Synthesis, photo-physical and DFT studies of ESIPT inspired novel 2-(2′,4′-dihydroxyphenyl) benzimidazole, benzoxazole and benzothiazole

Article abstract of DOI:10.1007/s10895-013-1228-4

Novel ESIPT inspired benzimidazole, benzoxazole and benzothiazole were synthesized from 2,4-dihydroxy benzoic acid and 1,2-phenelenediamine, 2-aminophenol, and 2-aminothiophenol respectively. The synthesized 2-(2′,4′-dihydroxyphenyl) benzimidazole, benzox

Full text of DOI:10.1007/s10895-013-1228-4

J Fluoresc
DOI 10.1007/s10895-013-1228-4
ORIGINAL PAPER
Synthesis, Photo-physical and DFT Studies of ESIPT
Inspired Novel 2-(2′,4′-Dihydroxyphenyl) Benzimidazole,
Benzoxazole and Benzothiazole
Vikas S. Patil & Vikas S. Padalkar & Abhinav B. Tathe &
Vinod D. Gupta & N. Sekar
Received: 17 January 2013 /Accepted: 30 April 2013
#
Springer Science+Business Media New York 2013
Abstract Novel ESIPT inspired benzimidazole, benzoxazole
and benzothiazole were synthesized from 2,4-dihydroxy
benzoic acid and 1,2-phenelenediamine, 2-aminophenol,
and 2-aminothiophenol respectively. The synthesized
2-(2′,4′-dihydroxyphenyl) benzimidazole, benzoxazole and
benzothiazole are fluorescent and the emission characteristic
are very sensitive to the micro-environment. They show a
single absorption and dual emission with large Stokes shift
originating from excited state intramolecular proton transfer.
The absorption-emission characteristics of all these com-
pounds are studied as a function of pH. The change in the
electronic transition, energy levels, and orbital diagrams of
synthesized compounds were investigated by the molecular
orbital calculation and were correlated with the experimental
spectral emission. Experimental absorption and emission
wavelengths are in good agreement with those predicted
using the Density Functional Theory (DFT) and Time-
Dependent Density Functional Theory (TD-DFT)
[B3LYP/6-31G(d)].
Introduction
There is a growing interest for environment sensitive mate-
rials due to potential applications in biotechnology such as
biomedical and biosensing areas [1, 2]. Particularly the
direct visualization of general biochemical events, specific
molecular targets, or defined biochemical processes are
indispensible for both in vitro [3–6] and, increasingly, in
vivo applications [5–8]. These applications, because of the
remarkable advances in imaging techniques, have begun to
use an array of different fluorescent labels, probes, and
sensors in multi-channel multiples studies [9–11] which
include the real-time analysis of cells and whole organisms
[12–15]. Dictated by the particular needs of several studies,
fluorescent dyes and probes with defined optical, chemical,
and biological properties are in high demand [16, 17]. Much
effort has been focussed on the development of optical pH
chemosensors [18–22] with specific attention paid to highly
sensitive indicators within the physiological pH range [23,
24]. Two-band wavelength ratiometric sensing with a single
dye is becoming increasingly important in this context [25]
to measure pH by a two-band wavelength-ratiometric sens-
ing both the acidic and basic states of the indicator should be
ideally fluorescent and exhibit shifted absorption and dual-
emission spectra. Obtaining two bands in emission spectra
requires finding a dye with complex photo physics that
exhibit an excited state reaction. In this case one of two
bands belongs to the initially excited state and the other to
the reaction product. If the molecule exhibits a fast (com-
pared to the rate of fluorescence emission) and reversible
excited state reaction, the two band ratiometric detection can
provide self-calibration of fluorescence signal which allows
compensation of all fluorescence quenching effects [26, 27].
The Excited State Intramolecular Proton Transfer
(ESIPT) reactions generally incorporates transfer of a
.
.
.
