Chemiluminescence properties of luminol related o-hydroxybenzimidazole analogues: Experimental and DFT based approach to photophysical properties

Article abstract of DOI:10.1016/j.dyepig.2014.08.009

Novel luminol-isoluminol derivatives containing o-hydroxyphenyl benzimidazole unit were synthesized from aromatic aldehydes and diaminophthalates followed by heating under reflux with hydrazine hydrate. The chemiluminescent properties were studied in hydr

Full text of DOI:10.1016/j.dyepig.2014.08.009

Dyes and Pigments 113 (2015) 189e199
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Chemiluminescence properties of luminol related
o-hydroxybenzimidazole analogues: Experimental and DFT based
approach to photophysical properties
Mininath S. Deshmukh, Nagaiyan Sekar*
Tinctorial Chemistry Group, Dyestuff Technology Department, Institute of Chemical Technology (Formerly UDCT), N.P. Marg, Matunga, Mumbai 400019,
Maharashtra, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Novel luminoleisoluminol derivatives containing o-hydroxyphenyl benzimidazole unit were synthesized
from aromatic aldehydes and diaminophthalates followed by heating under reflux with hydrazine hy-
drate. The chemiluminescent properties were studied in hydrogen peroxide, potassium hex-
acyanoferrate(III) and sodium hydroxide solution. The chemiluminescence properties were compared
with the standard luminol and isoluminol systems it was observed that the chemiluminescence prop-
erties of the novel derivatives were superior to luminol and isoluminol. Density Functional Theory
computations have been used in order to have a greater understanding of the structural, molecular,
electronic and photophysical properties. The experimental absorption and emission wavelength values
were in good agreement with the computed vertical excitation and emission values.
Received 5 May 2014
Received in revised form
1 August 2014
Accepted 10 August 2014
Available online 20 August 2014
Keywords:
Luminoleisoluminol
Chemiluminescence
Fluorescence
Solvatochromism
ESIPT
© 2014 Elsevier Ltd. All rights reserved.
TD-DFT
1. Introduction
because of their wide applications in molecular probes [10], lumi-
nescent materials [11,12], metal ion sensors [13e15], organic light
Compounds exhibiting chemiluminescence (CL) are extensively
used for various biomicroanalyses due to their high selectivity and
sensitivity [1]. In recent years, chemiluminescence has been an
attractive detection technique in the high-performance liquid
chromatography (HPLC) [2]. A convenient labelling reagent having
a luminol moiety has not been reported due to the fact that the CL
intensity is always low and the synthesis method is difficult. On the
other hand, various reagents containing an isoluminol moiety like
isoluminolisothiocyanate (ILITC) [3], N-aminobutyl-N-ethyl-
isoluminol (ABEI) [4,5], 4,5-diaminophthalhydrazide (DPH) [6] and
6-aminomethylphthalhydrazide (6-AMP) [7] have been widely
used for the microanalyses of biological compounds. The intro-
duction of electron donating groups (alkyl or aryl) on the amino
group of luminol causes a decrease in the CL intensity. On the other
hand, the electron donating group (alkyl or aryl) on the amino of
isoluminol causes an increase in the CL intensity [8,9].
emitting devices (OLEDs) [16] and molecular logic gates [17]. In the
case of barrierless excited state intramolecular proton transfer
(ESIPT), a covalently attached proton, typically of a hydroxyl or
amino group, in the electronically excited state migrates to a
neighbouring hydrogen-bonded atom less than 2 Å away [18]. A
large Stokes shift is a desired characteristic for fluorophores
because self-absorption or the inner filter effect can be avoided and
the fluorescence analysis can be enhanced [19]. It is difficult to
increase the Stokes shift of the conventional fluorophores by
chemical modification.
During photoexcitation to the first excited state, the acidity of
the acidic center (proton) and the basicity of the basic center
(imidazole nitrogen) increase because of a change in the charge
density of the chromophore. This leads to the migration of the
proton from the acidic centre to the basic center via the hydrogen
bond coordinate to give the phototautomer. The phototautomer
formed in the excited state emits light and thermally equilibrates
back to the ground state with the proton bound to its original
atom. Generally these processes are extremely fast and several
studies have shown subpicosecond reaction rates for a variety of
molecules [20,21]. In this process a significant amount of excited-
state energy is dissipated, the phototautomer fluoresces at a lower
In recent years, there has been an increasing interest in mole-
cules with excited state intramolecular proton transfer (ESIPT)
* Corresponding author. Tel.: þ91 22 3361 1111/2707; fax: þ91 22 3361 1020.
E-mail addresses: n.sekar@ictmumbai.edu.in, nethi.sekar@gmail.com (N. Sekar).
http://dx.doi.org/10.1016/j.dyepig.2014.08.009
0143-7208/© 2014 Elsevier Ltd. All rights reserved.

190
M.S. Deshmukh, N. Sekar / Dyes and Pigments 113 (2015) 189e199
energy with an unusually larger Stokes shift (up to 10,000 cmꢀ1
[22].
)
in sodium hydroxide to the 100 mL portion of a 10 nM solution of
each chemiluminophore. The chemiluminescence intensities were
recorded immediately after the injection of oxidizing agent solu-
tion. The integrated photon counts for 1 min after the injections
were explained as the chemiluminescence intensities.
Therefore, considering the wide applications and importance of
ESIPT inspired molecules and in continuation of our research work
on fluorescent materials [23e25] herein, we report the synthesis of
novel chemiluminescent ESIPT inspired substituted HPBI
(hydroxyphenyl benzimidazole) fluorophores and their photo-
physical properties are correlated with the computational results
obtained from DFT [B3LYP/6-31G(d)] and reported analogues.
2.3.2. Optimization of the chemiluminescence conditions
To establish the optimum chemiluminescence conditions the
concentrations of the oxidizing reagents (hydrogen peroxide, po-
tassium hexacyanoferrate(III) and sodium hydroxide) were varied
one at a time. The temporarily settled conditions were DMF solu-
tion of chemiluminophore, 5.0 mM hydrogen peroxide and 5.0 mM
2. Experimental section
2.1. Materials and methods
potassium hexacyanoferrate(III) prepared in 1.5
hydroxide.
M sodium
All the commercial reagents and solvents were used as received
or purified using standard procedures. The reaction was monitored
by TLC using 0.25 mm silica gel 60 F254 precoated plates, which
were visualized with UV light. Melting points were measured on
standard melting point apparatus from Sunder Industrial Product,
Mumbai, and are uncorrected. The FT-IR spectra were recorded on
Jasco 4100 using ATR accessory. 1H NMR spectra were recorded on
VARIAN Inc. (USA) 400/600-MHz instrument using TMS as an in-
ternal standard and DMSO-d6 as the solvent at room temperature.
Mass spectra were recorded on Finnigan mass spectrometer.
Simultaneous DSC-TGA measurements were performed out on SDT
Q 600 v8.2 Build 100 model of TA instruments Waters (India) Pvt.
Ltd. The UVevisible absorption and emission spectra were per-
formed on a PerkineElmer Lamda 25 and Varian Cary Eclipse at
room temperature. Quantum yields were obtained by using quinine
sulfate (0.54 in ethanol) as reference [26]. NaOH solution of the
2.3.3. Fluorescence of the CL reaction product
To 200
L of 10 mM potassium hexacyanoferrate(III) in 1.5 M NaOH and
L of 10 mM hydrogen peroxide were added and allowed to
mL of the 1 mM stock solution of chemiluminophores,
50
50
m
m
stand for 1 h at room temperature. The fluorescence emission
maxima, excitation and relative intensities of the resulting mix-
tures were recorded in the fluorescence spectrometer.
2.4. Synthesis and characterization
The synthetic schemes for the preparation of the o-hydrox-
ybenzimidazole derivatives are shown in Scheme 1. Dimethyl 4,5-
diaminophthalate (6) and dimethyl 4,3-diaminophthalate (6')
were prepared by the reported procedure [38] from 4-nitrophthalic
acid (1).
hydrogen peroxide and potassium hexacyanoferrate(III) as
oxidizing agent for chemiluminescence measurements.
a
2.4.1. General procedure for the preparation of benzimidazole
To a solution of diaminodimethylphthalate (6 or 6') (0.5g,
2.2 mmol), and o-hydroxybenzaldehyde (a or b or c) (2.2 mmol) in
ethanol (5 mL), Phosphorus trichloride (0.22 mL, 2.6 mmol) was
added drop wise over a period of 15 min.The temperature was
maintained at 40e45 ꢁC during complete addition. The mixture
was heated under reflux for 6 h. Solid product separated on pouring
reaction mass to crushed ice to afford dimethylphthalate HPBI
(7ae7b and 7'ae7'c). The product was recrystallized from ethanol.
