Solvent effect profiles of absorbance and fluorescence spectra of some indole based chalcones

Article abstract of DOI:10.1007/s10895-011-0926-z

The photophysical properties of a series of 3-(1' H-Indol-3′-yl)-1- phenylprop-2-en-1-one and its derivatives (indole chalcones) were studied in different solvents. Solvent effects on the absorption and fluorescence spectra were quantified using Reichardt

Full text of DOI:10.1007/s10895-011-0926-z

J Fluoresc (2011) 21:2213–2227
DOI 10.1007/s10895-011-0926-z
ORIGINAL PAPER
Solvent Effect Profiles of Absorbance and Fluorescence
Spectra of Some Indole Based Chalcones
Manju Kumari Saroj & Neera Sharma &
Ramesh C. Rastogi
Received: 17 May 2011 /Accepted: 28 July 2011 /Published online: 9 August 2011
#
Springer Science+Business Media, LLC 2011
Abstract The photophysical properties of a series of 3-(1′
H-Indol-3′-yl)-1-phenylprop-2-en-1-one and its derivatives
(indole chalcones) were studied in different solvents.
Solvent effects on the absorption and fluorescence spectra
were quantified using Reichardt’s and bulk solvent polarity
parameters and were complemented by the results of the
Kamlet-Taft treatment. The observed excited state dipole
moment was found to be larger than the ground state dipole
moment of these chalcones. The correlation of the
solvatochromic Stokes-shifts with the microscopic solvent
polarity parameter (ETN ) was found to be superior to that
obtained using bulk solvent polarity functions.
chalcone moiety markedly influences the biological activity
of the chalcone. Indole based chalcones show anti-cancer
[20], anti-inflammatory [21, 22], neuroprotective [23] and
anti-amoebic [24] properties. These compounds have also
been investigated as β-amyloid imaging probes [25].
Recent radiation and quantum chemical studies have shown
indole chalcones to be the ideal antioxidants [26]. These
types of chalcones are also used for the photogeneration of
reactive oxygen species and photo-induced plasmid DNA
cleavage [27].
The physicochemical and theoretical investigations of
indole based chalcones attract special attention towards
their excited and ground state properties because of the
presence of donor and acceptor moieties in chalcones. The
solvent can significantly influence the chemical and
physical properties of the solute. The solvents induce
changes in the electronic transitions of solutes (solvato-
chromism) based on the nature and the extent of the solute-
solvent interactions in ground and first excited states of
solutes (developed locally in the immediate vicinity) [28–
30]. The literature related to the systematic solvatochromic
study on this system is scarcely available [31]. This paper
reports the systematic solvatochromic study of the photo-
physical properties of some indole based chalcones for the
first time. Photophysical studies provide valuable informa-
tion about ground and excited states, which can be
exploited in the determination of excited state dipole
moments [32–36], and ground and excited state proton
transfer equilibrium reactions in aqueous [37, 38] and
micellar media [39–41].
.
.
.
Keywords Dipole moment Onsager radius Polarity
.
Excited state Solvatochromism
Introduction
Chalcones are a chemical class that shows paramount
importance in chemistry and biochemistry [1–5]. They
exhibit promising therapeutic efficacy for the treatment of
several diseases [6–9]. In addition, chalcones have been
extensively used for various optical applications including
photo-alignment layer of liquid crystal displays [10–13],
photo refractive polymers and fluorescent probes for
sensing the DNA [14–16] or metal ions [17–19].
Indole, the potent basic pharmacodynamic nucleus, has
been reported to possess a wide variety of biologically
active properties, and the substitution of indole ring into the
In this paper, solvent effects on the energy of excited and
ground states of 3-(1′H-Indol-3′-yl)-1-phenylprop-2-en-1-one
and its derivatives (Fig. 1) have been investigated by
absorption and fluorescence spectroscopy. The photophysical
properties of these derivatives in different solvents have been
:
:
M. K. Saroj N. Sharma R. C. Rastogi (*)
Department of Chemistry, University of Delhi,
Delhi 110007, India
e-mail: rc.rastogi@gmail.com