Keywords ESIPT Absorption-emission pH sensitive
TD-DFT
:
:
:
:
V. S. Patil V. S. Padalkar A. B. Tathe V. D. Gupta
N. Sekar (*)
Tinctorial Chemistry Group, Institute of Chemical Technology
(Formerly UDCT), N. P. Marg, Matunga,
Mumbai 400 019, Maharashtra, India
e-mail: n.sekar@ictmumbai.edu.in
V. S. Patil
e-mail: vikasudct@gmail.com
V. S. Padalkar
e-mail: vikaspadalkar@gmail.com

J Fluoresc
hydroxyl (or amino) proton to the carbonyl oxygen through
a pre-existing hydrogen bonding configuration [28]. The
resulting proton-transfer tautomer possesses significant dif-
ferences in structure and electronic configuration from its
corresponding normal species. The fundamental require-
ments of ESIPT process are the presence of intra-
molecular hydrogen bonding between the acidic proton,
basic moiety and the suitable geometry of molecular system
[29]. The acidic proton mostly used are −OH, −NH2 etc. and
basic centres are = N-, carbonyl oxygen (=C = O). A large
number of potential molecules [30] which show ESIPT are
reported in literature and their absorption-emission proper-
ties are also studied as a function of their environment.
A variety of molecules have intramolecular hydrogen
bonds (H-bonds) that may be photo induced to undergo proton
transfer [31]. ESIPT can be observed in variety of such mol-
ecules that contain both hydrogen donor and acceptor in close
proximity [32]. An intramolecular hydrogen bond generally
formed in the ground state will migrate to the neighboring
proton acceptor leading to a phototautomer [33]. In the general
family of 2-(2′ hydroxyphenyl benzimidazole), benzaoxazole,
benzthiazole and benztriazole the ESIPT phototautomer gives
rise to an emission with large stoke shift [34]. The high
intensity of fluorescence emission and large stokes shift due
to intramolecular proton transfer phenomena allow these mol-
ecules to have many interesting applications [35–37].
Molecules undergoing ESIPT exhibit dual fluorescence, one
a normal stoke shifted fluorescence band which originates
from a tautomer formed in the exited state (i.e. keto form)
and second due to excited enol form. Further the fluorescence
band maximum and the fluorescence quantum yield of the
tautomer fluorescence band are very sensitive, whereas those
of the normal band do not depend much on the environment. It
is commonly understood that the molecular structure of the
ESIPT compounds regulates their abnormal photo physical
properties to a considerable extent, in most cases exceeding
the influence of environment on them [34].
were prepared by the reaction of 2, 4-dihydroxy benzoic
acid 1 with 1,2-phenylenediamines or o-aminophenol or o-
aminothiophenol in the presences of polyphosphoric acid as
shown in Scheme 1. All synthesised compounds were
characterised by spectral analysis.
Synthesized benzimidazole, benzothiazole and
benzoxazole molecules which contain acidic hydroxy group
at 2′-position and 4′-position with respect to basic –N =
moiety. The location of these groups is such that there is
existence of intra-molecular hydrogen bonding in the
ground state. On excitation, the –N = moiety become strong-
ly basic and -OH group becomes strongly acidic. This leads
to the ESIPT and thus the formation of keto isomer (k1).
The ESIPT reactions of o-hydroxy and o-amino benzazoles
(benzimidazole, benzoxazole and benzothiazole) have been
studied extensively and it has been reported that o-amino
benzazoles have lower fluorescent quantum yield accompa-
nied by shorter Stokes shift as compared to the respective o-
hydroxy benzazoles [38]. This observation has been explained
in the light of the fact that NH2 group is low acidic compared
to the -OH group. In addition improving the basicity of = N-
on the azole ring will also help in accepting proton from the
−OH or −NH2 facilitating the ESIPT phenomenon. In this
paper we report ESIPT molecules where the basicity of = N-
group on the azole ring is enhanced by the presence of −OH
group on the phenyl ring at 2 and 4 position.
