Photofunctional self-assembled nanostructures formed by pery lene diimide-gold nanoparticle hybrids

Article abstract of DOI:10.1021/jp100662b

We report on the synthesis of organic dye-metal nanoparticle hybrids from two thiol-derivatized perylenediimide (PDI) ligands and 1.5 nm gold nanoparticles. The hybrids form spherical nanostructures when cast from 40% methanol/chloroform solution and tolu

Full text of DOI:10.1021/jp100662b

J. Phys. Chem. B 2010, 114, 14389–14396
14389
Photofunctional Self-Assembled Nanostructures Formed by Perylene Diimide-Gold
Nanoparticle Hybrids†
G. Santosh,‡ Elijah Shirman,‡ Haim Weissman,‡ Eyal Shimoni,§ Iddo Pinkas,| Yinon Rudich,⊥
and Boris Rybtchinski*,‡
Departments of Organic Chemistry, Chemical Research Support, Plant Sciences, and EnVironmental Sciences
and Energy Research, Weizmann Institute of Science, RehoVot 76100, Israel
ReceiVed: January 23, 2010; ReVised Manuscript ReceiVed: April 7, 2010
We report on the synthesis of organic dye-metal nanoparticle hybrids from two thiol-derivatized
perylenediimide (PDI) ligands and 1.5 nm gold nanoparticles. The hybrids form spherical nanostructures
when cast from 40% methanol/chloroform solution and toluene. The spherical aggregates are in the size
range 50-230 nm in 40% MeOH/CHCl3 mixture and 100-400 nm in toluene solution, as evidenced by
transmission electron microscopy (TEM). Scanning electron microscopy (SEM) measurements show that these
spherical aggregates are vesicles with a hollow interior. The π-π interactions of the perylenediimides are
the predominant driving force leading to the aggregation of the hybrids, whereby the sizes of the nanospheres
can be regulated via the PDI linker moiety and solvent choice. Femtosecond transient absorption studies of
the hybrids reveal complex photophysical behavior involving electron transfer from the gold nanoparticles to
the PDI moieties. This study shows that the formation of well-defined hybrid nanostructures as well as tuning
their sizes can be achieved through employing a combination of the capping ligand choice and regulating the
solvophobic interactions between the ligands.
Introduction
between the particle sizes and the quenching efficiencies.24 Other
studies indicate that gold nanoparticles act as very good
Self-assembly of metal nanoparticles has emerged as an
important area of research due to potential applications in
sensors,1 catalysis,2 and electronics.3 It has been demonstrated
that the size, shape, and complexity of the building blocks are
very important in determining the self-assembly aptitudes.4
Strategies to assemble nanoparticles into complex nanostructures
include the polymer-mediated “brick and mortar” approach,5
templates,6-9 and the dithiol-mediated nanoparticle aggrega-
tion.10-13 The last has been utilized to develop novel functional
nanomaterials.14,15 Yet, spontaneous assembly of nanoparticles,
whereby metal nanoparticles self-assemble under certain condi-
tions, is an attractive strategy due to its simplicity. For example,
Schaak and co-workers have reported on the spontaneous
aggregation of poly(vinylpyrrolidone)-stabilized rhodium nano-
particles into spherical aggregates and superlattices.16 Cao and
co-workers have demonstrated the use of solvophobic interac-
tions to control the formation of Fe3O4 superparticles that in
turn form near perfect superlattices.17 The self-assembly between
two different types of metal nanoparticles mediated by elec-
trostatic interaction leads to the formation of large diamond-
like crystals.18
quenchers for porphyrinic,25 fullerene,26 dansyl,27 fluorenyl,28
perylenediimide,29 and fluorescein30 fluorophores. Importantly,
aromatic chromophores can promote self-assembly due to
π-stacking and solvophobic interactions. Thus, aromatic dye-
capped gold nanoparticles may be very advantageous for
creation of photoactive assemblies. An insight into the self-
assembly and photonics of such systems would be of funda-
mental importance for developing new functional nanomaterials
and photocatalytic systems.31
Perylene diimides (PDIs) represent an important class of
aromatic dye molecules. The extended aromatic π-system of
the PDI results in strong hydrophobicity, excellent light absorp-
tion and luminescence characteristics, and advantageous redox
properties,32,33 making PDIs useful multifunctional components
in self-assembled light-harvesting materials.34,35 Molecular
systems containing PDI moieties have been reported to aggregate
in various solvents due to π-π-stacking and solvophobic
interactions.29,32-38 We have recently reported on adaptive
photofunctional fibers39 and gels40 that resulted from the self-
assembly of PEG containing derivatives of PDI in aqueous
medium.
Introduction of chromophores into the nanoparticle capping
layer may result in systems with novel photophysical properties.
For example, gold nanoparticle hybrids with pyrene show very
interesting photophysics, including photoinduced charge trans-
fer.19-23 Murray and co-workers have studied the dynamic and
static fluorescence-quenching processes of five different fluo-
rophores by gold nanoparticles, demonstrating the relationship
Herein we report on the spontaneous aggregation of PDI-
covered gold nanoparticles into spherical aggregates. The self-
assembly is based on solvophobic interactions, allowing control
over the size of the assemblies. These systems show complex
photophysical behavior, as evidenced by femtosecond transient
absorption studies.
† Part of the “Michael R. Wasielewski Festschrift”.
* Corresponding author. E-mail: boris.rybtchinski@weizmann.ac.il.
‡ Department of Organic Chemistry.