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J Fluoresc (2011) 21:2213–2227
Fig. 1 Structure of indole
chalcones
5'
O
4'
6'
2"
9'
1"
3'
2'
1
3
3"
2
7'
8'
1'
N
6"
4"
R
5"
H
R
Abbreviation
COMPOUNDS
3-(1′H-Indol-3′-yl)-1-phenylprop-2-en-1-one
3-(1′H-Indol-3′-yl)-1-p-tolyl-prop-2-en-1-one
1-(4′′-Hydroxy-phenyl)-3-(1H-indol-3-yl)-prop-2-en-1-one
1-(4′′-Chloro-phenyl)-3-(1H-indol-3-yl)-prop-2-en-1-one
3-(1H-Indol-3-yl)-1-(4′′-methoxyphenyl)-prop -2-en-1-one
3-(1H-Indol-3-yl)-1-(4′′-nitrophenyl)-prop-2-en-1-one
H
CH3
OH
Cl
OCH3
NO2
I
Me-I
OH-I
Cl-I
OMe-I
NO2-I
analyzed using microscopic and bulk solvent polarity
parameters and rationalized using the Kamlet-Taft treatment.
3-(1′H-Indol-3′-yl)-1-Phenylprop-2-en-1-One (I) A solution
of indole-3-carboxaldehyde (0.01 mol) and acetophenone
(0.01 mol) in 8–10 ml of ethylene glycol was treated with
1 ml of piperidine and refluxed for 20–30 min at 170–180 °C
and the progress of reaction was monitored by TLC in ethyl
acetate (30%)–petroleum ether (70%) solvent. After cooling
the reaction mixture, 5–10 ml of water and 1 ml of glacial
acetic acid were added; the crystals that deposited were
recrystallized from ethanol. Other derivatives were also
synthesized using the same procedure.
Experimental
Materials
Materials and reagents for the synthesis of 3-(1′H-indol-3′-yl)-
1-phenylprop-2-en-1-one derivatives were purchased from
Spectrochem Pvt. Ltd. and used without further purification.
All solvents used were of spectroscopic grade (Spectrochem/
Merck) and were checked for the absence of absorbing or
fluorescent impurities within the scanned spectral ranges.
The water used was triple distilled.
Spectral Data
3-(1′H-Indol-3′-yl)-1-Phenylprop-2-en-1-One (I)
IR (KBr, cm−1): 3428, 3145, 3051, 1640, 1591, 1438, 1367;
1H NMR (400 MHz, DMSO-d6, ppm): δ7.19–7.22 (d, J =
12 Hz, 1H), 7.44–7.46 (d, J = 8 Hz, 1H), δ7.50–7.55 (m,
2H), δ7.57–7.61 (m, 2H), 7.98 (s, 1H), δ7.99–8.04 (m, 5H),
δ11.89 (s, 1H); melting point: 172–174 °C.
Instrumentation
Absorption and fluorescence spectra were recorded by
using ANALYTIKA JENA UV WINASPECT SPEC-
TROD PC 250 spectrophotometer and FLUOROMAX
SPEX-3 JOBIN YVON HORIBA spectrofluorometer,
respectively. All measurements were done in a 1 cm
quartz cuvette. Absorption and fluorescence spectra
were recorded in freshly prepared 2×10−5 M and 1×
10−5 M solutions, respectively. The slit widths used were
1 nm and 4 nm for absorption and emission measure-
ments, respectively.
3-(1′H-Indol-3′-yl)-1-p-Tolylprop-2-en-1-One (Me-I)
1
IR (KBr, cm−1): 3424, 3186, 1638, 1580, 1437, 1355; H
NMR (400 MHz, DMSO-d6, ppm): δ2.36 (s, 3H), δ7.17–
7.21 (m, 2H), δ7.31–7.33 (d, J = 8 Hz, 1H), δ7.44–7.47 (m,
2H), δ7.58–7.62 (d, J = 16 Hz, 1H), δ7.98 (s, 1H), δ8.00–
8.06 (m, 4H), δ11.87 (s, 1H); melting point: 168–170 °C.
Synthesis
1-(4″-Hydroxyphenyl)-3-(1H-Indol-3-yl)-Prop-2-en-1-One
(OH-I)
The investigated 3-(1′H-indol-3′-yl)-1-phenylprop-2-en-1-
one derivatives were synthesized according to the proce-
dures reported in the literature [42] and their purity was
IR (KBr, cm−1): 3285, 3112, 2927, 1641, 1557, 1440,
1344; 1H NMR (400 MHz, DMSO-d6, ppm): δ4.57 (s,
1
confirmed by TLC, IR and H NMR.