Phtotophysical Properties
Synthesised compounds are highly fluorescent in solution
under irradiation of UV light; absorption and emission of
compounds were measured in water as well as in DMF. The
absorption and fluorescence emission spectra of these com-
pounds are shown in the Figs. 1, 2 and 3. All analyses were
performed at room temperature and concentration was 1×
10−6 M. The absorption and emission properties of com-
pound in water and DMF are summarised in Table 1. The
Stokes shift values of new compounds 5–7 are relatively
higher than the reported hydroxy benzimidazole,
benzoxazole and benzothiazole derivatives. The Stokes shift
for molecules that do not show structural change in the
excited state is generally found between 50 and 70 nm
[37]. In contrast, novel compounds 5–7 exhibit ESIPT
In this paper, we present the synthesis and characteriza-
tion of new ESIPT unit hydroxyl benzimidazole, oxazole
and thiazole. The absorption-emission characteristics of all
these compounds were studied as a function of pH. The
change in the electronic transition, energy levels, and orbital
diagrams of synthesized compounds were investigated by
the DFT computation and were correlated with the experi-
mental photo-physical properties.
N
H2N
HX
COOH
OH
PPA
X
+
HO
Result and Discussion
HO
OH
5 = X: NH
2 = X: NH
1
6 = X: 0
7 = X: S
3 = X: 0
4 = X: S
Chemistry
2-(2′,4′-Dihydroxyphenyl)benzimidazole benzimidazole,
benzoxazole and benzothiazole fluorescent derivatives 5–7
Scheme 1 Synthesis of 4-(1H-benzo[d]imidazole, thiazole, oxazole-2-yl)
benzene-1,3-diol

J Fluoresc
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
5
6
7
0
260
290
320
345
370
395
420
445
470
Fig. 3 Emission spectra of compound 6 and 7 in DMF
Wavelength (nm)
Fig. 1 Absorption of compound 5, 6 and 7 in DMF
HBO and 2-HBT. The emission of 2-HBI (354 nm and
463 nm) are same like its 2,4-di hydroxy benzimidazole
analogue 5 (374 and 444 nm). 2 (2′-Hydroxyphenyl
benzoxazole shows single broad emission (506 nm), how-
ever 2 (2′4′-hydroxyphenyl benzoxazole 6 shows well
splitted two emission shorter emission at 387 nm and longer
emission at 447 nm. Blue shift is observed for 2(2′,4′-
dihydroxy benzimidazole. In case of thiazole analogue 2,4-
dihydroxy substituted 7 shows three significant emission
maxima shorter emission at 371 nm, longer emission at
513 nm and in between emission at 459 nm with high
quantum yield (0.57), But 2-HBT shows dual emission,
prominent peak at 510 nm and shoulder peak at 535 nm.
Incorporation of hydroxy at 4-position with respect to azole
ring enhances the ESIPT process in terms of well defined
emission band.
phenomena with Stokes shift between 42 and 148 nm. The
occurrence of the dual emission of synthesised compound is
clearly reflected in emission spectrum. The spectrum con-
sists of two emission maxima ranging from 302 nm to
546 nm. Compounds 5–7 absorb in the range 317–335 nm
and emit in the range 371–513 nm in DMF. Compounds 4–6
show multiple emissions due to ESIPT process. Compound
5 shorter emission (shoulder peak) observed at 374 nm and
longer emission at 444 nm. In case of compound 6 shorter
emission at 387 nm and longer emission at 447 nm.
Compound 7 shows three emissions shortest at 371 nm,
and longer emission at 513 nm in between these two emis-
sions one more emission observed at 459 nm it may be due
to rotamers of compounds. Quantum yields of compounds
for short as well as longer emission were evaluated.
Quantum yields of compounds 5–7 for short as well as long
emission were evaluated and results are summarized in
Table 1. Results reveal that Quantum yield is high at short
emission as compared to longer emission.
Effect of pH
pH-Sensing is critical in chemistry and biology because it
usually plays a pivotal role in a variety of systems. The most
common pH sensors are glass electrodes. However, the
known limitations of the glass pH electrodes make them
unsuitable for certain applications like intracellular pH and
microscopy studies. In contrast to the electrochemical
methods, optical measurements based on fluorescent probes
have no such limitations like common pH glass electrodes
[42–46]. Moreover, fluorescent probes have their obvious
advantages in terms of sensitivity, selectivity, and more
convenient operations in many applications like remote
sensing with optical fibers [46].
