Estimation of excited state dipolemoments from solvatochromic shifts-effect of pH

Article abstract of DOI:10.1007/s10895-013-1334-3

Multicomponent, highly efficient, catalytic synthesis of some polysubstituted imidazole under solvent-free condition is reported. Characterization of polysubstituted imidazole have been carried out by X-ray diffraction (XRD) and spectral techniques. Electronic spectral studies reveal that their solvatochromic behavior depends not only on the polarity of the medium but also on the hydrogen bonding properties of the solvents. Specific hydrogen bonding interaction in polar solvents modulated the order of the two close lying lowest singlet states. The solvent effect on both the absorption and emission spectral results have been analyzed by multiple parametric regression analysis. Solvatochromic effects on the emission spectral position indicate the charge transfer (CT) character of the emitting singlet states both in a polar and a non polar environment. The fluorescence decays for the imidazole fit satisfactorily to a single exponential kinetics. The prototropic studies of N,N-dimethyl-4-(1,4,5-triphenyl-1H-imidazol-2-yl)naphthalen-1-amine (DTINA) reveal that two monocations [imidazole nitrogen protanated (MC1) and dimethylamino nitrogen protanated (MC2)] and a dication [both imidazole nitrogen and dimethylamino nitrogen protanated (DC)] are formed by protonation in both ground and excited states. These observations are in consistent with quantum chemical calculations.

Full text of DOI:10.1007/s10895-013-1334-3

J Fluoresc
DOI 10.1007/s10895-013-1334-3
ORIGINAL PAPER
Estimation of Excited State Dipolemoments
from Solvatochromic Shifts–Effect of pH
J. Jayabharathi & V. Kalaiarasi & V. Thanikachalam &
K. Jayamoorthy
Received: 25 September 2013 /Accepted: 20 November 2013
#
Springer Science+Business Media New York 2014
Abstract Multicomponent, highly efficient, catalytic synthe-
sis of some polysubstituted imidazole under solvent-free con-
dition is reported. Characterization of polysubstituted imidaz-
ole have been carried out by X-ray diffraction (XRD) and
spectral techniques. Electronic spectral studies reveal that their
solvatochromic behavior depends not only on the polarity of
the medium but also on the hydrogen bonding properties of
the solvents. Specific hydrogen bonding interaction in polar
solvents modulated the order of the two close lying lowest
singlet states. The solvent effect on both the absorption and
emission spectral results have been analyzed by multiple
parametric regression analysis. Solvatochromic effects on
the emission spectral position indicate the charge transfer
(CT) character of the emitting singlet states both in a polar
and a non polar environment. The fluorescence decays for the
imidazole fit satisfactorily to a single exponential kinetics.
The prototropic studies of N,N-dimethyl-4-(1,4,5-triphenyl-
1H-imidazol-2-yl)naphthalen-1-amine (DTINA) reveal that
two monocations [imidazole nitrogen protanated (MC1) and
dimethylamino nitrogen protanated (MC2)] and a dication
[both imidazole nitrogen and dimethylamino nitrogen
protanated (DC)] are formed by protonation in both ground
and excited states. These observations are in consistent with
quantum chemical calculations.
Introduction
Polysubstituted imidazole derivatives have been used as highly
sensitive fluorescent chemosensor for sensing, imaging metal ions
and play an important role in material science due to their
optoelectronic properties [1–4]. Its chelates with Ir3+ are major
components for organic light emitting diodes [5]. Since they exert
the properties of piezochromism, photochromism and
thermochromism, they are used in molecular photonics [6]. Im-
idazole derivatives possess diverse biological activities, play im-
portant role as versatile building blocks for the synthesis of natural
products [7–11]. Therefore, the synthesis of these imidazole
derivatives has attracted much attention in organic synthesis.
The influence of solvents on fluorescence spectra and quan-
tum yields may have several origins like perturbation, due to the
solvent refractive index and dielectric constant to hydrogen
bonding and complexation between the fluorophore and the
solvent [12]. These factors can change the energy difference
between the ground (S0) and the first excited (S1) states and can
shift the emission or affect quantum yield. Solvent moderated
shifts of the energy levels may enhance or inhibit radiationless
transitions to the ground state via proximity effect [13].
In this work, naphthyl substituted imidazole derivatives are
synthesized using indium trifluride (InF3) as a highly efficient
catalyst. The spectral characteristics of the synthesised imid-
azole in solvents of different polarity, hydrogen bonding
capacity and at different pH in aqueous media have been
discussed. The Lippert solvent parameter Δf, the normalized
ET(30) polarity scale, and the multiparameter Kamlet-Taft and
Catalan solvent scales are used to describe the solvent effect
on the fluorescence emission and stokes shift of imidazole
derivatives. We have also addressed the influence of solvents
on the photophysical properties of the synthesized molecules
.
.
.
.
Keywords Imidazole Indium trifluoride DFT CT
HOMO-LUMO
:
:
:
J. Jayabharathi (*) V. Kalaiarasi V. Thanikachalam
K. Jayamoorthy
Department of Chemistry, Annamalai University, Annamalainagar,
608 002 Tamilnadu, India
e-mail: jtchalam2005@yahoo.co.in
vac
abs
vac
flu
vac
vac
flu
in terms of hceυ , hceυ
and (hceυabs−hceυ ) with solvent