J Fluoresc (2011) 21:2213–2227
2215
1H), δ7.70–7.72 (d, J = 8 Hz, 1H), δ8.01–8.05 (m, 2H),
δ8.29–8.31 (m, 2H), δ8.44–8.48 (d, J = 16 Hz, 1H), δ8.78
(s, 1H), δ8.82–8.92 (m, 4H), δ12.68 (s, 1H); melting
point: 180–182 °C.
where na and nf are the absorption and fluorescence
maxima, n and D are the refractive index and the dielectric
constant of the solvents, respectively. The slopes S1 and S2
are expressed as
ꢀ
ꢁ
ꢀ
ꢁ
2
»
2 m» ꢀ m
ꢀ2 m ꢀ m2
2
1-(4″-Chlorophenyl)-3-(1H-Indol-3-yl)-Prop-2-en-1-One
(Cl-I)
S1 ¼
and
S2 ¼
ð3Þ
hcR3
hcR3
where, μ and μ* are excited and ground state dipole
moments of the solute molecule, respectively and ‘R’ is the
Onsager cavity radius. F1(D,n) and F2(D,n) are taken as
1
IR (KBr, cm−1): 3427, 3186, 1629, 1572, 1435, 1356; H
NMR (400 MHz, DMSO-d6, ppm): δ7.21–7.24 (m, 2H),
δ7.47–7.49 (m, 2H), δ7.55–7.57 (d, J = 8 Hz, 1H), δ7.62–
7.65 (d, J = 12 Hz, 1H), δ8.02 (s, 1H), δ8.05–8.11 (m, 4H),
δ11.91 (s, 1H); melting point: 178–180 °C.
ꢂ
ꢃ
D ꢀ 1 n2 ꢀ 1
ꢀ
2n2 þ 1
n2 þ 2
F1ðD; nÞ ¼
ꢁ
ð4Þ
D þ 2 n2 þ 2
3-(1H-Indol-3-yl)-1-(4″-Methoxyphenyl)-Prop -2-en-1-One
(OMe-I)
1
3=2ðn4 ꢀ 1Þ
F2ðD; nÞ ¼ F1ðD; nÞ þ
ð5Þ
2
ðn2 þ 2Þ
2
IR (KBr, cm−1): 3389, 3107, 1653, 1601, 1585, 1438, 1360;
1H NMR (400 MHz, DMSO-d6, ppm): δ3.80 (s, 3H),
δ7.03–7.05 (d, J = 8 Hz, 1H), δ7.18–7.20 (m, 2H),
δ7.44–7.45 (m, 2H), δ7.59–7.63 (d, J = 16 Hz, 1H),
δ7.96 (s, 1H), δ8.04–8.10 (m, 4H), δ11.83 (s, 1H);
melting point: 156–158 °C.
Plots of the Stokes-shifts (na ꢀ nf ) and 1/2(na þ nf )
against the bulk solvent polarity functions, (F1(D,n) and
F2(D,n)) for different solvents yield the slopes S1 and S2,
respectively. Assuming that the angle between μ and μ* is
small and the cavity radius, ‘R’ is same in both the ground
and the excited states and using the Eqs. 1, 2 and 3, the
ratio of the dipole moments in the excited and ground
states, which is given by the equation:
3-(1H-Indol-3-yl)-1-(4″-Nitrophenyl)-Prop-2-en-1-One
(NO2-I)
m» jS1 ꢀ S2j
IR (KBr, cm−1): 3368, 3103, 1654, 1559, 1420, 1420, 1341;
1H NMR (400 MHz, DMSO-d6, ppm): δ7.22–7.28 (m, 2H),
δ7.50–7.52 (m, 2H), δ7.61–7.64 (d, J = 12 Hz, 1H), δ8.09–
8.12 (d, J = 12 Hz, 1H), δ8.17 (s, 1H), δ8.31–8.37 (m, 4H),
δ12.03 (s, 1H); melting point: 226–228 °C.
¼
ð6Þ
m
jS1 þ S2j
It may be noted that most theories [52–57] of the solvent
effect on the location of the absorption (na) and fluorescence
(nf ) bands lead, in spite of different assumptions, to similar
expressions for na ꢀ nf and 1/2(na þ nf ) (Eqs. 1 and 2) with
the difference, however, that the applied solvent polarity
parameters, (F1(D,n) and F2(D,n)) differ significantly.
In the second method [58], the problem associated with
the error in estimating the Onsager radius ‘R’ was
minimized, since the ratio of two Onsager radii (RB/R) is
involved Eq. 7. The excited-state dipole moment is
determined by
Methods
Dipole Moment Calculation
The dipole moment of a molecule in the excited state is
determined by the effect of electric field (internal or
external) on the position change of the absorption and
fluorescence bands. Two methods depending on the internal
electric field (solvatochromism) were employed in the
present study.
In the first method, by employing the simplest quantum-
mechanical second order perturbation theory and taking
into account Onsager’s model, Kawski et al. [43–51] have
obtained the following expressions for (na ꢀ nf ) and 1/2
(na þ nf ):
h
i
2
3
Δnaf ¼ 11307:6 ðΔm=ΔmBÞ ðRB=RÞ ETN þ constant
ð7Þ
where Δnaf is the Stokes-shifts and ETN is the solvent-
polarity parameter proposed by Reichardt [59], that
correlates better than the traditionally used bulk solvent
polarity functions. ETN is based on the absorption wave-
number of a standard betaine dye in the solvent. Its value
for different solvents is given in Table 1 and is expressed as
na ꢀ nf ¼ S1F1ðD; nÞ þ C1
ð1Þ
ET ðsolventÞ ꢀ ET ðTMSÞ
ETN
¼
ð8Þ
ꢀ
ꢁ
1=2 na þ nf ¼ S2F2ðD; nÞ þ C2
ð2Þ
ET ðwaterÞ ꢀ ET ðTMSÞ

