Time resolved spectroscopic studies on a novel synthesized photo-switchable organic dyad and its nanocomposite form in order to develop light energy conversion devices

Article abstract of DOI:10.1166/jnn.2015.10290

UV-vis absorption, steady state and time resolved spectroscopic investigations in pico and nanosecond time domain were made in the different environments on a novel synthesized dyad, 3-(2-methoxynaphthalen-1-yl)-1-(4-methoxyphenyl)prop-2-en-1-one (MNTMA) in its pristine form and when combined with gold (Au) nanoparticles i.e., in its nanocomposite structure. Both steady state and time resolved measurements coupled with the DFT calculations performed by using Gaussian 03 suit of software operated in the linux operating system show that though the dyad exhibits mainly the folded conformation in the ground state but on photoexcitation the nanocomposite form of dyad prefers to be in elongated structure in the excited state indicating its photoswitchable nature. Due to the predominancy of elongated isomeric form of the dyad in the excited state in presence of Au Nps, it appears that the dyad MNTMA may behave as a good light energy converter specially in its nanocomposite form. As larger charge separation rate (k_CS ～ 4 × 10 s^-1) is found relative to the rate associated with the energy wasting charge recombination processes (k_CR ～ 3 × 10s^-1) in the nanocomposite form of the dyad, it demonstrates the suitability of constructing the efficient light energy conversion devices with Au-dyad hybrid nanomaterials.

Full text of DOI:10.1166/jnn.2015.10290

Article
Journal of
Nanoscience and Nanotechnology
Vol. 15, 5775–5784, 2015
Copyright © 2015 American Scientific Publishers
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Time Resolved Spectroscopic Studies on a Novel
Synthesized Photo-Switchable Organic Dyad and
Its Nanocomposite Form in Order to Develop
Light Energy Conversion Devices
Gopa Dutta (Pal)1, Abhijit Paul2, Somnath Yadav2, Munmun Bardhan3, Asish De4,
Joydeep Chowdhury5, Aindrila Jana6, and Tapan Ganguly6ꢀ∗
1Department of Physics, Gokhale Memorial Girl’s College, Kolkata 700020, India
2Department of Applied Chemistry, Indian School of Mines, Dhanbad 826004, India
3Saha Institute of Nuclear Physics, Salt Lake 700098, India
4Formerly Senior Professor of the Organic Chemistry Department of Indian Association
for the Cultivation of Science, Jadavpur, Kolkata 700032, India
5Department of Physics, Sammilani Mahavidyalaya, Bagha Jatin, Kolkata 700094, India
6School of Laser Science and Engineering, Jadavpur University, Jadavpur, Kolkata 700032, India
Delivered by Publishing Technology to: University of Waterloo
UV-vis absorption, steady state and time resolved spectroscopic investigations in pico and nanosec-
IP: 66.162.36.106 On: Tue, 03 Nov 2015 10:21:47
ond time domain were made in the different environments on a novel synthesized dyad, 3-(2-
Copyright: American Scientific Publishers
methoxynaphthalen-1-yl)-1-(4-methoxyphenyl)prop-2-en-1-one (MNTMA) in its pristine form and
when combined with gold (Au) nanoparticles i.e., in its nanocomposite structure. Both steady state
and time resolved measurements coupled with the DFT calculations performed by using Gaussian
03 suit of software operated in the linux operating system show that though the dyad exhibits mainly
the folded conformation in the ground state but on photoexcitation the nanocomposite form of dyad
prefers to be in elongated structure in the excited state indicating its photoswitchable nature. Due
to the predominancy of elongated isomeric form of the dyad in the excited state in presence of Au
Nps, it appears that the dyad MNTMA may behave as a good light energy converter specially in
its nanocomposite form. As larger charge separation rate (kCS ∼ 4 × 108
s
−1) is found relative to
the rate associated with the energy wasting charge recombination processes (kCR ∼ 3×105 s−1) in
the nanocomposite form of the dyad, it demonstrates the suitability of constructing the efficient light
energy conversion devices with Au-dyad hybrid nanomaterials.
Keywords: Hybrid Nanocomposite, The Dyad, Au Nanoparticles, Time Resolved Spectroscopy,
Charge Separation, Charge Recombination, Light Energy Converter.
1. INTRODUCTION
photosynthesis based on development of organic molecular
systems which will provide nonradiative processes through
photosensitized reactions like excitational energy transfer
and photoinduced electron transfer (PET) processes. Some
novel chemical approaches are being attempted lately to
design model compounds of artificial photosynthesis.10–15
We could claim these donor–acceptor linked systems are
artificial photosynthetic systems in true sense as neither
of the redox components is present in natural photo-
synthesis where chlorophyll special pair is present with
quinones. Moreover to avoid much of the complexity of
the natural photosynthetic apparatus, many workers in this
In recent years it has become a great challenge to
researchers to explore the solution of energy crisis which
is the key scientific problem in 21st century. Produc-
tion of hydrogen gas,1 a promising future fuel and the
studies on artificial photosynthesis2–9 are the most impor-
tant researches going on today. Apart from this type
of research, a number of other promising approaches,
have appeared in recent years for working on artificial
∗Author to whom correspondence should be addressed.
J. Nanosci. Nanotechnol. 2015, Vol. 15, No. 8
1533-4880/2015/15/5775/010
doi:10.1166/jnn.2015.10290
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Time Resolved Spectroscopic Studies on a Novel Synthesized Photo-Switchable Organic Dyad
Dutta (Pal) et al.
field restricted their focus to design the organic systems
for the artificial light energy conversion where the sur-
vival duration of the charge separated products resulted
from photoinduced charge separation reactions would be
enhanced by retarding the energy wasting charge recombi-
nation process. This has been done successfully by com-
bining the short chain organic dyads with semiconductor
nanoparticles.16 So, low-cost and at the same time energy-
efficient light energy conversion devices like components
of molecular photovoltaic cells, organic solar cells would
be helpful to every individual and the society as a whole
to fight against energy crisis.
