Molecular Control of Internal Crystallization and Photocatalytic Function in Supramolecular Nanostructures

Article abstract of DOI:10.1016/j.chempr.2018.04.002

Supramolecular light-absorbing nanostructures are useful building blocks for the design of next-generation artificial photosynthetic systems. Development of such systems requires a detailed understanding of how molecular packing influences the material's

Full text of DOI:10.1016/j.chempr.2018.04.002

Article
Molecular Control of Internal Crystallization
and Photocatalytic Function in Supramolecular
Nanostructures
Roman V. Kazantsev, Adam J.
Dannenhoffer, Taner Aytun,
Boris Harutyunyan, Daniel J.
Fairfield, Michael J. Bedzyk,
Samuel I. Stupp
s-stupp@northwestern.edu
HIGHLIGHTS
Development of light-harvesting
perylene monoimide
nanostructures
Controlling crystal phase and
growth kinetics by using sterics
and electrostatics
Highly active charge-transfer
excitons lead to enhanced
photocatalytic ability
The development of clean fuels is an exciting area of research with the goal of
reducing the world’s reliance on fossil fuels. In our system, nanoscale ribbons are
used to collect light and transfer the energy to a catalyst. This catalyst can be used
to convert protons into hydrogen gas. Hydrogen gas represents a high-energy and
clean-burning fuel that can be used in place of fossil fuels in many applications.
Kazantsev et al., Chem 4, 1–13
July 12, 2018 ª 2018 Elsevier Inc.
https://doi.org/10.1016/j.chempr.2018.04.002

Please cite this article in press as: Kazantsev et al., Molecular Control of Internal Crystallization and Photocatalytic Function in Supramolecular
Nanostructures, Chem (2018), https://doi.org/10.1016/j.chempr.2018.04.002
Article
Molecular Control of Internal
Crystallization and Photocatalytic Function
in Supramolecular Nanostructures
Roman V. Kazantsev,1,2,8 Adam J. Dannenhoffer,3,8 Taner Aytun, Boris Harutyunyan,2,5
3
Daniel J. Fairfield, Michael J. Bedzyk,3,5 and Samuel I. Stupp1,2,3,4,6,7,9,
3
*
SUMMARY
The Bigger Picture
Supramolecular light-absorbing nanostructures are useful building blocks for
the design of next-generation artificial photosynthetic systems. Development
of such systems requires a detailed understanding of how molecular packing
influences the material’s optoelectronic properties. We describe a series of crys-
talline supramolecular nanostructures in which the substituents on their mono-
meric units strongly affect morphology, ordering kinetics, and exciton behavior.
By designing constitutionally isomeric perylene monoimide (PMI) amphiphiles,
we studied the effect of side chain sterics on nanostructure crystallization. Mol-
ecules with short amine-linked alkyl tails rapidly crystallize upon dissolution in
water, whereas bulkier tails require the addition of salt to screen electrostatic
repulsion and annealing to drive crystallization. A PMI monomer bearing a
Supramolecular chemistry, which
focuses in part on our
understanding of many molecule
systems, can be a useful tool in the
development of novel strategies
for renewable and clean energy
technologies. In plants,
assemblies of light-absorbing
molecules arrange themselves in
configurations that optimize their
ability to efficiently collect light
from the sun and utilize that
energy for the production of
useful chemicals. The function of
these systems is critically
3
-pentylamine tail was found to possess a unique structure that results in
strongly red-shifted absorbance, indicative of charge-transfer exciton forma-
tion. This particular supramolecular structure was found to have an enhanced
ability to photosensitize a thiomolybdate catalyst ((NH
hydrogen gas.
4 2 3
) Mo S13) to generate
dependent on the interactions of
molecules within nanoscale
structures present in green leaves.
The long-term objective of this
work is to learn how to design soft
materials containing light-
INTRODUCTION
Non-covalent supramolecular polymerization with designed molecules is an effec-
1
tive bottom-up strategy to synthesize functional nanostructures and materials. In
the field of chromophore assemblies, many design strategies have been developed
harvesting molecules and
2
–8
to control the resulting nanoscale morphologies, as well as the optical and trans-
catalysts to create biomimetic
systems that create fuels and
other useful products by using
sunlight as the energy source.