J Fluoresc
polarity function. Our aim is also to investigate the effect of
protic solvents on the fluorescence characteristics of DTINA
and to understand the excited state prototropic equilibria of
DTINA. Finally, the results obtained from the steady state and
time resolved fluorescence studies have been rationalized with
theoretical calculations.
difference density plots and optimization of the excited states
were performed using time-dependent DFT (TD-DFT) using
B3LYP/6-31G (d,p) basis set. The ground and excited states
HOMO and LUMO frontier orbital’s of imidazole derivatives
were calculated by both DFT and TD-DFT methods at the
B3LYP/ 6-31(d,p) level.
InF3–Catalyzed Facile and Rapid Synthesis of Polysubstituted
Imidazole
Experimental
Spectral Measurements
A mixture of naphthaldehyde (1 mmol), benzil (1 mmol), aniline
(1 mmol), ammonium acetate (1 mmol) and InF3 (1 mol%) was
stirred at solvent-free conditions at 80 °C, progress of the
reaction was monitored by TLC. After completion of the reac-
tion, the mixture was cooled, dissolved in acetone and filtered.
The product was purified by column chromatography using
benzene: ethyl acetate (9:1) as the eluent.
The infrared spectra were recorded with an Avatar 330-Thermo
Nicolet FT-IR spectrometer. The proton spectra at 400 MHz
were obtained at room temperature using a Bruker 400 MHz
NMR spectrometer. Proton decoupled 13C NMR spectra were
also recorded at room temperature employing a Bruker
400 MHz NMR spectrometer operating at 100 MHz. The mass
spectra of the samples were obtained using a Thermo Fischer
LC-Mass spectrometer in FAB mode. The cyclic voltammetry
analyses were performed with CHI 630A potentiostap-
electrochemical analyzer at a scan rate of 100 mV s−1 using
0.1 M tetra-(n-butyl)-ammonium hexafluorophosphate as
supporting electrolyte with Ag/Ag+ (0.01 M AgNO3) as the
reference electrode and Pt electrode as the working electrode
under nitrogen atmosphere at room temperature.
The UV–vis absorption and fluorescence spectra were
recorded with PerkinElmer Lambda 35 spectrophotometer
and PerkinElmer LS55 spectrofluorimeter, respectively. Fluo-
rescence lifetime measurements were carried out with a nano-
second time correlated single photon counting (TCSPC) spec-
trometer Horiba Fluorocube-01-NL lifetime system with
NanoLED (pulsed diode exCitation source) as the excitation
source and TBX-PS as detector. The slit width was 8 nm and
the laser excitation wavelength was 280 nm. The fluorescence
decay was analyzed using DAS6 software. The quantum yield
for all the imidazole were measured in dichloromethane using
coumarin 47 in ethanol as the standard [14–16].
2-(naphthalen-1-yl)-4,5-diphenyl-1H-imidazole (1)
M.p. 258 °C., Anal. calcd. for C25H18N2: C, 86.68; H, 5.24; N,
8.09. Found: C, 86.63; H, 5.21; N, 8.03. 1H NMR (400 MHz,
CDCl3): δ 7.26–7.38 (m, 6H), 7.52–7.66 (m, 8H), 7.79 (d, Hz,
1H) , 7.93 (bs, 2H), 7.98 (d, 1H), 8.82 (d, 1H), 10.41 (s, 10.4,).
13C NMR (100 MHz, CDCl3): δ 125.11, 126.00, 126.37,
126.71, 127.20, 127.55, 127.88, 128.45, 128.69, 129.06,
129.75, 129.95, 134.93. MS: m/z. 346 [M+].
2-(naphthalen-1-yl)-1, 4, 5-triphenyl-1H-imidazole (2)
M.p. 248 °C., Anal. calcd. for C31H22N2: C, 88.12; H, 5.25; N,
6.63. Found: C, 88.08; H, 5.21; N, 6.60. 1H NMR (400 MHz,
CDCl3): δ 6.88 (d, 2H), 7.03 (q, 3H), 7.22–7.31 (m, 10H) 7.47
(t, 2H), 7.66 (d, 2H), 7.80 (d, 2H), 8.19–8.20 (d, J=5.6 Hz,
1H). 13C NMR (100 MHz, CDCl3): δ 124.63, 126.04, 126.27,
126.63, 126.69, 127.53, 127.74, 127.84, 127.98, 128.10,
128.19, 128.31, 128.47, 128.61, 129.31, 129.42, 129.90,
130.81, 131.08, 132.83, 133.61, 134.57, 136.63, 138.10,
146.18. MS: m/z. 422 [M+].
The radiative and non-radiative rate constants, kr and knr,
were deduced from the quantum yield (Φf) and lifetime (τ)
using the equation, Φf=Φisc{kr/(kr+knr)};Φisc is the intersys-
tem crossing yield, kr=Φf/t,knr=1/(τ)−Φf/(τ);(τ)=(kr+knr)−
1. Single crystal XRD was recorded in Agilent Xcalibur Ruby
Gemini diffractometer. The Radiation source is enhanced Mo
X-ray source. Graphite monochromator used is of 10.5081
pixels mm−1 for the detector resolution.
N,N-dimethyl-4-(1,4,5-triphenyl-1H-imidazol-2-yl)
naphthalen-1-amine (3)
M.p. 261 °C., Anal. calcd. for C33H27N3: C, 84.76; H, 6.25; N,
8.99. Found: C, 84.73; H, 6.23; N, 8.97. 1H NMR (400 MHz,
CDCl3): δ 6.89 (d, 2H), 7.07 (q, 3H), 7.24–7.36 (m, 10H) 7.50
(t, 2H), 7.74 (d, 2H), 7.85 (d, 1H), 8.30 (d, 1H), 2.81 (S, 6H).
13C NMR (100 MHz, CDCl3): δ 124.83, 126.24, 126.43,
126.83, 126.99, 127.83, 127.94, 127.99, 128.18, 128.30,
128.39, 128.41, 128.53, 128.74, 129.47, 129.62, 129.94,
130.91, 131.48, 132.93, 133.71, 134.77, 136.83, 138.70,
146.98. MS: m/z. 465 [M+].
Computational Details
The quantum chemical calculations were performed using the
Gaussian-03 [17]. The geometry was fully optimized with the
density functional theory (DFT) method, using B3LYP/6-31G
(d,p) basis set. Computations of the vertical excitations,