Recent work provides us the new information about the
ingenious properties of nanoparticles as well as core–shell
or hybrid nanocomposites16–19 in building materials where
the light (or solar) energy could be stored and potentially
utilized.
environments on this novel synthesized dyad in its pristine
form and when combined with gold (Au) nanoparticles
i.e., in its nanocomposite form. In the present work the
primary aim was to determine the rates associated with
charge-separation and energy destructive charge recom-
bination processes by using the steady state and time
resolved spectroscopic techniques. The experimental data
would help to reveal the suitability of the novel dyad
to acting as an artificial light energy conversion device.
Attempts were made to explore the better suitability of
the dyad whether in its pristine or nanocomposite form
to acting as a potential energy storage. In this article the
experimental details along with the results observed from
the steady state and time resolved spectroscopic techniques
and their interpretations in the light of the development
of efficient light energy conversion devices have been pre-
sented and discussed.
A better understanding of the various properties of these
nanostructures is needed in order to realize their full poten-
tials in the field of storage of energy. Our present and
future investigations are directed in this way along with
the other researchers.20–31
2. EXPERIMENTAL DETAILS
2.1. Materials
The method of synthesis and characterization of the dyad
3-(2-methoxynaphthalen-1-yl)-1-(4-methoxyphenyl)prop-
2-en-1-one (MNTMA) is given below in the Section 2.3.
The solvents acetonitrile (ACN) (SRL), and cyclohex-
ane (CH) of spectroscopic grade were purified follow-
ing the standard procedures and tested before use for
Recently we have synthesized a novel dyad (Fig. 1(a))
where the donor methoxynaphthalene is linked by a short
chain with an acceptor p-methoxyacetophenone.
UV-vis absorption, steady state and time resolved
spectroscopic investigations were made in the different
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the absence of any impurity emission in the concerned
wavelength region. ꢀ-Cyclodextrin (ꢀCD) (Aldrich) and
IP: 66.162.36.106 On: Tue, 03 Nov 2015 10:21:47
(a)
Copyright: American Scientific Publishers
methanol (MeOH) (E Merck, Germany) of spectroscopic
grade were used as received. Water was deionized using
a Millipore Milli-Q system. The solutions were prepared
by dissolving the appropriate amount of ꢀCD in water.
Because of the low solubility in water, the dyad was dis-
solved in pure MeOH and added to the water solution of
ꢀCD. The final methanol concentration in a mixture was
0.2% (v/v).
(b)
2.2. Spectroscopic Apparatus
Steady state UV-vis and fluorescence emission spectra of
dilute solutions (10−4–10−6 M) of the dyad were recorded
at the ambient temperature (296 K) using 1 cm path length
rectangular quartz cells by means of an UV-vis absorp-
tion spectrophotometer (Shimadzu UV-vis 2101PC) and
F-4500 fluorescence spectrophotometer (Hitachi), respec-
tively. Fluorescence lifetime measurements were carried
out by using the time correlated single photon counting
(TCSPC) method using HORIBA JOBIN YVON FLUO-
ROCUBE. The quality of fit was assessed over the entire
decay, including the rising edge, and tested with a plot
of weighted residuals and other statistical parameters e.g.,
the reduced ꢁ2 and the Darbin–Watson (DW) parameters.
The time resolution is ∼20 ps and the excitation wave-
length used was 375 nm with a diode laser. In some cases
NANOLED 280 was used for excitation with 280 nm light.
Figure 1. (a) Novel short chain dyad MNTMA; (b) high resolution
transmission electron micrograph (HRTEM) picture of the Au nanopar-
ticles (32 nm Gold nanoparticles, there is a size variation ranging from
29 nm to 37 nm).
5776
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Dutta (Pal) et al.
Time Resolved Spectroscopic Studies on a Novel Synthesized Photo-Switchable Organic Dyad
The femtosecond-resolved fluorescence decays were
measured using a femtosecond upconversion setup (FOG
100, CDP). The samples were excited at 375 nm using the
second harmonic of a mode-locked Ti-sapphire laser with
an 80 MHz repetition rate (Tsunami, Spectra Physics),
pumped by 10 W Millennia (Spectra Physics). The funda-
mental beam was frequency doubled in a nonlinear crystal
(1 mm BBO, ꢀ = 25ꢀ, ꢁ = 90ꢀ), ꢀ and ꢁ being the polar
and azimuthal angles. Using a gate pulse of the fundamen-
tal beam, the fluorescence emitted from the sample was
upconverted in a nonlinear crystal (0.5 mm BBO, ꢀ = 10ꢀ,
ꢁ = 90ꢀ). The upconverted light was dispersed in a dou-
ble monochromator and detected using photon counting
electronics. The instrument response time was determined
from the cross-correlation function due to sum frequency
generation between gate and excitation pulses. The cross-
correlation function was obtained using the Raman scatter-
ing from water, displaying a full width at half maximum of
approximately (FWHM) of 165 fs. The observed femtosec-
ond resolved decays were fitted using a Gaussian shape
for the exciting pulse.
ethyl acetate layers were washed with brine till neutral,
after which it was concentrated in vacuo. The residue
was then chromatographed over a column of silica-gel
(60–120 mesh) using 10% ethyl acetate-petroleum ether as
eluent to afford the (E)-3-(2-methoxynaphthalen-1-yl)-1-
(4-methoxyphenyl)prop-2-en-1-one (3, 3.1 g, 56%). M.p.
100–102 C; H-NMR (400 MHz, CDCl3): 3.83 (s, 3H ),
3.99 (s, 3 H), 6.96 (d, J = 9 Hz), 7.26 (d, J = 9 Hz),
7.36 (dd, J = 8, 8 Hz), 7.51 (dd, J = 8, 8 Hz), 7.76 (d,
J = 8 Hz), 7.82 (d, J = 9 Hz), 7.89 (d, J = 16 Hz), 8.07
(d, J = 9 Hz), 8.26 (d, J = 9 Hz),8.48 (d, J = 16 Hz); 13C-
NMR (75 MHz, CDCl3ꢂ: 55.6, 56.4, 112.9, 113.9, 117.7,
123.6, 124.0, 127.3, 127.5, 128.7, 129.1, 131.0, 131.6,
131.7, 133.1, 137.1, 157.0, 163.4, 189.7.