Here, we have synthesized
9
–11
port properties.
Several studies have also shown that changes in the motifs used
1
2,13
to drive assembly can lead to dramatic changes to the final nanostructure.
1
4
15
Examples with differences as small as a single carbon or the isomeric linkage
used have been previously demonstrated. When working with chromophore assem-
1
6–20
blies that possess highly ordered crystalline domains,
it should also be possible
molecules and developed
to control the crystal packing through slight modifications to the supramolecular
monomers, an effect that has already been documented for organic single crys-
methods to manipulate their
assembly into nanostructures that
are highly effective at collecting
light and participate in its
2
1–23
24
tals
and self-assembled monolayers.
Changes in internal order have especially dramatic effects on electronic functions of
organic materials. M u¨ llen and co-workers demonstrated the ability to increase
conversion to chemical energy.
2
5
charge-carrier mobility by tuning the twist of discotic molecular stacks, and our
group has used hydrogen bonding to modulate chromophore assembly and achieve
2
6
higher device performance in organic solar cells. The strong propensity of rylene-
based chromophores such as perylene diimide (PDI) and perylene monoimide (PMI)
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to p-p stack makes them ideal targets to investigate how molecular packing affects
7
,20,27–29
electronic properties in supramolecular systems.
One example from Adams
and co-workers revealed that the photoconductivity of PDI thin films could be
dramatically altered by controlling the extent of H and J aggregation of chrom-
3
0
phores. Our group recently developed amphiphilic PMI molecules that readily
3
1
self-assemble in water to form ribbon-shaped supramolecular assemblies. The
molecules stack in an antiparallel fashion within the cross-section of the ribbons, al-
lowing for strong interactions among molecular dipoles. This structural feature en-
ables crystallization of the molecules within the supramolecular assembly, greatly
increasing internal order compared with that of other chromophore nanostructures.
These PMI assemblies, therefore, possess unique optical and electronic properties,
such as H-aggregates with strongly red-shifted absorbance bands, suggestive of
2
2,23,32
high-mobility charge-transfer excitons. As with other organic crystals,
electronic properties of these PMI assemblies are highly sensitive to small changes
the
2
9,33
in the packing of chromophores within the nanostructures.
we found that crystalline packing of chromophore amphiphiles based on PMIs can
In our previous work,
3
3
7,34
be altered through side-chain modification at the imide or 9 position.
The latter
is of particular interest because it affords the opportunity to investigate different
crystal packing arrangements through small modular changes without affecting
the imide linkage that affords solubility in water. This should allow for the discovery
of an expanding library of PMI nanostructures with unique packing arrangements
and properties.
In this work, we characterize the structural and photophysical properties of PMI chro-
mophore amphiphiles (CAs) bearing alkyl tails attached via an amine at the 9 posi-
tion. Because of the large molecular dipole imparted by the amine, we studied
four sterically demanding alkyl groups to determine their effect on the internal order
of the supramolecular assemblies. We further explore how these differences in inter-
nal order affect the morphology of nanostructures, their electronic properties, and
capacity to photosensitize a proton reduction catalyst.
