Synthesis of some fluorescent benzimidazole derivatives using cobalt(ii) hydroxide as highly efficient catalyst-spectral and physico-chemical studies and ESIPT process

Article abstract of DOI:10.1039/c3pp50083g

Some fluorescent benzimidazole derivatives have been designed and synthesized using cobalt(ii) hydroxide as highly efficient catalyst. Synthesized compounds have been characterized by ¹H and ¹³C-NMR and mass spectral analysis. The so

Full text of DOI:10.1039/c3pp50083g

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Synthesis of some fluorescent benzimidazole derivatives using cobalt(II) hydroxide as
high efficient catalyst - Spectral, physico-chemical studies and ESIPT process
J. Jayabharathi*, V. Thanikachalam, K. Jayamoorthy
Department of Chemistry, Annamalai University, Annamalainagar 608 002, Tamilnadu, India.
* Address for correspondence
Dr. J. Jayabharathi
Professor of Chemistry
Department of Chemistry
Annamalai University
Annamalainagar 608 002
Tamilnadu, India.
Tel: +91 9443940735
Eꢀmail: jtchalam2005@yahoo.co.in
* Corresponding author Tel.: +91 9443940735
Eꢀmail address: jtchalam2005@yahoo.co.in

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Synthesis of some fluorescent benzimidazole derivatives using cobalt(II) hydroxide as
high efficient catalyst - Spectral, physico-chemical studies and ESIPT process
J. Jayabharathi*, V. Thanikachalam, K. Jayamoorthy
Department of Chemistry, Annamalai University, Annamalainagar 608 002, Tamilnadu, India.
Abstract
Some fluorescent benzimidazole derivatives have been designed and synthesized using
cobalt(II) hydroxide as high efficient catalyst. Synthesized compounds have been
1
characterized by H, 13CꢀNMR and mass spectral analysis. The solvent effect on the
absorption and fluorescence bands has been analyzed and supplement by computational
studies. Solvatochromic effects on the spectral position and profile of the stationary
fluorescence spectra clearly indicate the charge transfer (CT) character of the emitting singlet
states of all of the compounds studied both in a polar and a non polar environment. The
fluorescence decays for the benzimidazoles fit satisfactorily to a single exponential kinetics.
HOMO and LUMO orbital picture [DFT/B3LYP/6ꢀ31G(d,p)] evidence the existence of
excited state intramolecular proton transfer (ESIPT) in benzimidazole derivative containing
hydroxy group.
Keywords: benzimidazole; cobalt(II) hydroxide; DFT; ESIPT; HOMOꢀLUMO; CT.
*
Corresponding author. Tel.: +91 9443940735
Eꢀmail address: jtchalam2005@yahoo.co.in

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1. Introduction
Benzimidazole based chromophore have received increasing attention due to their
distinctive linear, nonꢀlinear optical properties and also due to their excellent thermal stability
in guestꢀhost systems [1]. Hydrogen bond plays an important role as a site specific interaction
in most of the acid–base chemical and biological systems. The electrostatic interactions such
as N–H…N, N–H...O, O–H…N, O–H…O [2, 3], and other similar interactions between
neutral H and O, N, etc. with bond length ranging from 1.5 to 2.4 Å [4] have been studied.
Excited state intramolecular proton transfer (ESIPT) has become a field of active research [5,
6] due to the practical applications of ESIPT exhibited by molecules as laser dyes [7],
photostabilizers [8], fluorescent probes in biology [9] and lightꢀemitting materials for
electroluminescent devices [10]. Phototautomerisation through ESIPT is of prime importance
in biological and chemical systems leading to experimental and theoretical interests [11ꢀ15].
Photophysical studies on bioactive molecules with phenolic function are of our interest; they
exhibit ESIPT.
The electronic transition in molecules, involving intramolecular charge transfer (ICT), is
very sensitive to the nature of the microenvironment around the solute and the spectral
parameters can be used for studying soluteꢀsolvent interaction at the microscopic level [16].
The CT character of fluorescent state of the biaryl molecules in polar solvents seems to be
well proved by the large Stokes shift, the large dipole moment change upon excitation and by
the timeꢀresolved fluorescence [17ꢀ20] and transient absorption studies [21]. In this article,
we report the elegant catalytic synthesis, characterisation and solvatochromism of
benzimidazole molecules with fluoroꢀ, methylꢀ, methoxyꢀ, trifluoromethylꢀ, hydroxyꢀ and
substituents along with the unsubstituted one. The solventꢀdependent photophysical
characteristics of benzimidazole derivatives were investigated by steadyꢀstate absorption and
emission spectroscopy as well as time resolved fluorometry. From these experiments, we

