Toward oriented surface architectures with three coaxial charge-transporting pathways

Article abstract of DOI:10.1021/ja405776a

We report a synthetic method to build oriented architectures with three coaxial π-stacks directly on solid surfaces. The approach operates with orthogonal dynamic bonds, disulfides and hydrazones, self-organizing surface-initiated polymerization (SOSIP), and templated stack-exchange (TSE). Compatibility with naphthalenediimides, perylenediimides, squaraines, fullerenes, oligothiophenes, and triphenylamine is confirmed. Compared to photosystems composed of two coaxial channels, the installation of a third channel increases photocurrent generation up to 10 times. Limitations concern giant stack exchangers that fail to enter SOSIP architectures (e.g., phthalocyanines surrounded by three fullerenes), and planar triads that can give folded or interdigitated charge-transfer architectures rather than three coaxial channels. The reported triple-channel surface architectures are as sophisticated as it gets today, the directionality of their construction promises general access to multichannel architectures with multicomponent gradients in each individual channel. The reported approach will allow us to systematically unravel the ultrafast photophysics of molecular dyads and triads in surface architectures, and might become useful to develop conceptually innovative optoelectronic devices.

Full text of DOI:10.1021/ja405776a

Article
pubs.acs.org/JACS
Toward Oriented Surface Architectures with Three Coaxial Charge-
Transporting Pathways
Giuseppe Sforazzini, Edvinas Orentas,† Altan Bolag, Naomi Sakai, and Stefan Matile*
Department of Organic Chemistry, University of Geneva, Geneva, Switzerland
S
* Supporting Information
ABSTRACT: We report a synthetic method to build oriented
architectures with three coaxial π-stacks directly on solid surfaces.
The approach operates with orthogonal dynamic bonds, disulfides and
hydrazones, self-organizing surface-initiated polymerization (SOSIP),
and templated stack-exchange (TSE). Compatibility with naphthale-
nediimides, perylenediimides, squaraines, fullerenes, oligothiophenes,
and triphenylamine is confirmed. Compared to photosystems
composed of two coaxial channels, the installation of a third channel
increases photocurrent generation up to 10 times. Limitations concern
giant stack exchangers that fail to enter SOSIP architectures (e.g.,
phthalocyanines surrounded by three fullerenes), and planar triads that
can give folded or interdigitated charge-transfer architectures rather than three coaxial channels. The reported triple-channel
surface architectures are as sophisticated as it gets today, the directionality of their construction promises general access to
multichannel architectures with multicomponent gradients in each individual channel. The reported approach will allow us to
systematically unravel the ultrafast photophysics of molecular dyads and triads in surface architectures, and might become useful
to develop conceptually innovative optoelectronic devices.
ization with weak noncovalent interactions and polymerization
with dynamic covalent disulfide exchange chemistry13 imple-
ments central lessons from protein chemistry. The obtained
ladderphane14 architecture X has been realized for central
stacks of naphthalenediimides (NDIs),12 perylenediimides
(PDIs),15 and oligothiophenes.16,17
INTRODUCTION
■
In nature, functional systems have complex structures of highest
sophistication.1 Little is known about what we would get from
similarly sophisticated organic materials, probably because of
the lack of general synthetic methods to prepare them. This is
still the case despite the promise of such methods to open the
door to the materials of the future and much efforts worldwide
to tackle this fundamental challenge in organic synthesis.2−10
To contribute to these efforts, we became interested in the
development of synthetic methods to grow functional multi-
component architectures directly on solid surfaces. Such
methods are necessary to build functional systems with
oriented gradients for cascade processes as in biological
systems. Our initial attempts with zipper assembly, although
successful, were synthetically too demanding to ensure rapid
progress toward general use.11 Facile access to complex
systems, without extensive synthetic effort, was finally secured
with self-organizing surface-initiated polymerization (SOSIP).12
For SOSIP, propagators X1 are recognized by initiators X2
or growing polymers on solid surfaces (Figure 1). Both X1 and
X2 contain a central aromatic core that is expected to form