J Fluoresc
Result and Discussion
experiments, it was clearly demonstrated that the solvent-free
condition is the best for imidazole synthesis. In the absence of
catalyst under solvent-free conditions at room temperature the
yield is very poor even after 24 h. To enhance the yield of the
desired product, the temperature of the reaction was increased to
200 °C, but no appreciable increment in the product yield was
observed. We found that presence of a catalytic amount of InF3
under solvent-free condition is the best for this synthesis; max-
imum yield (82 %) was obtained at 30 min on loading with
1 mol% of InF3 at 80 °C (Table 1). Moreover, InF3 can be
Catalytic Activity of Indium Trifluoride
Initially, we have carried out the condensation reaction in the
presence of an optimized molar ratio of InF3 (1 mol%),
naphthaldehyde (1 mmol), ammonium acetate (1 mmol), and
arylamine (1 mmol) in different solvents such as water, ethanol,
methanol, chloroform, and acetonitrile under refluxing and also
in solvent-free conditions at 80 °C (Scheme 1). From these
Scheme 1 Possible mechanism for IF3 catalytic synthesis of imidazoles

J Fluoresc
Table 1 Effect of catalyst and temperature in the synthesis of imidazole
derivatives
The ORTEP diagram is presented in Fig. 1. The imidazole unit
is close to being planar [maximum deviation=0.59 (17) A˚]
and forms dihedral angles of 99.4.98° (2) and −101.0° (27)
with the adjacent naphthyl and phenyl rings [C(2)-C(1)-
C(11)-N(2)=99.4°(2), C(15)-C(14)-N(1)-C(11)=−101.0°
(2)]. The dihedral angle between the latter two planes is
−6.9° (3). In the crystal, molecules are consolidated into a
three-dimensional architecture by π–π stacking interactions.
Optimization of 2 has been performed by DFT at B3LYP/6-
31G(d,p) level using Gaussian-03. All these XRD data are in
good agreement with the theoretical values (Table S1). How-
ever, 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 are
of isolated molecule in the gaseous phase and the XRD results
are of the molecule in the solid state.
Entry Temp (°C) Solvent
InF3
InF3 (mol%)
Time (min) Yield (%)
1
r.t
Solvent-free 130 (380) 70(Trace) 0.1
2
50
70
90
80
80
80
80
80
80
80
80
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Water
65(250)
30(70)
25(90)
30
72(22) 0.1
76(45) 0.1
80(53) 0.1
3
5
6
82
85
1
2
7
25
8
45
88 10
9
100
40
35
65
70
42
62
1
1
1
1
1
10
11
12
13
Ethanol
Methanol
50
Chloroform
Acetonitrile
140
95
Solvent Modulated Absorption and Emission Behavior
values in the parentheses corresponds to absence of catalyst
recovered and reused several times without significant loss of
activity. High product yields, shorter reaction time, low catalyst
loading and easy work-up procedure, make this procedure quite
simple and more convenient. Our methodology could be a valid
contribution to the existing processes of imidazole synthesis.
Room temperature absorption and the fluorescence of imid-
azole derivatives (1–3) have been carried out. The absorption
maxima of the studied compounds is shifted from 319 to
361 nm in solvents of varying polarity which corresponds to
π-π* transition of the molecule. The absorption is intense and
the peak position is sensitive to the polarity of the medium.
The magnitude of the shifts suggests that the ground state of
the molecule is polar.
XRD Characterization of 2-(naphthalen-1-yl)-1, 4,
5-triphenyl-1H-imidazole (2)
The emission spectra of 1–3 displayed in Fig. 2a, show a red-
shifted band in the region of 335–452 nm from hexane to water.
The fluorescence maximum is greatly affected by solvent polar-
ity where a red-shifted fluorescence is observed on increasing
2-(Naphthalen-1-yl)-1, 4, 5-triphenyl-1H-imidazole is a tri-
clinic crystal and crystallizes in the space group P‾1with cell
dimensions a=11.1280(2), b=14.8250(3) A, c=14.8820(2) A
Fig. 1 ORTEP diagram of 2-(naphthalen-1-yl)-1, 4, 5-triphenyl-1H-imidazole

