Synthesis, characterization, photophysical properties of a novel organic photoswitchable dyad in its pristine and hybrid nanocomposite forms

Article abstract of DOI:10.1166/jnn.2012.6208

In the present paper the method of synthesis and characterization of a novel organic dyad, 3-(1-Methoxy-3,4-dihydro-naphthalyn-2-yl-)-1-p-chlorophenyl propenone, have been reported. In this paper our main thrust is to fabricate new hybrid nanocomposites by combining the organic dyad with different noble metals, semiconductor nanoparticle and noble metal-semiconductor core/shell nanocomposites. In this organic dyad, donor part is 1-Methoxy-3, 4-dihydro-naphthalen- 2-carboxaldehyde with the acceptor p-chloroacetophenone. We have carried out steady state and time-resolved spectroscopic measurements on the dyad and its hybrid nanocomposite systems. Some quantum chemical calculations have also been done using Gaussian 03 software to support the experimental findings by theoretical point of view. Both from the theoretical predictions and NMR studies it reveals that in the ground state only extended (E-type or trans-type) conformation of the dyad exists whereas on photoexcitation these elongated conformers are converted into folded forms (Z- or cis-type) of the dyad, showing its photoswitchable character. Time resolved fluorescence spectroscopic (fluorescence lifetime by TCSPC method) measurements demonstrate that in chloroform medium all the organic-inorganic hybrid nanocomposites, studied in the present investigation, possess larger amount of extended conformers relative to folded ones, even in the excited singlet state. This indicates the possibility of slower energy destructive charge recombination rates relative to the rate processes associate with charge-separation within the dyad. It was found that in CHCl₃ medium, the computed charge separation rate was found to be ～10⁸ s^?1 for the dyad alone and other hybrid nanocomposite systems. The rate is found to be faster than the energy wasting charge recombination rate ～10^2-10 ¹ s^?1, as observed from the transient absorption measurements for the corresponding hybrid systems. It indicates the conformational geometry has a great effect on the charge-separation and recombination rate processes. The suitability for the construction of efficient light energy conversion devices especially with Ag-Dyad nanocomposite of all the systems studied here is hinted from the observed long ion-pair lifetime. Copyright

Full text of DOI:10.1166/jnn.2012.6208

Copyright © 2012 American Scientific Publishers
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Printed in the United States of America
Journal of
Nanoscience and Nanotechnology
Vol. 12, 4591–4600, 2012
Synthesis, Characterization, Photophysical
Properties of a Novel Organic Photoswitchable
Dyad in Its Pristine and Hybrid Nanocomposite Forms
Gopa Mandal1, Amrita Chakraborty2, Ujjal Kumar Sur3, Balaprasad Ankamwar4ꢀ∗,
Asish De2, and Tapan Ganguly2ꢀ∗ꢀ†
1Department of Physics, Gobardanga Hindu College, North 24 Parganas, 743273, India
2Indian Association for the Cultivation of Science Jadavpur, Kolkata 700032, India
3Department of Chemistry, Behala College, Kolkata 700060, India
4Bio-Inspired Materials Science Laboratory, Department of Chemistry, University of Pune, Ganeshkhind, Pune 411007, India
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In the present paper the method of synthesis and characterization of a novel organic dyad,
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3-(1-Methoxy-3,4-dihydro-naphthalyn-2-yl-)-1-p-chlorophenyl propenone, have been reported. In
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this paper our main thrust is to Sfaubnric,a0te7 nOecwt h2y0b1ri2d 0n4an:0oc1o:m21posites by combining the organic
dyad with different noble metals, semiconductor nanoparticle and noble metal-semiconductor
core/shell nanocomposites. In this organic dyad, donor part is 1-Methoxy-3, 4-dihydro-naphthalen-
2-carboxaldehyde with the acceptor p-chloroacetophenone. We have carried out steady state and
time-resolved spectroscopic measurements on the dyad and its hybrid nanocomposite systems.
Some quantum chemical calculations have also been done using Gaussian 03 software to support
the experimental findings by theoretical point of view. Both from the theoretical predictions and NMR
studies it reveals that in the ground state only extended (E-type or trans-type) conformation of the
dyad exists whereas on photoexcitation these elongated conformers are converted into folded forms
(Z- or cis-type) of the dyad, showing its photoswitchable character. Time resolved fluorescence
spectroscopic (fluorescence lifetime by TCSPC method) measurements demonstrate that in chloro-
form medium all the organic–inorganic hybrid nanocomposites, studied in the present investigation,
possess larger amount of extended conformers relative to folded ones, even in the excited singlet
state. This indicates the possibility of slower energy destructive charge recombination rates relative
to the rate processes associate with charge-separation within the dyad. It was found that in CHCl3
medium, the computed charge separation rate was found to be ∼108 s−1 for the dyad alone and
other hybrid nanocomposite systems. The rate is found to be faster than the energy wasting charge
recombination rate ∼102–101 s−1, as observed from the transient absorption measurements for the
corresponding hybrid systems. It indicates the conformational geometry has a great effect on the
charge-separation and recombination rate processes. The suitability for the construction of efficient
light energy conversion devices especially with Ag-Dyad nanocomposite of all the systems studied
here is hinted from the observed long ion-pair lifetime.
