N-2-Aryl-1,2,3-Triazoles: A Novel Class of Blue Emitting Fluorophores-Synthesis, Photophysical Properties Study and DFT Computations

Article abstract of DOI:10.1007/s10895-015-1580-7

Novel fluorescent 2-[4-(4,5-diphenyl-1H-imidazol-2-yl) phenyl]-2H-naphtho [1,2-d] [1,2,3] triazolyl derivatives were synthesized from 4-(4,5-diphenyl-1H-imidazol-2-yl) aniline and substituted naphthalen-2-amine. The photophysical properties of the three n

Full text of DOI:10.1007/s10895-015-1580-7

J Fluoresc
DOI 10.1007/s10895-015-1580-7
ORIGINAL PAPER
N-2-Aryl-1,2,3-Triazoles: A Novel Class of Blue Emitting
Fluorophores-Synthesis, Photophysical Properties Study
and DFT Computations
Vikas S. Padalkar1 & Sandip K. Lanke1 & Santosh B. Chemate1 & Nagaiyan Sekar1
Received: 9 March 2015 /Accepted: 4 May 2015
#
Springer Science+Business Media New York 2015
Abstract Novel fluorescent 2-[4-(4,5-diphenyl-1H-imidazol-
2-yl) phenyl]-2H-naphtho [1,2-d] [1,2,3] triazolyl derivatives
were synthesized from 4-(4,5-diphenyl-1H-imidazol-2-yl) an-
iline and substituted naphthalen-2-amine. The photophysical
properties of the three new fluorophores were evaluated in
acetonitrile, methanol, dimethylsulfoxide and N,N-
dimethylformamide solvents and were compared with the re-
ported analogs. The compounds show the absorption in the
ultraviolet region and the emission in the blue region. The
thermal stabilities of these compounds was evaluated by ther-
mogravimetric analysis. The solvatochromism data are used
for ground and excited state dipole moment determination of
the synthesized triazoles using Bakhshiev and Bilot-Kawski
correlations. The experimental absorption and emission were
compared with the theoretical data obtained by DFT and TD-
DFT computations and they are well in agreement with each
other.
Introduction
1,2,3-Triazole and 1,2,4-triazole are important classes of hetero-
cyclic compounds due to their wide applications in the biology
[1, 2] and as functional materials [1, 2]. 1,2,3-Triazoles are com-
mercially used as ultraviolet absorbers [3], fluorescent brighten-
ing agents and finishing agents [4, 5]. Since last two decades
triazoles are used as metal sensors [6], in optoelectronic devices
[6], chemosensors [7], anion sensor [8] and donor-acceptor chro-
mophores [9]. More recently triazole derivatives are reported as
sensor for nitric oxide [10], Cu (II) sensor [11], Hg (II) sensor
[11], Pd (II) sensor [11], live cell sensor [12], organic light emit-
ting diodes [13], Zn (II) sensor [14], and in the sensing of DNA
in human cells [15].
N-1-Substituted 1,2,3-triazoles are explored more and they
have been widely applied in materials chemistry [16–19]. N-2
Aryl analogues have more desirable photophysical properties as
compared N-1-aryl substituted triazoles [22]. N-1-Aryl triazole
A gave almost no emission, while N-2- aryl-triazole B showed
good emission [22]. Better properties are expected based on
literature and structural consideration [22]. However, N-2-aryl
analogs of 1,2,3-triazole have received very less attention
[20–22] and hence we have studied photophysics of 2-aryl
substituted triazoles.
.
.
.
Keywords Triazole Fluorescence Photophysics
.
.
.
Imidazole Thermal stability DFT TD-DFT
Electronic supplementary material The online version of this article
(doi:10.1007/s10895-015-1580-7) contains supplementary material,
which is available to authorized users.
* Nagaiyan Sekar
n.sekar@ictmumbai.edu.in
1
Tinctorial Chemistry Group, Institute of Chemical Technology, N. P.
Marg, Matunga, Mumbai 400 019, Maharashtra, India

J Fluoresc
The synthesis methods for N-2-aryl triazoles are dif-
ficult [23]. The most of the N-2-aryl-1,2,3-triazoles have
been synthesized through reactions involving SNAr
mechanism [23]. The substitution effect of N-aryl-1,2,
3-triazole is not discussed much more till date [23].