Further we compared emission properties of synthesised
compounds with its reported analogues 2(2′-hydroxyphenyl
benzimidazole (2-HBI) [39], 2(2′-hydroxyphenyl
benzoxazole (2-HBO) [40] and 2(2′-hydroxyphenyl
benzothiazole (2-HBT) [41]. Emission properties of
synthesised azoles 5–7 are same like reported 2-HBI, 2-
In the literature, various fluorescent compounds are
reported as sensors for pH in the acidic region (less than
4). Rhodamine derivatives are extensively used as pH sen-
sors because of their excellent photophysical properties like
long absorption, emission wavelength, high quantum yield
and good photostability. Fluorescent sensors for pH range
7–13 [46] are scarce. In this paper we report dual emitting
fluorophores which show a large stokes shift with good
fluorescence intensity and quantum yield. Considering the
Fig. 2 Emission spectra of compound 5 in DMF

J Fluoresc
Table 1 Absorption, emission and quantum yield of synthesized compounds 5–7 in DMF solvent
Compd
λmax, nm
λ
ems, nm
Stokes shift, cm-1 Quantum yield
at short emission (Long emission)
λ
ems, nm
Stokes shift, cm-1 Quantum yield at longer
emission
(Short emission)
5
6
7
326
335
317
374
387
371
3937
4011
4592
0.05
0.09
0.18
444
447
459
513
8152
7479
9759
12053
0.11
0.13
0.20
0.19
1max = absorption maxima; 1ems = emission maxima; ε = molar extinction coefficient
excellent properties of fluorophores we tested the effect of pH
on fluorescence emission. Effect of pH on fluorescence emis-
sion was studied from pH 7–13 and results are summarised in
Table 2. The results reveal that all the compounds are sensitive
towards pH. All compounds show dual absorption at different
pH with single emission, compounds emit approximately at
the same wavelength at all pH. Absorption and emission
spectra of compounds are shown in Figs. 4, 5 and 6.
ratio and results are summarised below, results reveals that
all the compounds shows similar absorption and emission
pattern in all the mixtures (Table 3).
Time Dependent Density Functional Theory
Structural Properties of Compounds 5–7
The structural changes due to ESIPT phenomenon in terms
of bond angle, bond distances and geometry are investigated
with the help of the optimized geometry at the B3LYP/6-
31G(d) level of theory as shown in Table 4, 5 and 6. It
clearly indicates that due to the intra-molecular hydrogen
bonding the molecule has a six-member ring conformation
in excited state. The molecular structures like stoichiometry,
framework group, degree of freedom and point group of
compounds remain the same in both the enol and keto forms
Tables 4, 5 and 6 for all compounds. In compound 5 bond
length [REnol (O7-C5); 1.341)] and [RKeto (O7-C5); 1.271]
and bond angle [AEnol (N13-C9-N4); 128 and Aketo (N13-C9-
N4);123] differ from each other in enol and keto form
Effect of Solvent Polarity
The investigation of solvent polarity on the absorption and
fluorescence characteristics of organic compounds has been
a subject of interest [47, 48]. Excitation of a molecule by
photon causes a redistribution of charges leading to confor-
mational changes in the excited state. The excited state is
stabilized in solvents; the stability of excited state depends
on polarities of solvent and dipole moment of compounds in
excited state causes blue and red shift [47, 48]. Effects of
solvent polarities on absorption and emission properties of
compounds were studied in mixture of water and DMSO
Table 2 Absorption and fluorescence spectra of compound 5–7 at various pH
pH
Compound 5
Compound 6
Compound 7
λ
max, nm
λ
ems, nm
λ
max, nm
λ
ems, nm
λ
max, nm
λems, nm
(intensity, a.u.)
(ε, L. mol-1. cm-1)
(intensity, a.u.)
(ε, L. mol-1. cm-1)
(intensity, a.u.)