Keywords: Hybrid Nanocomposite, Nanoparticle, Organic Dyad, Core/Shell Nanoparticle,
Photoinduced Electron Transfer, Charge Recombination.
1. INTRODUCTION
technological importance due to their potential applications
in light emitting diode (LED),10 photovoltaic cells,11ꢀ12
organic solar cells13–22 etc. The function of present day
solar cell can be understood by a mechanism known as
‘photoinduced electron transfer’ (PET)23ꢀ24 which takes
place when certain photoactive materials interact with
light. In the existing solar cells, PET takes place between
the organic molecules and nanocrystalline inorganic phase
Hybrid organic–inorganic nanocomposites have attracted
considerable interest due to their promising optoelec-
tronic properties.1–9 These hybrid systems have great
∗Authors to whom correspondence should be addressed.
†Present address: Department of Physics, Bengal Engineering and
Science University, Shibpur (BESUS), Howrah 711103, India.
J. Nanosci. Nanotechnol. 2012, Vol. 12, No. 6
1533-4880/2012/12/4591/010
doi:10.1166/jnn.2012.6208
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Photoswitchable Dyad in Its Pristine and Hybrid Nanocomposite Forms
Mandal et al.
to generate the charge carriers. So far, several electron
acceptors have been shown to yield efficient devices in
solar cell applications: conjugated polymers, fullerenes,
and inorganic nanocrystals. In the class of inorganic accep-
tors, metal oxides are among the most studied materi-
als. Titanium dioxide (TiO2ꢁ has been studied in several
forms: nanoparticles, porous networks, and in situ forma-
tion of titanium dioxide from a precursor.25 The mecha-
nism of charge separation at the organic polymer/inorganic
semiconductor interface has been the subject of intense
studies. Besides the application-oriented interest, funda-
mental studies on the coupling between Frenkel excitons
on organic molecules and Wannier excitons in inorganic
semiconductors have gained enormous attention.26–30 Non-
radiative energy transfer lies at the heart of many of the
optoelectronic properties of these hybrid materials.
2. EXPERIMENTAL DETAILS
2.1. Materials
The procedure of synthesis of the donor 1-methoxy-3,
4-dihydro-naphthalen-2-carboxaldehyde (1MNTCHO) and
the dyad 1-(4-chloro-phenyl)-3-(1-methoxy-3,4-dihydro-
naphthalen-2-yl) propenone (MNCADH) are given below
and p-chloroacetophenone (PCA) is supplied from
Aldrich, and used after distillation. Analytical Reagent
(A.R.) grade titanium isopropoxide (TIP) (Aldrich),
chloroauric acid (HAuCl4ꢁ, AgNO3 (Merck), acetylace-
tone (Merck), Ammonia solution (Rankem 25%) were
purchased for nanoparticle synthesis. 2-butanol (Merck),
2-propanol (Merck), acetic acid (SRL) purchased were
high purity grade. The aqueous solution was prepared
using Millipore water. The solvent chloroform (CHCl3ꢁ of
spectroscopic grade, purchased from Aldrich, have been
distilled under vacuum according to the standard proce-
dure and tested before use to verify the absence of any
This kind of hybrid organic–inorganic systems can pro-
vide a cost-effective alternative to conventional solar cell
made of inorganic material like silicon. This is known as
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impurity emission in the concerned wavelength region. All
the solutions prepared for room temperature measurements
were deoxygenated by purging with an argon gas stream
for about 30 minutes.
hybrid solar cell. As hybrid solar cell consists of both
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organic and inorganic materials, therefore it combines
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the unique properties of inorganic semiconSduunc,to0rs7 wOictht 2012 04:01:21
the film forming properties of the conjugated polymers.
Organic materials are inexpensive, easily processable and
their functionality can be tailored by molecular design and
chemical synthesis. On the other hand, inorganic semicon-
ductors can be fabricated in the form of nanoparticles by
offering the advantage of having high absorption coeffi-
cients and size tunability.
2.1.1. Synthesis and Characterization of the
Organic Dyad
The dyad 1-(4-chloro-phenyl)-3-(1-methoxy-3,4-dihydro-
naphthalen-2-yl) propenone (MNCADH) was synthesized
from commercially available 1-tetralone in three steps as
shown in the Scheme 1.
Lately our research group is involved in the development
of organic–inorganic nanocomposite systems by combin-
ing some organic dyads with nanoparticles31a or core/shell
nanoconjugates.31b In the dyad systems, the donor and
acceptor moieties are connected by short spacers, instead of
long-chain spacers which were used by several workers in
this field. The utility of short spacer was also discussed by
Ganguly et al.31a as well as Ohkubo et al.32 to attain long-
lived charge-separated species. The short spacer has some
advantages over long-chain spacer. Firstly, multi-step elec-
tron transfer process, which is responsible for tremendous
loss of energy, could be avoided here. Moreover, the syn-
thesis cost of short-chain dyad is much less than the long-
chain dyads, tetrads, pentads etc. Thus it could be expected
that the nanocomposites being combined with short-chain
dyad may be useful for the development of efficient and
low cost light energy conversion devices.
