Synthesis, spectral behaviour and photophysics of donor-acceptor kind of chalcones: Excited state intramolecular charge transfer and fluorescence quenching studies

Article abstract of DOI:10.1016/j.saa.2014.10.105

The spectral and photophysical properties of two chalcones containing electron donating and accepting groups with intramolecular charge transfer characteristics were synthesized and characterized by ¹H NMR, ¹³C NMR and X-ray crystall

Full text of DOI:10.1016/j.saa.2014.10.105

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (2014) xxx–xxx
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Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Synthesis, spectral behaviour and photophysics of donor–acceptor kind
of chalcones: Excited state intramolecular charge transfer and
fluorescence quenching studies
Mehboobali Pannipara a, Abdullah M. Asiri a,b, Khalid A. Alamry a, Muhammad N. Arshad a,b
,
Samy A. El-Daly a,c,
⇑
a Department of Chemistry, Faculty of Science, King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia
b Center of Excellence for Advanced Materials Research, King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia
c Department of Chemistry, Faculty of Science, Tanta University, 31527 Tanta, Egypt
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
ꢀ
Chalcones with electron donor
acceptor group has been studied.
The absorption and emission
spectrum were sensitive to solvent
polarity.
ꢀ
ꢀ
Fluorescence quantum yield highly
depend on polarity of solvents.
Both chalcones show fluorescence
quenching by silver nanoparticles.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 29 July 2014
Received in revised form 13 October 2014
Accepted 23 October 2014
Available online xxxx
The spectral and photophysical properties of two chalcones containing electron donating and accepting
groups with intramolecular charge transfer characteristics were synthesized and characterized by 1H
NMR, 13C NMR and X-ray crystallography. Both compounds show very strong solvent polarity dependent
changes in their photophysical characteristics, namely, remarkable red shift in the emission spectra
with increasing solvent polarity, large change in Stokes shift, significant reduction in the fluorescence
quantum yield; indicating that the fluorescence states of these compounds are of intramolecular charge
transfer (ICT) character. The solvent effect on the photophysical parameters such as singlet absorption,
molar absorptivity, oscillator strength, dipole moment, fluorescence spectra, and fluorescence quantum
yield of both compounds have been investigated comprehensively. For both dyes, Lippert–Mataga and
Reichardt’s correlations were used to estimate the difference between the excited and ground state
Keywords:
Chalcones
Photophysics
Effect of solvent polarity
Silver nanoparticles
dipole moments (Dl). The interactions of dyes with colloidal silver nanoparticles (Ag NPs) were also
studied in ethanol using steady state fluorescence quenching measurements. The fluorescence quenching
data reveal that dynamic quenching and energy transfer play a major role in the fluorescence quenching
of dyes by Ag NPs.
Ó 2014 Elsevier B.V. All rights reserved.
⇑
Corresponding author at: King Abdulaziz University, P.O. Box 80302, Jeddah 21589, Saudi Arabia. Tel.: +966 554953522.
E-mail address: samyeldaly@yahoo.com (S.A. El-Daly).
http://dx.doi.org/10.1016/j.saa.2014.10.105
1386-1425/Ó 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: M. Pannipara et al., Synthesis, spectral behaviour and photophysics of donor–acceptor kind of chalcones: Excited state
intramolecular charge transfer and fluorescence quenching studies, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2014), http://
dx.doi.org/10.1016/j.saa.2014.10.105

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M. Pannipara et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (2014) xxx–xxx
The present work aims at setting up a scheme which can
Introduction
efficiently predict the excited state intramolecular charge transfer
process applying steady state spectroscopy, quantum yield calcula-
tion with variation of polarity of the solvents. For this purpose, we
have synthesized and characterized two chalcone derivatives,
namely, 3-(1-Benzyl-1H-3-indol-3-yl)-1-naphthalen-2-yl-prope-
none and 3-(1-Benzyl-1H-3-indol-3-yl)-1-thiophene-3-yl-prope-
none; and studied the effect of solvents in detail. Lippert–Mataga
and Reichardt’s correlations were applied to calculate the differ-
ence between the excited and ground state dipole moments
Organic molecules featuring intramolecular charge transfer
(ICT) has gained a substantial research interest owing to its poten-
tial application in making photoelectronic and nonlinear optical
devices [1,2], chemical sensing [3], understanding photochemical
and photobiological processes [4,5]. Pi-conjugated organic fluoro-
phores containing donor acceptor groups that are separated by
an ethenyl or keto-vinyl bridge, exhibiting the phenomenon of dual
fluorescence are of special interest because the material properties
can be tuned and tailored by structural modification for their
potential applications in molecular electronics, dye sensitised solar
cells [6,7] and other all optical devices [8,9]. The highly polar
excited states and large charge separation exhibited by these
molecules are sensitive to changes in its micro environment such
as pH, viscosity, polarity, heat and light that can be exploited in
various fields.
(Dl). To explore the effect of metallic nanoparticles with the syn-
thesized fluorophores, we also investigate fluorescence quenching
by colloidal silver nanoparticles in ethanol using steady state
emission measurements.