functional, charge-transporting π-stacks in the final SOSIP
architectures X. These functional channels are embedded in
self-organizing, hydrogen-bonded networks from peptide-like
structures. Initiators further contain diphosphonate groups to
bind to indium tin oxide (ITO) surfaces and thiolates to initiate
the polymerization. For ring-opening disulfide-exchange
polymerization, propagators X1 are equipped with strained
cyclic disulfides. The envisioned combination of self-organ-
To expand the SOSIP approach toward more complex
architectures, templated self-sorting (TSS)18 and templated
stack exchange (TSE)16,17,19,20 have been developed. In TSS,
initiators are used as templates on the surface to grow uniform
stacks next to each other by co-SOSIP. This transcription of
information from 2D monolayers into 3D architectures can be
achieved with up to 97% fidelity.18 In TSE, the covalent
counterpart to TSS, SOSIP architectures are grown in the
presence of templates on the surface and along the π-stack.19
The templates along the SOSIP stack are attached with a
hydrazone bridge,21 that is a covalent dynamic bond22
orthogonal to the disulfides13 in X. The templates along the
SOSIP stack are then removed with hydroxylamine, and the
empty space left behind in architectures X3 is filled with
aldehydes Y of free choice. The result is a double-channel
architecture XY, characterized by two coaxial stacks of different
nature. We have demonstrated successful construction of such
double-channel architectures with NDIs,19 triphenylamines,19
PDIs,16 fullerenes,17 phthalocyanines,20 and porphyrins20 as the
second channel.
Received: June 9, 2013
Published: August 1, 2013
© 2013 American Chemical Society
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surface architectures will naturally contain defects. However,
experimental evidence exists that the dynamic covalent
chemistry used for SOSIP-TSE can minimize these defects by
self-repair.12 Triple-channel photosystems were fascinating
because they offer as surface architectures what has been
appreciated for decades as molecular triads24 in solution. Better
coverage of the solar spectrum as well as the long-lived charge
separation due to stepwise electron transfer are some of the
most attractive characteristics of triads. So far, little is known on
molecular triads in surface architectures,5j presumably because
general preparation methods were not available. In the
following, we describe our attempt to synthesize triple-channel
architectures containing several classical components of
optoelectronic systems, that is NDIs,3 PDIs,4 oligothiophenes,5
fullerenes,6 squaraines,7 phthalocyanines or porphyrins,8 and
triphenylamines10 (Figure 2). The results reveal exceptional
structural tolerance of the SOSIP-TSE approach and high
functional importance of the newly introduced third channel.
The main limitations concern giant stack exchangers and
completely planar triads; the best activities were obtained for
triad photosystems composed of NDIs, squaraines and
fullerenes (Figure 3) or NDIs, PDIs and triphenylamines.
RESULTS AND DISCUSSION
■
General Design. The synthesis of triad photosystems XYZ
was based on single-channel SOSIP architectures A with NDI
stacks (Figure 2). This selection was made because SOSIP with
NDI stacks is best understood. With a low-lying LUMO, this
central NDI stack can also act as an electron-transporting
channel in triple-channel architectures (Figure 2, bottom left).
To secure the necessary space next to the NDI stacks for
incoming stack exchangers, NDI templates on the ITO surface
were used because they were shown to occupy enough space to
allow nearly quantitative TSE of stack exchangers as large as
phthalocyanines, and function as hole blockers.20 Such size
tolerance is counterintuitive but understandable considering
that with two NDI templates per initiator the space reserved for
TSE is more than twice as large as that occupied by one central
NDI.
Along the channel X obtained from SOSIP, channel Y is
attached using the hydrazone chemistry developed for TSE
(Figure 2). This study focuses on triad architectures XYZ with
squaraines B,7 phthalocyanines C,8 and PDIs D4 in channel Y.
Phthalocyanines C are green chromophores with properties
similar to chlorophyll. Squaraines B have been introduced more
recently as a more compact, better soluble phthalocyanine
mimic. With a high-lying HOMO, squaraines B and
phthalocyanines C are hole transporters. The complementary
triad architecture with an electron-transporting channel in the
middle was explored with the red PDI D.