Results and Discussion
Perylene diimide (PDI) molecules play a dual role in our
strategy: first, they provide a self-assembly motif; second, the
PDI chromophores act as light harvesting and redox units that
§ Department of Chemical Research Support.
| Department of Plant Sciences.
⊥ Department of Environmental Sciences and Energy Research.
10.1021/jp100662b  2010 American Chemical Society
Published on Web 04/29/2010

14390 J. Phys. Chem. B, Vol. 114, No. 45, 2010
Santosh et al.
SCHEME 1: Synthesis of PDI Thiols 1 and 2
Applying this aggregation methodology to our hybrid systems,
we prepared methanol/chloroform solutions of 3. TEM images
of hybrid system 3 (Figure 2a) deposited on a carbon-coated
copper grid from a 40% methanol/chloroform solution revealed
the formation of well-defined spherical aggregates with an
average size of 156 ( 34 nm. The individual gold nanoparticles
can be clearly seen arranged in a matrix formed by the organic
chromophore (forming the lower contrast region surrounding
the nanoparticles). The average interparticle distance in the
spherical aggregates of 3 is 3.0 ( 0.8 nm. The images reveal
somewhat denser darker regions on the edges of the aggregates,
as expected for projections of spherical assemblies.
Further insight into the aggregate structure was gained using
scanning electron microscopy (SEM). Representative SEM
images of hybrid 3 (Figure 2b,c) confirm the spherical nature
of the aggregates. The spheres cluster into large networks,
indicating a propensity to secondary aggregation. The aggregate
average size is 150 ( 23 nm, in very good agreement with the
TEM data. The SEM analysis also provides evidence for the
hollow interior of the aggregates. Some of the aggregates that
are not fully formed show a mouth-like opening 40-50 nm in
size on their surface, revealing a hollow interior typical of
vesicles (Figure 2b).
may participate in photoinduced electron transfer with the
nanoparticles.
The hybrids synthesis was based on the commonly employed
ligand-exchange strategy.41 Accordingly, PDI ligands and gold
nanoparticles were chosen to facilitate the ligand-exchange
process. We synthesized PDIs bearing a thiol functional group
(Scheme 1), which forms very strong bonds with gold nano-
particles,42 and gold nanoparticles stabilized by triphenylphos-
phine (a relatively labile ligand43). The latter system is an
excellent precursor for synthesizing a wide variety of nanopar-
ticles by ligand exchange.44 Additionally, the gold nanoparticles
stabilized by triphenylphosphine are reported to be less than 2
nm in diameter,45 of comparable size with the PDI ligand,
enabling PDIs to play a pronounced role in the self-assembly
process.
The synthesis of the PDI thiol derivatives is outlined in
Scheme 1 (see Supporting Information for details). The mono-
substituted derivatives were selectively generated by adding 1
equiv of the base to an excess of the dithiol in THF, which
then reacted with 1 equiv of the monobromo-PDI, affording
the PDI thiols 1 and 2.
Apart from the spherical aggregates, it was also observed that
a small fraction of hybrid 3 (roughly about 5%) aggregates into
a regular two-dimensional array (Figure S7, Supporting Infor-
mation). These little islands of aggregates are also seen around
the edges of the three-dimensional structures. They exhibit an
average interparticle distance of 3 ( 0.8 nm and may represent
the precursors to the three-dimensional aggregate formation. It
appears that the nanoparticles arrange in spherical aggregates
mainly by the π-π and solvophobic interactions between the
PDIs covering the nanoparticle surfaces. Molecular modeling
(see Supporting Information) shows that in a π-stack model
between two molecules of 1, having alkyl chains that are
completely stretched, the distance between two terminal sulfur
atoms that are responsible for AuNP binding is 3.46 nm (Figure
S10a, Supporting Information). This model fits well the
interparticle distances calculated from the TEM images. The
nanoparticle aggregation displays a distribution of interparticle
distances (3 ( 0.8 nm) that could be due to the inhomogeneous
particle sizes and conformers of 1. Scheme 2 illustrates the
possible assembly pattern of the nanoparticles as a result of the
π-π stacking between PDIs.
Aggregation of the hybrid 3 occurs also when it is deposited
from toluene (the latter is known to be an aggregating solvent
when multi-PDI systems are involved47). Thus, hybrid 3 formed
spherical aggregates when deposited from toluene solutions, as
shown in Figure 3a. However, these spherical systems are larger
and less uniform (average aggregate size of 366 ( 147 nm)
than the ones obtained from chloroform/methanol mixtures.
Interestingly, the particles exist exclusively as the spherical
aggregates, and no free particles or smaller assemblies were
observed. The three-dimensional structure of these aggregates
is revealed by SEM (Figure 3b), where hybrid 3 shows spherical
aggregates with an average size of 436 ( 238 nm. In addition,
well-separated aggregates rather than their clusters (as in the
case of methanol/chloroform) were observed. This indicates that
it is possible to tune the size of nanoparticle aggregates and
their secondary aggregation by choice of the solvents from
which they are deposited.
The triphenylphosphine-stabilized gold nanoparticles (AuNP)
were synthesized following the literature procedure.46 The
transmission electron microscopy (TEM) analysis of the particles
showed an average particle size of 1.5 ( 0.3 nm. The PDI-
covered gold nanoparticles 3 and 4 were synthesized by the
ligand-exchange reaction between the AuNP and 1 and 2,
respectively, carried out at 1:20 (AuNP:PDI) molar ratio in
chloroform under inert atmosphere. The progress of the ligand
exchange was monitored by steady-state fluorescence over 2
days. A considerable amount of quenching of the PDI fluores-
cence was observed immediately after the addition of the PDI
thiols 1 and 2 to the nanoparticles (Figure 1). The fluorescence
of the PDI was quenched by 92% by 1 and 76% by 2,
respectively. The quenching is indicative of the formation of
the bond between the thiol and the gold nanoparticle. The excess
of the ligands was removed by a series of precipitations from
1
hexane followed by centrifugation. H NMR of the resulting
precipitate (Figure S5, Supporting Information) revealed a
spectrum that shows significant broadening of the PDI protons
of 1 and 2, signifying their attachment to the nanoparticle
surface.