Experimental
Chalcones symbolize a class of organic compounds in which
two aromatic planar rings are connected through a three carbon
Materials and methods
a, b-unsaturated carbonyl system, with significant biological
All solvents and chemicals used in this work were of spectro-
scopic grade obtained from Sigma–Aldrich and used without
further purification. Synthetic procedure and characterization of
silver nanoparticles are given in the supporting information and
TEM image of Ag NPs with absorption spectrum shown as an
inset in Fig. S1. Indole-3-carbaldehyde, 3-acetylthiophene and
2-acetonaphthone were purchased from Sigma–Aldrich.
activities have been observed over the past two decades including
anti-ulcer, anti-cancer, anti-mitotic, anti-inflammatory, anti-
malarial, anti-fungal, and anti-oxidant activities [10–13]. Apart
from these very important medicinal applications, chalcones have
interesting optical and spectral properties depending on the
substituent attached to the aromatic rings that find applications
in metal sensing, optoelectronic and nonlinear optical devices
[14,2]. The asymmetrically substituted electron-donating and
electron-accepting substituents in these molecules are connected
through a pi-conjugated system of alternate single and double
bonds and exhibit different degrees of charge transfer in the
ground and excited states. The photophysical properties of these
conjugate intramolecular charge transfer compounds strongly
depend on the nature of the substituent, solvent polarity and
temperature.
Metallic nanoparticles possess unique spectroscopic, electronic
and chemical properties due to their small size and high surface to
volume ratios that are different from those of the individual atoms
as well as their bulk counterparts. The optical properties of noble
metal nano particles have received considerable attention because
the surface plasmon absorption band of noble metal nanoparticles
appears in the visible region of the spectrum to provide important
contribution towards sensing and bio-medical applications. Inter-
action of metallic nanoparticles (NPs) with fluorophores has
become an active area of research for the last two decades with
applications ranging from material science to biomedical science
[15–17]. The fluorescence of a dye molecule is quenched or
enhanced in the close proximity of the metallic nanoparticles
and these phenomena can be used to probe the micro environment
of the fluorophore. The emission behaviour of a dye molecule can
be altered by using metallic nanoparticles and quenching or
enhancement of photoluminescence of a dye by silver nanoparti-
cles (AgNPs) depend upon the distance between the dye molecule
and NPs [18–20]. The quenching processes are of three types:
static, dynamic and by electron/energy transfer. In static quench-
ing, the decrease in emission intensity is caused by the adsorption
of dye molecule on the surface of the metallic NPs, forming a non
fluorescent complex between the fluorophore and quencher where
as in dynamic quenching, the reduction of emission intensity is due
to the direct interaction or collision of excited fluorophore with a
quencher during their excited state life time. The third type of
quenching process takes place by non-radiative energy/electron
transfer between the dye molecule and NPs [21,22]. The quenching
of fluorescence dominates over enhancement at shorter distances
and it is attributed to the efficient non-radiative energy transfer
between the dye molecule and the metallic NP [23].
Spectral measurements
All solvents and chemicals used in this work were of spectro-
scopic grade obtained from Sigma Aldrich and used without
further purification. Melting points were determined on a Gallenk-
amp melting point apparatus and the infrared (IR) spectra were
recorded on Shimadzu FT-IR 8400S infrared spectrophotometer
using the KBr pellet technique. The NMR (1H and 13C) spectra were
recorded on a Bruker DPX-600 at 600 MHz and 150 MHz, respec-
tively, using tetramethylsilane as the internal standard. The chem-
ical shift values are recorded on d scale and coupling constants (J)
in Hertz; Splitting patterns were designated as follows: s: singlet;
d: doublet; m: multiplet. UV–Vis electronic absorption spectra
was recorded on a Shimadzu UV-160A spectrophotometer, and
the steady-state fluorescence spectra were measured using Shima-
dzu RF 5300 spectrofluorphotometer using a rectangular quartz
cell of dimensions 0.2 cm ꢁ 1 cm. The emission was monitored at
right angle. The fluorescence quantum yield (/f) was measured
using an optically diluted solution of quinine sulfate as reference
according to Eq. (1):
Iu As n2u
/u ¼ /s ꢁ
ꢁ
ꢁ
ð1Þ
Is Au n2s
where /u, /s are the fluorescence quantum yields of the unknown
and standard, respectively, I is the integrated emission intensity;
A is the absorbance at excitation wavelength, and n is the refractive
index of the solvent. The subscript u and s refers to unknown and
standard, respectively.
Procedure for the synthesis of 1-Benzyl-1H-3-indolecarbaldehyde (2)
To a solution of indole-3-carbaldehyde (1.5 g, 0.010 mol) in
ethanol (50 mL), KOH pellets were added (0.69 g, 0.012 mol) and
the mixture were stirred at room temperature until total solubili-
sation. The ethanol was completely removed in vacuum and acetone
(50 mL) was added followed by benzyl bromide (0.010 mol, 1.2 mL).
A precipitate was formed instantly on addition of benzyl bromide
Please cite this article in press as: M. Pannipara et al., Synthesis, spectral behaviour and photophysics of donor–acceptor kind of chalcones: Excited state
intramolecular charge transfer and fluorescence quenching studies, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2014), http://
dx.doi.org/10.1016/j.saa.2014.10.105

M. Pannipara et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (2014) xxx–xxx
3
and the solid was filtered and the solution was concentrated in
vacuum and crystallized from ethanol to give 2 (2.1 g, 92%) as
brown solid.
ethanolic solution of NaOH (2 g in 20 mL ethanol) was stirred for
3–5 h at room temperature and allowed to stand overnight. The
solid product was collected by filtration, dried and recrystallized
from ethanol to give 3a (1.36 g, 82%) as yellow solid.