RESULTS AND DISCUSSION
X-Ray Characterization
We synthesized a series of amine-substituted CAs with propyl, isopropyl, n-pentyl,
and 3-pentyl tails (3a–3d) by using Buchwald-Hartwig coupling to a previously
reported bromo-PMI (1) (Scheme 1; see Supplemental Information for complete
synthetic details from commercially available materials). Dissolution of propyl, iso-
propyl, and n-pentyl CAs (8.7 mM) in water resulted in the formation of crystalline
supramolecular assemblies as observed by wide-angle X-ray scattering (WAXS) (Fig-
ure 1A). The scattering patterns for these three CAs were remarkably similar, indi-
cating the formation of closely related unit cells. In contrast to previously reported
1Department of Chemistry, Northwestern
University, Evanston, IL 60208, USA
2Argonne Northwestern Solar Energy Research
Center, Northwestern University, Evanston,
IL 60208, USA
3Department of Materials Science and
Engineering, Evanston, IL 60208, USA
4Department of Medicine, Northwestern
University, Chicago, IL 60611, USA
7
,31,33
CAs,
the addition of salt to screen electrostatic repulsion was not required
to achieve crystallization within the nanostructures. We hypothesize that this is
due to enhanced intermolecular attraction as a result of stronger molecular dipole
moments in amine-substituted PMIs. However, the 3-pentyl derivative revealed
5
Department of Physics and Astronomy,
Northwestern University, Evanston, IL 60208, USA
6Simpson Querrey Institute for
BioNanotechnology, Northwestern University,
Chicago, IL 60611, USA
ꢀ
clear WAXS scattering peaks only after annealing at 80 C in the presence of
5
0 mM NaCl, whereas the freshly dissolved assemblies were non-crystalline (Fig-
7Department of Biomedical Engineering,
ure 1A). Grazing incidence X-ray scattering (GIWAXS) on films of CA nanostructures
drop cast from water show similar scattering patterns as the solution WAXS, demon-
strating the crystalline nanostructures remain stable upon drying (Figure S1). Anneal-
ing of the propyl, isopropyl, or n-pentyl CA solutions in the presence of salt did not
alter their crystallinity (Figure S5), indicating that the molecules readily self-assemble
Northwestern University, Evanston, IL 60208, USA
8These authors contributed equally
9Lead Contact
*Correspondence: s-stupp@northwestern.edu
https://doi.org/10.1016/j.chempr.2018.04.002
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Scheme 1. Synthesis of Amine-Linked Alkyl-Tail Perylene Monoimide Chromophore Amphiphiles
3
a–3d.
into a thermodynamically stable crystalline packing arrangement upon dissolution
in water.
We observed that the WAXS pattern of annealed 3-pentyl was quite different from
the other three CAs, which must indicate a different type of unit cell and crystal pack-
ing arrangement in these assemblies. In order to determine the 3-pentyl CA unit cell,
the solution WAXS data were numerically fitted by modeling the PMI molecules as
thin uniform slabs with attached cylindrical and spherical motifs representing the
chains attached to the thin perylene slab through the imide nitrogen and 9-position
3
4
carbon (Figure 1B; see Supplemental Informationfor details). A schematic repre-
sentation of the crystal’s unit cell is shown in Figure 1C consisting of eight PMI mol-
ecules in a herringbone arrangement with antiparallel packing. We also note that
ꢀ
each molecule is declined 28.5 with respect to the normal of the ribbon. This partic-
ular packing arrangement is distinct from that of other CAs designed in our labora-
7
,31,33,34
tory previously,
suggesting there is an extensive library of structures that
could exist through further modification of the PMI monomers and the pathways
used to produce supramolecular structures. A single crystal of 3-pentyl CA was
grown and found to organize into an antiparallel packing arrangement, as observed
in water above, indicating its low energy nature in both solution and solid states (Fig-
ure S8). This single crystal structure was taken into account when determining the
best numerical fit for the solution data. The unit cells of propyl, isopropyl, and n-pen-
tyl could not be determined by our model because of an insufficient number of fea-
tures in the scattering patterns or lack of unique numerical solution for the unit cell.
However, we expect that these three CAs possess a different unit cell than the 3-pen-
tyl, given that the scattering peaks are distinct. We hypothesize that the 3-pentyl CA
crystallizes into a unique unit cell because of the increased steric demand of its bulky
substituent when molecules pack to form nanostructures. Furthermore, the offset
between adjacent molecules may act to reduce steric interactions among alkyl tails
extending from the 9 and imide positions of PMI when molecules assemble into
nanostructures.
