Kamlet-Taft and Catalan studies of some novel Y-shaped imidazole derivatives

Article abstract of DOI:10.1007/s10895-011-1009-x

Some novel Y-shaped imidazole derivatives were developed and characterized by NMR and mass spectral techniques. The photophysical properties of these imidazole derivatives were studied in several solvents. The Kamlet-Taft and Catalan's solvent scales were

Full text of DOI:10.1007/s10895-011-1009-x

J Fluoresc (2012) 22:737–744
DOI 10.1007/s10895-011-1009-x
ORIGINAL PAPER
Kamlet-Taft and Catalan Studies of Some Novel Y-Shaped
Imidazole Derivatives
J. Jayabharathi & V. Thanikachalam & K. Brindha Devi &
M. Venkatesh Perumal
Received: 28 May 2011 /Accepted: 20 October 2011 /Published online: 9 November 2011
#
Springer Science+Business Media, LLC 2011
Abstract Some novel Y-shaped imidazole derivatives
were developed and characterized by NMR and mass
spectral techniques. The photophysical properties of
these imidazole derivatives were studied in several
solvents. The Kamlet-Taft and Catalan’s solvent scales
were found to be the most suitable for describing the
solvatochromic shifts of the absorption and fluorescence
emission. The adjusted coefficient representing the
electron releasing ability or basicity of the solvent, Cβ
or CSB has a negative value, suggesting that the absorption
and fluorescence bands shift to lower energies with the
increasing electron-donating ability of the solvent. This
effect can be interpreted in terms of the stabilization of the
resonance structures of the chromophore. The observed
lower fluorescence quantum yield may be due to an
increase in the non-radiative deactivation rate constant.
This is attributed to the loss of planarity in the excited
state provided by the non co-planarity of the cinnamalde-
hyde ring attached to C(2) atom of the imidazole ring.
Such a geometrical change in the excited state leads to an
important Stokes shift, reducing the reabsorption and
reemission effects in the detected emission in highly
concentrated solutions.
Introduction
Recently, heterocyclic imidazole derivatives have attracted
considerable attention because of their unique optical
properties [1]. These compounds play very important role
in chemistry as mediators for synthetic reactions, primarily
for preparing functionalized materials [2]. Imidazole nucle-
us forms the main structure of some well-known compo-
nents of human organisms and also has significant
analytical applications such as laser [3], polymer stabilizer
[4], Raman filters [5] environmental probes in bio-
molecules [6], etc. by utilizing their fluorescence and
chemiluminescence properties.
Several factors contribute to the best laser performance of
pyrromethene with respect to rhodamine as laser dye [7]: (1)
low triplet-triplet absorption capacity at the lasing spectral
region, which reduces the losses in the resonator cavity; (2) a
poor tendency to self-aggregate in organic solvents, avoiding
the fluorescence quenching of the monomer emission by the
presence of aggregates in highly concentrated solutions, as
was observed in rhodamine dyes: (3) and their high photo-
stability [8, 9], which improves the lifetime of the laser
action with respect to that of rhodamines [10]. Owing to
these properties, PM dyes have been successfully incorpo-
rated into different solid matrixes (polymers, silica, etc.,) [11]
to develop solid-state syntonizable dye lasers.
.
.
.
Keywords NMR Kamlet-Taft Catalan parameters
Aggregation
Photophysical properties and laser characteristics of
compounds in liquid solutions and in polymeric matrixes
[
11], indicating that the laser behaviour is a consequence of
:
:
:
J. Jayabharathi (*) V. Thanikachalam K. B. Devi
M. V. Perumal
the photophysical properties. Indeed, the evolution of the
fluorescence wavelength and quantum yield with several
environmental factors was similar to that observed for the
laser band and efficiency, respectively. The Stokes shift
modifies the lasing performance [12] in highly concentrated
Department of Chemistry, Annamalai University,
Annamalainagar,
08 002, Tamilnadu, India
6
e-mail: jtchalam2005@yahoo.co.in

7
38
J Fluoresc (2012) 22:737–744
1
solutions because it affects the reabsorption and reemission
phenomena, which shifts the emission band to longer
wavelengths and reduces its efficiency [13]. In the present
paper, the photophysical properties of imidazole derivatives
H NMR (500 MHz, CDCl ): δ 9.87 (s, 1H), 8.09 (d, 1H),
3
1
3
8.01 (d, 1H), 7.98–6.79 (m, 15H). C (100 MHz, CDCl3):
δ 145.56, 136.48, 133.40–125.49 (Aromatic carbons). MS:
m/e 322.15, calcd 321.68.
