Physicochemical studies of chemosensor imidazole derivatives: DFT based ESIPT process

Article abstract of DOI:10.1016/j.saa.2011.12.053

A series of substituted imidazoles have been synthesized in very good yield under a solvent free condition using molecular iodine as the catalyst. An excited state intramolecular proton transfer (ESIPT) process in hydroxy imidazole has been studied using emission spectroscopy. DFT calculations on energy, dipole moment, charge distribution of the rotamers in the ground and excited states of the imidazole derivatives have been performed and discussed. DFT analysis about HOMO, HOMO-1, LUMO and LUMO+1 has been carried out and discussed. The energy barrier for the interconversion of two rotamers is too high in the excited state than the ground state that is shown by PES calculation. The molecular electrostatic potential surface (MEP) has also been employed to show the higher electron density at N(3) nitrogen. Fluorescence enhancement has been found in the presence of transition metal ions and this may result from the suppression of radiationless transitions from the n-π* state in the chemosensors.

Full text of DOI:10.1016/j.saa.2011.12.053

Spectrochimica Acta Part A 89 (2012) 168–176
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Physicochemical studies of chemosensor imidazole derivatives:
DFT based ESIPT process
Jayaraman Jayabharathi∗, Venugopal Thanikachalam, Karunamoorthy Jayamoorthy
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:
A series of substituted imidazoles have been synthesized in very good yield under a solvent free condition
using molecular iodine as the catalyst. An excited state intramolecular proton transfer (ESIPT) process
in hydroxy imidazole has been studied using emission spectroscopy. DFT calculations on energy, dipole
moment, charge distribution of the rotamers in the ground and excited states of the imidazole derivatives
have been performed and discussed. DFT analysis about HOMO, HOMO−1, LUMO and LUMO+1 has been
carried out and discussed. The energy barrier for the interconversion of two rotamers is too high in
the excited state than the ground state that is shown by PES calculation. The molecular electrostatic
potential surface (MEP) has also been employed to show the higher electron density at N(3) nitrogen.
Fluorescence enhancement has been found in the presence of transition metal ions and this may result
from the suppression of radiationless transitions from the n–␲* state in the chemosensors.
© 2012 Elsevier B.V. All rights reserved.
Received 20 October 2011
Received in revised form 9 December 2011
Accepted 21 December 2011
Keywords:
Greenway synthesis
NLO
HOMO–LUMO
MEP
Chemosensor
1. Introduction
imidazoles. Herein we focus light on the photophysical studies of
2-arylimidazole derivatives (1–4) and ESIPT process of hydroxy
Excited state intramolecular proton transfer (ESIPT) phenom-
ena have been investigated [1,2] due to the practical applications
of ESIPT exhibiting molecules used as laser dyes, photo stabilizers,
fluorescent probes in biology and light-emitting materials for elec-
troluminescent devices [2–4]. ESIPT occurs in molecules having a
phenolic hydroxy group with an intramolecular hydrogen bond to
the nearby hetero atom of the same chromophore. The proton in
the hydroxy group upon photo excitation migrates to the hetero
imidazole 3. Density functional theory (DFT) calculation [10] on
energy, dipole moment of various species, HOMO, HOMO−1, LUMO
and LUMO+1 energies, PES and molecular electrostatic potential
surface (MEP) studies have been performed and discussed.
2. Experimental
˚
atom at a distance <2 A to form the excited state tautomer. The
2.1. Optical measurements and composition analysis
ESIPT exhibiting substances have dual fluorescence, depending on
the molecular nature and external factors such as solvent polarity
and temperature.
