Synthesis and photo-physical characteristics of ESIPT inspired 2-substituted benzimidazole, benzoxazole and benzothiazole fluorescent derivatives

Article abstract of DOI:10.1007/s10895-011-0962-8

Novel 2-(1H-benzimidazol-2-yl)-5-(N,Ndiethylamino) phenol, 2-(1,3-benzoxazol-2-yl)-5-(N,Ndiethylamino) phenol, 2-(1,3-benzothiazol-2-yl)-5- (N,Ndiethylamino) phenol and their derivatives have been synthesized from p-N,N-diethyl amino salicylaldehyde with different substituted o-phenylenediamine or o-aminophenol or o-aminothiophenol and their photo-physical properties were studied. Effects of solvent polarity in the absorptionemission properties of synthesized compounds were investigated. All these compounds shows excited state intramolecular proton transfer pathway having single absorption and dual emission characteristics. The fluorescent compounds were characterised by FT-IR, ¹HNMR, ¹³C NMR and Mass spectral analysis. TGA analysis showed these compounds are thermally stable up to 200 °C.

Full text of DOI:10.1007/s10895-011-0962-8

J Fluoresc (2012) 22:311–322
DOI 10.1007/s10895-011-0962-8
ORIGINAL PAPER
Synthesis and Photo-Physical Characteristics of ESIPT
Inspired 2-Substituted Benzimidazole, Benzoxazole
and Benzothiazole Fluorescent Derivatives
Vikas S. Padalkar & Abhinav Tathe & Vinod D. Gupta &
Vikas S. Patil & Kiran Phatangare & N. Sekar
Received: 31 May 2011 /Accepted: 30 August 2011 /Published online: 29 September 2011
#
Springer Science+Business Media, LLC 2011
Abstract Novel 2-(1H-benzimidazol-2-yl)-5-(N,N-
diethylamino) phenol, 2-(1,3-benzoxazol-2-yl)-5-(N,N-
diethylamino) phenol, 2-(1,3-benzothiazol-2-yl)-5-(N,N-
diethylamino) phenol and their derivatives have been
synthesized from p-N,N-diethyl amino salicylaldehyde with
different substituted o-phenylenediamine or o-aminophenol
or o-aminothiophenol and their photo-physical properties
were studied. Effects of solvent polarity in the absorption-
emission properties of synthesized compounds were inves-
tigated. All these compounds shows excited state intra-
molecular proton transfer pathway having single absorption
and dual emission characteristics. The fluorescent compounds
were characterised by FT-IR, 1HNMR, 13C NMR and Mass
spectral analysis. TGA analysis showed these compounds are
thermally stable up to 200 °C.
visualisations of general biochemical events, specific
molecular targets or defined biochemical processes are
indispensible for both in vitro [1–4] and increasingly in
vivo applications [3–6]. In recent years, advances in
imaging technique have made these applications very
encouraging particularly in the use of an array of different
labels, probes and channels in multi-channel multiple
studies [7–9] and the real time analysis of cells and whole
organism [10–13].
Fluorescent molecules which can be incorporated into
probes with defined optical and chemical properties having
compatibility in bio-materials are becoming increasingly
important. Fluorescent molecules exhibiting dual emission
find applications in ratiometric sensing since two band
ratiometric detection can provide self calibration of fluores-
cence signal which allows compensation of all fluorescence
quenching effect [14, 15].
.
.
.
Keywords Dual emission ESIPT Solvatochromism
.
.
.
.
Fluorescence Photo-physics Benzimidazole
Study of photophysical properties of molecules exhibit-
ing excited state intra-molecular proton transfer reactions
has received much attention in recent years [16–25]. This is
not only because of their importance in understanding the
ultrafast reactions at the molecular level [26], but also
because of the fact that these molecules undergoing the
ESIPT process find applications ranging from polymer UV
stabiliser [27, 28], laser dyes [29–31] to photo-chromic
materials [32].
Benzoxazole Benzothiazole
Introduction
Stimuli-sensitive materials have wide applications in bio-
sensing and biomedical diagnostics. In particular, the direct
The fundamental requirements of ESIPT process are the
presence of intra-molecular hydrogen bonding between the
acidic proton, basic moiety and the suitable geometry of
molecular system. The acidic proton mostly used are -OH,
-NH2 etc. and basic centres are =N-, carbonyl oxygen
(=C=O). A large number of potential molecules [33–36]
which show ESIPT are reported in literature and their
absorption-emission properties are also studied as a
function of their environment.
:
:
:
:
V. S. Padalkar A. Tathe V. D. Gupta V. S. Patil
:
K. Phatangare N. Sekar (*)
Department of Intermediate and Dyestuff Technology,
Institute of Chemical Technology (Formerly UDCT),
N. P. Marg, Matunga,
Mumbai 400 019, Maharashtra, India
e-mail: n.sekar@ictmumbai.edu.in
N. Sekar
e-mail: nethi.sekar@gmail.com