Brown solid, m.p: 100–1020C; 1H NMR (600 MHz, DMSO-d6) d
5.4 (s, 2H), 7.25–8.12 (m, 9H), 8.55 (s,1H), 10.2 (s,1H); IR (KBr,
Yellow solid, m.p: 182–1840C; 1H NMR (600 MHz, CDCl3) d 5.37
(s, 2H), 7.25–8.12 (m, 9H), 8.55 (s, 1H), 7.73 (d, 1H, J = 15.4 Hz), 7.37
(d, 1H, J = 15.1 Hz), 7.32 (s, 1H), 7.55–7.61 (m, 3H), 7.18 (d, 2H,
J = 7.2 Hz), 8.16 (d, 1H, J = 8 Hz); 13C NMR (150 M Hz, CDCl3)
d 190.05, 138.5, 137.8, 136.4, 136.0, 135.2, 133.7, 132.7, 129.5,
129.3, 129.0, 128.8, 128.3, 128.1, 128.0, 127.8, 127.0, 126.7,
126.6, 124.7, 123.3, 121.7, 120.9, 118.1, 117.6, 113.6, 110.7, 50.6;
cmꢂ1):
tmax = 1652, 2954.
Procedure for the synthesis of 3-(1-Benzyl-1H-3-indol-3-yl)-1-
naphthalen-2-yl-propenone (BNP) (3a)
A
solution of 1-Benzyl-1H-3-indolecarbaldehyde (1 g,
IR (KBr, cmꢂ1):
tmax = 1648, 1540, 1180, 1020.
0.0042 mol) and 2-acetonaphthone (0.71 g, 0.0042 mol) in an
Scheme 1. Synthetic route to synthesized chalcones.
Fig. 1. Labelled diagram of 3a showing inter molecular hydrogen bonding using dashed lines.
Please cite this article in press as: M. Pannipara et al., Synthesis, spectral behaviour and photophysics of donor–acceptor kind of chalcones: Excited state
intramolecular charge transfer and fluorescence quenching studies, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2014), http://
dx.doi.org/10.1016/j.saa.2014.10.105

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Procedure for the synthesis of 3-(1-Benzyl-1H-3-indol-3-yl)-1-
thiophene-3-yl-propenone (BTP) (3b)
Brown solid, m.p: 160–1620C; 1H NMR (600 MHz, CDCl3) d 5.4
(s, 2H), 7.25–8.12 (m, 9H), 8.55 (s, 1H), 7.53 (d, 1H, J = 14.2 Hz), 7.26
(d, 1H, J = 15.1 Hz), 7.22 (s, 1H), 7.08 (d, 1H, J = 7.2 Hz), 7.12 (d, 1H,
J = 7.2 Hz); 13C NMR (150 M Hz, CDCl3) d 190.1, 150.1, 142.2,
140.1,137.8, 137.4, 130.1,130.4, 130.4, 127.8, 124.6, 127.6, 125.2,
129.4, 129.4, 128.5, 122.2, 112.4, 120.8, 110.4, 102.8, 50.2; IR
A solution of 1-Benzyl-1H-3-indolecarbaldehyde (1 g, 0.0042 mol)
and 3-acetylthiophene (0.53 g, 0.0042 mol) in an ethanolic solution
of NaOH (2 g in 20 mL ethanol) was stirred for 3–5 h at room
temperature and allowed to stand overnight. The solid product
was collected by filtration, dried and crystallized from ethanol to
give 3b (1.2 g, 86%) as brown solid.
(KBr, cmꢂ1):
tmax = 1648, 1540, 1180, 1020.
X-ray crystal structure of 3a
Single crystal of 3a suitable for X-ray analysis was obtained at
room temperature by slow evaporation of ethanol and water
mixture of the compound. One crystal of 3a (yellow 0.30 ꢁ 0.20 ꢁ
0.10 mm) was mounted on Agilent Supernova (Dual source)
Agilent Technologies Diffract meter, equipped with a graphite-
monochromatic Cu Ka radiation (k = 1.54184), to collect diffraction
data using CrysAlisPro software at 296 K. The structure solution
was achieved through SHELXS-97 [24] and refined by full-matrix
least-squares methods on F2 using SHELXL-97, in-built with X-Seed
[25]. All the C–H hydrogen atoms were positioned geometrically
and treated as riding atoms with
Caromatic–H = 0.93, Cmethyl–
H0.96 Å and refined using a riding model with Uiso (H) = 1.5 Ueq
(C) for methyl and Uiso (H) = 1.2 Ueq (C) for all other carbon
atoms. CCDC 980984 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge at
www.ccdc.cam.ac.uk/conts/retrieving.html or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
UK.
Results and discussion
Chemistry
We have synthesized and characterized two chalcone deriva-
tives having intramolecular charge transfer characteristics, namely,
3-(1-Benzyl-1H-3-indol-3-yl)-1-naphthalen-2-yl-propenone (3a)
and 3-(1-Benzyl-1H-3-indol-3-yl)-1-thiophene-3-yl-propenone (3b).
The changes in dipole moments were calculated using Lippert–
Mattaga and Reichardt’s correlations. The solvent effect on the
absorption and emission spectra were investigated in detail with
solvents having different polarity. Scheme 1 outlines the synthetic
route to the synthesis of two chalcone derivatives. Compound 2
was synthesized by N-alkylation indole-3-aldehyde with 1 equiva-
lent benzyl bromide in presence of NaOH as base and ethanol as
solvent. Compound 3a and 3b were obtained by Claisen–Schmidt
aldol condensation of compound 2 with 3-acetylthiophene and
2-acetonaphthone in presence of one equiv. of NaOH in ethanol.
