Comparative analysis to explore the suitability of a short chain dyad in its pristine and nanocomposite forms for designing artificial light energy conversion device

Article abstract of DOI:10.1166/jnn.2018.15533

From the UV-vis, steady state and time resolved spectroscopic investigations on the pristine dyad, dyad-spherical gold nanoparticles (GNP) and dyad-star shaped gold nanoparticles (GNS), it was observed that though in the ground state the dyad in its pristine form possesses trans-type (elongated and planar) isomer but on photoexcitation trans-form converts into cis-structure (folded). Interestingly, the dyad exhibits different behavior when it combines with GNP or GNS. In nanocomposite form, even on photoexcitation some ground state trans-structure still retains its identity in the excited state. The 60% of the trans-species remains unchanged in the excited state due to excitation of dyad-GNS system and possibly this configuration facilitates the hindrance of energy destructive charge recombination processes as in this conformer the donor and acceptor moieties tend to move far away from each other causing lack of overlapping of charge clouds within the two redox components. The dyad-GNS nanocomposite appears to be the best possible light energy conversion or storage device within the three system studied in the present investigation. Investigations are underway to examine how the degree of surface coverage of the dyad on the surface of gold nanoparticles affect its geometry or conformational changes on photoexcitation.

Full text of DOI:10.1166/jnn.2018.15533

Article
Journal of
Nanoscience and Nanotechnology
Vol. 18, 7873–7881, 2018
Copyright © 2018 American Scientific Publishers
All rights reserved
Printed in the United States of America
www.aspbs.com/jnn
Comparative Analysis to Explore the Suitability of a Short
Chain Dyad in Its Pristine and Nanocomposite Forms for
Designing Artificial Light Energy Conversion Device
1
2
1
3
2
Somnath Paul , Ishani Mitra , Rituparna Dutta , Munmun Bardhan , Mridul Bose ,
4
5
1ꢀ∗
Subrata Das , Mithu Saha , and Tapan Ganguly
1
School of Laser Science and Engineering, Jadavpur University, Jadavpur, Kolkata 700032, India
2
Department of Physics, Jadavpur University, Jadavpur, Kolkata 700032, India
3
Saha Institute of Nuclear Physics, Salt Lake, Kolkata 700064, India
4
Department of Spectroscopy, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India
5
State Key Laboratory of Physical Chemistry of Solid Surfaces, Department of Chemistry, Xiamen University, Xiamen-361005, P. R. China
From the UV-vis, steady state and time resolved spectroscopic investigations on the pristine dyad,
dyad-spherical gold nanoparticles (GNP) and dyad-star shaped gold nanoparticles (GNS), it was
observed that though in the ground state the dyad in its pristine form possesses trans-type (elon-
gated and planar) isomer but on photoexcitation trans-form converts into cis-structure (folded).
Interestingly, the dyad exhibits different behavior when it combines with GNP or GNS. In nanocom-
IP: 91.216.3.107 On: Fri, 11 Jan 2019 02:46:23
posite form, even on photoexcitation some ground state trans-structure still retains its identity in
Copyright: American Scientific Publishers
the excited state. The 60% of the trans-species remains unchanged in the excited state due to
Delivered by Ingenta
excitation of dyad-GNS system and possibly this configuration facilitates the hindrance of energy
destructive charge recombination processes as in this conformer the donor and acceptor moieties
tend to move far away from each other causing lack of overlapping of charge clouds within the two
redox components. The dyad-GNS nanocomposite appears to be the best possible light energy
conversion or storage device within the three system studied in the present investigation. Investiga-
tions are underway to examine how the degree of surface coverage of the dyad on the surface of
gold nanoparticles affect its geometry or conformational changes on photoexcitation.
Keywords: Short-Chain Dyad, Gold Nanoparticles, Nanocomposites, Time Resolved
Spectroscopy, Light Energy Converter, Trans- and Cis-Conformer.
1
. INTRODUCTION
systems, they exhibit efficient artificial light energy con-
version materials. The efficiencies of the dyads as artifi-
1
–4
3–5
Lately photoswitchable short-chain dyads are found to
be of very interesting topics for time-resolved spectro-
scopic research due to their wide applications in molecular
electronics, building artificial light energy converters, opti-
cal data storage systems etc. Earlier experimental inves-
tigations exhibit that various short-chain dyad systems
possess two types of conformers in the excited state and
only one conformer in ground state. In recent investi-
gations by steady state and time resolved spectroscopic
techniques it has been observed that when the short-chain
dyads combine with nanoparticles of noble metals such
as silver, gold and gold/silver coreshell nanocomposite
cial light energy converters have been examined from the
two important rate parameters associated with charge sep-
aration and energy destructive charge recombination pro-
cesses. On comparing the results observed in the cases
of the pristine dyads and the corresponding nanocompos-
ites where the dyads combine a nanoparticle i.e., adsorb
on the nanosurface, the conformational changes occur
5
–7
7
from folded nature to extended nature or vice-versa.
In recent studies with the dyad MNTMA, where methoxy-
naphthalene donor is attached with the electron accep-
7
tor p-methoxyacetophenone, it was observed that the
dyad, in its pristine form, does not suffer any confor-
mational changes even on photoexcitation. But when the
∗
Author to whom correspondence should be addressed.