NMR spectra have been recorded for the 2-arylimidazole deriva-
tives (1–4) on a Bruker 400 MHz instrument. The ultraviolet–visible
(UV–Vis) spectra have been measured on UV–Vis spectropho-
tometer (Perkin Elmer, Lambda 35) and corrected for background
due to solvent absorption. Photoluminescence (PL) spectra
Organic solvents are high on the list of damaging chemicals,
so designing of “green” experimental protocol is a challenge to
chemists to improve the quality of synthesis. Though there are
several methods reported in the literature [5–8] for the synthe-
sis of imidazoles, they suffer from disadvantages such as harsh
reaction conditions, poor yields, prolonged time period, use of haz-
ardous and expensive acid catalysts. The advantages associated
with eco-friendly iodine element, has been explored as a powerful
catalyst for various syntheses [9]. During the course of our studies
towards the development of new route to the synthesis, we wish to
report a simple and efficient method for the synthesis of substituted
have been recorded on
a (Perkin Elmer LS55) fluorescence
spectrometer. Solvents used for spectral measurements are
spectroscopic grade. Mass spectra have also been recorded
on
cence quantum yield has been measured in acetonitrile using
coumarin 47 as standard [11] according to the equation,
unk = ˚std(Iunk/Istd)(Astd/Aunk)(ꢀunk/ꢀstd)2, where Фunk, Фstd, Iunk
a Varian Saturn 2200 GCMS spectrometer. The fluores-
a
˚
,
Istd, Aunk, Фunk and Фstd are the fluorescence quantum yields, the
integration of the emission intensities, the absorbances at the exci-
tation wavelength and the refractive indexes of the corresponding
solution of the 2-aryl imidazole derivatives (1–4) and the standard,
∗
Corresponding author. Tel.: +91 9443940735.
E-mail address: jtchalam2005@yahoo.co.in (J. Jayabharathi).
respectively. The radiative (kr) [kr = ˚p/ꢁ] and non-radiative (knr)
1386-1425/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2011.12.053

J. Jayabharathi et al. / Spectrochimica Acta Part A 89 (2012) 168–176
169
Scheme 1. Experimental pathway for the synthesis of imidazole derivatives 1–4.
[knr = 1/ꢁ − ˚p/ꢁ] rate constants were calculated using the life time
(ꢁ) of the excited state.
126.96, 127.54, 128.23, 128.63–129.13, 130.84, 131.58, 135.68,
137.60, 145.96 (aromatic carbons). MS: m/e 296.37, calcd 297.37
[M+1].
2.2. Molecular electrostatic potential (MEP) calculation
2.3.2. 2-(2,4-Difluorophenyl)-4,5-diphenyl-1H-imidazole (2)
In the present work MEPs derive from a classical point charge
model. The electrostatic potential for each molecule is obtained
by moving a unit positive point charge across the van der Waals
A
mixture of 2,4-difluorobenzaldehyde (1 mmol), benzil
(1 mmol), ammonium acetate (2.5 mmol) and iodine (15 mol%)
were grined in a morter at room temperature for appropriate time.
The reaction was monitored by TLC and the reaction mixture was
treated with aqueous Na2S2O3; the formed crude was purified
by column chromatography using n-hexane:ethyl acetate (9:1) as
the eluent. Yield: 55%. mp = 167 ◦C, Anal. calcd. for C21H14F2N2:
C, 75.89; H, 4.25; N, 8.43. Found: C, 75.05; H, 4.14; N, 7.10. 1H
NMR (400 MHz, CDCl3): ı 7.23–8.14 (m, 13H), 12.67 (s, 1H). 13C
(100 MHz, CDCl3): ı 116.02, 116.19, 127.00–129.12 (aromatic car-
bons), 131.52, 135.59, 137.56, 141.63, 145.15. MS: m/e 332.05, calcd
333.35 [M+1].
surface and it is calculated [12] at various points j on this surface
ꢀ
using, Vj = qi/rji, where qi represents the partial charge of each
atom i, and rji is the distance between point’s j and atom i. Starting
from the 3D model of a molecule and its partial atomic charges,
the electrostatic potential is calculated for points on the molecular
surface.
2.3. Synthesis of 2-arylimidazole derivatives (1–4)
The greenway experimental pathway [13] used for the synthetic
mechanism of 2-aryl imidazole derivatives (1–4) has been given in
Scheme 1.