ꢀ
1
2.4. Preparation of Gold (Au) Nanoparticles of
Diameter 32 nm
Au nanoparticles were synthesized by the standard proce-
dure adopted by Fren et al.32 In a flux 50 ml deionized
water is added with 1.25 ml of 0.01 Molar Chloroau-
ric acid (HAuCl4) and then heated to boiling. After
that 625microliter of 1% trisodium citrate (1% = 1 gm
trisodium citrate in 100 ml distilled water) was added with
vigorous shaking. The solution changes colour, and turns
red indicating the formation of nanoparticles. We keep it
boiling for another 5–10 min keeping the volume level of
liquid same in the flux. After that the liquid cools down to
room temperature. The high resolution Transmission Elec-
tron Micrograph (HRTEM) was used to measure the size
of the Au nanoparticles (HRTEM picture of Au nanopar-
ticles is shown below in Fig. 1(b)). Average size of Au
nanoparticles is found to be around 32 nm diameter, there
is a size variation ranging from 29 nm to 37 nm diameter.
Nanosecond laser flash photolysis set up (Applied Pho-
tophysics) containing Nd:YAG (Lab series, Model Lab
150, Spectra physics) laser was used for the measurement
of transient absorption spectra. The sample was excited at
355 nm (FWHM = 8 nm) laser light. The other details are
given elsewhere.18
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All the solutions prepared for room temperature mea-
IP: 66.162.36.106 On: Tue, 03 Nov 2015 10:21:47
surements were deoxygenated by purging with an argon
Copyright: American Scientific Publishers
gas stream for about 30 minutes.
2.3. Synthesis and Characterization of the Organic
Dyad (E)-3-(2-methoxynaphthalen-1-yl)-1-(4-
methoxyphenyl)Prop-2-en-1-One (MNTMA)
Synthesis of (E)-3-(2-methoxynaphthalen-1-yl)-1-(4-meth-
oxyphenyl)prop-2-en-1-one (3).
2.5. Theoretical Computations
All DFT calculations were performed using Gaussian
03 suit of software operated in the linux operating sys-
tem. Becke’s three parameter hybrid exchange (B3) and
Lee–Yang–Parr correlation functional(LYP) were used for
the DFT calculations. The geometry optimization and
HOMO–LUMO surfaces were obtained from B3-LYP/6–
311 g (d, p) level of theory.
To a solution of 2-hydroxynapthaldehyde 1 (3.0 g,
17.4 mmol) in anhydrous acetone (30 mL) was added
anhydrous K2CO3 (2.97 g, 21.5 mmol) and methyliodide
(5.43 g, 38.2 mmol) and the resulting mixture was heated
to 45 ꢀC (bath temperature) under argon for 2 hours. After
completion of the reaction (monitored by TLC), the sol-
vent was removed in vacuo and the residue worked up in
the usual way using ethyl acetate and water to yield the
crude 2-methoxynapthaldehyde (3.0 gm). The crude 2 was
used for the next step without further purification.
The crude product 2 from the previous step was taken
in a RB flask and dissolved in MeOH (30 mL) fol-
lowed by addition of 4-methoxyacetophenone (2.42 g,
16.1 mmol) and NaOH (2.1 g, 5 mmol). The mix-
ture was then refluxed for 3 hours after which the
reaction was found to be completed as monitored by
TLC. The reaction mixture was then cooled and the
MeOH evaporated in vacuo. The resulting mixture was
worked up by addition of water followed by extrac-
tion with ethyl acetate (3 × 50 mL). The combined
3. RESULTS AND DISCUSSION
The present investigation is being carried out in continu-
ation with our previous studies10ꢃ12ꢃ13ꢃ18 on photoinduced
electron transfer reactions within organic dyad systems
consisting of donor and acceptor moieties joined by vari-
ous types of short spacers. This dyad (Fig. 1) has been cho-
sen to make our studies on similar systems comprehensive.
The modifications consist of (a) shifting the acceptor
moiety attached to the spacer to the ortho-position of the
methoxy group rather than in the position para-to it and
(b) modification of the structure of the “acceptor” part of
the dyad. The modification of the relative disposition of
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Time Resolved Spectroscopic Studies on a Novel Synthesized Photo-Switchable Organic Dyad
Dutta (Pal) et al.
the moieties viz. 1,2-rather than 1,4-position was intro-
duced to bring geometrical constraint and for examining
its effect on the energy wasting charge recombination pro-
cess. The chlorine atom present in the benzene ring of the
acceptor part of the dyad earlier examined17 by us was
replaced by methoxy group. It is known that the methoxy
group displays an electron attracting inductive effect and
electron donating mesomeric effect. When present in the
para position to a reactive center, it is the donor prop-
erty which predominates. The donor property of a func-
tional group manifests itself through delocalization of lone
pair of electrons through conjugated ꢀ-system. An essen-
tial criterion for delocalization is that the whole ꢀ-system
should be planar. The idea behind the bringing of the two
Figure 2. HOMO (left) and LUMO (right) surfaces of optimized geom-
components of the dyad closure together was to introduce
a geometrical constraint resulting in the destruction of the
planarity. This should result in the electron-withdrawing
inductive effect of the methoxy group gaining upper hand
over its electron donating mesomeric effect. This proposi-
tion was corroborated from the theoretical and experimen-
tal observations (vide infra). It is known that the methoxy
group, when present as a substituent in an aromatic ring
can exert both electron donating R effect and electron
withdrawing I effect. The former gets upperhand when it
is in the position para-to the reactive side chain as evi-
dent from the negative Hammett ꢁ value (−0.27). How-
ever, the prerequisite for delocalization of electron and the
etry of the dyad.