The structures of compounds were confirmed by IR, 1H NMR and
13C NMR.
Fig. 2. Electronic absorption spectra of (1 ꢁ 10ꢂ5 M) 3a (Panel A) and 3b (Panel B)
in different solvents.
Table 1
Spectral and photophysical parameters of 3a in different solvents.
ꢂ1
Mꢂ1 cmꢂ1
/
f
l12 Debye
ET(30) K cal molꢂ1
Df (D, n)
ETN
ꢀ
Solvents
kabs (nm)
kem (nm)
D
m
(cm
)
e
f
Toluene
Heptane
DMSO
Dioxane
THF
Ethyl Acetate
Acetonitrile
Chloroform
DMF
Butanol
Propanol
Ethanol
382
367
404
394
387
382
391
394
402
402
404
403
401
432
430
490
454
461
463
482
471
481
498
495
508
515
3029
3992
4344
3354
4147
4579
4828
4149
4085
4795
4550
5128
5520
27160
28860
28190
24540
29980
28750
27660
26380
26390
29520
29690
30090
31160
0.03
0.0092
0.35
0.029
0.047
0.04
0.56
0.58
0.56
0.52
0.64
0.61
0.61
0.57
0.53
0.64
0.63
0.63
0.68
6.75
6.70
6.93
6.61
7.27
7.03
7.10
6.87
6.72
7.40
7.35
7.35
7.62
33.9
31.1
45.1
36
37.4
38.1
45.6
39.1
43.8
50.2
49.2
51.9
55.4
0.0132
0.0004
0.263
0.021
0.210
0.199
0.304
0.148
0.274
0.263
0.274
0.288
0.308
0.099
0.006
0.441
0.164
0.210
0.230
0.472
0.259
0.404
0.506
0.570
0.654
0.762
0.41
0.137
0.267
0.263
0.306
0.314
0.18
Methanol
Please cite this article in press as: M. Pannipara et al., Synthesis, spectral behaviour and photophysics of donor–acceptor kind of chalcones: Excited state
intramolecular charge transfer and fluorescence quenching studies, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2014), http://
dx.doi.org/10.1016/j.saa.2014.10.105

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5
Single crystal structure of 3a
acceptor group constituted of a conjugated
p-electron system.
The red shifted band in polar solvents arises from the charge
transfer state and short wavelength emission band (ꢄ430 nm)
originate from a locally excited state and appears almost at the
same position as the maximum in non polar solvents (Fig. 3).
Moreover, in solvents having almost equal polarity like acetonitrile
Single crystal analysis of compound 3a reveals that the mole-
cule forms a monoclinic crystal pattern with P21/n space group.
The molecule contains naphthalene, indole and a phenyl ring sys-
tem in its skeleton. The structural studies along with the other
physical studies were carried out to know the three dimensional
behaviour of molecules and Vander-Walls interactions in a unit
cell. The naphthalene and indole moieties are making a curve with
the dihedral angle of 8.29(1)°. The phenyl ring is almost perpendic-
ular to naphthalene and indole systems and the dihedral angles are
81.12(1)° and 79.06(7)°, respectively. The molecules are further
stabilized by weak C–Hꢃ ꢃ ꢃO interactions and form inversion dimers
via the formation of two five membered ring motifs R12 (5) [26] as
shown in Fig. 1. The crystallographic data and refinement parame-
ters are available in Table S1 (supporting information) and the bond
lengths and bond angles are provided in Tables S2 and S3 (support-
ing information). In this intermolecular interaction, the oxygen atom
is situated at (1 ꢂ x, 1 ꢂ y, ꢂz) with respect to the donor carbon
atoms (C15 & C22) at (x, y, z) via H15 & H22B respectively (Table S4).
and methanol (expressed in
Df), compound 3a shows a red shift of
33 nm in methanol (ꢄ515 nm) from that of the emission band in
acetonitrile solvent (ꢄ482 nm) (Fig. 3) and compound 3b show a
Solvent effect on absorption and emission spectra of 3a and 3b
Absorption spectra of compound 3a and 3b (1 ꢁ 10ꢂ5 M) were
recorded in solvents of different polarity show slight variation in
the absorption maxima on going from polar aprotic solvents to
polar protic solvents. Representative absorption spectra of both
compounds recorded in some selective solvents are shown in
Fig. 2 and the corresponding spectral data are summarized Tables
1 and 2. An overall red shift of 38 nm is observed in the absorption
maxima of both compounds on going from heptane to methanol
suggesting some polar character in the ground state. In all alcoholic
solvents, both compounds show a red shifted broad absorption
band indicating that the allowed transition is
p–p* with charge
transfer character. On excitation at 365 nm, the emission spectrum
of both 3a and 3b show smooth correlation with increasing polar-
ities of the solvents (Fig. 3; Tables 1 and 2). A remarkable red shift
of 85 nm is observed for compound 3a on going from heptane to
methanol, where as the shift is 70 nm for compound 3b; suggest-
ing the involvement of photo induced intramolecular charge trans-
fer (ICT) in the singlet excited state from the electron donating
group to electron accepting keto group with larger dipole moment
in the excited state than in ground state. In case of both com-
pounds only one emission peak is observed in non polar solvents
like heptane and toluene at ꢄ430 nm, but in polar solvents a large
red shifted emission band is observed within the wavelength range
ꢄ455–515 nm for compound 3a and ꢄ446–499 nm for 3b in
addition to the short wave length band. This kind of dual emission
is characteristic of compounds containing electron donor and
Fig. 3. Emission spectra of (1 ꢁ 10ꢂ5 M) 3a (Panel A) and 3b (Panel B) in different
solvents.