Along this second channel B, C, or D, channel Z is attached
via an oxime bridge (Figure 2). As oximes are more stable
relatives of hydrazones,25 formation of dyads YZ followed by
TSE with SOSIP architecture X3 should give the triad
architecture XYZ without problem. Hole-transporting squar-
aines B and phthalocyanines C in channel Y are combined with
electron transporters in channel Z. In the fullerene E, this
powerful electron acceptor is wrapped in solubilizing TEG-
tails.17 The planar, core-expanded NDI F has attracted some
recent attention because a very low LUMO is combined with a
red-shifted absorption and excellent charge mobility in FETs.3e
With electron-transporting PDI B in channel Y, hole trans-
porters were placed in channel Z. A slightly deplanarized
Figure 1. The SOSIP-TSE approach to double-channel surface
architectures. Disulfide-exchange SOSIP yields the single-channel
architecture X, TSE with chemo-orthogonal hydrazone exchange yields
the double-channel architecture XY.
Nanostructured double-channel architectures are of interest
in artificial photosystems because the coaxial alignment of
transport pathways for electrons (e, n) and holes (h, p) with
the width of molecules is expected to ensure efficient
photoinduced charge separation at the maximized contact
area, whereas charge mobility in the separate channels should
remain high. Such architectures are referred to as supra-
molecular n/p-heterojunctions (SHJs).2,23 Whereas several
elegant approaches to SHJs have been reported in the recent
literature, SOSIP-TSE is unique in the sense that it provides
orientational control of SHJs relative to the surface. This
directionality is essential to engineer antiparallel redox gradients
into both channels.19,20 Oriented antiparallel gradients
(OMARGs) are expected to drive holes and electrons in
opposite directions after their generation upon irradiation, a bit
like in biological photosystems.1 Already two-component
gradients in OMARG-SHJ photosystems made by SOSIP-
TSE were sufficient to significantly reduce charge recombina-
tion efficiencies.19,20
At this point, “triple-channel” photosystems XYZ were the
obvious next big step to take toward architectures with higher
complexity (Figure 2). The term “triple-channel” is used here
to refer to architectures XYZ that are likely to contain three
coaxial charge-transporting pathways. This is done for
convenience only. Moreover, it is understood that all reported
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Figure 2. Triad architectures XYZ explored in this study, with energy levels of HOMO (bold) and LUMO (dashed) of electron (n) and hole (p)
transporting components (in eV against vacuum, normalized for −5.1 eV for Fc/Fc+).
Figure 3. Synthesis and full structure of triple-channel architecture ABE. (a) Pd(PPh3)2Cl2, n-Bu3SnH, p-NO2 phenol, (b) TFA.
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oligothiophene H5i is compared with a nonplanar triphenyl-
apparent hypochromism caused by the aggregation7e and the
sensitivity of squaraines to nucleophiles;7f thus yields based on
the absorbance values of neither the films nor the films
dissolved with 2-mercaptoethanol were meaningful. Therefore,
we report here only the yield relative to that of NDI-squaraine
double-channel photosystem AB* (Table 1). In light of the
amine G.10
Triad Architectures with Squaraines and Fullerenes.
Squaraines B have never been used before in SOSIP-TSE
architectures. To serve as central channel Y in triple-channel
architectures XYZ, the squaraines had to be equipped with a
protected alkoxyamine on one side for the covalent capture of
channel Z and an orthogonally protected aldehyde on the other
side for TSE with the single-channel SOSIP architecture X
(Figure 3). In squaraine B1, Alloc and acetal protection are
used for this purpose. Squaraine B1 was readily accessible by
the coupling of the known dicarboxylic acid7b with the
corresponding amines. Pd-mediated Alloc removal yielded the
desired squaraine B. Oxime formation with fullerene E followed
by acetal removal gave the desired squaraine-fullerene dyad BE
in good yield under very mild conditions.