The fluorescence properties of the triazoles depend on
the substitution pattern [22].
This literature results suggested that the N-2-aryl-triazoles
may adopt effective co-planar conformation between the tri-
azole ring and the N-2- aryl ring even with the presence of a
Bfree^ rotatable N-C bond [22]. This unique characteristic
suggested that N-2-aryl-triazole might be applied as new
building blocks to reach effective UV/blue-emitting com-
pounds by overcoming the challenges associated with the oth-
er small molecule systems.
triazole family N-1-substituted 1,2,3-triazoles and N-2-
substituted 1,2,3-triazoles emit in blue region by varying
substitution on benzene ring attached to triazole ring
[25–31]. Considering all these factors in this paper, three
novel 2-aryl-1,2,3-triazoles were synthesized as shown in
Scheme 1. The optical properties of these compounds
were investigated in methanol, acetonitrile, DMF and
DMSO. The photophysical properties of these
fluorophores were compared with that of the reported tri-
azole analogs [25–31]. The experimental photophysical
properties were compared with theoretical data obtained
by DFT and TD-DFT computations.
Experimental
Blue emitting compounds are commercially used as
fluorescent brightening agents. Coumarins are well known
fluorescent brighteners, but have less photostability and
their synthesis methods are tedious [24]. These limitations
were overcome by triazole fluorescent brighteners. In
Experimental Methods
Reagents and Analysis Methods p-Nitrobenzaldehyde,
benzil, ammonium acetate, Pd-C (10 %), hydrazinehydrate
Scheme 1 Synthesis of N-2-aryl triazoles 7a–7c

J Fluoresc
(99 %), 2-aminonaphthyl-substituted sulphonic acid and
solvents were purchased from Spectrochem Pvt. Ltd.
The FT-IR spectra were recorded on a Perkin-Elmer
Spectrum 100 FT-IR Spectrometer. 1H-NMR and 13C-
NMR spectra were recorded on VXR 300 MHz instru-
ment using TMS (trimethylsilane) as the internal stan-
dard. Mass spectra were recorded on Finnigan mass
spectrometer.
Ar- H), 8.02–8.04 (d, 2H, J=2.2, 6.0
Hz, Ar- H), 8.26–8.29 (dd, 2H, J=
2.2, 6.0Hz, Ar- H).
Synthesis of 4-(4,5-diphenyl-1H-imidazol-2-yl) aniline 4
Palladium-carbon catalyst (10 %) was added portion
wise over a period of 5–10 min lot wise to a hot
solution of 3 (1.0 g, 0.0032 mol) in ethanol (60 mL)
containing hydrazine hydrate (1.12 g, 0.022 mol). The
mixture was heated at reflux temperature for 1 h. The
hot solution was filtered through a Whatman paper No
41 to remove Pd and further filtrate was filtered
through silica gel (10 g) and solvent was evaporated.
Pure product was obtained and used without
purification.
UV Absorption Spectra Absorption spectra were recorded
on a Perkin-Elmer Lamda-25 spectrophotometer in 1 cm-
length quartz cell with resolution of 0.1 nm. All the samples
were measured from 1×10−6 mol L−1 solution of N-2-aryl-1,2,
3-triazole in methanol, acetonitrile, DMF and DMSO with the
wavelength range between 300 and 700 nm.
Fluorescence Emission Spectra Fluorescence emission
spectra were measured on Varian Cary Eclipse fluorescence
spectrophotometer with slit width of 2.5 nm. The emission
spectra were obtained from 1×10−6 mol L−1 solution. The
excitation wavelength was absorption maxima of the corre-
sponding compound.
Yield
0.69 g, 76 %. m.p.: 243oC
Decomposes.
FT-IR (KBr)
1465, 1610, 2930, 3205, 3430 cm−1.
3.84 (s, 2H, -NH2), 6.62–6.65 (dd,
2H, J=1.5, 6.0Hz, Ar- H), 6.81–6.83
(dd, 2H, J=1.5, 6.0Hz, Ar- H), 7.02–
7.04 (dd, 4H, J=1.5, 6.0Hz, Ar- H),
7.11–7.14 (dd, 2H, J=1.5, 9.0Hz,
Ar- H) 7.62–7.65 (dd, 2H J=9.0Hz,
Ar- H), 8.97–8.99 (d, 2H).