2.3.3. 5-Bromo-2-(4,5-diphenyl-1H-imidazol-2-yl)phenol (3)
A mixture of 5-bromo-2-hydroxybenzaldehyde (1 mmol), ben-
zil (1 mmol), ammonium acetate (2.5 mmol) and iodine (15 mol%)
were grined in a morter at room temperature for appropriate time.
The reaction was monitored by TLC and the reaction mixture was
treated with aqueous Na2S2O3, the formed crude was purified by
column chromatography using n-hexane:ethyl acetate (9:1) as the
eluent. Yield: 55%. mp = 143 ◦C, Anal. calcd. for C21H15BrN2O: C,
64.46; H, 3.86; N, 7.16. C, 64.96; H, 3.95; N, 7.61. 1H NMR (400 MHz,
CDCl3): ı 6.93–8.05 (m, 13H), 12.97 (s, 2H). 13C (100 MHz, CDCl3): ı
113.36–130.54 (aromatic carbons), 146.33, 157.17. MS: m/e 390.00,
calcd 391.26 [M+1].
2.3.1. 2,4,5-Triphenyl-1H-imidazole (1)
A mixture of benzaldehyde (1 mmol), benzil (1 mmol), ammo-
nium acetate (2.5 mmol) and iodine (15 mol%) were grined in a
morter at room temperature for appropriate time. The reaction
was monitored by TLC and the reaction mixture was treated with
aqueous Na2S2O3, the formed crude was purified by column chro-
matography using n-hexane:ethyl acetate (9:1) as the eluent. Yield:
60%. mp = 121 ◦C, Anal. calcd. for C22H16N2: C, 85.11; H, 5.44; N,
9.45. Found: C, 84.92; H, 5.39; N, 9.69. 1H NMR (400 MHz, CDCl3): ı
7.23–8.10 (m, 15H), 12.66 (s, 1H). 13C (100 MHz, CDCl3): ı 125.67,

170
J. Jayabharathi et al. / Spectrochimica Acta Part A 89 (2012) 168–176
Fig. 1. Excitation spectra of the imidazole derivatives 1–4.
2.3.4. 2-(2-Methoxyphenyl)-4,5-diphenyl-1H-imidazole (4)
mixture of 2-methoxybenzaldehyde (1 mmol), benzil
3. Results and discussion
A
(1 mmol), ammonium acetate (2.5 mmol) and iodine (15 mol%)
were grined in a morter at room temperature for appropriate time.
The reaction was monitored by TLC and the reaction mixture was
treated with aqueous Na2S2O3, the formed crude was purified by
column chromatography using n-hexane:ethyl acetate (9:1) as
the eluent. Yield: 50%. mp = 178 ◦C, Anal. calcd. for C22H18N2O:
C, 80.96; H, 5.56; N, 8.58. Found: C, 80.00; H, 5.06; N, 8.41. 1H
NMR (400 MHz, CDCl3): ı 3.87 (s,3H), 6.97–7.87 (m, 14H), 12.67 (s,
1H). 13C (100 MHz, CDCl3): ı 55.37, 114.31, 127.79, 126.76–132.45
(aromatic carbons), 146.10, 160.20, 167.30. MS: m/e 327.50, calcd
327.39 [M+1].
The 2-arylimidazole derivatives (1–4) fluoresce strongly in solu-
tions at room temperature. Their luminescence excitations spectra
(Fig. 1) are in coincidence with their absorption spectra and dif-
fer from their emission spectra (Fig. 2). Hence, the fluorescence of
the 2-arylimidazole derivatives deserves for discussion. Fluores-
cence band maxima (ꢂflu) fluorescence excitation maxima (ꢂexc
)
and Stokes shifts (ꢃꢄss) have been presented in Table 1. The fluo-
rescence spectra of 3 in dioxane display an abnormal Stokes-shifted
emission band at shorter wavelength (410.8 nm) and one small
shoulder peak at longer wavelength (460.0 nm). This result shows
that in aprotic solvent, the hydroxy 2-arylimidazole exists as two
Fig. 2. Emission spectra of the imidazole derivatives 1–4.

