Synthesis, structure, luminescent and intramolecular proton transfer in some imidazole derivatives

Article abstract of DOI:10.1007/s10895-010-0747-5

A group of novel 2-aryl imidazole derivatives were synthesized and characterized by NMR spectra, X-ray, mass and CHN analysis. An excited state intramolecular proton transfer (ESIPT) process in hydroxy imidazoles (dmip and dmtip) have been studied using e

Full text of DOI:10.1007/s10895-010-0747-5

J Fluoresc (2011) 21:595–606
DOI 10.1007/s10895-010-0747-5
ORIGINAL PAPER
Synthesis, Structure, Luminescent and Intramolecular
Proton Transfer in Some Imidazole Derivatives
Jayaraman Jayabharathi & Venugopal Thanikachalam &
Natesan Srinivasan & Kanagarathinam Saravanan
Received: 13 July 2010 /Accepted: 5 October 2010 /Published online: 3 November 2010
#
Springer Science+Business Media, LLC 2010
Abstract A group of novel 2-aryl imidazole derivatives
were synthesized and characterized by NMR spectra, X-ray,
mass and CHN analysis. An excited state intramolecular
proton transfer (ESIPT) process in hydroxy imidazoles
(dmip and dmtip) have been studied using emission
spectroscopy and it was detected that the two distinct
ground state rotamers of I and II are responsible for the
normal and the tautomer emission respectively. In hydro-
carbon solvent, the tautomer emission predominates over
the normal emission for both dmip and dmtip. This reveal
that rotamer II is responsible for the tautomer emission and
it is stabler than rotamer I which causes the normal
emission. In alcoholic solvent like ethanol, a dramatic
enhancement of normal emission is observed which was
due to increased solvation, the more polar rotamer I become
stabler than rotamer II. In dioxane—water mixtures it is
observed that the addition of water inhibits the ESIPT
process due to the formation of the intermolecular hydrogen
bonding involving water. DFT calculations on energy,
dipole moment, charge distribution of the rotamers in the
ground and excited states of the imidazole derivatives were
performed and discussed. PES calculation indicates that the
energy barrier for the interconversion of two rotamers is too
high in the excited state than the ground state.
Introduction
Excited state intramolecular proton transfer (ESIPT) phe-
nomena have been investigated [1–13] in the past decades
due to the practical applications of ESIPT exhibiting
molecules as laser dyes [1], photostabilizers [11], fluores-
cent probes in biology [13] and light-emitting materials for
electroluminescent devices [6, 7]. ESIPT typically occurs in
aromatic 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 photoexcitation migrates to the hetero atom at a
distance <2Å to form the excited state tautomer. The two
distinct intramolecular hydrogen bonded rotamers I and II
(Fig. 1) have been detected experimentally in the ground
state [3] by the typical ESIPT exhibiting 2-(2′-hydroxy-
phenyl)benzazole derivatives. It is established that only
rotamer II undergoes ESIPT to form the excited state
phototautomer III, which transmits a long wavelength
emission with a large Stokes shift of 200 nm, while rotamer
I is responsible for the short wavelength normal emission. It
is therefore possible for the ESIPT exhibiting substances to
have dual fluorescence, depending on the molecular nature
and the external factors such as solvent polarity and
temperature.
.
.
.
.
Keywords NMR X-ray ESIPT PES CIS
The ESIPT process involves very fast transfer of the
hydroxyl (or amino) proton to an acceptor such as carbonyl
oxygen or nitrogen atom in an excited state of a molecule,
resulting in the formation of the keto tautomer (III) from the
initial enol tautomer (II) [14, 15]. Then the excited keto
tautomerluminesce, undergoing a transition to the ground
state, the reverse proton transfer takes place and the
molecule returns to the enol form. The ESIPT process
usually manifested in steady state spectra in an significantly
large Stokes shift of the fluorescence spectrum relative to
:
:
:
J. Jayabharathi (*) V. Thanikachalam N. Srinivasan
K. Saravanan
Department of Chemistry, Annamalai University,
Annamalainagar 608 002 Tamilnadu, India
e-mail: jtchalam2005@yahoo.co.in

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J Fluoresc (2011) 21:595–606
Fig. 1 Various rotameric forms of dmip (4) and dmtip (5)
the initial absorption spectrum [16–22]. In the present
paper, we will focus the light on the photophysical studies
of 2-aryl imidazole derivatives (1–6), ESIPT process of
hydroxy imidazoles (dmip (4) and dmtip (5)) by comparing
their photophysical properties and Density Functional
Theory (DFT) calculations on energy and dipole moment
of various species, NLO property, HOMO-LUMO energies
and PES studies of the two rotamers I and II were
performed and discussed.