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J Fluoresc (2012) 22:311–322
It has been observed that Stokes shifts are higher when
Gradients were calculated for each unknown compound and
for standard. All the measurements were done by keeping the
parameters such as solvent and slit width constant. Relative
quantum yield of all synthesized compounds 4a–4i were
calculated by using the Formula 1 [55].
molecules contain benzimidazole, benzothiazole, benzox-
azole and benzotriazole units. The high intensity of
fluorescence emission and large Stokes shift are due to
the excited state intramolecular proton transfer phenomenon
[37–52] which allows these molecules to have many
interesting applications [53].
In this paper, we report the synthesis and characterization of
novel hydroxy benzimidazole, benzoxazole and benzothiazole
derivatives. These compounds have interesting absorption-
emission characteristics and solvatochromism properties.
Formula 1: Relative fluorescence quantum yield
À
Á
Φx ¼ ΦSTðGradx=GradSTÞ h2x=h2ST
Where:
ΦX
ΦST
Quantum yield of unknown sample
Quantum yield of standard used
GradX Gradient of unknown sample
GradX Gradient of standard used
Experimental Section
h2ST
h2X
Refractive index of solvent for standard sample
Refractive index of solvent for sample
Materials and Equipments
The fluorescence quantum yields of 4a–4i were recorded
in ethanol as well as acetonitrile at room temperature. The
compounds 4a–4i showed similar fluorescence behaviour
with respect to fluorescence quantum yield in both the
solvents, the details are given in the Table 1.
All commercial reagents and solvents were procured from
s.d.fine chemicals (India) and were used without purifica-
tion. 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 Perkins-Elmer 257 spectrometer
Synthesis and Characterisation
Synthesis of 4-(N,N-diethyl amino)-2-hydroxybenzaldehyde (2)
1
using KBr discs. H NMR 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 spectro-
photometer; UV-Visible emission spectra were recorded on
JASCO – FP 1520. Simultaneous DSC-TGA measurements
were performed on simultaneous DSC-TGA Waters (India)
Pvt Ltd. DFT calculations were performed on a HP
workstation XW 8600 with Xeon processor, 4 GB RAM
and Windows Vista as operating system. The software
package used was Guassian 03 W. The ground state
geometry was optimized at B3LYP level of theory and 6–
31 G (d) as basis set and excited state geometry was optimized
by TD-DFT with B3LYP/6–31 G (d) basis set 3 [54].
Phosphorous oxychloride (POCl3) (2.75 ml, 0.03 mol) was
slowely added to dimethylformamide (DMF) (3.65 ml,
0.05 mol) at 5–10 °C under the stirring. To this cooled
reagent 3-(N,N-diethyl amino) phenol (0.01 mole) was
added by dissolving it in to DMF (6 ml) under the stirring
and the resulting mixture was heated at 75 °C for 4 h. The
reaction mixture was cooled to room temperature and then
poured into ice cold water (60 ml). Reaction mass was
Table 1 Quantum yield of compounds 4a - 4i in ethanol and
acetonitrile
Compounds
Quantum yield ethanol
Quantum yield acetonitrile
4a
4b
4c
4d
4e
4f
0.105
0.188
0.122
0.158
0.121
0.139
0.124
0.118
0.136
0.094
0.209
0.139
0.168
0.121
0.130
0.131
0.098
0.132
Photo-Physical Properties
An effective fluorescent dye for biological application
should have good fluorescence intensity, high quantum
yield and high photostability. Quantum yield of compounds
4a–4i were determined by using anthracene as standard.
Absorption and emission characteristics of standard as well
as unknown samples were measured at different concentra-
tion of unknown samples and standard at (2, 4, 6, 8 and
10 ppm level). Absorbance intensity values were plotted
against emission intensity values. A linear plot was obtained.
4g
4h
4i
lmax and lem were measured in nm
Samples were prepared in ethanol and acetonitrile
Analyses were carried out at room temperature

J Fluoresc (2012) 22:311–322
313
neutralised with sodium carbonate, brown colored solid
separated out. Separated product was filtered and washed
with cold water, dried and crystallised from ethanol to
afford the pure product 2 (m.p. 62 °C) (lit. 62–64 °C [56])
1H NMR (DMSO- d6, 400 MHz): δ 1.17 (t, 6H), 3.45
(q, 4H), 6.42 (s, 1H), 6.53–6.55(d, 1H, J=8.8 Hz, Ar-H),
7.83–786 (d, 1H, J=8.8 Hz, Ar-H), 8.03–8.05 (d, 1H, J=
9.2 Hz, Ar-H),8.25–8.28 (d, 1H, J=10.8, 2.0 Hz, Ar-H),
8.50–8.55 (d, 1H, J=2.0 Hz, Ar-H), 13.00 (s, 1H).
LC-MS: (M+1: 327.3, 97.99%).
General Procedure for Synthesis of 2-substituted
Benzimidazole, Benzoxazole and Benzothiazole (3)
2-(5-Amino-1H-benzimidazol-2-yl)-5-(N,N-diethylamino)
Phosphorus trichloride (0.33 mol) was added dropwise to a
solution of the p-N,N-diethyl salicylaldehyde (0.33 mol)
and substituted 1,2-phenylenediamine or o-aminophenol
and o-aminothiophenol (0.33 mol) in ethanol (50 ml).
During addition of PCl3 maintained the temperature 40–
45 °C. The mixture was heated at 60 °C for 4 h, reaction
was monitored by thin layer chromatography. The reaction
mass was cooled to room temperature and made alkaline to
pH 8 with aqueous 20% sodium bicarbonate solution. Reaction
mass was concentrated under vacuum and the solid obtained
was collected and crystallized from isopropyl alcohol.
phenol (4c)
M.p. 272 °C
FT-IR (KBr): 3417, 3285, 2971, 1642, 1575, 1484,
1270, 1124, 1077, 784 cm−1.
1H NMR (DMSO-d6, 400 MHz): δ 1.12 (t, 6H), 3.35
(q, 4H), 5.30 (s, 1H), 6.17 (s, 2H), 6.18–6.33 (d, 1H, J=8.8,
2.4 Hz, Ar-H), 6.52–6.55 (d, 1H, J=8.8, 2.0 Hz, Ar-H), 6.69
(s, 1H, Ar-H), 7.22–7.24 (d, 1H, J=8.4 Hz, Ar-H), 7.69–7.77
(d, 1H, J=8.8 Hz, Ar-H), 8.16 (s, 1H, Ar-H), 12.63 (s, 1H).
LC-MS: (M+1: 297.3, 98.76%).
General Procedure for Synthesis of 2-substituted
2-(1,3-Benzoxazol-2-yl)-5-(N,N-diethylamino)phenol (4d)
Benzimidazole, Benzoxazole and Benzothiazole (4a–4i)
M.p. 268 °C
Palladium-carbon catalyst (10%) was added portion-wise
over a period of 10 min to a hot solution of compound 3
(1.0 g, 0.28 mol) in ethanol (50 ml) containing hydrazine
hydrate (0.007 g, 1.96 mol). The mixture was heated under
reflux for 1 h. The hot solution was filtered through a
Whatman paper to remove Pd-C and further filtered
through silica gel (5 g) to minimise excess hydrazine
hydrate. Filtrate was concentrated under reduced pressure to
afford pure products 4a–4i, and they were further analyzed
without further purification.
FT-IR (KBr): 3013, 1656, 1575, 1484, 1287, 1270,
1124, 1077, 784 cm−1.
1H-NMR (400 MHz): δ 1.16 (t, 6H), 3.41 (q, 4H), 5.61
(s, 1H), 5.87–5.89 (d, 1H, J=8.8 Hz, Ar-H), 6.08–6.10 (d,
1H, J=8.8 Hz, Ar-H), 6.42–6.44 (d, 1H, J=8.8 Hz, Ar-H),
6.78 (s, 1H, Ar-H), 6.89–6.91(d, 1H, J=8.8 Hz, Ar-H), 7.11–
7.13 (d, 1H J=8 Hz, Ar-H), 7.23 (s, 1H, Ar-H), 7.59 (s, 1H,
Ar-H), 8.19 (s, 1H, Ar-H), 8.63 (s, 1H, Ar-H), 11.23 (s, 1H).
LC-MS: (M+1: 283.3, 96.25%)
5-(N,N-Diethylamino)-2-(6-nitro-1,3-benzoxazol-2-yl)
Spectral Data of Synthesized Compounds (4a–4i)
phenol (4e)
2-(1H-Benzimidazol-2-yl)-5-(N,N-diethylamino) phenol (4a)
M.p. 218 °C
FT-IR (KBr): 2987, 1635, 1530, 1350, 1145, 1098, 847,
742 cm−1.
M.p. 192 °C
FT-IR (KBr): 2975, 1620, 1518, 1145, 817, 733 cm−1.
1H NMR (DMSO- d6, 400 MHz): δ 1.13 (t, 6H), 3.32
(q, 4H), 6.17 (s, 1H), 6.30–6.52 (d, 1H, J=8.8 Hz, Ar-H),
6.98–7.00 (d, 2H, J=8.8 Hz, Ar-H), 7.57–7.59 (d, 1H, J=
8 Hz, Ar-H), 7.83–7.85 (d, 2H, J=8 Hz, Ar-H), 8.23–8.25
(d, 1H, J=8 Hz, Ar-H), 12.13 (s, 1H).
1H-NMR (400 MHz): δ 1.22 (t, 6H), 3.40 (q, 4H), 5.62
(s, 1H), 5.98–6.00 (d, 1H, J=8 Hz, Ar-H), 6.12–6.14 (d,
1H, J=8 Hz, Ar-H), 6.45–6.47 (d, 1H, J=8.8 Hz, Ar-H),
6.80 (s, 1H), 6.91–6.93 (d, 1H, J=8.8 Hz, Ar-H), 7.14–7.16
(d, 1H J=8 Hz, Ar-H), 7.26 (s, 1H, Ar-H), 7.60 (s, 1H,
Ar-H), 8.16 (s, 1H, Ar-H), 8.65 (s, 1H), 11.37 (s, 1H).
LC-MS: (M+1: 328.3, 97.12%)
LC-MS: (M+1: 282.3, 97.89%).
5-(N,N-Diethylamino)-2-(5-nitro-1H-benzimidazol-2-yl)
5-(N,N-Diethylamino)-2-(5-nitro-1,3-benzoxazol-2-yl)
phenol (4b)
phenol (4f)
M.p.264 °C
M.p. 240 °C Decomposes
FT-IR (KBr): 2991, 1620, 1520, 1338, 1149, 946, 817,
FT-IR (KBr): 2987, 1632, 1535, 1347, 1147, 1100, 845,
740 cm−1.
733 cm−1.