PDIs are known to stack by π-π interactions in different
solvent environments. This phenomenon had previously been
exploited to develop diverse self-assembled structures of dif-
ferent morphologies and sizes.32-35,47-51 One of the methodolo-
gies uses addition of methanol (an aggregating solvent for PDI)
to a solution of PDI in a disaggregating solvent such as
chloroform, resulting in aggregation.52
It should be noted that when the hybrid particles were
deposited from a pure chloroform solution, no formation of
spherical aggregates was observed. Only a small fraction of the

Photofunctional Self-Assembled Nanostructures
J. Phys. Chem. B, Vol. 114, No. 45, 2010 14391
Figure 1. Fluorescence quenching of (a) 1 and (b) 2 followed in time after the addition of gold nanoparticles to the PDI-thiol (1:20 molar ratio)
in chloroform.
Figure 2. (a) TEM and (b)-(c) SEM images of spherical aggregates of 3 deposited from 40% methanol/chloroform. Insets show images at higher
magnification. Individual AuNP can be seen in (c).
entire grid displayed domains (Figure S9, Supporting Informa-
tion) with less-ordered two-dimensional formations of the
nanoparticles.
An additional control experiment was done to examine if the
precursor gold nanoparticles covered with triphenylphosphine
ligand show aggregation behavior. Accordingly, these gold

14392 J. Phys. Chem. B, Vol. 114, No. 45, 2010
Santosh et al.
SCHEME 2: Slice of the Vesicle (Seen in Figure 2)a
hydrodynamic diameter (DH) of hybrid 3 in chloroform was
found to be 38 ( 4 nm with a small contribution from aggregates
with DH ) 748 nm. In contrast, 3 and 4 in other solvents
exhibited aggregates of much larger sizes. The hydrodynamic
diameters (DH) of hybrid 3 in 40% MeOH/chloroform and
toluene were found to be 192 ( 20 and 854 ( 228 nm,
respectively, and the DH values for hybrid 4 were found to be
191 ( 19, 270 ( 31, and 310 ( 57 nm, respectively, in
chloroform, 40% MeOH/chloroform, and toluene solutions with
small contributions from aggregates measuring less than 20 nm.
Overall, DLS indicates that polydisperse, relatively large
aggregates exist in solution, while further analysis is complicated
by the polydisperse nature of the systems and the fact that DLS
may overestimate the contribution from large aggregates.
Unfortunately, the systems in solution could not be studied by
X-ray scattering techniques due to their lack of stability under
the experimental conditions.
We note that DLS indicates aggregation in all solvents, while
upon drying, TEM and SEM show that ordered spherical
structures form only in several cases, most probably due to the
interplay of solution-phase self-assembly and drying. Neverthe-
less, the formation of the spherical structures in the solid state
is highly reproducible and insensitiVe to the aging of the solution
from which they are deposited (aging times from 1 h to 3 days
result in almost identical systems). Furthermore, control over
the assembly size can be achieved by the organic ligand and
solvent choice. Thus, although a complex interplay of preaggre-
gation in solution and solvent evaporation is involved in the
formation of spherical assemblies, it represents a reliable
methodology for creation of spherical gold-PDI nanohybrids.
Apparently, van der Waals and solvophobic interactions as well
as surface tension are important factors for the formation of
hollow spheres in the solid state.
a The nanoparticles (brown balls) are assembled on the face of the
slice by the π-π stacking between PDIs (red rectangles; see also Figure
S10, Supporting Information). The π-π stacking is shown only on
the front face for the sake of clarity. PDIs are shown only for the
stacking pattern.
nanoparticles were deposited from 40% methanol/chloroform
solution and were analyzed by TEM. There were no aggregates
observed for hybrids 3 and 4, confirming that the PDI molecules
on the surface of the nanoparticles in a solvophobic environment
are responsible for the nanosphere formation.
The aggregation behavior of hybrid 4 is similar to that of 3.
The TEM images of the samples deposited from 40% methanol/
chloroform solution (Figure 4a) reveal spherical aggregates, 113
( 40 nm in diameter. The SEM images (Figure 4b) confirm
the spherical nature of these aggregates, which are found to be
107 ( 40 nm in diameter. The average interparticle distance
within the aggregates is 2.2 ( 0.5 nm. The aggregation appears
to be very effective as almost all of the material existed as
spherical aggregates in contrast to 3, which showed a small
fraction (ca. 5%) of less-ordered aggregates. Vesicular nature
of the aggregates is evident from the SEM, where the hollow
interior of the spheres is revealed by several not fully formed
spheres (Figure 4b, inset). Molecular models show that in a
π-stacked model of two molecules of 2 (Figure S10b, Supporting
Information) the distance between the thiol sulfur atoms is 2.47
nm. This distance agrees well with the average interparticle
separation, as measured from TEM images.
