Synthesis, spectral studies and solvatochromic analysis of novel imidazole derivatives

Article abstract of DOI:10.1016/j.saa.2011.12.060

Bioactive imidazole derivatives were synthesized and characterized by spectral techniques. The photophysical properties of imidazole derivatives were studied in several solvents. The observed spectral shift is attributed to a loss of planarity in the exci

Full text of DOI:10.1016/j.saa.2011.12.060

Spectrochimica Acta Part A 89 (2012) 194–200
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Synthesis, spectral studies and solvatochromic analysis of novel imidazole
derivatives
Jayaraman Jayabharathi∗, Venugopal Thanikachalam, Natesan Srinivasan,
Marimuthu Venkatesh Perumal
Department of Chemistry, Annamalai University, Annamalainagar, Tamilnadu 608 002, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Bioactive imidazole derivatives were synthesized and characterized by spectral techniques. The photo-
physical properties of imidazole derivatives were studied in several solvents. The observed spectral shift
is attributed to a loss of planarity in the excited state provided by the non-co-planarity of the aryl rings
attached to C(2) and N(1) atoms of the imidazole ring. The observed solvatochromic shifts were analyzed
in detail by Kamlet–Taft and Catalan parameters. The interaction between bioactive imidazole derivative
and bovine serum albumin (BSA) was also investigated.
Received 19 October 2011
Received in revised form 9 December 2011
Accepted 21 December 2011
Keywords:
Imidazole
Polarisability
Catalan parameters
Binding constant
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
with protein because of its accuracy, sensitivity, rapidity and con-
venience of usage [9].
Recently, heterocyclic imidazole derivatives have attracted con-
siderable attention because of their unique optical properties [1]
and their usage for preparing functionalized materials [2]. Imi-
dazole nucleus forms the main structure of human organisms,
i.e., the amino acid histidine, Vitamin B12, a component of DNA
base structure and also has significant analytical applications
utilizing their fluorescence and chemiluminescence properties
[3].
Serum albumins are the most abundant proteins in plasma
[4], one of the biological functions of albumins is their ability to
carry drugs as well as endogenous and exogenous substances, the
binding capacity and sites of albumins have been characterized
[5]. Among the serum albumins, BSA has a wide range of physi-
ological functions involving the binding, transport and delivery of
fatty acids, porphyrins, steroids, etc. and the binding properties of
BSA and drugs were investigated by many researchers [6,7]. For
the study of structural and dynamical information of protein cav-
ities and the binding interaction of foreign species, steady state
and time-resolved fluorescence spectroscopy have attracted much
more attention for many research groups [8]. Fluorescence quench-
ing is an important method to study the interaction of substances
In this present paper, the photophysical properties of imi-
dazole derivatives 1–3 were investigated in a wide variety of
solvents, including apolar, polar and protic solvents. The solvent
effects on the absorption and fluorescence bands are analyzed
by a multi-component linear regression in which several solvent
parameters are simultaneously analyzed. We also exploited the
detailed investigation of BSA–imidazole association using fluores-
cence and UV–Vis absorption studies and evaluated the binding
parameters.
2. Experimental
2.1. Materials and methods
Butane-2,3-dione
(Sigma–Aldrich
Ltd.),
benzaldehyde,
4-fluorobenzaldehyde, p-hydroxybenzaldehyde (S.D. fine.), m-
methoxy aniline and all other reagents were used without further
purification. Bovine Serum Albumin (BSA) was purchased from
Sigma–Aldrich Company, Bangalore and used. All the BSA solutions
were prepared in the Tris–HCl buffer solution (0.05 mol L−1 Tris,
0.15 mol L−1 NaCl, pH 7.4) and was kept in the dark at 303 K. Tris
base (2-amino-2-(hydroxymethyl)-1,3-propanediol) had a purity
of not less than 99.5% and NaCl, HCl and other starting materials
were all of analytical purity and doubly distilled water was used
throughout.
∗
Corresponding author. Tel.: +91 9443940735.
E-mail address: jtchalam2005@yahoo.co.in (J. Jayabharathi).
1386-1425/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2011.12.060