(ε, L. mol-1.cm-1)
7
315 (0.786)
410 (511)
291 (0.323)
336 (0.392)
450 (186)
294 (0.405)
345 (0.552)
404 (722)
8
291 (0.562)
315 (0.735)
410 (475)
398 (380)
404 (235)
404 (239)
400 (249)
406 (249)
291 (0.331)
330 (0.443)
450 (57)
291 (0.398)
315 (0.450)
416 (223)
408 (305)
406 (372)
400 (481)
400 (454)
404 (722)
9
294 (0.612)
330 (0.703)
291 (0.297)
333 (0.423)
448 (153)
448 (226)
443 (223)
450 (214)
450 (236)
291 (0.420)
330 (0.465)
10
11
12
13
294 (0.777)
339 (0.779)
294 (0.338)
336 (0.466)
291 (0.408)
330 (0.519)
294 (0.681)
342 (0.822)
294 (0.339)
375 (0.589)
294 (0.376)
351 (0.512)
294 (0.735)
342 (0.870)
294 (0.354)
375 (0.604)
294 (0.342)
351 (0.483)
294 (0.727)
342 (0.879)
294 (0.305)
375 (0.569)
294 (0.405)
345 (0.552)
1max = absorption maxima; 1ems = emission maxima; ε = molar extinction coefficient

J Fluoresc
Fig. 4 Absorption and
fluorescence spectra of
compound 5 Vs pH
1
0.8
0.6
0.4
0.2
0
600
500
400
300
200
100
0
pH 7
pH 7
pH 8
pH 8
pH 9
pH 9
pH 10
pH 11
pH 12
pH 13
pH 10
pH 11
pH 12
pH 13
255 285 315 345 375 405
Wavelength (nm)
Wavelength (nm)
respectively, change in bond length clearly indicates ESIPT
phenomenon observed in synthesised molecules. Similar trends
observed for molecules 6 and 7. Compounds 5–7 are roughly
planar in enol and keto forms with dihedral angle is apprometly
0° between two rings and facilitate excited state intra-molecular
hydrogen transfer. In the enol form lone pair present on oxygen
is involved in resonance. In this process single bond character is
converted in to double bond character, bond length between O-
H decreases as compared to keto form, this trend observed in all
compounds. In ESIPTcompounds change in bond length, bond
angle and dihedral angle due to excited state intramolecular
phenomenon for all compounds were summarised in Tables 4,
5 and 6. The bond length O-C of hydroxy is different for the
enol and keto forms, the change in bond length clearly indicates
that hydrogen is transferred in excited state and the enol form is
(single bond, C-O) converted in keto form (double bond, C =
O) by ESIPT. For the ESIPT process, the geometry between
two rings is also very important.
shows single emission in between 404–410 nm. Experimental
emissions were compared with TDDFT emission. The differ-
ence between experimental mission and TD DFT emission is
40 nm. Compounds show dual emission in DMSO, short emis-
sion is well agreement with TDDFT emission. In water as well
as in DMSO excitation is from HOMO-LUMO with almost
same population. Observed absorption, emission, computed
vertical excitations and emission are summarized in Table 7.
Experimental
General
All commercial reagents and solvents were procured from s. d.
fine chemicals (India) and were used without purification. The
reactions were monitored by TLC using on 0.25 mm E-Merck
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 a
Vertical Excitation and Emission Study of Compounds 5–7
in Water and DMSO
1
Perkins-Elmer 257 spectrometer using KBr discs. H NMR
Compounds are fluorescent in solution on irradiation with UV
light. Photophysical properties of compounds 5–7 were evalu-
ated in water and DMSO solvents. Observed absorption and
fluorescence properties were compared with absorption and
emission obtained by DFT and TDDFT. Experimental observed
absorption values of compounds are well in agreement with
computed vertical excitation obtained by DFT. Compounds
spectra were recorded on VXR 300-MHz instrument using
TMS as an internal standard. LC-MS were recorded on
FINNIGAN LCQ ADVANTAGE MAX instrument from
Thermo Electron Corporation (USA). The absorption spectra
of the compounds were recorded on a Spectronic Genesys 2
UV-Visible spectrophotometer; UV-Visible emission spectra
were recorded on JASCO – FP 1520.