Hybrid 4, when deposited from a toluene solution, revealed
a very different aggregation picture: dense aggregated networks
extending to several hundreds of nanometers in size with no
specific shape. TEM also revealed relatively less dense giant
networks of the hybrid particles, as shown in Figure S8a
(Supporting Information). There were a few spherical aggregates
(Figure S8b).
Photophysical Studies
To gain insight into the photophysics of AuNP-PDI hybrids,
we performed studies that address the dynamics of the systems
following photoexcitation. As mentioned above, we observed
quenching of PDI fluorescence in hybrids 3 and 4 in chloroform.
Quenching of fluorescence can be explained by energy transfer
(including excimer formation), charge separation, or other
nonradiative decay pathways competing with the emission.
Energy transfer from PDI to other entities is less probable due
to the absence of chromophores that have absorption red-shifted
from PDI.
In earlier studies, which utilized gold nanoparticles larger than
2 nm and organic chromophores with relatively low oxidation
potential, an electron transfer from the organic capping layer
to the nanoparticles has been observed.19,22,23 In the current study
we utilized gold nanoparticles less than 2 nm in diameter, also
known as nanoclusters, potentially being able to serve as both
electron donors and acceptors.53 The Gibbs free energy change
for photoinduced formation of an ion pair (∆Gcs) from two
coupled neutral redox components can be estimated using
eq 1:54,55
The linker moieties in the PDI-thiols 1 and 2 influence the
assembly process. Thus, in chloroform/methanol systems the
aggregates of 3 were somewhat larger in average size than
those of 4, which appears to be due to the longer linker in
the case of 3.
In addition to solid-state assemblies, we studied the solution-
phase aggregates of 3 and 4 by dynamic light scattering (DLS).
The results are presented in Table 1. The DLS data suggest
that the hybrids exist as aggregates in all solvents. The
e2
εsrDA
∆Gcs ) Eox - Ered - Es -
+
1
1
1
1
εP
e2
+
-
(1)
(
)(
)
2rD
2rA εS
where Eox and Ered are the oxidation potential of the donor and
the reduction potential of the acceptor, respectively, Es is the

Photofunctional Self-Assembled Nanostructures
J. Phys. Chem. B, Vol. 114, No. 45, 2010 14393
Figure 3. (a) TEM and (b) SEM images of spherical aggregates of 3 deposited from toluene. The inset in (a) shows the image at higher magnification.
Figure 4. TEM image (a) and SEM image (b) of spherical aggregates of 4 deposited from 40% methanol/chloroform. The inset shows the image
at higher magnification, several hollow spheres are present.
TABLE 1: Comparison of Shape and Sizes of Aggregates of 3 and 4 in Different Solvents Measured by Electron Microscopy
and Dynamic Light Scattering Experiments
aggregate size (nm)
hybrid
solvent
CHCl3
40% MeOH/CHCl3
toluene
CHCl3
40% MeOH/CHCl3
toluene
shape of aggregate
TEM
SEM
DLS (DH)a
3
3
3
4
4
4
random
38 ( 4b
spherical
spherical
random
spherical
giant network
156 ( 34
366 ( 147
150 ( 23
436 ( 238
192 ( 20b
854 ( 228
191 ( 19b
270 ( 31b
310 ( 57b
113 ( 40
.100
107 ( 40
.100
a Hydrodynamic diameter. b Denotes the average aggregate size with the major contribution to the scattering intensity.
lowest excited singlet-state energy of the system, e is the charge
of the electron, rDA is the ion pair distance, rD and rA are the
ionic radii, εs is the dielectric constant of a particular solvent,
and εP is the dielectric constant of the solvent in which the
electrochemistry was done.
According to the reported studies, gold nanoclusters of size
similar to that used for the current study, protected with different
aromatic and aliphatic thiols, are oxidized electrochemically at
0.2, 0.5, and 0.7 eV vs SCE (the first, the second, and the third
oxidation potentials, respectively).30,53,56 The first reduction
potential of PDI is -0.65 eV (Figure S20, Supporting Informa-
tion). The electrostatic barrier and the reorganization energy,
the two last terms in eq 1, are almost zero for polar solvents.
The total energy available from the absorption of photon by 1
and 2 is about 2 eV. Accordingly, the change in the standard
Gibbs energy for the photoinduced charge separation (∆Gcs) in
the PDI-AuNP hybrids in a polar environment are about -1.15,
-0.85, and -0.65 eV, respectively (for the first, the second,
and the third electron transfer from AuNP to the PDIs in the
capping layer). These negative changes in the Gibbs energy
imply feasible photoinduced transfer of up to three electrons
from the gold nanocluster to the PDI ligands. Conversely,
according to the same studies, the same gold nanoclusters can
be reduced at -0.15 and -0.4 eV with one and two electrons,
respectively. The oxidation potentials of compounds 1 and 2
should be relatively high (above 1.5 eV29), and could not be
observed in our electrochemical experiments (Figure S20,
Supporting Information). Thus, in polar media the free energy
change is negative at least for one electron transfer from the
PDI covering layer to gold nanocluster (∆Gcs ∼ -0.4 eV). The

14394 J. Phys. Chem. B, Vol. 114, No. 45, 2010
Santosh et al.
TABLE 2: Time Constants of the Kinetic Components of
SADS Evolution of Hybrid 3 in Different Media Obtained
Using Target Fit Analysis
hybrid 3
CHCl3/MeOH
CHCl3
toluene
τ1 (ps)
τ2 (ps)
τ3 (ps)
5
66
>2000
6
54
>2000
4.5
50
>2000
consistent with the shorter (1 vs 1.6 nm) and partially aromatic
nature of the linker coupling the redox components. Unfortu-
nately, hybrid 4 was partially decomposed under the illumination
conditions used in these experiments. Therefore, we present only
the data analysis regarding hybrid 3 that was stable over time
under laser light. The time constants of the kinetic components
obtained by target analysis are summarized in Table 2, the
representative spectra, representative kinetic traces, and the
SADS are shown in Figures 5-7 and S11-S19 (Supporting
Information).