2.2. Computational methods
The quantum chemical calculations were performed using
Gaussian 09 [27] software at the B3LYP/6-31G(d) level of theory.
The ground state (S0) geometry of the synthesized compounds has
been obtained by full optimization of the structural parameters
using DFT employing 6-31G(d) basis set [28]. The B3LYP method
combines Becke's three parameter exchange functional (B3) [29]
with the nonlocal correlation functional by Lee, Yang, and Parr
(LYP) [30]. The vibrational frequencies at the optimized structures
were computed using the same method to verify that the optimized
structures correspond to local minima on the energy surface [31].
The vertical excitation energy and oscillator strengths at ground
state equilibrium geometries were calculated using TD-DFT at the
same level of theory [32e34]. The low-lying first singlet excited
states (S1) of the compounds were relaxed using TD-DFT to obtain
the minimum energy geometry. The difference between the en-
ergies of the optimized geometries at the first singlet excited state
and the ground state was used in computing the emissions [35,36].
Frequency computations were also carried out on the optimized
geometry of the low-lying vibronically relaxed first excited state of
conformers to ascertain the validity of structure and no imaginary
frequencies were found. All the computations in solvents of
different polarities were carried out using the Polarizable Contin-
uum Model (PCM) [37].
2.4.1.1. Dimethyl 2-(4-(diethylamino)-2-hydroxyphenyl)-1H-benzo
[d]imidazole-5,6-dicarboxylate (7a). Yield: 77%; Melting point:
212e214 ꢁC; FT-IR: 3326 (eOH), 2953 (eNH), 1720 (eC]O), 1608,
1556 (C]C, C]N ring stretching), 1271, 1195 (CeO stretching)
cmꢀ1. 1H NMR (DMSO-d6):
d 13.09 (s, 1H, eOH), 12.36 (s, 1H, eNH),
7.86 (s, 2H, AreH), 7.81 (d, 1H, AreH, J ¼ 9.0 Hz), 6.40 (d, 1H, AreH,
J ¼ 7.5 Hz), 6.20 (s, 1H, AreH), 3.81 (s, 6H, eOCH3), 3.40 (q, 4H,
eNCH2), 1.13 (t, 6H, eCH3); 1H NMR (D2O exchange):
d 7.82 (s, 2H,
AreH), 7.74 (d, 1H, AreH, J ¼ 9.0 Hz), 6.35 (d, 1H, AreH, J ¼ 8.0 Hz),
6.15 (s, 1H, AreH), 3.78 (s, 6H, eOCH3), 3.33 (q, 4H, eNCH2), 1.08 (t,
6H, eCH3); 13C NMR (DMSO-d6, 125.8 MHz):
d 168.29, 168.18,
160.24, 156.68, 151.26, 145.24, 144.97, 128.43, 118.34, 117.23, 112.56,
104.36, 100.06, 97.93(s), 52.43(s), 44.14(s), 12.88(s); HRMS: Calcd
for C21H24N3O5 [MþH]þ 398.1716, found 398.2210; Mass m/z:
398.20 [MþH]þ.
2.4.1.2. Dimethyl 2-(2-hydroxyphenyl)-1H-benzo[d]imidazole-5,6-
dicarboxylate (7b). Yield: 72%; Melting point: 240e242 ꢁC; FT-
IR: 3321(eOH), 2945(eNH), 1738, 1686 (eC]O), 1639, 1611,
1557 (C]C, C]N ring stretching), 1242, 1213 (CeO stretching)
cmꢀ1; 1H NMR (DMSO-d6): (eOH and eNH proton exchange with
2.3. Examination of the chemiluminescent properties
2.3.1. Standard procedure
The chemiluminescence reaction was initiated by the simulta-
solvent)
d
8.24 (d, 1H, AreH, J ¼ 8.0 Hz), 8.07 (s, 2H, AreH), 7.51
neous automatic injections of 100
mL of hydrogen peroxide solution
(t, 1H, AreH, J ¼ 8.0 Hz), 7.19 (d, 1H, AreH, J ¼ 8.0 Hz), 7.08 (t, 1H,
and 100 mL of potassium hexacyanoferrate(III) solution dissolved
AreH, J ¼ 8.0 Hz), 3.85 (s, 6H, eOCH3); 1H NMR (D2O exchange):

M.S. Deshmukh, N. Sekar / Dyes and Pigments 113 (2015) 189e199
191
Scheme 1. Synthesis of dimethyl phthalate and phthalazine substituted HPBI derivatives (7ae7b, 7'ae7'c, 8ae8b and 8'ae8'c).
d
8.03 (s, 2H, AreH), 8.01 (d, 1H, AreH, J ¼ 8.0 Hz), 7.49 (t, 1H,
156.00, 150.65, 139.29, 137.19, 128.29, 123.61, 121.61, 112.13, 104.29,
99.84, 97.91(s), 52.73(s), 44.12(s), 12.89(s); HRMS: Calcd for
AreH, J ¼ 8.0 Hz), 7.10 (d, 1H, AreH, J ¼ 8.5 Hz), 7.06 (t, 1H, AreH,
J ¼ 8.0 Hz), 3.81 (s, 6H, eOCH3); 13C NMR (DMSO-d6, 125.8 MHz):
C
21H24N3O5, [MþH]þ 398.1716, found 398.2211; Mass m/z: 398.20
d
167.03(s), 157.81, 151.45, 134.73, 134.37, 129.04, 127.88,
[MþH]þ.
119.80(s), 117.32(s), 115.29(s), 109.77, 52.79(s); HRMS: Calcd for
C
17H15N2O5, [MþH]þ 327.0981, found 327.1432; Mass m/z: 327.10
[MþH]þ.
2.4.1.4. Dimethyl 2-(2-hydroxyphenyl)-1H-benzo[d]imidazole-4,5-
dicarboxylate (7'b). Yield: 70%; Melting point: 192e194 ꢁC; FT-IR:
3316 (eOH), 2967 (eNH), 1711 (eC]O), 1633, 1558 (C]C, C]N
2.4.1.3. Dimethyl 2-(4-(diethylamino)-2-hydroxyphenyl)-1H-benzo
[d]imidazole-4,5-dicarboxylate (7'a). Yield: 76%; Melting point:
208e210 ꢁC; FT-IR: 3365 (eOH), 2797 (eNH), 1731 (eC]O), 1608,
1558 (C]C, C]N ring stretching), 1316, 1246 (CeO stretching)
ring stretching), 1306, 1264 (CeO stretching) cmꢀ1
(DMSO-d6): (eOH and eNH proton exchange with solvent)
;
1H NMR
8.17
d
(d, 1H, AreH, J ¼ 7.5 Hz), 7.86 (dd, 2H, AreH, J ¼ 8.5 Hz), 7.47 (t, 1H,
AreH, J ¼ 8.5 Hz), 7.10 (d, 1H, AreH, J ¼ 7.5 Hz), 7.06 (t, 1H, AreH,
J ¼ 8.0 Hz), 3.93 (s, 3H, eOCH3), 3.85 (s, 3H, eOCH3); 1H NMR (D2O
cmꢀ1; 1H NMR (DMSO-d6):
d 13.17 (s, 1H, eOH), 12.41 (s, 1H, eNH),
7.79 (d, 1H, AreH, J ¼ 9.0 Hz), 7.75 (d, 1H, AreH, J ¼ 7.5 Hz), 7.64 (d,
1H, AreH, J ¼ 8.0 Hz), 6.39 (d, 1H, AreH, J ¼ 9.0 Hz), 6.17 (s, 1H,
AreH), 3.89 (s, 3H, eOCH3), 3.83 (s, 3H, eOCH3), 3.38 (q, 4H,
exchange):
d
8.00 (d, 1H, AreH, J ¼ 8.0 Hz), 7.81 (dd, 2H, AreH,
J ¼ 8.5 Hz), 7.44 (t, 1H, AreH, J ¼ 8.5 Hz), 7.06 (t, 1H, AreH,
eNCH2), 1.13 (t, 6H, eCH3); 1H NMR (D2O exchange):
d 7.74 (d, 1H,
J ¼ 6.0 Hz), 7.04 (d, 1H, AreH, J ¼ 8.5 Hz), 3.89 (s, 3H, eOCH3), 3.80
AreH, J ¼ 6.0 Hz), 7.70 (d, 1H, AreH, J ¼ 6.0 Hz), 7.64 (d, 1H, AreH,
J ¼ 7.5 Hz), 6.34 (d, 1H, AreH, J ¼ 9.0 Hz), 6.13 (s, 1H, AreH), 3.86 (s,
3H, eOCH3), 3.78 (s, 3H, eOCH3), 3.32 (q, 4H, eNCH2), 1.07 (t, 6H,
(s, 3H, eOCH3); 13C NMR (DMSO-d6,125.8 MHz):
d 166.53(s),158.21,
153.16, 136.81, 134.21, 129.09, 125.30, 124.22, 119.88(s), 117.60(s),
115.62, 111.13, 53.24(s); HRMS: Calcd for C17H15N2O5, [MþH]þ
327.0981, found 327.1435; Mass m/z: 327.10 [MþH]þ.