J. Jayabharathi et al. / Spectrochimica Acta Part A 89 (2012) 168–176
171
Table 1
Photophysical datas of the imidazole derivatives 1–4.
Solvent
1
2
3
4
ꢂflu
ꢂexc
ꢃꢄss
ꢂflu
ꢂexc
ꢃꢄss
ꢂflu
ꢂexc
ꢃꢄss
ꢂflu
ꢂexc
ꢃꢄss
Hexane
360.5
365.6
370.1
375.9
380.8
378.1
381.4
383.5
384.3
388.5
389.2
391.0
309.0
309.6
310.0
311.2
314.1
312.9
319.0
322.5
328.1
329.1
335.0
337.0
4665
5000
5249
5418
5597
5258
5011
5067
4541
4526
4220
4151
383.4
383.9
384.2
384.3
384.9
385.1
385.2
386.2
386.3
388.1
397.8
411.9
303.0
304.1
305.2
306.1
309.4
311.2
316.0
322.0
324.5
341.8
342.5
346.0
7019
6966
6932
6755
6466
6197
5866
5279
5042
3689
4239
4359
409.2, 458.0(sh)
409.9, 459.0(sh)
410.8, 460.0(sh)
415.3, 462.0(sh)
417.0, 463.5(sh)
418.0, 465.0(sh)
423.2
323.2
323.4
321.0
325.2
326.5
327.0
322.5
320.5
319.0
320.0
322.0
319.0
6503
6535
6588
6709
6760
6742
7543
7504
7941
8161
8289
8139
355.0
360.7
364.9
370.0
376.4
383.8
388.1
390.6
398.7
419.1
421.8
423.1
308.0
310.4
311.1
319.8
320.6
322.0
330.1
340.5
345.2
389.7
392.2
397.0
4256
4576
4843
4390
4761
5185
4555
3949
3980
1728
1494
1440
Cyclohexane
1,4-dioxane
Benzene
Diethylether
Chloroform
Ethyl acetate
Dichloromethane
1-Butanol
420.6
401.0
398.2
396.0
Ethanol
Methanol
Acetonitrile
429.6
To evidence ESIPT mechanism in 3, we have performed DFT cal-
culation to know the electron density of the keto and enol isomers
of 3 in the ground and the excited states (Table 2). The results
reveal that excitation of enol isomer leads to an increase in the
electron density at the N(5) nitrogen atom and decreases the same
at the oxygen atom resulting in ESIPT. As a consequence of ESIPT,
the excited keto isomer is formed. The excited keto isomer emits
luminescence and returns to the ground state keto form. This is
characterized by a large positive charge at the N(5) nitrogen atom
and negative charge at the oxygen atom. As a result, a reverse pro-
cess occurs in the ground state of the molecule producing the enol
form.
3.1. Driving force for ESIPT process
The existence of intramolecular hydrogen bond in hydroxy imi-
dazole 3 is confirmed by the presence of the singlet at 12.97 ppm
in the 1H NMR spectra (Fig. 5) which is a typical signal for hydro-
gen bonded hydrogen atom. In order to reveal the contribution of
the intramolecular hydrogen bonding in the hydroxy imidazole 3
to their optical properties, the fluorescence spectra of compound 3
and its methoxy derivative 4 have been measured in dioxane sol-
vent under identical condition (Fig. 6). A dual fluorescence detected
for 3 with emission peaks centred at 460 and 410.8 nm, respec-
tively. The emission peak at shorter wavelength (410.8 nm) is
assigned to rotamer I and that at the longer wavelength (460 nm) is
assigned to rotamer II. Compound 4 exhibits emission peak only at
364.9 nm. The absence of additional peak at longer wavelength con-
firms the absence of intramolecular hydrogen bond in compound
4. It is further evident that intramolecular hydrogen bonding is the
driving force for ESIPT and the dual fluorescence behaviour of 3.