J. Nanosci. Nanotechnol. 2018, Vol. 18, No. 11
1533-4880/2018/18/7873/009
doi:10.1166/jnn.2018.15533
7873

Comparative Analysis to Explore the Suitability of a Short Chain Dyad in Its Pristine and Nanocomposite Forms
Paul et al.
dyad combines with the noble nanometals like gold (Au),
silver (Ag) and Au@Ag core–shell nanocomposite sys-
tems, significant conformational changes in the excited
states occur. In these changes, the elongated conformer
of the dyad facilitates the lowering of the energy destruc-
tive charge recombination rate processes as the two redox
components donor and acceptor move far away from each
other minimizing the overlapping of the charge clouds of
both the redox components. In the present investigation,
a novel synthesized dyad, organic (E)-4-(((9H-fluorene-
and excited state conformers of the pristine dyad and its
nanocomposite forms when the dyad combines with the
GNP and GNS nanoparticles. In the present paper the
results have been discussed in light of energy conver-
sion devices where rates of charge separation and energy
destructive charge recombination processes could be con-
trolled and modulated conformationally.
2
. EXPERIMENTAL DETAILS
.1. Materials
2
2
yl)imino)methyl)-N ,N-dimethylaniline (NNDMBF) has
The method of synthesis and characterization of the short-
chain dyad (E)-4-(((9H-fluorene-2-yl)imino)methyl)-N,N-
dimethylaniline (NNDMBF) is given below. The solvents
acetonitrile (ACN) (SRL), and cyclohexane (CH) (Sigma
Aldrich) of spectroscopic grade were purified following
the standard procedures and tested before use for the
absence of any impurity emission in the concerned wave-
length region. Water was deionized using a Millipore
Milli-Q system. The solutions were prepared by dissolving
the appropriate amount of dyad in ACN and CH.
been chosen where N ,N -dimethyl amino donor (NNDMB)
is being attached with the acceptor fluorene (F) (Fig. 1(a)).
Our primary aim is to perform comparative analysis to
look at the suitability of designing efficient artificial light
energy converter when it is in its pristine form and when
combined with spherical gold (GNP) and gold nanostar
(
GNS) nanoparticles. From the molecular structure of the
dyad, one can presume the possibilities of the two con-
formers: Trans- and Cis- (Fig. 1(b)) of the dyad.
UV-vis absorption, steady state, time resolved spec-
troscopic (fluorescence lifetime and laser flash photoly-
sis) investigations along with theoretical predictions have
been done to reveal the nature of the prevailing ground
2
.2. Spectroscopic Apparatus
UV-vis absorption and steady state fluorescence emission
−4
−6
spectra of dilute solutions (10 –10 M) of the dyad
were recorded at the ambient temperature (296 K) using
1 cm path length rectangular quartz cells by means of
(
a)
an UV-vis absorption spectrophotometer (JASCO UV-Vis
IP: 91.216.3.107 On: Fri, 11 Jan 2019 02:46:23
Copyright: American S ca ibe s no tr i pf i tci o Pn us bp lei sc ht r eo rms eter, Model: V-630) and JASCO spec-
Delivered byt rI no flg ue on rt iam eter Model: 8200) respectively. All the picosec-
ond resolved fluorescence lifetimes were measured by
using a commercially available time-correlated single-
photon counting (TCSPC) setup from Edinburgh instru-
(
b)
8.4 in NMR
ments. Picosecond excitation pulses from the picoquant
diode laser were used at 375 nm and 440 nm with an
IRF of 50 ps. A microchannel-plate-photomultiplier tube
A
H
CH3
CH3
N
N
(
MCP-PMT; Hammamatsu Photonics, Kyoto, Japan) was
Trans
used to detect the photoluminescence from the sample
after dispersion through a monochromator. The observed
fluorescence decays were fitted by using a nonlinear
least-square fitting procedure to a function with software
F900 from Edinburgh Instruments (Livingston, U.K). The
quality of the curve fitting was evaluated by reduced chi-
square and residual data. Nanosecond laser flash photoly-
sis (Applied Photophysics) containing Nd:YAG laser (Lab
series, Model Lab 150, Spectra physics) was used to mea-
sure transient absorption decays.
CH3
N
CH3
N
H
8
.4 in NMR
Cis
(
c)
(d)
All the solutions prepared for room temperature mea-
surements were deoxygenated by purging with an argon
gas stream for about 30 minutes.
2
.3. Synthesis and Characterization of the Organic
Dyad (E)-4-(((9H-fluorene-2yl)imino)methyl)-
N,N-Dimethylaniline (NNDMBF)
Figure 1. (a) Molecular structure of the dyad NNDMBF. (b) Trans- and
Cis-structure of the dyad NNDMBF. (c) High resolution Transmission
Electron Microscopy (HRTEM) picture of GNP. (d) HRTEM image of
GNS.
The synthesis procedure of the investigated compound
is given in Scheme 1. In brief, 2-amino fluorene 3 has
7874
J. Nanosci. Nanotechnol. 18, 7873–7881, 2018

Paul et al.
