f while
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dye 3b shows highest band gap of 3.92 eV (Table S1). This is because the dye 3g shows more
π conjugation as compared to the dye 3b. The results reveal that lengthening of the π-
systems and electron-donating and electron accepting ability of chromophore leads to
lowering of the HOMO-LUMO energy band gap.
<< Please Insert: Fig. 7. HOMO (H) – LUMO (L) energy level diagram of dyes 3a-3g in
DMF solvent.>>
Table 2 reveals that the charge transfer transition from S0 to S1 state was observed for the
dyes 3a, 3b, 3c, 3f and 3g from N-phenyl pyrazole to acceptor unit like dicyanovinyl,
carboethoxy cyanovinylene etc. The major contribution for the charge-transfer transition
arises from the HOMO-1 to LUMO excitation. Due to increase in conjugation lowering of the
HOMO-LUMO energy gap is observed.
<< Please Insert: Table 2. Electronic distribution in the selected molecular orbitals of the
dyes 3a-3g.>>
3.3.3. Electronic vertical excitation and emission spectra (TDDFT)
The optimized structures of the pyrazole “push-pull” dyes at the ground state in solvents of
various polarities were subjected to TD-DFT. At least 10 excited states were calculated for
each molecule. The computed energies of the vertical excitations, oscillator strength and their
orbital contributions are listed in Table S2. The results of DFT and TD-DFT suggest that
there is negligible influence of the change in solvent polarity on the absorption spectra of the
pyrazole “push-pull” dyes. There is no positive or negative solvotochromism observed for the
dyes 3a-3f and TD-DFT calculation also supports the experimental results. TD-DFT results
did not show any solvotochromism. But noticeable positive solvotochromism was observed
for the dye 3g which showed the highest λmax value in DMSO at 432 nm and a lower value in
toluene at 422 nm. Here, the computed absorption values for the pyrazole dyes are in better
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agreement with the experimental values. The representative computed absorption and
emission spectra for the pyrazole dyes 3a-3g in DMF solvent are shown in Fig.8.
<< Please Insert: Fig. 8. Simulated absorption (a) emission spectra (b) in DMF of the
investigated pyrazole dyes 3a-3g at the TD-B3LYP/6-311G(d) level. >>
The dyes 3a to 3f showed less deviation in the range of -4.6 to 5.5% in all the solvents.
However, the dye 3g showed more deviation of about 5.3 to 7.9% as compared to the dyes
3a-3f. This is because of longer conjugation path length and more charge transfer
characteristics of the dye 3g. The emission spectra of the dyes 3a-3g were calculated using
TD-B3LYP/6-311G(d) and the computed emissions are in good agreement with the
experimental emission. The dye 3e shows less deviation of 0.0% in ethanol and more
deviation of 4.7% in 1,4-dioxane. Similarly the dye 3f shows less deviation in the range of -
4.4 to -2.5% and which is quite good for this basis set. For the other dyes such as dyes 3a and
3g more deviations of about 7.9-12.7 and -7.6 -12.7 respectively were observed.
3.3.4. TICT Characteristics
To have more insight into the excited state and to ascertain whether this molecules are
showing intramolecular charge transfer characteristics or twisted intramolecular charge
transfer we have used Rettig equation, which is the plot of emission wavenumber of emission
(ῡꢀ) versus the Rettig function (∆ꢁ ) [64,65]. The Rettig equation is
ꢂ
ꢆ
ꢃꢄ
ꢅ
ῡꢀ = ꢇꢈꢉ ∆ꢁ + ꢋ
(1)
ꢂ
ꢊ
where,
ῡꢀ = emission (cm-1)
c = velocity of light in vacuum,
a = Onsager cavity radius,
b = constant,
∆ꢁ = orientation polarizability by Rettig,
ꢂ
µe = excited-state dipole in the excited-state geometry,
The Rettig function is defined as
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ꢆ
ꢐ ꢍꢎ
∆ꢁ = ꢌєєꢍꢏꢎꢃ
−
ꢒ
(2)
ꢂ
ꢆ
ꢃꢐ ꢏꢑ
Where, є is dielectric constant, and n is the refractive index of respective solvent.
To understand TICT behaviour we have obtained Rettig’s plot (Fig. 9) and it was observed
that the dyes 3b, 3c, 3f and 3g show linear relationship with most of the nonpolar to polar
solvent and the correlation coefficient is 0.9510, 0.9102, 0.9578 and 0.9179 respectively.
Hence the linear behaviour of the dyes 3b, 3c, 3f and 3g shows that they exhibit twisted
intramolecular charge transfer characteristics. The DFT study also support this observation.
The N-phenyl ring present on pyrazole of the dye 3g shows the dihedral angle N14-N15-C3-C4,
at the ground state 25.04o which is reduced to 1.2o at the excited state (Fig. 4) Moreover, the
dihedral angle N14-N15-C3-C4 of the dye 3c is 27.4o which reduced to 8.66o at the excited state
(Fig. S8). Similarly the dye 3f shows N17-N16-C6-C5, 24.6 which is reduced to 2.1o (Fig. S9)
This observation indicates that the electron rich nitrogen of pyrazole containing phenyl ring
get twisting and gets more planar geometry in the excited state. Because of planarity the
charge transfer is more feasible in the excited state. So we conclude that the dyes show more
or less twisted intramolecular charge transfer (TICT) characteristics.