eCH3); 13C NMR (DMSO-d6, 125.8 MHz):
d 167.55, 167.57, 160.41,

192
M.S. Deshmukh, N. Sekar / Dyes and Pigments 113 (2015) 189e199
2.4.1.5. Dimethyl
2-(2,4-dihydroxyphenyl)-1H-benzo[d]imidazole-
for C19H20N5O3, [MþH]þ 366.1566, found 366.1534; Mass m/z:
4,5-dicarboxylate (7'c). Yield: 80%; Melting point: 242e244 ꢁC; FT-
IR: 3387, 3286 (eOH), 2955 (eNH), 1731, 1695 (eC]O), 1633, 1558
(C]C, C]N ring stretching), 1306, 1264 (CeO stretching) cmꢀ1; 1H
366.19 [MþH]þ.
2.4.2.4. 2-(2-Hydroxyphenyl)-7,8-dihydro-3H-imidazo[4,5-f]phtha-
lazine-6,9-dione (8'b). Yield: 78%; Melting point: 192e194 ꢁC; FT-
IR: 3383 (eOH), 2855 (eNH), 1659 (eC]O amide), 1608, 1589
(C]C, C]N ring stretching), 1348, 1294 (CeO stretching) cmꢀ1; 1H
NMR (DMSO-d6): (Two eOH proton is exchange in solvent)
d 10.69
(br s, 1H, eNH), 8.06 (d, 1H, AreH, J ¼ 8.5 Hz), 7.89 (d, 1H, AreH,
J ¼ 8.5 Hz), 7.81 (d, 1H, AreH, J ¼ 8.5 Hz), 6.61 (s, 1H, AreH), 6.53
(dd, 1H, AreH, J ¼ 2.5 Hz and 6.5 Hz), 3.94 (s, 3H, eOCH3), 3.81 (s,
NMR (DMSO-d6): d 12.62 (br s,1H, eOH),11.95 (br s,1H, eNH),11.71
3H, eOCH3); 1H NMR (D2O exchange):
d
7.85 (d, 1H, AreH,
(br s, 2H, eNH), 8.46 (br s, 1H, AreH), 8.17 (br s, 1H, AreH), 7.87 (br
s, 1H, AreH), 7.40 (t, 1H, AreH, J ¼ 8.5 Hz), 7.08 (d, 1H, AreH,
J ¼ 8.0 Hz), 7.03 (t, 1H, AreH, J ¼ 8.5 Hz); 1H NMR (D2O exchange):
J ¼ 8.5 Hz), 7.82 (d, 1H, AreH, J ¼ 8.5 Hz), 7.76 (d, 1H, AreH,
J ¼ 8.5 Hz), 6.51 (dd, 1H, AreH, J ¼ 2.5 Hz and 6.0 Hz), 6.47 (d, 1H,
AreH, J ¼ 2.5 Hz), 3.89 (s, 3H, eOCH3), 3.79 (s, 3H, eOCH3); 13C NMR
d
8.27 (br s, 1H, AreH), 8.15 (d, 1H, AreH, J ¼ 8.0 Hz), 7.93 (br s, 1H,
(DMSO-d6, 125.8 MHz):
d
166.64, 165.57, 164.31, 160.45, 152.15,
AreH), 7.41 (t, 1H, AreH, J ¼ 7.5 Hz), 7.08 (d, 1H, AreH, J ¼ 8.0 Hz),
7.03 (t, 1H, AreH, J ¼ 7.5 Hz); HRMS: Calcd for C15H11N4O3, [MþH]þ
295.0831, found 295.0808; Mass m/z: 295.30 [MþH]þ.
135.33, 131.41, 125.47, 119.50, 115.98, 109.27, 103.34(s), 101.04(s),
53.43, 53.18; HRMS: Calcd for C17H15N2O6, [MþH]þ 343.0930, found
343.1392; Mass m/z: 343.67 [MþH]þ.
2.4.2.5. 2-(2,4-Dihydroxyphenyl)-7,8-dihydro-3H-imidazo[4,5-f]
phthalazine-6,9-dione (8'c). Yield: 86%; Melting point: 180e182 ꢁC;
FT-IR: 3332 (eOH), 2980 (eNH), 1687 (eC]O amide), 1618, 1588
(C]C, C]N ring stretching), 1374, 1252 (CeO stretching) cmꢀ1. 1H
NMR (DMSO-d6): (eOH and eNH proton is exchange in solvent)
2.4.2. General procedure for the preparation of phthalazine
The solution of dimethylphthalate HPBI (7a,7b and 7'ae7'c)
(1 mmol) and hydrazine hydrate (1.5 mL) in methanol (5 mL) was
heated under reflux in presence of triethylamine (1.5 mL) for 2 h.
The product precipitated during the reaction. The crude product
was recrystallized from N,N'-dimethylacetamide to give the com-
pounds 8a,8b and 8'ae8'c respectively. The low solubility of this
compounds made the 13C NMR characterization of these substrates
not possible. Only the IR, 1H NMR, mass and HRMS are reported.
d
8.11 (d,1H, AreH, J ¼ 8.5 Hz), 7.96 (d,1H, AreH, J ¼ 8.5 Hz), 7.81 (d,
1H, AreH, J ¼ 8.5 Hz), 6.50 (s, 1H, AreH), 6.35 (dd, 1H, AreH,
J ¼ 1.5 Hz and J ¼ 7.0 Hz); 1H NMR (D2O exchange):
d 8.09 (d, 1H,
AreH, J ¼ 8.5 Hz), 7.97 (d, 1H, AreH, J ¼ 8.5 Hz), 7.87 (d, 1H, AreH,
J ¼ 8.5 Hz), 6.56 (s, 1H, AreH), 6.37 (d, 1H, AreH, J ¼ 7.5 Hz); HRMS:
Calcd for C15H11N4O4, [MþH]þ 311.0780, found 311.1016; Mass m/z:
310.14 [M]þ.
2.4.2.1. 2-(4-(Diethylamino)-2-hydroxyphenyl)-6,7-dihydro-1H-imi-
dazo[4,5-g]phthalazine-5,8-dione (8a). Yield: 88%; Melting point:
186e188 ꢁC; FT-IR: 3233 (eOH), 2960, 2869 (eNH), 1644 (eC]O
amide), 1609, 1531 (C]C, C]N ring stretching), 1315, 1252 (CeO
3. Results and discussion:
3.1. Synthetic strategy
stretching) cmꢀ1; 1H NMR (DMSO-d6):
d 13.17 (br s, 1H, eOH), 12.53
(br s, 1H, eNH), 11.30 (br s, 2H, eNH), 8.11 (br s, 2H, AreH), 7.85 (d,
1H, AreH, J ¼ 9.0 Hz), 6.43 (dd, 1H, AreH, J ¼ 2.0 Hz and J ¼ 7.0 Hz),
6.21 (d, 1H, AreH, J ¼ 2.5 Hz), 3.41 (q, 4H, eNCH2), 1.14 (t, 6H,
In the first step, dimethyl-4-nitrophthalate 1 was synthesized by
esterification of 4-nitrophthalic acid. Compound 1 on reduction
using H2/PdeC (10%) gave the compound 2, which on further
acetylation using acetic anhydride in toluene yielded the inter-
mediate 3. Nitration of the intermediate of 3 with fuming HNO3 and
H2SO4 gave a mixture of 4 and 4'. The mixture of compounds 4 and
4' was separated by fractional crystallization in methanol and CCl4,
which on deacetylation in concentrated H2SO4 gave the in-
termediates 5 and 5'. Nitro groups of compound 5 and 5' were
reduced by using H2/PdeC (10%) gave the compound (6 and 6'),
which on further condensation with o-hydroxybenzaldehyde (a-c)
in absolute ethanol containing a catalytic amount of PCl3 gave
7ae7b and 7'ae7'c. Compounds 7a, 7b and 7a'e7c' was further
heated under reflux in hydrazine hydrate, triethyl amine in meth-
eCH3); 1H NMR (D2O exchange):
d 8.14 (br s, 2H, AreH), 7.81 (d, 1H,
AreH, J ¼ 9.0 Hz), 6.37 (dd, 1H, AreH, J ¼ 2.0 Hz and J ¼ 7.0 Hz), 6.18
(d, 1H, AreH, J ¼ 2.0 Hz), 3.37 (q, 4H, eNCH2), 1.11 (t, 6H, eCH3);
HRMS: Calcd for C19H20N5O3, [MþH]þ 366.1566, found 366.2037;
Mass m/z: 366.22 [MþH]þ.