Triad photosystem ABE was then obtained from A by TSE.
The templates along the NDI stacks were first removed with
excess hydroxylamine following the reported procedures
(Figure 1, X = A). The obtained architecture A3 was then
immersed into a 4 mM solution of dyad BE in DMSO for the
covalent capture of the aldehydes BE by the reactive hydrazides
along the NDI stacks in A3. The formation of triad
photosystem ABE by TSE was monitored by absorption
spectroscopy. The absorption of the dyad BE in solution is
dominated by the strong squaraine band at λmax = 660 nm
(Figure 4a, dashed). The signature of fullerenes is a steady
Table 1. Summary of Synthesis and Activity of Triad
Photosystems
a
b
c
d
XYZ
MW
shape
planar
yield
J
g
A
0.1
0.9
9.9
e
AB*
ABE
ABF
AC*
ACE
AD*
ADG
ADH
892
2137
1485
1426
5326
802
planar
100%
e
nonplanar
planar
85%
e
82%
0.04
0.27
f
g
planar
nd
g
nonplanar
planar
2%
1.1
g
96%
92%
87%
3.7
g
973
nonplanar
planarizable
9.8
g
1034
1.2
h
g
AD*/G
60%
4.6
a
For the structure of each component, see Figure 2. The thickness of
films was ∼20 nm, estimated on the basis of the absorbance of
b
photosystems A (∼0.07 at 380 nm). Molecular weights of stack
c
d
exchangers. TSE yield. Photocurrent densities (μA cm−2) generated
upon irradiation with the simulated sunlight (28 mW cm−2) in the
e
presence of ascorbic acid. TSE yield relative to that of AB* system.
f
Not determined. Analogous double-channel system with oxygen-
substituted NDI as X gave quantitative yield.20 TEOA was used
g
instead of ascorbic acid. Lower photocurrents were observed with
h
ascorbic acid. TSE yield of D* after 1 day of TSE.
previous observation with large stack exchangers like
phthalocyanines, the TSE of smaller and well soluble squaraine
B* is expected to proceed in nearly quantitative yield. If so, the
TSE yield of squaraine-fullerene BE calculates to 85%. In any
case, similar TSE yield observed for squaraine B* and
squaraine-fullerene conjugate BE demonstrated the remarkable
tolerance of TSE to structural variations, and provided support
for the existence of triad photosystem ABE as depicted in
Figure 3. AFM images of triad architecture ABE showed
characteristic features of SOSIP architectures: smooth surfaces
in height images and low-defect long-distance self-organization
down toward the molecular level in phase-contrast images
(Supporting Information, Figure S4). Thus, evidently the highly
ordered structure is intact after TSE with large dyads.
Photocurrent generation was evaluated with the triad
photosystem ABE as a working electrode, a Pt electrode as
cathode, and ascorbic acid as mobile hole acceptor in solution
(Figure 5, Table 1). These are very simple, nonoptimized
conditions, suited for studies on conceptual innovation from
the point of view of supramolecular organic chemistry. Results
obtained under identical conditions give trends needed to draw
valid conclusions but are naturally not comparable to results
from optimized devices.
Figure 4. (a) Absorption spectra of dyad BE (blue dashed line),
single-channel architecture A3 (gray dotted line), and triple-channel
architecture ABE (solid purple line). (b) Action spectra of triad
architecture ABE (solid purple line, filled circles) in comparison to
double-channel architectures AB* (blue dashed line, empty circles)
and AE (green hatched line, filled squares). Incident photon-to-current
efficiencies (IPCEs) were normalized against that of AB* at 660 nm.
increase in absorption beginning around 450 nm. The
absorption spectrum of the SOSIP architecture A3 before
TSE shows NDI bands below 400 nm, including partially
resolved vibrational finestructures (Figure 4a, dotted). These
finestructures are still visible after TSE along with the above-
mentioned signatures of squaraines and fullerenes (Figure 4a,
solid). Thus, these results indicated the good incorporation of
dyads BE in the photosystems.