Table 2
Spectral and photophysical parameters of 3b in different solvents.
ꢂ1
Mꢂ1 cmꢂ1
/
f
l12 Debye
ET(30) K cal molꢂ1
Df (D, n)
ETN
ꢀ
Solvents
kabs (nm)
kem (nm)
D
m
(cm
)
e
f
Toluene
Heptane
DMSO
Dioxane
THF
Ethyl Acetate
Acetonitrile
Chloroform
DMF
Butanol
Propanol
Ethanol
379
356
391
370
370
370
379
386
384
393
391
395
390
430
429
461
430
431
446
464
461
457
479
475
495
499
3129
4780
3883
3771
3825
4606
4834
4215
4160
4568
4523
5114
5601
26150
31520
27650
31270
31900
29370
28410
25820
28710
29330
31030
30330
31490
0.051
0.021
0.32
0.041
0.036
0.049
0.18
0.11
0.17
0.22
0.24
0.54
0.58
0.55
0.59
0.59
0.57
0.59
0.54
0.58
0.60
0.63
0.61
0.65
6.60
6.60
6.73
6.79
6.80
6.69
6.88
6.66
6.88
7.05
7.21
7.13
7.34
33.9
31.1
45.1
36
37.4
38.1
45.6
39.1
43.8
50.2
49.2
51.9
55.4
0.0132
0.0004
0.263
0.021
0.210
0.199
0.304
0.148
0.274
0.263
0.274
0.288
0.308
0.099
0.006
0.441
0.164
0.210
0.230
0.472
0.259
0.404
0.506
0.570
0.654
0.762
0.27
0.24
Methanol
Please cite this article in press as: M. Pannipara et al., Synthesis, spectral behaviour and photophysics of donor–acceptor kind of chalcones: Excited state
intramolecular charge transfer and fluorescence quenching studies, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2014), http://
dx.doi.org/10.1016/j.saa.2014.10.105

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M. Pannipara et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (2014) xxx–xxx
red shift of 35 nm. This clearly indicates that apart from polarity
effect, hydrogen bonding effect is also responsible for the red
shift with increasing hydrogen bonding ability of solvents from
1-propanol to methanol.
alcoholic solvents due to enhanced radiationless processes. The
reduction in /f going from DMSO to polar protic solvents like
MeOH (positive solvatokinetic effect) can be attributed to strong
ICT interaction, the /f value decrease strongly in highly proton
donor solvents. This effect is due to inter molecular hydrogen
bonding interaction between solvent and excited dyes. Hydrogen
bonding will induce fluorescence quenching due to the enhance-
ment of intersystem crossing, strong internal conversion and
vibrational deactivation [29–32].
Fluorescence quantum yield of 3a and 3b in different solvents
Fluorescence quantum yield values give an insight on the
photophysical properties of an unknown fluorescent molecule.
The fluorescence quantum yield (/f) of both compounds are
strongly influenced by the polarity and hydrogen bonding ability
of the solvents as shown in Tables 1 and 2. The fluorescence quan-
tum yield can be correlated with ET(30) of the solvent, where
ET(30) is the solvent polarity parameter introduced by Reichardt
[27], that considers interactions such as solvent polarizability
and hydrogen bonding besides those of a specific nature. As
depicted in Fig. 4, /f increases with increase in the polarity of the
solvent (expressed as ET(30)); reaching maximum in polar aprotic
solvents like acetonitrile for 3a and in DMSO for that of 3b with a
drop on further increase in the solvent polarity such as in alcoholic
solvents. The increase in /f (negative solvatokinetic effect) with
charge transfer character was explained by several mechanisms
such as proximity effect and conformational changes [28]. Hydro-
gen bonding between solvent molecules and the carbonyl group of
fluorophore is accounted for the reduction of /f in highly protic
Estimation of dipole moments using solvatochromic methods
The degree of charge distribution in a molecule used to clarify a
variety of physical and chemical properties. Lippert–Mataga’
Eqs. (2) and (3) [33,34], which is based on the correlation of energy
difference between the ground and excited states (Stokes’ shift)
with the solvent orientation polarizability (Df), can be used to
investigate the change in dipole moment between the excited
singlet state and ground state.
2
2ðle
ꢂ
lg
Þ
ꢀ
D
D
mst
¼
D
f þ Const:
ð2Þ
ð3Þ
hca3
e
2
ꢂ 1
n2 ꢂ 1
f ¼
ꢂ
e
þ 1 2n2 þ 1
Fig. 4. Plot of ET(30) versus fluorescence quantum yield for 3a (Panel A) and 3b
(Panel B).
Fig. 5. Correlations of the Stokes’ shift with the solvent polarity function (
(Panel A) and 3b (Panel B).