3.2. Steady State and Time Resolved Spectroscopic
Measurements on the Dyad in Its Pristine and
Nanocomposite Forms
When the unlinked donor methoxynaphthalene (MNT) and
the acceptor p-methoxy acetophenone (MA) of the present
dyad (MNTMA) interact with each other, the UV-vis spec-
tra show only the superposition of the corresponding spec-
tra of the individual redox components. This indicates the
lack of ground state complex formation. But when the
donor MNT is linked with the acceptor MA via a short
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spacer (Fig. 1), i.e., when the dyad is formed, a broad
consequent manifestation of R effect is that the molecule
IP: 66.162.36.106 On: Tue, 03 Nov 2015 10:21:47
absorption band, as expected, is observed in CH solvent
spanning between 350 to 400 nm peaking at around 370 nm
region (Fig. 3). The band is found to be of charge transfer
(CT) nature as confirmed from the observed red shift with
the increase of the polarity of the surrounding solvent.
This observation corroborates our proposition made
from the theoretical analysis that methoxy functionality
attached with the benzene ring of the acetophenone side
behaves as an electron accepting group. On excitation of
the CT absorption band, a CT fluorescence spectrum was
Copyright: American Scientific Publishers
should be planar. As evident from theoretical calculation,
the dyad molecule is not planar. The electron attraction by
the carbonyl group would have been attenuated had the
methoxy group exerted its electron donating effect. Since
this is not the case, the acetophenone part of the dyad, as a
whole behaves as electron acceptor, which actually reflects
in Figure 2 (LUMO surface).
3.1. Theoretical Approach
Theoretical computations were done on the optimized
ground state geometry of the dyad and the HOMO–LUMO
surface by using B3-LYP/6-311g(d,p) level of theory (for
details, see Section 2.5). Figure 2 reproduces the HOMO
and LUMO surfaces of the dyad.
1.50
1.25
1.00
The optimized structure (Fig. 2) clearly shows the non-
planar geometry of the ground state dyad. As it was seen
from the Figure 2, though in HOMO orbital the elec-
tron cloud is localized on the donor part but in LUMO
hyper surface the electron cloud shifts from the donor
to the acceptor sites via the spacer. This indicates the
dyad is of donor–spacer–acceptor type and hence methoxy
group attached in the benzene ring of the acetophenone
side should be acting here as acceptor group due to
non-planarity structure (similar to the folded type) of
the ground state dyad. Further, spectroscopic investiga-
tions strengthening the views made from the theoretical
approaches have been described below.
1
0.75
0.50
2
0.25
0.00
250
275
300
325
350
375
Wavelength/nm
Figure 3. UV-vis absorption spectra of MNTMA in (1) CH, (2) ACN.
J. Nanosci. Nanotechnol. 15, 5775–5784, 2015
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Dutta (Pal) et al.
Time Resolved Spectroscopic Studies on a Novel Synthesized Photo-Switchable Organic Dyad
350
300
250
200
150
100
50
from the entire fluorescence emission band envelop (differ-
ent monitoring wavelengths were used) originate primarily
from one species whose lifetime is about 3 ns (Table I).
In triple exponential fitting of the decay, when the exci-
tation was made in the CT fluorescence by using 375 nm
1
2
λ~375 nm
wavelength, the components of 1 ns and ps species are
observed to be much smaller than 3 ns lifetime species.
However, it has been shown below that ps component
gradually becoming dominating component when the dyad
combined with gold nanoparticles. In such a situation also
1 ns component remains significantly small relative to the
other two components: one of ps and other 4 ns, as appar-
2
1
0
400
450
500
550
600
650
ent from the corresponding fractional contributions. This
1 ns component seems to be due to the quenched donor part
as its presence was clearly observed with the unquenched
donor Methoxynaphthalene (ꢅ ∼ 11 ns) when the dyad was
excited in the donor region (∼280 nm, Table I). When
the CT absorption was excited by using 375 nm light, this
1 ns component becomes significantly small, since this CT
band resides in the longer wavelength region, far away from
the absorption domain of the donor molecule (∼280 nm).
The situation becomes entirely different when the dyad is
combined with gold nanoparticles. Here on 375 nm excita-
tion, ps order emitting species significantly develops at the
expense of the longer lived ones (∼4 ns) (Table I), espe-
cially at the longer wavelength side (∼550 nm) of the CT
emission band. In Figure 5(b) the fluorescence upconver-
sion profile shows significantly the picosecond component
Wavelength/nm
Figure 4. CT fluorescence band of the dyad (MNTMA) with ꢀexe
=
375 nm. 1: CH; 2: ACN.
obtained at around 450 nm region (Fig. 4). This band
undergoes expectedly large bathochromic (red) shift with
increase of the polarity of surrounding solvent from CH
(ꢁs ∼ 2) to ACN (ꢁs ∼ 37ꢂ5).
Thus, when the donor and acceptor are present within
an intramolecular system (such as dyad), the alignment of
both the chromophoric units are such that the formations
of CT absorption as well as CT fluorescence bands will be
facilitated. The similar observations were made before for
similar type of dyad systems13ꢃ17 where the acceptor part
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was p-chloroacetophenone (PCA).
IP: 66.162.36.106 On: Tue, 03 Nov 2015 10:21:47
From fluorescence lifetime measurements it was
and does not exhibit the long-lived ns emission (3–4 ns).
This finding is consistent with that observed from TCSPC
measurements where relatively larger fractional contribu-
tion of the ps species (∼85%) than the longer-lived ns
component (12%) at 550 nm region (Table I) of the CT
fluorescence band, specially when the dyad combined with
gold nanoparticles, is apparent.
