Synthesis, spectral studies and solvatochromism of some novel benzimidazole derivatives-ESIPT process

Article abstract of DOI:10.1016/j.saa.2012.12.037

Some novel benzimidazole derivatives were synthesized and characterized by ¹H, ¹³C NMR mass and elemental analysis. XRD analysis was carried out for 1-(4-methylbenzyl)-2-p-tolyl-1H-benzo[d]imidazole. The solvent effect on the absorpt

Full text of DOI:10.1016/j.saa.2012.12.037

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 105 (2013) 223–228
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Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Synthesis, spectral studies and solvatochromism of some novel benzimidazole
derivatives – ESIPT process
⇑
J. Jayabharathi , V. Thanikachalam, K. Jayamoorthy, N. Srinivasan
Department of Chemistry, Annamalai University, Annamalainagar 608 002, Tamil Nadu, India
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
"
"
"
"
Absorption and emission spectral
studies have been carried out.
HOMO–LUMO – probable charge
transfer taking place.
Formation of the keto-isomer due to
ESIPT.
Competition of intra and
intermolecular hydrogen bonding.
a r t i c l e i n f o
a b s t r a c t
Some novel benzimidazole derivatives were synthesized and characterized by H, 13C NMR mass and ele-
mental analysis. XRD analysis was carried out for 1-(4-methylbenzyl)-2-p-tolyl-1H-benzo[d]imidazole.
The solvent effect on the absorption and fluorescence bands has been analyzed. The energetic analysis
of the potential energy surface (PES), HOMO and LUMO levels [DFT/B3LYP/6-31G(d,p)] evidenced the
existence of excited state intramolecular proton transfer (ESIPT) in hydroxy benzimidazole derivative.
Ó 2012 Elsevier B.V. All rights reserved.
1
Article history:
Received 12 October 2012
Received in revised form 2 November 2012
Accepted 11 December 2012
Available online 20 December 2012
Keywords:
Photophysical study
DFT
PES
XRD
ESIPT
Introduction
pertensive, antiviral, antifungal, antioxidants and antihistamines
among others [4–8]. They are important intermediates in many or-
Heterocyclic imidazole derivatives have attracted considerable
attention because of their unique optical properties [1] and used
for preparing functionalized materials [2]. Benzimidazole based
chromophores have received increasing attention due to their dis-
tinctive linear, non-linear optical properties and also due to their
excellent thermal stability in guest–host systems [3]. Functional-
ized benzimidazoles represent an important class of N-containing
heterocyclic compounds and have received considerable attention
in recent times because of their applications as antiulcer, antihy-
ganic reactions [9,10] and act as ligands to transition metals for
modeling biological systems [11,12].
Excited state intramolecular proton transfer (ESIPT) has become
a field of active research [13–17] due to the practical applications
of ESIPT exhibiting molecules as laser dyes [18], photo stabilizers
[19], fluorescent probes in biology [20] and light-emitting materi-
als for electroluminescent devices [21–23]. ESIPT occurs in aro-
matic molecules having a phenolic hydroxy group with an
intramolecular hydrogen bond to the nearby hetero atom of the
same chromophore. Mechanism of ESIPT process is currently being
used to understand the photophysics of some molecules that show
such interesting characteristics as ultraviolet stabilization [24],
⇑
Corresponding author. Tel.: +91 9443940735.
E-mail address: jtchalam2005@yahoo.co.in (J. Jayabharathi).
1
386-1425/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2012.12.037