We have synthesized a short-chain organic dyad and
developed several hybrid organic–inorganic nanocompos-
ites by combining this dyad with Au, Ag, TiO2 and
Ag@TiO2 core/shell nanoparticles. The results observed
from our steady state and time-resolved spectroscopic
measurements can be utilized to evaluate the capability of
these hybrid nanocomposites to fight against energy crisis
in the form of artificial light energy conversion devices.
Vilsmeier-Haack reaction using freshly distilled phos-
phorous oxychloride and N,N-dimethylformamide (freshly
distilled from calcium hydride), smoothly converted 1-
tetralone (also freshly distilled) to 1-chloro-3,4-dihydro-
naphthlene-2-carboxaldehyde. The chlorovinyl compound
obtained as lightly coloured oil could be character-
ized from spectral data and was sufficiently pure for
the next step which consisted of nucleophilic displace-
ment of chlorine by methoxy through treatment with
methanolic sodium methoxide. The resulting 1-methoxy-
3,4-dihydro-naphthalene-2-carboxaldehyde (1MNTCHO)
was characterized from its spectral data. Subjecting
the dihydro compound to aldol condensation with
4-chloroacetophenone in the presence of sodium hydrox-
ide resulted the target molecule, 1-(4-chloro-phenyl)-3-(1-
methoxy-3,4-dihydro-naphthalen-2-yl) propenone. It was
found that from the coupling constants of the olifinic pro-
tons (J = 15 Hz) that the dyad was present exclusively as
trans geometrical isomer.
(a) Synthesis of 1-Chloro-3,4-Dihydro-Naphthalene-2-
Carboxaldehyde. Freshly distilled phosphorous oxychlo-
ride (3 g, 5.4 ml) was added drop wise to a magnetically
stirred dry DMF under anhydrous condition and external
cooling (ice-salt bath) at such rate that the temperature
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Mandal et al.
Photoswitchable Dyad in Its Pristine and Hybrid Nanocomposite Forms
OMe
Cl
O
CHO
CHO
(i) POCl3
(ii) DMF
(iii) NaOMe
(iv) MeOH
(v) 4-
chloroaccetophenone
(vi) NaOH
Cl
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O
Scheme 1. The synthesis of the dyad 1-(4-chloro-phenyl)-3-(1-methoxy-3,4-dihydro-naphthalen-2-yl) propenone (MNCADH) from commercially
available 1-tetralone.
ꢀ
ꢀ
remains between −5 C to 0 C. After the addition was
reaction mixture was poured into crushed ice and acidi-
fied with dilute hydrochloric acid. After extraction with
ether, the ethereal layer has washed with water and dried
over sodium sulfate. Removal of solvent left a dark red
oil, which indicated a mixture of the desire product and
some uncharacterized impurities from the thin layer chro-
complete, the reaction mixture was stirred at 0 ꢀC for
ꢀ
30 minutes and at 80 C for 90 minutes. After cooling to
room temperature, the reaction mixture was poured into
cold aqueous solution of sodium acetate (25%, 50 ml).
After extraction with dichloromethane, the organic layer
was washed with water and dried over sodium sulfate.
Removal of solvent left reddish oil (3.16 gm) which was
1
matography and H NMR spectrum. The pure compound
was isolated from column chromatoghaphy of the crude
product upon elution with ethyl-acetate-petroleum ether
1
sufficiently pure for the next stage. The H NMR spec-
1
trum in CDCl3, showed all the peaks characteristic of the
designed product. ꢂ (ppm) 10.29 (s, 1H, CHO) 7.79 (dd,
1H, H-8, J = 8.1, 1.2) 7.54–7.07 (m, 3H, H-5, H-6, H-7)
2.85–2.76 (m, 2H, H-3), 2.68–2.55 (m, 2H, H-4).
(5:95) and was characterized from H NMR spectrum.
IR (neat) 1656 cm−1. H NMR (CDCl3) ꢂ (ppm) 10.2
1
(s, 1H, CHO) 7.49 (dd, 1H, J = 7.8, 1.5, H-8), 7.303-7.105
(m, 2H, H-5, H-6, H-7) 3.83 (s, 3H, OMe) 2.71(m, 2H,
H-3), 2.46 (m, 2H, H-4).
(b)
1-Methoxy-3,4-Dihydro-Naphthalene-2-Carbox-
aldehyde. Sodium (0.36 gm, 1.55 mmole) was added in
small portions to Mg-dry methanol (25 ml) under anhy-
drous condition and external cooling (ice bath). A solution
(c)
1-(4-Chloro-Phenyl)-3-(1-Methoxy-3,4-Dihydro-
Naphthalen-2-yl) Propenone. Sodium (0.07 gm, 0.003 M)
was dissolved in dry methanol under magnetic stirring in
an argon atmospheres with external cooling (ice bath). To
the methanolic sodium methoxide, 4-chloroaccetophenone
(0.464 gm, 0.003 M) was added from a syringe over
15 minutes. The reaction mixture was stirred for 45 minutes
in an inert atmosphere and after this period 1-methoxy-3,
4-dihydro-naphthalene-2-carbaldehyde, dissolved in dry
of
1-chloro-3,4-dihydro-naphthalene-2-carboxaldehyde
(3 g, 1.55 mmole) in Mg-dry methanol (20 ml) was added
drop wise to the magnetically stirred methanolic sodium
methoxide. The reaction mixture was further stirred for
30 minutes after the addition was complete followed by
heating under reflux for another 1 hr. After cooling, the
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Photoswitchable Dyad in Its Pristine and Hybrid Nanocomposite Forms
Mandal et al.