In comparison to the absorbance of NDI before TSE, the
absorbance of squaraine at 672 nm was about six times higher.
This value was comparable to that obtained by TSE of
squaraine B*. Estimation of TSE yield was complicated by the
To assess the significance of triad photosystem ABE under
these conditions, controls AB* with NDI-squaraine dyads and
AE with NDI-fullerene dyads were therefore prepared by
SOSIP-TSE. Gratifyingly, triad photosystem ABE generated
much more photocurrent than the sum of dyad controls
(Figure 5, Table 1). Photocurrent generation could be repeated
without significant changes of the kinetic profile. The initial
photocurrent decay with triad photosystem ABE originates thus
from saturation rather than from instability of the photosystem.
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The target NDI F, equipped with a solubilizing alkyl chain
and an aldehyde for hydrazone or oxime formation, was
synthesized following the procedure reported last year.16 The
squaraine dyad BF was prepared by oxime formation and
deprotection of the aldehyde as outlined for BE (Figure 3).
Stack exchange with SOSIP architecture A gave triad
photosystem ABF in high yield (Table 1). However, in sharp
contrast to the outstanding performance of ABE, photocurrent
generation by triad photosystem ABF was extremely poor
(Figure 5). The addition of the third component F inhibited
rather than enabled photocurrent generation by double-channel
photosystem AB*.
One possible explanation of the complementary behavior of
triad photosystems ABE and ABF considers that the third
component in the former is nonplanar, whereas the one in the
latter is planar. With only planar components, triple-channel
architectures (Figure 6a) could become unfavorable compared
Figure 5. (a) Photocurrent density J generated by triad architectures
ABF and ABE and dyad architectures AB* and AE in response to
irradiation with a solar simulator (power, 28 mW cm−2; 50 mM
ascorbic acid; 0.1 M Na2SO4 at 0 V vs Ag/AgCl). (b) Same by triad
architectures ADG and ADH and dyad architectures AD* and AH in
response to irradiation with a solar simulator (power, 28 mW cm−2; 50
mM TEOA, 0.1 M Na2SO4 at 0 V vs Ag/AgCl).
Similar profiles have been observed previously, but the origin of
saturation remains unknown. Charge recombination and poor
charge injection into electrode or mobile carrier are reasonable
possibilities.
Action spectra report the efficiency of photocurrent
generation at specific wavelength and thus reveal the
contribution of the individual components. Without strong
chromophores, the double-channel photosystem AE generated
photocurrent only at high energy (Figure 4b◆). Photocurrent
generation as such by photosystem AE was interesting because
it contains only electron transporters, NDI, and fullerenes.
Known absence of electron transfer between the two supported
their individual contribution to photocurrent generation.3f In
contrast, double-channel photosystem AB* is equipped with
excellent electron- and hole-transporting channels. Poor
photocurrent generation suggested that charge separation
between NDI and squaraine is not efficient, although several
other explanations cannot be excluded at this point. Consistent
with this explanation, weak IPCE was found for the squaraine
chromophore in the action spectrum of photosystem AB*
(Figure 4b○). In comparison, greatly increased IPCE of
squaraines and fullerenes in photosystem ABE (Figure 4b●)
indicated the improved photoinduced charge separation to be
the cause of an overall increased photocurrent, and not only a
better light harvesting (Figure 5, Table 1). The significant
increase in photocurrent generation from dyad photosystem
AB* to triad photosystem ABE provided a powerful example
for the functional relevance of the here-introduced triple-
channel architecture (Figures 4b and 5).
Figure 6. Inactivity of photosystems obtained by SOSIP-TSE with
completely planar dyads might indicate the formation of (b) folded or
(c) interdigitated donor−acceptor systems rather than (a) triple-
channel architectures.
to alternatives with donor−acceptor complexes obtained by
folding or interdigitation (Figures 6b,c). Architectures with
donor−acceptor complexes should excel with charge separation
but suffer from poor charge mobility. They could thus explain
the poor photocurrent generation of triad photosystems ABF
(Figure 5, Table 1). Although other interpretations remain
possible (e.g., better charge separation from B to E than from B
to F), the inactivity of photosystems ABF could thus suggest
that the TSE-SOSIP with fully planar dyads similar to BF does
not yield triple-channel architectures but folded or interdigi-
tated alternatives dominated by donor−acceptor complexes.