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J Fluoresc (2012) 22:311–322
Scheme 1 Synthesis of
2-substituted benzimidazole,
benzoxazole and benzothiazole
compounds 4a–4i
1H-NMR (400 MHz): δ 1.23 (t, 6H), 3.42 (q, 4H), 5.59
(s, 1H), 5.88–5.90 (d, 1H, J=8 Hz, Ar-H), 5.98–6.00 (d,
1H, J=8 Hz, Ar-H), 6.16–6.18 (d, 1H, J=8.8 Hz, Ar-H),
6.95 (s, 1H), 7.06–7.08 (d, 1H, J=8.8 Hz, Ar-H), 7.12–7.14
(d, 1H J=8 Hz, Ar-H), 7.44 (s, 1H), 8.63 (s, 1H), 9.16 (s,
1H), 11.03 (s, 1H), 11.77 (s, 1H).
2-(1,3-Benzothiazol-2-yl)-5-(N,N-diethylamino)phenol (4i)
M.p. 168 °C.
FT-IR (KBr): 2875, 1630, 1618, 1456, 1342, 1135, 812,
743 cm−1.
1H-NMR (400 MHz): δ 1.21 (t, 6H), 3.41 (q, 4H), 6.27
(s, 1H), 7.26–7.28 (d, 1H, J=8.0 Hz, Ar-H), 7.29–7.31
(d, 1H, J=8.8, 2.0 Hz, Ar-H), 7.44–7.46 (d, 1H, J=8.8,
2.0 Hz, Ar-H), 7.80–7.86 (d, 2H, J=8.0, 2.0 Hz, Ar-H),
12.56 (s, 1H).
LC-MS: (M+1: 328.3, 98.62%).
2-(6-Amino-1,3-benzoxazol-2-yl)-5-(N,N-diethylamino)
phenol (4g)
LC-MS: (M+1: 299.4, 98.67%).
M.p.269 °C Decomposes
FT-IR (KBr): 3427, 3281, 2967, 1643, 1560, 1487,
1267, 1124, 1067, 774 cm−1.
Results and Discussion
1H-NMR (400 MHz): δ 1.10 (t, 6H), 3.31 (q, 4H), 5.90
(s, 1H), 6.14 (s, 2H), 6.24–6.26 (d, 1H, J=8.8, 2.4 Hz,
Ar-H), 6.59–6.61 (d, 1H, J=8.8, 2.0 Hz, Ar-H), 6.74 (s,
1H), 7.27–7.29 (d, 1H, J=8.4 Hz, Ar-H), 7.71–7.73 (d, 1H,
J=8.8 Hz, Ar-H), 8.23 (s, 1H), 12.13 (s, 1H).
LC-MS: (M+1: 298.3, 96.62%).
2-Substituted benzimidazole, benzoxazole and benzothia-
zole fluorescent derivatives 4 were prepared by the reaction
of 4-(N,N-diethyl amino)-2-hydroxybenzaldehyde 2 with
substituted 1,2-phenylenediamines or o-aminophenol or o-
aminothiophenol in the presences of phosphorus trichloride
2-(5-Amino-1,3-benzoxazol-2-yl)-5-(N,N-diethylamino)
Table 2 Synthesized
benzimidazole, benzoxazole and
benzothiazole derivatives
phenol (4h)
Compound
X
R
M.p. 250 °C Decomposes.
4a
4b
4c
4d
4e
4f
-NH
-NH
-NH
O
H
FT-IR (KBr): 3431, 3290, 3013, 1656, 1567,
1484, 1276, 1260, 1227, 1068, 780 cm−1.
1H-NMR (400 MHz): δ 1.09 (t, 6H), 3.32 (q,
4H), 5.93 (s, 1H), 6.21 (s, 2H), 6.23–6.25 (d,
1H, J=8.8, 2.4 Hz, Ar-H), 6.61–6.63 (d, 1H, J=
8.8, 2.0 Hz, Ar-H), 6.74 (s, 1H), 7.29–7.31 (d, 1H,
J=8.4 Hz, Ar-H), 7.69–7.71 (d, 1H, J=8.8 Hz,
Ar-H), 8.16 (s, 1H), 12.09 (s, 1H).
3-NO2
3-NH2
H
O
4-NO2
5-NO2
4-NH2
5-NH2
H
O
4g
4h
4i
O
O
S
LC-MS: (M+1: 298.3, 95.62%).