D
f) for 3a
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intramolecular charge transfer and fluorescence quenching studies, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2014), http://
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M. Pannipara et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (2014) xxx–xxx
7
ꢀ
Dmst is the difference between the absorption and emission
ET ðsol
v
32:4
entÞ ꢂ 30:7
where
ETN
¼
ð5Þ
maxima expressed in wave numbers (cmꢂ1), respectively, h is
Planck’s constant, c is the speed of light in vacuum, a is the Onsager
where kmax corresponds to the peak wavelength in the red region of
the intramolecular charge transfer absorption of the betaine dye. In
this method, change in dipole moment is calculated by correlating
the Stokes shift of the fluorophore to ENT (Fig. 6) according to Eq. (6).
cavity radius,
of the solvent, respectively,
excited and ground state, respectively, and
e
and n are the dielectric constant and refractive index
e and g are the dipole moments in the
f is the orientation
l
l
D
polarizability of the solvent which measures both electron mobility
and dipole moment of the solvent molecule. The Onsager cavity
radii (a) from molecular volume of molecules is calculated by using
Suppan’s Eq. (4) [35]:
ꢀ
ꢁ
2
ꢂ
ꢃ
3
D
l
aD
a
D
¼ 11307:6
ENT þ Const:
ð6Þ
ꢀ
m
D
lD
D
l
is the difference between the excited and ground state dipole
moments of the probe molecule and D is the change in the dipole
ꢀ
ꢁ
1=3
D
l
3M
dN
a ¼
ð4Þ
moment of the betaine dye; a (taken as 4.35 Å) and aD are the
Onsager cavity radii a of the probe molecule and betaine molecule
respectively. Since the values of aD and lD are known (6.2 Å and 9
Debye, respectively) the change in dipole moment is calculated
using Eq. (7),
4
p
where d is the density (obtained from crystallographic data) of dye,
M is the molecular weight of dye and N is Avogadro,s number. Fig. 5
shows the plots of Stokes shift versus the orientation polarization
(Df), and the change in dipole moment calculated for 3a from the
"
#
1=2
81m
slop of this plot and cavity radius (a) is found to be 8.07 D and that
of 3b is 7.75 D. The data in polar protic solvents were excluded to
avoid specific solute–solvent interactions (hydrogen bonding).
The linear correlation of Lippert–Mataga plot supports the occurrence
of photo induced intramolecular charge transfer in both compounds.
D
l
¼
ð7Þ
3
ð6:2=aÞ ꢁ 11307:6
where m is the slope of linear plot of ENT vs. Stokes shift (Fig. 6) and
the value for
The value of
Dl
is found to be 3.67 D for 3a and that of 3b is 3.26 D.
obtained by Lippert–Mataga’s equation is higher
D
l
In addition, the change in dipole moment (Dl) between the excited
singlet and ground state was also investigated using solvatochromic
shift method [27,36], making use of the dimensionless microscopic
solvent polarity parameters ENT given by the Eq. (5):
Fig. 7. (A) Emission spectra of 1 ꢁ 10ꢂ5 mol Lꢂ1 of 3a in EtOH in presence of
different concentrations of Ag NPs. The concentrations of Ag NPs at decreasing
emission intensity are 0.0, 39, 78, 117, 156, 195, 234, and 312 pM (kex = 380 nm).
(B) Emission spectra of 1 ꢁ 10ꢂ5 mol Lꢂ1 of 3b in EtOH in presence of different
concentrations of Ag NPs. The concentrations of Ag NPs at decreasing emission
intensity are 0.0, 39, 117, 156, 195, 273, and 312 pM (kex = 380 nm).
Fig. 6. Correlation of the Stokes’ shift with ENT for 3a (Panel A) and 3b (Panel B).
Please cite this article in press as: M. Pannipara et al., Synthesis, spectral behaviour and photophysics of donor–acceptor kind of chalcones: Excited state
intramolecular charge transfer and fluorescence quenching studies, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2014), http://
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8
M. Pannipara et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (2014) xxx–xxx
than that obtained by dimensionless microscopic solvent polarity
parameters ENT , because in Lippert’s plot only the dipole–dipole
interactions are taken into account and does not consider the polar-
izability of solute molecules.
The ground to excited state transition dipole moment (l12) of
3a and 3b in different solvents was calculated using Eq. (8) [37]:
experiments. Interaction of silver nanoparticles with 3a and 3b
were investigated by fluorescence quenching measurements with
variable concentrations of Ag NPs in ethanol. The emission spectra
of 1 ꢁ 10ꢂ5 mol Lꢂ1 solution of 3a and 3b in ethanol with different
concentrations of Ag NPs are shown in Fig. 7. On increasing the
concentration of Ag NPs, the emission spectrum of 3a and 3b
remains unaltered in wavelength but a substantial decrease in
fluorescence intensity was observed; which rules out the possibil-
ity of ground state complex formation between dyes and Ag NPs.
The fluorescence quenching behaviour can be analyzed by the
well-known Stern–Volmer equation. The Stern–Volmer quenching
constant (Ksv) for 3a and 3b using Ag NPs as quencher were
obtained from the Stern–Volmer Eq. (10) [39]:
f
l212
¼
ð8Þ
4:72 ꢁ 10ꢂ7Emax
where Emax is the energy maximum absorption in cmꢂ1 and f is the
oscillator strength which shows the effective number of electrons
whose transition from ground to excited state gives the absorption
area of the electronic spectrum. The experimental oscillator
strength values were calculated using Eq. (9) [38]:
Z
Io
I
¼ 1 þ Ksv½Agoꢅ
ð10Þ
f ¼ 4:32 ꢁ 10ꢂ9
eð
m
Þd
m
ð9Þ
ꢀ
ꢀ
where Io and I are the fluorescence intensities in the absence and
presence of the quencher concentration [Ago]. The Stern–Volmer
plot for 3a and 3b (Fig. 8) were found to be linear with correlation
coefficient (R2) equal to 0.99. From the slopes of the linear plots, Ksv
values were calculated as 2.34 ꢁ 109 Mꢂ1 for 3a and 1.52 ꢁ 109 Mꢂ1
for 3b. The significant overlap between the emission spectrum of
both dyes with the absorption of spectrum of Ag NPs (Fig. 9) reveal
the possibility of non-radiative energy transfer from dyes to Ag NPs
according to Forster’s theory [40]. Thus, the Stern–Volmer plot and
the spectral overlap between the both compounds and Ag NPs
indicate the dynamic nature of the quenching process.
