Synthesis and photophysics of some novel imidazole derivatives used as sensitive fluorescent chemisensors

Article abstract of DOI:10.1007/s10895-010-0690-5

Some novel imidazole derivatives were developed for highly sensitive chemisensors for transition metal ions. Since these compounds are sensitive to different external stimulations such as UV irradiation, heat, increasing pressure and changing the environm

Full text of DOI:10.1007/s10895-010-0690-5

J Fluoresc (2011) 21:65–80
DOI 10.1007/s10895-010-0690-5
ORIGINAL PAPER
Synthesis and Photophysics of Some Novel Imidazole
Derivatives Used as Sensitive Fluorescent Chemisensors
Kanagarathinam Saravanan & Natesan Srinivasan &
Venugopal Thanikachalam & Jayaraman Jayabharathi
Received: 19 February 2010 /Accepted: 21 June 2010 /Published online: 10 July 2010
#
Springer Science+Business Media, LLC 2010
Abstract Some novel imidazole derivatives were devel-
oped for highly sensitive chemisensors for transition metal
ions. Since these compounds are sensitive to different
external stimulations such as UV irradiation, heat, increas-
ing pressure and changing the environmental pH causing
colour change and so they can be used as a ′multi-way′
optically switchable material. A prominent fluorescence
enhancement was found in the presence of transition metal
ions such as Hg2+, Pb2+ and Cu2+ and this was suggested
to result from the suppression of radiationless transitions
from the n-π* state in the chemisensors. The existence of
C-H….O intramolecular hydrogen bonding in dmphnpi is
confirmed by the Natural Bond Orbital analysis (NBO).
The Mulliken, NBO charge analysis and the HOMO-
LUMO energies were also calculated. The electric dipole
moment (μ) and the first-hyperpolarisability (β) value of
the investigated molecules have been studied both experi-
mentally and theoretically which reveal that the synthesized
molecules have microscopic non-linear optical (NLO)
behaviour with non-zero values. Ground and excited state
DFT calculation were carried out in order to find out dipole
moment and energy.
Introduction
Recently, heterocyclic imidazole derivatives have attracted
considerable attention because of their unique optical
properties [1–3]. These compounds play very important
role in chemistry as mediators for synthetic reactions,
primarily for preparing functionalized materials [4–9].
Imidazole nucleus forms the main structure of some well-
known components of human organisms, i.e., the amino
acid histidine, vitamin B12, a component of DNA base
structure, purines, histamine and biotin and present in
structure of many natural or synthetic drug molecules, i.e.,
azomycin, cimetidine and metronidazole and also have
significant analytical applications utilizing their fluores-
cence and chemiluminescence properties [10–12].
Some basic processes, such as excited state intermo-
lecular proton transfer (ESIPT) have been carried out for
imidazole derivatives [13–15]. An important property
that makes imidazole derivatives more attractive as a
chelator is the appreciable change in its fluorescence
upon metal binding. Therefore, imidazole derivatives
have been used to construct highly sensitive fluorescent
chemisensors for sensing and imaging of metal ions and
its chelates in particular those with Ir3+ are major
components for organic light emitting diodes [2, 3] and
are promising candidates for fluorescent chemisensors for
metal ions. In the present paper we summarized a
comprehensive study of the imidazole derivatives and
their structures were characterized by NMR spectra.
Photophysical and photochemical studies of imidazole
derivatives and their chelates were analysed and dis-
cussed. Theoretical calculations were carried out by using
Gaussian-03 progam [16, 17] to supplement the experi-
mental results.
.
.
.
Keywords NMR Emission kinetics Chemisensors
.
NLO HOMO-LUMO
:
:
:
K. Saravanan N. Srinivasan V. Thanikachalam
J. Jayabharathi (*)
Department of Chemistry, Annamalai University,
Annamalainagar 608 002 Tamilnadu, India
e-mail: jtchalam2005@yahoo.co.in