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determined the position of the spectral maxima (λabs, λex and λemi), the stokes and lifetime (τ),
and the rate constants of radiative (kr) and nonradiative (knr) deactivation of benzimidazole
derivatives. We used the Lippert solvent parameter ꢁf, the normalised ET(30) polarity scale,
and the multiparameter KamletꢀTaft and Catalan solvent scales to describe the solvent effect
on the fluorescence emission maxima and stokes shift of benzimidazole derivatives. We have
also addressed the influence of solvents on the photophysical properties of the synthesized
molecules in terms of
(
) with solvent polarity function.
ꢀ
,
and
Single crystal Xꢀray diffraction (XRD) analysis and density functional theory (DFT)
calculations support the ESIPT in hydroxy benzimidazole derivative.
2. Experimental
2.1. Spectral measurements
The proton spectra at 400 MHz were obtained at room temperature using a Bruker 400 MHz
NMR spectrometer. Proton decoupled 13C NMR spectra were also recorded at room
temperature employing a Bruker 400 MHz NMR spectrometer operating at 100 MHz. The mass
spectra of the samples were obtained using a Thermo Fischer LCꢀMass spectrometer in fast
atom bombardment (FAB) mode. The UVꢀvis absorption and fluorescence spectra were
recorded with PerkinElmer Lambda 35 spectrophotometer and PerkinElmer LS55
spectrofluorimeter, respectively. Fluorescence lifetime measurements were carried out with a
nanosecond time correlated single photon counting (TCSPC) spectrometer Horiba
Fluorocubeꢀ01ꢀNL lifetime system with NanoLED (pulsed diode excitation source) as the
excitation source and TBXꢀPS as detector. The slit width was 8 nm and the laser excitation
wavelength was 280 nm. The fluorescence decay was analyzed using DAS6 software. The
photoluminescence (PL) quantum yield for all benzimidazoles were measured in
dichloromethane using coumarin 47 in ethanol as the standard. Single crystal XRD was
recorded in Agilent Xcalibur Ruby Gemini diffractometer. Enhanced Mo Xꢀray source was

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used as radiation source. Graphite monochromator with 10.5081 pixels mmꢀ1 was used as
detector resolution.
2.2. General procedure for the catalytic synthesis of benzimidazole derivatives
A mixture of Nꢀphenylꢀoꢀphenylenediamine (1 mmol), corresponding aldehyde (1 mmol),
ammonium acetate (2.5 mmol) and cobalt(II) hydroxide (10 or 15 mol %) was refluxed at
80º C in ethanol for different time intervals. The reaction was monitored by thin layer
chromatography (TLC) and purified by column chromatography using petroleum ether as the
eluent.
2.2.1. 1,2ꢀDiphenylꢀ1Hꢀbenzo[d]imidazole (1)
Mp = 107°C, C19H14N2: 1H NMR (400 MHz, CDCl3): δ7.24ꢀ7.33 (q, 7H), 7.45ꢀ7.47 (d, 3H),
7.53ꢀ7.57 (d, 2H), 7.88ꢀ7.90 (d, 1H). 13C NMR (100 MHz, CDCl3): δ 110.52, 119.89, 123.04,
123.39, 127.44, 128.35, 128.59, 129.26, 129.50, 129.90, 129.98, 137.00, 137.24, 143.03,
152.43. MS: m/e 271.26, calcd m/e 271.12 [M+1].
2.2.2. 2ꢀ(4ꢀFluorophenyl)ꢀ1ꢀphenylꢀ1Hꢀbenzo[d]imidazole (2)
1
Mp = 96°C, C19H13FN2: H NMR (400 MHz, CDCl3): δ6.98ꢀ7.02 (T, 2H), 7.26ꢀ7.36 (m,
4H), 7.50ꢀ7.55 (m, 6H), 7.87ꢀ7.88 (d, 1H). 13C NMR (100 MHz, CDCl3): δ 110.50, 115.44,
115.64, 119.83, 123.13, 123.47, 126.14, 126.18, 127.43, 128.75, 130.01, 131.41, 131.49,
136.84, 137.19, 142.90, 151.47, 162.16, 164.65. MS: m/e 289.17, calcd m/e 289.11 [M+1].
2.2.3. 2ꢀ(4ꢀmethylphenyl)ꢀ1ꢀphenylꢀ1Hꢀbenzo[d]imidazole (3)
Mp = 111°C, C20H16N2: 1H NMR (400 MHz, CDCl3): δ2.33 (s, 3H), 7.09ꢀ7.11 (d, 2H), 7.24ꢀ
13
7.26 (m, 2H), 7.30ꢀ7.32 (m, 3H), 7.44ꢀ7.50 (m, 5H), 7.87ꢀ7.89 (s, 1H). C NMR (100 MHz,
CDCl3): δ 21.42, 110.41, 119.73, 122.93, 123.18, 127.05, 127.48, 128.53, 129.07, 129.37,
129.88, 137.14, 137.24, 139.62, 143.02, 152.60. MS: m/e 285.30, calcd m/e 285.13 [M+1].