<< Please Insert: Fig. 9. Rettig’s plot of dyes 3b, 3c, 3f and 3g in solvent of varying
polarities.>>
3.4. Non Linear Optical (NLO) properties by experimental and DFT Method
Non-linear optics requires a non-centrosymmetric material. Consequently, tremendous effort
has been taken to develop polar materials based on a push-pull conjugated system.
Computational studies have suggested push-pull chromophores having twisted π-systems
with charge-separated ground state structures show large first-order hyperpolarizabilities
(βo)[66]. These pyrazole based dyes showing charge transfer characteristics, particularly
TICT. As revealed by Rettig plot these molecules are showing TICT and which is a
requirement for NLO properties. Therefore we were interested in understanding the NLO
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properties from solvatochromic shifts. In this paper we have estimated both the experimental
and theoretical values for non-linear optical properties. The experimentally derived values for
oscillator strength (f), transition dipole moment (µge), linear polarizability (αxx) and first
hyperpolarizability (βxxx) can be obtained using reported method [67] and are summarized in
Table 3 The dipole moment (ꢀ), mean polarizability (α0), polarizability anisotropy (ꢁα),
.
static first hyperpolarizability (β0) and second hyperpolarizability (γ) for the pyrazole based
dyes 3a-3g were calculated using B3LYP/6-31G(d) on the basis of the finite field approach
[32,68] which are summarized in Table S3.
Here, we have correlated the theoretically derived values with experimentally derived values
of the linear polarizability (αxx), first hyperpolarizability (βxxx) and second hyperpolarizability
(γ) for the pyrazole based push pull dyes 3a-3g. Among 3a-3g, in accordance with the
relative donor and acceptor strength of chromophore the hyperpolarizability value varies. The
computed first hyperpolarizability β0, of dyes was found to be ranging from 47.04, 50.01,
145.68, 39.93, 57.22, 64.43 and 296.35 x 10-30 e.s.u. for dyes 3a-3g respectively in DMSO.
The computed first hyperpolarizability (β0 ) value for urea are 0.38 x 10-30 e.s.u. In
comparison to urea these dyes show 68-780 times greater β0 value. The dyes 3c and 3g have
longer conjugation as compared to the dyes 3a-3b and 3d-3f and hence as expected it shows
higher β0 value. The computed value for second hyperpolarizability (γ) for the dyes 3a-3g are
found to be in the range of 115.38 to 1151 x 10-36 e.s.u which is 169-1692 times higher than
urea (computed γurea = 0.68 x 10-36 e.s.u.). The experimental findings also support the
theoretical results. But there are a number of parameters involved for calculating the
hyperpolarizability using the experimental method hence the experimental trend is somewhat
different than the theoretical one. Table 3 reveals that the experimentally calculated values
are found to be closer to the computed results.
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In general, the hyperpolarizability mainly depends on conjugation path length and strength of
electron donor and electron acceptor groups. The detailed theory used for calculating
theoretical and experimental values for non-linear optical properties are provided in
supporting information.
<< Please Insert: Table 3. Experimentally estimated linear and non-linear optical properties
of dyes 3a-3g in solvents of varying polarities.>>
4. Conclusion
In conclusion, we have synthesized pyrazole based D-π-A dyes through the rational design of
molecular structures involving the cyclization and Knoevenagel condensation reaction. These
1
dyes were fully characterized by FT-IR, H NMR, 13C NMR, Mass spectral analysis. These
dyes with extended conjugation show full-color-tunable solid-state emissions in blue to red
region (λem: 441–604 nm). The theoretical data obtained using DFT and TD-DFT with
B3LYP/6-311G(d) level are well agreement with the experimental result. The first
hyperpolarizability and static second order hyperpolarizability were calculated using finite
field approach. It was found that these dyes possess large second-order nonlinear optical
characteristics, which is mainly because of strong donor property of the electron rich pyrazole
unit and different acceptor groups separated by π bonds. Finally, DFT calculations proved to
be a useful tool to analyze the molecular structure and the non-linear optical (NLO)
properties of the new materials.
Acknowledgements
The authors are thankful to SAIF-Indian Institute of Chemical Technology, Bombay for
recording the mass spectra. Sandip K Lanke is thankful to UGC-CSIR, New Delhi for
providing junior and senior research fellowships.
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Tables All
List of Tables
Page No.
Table 1. Photophysical data of 3a-3g in selected solvents of varying polarities. ……….(2)
Table 2. Electronic distribution in the selected molecular orbitals of the dyes 3a-3g…..(3)
Table 3. Experimentally estimated linear and non-linear optical properties of dyes
3a-3g in solvents of varying polarities.
…………………………(4)
1