66
J Fluoresc (2011) 21:65–80
Experimental
Using the x, y and z components, the magnitude of the
first hyperpolarizability tensor can be calculated by
Materials and methods
ꢀ
ꢁ
1=2
btot
¼
b2x þ b2y þ b2z
ð2Þ
1,2-dione (Sigma-Aldrich Ltd.), substituted benzaldehydes
(S.D. fine.) and all the other reagents used without further
purification.
The complete equation for calculating the magnitude of first
hyperpolarizability from Gaussian-03 output is given as
follows:
Optical measurements and composition analysis
"
#
1=2
2
2
ðbxxx þ bxyy þ bxzzÞ þ ðbyyy þ byzz þ byxx
Þ
btot
¼
2
NMR spectra were recorded for 1–4 on a Bruker
400 MHz. 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 2,200 GCMS
spectrometer.
þ ðbzzz þ bzxx þ bzyy
Þ
ð3Þ
All the electric dipole moment and the first hyper-
polarizabilities are calculated by taking the Cartesian
coordinate system (x, y, z)=(0, 0, 0) at own center of mass
of the compound.
General procedure for the synthesis of imidazole
Non-linear optical measurements
derivatives (1–4)
The non-linear optical conversion efficiencies were parted
using a modified set up of Kurtz and Perry. A Q-switched
Nd:YAG laser beam of wavelength of 1,064 nm was used
with an input power of 4.1 mJ/pulse width of 10 ns,
scattering geometry 90°, the repetition rate being 10 Hz,
monochromater Jobin Youon Triax 550, slit width
0.5 mm, focal length of focussing lense 20 cm, PMT
model number XP2262B used in Philips photonics, power
supply for PMT is 1.81 KU/mA with oscilloscope
Jektronix TDS 3052B.
The experimental procedure was used as the same as
described in our recent paper [18]. The various substituted
imidazole derivatives were synthesized from an unusual
four components assembling of 1,2-dione, ammonium
acetate, arylamine and an arylaldehyde.
4,5-Dimethyl-2-phenyl-1H-imidazole (dpi) , (1)
Yield: 50%. mp 96°C, Anal. calcd. for C11H12N2: C, 76.71;
1
H, 7.02; N, 16.27. Found: C, 75.25; H, 6.08; N, 17.08. H
NMR (400 MHz, CDCl3): δ 7.80 (d, J=8.0 Hz, 2H), 7.30
(t, J=8.0 Hz, 2H), 7.25 (t, J=7.6 Hz, 1H), 3.5 (bs, 1H), 2.12
(s, 6H). 13C (100 MHz, CDCl3): δ 10.5, 130.6, 126.0,
129.9, 132.7. MS: m/z 172.09, calcd 172.10.
Computational details
Quantum mechanical calculations were used to carry out
the optimized geometry, NLO, NBO, HOMO-LUMO
analysis and TD-DFT with Guassian-03 program using
the Becke3-Lee-Yang-Parr (B3LYP) functional supple-
mented with the standard 6-31G(d,p) basis set [16, 17].
As the first step of our DFT calculation for NLO analysis,
the geometry taken from the starting structures were
optimized and then, the electric dipole moment μ and β
tensor components of the studied compounds were
calculated by using B3LYP method at the 6-31G (d, p)
basis set level, which has been found to be more than
adequate for obtaining reliable trends in the first hyper-
polarizability values.
4,5-Dimethyl-1,2-diphenyl-1H-imidazole (ddi), (2)
Yield: 48%. mp 102°C, Anal. calcd. for C17H16N2: C,
82.21; H, 6.45; N, 11.28. Found: C, 82.07; H, 6.32; N,
11.04. 1H NMR (400 MHz, CDCl3): δ 2.07 (s, 3H), 2.35 (s,
3H), 7.22 (m, 5H), 7.37 (m, 2H), 7.48 (m, 3H). 13C NMR
(100 MHz, CDCl3): δ 9.50, 12.58, 71.50, 114.2, 117.3,
128.20, 129.58, 144.35. MS: m/z 248.03, calcd. 248.13.
4,5-Dimethyl-1-phenyl-2-(p-fluorophenyl)-1H-imidazole
(dmpfpi), (3)
We report the βtot (total first hyperpolarizability) for the
investigated molecules 1−4 and the components of the first
hyperpolarizability can be calculated using Eq. 1:
Yield: 40%. mp 125°C, Anal. calcd. for C17H15FN2: C,
76.67; H, 5.68; N, 10.52. Found: C, 75.01; H, 5.36; N,
P
1
8.98. H NMR (400 MHz, CDCl3): δ 1.99 (s, 3H), 3.85
bi ¼ biii þ 1=3
ðbijj þ bjij þ bjjiÞ
ð1Þ
(s, 3H), 6.85-7.35 (aromatic protons). 13C (100 MHz,
i¼j