J Fluoresc
Fig. 2 a Emission profile of 3 in different solvents; b νss cm−1 Vs ET(30)
the solvent polarity. The effect of polarity of the medium on the
fluorescence maximum is more intense than that on the absorp-
tion maximum. This observation suggests that the emitting state
is more polar than the ground state [18–20]. The emission yields
are more prominent in polar solvents compared to that of non-
polar solvents. The linear variation of the Stokes shift with
ET(30) has been shown in Fig. 2b., where a double linear
correlation is obtained [21]. Polar protic solvents fall on a
separate line indicating that the mode of solvation of the emitting
state is different from that in the other polar aprotic solvents. For
polar protic solvents, gradual increment of Stokes shift is due to
intermolecular hydrogen bonding interactions. The studied com-
pounds exhibit overall increase of Stokes shift from non-polar to
polar aprotic solvents mainly due to combined effect of increas-
ing the polarity of the medium and intramolecular charge trans-
fer (CT) state.
composition has been monitored and the spectra are illustrated
in Fig. 3a. The addition of water to methanol–water mixture
remarkably enhances the fluorescence of 1–3 with red shift.
This effect may be described by the combined effect of hy-
drogen bonding and polarity of the medium. Mixing of two
closely lying lowest singlet states (n, π*and π, π*) of imid-
azole derivatives favors the intersystem crossing [22]. As the
polarity of the medium is increased, intermolecular hydrogen
bonding interaction in the excited state stabilized the π, π*
state and enhancing the energy between the two states, which
diminishes the mixing between the states (Scheme 2). As a
result intersystem crossing from S1 to T1 decreases and an
enhancement of fluorescence is observed.
The behavior of imidazole derivatives toward different
solvent polarities may be interpreted in terms of the difference
in the ground state and the excited state dipole moments. The
spectral shift in fluorescence band may be attributed to the
interaction between the dipole moment of the solute and the
polarizability of the solvent. The extent of charge separation
In order to obtain an insight about specific solvent–
fluorophore interactions, the influence on fluorescence emis-
sion of 1–3 in methanol–water mixtures of different
Fig. 3 a Emission profile of 3 in
MeOH as function of water
composition; b Lippert–Mataga
plot

J Fluoresc
Scheme 2 Schematic
demonstration of modulation of
the close-lying lowest singlet n,
π* and π, π* states with solvent
polarity in the excited state
on electronic excitation of 1–3 have been determined by
measuring the change in the dipole moment (Δμ=μe–μg)
utilizing the spectral shift between the absorption and emis-
sion maxima as a function of solvent polarity. According to
Lippert–Mataga equation [23] (1):
where f(D,n)=(D–1)/(2D +1)–(n2–1)/(2n2+1), which indi-
cates the orientation polarizability and depicts polarity param-
eter of the solvent , n is refractive index, D is dielectric
constant, μe and μg are dipole moments of the species in S1
and S0 states, respectively, h is Planck’s constant, c is velocity
of light and a is Onsager’s cavity radius. The Lippert-Mataga
plot is linear for the non-polar and polar / aprotic solvents as
shown in Fig. 3b; correlation is satisfactory. The geometrical
optimization of 1–3 was done by DFT method using
Gaussian-03 [17] to calculate the μg. Using μg value, 4.50D
2
3
ꢀ
ꢁ
2
2 μe−μg
6
4
7
5
v− ¼ v− −v− ¼ const þ
f ðD; nÞ
ð1Þ
ss
ab fl
hca3
Table 2 Adjusted coefficients ((υa)0, ca, cb and cc) for the multilinear
regression analysis of the absorption (υab) and fluorescence (υfl)
wavenumbers and Stokes Shift (Δυss) of imidazole derivatives (1–3)
with solvent polarity/polarizability, and acid and base capacity using the
Taft (π*, α and β) and the Catalan (SPPN, SA and SB) scales
1
(υx)
(υx)0cm−1
(π*)
c α
c β
λab
(3.04 0.03)×104
(2.63 0.05)×104
(0.41 0.01)×104
(υx)0cm−1
−(8.74 2.73)×103
(16.80 2.18)×103
(21.92 1.94)×103
−(100.82 16.56)×103
−(13.70 2.41)×103
λfl
−(11.73 1.52)×103
Δυss=υab–υfl
(2.98 0.89)×103
N
−(5.12 0.19)×103
cSA
(2.88 0.11)×103
(υx)
cSPP
cSPP
cSPP
cSB
λab
−(3.00 0.02)×104
(2.57 0.03)×104
(0.42 0.01)×104
(υx)0cm−1
(3.08 0.03)×104
(2.53 0.04)×104
(0.54 0.02)×104
(υx)0cm−1
(3.05 0.02)×104
(2.49 0.03)×104
(0.55 0.01)×104
(υx)0cm−1
(3.08 0.03)×104
(2.54 0.04)×104
(0.54 0.01)×104
(υx)0cm−1
−(7.98 1.66)×103
−(10.40 2.60)×103
(2.41 0.03)×103
(π*)
(12.11 2.79)×103
(15.09 1.12)×103
−(2.98 0.59)×103
−(6.30 1.39)×103
−(7.68 1.89)×103
(1.37 0.67)×103
λfl
Δυss=υab−υfl
(υx)
c α
c β
λab
−(10.14 1.62)×103
(24.33 3.70)×103
(29.13 4.36)×103
−(4.79 0.19)×103
−(18.21 3.84)×103
−(20.88 3.83)×103
(2.66 0.75)×103
λfl
−(12.81 1.06)×103
Δυss=υab−υfl
(υx)
(2.67 0.55)×103
N
2
3
cSA
cSB
λab
−(12.69 2.21)×103
−(13.97 2.62)×103
(1.28 0.36)×103
(π*)
(27.44 3.55)×103
(27.43 3.43)×103
(0.01 0.48)×103
−(16.31 3.58)×103
−(15.95 2.12)×103
−(0.35 0.25)×103
λfl
Δυss=υab−υfl
(υx)
c α
c β
λab
−(8.44 2.52)×103
(18.51 3.36)×103
(24.13 2.94)×103
−(13.54 2.58)×103
−(17.29 3.72)×103
(3.74 1.411)×103
λfl
−(11.12 3.61)×103
Δυss=υab–υfl
(2.68 1.42)×103
N
−(5.61 1.75)×103
(υx)
cSA
cSB
λab
(3.05 0.02)×104
(2.50 0.02)×104
(0.55 0.01)×104
−(10.30 3.62)×103
−(10.49 2.69)×103
(0.18 0.02)×103
(18.72 3.95)×103
(15.93 2.88)×103
(2.79 0.11)×103
−(10.26 2.52)×103
−(8.23 1.45)×103
−(2.02 0.01)×103
λfl
Δυss=υab–υfl