Triad Architectures with Phthalocyanines and Full-
erenes. The excellent yield obtained for the synthesis of triad
architecture ABE confirmed that TSE tolerates structural
changes remarkably well. To explore the limitations of TSE
with regard to the size of the stack exchangers, phthalocyanines
appeared ideal. Phthalocyanines are the big brothers of
squaraines.7,8 Efficient TSE with simple phthalocyanines C*
has been achieved previously,20 but the resulting double-
channel architectures (AC*) produced as little photocurrent as
the squaraine analogues AB* (Table 1).
Triad Architectures with Squaraines and Core-
Expanded NDIs. The core-expanded NDI F was considered
as replacement of fullerenes E in triad architectures. Both are
electron acceptors and can transport electrons along their
stacks. The LUMO of core-expanded NDI F is very low, clearly
lower than that of fullerene E but also below that of
unsubstituted NDIs A used in the SOSIP stack (Figure 2,
bottom left). Different to fullerene E, NDI F absorbs strongly
at long wavelength.
Giant stack exchangers CE were synthesized to explore the
possibility of engineering phthalocyanine−fullerene dyads into
triple-channel SOSIP-TSE architectures (Figures 2,7). The
phthalocyanine−fullerene stack exchangers CE were prepared
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Figure 7. Synthesis and schematic structure of incomplete triad architecture ACE, with possible charge-transfer pathways and absorption spectra of
dyad CE (green dashed line), single-channel architecture A3 (gray dotted), and incomplete triad architecture ACE (green solid).
analogously to the squaraine−fullerene dyads BE, using Fmoc
rather than Alloc protection of the bifunctional phthalocyanine
precursor. The attachment of the three fullerenes E and the
final aldehyde deprotection to give CE were confirmed by
MALDI mass spectrometry, whereas NMR spectra of these
larger systems were broad in all solvents and thus not very
informative. According to the absorption spectra, TSE with the
giant exchanger CE occurred in only 2% yield (Figure 7). The
phthalocyanine band at low energy did also not increase upon
disassembly of photosystem ACE with 2-mercaptoethanol, thus
excluding hypochromism in the solid to obscure better TSE
yield.
This poor yield for TSE suggested that the phthalocyanine−
fullerene stack exchangers CE are too big to enter the pores
available along the NDI stacks produced by SOSIP. Most likely,
TSE occurs only at the surface of the architectures (Figure 7).
Considering this incomplete triple-channel architecture, it was
remarkable to find that the presence of the fullerenes in ACE
nevertheless caused a drastic increase of the global photo-
current compared to double-channel control AC* (Table 1).
To explain this increase, long-distance electron transfer from
fullerenes at the surface to the ITO electrode appeared quite
unlikely. Photocurrent generation by the phthalocyanines C
thus should occur by photoinduced electron transfer first to the
fullerene E and then from there into the NDI stack for
transport to the ITO (Figure 7). The LUMO of NDI A below
that of fullerene E is in agreement with this interpretation.
Nevertheless, the most important lesson learned with photo-
systems ACE was that there is an upper limit for SOSIP-TSE;
giant stack exchangers are not accepted and react only at the
surface.
NDI stack obtained by SOSIP. Important for high yielding TSE
was the insertion of solubilizing leucines at both sides of the
PDI. The high activity of photosystem AD* was surprising
because stacks of unsubstituted NDIs and PDIs are both known
to transport electrons. Inspection of their HOMO and LUMO
energy levels suggested that the red PDI stacks in photosystem
AD* should transport holes rather than electrons (Figure 2,
bottom left). Ongoing analysis of this architecture by ultrafast
photophysics implies that this interpretation is correct
(Vauthey, E.; Sakai, N.; Matile, S., unpublished results). This
unusual role of the PDI channels in photosystem AD*
suggested that the addition of a more conventional hole-
transporting channel could give even better activities.