J Fluoresc (2011) 21:65–80
67
CDCl3): δ 7.0, 12.5, 110.0, 116.5, 122.0 129.1, 135.2,
145.0, 170.0. MS: m/z 266.12, calcd. 266.0.
(pH 11). The red shift of the maximum emission in acid
medium and blue shift of maximum emission in basic
medium may be due to the presence of increased resonance
interaction of the π-cloud of the phenyl ring attached to the
carbon of the imidazole ring and the lone pair of nitrogen
(N4) of the imidazole ring [19] (Fig. 2). The resonance
interaction increases if the lone pair of electrons and the π-
cloud is parallel to each other and in order to understand the
coplanarity, optimization of imidazole derivatives 1–4, have
been performed by DFT at B3LYP/6-31G(d,p) using
Gaussian-03 (Table 2) which also strongly supports the
existence of increased resonance interaction of the π-cloud
of the phenyl ring attached to the carbon of the imidazole
ring and the lone pair of nitrogen of the imidazole ring.
From the optimized structures it was found that there is
poor coplanarity between the phenyl ring attached to the
nitrogen and carbon of the the imidazole ring (2 and 3). The
imidazole ring is essentially planar and the angle of tilting of
the phenyl ring attachéd to the carbon of the imidazole ring is
C5-N4-C8-C9 (-26.68 (1), -27.59 (2), -25.25 (3) and -28.23
(4) ) and the phenyl ring attached to the nitrogen of the
imidazole ring is tilted from the plane of the imidazole
ring by an angle of C5-N4-C24-C25 (-69.68º (2), -65.59º
(3) and -62.65º(4) ). However the angle of tilting is higher
in magnitude in the case of phenyl ring attached the
nitrogen and the bond lengths of them lie between a C-C
single bond (about 1.54 Å ) and a C = C double bond (about
1.33 Å) and are close to each other. In very basic acetonitrile
solution, the imidazole derivatives ddi (2) and dmpfpi (3)
4,5-Dimethyl-1-phenyl-2-(2′-hydroxy-5′-nitrophenyl)
-1H-imidazole (dmphnpi), (4)
Yield: 50%. mp 145°C, Anal. calcd. for C17H15N3O3: C,
66.01; H, 4.85; N, 13.59. Found: C, 65.89; H, 4.76; N,
12.83. 1H NMR (400 MHz, CDCl3): δ 2.14 (s, 6H), 5.8 (m,
5H), 7.23 (s, 1H), 7.0 (d, J=7.5 Hz, 1H), 6.8 (d, J=7.0 Hz,
1H), 9.23(s,1H). 13C (100 MHz, CDCl3): δ 11.0, 131.7,
147.0, 118.0, 127.9, 155.9, 121.9, 115.4, 130.2. MS: m/z
309.0, calcd. 309.08.
Results and discussion
Photophysical properties of imidazole derivatives
1–4 in solution
Absorption and emission properties of imidazole deriva-
tives 1–4 have been studied in various solvents. The
absorption band maxima 1max, emission band maxima 1max
(em) and the associated Stokes shift (ΔE) together with the
fluorescence quantum yields (Фf) are shown in Table 1.
It is evident from Fig. 1a–c that 1abs changes slightly
when pH of the solution increases but the emission bands
are red shifted in acidic acetonitrile solution (pH 2) and
blue shifted in the case of very basic acetonitrile solution
Table 1 Photophysical data of imidazole derivatives 1–4
Solvent
1max (nm) 1f (nm) Stokes 1max (nm) 1f (nm) Stokes 1max (nm) 1f (nm)
Stokes 1max (nm) 1f (nm)
shift
Stokes
shift
shift
shift
Hexane
282.0
288.0
285.3
284.2
361.5
368.8
363.0
360.0
384.0
368.0
7798.5
284.3
284.3
288.2
314.0 3327.0
281.5
284.6
385.0 9552.5 284.5
Non-fluorescent
Benzene
7607.3
7502.6
7408.7
8765.2
7988.5
368.0 7524.2
381.6 9005.8
360.7 7216.7
380.5 8806.5
360.0 7628.0
362.3 7571.9
361.2 7604.6
363.0 7053.9
331.0 4931.7 240.1
286.5
Chloroform
Ethyl acetate
245.0
284.0
227.0
286.2
285.0
240.0
283.2
283.0
377.1 8794.9 285.3
367.4 8117.4 284.5
Dichloro
methane
229.0
287.3
229.0
282.1
226.5
283.3
227.0
282.4
287.5
229.0
283.3
223.5
280.4
212.2
277.5
210.0
277.3
239.0
282.1
381.5 9079.6 229.0
286.2
n-butanol
226.2
282.4
232.7
284.3
226.2
238.4
289.0
378.6 9253.1 283.2
Ethanol
364.0 21389.0
364.0 8563.5 209.6
282.1
Methanol
Acetonitrile
362.0
383.2
7786.5
8686.6
363.0 8511.7 285.1
355.0 8819.3 231.5
(sh) 375.6
283.75

68
J Fluoresc (2011) 21:65–80
Fig. 1 Normalised absorbance
and emission spectra of 1–4 at
various pH solutions
almost inhibits fluorescence. In the case of dmphnpi 4, two
clear isobestic points observed at 325 and 372 nm (Fig. 3),
suggest the presence of two possible species (4a and 4b,
Scheme 1) and these observations show that compound
dmphnpi 4 undergo deprotonation in basic pH [20].
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. 4a–c). Lippert-Mataga [21] plot was
constructed for the normal fluorescence spectrum of
imidazole derivatives (1–3) (Fig. 5a) using the following
equation.
Solvent effects on the absorption and fluorescence
properties
"
#
2
2ðme ꢀ mgÞ
nꢀss ¼ naꢀb ꢀ nfꢀl ¼ const þ
f ðD; nÞ
ð4Þ
All these imidazole derivatives 1–3 show solvatochromism
(Table 1) i.e., changes in the polarity of the solvents, charge
hca3

J Fluoresc (2011) 21:65–80
69
constant; c, velocity of light and a, Onsagar’s cavity radius.
Stokes shifts were calculated from lfmax rather than 0-0-
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 S0 state and large geometrical changes have
takes place in the species when excited to S1 state and the
large Stokes shift may be explained by the presence of
intermolecular hydrogen bonding of imidazole nitrogen
(N2) with polar solvent molecules leading to the stabiliza-
tion of solvated isomers of 1–3 (Fig. 5b).
Quantum yield and photochemical properties of imidazole
derivatives 1–3
The fluorescence quantum yield for compounds 1−3
(Table 3) were measured in acetonitrile, using coumarin
47 in ethanol as a standard according to the equation,
Fig. 2 Excited state optimized structure of dpi 1
ꢂ
ꢃꢂ
ꢃꢂ
ꢃ
2
Iunk
Istd
Astd
hunk
hstd
6unk ¼ 6std
ð5Þ
Aunk
where f ðD; nÞ ¼ ðD ꢀ 1Þ=ð2D þ 1Þ ꢀ ðn2 ꢀ 1Þ=ð2n2 þ 1Þ,
indicates the orientation polarizability and depicts polarity
parameter of the solvent [22], n is refractive index, D is
dielectric constant, μe and μg are dipole moments of the
species in S1 and S0 states, respectively, h, Planck’s
where Фunk, Фstd, Iunk, Istd, Aunk, Фunk and Фstd are the
fluorescent quantum yields, the integration of the emission
intensities, the absorbances at the excitation wavelength
Table 2 Calculated parameters
of imidazole derivatives 1–4 in
Characteristics
dpi (1)
ddi (2)
dmpfpi (3)
dmphnpi (4)
the ground and excited states
S0 state
E (kcal/mol)
μg (D)
0.00
3.30
0.00
0.00
3.66
0.00
4.14
0.95
Excited state DFT
E (kcal/mol)
μg (D)
1.00
8.46
1.52
4.02
1.26
9.85
1.45
10.04
Dihedral angle (°)
(C25-C24-N4-C5)
(C9-C8-C3-N2)
(C11-C8-C3-N2)
(C9-C8-C3-N4)
(C11-C8-C3-N4)
Bond length (Å)
N2-C3
−26.68
0.0002
−27.59
0.0021
−25.25
0.0028
−28.23
0.0031
−180.00
179.99
−179.89
178.29
−178.31
178.86
−178.99
178.98
-0.0032
−0.0068
−0.0081
−0.0091
1.3773
1.4061
1.3854
1.4442
1.4428
1.3885
1.4054
1.3888
1.4481
1.4462
1.3772
1.4089
1.3828
1.4485
1.4472
1.3828
1.4091
1.3891
1.4500
1.4482
C3-N4
C8-C3
C9-C8
C8-C11