1H NMR (300 MHz
CDCl3) (δ: ppm)
Quantum Yield The quantum yields of the compounds 7a-7c
were evaluated in methanol, acetonitrile, DMF and DMSO.
The comparative method [32, 33] was used for quantum yield
determination. Anthracene (ϕ: 0.27 in ethanol) was used as
the standard.
General Procedure for Preparation of o-Amino azo
Compounds 6
Thermal Stability The thermogravimetric studies have been
carried out in the temperature range 50–600oC under nitrogen
gas at a heating rate of 10oC min−1 on Water India TGA
instrument.
4-(4,5-Diphenyl-1H-imidazol-2-yl) aniline 4 (2 g,
0.008 mol) was dissolved in HCl (2 mL) with stirring
and cooled to 0–5 °C. It was diazotized with sodium
nitrite (0.69 g, 0.010 mol). The excess of nitrous acid
was destroyed by the addition of urea. 2-Aminonaphthyl-
substituted sulphonic acid 5 (1.78 g, 0.008 mol) was
neutralized with 10 % aqueous sodium carbonate and
the diazo compound was added to it slowly maintain-
ing the pH 6 of the solution adding sodium acetate.
The solution was stirred at 10oC for 2 h and then at
30oC for 1 h. It was finally heated at 70oC for 3 h, the
red o-amino azo compound 6 was filtered, washed with
dilute sodium carbonate solution and dried. Yield: 72–
84 %
Synthesis of 2-(4-nitrophenyl)-4,5-diphenyl-1H-imidazole 3
A mixture of benzil (2.1 g, 10 mmol) and ammonium
acetate (7.7 g, 0.01 mol) in glacial acetic acid
(20 mL) was stirred at 80oC for 1 h under nitrogen
atmosphere. p-Nitro benzaldehyde (1.5 g, 0.01 mol) in
glacial acetic acid (10 mL) was added drop wise over a
period of 15 min at the same temperature and stirred for
another 3 h. The resulting homogenous solution was
poured over crushed ice (100 g). The yellow precipitate
was collected by filtration and washed with cold water,
then dried under vacuum. The crude product was recrys-
tallized from acetone to afford the pure 3.
Yield
2.83 g, 83 %. m.p.: 209–210oC.
1443, 1518, 1600, 3394 cm−1.
7.26 (s, 1H, J=6.0Hz, Ar -H), 7.34–
7.36 (d, 6H, J=6.0Hz), 7.54 (s, 4H,
General Procedure for the Formation of Triazols 7
FT-IR (KBr)
1H NMR (300 MHZ
CDCl3) (δ: ppm)
The o-amino azo compounds 6 (0.01 mol) was dis-
solved in DMF (5 mL) and basic cupric acetate

J Fluoresc
Synthesis of 2-[4-(4,5-diphenyl-1H-imidazole-2-yl)] phenyl]
-2H-naphtho {1,2-d] [1,2,3] triazole (7c)
(0.012 mol) solution was added. The mixture was heat-
ed at 120 °C and air was rapidly bubbled into it. After
15–30 min, the red color was discharged and the mix-
ture was poured to ice cold water (10 mL). The sepa-
rated solid was filtered, washed well with water and
dried in oven at 60 °C for 10 h to yield the crude
triazolyl compounds, which were further recrystallized
from DMF. Yield: 79–87 %
Yield
m.p.
87 %
> 300oC
FT-IR (KBr)
3441, 3119, 1581, 1489, 1449, 1184,
1032, 991, 836, 701.
7.23 (d, 1H, J=7.8Hz, Ar-H), 7.29–7.31
1H NMR (300
MHz DMSO) (t, 2H, J=1.6, 7.8Hz, Ar-H), 7.34-7.37 (m,
8H, J=1.6, 7.8Hz, Ar-H), 7.47–7.51 (dd,
5H, J=1.8, 7.8Hz, Ar- H), 7.87–7.89 (d,
2H, J=7.8Hz, Ar-H), 8.04–8.07 (d, 2H,
J=1.6, 7.8Hz, Ar-H), 8.77 (s, 1H).
109.3, 119.0, 123.3, 124.7 (s), 125.9,
MHz DMSO) 126.1, 127.0 (s), 127.4, 127.9 (s), 128.4,
128.9 (s), 129.0, 129.3, 129.8, 131.8,
2-[4-(4,5-Diphenyl-1H-imidazole-2-yl) phenyl]-2H-naphtho
[1,2-d] [1,2,3] triazole-7-sulphonic acid (7a)
Yield
m.p.