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J Fluoresc (2011) 21:2213–2227
Table 1 Various solvent param-
eters used in the
ETN
Solvent
ET(30) kcal/mol
F1(D,n)
F2(D,n)
α
β
π*
solvatochromic studies
n-Hexane
Cyclohexane
1,4-Dioxane
THF
31.0
30.9
36.0
37.4
38.1
40.7
43.2
43.3
45.6
48.4
49.7
51.9
55.4
56.3
63.1
0.009
0.006
0.164
0.207
0.228
0.309
0.386
0.389
0.460
0.546
0.586
0.654
0.762
0.790
1.000
−0.003
−0.003
0.041
0.549
0.489
0.590
0.836
0.688
0.859
0.779
0.750
0.813
0.855
0.836
0.913
0.254
0.288
0.307
0.551
0.498
0.583
0.710
0.608
0.664
0.646
0.647
0.653
0.651
0.710
0.683
0.00
0.00
0.00
0.00
0.00
0.13
0.00
0.42
0.19
0.76
0.84
0.83
0.98
0.90
1.17
0.00
0.00
0.37
0.55
0.45
0.10
0.69
0.93
0.40
0.84
0.84
0.77
0.66
0.52
0.47
−0.011
0.000
0.490
0.550
0.450
0.730
0.880
0.410
0.660
0.480
0.470
0.540
0.600
0.920
1.090
Ethyl acetate
DCM
DMF
tert-Butanol
Acetonitrile
2-Propanol
1-Butanol
Ethanol
Methanol
Ethylene glycol
Water
ΔμB (=9D) and RB (= 6.2Å) are the dipole-moment
changes upon excitation and the Onsager cavity radius,
respectively, of betaine dye [59, 60], whereas Δμ and ‘R’
are the corresponding quantities for the molecule of interest
(indole chalcones). Thus, the plot of Δnaf (Stokes-shifts)
versus ETN (Eq. 7) can be used to obtain the change in the
dipole moment (Δμ) upon excitation.
Parametric Method 3 (PM3) semiempirical molecular-
orbital calculations were carried out to obtain the optimized
geometries of all the chalcone molecules studied here. The
Onsager cavity radii of all molecules have been taken to be
40% of the longest axis in the optimized geometry [61].
The quantum chemical package HyperChem Release 5.1
Pro [62] was used for the semiempirical molecular-orbital
calculations.
interpreted as solute properties. The coefficient, s is related
to the solute polarity/polarizability character, a describes its
tendency to accept hydrogen bond from the solvent and b
measures its property to donate hydrogen bond to the
solvent. The coefficients s, a and b indicate the suscepti-
bility of ν to a change in the corresponding parameter.
The term δ used in Eq. 9 is a ‘polarizability correction
term’, whose value depends on the class of solvents used;
for aromatic solvents δ=1, for polyhalogenated solvents δ=
0.5 and for all other solvents δ=0 [68]. The term d is
interpreted as an indicator of the change in direction of the
molecular dipole moment in going from initial to transition
state, is zero for all spectra that are shifted bathochromi-
cally with increasing solvent dipolarity [69]. So, the Kamlet
and Taft Eq. 9 reduces to Eq. 10.
n ¼ no þ sp» þ aa þ bb
ð10Þ
Kamlet-Taft Treatment: Solvatochromic Comparison
The corresponding parameters for different solvents were
taken from literature [59, 70] and are given in Table 1. All
least-squares fit analyses were carried out with Microsoft
EXCEL.
To find the information about the individual contribution of
different solvent effects, a multiparametric approach,
known as solvatochromic comparison method or linear
solvation energy relationship (LSER) proposed by Kamlet,
Taft and others [63, 64], has been used. In the general
expression (9),
Results and Discussion
ꢄ
ꢅ
n ¼ no þ s p» þ dd p» þ aa þ bb
ð9Þ
Absorption and Emission Properties
where νo is the vapor phase wave-number (independent of
solvent effects) and the values of ν are absorption/
fluorescence band maxima in a solvent. The empirical
parameters π*, α and β are a measure of the polarity/
polarizability character of the solvent, its hydrogen-bond
donor (HBD) and hydrogen-bond acceptor (HBA) capaci-
ties, respectively [65–67]. The coefficients s, a and b are
Absorption and fluorescence spectra of 3-(1′H-indol-3′-yl)-
1-phenylprop-2-en-1-one (I) and its derivatives were
recorded in solvents of different polarity. Absorption and
fluorescence spectra of I in different solvents are shown in
Fig. 2 and the corresponding spectral data of all I
derivatives have been summarized in Tables 2 and 3.

J Fluoresc (2011) 21:2213–2227
2217
Fig. 2 a Electronic absorption
spectra of I in different solvents.
b Fluorescence emission spectra
of I in different solvents.
* Fluorescence
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
n-Hexane
(a)
O
Cyclohexane
1,4-Dioxane
THF
A1
N
H
intensity = 650,000×2
A4
Ethyl acetate
DCM
DMF
A2
t-Butanol
Acetonitrile
2-Propanol
1-Butanol
Ethanol
A3
Methanol
Ethylene glycol
Water
210
260
310
360
410
460
510
Wavelength/nm
800000
700000
600000
500000
400000
300000
200000
100000
0
1,4-Dioxane
THF
(b)
O
Ethyl acetate
DCM
N
H
DMF
tert-Butanol
Acetonitrile
2-Propanol
1-Butanol
Ethanol
Methanol
Ethylene glycol*
Water
395
445
495
545
595
645
695
Wavelength/nm
Absorption Properties
polarity of the solvent as well as the various substituents on
the phenyl ring. Absorption band A4 (~340–400 nm) shows
a large bathochromic shift (30–55 nm, Table 2) with
increase in polarity. Bands A2 and A3 (~250–285 nm)
show the less sensitivity towards both solvent polarity and
the substituents, as compared to band A4 (Table 2).
The wavelengths, λA1, λA2 and λA3 of the derivatives are
the characteristic absorption bands of the benzoyl group
and the indole moiety due to π−π* transitions [71, 72]. The
wavelength, λA4 that shows the large bathochromic shift
indicates the charge-transfer (CT) interaction among the
indole, phenyl and α, β-unsaturated ketone parts of the
The absorption spectra of all 3-(1′H-indol-3′-yl)-1-phenyl-
prop-2-en-1-one derivatives consist of four absorption
bands (Fig. 2a) (Table 2). It can be clearly seen from the
Figs. 2a, 3a and b that the third absorption band (A3) is
merging with the second absorption band (A2) and appears
as shoulders. However, the first band (A1) and fourth band
(A4) appear clearly as distinct absorption bands in all the
solvents.
Band A1 is located in the range 220–230 nm in all the
derivatives and does not show any change towards the