methanol (10 ml) was added from a syringe over 10 min-
utes. It was stirred further for 30 minutes followed by
heating under reflux for 5 hrs. After cooling, methanol was
removed under reduced pressure. The reaction mixture
was extracted with ether and the etherial layer was washed
with water. After drying over sodium sulfate, solvent was
removed. The pure compound was isolated from the crude
residue chromatographically over a silica gel column. The
dyad was eluted with ethyl-acetate-petroleum ether (2:98)
and was characterized spectroscopically. Yield 0.5 gm
(55%).
addition of toluene. The precipitate was washed repeatedly
with toluene and redissolved in 2-propanol. Dry powders
were not redispersible (thus, the 2-propanol dispersions
contained traces of toluene). The solutions (mother liquor
as well as redispersed particles) were stable for over two
months. In the present study, fresh colloidal Ag@TiO2
dispersed in chloroform was prepared before each set of
measurements. The stock solution was diluted with chlo-
roform to obtain the desired concentrations of Ag@TiO2.
No attempts were made to exclude the traces of 2-propanol
(∼0.4%) present in the colloidal Ag@TiO2 and it was con-
firmed separately that the presence of 2-propanol did not
affect the photochemical measurements.
The size of the Ag@TiO2 core shell nanoparticles was
also estimated from high resolution Transmission Electron
Micrograph (TEM). The core (Ag) diameter varies in the
range of 20–30 nm (Fig. 1(E)). The shell (TiO2ꢁ thickness
is 2–6 nm. The average diameter of Ag@TiO2 nanoparticle
is found to be <40 nm.
1
IR ꢃmax 1726 cm−1. H NMR (CDCl3) 8.21 (d, 1H,
H-3, J = 15.6), 7.92 (d, 1H, H-, J = 8.4) 7.5–7.19 (m,
8H, aromatic protons), 6.96 (d, 1H, H-2, J = 15.3) 3.802
(s, 3H, OMe) 2.88 (m, 2H, H-3 or H-4), 2.61 (m, 2H, H-3
or H-4).
Preparation of Ag Nanoparticles: Ag nanoparticles were
synthesized from Murraya Koenigi leaf extract33 as the
natural source of reducing and capping biomolecules. The
broth used for the reduction of Ag+ ions to Ag was pre-
pared by taking 10 gm of thoroughly washed Murraya
koenigi leaves in a 500 ml Erlenmeyer flasks with 40 ml
of sterile distilled water and boiled it for 15 min. On cool-
ing to room temperature, it was filtered through four fold
muslin cloth and used for the synthesis of silver nanopar-
ticles. In a typical experiment, 0.4 ml of broth was added
to 9 ml of 10−3 M aqueous silver nitrates solution fol-
lowed by addition of 0.04 ml (0.625%) ammonia solution.
After shaking, it was kept for 6 hours at room temperature
followed by centrifugation at 10000 rpm for 25 min. The
pellets were redispersed in millipore water and centrifuged
again at 10000 rpm for 10 min and collected pellets were
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Preparation of Au Nanoparticles: Au nanoparticles
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were prepared using Lemongrass extract.34 Tamarind leaf
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extract35 and Emblica officinalis fruit extract36 as natural
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source of reducing and capping biomolecules. Briefly, the
broth used for the reduction of Au3+ ions to Au0 was pre-
pared by taking 50 g of thoroughly washed tamarind leaves
as earlier reported by Ankamwar et al.35 in a 500 mL
Erlenmeyer flask with 200 mL of sterile distilled water;
this mixture was then boiled for 5 min. 10 mL of this broth
was added to 45 mL of 1 × 10−3 M aqueous chloroauric
acid (HAuCl4ꢁ solution at room temperature. The detailed
synthesis of this nanoparticle is described elsewhere.35 The
size of the Au nanoparticles was estimated from High Res-
olution Transmission Electron Microscope (HRTEM) and
also from Atomic Force Microscope (AFM). Figure 1(B)
illustrates the HRTEM and AFM images of the synthe-
sized single nanoparticle. From both the TEM and AFM
images, it can be seen that Au nanoparticles are predom-
inantly triangular in morphology. AFM analysis (surface
profile analysis) of one single Au nanotriangle indicated
that the particle has a thickness of 5 nm and edge length
of 100 nm.
ꢀ
dried in hot air oven at 65 C for 6 hrs and character-
ized. The size of the Ag nanoparticle was estimated from
HRTEM. Figure 1(C) shows the HRTEM images of the Ag
nanoparticles. The TEM image shows that particle diame-
ter varies in the range of 20–40 nm. The average diameter
of silver nanoparticle is found to be ∼30 nm.