Comparative Analysis to Explore the Suitability of a Short Chain Dyad in Its Pristine and Nanocomposite Forms
H C
CH3
3
N
CHO
CH COOH
Pd/C, H2
MeOH
4
3
CH3
N
NO2
NH EtOH, RT
N
HNO3
2
2h
CH3
1
2
3
5
86 %
4
2 %
71 %
Scheme 1. Synthesis of the investigated dyad.
been synthesized according to the literature procedure.8
A mixture of 3 (1.5 mmol) and 4-(dimethylamino)
benzaldehyde 4 (1.5 mmol) was taken in a round bottomed
flask, 7 mL of anhydrous ethanol was added to it. The
reaction mixture was stirred at room temperature for 2 h.
After completion of the reaction (monitored by thin layer
chromatography, TLC), the solvent was removed under
high vacuum and crude reaction mass was washed several
times with hexane to get the pure product 5 (Yield-86%).
by drop which changes the solution color from light yel-
low to light red. This seed solution kept for another 2 hrs
before using for the next synthesis step when the color of
the solution turns to deep red.
In the next step mono dispersed GNS was synthesized
with average size 45± 3 nm. To synthesize GNS, 0.049 gm
−3
of CTAB was dissolved in 45 mL water (42ꢂ8 ×10 M)
bymild heating with constant slow stirring. To this solu-
−2
tion 2 mL of 10 M HAuCl solution was added and
4
1
13
H and C NMR spectra were recorded on Bruker
AVIII-500. Chemical shifts were reported in ppm. Cou-
stirred for additional 1 min. Thereafter by 300 ꢃL of
−
2
10 M freshly prepared AgNO solution was added gen-
3
1
−1
pling constants (J values) were reported in Hertz.
H
tly, mixed well and then 320 ꢃL 10 M ascorbic acid was
1
3
NMR chemical shifts were referenced to CDCl3. C NMR
addeddrop-wise. As the solution became colorless, imme-
diately 500 ꢃL of gold nanoseed solution (as prepared)
was added at a time. The solution became deep blue within
a minute and left still for 2 hrs before using it for the
chemical shifts were referenced to CDCl . High-resolution
3
mass spectra (HRMS) were recorded on a Bruker En Apex
Ultra 7.0T FT-MS mass spectrometer.
IP: 91.216.3.107 On: Fri, 11 Jan 2019 02:46:23
1
H NMR (500 MHz, CDCl ꢀ: ꢁ =C 2o. 9p 0y r i( gs ,h t6: HA )m, e3 .r 8i c1 an S sc pi ee cn tt ri of i sc c Po pu i bc li is nhv ee rs st igation. Removal of unbound CTAB
3
Delivered byf rI on mg e tnh tea solution has been done by centrifugation. Three
(
7
(
3
1
1
3
s, 2H), 6.62 (d, 2H, J = 8ꢂ5 Hz), 7.13–7.18 (m, 2H),
.24–7.28 (m, 2H), 7.41 (d, 1H, J = 7ꢂ5 Hz), 7.64–7.69
times controlled centrifugation have done for complete
removal of CTAB from the solution without disturbing the
surface-CTAB which is necessary for particle stabilization.
Purified GNS then used for spectroscopic measurement.
High resolution TEM picture of GNS is shown in the
Figure 1(c). The nano-templating method with a unique
nano-crystal growth mechanism has been used for the
single-pot production of variable size (110–360 nm). In the
first growth step 45 ± 3 nm gold nanopopcorns (GNPops)
have been synthesized from 4ꢂ3 ± 1ꢂ4 nm spherical gold
nano-seeds. In the second step, the template GNPops has
been used for control overgrowth, then controlling the vis-
cosity of surfactant the bigger GNS (average petal length:
60–100 nm) are produced. The detailed synthesis proce-
dure about the gold nanoseeds, GNPops and GNS has been
1
3
m, 4H), 8.31 (s, 1H). C NMR (125 MHz, CDCl ꢀ: ꢁ =
3
6ꢂ9, 40.2, 111.6, 117.5, 119.5, 120.2, 120.3, 124.5, 125.0,
26.2, 126.8, 130.4, 138.9, 141.6, 144.4, 151.9, 152.4,
59.6. ESI-HRMS m/z: Calcd. for [C H NO +H]+:
2
2
20
2
13.1706; Found: 313.1701.
2
.4. Preparations of Spherical Gold (GNP)
Nanoparticles (nps) of Diameter 25 nm and
Gold Nanostar (GNS)
1
00 mL deionized distilled water were taken in a 3-neck
−2
round bottomed flask. 1.25 mL 10 M HAuCL · 3H O
4
2
(
Sigma Aldrich: 520918-1G) added into the water and
warm it till the whole volume of water started boiling
vigorously under constant stirring and water condensation.
Once the water starts boiling, 0.75 ml of 1% trisodium cit-
rate (TSC) was added drop by drop to form 25 nm sized
spherical gold nanoparticles.
9
ꢄ10
reported elsewhere.
2.5. Theoretical Computations
Goldnanostar particles were synthesized using a two-
step procedure. In the first step we synthesized spherical
All DFT calculations were performed using Gaussian 03
suit of software operated in the linux operating system.
1
1
gold nanoseed by adding 0.5 mL of 10–2 M HAuCl solu-
Becke’s three parameter hybrid exchange (B3) and Lee–
4
1
2
tion with 20 mL water in a small vial. Next 0.2 mL of
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 gꢅdꢄpꢀ level of theory.