Copyright: American Scientific Publishers
observed that (Table I) when CT absorption band is excited
(∼375 nm), mainly two lifetimes, one is the fast compo-
nent (∼ps order) and the other due to longer lived species
or conformation (in ns order) were observed (Table I,
Figs. 5(a) and (b), Fig. 5(b) is for picoseconds lifetime
measurements done by femtosecond upconversion process
to confirm ps species observed from the TCSPC measure-
ments of the decay of the Fig. 5(a)).
The steady state CT emission spectra of the dyad exhibit
the similar effect in presence of the gold nanoparticles as it
was apparent from time resolved measurements. A devel-
opment of a band at the longer wavelength side (∼500 to
However, from fractional contribution, fi, it is appar-
ent that ps contribution is negligibly small and the decays
Table I. Time resolved fluorescence data of the dyad MNTMA in absence and presence of noble Au nanoparticles at the different excitation and
emission wavelengths at the ambient temperature.
Conc Au Np (mol dm−3) or presence of ꢄ CD
ꢀexc/nm
ꢀem/nm
ꢅ1
f1
ꢅ2/ns
f2
ꢅ3/ns
f3
ꢆ2
kcs/s−1
0
280
375
358
448
460
500
550
1.8 ns
25 ps
28 ps
–
0.22
0.05
0.04
–
11.0
1.1
1.2
–
0.78
0.03
0.06
–
1.05
1.12
1.11
1.08
1.10
4ꢂ6×108
3.0
3.0
3.1
3.0
0.92
0.90
1.0
–
–
–
–
1.0
×10−5 (Au)
ꢄ CD
280
375
358
450
530
550
1.9 ns
30 ps
35 ps
30 ps
0.11
0.05
0.55
0.85
10.6
1.3
0.8
0.6
0.89
0.03
0.02
0.03
1.09
1.11
1.09
1.02
4ꢂ3×108
4.2
4.2
4.1
0.92
0.43
0.12
375
500
530
550
38 ps
39 ps
39 ps
0.26
0.38
0.50
–
–
–
–
–
–
4.14
4.10
4.10
0.74
0.62
0.50
1.05
1.05
1.12
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Time Resolved Spectroscopic Studies on a Novel Synthesized Photo-Switchable Organic Dyad
Dutta (Pal) et al.
(a) 4500
4000
(a)
1
3500
3000
2500
λ ~ 375 nm
2
2000
1500
1000
500
0
2
1
400
450
500
550
600
650
Wavelength/nm
(b) 400
350
(b)
1
300
250
λ
ex ~ 375 nm
2
200
3
150
100
3
2
1
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Copyright: American Scientifi0c Publishers
400
450
500
550
600
650
700
Wavelength/nm
Figure 6. (a) CT fluorescence spectra of the dyad in presence of Au
nano particle Without nano particle, (2) with Au nano particle (conc.
8 × 10−6 M); (b) CT fluorescence spectra of the dyad in presence of
ꢃCD, the conc. (M) being in 1: 0 M; 2: 1×10−4 M; 3: 1×10−3 M.
Figure 5. (a) Fluorescence decay of the dyad (red) in ACN ꢀex
=
375 nm, ꢀem = 500 nm, the fast (blue) decaying component represents
impulse response (diode laser) (ꢁ2 ∼ 1ꢂ08); (b) Picosecond lifetime mea-
surements by femtosecond upconversion technique (see details in the
experimental section) (deconvolution of the decay shows the magnitude
of the fluorescence lifetime component is ∼11 ps at 550 nm of the CT
fluorescence produced by excitation with 375 nm light (second harmonic
of Ti: Sapphire laser). (Sample was MNTMA dyad combined with Au
nanoparticles).
4 ns component. This long-lived ns species was primarily
observed in case of pristine dyad only. From picosecond
lifetime measurements by femtosecond upconversion tech-
nique (see details in the experimental section) deconvolu-
tion of the decay shows the magnitude of the fluorescence
lifetime component is ∼11 ps at 550 nm of the CT fluo-
rescence (Fig. 5(b)) produced by excitation with 375 nm
light (second harmonic of Ti: Sapphire laser). Thus, the
presence of an excited state conformer having fast decay
component of the order of picosecond, as observable from
TCSPC technique, is further confirmed from upconver-
sion technique of femtosecond resolution. As extraction of
25 ps decay time from TCSPC decay profile may not be
accurate enough as this falls on the order of the calibra-
tion factor or within time resolution, confirmation of 25 ps
species was necessary from femtosecond up conversion
technique.
550 nm) of the CT fluorescence is clearly observed when
the dyad combined with gold nanoparticles (Fig. 6(a)).
Thus in nanocomposite form of the dyad, comparing
the time resolved spectra with the steady state one, it
is apparent that another new emitting species is formed
whose lifetime is of the order of picosecond (ps). The
decrement of fractional contribution associated with the
longer lifetime component with concomitant increase of
the contribution of ps one as one approaches to the red
region of the emission band, provides us the supporting
evidences in favor of generation of the new excited con-
formers in nanocomposite systems at the expense of the
To reveal the nature of the fast decaying conformer,
which is found to generate effectively in nanocomposite
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J. Nanosci. Nanotechnol. 15, 5775–5784, 2015

Dutta (Pal) et al.
Time Resolved Spectroscopic Studies on a Novel Synthesized Photo-Switchable Organic Dyad
OMe
CHO
O
CHO
MeO
COCH3
OH
OMe
Mei, K2CO3
OMe
acetone, 45 ºC
NaOH, MeOH, reflux
1
2
3
Scheme 1. Synthesis of dyad 3.
form of the dyad, the steady state intensity and life-
times of CT fluorescence were measured in presence of
ꢀ-cyclodextrin (ꢀCD). As it was reported earlier,33 in pres-
ence of ꢀCD due to formation of inclusion complex there
will be preponderance of elongated type isomer of a dyad
which will be formed from its folded isomers (Scheme 2).