70
J Fluoresc (2011) 21:65–80
Fig. 3 Absorption spectrum of
dmphnpi 4 at different apparent
pH solutions
and the refractive indexes of the corresponding solution for
the samples and the standard, respectively.
The quantum yield has a tendency to shift to higher with
increasing the maximum emission peak wavelength and the
observed features of dpi 1 in the solution are determined by
specific interactions of the NH hydrogen.
It is evident that the main decay route of the excited state
in these compounds and their fluorescence and non-
radioactive decay with minor contribution on from the
photochemical decay route.
From the view point of the relationship between maximum
emission peak wavelength of fluorescent spectra and decay
rate constants, two trends are evident for the imidazole
derivatives 1−3. The radiative rate constants (kr) increases
as the maximum emission peak shift to red and the non-
radiative decay rate constant (knr) decreases as the
maximum emission peak shift to red.
Imidazole derivatives 1–3 as fluorescent chemisensors
The observed non-radiation emission may be due to n-π*
transition [23]. Imidazole derivatives have been used to
construct highly sensitive fluorescent chemisensors for
sensing and imaging of metal ions and its chelates in
particular those with Ir3+ are major components for organic
light emitting diodes [2, 3] and are promising candidates for
fluorescent chemisensor for metal ions, if their radiationless
channel could be blocked by metal binding. In the presence
of transition metal ions such as Hg2+, Pb2+ and Cu2+, an
enhancement of fluorescence was observed to a different
kr ¼ 6p=:C
ð6Þ
knr ¼ 1=t ꢀ 6p=C
ð7Þ
C ¼ ðkr þ knrÞꢀ1
Where kr, knr are the radiative and non-radiative
deactivation, τf is the life time of the S1 excited state.
ð8Þ
Scheme. 1 The possible depro-
tonating equilibrium of dpi 1
and dmphnpi 4

J Fluoresc (2011) 21:65–80
71
Fig. 4 Absorption and emission
spectra of 1–4 at different
solvents
extent (Table 4) which shows that the metal ions binding,
blocks the radiationless decay channel in 1–3 ( Fig. 6). The
radiative (kr) and non-radiative(knr) rate constants were
calculated for ddi 2 in Cu2+ in ACN. 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 radiation constant remains unchanged within
the experimental error. This means that the radiationless
decay in ddi 2 is indeed blocked upon metal binding and
the emission of chelate (Cu2+ ddi) orginates from π-π* state
[23].
Halochromisim of imidazole derivative dmphnpi
(4) in acetonitrile solution
We have measured the absorption spectra of dmphnpi 4 in
aprotic solvent acetonitrile at different pH value (Fig. 1a–c)
and it was found that the imidazole compound 4 show

72
J Fluoresc (2011) 21:65–80
Table 4 Absorption and Fluorescence spectral data of Hg2+, Cu2+
,
and Pb2+ chelates in acetonitrile
Compound
Chemisensor
1em (nm)
1abs (nm)
dpi (1)
1
226/287.5
226/289.0
227/288.0
226/289.0
226/286.0
228/288.0
228/289.0
281.0
383
395
393
384
363
373
378
366
375
390
391
390
1+Cu2+
1+Hg2+
1+Pb2+
2
2+Cu2+
2+Hg2+
2+Pb2+
3
ddi (2)
dmpfpi(3)
239/282.1
238/282.0
238/283.0
239/283.0
3+Cu2+
3+Hg2+
3+Pb2+
as the pH of the solution increases, the absorption bands
disappeared and new absorption bands appeared at 286 and
450 nm. There are two isobestic points observed approx-
imately at 325 and 372 nm which may be due to the two
deprotonation steps shown in Scheme 1. The first deproto-
nation step around pH 3.9 is attributed to the loss of phenolic
proton forming [4-H]¯and the second deprotonation step
occurs at pH 9.3 is attributed to the formation of doubly
deprotonated system [4-2H]2- (Scheme 1). Comparison of
dmphnpi 4 with a system dpi 1 that lacks the phenolic
proton, one can clearly see that the first deprotonation step
of dpi 1 is almost identical to the second deprotonation step
of dmphnpi 4, supporting our assumption that the deproto-
nation step at basic pH of the imidazole ring.
Intramolecular hydrogen bonding (C-H….O) in dmphnpi
(4)
Generally a hydrogen bond is formed when the hydrogen
atom of a covalent bond of a proton donor molecule
interacts with a lone pair electron of an atom of a proton
acceptor. Recently, it has been established that a C-H group
can be a hydrogen bond donor and although the C-H….O
interactions are considered as weak, they form 20–25% of
the total number of hydrogen bonds constituting the second
Fig. 5 (a) Plot of Stokes shifts versus ET(30) parameters. (b)
Stabilization of solvated isomers of 1–3. (c) Plot of ln knr versus the
energy gap
halochromic behaviour dependent on the pH. The absorp-
tion band was red shifted with increasing pH and the
absorption peak (1max) was located at 286 and 450 nm i.e.,
Table 3 Quantum yield and emission kinetics of imidazole derivatives 1–4
Compound
1max (nm)
ff
τf (ns)
kr (ns-1)
knr (ns−-1)
HOMO (a.u)
LUMO (a.u)
Eg (a.u)
dpi (1)
383.2
363.0
375.6
–
0.92
0.65
0.80
–
1.70
1.44
1.61
–
0.54
0.45
0.50
–
0.05
0.24
0.12
–
−0.2784
−0.2794
−0.2860
−0.2705
0.1101
0.1157
0.1110
0.1101
0.388
0.395
0.396
0.381
ddi (2)
dmpfpi (3)
dmphnpi (4)