2.4.2.2. 2-(2-hydroxyphenyl)-6,7-dihydro-1H-imidazo[4,5-g]phtha-
lazine-5,8-dione (8b). Yield: 78%; Melting point: 250e252 ꢁC; FT-
IR: 3213 (eOH), 2965, 2866 (eNH), 1636 (eC]O amide), 1620,
1540 (C]C, C]N ring stretching), 1350, 1268 (CeO stretching)
cmꢀ1 1H NMR (DMSO-d6):
; d 13.56 (s, 1H, ]OH), 12.52 (s, 1H, ]
NH), 11.38 (br s, 2H, ]NH), 8.29 (s, 2H, AreH), 8.16 (dd, 1H, AreH,
J ¼ 1.5 Hz and J ¼ 6.5 Hz), 7.46 (t, 1H, AreH, J ¼ 8.5 Hz), 7.08 (m, 2H,
anol to yield desired D-p-A fluorescent compounds 8ae8b and
8'ae8'c respectively.
AreH); 1H NMR (D2O exchange):
d 8.30 (s, 1H, AreH), 8.12 (s, 1H,
The structures of the compounds were confirmed by FTIR, 1H
NMR, and mass spectral analysis. The protons corresponding to
eOH and eNH are not observed in the 1H NMR spectrum of 7b
which is due to the exchange of protons with the solvent, but in the
AreH), 8.09 (dd, 1H, AreH, J ¼ 1.5 Hz and J ¼ 6.5 Hz), 7.46 (t, 1H,
AreH, J ¼ 8.5 Hz), 7.05 (m, 2H, AreH); HRMS: Calcd for C15H11N4O3,
[MþH]þ 295.0831, found 295.0805; Mass m/z: 295.27 [MþH]þ.
next step for 8b both signals (eOH, eNH) were observed (
d 13.56
2.4.2.3. 2-(4-(Diethylamino)-2-hydroxyphenyl)-7,8-dihydro-3H-imi-
dazo[4,5-f]phthalazine-6,9-dione (8'a). Yield: 90%; Melting point:
188e190 ꢁC; FT-IR: 3312 (eOH), 2977 (eNH), 1706 (eC]O amide),
1607,1581 (C]C, C]N ring stretching),1347,1261 (CeO stretching)
and 12.52) and similar observation is observed in the case of
d
compounds 7'b and 8'b. Compounds 7'c and 8'c both the eOH and
eNH signal was not observed but all the compounds mass spectra is
in good agreement with molecular weight.
cmꢀ1 1H NMR (DMSO-d6):
; d 11.79 [br s, 3H (2H, eNH and 1H,
eOH)], 8.29 (s, 1H, eNH), 8.19 (br s,1H, AreH), 8.01 (br s,1H, AreH),
7.81 (br s, 1H, AreH), 6.37 (dd, 1H, AreH, J ¼ 2.5 Hz and J ¼ 6.5 Hz),
6.23 (s, 1H, AreH), 3.34 (q, 4H, eNCH2), 1.14 (t, 6H, eCH3); 1H NMR
3.2. ESIPT phenomenon
The synthesized substituted HPBIs (linear and angular) 7ae7b,
7'ae7'c and 8ae8c, 8'ae8'c contains an acidic eOH group at the 2-
position with respective to the benzimidazole ring. The position of
the acidic proton of the eOH group and the benzimidazole ]Ne
(D2O exchange):
d 8.15 (br s, 1H, AreH), 7.02 (d, 1H, AreH,
J ¼ 7.0 Hz), 7.85 (br s, 1H, AreH), 6.38 (d, 1H, AreH, J ¼ 8.0 Hz), 6.22
(s, 1H, AreH), 3.35 (q, 4H, eNCH2), 1.11 (t, 6H, eCH3); HRMS: Calcd

M.S. Deshmukh, N. Sekar / Dyes and Pigments 113 (2015) 189e199
193
atom is such that there is an existence of the intramolecular
hydrogen bonding in the ground state. On excitation the basicity of
]Ne moiety of the benzimidazole ring increases and the acidity of
the hydroxy proton increases. The excitation of the ground state
enol (E-Enol) to the excited enol (E-Enol*) is due to the absorption
of photon. This E-Enol* form either undergoes ESIPT to form the
excited state keto form (K-Keto*) or emits a radiation and returns to
the ground state enol form (E-Enol). The excited state keto form (K-
Keto*) emits radiation and comes to the ground state keto form (K-
Keto) (Fig. 1). But in the case of the barrierless ESIPT, typically the
proton of the hydroxyl or amino group, in the excited state migrates
to a basic unit is very fast because the distance between acidic
proton and basic unit is less than 2 Å. In this way the same molecule
either shows broad single emission or dual emission with a large
Stokes shift in solvents of different polarity.
exist. The absence of long wavelength absorption in the absorptive
species is also indicative of the non-existence of the keto form.
3.4. Photo-physical properties
The photophysical behaviour of the compounds has been
examined by measuring absorption and fluorescence spectra in
different solvents (7ae7b, 7'ae7'c in acetone, dichloromethane,
methanol, acetonitrile, N,N'-dimethylformamide and compounds
8ae8b, 8'ae8'c in N,N'-dimethylformamide, dimethyl sulphoxide,
acetic acid) (Table 1, Table S8eS11).
All of the compounds show a well-resolved absorption profile in
the range of 334e410 nm with molar extinction coefficient values
(ε) in agreement with pep* transitions (Fig. 3, Fig. S1eS10). All the
compounds show single absorption in the solvents studied. The
theoretical vertical excitation values of the compounds are in good
agreement with the experimental absorption; the difference be-
tween the theoretical and experimental values is less than 45 nm in
all the solvents studied.
3.3. Rotamers and conformers and their ground state energy
The substituted linear and angular HPBIs contain a fused
imidazole ring (at 3,4 or 4,5-position of dimethylphthalate) having
a free phenyl ring with o-hydroxyl group which is participating in
the proton transfer at the excited state (ESIPT) [39]. In the ground
state (S0), a linear and angular HPBI derivative can exist in equi-
librium with several different conformers arising from tautom-
erism and rotamerism (Fig. 2). Generally planar syn form (denoted
Esyn) features an intramolecular hydrogen bond between the acidic
hydroxyl (eOH) group and the basic nitrogen (]N) atom [40]. The
Esyn conformer can undergo proton transfer (PT) to form its keto
tautomer (denoted Ksyn). These two conformers undergo rotame-
rization to form their non planar anti forms (denoted Eanti and
Kanti, respectively). The Eanti form has its eOH group rotated to the
opposite side, relative to that of the planar form. In a protic solvent,
the Eanti conformer can form hydrogen bonds with solvent mole-
cules, but the probability of proton transfer happens to be very
remote. DFT computations indicate that in all the solvents, the Esyn
form is more stable than the Ksyn and Eanti enol form, which is
preferred in the ground state (Tables S1eS7). In the ground state
keto forms are prohibitively higher in energy such that they do not
The red shifted absorption maxima indicates that the o-hydroxy
and N,N-diethylamino moiety present in the compounds 7a, 7'a, 8a,
8'a (364 nm, 385 nm, 385 nm, 390 nm) has a stronger electron
donating effect than in 7b, 7'b, 8b, 8'b (343 nm, 349 nm, 342 nm,
356 nm) devoid of the N,N-diethylamino group respectively. These
results also showed that the compounds 7a, 7'a, 8a, 8'a exhibited
red shifted emission as compared to the compounds 7b, 7'b, 8b,
8'b. The introduction of electron donor group in the C2', C4' posi-
tions induced intramolecular charge transfer and mesomeric dipole
moment. When compared with unsubstituted analogues [25] of the
molecules 7a, 7'a, 8a, 8'a and 7b, 7'b, 8b, 8'b latter have shown a
red shifted absorption. The electron withdrawing effect of diester
(7ae7b, 7'ae7'c) and cyclic amide (8ae8b, 8'ae8'c) groups have
induced efficient donor to acceptor charge transfer. The molecules
7a, 7'a, 8a, 8'a and 7b, 7'b, 8b, 8'b absorbs at longer wavelength in
DMF as compared to their unsubstituted analogue which absorb at
229 nm and 246 nm respectively. It is also observed that molecules
7a, 7'a, 8a, 8'a absorbs at longer wavelength than the nitro
substituted (333 nm) analogue. Nitro group being strong electron
Fig. 1. Excited state intramolecular proton transfer (ESIPT) pathway.

