Lateral self-sorting on surfaces: A practical approach to double-channel photosystems

Article abstract of DOI:10.1021/ja204020p

We report that self-sorting during self-organizing surface-initiated copolymerization (co-SOSIP) provides facile access to oriented multicomponent architectures. Alternate lateral and uniform axial self-sorting into formal supramolecular n/p-heterojunction photosystems is found to generate up to 40 times more photocurrent. More or less topological matching gives rise to alternate axial self-sorting into inactive charge-transfer complexes or uniform lateral sorting into the less active macrodomains, respectively. Experimental support for self-repair during co-SOSIP is reported. Initiators on the surface are shown to serve as templates for the self-sorting into multichannel architectures of freely variable composition.

Full text of DOI:10.1021/ja204020p

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pubs.acs.org/JACS
Lateral Self-Sorting on Surfaces: A Practical Approach
to Double-Channel Photosystems
Marco Lista, Jetsuda Areephong, Naomi Sakai, and Stefan Matile*
Department of Organic Chemistry, University of Geneva, Geneva 1211, Switzerland
S Supporting Information
b
(Figure 1D).1,3 Their usefulness will be determined by lateral
self-sorting. Uniform lateral self-sorting will produce photo-
systems A (psA) with large domains that have interfaces that are
too small to work together and give high activity (Figure 1A).
This situation can correspond to phase segregation. Alternate lateral
self-sorting at preserved uniform axial self-sorting will afford the
coaxial n- and p-channels of supramolecular n/p-heterojunction
(SHJ) psB (Figure 1B). With maximal photoinduced charge
separation at a molecular-level n/p-contact area and high charge
mobility in the coaxial n- and p-channels, SHJs have been pro-
posed to combine the advantages of current organic solar cells
while bypassing their shortcomings.1,3
Alternate axial self-sorting, also referred to as social self-
sorting,8a,e is supported by aromatic donorÀacceptor interactions
(or “charge-transfer complexes”, Figure 1C,E).3,4,9 This process
is undesirable. It leads to psC that is inactive because charge
translocation along alternate donorÀacceptor stacks is very poor.
To elaborate on self-sorting by co-SOSIP, we synthesized
initiators and propagators 1À11 (Figure 1). Detailed procedures
and analytical and spectroscopic data can be found in the Supporting
Information.11 They are all composed of functionalizing, self-
organizing, and polymerizing subunits.7 In the functional subunits,
naphthalenediimides (NDIs)12 of different colors and redox
levels are envisioned to self-organize into n- and p-transporting
π-stacks. Ordered π-stacking is supported by hydrogen-bonded
networks and variable alkyl tails in the self-organizing subunits
of propagators 1À9.4,9 Initiators 10 and 11 are equipped with
two diphosphonate feet for a well-defined deposition on indium
tin oxide (ITO) and activated with (S,S)-dithiothreitol (DTT).
Molecular recognition of propagators 1À9 by activated initiators
on the surface is thought to prepare for base-catalyzed ring-opening
disulfide exchange10 polymerization. The growth of oriented
ladderphane13 brushes by SOSIP has been characterized in detail
on both structural and functional levels.7
ABSTRACT: We report that self-sorting during self-orga-
nizing surface-initiated copolymerization (co-SOSIP) pro-
vides facile access to oriented multicomponent
architectures. Alternate lateral and uniform axial self-sorting
into formal supramolecular n/p-heterojunction photosys-
tems is found to generate up to 40 times more photocurrent.
More or less topological matching gives rise to alternate axial
self-sorting into inactive charge-transfer complexes or uni-
form lateral sorting into the less active macrodomains,
respectively. Experimental support for self-repair during
co-SOSIP is reported. Initiators on the surface are shown
to serve as templates for the self-sorting into multichannel
architectures of freely variable composition.
ser-friendly access to oriented and ordered 3D architectures
U
on surfaces will be essential to build the materials of the future,
including organic optoelectronic devices such as solar cells.1À3
Today, thisisoneofthekeychallengesinsupramolecularchemistry.