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J Fluoresc (2011) 21:2213–2227
2219
Table 3 Fluorescence maxima of indole chalcone derivatives in
solvents of different polarities
[compare λA4 (ethanol) = 388 nm vs. λA4 (acetonitrile) =
370 nm, Table 2].
Solventa
Fluorescence maximum (λFlu/nm)
OMe-I NO2-Id
Emission Properties
I
Me-I
OH-I Cl-I
The longest wavelength absorption maxima band (A4) was
used as excitation wavelength to record the emission
spectra. For all derivatives of the indole chalcones, a
completely non-structured intense band was observed in all
solvents in the range of 430–580 nm (Fig. 2b). Fluores-
cence intensity was enhanced in polar solvents as compared
to the nonpolar solvents. Fluorescence was almost absent in
highly nonpolar solvents (n-hexane and cyclohexane).
Fluorescence intensity in ethylene glycol was two-three
times greater than in other solvents. Thus, the fluorescence
intensity that was dissipated in other solvents remained
unaffected in ethylene glycol due to the viscous medium.
On increasing solvent polarity, the emission became
structureless and an appreciable bathochromic shift was
detected with a band enlargement (Table 3). Such a
behavior indicated the stabilization of the highly dipolar
excited state in polar solvents.
The emission maximum of I derivative underwent a red
shift of 74 nm on changing the solvent from 1,4-dioxane to
water (Fig. 4). A considerable red shift of the spectral
position for these derivatives, and the increase of the
Stokes-shifts and emission bandwidth with increasing
solvent polarity, point towards the distinct charge-transfer
character of the fluorescent states. These effects clearly
indicate the values of the excited-state dipole moments to
be higher than those of the ground state (Section Excited
State Dipole Moments). This behaviour points to a strong
intramolecular charge transfer (ICT) character of the
excited state.
n-Hexane
Cyclohexane
1,4-Dioxane
THF
NFb NF
NSc
NS
NF
NF
NS
NS
553
578
580
NF
NF
540
NF
538
537
498
NF
NF
520
NF
NF
NF
NF
441
445
444
444
468
464
478
475
478
490
499
441
445
441
449
459
458
470
469
472
482
497
502
513
432
433
437
444
449
458
464
459
465
470
487
492
511
445
455
450
460
480
490
490
494
494
501
503
515
512
430
441
437
443
452
466
464
464
470
480
492
495
515
Ethyl acetate
DCM
DMF
tert-Butanol
Acetonitrile
2-Propanol
1-Butanol
Ethanol
Methanol
Ethylene glycol 505
Water 515
a Solvents are listed in order of increasing dielectric constant values
b No fluorescence in this solvent
c Not soluble in this solvent
d Not used in dipole moment calculation and Kamlet-Taft treatment
due to non-fluorescent nature
molecule. No clear transitions of n−π* type have been
observed, since they are possibly masked by the more
intense π−π* type transition (∈ ≈ 104 M−1cm−1) [73].
Assignment of A4 as CT transition is based on the presence
of red-shifted band, on increasing the solvent polarity
(Table 2).
Absorption spectral data suggest the ground-state struc-
ture in all the molecules to be strongly sensitive to solvent
environment. As the polarity of the solvent is increased
from n-hexane to water, the absorption profile becomes
broad and structureless, suggesting larger solute-solvent
interactions in highly polar solvents. Attention is focused
on the highly red-shifted intense long-wavelength band A4
observed for the same molecule in different solvents
ðlnꢀhexane ¼ 346nm ꢀ lwater ¼ 391nmÞ, which has been
assigned to charge transfer from the donor groups (>N-H,
R = OH, OMe, etc., Fig. 1) to the carbonyl group.
In compound OH-I, for instance, absorption maxima of
band A4, in methanol, ethanol, propanol and butanol is at
385, 388, 390 and 392 nm, respectively. This suggests the
occurrence of a substantial increase in delocalization of
charge with ionization of the hydroxyl group caused by the
increase in the solvent’s basicity [74]. Thus, in hydroxylic
solvents the contribution of the charge from the ionized
hydroxyl (conjugate base) causes extra charge transfer
Looking at the molecular structure of these derivatives,
one can see that it has electron donor and acceptor pairs
offering two resonating structures (structures ‘A’ and ‘B’,
Fig. 5), as a result of the ICT. The resonance structure ‘A’ is
characterized by the participation of the electron lone pair
of N atom of the chromophore towards carbonyl group and
the structure ‘B’ shows the donation of electron density
from phenyl ring to the carbonyl group.
The effect of electron withdrawing substituents on I
derivatives can be clearly seen in the cases of Cl-I and
NO2-I, as they show bathochromic shifts as compared to
other derivatives in absorption and fluorescence spectra.
The lack of fluorescence in nitro derivative can be
explained on the basis of the strong electron-withdrawing
capacity of the nitro group (Table 3). Aromatic nitro
compounds are in general only weakly fluorescent or
completely nonfluorescent, due to the efficient nonradiative
decay processes such as singlet-triplet intersystem crossing
and internal conversion [75].