194
M.S. Deshmukh, N. Sekar / Dyes and Pigments 113 (2015) 189e199
Fig. 2. Possible conformers and tautomers of linear and angular Benzimidazole.
withdrawing group than diester and amide was expected to show a
red shift in absorption [25]; however the diester and cyclic amide
group affords rigidity and planarity to the molecules and proved to
be an efficient electron withdrawer.
density is redistributed equally throughout the molecules, which
all are in one plane (Fig. 4, Fig. S11eS19).
Compound 8'c, taken as an example, in this case of polar solvent
like DMF, HOMO/LUMO (98%) transition is responsible for vertical
excitation located at 361 nm with oscillator strength (f) 0.731
(Fig. 4), which corresponds to the experimentally observed ab-
sorption at 356 nm. A similar trend is observed in all compounds in
all other solvents. The solvatochromism study reveals that the ab-
sorption wavelength of the compounds 7a, 7b, 7'a, 7'c, 8b, 8'a are
sensitive to solvent polarity. However, the absorption intensity is
insensitive to the solvent polarity.
The intense absorption of the phthalate and phthalazine (HPBI)
can be assigned to HOMOeLUMO transition in all solvents. The
frontier molecular orbital (FMO) 105, 85, 89, 96, 76, 80 is HOMO and
FMO 106, 86, 90, 97, 77, 81 is LUMO in all cases of compounds
7ae7'a, 7be7'b, 7'c, 8ae8'a, 8be8'b, 8'c respectively. It also
observed that in phthalate (7ae7b, 7'ae7'c) HPBI, eOH, eN(C2H5)2
acts as an electron donor; HOMOeLUMO transition is accompanied
by the transfer of electrons toward the dimethyl phthalate core. It is
observed that in the HOMO the electron density is distributed on
the o-hydroxyphenyl ring and benzimidazole core. While in the
phthalazine (8ae8b, 8'ae8'c) HPBI, electron density is concen-
trated on the phthalazine nitrogen atoms in the HOMO. On exci-
tation of both phthalate and phthalazine to the LUMO, the electron
3.5. Emission and solvatofluorism study
The solvatofluorism study reveals that compound 8'c shows
dual emission in DMF and DMSO, while compounds 7a,7b, 7'a,
7'b, 7'c, 8a, 8b, 8'a, 8'b showed broad single emission in all
Table 1
Observed UVeVisible absorption-emission and computed vertical excitation of the compounds 7ae7b, 7'ae7'c, 8ae8b and 8'ae8'c in DMF.
Comp.
Experimental
Stokes shift
Computed (TD-DFT)
%De
F
Verticalc excitation (nm)
fd
Orbital contribution
a
lmax nm
lmaxb nm Intensity (au)
7a
364 (43,670)
343 (36,186)
385 (29,378)
349 (24,124)
346 (29,070)
385 (66,795)
342 (21,756)
390 (53,290)
356 (46,158)
356 (30,380)
507 (9.03)
143
123
130
138
99
104
97
130
116
49
350
327
379
355
362
360
335
387
355
361
0.670
0.652
0.585
0.503
0.644
0.690
0.596
0.682
0.551
0.731
H / L (95%)
H / L (97%)
H / L (98%)
H / L (98%)
H / L (98%)
H / L (97%)
H / L (97%)
H / L (98%)
H / L (98%)
H / L (98%)
3.9
4.7
1.7
1.6
4.5
6.5
2.1
0.8
0.3
1.4
0.08
0.07
0.06
0.08
0.05
0.12
0.15
0.11
0.12
0.13
7b
7'a
7'b
7'c
8a
8b
8'a
8'b
8'c
462 (26.42)
515 (10.34)
487 (62.45)
445 (13.19)
489 (55.32)
439 (28.21)
520 (27.80)
469 (100.8)
405 (24.34)
515 (20.18)
164
F
: Quantum yield in various solvents.
a
Experimental absorption wavelength.
Experimental emission wavelength.
Computed vertical excitation.
Oscillator strength.
b
c
d
e
D% Deviation between experimental absorption and vertical excitation computed by DFT.

M.S. Deshmukh, N. Sekar / Dyes and Pigments 113 (2015) 189e199
195
Table 2
Observed and the computed emission of the compounds 7ae7b, 7'ae7'c, 8ae8b and
8'ae8'c in DMF.a
Comp. Experimental
emissionb (nm)
TD-DFT
%Dd
%De
emissionc (nm)
Short
Long
Short
Long
wavelength wavelength wavelength wavelength
7a
507
462
515
487
445
489
439
520
469
405
e
433
365
521
395
403
439
362
522
392
399
458
449
531
521
511
456
461
527
520
503
14.6
21.0
1.2
18.9
9.4 14.8
10.2
17.5
0.4
16.4 10.9
1.5 2.3
9.7
2.8
3.1
7.0
7b
7'a
7'b
7'c
8a
8b
8'a
8'b
8'c
e
e
e
e
e
6.7
5.0
1.3
e
e
e
515
a
Analysis were carried out at room temperature (25 ꢁC).
Observed experiment emission.
Computed using TD-DFT method with the B3LYP/6-31G(d).
b
c
Fig. 3. Absorption spectra of compounds 7ae7b, 7'ae7'c, 8ae8b and 8'ae8'c in DMF.
d
% Deviation between experimental short emission and computed enol emission
by TD-DFT.
e
% Deviation between experimental long emission and computed keto emission
by TD-DFT.
solvents (Table 2, Table S12eS15, Fig. 5, Fig. S1eS10). It is well-
known that the long wavelength emission is attributed to the
excited state keto tautomer originating from the excited state
enol tautomer in the ESIPT molecules [19]. The driving force
behind the proton transfer is the associated intrinsic extra sta-
bilization at the exited state of the keto tautomer. Also, the ge-
ometry consideration favours the keto form in the excited state.
In compounds 7a,7b, 7'a, 7'b, 7'c, 8a, 8b, 8'a, 8'b a broad single
emission may be due to the barrierless proton transfer because
the computed enol and keto emission values are in the range of
broad emission obtained experimentally and the distance be-
tween eOH proton and benzimidazole nitrogen is less than 2 Å
(Fig. S11eS19).
3.6. Relative quantum yield of compounds
The fluorescence quantum yields of the synthesized compounds
were determined in different solvents are illustrated in Table 1,
Table S8eS11. The fluorescence quantum yields of the compounds
are largely dependent on the nature of the substituent and the
solvent polarity. Here, decrease in the solvent polarity strongly
enhances the fluorescence quantum yields. The compounds 7ae7b
and 7'ae7'c showed higher quantum efficiencies in non-polar
solvents whereas in polar solvents the quantum yield values were
Fig. 4. Representation of the UVevisible absorption and emission of the compound 8'c in DMF. The vertical excitation related calculations were based on optimized ground state
geometry and the emission based calculations were based on optimized excited state geometry at B3LYP/6-31G(d). The HOMO and LUMO energy levels in the excited state are
different from those in the ground state. The FMO involved in the vertical excitation (UVevisible absorption: one blue line and emission: two red lines). (For interpretation of the
references to colour in this figure legend, the reader is referred to the web version of this article).