1
and 2 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.
The fluorescence quantum yield and the Stokes shift are
analyzed to look for the best conditions to improve the
lasing efficiencies of these derivatives.
4,5-Diphenyl-2(E)-Styryl-1-p-Tolyl-1H-Imidazole (2)
Yield: 55%. mp : 168 °C, Anal. calcd. for C H N : C,
3
0 24 2
87.35; H, 5.86; N, 6.79. Found: C, 86.57; H, 5.14; N, 7.57.
1
H NMR (500 MHz, CDCl ): δ 2.33 (s, 3H), 6.84 (d, 2H),
3
1
3
8.01–7.03 (m, 19H). C (100 MHz, CDCl ): δ 24.1,
3
110.16, 122.4, 127.45–135.89 (Aromatic carbons). MS: m/e
412.19, calcd 410.57.
Experimental
4-((E)-2-(4,5-Diphenyl-1-p-Tolyl-1H-Imidazol-2-yl)vinyl)
phenol (3)
Materials and Methods
Yield: 55%. mp : 165 °C, Anal. calcd. for C H N O: C,
3
0 24 2
Benzil (Sigma-Aldrich Ltd.), cinnamaldehyde, p-hydroxy
cinnamaldehyde, p-methyl aniline (S.D. fine.) and all the
other reagents were used without further purification.
84.08; H, 5.65; N, 6.54. Found: C, 83.13; H, 5.02; N, 7.33.
1
H NMR (500 MHz, CDCl ): ): δ 2.31 (s, 3H), 5.31 (s,
3
1
3
1H), 6.89 (d, 2H), 8.05–6.98 (m, 18H). C (100 MHz,
CDCl ): δ 24.1, 110.16, 122.4, 127.45–135.89 (Aromatic
3
Optical Measurements and Composition Analysis
carbons). MS: m/e 428.19, calcd 426.97.
NMR spectra were recorded on a Bruker 400 MHz
instrument. The ultraviolet–visible (UV–vis) spectra were
measured on UV–vis spectrophotometer (Perkin Elmer,
Lambda 35) and corrected for background due to solvent
absorption. Photoluminescence (PL) spectra were recorded
on a (Perkin Elmer LS55) fluorescence spectrometer. MS
spectra were recorded on a Varian Saturn 2200 GCMS
spectrometer.
Results and Discussion
Steric Hindrance in Imidazoles: X-Ray Analysis
DFT calculation of Y-shaped imidazole derivatives reveal
that the imidazole ring is essentially planar [27] and makes
dihedral angle around 166.0° with the cinnamaldehyde ring.
Three key twists, designated as α, β and g have been
examined. α is used to indicate the twist of imidazole ring
from the aromatic six-membered ring at C(2), β and g are
used for twists of imidazole ring from phenyls at C(5) and
C(4) positions, respectively (Fig. 1a). By comparison of
results in Table 1 several additional interesting structural
features can be concluded that the g twist is always smaller
than the β twist. The β twist originates from the interaction
of substituent at N(1) of the imidazole with the phenyl
group at C(5) whereas the g twist is a result of the
interaction of the phenyl group at C(4) with the other one at
C(5). The β twist always increases upon the substitution at
N(1) of the imidazole derivatives compared to the parent
counterparts and the substitution increases the β twist but
decreases the g twist (Fig. 1b). The present structural
information allows us to further explore the correlation
between structural features and fluorescent property. It
reveals that α twist is correlated with fluorescent property,
the larger α twist; the more drops the fluorescence quantum
yield. Such a clear correlation indicates the importance of
coplanarity between imidazole and the aromatic ring at C
Computational Details
Quantum mechanical calculations were used to carry out
the optimized geometry, with Guassian-03 program using
the Becke3-Lee-Yang-Parr (B3LYP) functional supple-
mented with the standard 6–31 G(d,p) basis set [14].