Microscopy
Cryogenic transmission microscopy (cryo-TEM) was used to examine the nanostruc-
ture morphology of the crystalline assemblies. Dissolution of propyl CAs in water re-
sulted in the formation of supramolecular scrolls that are 120 G 10 nm in width
and ꢁ600 nm in length (Figure 2A). Isopropyl CAs formed instead ꢁ500-nm-wide
sheets that curved at the edges, whereas n-pentyl CAs formed smaller curved rib-
bons in both length and width (Figures 2B and 2C). Freshly dissolved 3-pentyl CA
solutions showed the presence of thin fibers that were several hundred nanometers
in length, in contrast to the roughly ꢁ750-nm-wide ribbons that were several
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Figure 1. X-Ray Scattering of 3a–3d
(
(
A) WAXS patterns of CA aqueous solutions (7.25 mM).
B) Experimental and fitted WAXS curves of 3-pentyl CA solutions (7.25 mM) with peaks indexed for
the full range of q values.
C) Projection of the basis of 3-pentyl CA onto the plane of the 2D sheet with selected parameter
(
values determined from the analysis (longer outlined rectangles represent the perylene fused rings;
solid and dashed wedges represent the carboxypentyl chain above and below the 2D plane,
respectively).
4
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Figure 2. Visualization of Nanostructures
Cryo-TEM images of propylamine (A), isopropylamine (B), n-pentylamine (C), 3-pentylamine freshly
dissolved (8.7 mM; white arrows indicate the location of nanostructures) (D), and 3-pentylamine
annealed (7.25 mM) in 50 mM NaCl (E). AFM images of propyl (F), isopropyl (G), n-pentyl (H), 3-
pentyl freshly dissolved (I), and 3-pentyl annealed (J) CAs are shown. All samples for AFM images
were diluted 103 from samples used for TEM and then spun cast onto a silicon substrate.
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Figure 3. Visible Absorbance of 3a–3d
(
A) Absorbance spectra of propyl, isopropyl, n-pentyl, 3-pentyl annealed, and 3-pentyl derivatives
dissolved as monomers in DMSO (8.7 mM).
B) Energy-level diagram showing energy splitting due to transition dipole coupling and splitting
(
due to Frenkel exciton (FE) and charge-transfer (CT) state mixing. The resulting CT-exciton state is
lower in energy than that corresponding to the monomer energy (black arrow). The transitions to
the FE and CT-exciton states (blue and red arrows) are indicated in the absorbance spectra in (A) by
blue and red dotted lines.
micrometers long observed after annealing in 50 mM NaCl solutions (Figures 2D
and 2E). This structural transformation of 3-pentyl CA and the morphologies of the
other CAs were confirmed by atomic force microscopy (AFM) (Figures 2F–2J). The
heights of the nanostructures are consistent with an antiparallel packing of CA mol-
3
1,34
ecules, as observed previously for other PMI nanostructures.
Optical Properties
The optical absorbance behavior of organic crystals is highly sensitive to sub-
2
9,35–38
˚
Angstr o¨ m displacements in molecular packing arrangements.
As the crystal
packing arrangement between propyl, isopropyl, and n-pentyl are similar, the opti-
cal response of the assemblies is also expected to be the same. Indeed, we observe
similar absorbance maxima at 535 nm for the propyl, isopropyl, and n-pentyl CAs.
Furthermore, all three absorbance spectra possess line shapes that indicate the for-
3
9
mation of vibrationally decoupled excitons, as observed previously for crystalline
3
1
CA assemblies (Figure 3A). On the other hand, the absorbance spectrum of 3-pen-
tyl CA was dramatically different from the other three. In the non-crystalline state,
l
max appeared at 583 nm, and the spectrum resembled that of other non-crystalline
3
1
PMI assemblies (Figure S4). Upon crystallization, we observed the appearance of a
strong low-energy absorbance feature at 750 nm that was red-shifted in relation to
the monomer absorbance (Figure 3A). Given that the absorbances for all PMI mono-
mers were virtually identical in effective organic solvents to disaggregate chromo-
phores such as DMSO, in which no assembly is expected, the spectral differences
observed arise exclusively from differences in molecular packing of the various crys-
talline structures (Figure S3).