J Fluoresc (2012) 22:311–322
315
Fig. 1 Excited state proton
transfer reaction pathways
in ethanol as shown in Scheme 1 and preparative details are
presented in Table 2.
the basicity of = N- group on the azole ring is enhanced by
the presence of N,N-diethyl amino group on the phenyl ring
containing -OH group. Quantum yield of 2-(1H-benzimidazol-
2-yl)-5-(N,N-diethylamino) phenol 4a is (0.105) high com-
pared to the compound where -NH2 group is present in place
of hydroxyl group (0.027) in ethanol. Synthesized com-
pounds 4a–4i show single absorption and dual emission with
large Stokes shift. Single absorption and dual emission are
due to ESIPT phenomenon as shown in Fig. 1.
Synthesized benzimidazole, benzothiazole and benzox-
azole molecules which contain acidic hydroxy group at 2′-
position and N,N′- diethyl group at 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
strongly basic and -OH as well as p-N N′-diethyl group
becomes strongly acidic. This leads to the excited state
intra-molecular proton transfer (ESIPT) and thus the
formation of keto isomer (k1).
UV–Vis Absorption-Emission Analysis
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 [57]. 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
Synthesised novel benzimidazole, benzoxazole and benzo-
thiazole 4a–4i compounds are fluorescent in solution as
well as in the solid state when irradiated with UV light.
Compounds 4e and 4i show solid state fluorescence and the
other compounds show fluorescence in solution. As against
the known compounds devoid of N,N-diethylamino group
[58] the synthesized new compounds 4a–4i show high
fluorescence emission intensity with high Stokes shift value.
This property attributed due to the intra-molecular proton
Fig. 3 Emission spectra of compound 4a–4i in acetonitrile
Fig. 2 Absorption spectra of compounds 4a–4i in acetonitrile

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J Fluoresc (2012) 22:311–322
Table 3 Absorption-Emission with molar extinction coefficient at
1×10−6 M concentration of compounds 4a–4i in acetonitrile
Compound lmax absorbance in lmax emission in Stokes shift
nm (intensity)
nm (intensity)
Fig. 4 Resonating effect of p-N,N-diethyl group which extend the
conjugation
4a
4b
4c
4d
4e
4f
218 (0.531)
296(0.597)
396(0.032)
404(0.189)
480(0.113)
352(0.461)
526(0.903)
412(0.298)
482(0.257)
310(0.298)
482(0.257)
316(0.189)
374(0.461)
434(1.180)
532(0.946)
388(0.455)
542(1.643)
398(1.146)
526(0.800)
078
178
071
147
106
280
088
158
076
248
052
110
152
250
055
209
134
262
333(0.452)
246(0.364)
324(0.444)
234(0.368)
264(0.550)
282(0.366)
333(0.056)
264(0.056)
4a is (0.105) higher compared to the parallel compound
devoid of N,N-diethylamino group (0.005) in ethanol.
The absorption and fluorescence emission spectra of
these compounds are shown in the Figs. 2 and 3
respectively. All analyses were performed at room temper-
ature in acetonitrile as a solvent and the compounds
concentration are 1×10−6 M. Table 3 shows the absorption
and emission values and the corresponding Stokes shifts for
the compound 4a–4i. The Stokes shift values of new
compounds 4a–4i 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 [60]. In contrast, novel
compounds 4a–4i that exhibit ESIPT phenomenon with
Stokes shift in between 50 and 280 nm. The occurrence of
the dual emission of synthesised compound is clearly
reflected in emission spectrum. The spectrum consists of
two emission maxima ranging from 302 to 546 nm.
4g
4h
4i
lmax and lem were measured in nm
Samples were prepared in acetonitrile
Analyses were carried out at room temperature
transfer reaction as well as basic p-N,N′-diethyl group present
in the system. The quantum yield of compound 4a which
contains a N,N-diethylamino group has been compared with
a parallel compound devoid of N,N-diethylamino group,
which has been synthesized by our group [59]. Quantum yield
of 2-(1H-benzimidazol-2-yl)-5-(N,N-diethylamino) phenol
UV-Visible Absorption and Fluorescence Emission
of Compound 4a and 2-(1H-benzimidazol-2-yl) phenol
in Different Solvent (Solvatochromism)
To evaluate the effect of solvent polarity on absorption-
emission properties of synthesized compounds absorption,
Table 4 Effect of solvent polarity
on photo-physical properties of
compound 4a
Solvent
DCM
labs nm (Intensity)
lem nm (Intensity)
Stokes Shift
Quantum Yield
0.1009
213(1.324)
282 (0.657)
370 (0.093)
69
157
1,4 – Dioxane
Acetonitrile
Ethyl acetate
Ethanol
215(1.287)
218(1.531)
218(1.439)
222(0.989)
224(1.568)
229(1.893)
232(2,173)
287 (0.445)
378 (0.234)
72
163
0.0930
0.094
296 (0.597)
396 (0.032)
78
178
310 (0.632)
405 (0.043)
108
187
0.122
312 (0.567)
417 (0.012)
90
195
0.1050
0.1164
0.0789
0.1323
Methanol
DMF
321(1.214)
423 (0.037)
97
199
328 (0.098)
429 (0.056)
99
200
DCM Dichloromethane, DMF
Dimethyforamide, DMSO
Dimethyl sulphoxide
DMSO
333 (0.076)
435 (0.087)
101
203