79 %
13C NMR (100
> 300oC
FT-IR (KBr)
3439, 3115, 1612, 1570, 1488, 1453,
1181, 1028, 988, 833, 775, 699 cm−1.
7.28 (d, 1H, J=7.5Hz, Ar-H), 7.33–7.37
1H NMR (300
134.8 (s), 136.1, 137.2, 168. 0.
Mass (m/z)
464.33 (M+163 %) 465.30 (100 %),
332.78 (47 %), 268.79 (12 %).
MHz DMSO) (t, 2H, J=1.8, 7.5Hz, Ar-H), 7.39–7.47
(m, 8H, J=1.8, 7.5Hz, Ar-H), 7.53–7.56
(dd, 4H, J=1.8, 7.5Hz, Ar- H) 7.94–7.97
(d, 2H, J=7.5Hz, Ar-H), 8.08–8.10 (d,
2H, J=1.8, 7.5Hz, Ar-H), 8.98 (s, 1H),
12.5 (s, 1H).
Results and Discussion
13C NMR (100
112.2, 124.0, 126.0, 126.7(s), 127.0,
Chemistry
MHz DMSO) 127.3, 127.6 (s), 128.1, 128.4 (s), 129.0,
129.2 (s), 129.7, 130.3, 132.4, 134.8,
Three new triazoles were prepared by the cyclisation, reduc-
tion, diazotization and oxidation reactions Scheme 1. The re-
action of 4-nitrobenzaldehyde with benzil in the presence of
ammonium acetate in acetic acid at 80oC to afford the com-
pound 3 in good yield. The compound 3 on Pd-C/ hydrazine
hydrate reduction in ethanol at reflux temperature gave the
amine 4, which on further diazotization using sodium nitrite
gave the corresponding diazonium salt. The diazonium salt 4a
was coupled with different substituted naphthalen-2-amine at
low temperature to give the diazo compounds 6. The triazoles
7a–7c was obtained by air oxidation of the compound 6 in
basic cupric acetate solution at 120oC in DMF. The structures
of the compounds were confirmed by spectral analysis.
138.2(s), 139.0, 141.2, 179. 4.
543.34 (M+123 %) 542.40 (100 %),
356.52 (12 %), 260.34 (46 %)
Mass (m/z)
Synthesis of 2-[4-(4,5-diphenyl-1H-imidazol-2-yl) phenyl]
-2H-naphtho [1,2-d] [1,2,3] triazole-6-sulphonic acid (7b)
Yield
m.p.
FT-IR (KBr)
83 %
> 300oC
3432, 3106, 1618, 1573, 1492, 1458,
1179, 1031, 991, 778 cm−1.
7.31 (d, 1H, J=8.0Hz, Ar-H), 7.34 (t,
1H NMR (300
MHz DMSO) 2H, J=1.6, 8.0Hz, Ar-H), 7.43–7.46 (m,
8H, J=1.6, 8.0, Hz, Ar-H), 7.50–7.53
(dd, 4H, J=1.6, 8.0Hz, Ar- H) 7.91–7.94
(d, 2H, J=8.0Hz, Ar-H), 8.06–8.08 (d,
2H, J=1.6, 8.0Hz, Ar-H), 8.87 (s, 1H),
12.7 (s, 1H).
112.0, 123.7, 125.9, 126.0 (s), 127.4,
MHz DMSO) 127.7, 127.9 (s), 128.0, 128.3 (s), 129.2,
129.6 (s), 130.0, 130.7, 132.6, 134.9,
Photophysical Properties
N-2-Aryl-1,2,3-triazoles are good fluorescent fluorophores
compared to the N-1-Aryl-1,2,3-triazoles [22]. The
photophysical properties of the N-2-Aryl-1,2,3-triazoles are
not studied intensively. In the literature, very few reports are
available which describe the photophysical properties of the N-
2-aryl-1,2,3-triazoles [22]. In the present paper, we discuss the
photophysical properties of the novel N-2-aryl-1,2,3-triazoles
7a–7c. The photophysical properties are compared with the re-
ported analogs [25–31]. The effect of the substituents on the
13C NMR (100
138.4 (s), 139.7, 141.9, 181.3.