J. Jayabharathi et al. / Spectrochimica Acta Part A 89 (2012) 194–200
195
and fluorescence spectrometer (Perkin Elmer LS55), which was cor-
rected for background due to solvent absorption. MS spectra were
recorded on a Varian Saturn 2200 GCMS spectrometer.
2.3. Computational details
Quantum mechanical calculations were carried out using
Gaussian-03 program [10]. As the first step of our DFT calculation,
the geometry taken from the starting structure was optimized.
Fig. 1. Optimized structure of the imidazole derivatives 1–3.
2.4. General procedure for the synthesis of 2-aryl imidazole
derivatives (1–3)
2.2. Optical measurements and NMR spectral analysis
NMR spectra were recorded on a Bruker 400 MHz instrument.
The UV–Vis spectra and photoluminescence (PL) spectra were mea-
sured on UV–Vis spectrophotometer (Perkin Elmer, Lambda 35)
The experimental procedure was used as the same as described
in our recent papers [11–20]. The 2-aryl imidazole derivatives (1–3)
Table 1
Photoluminescence spectral data of various solvents and solid emission spectra of 1-3.
Solvent
Absorptiona (ꢀ, nm)
Emissionb (ꢀ, nm)
Stokes shift (cm−1
)
1
2
3
1
2
3
1
2
3
n-Hexane
1,4-Dioxane
Benzene
Chloroform
Ethyl acetate
Dichloromethane
Butanol
Ethanol
Methanol
Acetonitrile
282
287
284
283
290
282
290
292
289
288
284
285
287
285
288
284
289
290
288
287
286
288
287
287
290
286
291
292
287
288
365
366
372
366
370
364
370
369
368
373
369
369
365
369
367
368
370
371
368
365.4
370
368
371
372
368
369
371
372
369
364
8064
7521
8373
8013
7456
7988
7456
7183
7465
7913
8111
8017
7467
8002
7474
8037
7575
7529
7585
7452
7938
7548
7889
7961
7309
7865
7410
7365
7743
7250
a
UV–Vis absorption measured in solution concentration = 1 × 10−5 M.
Photoluminescence measured in solution concentration = 1 × 10−4 M.
b
Fig. 2. Absorption solvatochromism of 1–3 in various solvents.

196
J. Jayabharathi et al. / Spectrochimica Acta Part A 89 (2012) 194–200
were synthesized from an unusual four components assembling
of butane-2,3-dione, ammonium acetate, substituted anilines and
substituted benzaldehydes.
with the substitution at C(2) with a minimum loss of fluorescence
property.
The solvent effect (Table 1) on ꢁabs (Fig. 2), ꢁem, (Fig. 3) and ꢂꢁss
can also be described on the basis of a multi-linear expression (Eq.
(1)):
3. Results and discussion
y = y0 + aA + bB + cC
(1)
3.1. Optimized geometry
in which y0 stands for the physicochemical property of interest
in the absence of solvent (i.e., in the gas phase); a, b, and c are
adjustable coefficients that reflect the dependency of the physic-
ochemical property (y) in a given solvent on the various (A, B, C)
solvent parameters. At least two different sets of solvent scales
can be found in the literature to characterize these solvent proper-
ties. In the present analysis, the polarity/polarizability, the acidity
and the basicity of the solvent are considered. Kamlet and Taft
[21] put forward the ꢃ*, ˛ and ˇ parameters to characterize the
polarity/polarizability, the acidity and the basicity of a solvent
respectively (Eq. (2)). Conversely, Catalan et al. proposed an empir-
ical solvent scales for polarity/polarizability [22–25] (SPP), acidity
[23] (SA) and basicity [23,24] (SB) to describe the respective prop-
erties of a given solvent (Eq. (3)).
Density functional theory (DFT) calculations of the imidazole
derivatives 1–3 reveals that the fused imidazole ring system is
essentially planar. The imidazole ring makes dihedral angles of
70.01◦ and 52.86◦ with the phenyl ring attached to N(1) nitrogen
and the phenyl ring attached to C(2) carbon, respectively. The dihe-
dral angle between the two phenyl rings is found to be 62.02◦ Fig. 1.
The key twist (˛) is used to indicate the twist of imidazole ring from
the aromatic six-membered ring at C(2). It reveals that ˛ twist is
correlated with fluorescent property, the larger ˛ twist, the more
drops the fluorescence quantum yield [11]. Such a clear correla-
tion indicates the importance of co-planarity between imidazole
and the aromatic ring at C(2). This correlation can be ascribed to
the conjugation rigidity. When the two adjacent aromatic species
are in a coplanar geometry, the p-orbitals from the C C bond con-
necting the two species will have maximal overlapping and the
two rings will have a rigid and delocalized conjugation, as the
result, the bond is no longer a pure single bond, as evident from
the X-ray data [19]. When the two rings are deviated from each
other, the p-orbital overlapping will be reduced. The partial con-
jugation will lead to less rigid structure; therefore, radiationless
twist motion will deactivate the emitting state, leading to the low
quantum yield efficiency. Thus, it is suggested that fluorophores
y = y0 + a˛˛ + bˇˇ + cꢃ∗ꢃ ∗ (Kamlet–Taft)
y = y0 + aSASA + bSBSB + cSPPSPP (Catalan)
(2)
(3)
The dominant coefficient affecting the absorption and fluores-
cence band of imidazole derivatives 1–3 is that describing the
polarity/polarizability of the solvent [cꢃ* or cSPPN] having a posi-
tive value, corroborating the solvatochromic shifts with the solvent
polarity. For 1–3, the adjusted coefficient representing the elec-
tron releasing ability or basicity of the solvent, cˇ or cSB has the
Fig. 3. Emission solvatochromism of 1–3 in various solvents.