J Fluoresc
Fig. 4 a Fluorescence maxima
and the ET(30); b Δ (ΔGsolv) Vs
ET(30)
(1), 4.3 D (2) and 5.0 D (3) obtained from the DFTcalculation
and the slope of Lippert–Mataga plot, the value of μe calcu-
lated is in the range, 6. 8–10 D for the studied imidazole.
Multiple linear regression analysis is performed to identify
the different modes of solvation determining the absorption and
emission energies. Kamlet and Taft [24] put forward the π*, α
and β parameters to characterize the polarity/polarizability, the
acidity and the basicity of a solvent respectively. Conversely,
Catalan et al. [25] proposed an empirical solvent scales for
polarity/polarizability (SPP), acidity (SA) and basicity (SB) to
describe the respective properties of a given solvent
The dominant coefficient affecting the absorption and fluo-
rescence band of imidazole derivatives (1–3) are displayed in
Table 2. Negative values of solvent dipolar interaction (π*)
and hydrogen bond accepting property (β) indicate these two
parameters contribute to the stabilization of both the ground
and the excited states of imidazole derivatives. The calculated
ratio of β over π* [1.24(υabs) & 1.17(υemi) (1), 1.60(υabs) &
1.55(υemi) (2) and 1.80 (υabs) & 1.63(υemi) (3)] reveal that
interactions between imidazole derivatives and solvents with
hydrogen bond accepting property (β) predominate in the
excited state. A good linear variation is also obtained between
the fluorescence maxima and ET (30) values [26] as shown by
Fig. 4a. The free energy change of solvation and reorganiza-
tion energies of 1–3 in various solvents have been estimated.
y ¼ y0 þ aαα þ bββ þ cπà πà ðKamlet−TaftÞ
y ¼ y0 þ aSASA þ bSBSB þ cSPPSPP ðCatalanÞ
ð2Þ
ð3Þ
Fig. 5 a Quantum yield with
ET(30); b Lifetime curves of 1–3
in ethanol; c log (kr/knr) Vs
ET(30)