In presence of ꢀCD, the enhancement of 500 nm region
of the CT emission band (Fig. 6(b)) of the dyad MNTMA
is observed, similar to the findings made with the addition
of gold nanoparticles (Fig. 6(a)). Moreover, from fluores-
cence lifetime measurements (Table I) it was observed that
ps components generate, similar to the situation observed
when the dyad combines with Au nanoparticles, in pres-
ence of ꢀCD and the associated fractional contribution
increases in the similar fashion towards the red region of
the CT fluorescence band with the simultaneous reduction
of the f values of the longer component. Thus, it could
(last column of Table I) also indicates in favor of formation
of extended conformation in this nanocomposite environ-
ment.
3.3. Fluorescence Excitation Spectra
The fluorescence excitation spectra of the two fluorescence
bands, one at 450 nm (region of primarily Z-type or folded
isomer) and the other at 510 nm (region of considerable
amount of E-isomer) of the dyad MNTMA in presence of
gold nanoparticles were measured and are shown in the
Figure 7.
The observed similar excitation spectra, which corre-
spond well to the UV-vis spectra of CT absorption band of
the dyad (Fig. 3), demonstrate that both the emitting species
(E- and Z-isomers) originate mainly from the ground state
Z-isomeric species of the dyad MNTMA. Thus the possi-
bility of formations of some unwanted bi-products in pres-
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be inferred that the shorter (∼ps order) component of the
ence of gold nanoparticles appears to be slim.
Thus, this novel synthesized dyad is unique in that
IP: 66.162.36.106 On: Tue, 03 Nov 2015 10:21:47
fluorescence lifetime should correspond to the elongated
Copyright: American Scientific Publishers
isomeric species (E-type) of the dyad and the longer one
respect that the similar short chain dyads, earlier studied,
exhibit mainly extended conformers in the ground state
and on photoexcitation due to their photoswitchable char-
acter becomes of folded nature in the excited states. The
situation is just opposite in this particular dyad MNTMA.
Its switching over behavior to elongated conformer on
photoexcitation in presence of gold nanoparticles, from its
folded (Z) conformations in the ground state makes it the
should correspond to the folded (Z-form) isomeric species.
As we have discussed above, the dyad in the ground state
possesses primarily folded type species. So the dyad in
its nanocomposite form exhibits photoswitchable character
and its elongated form produced on photoexcitation should
inhibit the energy wasting charge recombination process
as in this type of conformation the donor and acceptor
being far away from each other suffer lack of overlapping.
A slight decrement (though order remains same ∼108) in
charge separation rate kcs in presence of gold nanoparticles
800
700
1
600
500
400
300
200
100
0
2
3
350
360
370
380
390
400
Wavelength/nm
Scheme 2. “Cis” means folded type (Z-form), “Trans” corresponds to
elongated or E-type isomeric species
Figure 7. Fluorescence excitation spectra of MNTMA dyad in ACN.
Montoring emission wavelengths in: 1: 456 nm; 2: 500 nm; 3:510 nm.
J. Nanosci. Nanotechnol. 15, 5775–5784, 2015
5781

Time Resolved Spectroscopic Studies on a Novel Synthesized Photo-Switchable Organic Dyad
Dutta (Pal) et al.
(a)
promising candidate for the development of artificial light
energy converter and storage device.
0.0048
0.0042
0.0036
0.0030
0.0024
0.0018
0.0012
0.0006
0.0000
2.9
4.9
6.9
9.9
24.9
Our next approach would be to measure the charge
recombination rate by using laser flash photolysis (pump-
probe) technique by using the third harmonic of Nd:YAG
laser system as an excitation profile and a probe for
monitoring the transients. The transient absorption spectra
would be measured at the different delay times between
the exciting pulse and the analyzing or probe pulses.
Special attention would be taken to analyze the tran-
sient absorption spectra and the corresponding absorption
decays of the dyad without and in presence of Au nanopar-
ticles i.e., in its pristine and nanocomposite forms.
400
450
500
550
600
Wavelength/nm
3.4. Transient Absorption Measurements by
(b)
4.9 µS
9.9 µS
24.9 µS
Nanosecond Laser Flash Photolysis Technique
0.0030
0.0025
0.0020
0.0015
0.0010
0.0005
The transient absorption spectra of the dyad MNTMA
in ACN were measured by the direct excitation of CT
absorption band at 355 nm (using the third harmonic
of Nd: YAG laser system), without (Fig. 8(A)) and
in presence of Au nanoparticles (Fig. 8(B)). From the
Figure 8(A), it is apparent that in absence of Au nanopar-
ticles the dyad MNTMA exhibits initially a broad transient
band at around 400 nm to 500 nm domain, peaking at
about 420 nm with a shoulder at 450 nm region.
With increase of delay times between the exciting
and the probe pulses there occurs an intensity inversion
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0.0000
IP: 66.162.36.106 On: Tue, 03 Nov 2400105 10:21:47
425
450
475
500
525
550
between the 420 nm and 450 nm bands (Fig. 8(A)). It
Copyright: American Scientific Publishers
Wavelength/nm
is apparent that the 450 nm band is developing at the
expense of the 420 nm one. These observations clearly
indicate that the two different species are involved. It is
confirmed that 420 nm band of MNTMA is responsible for
the donor cationic species (MNT+). This was confirmed
from the artificial production of oxidized species (MNT+)
of the donor MNT by constant current charger (model DB
300 DB Electronics, India). The donor cation, produced by
electrochemical oxidation of MNT was found to absorb at
420 nm region, as observed from the conventional UV-vis
absorption spectrophotometer. Following the observations
made earlier by Baba et al.34 the 450 nm band could be
assigned to the monomeric triplet of the donor MNT.
It can be surmised that the excited singlet CT species
formed from the excitation of ground state CT complex
undergoes relaxation to the corresponding triplet state by
the intersystem crossing (ISC). Since ISC rate generally
varies between 1011 and 108 s−1 and as the delays used
in the nanosecond laser flash photolysis measurements are
of microsecond (ꢀs) order, so, as expected, no rise time
indicating the generation of the triplet at 450 nm region
from the charge recombination processes is observed.