196
M.S. Deshmukh, N. Sekar / Dyes and Pigments 113 (2015) 189e199
3.7. Chemiluminescence study
The distinct chemiluminescence properties of iso-
luminoleluminol type of compounds 8ae8b and 8'ae8'c were
examined in order to evaluate their use as highly chemilumines-
cent probes. These compounds 8ae8b and 8'ae8'c produce
chemiluminescence by reaction with hydrogen peroxide in the
presence of potassium hexacyanoferrate(III) in alkaline medium
(Fig. 6). The oxidizing reagent concentrations have a major influ-
ence on the chemiluminescence intensity (Fig. 7); 2.5e5 mM
hydrogen peroxide, 2.5e5 mM potassium hexacyanoferrate(III) and
0.5e2.0 M sodium hydroxide give the highest chemiluminescence
intensity. After the addition of the oxidising agents, the chem-
iluminescence intensity quickly reached a maximum in a few sec-
onds and then almost disappeared within one minute.
Under these optimum reaction conditions, the chem-
iluminescence intensities of the compounds were determined and
summarised in Table 3. In the compounds 8ae8b and 8'ae8'c
chemiluminescence generation were found approximately 3.5e7.3
times higher than that of isoluminol, and the intensities corre-
sponded to 0.8e1.8 times of the value obtained with luminol itself.
The time dependence of the chemiluminescence reaction of the
compounds 8ae8b and 8'ae8'c and luminoleisoluminol is illus-
trated in Fig. 8. The chemiluminescence generation is initiated by
the addition of the alkaline solution of hydrogen peroxide and
potassium hexacyanoferrate(III) to the stock solution. After the
addition of the oxidizing reagents the chemiluminescence intensity
reached their maxima within 1.5 s, and then decreased rapidly. The
chemically excited 3-aminophthalate and 4-aminophthalate ion
produced during the oxidizing reaction has been shown to be the
light emitter in the chemiluminescence of the luminol (8'ae8'c)
and isoluminol (8ae8b). Therefore, in the newly synthesized
luminoleisoluminol type chemiluminophores, the expected light
emitting species could be the corresponding dicarboxylate ions
(Fig. 6). The fluorescent properties like excitation and emission
maxima of the fluorescence and its relative intensities of the spe-
cies, on the completion of the chemiluminescence reaction are
measured, and compared with the conventional lumi-
noleisoluminol analogues (Table 3). The fluorescence intensities of
the compounds 8ae8b and 8'ae8'c dicarboxylate ions were
4.11e10.80 and 4.6e12.09 times larger than that of 4-
aminophthalate anion of the isoluminol and 3-aminophthalate
anion of the luminol respectively. These results proposed that the
efficiency of the chemiluminescence intensity of the iso-
luminoleluminol type chemiluminophores is mostly dependent on
the fluorescence intensities of the light-emitting species produced
during the chemiluminescence reaction. Otherwise, the
Fig. 5. Emission spectra of compounds 7ae7b, 7'ae7'c, 8ae8b and 8'ae8'c in DMF.
Fig. 6. Chemiluminescence reactions of isoluminol-related compound (8a).
invariably found to be low. The quantum yields of the compounds
8ae8b and 8'ae8'c are recorded only in polar solvent due to their
solubility. The compounds 7ae7b and 7'ae7'c showed the highest
quantum yield in DCM. The decreasing order is 7'b (0.15) > 7a
(0.14) ¼ 7b (0.14) > 7'a (0.07) ¼ 7'c (0.07). The quantum yield values
were the lowest in acetonitrile in the increasing order: 7'c
(0.04) < 7'a (0.05) < 7b (0.06) ¼ 7'b (0.06) < 7a (0.07). While for the
compounds 8ae8b and 8'ae8'c the quantum yield is higher in DMF
the decreasing order is 8b (0.15) > 8'c (0.13) > 8a (0.12) ¼ 8'b
(0.12) > 8'a (0.11) and lowest in acetic acid increasing order: 8a
(0.04) ¼ 8'b (0.04) ¼ 8'c (0.04) < 8b (0.07) ¼ 8'a (0.07). The com-
pounds 7ae7b, 7'ae7'c in acetonitrile, DMF and 8ae8b, 8'ae8'c in
acetic acid showed very less emission intensity. The positive sol-
vatokinetic behaviour suggests that a highly polar excited-state
population charge transfer state and a non-radiative decay was
prominent in these compounds [35].
100
90
80
100
90
80
100
90
80
70
60
50
40
30
20
10
0
70
60
50
40
30
20
10
0
70
60
50
40
30
20
10
0
0
5
10
15
20
25
30
0
5
10
15
20
25
30
0
0.5
1
1.5
2
2.5
3
H2O2 (mM)
K3Fe(CN)6 (mM)
NaOH (M)
Fig. 7. Effects of concentrations of hydrogen peroxide, potassium hexacyanoferrate(III) and sodium hydroxide on integrated chemiluminescence intensities of compounds Luminol,
Isoluminol, 8ae8b and 8'ae8'c (a run time of 60 s).

M.S. Deshmukh, N. Sekar / Dyes and Pigments 113 (2015) 189e199
197
Table 3
summarized in (Table S16eS23). In the case of compound 7a in
methanol, the hydrogen bonding distance between the N28eH30
decreases by 0.037 Å from the ground state enol (1.728 Å) to the
excited state enol (1.691 Å), The bond distances between O29eH30
Fluorescence excitation and emission maxima and relative intensities (R.F.I.) and
Relative chemiluminescence intensities (RCI) of the dicarboxylate anions corre-
sponded to the chemiluminophores.
Chemiluminophore
Excitation (nm)
Emission (nm)
R.F.I.a
R.C.I.b
8a
8b
8'a
8'b
8'c
Isoluminol
Luminol
389
394
385
363
375
274
316
503
486
511
501
498
470
425
1136
432
1209
467
183
89
176
80
576
139
24
105
100a
100b
a
Integrated chemiluminescence intensity of luminol in DMF was taken as 100.
Integrated fluorescence intensity of luminol in DMF was taken as 100.
b
chemiluminescence spectra of isoluminol and luminol type com-
pounds and the fluorescence emission spectra of the corresponding
phthalate ion are almost identical [41,42]. Thus, compounds 8ae8b
and 8'a, 8'c were expected to generate chemiluminescence at
longer wavelength as compared to the isoluminol and luminol
respectively, which means that the use of these compounds as the
chemiluminescence labelling reagents might be profitable.
3.8. Structural properties of compounds 7ae7b, 7'ae7'c, 8ae8b
and 8'ae8'c
In the ESIPT process the proton is transferred in the excited
state, after excitation of the enol form to first excited state singlet
undergoes redistribution of electron densities leading to a different
geometry favouring a proton transfer and ultimately forms the
excited state keto, which becomes an emissive species fluorescing a
longer wavelength. In the excited state, the hydrogen attached to
oxygen approaches to the basic nitrogen of the imidazole unit via a
six-membered hydrogen bonding framework for the transfer of
proton. Planarity of the molecule is also a major factor for the
proton transfer in the excited state enol. This ESIPT phenomenon is
explained by using the TD-DFT computation. Changes in bond
lengths, bond angles, Mulliken charges of the ground and excited
state of the optimized geometries of the enol as well as keto forms
are explained with the help of TD-DFT.
As a representative example of the dicarboxylate HPBI, the
structural view of compound 7a in the ground and the excited state
of the enol as well as the keto form are presented in Fig. 9 and the
structures of compounds 7b, 7'ae7'c, 8ae8b and 8'ae8'c are
Fig. 8. Time dependence of the chemiluminescence reaction of the Isoluminol, luminol
and its analogue (8ae8b) and (8'ae8'c).
Fig. 9. Structural view of compound 7a in ground and excited state of enol as well as
keto form.

198
M.S. Deshmukh, N. Sekar / Dyes and Pigments 113 (2015) 189e199
Table 4
Dipole moment (m in Debye) of compounds 7ae7b, 7'ae7'c, 8ae8b and 8'ae8'c.