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2.2.4. 2ꢀ(4ꢀmethoxyphenyl)ꢀ1ꢀphenylꢀ1Hꢀbenzo[d]imidazole (4)
1
Mp = 118°C, C20H16N2O: H NMR (400 MHz, CDCl3): δ3.80 (s, 3H), 6.81ꢀ6.82 (d, 2H),
7.23ꢀ7.26 (m, 2H), 7.31ꢀ7.33 (m, 3H), 7.47ꢀ7.52 (m, 5H), 7.85ꢀ7.87 (s, 1H). 13C NMR (100
MHz, CDCl3): δ 55.29, 110.32, 113.77, 119.55, 122.32, 122.89, 123.02, 127.50, 128.53,
129.90, 130.92, 137.21, 143.00, 152.41, 160.51. MS: m/e 301.21, calcd m/e 301.13 [M+1].
2.2.5. 2ꢀ(4ꢀ(Trifluoromethyl)phenyl)ꢀ1ꢀphenylꢀ1Hꢀbenzo[d]imidazole (5)
1
Mp = 108°C, C20H13F3N2O: H NMR (400 MHz, CDCl3): δ7.26ꢀ7.29 (m, 1H), 7.30ꢀ7.35
(m, 3H), 7.37ꢀ7.39 (m, 1H), 7.51ꢀ7.58 (m, 5H), 7.69ꢀ7.71 (d, 2H), 7.90ꢀ7.92 (d, 1H). 13C
NMR (100 MHz, CDCl3): δ 110.68, 120.15, 122.51, 123.40, 123.99, 125.25, 125.29, 125.33,
127.40, 129.01, 129.67, 130.16, 131.00, 131.32, 133.45, 136.61, 137.35, 142.90, 150.68. MS:
m/e 339.38, calcd m/e 339.11 [M+1].
2.2.6. 2ꢀ(1ꢀPhenylꢀ1Hꢀbenzo[d]imidazolꢀ2ꢀyl)phenol (6)
Mp = 114°C, C19H14N2O: 1H NMR (400 MHz, CDCl3): δ6.51ꢀ6.50 (t, 1H), 6.84ꢀ6.86 (d, 1H),
7.08ꢀ7.12 (q, 2H), 7.21ꢀ7.28 (m, 3H), 7.33ꢀ7.37 (t, 1H), 7.40ꢀ7.43 (m, 2H), 7.60ꢀ7.65 (m,
3H), 7.81ꢀ7.83 (d, 1H), 13.56 (s, 1H). 13C NMR (100 MHz, CDCl3): δ 110.42, 112.37,
118.05, 118.06, 118.71, 123.48, 123.78, 127.33, 127.95, 129.58, 130.43, 131.45, 136.56,
137.21, 140.04, 150.82, 159.65. MS: m/e 287.26, calcd m/e 287.11 [M+1].
3. Results and discussion
3.1 Crystal structure of 2ꢀ(1ꢀphenylꢀ1Hꢀbenzo[d]imidazolꢀ2ꢀyl)phenol (6)
2ꢀ(1ꢀPhenylꢀ1Hꢀbenzo[d]imidazolꢀ2ꢀyl)phenol is a triclinic crystal and crystallizes in the
space group P1. The ORTEP diagram of 6 is presented in Figure 1. The benzimidazole unit
is close to being planar [maximum deviation = 0.0253 (11) A ˚] and forms dihedral angles of
68.98 (6) and 20.38 (7)º with the adjacent phenyl and benzene rings; the dihedral angle
between the latter two planes is 64.30 (7) º. In the crystal, molecules are linked by C—H…N