Preparation of TiO2 Nanoparticles: The procedure for
synthesizing TiO2 nanoparticles (Fig. 1(D)) is described in
our earlier paper.31a
Preparation of Ag@TiO2 Nanoparticles: To synthesize
the Ag@TiO2 nanocomposites,31b a solution containing
equimolar (19.9 mM) amounts of titanium isopropoxide
and acetylacetone in 2-propanol was prepared. A clear
solution was formed upon mild sonication. Another solu-
tion of 8.80 mM AgNO3 and 13.88 M H2O in DMF was
prepared. A 40 ml sample of the first solution and 20 ml
of the second solution were mixed and stirred for about
10 min. The mixture was transferred to a heating mantle
and refluxed for 45 min. The solution became green-black
in the case of Ag. The color change was gradual in the
case of Ag. Further refluxing of the solution resulted in
the formation of a precipitate, which could be dispersed by
sonication. The colloidal material was precipitated by the
2.2. Spectroscopic Measurements
At the ambient temperature (296 K) steady state UV-vis
electronic absorption and fluorescence emission spec-
tra of dilute solutions (10−4–10−6 M) of the samples
were recorded by using 1 cm path length rectangular
quartz cells by means of an absorption spectrophotome-
ter (Shimadzu UV-Vis 2101PC) and F-4500 fluorescence
spectrophotometer (Hitachi), respectively. Fluorescence
lifetimes were measured by using a Time-Correlated Sin-
gle Photon Counting (TCSPC) method by using HORIBA
JOBIN YVON FLUOROCUBE. The goodness of fit has
been assessed with the help of statistical parameters ꢄ2
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Mandal et al.
Photoswitchable Dyad in Its Pristine and Hybrid Nanocomposite Forms
1-MNTCHO
PCA
(A)
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(B)
(C)
TiO2
Ag
(D)
(E)
Fig. 1. (A) Molecular structure of the compounds MNCADH, PCA and 1 MNTCHO. (B) HRTEM micrograph of a single gold nanotriangle; AFM
image of the biologically synthesized gold nanotriangle along with the surface height profile. (C) Representative TEM images of MK leaf extract-
reduced silver nanoparticles. (D) HRTEM picture of a single TiO2 nanoparticle. (E) TEM picture of Ag@TiO2 core/shell nanoparticles.
and DW. All the solutions prepared for room tempera-
ture measurements were deoxygenated by purging with
an argon gas stream for about 30 min. The size of
the synthesized nanoparticles was estimated by using
HRTEM (JEOL, Model JEM-2010). AFM image of Au
nanoparticle was obtained using a multimode AFM (Vecco
Metrology, Autoprobe diCP-II, Model No. AP0100) at
ambient temperature with the help of silicon (Si) probes
(RTESPA-M, Vecco, Santa Barbara, CA) in tapping (NC-
AFM) mode. Long tips (aspect ratio 4:1) cantilever (Mate-
rial: 1–10 ꢅ-cm phosphorous (n) doped Si) with spring
constants ranging from 20 to 80 N/m and resonance fre-
quencies of 245–285 kHz were used to image the surface
morphology of the nanoparticle. Proscan Image Processing
Programme (PIPP) software provided by the manufacturer
was used to measure the thickness and edge length of the
Au nanoparticle.
2.3. Laser Flash Photolysis (LFP)
Nanosecond flash photolysis set-up (LFP) (Applied Photo-
physics) 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 ns) laser light. Transients were moni-
tored through absorption of light from a pulsed xenon lamp
(250 W). The photomultiplier (IP28) output was fed into
a Tektronix oscilloscope (TDS 3054B, 500 MHz, 5 Gs/s),
and the data were transferred to a computer using the
TekVISA software. The software Origin 6.0 was used for
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Photoswitchable Dyad in Its Pristine and Hybrid Nanocomposite Forms
Mandal et al.
curve fitting. The solid curves are obtained by connecting
the points by using B-Spline option. The samples were
deaerated by passing pure Argon gas stream for 30 min
prior to each experiment. No degradation of the samples
was observed during the experiment.
confirmed from solvent effect studies. It is observed that
with increasing the polarity of the solvents emission max-
imum of MNCADH at 490 nm clearly shifts towards red
side, the shift being considerably larger than that observed
in the solvent shifts of the absorption bands. This con-
firms the charge transfer nature of the fluorescence emis-
sion band produced due to the excitation of the ground
state CT absorption spectrum.
3. RESULTS AND DISCUSSION
From all the experimental measurements it is demon-
strated that dyad MNCADH is dipolar in character
and intramolecular charge separation occurs within the
donor 1MNTCHO and acceptor PCA parts of the dyad
MNCADH after photoexcitation of the ground state CT
band.
3.1. UV-Vis Absorption and Steady State
Fluorescence Emission Spectra in Different
Solvents at the Ambient Temperature
UV-vis absorption spectra of the novel synthesized dyad
MNCADH (Fig. 1(A)) have been recorded in various
solvents including polar aprotic acetonitrile (ACN). It is
apparent from the Figure 2 that in case of the dyad a new
broad absorption band (ꢆabs of peak position ∼375 nm)
is developed. This band is red shifted compared to the
absorption bands of donor (ꢆabs∼317 nm) and acceptor
As it was observed earlier by our research group,37
similar isosteric systems exhibit primarily elongated
i.e., E-type conformation in the ground state but upon
photoexcitation extended conformation converts into the
form of folded nature. The folded conformation facilitates
the spatial overlapping of the orbitals of the donor and
acceptor molecules which results in occurrence of energy
destructive charge recombination process efficiently. In the
case of the present dyad MNCADH also, NMR study
shows the existence of only one conformer of trans nature,
as evidenced from the value of the coupling constant, J.