Compounds
aEnol mg
bEnol me
bEnol me
aEnol mg
aKeto mg
bKeto me
bKeto me
aKeto mg
cExperimental
%D
Dipole moment
7a
4.16
4.35
5.76
4.90
6.32
4.58
3.80
3.22
3.40
4.86
4.02
4.79
6.54
4.57
6.89
4.95
3.56
5.67
3.03
4.58
0.97
1.10
1.14
0.93
1.09
1.08
0.94
1.76
0.89
0.94
6.15
3.76
5.40
4.95
4.43
4.88
4.47
3.71
4.64
3.34
8.97
3.92
4.87
4.56
4.35
5.02
4.12
3.87
4.43
3.02
1.46
1.04
0.90
0.92
0.98
1.03
0.92
1.04
0.95
0.90
1.83
0.41
0.02
3.17
0.33
1.5
47
63
98
71
70
28
73
69
22
15f
9g
7b
7'a
7'b
7'c
8a
8b
8'a
8'b
8'c
0.25
5.6
0.7
0.8d
1.03e
a
Dipole moment of compound in ground state.
Dipole moment of compound in excited state.
Experimental dipole moment obtained from solvatochromism data using Bakhshiev and KawskieChammaeViallet correlations.
Dipole moment at short wavelength emission obtained from solvatochromism data using Bakhshiev and KawskieChammaeViallet correlations.
Dipole moment at long wavelength emission obtained from solvatochromism data using Bakhshiev and KawskieChammaeViallet correlations.
(%D) % Deviation between experimental and computed dipole moment for short wavelength emission.
b
c
d
e
f
g
(%D) % Deviation between experimental and computed dipole moment for long wavelength emission.
increase by 0.006 Å from the ground state enol (0.998 Å) to the
excited enol (1.004 Å). While the charges on N28, O29 and H30 were
increased by 0.005jej, 0.001jej and 0.002jej from the ground state
enol to the excited state enol respectively. The charges on N28 in-
crease more than O29 in the excited state enol which means that the
acidic proton (H30) approaches near to the basic nitrogen (N28) via
hydrogen bonding. The bond distance between O29eC21 and
which depend both on the relative permittivity (ε) and refractive
index ( ) of the solvent. The polarity function of solvent has been
taken from the literature [49].
h
The dipole moment ratio of the excited state to the ground state
of the compounds 7ae7b, 7'ae7'c, 8ae8b and 8'ae8'c were
calculated by using the solvatochromic data and polarity function
of the solvents. The experimental dipole moment values were
compared with the computed dipole moments in the vacuum
phase by DFT and TD-DFT are summarized in Table 4. The dipole
moment obtained from long wavelength emission data compared
with ratio of dipole moment of excited state keto and ground state
keto form in vacuum phase. These results clearly signify that a large
difference was observed between the dipole moment obtained
from solvatochromism data and the computed dipole moment.
N28eC7 increases by 0.006 Å and 0.030 Å from the ground state
enol (1.349 Å and 1.333 Å) to the excited state enol (1.356 Å and
1.364 Å) which is decreased by 0.086 Å and 0.020 Å in the case of
excited keto form (1.269 Å and 1.343 Å) respectively. In addition to
this, the bond angle H30eO29eC21 and H30eN28eC7 is decreases by
0.59ꢁ and 1.70ꢁ from the ground state enol (108.01ꢁ and 98.47ꢁ) to
the excited state enol (107.42ꢁ and 96.77ꢁ) respectively. In this
pathway, proton (H30) approaches the nitrogen (N28) of imidazole
unit in the excited state enol form. The H30 transfer to N28 leads to
formation excited state keto form with N28eH30 and O29eH30 bond
distance are 1.023 Å and 1.897 Å, which further returns to the
ground state keto conformer with bonding distance are 1.044 Å and
1.703 Å respectively. The above observations indicate that the
proton (H30) approaches to oxygen (O29), the hydrogen bonding
distance in excited state keto form decreases by 0.194 Å from 1.897
to 1.703 Å and it is immediately converted to the ground state keto
form then to the ground state enol form. Similar structural
behaviour is observed for the compounds 7b, 7'ae7'c, 8ae8b and
8'ae8'c.
4. Conclusion
In this paper we have reported new chemiluminescent ana-
logues containing an inbuilt ESIPT residue and their photophysical
and chemiluminescence properties by reaction with hydrogen
peroxide in the presence of potassium hexacyanoferrate (III) in an
alkaline medium. Compounds 8a, 8b, 8'a, 8'b and 8'c can be used
promising candidate for chemiluminescent probes. Photophysical
properties of the compounds were supported by DFT and it was
observed that the computational results are in good agreement
with the theoretical observations. All the compounds except 8'c
show a single broad emission peak in polar as well as in non-polar
solvents. The barrierless proton transfer is proved by DFT;
computed enol and keto emission values are in the range of broad
emission obtained experimentally and the distance between acidic
proton and basic nitrogen is less than 2 Å.
3.9. Effect of solvent polarity on the ground and excited state dipole
moments
The fluorescence properties and dipole moment are affected by
the solvent polarity because of their interaction with solute or
hydrogen bonding with heteroatom present in the solute. Geom-
etry of the compound at the ground and the excited state decide
their electronic behaviour. The effect of the solvent polarity on the
photophysical properties clearly indicates that in the excited state
the dipolar characteristics of compounds change, which means that
solvatochromic data gives an efficient tool to understand the
change in dipole moment in the first excited state.
Acknowledgements
The authors are greatly thankful to TIFR, SAIF-I.I.T. Mumbai for
recording the 1H NMR, 13C NMR and Mass spectra. One of the au-
thors Mininath S. Deshmukh is grateful to CSIR, New Delhi for the
award of a research fellowship.
Here, we report the ground and excited state dipole moments of
the compounds 7ae7b, 7'ae7'c, 8ae8b and 8'ae8'c by using
Bakhshiev [43] and KawskieChammaeViallet correlations [44,45].
This method is based on a linear relation between the absorption,
emission maxima and solvent polarity functions (ENT ) [46e48]
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.dyepig.2014.08.009.

M.S. Deshmukh, N. Sekar / Dyes and Pigments 113 (2015) 189e199
199
yl)phenol: experimental and DFT based approach to photophysical properties.
Tetrahedron 2013;69:1767e77.
References
[24] Patil VS, Padalkar VS, Phatangare KR, Gupta VD, Umape PG, Sekar N. Synthesis
of new ESIPT-fluorescein: photophysics of pH sensitivity and fluorescence.
J Phys Chem A 2011;116:536e45.
[25] Padalkar V, Tathe A, Gupta V, Patil V, Phatangare K, Sekar N. Synthesis and
photo-physical characteristics of ESIPT inspired 2-Substituted benzimidazole,
benzoxazole and benzothiazole fluorescent derivatives. J Fluoresc 2012;22:
311e22.
[26] Brouwer AM. Standards for photoluminescence quantum yield measurements
in solution (IUPAC technical report). Pure Appl Chem 2011;83:2213e28.
[27] Frisch MJ, Trucks GW, Schlegel HB, Scuseria GE, Robb MA, Cheeseman JR, et al.
Gaussian 09 C.01; 2009.
[28] Treutler O, Ahlrichs R. Efficient molecular numerical integration schemes.
J Chem Phys 1995;102:346.
[1] Todoroki K, Ohba Y, Zaitsu K. Chemiluminescence characteristics of N-(4-
substituted Benzyl)isoluminol. Chem Pharm Bull (Tokyo) 2000;48:2011e3.
[2] Yoshida H, Ureshino K, Ishida J, Nohta H, Yamaguchi M. Chemiluminescent
properties of some luminol related compounds (II). Dye Pigment 1999;41:
177e82.
[3] Spurlin SR, Cooper MM. A ChemiluminesClent precolumn labelling reagent for
high-performance liquid chromatography of amino acids. Anal Lett 1986;19:
2277e83.
[4] Kawasaki T, Maeda M, Tsuji A. Chemiluminescence high-performance liquid
chromatography using N-(4-aminobutyl)-N-ethylisoluminol as a precolumn
labelling reagent. J Chromatogr A 1985;328:121e6.
[5] Steijger OM, Lingeman H, Brinkman UAT, Holthuis JJM, Smilde AK,
Doornbos DA. Liquid chromatographic analysis of carboxylic acids using N-(4-
aminobutyl)-N-ethylisoluminol as chemiluminescent label: determination of
ibuprofen in saliva. J Chromatogr B Biomed Sci Appl 1993;615:97e110.
[6] Ishida J, Yamaguchi M, Nakahara T, Nakamura M. 4,5-Diaminophthalhy dra-
[29] Becke A. A new mixing of Hartree-Fock and local density functional theories.
J Chem Phys 1993;98:1372.
[30] Lee C, Yang W, Parr RG. Development of the Colle-Salvetti correlation-energy
formula into a functional of the electron density. Phys Rev B 1988;37:785e9.