Organic assemblies are well known to display both blue- and red-shifted absorbance
peaks relative to the monomer absorbance maxima. These features commonly result
from the long-range coulombic coupling of the chromophore’s transition dipole
4
0–43
moments (TDMs) and lead to the well-known H- and J-aggregates.
For organic
crystals, short-range, nearest-neighbor charge transfer (CT) coupling between over-
lapping p-orbital wavefunctions can also play a significant role in the overall exciton
4
4,45
character of the ensemble system.
TDM coupling is primarily influenced by large
6
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shifts in the orientation of chromophores in space, e.g., head to tail or face to face,
3
5
˚
whereas CT coupling is highly sensitive to sub-Angstr o¨ m molecular displacements.
Although some orbital overlap is certainly present in non-crystalline systems, the
disordered nature of the aggregate prevents a consistent CT coupling phase be-
tween each chromophore pair. CT coupling can only become a pronounced,
ensemble-wide effect in crystalline systems because the orbital orientation between
pairs of molecules are identical across the crystal. In certain cases, these two modes
of coupling can mix, creating an entirely new electronic structure with so-called CT
3
6,46,47
excitons.
feature to appear even in H-aggregated systems (Figure 3B), which has been previ-
When this happens, it is possible for a new low-energy absorbance
4
7,48
ously observed in perylene diimide-based crystals.
Analyzing the spectral data
in our work from this perspective, the absorbance spectra of propyl, isopropyl,
and n-pentyl CAs show a strongly blue-shifted peak common for H aggregates (Fig-
ure 3B, blue arrow), and the lack of a strong red-shifted absorbance feature indicates
little mixing between the TDM and CT states. The unique packing environment of
3
-pentyl crystals modifies the CT coupling between monomers resulting in greatly
enhanced TDM/CT mixing. This mixing is manifested as a strong absorbance feature
that is red-shifted in relation to the monomer absorbance (Figure 3B, red arrow). This
red-shifted feature cannot be caused by J-like transition dipole coupling because the
3
-pentyl CA molecules are still locked in a face-to-face stacking arrangement.
Together, the data suggest that the molecular packing in the 3-pentyl CA crystal
phase enables a unique intermolecular coupling environment compared with its
non-crystalline state or the other crystalline CAs studied here. The unique red-shifted
peak also gives a distinct spectral signature only present in the 3-pentyl crystal.
Phase Transition
In order to understand the non-crystalline to crystal phase transition of 3-pentyl CAs,
variable temperature (VT) X-ray, light-scattering, and absorbance techniques were
used to monitor the transformation in situ. The transition can be readily tracked by
VT-absorbance spectroscopy (VT UV-vis) by monitoring the 750 nm feature that is
ꢀ
unique to this crystalline phase. Upon heating a solution of 3-pentyl at 1 C/min, a
ꢀ
sharp increase in the absorbance intensity was observed around 70 C (Figure 4A),
corresponding to the appearance of the crystalline phase. Upon cooling, the absor-
bance intensity remains constant, indicating the stability of that phase. WAXS pat-
terns were collected on solutions in a temperature-controlled capillary heated to
ꢀ
8
0 C over 20 min. During this time interval, scattering peaks corresponding to for-
mation of the crystalline phase appeared gradually (Figure 4B). Small-angle X-ray
scattering (SAXS) traces collected over this same time interval showed a progression
of the Porod slope in low q region to a final value of ꢂ2, consistent with the formation
of 2D nanostructures observed by cryo-TEM (Figure S9). This correlation between
the appearance of diffraction peaks by WAXS and a ꢂ2 Porod slope in SAXS is
consistent with nucleation and growth of flat crystalline ribbons. Finally, VT-dynamic
light scattering (VT-DLS) showed an irreversible increase in scattering upon anneal-
ing, further supporting the morphological transition to larger 2D structures observed
in SAXS experiments (Figure 4C).