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and C—H…O hydrogen bonds, and consolidated into a threeꢀdimensional architecture by π–
π stacking interactions, with a centroid – centroid distance of 3.8428 (12) Aº [22].
Intramolecular hydrogen bonding shown by the ORTEP initiates us to study the ESIPT.
Optimization of 6 have been performed by DFT at B3LYP/6ꢀ31G(d,p) level using Gaussianꢀ
03. All these XRD data are in good agreement with the theoretical values (Table S1).
However, from the theoretical 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 are of isolated molecule in the
gaseous phase and the XRD results are of the molecule in the solid state.
3.2. Effect of the catalytic activity of cobalt(II) hydroxide
Initially, we have carried out the condensation reaction of Nꢀphenylꢀoꢀphenylenediamine
with substituted benzaldehyde in ethanol for 24 h with stirring, in the absence of catalyst and
observed only trace amount of products. In order to improve the yield and shorten the
reaction time, we have carried out the reaction using different amounts of the catalysts at
different time intervals (Table 1). It was observed that the amount of catalyst plays a
significant role in controlling the activity of the catalyst. Among the various amounts of
catalyst used, 15 mol % was found to be the best in ethanol medium. A possible mechanism
is that the reaction proceeds via the activation of aldehyde by Co(OH)2 followed by imine
formation. The resulted imine further reacts with another amine group of diamine leading to
the formation of dihydroimidazole which subsequently undergoes aromatization under the
oxidative conditions to give the benzimidazole as shown in Scheme 1. The present catalyst is
novel and efficient. The salient features of this protocol are high product yields, shorter
reaction time, low catalyst loading and easy workꢀup procedure, which make this procedure
quite simple, more convenient and environmentally benign. Hopefully, our methodology

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could be a valid contribution to the existing processes in the field of benzimidazoles
synthesis.
3.3. Solvatochromism of benzimidazole derivatives (1-6)
Room temperature absorption and emission of benzimidazole derivatives (1-6) have been
studied in different solvents and the spectra are displayed in Figures 2 and 3, respectively.
Red shifts in the fluorescence bands of the benzimidazole derivatives were observed on
increasing the solvent polarity. This is because the distortion of the geometry in the excited
state which implies a decrease of the resonance energy; the fluorescence band is
bathochromically shifted to a larger extent. Moreover, the loss of planarity in the excited state
of the benzimidazole derivative could explain the lower fluorescence quantum yield in apolar
solvents owing to an increase in the nonꢀradiative processes. Blue shifts were observed with
increase of solvent polarity in the absorption band of the benzimidazole derivatives. The
shape of the absorption band is independent of the imidazole concentration suggesting the
poor aggregation which is a required behavior for the performance of lasers [23].
A good linear correlation is obtained between the fluorescence maxima and the ET(30) values
[24] (Figure 4a). Figure 4b is the constructed Lippert–Mataga plot [25] by using the relation
SS = [2(ꢂe ꢀ ꢂe) / hca3] ꢁf + νssº
(1)
where SS is the Stokes shift, the superscript o indicates the absence of solvent, ꢃg and ꢃe are
dipole moments in the ground state and excited state respectively and a is the Onsager cavity
radius. The orientation polarizability ꢁf is defined as
ꢁf = [(ε ꢀ1) / (2ε + 1)] – [(n2 ꢀ 1) / (2n2 + 1)
(2)
where ε and n are solvent dielectric constant and refractive index, respectively. The
geometrical optimization of benzimidazole derivatives was done by DFT method using
Gaussian 03 software to calculate the ꢃg [26]. Using the ꢃg value, obtained from the DFT
calculation [26], 3.36 D (1), 3.02 D (2), 3.38 D (3), 2.60 D (4), 3.40 D (5) and 3.06 D (6), and

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the slope of Lippert–Mataga plot the value of ꢃe is calculated. The calculated ꢃe values are in
the range of 12ꢀ14 D.
Effect of the variation of quantum yield (φf) with solvent polarity parameter ET (30) is shown
in Figure 5a. It is clear from the plot that fluorescence quantum yield of benzimidazole
derivatives decreases with increasing polarity of the solvents. The significant decreases of
quantum yield observed upon increasing the protic nature of the medium suggest the
differential contribution of charge transfer and hydrogen bonding interactions [27]. Radiative
(kr) and nonradiative (knr) rate constants are calculated from fluorescence quantum yield and
lifetime values of benzimidazole derivatives in different solvents to understand the effect of
solvation on the dynamics of the excited state. A typical set of kr and knr values are given in
Table 2. Figure 5b is the semi logarithmic plot of kr/knr versus solvent polarity parameter
ET(30). Upon increasing the polarity the logarithm ratio of radiative to nonradiative rate
decreases. It indicates that the radiative and nonradiative rates are more sensitive toward
protic solvents. It may be that the hydrogen bonding interaction in polar protic environments
enhances the stabilization of the S1 state of benzimidazole derivatives; as a result, the
nonradiative relaxation rate increases [27].
The free energy change of solvation and reorganization energies of 1-6 in various solvents
have been estimated (Table 3). According to Marcus [28],
E (A) = ∆Gsolv + λ1 and E (F) = ∆Gsolv ꢀ λ0 , where E(A) and E(F) are absorption and
fluorescence band maxima in cm−1, respectively, ∆Gsolv is the difference in free energy of the
ground and excited states in a given solvent and λ represents the reorganization energy . The
free energy change of solvation and reorganization energies of 1-6 in various solvents have
been estimated (Table 3). The definite values of reorganisation energy confirmed the
interaction between low frequency motions such as reorientation of solvent cell with low and
medium frequency nuclear motion of the solute. Under the condition that λ0 λ1 λ, we get,