−
2
2
ꢂ5 × 10 M Trisodium citrate solution was added drop-
wise and left for 1 min to mix well. Then freshly prepared
−
0 ꢃL of 10 M ice cold NaBH solution was added drop
4
1
6
J. Nanosci. Nanotechnol. 18, 7873–7881, 2018
7875

Comparative Analysis to Explore the Suitability of a Short Chain Dyad in Its Pristine and Nanocomposite Forms
Paul et al.
3
. RESULTS AND DISCUSSION
intramolecular interaction between the donor NNDMB and
the acceptor Fluorene (F) leads to the formation of Charge
transfer band (broad band at 365 nm region). This indi-
cates that within the dyad, the alignment of the donor and
the acceptor moieties are such that it facilitates the forma-
tion of CT spectra. Figure 3(a) indicates that in the ground
state, as proposed by the theoretical predictions, two CT
states are formed, one at 365 nm of stronger nature and
another one of very weak type (tail like appearance) at
around 440 nm. Following the observations made from the
theoretical computations, the stronger one may be due to
trans-type conformation and the weaker one should orig-
inate from another type of conformer of the dyad which
in high probability may be of cis-type conformation. With
addition of GNP (Fig. 3(a)) or GNS (Fig. 3(b)), 365 nm
band tends to diminish gradually with the concomitant
development of 440 nm band. It appears that when the
dyad adsorbs on the surface of GNP or GNS, band associ-
ated with the cis-conformation develops, peaking at about
440 nm, and tends to prevail over trans-isomers through
interconversion (trans to cis) process in the ground state.
As spr bands of GNP and GNS reside at around 523 nm
region (inset in Fig. 3(a)), 440 nm band can be safely inter-
preted due to formation of a new CT conformer (possibly
cis) from the trans-form. Moreover, the concentrations of
3
.1. Theoretical Predictions, UV-Vis Absorption and
Steady State Fluorescence Measurements
In the present dyad NNDMBF, since there is no proton on
the N-atom (Fig. 1(a)), which is attached with fluorene and
double bond, we cannot predict the possible ground state
conformation of the dyad from coupling constant value
derived by the NMR measurements. From the theoretical
computations on ground state optimized geometry of the
dyad NNDMBF by using BL-LYP/6-311 gꢅdꢄpꢀ level of
theory on HOMO–LUMO surfaces it shows that there may
be the two types of conformers Cis- and Trans-, in which
the latter form is found to be more stable in the ground
state. Thus the above theoretical predictions suggest that
both cis- and transforms of the dyad exist in the ground
state (Fig. 2) with the preponderance of the latter.
UV-vis absorption spectra of the pristine dyad in ACN
solvent shows a broad long wavelength band at around
3
65 nm region (curve 1 of Fig. 3(a)) with long tail extend-
ing from 450 nm wavelength. The charge transfer nature of
65 nm band was confirmed from solvent effect (Fig. 3(c)).
3
This CT band originates due to partial transfer of elec-
tron from the highest occupied molecular orbital (HOMO)
of the donor chromophore (D) to the lowest unoccupied
molecular orbital (LUMO) of the acceptor fluorene (A).
From the Figure 3(d), it is observed that the UV-vis
absorption spectra of the mixture of the free donor and
acceptor components (when not present in the dyad) in
ACN medium correspond to the superposition of the corre-
sponding spectra of the donor and the acceptor molecules.
This indicates the lack of formations of any interrmolec-
ular ground state complex between the donor NNDMB
and acceptor (F). But, however, in case of the dyad,
−7
GNP as well as GNS nps were so chosen (∼10 M) that
the absorbance of spr band becomes negligible.
IP: 91.216.3.107 On: Fri, 11 Jan 2019 02:46:23
On exciting the CT absorption band (CT nature has been
Copyright: American Scientific Publishers
conformed from the solvent polarity effect) at 365 nm
region, CT fluorescence band peaking at about 470 nm was
found (Figs. 4(a and b)) along with a very weak shoulder-
like band at 550 nm. This 550 nm band becomes promi-
nent with increase of concentrations of GNP (Fig. 4(a)) or
GNS (Fig. 4(b)). The above findings demonstrate that even
with the excitation by using 365 nm light, where trans-
form predominates, cis-isomers appear to be formed in
the excited state. This observation demonstrates that pho-
toswitchable conversion of the trans-dyad to the cis-form
facilitates when combined with gold nanoparticles, GNP
and GNS.
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(
a)
When the excitation was made by 450 nm wavelength
(
the region of cis-isomer), the CT fluorescence emission
mostly originates from 550 nm region (Fig. 4(c)) indicat-
ing in this region mostly excited cis-isomeric species of
the dyad predominates.
(
b)
From this figure it is apparent that though the pris-
tine dyad (curve 1 of Fig. 4(c)) exhibits very weak emis-
sion at 550 nm but the emission becomes significantly
strong when the dyad adsorbs the surface of GNP or
GNS. This indicates in this environment formation of
cis-isomer facilitates largely and the production of cis-
species in the excited state occur directly from the ground
state cis-isomer and also from ground trans-isomer through
interconversion process. This observation indicates the
photoswitchable character of the dyad NNDMBF.
Figure 2. (a) Cis form of dyad NNDMBF in vacuum; (b) Trans form
of dyad NNDMBF in vacuum Trans-form of the dyad appears to be more
planar than the cis-one.