J Fluoresc (2012) 22:311–322
317
Effect of Viscosity on fluorescence emission
Table 5 Effect of solvent polarity on photo-physical properties of
2-(1H-benzimidazol-2-yl) phenol
250
210
170
130
90
4a
4b
4c
4d
4e
4f
Solvent
labs nm
lem nm
Stokes shift
(intensity)
(intensity)
DCM
237 (0.087)
243 (0.543)
243 (0.091)
246 (0.212)
246 (0.431)
243 (0.068)
246 (0.302)
249 (0.503)
272 (0.145)
320 (0.10)
35
73
44
69
58
75
33
82
42
76
47
87
50
75
52
84
4g
4h
4i
1,4 – Dioxane
Acetonitrile
Ethyl acetate
Ethanol
287 (0.149)
312 (0.094)
301 (0.323)
318 (0.037)
279 (0.152)
328 (0.056)
278 (0.097)
322 (0.034)
290 (0.114)
330 (0.032)
296 (0.078)
321 (0.034)
301 (0.260)
333 (0.100)
50
0
2
4
6
8
10
12
% of Glycerin in Acetonitrile
Fig. 6 Effect of viscosity on fluorescence emission of compound 4a–4i
spectrum of 4a is affected by a change in polarity and
hydrogen bonding capacity of the solvent. The absorption-
emission spectrum of compound 4a is slightly red shifted in
polar solvents relative to that of non-polar solvent. Red
shift increases as the polarity of the solvent increases. In
dichloromethane and 1,4-dioxane the absorption—emission
shifted towards the blue region and in DMF and DMSO red
shift is observed as compared to acetonitrile. Effect of
solvent polarity on quantum yield of 4a was also studied in
different solvent. Quantum yield results show that quantum
yield is sensitive towords the polarities of solvent and
results are summarized in Table 4. A previous investigation
of unsubstituted benzimidazole, benzoxazole and benzo-
thiazole derivatives has suggested that the long wavelength
Methanol
DMF
DMSO
DCM Dichloromethane, DMF Dimethyforamide, DMSO Dimethyl
sulphoxide
emission properties of compound 4a is studied in eight
different solvents of varying polarity and hydrogen bonding
capability as shown in Table 4. The absorption-emission
Fig. 5 a Effect of different
solvent polarity on absorption
of compound 4a (Day light).
b Effect of different solvent
polarity on emission of
compound 4a (UV- light)

318
J Fluoresc (2012) 22:311–322
Table 6 Thermal Gravimetric
Analysis of Compound 4a–4i
compound is far less than compound 4a, this clearly
indicates that p-N,N-diethyl group plays an important role
to enhance the fluorescence properties due to the delocal-
isation of lone pair of electrons present on nitrogen.
Absorption and emission properties of 2-(1H-benzimidazol-
2-yl) phenol are not sensitive towards the polarity of solvents
as shown in Table 5, which may be due to absence of
electron donating group on phenyl ring.
In polar solvent observed Stokes shift of 4a is higher as
compared to non polar solvent, the difference in Stokes
shift between polar and nonpolar solvent is significant. But
in case of 2-(1H-benzimidazol-2-yl) phenol devoid of N,N-
diethylamino group such a definite trend was not observed
from nonpolar to polar solvent. The absorption and
emission maxima with Stokes shift of the compound 4a in
different solvents are summarised in Table 4 and for 2-(1H-
benzimidazol-2-yl) phenol in Table 5. The changes in
fluorescence intensity with change in solvent polarity of
compound 4a is as shown in Fig. 5a and b.
Compound
TGA
4a
4b
4c
4d
4e
4f
158 (95.97%)
155 (98.94%)
149 (99.22%)
195 (100.0%)
208 (99.11%)
226 (99.98%)
273 (98.41%)
186 (95.04%)
226 (99.59%)
4g
4h
4i
a TGA measured in °C
transition is localised on the phenyl ring and is π-π* in
nature. The moment corresponding to this transition is
polarised along the axis, since band shape for compound 4a
are almost similar to those of unsubstituted benzimidazole,
benzoxazole and benzothiazole except for the red shift. The
red shift in the absorption-emission characteristic of novel
compound 4a is explained by the resonance effect of the
p-N,N′-diethyl group which extends the conjugation
leading to structure II (Fig. 4).
Effect of Viscosity on Fluorescence Emission
Structure II is more rigid than structure I and being polar
is more stable in polar solvent, As a result the fluorescence
emission spectrum of compound 4a shows red shift in polar
solvents, and this is due to conjugation of electron donor N,
N-diethyl group with electron acceptor imidazole ring
Table 4. In case of 2-(1H-benzimidazol-2-yl) phenol devoid
of N,N-diethylamino group at para position, absorption and
emission characteristics as a function of polarity of solvent
were also studied in different nonpolar and polar solvents.
2-(1H-Benzimidazol-2-yl) phenol shows single absorption
and dual emission due to acidic -OH group at 2-position
with respect imidazole ring. Observed stokes shift of this
In order to understand the sensitivity of fluorescence of
compounds 4a–4i to the microenvironment, particularly
viscosity fluorescence measurements were carried out
under varying viscous environments with added glycerin.
Fluorescence emission of compound 4a–4i before and
after addition of glycerine were recorded at different
concentration of glycerine (2, 4, 6, 8, and 10%), results
reveals that the compounds 4a–4i are sensitive towards the
viscosity. Fluorescence intensity of compounds 4a–4i
increases with increase in percentage of glycerine. Emission
intensity against% of glycerine in acetonitrile is shown in
Fig. 6.
Fig. 7 TGA Plots of
compounds 4a–4i
120
100
80
60
40
20
0
– – – – 4a
–– –– – 4b
– – – – 4c
––– – – 4d
––––– – 4e
––– ––– 4f
––––––– 4g
– – – – 4h
––––– · 4i
-20
0
100
200
300
400
500
600
Universal V4.2E TA Instruments
Temperature (°C)