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E (A) + E (F) = 2∆Gsolv; E (A) – E (F) = 2 λ. The ∆G solv of 1-6 is maximum for hexane since
it is purely nonꢀpolar and also α and β values of hexane are zero. The ∆Gsolv is minimum in
water. The difference between these values (water and hexane) should give the free energy
_
change required for hydrogen bond formation. The plot of ∆ (∆Gsolv) = (∆Ghex ∆Gwater
)
versus ET(30) is depicted in Figure 6a. The difference in free energy of solvation in hexane
and different hydrogen bonding solvents, i.e., ∆Gsolv of 1-6 follow the order of the hydrogen
bond energy [29]. In the aprotic solvents the values are small and interaction of imidazole
derivatives with those solvents is purely out of dipolar interactions in the excited state. The
reorganization energy values of 1-6 have also been determined in different solvents.
The solvatochromic effects on the benzimidazole derivatives (1-6) were analysed by
employing the multilinear expression proposed by Catalan. It involves a generalized
treatment of the solvent effect based on the acidity (SA), basicity (SB), polarizability (SP),
and dipolarity (SdP) given in the following equation [30, 31],
y = yo + a SA +b SB + c SP + d SdP
(3)
where yo represents the physiochemical property of interest in the gas phase; a, b, c and d are
the estimated coefficients that describe the dependence of the physiochemical property.
Table 4 presents the coefficients of multiꢀparameter correlation. The absorption and
fluorescence bands of the benzimidazoles show that the solvatochromic shifts are associated
with the solvent polarity. The emission of imidazole derivative with pꢀtolyl, pꢀmethoxy and
pꢀtrifluoromethyl substituents is less affected by the Hꢀdonor capacity of the solvent. The
absorbance of all the imidazole derivatives studied is insensitive to the electron releasing
ability or basicity of the solvent (Cβ or CSB). However, pꢀfluoroꢀ and oꢀhydroxyꢀsubstituted
imidazoles along with the parent molecule are highly susceptible to the Hꢀdonor capacity of
the solvent. The susceptibility of the hydroxy compound is double that of pꢀfluoro
benzimidazole and the parent benzimidazole derivatives. The large solvatochromic effect

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shown by the hydroxyl molecule is due to intermolecular hydrogen bonding. The less
significant solvent effect experienced by pꢀmethyl, pꢀmethoxy and pꢀtrifluoromethylꢀ
substituted molecules cannot be due to electronic effect but may be because of steric effect.
Figure 6b presents the possible resonance stabilization of the studied molecules. In the case
of pꢀfluoro substituted molecule and the parent molecule the charge on the phenyl carbon
attached to the substituent could be stabilized by acidic solvents but the same is sterically
hindered by pꢀmethyl, pꢀmethoxy and pꢀtrifluoromethyl substituents. This resonance structure
is predominant in the S1 state and the stabilization of the S1 state with the Hꢀdonor capacity of
the solvent is more important than that of the S0 state. Consequently, the energy gap between
S1 and S0 states is decreased. With increase of the Hꢀdonor capacity of the solvent,
fluorescence are shifted to longer wavelengths which explains the solvatochromic shifts.
An interesting result is provided by the blue shift of the CT absorption bands with increasing
solvent polarity (Figure 7a) [32, 33]. With the assumption that point dipole is at the center of
the spherical cavity and the mean solute polarizability (α) to be insignificant, it follows,
hc
– 2 µg (µe - µg) /
where µg and µe are the dipole moments of the solute in the ground and excited states,
correspondingly, νabs and are the spectral positions of a solventꢀequilibrated absorption
[(ε – 1/ 2 ε + 1) – ½ (n2 ꢀ1/ 2n2 + 1)]
(4)
abs ≈
maxima and the value extrapolated to the gasꢀphase, respectively, ao is the effective radius of
the Onsager cavity, [34] and ε and n are the static dielectric constant and the refractive index
of the solvent, respectively. In the case of CT absorption bands, eqn 4 is used to determine
the values of µg (µe - µg) /
and
. The linear correlation reveal that the blue shift of the
CT absorption band is probably due to the fact that the dipolar molecule cannot be
approximated by the point dipole situated in the center of the cavity. The ground state dipole
moment is mostly determined by polar fragment of benzimidazole derivatives lying far from
the center of the molecule (Figure 7b). It leads to an increase of the effective Onsager