2220
J Fluoresc (2011) 21:2213–2227
Fig. 3 a Electronic absorption
spectra of NO2-I in different
solvents. b Electronic absorption
spectra of OH-I in different
solvents
(a)
O
A1
N
H
NO2
A2
A4
A3
(b)
O
N
OH
H
A1
A4
A2
A3
Excited State Dipole Moments
between the ground and excited states. For estimation
of the excited state dipole moments, Stokes-shifts have
been calculated for all the molecules from their absorp-
tion (band λA4 in Table 2) and fluorescence maxima
(Table 3) in different solvents. The data in hydroxylic
solvents were excluded to avoid specific solute-solvent
interactions [76, 77].
Solvatochromic shifts caused by general (non-specific)
solvent effects are often described by solvatochromic
method, which relates the energy difference between
absorption and emission maxima to the dielectric constant
and refractive index.
The solvatochromic method has been used for the
determination of the difference in the dipole moment
Figure 6 shows the plots of Stokes-shifts for Me-I versus
solvent polarity functions, F1(D,n), F2(D,n) and ETN . The

J Fluoresc (2011) 21:2213–2227
2221
Fig. 4 Normalized absorbance
and fluorescence spectra of I
1.2
1
n-Hexane
Cyclohexane
1,4-Dioxane
Tetrahydrofuran
Ethyl acetate
Dichloromethane
DMF
0.8
0.6
0.4
0.2
0
tert-Butanol
Acetonitrile
2-Propanol
1-Butanol
Ethanol
Methanol
Ethylene glycol
Water
295
345
395
445
495
545
595
645
695
Wavelength/nm
values of correlation coefficients obtained from the linear
least squares fit analysis of the Stokes-shifts versus bulk
solvent polarity functions, F1(D,n) and F2(D,n) as well as
microscopic solvent polarity parameter ETN, are summarized
in Table 4.
Onsager cavity radii ‘R’ have been calculated theoreti-
cally in gaseous-phase for all the systems and are given in
Table 5. The slopes S1 and S2 obtained from the plots of
Eqs. 1 and 2 and the ratios of dipole moments (μ*/μ)
calculated using Eq. 6 are summarized in Table 5. The
changes in dipole moment (Δμ) values, derived from the
slopes (m) obtained from the plots of Stokes-shifts versus
ETN using Eq. 7, are given in Table 5.
The positive value of Δμ of all systems studied here
suggests considerable amount of charge transfer from the
donor to the carbonyl acceptor moieties, which leads to an
increase in the polarity of the excited state and consequent-
ly, the intensification of the donor-acceptor type of
intramolecular interaction.
The increase in the dipole moment values for indole
chalcone derivatives upon excitation can be explained in
terms of the nature of emitting state (ICT). ICT can be
explained on the basis of resonance structures as shown
in Fig. 5. The contributing resonance structures arise out
of the delocalization of the π-electrons from either the
indole or phenyl ring to the carbonyl group. The negative
charge on the carbonyl favors the proposed delocalization
(Fig. 5). This charge transfer accompanying excitation to
lowest excited singlet-state usually results in the excited
molecule having larger dipole moment than the ground-
state molecule.
Resonance structures, A and B (Fig. 5), both are
probable according to the structure of indole chalcones.
Resonance structure ‘B’ dominates in the excited state with
substituents having electron donating capacity (less partic-
ipation of the electron lone pair of the N-atom), explaining
large change in dipole moment upon excitation. Resonance
structure ‘A’ dominates in the excited state with substituents
Fig. 5 Possible resonance struc-
tures of indole chalcone
derivatives
.
.
.
.
O
O
..
N
N
R
R
A
H
H
.
.
.
.
O
O
..
..
..
N
N
R
R
B
H
H