We used VT UV-vis to investigate the activation energy barrier that controls the
nucleation and growth kinetics of crystalline 3-pentyl CA nanostructures. We carried
out isothermal heating experiments by preparing non-crystalline 3-pentyl CA sam-
ꢀ
ꢀ
ꢀ
ꢀ
ples in 50 mM NaCl and annealing at 70 C, 75 C, 80 C, and 85 C while monitoring
the absorbance at 750 nm. The phase transition revealed a linear dependence of the
logarithm of time with heating temperature (Figure 4D, inset), allowing us to use pre-
viously established kinetic models to determine an activation barrier of 85 G 8 kJ/
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Figure 4. Phase Transformation of 3d
(
A) Absorbance at 750 nm as a function of temperature for 7.25 mM 3-pentylamine solutions containing 50 mM NaCl (heating and cooling rates
ꢀ
were 1 C/min).
ꢀ
(
(
(
(
B) WAXS curves for 3-pentylamine solutions (7.25mM) containing 50 mM NaCl obtained at different temperatures. The samples were heated to 80 C
top scan), and scans were collected every 30 s
ꢀ
C) DLS intensity as a function of temperature for a 7.25 mM 3-pentylamine solution containing 50 mM NaCl (heated at 1 C/min).
D) Arrhenius plot for calculating activation energy from isothermally annealed samples of 3-pentyl CA by using Equation S1 in the Supplemental
Information. Error bars represent the standard deviation from n = 3 samples. The inset shows the average x(t) as a function of time for three 0.87 mM
solutions of 3-pentylamine CA, where x(t) represents the percentage conversion of 3-pentyl CA from the non-crystalline (x(t) = 0) to crystalline (x(t) = 1)
form of the assemblies.
ꢀ
(
E) DSC of 3-pentylamine CA solutions containing 50 mM NaCl heated at 80 C ([3-pentylamine] = 7.25 mM) revealing a phase transition with an enthalpy
change of ꢂ6.5 kJ/mol.
F) Schematic representation of the energy landscape of 3-pentylamine nanostructures shows the transition from non-crystalline aggregates to
(
crystalline nanoscale ribbons. The activation energy E
DS and DG are only represented schematically.
a
and enthalpy DH were directly measured, but the entropic contribution DS was not. Thus, both
7
,49
mol (Figure 4D; see Supplemental Information for kinetic model details).
This
barrier represents the energy needed for nucleus formation and subsequent growth
into crystalline ribbons. We also used differential scanning calorimetry (DSC) to mea-
ꢀ
sure the DH value associated with crystallization at 70 C and found it to be equal to
ꢂ6.5 kJ/mol (see Figure 4E). We believe crystallization within the nanostructures has
both an enthalpic and entropic driving force because CA molecules in the less-or-
dered assemblies are likely to be hydrated. Both factors, as well as the activation en-
ergy, are represented in the schematic energy landscape shown in Figure 4F.
Supramolecular Landscape
Because attractive p-p stacking and dipole-dipole forces that drive CA crystalliza-
tion must be balanced by electrostatic repulsion among CA head groups, one
expects that the solution ionic strength will greatly influence crystal formation in
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3
-pentyl assemblies as a result of electrostatic screening effects. Indeed, we found
that below 47 mM ionic strength, a phase transition to the crystalline structure after
annealing did not occur, as indicated by absorbance experiments (Figure S10). This
result suggests that a significant activation energy is necessary to nucleate crystalline
assemblies without electrostatic screening by dissolved ions. Thus, at 47 mM ionic
strength, the supramolecular system does not order into a periodic lattice but retains
the ordering characteristic of the hydrophobic collapse of amphiphilic molecules in
water. We investigated this further by pre-forming 3-pentyl CA crystalline assem-
blies and then diluting the surrounding water to reduce the ionic strength. We found
the system retained the absorbance spectrum characteristic of the crystalline nano-
structures above 12 mM ionic strength, but lost the distinctive red-shifted peak
below 6 mM (Figure S11). Thus, the crystalline phase, once formed under conditions
of high ionic strength, is either thermodynamically stable or in a deep kinetic trap
even at low ionic strength. The results also suggest that electrostatic repulsion
between monomeric units creates an energy barrier for crystalline nuclei to form.