Fig. 5 Absorption and
fluorescence spectra of
compound 6 Vs pH
250
0.7
pH 7
pH 8
pH 9
pH 10
pH 11
pH 12
pH 13
pH 7
pH 8
pH 9
pH 10
pH 11
pH 12
pH 13
0.6
200
150
100
50
0.5
0.4
0.3
0.2
0.1
0
0
258 288 318 348 378 408
Wavelength (nm)
344 374 404 434 464 494 524 554 584
Wavelength (nm)

J Fluoresc
Fig. 6 Absorption and
fluorescence spectra of
compound 7 Vs pH
0.6
0.5
0.4
0.3
0.2
0.1
0
pH 7
pH 8
pH 9
pH 10
pH 11
pH 12
pH 13
800
700
600
500
400
300
200
100
0
pH 7
pH 8
pH 9
pH 10
pH 11
pH 12
pH 13
264 294 324 354 384 414
349369389409429449469489
Wavelength (nm)
Wavelength (nm)
Computational Methods
computed using the same method to verify that the
optimized structures correspond to local minima on the
energy surface. The vertical excitation energies at the
ground-state equilibrium geometries were calculated with
TD-DFT [55]. The low-lying first singlet excited state
(S1) of each tautomer was relaxed using the TD-DFT to
obtain its minimum energy geometry. The difference
between the energies of the optimized geometries at the
first singlet excited state and the ground state was used
in computing the emissions [56, 57]. All electronic struc-
ture computations were carried out using the Gaussian 09
program [58].
Compounds 5–7 were studied to find out electronic state and
energy level by DFT computation methods. The ground
state geometry of the compounds 5–7 in their Cs symmetry
were optimized using the tight criteria in the gas phase using
Density Functional Theory (DFT) [49]. The functional used
was B3LYP. The B3LYP method combines Becke’s three
parameter exchange functional (B3) [50] with the nonlocal
correlation functional by Lee, Yang and Parr (LYP) [51].
The basis set used for all atoms was 6-31G(d) [52–54]. The
vibrational frequencies at the optimized structures were
Table 3 Effect of solvent polarity on absorption and emission properties of synthesized compounds
Compounds
5
Solvent ratio (Water/DMSO)
20 % Water: 80% DMSO
λ
max, nm
λ
ems, nm
Stokes shift, cm-1
319
319
316
316
334
334
334
334
322
319
334
319
358
446
355
440
356
432
356
420
389
443
358
453
389
447
390
443
358
411
363
428
389
447
355
416
3415
8926
3179
8621
3556
8497
3556
7836
4233
7367
2007
7865
4233
7569
4299
7367
3123
6725
3800
7983
4233
7569
3179
7310
40 % Water: 60% DMSO
60 % Water: 40% DMSO
80 % Water: 20 % DMSO
20 % Water: 80% DMSO
40 % Water: 60% DMSO
60 % Water: 40% DMSO
80 % Water: 20 % DMSO
20 % Water: 80% DMSO
40 % Water: 60% DMSO
60 % Water: 40% DMSO
80 % Water: 20 % DMSO
6
7

J Fluoresc
Table 4 Structural properties of compound 5
Properties
5a (Enol)
5b (Keto)
Stoichiometry
Framework group
Point group
C13H10N2O2
C1(×(C13H10N2O2)
C1
C13H10N2O2
C1(×(C13H10N2O2)
C1
Bond distances
Bond
Bond length
1.341
Bond
Bond length
1.271
R(O7-C5)
R(O7-C5)
R(N13-C9)
R(C1-O8)
1.328
R(N13-C9)
R(C1-C8)
1.354
1.363
1.364
Bond angle
Angle
Bond angle
123
Angle
Bond angle
123
A(N13-C9-C4)
A(O7-C5-C4)
A(N13-C9-C4-C5)
A(N13-C9-C4)
A(O7-C5-C4)
A(N13-C9-C4-C5)
122
122
Dihedral angle
0.01
0.00
Bond angle in degree,
Bond length in o A Unit
Table 5 Structural properties of compound 6
Properties
6a (Enol)
6b (Keto)
Stoichiometry
Framework group
Point group