Available approaches such as layer-by-layer assembly give poor
organization on the molecular level and little control over the
lateral dimension.2 Solution processing methods are intrinsically
incompatible with directionality,3,4 and surface-initiated poly-
merization seems to stop working as soon as the involved chemistry
becomes more ambitious with regard to structure or function.5
Zipper assembly has been introduced to successfully build
supramolecular n/p-heterojunctions with oriented double-channel
gradients.6 However, zipper assembly will never be of practical
use because the synthetic organic chemistry involved is much too
demanding. Recently we have introduced self-organizing surface-
initiated polymerization (SOSIP) to do the impossible and provide
user-friendly, low-cost, high-speed access to oriented and ordered
multicomponent photosystems on solid surfaces.7 However, to
use SOSIP for the directional construction of multicomponent
architectures, control over self-sorting will be needed. Here we
report methods for lateral and axial, uniform and alternate self-
sorting on surfaces (a process that is maybe best imagined as 3D
Tetris on the molecular level with remote control). The intro-
duced approach is shown to yield double-channel photosystems
with more than 40 times increased activity.
Activities were determined from the photocurrent generation
in a wet setup analogue to dye-sensitized solar cells, using SOSIP
photosystems as anodes, a Pt electrode as cathode, and trietha-
nolamine (TEOA) as mobile hole acceptor in 100 mM aqueous
Na2SO4.7 The incident photon-to-current efficiency (IPCE) at
470 nm in the obtained action spectra was normalized against the
transmission at 470 nm, the absorption maximum of the most
relevant yellow NDIs 1À6 (Figure S3). For purely yellow psD
grown from initiator 10 and propagators 1À6, the normalized
activity at 470 nm was independent of the length of the alkyl
tail in the self-organizing subunits (Figure 2, Â). This value was
In principle, the directional assembly of π-stacks of p- and
n-type aromatics can occur either randomly or with uniform or
alternate axial and lateral self-sorting.4,8À10 Uniform or “narcissistic”8a,d
axial self-sorting of π-stacks produces electron (eÀ or n) and
hole (h+ or p)-transporting pathways on the molecular level
Received: May 2, 2011
Published: June 16, 2011
r
2011 American Chemical Society
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Figure 3. (A) Activity Y470 as a function of the absorption ΔA of a blue
layer from SOSIP with 7 only inserted after co-SOSIP with 1 and 7
at 25 °C and before co-SOSIP with 1 and 7 at 25 (b) and 40 °C (O).
(B) Fractional absorption and activity Y as a function of the mole
fraction x = [11]/([11] + [10]) deposited on ITO before co-SOSIP
with 1 and 7 at their cSOSIP
.
maximum of blue NDIs, 620 nm. However, co-SOSIP of the yellow
propagator 1 in the presence of the inactive blue propagator 7
caused a more than 7-fold increase in activity (Figure 2, O).
Co-SOSIP of the yellow propagator 6 together with 7 caused a
complementary decrease in activity (Figure 2, O). These trends
suggested that increasing structural similarity induces the con-
traction of less active macrodomains in psA toward more active
SHJ-type psB. The macrodomains in psA were directly visible
under microscope (Figure S4).
The highest activity indicative for SHJ-like psB was found for
co-SOSIP of propagators 1 and 7 with identical self-organizing
subunits. This implied that the structural differences between
blue and yellow NDIs alone were sufficient to avoid alternate axial
self-sorting into inactive psC. To increase the similarity between
the two partners, blue propagator 8 without hydrogen-bond donors
in the NDI core was synthesized.12 Co-SOSIP of 8 with yellow
propagators 1À6 gave maximal activity with yellow propagators
3 with intermediate propyl chains, whereas activities with highly
similar and highly different self-organizing subunits were low
(Figure 2, b). This suggested that at least partial alternate axial
self-sorting into “charge-transfer complex” psC occurs with blue
and yellow NDIs 8 and 1. This unfavorable sorting could be
suppressed by the addition of either hydrogen-bond donors as
in 7 (with 1, Figure 2, O) or increasingly different hydrophobic
subunits as in 3 (with 8, Figure 2, b) to give uniform axial and
alternate lateral self-sorting into psB for highest activity. Incom-
plete inactivation of yellow propagators with longer tails 4À6
by the blue partner 8 suggested that increasing similarity in the
NDI core hinders complete transition toward uniform lateral
self-sorting into psA.