Fig. 3. Intramolecular hydrogen bonded isomeric forms of the imidazole derivative
3.
different intramolecular hydrogen bonded isomeric forms I and II
(Fig. 3). Excitation of the isomer II should lead to the formation of
the keto-isomer III due to ESIPT (Fig. 4), while excitation of the iso-
mer I must yield the normal emission. This reveals that only isomer
II of these molecules is stable under the experimental conditions.
Stokes shift is important for a fluorescent sensor. The higher stokes
shift value supplies very low background signals and allows the
usage of the material in construction of a fluorescence sensor [13].
However in hydroxylic solvent (EtOH), a short wavelength emis-
sion band appears for 3 which is absent in the fluorescence spectra
of 1, 2, 4 and 5 (Fig. 2). This result corresponds to the data
obtained earlier for compounds demonstrating ESIPT [14–16]. This
is explained by the presence of intermolecular hydrogen bond-
ing with solvent molecule leading to the stabilization of solvated
isomer IV in which ESIPT is impossible.
3.2. Competition of intra and intermolecular hydrogen bonding
in binary solvents
The competition of intra and intermolecular hydrogen bond-
ing in 3 has been studied in dioxane–water mixture. A series of
solutions in dioxane–water mixture with different water fraction
but identical concentrations of 3 have been made for spectroscopic
measurements. In pure dioxane, hydroxy imidazole 3 exhibits both
normal and tautomer emission peaks at 410.8 and 460 nm, respec-
tively. The tautomer emission intensity decreases relatively to that
Table 2
Electron density of atoms N(3) oxygen for the imidazole derivative 3.
Atom
II
II*
III
0.312
−0.698
N(3)
O
−0.394
−0.571
−0.456
−0.528
*
Corresponds to excited state of II.
Fig. 4. ESIPT mechanism of the imidazole derivative 3.

172
J. Jayabharathi et al. / Spectrochimica Acta Part A 89 (2012) 168–176
Fig. 5. Expanded 1H NMR spectrum of the imidazole derivative 3.
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 appeared at 430.0 nm in addition to the nor-
mal 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 bonding between 3 and water is taken into account. In the
initial stage, the presence of small amount of water in dioxane solu-
tion must give rise to solvation of 3. The intermolecular hydrogen
bonding between 3 and water definitely disrupts the ground state
intramolecular hydrogen bonding rotamers I and II but increases
the quantity of species IV, in which ESIPT and tautomer formation
are inhibited. Consequently the tautomer emission decreases with
the addition of water and finally vanished [17].
Fig. 7. Competition of intra and intermolecular hydrogen bonding of the imidazole
derivative 3 in binary solvents.
3.3. Density functional theory calculation
In the present study, theoretical calculations [18–20] have been
used to support ESIPT process. The ground-state geometries of the
three species, I, II, and III of 3 have been optimized using the DFT/6-
31G(d,p) method. The energy of the excited state has also been
calculated using the standard CIS method. Table 3 gives the energies
and dipole moment of the species I, II, and III in the ground and the
excited states. For better understanding the ESIPT mechanism in 3,
we have performed DFT calculation to know the electron density of
the keto and enol isomers of 3 in the ground and the excited states
(Table 2).
In the excited state the nitrogen atom becomes richer in elec-
trons than the hydroxylic oxygen atom. The ␲-electron densities
in the excited state are the driving force for the intramolecular
proton transfer from the hydroxylic group to the nitrogen atom.
The potential energy surface (PES) curves (Fig. 8a and b) for the
Table 3
Relative energies and dipole moment for the imidazole derivative 3.
Rotamer
Ground state
Excited state
ꢅ (D)
E (eV)
ꢅ (D)
E (eV)
I
II
III
4.81
4.55
6.22
0.05 (0.00)
0.09 (0.03)
0.51 (0.32)
9.25
9.01
4.01
4.00 (3.52)
4.52 (4.02)
3.82 (3.00)
Fig. 6. Fluorescence spectra of the imidazole derivatives 3 and 4 in dioxane.