To discriminate the S1 appearance from other components
contributing to the fsTA signal of 3, a control NIR fsTA
experiment on compound 1 was performed in chloroform
(Figures S11-S13, Supporting Information). The evolution of
the fsTA signal of the free ligand can be described by a
combination of two SADS. One (τ > 2 ns) with a major
contribution (∼90%) is a superposition of the S1 positive
absorption in the region red-shifted from 700 nm accompanied
by the stimulated emission, which appears as a negative signal
in the 630-700 nm spectral range. An additional minor (10%)
nonradiative decay component (τ ∼ 100 ps) is contributing to
the disappearance of the fsTA signal. This is consistent with
the measured fluorescence life time (FLT) (single 8.1 ns
component; see Figure S22, Supporting Information). The SADS
associated with 100 ps decay component shows a positive
absorption at wavelengths greater than 700 nm and lacks the
negative (radiative) part.
Figure 5. Representative spectra of hybrid 3 in toluene. The sample
was excited at 575 nm (∼2 mJ cm-2).
prediction of the free energy change for the charge separation
process occurring in a nonpolar environment is more compli-
cated, and substantially more positive values are expected.57,58
This will lower the previously obtained highly negative values
of ∆Gcs for nanocluster oxidation and will significantly decrease
the predicted driving force for the photoinduced nanocluster
reduction. Overall, the photoinduced electron transfer in the
reported PDI-AuNP hybrids in both directions is theoretically
possible and should be sensitive to the polarity of the immediate
environment, favoring oxidation of the nanocluster due to more
negative ∆Gcs.
To follow the electron dynamics resulting from photoexci-
tation of the hybrids, we carried out preliminary femtosecond
transient absorption (fsTA) studies in the visible (vis) and near
infrared (NIR) spectral ranges (470-750 and 635-1000 nm,
respectively). The NIR spectral range is better suited than the
normal visible range (400-700 nm) to follow and identify the
evolution of the excited states of PDIs since almost all known
species of PDIs in the excited states absorb in this region and
have characteristic features there.35 We note that in our NIR
experiments an 808 nm notch filter was used to block the
fundamental frequency (800 nm) seeding the system, while in
the visible region experiments we used a 750 nm short pass
filter to block the fundamental. This produced a discontinuity
in the measured spectrum in the ∼745-830 nm region. The
experiments were performed on hybrids 3 and 4 in chloroform,
chloroform/methanol 60/40 v/v mixture, and toluene. The
experimental data were subjected to a global and target fit
analysis to obtain the species-associated difference spectra
(SADS) and the kinetic components of their evolution.59
Illustratively, the SADS obtained from the analysis of the fsTA
data for the same samples in different experiments (one in the
visible range and another in the NIR using slightly different
setup) match spectrally in the overlap region (635-745 nm) of
the two experiments and show very similar kinetic time
constants, indicating the stability of the experimental system
and the consistency of the data analysis methodology. The
discontinuity region is marked by gray color in Figures 5, 7,
S11, S13, S14, S16, S17, and S19 (Supporting Information).
The spectra in this region were interpolated.
The target analysis of the evolution of the fsTA signal of
hybrid 3 resulted in a satisfactory fit when performed in
accordance with the energy level diagram depicted in Scheme
3. The SADS obtained by this analysis (Figure 7) were identified
as (1) the first excited state S1 (having characteristic spectral
appearance as discussed earlier), (2) an intermediate state
spectrally resembling the S1 (designated as IS1; possibly an
exciton-like state formed due to the π-stacking of PDIs or a
vibrationally relaxed sate), and (3) an additional state not
Our studies reveal complex photoinduced behavior within 3
and 4, which is sensitive to the bridging moiety of these hybrid
systems. The target fit analysis of the experimental data shows
formation of three major photoinduced species. These species
have absorption peaks red-shifted from ∼720 nm, a spectral
region characteristic for the absorption features of PDI in the
first excited state (S1) and the radical anion of PDI (PDI•-).35
The overall decay kinetics of 4 were faster than those of 3,
Figure 6. Decay kinetics of hybrid 3 in toluene pumped at 575 nm
(∼2 mJ cm-2) and probed at the absorption peaks.

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J. Phys. Chem. B, Vol. 114, No. 45, 2010 14395
recombination. Slow conversion of S1 into CS through IS1, in
comparison with the direct S1 f CS transition and the relatively
long decay of the CS state, gives rise to a local maximum in
the kinetic profiles of hybrid 3 at ∼720 nm, ∼100-200 ps
after the excitation (see Figure 6). We note that our analysis
describes the photophysics of the assembly, and therefore all
time constants represent averaged values for many different
individual hybrids and conformations. Further characterization
of the photoinduced electron dynamics in PDI-AuNP hybrids
including additional experimental methodologies is currently
underway.