J Fluoresc (2012) 22:311–322
319
Fig. 8 a Optimized structural
model of compound 4a
(Enol form). b Optimized
structural model of compound
4a (Keto form)
Thermal Stability
that the thermal stability of the 4a–4i decreases in the order
4g>4f>4i>4e>4d>4h>4a>4b>4c Table 6. Above 149 °C,
the thermo gravimetric curve of 4a–4i shows continuous
significant weight loss. TGA analysis curves of compound
4a–4i are shown in Fig. 7.
In order to give more insight into the compounds 4a–4i the
thermal studies have been carried out using thermogravi-
metric techniques (TGA). The thermogravimetric analyses
have been carried out in the temperature range 50–600 °C
under nitrogen. The TGA results in Table 6 indicates that
the frame work of the synthesized compound is stable up to
149 °C. Above 149 °C the thermo gravimetric curve of the
synthesized compounds show a major loss in weight. The
comparisons of the Td (decomposition temperature) showed
Structural Properties of Compound 4a
The structural changes Fig. 8 (8a and 8b) due to ESIPT
phenomenon in terms of bond angle, bond distances and
geometry are investigated by using Gaussian 03 software
Table 7 Structural properties
of compound 4a
Properties
4a (Enol)
4a (Keto)
Stoichiometry
Framework Group
Point Group
C17H19N3O
C1(×(C17H19N3O)
C1
C17H19N3O
C1(×(C17H19N3O)
C1
Bond Distances
Bond
Bond Length
1.3444
Bond
Bond Length
1.2755
R(15–21)
Angle
R(15–21)
Angle
Bond Angle
Bond Angle
122.443
Bond Angle
122.301
A(10,15,21)
A(14,15,21)
A(10,15,21)
A(14,15,21)
Bond angle in degree,
Bond length in °A Unit
117.145
121.390

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J Fluoresc (2012) 22:311–322
package as shown in Table 7. It clearly indicates that due to
the intra-molecular hydrogen bonding the molecule has a
six-member ring conformation in excited state. The main
feature of the molecular structures like stiochiometry,
framework group, degree of freedom and point group of
compound 4a are same in both enol and keto forms which
are shown in Table 7, while bond length [REnol (15–21);
1.344)] and [RKeto (15–21); 1.275] and bond angle [AEnol
(10-15-21); 122.443 and AEnol (14-15-21);117.145] and
[AKeto (10-15-21); 122.301 and AKeto (14-15-
21);121.390] differ from each other in enol and keto form
respectively. Compound 4a is roughly planar in enol form
with dihedral angle 0.0139° between N9-C8-C10-C15 and
facilitate excited state intra-molecular hydrogen transfer.
In enol form the bond C15-O21 is a single bond having
bond length 1.3442°A, while in keto form double bond
character of C15-O21 bond increases and bond length
decrease bond length 1.2755°A.
(iii) Synthesized compounds exist in to two form keto (K)
and enol (E) form. Compounds are planar in enol
form and non-planar in keto form.
(iv) Synthesised compounds have good thermal properties.
(v) Existences of Excited States Intramolecular Proton
Transfer (ESIPT) were confirmed by Density Func-
tion Theory (DFT).
Acknowledgement The author is greatly thankful to I.I.T. Mumbai
1
for recording the H-NMR and Mass spectra.
References
1. Johnsson N (2007) Chemical tools for biomolecular imaging K.
ACS Chem Biol 2:31–38
2. Unciti-Broceta A, Díaz-Mochón J, Mizomoto H, Bradley M
(2008) pH sensing in living cells using fluorescent microspheres. J
Comb Chem 10:179–184
3. Domaille D, Que E, Chang C (2008) Synthetic fluorescent sensors
for studying the cell biology of metals. J Nat Chem Biol 4:168–
175
Density Function Theory Calculation
4. Giepmans B, Adams S, Ellisman M, Tsien R (2006) The
fluorescent toolbox for assessing protein location and function.
Science 312:217–224
5. Weissleder R, Ntziachristos V (2003) Shedding light onto live
molecular targets. Nat Med 9:123–128
6. Zhang J, Campbell R, Ting A, Tsien R (2002) Creating new
fluorescent probes for cell biology. Nat Rev Mol Cell Biol 3:906–
918
7. Diaz-Mochón J, Tourniaire G, Bradley M (2007) Microarray
platforms for enzymatic and cell-based assays. Chem Soc Rev
36:449–457
8. Pernagallo S, Unciti-Broceta A, Diaz-Mochón J, Bradley M
(2008) Deciphering cellular morphology and biocompatibility
using polymer microarrays. Biomed Mater 3:34112–34118
9. Unciti-Broceta A, Diezmann F, Ou-Yang C, Fara M, Bradley M
(2009) Synthesis, penetrability and intracellular targeting of
fluorescein-tagged peptides and peptide–peptoid hybrids. Bioorg
Med Chem 17:959–966
10. Lavis L, Raines R (2008) Bright ideas for chemical biology. ACS
Chem Biol 3:142–155
11. Hell S (2003) Toward fluorescence nanoscopy. Nat Biotechnol
11:1347–1355
Density functional theory calculations were performed to
optimise the geometry of compound 4a. Ground state
geometry was optimised with B3LYP level of theory and 6–
31/G (d) basic set. Excited state geometry was optimised by
time dependent-DFT with B3LYP level and 6–31/G (d)
basic set. It is observed that the enol form of 4a is stabilised
by 79.40 Kcal/mole than the ground state keto form and so
it exists only in enol form (supported by UV-Visible
spectrum with single absorption and dual emission).
The dihedral angle of almost zero between two rings
N9-C8-C10-C15: 0.013 and the distance between N9-H40
(1.715) at ground state suggests a strong hydrogen bonding.
As enol form goes to excited state the change in dipole
moment has been observed from 6.975 to 6.737 and
negative charge on O21 decreases from −0.69 to −0.68,
whereas negative charge on N9 is −0.72 to −0.71. The
structure remains planar in excited state with N9-H40
distance of 1.732 this distance suggests a strong H-bonding.
These conditions facilitate the ESIPT phenomenon.
12. Weissleder R (2006) Molecular imaging in cancer. Science
312:1168–1171
13. Alexander L, Dhaliwal K, Simpson J, Bradley M (2008) Dunking
doughnuts into cells—selective cellular translocation and in vivo
analysis of polymeric micro-doughnuts. Chem Commun 3507–
3509
Conclusion
14. Peng X, Wu Y, Fan J, Maozhong T, Keli H (2005) Colorimetric
and ratiometric fluorescence sensing of fluoride: tuning selectivity
in proton transfer. J Org Chem 70(25):10524–10531
15. Shi, Lim J, Ha T (2010) Acidification of the oxygen scavenging
system in single-molecule fluorescence studies: in situ sensing with a
ratiometric dual-emission probe. Anal Chem 82(14):6132–6138
16. Kasha M (1986) Proton-transfer spectroscopy, Perturbation of the
tautomerization potential. J Chem Soc Faraday Trans II 82:2379–
2392
The above study proves that,
(i) Synthesized new compounds shows single absorption
and dual emission due to excited state intra-molecular
proton transfer.
(ii) Fluorescence properties of the synthesized compound
depend on solvent polarity, in non-polar solvent it
absorbs and emits in blue region and polar solvent it
shows red shift.
17. Arnaut L, Formoshinho S (1993) Excited-state proton transfer
reactions I. Fundamentals and intermolecular reactions. J Photo-
chem Photobiol A: Chem 75:1–20