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J. Jayabharathi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 105 (2013) 223–228
stimulated radiation production [25], information storage [26],
fluorescent solar concentrators [27] and environmental probes in
biomolecules [28] and organized assemblies [29]. ESIPT process
in electronically excited singlet states can be easily recognized by
the appearance of a second band to the red of the normal band
in the fluorescence spectrum. Normal band is due to emission from
S1 state of the molecule in the form which is thermodynamically
most stable in S0 state and second band is due to emission from
tautomer, formed by ESIPT. In this paper we have reported
synthesis, characterization, solvatochromism and ESIPT process
of some benzimidazole derivatives.
Experimental
Spectral measurements
The ultraviolet–visible (UV–vis) spectra of the benzimidazole
derivatives were measured in an UV–vis spectrophotometer (Per-
kin Elmer Lambda 35) and corrected for background absorption
due to solvent. Photoluminescence (PL) spectra were recorded on
a (Perkin Elmer LS55) fluorescence spectrometer. NMR spectra
were recorded on Bruker 400 MHz NMR spectrometer. Mass spec-
tra were recorded on a Varian Saturn 2200 GCMS spectrometer.
General procedure for the synthesis of benzimidazole derivatives
A mixture of corresponding aldehyde (2 mmol), o-phenylenedi-
amine (1 mmol) and ammonium acetate (2.5 mmol) has been re-
fluxed at 80 °C in ethanol. The reaction was monitored by TLC
and purified by column chromatography using petroleum ether:-
ethyl acetate (9:1) as the eluent. The chemical structures of all
benzimidazoles have been provided in Fig. 1.
Results and discussion
Fig. 1. Chemical structures of benzimidazole derivatives 1–6.
Absorption and emission spectra of benzimidazole derivatives
peak at 346.8 nm. Excitation of isomer I lead to the formation of
keto-isomer II (Fig. S1) due to ESIPT (Fig. 4).
Figs. 2 and 3 show the absorption spectrum and emission spec-
tra of benzimidazole derivatives (1–6) respectively (Table 1). The
bathochromic shifts were observed in the fluorescence band of
the benzimidazole derivatives with increasing the solvent polarity.
Because the distortion of the geometry in the excited state implies
a decrease in the resonance energy, the fluorescence band is batho-
chromically shifted to a higher extent. Moreover, the loss of planar-
ity in the excited state of the benzimidazole derivative could
explain the lower fluorescence quantum yield in apolar solvents
owing to an increase in the non-radiative processes. However,
the shape of the absorption band is independent of the imidazole
concentration suggesting the poor aggregation and it is a suitable
behavior in the performance of active media of lasers. Indeed, high
concentration is necessary to bring about laser action, and the
presence of aggregates could drastically reduce the fluorescence
quantum yield owing to the efficient quenching of the monomer
fluorescence by the aggregates. However, experimental data indi-
cate that the fluorescence band is shifted to lower energies by
increasing the concentration. This bathochromic shift is attributed
to reabsorption and reemission phenomena [30] and this result
corroborates the importance of registering photophysical proper-
ties in dilute solutions.
However in hydroxylic solvent (EtOH), a short wavelength
emission band appears for 4 which is absent in the fluorescence
spectra of 1, 2, 3, 5 and 6 (Fig. 3). This result corresponds to the re-
sults obtained earlier for compounds demonstrating ESIPT [31,32].
Absence of dual emission in these compounds explained by the
presence of intermolecular hydrogen bonding with solvent mole-
cule leading to the stabilization of solvated isomer III in which
ESIPT is impossible.
Existence of ESIPT mechanism in hydroxy benzimidazole deriv-
ative (4) was supported by DFT calculation. The electron density of
keto and enol isomers of 4 in the ground and excited states calcu-
lated by B3LYP/6-31G(d,p) method was tabulated (Table 2). Excita-
tion of enol isomer leads to increase the electron density at N(15)
nitrogen atom and decreases the same at oxygen atom resulting in
ESIPT. As a consequence of ESIPT, excited keto isomer is formed.
The excited keto isomer emits luminescence and returns to ground
state keto form (III). This is characterized by a large positive charge
at N(15) nitrogen atom and negative charge at oxygen atom. As a
result, a reverse process occurs in ground state of the molecule
producing enol form.
ESIPT process
Driving force for ESIPT process
The fluorescence spectra of 4 in dioxane display an abnormal
The existence of intramolecular hydrogen bond in hydroxy
Stokes-shifted emission band at 329.3 nm and one small shoulder
benzimidazole 4 is confirmed by the presence of the singlet at

J. Jayabharathi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 105 (2013) 223–228
225
Fig. 2. Absorption spectra of the benzimidazole derivatives 1–6.
Fig. 3. Emission spectra of the benzimidazole derivatives 1–6.
1
2.97 ppm in the 1H NMR which is a typical signal for hydrogen
The emission peak at shorter wavelength (329.3 nm) is assigned to
rotamer I and that at the longer wavelength (346.8 nm) is assigned
to rotamer II. Whereas compound 6 exhibits emission only at
342.1 nm. The absence of additional peak confirms absence of
intramolecular hydrogen bond in 6. It is further evident that intra-
molecular hydrogen bonding is the driving force for ESIPT and dual
fluorescence behavior of hydroxy benzimidazole derivative 4.
bonded hydrogen atom. In order to reveal the contribution of the
intramolecular hydrogen bonding in hydroxy benzimidazole 4 to
their optical properties, the fluorescence spectra of 4 and its meth-
oxy derivative 6 in dioxane solvent under identical condition
(Fig. 5) have been measured. A dual fluorescence detected for 4
with emission peaks centered at 346.8 and 329.3 nm respectively.