7876
J. Nanosci. Nanotechnol. 18, 7873–7881, 2018

Paul et al.
a)
Comparative Analysis to Explore the Suitability of a Short Chain Dyad in Its Pristine and Nanocomposite Forms
(
b) 0.5
(
0
0
0
0
0
.4
.3
.2
.1
.0
1
4
4
1
3
00
400
500
600
Wavelength/nm
(
d)
(
c) 1
1.2
0
0
0
0
0
0
0
0
0
.40
.35
.30
.25
.20
.15
.10
.05
.00
0
.5
0.4
0.3
0.2
0.1
0.0
Dyad NNDMBF in CH and ACN
Donor
1
0
0
.0
.8
.6
1
2
. CH
. ACN
Acceptor
1
2
2
00
300
400
200
300
400
Wavelength/nm
Wavelength/nm
IP: 91.216.3.107 On: Fri, 11 Jan 2019 02:46:23
0
.4
Copyright: American Scientific Publishers
Delivered by Ingenta
Donor + Acceptor
0
.2
0
3
00
350
400
450
500
550
600
2
00
300
400
500
Wavelength/nm
Wavelength/nm
−5
Figure 3. (a) UV-vis absorption spectra of the dyad NNDMBF (conc. 10 M in ACN) in presence of different concentrations of GNP. The conc. (M)
−7
−7
of GNP in 1:0; 2:3 × 10 ; 3:6 × 10 ; (inset: SPR band of GNP in millipore water). (b) UV-vis absorption spectra of the dyad NNDMBF (conc.
−
5
−7
−7
−7
10
M in ACN) in presence of different concentrations of GNS. The conc. (M) of GNP in 1:0; 2:1 ×10 ; 3:4×10 ; 4:6×10 . (c) CT nature of
the 365 nm absorption band of the dyad is confirmed from solvent effect (red shift in highly polar solvent ACN relative to the peak position observed
in CH). (d) UV-vis absorption spectra of the mixture of the donor and the acceptor in ACN medium. Inset: (right) UV-vis spectra of the acceptor and
(left) UV-vis spectra of the donor only.
3
.2. Fluorescence Lifetime Measurements by
TCSPC Techniques
of lifetime 15 ps and a very weak (as evidenced from
the observed very low value of fractional contribution, f )
slower (∼1ꢂ5 ns) lifetime component was also apparent
(Fig. 5(a), Table I).
Measurements of the fluorescence lifetimes (Table I) of
the dyad were made by using the time correlated single
photon counting (TCSPC) technique. By monitoring the
different positions of the steady state fluorescence spectra
the possibility of formations of another type of conformer,
apart from trans, in the excited state due to photoexcita-
tion has been examined. These findings further provide us
the idea that the present short-chain dyad possesses pho-
toswitchable character.
When excitation was made at 375 nm (using diode
laser) in case of the pristine dyad NNDMBF, it was
found that at the monitoring wavelength 480 nm (the
peak region), majority emission originates from a species
On monitoring at the 550 nm region (Fig. 5(b)), only
faster component of lifetime of picoseconds (ps) order was
observed. To assign unambiguously the origin of these two
lifetimes, we measured the fluorescence lifetime of the
dyad in presence of GNP or GNS (nanocomposite sys-
tems) by using the excitation wavelength at 440 nm, the
region of mostly cis-isomer. By monitoring at the dif-
ferent positions of the steady state fluorescence band on
the longer wavelength side of the excitation wavelength
440 nm, only picoseconds component was observed. This
it appears that the picoseconds species, in high probability,
J. Nanosci. Nanotechnol. 18, 7873–7881, 2018
7877

Comparative Analysis to Explore the Suitability of a Short Chain Dyad in Its Pristine and Nanocomposite Forms
Paul et al.
(
b) 200
(
a) 800
1
1
1
1
80
60
40
20
7
6
5
4
3
2
1
00
00
00
00
00
00
00
0
3
1
3
1
100
8
6
4
2
0
0
0
0
0
4
50
500
550
600
650
700
5
00
600
Wavelength/nm
Wavelength/nm
(c) 450
4
3
3
2
2
1
1
00
50
00
50
00
50
00
3
1
IP: 91.216.3.107 On: Fri, 11 Jan 2019 02:46:23
5
0
Copyright: American Scientific Publishers
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0
5
50
600
650
700
Wavelength/nm
−7
−6
Figure 4. (a) Fluorescence CT emission spectra (exc ∼ 365 nm) of dyad NNDMBF in presence of GNP nps of conc. (M) in 1:0; 2:6×10 ; 3:10
at the ambient temperature. (b) Fluorescence CT emission spectra (exc ∼ 365 nm) of dyad NNDMBF in presence of GNS nps of conc. (M) in 1:0;
−
7
−7
2
:4 × 10 ; 3:6 × 10 at the ambient temperature. (c) Fluorescence emission spectra (exc ∼ 450 nm) of dyad NNDMBF in presence of GNP of
−7 −7
conc. (M) in 1:0; 2:1×10 ; 3:6×10 at the ambient temperature.
should originate from cis-conformer. Another interesting
findings are that in presence of both GNP and GNS
trans-isomer primarily dominates in the ground state
but on photoexcitation, most of the trans compo-
nents convert (photoconversion) to the cis-form in the
excited level. However this photoconversion is largely
impeded when the dyad adsorbs the GNP and GNS
nanoparticles.