C13H9NO3
C1(×(C13H9NO3)
C1
C13H9NO3
C1(×(C13H9NO3)
C1
Bond distances
Bond
Bond length
1.343
Bond
Bond length
1.270
R(O16-C3)
R(N8-C7)
R(O16−C3)
R(N8-C7)
1.311
1.345
R(C1-O17
Angle
)
1.362
R(C1-O17
)
1.364
Bond angle
Bond angle
126
Angle
Bond angle
124
A(N8-C7-C4)
A(C4-C3-O16
A(N8-C7-C4)
A(C4-C3-O16
)
122
)
122
Dihedral angle
A(N8-C7-C4-C3)
0.00
A(N8-C7-C4-C3)
0.01
Bond angle in degree,
Bond length in o A Unit

J Fluoresc
Table 6 Structural properties of compound 7
Properties
7a (Enol)
7b (Keto)
Stoichiometry
Framework group
Point group
C13H9NSO2
C1(×(C13H9NSO2)
C1
C13H9NSO2
C1(×(C13H9NSO2)
C1
Bond distances
Bond angle
Bond
Bond length
1.311
Bond
Bond length
1.347
R(N10-C9)
R(O7-C4)
R(N10-C9)
R(O7-C4)
1.341
1.270
R(C2-O8)
1.361
R(C2-O8)
1.361
Angle
Bond angle
123
Angle
Bond angle
122
A(N10-C9-C5)
A(C4-C5-C9)
A(O7-C4-C5)
A(N10-C9-C5-C4)
A(N10-C9-C5)
A(C4-C5-C9)
A(O7-C4-C5)
A(N10-C9-C5-C4)
119
117
122
122
Dihedral Angle
0.00
0.00
Bond angle in degree,
Bond length in o A Unit
Relative Quantum Yield Calculations
[59–63]. Absorption and emission characteristics of the
standard and the compounds in polar as well as non-polar
solvents were measured at different concentrations at (2, 4,
6, 8, and 10 ppm level). Emission intensity values were
plotted against absorbance values and linear plots were
Quantum yield of synthesized compounds in different sol-
vents were calculated by using anthracene as reference
standard in different solvents using the comparative method
Table 7 Observed UV-visible absorption, emission and computed vertical excitation and emission of compounds 5–7 in water and DMSO
Compd Experimental absorption, Vertical % Deviation Assignment
Observed emission, Stokes shift, cm-1 TD-DFT emission,
f
λmax, nm
excitation
(TDDFT)
λ
ems, nm ems, nm
λ
nm eV
5a
6a
7a
5b
315
336
294
311
307 4.0257 0.63 2.5
321 3.9801 0.73 4.4
333 3.7207 0.75 13
311 3.9741 0.81 0.0
H→L (0.6325) 410
H→L (0.7019) 450
H→L (0.6463) 404
7356
7540
9261
367
375
357
367
H→L (0.6530) 360c
4377
9733
446d
6b
7b
334
322
315 3.9305 0.88 5.6
335 3.6941 0.80 4.0
H→L (0.6469) 366c
2618
7519
375
394
446d
H→L (0.6502) 381c
4809
5014
384d
a Absorption and emission were measured in Water solvent
b Absorption and emission were measured in DMSO solvent
c Observed emission at shorter wavelength
d Observed emission at longer wavelength
f: Oscillator Strength

J Fluoresc
obtained. Gradients were calculated for compounds in each
solvent and for the standards. All the measurements were
done by keeping the parameters constant such as solvent and
slit width. Relative quantum yields of synthesized com-
pounds in different solvents were calculated by using
Eq. 1[59–63].
137 mL of water) acidified with 1:1 HCl (40 mL) at 10 °C. This
solution was kept overnight at 0 °C to give the product 6.
Compound was recrystallized from ethanol.
Yield: 61 %, Melting point = >300 °C.
1H-NMR (CD3)2SO: δ 6.25 (m, 2H), 6.82 (d, 1H J=8.9
Hz), 6.95 (d, 1H J=9.1 Hz), 7.59 (d, H), 7.82 (m, 2H). 10.60
(s, 2H) ppm.
2
Gradx
ηx
FT-IR (KBr1) : 3341 (phenolic O-H stretching), 1696
(amide C = O), 1651 (imine C = N), 1583 (aromatic C = C),
1501, 1448, 1422, 1215 (phenol C-O) cm−1.