Mass (m/z)
543.30 (M +148 %) 542.36 (100 %),
355.12 (29 %), 260.00 (39 %)

J Fluoresc
photophysical properties are highlighted in the comparison
study. The DFT and TD-DFT computation were performed to
estimate theoretical absorption and emission. The experimental
results were compared with computed data and experimental
results are in good agreements with theoretical data.
uents attached to the para position with respect to the
triazole ring show significant change in the absorption
and the emission. The different electron donor, acceptor
and extended conjugated groups are considered for the
comparison 8–11. Electron donor and extended conjuga-
tion groups show the red shifted absorption as well as
the emission as compared to the electron acceptor group
attached to 2-aryl position. The compound 8 shows a
blue shifted absorption and emission compared to the
compounds 9 and 11.
Substituent’s Effect on Photophysical Properties
The electron donor and the acceptor groups tune the
absorption and the emission properties [29]. The substit-
The planarity and extended conjugation or push-pull-type
of chromophores gives bathochromic shift because of the ef-
fective aromatic conjugation, usually these compounds pos-
sess a low energy gap between the excited and the ground
states, thus, giving emission at a longer wavelength [22].
The effect of the fused groups on optical properties attached
to p-position with respect to the triazole ring is also considered
in the present study. Three different fused triazoles 12–14
were compared. The compounds 12–14 show red shifted ab-
sorption and the emission compared to compounds 8–11.

J Fluoresc
The electron donor or fused system on the naphthalene ring
shows red shifted absorption and emission compared to the
other substituents on the triazoles. The triazole 15 contains
electron donor carbazole type of ring fused with triazole ring,
which helps in a better delocalization and hence red shifted
absorption and emissions are observed.
properties of the compound 7b are similar to the compound
7a. The compound 7c shows absorption between 353 and
361 nm. The compounds 7a–7c show red shifted absorption
in DMSO and DMF. The compounds 7a–7c are fluorescent in
solution. The compounds show emission in the visible region
in between 401 and 431 nm. The emission properties of the
compounds depend on solvent polarity. In polar solvents
(DMF and DMSO) the compounds show red shift as com-
pared to the other solvents, methanol and acetonitrile. The
quantum efficiency of the compounds 7a–7c are between 13
and 19 % in the studied solvents.
Computational Methodology
DFT method was used for the ground state optimization, while
for the excited state optimization, TD-DFT and Configuration
Interaction Singles (CIS) methods were employed. The hybrid
functionals namely B3LYP (Becke3-Lee-Yang-Parr hybrid
functional) [34–37] was used. The 6-31G(d) basis set was
used for all the atoms and later was ascertained in the literature
[38]. The Polarizable Continuum Model (PCM) [39] was used
to optimize the ground and excited state geometries in sol-
vents. On the basis of the optimized ground state structures
absorption characteristics, oscillator strengths and configura-
tion were systematically evaluated using TD-DFT with polar-
izable continuum model PCM model. The solvents used were
methanol (MeOH), acetonitrile (ACN), N,N-dimethyl form-
amide (DMF), dimethylsulfoxide (DMSO). All computations
were performed with the Gaussian 09 package [40].
The calculated excitation spectra using B3LYP functional
at 6-31G(d) levels in solvents of varying polarity, oscillator
strength, and their orbital contribution are shown in Table 2.
The vertical excitation computed by TD-DFT is compared
with the experimental absorption maxima. The chromophore
7a has absorption at 353 nm in MeOH, its calculated vertical
excitation at 6- 31G(d,) level is 353 nm and it is well in agree-
ment with the experimental value. The experimental absorp-
tion maxima for 7a is 354 nm in acetonitrile, 360 nm in DMF
and 354 nm in DMSO, while the computed vertical excita-
tions are 350 nm in acetonitrile and 349 nm in DMF and
Novel triazoles 7a–7c contains sulphonic acid on the naph-
thalene ring and phenyl groups on the imidazole ring. These
compounds are novel class of blue emitting compounds which
emits at wavelengths similar to the reported analogs 8–15. The
compounds 7a–7c show absorption peaks around 350 nm in
ultraviolet region. The absorption wavelengths were used as
excitation wavelength for the fluorescence measurements.