J Fluoresc (2011) 21:65–80
73
[27] indicating the existence of the C-H….O interaction in 4.
The calculated C-H….O angle nearly equal to 103.6° is within
the angle limit, as the interaction path is not necessarily be linear
as several C-H….O hydrogen bonding have a bend structure.
Potential Energy Surface (PES) scan for conformation
of hydroxy group in dmphnpi (4)
The potential energy surface scan about C25-C27-O34-H35 is
performed for O-H bond using B3LYP/6-31G(d,p) level of
theoretical approximation for dmphnpi 4. The torsional angle
of C25-C27-O34-H35 for dmphnpi 4 is also relevant
coordinate for conformation flexibility within the molecule.
During the calculation all the geometrical parameters were
simultaneously relaxed while the C25-C27-O34-H35 torsion-
al angles were varied in steps of 0°, 10°, 20°, 30°……..360°.
The potential energy surface diagram clearly reveals that the
minimum energy conformation corresponds to the one in
which –OH group is oriented towards nitrogen atom N4 and
is formed by the rotation of 0° or 360° (Fig. 7).
Second Harmonic Generation (SHG) studies of imidazole
derivatives 1–4
Second harmonic signal of 2 mV, 1.6 mV and 1.7 mV were
obtained for imidazole derivatives 1–4 by an input energy
Fig. 6 Fluorescence chemisensors blocking by metal ion binding
most important group [24]. These interactions have been
shown to be of greater importance in biological systems in
order to elucidate the structure-activity relationship [25].
The C-H…O interaction have been identified from DFT
calculation, in which the C-H donor group is strengthened,
shortened and blue shifted in the stretching vibrational
wave number [26]. The intramolecular H….O distances of
H16-O23 and H31-O23 is found to be 2.25° Å and 2.252 Å
respectively. The intramolecular C-H….O distance of C3-
H24…O23 is found to be 2.53 (1). These distances are
significantly shorter than that of the van der Waals
separation between the O atom and the H-atom (2.72 Å)
Fig. 7 Energy-minimized molecular modeling structures of dmphnpi 4