J Fluoresc
Fig. 7 CT Fluorescence maxima Vs solvent polarity function
Fluorescence quantum yield (φf) was measured in solvents
of different polarity. The variation of φf with solvent polarity
parameter ET (30) is depicted in Fig. 5a. A close look at the
results shows that the φf values in various solvents are sensi-
tive towards solvent polarity. The remarkable increase of φf in
polar protic medium is compared with that in polar aprotic
medium. This may be due to differential contribution of CT
and hydrogen bonding interactions. Radiative (kr) and
nonradiative (knr) rate constants are calculated from fluores-
cence quantum yields and lifetime values (Fig. 5b) in different
solvents to understand the effect of solvation on the dynamics
of the excited state. The logarithm of (kr/knr) is plotted against
the solvent polarity parameter ET (30) which is shown in
Fig. 5c. Two different straight lines are obtained, one for
aprotic solvents and the other for protic solvents. In both the
cases, upon increasing the polarity the logarithm ratio of
radiative to nonradiative rate decreases but a steeper slope is
obtained in the case of protic solvents. It indicates that the
radiative and nonradiative rates are more sensitive toward
protic solvents. It may be that the hydrogen bonding interac-
tion in polar protic environment enhances the stabilization of
the S1 state as a result the nonradiative relaxation rate
increases [29].
Fig. 6 CT Absorption maxima Vs solvent polarity function
A c c o r d i n g t o M a r c u s [ 2 7 ] , E ( A ) = Δ G s o l v
+
λ1 and E(F)=ΔGsolv−λ0, where E(A) and E(F) are absorp-
tion and fluorescence band maxima in cm−1, respectively,
ΔGsolv is the difference in free energy of the ground and
excited states in a given solvent and λ represents the reorga-
nization energy . The free energy changes of solvation and
reorganization energies of 1–3 in various solvents have
been estimated. Under the condition that λ0≈λ1≈λ, we
get, E(A)+E(F)=2ΔGsolv;E(A)−E(F)=2 λ. The ΔG
1–3 is maximum for hexane since it is purely non-polar
and also α and β values of hexane are zero. The ΔG is
minimum in water. The difference between these values
(water and hexane) should give the free energy change
required for hydrogen bond formation. The plot of
Δ(ΔGsolv)=(ΔGhex_ΔGwater) versus ET(30) has been
depicted in Fig. 4b. The difference in free energy of sol-
vation in hexane and different hydrogen bonding solvents
(i.e. ΔG solv) of 1–3 follow the order of the hydrogen bond
energy [28]. In the aprotic solvents the values are small and
interaction of imidazole derivatives with those solvents is
purely due to dipolar interactions in the excited state. The
reorganization energy values of 1–3 have also been deter-
mined in different solvents. The definite values of reorga-
nization energy confirmed the interaction between low fre-
quency motions such as reorientation of solvent cell with
low and medium frequency nuclear motion of the solute.
of
solv
solv
An interesting result is provided by the effect on the shift of
the CT absorption bands with increasing solvent polarity
(Fig. 6) [30, 31]. With the assumption that point dipole is at
the center of the spherical cavity and the mean solute polariz-
ability (α) to be insignificant, it follows,
hceυabs≈hceυvac−2 μg μe−μg =α3 ðε−1=2ε þ 1Þ−1 n2−1=2n2 þ 1
À
Á
Â
ꢂ À
ÁÃ
abs
o
2
ð4Þ
Table 3 Slopes and intercepts of the solvatochromic plots of the CT
Fluorescence of the imidazoles (1–3)
where μg and μe are the dipole moments of the solute in
the ground and excited state, correspondingly, νabs and eυavbacs
are the spectral positions of a solvent-equilibrated absorp-
tion maxima and the value extrapolated to the gas-phase,
respectively, ao is the effective radius of the Onsager
cavity, [32] and ε and n are the static dielectric constant
and the refractive index of the solvent, respectively. In the
case of the well-separated CT absorption bands, Eq. 4 is
Compound Nonpolar solvents
and Polar solvents
Polar solvents
Nonpolar
solvents and
Polar solvents
μe(μe
μe(μe
μe(μe−μg)/ μ, D
hceυvabasc; eV
hceυvabasc; eV
−μg)/a3o
−μg)/a3o a3o eV
1
2
3
3.48
3.56
3.38
1.13
1.35
1.66
3.61
3.93
3.63
1.23
1.45
1.86
1.407
1.509
1.023
6.00
6.65
8.09
used to determine the values of μg(μe−μg)/α3o and eυ
vac
abs
and the values are displayed in Table 3. In the excited state,

J Fluoresc
Table 4 Frontier Orbital Energies along with dipole moment of 1–3 and MC1, MC2 and DC
Comd.
Dipole Moment μe/μg D
HOMO ev
LOMO ev
HOMO-1 ev
LOMO+1 ev
EHOMO-ELUMO ev
1
4.5/8.8
4.3/7.9
5.0/9.9
22.6
−8.54(4.31)
−8.56(4.52)
−7.35(3.96)
−9.22
−10.18
−5.85
−0.89(3.20)
−5.88(2.02)
2.39(1.01)
−1.33
−0.60
−5.23
−8.95
−7.64
−7.62
−10.34
−11.13
−8.65
0.00
3.11
1.07(1.11)
2.68(2.50)
4.96(2.95)
7.89
2
3
3.10
MC1
MC2
DC
−0.08
−0.27
−5.08
11.4
9.58
20.6
0.62
values in the parentheses are experimental
the negative and positive ends of the electric dipole are
localized nearly in the centers of the donor and acceptor
fragments respectively. A considerable shift of their spectral
position and the increase of the Stokes shift and enlarge-
ment of the emission bandwidth with increasing solvent
polarity in fluorescence spectra point to the CT character
of the fluorescent states and clearly indicate that the abso-
lute values of μe are much higher than those of μg. The
excited state dipole moments μe can be estimated by the
fluorescence solvatochromic shift method due to the fact
that the excited states live sufficiently long with respect to
the orientation relaxation time of the solvent [33–36].
Under the same assumptions as used for expression 4, it
follows that
hceυflu≈hceυvac−2μe μe−μg =αo3 ðε−1=2ε þ 1Þ−1 n2−1=2n2 þ 1
À
Á
Â
ꢂ À
ÁÃ
flu
2
ð5Þ
vac
flu
where eυflu and eυ
are the spectral positions of the solvent
equilibrated fluorescence maxima and the value extrapolated
to the gas-phase, respectively. The compounds studied show a
satisfying linear correlation between the energy hceυflu and the
solvent polarity function in a polar environment and also in all
the solvents (Fig. 7) [37]. The values of μe(μe−μg)/α3o are
Scheme 3 Possible mono and
dictations