J. Jayabharathi et al. / Spectrochimica Acta Part A 89 (2012) 194–200
197
Table 2
Adjusted coefficients ((ꢄx)0, ca, cb and cc) and correlation coefficients (r) for the multilinear regression analysis of the absorption ꢄab and fluorescence ꢄfl wavenumbers
and stokes shift (ꢂꢄss) of Schiff base derivatives 1–3 with the solvent polarity/polarizability, and the acid and base capacity using the Taft (ꢃ*, ˛ and ˇ) and the Catalan
(SPPN, SA and SB) scales.
Kamlet–Taft
(ꢄx)0 (cm−1
)
(ꢃ*)
c˛
cˇ
r
ꢀab
ꢀfl
(3.54 0.007) × 104
(2.73 0.003) × 104
(7.98 0.006) × 104
−(2.02 1.40) × 103
−(4.44 6.93) × 103
(2.42 1.29) × 103
(0.36 1.58) × 103
(1.10 2.25) × 103
−(1.07 4.22) × 103
cSA
(0.81 3.57) × 103
−(7.42 1.76) × 103
(8.23 3.29) × 103
cSB
0.92
0.93
0.88
r
ꢂꢄss = ꢄab − ꢄfl
Catalan
N
(ꢄx)0 (cm−1
)
cSPP
1
2
3
ꢀab
ꢀfl
(3.49 0.001) × 104
(2.71 0.001) × 104
(7.84 0.009) × 104
(5.90 7.62) × 103
(1.60 5.51) × 103
(4.30 4.01) × 103
−(3.58 3.44) × 103
−(8.79 2.32) × 103
−(2.70 1.69) × 103
(4.11 3.83) × 103
(1.00 2.41) × 103
(3.10 1.75) × 103
0.56
0.66
0.74
ꢂꢄss = ꢄab − ꢄfl
Kamlet–Taft
(ꢄx)0 (cm−1
)
(ꢃ*)
c˛
cˇ
r
ꢀab
ꢀfl
(3.51 0.003) × 104
(2.71 0.002) × 104
(8.04 0.002) × 104
−(1.07 7.27) × 103
(2.59 4.02) × 103
−(3.66 4.43) × 103
(0.28 2.37) × 103
−(6.93 1.31) × 103
(0.74 1.41) × 103
cSA
(0.21 1.84) × 103
(4.39 1.02) × 103
−(4.18 1.12) × 103
cSB
0.94
0.93
0.98
r
ꢂꢄss = ꢄab − ꢄfl
Catalan
N
(ꢄx)0 (cm−1
)
cSPP
ꢀab
ꢀfl
(3.50 0.009) × 104
(2.71 0.006) × 104
(7.76 0.001) × 104
(2.71 4.00) × 103
(4.62 2.53) × 103
(2.25 6.23) × 103
−(1.86 1.68) × 103
−(5.64 1.06) × 103
−(1.29 2.62) × 103
(2.21 1.75) × 103
(7.75 1.10) × 103
(1.44 2.76) × 103
0.61
0.56
0.71
ꢂꢄss = ꢄab − ꢄfl
Kamlet–Taft
(ꢄx)0 (cm−1
)
(ꢃ*)
c˛
cˇ
r
ꢀab
ꢀfl
(3.49 0.006) × 104
(2.69 0.003) × 104
(7.93 0.005) × 104
−(4.85 1.13) × 103
(2.55 6.73) × 103
−(3.04 8.87) × 103
(4.86 3.70) × 103
−(6.36 2.19) × 103
(5.87 2.89) × 103
cSA
(3.17 2.29) × 103
(3.87 1.77) × 103
−(3.55 2.25) × 103
cSB
0.84
0.84
0.92
r
ꢂꢄss = ꢄab − ꢄfl
Catalan
N
(ꢄx)0 (cm−1
)
cSPP
ꢀab
ꢀfl
(3.47 0.008) × 104
(2.71 0.007) × 104
(7.76 0.001) × 104
(2.95 3.27) × 103
(2.46 3.06) × 103
(3.19 5.61) × 103
−(2.08 1.37) × 103
−(2.21 1.29) × 103
−(1.86 2.36) × 103
(2.59 1.43) × 103
(4.33 1.34) × 103
(2.15 2.43) × 103
0.71
0.85
0.58
ꢂꢄss = ꢄab − ꢄfl
of the bimolecular without quencher (ꢅ0 = 10−8 s) [27] and the
concentration of the quencher, respectively. Obviously, Ksv = kqꢅ0,
hence, Eq. (4) was applied to determine KSV by linear regression
of a plot of F0/F vs. [Q]. The Stern–Volmer plot of F0/F against