MASS: m/z: 243 (M+).
fx ¼ Φst ꢀ
ꢀ
ð1Þ
2
Gradst ηst
Where:
Φx
Quantum yield of compound
Φst
Quantum yield of standard sample
Synthesis of (1H-benzo[d]oxazole-2-yl)benzene-1,3-diol (7)
Gradx Gradient of compound
Gradst Gradient of standard sample
2,4-Dihydroxy benzoic acid 1 (10 g, 64.9 mmol), and o-
aminophenol 4 (7.07 g, 64.9 mmol) were mixed in
polyphosphoric acid (121.94 g). The mixture was stirred at
150 °C for 4 h and then it was allowed to cool to room
temperature, poured into 1,200 mL ice-cold water with
constant stirring. Dark green precipitate was obtained. It
was filtered and dissolved in cold solution of 10 %
Na2CO3 (15.75 g dissolved in 137 mL of water) acidified
with 1:1 HCl (40 mL) at 10 °C. This solution was kept
overnight at 0 °C to give the product 7. Compound was
recrystallized from ethanol.
ηx
Refractive index of solvent used for synthesized
compound
ηst
Refractive index of solvent used for standard
sample
Synthesis of Compounds 5–7
Synthesis of 4-(1H-benzo[d]imidazol-2-yl)benzene-1,3-diol
(5)
2,4-Dihydroxy benzoic acid 1 (10 g, 64.9 mmol), and o-
phenylenediamine 2 (7.01 g, 64.9 mmol) were mixed in
polyphosphoric acid (121.94 g). The mixture was stirred at
250 °C for 4 h and then it was allowed to cool at room
temperature, poured into 1,200 mL ice-cold water with constant
stirring. Dark brown precipitate was obtained which was fil-
tered and dissolved in cold solution of 10 % Na2CO3 (15.75 g
dissolved in 137 mL of water) acidified with 1:1 HCl (40 mL)
at 10 °C. This solution was kept overnight at 0 °C to give the
final product 5. Compound was recrystallized from ethanol.
Yield 54 %, Melting point = >300 °C.
Yield: 63 %, Melting point = >300 °C.
1H-NMR (CD3)2SO: δ 6.87 (s, 1H), 6.9 (s, 2H),
7.56 (d, H, J=7.3 Hz), 7.89 (d, H, J=7.3 Hz), 8.2 (s, 2H),
11.71(s, 1H) ppm.
FT-IR (KBr) : 3315 (phenolic O-H stretching), 1679
(amide C = O), 1647 (imine C = N), 1611 (aromatic C = C),
1567, 1237 (phenol C-O) cm−1.
MASS: m/z: 227 (M+).
Conclusion
1H NMR (CD3)2SO: δ 6.88 (d, 1H, J=6.4 Hz), 6.88 (d,
1H, J=6.4 Hz), 7.05 (s, 1H), 7.34 (s, 2H), 7.71 (s, 2H), 8.07
(s, 1H), 11.2 (s, 1H) ppm.
Novel dual emitting 2-substituted benzimidazole,
benzothiazole and benzoxazole were synthesised. The
photophysical properties, absorption and emission of the
compounds 5–7 in DMF were obtained experimentally and
correlated with the computational results. Computed absorp-
tion and emission of the compounds are in good agreement
with the experimental results. Structural properties of the
compounds were evaluated for the keto as well as enol form,
and the results clearly indicate that the ESIPT process is
observed in the present compounds. The effect of pH and
polarity of solvents on the photophysical properties were
studied at pH range 7–13 and the results reveal that the
compounds are sensitive towards pH and solvent polarity.
FT-IR (KBr): 3438 (phenolic O-H), 3322 (N-H), 1651
(imine C = N), 1571 (aromatic C = C), 1516, 1421, 1389,
1203 (phenol C-O) cm−1.
Mass: m/z 226 (M+).
Synthesis of (1H-benzo[d]thiazole-2-yl)benzene-1,3-diol (6)
2,4-Dihydroxy benzoic acid 1 (10 g, 64.9 mmol), and o-
aminothiophenol 3 (8.11 g, 64.9 mmol) were mixed in
polyphosphoric acid (121.94 g). The mixture was stirred at
150 °C for 4 h and then it was allowed to cool to room
temperature, poured into 1,200 mL ice-cold water with constant
stirring. Dark green precipitate was obtained. It was filtered and
dissolved in cold solution of 10 % Na2CO3 (15.75 g dissolved in
Acknowledgments Vikas Patil and Vikas Padalkar are thankful to
the Institute of Chemical Technology, Mumbai, India.

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