74
J Fluoresc (2011) 21:65–80
of 4.1 mJ/pulse. But the standard KDP crystal gave a SHG
signal of 110 mV/pulse for the same input energy. The
second order nonlinear efficiency will vary with the particle
size of the powder sample [28]. Higher efficiencies are
expected to be achieved by optimizing the phase matching
[29]. On a molecular scale, the extent of charge transfer
(CT) across the NLO chromophore determines the level of
SHG output, the greater the CT, the larger the SHG output.
UV-visible spectroscopy Vs NLO activity in imidazole
derivatives 1–4
It can be very helpful in the investigation of NLO materials
making it possible to check, apart from NLO responses,
also spectroscopic absorbances in the appropriate wave-
length. Thus, the wavelengths obtained by UV-visible
spectral analysis can be helpful in planning the synthesis
of the promising NLO materials only [34]. The solution
electronic absorption spectral studies of compound designed
to possess NLO properties are important for two specific
reasons. First, it is necessary to know the transparency
region. Second, the solvatochromic behaviour of the sample
is generally considered as indicative of high molecular first
hyperpolarizability [34–38].
To determine the transference region and hence to know
the suitability of these compounds 1–4 for microscopic
nonlinear optical applications, the UV-visible spectra have
been recorded by using the spectrometer in the range of
190-1,100 nm. Table 1 revealed that these compounds show
absorption spectra in the UV region between 210 and
290 nm. The increased transparency in the visible region
might enable the microscopic NLO behaviour with non-
zero values [28, 39, 40]. All the absorption bands are due to
π→π* transitions. The β values (Table 5) computed here
might be correlated with UV-visible spectroscopic data in
order to understand the molecular-structure and NLO
relationship in view of a future optimization of the
microscopic NLO properties. Therefore, the validity of
B3LYP/6-31G(d,p) approximation used in all the computa-
tions here might also be illustrated by analyzing the
relationship between calculated β values and measured
values of 1max (Table 1). The band at around 280 nm
exhibits a solvatochromic shift, characteristic of a large
dipole moment (Table 5) and frequently suggestive of a
large hyperpolarizability (Tables 5). These compounds
show red shift in absorption with increasing solvent
polarity, accompanied with the upward shifts non-zero
values in the β-components [21]. From β-values it was
found that all these imidazole compounds are polar having
non-zero dipole moment and such compounds have large
hyperpolarizabilities and hence have rather well microscop-
ic NLO behaviour [28, 30, 35, 36].
Hyperpolarizability of imidazole derivatives 1–4
by DFT method
In addition to well known empirical rules to estimate
qualitatively the microscopic nonlinear response in imidazole
derivatives 1–4, DFT calculation is a more accurate
prediction of the NLO activity [30, 31]. The value of second
order optical susceptibility in a given NLO system depends
on the molecular hyperpolarizability (β), the number of
chromophores and the degree of noncentrosymmetry. From
Table 5, it was suggested that these compounds 1−3 are polar
having non-zero dipole moment, hyperpolarizabilities and
hence have well microscopic NLO behaviour [32–34].
Table 5 Electric dipole moment (D), Polarizability (α) and Hyper-
polarizability (βtotal) of 1–4
Parameter
dpi (1)
ddi (2)
dmpfpi (3) dmphnpi (4)
Electric dipole moment (D)
μx
0.4131
0.0487
−2.0219
−3.0449
2.0005
−1.7984
3.7337
−0.0011
4.1442
μy
−3.2726
−0.0078
3.2985
0.9081
−0.2707
0.9489
μz
μtotal
3.6551
Polarizability (α)
αxx
168.02
−0.760
0.002
184.04
−2.432
131.160
1.218
173.57
−0.589
−0.232
−0.005
−4.063
57.992
56.43
177.81
−3.07
131.833
0.001
αxy
αyy
αxz
0.062
αyz
119.264
43.391
70.47
−0.044
62.236
18.68
0.002
αzz
55.637
18.05
α x 10−24 (esu)
Hyperpolarizability (βtotal
)
βxxx
−594.199
−8.630
282.650 −346.888
−709.208
−45.492
18.718
−15.776
−4.738
−0.228
−0.256
13.414
3.773
βxxy
45.587
48.298
−4.642
−10.494
34.811
−30.3511
2.009
βxyy
51.470
Natural Bond Orbital (NBO) analysis of imidazole
derivatives 1–4
βyyy
−38.296
−0.002 −155.959
βxxz
βxyz
0.212
−0.002.
−13.619
−8.630
−0.004
48.26
53.604
−16.659
26.143
−37.297
13.261
34.62
20.806
−5.084
8.328
NBO analysis was performed to identify and confirm the
possible intra and intermolecular interactions between the
units that would form the hydrogen bonding. A useful
aspect of the NBO method is that it gives information about
interactions in both filled and virtual orbital spaces that
could enhance the analysis of intra and intermolecular
βyyz
βxzz
βyzz
−5.084
0.254
βzzz
β x 10−31 (esu)
0.454
50.01
58.72

J Fluoresc (2011) 21:65–80
75
Table 6 Significant donor-acceptor interactions of imidazole deriva-
tives 1–4 and their second-order perturbation energies (kcal/mol)
B3LYP/6-31G(d,p) level in order to elucidate the intramo-
lecular, rehybridization and delocalization of electron density
within the molecule. The intramolecular interaction are
formed by the orbital overlap between σ (C-C) and σ* (C-
C) bond orbital which results intramolecular charge transfer
(ICT) causing stabilization of the system. These interactions
are observed as increase in electron density (ED) in C-C
antibonding orbital that weakens the respective bonds.
Several donor-acceptor interactions are observed in
imidazole derivatives 1–4 and among the strongly occupied
NBOs, the most important delocalisation sites are in the π
system and in the lone pairs (n) of the nitrogens, oxygen
and fluorine of the imidazole derivatives 1–4 . The σ
system shows some contribution to the delocalization and
almost the donor-acceptor interactions are same in all these
compounds and the important contributions to the dilocal-
ization corresponds to the donor-acceptor interactions are
LPN4 → N2 – C3, LPN4 → C1-C5, N2-C3 → C1-C5,
C12-C13 → C8-C11 (3) , LPN4 → C9-C10 and C8-C16 →
Donor
Acceptor dpi (1) ddi (2) dmpfpi (3) dmphnpi (4)
C8-C11
C8-C11
C8-C11
N2-C3
18.20
18.49
17.76
19.40
19.76
20.34
19.06
11.9
8.79
10.94
31.28
–
14.39
20.61
20.54
19.80
19.05
11.77
23.62
33.31
30.07
18.50
21.62
19.21
308.69
–
14.50
18.84
19.65
20.62
18.19
11.36
26.44
36.35
31.60
19.68
21.56
–
C9-C10
C12-C13 20.51
C12-C13 C8-C11
C12-C13 C9-C10
19.61
19.40
11.44
27.57
38.90
32.33
20.23
LPN-2
LPN-4
LPN-4
N2-C3
C9-C10
C9-C10
C3-N4
C1-C5
N2-C3
C1-C5
C8-C11
C12-C13 20.89
–
C14-C17 C15-C16
C12-C13 C8-C11
–
–
–
–
–
–
–
–
C8-C11
C9-C10
–
280.88
–
C25-C26 C27-C30
C25-C26 C28-C29
19.58
20.35
–
–
–
Table 7 Percentage of p-character on each natural atomic hybrid of
the Natural Bond Orbital (NBO) of imidazole derivatives 1–4
interactions. The second order Fock matrix was carried out
to evaluate the donor–acceptor interactions in the NBO
analysis [41]. The interactions result is a loss of occupancy
from the localized NBO of the idealized Lewis structure
into an empty non-Lewis orbital. For each donor (i) and
acceptor (j), the stabilization energy E(2) associated with
the delocalization i → j is estimated as
NBO
Atom
dpi (1)
ddi (2)
dmpfpi (3)
dmphnpi (4)
C1-N2
C1
N2
C1
C5
N2
C3
73.05
71.31
99.92
99.93
99.72
99.92
–
73.10
70.01
99.90
98.87
99.81
99.78
–
73.16
71.35
99.92
99.92
99.68
99.89
–
73.04
71.33
62.23
60.78
99.71
99.88
–
C1-C5
N2-C3
2
Fði; jÞ
Eð2Þ ¼ $Eij¼q
ð9Þ
i "j ꢀ "i
N2-C3
N2
C3
C8-C11
C8
99.97
99.97
–
99.96
99.95
–
99.95
99.95
–
99.97
99.96
–
Where qi is the donor orbital occupancy, εi and εj are
diagonal elements and F(i, j ) is the off diagonal NBO Fock
matrix element. Natural bond orbital analysis provides an
efficient method for studying intra and intermolecular
bonding and interaction among bonds and also provides a
convenient basis for investigating charge transfer or
conjugative interaction in molecular systems. Some elec-
tron donor orbital, acceptor orbital and the interacting
stabilization energy resulted from the second-order micro-
perturbation theory are also reported [42]. The larger the
E(2) value, the more intensive is the interaction between
electron donors and electron acceptors, i.e., the more
donating tendency from electron donors to electron acceptors
and the greater the extent of electron density transfer (EDT)
or hyperconjugative interaction of the whole system.
Delocalization of electron density between occupied Lewis-
type (bond or lone pair) NBO orbitals and formally
unoccupied (antibond or Rydgberg) non-Lewis NBO orbitals
correspond to a stabilizing donor–acceptor interaction. NBO
analysis have been performed on the molecule at the DFT/
C11
C8-C11
C8-C11
C9-C10
C8
C11
C8
C11
–
–
–
–
C9
99.96
99.96
–
99.95
99.96
–
99.95
99.96
–
99.96
99.96
–
C10
C9-C10
C9
C10
C12-C13
C12
C13
99.96
99.96
–
99.96
99.96
–
99.95
99.95
–
99.96
99.96
–
C
12-C13
C12
C13
4N2
N2
N4
N2
C3
–
65.24
99.91
99.72
99.92
–
68.30
77.45
99.81
99.78
–
65.46
96.78
99.68
99.68
99.95
–
65.32
98.30
99.71
99.88
–
4N4
N2-C3
20F
LPO 14
–
–
–
97.81