From the coupling constants of the olefinic protons (J =
15 Hz) it was inferred that the dyad present in the ground
state is exclusively trans geometrical isomer or E-type
(elongated nature).
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(ꢆabs∼249 nm) measured in the same solvent ACN. More-
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over, no broad red shifted band appears in the UV-vis spec-
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acceptor). Instead the spectra show the superposition of the
corresponding spectra of the donor and acceptor moieties.
Further, from Figure 2 it appears that with increase of the
polarity of the medium, the long wavelength absorption
band of MNCADH at 375 nm suffers red shift indicating
the charge transfer (CT) nature of 375 nm absorption band
of the dyad.
When the CT absorption band is excited at 375 nm,
a fluorescence emission spectrum of MNCADH peaking
at about 490 nm is developed. Thus this band should be
assigned as CT emission band. This proposition is also
From quantum chemical calculations at DFT level
using Gaussian 03 software it was observed though the
energy difference between trans and cis conformer is
4.7 Kcal/mole but the trans-cis interconversion energy bar-
rier is found to be 64 Kcal/mol in ground state at the HF/6-
31 g level. This ground state high energy barrier may be
responsible for the hindrance of the trans-cis interconver-
sion resulting in the formation of exclusively trans isomer
only. This is actually observed from NMR study.
1MNTCHO
PCA
CHCl3
Cyclohexane
MNCADH
ACN
MeOH
3.2. Time Resolved Fluorescence
Measurements by TCSPC Method
300
400
Wavelength
Time resolved fluorescence measurements on the dyad
clearly demonstrate the presence of two distinct species
which might be due to the two different conformers: elon-
gated (E-type) and folded (Z-type) nature. The best fit of
the measured fluorescence decays exhibit the two differ-
ent components, one around 1.7 ns and the other 0.025 ns
(i.e., 25 ps order) (Table I). The interesting behavior is
observed in presence of ꢇ-cyclodextrin (CD). It is appar-
ent that with increase of CD concentrations (Table I) the
fractional contribution associated with the longer compo-
nent increases with the concomitant reduction of the corre-
sponding parameter associated with the shorter ps species.
As it is well known38 that encapsulations of the dyad in
280
350
420
Wavelength (nm)
Fig. 2. Absorption spectra of MNCADH in different solvents. Inset:
Absorption spectra of donor 1 MNTCHO, acceptor PCA and dyad
MNCADH in ACN solvent.
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Mandal et al.
Photoswitchable Dyad in Its Pristine and Hybrid Nanocomposite Forms
Table I. Changes of fractional contributions (fiꢁ associated with fluorescence lifetimes of the different conformers (E- and Z-type) of the dyad
in presence of CD and different noble metals, semiconductor nanoparticle TiO2 and Ag@TiO2 core/shell nanocomposite at the ambient temperature
(ꢆexc∼375 nm, ꢆem∼490 nm).
∗∗
Dyad MNCADH in different media
ꢈ1/ns
f1
ꢈ2/ns
f2
ꢈ
kCR(S−1ꢁ (∼1/ꢈip)
ip
MeOH
1.78
1.77
1.80
0.11
0.19
0.25
0.025
0.023
0.027
0.89
0.81
0.75
MeOH+0.48 mM ‘CD
MeOH+0.89 mM CD
CHCl3
1.48
1.53
1.19
1.50
1.42
0.79
0.74
1.0
0.57
0.64
0.053
0.049
—
0.050
0.053
0.21
0.26
—
0.43
0.36
8 ms
33 ms
11 ms
0.13×103
0.30×102
0.91×102
CHCl3 +Au
CHCl3 +Ag
CHCl3 +TiO2
CHCl3 +Ag@TiO2
∗∗The values (obtained from transient absorption decays) (see text) of ion-pair lifetime (ꢈipꢁ and approximate rate of charge recombination processes kCR (≈1/ꢈipꢁ for the
dyad MNCADH in CHCl3 and when combined with noble metal Ag nanoparticles and core/shell Ag@TiO2 nanocomposite system.
CD cavity populate artificially the elongated conformation
in the excited CT state due to inclusion complex forma-
tion, it appears that the longer component corresponds to
the extended (E-) conformers whereas the shorter compo-
organic dyad system MNCADH form hybrid nanocompos-
ite systems.
3.3.2. Time Resolved Fluorescence
Delivered by Ingenta to:
nent having ps order should be due to folded or Z-type
Rice University, Fondren Library
species.
From Table I it is apparent that if the nature of the
solvent is changed from MeOH to non-hydrogen bond-
IP : 93.80.195.225
Sun, 07 Oct 2012 04:01:21
ing solvent CHCl3, the majority dyad species remain-
ing in the excited state possess extended conformation
(79%). Thus, even after photoexcitation, the photoswitch-
able dyad prefers to retain its elongated conformation
in CHCl3 environment. The situation was different when
the surrounding medium was MeOH where folded con-
formational species in the excited state were found to
dominate from time resolved studies. Thus the CHCl3
appears to be much better medium relative to MeOH as
the former medium hinders the formations of folded struc-
ture which facilitates the energy wasting and destructive
charge recombination process. Further, in CHCl3 medium
when the dyad was combined with noble metal nanopar-
ticles interesting observations were made. In case of Au
nanoparticles, the percentage of the folded and extended
conformations remain more-or-less same as they were in
absence of this noble metal nanoparticles. When the other
noble metal Ag is combined with the dyad, significant
increase of extended conformations (f1∼100%) indicates
that this organic–inorganic nanocomposite may serve as a
better light energy conversion devices relative to the device
which could be built by replacing Ag with Au. It is known
that folded conformation favors the energy wasting charge
recombination process due to orbital overlapping of the
two reactants (donor and acceptor) being relatively closer
to each other.