The kinetics of the CS-state formation from S1 and from the
IS1 state exhibits poor solvent dependence. It may be due to
the fact that π-stacking of the PDIs expels the solvent molecules
from the vicinity of the aromatic part of PDIs (3 is aggregated
at all solvent conditions), thus decreasing the effect of solvent
polarity, whereas the actual permittivity of a medium at short
(nanometer) distances from the nanoparticle under excitation
can be highly affected by the nanoparticle.19
In conclusion, hybrid gold nanoparticles covered by perylene-
diimide ligands (PDI) were synthesized. These hybrids aggregate
into spherical nanostructures with a size that can be tuned by
the choice of PDI ligand and solvent. Although the photoinduced
dynamics in the hybrid self-assembled systems is complicated
by the complex structure of the arrays, the polydispersity of
the systems, and the difficulty in changing the environment
polarity, transient absorption investigation of the hybrids
indicates a photoinduced electron transfer from the gold
nanoparticles to PDIs. Our study on the hybrids’ self-assembly
shows that the formation of well-defined nanostructures as well
as tuning their sizes can be achieved by employing a combina-
tion of the capping ligand choice and regulating the solvophobic
interactions between the ligands. This, combined with the photo-
and redox-active nature of the ligands, demonstrates the
possibility to create and optimize functional nanostructures via
a simple variation in the ligand linker moiety and the solvent
in the systems based on nanoparticle-dye hybrids.
Figure 7. SADS (species-associated difference spectra) obtained by
global analysis of the transient absorption data for hybrid 3 in toluene.
The “short λ experiment” and the “long λ experiment” in the legend
refer to the experiments in which the detection was performed in the
450-750 nm and 635-1000 nm ranges, respectively. States corre-
sponding to the energy diagram (Scheme 3): S1, black trace; IS1, red
trace; CS, blue trace. The rate constants of the kinetic components
associated with the SADS obtained by independent analysis of the
experiments acquired at different spectral ranges are mentioned in the
legend.
SCHEME 3: Suggested Energy Level Scheme for
Hybrid 3a
a The kinetic components found for different samples fall within the
presented ranges.
observed for the free ligand (characterized by strong absorption
in the 700-750 nm range). On the basis of the steady-state
fluorescence measurements and considering the redox potentials
of the hybrids’ components, the newly formed state 3 appears
to represent charge separation (see below). Therefore, we
designated this state as charge separated (CS). While theoreti-
cally, the light-driven electron transfer can occur in both
directions in the PDI-AuNP hybrid (see above), we believe
that our measurements point to formation of PDI•-, indicating
electron transfer from AuNP to PDI. Thus, the spectral appear-
ance in the 700-900 nm range (peaking at ∼720 nm),
characteristic of PDI•-, the absence of a positive signal in the
550-650 nm range attributed to the PDI+,60 and the absence
of absorption features associated with PDI in the triplet state61-63
support the formation of PDI•-.64-68 Chemical reduction of
compound 1 using sodium dithionate (Figure S22, Supporting
Information) showed the appearance of the 715 nm peak as the
main absorption feature of PDI•-, consistent with our analysis.
To summarize the global fit analysis (Scheme 3 and Figure
7), the S1 state of PDI is created during the pump pulse and
decays within a few picoseconds (τ ∼ 5 ( 3 ps), populating
the IS1 and the CS states. The IS1 is being converted into the
CS within tens of picoseconds (τ ) 50 ( 20 ps); therefore it
appears as a superposition of the S1-like state at wavelengths
longer than 830 nm and the rising CS signal at wavelengths
shorter than 750 nm, where the S1 absorption is relatively low,
thus appearing as a negative feature peaking at ∼720 nm.
Consequently, the CS state converts to the ground state within
>2 ns (nonradiatively), a process that can be assigned to a charge
Acknowledgment. This work was supported by grants from
the MINERVA Foundation, Divadol Foundation, Yossie and
Dana Hollander, and Nathan Minzly. Transient absorption
studies were performed at the Dr. J. Trachtenberg laboratory
for photobiology and photobiotechnology (Weizmann Institute)
and were supported by a grant from Ms. S. Zuckerman (Toronto,
Canada). The cryo-EM studies were conducted at the Irving
and Cherna Moskowitz Center for Nano and Bio-Nano Imaging
(Weizmann Institute). B.R. holds the Abraham and Jennie
Fialkow Career Development Chair. Y.R. acknowledges support
by the Helen and Martin Kimmel Award for Innovative
Investigation.
Supporting Information Available: Experimental details
of synthesis and characterization of 1 and 2, additional electron
microscopy images, UV/vis, fluorescence, and NMR spectra,
molecular models, transient absorption data, cyclic voltammo-
grams, and emission lifetime measurements. This material is
available free of charge via the Internet at http://pubs.acs.org.
References and Notes
(1) Oh, M.; Mirkin, C. A. Nature 2005, 438, 651–654.
(2) Mohr, C.; Hofmeister, H.; Radnik, J.; Claus, P. J. Am. Chem. Soc.
2003, 125, 1905–1911.
(3) Daniel, M.-C.; Astruc, D. Chem. ReV. 2003, 104, 293–346.
(4) Srivastava, S.; Kotov, N. A. Soft Matter 2009, 5, 1146–1156.
(5) Boal, A. K.; Ilhan, F.; DeRouchey, J. E.; Thurn-Albrecht, T.;
Russell, T. P.; Rotello, V. M. Nature 2000, 404, 746–748.

14396 J. Phys. Chem. B, Vol. 114, No. 45, 2010
Santosh et al.
(6) Lim, S. I.; Zhong, C.-J. Acc. Chem. Res. 2009, 42, 798–808.
(7) Maye, M. M.; Chun, S. C.; Han, L.; Rabinovich, D.; Zhong, C.-J.