J Fluoresc
Table 5 Electronic
Spectral data of 3 and
their mono and dications
along with cation
abundance
Species
λabs
λexci
λemi
3
350
306
432
317
351
306
432
317
452
424
570
436
MC1
MC2
DC
pka
3.25
4.50
0.20
Cation abundance
MC1≫MC2
MC2>MC1
DC>MC
Fig. 8 Absorption spectra of 3 for neutral-monocation equilibrium
displayed in Table 3. The values extracted from the data
measured in polar media are somewhat larger than those
resulting from the analysis of the data obtained for the
whole range of the solvents. This finding can be ex-
plained only by the dependence of the electronic struc-
ture of the fluorescent states on solvation. Due to a
relatively small energy gap between the lowest internal
charge transfer (ICT) states and the states excited local-
ly in the nonpolar solvents, which leads to increase of
the contribution of the (π, π *) character to the wave
function of the CT states. It leads to a lowering of
energy with respect to a pure CT state because of a
stabilizing character of such interactions and red shift
obtained in the fluorescence spectra.
Under the assumption that the CT fluorescence corre-
sponds to the state reached directly upon excitation, the quan-
tity (μe−μg)/α3o can be evaluated from the solvation effects on
the Stokes shift,
ꢀ
ꢁ
ꢀ
ꢁ
vac
flu
hc eυabs−eυflu ¼ hc hceυvabasc−hceυ
þ 2 μe−μg =α3o ðε−1=2 ε þ 1Þ−1 n2−1=2n2 þ 1
ð6Þ
À
Á
Â
ꢂ À
ÁÃ
2
2
The compounds studied show a satisfying linear correlation
between the energy hceυabs−hceυflu and the solvent polarity
function in a polar environment and also in all the solvents
studied; the values of (μe−μg)/α3o are 1.407 eV (1), 1.509 eV
(2), and 1.023 eV (3). The Eqs. 4–6 relate the measured
quantities to the excited state dipole moments μe. Under the
assumption that μe≫μg and with the effective spherical radius
of the molecules a0, 5.21 A° (1), 5.27 A° (2), and 5.29 A° (3),
as estimated from the molecular dimensions of the compounds
calculated by molecular mechanics, Eqs. 5 and 6 yield very
similar values of μe being in the range of 6.00 D (1), 6.65 D
(2) and 8.09 D (6) for the studied molecules.
The electrochemical properties of the imidazole (1–3) have
been examined by cyclic voltammetry and the redox poten-
tials have been measured from the plot potential versus cur-
rent. The energies of the highest occupied molecular orbital
(HOMO) and lowest unoccupied molecular orbital (LUMO)
have
been
calculated
using
the
relation,
oxi;
EHOMO ¼ 4:8 þ E
1
E
LUMO
¼ EHOMO–1239=λonset and
=
2
the calculated values are given in Table 4. The LUMO
Fig. 9 a Emission spectrum of 3
at excitation 336 nm b Emission
spectrum of 3 at excitation
432 nm

J Fluoresc
emission peak at 452 nm corresponding to the neutral molecule
is predominantly observed when excited at 397 nm. The inten-
sity of the neutral fluorescence band decreases with increase in
acid concentration. Excitation at 432 nm produces an additional
emission at 570 nm which is shown in Fig. 9b. Decreasing the
pH increases the intensity of the emission at 570 nm. All these
interesting results are displayed in Table 5 and confirmed two
types of monocations are formed in both ground and excited
states. Protonation of the dimethylamino nitrogen leads to a
blue shift and protonation of the imidazole nitrogen leads to a
red shift [38–40]. In DTINA, due to an increase in charge flow
from the dimethylamino nitrogen to the imidazole nitrogen,
protonation predominantly occurs at the imidazole nitrogen
and a small amount of protonation occurs at the
dimethylamino nitrogen [41]. Therefore the blue shifted
band at 306 nm in the absorption maximum and the
corresponding fluorescence maximum at 424 nm can be
assigned to the monocation formed by the protonation of
dimethylamino nitrogen, i.e. MC1 (Scheme 3) whereas the
red-shifted absorption and the fluorescence spectra are
assigned to the monocation formed by the protonation of
the imidazole nitrogen (MC2). Decreasing the pH of a
solution below 2 causes a decrease in the absorbance of
MC1 and the absorption spectrum of MC2 also undergoes a
bathochromic shift with an increase in absorbance as
displayed in Fig. 10 At pH 0.20, absorption peak is observed
at 317 nm and the emission band at 360 nm can be assigned to
DC. These changes are consistent with a shift of the equilib-
rium towards the dication from the monocation. As expected it
is red shifted with respect to MC1 and blue shifted compared
to MC2.
Fig. 10 Absorption spectra of 3 for monocation-dication equilibrium
energies have been deduced from the HOMO energies and the
lowest-energy absorption edges of the UV–vis absorption
spectra. The calculated energy gap (Eg=EHOMO−ELUMO) of
1–3 are 1.11, 2.50 and 2.95 eV. Therefore, the HOMO stabil-
ity and the emission energy gap are controlled by the nature
and substituent present in the imidazole moiety.
Prototropic Equilibria
Upon protonation of the basic nitrogens in DTINA, the formed
possible mono and dictations are presented in Scheme 3. De-
creasing the pH two new peaks at 306 and 432 nm appear in the
absorption spectra with two isobestic points at 336 nm and
397 nm. Figure 8, presents the absorption spectra. This indi-
cates the formation of two types of monocations. Excitation at
336 nm resulted two emission peaks at 424 and 452 nm with a
isoemissive point at 434 nm which confirmed the equilibrium
between the neutral form and the monocation (Fig. 9a). The
Fig. 11 MEP diagram