where
e
is the numerical value for molar decadic extinction coeffi-
cient measured in dm3 molꢂ1 cmꢂ1 and
m
is the numerical value of
ꢀ
the wave number measured in cmꢂ1. The values of f and
l12 are listed
in Table 1 and indicate that the So ? S1 transition is strongly allowed.
Fluorescence quenching study of 3a and 3b by silver nanoparticles
The nature of molecular interaction and microenvironment of
fluorophores in solution can be studied by fluorescence quenching
Fig. 8. Stern–Volmer plot for fluorescence quenching of 3a (Panel A) and 3b (Panel
B) by Ag NPs in ethanol.
Fig. 9. Spectral overlap of (a) absorption spectrum of silver nanoparticles with and
(b) emission spectrum of 3a (Panel A) and 3b (Panel B).
Please cite this article in press as: M. Pannipara et al., Synthesis, spectral behaviour and photophysics of donor–acceptor kind of chalcones: Excited state
intramolecular charge transfer and fluorescence quenching studies, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2014), http://
dx.doi.org/10.1016/j.saa.2014.10.105

M. Pannipara et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (2014) xxx–xxx
9
identified by spectroscopy and single crystal X-ray crystallography.
It is inferred from the absorption and emission spectra that
emissive states of both compounds are of intramolecular charge
transfer characteristics. For both the dyes the
uf value shows sharp
increase with increase in the polarity of the aprotic solvents and
decrease in alcoholic solvents due to hydrogen bonding between
solute and solvent. Physicochemical parameters such as molar
absorptivity, oscillator strength, transition dipole moment and
fluorescence quantum yield of both dyes have been calculated.
The interactions of dyes with colloidal silver nanoparticles in
ethanol have also been studied using fluorescence techniques.
From the fluorescence quenching data, dynamic quenching and
energy transfer from excited dyes to Ag NPs play a major role in
the fluorescence quenching of dyes by Ag NPs.
Acknowledgments
The authors thank the Center of Excellence for Advanced Mate-
rials Research at King Abdul Aziz University for providing the
research facilities.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.saa.2014.10.105.
References
[1] A.C. Arias, J.D. MacKenzie, I. McCulloch, J. Rivnay, A. Salleo, Chem. Rev. 110
(2010) 3–24.
[2] K.M. Rahulan, S. Balamurugan, K.S. Meena, G-Y. Yeap, C.C. Kanakam, Opt.
LaserTechnol. 56 (2014) 142–145.
[3] Q. Li, M. Peng, H. Li, C. Zhong, L. Zhang, X. Cheng, X. Peng, Q. Wang, J. Qin, Z. Li,
Org. Lett. 14 (2012) 2094–2097.
[4] Z.R. Grabowski, K. Rotkiewicz, W. Rettig, Chem. Rev. 103 (2003) 3899–4302.
[5] F.A.S. Chipem, A. Mishra, G. Krishnamoorthy, Phys. Chem. Chem. Phys. 14
(2012) 8775–8790.
[6] A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo, H. Pettersson, Chem. Rev. 110 (2010)
6595–6663.
[7] H. Tian, X. Yang, R. Chen, R. Zhang, A. Hagfeldt, L. Sun, J. Phys. Chem. C 112
(2008) 11023–11033.
Fig. 10. Benesi–Hildebrand plot for the adsorption of 3a on Ag NPs in ethanol (Panel
A) and 3b (Panel 3B).
[8] Y. Suzuki, K. Komatsu, T. Kaino, Y. Honda, Opt. Mater. 2 (2003) 521–524.
[9] Y. Shi, W. Lin, D.J. Olson, J.H. Bechtel, H. Zhang, W.H. Steier, et al., Appl. Phys.
Lett. 77 (2000) 1–3.
[10] M. Liu, P. Wilairat, M.L. Go, J. Med. Chem. 44 (2001) 4443.
[11] S.N. López, M.V. Castelli, S.A. Zacchino, J.N. Dominguez, G. Lobo, J. Charris-
Charris, J.C. Cortes, J.C. Ribas, C. Devia, A.M. Rodriguez, R.D. Enriz, Bioorg. Med.
Chem. 9 (2001) 1999.
The interaction between adsorbed and unadsorbed dye mole-
cules was further investigated by determining the apparent associ-
ation constant (Kapp
)
using Benesi–Hildebrand method [41],
[12] J.H. Wu, X.H. Wang, Y.H. Yi, Bioorg. Med. Chem. Lett. 13 (2003) 1813.
[13] L. Mathiesen, K.E. Malterud, R.B. Sund, Planta Med. 61 (1995) 515.
[14] T.A. Fayed, Chem. Phys. 324 (2006) 631–638.
[15] R. Chhabra, J. Sharma, H. Wang, S. Zou, S. Lin, H. Yan, S. Lindsay, Y. Liu,
Nanotechnology 20 (2009) 485201.