J. Am. Chem. Soc. 2002, 124, 4958–4959.
(8) Maye, M. M.; Lim, I. I. S.; Luo, J.; Rab, Z.; Rabinovich, D.; Liu,
T.; Zhong, C.-J. J. Am. Chem. Soc. 2005, 127, 1519–1529.
(9) Maye, M. M.; Luo, J.; Lim, I. I. S.; Han, L.; Kariuki, N. N.;
Rabinovich, D.; Liu, T.; Zhong, C.-J. J. Am. Chem. Soc. 2003, 125, 9906–
9907.
(10) Brust, M.; Schiffrin, D. J.; Bethell, D.; Kiely, C. J. AdV. Mater.
1995, 7, 795–797.
(11) Andres, R. P.; Bielefeld, J. D.; Henderson, J. I.; Janes, D. B.;
Kolagunta, V. R.; Kubiak, C. P.; Mahoney, W. J.; Osifchin, R. G. Science
1996, 273, 1690–1693.
(12) Brust, M.; Bethell, D.; Kiely, C. J.; Schiffrin, D. J. Langmuir 1998,
14, 5425–5429.
(13) Hussain, I.; Wang, Z.; Cooper, A. I.; Brust, M. Langmuir 2006,
22, 2938–2941.
(14) Klajn, R.; Bishop, K. J. M.; Fialkowski, M.; Paszewski, M.;
Campbell, C. J.; Gray, T. P.; Grzybowski, B. A. Science 2007, 316, 261–
264.
(15) Klajn, R.; Gray, T. P.; Wesson, P. J.; Myers, B. D.; Dravid, V. P.;
Smoukov, S. K.; Grzybowski, B. A. AdV. Funct. Mater. 2008, 18, 2763–
2769.
(16) Ewers, T. D.; Sra, A. K.; Norris, B. C.; Cable, R. E.; Cheng, C.-
H.; Shantz, D. F.; Schaak, R. E. Chem. Mater. 2005, 17, 514–520.
(17) Zhuang, J.; Wu, H.; Yang, Y.; Cao, Y. C. J. Am. Chem. Soc. 2007,
129, 14166–14167.
(18) Kalsin, A. M.; Fialkowski, M.; Paszewski, M.; Smoukov, S. K.;
Bishop, K. J. M.; Grzybowski, B. A. Science 2006, 312, 420–424.
(19) Ipe, B. I.; Thomas, K. G. J. Phys. Chem. B 2004, 108, 13265–
13272.
(20) Schneider, G.; Decher, G.; Nerambourg, N.; Praho, R.; Werts,
M. H. V.; Blanchard-Desce, M. Nano Lett. 2006, 6, 530–536.
(21) Werts, M. H. V.; Zaim, H.; Blanchard-Desce, M. Photochem.
Photobiol. Sci. 2004, 3, 29–32.
(38) Dong, B.; Galka, C. H.; Gade, L. H.; Chi, L.; Williams, R. M.
Nanopages 2006, 1, 325–338.
(39) Baram, J.; Shirman, E.; Ben-Shitrit, N.; Ustinov, A.; Weissman,
H.; Pinkas, I.; Wolf, S. G.; Rybtchinski, B. J. Am. Chem. Soc. 2008, 130,
14966–14967.
(40) Krieg, E.; Shirman, E.; Weissman, H.; Shimoni, E.; Wolf, S. G.;
Pinkas, I.; Rybtchinski, B. J. Am. Chem. Soc. 2009, 131, 14365–14373.
(41) Brown, L. O.; Hutchison, J. E. J. Am. Chem. Soc. 1997, 119, 12384–
12385.
(42) Dahl, J. A.; Maddux, B. L. S.; Hutchison, J. E. Chem. ReV. 2007,
107, 2228–2269.
(43) Warner, M. G.; Hutchison, J. E. Nat. Mater. 2003, 2, 272–277.
(44) Woehrle, G. H.; Brown, L. O.; Hutchison, J. E. J. Am. Chem. Soc.
2005, 127, 2172–2183.
(45) Rapoport, D. H.; Vogel, W.; Colfen, H.; Schlogl, R. J. Phys. Chem.
B 1997, 101, 4175–4183.
(46) Weare, W. W.; Reed, S. M.; Warner, M. G.; Hutchison, J. E. J. Am.
Chem. Soc. 2000, 122, 12890–12891.
(47) Ahrens, M. J.; Sinks, L. E.; Rybtchinski, B.; Liu, W.; Jones, B. A.;
Giaimo, J. M.; Gusev, A. V.; Goshe, A. J.; Tiede, D. M.; Wasielewski,
M. R. J. Am. Chem. Soc. 2004, 126, 8284–8294.
(48) Balakrishnan, K.; Datar, A.; Naddo, T.; Huang, J.; Oitker, R.; Yen,
M.; Zhao, J.; Zang, L. J. Am. Chem. Soc. 2006, 128, 7390–7398.
(49) Kazunori, S.; Norifumi, F.; Seiji, S. Angew. Chem., Int. Ed. 2004,
43, 1229–1233.
(50) Schenning, A. P. H. J.; v. Herrikhuyzen, J.; Jonkheijm, P.; Chen,
Z.; Wu¨rthner, F.; Meijer, E. W. J. Am. Chem. Soc. 2002, 124, 10252–10253.
(51) van Herrikhuyzen, J.; Syamakumari, A.; Schenning, A. P. H. J.;
Meijer, E. W. J. Am. Chem. Soc. 2004, 126, 10021–10027.
(52) Yang, X.; Xu, X.; Ji, H.-F. J. Phys. Chem. B 2008, 112, 7196–
7202.