J Fluoresc (2012) 22:311–322
321
18. Formoshinho S, Arnaut LG, Formoshinho S, Arnaut LG (1993)
Excited-state proton transfer reactions II. Intramolecular reactions.
J Photochem Photobiol A: Chem 75:21–48
19. Douhal A, Lahmani F, Zewil A (1996) Proton-transfer reaction
dynamics. Chem Phys 207:477–498
hydroxyphenyl)benzimidazole and benzoxazole: Effect of rota-
merism and hydrogen bonding. J Phys Chem 98:9126–9132
38. Kelly R, Schulman S, Schulman S (1988) Molecular luminisons
spectroscopy, methods and applications Part −2, Willy Interscience
New York, Chapter 6
20. Herek J, Pederson S, Banares L, Zewil A (1992) Fluorescence
anisotropy controlled by light quenching. J Phys Chem 97:9046–
9061
39. Ghiggino K, Scully A, Leaver I (1986) Effect of solvent on
excited-state intramolecular proton transfer in benzotriazole
photostabilizers. J Phys Chem 90:5089–5093
21. Mosquera M, Rodriguez M, Rodriguez-Prieto F (1997) Physico-
chemical principles of the creation of highly efficient organic
luminophores with anomalously high Stokes’ shifts. J Phys Chem
A 101:2766–2772
22. Krieg R, Eitner A, Guenther W, Schuerer C, Lindenau J,
Halbhuber K (2008) N,N-dialkylaminostyryl dyes: specific and
highly fluorescent substrates of peroxides and their application in
histochemistry. J J Mol Histol 39(2):169–191
23. Cheng YM, Pu SC, Hsu CJ, Lai CH, Pi T (2006) Femtosecond
dynamics on 2-(2′-hydroxy-4′-diethylaminophenyl) benzothiazole:
solvent polarity in the excited state proton transfer. Chem Phys
Chem 7(6):1372–1381
24. Jangwon S, Kim S, Park S, Soo Y (2005) Tailoring the excited-
state intramolecular proton transfer (ESIPT) fluorescence of 2-(2′-
hydroxyphenyl) benzoxazole derivatives. Bull Korean Chem Soc
26(11):1706–1710
40. Kosower E, Huppert D (1986) Excited state electron and proton
transfers. Ann Rev Phys Chem 37:127–156
41. Law K, Shoham J (1995) Photoinduced proton transfers in 3,5-di-
tert-butylsalicylic acid. J Phys Chem 99:12103–12108
42. Goodman J, Brus L (1978) Proton transfer and tautomerism in an
excited state of methyl salicylate. J Am Chem Soc 100:7472–
7474
43. Herek J, Pedersen S, Banares L, Zewali A (1992) Femtosecond
real-time probing of reactions. IX. Hydrogen-atom transfer. J
Chem Phys 97:9046–9061
44. Formosinho S, Arnaut J (1993) Electrically generated intramolecular
proton transfer: electroluminescence and stimulated emission from
polymers. J Photochem Photobiol A Chem 75:21–48
45. Segala M, Dorirngues N, Livotto V, Stefani
V (1999)
Heterocyclic dyes displaying Excited-State Intramolecular
Proton-Transfer reactions (ESIPT): computational study of the
substitution effect on the electronic absorption spectra of 2-(2′-
hydroxyphenyl)benzoxazole derivatives. J Chem Soc Perkin
Trans-2 2:1123–1128
25. Vazquez SR, Rodriguez MCR, Mosquera M (2008) Rotamerism,
tautomerism, and excited-state intramolecular proton transfer
in 2-(4′-N, N-diethylamino-2′- hydroxyphenyl)benzimidazoles:
Novel benzimidazoles undergoing excited-state intramolecular
coupled proton and charge transfer, et al. J Phys Chem A 112
(3):376–387
46. Elguero J, Katriezky R, Denisko O (2000) Prototropic tautomerism
of heterocycles: heteroaromatic tautomerism - General overview and
methodology. Adv Heterocycl Chem 76:1–84
26. Barbara P, Walsh P, Brus L (1989) Picosecond kinetic and
vibrationally resolved spectroscopic studies of intramolecular
excited-state hydrogen atom transfer. J Phys Chem 93:29–34
27. Catalan J, Fabero F, Claramunt R, Santa M, Foces-Foces M, Cano
F (1992) Chem Inform New UV stabilizers: 3- and 5-(2′-
Hydroxyphenyl) pyrazoles. J Am Chem Soc 114:5039–5048
28. Stueber G, Kieninger M, Schetter H, Busch W, Goella B, Franke
J, Kramer H, Horer H, Henele S (1995) Deactivation processes of
polymeric and monomeric UV- absorbers of the benzotriazole and
triazine class. J Phys Chem 99:10097–10109
29. Chou P, McMorrow D, Aartsma T, Kasha M (1984) The proton-
transfer laser. Gain spectrum and amplification of spontaneous
emission of 3-hydroxyflavone. J Phys Chem 88:4596–4599
30. Chou P, Martinej M (1993) The observation of solvent-dependent
proton-transfer/charge-transfer lasers from 4′-diethylamino-3-
hydroxyflavone. J Chem Phys Lett 204:395–399
47. Arnaut L, Formosinho S (1993) Excited-state proton transfer
reactions. Fundamentals and intermolecular reactions. J Photo-
chem Photobiol A Chem 75:1–20
48. Formosinho S, Arnaut L (1993) Excited-state proton transfer
reactions II. Intramolecular reactions. J Photochem Photobiol A
Chem 75:21–48
49. Rios N, Rios M (1998) Ab initio study of the hydrogen bond and
proton transfer in 2-(2′-hydroxyphenyl) benzothiazole and 2-(2′-
Hydroxyphenyl) benzimidazole. J Phys Chem A 102:1560–1567
50. Vollmer F, Rettig W, Birckner E (1994) Switching between charge
and proton-transfer emission in the excited state of a substituted 3-
hydroxyflavone. J Fluorescence 4:65–72
51. Minkin V, Garnovski A, Elguero J, Katriezky A, Denisko O
(2000) The tautomerism of heterocycles: five-membered rings
with two or more heteroatoms. Adv Heterocycl Chem 76:157–323
52. Doroshenko A, Posokhov E, Verezubova A, Ptyagina L (2000)
Excited state intramolecular proton transfer reaction and
luminescent properties of the ortho-hydroxy derivatives of 2,5-
diphenyl-1,3,4-oxadiazole. J Phys Org Chem 13:253–265
53. Demchenko A (2009) Introduction to fluorescence sensing,
SpringeScience+Business Media B.V.
31. Chou P, Martinej M, Clements J (1993) Reversal of excitation
behavior of proton-transfer vs. charge-transfer by dielectric
perturbation of electronic manifolds. J Phys Chem 97:2618–2622
32. Durr H, Bouas LH (1990) Photochromism moleules and system
elsevier, Amsterdam
33. Sinha H, Dogra S (1986) Ground and excited state prototropic
reactions in 2-(o-hydroxyphenyl) benzimidazole. Chem Phys
102:337–347
34. Das K, Mjumder D, Bhattacharyya K (1992) Excited-state
intramolecular proton transfer and rotamerism of 2-(2′-
hydroxyphenyl) benzimidazole. Chem Phys Lett 198:443–448
35. Douhal A, Amat-Guerri F, Lillo M, Acuna A (1994) Proton
transfer spectroscopy of 2-(2′-hydroxyphenyl) imidazole and 2-
(2′-hydroxyphenyl) benzimidazole dyes. J Photochem Photobiol
A: Chem 78:127–138
36. Rios M (1995) Initio study of ground and excited state proton
transfer in 2-(2′-hydroxyphenyl) benzoxazole. J Phys Chem
99:12456–12460
37. Das K, Sarkar N, Ghosh A, Majumdar D, Nath D, Bhattacharya K
(1994) Excited-State Intramolecular Proton Transfer in 2-(2-
54. Frisch MJ, Trucks GW, Schlegel HB, Scuseria GE, Robb MA,
Cheeseman JR, Montgomery JA, Vreven T Jr, Kudin KN, Burant
JC, Millam JM, Iyengar SS, Tomasi J, Barone V, Mennucci B,
Cossi M, Scalmani G, Rega N, Petersson GA, Nakatsuji H, Hada
M, Ehara M, Toyota K, Fukuda R, Hasegawa J, Ishida M,
Nakajima T, Honda Y, Kitao O, Nakai H, Klene M, Li X, Knox
JE, Hratchian HP, Cross JB, Bakken V, Adamo C, Jaramillo J,
Gomperts R, Stratmann RE, Yazyev O, Austin AJ, Cammi R,
Pomelli C, Ochterski JW, Ayala PY, Morokuma K, Voth GA,
Salvador P, Dannenberg JJ, Zakrzewski VG, Dapprich S, Daniels
AD, Strain MC, Farkas O, Malick DK, Rabuck AD, Raghavachari
K, Foresman JB, Ortiz JV, Cui Q, Baboul AG, Clifford S,
Cioslowski J, Stefanov BB, Liu G, Liashenko A, Piskorz P,
Komaromi I, Martin RL, Fox DJ, Keith T, Al-Laham MA, Peng
CY, Nanayakkara A, Challacombe M, Gill PMW, Johnson B,