3.3. Effect of Noble Metal Nanoparticles,
Nano Semiconductor TiO2 and Noble
Metal-Semiconductor Nanocomposites on
the Different Conformers, E- and Z-, of the
Synthesized Photoswitchable Dyad
3.3.1. Steady State Fluorescence Measurements
Similar spectral behavior of the dyad i.e., gradual quench-
ing of fluorescence intensity of MNCADH with increasing
the concentrations of Au, Ag, TiO2, Ag@TiO2 core/shell
nanoparticles has been observed. Figure 3 exhibits the low-
ering of emission intensity of MNCADH with addition
of different concentrations of TiO2. The above phenom-
ena indicate that these nanoparticles when combine with
1
50
7
The reverse situation from Ag nanocomposites was
observed when the dyad is combined with semiconductor
TiO2 nanoparticles (Table I). For this composite system,
the folded conformation grows significantly (f2 increases)
at the expense of the elongated conformational species
(lowering of f1ꢁ as it evidenced from the changes in the
fractional contributions associated with the fluorescence
0
450
500
550
600
Wavelength (nm)
Fig. 3. Emission spectra of MNCADH after addition of different con-
centration of TiO2 nanoparticles. (1) 0, (2) 5.6 mM, (3) 16.8 mM,
(4) 27.8 mM, (5) 38 mM, (6) 0.058 M, (7) 0.08 M.
J. Nanosci. Nanotechnol. 12, 4591–4600, 2012
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Photoswitchable Dyad in Its Pristine and Hybrid Nanocomposite Forms
Mandal et al.
lifetimes of the two different species. The similar role
of TiO2 was also reported earlier by our research group
in case of another photoswitchable dyad MNBA.39 Thus
the noble metal nanoparticles Ag and the semiconductor
TiO2 nanoparticles behave differently. However, when the
dyad is combined with Ag (metal)-TiO2 (semiconductor)
core/shell nanocomposite system (Ag@TiO2ꢁ, the situa-
tion looks much better from only TiO2 system. The con-
centrations of the folded species appear to be dropped
down and become significantly smaller than that of the
extended conformational species. Thus the presence of the
metal Ag nanoparticles plays a constructive role in this
core/shell nanocomposite in inhibiting energy destructive
charge recombination process and upgrading the dyad–
core/shell nanocomposite system to a better artificial light
energy conversion device. The hybrid of Ag and organic
dyad shows nearly 100% elongated conformations of the
latter. This predicts the dyad when combined with the
metal Ag, chance of charge recombination becomes very
of Nd:YAG laser was used to excite the ground CT band
of the dyad. It appears from the transient absorption decay
profiles that the dyad when combined with the noble metal
Ag nanoparticles, the ion-pair lifetime, ꢈip becomes sig-
nificantly larger (∼33 ms) than the situation when the
former combines with Ag@TiO2 core/shell nanocompos-
ites (11 ms, Table I). Thus in presence of the noble
metal Ag nanoparticles the survival duration of the charge-
separated species within the dyad significantly increases.
This observation appears to be due to the elongated con-
formation of the dyad which is largely dominating in this
environment, as it is observed from the time resolved
fluorescence studies discussed above. Thus, the energy
destructive kCRꢊ∼1/ꢈipꢁ possesses slowest rate in case of
the hybrid Ag-dyad nanocomposite system (Table I). It
indicates the suitability for the construction of efficient
light energy conversion devices especially with Ag-Dyad
nanocomposite of all the systems studied here.
As the transient absorption decay profiles have been
measured by using nanosecond laser flash photolysis tech-
niques by using the delays in microsecond orders, it
could be inferred that the excited singlet charge-separated
species formed from the excitation of ground state CT
complex relaxes to the corresponding triplet state by the
intersystem crossing (isc) process. In the CT triplet level,
further charge separations lead to the formations of charge-
separated species whose level becomes further stabilized
and closer to TiO2 conduction band (Scheme 2). It appears
that the partially charged donor, 1 MNTCHO, in CT triplet
level, may prefer to donate electrons to the conduction
band of TiO2 rather than to the acceptor PCA. From ener-
getic consideration this route for electron transfer appears
to be possible. However, there is a possibility that from
TiO2 some electrons may return back to the triplet level
of charge-separated species causing recombination with
the donor cation. In Ag-dyad nanocomposite, due to the
accumulation of the electrons in the much lower Fermi
level of Ag, charge recombination process will be sig-
nificantly impeded. This is probably the reason for the
observed significant enhancement of the longivity of the
charge-separated species within the dyad MNCADH when
combined with noble metal Ag nanoparticles only.