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J. Jayabharathi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 105 (2013) 223–228
Table 1
Absorption and emission spectral data of 1–6 in various solvents.
Solvent
Absorptiona (l, nm)
Emissionb (l, nm)
1
2
3
4
5
6
1
2
3
4
5
6
Hexane
293.5
292.9
292.7
291.2
291.7
290
289.9
289.7
287.5
291.4
286.9
288.5
292
290.6
290
292.7
289
290.7
291.6
289.7
288.8
287.7
289.7
283.6
294.7
294.1
293.1
292
291.7
288.7
277.6
273
263.5
261.4
283.6
273.7
296.4
295.6
292.2
293.3
292.5
293.9
293.7
292.5
292.6
291.4
289.6
288.5
295.7
294.7
293.7
294.1
293
292.7
292.7
290.5
291.6
290.6
288.6
287.5
297.7
296.7
294.7
296.1
293.7
295.7
294.1
293.7
292.7
291.7
289.7
289.1
323
335.2
339.4
341.4
343.1
346.6
346
347.9
353.1
354.6
356.2
354.8
357.1
333.5
337.9
346.6
353.5
360.1
365.7
373.7
381.6
385.8
387.5
391.8
393.8
328.6(sh), 345.5
329.3(sh), 346.8
329.7(sh), 347.5
330.6(sh), 348.4
331.7(sh), 349.9
347.7
351.6
351.7
353.4
355.3
340.6
339.3
346.7
334.6
360.7
358.9
348.6
351.3
353.4
357.7
359.7
363.7
344.6
342.1
346.9
334.2
364.9
347.7
351.6
354.4
359.8
361.1
363.7
366.6
1
,4-Dioxane
333.1
338.1
339.5
358.9
357.6
361.7
363.2
365.5
364.6
370.9
373
Benzene
Toluene
Chloroform
Ethyl acetate
Dichloromethane
DMSO
1
2
-Butanol
-Propanal
Methanol
Acetonitrile
359.7
362.6
a
À5
Solution concentration = 1 Â 10 M.
b
À8
Solution concentration = 1 Â 10 M.
Table 3
Relative energies and dipole moment of 4.
Rotamer
Ground state
(D)
Excited state
l
E (eV)
l
(D)
E (eV)
I
4.85
0.07
9.23
4.02
(
0.00)
(3.50)
II
4.57
0.08
9.02
4.54
(
0.03)
(4.01)
calculated using standard CIS method. Table 3 gives the energies
and dipole moment of the species I and II in the ground and the ex-
cited states.
Fig. 4. ESIPT mechanism of the benzimidazole derivative 4.
In the excited state the azomethine nitrogen atom becomes ri-
cher in electrons than the hydroxylic oxygen atom. The p-electron
densities in the excited state are the driving force for the intramo-
lecular proton transfer from the hydroxylic group to the azome-
thine nitrogen atom. The potential energy surface (PES) curves
Table 2
Electron density of atoms N(15) oxygen of 4.
IIa
Atom
II
III
(Fig. 6a and b) for the interconversion of isomers I and II of 4 in
N(15)
O(35)
À0.397
À0.572
À0.458
À0.529
0.315
À0.699
the ground state for isolated molecule is 4.6 kcal/mol and that in
ethanol is 3.7 kcal/mol. The corresponding value in the excited
state for the isolated molecule is 12.7 kcal/mol and that in ethanol
is 10.1 kcal/mol, respectively. The barrier for interconversion in the
excited state is much higher than that in the ground state.
a
Corresponds to excited state.
Competition of intra and intermolecular hydrogen bonding in binary
solvents
The competition of intra and intermolecular hydrogen bonding
in 4 has been studied in dioxane–water mixture. In pure dioxane,
hydroxy benzimidazole 4 exhibits both normal and tautomer emis-
sion at 329.3 and 346.8 nm respectively. The tautomer emission
intensity decreases relatively to that of the normal emission with
the addition of water. When water fraction is increased to 20%
(
v/v), the tautomer emission almost vanished. With further
increasing water fraction to 40% (v/v), a new emission band ap-
peared at 337.5 nm in addition to the normal emission. This band
intensity increased with increasing water fraction and turned to
the main emission when water fraction is 80% (v/v) (Fig. 7). It
shows that the tautomer emission decreases with increasing water
fraction in the mixed solvent if the intermolecular hydrogen bond-
ing between 4 and water is taken into account. In the initial stage,
presence of small amount of water in dioxane solution must give
rise to solvation of 4. The intermolecular hydrogen bonding
Fig. 5. Fluorescence spectra of the benzimidazole derivatives 4 and 6 in dioxane.
Evidence for ESIPT
In the present study, theoretical calculations have been used to
support ESIPT process [33–35]. The ground-state geometries of
three species, I and II of 4 have been optimized using DFT/B3LYP/
6
-31G(d,p) method. The energies of excited state have also been
between 4 and water definitely disrupts the ground state