(
Table I) contribution from the faster component (∼ps
order) decreases along with concomitant increase of the
slower component (in ns order) in the peak position of
4
80 nm of the dyad fluorescence emission band produced
by the excitation at 375 nm.
In case of GNS, about 60% trans-form of the dyad exists
even on photoexcitation (Table I), contrary to the situation
observed in case of its pristine form where on excitation
most of the ground state trans-isomer convert to the excited
state cis-structure.
Hence the above findings demonstrate that when the
trans-type dyad in its pristine form is excited using 375 nm
wavelength, most ground state trans isomers convert to the
cis-type isomers having faster component (∼ps) lifetime.
Thus the dyad NNDMBF appears to behave as photo-
switchable dyad. However, when the dyad combines with
GNP or GNS nanoparticles, significant amounts of slow
component species, appearing to possess trans-type struc-
ture, are found to be present along with the faster compo-
nent which corresponds to cis-isomer.
This presumption was further corroborated from the
observed excitation fluorescence spectra.
The fluorescence excitation spectra of the pristine dyad
and its nanocomposite forms with GNP and GNS were
measured by monitoring at 465 nm and 550 nm wave-
lengths (Figs. 6(a–c)). In the cases of the monitoring wave-
lengths 550 nm and 465 nm (Figs. 6(a and b)) the observed
excitation spectra correspond well to the CT absorption
Thus the experimental observations made from the
fluorescence lifetime measurements reveal that though
7878
J. Nanosci. Nanotechnol. 18, 7873–7881, 2018

Paul et al.
Comparative Analysis to Explore the Suitability of a Short Chain Dyad in Its Pristine and Nanocomposite Forms
Table I. Fluorescence lifetime data of the the short chain dyad
NNDMBF in absence (pristine) and presence of GNP and GNS at the
different excitation and emission wavelengths at the ambient temperature.
Values in parentheses besides lifetimes correspond to fractional contri-
butions of the particular species in the total steady state fluorescence
emission.
with the cis-ones. The effect is found to be more pro-
nounced in case of GNS when on photoexcitation 60%
ground state trans-isomer do not suffer photoconversion
(
Table I). Hence in the case of the present dyad, adsorption
with the GNP or GNS nps facilitates relatively planar con-
figuration (trans-isomer) in the excited state. This is oppo-
site to the situation observed in the ground state when in
presence of gold nanoparticles the trans-species converts to
cis-one. As in the excited state in nanocomposite forms of
the dyad the formations of trans-structure facilitates where
the redox components i.e., donor and acceptor moieties
will tend to move away from each other, this geometry
will lower down energy destructive charge recombination
Samples
ꢆexc (nm) ꢆem (nm) ꢇ /ns (f )
ꢇ (f )
ꢈ2
1
1
2
2
Dyad in ACN
375
480
50
1.50 (0.06) 15 ps (0.94) 1.088
5
–
–
19 ps (∼1.0) 1.068
15 ps (1.0) 1.15
440
550
480
550
550
480
550
550
Dyad+GNP
375
1.51 (0.16) 20 ps (0.84) 1.080
−
6
(
1×10 M)
–
–
20 ps (1.0)
25 ps (1.0)
1.09
1.16
440
Dyad+GNS
4×10 M)
375
1.59 (0.60) 22 ps (0.40) 1.138
−5
13
(
–
–
22 ps (∼1.0) 1.18
rates. From the present investigations it seems that the
440
28 ps (∼1.0) 1.178
dyad in its nanocomposite form may exhibit better light
energy conversion device than its pristine form due to pre-
dominance of elongated structure. To corroborate this view
transient absorption decay measurements were made, the
details of the observed results are given below.
band at 365 nm region. This was observed in the case of
the pristine dyad in ACN.
This observation infers that in the case of the pristine
dyad, the excited cis-species which are mostly formed due
to photoconversion originate from the ground state trans-
isomers. In the case of the nanocomposite dyad (Fig. 6(c))
also, the excitation spectra clearly show that cis-forms in
the excited state are produced from the ground trans-state
on photoexcitation i.e., through interconversion processes.
However, presence of weak 440 nm band in the excita-
tion spectra (possesses clear resemblance with the corre-
sponding absorption spectra of the nanocomposite dyad)
3.3. Transient Absorption Decay Measurements
To observe effectively the slowing down of the energy
wasting charge recombination rate (kCRꢀ, the transient
absorption decay was measured for the pristine dyad and
dyad-GNS systems. As in the later nanocomposite system
majority (∼60%) excited species is in the trans form hav-
ing elongated structure where longer ion-pair lifetime (ꢇ ꢀ
ip
IP: 91.216.3.107 On: Fri, 11 Jan 2019 02:46:23
(slower charge recombination rate) could be expected rela-
tive to the pristine dyad which possesses a folded geometry
(cis-conformer) in the excited state.
Copyright: American Scientific Publishers
indicates that some direct conversion of ground state cis
to excited cis is also possible, especially for the nanocom-
posite dyad system.