A critical ionic strength of 47 mM must be reached to sufficiently screen the electro-
static repulsion and allow nucleation and growth to occur (changes in the energy
landscape as a function of ionic strength are summarized in Figure S12).
Our data suggest that antiparallel crystalline packing of the PMI molecules investi-
gated are strongly affected by steric interactions between alkyl chains at the imide
and 9 positions. In this work, two pairs of constitutional isomers (propyl/isopropyl
and n-pentyl/3-pentyl) were used to investigate this effect. When steric demand is
low (propyl/isopropyl), the assemblies formed upon dissolution in water have iden-
tical, thermodynamically favorable crystalline packing and absorbance spectra. As
suggested above, interactions among these substituents plus other attractive inter-
actions (p-stacking and dipolar) overcome electrostatic barriers to crystallization in
these light absorbing amphiphiles. Lengthening the tail increases the competitive ef-
fect of steric bulk on nanostructure crystallization. With the n-pentyl tail, assemblies
resemble those of propyl/isopropyl in crystal packing and absorbance, but the nano-
structures are smaller likely as a result of increased steric interaction from the two
additional carbons. In the most sterically demanding case, the 3-pentyl CAs lose
the ability to crystallize upon initial dissolution, indicating that steric interactions frus-
trate the molecule’s inherent propensity to crystallize. Once crystallization of the
3
-pentyl CA is achieved, it forms a distinct packing arrangement from the rest of
the CAs studied here. Because all four CA molecules differ only in the structure of
their alkyl tails, we conclude that steric interactions among 3-pentyl substituents
are responsible for the electrostatic sensitivity of this compound to organize into crys-
talline assemblies. This information guides molecular design to optimize synthesis
versus function in these light-absorbing chromophores for photocatalysis.
Photocatalytic Hydrogen Production
Having established differences between the crystalline packing and the electronic
properties of the assemblies investigated, we explored how variances in supramo-
lecular structure affect the ability of these light-absorbing scaffolds to photosensitize
a proton reduction catalyst. The various CAs were tested by first forming hydrogels
with the addition of the polycation (polydiallyl dimethyl ammonium chloride) and
illuminating them in the presence of a thiomolybdate cluster (proton reduction cata-
lyst; Figure 5, inset) and ascorbic acid (source of protons and sacrificial electron
donor). We note that the larger nanostructures of the 3-pentyl CA lead to the forma-
tion of more robust hydrogels. However, a more detailed study of how morphology
affects H
2
production is beyond the scope of this paper because it requires systems
without the complication of different crystalline phases. The crystalline 3-pentyl CA
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Figure 5. Photocatalytic Hydrogen Production
Turnover numbers for the photocatalytic synthesis of hydrogen within hydrogels prepared with
propyl, isopropyl, n-pentyl, freshly dissolved 3-pentyl, and annealed 3-pentyl nanostructures as the
light-harvesting assemblies and poly (diallyl dimethylammonium) chloride. Error bars represent the
standard deviation from n = 4 samples. The hydrogels contain ascorbic acid as a sacrificial electron
donor, and the inset shows the structure of the molybdenum sulfide cluster used as a catalyst.
outperformed all other CAs studied, yielding nearly three times more H
tical experimental conditions with a maximum turnover number of around 2,000
Figure 5). We observe that there is a direct correlation between the appearance
of the red-shifted absorbance feature in 3-pentyl CA crystals and the enhancement
in observed H production. Theoretical work has suggested that the ability of an
exciton to diffuse and separate in an organic crystal is directly tied to mixing of
2
under iden-
(
2
2
9,35
TDM and CT states.
Given that the spectral features of 3-pentyl CA assemblies
are in excellent agreement with theoretical absorbance spectra of highly mobile CT
2
9
excitons, the excitation energy can likely charge separate and diffuse larger dis-
3
5
tances before being transferred to a catalyst on the surface of the nanostructure.