Optical Measurements and Composition Analysis
NMR spectra were recorded for the 2-aryl imidazole
derivatives (1–6) on a Bruker 400 MHz. The ultraviolet–
visible (UV–Vis) spectra were measured on UV–Vis
spectrophotometer (Perkin Elmer, Lambda 35) and cor-
rected for background due to solvent absorption. Photo-
luminescence (PL) spectra were recorded on a (Perkin
Elmer LS55) fluorescence spectrometer. MS spectra were
recorded on a Varian Saturn 2200 GCMS spectrometer.
Experimental
Non-Linear Optical Measurements
Materials and Methods
Its second harmonic generation efficiency was assessed by
Kurtz powder technique [23] at IISc., Bangalore, India. It is
a well established tool to evaluate the conversion efficiency
of nonlinear optical materials. A Q-switched Nd:YAG laser
operating at the fundamental wavelength of 1,064 nm,
Butane-2,3-dione (Sigma-Aldrich Ltd.), p-tolualdehyde
(S.D. fine.), salicylaldehyde, o-anisaldehyde and all other
reagents used without further purification.

J Fluoresc (2011) 21:595–606
597
generating about 4.1 mJ and pulse width of 8 ns was used
for the present experimental study. The input laser beam
was passed through an IR reflector and then incident on the
powder form of the specimen, which was packed in a glass
capillary tube. The output energy was detected by a
photodiode detector integrated with oscilloscope assembly.
Computational Details
Quantum mechanical calculations were used to carry out
the optimized geometry, NLO, PES, HOMO-LUMO
energies and excited state parameters by CIS method with
Guassian-03 program using the Becke3-Lee-Yang-Parr
(B3LYP) functional supplemented with the standard 6–
31G(d,p) basis set [24]. As the first step of our DFT
calculation, the geometry taken from the starting structure
was optimized and for NLO analysis, the electric dipole
moment μ and β tensor components of the imidazole
derivatives (1–6) were calculated.
Fig. 2 Excitation spectra of 2-aryl imidazole derivatives 1–6
4,5-Dimethyl-1-(p-tolyl)-2-phenyl-1H-imidazole (dmppi), (1)
The βtot (total first hyperpolarisability) for the 2-aryl
imidazole derivatives (1–6) and the components of the first
hyperpolarisability can be calculated using the following
equation,
Yield: 40%. mp 125 °C, Anal. calcd. for C18H8N2: C,
82.41; H, 6.92; N, 10.68. Found: C, 82.02; H, 6.69; N,
1
10.70. H NMR (400 MHz, CDCl3): δ 2.42 (s, 3H), 2.31
(s, 3H), 2.02 (s, 3H), 7.02–7.36 (aromatic protons). 13C
(100 MHz, CDCl3): δ 9.57, 12.76, 21.19, 125.45, 127.54,
127.63, 127.97, 128.01, 130.11, 130.92, 133.41, 135.34,
138.37, 145.15. MS: m/z 262.00, calcd. 262.35.
X
bi ¼ biii þ 1=3
ðbijj þ bjij þ bjjiÞ
ð1Þ
i¼j
Using the x, y and z components, the magnitude of the
first hyperpolarisability tensor can be calculated by,
2-(4-fluorophenyl)-4,5-dimethyl-1-p-tolyl-1H-imidazole
(fpdmti) (2)
1=2
btot ¼ ðb2x þ b2y þ bz2Þ
ð2Þ
Yield: 40%. mp 130 °C, Anal. calcd. for C18H17FN2: C,
77.12; H, 6.11; N, 9.99. Found: C, 77.00; H, 5.98; N, 9.03.
1H NMR (400 MHz, CDCl3): δ 2.01 (s, 3H), 2.29 (s, 3H),
2.43 (s, 3H), 6.87–7.34 (aromatic protons). 13C (100 MHz,
The complete equation for calculating the magnitude of first
hyperpolarisability from Gaussian-03 output is given as follows,
2
2
btot ¼ ½ðbxxx þ bxyy þ bxzzÞ þ ðbyyy þ byzz þ byxx
Þ
ð3Þ
2 1=2
þ ðbzzz þ bzxx þ bzyyÞ ꢀ
ꢀ
ꢁ
a ¼ axx þ ayy þ azz 1=3
ð4Þ
All the electric dipole moment and the first hyperpolaris-
ability were calculated by taking the Cartesian coordinate
system (x, y, z)=(0, 0, 0) at own center of mass of the
compounds (1–6).
General Procedure for the Synthesis of 2-aryl Imidazole
Derivatives (1–6)
The experimental procedure was used as the same as
described in our recent papers [25, 26]. The 2-aryl imidazole
derivatives (1–6) were synthesized from an unusual four
components assembling of Butane-2,3-dione, ammonium
acetate, substituted anilines and substituted benzaldehydes.