From the transient absorption decays of the donor
cationic species at 420 nm (Fig. 9) without (pristine) and
with the addition (nanocomposite form) of Au nanopar-
ticles, the energy destructive charge recombination rates
(kCR) were determined. From the decays it is apparent that
Figure 8. (A) Transient absorption spectra of the dyad MNTMA in
ACN at the different delay times (in pristine form) (The ꢀs delays used
are shown at the top right-hand corner); (B) Transient absorption spectra
of the dyad MNTMA in ACN at the different delay times in presence of
Au nanoparticles (in nanocomposite form).
the decay becomes slower when the dyad combines with
the Au nanoparticles. From the analysis of the decays, ion-
pair lifetime (ꢁip) of the dyad was found to be enhanced
by 3 times in case of its nanocomposite form relative to its
pristine form (from 1.1 ꢀs to 3 ꢀs ). Thus when the dyad
combines with Au nanoparticles, the survival duration
of the charge separated species increases. Approximated
charge recombination rates were computed (kCR ∼ 1/ꢁip)
and the energy destructive charge recombination rate kCR
is found to be slowed down (3ꢂ3×105 s−1) in case of Au-
dyad hybrid nanocomposite system in comparison to the
pristine form of the dyad (∼ 9ꢂ0 ×105 s−1).
From all the experimental observations it could be
inferred that in case of the nanocomposite system as the
formations of the elongated conformations of the dyad
facilitate in the excited state (in the ground state the dyad
shows only folded conformer), the two redox components
will be at a far apart position lacking the overlapping
between them. This particular geometry will impede the
energy destructive charge recombination process between
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Dutta (Pal) et al.
Time Resolved Spectroscopic Studies on a Novel Synthesized Photo-Switchable Organic Dyad
1.6
1.4
1.2
1.0
0.8
0.6
isomeric form of the dyad in the excited state in pres-
ence of Au Nps, it appears that the dyad MNTMA may
behave as a good light energy converter in its nanocom-
posite form. As the larger charge separation rate (kCS ∼ 4×
108 s−1) is observed relative to the rate associated with the
energy wasting charge recombination processes (kCR ∼ 3×
105 s−1) in the nanocomposite form of the dyad, it demon-
strates the possibility of constructing the efficient light
energy conversion devices with Au-dyad hybrid nanoma-
terials. Further investigation on the dyad system to design
nanocomposite light energy converter using the other nano
noble metal silver is underway.
2
0.4
1
0.2
Acknowledgments: The financial assistances provided
0
10
20
30
40
50
by the Department of Science and Technology (DST), New
Delhi, India (project No.: SR/S2/CMP-0045/2012) in the
form of grants and fellowships are gratefully acknowl-
edged. The authors also thank Mr. Subrata Das of the
Department of Spectroscopy, IACS for assisting in the
time-resolved fluorescence measurements. We are very
much grateful to Professor Samir K. Pal of S. N. Bose
National Centre for Basic Sciences, Salt lake, Kolkata for
allowing us to use the femtosecond upconversion tech-
nique. We are also thankful to his student Dr. Subrata
Batabyal for helping in the measurements of picosecond
lifetimes by using this femtosecond upconversion instru-
Time in µ sec
Figure 9. Comparative analysis of the transient absorption decays of
the donor cations at 420 nm (1) without and (2) in presence of Au
nanoparticles at the ambient temperature.
the redox components. Thus the survival duration of the
charge separated species would be increased what we actu-
ally observed above.
In addition to this effect, as the charge separa-
tion rate (∼ 4 × 108
s
−1) is much faster than the CR
rates (∼ 3 ×105 s−1) in nanocomposite environment of
the dyad, it indicates the possibility of constructing
ment. Tapan Ganguly specially thanks the Department of
Delivered by Publishing Technology to: University of Waterloo
the efficient light energy conversion devices specially
Science and Technology (DST), New Delhi, India, for
IP: 66.162.36.106 On: Tue, 03 Nov 2015 10:21:47
with Au-dyad nanocomposite systems. The suitability
of constructing light energy conversion device using
the Au-Dyad nanocomposite mainly originates from the
changed dyad conformations to elongated nature on pho-
toexcitations and the present results seemingly indicate
that Fermi level of Au nanoparticles has no significant role
in slowing down the charge recombination rate processes
unlike our observations made earlier in case of other short-
chain dyads.
Copyright: American Scientific Publishers
the Nano-Mission project (no. SR/NM/NS-51/2010) for
supporting the various research works conducted in his
photophysics and photochemistry group. Tapan Ganguly
gratefully acknowledges. All India Council for Technical
Education (AICTE), New Delhi for awarding the Emer-
itus fellowship and providing the contingency grant for
research.
References and Notes
Our future efforts would be to combine the present
organic short-chain dyad with various nanoparticles or
core–shell of noble metal and semiconductor nanocompos-
ite systems. In this way efficient and at the same time
cost-effective and biocompatible light energy conversion
devices could be designed to fight against energy crisis.
1. P. P. Edwards, V. L. Kuznetsov, W. I. F. David, and N. P. Brandon,
Energy Policy 36, 4356 (2008).
2. S. Bhattacharyya, T. K. Pradhan, A. De, S. R. Chaudhury, A. K. De,
and T. Ganguly J. Phys. Chem. A 110, 5665 (2006).
3. H. Imahori, M. E. El-khouly, M. Fujitsuka, O. Ito, Y. Sakata, and
S. Fukuzumi J. Phys. Chem. A. 105, 325 (2001).
4. H. Imahori, H. Norieda, H. Yamada, Y. Nishimura, I. Yamazaki,
Y. Sakata, and S. Fukuzumi J. Am. Chem. Soc. 123, 100 (2001).
5. J. A. Hutchison, T. D. M. Bell, T. Ganguly, K. P. Ghiggino, S. J.
Langford, N. R. Loken, and M. N. Paddon-Row J. Photochem. Pho-
tobiol., A: Chemistry 197,220 (2008); P. Mandal, T. Sahu, T. Misra,
S. K Pal, and T. Ganguly J. Photochem. Photobiol. A: Chem. 188,
235 (2007); T. Sahu and T. Ganguly, Indian Journal of Radiation
Research 3, 138 (2006).