[31] Li H, Niu L, Xu X, Zhang S, Gao F. A comprehensive therotical investigation of
intramolecular proton transfer in the excited states for some newly-designed
diphenylethylene derivatives bearing 2-(2-Hydroxy-Phenyl)-Benzotriazole
Part. J Fluoresc 2011;21:1721e8.
[32] Bauernschmitt R, Ahlrichs R. Treatment of electronic excitations within the
adiabatic approximation of time dependent density functional theory. Chem
Phys Lett 1996;256:454e64.
[33] Furche F, Rappoport D. Density functional theory for excited states: equilib-
rium structure and electronic spectra. In: Olivucci M, editor. Computational
Photochemistry. Amsterdam: Elsevier; 2005. p. 93.
[34] Wiberg KB. Ab initio molecular orbital theory by W. J. Hehre, L. Radom, P. v. R.
Schleyer, and J. A. Pople, John Wiley, New York, 548pp. Price: 79.95 (1986).
J Comput Chem 1986;7:379.
[35] Lakowicz JR. Principles of fluorescence spectroscopy. 2nd ed. New York:
Kluwer Academic; 1999.
[36] Valeur B. Molecular fluorescence: principles and applications. Weinheim:
Wiley-VCH Verlag GmbH; 2001.
[37] Cossi M, Barone V, Cammi R, Tomasi J. Ab initio study of solvated molecules: a
new implementation of the polarizable continuum model. Chem Phys Lett
1996;255:327e35.
zide as a highly sensitive chemiluminescence reagent for a-keto acids in liquid
chromatography. Anal Chim Acta 1990;231:1e6.
[7] Ishida J, Yakabe T, Nohta H, Yamaguchi M. 6-Aminomethylphthalhydrazide as
highly sensitive chemiluminescence derivatization reagent for 5-
a
hydroxyindoles in liquid chromatography. Anal Chim Acta 1997;346:175e81.
[8] Gundermann K-D, Drawert M. Konstitution und Chemilumineszenz, I. Steri-
sche Resonanzhinderung bei alkylierten Amino-phthalhydraziden. Chem Ber
1962;95:2018e26.
[9] Gundermann K-D, Horstmann W, Bergmann G. Konstitution und Chem-
ilumineszenz, II. Synthese und Chemilumineszenz-Verhalten von 7-
€
Dialkylamino-naphthalin-dicarbonsaure-(1.2)-hydraziden. Justus Liebigs Ann
Chem 1965;684:127e41.
[10] Wu J, Liu W, Ge J, Zhang H, Wang P. New sensing mechanisms for design of
fluorescent chemosensors emerging in recent years. Chem Soc Rev 2011;40:
3483e95.
[11] Kwon JE, Park SY. Advanced organic optoelectronic materials: harnessing
excited-state intramolecular proton transfer (ESIPT) process. Adv Mater
2011;23:3615e42.
[12] Park S, Kwon JE, Kim SH, Seo J, Chung K, Park S-Y, et al. A White-Light-
Emitting molecule: frustrated energy transfer between constituent emitting
centers. J Am Chem Soc 2009;131:14043e9.
[38] Williams RL, Shalaby SW. Heterocyclic studies: new heterocyclic ring systems
[13] Taki M, Wolford JL, O'Halloran TV. Emission ratiometric imaging of intracel-
lular zinc: design of a benzoxazole fluorescent sensor and its application in
two-photon microscopy. J Am Chem Soc 2003;126:712e3.
[14] Ohshima A, Momotake A, Arai T. A new fluorescent metal sensor with two
binding moieties. Tetrahedron Lett 2004;45:9377e81.
from isomeric dimethyl diaminophthalates.
J Heterocycl Chem 1973;10:
891e8.
[39] Magde D, Wong R, Seybold PG. Fluorescence quantum yields and their rela-
tion to lifetimes of rhodamine 6G and fluorescein in nine solvents: improved
absolute standards for quantum yields. Photochem Photobiol 2002;75:
327e34.
[40] Tsai H-HG, Sun H-LS, Tan C-J. TD-DFT study of the excited-state potential
energy surfaces of 2-(20-Hydroxyphenyl)benzimidazole and its amino de-
rivatives. J Phys Chem A 2010;114:4065e79.
[41] Yoshida H, Nakao R, Nohta H, Yamaguchi M. Chemiluminescent properties of
some luminol-related compounds d Part 3. Dye Pigment 2000;47:239e45.
[42] Ishida J, Takada M, Yakabe T, Yamaguchi M. Chemiluminescent properties of
some luminol related compounds. Dye Pigment 1995;27:1e7.
[43] Bakhshiev N. Universal intermolecular interactions and their effect on the
position of the electronic spectra of molecules in two-component solutions.
Opt Spectrosc 1962;13:24.
^
[15] Rodembusch FS, Brand FR, Correa DS, Pocos JC, Martinelli M, Stefani V.
Transition metal complexes from 2-(20-hydroxyphenyl)benzoxazole: a spec-
troscopic and thermogravimetric stability study. Mater Chem Phys 2005;92:
389e93.
ꢀ
[16] Ríos Vazquez S, Ríos Rodríguez MC, Mosquera M, Rodríguez-Prieto F. Excited-
state intramolecular proton Transfer in 2-(3‘-Hydroxy-2‘-pyridyl)benzox-
azole. Evidence of coupled proton and charge transfer in the excited state of
some o-Hydroxyarylbenzazoles. J Phys Chem A 2007;111:1814e26.
[17] Luxami V, Kumar S. Molecular half-subtractor based on 3,3[prime or minute]-
bis(1H-benzimidazolyl-2-yl)[1,1[prime or minute]]binaphthalenyl-2,2[prime
or minute]-diol. New J Chem 2008;32:2074e9.
[18] Fahrni CJ, Henary MM, VanDerveer DG. Excited-state intramolecular proton
Transfer in 2-(2‘-Tosylaminophenyl)benzimidazole. J Phys Chem A 2002;106:
7655e63.
[19] Zhao J, Ji S, Chen Y, Guo H, Yang P. Excited state intramolecular proton transfer
(ESIPT): from principal photophysics to the development of new chromo-
phores and applications in fluorescent molecular probes and luminescent
materials. Phys Chem Chem Phys 2012;14:8803e17.
[20] Frey W, Elsaesser T. Femtosecond intramolecular proton transfer of vibra-
tionally hot molecules in the electronic ground state. Chem Phys Lett
1992;189:565e70.
[44] Kawski A. Dipolmomente einiger Naphthole im Grund- und Anregungszu-
stand. Naturwissenschaften 1964;51:82e3.
[45] Chamma A, Viallet PC. Determination du moment dipolaire d’une molecule
dans un etat excite singulet. Acad Sci Paris Ser C 1970;270:1901e4.
[46] Biradar DS, Siddlingeshwar B, Hanagodimath SM. Estimation of ground and
excited state dipole moments of some laser dyes. J Mol Struct 2008;875:
108e12.
[47] Nowak K, Wysocki S. An experimental and theoretical study of dipole mo-
ments of N-[4-(9-acridinylamino)-3-methoxyphenyl]methanesulfonamide.
Spectrochim Acta Part A Mol Biomol Spectrosc 2008;70:805e10.
[48] Sharma N, Jain SK, Rastogi RC. Solvatochromic study of excited state dipole
moments of some biologically active indoles and tryptamines. Spectrochim
Acta Part A Mol Biomol Spectrosc 2007;66:171e6.
[21] Schwartz BJ, Peteanu LA, Harris CB. Direct observation of fast proton transfer:
femtosecond photophysics of 3-hydroxyflavone.
3591e8.
J Phys Chem 1992;96:
[22] Henary MM, Wu, Cody J, Sumalekshmy S, Li J, Mandal S, et al. Excited-state
intramolecular proton Transfer in 2-(2‘-Arylsulfonamidophenyl)benzimid-
azole Derivatives: the effect of donor and acceptor substituents. J Org Chem
2007;72:4784e97.
[49] Reichardt C. Solvents and related titles from WILEY-VCH organic synthesis
workbook II chemical synthesis using supercritical fluids. Upd. 3rd ed. WILEY-
VCH Verlag; 2003.
[23] Phatangare KR, Gupta VD, Tathe AB, Padalkar VS, Patil VS, Ramasami P, et al.
ESIPT inspired fluorescent 2-(4-benzo[d]oxazol-2-yl)naphtho[1,2-d]oxazol-2-