General Procedure for the Synthesis of Imidazole
Derivatives 1–3
The experimental procedure was used as the same as
described in our recent papers [15–26]. The imidazole
derivatives were synthesized from an unusual four compo-
nents assembling of benzil, ammonium acetate, aniline and
the corresponding aldehydes (Scheme-1).
4
,5-Diphenyl-2(E)-Styryl-1H-Imidazole (1)
Yield: 55%. mp : 260 °C, Anal. calcd. for C H N : C,
2
3 18 2
8
5.68; H, 5.63; N, 8.69. Found: C, 84.89; H, 5.23; N, 7.93.

J Fluoresc (2012) 22:737–744
739
2
2
where f ðD; nÞ ¼ ðD ꢀ 1Þ=ð2D þ 1Þ ꢀ ðn ꢀ 1Þ=ð2n þ 1Þ,
indicates the orientation polarizability and depicts polarity
parameter of the solvent [29], n is refractive index, D is
dielectric constant, μ and μ are dipole moments of the
e
g
species in S1 and S0 states, respectively, h, Planck’s
constant; c, velocity of light and a, Onsagar’s cavity radius.
Stokes shifts were calculated from lf
rather than 0-0-
max
emission transition. The Lippert-Mataga plot is linear for
the non-polar and polar / aprotic solvents with excellent
correlation coefficient.
Large stokes shifted fluorescence band suggest that this
emission has originated from the species which is not
present in the S state and large geometrical changes have
0
takes place in the species when excited to S state and the
large stokes shift may be explained by the presence of
intermolecular hydrogen bonding of imidazole nitrogen
1
Fig. 1 (a) The key α twist of imidazole ring at C(2) (b) Molecular
modeling of the imidazole derivative (3) by DFT/B3LYP/6–31 G(d,p)
(N2) with polar solvent molecules leading to the stabiliza-
tion of solvated isomers of 1–3 (Fig. 3).
(
2). This correlation can be ascribed to the conjugation
UV–vis absorption and fluorescence spectra (Table 2) of
the Y-shaped imidazole derivatives reveal that in apolar
solvents, the main absorption band is centered around
rigidity. When the two adjacent aromatic species are in a
coplanar geometry, the p-orbitals from the C–C bond
connecting 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 of (1). The
present bond distance of C2–C21 is 1.454(4) Å (Table 1) is
shorter than the regular single bond distance between two
sp2 carbons (1.48Å) [27], because of delocalization.
300 nm, whereas the fluorescence spectrum is centered
around 370 nm. The fluorescence spectrum of the imidaz-
ole derivative 3 is shifted to lower energies with respect to
parent compound 1 in common solvents. These spectral
shifts are attributed to the higher inductive electron-
acceptor character of the fluorine atom located at C(13)
carbon atom. In the case of alcoholic solvents, bath-
ochromic shifts were observed for both absorption and
emission providing large Stokes shift. These results suggest
Solvent Effects on the Absorption and Fluorescence
Properties
an important geometrical rearrangement in the S excited
1
state and these observations are in good agreement with the
literature report [30].
All these imidazole derivatives show solvatochromism
(
Table 2) i.e., changes in the polarity of the solvents,
The higher bathochromic shifts were observed in the
fluorescence band of the imidazole derivative 3 with
respect to the parent 1 can be interpreted by the
Brunings-Corwin effect (Twisted geometry of the imid-
azole derivative lead the observed bathochromic shift)
charge transfer takes place and causing colour changes. The
position of the longer wavelength absorption and emission
bands in the spectra was determined in several protic and
aprotic solvents (Fig. 2). Lippert-Mataga [28] plot was
constructed for the normal fluorescence spectrum of
imidazole derivatives (1–3) (Fig. 3) using the following
equation.