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reaction field [35] and to a corresponding increase in the ground state solvation energies and
blue shift observed in the absorption spectra. In the excited state, the negative and positive
ends of the electric dipole are localised nearly in the centres of the donor and acceptor
fragments of the benzimidazole derivatives, respectively (Figure 7b). In fluorescence spectra,
the observed red shift and the increase of the Stokes shift with increasing solvent polarity
clearly indicate that the absolute values of µe are much higher than those of µg [36ꢀ38]. Under
the same assumption as used for expression 4, it follows that
hc
– 2 ꢂe (ꢂe ꢀ ꢂg) /
[(ε – 1/ 2 ε + 1) – ½ (n2 ꢀ1/ 2n2 + 1)]
(5)
flu ≈
where,
and are the spectral positions of the solvent equilibrated fluorescence
flu
maxima and the value extrapolated to the gasꢀphase, respectively. The compounds studied
show a satisfying linear correlation between the energy hc and the solvent polarity
flu
function in a polar environment and also in all the solvents studied [39] (Figure 7c) the values
of ꢂe (ꢂe ꢀ ꢂg) / are displayed in Table 5. The values, extracted from the
data measured in polar media, are somewhat larger than those resulting from the analysis of
the data obtained for the whole range of the solvents. This finding can be explained only by
the dependence of the electronic structure of the fluorescent states on solvation. Due to a
relatively small energy gap between the lowest internal charge transfer (ICT) states and the
states excited locally in the nonpolar solvents, which leads to increase of the contribution of
the (π, π*) character to the wave function of the CT states. It leads to a lowering of energy
with respect to a pure CT state because of a stabilizing character of such interactions and red
shift obtained in the fluorescence spectra.
Under the assumption that the CT fluorescence corresponds to the state reached directly upon
2
excitation, the quantity (ꢂe ꢀ ꢂg)
Stokes shift,
/
can be evaluated from the solvation effects on the

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hc(
flu) = hc(
)
+ 2 (ꢂe ꢀ ꢂg)2 /
[(ε – 1/ 2 ε + 1) – ½ (n2 ꢀ1/ 2n2 + 1)]
(6)
abs ꢀ
ꢀ
The compounds studied show a satisfying linear correlation between the energy hc
ꢀ
abs
hc
and the solvent polarity function in a polar environment and also in all the solvents
flu
studied, the values of (ꢂe ꢀ ꢂg) /
are 1.407 eV (1), 1.509 eV (2), 1.115 eV (3), 1.358 eV
(4), 1.362 eV (5) and 1.023 eV (6).The equations 4ꢀ6 relate the measured quantities to the
excited state dipole moments ꢂe. Under the assumption that ꢂe >> ꢂg and with the effective
spherical radius of the molecules a0, 5.21 Aº (1), 5.27 Aº (2), 5.30 Aº (3), 5.39 Aº (4), 5.52 Aº
(5) and 5.29 Aº (6), as estimated from the molecular dimensions of the compounds calculated
by molecular mechanics, equations 5 and 6 yield very similar values of ꢂe being in the range
of 12.4 D (1), 13.08 D (2),13.20 D (3), 12.83 D (4), 12.32 D (5) and 10.84 D (6) for the
studied molecules.
3.4. ESIPT process
The emission spectra of 2ꢀ(1ꢀPhenylꢀ1Hꢀbenzo[d]imidazolꢀ2ꢀyl)phenol (6) in dioxane exhibit
a dual emission. Excitation of isomer I lead to the formation of ketoꢀisomer II shown in
Figure 8. The dual emission is due to ESIPT displayed in Figure 9. However, in hydroxylic
solvent a short wavelength emission band appears for 6 which is absent in the emission
spectra of other molecules (Figure 3). This result is similar to those reported in literature [40].
Absence of dual emission by 6 in hydroxyl solvents is explained by the presence of
intermolecular hydrogen bonding with solvent molecule leading to the stabilization of
solvated isomer III in which ESIPT is impossible.
Existence of ESIPT mechanism in hydroxybenzimidazole derivative, 2ꢀ(1ꢀPhenylꢀ1Hꢀ
benzo[d]imidazolꢀ2ꢀyl) phenol (6) is supported by DFT calculations. The electron density of
keto and enol isomers of 6 in the ground and excited states calculated by B3LYP/6ꢀ31G(d,p)
method is presented in Table 6. Excitation of enol isomer leads to increase of the electron
density at azomethine nitrogen atom (3) and decrease of the same at oxygen atom (26)