Delivered by Ingenta
In transient absorption measurements transient of
anionic species of the acceptor fluorene could be detected
by nanosecond laser pump probe technique using the
different microsecond time delays between the pump
As the theoretical predictions along with the steady state
and time resolved spectral measurements clearly show
that though in the ground state trans-isomer of the dyad
prevails but on photoexcitation cis-isomeric species of
the dyad predominates and this cis-form having folded
nature generates from the relatively planar trans conformer
through interconversion processes. However, in presence
of GNP and GNS, the photoconversion is somewhat hin-
dered resulting the presence of excited trans-isomers along
and probe pulses. Ganguly and Durocher1 and others
4
15
reported the existence of fluorene anionic species at around
540 nm region. The triplet ion-pair complex is formed
by intersystem crossing from the corresponding excited
singlet state which was initially formed by the laser
photoexcitation of the ground state CT absorption band
(
a)
(b)
Figure 5. (a) Fluorescence decay of the dyad (yellow) in ACN ex = 375 nm, em = 480 nm, the fast (blue) decaying component represents impulse
2
response (diode laser) (ꢈ ∼ 1ꢂ088). The residual is also shown. (b) Fluorescence decay of the dyad (yellow) in ACN ex = 375 nm, em = 550 nm, the
2
fast (blue) decaying component represents impulse response (diode laser) (ꢈ ∼ 1ꢂ068). The residual is also shown.
J. Nanosci. Nanotechnol. 18, 7873–7881, 2018
7879

Comparative Analysis to Explore the Suitability of a Short Chain Dyad in Its Pristine and Nanocomposite Forms
Paul et al.
(
a) 40
(b)
2
0
0
3
00
400
Wavelength/nm
Wavelength/nm
(c)
5
0
0
IP: 91.216.3.107 On: Fri, 11 Jan 2019 02:46:23
4
00
500
Copyright: American Scientific Publishers
Wavelength/nm
Delivered by Ingenta
Figure 6. (a) Fluorescence excitation spectra of pristine dyad in ACN (monitoring wavelength at 550 nm), (b) Fluorescence excitation spectra
of pristine dyad in ACN (monitoring wavelength at 465 nm), (c) Fluorescence excitation spectra of dyad-GNP nanocomposite system (monitoring
wavelength at 550 nm).
1(
ꢁ+ ꢁ−
D -sp-A ) (trans-type structure). The ground CT (of
conversion or storage device where the majority dyads on
photoexcitation retains its trans-structure which effectively
slows down the energy destructive charge recombination
processes within the redox centers.
extended and planar structure) on excitation by using the
third harmonic of ND:YAG laser goes to excited CT
1
ꢁ+
ꢁ− ∗
+
level (D -sp-A ꢀ from where it relaxes to the charge-
1
− ∗
separated state (D -sp-A ꢀ . It moves to the triplet state
D -sp-A ꢀ through intersystem crossing (ISC). The
In GNS due to its special morphology (tips extend-
ing from the gold core) there is relatively lesser cov-
erage area for adsorption of the dyad on gold surface
relative to GNP. From the steady state and time resolved
spectroscopic results it appears that due to larger cover-
age on the GNP, relative to GNS, the dyad prefers to
alter its conformation, on photoexcitation, from elongated
(tran-form) to cis-form (∼84%) which is folded in nature.
The similar change of conformation (close packing) was
3
+
− ∗
(
transient absorption decays (Fig. 7 (upper)–(lower)) were
measured and ion-pair lifetimes (ꢇ ) derived from the
ip
decays were compared for the pristine dyad (Fig. 7, upper
curve) and dyad-GNS (lower curve of Fig. 7) nanocom-
posite system. The ꢇip was found to be significantly large
(
∼2.4 ꢃs) in case of dyad-GNS syatem relative to the
pristine dyad where the ion-pair lifetime observed to be
0.4 ꢃs. Thus the above transient absorption decay mea-
1
6
∼
observed in cases of polymeric chain systems when the
degree of surface coverage area of the polymer on the
nanoparticles increases. Thus in our studies, by changing
the morphologies of the gold nanoparticles (from spherical
to star-shaped) we can control the coverage of the dyad
on the nanosurface and this shows the geometry (or con-
formation) of the dyad (84% cis and 40% cis-formations
from trans-isomer were observed in cases of GNP and
GNS respectively) is dependent on the surface coverage.
surements demonstrate that when the dyad adsorbs the
gold nanoparticle GNS, the charge recombination rate
slows down due to the preponderance of trans-isomers
over cis-one. The later possessing folded structure suffers
larger charge recombination rates within the donor and
acceptor moieties being in close proximity with each other.
It may be presumed that within the three systems stud-
ied, dyad-GNS appears to be the best artificial light energy
7880
J. Nanosci. Nanotechnol. 18, 7873–7881, 2018

Paul et al.
Comparative Analysis to Explore the Suitability of a Short Chain Dyad in Its Pristine and Nanocomposite Forms
0
0
0
0
0
0
0
.006
.005
.004
.003
.002
.001
.000
presumed as a most suitable material out of the three
systems studied to building efficient light energy conver-
sion or storage device. Transient absorption decay mea-
surements further corroborate our views. Investigations are
underway to examine how the degree of surface coverage
of the dyad on gold nanoparticles affect its geometry or
conformational structure on photoexcitation.