322
J Fluoresc (2012) 22:311–322
Chen W, Wong MW, Gonzalez C, Pople JA (2004) Gaussian, Inc.,
Wallingford CT
isothiocyanates as new fluorescent probes for proteins. J
Photochem Photobio A: Chem 149:217–225
55. Williams ATR, Winfield SA, Miller JN (1983) Relative fluores-
cence quantum yields sing a computer controlled luminescence
spectrometer. Analyst 108:1067–1071
59. Padalkar V, Patil V, Phatangare K, Gupta V, Umape P, Sekar N
(2011) Indion 190 resin: efficient, environmentally friendly, and
reusable catalyst for synthesis of benzimidazoles, benzoxazoles,
and benzothiazoles. Green Chem Lett Rev. (doi:10.1080/
17518253.2011.585666, Article in Press)
60. Dey J, Dogra S (1991) Solvatochromism and phototropism of 2-
(3′-hydroxyphenyl) benzoxazole and 2-(4′-hydroxyphenyl)
benzoxazole in the excited singlet state J. Photochem Photobio
A: Chem 59:307–321
56. British patent 914347, 1963
57. Sandra S, Dogra S (1999) Excited state intramolecular protons
transfer in 2-(2′-N-palmitoyl-aminophenyl) benzimidazole: effect
of carbonyl group. J Mol Struct 476:223–233
58. Holler M, Campo L, Brandelli A, Stefani V (2002) Synthesis and
spectroscopic characterization of 2-(2′-hydroxyphenyl) benzazole