[imidazole] gives by linear regression (Fig. 6) which indicates the
existence of single quenching (dynamic or static). The obtained kq
values are in the range of 1012 L mol−1 s−1 which far exceed the
diffusion controlled rate 2.0 × 1010 L mol−1 s−1, [25,26,28] confirm-
ing that the fluorescence quenching does not involve the diffusion
process but occurs statically by a specific interaction between BSA
and imidazole and the quenching mechanism mainly arise from
BSA–imidazole (Fig. 7) complex formation.
large negative values (Table 2), suggesting solvent basicity play an
important role in absorption and fluorescence displacementswhich
can be due to the existence of positive charge on the imine carbon
so that the basic solvent stabilize the structures and cause solva-
tochromic shifts (Fig. 4). The existence of good correlations (Fig. 5a
and b) between the absorption and fluorescence wavenumbers cal-
culated by the multicomponent linear regression by employing
the Taft-proposed solvent parameters and the obtained correla-
tion coefficients are tabulated in Table 2. The two sets of solvent
parameters give qualitatively similar results.
3.2. Fluorescence quenching mechanism
Fluorescence quenching [26] is described by the Stern–Volmer
equation:
3.3. Calculation of binding parameters
F0
Apparent binding constant kA and binding sites n [28] can be
obtained from
= 1 + kqꢅ0[Q] = 1 + Ksv[Q]
(4)
F
where F0 and F are the fluorescence intensities before and
after the addition of the quencher, respectively. kq, KSV, ꢅ0 and
[Q] are the quenching rate constant of the bimolecular, the
Stern–Volmer dynamic quenching constant, the average lifetime
log(F0 − F)
= log KA + n log[Q]
(5)
F
where F0 and F are the fluorescence intensities before and after the
addition of the quencher, [Q] is the total quencher concentration.
By the plot of log (F0 − F)/F vs. log [Q], the number of binding sites n
and binding constant KA can be obtained as 2.08 × 104 M−1 (301 K)
and binding sites “n” (1.16) (301 K) of imidazole with BSA from the
intercept and slope. The value of ‘n’ (equal to one) indicates that
there may be a single class binding site in the neighbourhood of
the tryptophan residues in BSA.
3.4. Energy transfer between imidazole and BSA
The distance between the BSA and the interacted imidazole
was estimated by Forester’s non-radiative energy transfer (FRET)
and the overlap fluorescence spectra of imidazole and the UV–Vis
absorption spectra of BSA were shown in Fig. 8.
Fig. 4. Resonance structures (a and b) of the imidazole chromophore (2).