[
31]. The distortion of the geometry in the excited state
implies a decrease in the resonance energy, the fluores-
cence band is bathochromically shifted to a higher extent
than the absorption band. Moreover, the loss of planarity
in the excited state of the imidazole derivative could
explain the lower fluorescence quantum yield in apolar
solvents owing to an increase in the non-radiative
processes. Taking into account the resonance structures
of the imidazole chromophore, we observed that the
resonance structure “b” has the largest charge separation
along the short molecule axis. Consequently, its contribu-
"
#
2
2
ðm ꢀ m Þ
ꢀ
ꢀ
ꢀ
e
g
n ¼ n ꢀ n ¼ const þ
f ðD; nÞ
ð4Þ
ss
ab
fl
3
hca
Table 1 Deviation parameters (°) of imidazole from other rings
Compound
(α)
(β)
(g)
ϕ
f
τ
f
(ns)
1
2
3
−43.08
43.16
41.90
8.02
7.90
5.29
166.06
178.06
179.28
0.09
0.12
0.15
1.3
tion would be more important in the S ground states than
0
1.4
1.5
in the S excited states. Thus, the polar solvents would
1
stabilize the S state more extensively than the S state,
0
1

7
40
J Fluoresc (2012) 22:737–744
Table 2 Photo physical data of imidazole derivatives 1–3
Solvents
1
2
3
ss (cm−
1
)
lmax (nm)
l
(nm)
υ
ss (cm−1
)
l
max (nm)
l
f
(nm)
υ )
ss (cm−1
l
max (nm)
l
f
(nm)
υ
f
Hexane
310.0
309.0
305.7
308.0
309.0
306.0
308.0
295.0
297.0
303.0
305.0
304.0
368.0
368.0
372.0
385.0
369.0
380.0
369.0
366.0
377.0
379.0
376.0
379.0
5084
5188
5830
6493
5262
6363
5367
6575
7144.
6618
6191
6509
351.0
348.0
350.0
348.0
350.0
348.0
348.0
344.0
344.0
344.7
349.0
344.5
405.0
406.0
409.0
414.0
408.0
413.0
408.0
418.0
417.0
413.5
407.0
415.0
3798
4105
4121
4581
4061
4522
4225
5146
5088
4826
4083
4931
360.0
362.0
369.0
370.0
362.0
368.0
363.0
370.0
368.4
368.0
371.0
368.5
410.0
409.0
420.0
425.0
411.0
422.0
411.0
428.0
423.0
424.0
423.0
429.0
3387
3174
3290
3497
3293
3477
3217
3662
3503
3589
3313
3827
Benzene
1
,4-Dioxane
Diethyl ether
Chloroform
Ethyl acetate
Dichloromethane
1
-propanol
Ethanol
Methanol
Acetonitrile
DMSO
thereby increasing the energy gap between both states and
explaining the solvatochromic shifts [30].
where (XYZ)
solvent and C
0
is the physicochemical property in an inert
, C , C and so forth are the adjusted
a
b
c
To analyze the solvatochromic effects, we checked
several methods [32–34]. Neither the absorption nor the
fluorescence wavenumbers linearly correlate with the
coefficients that reflect the dependence of the physico-
chemical property (XYZ) on several solvent properties.
Solvent properties that mainly affect the photophysical
properties of aromatic compounds are polarity, H-bond
donor capacity and electron donor ability. Different scales
for such parameters can be found in the literature, Taft et al.
[37] propose the π* (dipolarity/polarizability), α (acidity )
and β (basicity) scales, whereas more recently Catalan et al.
2
Lippert parameter Δf( ,n ) [35], which considers the
solvent polarity/polarizability or with the Reichardt param-
N
eter ET (30) [33, 36], which takes into account several
solvent properties (polarity and H-bond donor capacity) in a
common parameter. For these reasons, a multi-parameter
correlation analysis is employed in which a physicochem-
ical property is linearly correlated with several solvent
parameters by means of Eq. 1:
N
[38] suggest the SPP , SA and SB scales to describe the
polarity/polarizability, the acidity and basicity of the
solvents respectively.