Fig. 3 Emission spectra of 2-aryl imidazole derivatives 1–6

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ΔE cm−1
Table 1 Photophysical spectral
parameters of 2-arylimidazole
derivatives (1–6) in dioxane
Compound
labs (nm)
lexi (nm)
lflu (nm)
dmppi (1)
fpdmti (2)
dmdmppi (3)
dmip (4)
290.5 (303.9)
291.5 (280.0)
303.9 (274.1)
248.0 (231.0)
238.5 (227.0)
245.0 (298.5)
290.0 (303.0)
292.0 (282.0)
304.0 (272.0)
248.5 (231.0)
239.0 (226.5)
244.5 (290.0)
362.0 (359.7)
365 (360.4)
6799 (5104.6)
6908 (7967)
360.0 (360.0)
470.0, 500.0 (sh) (453.0)
478.0, 510.0 (sh) (422.0)
460.0 (452.0)
5128 (8705)
19046 (21215)
21008.2 (20356.0)
19077 (11377)
dmtip (5)
Values in the parenthesis are
recorded in ethanol
mpdmti (6)
CDCl3): δ 9.53, 12.69, 21.18, 114.92, 115.10, 125.44,
127.16, 129.89, 130.20, 133.35, 135.13, 138.57, 144.28,
161.26, 163.23. MS: m/z 280.00, calcd. 280.14.
115.6, 121.56, 122.32, 123.62, 129.80, 130.08, 135.29,
145.82. MS: m/z 292.00, calcd. 292.16.
4,5-dimethyl-1-(3,5-dimethylphenyl)-2-phenyl-1H-imidazole
Results and Discussion
(dmdmppi) (3)
All 2-aryl imidazole derivatives (1–6) fluoresces strongly in
solutions at room temperature. Their luminescence excita-
tion spectra (Fig. 2) are in coincide with their absorption
spectra and differ from their emission spectra (Fig. 3).
Therefore, the fluorescence of the 2-aryl imidazole deriv-
atives (1–6) was taken for discussion. Fluorescence band
Yield: 40%. mp 142 °C, Anal. calcd. for C19H20N2: C,
82.57; H, 7.29; N, 10.14. Found: C, 82.00; H, 6.89; N,
1
9.72. H NMR (400 MHz, CDCl3): δ 1.98 (s, 3H), 2.27 (s,
3H), 2.29 (s, 6H), 6.77–7.36 (aromatic protons). 13C
(100 MHz, CDCl3): δ 9.65, 12.77, 21.27, 125.54, 125.59,
127.59, 127.97, 128.03, 130.02, 130.07, 133. 53, 137.88,
139.33, 144.98. MS: m/z 276.00, calcd. 276.16.
maximum (lflu) fluorescence excitation maximum (lexc
)
and Stokes shifts (ΔE) are presented in Table 1 for 1–6.
It maybe suggested that in aprotic solvents, the hydroxy
substituted 2-aryl imidazoles namely dmip (4) and dmtip
(5) exists as two different intramolecular hydrogen bonded
isomers I and II (Fig. 4). Excitation of the isomer II should
lead to the formation of the keto-isomer III due to ESIPT
(Fig. 1), while excitation of the isomer I must yield the
normal emission. But, we found out that the fluorescence
spectra of dmip (4) and dmtip (5) in dioxane contain an
abnormal Stokes-shifted emission band and one small
shoulder peak at higher wavelength which reveal that only
the isomer II of these molecules is stable under these
conditions. Stokes shift is important for a fluorescent
2-(4,5-dimethyl-1H-imidazol-2-yl)phenol (dmip) (4)
Yield: 45%. mp 115 °C, Anal. calcd. for C11H12N2O: C,
70.19; H, 6.43; N, 14.88. Found: C, 70.00; H, 5.89; N,
1
14.00. H NMR (400 MHz, CDCl3): δ 2.27 (s, 3H), 2.30
(s, 3H), 6.79–7.31, (aromatic protons), 10.30 (s, 1H). 13C
(100 MHz, CDCl3): δ 9.53, 12.26, 116.40, 121.00, 127.23,
130.53, 146.08, 154.30. MS: m/z 188.00, calcd. 188.09.
2-(4,5-dimethyl-1-p-tolyl-1H-imidazol-2-yl)phenol
(dmtip) (5)
Yield: 42%. mp 150 °C, Anal. calcd. for C18H18N2O: C,
77.67; H, 6.52; N, 10.06. Found: C, 77.00; H, 5.89; N,
1
9.82. H NMR (400 MHz, CDCl3): δ 2.20 (s, 3H), 2.31
(s, 3H),2.40 (s, 3H), 6.88–7.30 (aromatic protons), 11.62
(s, 1H). 13C (100 MHz, CDCl3): δ 9.53, 12.69, 21.18,
116.20, 118.00, 121.00, 127.23, 130.00, 134.48, 137.90,
144.31, 155.28. MS: m/z 278.00, calcd. 278.14.