76
J Fluoresc (2011) 21:65–80
C9-C10 (4) (Table 6). NBO analysis clearly manifests the
evidence of the intramolecular hydrogen bonding by charge
transfer from LpN4 to antibonding orbital of C9-C10 shows
that the hydroxy group is inclined towards the nitrogen
atom N4 side (2 and 3).
the NBO and Mulliken methods and the charges are
displayed in Table 8. The two methods predict the same
tendencies i.e., compared to nitrogen atom N4, N2 nitrogen
atom is acidic in all the imidazole derivatives i.e., the
hydrogen atom bonded to N2 is easily replaceable (1) and
the nitrogen atom N4 is basic site [44]. From the Table 2,
one can see that the excited molecule leads to increase in
the electron density at N2 nitrogen atom and then the
excited molecule emits luminescence and turns to the
ground state.
The percentage of p-character [43] in each NBO natural
atomic hybrid orbital is presented in Table 7. The C1-N2
bond of imidazole ring has a p-character around 70–73%.
However, for all other carbon hybrids in benzene ring,
almost 100% p-character was observed. Finally, the σ lone
pair on nitrogen has very strong deviation to around 65% in
all the compounds whereas in the case of N2-C3 bond, the
% of p-character on N2 and C3 atoms are strongly
increased to around 99% but the σ lone pair on nitrogen
atom N4 has a increase to about 96.8–99.9% in all the
compounds except compound 2 where the p-character is
reduced to 77.51%. The charge distribution shows that the
more negative charge is concentrated on nitrogens, fluorine
and oxygen whereas the partial positive charge resides at
hydrogens. The charge distributions of 1–4 calculated by
HOMO-LUMO energies of imidazole derivatives
1-3 by DFT method
Molecular orbital and their properties, like energy are very
useful for physicists and chemists and their frontier electron
density used for predicting the most reactive position in π-
electron systems and also explained several types of
reaction in conjugated system [45]. Moreover, eigen values
of LUMO and HOMO and their energy gap reflect the
Table 8 The charge distribution
Atom
dpi (1)
ddi (2)
dmpfpi (3)
dmphnpi (4)
calculated by Mulliken and
natural bond orbital (NBO)
methods for imidazole
derivatives 1–4
NBO
Mulliken
NBO
Mulliken
NBO
Mulliken
NBO
Mulliken
1C
0.1869
0.1869
0.2005
−0.5571
0.4167
−0.7100
0.1393
−0.4581
−0.4565
−0.0264
−0.1631
−0.2162
−0.1749
−0.2169
−0.1849
−0.2045
−0.1489
0.1085
−0.1937
−0.2032
−0.1773
–
0.2005
−0.5571
0.4167
−0.7100
0.1393
0.0075
0.0754
−0.0264
−0.0846
−0.0136
−0.0563
−0.0160
−0.0161
0.0058
0.0884
0.1085
0.0115
0.0029
0.0420
–
0.1766
−0.5218
0.4106
−0.6186
0.2767
−0.3784
−0.3852
0.0696
−0.0963
−0.1344
−0.1025
−0.1368
0.3313
–
0.1766
−0.5218
0.4106
−0.6186
0.2767
−0.083
−0.0247
0.0696
0.0227
−0.0270
0.0048
−0.0308
0.3314
–
0.1826
0.1825
2N
−0.5361
0.4546
−0.5361
0.4546
−0.5252
−0.5252
3C
0.4270
0.4270
4N
−0.6531
0.2922
−0.3872
0.2922
−0.6203
−0.3420
5C
0.2828
0.2828
6C
−0.3732
−0.3835
−0.0116
−0.0298
−0.1022
0.0041
0.3756
−0.0156
7C
−0.3837
−0.0330
8C
0.1022
0.0798
0.0798
9C
−0.1111
−0.0898
−1.2227
−0.0925
−0.1166
−0.0008
10C
11C
12C
13C
25C
26C
27C
28C
29C
30C
14C
15C
16C
17C
18C
19C
20F
−0.0024
−0.0647
−0.0091
0.00
–
−0.0893
−0.0024
0.2302
0.2302
−0.0943
−0.0044
−0.0825
−0.0886
−0.0019
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
0.2160
−0.0760
−0.0973
−0.0947
−0.0948
−0.0791
−0.0791
0.2160
0.0336
0.0014
0.0041
0.0008
0.0127
−0.3299
–
–
–
–
–
–
–
–
–
–
−0.6080
−0.2770
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–