[16] M.C. Daniel, D. Astruc, Chem. Rev 104 (2004) 293.
[17] W. Deng, E.M. Goldys, Langmuir 28 (2012) 10152.
[18] Y. Fu, J. Zhang, J.R. Lakowicz, J. Fluoresc. 17 (2007) 811.
[19] S. Kalele, A.C. Deshpande, S.B. Singh, S.K. Kulkarni, Bull. Mater. Sci. 31 (2008)
541.
[20] G. Raino, T. Stoferle, C. Park, H.C. Kim, T. Topuria, P.M. Rice, I.J. Chin, R.D. Miller,
R.F. Mahrt, ACS Nano 5 (2011) 3536.
[21] M. Swierzewska, S. Lee, X. Chen, Phys. Chem. Chem. Phys. 13 (2011) 9929–
9941.
[22] D.J. Maxwell, J.R. Taylor, S. Nie, J. Am. Chem. Soc. 124 (2002) 9606.
[23] H. Chung, P. Leung, D. Tsai, Optic. Exp. 22 (2013) 26483–26492.
[24] G.M. Sheldrick, Acta Cryst. Sec. A 64 (2007) 112–122.
[25] L.J. Barbour, J. Supra. Chem. 1 (2001) 189–191.
[26] J. Bernstein, R.E. Davis, L. Shimoni, N.L. Chang, Angew. Chem. Int. Ed. Engl. 34
(1995) 1555–1573.
[27] C. Reichardt, Chem. Rev. 94 (1994) 2319–2358.
[28] K. Rurack, M.I. Dekhtyar, J.L. Bricks, U. Resch-Genger, W. Retting, J. Phys. Chem.
A 103 (1999) 9626–9635.
Eqs. (11)–(13).
Dye þ Ag ! ½Dye ꢃ ꢃ ꢃ Agꢅ
Kapp ¼ ½Dye ꢃ ꢃ ꢃ Agꢅ=½Dyeꢅ½Agꢅ
ð11Þ
ð12Þ
1
1
1
Kapp
1
¼
þ
ꢁ
ð13Þ
Fo ꢂ F ðFo ꢂ F0Þ
ðFo ꢂ F0Þ½Agꢅ
where Kapp is the apparent association constant, Fo is the initial fluo-
rescence intensity of dye molecules, F0 is the fluorescence intensity
of Ag adsorbed dye and F is the observed fluorescence intensity at
its maximum. The calculated values of Kapp obtained from the plot
of 1/Fo–F vs. 1/[Ag] (Fig. 10) are found to be 6.48 ꢁ 108 for 3a and
2.0 ꢁ 109 Mꢂ1 for 3b. The higher value of Kapp indicates the strong
association between the dyes and Ag NPs.
Conclusion
[29] J.B. Birks, Organic Molecular Photophysics, John Wiley and Sons, New York,
1973.
[30] S.A. El-Daly, A.M. Asiri, S.A. Khan, K.A. Alamry, J. Lumin. 134 (2013) 819–824.
[31] Z. Xu, G. Bai, C. Dong, Spectrochim. Acta Part A 62 (2005) 987–990.
[32] T.A. Fayed, M.A. El-morsi, M.N. El-Nahass, J. Photochem. Photobiol. A: Chem.
224 (2011) 38–45.
In summary, we have synthesized two chalcone derivatives,
namely 3-(1-Benzyl-1H-3-indol-3-yl)-1-naphthalen-2-yl-propenone
and 3-(1-Benzyl-1H-3-indol-3-yl)-1-thiophene-3-yl-propenone, with
push–pull type system and structure of these compounds were
[33] E. Lippert, Z. Elektrochem 61 (1957) 962–975.
Please cite this article in press as: M. Pannipara et al., Synthesis, spectral behaviour and photophysics of donor–acceptor kind of chalcones: Excited state
intramolecular charge transfer and fluorescence quenching studies, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2014), http://
dx.doi.org/10.1016/j.saa.2014.10.105

10
M. Pannipara et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (2014) xxx–xxx
[34] N. Mataga, T. Kubota, Molecular Interactions and Electronic Spectra, Marcel
Dekker, New York, 1970, pp. 371–410.
[35] P. Suppan, Chem. Phys. Lett. 94 (1983) 272.
[38] P. Gordon, P. Gregory, Organic Chemistry in Colour, Chimia, Moskva, 1987.
[39] J.R. Lakowicz, Principles of Fluorescence Spectroscopy, third ed., Springer, New
York, 2006.
[36] M. Ravi, T. Soujanya, A. Samanta, T.P. Radhakrishnan, J. Chem. Soc. Faraday
Trans. 91 (1995) 2739–2742.
[40] T. Förster, in: O. Sinanoglu (Ed.), Modern Quantum Chemistry, Academic Press,
New York, 1996.
[37] B.J. Coe, J.A. Harris, I. Asselberghs, K. Clays, G. Olbrechts, A. Persoons, J.T. Hupp,
R.C. Johnson, S.J. Coles, M.B. Hursthouse, K. Nakatani, Adv. Funct. Mater. 12
(2002) 110–116.
[41] H.A. Benesi, J.H. Hildebrand, J. Am. Chem. Soc. 71 (1949) 2703–2707.
Please cite this article in press as: M. Pannipara et al., Synthesis, spectral behaviour and photophysics of donor–acceptor kind of chalcones: Excited state
intramolecular charge transfer and fluorescence quenching studies, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2014), http://
dx.doi.org/10.1016/j.saa.2014.10.105