198
J. Jayabharathi et al. / Spectrochimica Acta Part A 89 (2012) 194–200
Fig. 5. (a) Correlation between the experimental absorption wavenumber with the predicted values obtained by a multicomponent linear regression using the ꢃ*, ˛ and
ˇ-scale (Taft) solvent parameters. (b) Correlation between the experimental fluorescence wavenumber with the predicted values obtained by a multicomponent linear
regression using the ꢃ*, ˛ and ˇ-scale (Taft) solvent parameters.
R06 = 8.8 × 1023[ꢆ2n−4˚DJ(l)] in Å 6
(7)
(8)
According to Forester’s non-radiative energy transfer theory, the
energy transfer efficiency (E) is defined by the following equations:
ꢀ
∞
R06
(R06 + r06)
F
FD(ꢀ) εA(ꢀ)ꢀ4 dꢀ
E = 1 −
=
(6)
J(ꢀ) =
F0
0

J. Jayabharathi et al. / Spectrochimica Acta Part A 89 (2012) 194–200
199
Fig. 6. Stern–Volmer plot of F0/F against [imidazole].
Fig. 8. Overlapping of fluorescence spectra of BSA with absorption spectra of 3.
In the present case, ꢆ2 = 2/3, n = 1.16 ˚D = 0.22 and from the avail-
able data, it results that J(ꢀ) = 3.18 × 10−15 cm3 L mol−1, E = 0.61,
R0 = 2.95 nm and r = 1.9 nm were calculated. The donor-to-acceptor
distance is less than the 8 nm, and the long distance indicates that
the quenching mechanism is not a dynamic one, which is strong
evidence for static quenching and energy could transfer from BSA
to imidazole (3) [16].
3.5. Synchronous fluorescence spectroscopic studies of BSA
The synchronous fluorescence spectra can provide the infor-
mation about the molecular environment in the vicinity of the
chromophore molecules [29]. The fluorescence of BSA comes
from the tyrosine, tryptophan and phenylalanine residues. The
spectrum of BSA was sensitive to the micro-environment of
these chromophores and it allows non-intrusive measurements
of protein in low concentration under physiological conditions.
Hence spectroscopic methods were usually applied to the study
of conformation of serum protein. In synchronous fluorescence
spectroscopy, according to Miller [30], distinction of the differ-
ence between excitation wavelength and emission wavelength
(ꢂꢀ = ꢀemi − ꢀexc) reflects the spectra of a different nature of chro-
mophores, with large ꢂꢀ values such as 60 nm, the synchronous
fluorescence of BSA is characteristic of tryptophan residue and with
small ꢂꢀ values such as 15 nm is characteristic of tyrosine [31]. The
synchronous fluorescence spectra of BSA with various concentra-
tions of the imidazole derivative (3) were recorded at ꢂꢀ = 15 nm
and ꢂꢀ = 60 nm (Fig. 9a and b), respectively. From the synchronous
spectra it results out that, at ꢂꢀ = 15 nm, the emission wavelength
Fig. 7. Interaction between BSA and imidazole and the formation of BSA–imidazole
complex.
FD(ꢀ) is the corrected fluorescence intensity of the donor at wave-
length ꢀ to (ꢀ + ꢂꢀ), with the total intensity normalized to unity
and εA(ꢀ) is the molar extinction coefficient of the acceptor at
wavelength ꢀ. The Forester distance (R0) has been calculated by
assuming random orientation of the donor and acceptor molecules.
Fig. 9. Synchronous fluorescence spectra of BSA in the presence and absence of imidazole (a) at ꢂꢀ = 15 nm (b) at ꢂꢀ = 60 nm.

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J. Jayabharathi et al. / Spectrochimica Acta Part A 89 (2012) 194–200
(corresponding to tyrosine residues) is blue-shifted from 362 to
354 nm with increasing concentration of imidazole. At the same
time, for the fluorescence emission recorded at ꢂꢀ = 60 nm the
emission wavelength was decreased regularly, but no significant
change in wavelength was observed. It suggests that the interaction
of imidazole with BSA affects the conformation of tyrosine residue
and not the tryptophan micro-region. The tyrosine fluorescence
spectrum (ꢂꢀ = 15 nm) may represent that the conformation of
BSA is somewhat changed, leads to the polarity around Tyr residues
changes and the hydrophobicity increases [32]. This is because tyro-
sine contains one aromatic hydroxyl group unlike tryptophan and
tyrosine can undergo an excited state ionization, resulting in the
loss of the proton on the aromatic hydroxyl group. The hydroxyl
group can dissociate during the lifetime of its excited state, lead-
ing to quenching. Hence the aromatic hydroxyl group present in
the tyrosine residues is responsible for the interaction of BSA with
imidazole.
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One of the authors Dr. J. Jayabharathi, Associate Professor in
Chemistry, Annamalai University is thankful to Department of Sci-
ence and Technology [No. SR/S1/IC-73/2010] and University Grants
commission (F. No. 36-21/2008 (SR)) for providing funds to this
research study.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.saa.2011.12.060.