Figure 4 shows the obtained correlation between the
absorption and fluorescence wavenumbers calculated by the
ðXYZÞ ¼ ðXYZÞ þ C A þ C B þ C C þ . . .
ð1Þ
0
a
b
c
Fig. 2 Absorption and fluorescence spectra of 3 in various solvents

J Fluoresc (2012) 22:737–744
741
Fig. 3 Solvation of imidazole derivative and Lippert mataga plot of 1–3
multi-component linear regression employing the Taft-
proposed solvent parameters and the experimental
values listed in Table 3. Table 3 lists the obtained
adjustment and correlation coefficients by the Taft and
Catalan parameters. The dominant coefficient affecting the
absorption and fluorescence bands of Y-shaped imidazole
derivatives 1–3 is that describing the polarity/polarizabil-
ity of the solvent, Cπ* or CSPP having a positive value,
corroborating the above-mentioned solvatochromic shifts
with the solvent polarity. The coefficient controlling the
acidity does not play an important role in absorption and
fluorescence displacements. The adjusted coefficient rep-
resenting the electron releasing ability or basicity of the
solvent, C or C has a negative value, suggesting that
the absorption and fluorescence bands shift to lower
energies with the increasing electron-donating ability of
the solvent. This effect can be interpreted in terms of the
stabilization of the resonance structures of the chromo-
phore (Fig. 5). Resonance structure “b” has the positive
charge located at the nitrogen atom and it will be
stabilized in basic solvents because this resonance struc-
β
SB
N
H-donor capacity or acidity of the solvent, C or CSA, is
α
the lowest coefficient (Table 3), therefore, the solvent
ture is predominant in the S state, as discussed above and
1
Fig. 4 Correlation between the experimental absorption and fluorescence wavenumber with the predicted values obtained by a multicomponent
linear regression using the π*, α and β-scale (Taft) solvent parameters for 1–3

7
42
J Fluoresc (2012) 22:737–744
Table 3 Adjusted Coefficients ((υ
Coefficients (r) for the Multilinear Regression Analysis of the
Absorption υab and Fluorescence υfl Wavenumbers and Stokes Shift
x
)
0
, c
a
, c
b
and c
c
) and Correlation
(Δυss) of imidazole derivatives 1–3 with the Solvent Polarity/
Polarizability, and the Acid and Base Capacity Using the Taft (π*, α
and β) and the Catalan (SPP , SA and SB) Scales
N
cm−
1
(π*)
c
c
r
Kamlet-Taft
x
(υ )
0
α
β
4
4
4
3
3
3
3
3
3
3
1
2
3
l
l
ab
fl
(3.23±0.01)×10
(2.71±0.01)×10
(3.78±2.17)×10
(2.40±2.05)×10
−(12.03±7.24)×10
−(16.87±7.85)×10
(11.41±5.82)×10
0.87
0.84
0.91
r
(16.68±5.50)×10
3
3
Δυss=υab – υfl
(0.52±0.01)×10
(1.38±2.31)×10
(4.84±7.73)×10
−(5.27±6.21)×10
cm−
1
c
SPP
N
c
SA
c
SB
Catalan
x
(υ )
0
4
4
4
3
3
3
3
3
l
l
ab
fl
(3.25±0.01)×10
(2.67±0.02)×10
(0.22±6.66)×10
(7.94±9.44)×10
(20.33±17.15)×10
−(31.72±17.62)×10
(22.95±40.21)×10
0.90
0.79
0.54
r
3
3
−(27.87±39.16)×10
3
3
Δυss=υab – υfl
(0.58±0.03)×10
−(7.71±11.39)×10
(π*)
(48.18±47.21)×10
−(54.66±48.51)×10
cm−
1
c
c
β
Kamlet-Taft
x
(υ )
0
α
4
4
4
3
3
3
l
l
ab
fl