Exciton diffusion is expected to occur to a far smaller degree in non-crystalline
-pentylamine assemblies and crystalline CA assemblies of propyl, isopropyl, and
3
n-pentyl that do not bear a red-shifted CT exciton. These results demonstrate that
crystal packing and its influence on TDM/CT mixing are important parameters for
the design of light-absorbing supramolecular scaffolds for photocatalysis.
Conclusions
We have studied several visible-light-absorbing chromophore amphiphiles that
assemble in water to create supramolecular scaffolds. In the process of tuning steric
interactions among the supramolecular monomers forming these scaffolds, we iden-
tified unique morphologies and crystal packing modes that yield highly active as-
semblies for photocatalysis. Only crystalline assemblies that support charge-transfer
excitons seem to be particularly useful for photocatalytic activity. Thus, it is impor-
tant to understand how rational design of monomers can be used to create the
most functional supramolecular nanostructures. In the future, atomistic simulations
may accelerate the search for nanostructures with optimal supramolecular ordering.
EXPERIMENTAL PROCEDURES
Full experimental details are provided in the Supplemental Information.
DATA AND SOFTWARE AVAILABILITY
The accession number for 9-(N-3-pentylamino)-N-(hexanoic acid) perylene-3,4-
dicarboximide is CCDC: 1578290.
10
Chem 4, 1–13, July 12, 2018

Please cite this article in press as: Kazantsev et al., Molecular Control of Internal Crystallization and Photocatalytic Function in Supramolecular
Nanostructures, Chem (2018), https://doi.org/10.1016/j.chempr.2018.04.002
SUPPLEMENTAL INFORMATION
Supplemental Information includes Supplemental Experimental Procedures, 44
figures, 1 table, 3 schemes, and 1 data file and can be found with this article online
at https://doi.org/10.1016/j.chempr.2018.04.002.
ACKNOWLEDGMENTS
This work was supported as part of the Argonne-Northwestern Solar Energy Research
Center, an Energy Frontier Research Center funded by the Basic Energy Sciences pro-
gram of the US Department of Energy Office of Science under award no. DE-
SC0001059. Use of the Advanced Photon Source (APS) was supported by the Basic
Energy Sciences program of the US Department of Energy Office of Science under con-
tract no. DE-AC02-06CH11357. Variable temperature X-ray experiments were per-
formed at the DuPont-Northwestern-Dow Collaborative Access Team (DND-CAT)
located at Sector 5 of the APS. DND-CAT is supported by Northwestern University,
E.I. DuPont de Nemours & Co., and the Dow Chemical Company. WAXS experiments
were conducted at BioCARS Sector 14 at the APS and were supported by grants from
the National Center for Research Resources (5P41RR007707) and the National Institute
of General Medical Sciences (8P41GM103543) from the National Institutes of Health.
GIXS data were collected at Sector 8-ID-E of the APS. We thank the Biological Imaging
FacilityatNorthwesternfortheuseofTEMequipmentandtheElectronProbeInstrumen-
tation Center facilities of the Northwestern University Atomic and Nanoscale Character-
ization Experimental Center for the use of SEM equipment. NMRand massspectroscopy
equipment at the Integrated Molecular Structure Education and Research Center was
supported by the National Science Foundation under CHE-9871268. We thank Char-
lotte Stern for assistance with X-ray crystal structure determination and Oliver Dumele
for helping format the Supplemental Information.
AUTHOR CONTRIBUTIONS
R.V.K. and A.D. conceived the project. A.D. conducted the synthesis and physical
characterization and data analysis. R.V.K. was involved in the physical characteriza-
tion and data analysis. T.A. performed AFM measurements. B.H. completed the
X-ray analysis. D.J.F. collected GIWAXS data. M.J.B and S.I.S. provided advice
and helped with writing the manuscript.
DECLARATION OF INTERESTS
The authors declare no competing interests.
Received: October 25, 2017
Revised: November 28, 2017
Accepted: April 4, 2018
Published: May 3, 2018
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