2-(2-methoxyphenyl)-4,5-dimethyl-1-p-tolyl-1H-imidazole
(mpdmti) (6)
Yield: 42%. mp 169 °C, Anal. calcd. for C19H20N2O: C,
78.05; H, 6.89; N, 9.58. Found: C, 78.00; H, 6.29; N, 9.82.
1H NMR (400 MHz, CDCl3): δ 2.20 (s, 3H), 2.31 (s, 3H),
2.45 (s, 3H), 3.95 (s, 3H), 6.83–7.37 (aromatic protons).
13C (100 MHz, CDCl3): δ 9.45, 11.70, 24.51, 55.82, 114.8,
Fig. 4 Excitation and emission spectra of dmip (4) and dmtip (5)

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Table 2 Electron density of atoms N(5) and O for dmip (4) and dmtip
(5)
As a result, a reverse process occurs in the ground state of
the molecule producing an initial molecule in the enol form.
Compound
dmip (4)
Atom
II
II*
III
Correlation Between Intramolecular Hydrogen Bonding
and ESIPT Process
N(5)
O
−0.390
−0.571
−0.330
−0.458
−0.432
−0.523
−0.402
−0.435
0.312
−0.692
0.304
dmtip (5)
N(5)
O
The existence of intramolecular hydrogen bond in hydroxy
substituted imidazoles namely dmip (4) and dmtip (5) is
confirmed by the presence of the singlet at 10.30 ppm
(dmip) and 11.62 ppm (dmtip) in the 1H NMR spectra
which is a typical signal for hydrogen bonded hydrogen
atom. In order to reveal the contribution of the intramolec-
ular hydrogen bonding in the hydroxy imidazoles dmip (4)
and dmtip (5) to their optical properties, the fluorescence
spectra of compounds dmdip (5) and its derivative mpdmti
(6) were measured in dioxane solvent under identical
condition (Fig. 5). A dual fluorescence was detected for
dmtip (5) with emission peak centered at 478 and 510 nm
respectively. The emission peak at shorter wavelength at
478 nm is assigned to rotamer I and longer wavelength
band at 510 nm is assigned to rotamer II [3] whereas
compound mpdmti (6) exhibits emission peak only at
460 nm, absence of additional peak at longer wavelength
confirms that absence of intramolecular hydrogen bond in
compound mpdmti (6) which further evident that intramo-
lecular hydrogen bond is the essential driving force for
ESIPT and the dual fluorescence behaviour of dmtip (5).
−0.526
*Excited state enol isomer
sensor. The higher stokes shift value supplies very low
background signals and resultantly allows the usage of the
material in construction of a fluorescence sensor [27].
However, in the case of hydroxyl containing solvent
(EtOH), a short wavelength emission band appears for
dmip (4) and dmtip (5) which is absent in the fluorescence
spectra of 1–3 and 6 (Fig. 4). This result corresponds to the
data obtained earlier [26, 28] for compounds demonstrating
ESIPT and can be explained by the presence of intermo-
lecular hydrogen bonding with solvent molecule leading to
the stabilization of solvated isomer IV in which ESIPT is
impossible.
For better understanding the ESIPT mechanism in dmip
(4) and dmtip(5), we performed DFT calculation of electron
density for the keto and enol isomers of the dmip (4) and
dmtip(5) molecules in the ground and the excited states
(Table 2) which reveal that excitation of enol isomer leads
to an increase in the electron density at the N (5) atom and
decrease at the O atom resulting in ESIPT and formation of
the excited keto isomer in excited state. Then the excited
keto isomer emits luminescence and returns to the ground
state keto form, which is characterized by a large positive
charge at the N (5) atom and negative charge at the O atom.
Competition of Intra and Intermolecular Hydrogen Bonding
in Binary Solvents
The competition of intra and intermolecular hydrogen
bonding in dmip (4) and dmtip (5) derivatives were studied
in dioxane-water. In this binary-solvent solution, dioxane is
Fig. 6 Fluorescence spectra of dmtip (5) in dioxane-water mixed
Fig. 5 Fluorescence spectra of dmtip (5) and mpdmti (6)
solvents

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Table 3 Relative energiesa (eV)
and dipole moments (D) in the
ground and excited state for
rotamers I, II and III
Compound
Rotamers
Ground state
Excited state
μ (D)
E(eV)
E(eV)
μ(D)
dmip (4)
dmtip (5)
I
3.62
3.23
5.49
4.03
3.62
5.28
0.04 (0.00)
0.08 (0.03)
0.58 (0.32)
0.01 (0.00)
0.04 (0.02)
0.42 (0.28)
3.89 (3.42)
4.12 (3.76)
3.52 (3.14)
4.23 (4.02)
3.92 (3.46)
3.56 (3.04)
8.25
8.02
4.56
7.84
8.52
3.45
II
III
I
a Relative energies are calculated
with respect to the ground state
minimum energy form in ethanol.