J. Jayabharathi et al. / Spectrochimica Acta Part A 89 (2012) 168–176
173
Fig. 8. The potential energy surface curves for the interconversion of isomers I and II of the imidazole derivative 3 in the ground and excited states.
interconversion of isomers I and II of 3 in the ground state for iso-
lated molecule is 4.5 kcal/mol and that in ethanol is 3.6 kcal/mol.
The corresponding value in the excited state for the isolated
molecule is 12.8 kcal/mol and that in ethanol is 10.0 kcal/mol,
respectively. The barrier for interconversion in the excited state
is much higher than that in the ground state (Table 3).
the MEP surface corresponds to a potential halfway between the
two extremes red and dark blue colour [21].
3.5. HOMO–LUMO energies of imidazole derivatives 1–4 by DFT
method
The energy gap between the highest occupied molecular orbital
(HOMO) and the lowest unoccupied molecular orbital (LUMO) has
been used to prove the bioactivity from intramolecular charge
transfer (ICT) and also reflect the chemical activity of the molecule
[22]. The HOMO–LUMO energy gap (Table 4) of 1–4 have been cal-
culated by B3LYP/6-31G(d,p) and from the HOMO–LUMO orbital
picture (Fig. 10) it was found that the HOMO is located on the imi-
dazole ring (not on the two phenyl rings attached to the C3 and
C4 carbons of the imidazole rings) and the phenyl ring attached to
the C2 carbon of the imidazole ring and the LUMO located on the
imidazole ring (the two phenyl rings attached to the C3 and C4 car-
bons of the imidazole rings with C2 carbon). The HOMO → LUMO
3.4. Molecular electrostatic potential map (MEP)
In order to prove the higher electron density at N(3), we have
performed DFT calculation to get the molecular electrostatic poten-
tial (MEP) for 1–4. The MEP map (Fig. 9) shows that oxygen, nitrogen
and fluorine atoms represent the most negative potential region
(dark red). (For interpretation of the references to colour in text,
the reader is referred to the web version of this article.) The nitro-
gen atom seems to exert comparatively small negative potential as
compared to oxygen atom. The predominance of green region in
Fig. 9. MEP surface diagrams of the imidazole derivatives 1–4.

174
J. Jayabharathi et al. / Spectrochimica Acta Part A 89 (2012) 168–176
Fig. 10. HOMO–LUMO energy level diagrams of the imidazole derivatives 1–4.
transition implies that intramolecular charge transfer takes place
within the molecule [23,24].
3.6. Fluorescent chemosensor
Therefore, introduction of an electron donating substituent into
the phenyl ring attached to the C2 carbon raises the energy of the
HOMO resulting in a red-shift of the emission. On the other hand,
the introduction of the electron withdrawing substituent into the
aldehydic phenyl ring lowers the energy of the HOMO, and leads to
the blue shift of the emission. The calculated energy gap explains
the eventual charge transfer interactions within the molecule.
The optimized HOMO−1, HOMO, LUMO and LUMO+1 molecular
orbitals of 1–4 revel that HOMO−1 and HOMO have identical nodal
patterns, i.e., nodal between the phenyl and imidazole ring por-
tions of the orbital. Therefore the ␲-interaction between two rings
is anti-bonding in character. From Table 4, it was concluded that the
lowest energy transition (S0 → S1) corresponds to HOMO → LUMO
transition having higher oscillator strength and the higher energy
transition (S0 → S2) corresponds to (HOMO−1 → LUMO) transition
having lower oscillator strength.
Imidazole derivatives have been used to construct highly sen-
sitive fluorescent chemosensors for sensing and imaging of metal
ions and its chelates in particular those with Ir3+ are major compo-
nents for organic light emitting diodes [26–30]. They are promising
candidates for fluorescent chemosensors for metal ions such as
Mn2+, Pb2+, Co2+, Zn2+, Hg2+ and Cu2+, if their radiationless channel
could be blocked by metal binding. The fluorescence enhancement
was observed (Table 5) to a different extent which shows that the
metal ions binding blocks the radiationless decay channel (Fig. 11).