J Fluoresc (2011) 21:65–80
77
chemical activity of the molecule. Recently the energy gap
between highest occupied molecular orbital (HOMO) and
the lowest unoccupied molecular orbital (LUMO) have
been used to prove the bioactivity from intramolecular
charge transfer (ICT) [46, 47]. The HOMO-LUMO energy
gap for 1–3 were calculated by B3LYP/6-31G(d,p) and the
HOMO-LUMO orbital picture of dpi 1 is given in Fig. 8.
From the HOMO-LUMO orbital picture it was found that
the filled π orbital (HOMO) is mostly located on the
aldehydic phenyl ring of the imidazole derivative and the
unfilled π* orbital (LUMO) on the imidazole ring with
aniline phenyl ring. Therefore introduction of an
electron-donating substituent into the aniline phenyl ring
raises the energy of the LUMO resulting in a blue-shift
of the emission. On the otherhand, the introduction of
the electron-donating substituent into the aldehydic
phenyl ring raises the energy of the HOMO, leading to
the red shift of the emission [48, 49].
(Fig. 5c) shows linear relationship.The decrease in the
HOMO and LUMO energy gap explains the probable
charge transfer (CD) taking place inside the chromophore.
The energy gap law predicts that the rate of non-radiative
decay increases when the energy gap separating the ground
and excited state decreases. This relation is based on the
vibrational overlap between the ground state and excited
state and knr is a function of Frank-Condon overlap integral.
In the cases of compounds having similar excited states and
vibratonal coupling, a simplified form of the energy gap
law is obtained that predicts a linear relationship between ln
(knr) and the energy gap. This correlation suggests that
decay rate constants are due to the energy gap of the
compounds [50] and from the emission spectra it is
observed that the emission of 1–3 is due to π-π* transition.
A good correlation was found between the computed and
experimental wave numbers (Fig. 9, Table 9) and calculated
wave numbers obtained by DFT/B3LYP/6-31G(d,p) method
[51, 52]. Experimental C=N and C-N stretching wave-
numbers have been lowered due to conjugation and π-
electron delocalization.
The energy gap (Eg) of all the reported compounds 1−4
were calculated from the HOMO and LUMO levels. The
plot of ln(knr) versus the energy gap for compounds 1–3
Fig. 8 HOMO-LUMO orbital
picture of dpi 1

78
J Fluoresc (2011) 21:65–80
Fig. 9 Graphical correlation between experimental and theoretical absorption frequencies of 1–4
Conclusion
emission towards Cu2+ ion. We suggest that ddi (2)
operates under the mechanism that metal binding block
the radiationless n-π* transition in ddi (2). Further work we
focused that the increased transparency of these imidazole
derivatives in the visible region might enable the micro-
scopic NLO behaviour with non-zero values and all the
We have developed Imidazole derivative as a new set of
fluorescent chemisensors for transition metal ions Hg2+,
Pb2+ and Cu2+. Especially the imidazole derivative ddi (2)
shows a highly sensitive and selective response in its
Table 9 Experimental and theoretical IR spectral data (cm−1) of imidazole derivatives 1–4
dpi (1)
ddi (2)
dmpfpi (3)
dmphnpi (4)
Assignment
Experimental Theoretical
Experimental Theoretical Experimental Theoretical Experimental Theoretical
( υ, cm−1
)
( υ, cm−1
)
( υ, cm−1
)
( υ, cm−1
)
( υ, cm−1
)
( υ, cm−1
)
( υ, cm−1
)
( υ, cm−1
)
1604
987
3345
–
1636
991
3682
−
1599
975
−
1637
998
−
1603
1030
−
1605
1049
−
1608
1103
–
1608
1105
–
νC=N
νC−N
νN-H
–
–
–
–
1642
1653
νN−OH
νC=C
1412,1489
755,827
1415,1474
1400,1542
1402,1541
955,846
1422,1514
727,674
1420,1528
738,690
1434,1523
820,727
1451,1536
829,738
772,870, 1511,1463 944,823
νC-H, νCH3

J Fluoresc (2011) 21:65–80
79
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Acknowledgements One of the author Dr. J. Jayabharathi, Reader
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