Values in the parenthesis are
recorded in ethanol
II
III
a non polar and weakly hydrogen bonding solvent in which
normal emission is weak while water is a protic but poor
solvent for most of the organic molecules. For this purpose,
a series of solutions in dioxane-water mixture with different
water fraction but identical concentrations of dmtip (5)
were made for spectroscopic measurements. As shown by
the normalized fluorescence spectra (Fig. 3), dmtip(5)
exhibits both normal and tautomer emission peaks at 478
and 510 nm respectively in pure dioxane. With the addition
of water, the tautomer emission intensity decreases rela-
tively to that of the normal emission. 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 at 445 nm appeared in addition
to the normal emission. This band intensity increased with
increasing water fraction and turned to be the main
emission when water fraction is high up to 80% (v/v)
(Fig. 6). It is understandable that the tautomer emission
decreases with increasing water fraction in the mixed
solvent if the intermolecular hydrogen bonding between
dmtip (5) and water is taken into account. In the initial
stage, the presence of small amount of water in the dioxane
solution must give rise to solvation of dmtip (5). The
intermolecular hydrogen bonding between dmtip (5) 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 emis-
sion decreases with addition of water and finally vanished.
Density Functional Theory Calculation
The ESIPT phenomenon has inspired a large number of
theoretical calculations [28–30]. The ground-state geome-
tries of the three species, I, II, and III for both dmip (4) and
dmtip(5) were optimized using the DFT/6-31G (d,p)
method. Complete optimization of all the geometrical
parameters gives the ground state energy of each species.
The energy of the excited state was calculated using the
standard CIS method. Table 3 gives the energies and dipole
Fig. 7 a. PES for the interconversion of the rotamers of dmip (4) in
the ground state. b. PES for the interconversion of the rotamers of
dmip (4) in the excited state

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Table 4 Quantum yield and
thermodynamic parameters of
imidazole derivatives 1–6
Compound
ff
τf (ns)
kr (ns−1
)
knr (ns−1
)
HOMO (a.u)
LUMO (a.u)
Eg (a.u)
dmppi (1)
fpdmti (2)
dmdmppi (3)
dmip (4)
0.71
0.44
0.60
0.42
0.49
0.51
1.42
1.44
1.61
1.72
1.35
1.31
0.50
0.30
0.37
0.24
0.30
0.40
0.20
0.39
0.25
0.34
0.44
0.36
−0.1984
−0.2794
−0.2059
−0.1997
−0.1894
−0.2648
−0.0168
−0.1157
−0.0441
−0.0297
−0.0183
−0.099
0.1816
0.1637
0.1619
0.1770
−0.1711
0.1658
dmtip (5)
mpdmti (6)
moment of the species I, II, and III in the ground and the
excited state. For better understanding the ESIPT mecha-
nism in dmip (4) and dmtip(5), we performed DFT
calculation of electron density for the keto and enol isomers
of the dmip (4) and dmtip(5) in the ground and the excited
states (Table 2).
In the excited state the nitrogen atom becomes richer in
electrons than the hydroxylic oxygen atom. This redistri-
bution of the π-electron densities in the excited electronic
state is the driving force for the intramolecular proton
transfer from the hydroxylic group to the nitrogen atom.
The potential energy (PE) curves (Fig. 7a and b) for the
interconversion of isomers I and II of dmtip (5) in the
ground and the excited states reveal that for isolated dmtip
(5) in the ground state the barrier for interconversion is
4.5 kcal/mol and 3.4 kcal/mol in ethanol. In the excited
state the barrier for dmtip (5) in the isolated molecule and
in the alcohol medium are 12.5 and 9.8 kcal/mol,
respectively. The barriers for interconversion in the excited
state is much higher than that in the ground state (Table 3).