The compounds show emission in blue region between 400
and 430 nm with considerable Stokes shift and quantum effi-
ciency. It is seen that the absorption-emission characteristics
of the compounds 7a–7c are nearly same. The absorption and
emission spectra of the compounds 7a–7c are shown in
Figs. 1, 2 and 3 and photophysical data are summarized in
Table 1. The compound 7a absorbs between 353 and
360 nm in acetonitrile, methanol, DMF and DMSO. In DMF
and DMSO compound 7a shows red shifted absorption as
compared to methanol and acetonitrile. The absorption
Fig. 1 Absorption and emission
a
b
spectra of compound 7a, a
Absorption spectra b Emission
spectra
0.08
0.06
0.04
0.02
0
DMF
DMF
DMSO
Methanol
Acetonitrile
DMSO
Acetonitrile
Methanol
800
600
400
200
0
325
350
375
400
425
375
400 425
450 475 500
Wavelength(nm)
Wavelength(nm)

J Fluoresc
Fig. 2 Absorption and emission
spectra of compound 7b, a
Absorption spectra b Emission
spectra
a
b
0.08
0.06
0.04
0.02
0
DMF
DMSO
Acetonitrile
Methanol
DMF
DMSO
800
600
400
200
0
Methanol
Acetonitrile
325
350
375
400
425
375 400 425 450 475 500
Wavelength(nm)
Wavelength(nm)
DMSO. It is observed that the chromophore 7a shows less
deviation of 3 nm in methanol while more deviation of
14 nm is observed in DMSO. The chromophore 7b shows
more deviation of 14 nm in methanol while less deviation of
11 nm in DMSO. The chromophore 7c shows highest devia-
tion as compared to chromophore 7a and 7c which is in the
range of 18–25 nm, in methanol solvent it is 18 nm, and 25 nm
in DMSO. In the case of emission a large difference was
observed between experimental emission and TD-DFT emis-
sion. The compounds 7a and 7b show more deviation as com-
pared to the compound 7c. A large difference was observed
for the compound 7a in acetonitrile (87 nm). The major orbital
contribution for the charge transfer transition arises from
HOMO to LUMO+1 for the compounds 7a and 7b and it is
98 % while for the compound 7c the major orbital contribution
is HOMO-1 to LUMO and it is 94 %. The frontier molecular
orbital diagrams for the compounds 7a–7c in acetonitrile sol-
vent are shown in Table 3. In the case of HOMO-1 the electron
density is distributed over the complete structure, while in the
case of HOMO the electron density is distributed over the
diphenyl and the imidazole core. In the case of LUMO the
electron density is distributed over the triazole and the naph-
thalene units, while the electron density is on the diphenyl and
the naphthalene units for LUMO+1 orbitals. Thus, the absorp-
tion leading to the electronic excitation from the HOMO or
HOMO-1 to the LUMO+1 or LUMO orbital can constitute to
the charge transfer from the trizole unit to the imidazole con-
taining two phenyl units.
The optimized bond angles and dihedral angles at B3LYP/
6–31 g (d) level are summarized in SI Table 1. The compound
7a shows bond angle N(17)-N(16)-C(8), 121.8° at the ground
state but at the excited state it is slightly higher (122.7°) but
significant change is observed for the dihedral angle C(12)-
C(13)-C(33)-N(36) between the central benzene ring and the
imidazole segment. The dihedral angle of 7a in acetonitrile at
the ground state is 173.3o while in the excited state it is
175.68° which indicates that imidazole core attached with
benzene segment is more planar in the singlet excited state
as compared to the ground state. But there is very small
change observed for the central benzene core and the triazole
segment which indicates that a small change is observed for
planarity between central benzene core and triazole segment.
A similar trend is observed for the compound 7a in the other
solvents like DMF, DMSO and MeOH. The dihedral angle,
N(37)-C(34)-C(49)-C(50) show 2o change in the excited state.
This indicates a slight twisting occuring when the molecule is
excited; the two phenyl rings attached to the imidazole core
become twisted. The trend is similar in all the other solvents.