The radiative (kr) and non-radiative (knr) rate constants have been
calculated in Cu2+ in acetonitrile. The observed non-radiative emis-
sion (knr) may be due to n–␲* transition [25]. It was found that
the substantial increase in the quantum yield in the presence of
Cu2+ is due to dramatic decrease in nonradiative transition whereas
the radiative constant remains unchanged within the experimen-
tal error. That is the radiationless decay due to n–␲* transition
Table 4
HOMO–LUMO energies of the imidazole derivatives 1–4.
Compound
ꢆ
HOMO−1 (eV)
HOMO (eV)
LUMO (eV)
LUMO+1 (eV)
Eg (EHOMO−1 − ELUMO) (eV)
Eg (EHOMO − ELUMO) (eV)
1
2
3
4
0.41
0.52
0.60
0.63
−0.345
−0.348
−0.308
−0.334
−0.301
−0.304
−0.293
−0.302
0.001
−0.010
−0.193
−0.193
−0.014
−0.028
−0.200
−0.200
0.346 (0.410)
0.358 (0.201)
0.115 (0.086)
0.141 (0.001)
0.302 (0.505)
0.293 (0.219)
0.100 (0.268)
0.102 (0.115)
Values in the parenthesis corresponds to oscillator strength.
Table 5
Emission spectral data of 1–4 with Cu2+ metal ion at different concentrations.
Concentration of Cu2+ metal (M)
1 ꢂemi (nm)
2 ꢂemi (nm)
3 ꢂemi (nm)
4 ꢂemi (nm)
0
388.5 (440.5)
420.4 (536.4)
420.3 (602.4)
420.5 (611.9)
420.3 (622.5)
420.1 (638.9)
420.2 (656.6)
385.2 (446.7)
400.7 (598.4)
400.6 (618.6)
400.6 (630.8)
400.8 (641.7)
400.7 (662.4)
400.8 (675.5)
428.2 (363.7)
448.4 (524.4)
448.5 (538.5)
448.2 (551.7)
448.3 (566.1)
448.2 (577.6)
448.5 (590.5)
388.1 (183.6)
420.5 (293.6)
420.4 (296.7)
420.5 (301.4)
420.6 (333.8)
420.3 (354.4)
420.5 (369.5)
1 × 10−5
5 × 10−5
1 × 10−4
5 × 10−4
1 × 10−3
5 × 10−3

J. Jayabharathi et al. / Spectrochimica Acta Part A 89 (2012) 168–176
175
Fig. 11. Fluorescence chemisensors blocking by metal ion binding.
that is blocked upon metal binding and the emission of chelate
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Intermolecular hydrogen bonding of hydroxy imidazole with
water giving rise to IV impedes the ESIPT process, resulting in an
increase in the quantum yield of normal emission at the expense
of tautomer emission. Fluorescence enhancement has been found
in the presence of transition metal ions. This resulted from the
suppression of radiationless transitions from the n–␲* state in
the chemosensors. DFT calculation supports the ␲-electron den-
sities in the excited electronic state that is the driving force for the
intramolecular proton transfer from the hydroxylic group to the
N(3) nitrogen atom. PES calculation reveals that the barriers for
interconversion in the excited state are much higher than that in
the ground state. HOMO–LUMO energies and energy gap reveal the
existence of intramolecular charge transfer within the molecule.
MEP map supports the higher electron density at N(3) nitrogen
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One of the authors Dr. J. Jayabharathi, Associate professor,
Department of Chemistry, Annamalai University is thankful to
Department of Science and Technology [No. SR/S1/IC-07/2007] and
University Grants Commission (F. No. 36-21/2008 (SR)) for provid-
ing funds to this research work.
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