Acknowledgments: Tapan Ganguly expresses his grat-
itude to the University grant commission (UGC),
New Delhi, India for awarding Emeritus Fellow-
ship and providing contingency grant for research
purpose (F.6-6/2014-15/EMERITUS-2014-15-GEN-3976).
0
.0000000
0.0000005
0.0000010
0.0000015
MB expresses her sincere gratitude to the DST-SERB
project (YSS/2015/000589), DST, Govt of India. 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 picosecond resolved fluores-
cence lifetimes by using a commercially available time-
correlated single-photon counting (TCSPC) setup from
Edinburgh instruments. We are also thankful to his stu-
dent Dr. Priya Singh for helping in the measurements of
picosecond lifetimes by using this TCSPC instrument.
time in s
0
0
0
0
0
0
0
.006
.005
.004
.003
.002
.001
.000
References and Notes
1. S. Fukuzumi, K. Ohkubo, H. Imahori, J. Shao, Z. Ou, G. Zheng,
Y. Chen, R. K. Pandey, M. Fujitsuka, O. Ito, and K. M. Kadish,
IP: 91.216.3.107 On: Fri, 11 Jan 2019 02:46:23
J. Am. Chem. Chem. Soc. 123, 10676 (2001).
0
.000000 0.000002 0.000004 0.000006 0.000008 0.000010
Copyright: American Scientific Publishers
2
. S. Bhattacharya, T. K. Pradhan, A. De, S. Roy Chowdhury, A. K.
De, and T. Ganguly, J. Phys. Chem. A 110, 5665 (2006).
time in s
Delivered by Ingenta
3
. G. Zaragoza-Galán, J. Ortíz-Palacios, B. X. Valderrama, A. A.
Camacho-Dávila, D. Chávez-Flores, V. H. Ramos-Sánchez, and
E. Rivera, Molecules 19, 352 (2014).
Figure 7. (upper) The transient absorption decay of the pristine dyad
2
at 540 nm (anionic acceptor fluorene) (r ∼ 0ꢂ98) (lower). The transient
absorption decay of the nanocomposite system of dyad-GNS at 540 nm
2
4. E. Allard, J. Cousseau, J. Ordúna, J. Garín, H. Luo, Y. Araki, and
O. Ito, Phys. Chem. Chem. Phys. 4, 5944 (2002).
(
r ∼ 0ꢂ99) corresponding to anionic species of acceptor fluorene.
5
. G. Dutta (Pal), P. Chakraborty, S. Yadav, A. De, M. Bardhan,
P. Kumbhakar, S. Biswas, H. S. DeSarkar, and T. Ganguly,
J. Nanosci. Nanotechnol. 16, 7411 (2016).
However, further investigations are underway to examine
quantitavely how the degree of surface coverage of the
dyad on gold nanoparticles affect its geometry or confor-
mational changes.
6. G. Pal, A. Paul, S. Yadav, M. Bardhan, A. De, J. Chowdhury,
A. Jana, and T. Ganguly, J. Nanosci. Nanotechnol. 15, 5775 (2015).
7
8
9
. G. Dutta (Pal), S. Paul, M. Bardhan, A. De, and T. Ganguly, Spec-
trochimica Acta Part A 180, 168 (2017).
. K. K. Park, H. Jung, T. Lee, and S. K. Kang, Bull Korean Chem.
Soc. 31, 984 (2010).
. M. Bardhan, B. Satpati, T. Ghosh, and D. Senapati, J. Mater. Chem.
C 2, 3795 (2014).
4
. CONCLUSIONS
Steady state and time resolved spectroscopic investigations
on the pristine dyad and the nanocomposites where the
dyad combines with gold nanoparticles of two different
morphologies: spherical and star shaped demonstrate that
nanocomposite systems appear to be better light energy
conversion device than the pristine dyad. This occurs as
the photoconversion of trans-(stable in the ground state)
to the cis-form on photoexcitation hinders to some extent
in nanocomposites and majority of ground trans-isomer
retains its configuration even in the excited state. As within
the three systems studied in the present investigation larger
amount of ground trans structures retain on photoexci-
tation in case of dyad-GNS system, this one could be
10. G. Dutta (Pal), S. Paul, M. Bardhan, and T. Ganguly, J. Nanosci.
Nanotechnol. 18, 2943 (2018).
11. A. D. Backe, J. Chem. Phys. 98, 5648 (1993).
12. C. Lee, W. Yang, and G. Parr, Phys. Rev. B 37, 785 (1988).
13. N. J. Heada, J. Thomasa, M. J. Shepharda, M. N. Paddon-Row, T. D.
M. Bell, N. M. Cabral, and K. P. Ghiggino, J. Photochem Photobiol.
A: Chemistry 133, 105 (2000).
14. T. Ganguly, D. K. Sharma, S. Gauthier, D. Gravel, and G. Durocher,
J. Phys. Chem. 96, 3757 (1992).
1
1
5. K. Nakamura, J. Am. Chem. Soc. 102, 7846 (1980).
6. S. Stolnik, B. Daudali, A. Arien, J. Whetstone, C. R. Heald, M. C.
Garnett, S. S. Davis, and L. Illum, Biochimica et Biophysica Acta
1514, 261 (2001).
Received: 6 September 2017. Accepted: 9 January 2018.
J. Nanosci. Nanotechnol. 18, 7873–7881, 2018
7881