2222
J Fluoresc (2011) 21:2213–2227
Fig. 6 Stokes-shifts for Me-I
derivatives vs. the solvent po-
larity functions, F1(D,n),
F2(D,n) and ETN
7000
6000
5000
4000
3000
2000
1000
0
60000
55000
50000
45000
40000
35000
30000
y = 1036.6x + 4361.6
R = 0.708
y = -5447.5x + 52187
R = 0.841
0.0 0.2 0.4 0.6 0.8 1.0 1.2
0.0
0.2
0.4
0.6
0.8
F1 (D,n)
F2 (D,n)
8000
y = 3602.4x + 3900.4
R = 0.972
7000
6000
5000
4000
3000
2000
1000
0
0
0.2
0.4
0.6
ETN
having electron withdrawing capacity, explaining large
change in dipole moment upon excitation.
There is improvement in correlation coefficients obtained
for the Stokes-shifts data using ETN parameter (see Table 4)
except for I and I-OH. Hence, the solvatochromic method
based on microscopic solvent polarity parameter ETN seems
to be more reliable in estimating excited state dipole
moments than the one involving solvent polarity functions
F1(D,n) and F2(D,n).
The trend observed in the values of Δμ for different I
derivatives is in accordance with the ionization potential
energies (IP) of the substituted benzene ring [78]. IP values
of only those I derivative can relate with the Δμ values,
which follow the resonance structure ‘B’ in the excited
state. In correlation with the IP values, OMe-I having
lowest IP value shows the highest Δμ value as compared to
the other I derivatives. Following the trend of IP values, the
Cl-I derivative should have lowest Δμ values but it is high
and almost similar to OMe-I. This unusual change in the
dipole moment signifies that the resonance structure formed
by the Cl-I derivative is different from the other I
derivatives. Thus the resonance structure ‘A’ dominates in
the excited state of Cl-I derivative.
Kamlet and Taft Treatment
The distinct solvatochromic behavior of all I derivatives in
protic and non-protic media must reflect different solvent
effects in these environments. The absorption and emission
wavelength maxima obtained in various solvents have been
analyzed using the Kamlet and Taft treatment [63]. This
approach separates the dielectric effects of solvents (π*),
hydrogen-bond donor ability (α), and hydrogen-bond accep-
tor ability (β) of the solvents on the spectral properties.
Figure 7 shows the possible specific solute-solvent
interactions of indole chalcone derivatives. To get informa-
tion about the various solute-solvent interactions that
control the solvent-induced spectral shifts, the effect of
solvents on the absorption and emission energies is
quantified by the multiple regressions and the results are
presented in Tables 6 and 7.
Table 4 Correlation coefficients for the fits of Stokes-shifts of indole
chalcone derivatives against the solvent polarity functions, ETN , F1(D,n)
and F2(D,n)
Molecule
Correlation coefficients
ETN
F1 (D,n)
F2 (D,n)
I
0.752
0.972
0.629
0.903
0.988
0.620
0.708
0.666
0.648
0.910
−0.813
−0.841
−0.855
−0.895
−0.902
Me-I
OH-I
Cl-I
OMe-I
The formation of various specific hydrogen bonds
between the solvent and the solvatochromic probe may
possibly explain the UV–vis shifts. The contribution of
each of the specific interactions (solvent-carbonyl oxygen

J Fluoresc (2011) 21:2213–2227
2223
Table 5 The change (Δμ) and the ratio (μ*/μ) of dipole moments of indole chalcones
Cavity radius,c R/Å
Slope, md
IPe/eV
(μ*/μ)f
Δμg/D
a
b
Molecule
S1
-S2
Longest axis/Å
I
1304
1037
1062
1452
1270
–
6425
5447
5504
7108
6144
–
9.74
10.85
10.62
11.60
11.80
–
3.90
4.34
4.25
4.64
4.72
–
3714
3602
2522
5263
3496
–
9.24
8.83
8.49
9.07
8.22
–
1.51
1.47
1.48
1.51
1.52
–
2.57
2.97
2.41
3.98
3.32
–
Me-I
OH-I
Cl-I
OMe-I
NO2-Ih
a Calculated from the plots of Stokes-shifts versus F1 (D,n) using Eq. 1
b Calculated from the plots of Stokes-shifts versus F2 (D,n) using Eq. 2
c Onsager cavity radius (40% of the longest axis in the optimized geometry of the molecule) obtained by using semiempirical calculations [61]
d Slope (m) = 11307.6[(Δμ/ΔμB)2 (RB/R)3 ], calculated from the plot of Stokes-shifts vs. ETN using Eq. 7
e Ionization Energies (IP) of gas-phase molecules of substituted benzene ring
f Calculated using Eq. 6 from experimental values of S1 and S2
g Calculated using Eq. 7 from experimental value m
h Not used in dipole moment calculation and Kamlet-Taft treatment due to non-fluorescent nature
atom, solvent/indole ring nitrogen atom and solvent-indole
ring hydrogen atom) can have a different effect by changing
the nature and geometry of the solvent [79]. This behavior
explains the average correlation coefficients of the multiple
square analyses, because each solvent most likely interacts
in a different way with the probe (>NH and > C = O).
The possible hydrogen bonds involved in the ground and
the excited states of I derivatives are shown in Fig. 8. These
derivatives can accept hydrogen bonds at the nitrogen lone
pair (type X) and at the carbonyl group (type Z) from
hydrogen bond donating solvents. These derivatives can also
show hydrogen bond formation with hydrogen bond accept-
ing solvents from the H atom on the NH group (type Y) [80].
The negative values of a, b and s indicate that all the three
parameters π*, α and β, contribute to the stabilization of the
ground state as well as the excited state (Tables 6 and 7) [81].
of N atom in indole ring participates in the formation of
hydrogen bond with solvents. The lower negative value of
a for all the derivatives designates the lowest HBA ability
(Table 6) [76, 82].
For all the derivatives, there is a stabilizing term b for
the ground state caused by donating hydrogen bonds from
the NH group (Table 6) [81]. This is due to the two factors,
first is the weak strength of the hydrogen bond of type X
and type Z and second is the strong hydrogen bond strength
of type Y (Fig. 8) [80].
Emission Process
The more negative values of a in emission process as
compared to the excitation process reveal that the HBA
ability is more in excited state [76, 81]. This is because of
the strengthening of the hydrogen bond on the carbonyl
group (type Z). Hydrogen bond of type Z is thus stronger
than the hydrogen bonds of type X and type Y (Fig. 8 and
Table 7) [80].
Excitation Process
The more negative values of b point toward the higher
HBD ability of the molecules, as the lone pair of electrons
Table 6 Multiparametric correlation of Kamlet-Taft parameters using
the LSER equation, n ¼ no þ sp» þ aa þ bb towards the spectral
properties of indole chalcone derivatives in excitation processes
S
H
α
O
Molecule
vo
a
b
s
ra
I
28803
28948
28836
28280
29052
−654
−824
−1232
−522
−1058
−2203
−2090
−2015
−2334
−2386
−1573
−1651
−1090
−1728
−1411
0.979
0.977
0.940
0.979
0.974
Me-I
OH-I
Cl-I
OMe-I
S
α
N
R
H
β
H
S
Fig. 7 Possible specific solute-solvent interactions of indole chalcone
derivatives (S and HS = Solvent)
a Correlation coefficients of indole chalcones in their respective LSER
equations