4. CONCLUSIONS
From UV-vis absorption, steady state and time resolved
spectroscopic measurements coupled with theoretical
approaches, it has been inferred that the present short chain
dyad MNTMA behaves like a photoswitchable compound,
the effect of which becomes more pronounced when com-
bined with noble metal Au nanoparticles. Au-nanoparticles
act as very effective tool to recognize the presence of the
two emitting isomeric species (elongated and folded) of
the dyad though only one conformer of folded nature exists
in the ground state. Due to the predominancy of elongated
6. T. Kiyonaga, T. Mitsui, M. Torikoshi, M. Takekawa, T. Soejima, and
H. Tada J. Phys. Chem. B 110, 10771 (2006).
7. T. Hirakawa and P. V. Kamat, Langmuir 20, 5645 (2004).
8. T. Hirakawa and P. V. Kamat, J. Am. Chem. Soc. 127, 3928 (2005).
9. V. Alt, T. Bechert, P. Steinrücke, M. Wagener, P. Seidel,
E. Dingeldein, E. Domann, and R.Schnettler, Biomaterials 25, 4383
(2004).
J. Nanosci. Nanotechnol. 15, 5775–5784, 2015
5783

Time Resolved Spectroscopic Studies on a Novel Synthesized Photo-Switchable Organic Dyad
Dutta (Pal) et al.
10. S. Bhattacharya, J. Chowdhury, and T. Ganguly J. Lumin. 130, 1924
(2010).
22. S. Eustis and M. A. El-Sayed Chem. Soc. Rev. 35, 209 (2006).
23. S. Devi and M. Srivastva Indian J. Phys. 84, 1561 (2010).
11. M. Bardhan, P. Mandal, A. De, and A. K. De, J. Chowdhuri, and
T. Ganguly J. Lumin. 130, 932 (2010).
24. H. Nemec, J. Rochford, O. Taratula, E. Galoppini, P. Kuzel,
T. Polivka, A. Yartsev, and V. Sundstrom, Phys Rev. Letts. 104
(2010).
12. K. Ohkubo, H. Kotani, J. Shao, Z. Ou, K. M. Kadish, G. Li, R. K.
Pandey, and S. Fukuzumi, Angew Chem. Int. Ed. 43, 853 (2004).
13. M. Maiti, T. Misra, T. Bhattacharya, C. Basu (nee Deb), A. De, S. K.
Sarkar, and T. Ganguly, J. Photochem. Photobiol. A: Chemistry 152,
41 (2002).
14. T. Ganguly and S. K. Pal, Journal of Chin Chem Soc. (JCCS) 53,
219 (2006).
15. P. Mandal, T. Misra, A. De, S. Ghosh, S. R. Chaudhury,
J. Chowdhury, and T. Ganguly, Spectrochimica Acta Part A 66, 534
(2007).
25. H. B. Gray and J. R. Winkler, Chem. Phys. Lett. 483,
(2009).
1
26. M. R. Hartings, I. V. Kurnikov, A. R. Dunn, J. R. Winkler,
H. B. Gray, and M. A. Ratner, Coord. Chem. Rev. 254, 248
(2010).
27. H. Imahori, K. Tamaki, Y. Araki, Y. Sekiguchi, O. Ito, Y. Sakata,
and S. Fukuzumi, J. Am. Chem. Soc. 124, 5165 (2002).
28. A. Burke, S. Ito, H. Snaith, U. Bach, J. Kwiatkowski, and
M. Grätzel, Nano Lett. 8, 977 (2008).
16. G. Mandal, S. Bhattacharya, and T. Ganguly, J. Nanosci. Nanotech-
nol. 10, 579 (2010).
17. G. Mandal, S. Bhattacharya, S. Das, and T. Ganguly, J. Nanosci.
Nanotechnol. 12, 187 (2012).
18. G. Mandal, A. Chakraborty, U. K. Sur, B. Ankamwar, A. De, and
T. Ganguly, J. Nanosci. Nanotechnol. 12, 4591 (2012).
19. A. Chakraborty, S. Chakraborty, and T. Ganguly, Ind. J. Phys.
87, 1113 (2013).
20. M. C. Scharber, D. Mühlbacher, M. Koppe, P. Denk, C. Waldlauf,
A. J. Heeger, and C. J. Brabec, Adv. Mater. 18, 789 (2006).
21. C. Brabec, V. Dyakonov, and U. Scherf (eds.), Organic Photovoltaics,
Wiley-VCH, Weinheim, Germany (2008).
29. H. J. Lee, J.-H. Yum, H. C. Leventis, S. M. Zakeeruddin, S. A.
Haque, P. Chen, S. Il Seok, M. Grätzel, and Md. K. Nazeeruddin,
J. Phys. Chem. C 112, 11600 (2008).
30. P. V. Kamat, K. M. Guldi, F. D’Souza, S. Fukuzumi (eds.), Proceed-
ings of the Electrochemical Society, The Electrochemical Society,
Pennington, NJ (2004), Vol. 14, ISBN 1-56677-451-9.
31. F. D’Souza and O. Ito Chem. Soc. Rev. 41, 86 (2012).
32. G. Frens, Nature Physical Science 241, 20 (1973).
33. T. Misra, T. Bhattacharya, S. K. Pal, A. De, R. D. Saini, and
T. Ganguly, Chem. Phys. Lett. 382, 167 (2003).
34. T. Takemura, K. Hara, and H. Baba, Bull. Chem. Soc. Jpn. 44, 977
(1971).
Received: 26 March 2014. Accepted: 7 July 2014.
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Copyright: American Scientific Publishers
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