Quantum Yield and Thermodynamic Parameters of 2-aryl
Imidazole Derivatives (1-6)
The fluorescence quantum yield for compounds 1–6
(Table 4) were measured in acetonitrile using coumarin 47
Fig. 8 HOMO-LUMO orbital
picture of dmppi (1)

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Table 5 Electric dipole moment
(D), polarizability (α) and
hyperpolarizability (βtotal) of
(1–6)
Parameter
dmppi (1)
fpdmti (2)
dmdmppi (3)
dmip (4)
dmtip (5)
mpdmti (6)
Electric dipole moment (D)
μx
4.1575
−1.6667
−3.6836
−1.4874
4.3081
−2.8610
−1.0621
−1.1603
3.2649
−0.4584
0.8072
−0.4015
−0.8858
1.0729
−0.0538
0.4505
0.4463
0.8430
μy
−0.2181
0.2560
4.1711
μz
−1.2426
1.5510
μtotal
−0.2144
Polarizability (α)
αxx
168.02
−0.760
0.002
323.342
−3.123
264.943
12.572
5.881
311.922
4.169
321.177
−1.469
286.834
−8.127
0.767
180.5289
34.0782
133.8131
−39.1104
−34.5488
614.3149
45.876
229.4428
37.9341
99.4140
−43.7296
−8.5868
640.1431
47.869
αxy
αyy
250.777
0.188
αxz
0.062
αyz
119.264
43.391
70.47
26.507
114.336
33.4455
αzz
103.467
34.1725
96.133
34.7847
α×10−24 (esu)
Hyperpolarizability (βtotal
)
βxxx
889.0866
1.1910
433.7337
−125.9418
−69.0532
−273.7393
−175.4702
75.9236
105.6872
−94.7286
−67.0422
43.4894
−51.1599
37.0878
20.9672
86.448
43.1135
−46.0440
110.5338
149.2485
−21.7770
133.0105
43.1935
33.5059
−28.5833
−14.8611
−93.1900
−29.4058
45.4424
−68.8788
−29.6330
−3.992
βxxy
βxyy
2.2085
βyyy
11.1018
53.4965
37.1521
60.8972
63.8108
478.3876
21.3076
−8608.0030
73.590
βxxz
143.6161
−11.4331
−26.3634
−20.0420
−23.8558
−199.3893
75.6105
βxyz
βyyz
46.0494
172.1898
539.3020
202.4031
−7631.1062
64.836
βxzz
43.0301
119.2973
10.2854
βyzz
−69.0634
−30.1211
55.4131
−23.6013
−9.3178
43.4238
βzzz
β×10−31 (esu)
−52.7186
45.6805
as a standard according to the equation,
Where kr, knr are the radiative and non-radiative deactiva-
tion, τf is the life time of the S1 excited state. From the view
point of the decay rate constants, two trends are evident for
the imidazole derivatives 1−6. From the Table 4 it was
suggested that the radiative rate constant (kr) increases
when the imidazoles (1,3 and 5) are substituted by electron
releasing substituents like methyl and methoxy whereas in
the case of fluoro substituted imidazole (2) the radiative rate
constant (kr) decreases.
ꢂ
ꢃꢂ
ꢃꢂ
ꢃ
2
Iunk
Istd
Astd
hunk
hstd
6unk ¼ 6std
ð5Þ
Aunk
Where Фunk, Фstd, Iunk, Istd, Aunk, Фunk and Фstd are the
fluorescence quantum yields, the integration of the emis-
sion intensities, the absorbances at the excitation wave-
length and the refractive indexes of the corresponding
solution of the 2-aryl imidazole derivatives (1–6) and the
standard, respectively.
HOMO-LUMO Energies of Imidazole Derivatives 1–6
by DFT Method
The main decay route of the excited state of these
compounds (1–6) and their radiative and non-radiative
decay are studied in detail. The radiative (kr) and non-
radiative (knr) rate constants are calculated using the
following equations
The Eigen values of HOMO, LUMO and their energy gap
reflect the chemical activity of the molecule and 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) [31–33]. The HOMO-
LUMO energy gap for 1–6 were calculated by B3LYP/6-
31G(d,p) and from the HOMO-LUMO orbital picture
(Fig. 8) of dmppi (1), it was found that the filled π orbital
(HOMO) is mostly located on the phenyl ring attached to
kr ¼ 6p=t
ð6Þ
ð7Þ
ð8Þ
knr ¼ 1=t ꢁ 6p=t
t ¼ ðkr þ knrÞꢁ1

J Fluoresc (2011) 21:595–606
603
the carbon of the imidazole derivative and the unfilled π*
orbital (LUMO) located on the imidazole ring with the
phenyl ring attached to the nitrogen of the imidazole ring.
Therefore, introduction of an electron-donating substituent
into the phenyl ring attached to the nitrogen 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 [34, 35]. The calculated energy gap explains
the eventual charge transfer interactions within the molecule.