Triphenylamines (TPAs) G were selected to explore these
triple-channel architectures because their nonplanar, propeller-
like structure should disfavor backfolding or interdigitation into
inactive donor−acceptor architectures (Figure 6). TSE
proceeded in almost quantitative yield to give triad photo-
system ADG (Figure 2, Table 1). AFM images of ADG were
consistent with the intact SOSIP architecture (Supporting
Information, Figure S4). As expected, the addition of the hole-
transporting TPA-channel G increased the photocurrent
generated by the already quite active double-channel photo-
system AD* around 2.5 times (Table 1). Improved charge
separation was evident in the action spectra, in which the
activity of PDIs increased almost four times in triad
photosystem ADG compared to double-channel photosystem
AD* (Figure 8a).
Unlike photosystems ABE and AB*, the increase in
photocurrent of photosystems ADG and AD* was not
proportional to the increase in IPCE. The explanation of this
inconsistency was easily found in the bimolecular recombina-
tion loss efficiency ηBR (Figure 9).26 By measuring the
dependence of photocurrent on the irradiation power, ηBR
values of triad systems ADG and ABE were calculated to be
47% and 3%, respectively. Despite efficient charge separation,
Triad Architectures with NDIs, PDIs, and Triphenyl-
amines. Among several double-channel architectures tested,
photosystem AD* generated unusually high photocurrent
(Figure 2, Table 1). In this photosystem, a stack of
unsubstituted PDIs D is introduced by TSE along the original
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photocurrents observed for AD*/G and AD* are consistent
with this interpretation (Table 1). Thus, these results
demonstrated that the organization in photosystem ADG is
essential for function, that is, provided corroborative evidence
for existence and relevance of triple-channel architectures.
Triad Architectures with NDIs, PDIs, and Oligothio-
phenes. As another class of classical hole transporters,
oligothiophenes5 are generally planar, and their affinity to
PDIs is known.4e To possibly hinder this donor−acceptor
interactions, we chose twisted but planarizable oligothiophene
H.5i Planarity would be eventually desired for better stacking
interactions with the other oligothiophenes to secure good
charge mobility. To our disappointment, strong affinity
between PDI and twisted oligothiophene was already evident
during the preparation of DH, which was obtained exclusively
as a syn-oxime isomer. As anti is the more stable
configuration22a and 1H NMR spectra indicated folded
conformation, these findings suggested the existence of
donor−acceptor type stacking interactions under the reaction
conditions. The TSE of PDI-oligothiophene dyads DH
proceeded in high yield, but the obtained triad photosystem
ADH generated clearly lower photocurrent compared to the
double-channel photosystem AD* (Figure 5, Table 1). Action
spectra further demonstrated the inactivation of PDIs by the
attachment of the oligothiophenes (Figure 8b). Thus, these
results provided corroborative evidence for the existence and
deactivating effect of donor−acceptor type interactions, which
should be dealt with by applying self-sorting principles,27,18 that
is, the introduction of additional elements of molecular
recognition which favor the formation of segregated rather
than mixed stacks.
Figure 8. Action spectra of (a) triad architecture ADG (filled circles)
in comparison to double-channel architectures AD* (empty circles)
and (b) ADH (filled circles) in comparison to AD* (empty circles)
and AH (empty squares). IPCE are normalized relative to that of AD*
at 520 nm.
CONCLUSIONS
■
We have shown that triad photosystems are easily accessible by
self-organizing surface-initiated polymerization (SOSIP) and
templated stack exchange (TSE). The best triad architectures
found are composed of NDIs, squaraines, and fullerenes (i.e.,
ABE) or NDIs, PDIs and triphenylamines (i.e., ADG). These
findings are important because they provide general access to
the systematic study of molecular dyads and triads in ordered
and oriented multicomponent surface architectures by ultrafast
photophysics. Moreover, the disclosed synthetic method might
become useful to develop conceptually innovative optoelec-
tronic devices.