2224
J Fluoresc (2011) 21:2213–2227
Table 7 Multiparametric correlation of Kamlet-Taft parameters using
the LSER equation, n ¼ no þ sp» þ aa þ bb towards the spectral
properties of indole chalcone derivatives in emission processes
Conclusions
The photophysical properties of these indole chalcone
derivatives are reported and discussed by examining
solvent effects on the absorption and fluorescence
spectra. These derivatives show positive solvatochrom-
ism with the increase in the polarity of the solvent. This
study has brought into focus the single emission that
originates from ICT excited states. The changes displayed
by the photophysical properties of this molecule in
different solvents can be explained in terms of electron
distribution and hydrogen bonding participation in the
stabilization of these molecules in ground and excited
states.
In the LSER equation, parameters α, β and π* play
important roles in the mechanism of the interactions
between these probes and solvents. These derivatives show
hydrogen bond accepting ability in ground state and
hydrogen bond donating ability in excited state. The lowest
singlet excited state of these molecules has larger dipole
moment than that of the ground state. Based on multi-
parametric regression analysis, it appears that the excited
state stabilization is mainly determined by the dipolar
interactions and hydrogen bond accepting ability of
solvents. Dipolar interaction contributes usually to high
value of μ*/μ and Δ μ more in the excited state than in the
ground state.
Molecule
vo
a
b
s
ra
I
24007
23596
23927
23535
24124
−1697
−1848
−1938
−1437
−1958
−1025
−247
−140
−1636
−706
−2187
−1887
−1890
−1845
−1929
0.902
0.937
0.924
0.864
0.930
Me-I
OH-I
Cl-I
OMe-I
a Correlation coefficients of indole chalcones in their respective LSER
equations
The more negative values of b in case of Cl-I show that
the HBD ability is more in excited state, thus favoring
structure ‘A’ as compared to the other derivatives with the
less negative value of b, thus favoring structure ‘B’ in
excitation process (Fig. 5 and Table 7). On comparing the
values of b for all the derivatives in excitation process, the
OH-I shows the lowest negative value of b because of
electron donating tendency of both NH and OH groups
(Table 7) [80, 81].
In emission process, the large negative values of s of these
compounds indicate that excited state has large ICT character.
The absolute value of s shows that excited state is more polar
and sensitive to the environment than the ground state as
evident from μ*/μ and Δμ values (Tables 5 and 7).
Fig. 8 Simplified view of the
possible hydrogen bonds in-
volved in the ground and the
excited states of I derivatives.
Hydrogen bonds may be formed
on the nitrogen lone pair (type
X) and the carbonyl group (type
Z) from hydrogen bond donat-
ing solvents, and hydrogen
S
S
H
Z
Z
H
O
O
hv
..
X
N
..
N
..
R
atoms on the NH group (type Y)
from hydrogen bond accepting
solvents (S and HS = Solvent)
R
H
H
Y
H
Y
S
S
S
Excited state 1
Ground state
S
Z
H
S
H
Z
O
O
hv
..
X
N
H
..
N
R
X
H
..
Y
R
H
S
H
Y
S
S
S
Excited state 2
Ground state

J Fluoresc (2011) 21:2213–2227
2225
Acknowledgements The financial support from University of Delhi
under the Scheme “To strengthen R & D Doctoral Research Program”
is gratefully acknowledged. Manju K. Saroj is thankful to the
University Grants Commission (UGC), New Delhi for the financial
assistance. The authors thank the Institute of Genomics and Integrative
Biology (IGIB, CSIR), Delhi for providing access to its Central
Instrumentation Facility.
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