Delivered by Ingenta to:
slim as majority contribution in emission originating from
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extended conformation where donor and acceptor being
IP : 93.80.195.225
far apart suffer lack of overlapping with each other. The
Sun, 07 Oct 2012 04:01:21
observation indicates the suitability for the construction of
efficient light energy conversion devices taking especially
Ag-dyad nanocomposite of all the systems studied in the
present investigation.
3.3.3. Transient Absorption Spectra by Nanosecond
Laser Flash Photolysis
The transient absorption decay profiles of the dyad were
measured firstly in chloroform (Fig. 4) and then in mix-
tures of chloroform-Ag nanoparticles and chloroform-
Ag@TiO2 core/shell nanocomposites. The third harmonic
–4.8
–5.4
0.009
–6.0
–6.6
–7.2
0.006
0.0 9.0x10–7 1.8x10–6 2.7x10–6
Time / sec
In CHCl3 medium the fluorescence lifetime of the free
donor 1MNTCHO is determined to be 1.62 ns (ꢈf ) and
the quenched lifetime of the donor when present in dyad
is observed to be 1.18 ns (ꢈqꢁ. The charge separation rate
kCS is estimated to be 2.3×108 s−1 from the well known
relation:
0.003
0.000
kCS = 1/ꢈq −1/ꢈf
0.0
3.0x10–6
6.0x10–6
9.0x10–6
In all the hybrid nanocomposite systems i.e., when
the dyad is combined with noble metal/semiconductor
systems, the charge-separation rates kCS were found to
be nearly the same order of magnitude (∼108 s−1ꢁ and
the charge recombination rates, as observed from tran-
sient absorption measurements by laser flash photolysis
Time/sec
Fig. 4. Transient absorption decay profile of the dyad MNCADH when
combined with noble metal Ag nanoparticles (Inset: plot of ln (ꢉA) as a
function of delay times, obtained by subtracting the constant value of an
absorbance at long delay time from the observed decay curve).
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Mandal et al.
Photoswitchable Dyad in Its Pristine and Hybrid Nanocomposite Forms
1(D+δ-A–δ)*
ISC
3(D+δ-A–δ)*
e–
3(D+…A–)*
e–
(Donor
recombination)
e-
CB
Ag
TiO2
e–
VB
D+δ-A–δ
(Ground state CT of organic dyad)
Scheme 2. Pathways of charge separation and charge recombination within the dyad MNCADH and Ag@TiO2 core/shell nanocomposite systems.
technique vary within 103 to 102 s−1. The slowest charge
species (∼33 ms) within the dyad MNCADH. Ag-Dyad
Delivered by Ingenta to:
recombination rate (∼0.3×102 s−1ꢁ was observed, as dis-
cussed above, for Ag-Dyad hybrid nanocomposite system.
Thus all the systems studied here appear to be good can-
hybrid nanocomposite appears to be the best candidate
Rice University, Fondren Library
amongst all the systems studied here for construction of
IP : 93.80.195.225
efficient light energy conversion devices.
Sun, 07 Oct 2012 04:01:21
didates, from the appearance of faster charge-separation
and much slower energy destructive charge recombina-
tion rates, for designing efficient light energy conversion
devices. The suitability for the construction of efficient
light energy conversion devices especially with Ag-dyad
nanocomposite of all the systems studied here is apparent
from the present investigation.
Acknowledgments: The authors thank Mr. Subrata Das
of the Department of Spectroscopy for his active assis-
tances in fluorescence lifetime measurements by time-
correlated single photon counting technique. The authors
express their gratitude to Professor Samita Basu of
Saha Institute of Nuclear Physics, Salt lake, Kolkata
for helping in the measurements of transient absorption
spectra by using laser flash photolysis technique. BA
expresses his appreciation to BCUD (BCUD/OSD/390;
dated 16/11/2010), University of Pune for financial sup-
port. UKS acknowledges financial support from the project
funded by the UGC, New Delhi (Grant No. PSW-038/10-
11-ERO). TG specially thanks Department of Science and
Technology (DST), New Delhi, India for Nano-Mission
project (No: SR/NM/NS-51/2010) for supporting the vari-
ous research work conducted in his photophysics and pho-
tochemistry group.
4. CONCLUSIONS
In this paper we have shown that organic dyad molecules
could become potential light energy harvesting materi-
als in the presence of nanocomposites. Though Ag@TiO2
core/shell nanocomposite when combined with the dyad
facilitates the conversions of majority Z-conformers of
this dyad, present in the excited state, to E-type conform-
ers, but the situation becomes more effective when the
dyad combines with only noble metal Ag nanoparticles.
Time resolved fluorescence studies indicate that in the
hybrid of Ag-Dyad nanocomposite system nearly 100%
elongated conformations persist even in the excited state.
Among the nanocomposite systems studied in the present
investigation, it appears from the transient absorption mea-
surements that the dyad when combined with the noble
metal Ag nanoparticles, the ion-pair lifetime, ꢈip becomes
significantly large (∼33 ms). This indicates the rate asso-
ciated with energy destructive charge recombination pro-
cesses kCRꢊ∼1/ꢈipꢁ is slowed down when the medium is
changed from chloroform to the mixture of chloroform and
Ag. Possibly due to the accumulation of the electrons in
the much lower Fermi level of Ag, charge recombination
process has been significantly impeded causing significant
enhancement of the longivity of the charge-separated
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