(53) Murray, R. W. Chem. ReV. 2008, 108, 2688–2720.
(54) Marcus, R. A. J. Chem. Phys. 1956, 24, 966–978.
(55) Rehm, D.; Weller, A. Isr. J. Chem. 1970, 8, 259–271.
(56) Chen, S.; Ingram, R. S.; Hostetler, M. J.; Pietron, J. J.; Murray,
R. W.; Schaaff, T. G.; Khoury, J. T.; Alvarez, M. M.; Whetten, R. L. Science
1998, 280, 2098–2101.
(57) Greenfield, S. R.; Svec, W. A.; Gosztola, D.; Wasielewski, M. R.
J. Am. Chem. Soc. 1996, 118, 6767–6777.
(58) Closs, G. L.; Miller, J. R. Science 1988, 240, 440–447.
(59) Hippius, C.; van Stokkum, I. H. M.; Zangrando, E.; Williams, R. M.;
Wu¨rthner, F. J. Phys. Chem. C 2007, 111, 13988–13996.
(60) Kircher, T.; Lohmannsroben, H. G. Phys. Chem. Chem. Phys. 1999,
1, 3987–3992.
(61) Ford, W. E.; Kamat, P. V. J. Phys. Chem. 1987, 91, 6373–6380.
(62) Hippius, C.; van Stokkum, I. H. M.; Zangrando, E.; Williams, R. M.;
Wykes, M.; Beljonne, D.; Wu¨rthner, F. J. Phys. Chem. C 2008, 112, 14626–
14638.
(63) Veldman, D.; Chopin, S. M. A.; Meskers, S. C. J.; Groeneveld,
M. M.; Williams, R. M.; Janssen, R. A. J. J. Phys. Chem. A 2008, 112,
5846–5857.
(64) Wolffs, M.; Delsuc, N.; Veldman, D.; van Anh, N.; Williams, R. M.;
Meskers, S. C. J.; Janssen, R. A. J.; Huc, I.; Schenning, A. P. H. J. J. Am.
Chem. Soc. 2009, 131, 4819–4829.
(65) Gosztola, D.; Niemczyk, M. P.; Svec, W.; Lukas, A. S.; Wasielews-
ki, M. R. J. Phys. Chem. A 2000, 104, 6545–6551.
(66) Marcon, R. O.; Brochsztain, S. J. Phys. Chem. A 2009, 113, 1747–
1752.
(67) Shirman, E.; Ustinov, A.; Ben-Shitrit, N.; Weissman, H.; Iron,
M. A.; Cohen, R.; Rybtchinski, B. J. Phys. Chem. B 2008, 112, 8855–
8858.
(68) Van Anh, N.; Schlosser, F.; Groeneveld, M. M.; van Stokkum,
I. H. M.; Wu¨rthner, F.; Williams, R. M. J. Phys. Chem. C 2009, 113,
18358–18368.
(22) Thomas, K. G.; Kamat, P. V. J. Am. Chem. Soc. 2000, 122, 2655–
2656.
(23) Ipe, B. I.; Thomas, K. G.; Barazzouk, S.; Hotchandani, S.; Kamat,
P. V. J. Phys. Chem. B 2001, 106, 18–21.
(24) Cheng, P. P. H.; Silvester, D.; Wang, G.; Kalyuzhny, G.; Douglas,
A.; Murray, R. W. J. Phys. Chem. B 2006, 110, 4637–4644.
(25) Imahori, H.; Kashiwagi, Y.; Endo, Y.; Hanada, T.; Nishimura, Y.;
Yamazaki, I.; Araki, Y.; Ito, O.; Fukuzumi, S. Langmuir 2003, 20, 73–81.
(26) Sudeep, P. K.; Ipe, B. I.; Thomas, K. G.; George, M. V.; Barazzouk,
S.; Hotchandani, S.; Kamat, P. V. Nano Lett. 2001, 2, 29–35.
(27) Aguila, A.; Murray, R. W. Langmuir 2000, 16, 5949–5954.
(28) Gu, T.; Whitesell, J. K.; Fox, M. A. Chem. Mater. 2003, 15, 1358–
1366.
(29) Haas, U.; Thalacker, C.; Adams, J.; Fuhrmann, J.; Riethmu¨ller, S.;
Beginn, U.; Ziener, U.; Mo¨ller, M.; Dobrawa, R.; Wu¨rthner, F. J. Mater.
Chem. 2003, 13, 767–772.
(30) Templeton, A. C.; Cliffel, D. E.; Murray, R. W. J. Am. Chem. Soc.
1999, 121, 7081–7089.
(31) Gust, D.; Moore, T. A.; Moore, A. L. Acc. Chem. Res. 1993, 26,
198–205.
(32) Wu¨rthner, F. Chem. Commun. 2004, 1564–1579.
(33) Chen, Z.; Lohr, A.; Saha-Moller, C. R.; Wu¨rthner, F. Chem. Soc.
ReV. 2009, 38, 564–584.
(34) Wasielewski, M. R. Acc. Chem. Res. 2009, 42, 1910–1921.
(35) Wasielewski, M. R. J. Org. Chem. 2006, 71, 5051–5066.
(36) Hoeben, F. J. M.; Jonkheijm, P.; Meijer, E. W.; Schenning,
A. P. H. J. Chem. ReV. 2005, 105, 1491–1546.
(37) Gade, L. H.; Galka, C. H.; Williams, R. M.; De Cola, L.; McPartlin,
M.; Dong, B.; Chi, L. Angew. Chem., Int. Ed. 2003, 42, 2677–2681.
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