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resulting ESIPT. As a consequence of ESIPT, excited keto isomer is formed. The excited
keto isomer returns to the ground state keto form (III) with the emission of luminescence.
This is supported by DFT analysis which provides a large positive charge at N(3) nitrogen
atom and negative charge at oxygen atom. As a result, a reverse process occurs in the ground
state of the molecule producing enol form. The electron density at azomethine nitrogen (3) of
structure II is increased on excitation. The electron density of II and II* are ꢀ0.395 e.u and ꢀ
0.455 e.u respectively. i.e., the basicity is enhanced on excitation. Similarly the acidity is
decreased on excitation. The electron density at oxygen (26) of structure II decreases from ꢀ
0.570 to ꢀ0.528 i.e., the acidity is increased.
3.5. Driving force for ESIPT process
The existence of intramolecular hydrogen bonding in 2ꢀ(1ꢀPhenylꢀ1Hꢀbenzo[d]imidazolꢀ2ꢀ
yl)phenol (6) is confirmed by single crystal XRD (Figure 1) and also by the presence of
singlet at 13.56 ppm in the 1H NMR spectrum shown in (Figure 10), which is a typical signal
for hydrogen bonded hydrogen atom. The fluorescence spectra of 6 and benzimidazole
derivative 1 (not having hydroxyl group) in dioxane under identical condition reveal the
contribution of the intramolecular hydrogen bonding in 6. A dual fluorescence is exhibited by
6 whereas other compounds exhibit only single emission. The emission of 6 at shorter
wavelength is assigned to rotamer I and that at the longer wavelength is assigned to rotamer
II. The absence of additional peak in 1-5 confirms absence of intramolecular hydrogen bond.
It is further evident that intramolecular hydrogen bonding is the driving force for ESIPT and
dual fluorescence behaviour of 6.
3.6. HOMOꢀLUMO electron density vs ESIPT
The electron density of HOMO and LUMO of 2ꢀ(1ꢀPhenylꢀ1Hꢀbenzo[d]imidazolꢀ2ꢀyl)phenol
(6) shown in Figure 11 can throw some light on the excited state proton transfer processes.
The HOMO of enol form predicts that intramolecular hydrogen bonded (IMHB) ring system

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possess bonding character over the O(26) H(26) and C(2) C(21) atoms. Both oxygen O(26)
and azomethine nitrogen N(3) have bonding character and the electron density on oxygen
(O26) is large. The electron density of HOMO of keto tautomer around IMHB ring shows
antibonding character over the C(26) O(26) and N(3) H(26). Again, comparison of the
HOMOs of enol and keto forms shows that the electron density at the oxygen of keto form is
large. The HOMO electron density of both the enol and keto forms indicates less electron
density over the phenyl ring. By considering the LUMO of enol and keto forms, LUMO of
the enol tautomer possesses high electron density on azomethine N(3). After tautomerization,
the LUMO of ketoꢀtautomer still shows larger electron density on azomethine nitrogen and
comparatively smaller density on oxygen. Thus it favours the transfer of a proton from
oxygen to azomethine nitrogen in the excited state. Again our calculation of electron density
over the proton transfer coꢀordinate in the excited states (i.e., LUMO electron densities)
shows that there is a shift of πꢀelectron distribution from O(26) to azomethine N(3). Analysis
of the electron density of HOMO for enol tautomer predicts that azomethine nitrogen N(3)
has slightly higher bonding character than oxygen. On the other hand, HOMO electron
density of keto tautomer shows greater bonding character of oxygen than azomethine
nitrogen and hence transfer of proton from oxygen O(26) to azomethine nitrogen N(3) in the
ground state is quite impossible.
3.7. Fluorescence lifetime and quantum yield
The time correlated single photon counting (TCSPC) results fit to single exponentials decay
(Figure 12), DAS6 software was used for the fit and the χ2 values are less than 1.2. The
fluorescence lifetime measurements of all benzimidazole derivatives were made with laser
excitation set at 270 nm and the fluorescence signal was measured at the emission
wavelength of individual compound. The deduced parameters are listed in Table 2. The
absolute PL quantum yields were measured by comparing fluorescence intensities (integrated