An interesting result was observed for the compound 7b - the
dihedral angle, N(17)-N(16)-C(8)-C(9) between the triazole
and benzene ring at the ground state is 174.7° but in the singlet
excited state it is changed by 5°. The dihedral angle, N(17)-
N(16)-C(8)-C(10) at the ground state is 5.2° and in the excited
state it is change to 0.6° which confirms that, the triazole core
attached to the central benzene segment is more planar in the
excited state as compared to the ground state. The compound
Fig. 3 Absorption and emission
spectra of compound 7c, a
Absorption spectra b Emission
spectra
a
b
0.08
DMF
DMSO
Methanol
Acetonitrile
DMF
DMSO
Acetonitrile
Methanol
800
600
400
200
0
0.06
0.04
0.02
0
325
350
375
400
425
375
400
425
450
475
500
Wavelength(nm)
Wavelength(nm)

J Fluoresc
Table 1 Photophysical properties of compounds 7a–7c
Absorption (nm) Emission (nm) Stokes Shift (nm) Stokes Shift (cm−1
Compounds Solvents
)
aQuantum Yields (%) TD-DFT Emission (nm)
7a
7b
7c
Methanol
353
431
403
413
418
416
406
410
418
426
401
415
418
78
49
53
55
63
51
52
59
73
47
56
57
5200
3400
3500
3600
4300
3500
3500
3500
3900
4800
3300
3800
16
17
14
17
19
18
17
13
17
19
18
19
494
490
494
493
490
490
490
490
467
467
467
467
Acetonitrile 354
DMF
360
363
353
DMSO
Methanol
Acetonitrile 355
DMF
358
359
353
DMSO
Methanol
Acetonitrile 354
DMF
359
361
DMSO
λmax and λem were measured in nm and solvent used are methanol, acetonitrile, dimethylsulfoxide, and dimethylformamide, Measurements were carried
out at room temperature
a Anthracene is used as standard
7b in acetonitrile shows dihedral angle, C(12)-C(13)-C(32)-
N(35) 178.5 in the ground state and it is reduced to 3o in the
excited state. Similarly the dihedral angle, C(12)-C(13)-
C(32)-N(36) at the ground state is 1.9° increased to 4° in the
excited state. It clearly shows that the central benzene core
attached to the imidazole segment is more planar in the ground
state as compared to the excited state. But the two imidazole
containing phenyl rings exhibit little change 1 or 2o in the
excited state. A similar trend was observed in the other polar
solvents like MeOH, DMF and DMSO. The compound 7c
shows the dihedral angle, N(17)-N(16)-C(8)-C(9) at the
ground state 178.9° but in the excited state it is changed by
1o . The dihedral angle, N(17)-N(16)-C(8)-C(10) at the ground
state is 1.1° and in the excited state it is changed to 0.5° which
confirms that the triazole core attached to the central benzene
segment is closer to planarity in the excited state as compared
to the ground state. But it is noteworthy that the compound 7c
in acetonitrile shows a dihedral angle, C(12)-C(13)-C(32)-
N(30) 169.4° in the ground state and it is increased up to 6o
in the excited state. While the dihedral angle, C(12)-C(13)-
Table 2 Observed absorption and computed vertical excitation for compounds 7a–7c in various solvent
Structure
Solvent
aExp.
TD-B3LYP/6-31G(d)
Vartical excitation
λmax
Oscillator strength ( f )
cOrbital contribution
bnm
eV
7a
7b
7c
Methanol
Acetonitrile
DMF
353
354
360
363
353
355
358
359
353
354
359
361
350
350
350
349
367
367
367
368
335
335
336
336
3.5462
3.5465
3.5465
3.5473
3.3740
3.3741
3.3730
3.3737
3.7011
3.7001
3.6949
3.6954
0.0162
0.0165
0.0173
0.0175
0.0929
0.0928
0.0920
0.0923
0.2457
0.2457
0.2468
0.2461
H→L+1 (95 %)
H→L+1 (95 %)
H→L+1 (95 %)
H→L+1 (95 %)
H→L+1 (98 %)
H→L+1 (98 %)
H→L+1 (98 %)
H→L+1 (98 %)
H-1→L (94 %)
H-1→L (94 %)
H-1→L (94 %)
H-1→L (94 %)
DMSO
Methanol
Acetonitrile
DMF
DMSO
Methanol
Acetonitrile
DMF
DMSO
a Experimental absorption in nm; b Vertical excitation (theoretical absorption in nm); c Electronic transitionsy

J Fluoresc
Table 3 Electronic distribution in the selected molecular orbitals of compound 7a–7c in acetonitrile solvent
C(32)-N(31) at the ground state is 10.9° it is reduced to 6° in
the excited state. This clearly indicates that the central benzene
core attached to the imidazole segment is more planar in the
excited state as compared to the ground state. But the imidaz-
ole attached to two phenyl ring shows little change up to 1o in
the excited state. All the compounds show a similar behavior
in all the other solvents. The optimized structures of the com-
pounds 7a–7c are summarized in SI Fig. 1.
solvent medium [48, 49]. Hence solvatochromism and
solvatofluorism are found to be excellent tools to eval-
uate the ground state and excited state dipole moments
of the short-lived species [50–59].