J. Jayabharathi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 105 (2013) 223–228
227
Fig. 6. The potential energy surface curves for the interconversion of isomers I and II of the benzimidazole derivative 4 in the ground and excited states.
from O(35) to N(15). Analysis of electron density of HOMO for enol
tautomer predicts N(15) has slightly higher bonding character than
O(35). On the other hand, HOMO electron density of keto tautomer
shows greater bonding character of O(35) than N(15) and hence
transfer of proton from O(35) to N(15) in the ground state (ESIPT)
is quite impossible.
Crystal structure
Crystalline benzimidazole derivative (5) [37] are triclinic crys-
ꢀ
tal. It crystallizes in the space group P1. The cell dimensions are
a = 9.6610 Å, b = 10.2900 Å, and c = 17.7271 Å. ORTEP diagram of
(5) (Fig. S3) shows that the benzimidazole ring is essentially pla-
Fig. 7. Competition of intra and intermolecular hydrogen bonding of the benz-
imidazole derivative 4 in binary solvents.
nar. The dihedral angles between the planes of the benzimidazole
and the benzene rings of the 4-methylbenzyl and the p-tolyl
groups are 76.64(3)° and 46.87(4)°, respectively, in molecule A.
The corresponding values in molecule
B are 86.31(2)° and
intramolecular hydrogen bonding rotamers I and II but increases
the quantity of species III, in which ESIPT and tautomer formation
are inhibited. Consequently the tautomer emission decreases with
addition of water and finally vanished [36].
3
9.14(4)°. The dihedral angle between the planes of the two ben-
zene rings is 73.73(3)° and 80.69(4)° in molecules A and B, respec-
tively. Optimization of 5 have been performed by DFT at B3LYP/6-
3
1G(d,p) using Gaussian-03. All these XRD data are in good agree-
ment with the theoretical values (Table 4). However, from the the-
oretical values it can be found that most of the optimized bond
lengths, bond angles and dihedral angles are slightly higher than
that of XRD values. These deviations can be attributed to the fact
that the theoretical calculations were aimed at the isolated mole-
cule in the gaseous phase and the XRD results were aimed at the
molecule in the solid state.
HOMO–LUMO energies of benzimidazole derivative by DFT method
Analysis of the electron density of HOMO and LUMO (Fig. S2) of
4
can through some light on the ground and excited states proton
transfer processes. HOMO of enol form predicts that intramolecu-
lar hydrogen bonded (IMHB) ring system posses bonding character
over the O(35) H(39) and C(10) C(8) atoms, where as C(7) is anti-
bonding character. Both the hydroxyl oxygen [O(35)] and nitrogen
Conclusion
[
N(15)] have bonding character, with a larger electron density over
the hydroxyl oxygen(O(35)). Electron density of HOMO of keto tau-
tomer around IMHB ring shows antibonding character over the
H(36) O(37), O(3) C(5) and N(15) H(36) atoms and bonding charac-
ter over the C(10) C(8) C(7) atoms. Again comparison of HOMO of
enol form with HOMO of keto form shows much larger electron
density on O(35). In the later case HOMO electron density for both
the enol and keto form shows less electron density over the phenyl
ring.
By considering LUMO of enol and keto forms, LUMO of the enol
tautomer possess high electron density on O(35). After tautomer-
ization, LUMO of keto-tautomer still shows larger electron density
on N(15) and comparatively smaller density on O(35). Thus it fa-
vors the transfer of a proton from O(35) to N(15) in the excited
state (ESIPT). Again our calculation of electron density over the
proton transfer co-ordinate in the excited states (i.e., LUMO elec-
tron densities) show that there is a shift of p-electron distribution
Intermolecular hydrogen bonding of hydroxy benzimidazole
derivative with water giving rise to III inhibits the ESIPT process,
resulting in an increase in the quantum yield of normal emission
at the expense of the tautomer emission. Electron density and
HOMO–LUMO analysis supports the ESIPT process in hydroxy
benzimidazole derivative. Analysis of HOMO, LUMO
p-electron
density shows that the electron density on the hydroxyl oxygen
(O(35)) is quite high compare to the N(15) of the enol tautomer.
Again, after the transfer of proton along the proton transfer co-
ordinate shows the same oxygen atom, i.e., (O(35)) has higher elec-
tron density than N(15) in the keto form. This explained the non-
viability of ground state electron transfer. On the other hand, the
LUMO electron density shows a drift of electron cloud from
O(35) to N(15) along the proton transfer co-ordinate and explain-
ing the viability of ESIPT process. The potential energy surface
curves for the interconversion of isomers I and II of 4 shows the