Second Harmonic Generation (SHG) Studies of 2-aryl
Imidazole Derivatives (1–6)
Second harmonic signal of 75 mV, 12 mV, 16 mV, 23 mV,
25 mV and 17 mV were obtained for imidazole derivatives
1–6 by an input energy 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 [36]
and higher efficiencies are achieved by optimizing the
phase matching [37]. On a molecular scale, the extent of
charge transfer (CT) across the NLO chromophore deter-
mines the level of SHG output, the greater the CT, the
larger is the SHG output.
Fig. 10 The packing of the title compound, viewed down the axis.
Dashed lines indicate hydrogen bonds. H atoms not involved in
hydrogen bonding have been omitted
Hyperpolarizability of Imidazole Derivatives 1–6 by DFT
Method
In addition to well known empirical rules to estimate
qualitatively the microscopic nonlinear response in imidazole
derivatives 1–6, DFT calculation is a more accurate
prediction of the NLO activity [38, 39]. 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 1–6 are polar
having non-zero dipole moment, hyperpolarizabilities and
hence have well microscopic NLO behaviour [40–42]. To
determine the transference region and hence to know the
suitability of these compounds 1–6 for microscopic nonlinear
optical applications, the UV-visible spectra have been
recorded by using the spectrometer in the range of 190–
1,100 nm. These compounds show absorption spectra in the
UV region between 210 and 290 nm. The increased
transparency in the visible region might enable the micro-
scopic NLO behaviour with non-zero values [27, 43, 44]. All
the absorption bands are due to π→π* transitions. The β
values (Table 5) computed here might be correlated with
Fig. 9 ORTEP diagram of dmppi (1)

604
J Fluoresc (2011) 21:595–606
Table 6 Selected Bond lengths (Å), Bond angles (°) and torsional angles (°) of dmppi (1)
Bond connectivity
Experimental XRD (Å)
Bond angles
Experimental XRD (°)
Torsional angles
Experimental XRD(°)
N1-C2
1.372(1.380)
1.388(1.407)
1.443(1.429)
1.322(1.306)
1.373(1.395)
1.467(1.474)
1.359(1.354)
1.498(1.495)
1.485(1.496)
1.382(1.384)
1.380(1.385)
1.386(1.387)
1.387(1.390)
1.385(1.391)
1.511(1.516)
1.385(1.385)
1.390(1.392)
1.385(1.394)
1.383(1.387)
1.380(1.385)
1.372(1.388)
1.373(1.383)
0.930(1.073)
0.930(1.074)
0.930(1.074)
0.930(1.073)
0.960(1.086)
0.960(1.083)
0.960(1.084)
0.930(1.069)
0.930(1.073)
0.930(1.073)
C2-N1-C5
107.2(107.0)
128.6(128.1)
123.0(124.5)
106.4(107.6)
110.4(110.1)
126.7(126.4)
122.8(123.4)
110.4(109.4)
121.3(120.0)
128.3(130.6)
105.6(105.9)
123.0(122.0)
131.4(132.1)
119.9(120.1)
119.2(119.9)
120.8(120.0)
119.2(119.8)
121.2(120.9)
118.1(118.5)
120.3(120.9)
121.6(120.6)
121.6(121.0)
119.0(119.8)
123.8(123.5)
118.1(117.5)
118.1(119.0)
120.7(120.3)
120.1(120.4)
119.4(119.5)
120.6(120.3)
121.4(120.5)
120.0(119.7)
C5-N1-C2-N3
C5-N1-C2-C21
C11-N1-C2-N3
−0.4(−0.1)
−176.6(−179.6)
−167.5(−173.4)
16.3(6.2)
−0.1(−0.1)
−178.7(−179.1)
167.9(173.9)
−10.7(−5.5)
−119.5(−111.6)
63.6(68.7)
N1-C5
C2-N1-C11
C5-N1-C11
C2-N3-C4
N1-C11
N3-C2
C11-N1-C2-C21
C2-N1-C5-C4
N3-C4
N1-C2-N3
C2–C21
C4-C5
N1-C2-C21
N3-C2-C21
N3-C4-C5
C2-N1-C5-C51
C11-N1-C5-C4
C11-N1-C5-C51
C2-N1-C11-C12
C2-N1-C11-C16
C5-N1-C11-C12
C5-N1-C11-C16
C4-N3-C2-N1
C4-C41
C5-C51
N3-C4-C41
C5-C4-C41
C11-C12
C11-C16
C12-C13
C13-C14
C14-C15
C14-C17
C15-C16
C21-C22
C21-C262
C22-C23
C23-C24
C24-C25
C25-C26
C12-H12
C13-H13
C15-H15
C16-H16
C17-H17A
C17-H17B
C17-H17C
C22-H22
C23-H23
C24-H24
N1-C5-C4
75.3(75.0)
N1-C5-C51
C4-C5-C51
−101.6(−103.7)
0.7(0.2)
N1-C11-C12
N1-C11-C16
C12-C11-C16
C11-C12-C13
C12-C13-C14
C13-C14-C15
C13-C14-C17
C15-C14-C17
C14-C15-C16
C11-C16-C15
C2-C21-C22
C2-C21-C26
C22-C21-C26
C21-C22-C23
C22-C23-C24
C23-C24-C25
C24-C25-C26
C21-C26-C25
C11-C12-H12
C4-N3-C2-C21
C2-N3-C4-C5
177.1(179.8)
−0.8(−0.3)