Remarkable structural tolerance was found for the synthesis
of triad architectures by SOSIP-TSE, and the observed trends
are highly consistent and can be rationalized convincingly.
Limitations for SOSIP-TSE were discovered only for giant or
insoluble stack exchangers (i.e., ACE). As expected, photo-
currents were greatly improved by the installation of a third
component as long as the two chromophores used form
separate stacks. This is unproblematic as long as one of the two
is not planar (fullerenes in ABE, triphenylamines in ADG).
Photocurrent inhibition indicates that TSE with fully planar
donor−acceptor dyads can lead to folded or interdigitated
rather than triple-channel architectures (i.e., ABF). Relatively
weak chromophore deplanarization did not solve the problem,
at least not for the oligothiophenes tested (i.e., ADH).
Considering the confined space available for TSE, preference
for folded or interdigitated architectures over the larger triple-
channel architectures is reasonable. However, it should be easily
possible to suppress folding and interdigitation of planar
donor−acceptor dyads by incorporating self-sorting units. Once
Figure 9. Dependence of short circuit current densities (JSC) on light
intensities (I) for triad photosystems ABE (purple filled circles) and
ADG (red empty circles) with bimolecular charge recombination
efficiencies ηBR indicated in percent.
the relatively high charge recombination in triad system ADG
reduced the overall photocurrent generation. We interpreted
the difference in ηBR values to result from different charge
mobilities of two systems. Although both are nonplanar,
electron-rich triphenylamines do not self-assemble as well as
electron-poor fullerenes to give good charge-transporting
channels.
As a control experiment, TSE was performed in the presence
of the two stack exchangers D* and G instead of dyad DG. As
expected from the differences in reactivities and stabilities of
aliphatic and aromatic aldehyde hydrazones, fast incorporation
of D* followed by a slow exchange of D* with G were
observed. Such kinetics should result in the formation of
vertically phase-separated second channels composed of mostly
pure PDI stacks with TPA stacks only near the surface. Similar
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Journal of the American Chemical Society
Article
R.; Marder, S. R. Adv. Mater. 2010, 23, 268−284. (e) Gao, X.; Di, C.-
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sorting is accomplished, triple-channel architectures seem to be
easily accommodated thanks to the not so small space reserved
for TSE by two NDI templates per initiator. Moreover, the
SOSIP methodology used to build the central stack is not
limited to NDIs (e.g., PDIs, oligothiophenes),15,16 all
components of the triad can therefore be quite easily varied.
Judged from results with simpler systems,19,20 the installation of
oriented, antiparallel redox gradients in the two channels added
by TSE should be very straightforward. Taken together, the
results reported in this study demonstrate that the here-
introduced triple-channel strategy is a powerful general method
to build highly active multicomponent surface architectures of
highest sophistication. Interesting topics for the future include
the construction of triple-channel systems with refined
acceptor−chromophore−donor triads, the expansion toward
tetrads and beyond, the prevention of folded and interdigitated
architectures (e.g., templated self-sorting),18 the development
of co-TSE (e.g., different kinetics, self-sorting),18 and so on.
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ASSOCIATED CONTENT
* Supporting Information
Experimental details. This material is available free of charge via
the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
stefan.matile@unige.ch
■
Present Address
†E.O.: Department of Chemistry, Vilnius University, Vilnius,
Lithuania.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank J.-F. Nierengarten (Strasbourg) for advice, A. Fin, H.
Shaw, A. Cherix, and D.-H. Tran for contributions to synthesis,
D. Jeannerat, A. Pinto, and S. Grass for NMR measurements,
the Sciences Mass Spectrometry (SMS) platform for mass
spectrometry services, and the University of Geneva, the
European Research Council (ERC Advanced Investigator), the
National Centre of Competence in Research (NCCR)
Chemical Biology, and the Swiss NSF for financial support.
E.O. acknowledges a Sciex, G. S. a Curie Fellowship.
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