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J. Jayabharathi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 105 (2013) 223–228
Table 4
Selected bond lengths (Å), bond angles (°) and torsional angles (°) of 5.
Bond lengths (Å)
Experimental XRD (Å)
Bond angles (°)
Experimental XRD (°)
Torsional angles (°)
Experimental XRD (°)
N1A–C2A
N1A–C8A
N1A–C1A
N3A–C2A
N3A–C9A
N1B–C2B
N1B–C8B
N1B–C1B
N3B–C2B
N3B–C9B
C1A–C11A
C2A–C21A
C7A–C8A
C8A–C9A
C14A–C17A
C24A–C27A
1.3782(1.4972)
1.3833(1.4584)
1.4537(1.4700)
1.3163(1.3671)
1.3881(1.4606)
1.3795(1.4972)
1.3863(1.4584)
1.4550(1.4700)
1.3174(1.3671)
1.3866(1.4606)
1.5135(1.5400)
1.4729(1.5400)
1.3921(1.3862)
1.3999(1.4763)
1.5089(1.5400)
1.5078(1.5400)
C2A–N1A–C8A
C2A–N1A–C1A
C8A–N1A–C1A
C2A–N3A–C9A
C2B–N1B–C8B
C2B–N1B–C1B
C8B–N1B–C1B
C2B–N3B–C9B
N1A–C1A–C11A
N3A–C2A–N1A
N3A–C2A–C21A
N1A–C2A–C21A
N1A–C8A–C3A
N1A–C8A–C9A
N3A–C9A–C4A
N3A–C9A–C8A
106.23(101.69)
128.33(113.71)
124.79(113.33)
104.77(105.49)
106.27(101.69)
129.32(113.71)
123.98(113.33)
105.12(105.41)
115.07(109.47)
113.25(113.53)
123.44(123.23)
123.30(123.24)
131.76(130.72)
103.49(108.25)
129.91(130.69)
110.26(108.25)
C2A–N1A–C1A–C11A
C8A–N1A–C1A–C11A
C9A–N3A–C2A–N1A
C9A–N3A–C2A–C21A
C8A–N1A–C2A–N3A
C1A–N1A–C2A–N3A
C8A–N1A–C2A–C21A
C1A–N1A–C2A–C21A
C9A–C4A–C5A–C6A
C2A–N1A–C8A–C7A
C1A–N1A–C8A–C7A
C2A–N1A–C8A–C9A
C1A–N1A–C8A–C9A
C6A–C7A–C8A–N1A
C2B–N1B–C1B–C11B
C8B–N1B–C1B–C11B
C9B–N3B–C2B–N1B
C9B–N3B–C2B–C21B
C8B–N1B–C2B–N3B
C1B–N1B–C2B–N3B
C8B–N1B–C2B–C21B
C1B–N1B–C2B–C21B
C9B–C4B–C5B–C6B
C2B–N1B–C8B–C7B
C1B–N1B–C8B–C7B
C2B–N1B–C8B–C9B
C1B–N1B–C8B–C9B
C6B–C7B–C8B–N1B
109.45(À177.75)
À81.12(À62.25)
À0.02(À13.69)
À178.71(165.91)
0.59(17.54)
171.55(139.72)
179.28(À162.06)
À9.75(À39.88)
0.1(À0.1581)
178.45(166.15)
7.08(43.71)
À0.88(13.70)
À172.25(136.15)
179.75(176.19)
108.10(À177.75)
À79.66(À62.25)
À0.86(À13.69)
177.35(165.91)
1.24(17.54)
173.85(139.72)
À176.90(162.06)
À4.29(À39.88)
0.25(À0.1581)
176.39(166.15)
3.28(43.71)
À1.05(À13.70)
À174.15(À136.15)
177.76(176.19)
Values within the parenthesis corresponds to theoretical values.
barrier for interconversion in the excited state is much higher than
that in the ground state.
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[
Appendix A. Supplementary material
6
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.saa.2012.12.037.
[
[
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