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areas) of a standard sample (Coumarin 46) and the unknown sample using the formula



2
Iunk Astd ηunk
Фunk = Ф
where, Фunk is the fluorescence quantum yield of the

 
 


std 
Istd Aunk ηstd
 


sample, Фstd is the fluorescence quantum yield of the standard; Iunk and Istd are the integrated
emission intensities of the sample and the standard, respectively. Aunk, and Astd are the
absorbances of the sample and the standard at the excitation wavelength, respectively. ηunk
and ηstd are the indexes of refraction of the sample and standard solutions. The radiative and
nonꢀradiative decay of the excited state of 1-6 have been obtained using the quantum yields
and lifetimes and are listed in Table 2. The formula employed to calculate the radiative (kr) and
nonꢀradiative (knr) rate constants is kr = Φp/τ; knr = (1/τ) ꢀ (Φp/τ); τ = (kr + knr)ꢀ1, where kr and
knr are the radiative and nonꢀradiative deactivation, τf is the lifetime of the S1 excited state.
Table 2 shows that all the molecules synthesized have moderate quantum yields. Perusal of the
radiative and nonꢀradiative rate constants shows that in most of the cases the radiative emission is
predominant over nonꢀradiative transitions. Furthermore, Table 2 shows that the nonꢀradiative
rate constants of the methyl and methoxy substituted molecules are larger than those of the
others. This could be the electron releasing nature of the methyl and methoxy substituents.
4. Conclusion
In this article we report an elegant synthesis of some fluorescent benzimidazole derivatives
under mild reaction conditions with inexpensive cobalt(II) hydroxide as catalyst. In addition,
the overall yields of the compounds are not less than 75%, is a promising synthesis for
scaling up. Observed blue shift of absorption CT band and red shift of emission band have
been discussed. The difference in free energy of solvation in polar protic and aprotic solvent
corresponds to the hydrogen bond energy. The compounds studied show a satisfying linear
correlation between the energy hc abs, hc flu, hc
ꢀ hc
and the solvent polarity
flu
abs
function in a polar environment. All the studied compounds are having moderate quantum

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efficiency. Excited state depopulation kinetic studies have also been carried out. Hydroxyꢀ
substituted benzimidazole shows a single prominent emission in polar solvents and dual
emission in nonꢀpolar solvents due to ESIPT. HOMOꢀ LUMO analysis and crystal structure
support the ESIPT.
Acknowledgments
One of the authors Prof. J. Jayabharathi is thankful to DST [No. SR/S1/ICꢀ73/2010] and
DRDO (NRBꢀ213/MAT/10ꢀ11) for providing funds to this research study. Mr. K.
Jayamoorthy is thankful to DST [No. SR/S1/ICꢀ73/2010] for providing fellowship.

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Figure Captions
Figure 1. ORTEP diagram of 2ꢀ(1ꢀphenylꢀ1Hꢀbenzo[d]imidazolꢀ2ꢀyl)phenol
Figure 2. Absorption spectra of 1ꢀ6 in different solvents
Figure 3. Emission spectra of 1ꢀ6 in different solvents
Figure 4. (a) Fluorescence maxima and the ET(30); (b) Lippert–Mataga plot
Figure 5. (a) Quantum yield with ET(30); (b) log (kr/knr) Vs ET(30)
Figure 6. (a) ∆ (∆Gsolv) Vs ET(30); (b) Resonance structure of the Chromophoric cores
Figure 7. (a) CT Absorption maxima Vs solvent polarity function; (b) Direction of ground
and excited state dipole; (c) CT Fluorescence maxima Vs solvent polarity function
Figure 8. Various tautomeric and solvated forms of 6
Figure 9. Suggested ESIPT and interconversion mechanism
Figure 10. Proton NMR spectrum of 2ꢀ(1ꢀphenylꢀ1Hꢀbenzo[d]imidazolꢀ2ꢀyl)phenol
Figure 11. HOMOꢀLUMO contour maps calculated by DFT/B3LYP/6ꢀ31G(d,p)
Figure 12. Time resolved single exponential decay curves
Scheme.1. Possible mechanism of catalytic synthesis of benzimidazoles

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