177.3(179.4)
18.9(26.3)
C2-N3-C4-C41
N1-C2-C21-C22
N1-C2-C21-C26
N3-C2-C21-C22
N3-C2-C21-C26
N3-C4-C5-N1
−164.6(−156.1)
−156.9(−154.1)
19.6(23.4)
0.5(0.2)
N3-C4-C5-C51
C41-C4-C5-N1
C41-C4-C5-C51
N1-C11-C12-C13
C16-C11-C12-C13
N1-C11-C16-C15
C12-C11-C16-C15
C11-C12-C13-C14
C12-C13-C14-C15
C12-C13-C14-C17
C13-C14-C15-C16
C17-C14-C15-C16
C14-C15-C16-C11
C2-C21-C22-C23
179.0(179.0)
−177.3(−179.4)
1.1(1.3)
−176.4(−179.9)
0.4(0.2)
177.6(179.8)
0.7(0.4)
−0.9(−0.1)
0.3(0.1)
179.5(179.3)
0.9(0.1)
−178.3(−179.0)
−1.4(−0.4)
176.8(178.2)
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 computations here might also be illustrated by analyzing
the relationship between calculated β values and measured
values of lmax (Table 1). The band at around 285 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, accom-
panied with the upward shifts non-zero values in the
β-components [20].
X-Ray Analysis
Crystal Structure
Crystalline imidazole derivatives dmppi (1) and fpdpi (2)
[45] are monoclinic crystals. dmppi (1) (Fig. 9) crystallizes
in the space group pn and cell has dimensions of a=9.809
Å, b=7.700Å, c=19.907Å. ORTEP diagram of dmppi (1)

J Fluoresc (2011) 21:595–606
605
(Fig. 9) shows that the imidazole ring is essentially planar.
The imidazole ring makes dihedral angles of 70.91 (19°)
and 19.8 (2°) with the p-tolyl ring (C11-C16) attached to
the nitrogen of the imidazole nucleus and phenyl ring (C21-
C26) attached to the carbon of the imidazole nucleus
respectively. The dihedral angle between the two phenyl
rings is 74.59 (18°). The crystal packing is stabilized by
C12-H12···N3 (2-x, 2-y, -z) and C16-H16···N3 (2-x, 1-y, -z)
intermolecular hydrogen bonds (Fig. 10). From XRD data,
the shortening of bond angles C2-N1-C5, C2-N3-C4 and
N1-C5-C4 and increase of bond angles C2-N1-C11, C5-
N1-C11, N1-C2-C21, N3-C2-C21, C5-C4-C41and C2-
C21-C22 from 120° exactly at the substitution of the
imidazole nucleus. This asymmetry of dihedral angles and
bond angles reveal the conjugation of the phenyl ring
attached to the carbon with the imidazole nucleus.
Optimization of dmppi (1) have been performed by DFT
at B3LYP/6-31 G(d,p) using Gaussian-03 (Table 6). All
these XRD data are in good agreement with the theoretical
values (Table 6). However, from the theoretical values it
can be found that most of the optimized bond lengths, bond
angles and dihedral angles are slightly higher than that of
XRD values. These deviations can be attributed to the fact
that the theoretical calculations were aimed at the isolated
molecule in the gaseous phase and the XRD results were
aimed at the molecule in the solid state.
proton transfer from the hydroxylic group to the nitrogen
atom. PES calculation reveals that the barriers for intercon-
version in the excited state are much higher than that in the
ground state. The solution grown good quality of dmppi
single crystal belongs to monoclinic crystal system having
the space group of Pn. Further work we focused the
increased transparency of imidazole derivatives in the
visible region might enable the microscopic NLO behavior
with non-zero values and all the absorption bands are due to
only π-π* transition. The higher value of specific heat
shows that these compounds are more resistant to laser
damage threshold compared to potassium dihydrogen
phosphate. Its phase matching and relative second harmonic
generation efficiency was determined by using Nd: YAG
laser as a source. Its electric dipole moments, polarizability,
hyperpolarisability and HOMO-LUMO energies studies
were determined by DFT calculation.
Acknowledgment One of the author Dr. J. Jayabharathi, Reader in
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 providing fund to
this research work.
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