J Fluoresc
Fig. 12 HOMO-LUMO contour maps calculated by DFT/B3LYP/6-31G(d,p)
Protonation Vs Computational Studies
In compound 1, the HOMO is located on the imidazole ring,
both phenyl rings at C12, C13 carbon atoms of the imidazole
ring and partly on the naphthyl ring at C11 carbon whereas the
LUMO is located partly on the phenyl rings at C12, C13
carbon atoms and partly on the naphthyl ring at C11 carbon.
In compound 2, while the HOMO is on the imidazole ring and
partly on the phenyl ring at N1, the LUMO is located only on
the phenyl ring at C11 of the imidazole ring. For 3, the
HOMO is on the imidazole ring and on the N,N-
dimethylaminonaphthyl ring at C11carbon and the LUMO is
imidazole ring and on the phenyl rings at C12, C13 carbon
atoms of the imidazole ring. The electron density analysis on
the HOMO-LUMO orbital supports the flow of electron from
dimethylamino group to imidazole nitrogen. The HOMO→
LUMO transition implies that intramolecular charge transfer
takes place within the molecule. The energy gap (Eg) of
imidazole has been calculated from the HOMO and LUMO
energy levels. The energy gap explains the probable charge
transfer (CD) inside the chromophores.
In order to supplement the experimental predominant site of
protonation, the charge distribution was calculated by the nat-
ural bond orbital (NBO) and Mulliken methods [DFT/B3LYP/
6-31G (d,p)]. These two methods predict the same tendencies.
The calculated charges for imidazole nitrogen (—N==)
[−0.57(1); −0.61(2); −0.73(3)] and for dimethylamino nitro-
gen (—NMe2) [−0.55(3)]. The charge distribution shows that
the more negative charge is concentrated on imidazole nitrogen
atom whereas the partial positive charge resides at hydrogens.
Among the imidazole and dimethylamino nitrogen atoms, im-
idazole nitrogen atom is considered as more basic site
The higher electron density at the nitrogen atoms (−N=) in
imidazole was also supported by molecular electrostatic po-
tential (MEP) which is shown in Fig. 11. The MEP map
clearly suggests that nitrogen atoms represent the most nega-
tive potential region (dark red) The hydrogen atom attached to
the six membered ring bear the maximum positive charge
(blue region). The predominance of green region in the MEP
surface corresponds to a potential halfway between the two
extremes red and dark blue color. In compound 3 the imidaz-
ole nitrogen atom seems to exert comparatively more negative
potential as compared to dimethylamino nitrogen atom. The
MEP clearly confirms the existence of the electrophyllic ac-
tive center nitrogens characterized by red color.
In MC1, HOMO is located on the entire molecule whereas
LUMO is partly on the naphthyl ring at C11 carbon and on
imidazole ring. For MC2 and DC, HOMO is located partly on
the naphthyl ring at C11 carbon and LUMO is partly on
imidazole ring, phenyl ring at N1 and both phenyl rings at
C12, C13 carbon atoms of the imidazole ring.
Further the electron density distribution has been studied
using the frontier molecular orbitals by DFT analysis. The 3D
plots of the frontier orbitals HOMO and LUMO for the
imidazole 1–3 are shown in Fig. 12. The HOMO orbital acts
as an electron donor and the LUMO orbital acts as an acceptor.
Conclusion
In this work, we found a thermal exposure synthetic method for
polysubstituted imidazole that is simple, efficient and green via a

J Fluoresc
16. Jayabharathi J, Thanikachalam V, Sathishkumar R, Jayamoorthy K
(2013) Spectrochim Acta Part A: 101:249–253
multicomponent one-pot reaction in the presence of InF3 as an
inexpensive and eco-friendly catalyst in solvent-free conditions.
Photophysical studies reveal that the solvatochromic behavior of
imidazole derivatives depend not only the polarity of the medi-
um but also the hydrogen bonding properties of the solvents.
The quantum yield decreases in polar protic solvents due to an
increase in the nonradiative rate by hydrogen bonding interac-
tions. Kamlet–Taft analysis shows that in the excited state,
imidazole derivatives form a stable complex with solvents with
high hydrogen bond acceptance abilities and low hydrogen
bond donor character. The compounds studied show a satisfying
linear correlation between the energy hceυabs , hceυflu , hceυabs−hc
eυflu and the solvent polarity function in a polar environment.
Hydrogen bonding interaction in protic polar environment en-
hances the stabilization of S1 state which leads to reduce the
mixing between the states and as a result decrease of non-
radiative relaxation is observed. The theoretical results based
on density functional theory are in good agreement with exper-
imental data.
17. Frisch MJ, Trucks GW, Schlegel HB, Scuseria GE, Robb MA,
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