In this paper, we evaluated the ratio of ground to excited
state dipole moment of the synthesized triazoles using
Bakhshiev [60] and Bilot-Kawski correlations [61, 62]. The
detailed theory for the calculation of dipole moment is de-
scribed in SI (Supporting Information).
Dipole Moment Determination by Solvatochromic Method
Calculations of Dipole Moment The solvent polarity func-
tions f1(ε,η) and f2(ε,η) for each solvent were calculated by
substituting the respective values for dielectric constant and
refractive index in equations 4 and 7 (SI) the values obtained
are tabulated in SI Table 2. Graph of f1(ε,η) verses Stokes shift
The properties of solvents such as dielectric constant, refrac-
tive index and permittivity are very important for study the
physicochemical process of the molecules. These properties of
solvent induce changes in the electronic transitions of solutes
(solvatochromism) depending on the nature and the degree of
interaction between the solute solvent in ground and first ex-
cited states of solutes (developed locally in the immediate
vicinity) [41, 42]. In short we can say that change in the dipole
moment of short-lived species (excited state) is considerably
affected by properties of solvent and can be clearly indicated
by solvatochromism and solvatofluorism. The dipole moment
of electronically excited species is of great importance in rev-
elation of the nature of excited state, designing non linear
optical material and parameterization of semi-empirical
quantum chemical procedures of these states [43]. A
most accepted technique for dipole moment calculation
is based on Lippert-Mataga equation [44–47]. This tech-
nique is based on the absorption and emission maxima
with polarity functions of solvents, expressed by the
relative permittivity (ε) and refractive index (η) of the
in cm−1 is plotted and calculated m1 as slope 1, as well as
ꢀ
−1
Vf
graph of f2(ε,η) verses Vaþ Þ=2 in cm were plotted to
get values of function m2 as slope 2, the obtained values are
tabulated in SI Table 3. Further the ratio of the excited
state dipole moment to the ground state dipole moment
is calculated from equation 12 by substituting the values
Table 4 Excited state and ground state dipole moment (in Debye) ratio
value for dyes 7a–7c
|m1+m2| |
|m1−m2|
μe
μg
Compound
7a
7b
7c
2695
3515
6948
85
1091
2655
0.031
0.310
0.382

J Fluoresc
Fig. 4 Thermogravimetric curve
of compounds 7a–7c
of m1 and m2 and summarized in Table 4. The calcu-
lated values demonstrate that the compounds 7a–7c has
more polar ground state than the excited state indicating
that the charge transfer processes occurs in the ground
state rather than in the exited state.
states which were confirmed by the structure optimization
by DFT and TD-DFT. The experimental photophysical prop-
erties were correlated with the theoretical data and they are in
good agreements. The maximum deviation between experi-
mental absorption and vertical excitation is 25 nm. The com-
pounds are thermally stable up to 300oC.
Thermal Stability Study
Acknowledgments Vikas Padalkar and Santosh Chemate are thankful
to the Institute of Chemical Technology, Mumbai, India for the encour-
agement. Sandip Lanke is thankful to UGC-CSIR for Senior Research
Fellowship.
In addition to a large Stokes shift and quantum efficiency
thermal stability is also an important parameter for the com-
pounds which are applicable in high-technology. The TGA
curves of the compounds are shown in Fig. 4. The TG results
indicate that the framework of the synthesized compounds are
stable up to 3000C. Above 3000C the TG curves of the syn-
thesized compounds show a loss in weight. The compound 7c
shows complete weight loss around 451oC, while the com-
pounds 7a and 7b show 50 % weight loss around 450oC.
The compounds 7a and 7b does not show complete weight
loss even up to 600oC. The comparisons of the Td (decompo-
sition temperature) showed that the thermal stability of 7a–7c
decreases in the order 7a>7b>7c. Thermal stability and their
plausible degradation scheme are presented in Fig. 4.
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