(2.86±0.00)×10
(2.46±0.00)×10
−(1.66±0.88)×10
(6.08±2.94)×10
−(4.16±2.36)×10
0.87
0.90
0.93
r
3
3
3
(0.54±1.01)×10
−(3.81±3.37)×10
(9.89±4.61)×10
(2.75±2.71)×10
3
3
3
Δυss=υab – υfl
(0.40±0.00)×10
−(2.20±0.46)×10
−(6.90±3.70)×10
cm−
1
c
SPP
N
c
c
SB
Catalan
x
(υ )
0
SA
4
4
4
3
3
3
l
l
ab
fl
(2.87±0.00)×10
(2.44±0.01)×10
−(0.46±1.97)×10
(7.90±8.18)×10
−(10.15±8.40)×10
(23.94±15.81)×10
0.84
0.66
0.78
r
3
3
3
(2.93±3.71)×10
−(20.06±15.40)×10
3
3
3
Δυss=υab – υfl
(0.42±0.01)×10
−(3.39±5.06)×10
(π*)
(27.97±20.96)×10
−(34.10±21.53)×10
cm−
1
c
c
β
Kamlet-Taft
x
(υ )
0
α
4
4
4
3
3
3
3
3
3
3
l
l
ab
fl
(2.76±0.00)×10
(2.47±0.00)×10
−(4.66±0.80)×10
−(3.58±1.15)×10
−(1.08±1.04)×10
(10.95±2.62)×10
−(7.45±2.15)×10
−(3.77±3.10)×10
−(3.68±2.80)×10
0.94
0.95
0.79
r
3
(6.03±3.86)×10
3
Δυss=υab – υfl
(0.42±0.01)×10
(4.92±3.49)×10
cm−
1
c
SPP
N
c
SA
c
SB
Catalan
x
(υ )
0
4
4
4
3
3
3
3
3
3
l
l
ab
fl
(2.73±0.01)×10
(2.40±0.01)×10
(0.34±0.00)×10
(1.53±6.47)×10
(3.74±7.82)×10
−(10.29±26.82)×10
−(22.68±32.43)×10
(12.06±27.54)×10
(25.46±33.31)×10
0.75
0.34
0.60
3
3
3
Δυss=υab – υfl
−(2.21±2.38)×10
(12.39±9.85)×10
−(13.40±10.12)×10
the stabilization of the S state with the solvent basicity
reduction in the reabsorption and reemission losses, owing
to the higher stokes shift. From this photophysical studies,
polar solvents are recommended to obtain the highest
optical efficiencies in liquid media.
1
would be more important than that of the S0 state.
Consequently, the energy gap between the S1 and S0
states decreases and the absorption and fluorescence
wavelengths shift to longer wavelengths with increasing
solvent basicity.
Conclusions
The presence of cinnamaldehyde ring at C(2) in the Y-
shaped imidazole chromophore core originates a distortion
from planarity in the imidazole units, mainly in the excited
state, which leads to an increase in the rate constant of non-
radiative deactivation and in the stokes shift. Both photo-
physical factors have an opposite effect on the lasing
efficiency. Thus, the increase in the loss of the resonator
cavity due to the augmentation in the non-radiative
processes could be compensated to some extent by a
Fig. 5 Resonance structures of the imidazole chromophore (3)

J Fluoresc (2012) 22:737–744
743
Acknowledgements One of the author Dr. J. Jayabharathi, Associ-
ate Professor in Chemistry, Annamalai University is thankful to
Department of Science and Technology [No. SR/S1/IC-07/2007],
University Grants commission (F. No. 36-21/2008 (SR)) for providing
fund to this research work.
17. Gayathri P, Jayabharathi J, Srinivasan N, Thiruvalluvar A,
Butcher RJ (2010) 2-(4-Fluorophenyl)-4,5-dimethyl-1-(4-methyl-
phenyl)-1H-imidazole. Acta Crystallogr E 66:o1703
18. Jayabharathi J, Thanikachalam V, Venkatesh Perumal M,
Saravanan K (2011) Displacement reaction using ibuprofen
in a mixture of bioactive imidazole derivative and bovine
serum albumin–a fluorescence quenching study. J Fluoresc.
doi:10.1007/s10895-011-0878-3
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