To probe for eventual self-repair, co-SOSIP with propagators
1 and 7 was briefly interrupted by dipping into solutions with the
blue inhibitor 7 only and then restarted at either 25 or 40 °C
(Figure 3A). The thickness of the blue barrier placed on top of
the alternate lateral psB was measured by the additional blue
absorption ΔA in psE. Post-barrier co-SOSIP experiments at 40 °C
resulted in a roughly linear decrease in activity with increasing
barrier thickness (Figure 3A, O), whereas post-barrier co-SOSIP
at 25 °C was clearly nonlinear (Figure 3A, b). In the absence of
thiol nucleophiles and base catalysts, co-SOSIP architectures are
stable, also at 40 °C.
Figure 1. Lateral (AÀC) and axial (D,E) self-sorting into less active
macrodomains A (A), more active SHJs B (B), and inactive “charge-
transfer complexes” C (C) by co-SOSIP of yellow propagators 1À6 with
various partners (7À9) on initiators 10 or 11 (F). SOSIP is accom-
plished by (i) initiator deposition on ITO, (ii) activation with DTT, and
(iii) ring-opening disulfide exchange with recognized propagators for
(iv) continuing polymerization.
Figure 2. Activity Y470 as a function of the number of carbons in the
alkyl tail of yellow propagators 1À6 after SOSIP alone (Â) or together
with blue propagators 7 (O) and 8 (b) on initiator 10 at their cSOSIP
.
Y
470 = {IPCE470/(1 À T470)}/{IPCE470/(1 À T470)}0, i.e., the incident
photon-to-current conversion efficiency IPCE normalized against the
transmittance T at 470 nm and a reference value (0), here the average
value obtained for single-component SOSIP with 1À6 (Â).
taken as reference activity Y470 = 1 for further studies on self-
sorting with co-SOSIP.
Photosystems made exclusively with propagator 7 were essen-
tially inactive, not only at 470 nm but also at the absorption
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gave exponentially increasing activity with decreasing structural
differences (Figure 4A, O). This trend was exactly the same as
with the blue propagator 7, indicating that the transition from
uniform toward alternate lateral self-sorting into SHJ photo-
systems with increasing structural similarity can be generalized
(Figure 4A, b). SHJ photosystem H was 40 times more active
than single-component psD (Figure 4A, O vs Â). Moreover, psH
was clearly more active than psB, suggesting that yellow NDIs
transport holes better than electrons (Figure 4A, b vs O, and
Figure 4B,C).
In summary, these results identify self-sorting during co-
SOSIP as a promising, surprisingly reliable, and general strategy
to build complex surface architectures with very little effort in a
rational manner. Current studies focus on multichannel systems
with multicomponent gradients of the highest possible sophisti-
cation and the application to n- and p-stacks of confirmed
relevance in practice.
Figure 4. (A) Activity Y470 as a function of the number of carbons in
the alkyl tail of yellow propagators 1À6 after SOSIP alone (Â) or
together with propagators 7 (b) and 9 (O) at their cSOSIP. (B) Energy
diagram for photocurrent generation with 1 and 9 (O). (C) Same for 1
and 7 (b), with HOMO (—) and LUMO (---) energy levels given in eV
against vacuum.
’ ASSOCIATED CONTENT
S
The nonlinear response to barriers could indicate that the blue
barrier is removed by self-repair during post-barrier co-SOSIP
at 25 °C. The transition to linear response for post-barrier
co-SOSIP under at least partial thermal denaturation at 40 °C
could demonstrate that molecular recognition and self-organization
are needed for self-repair to occur. Identical trends have been
observed with error correction for gene repair.14 Moreover, lessons
from protein folding as well as surface reactivation experiments
with SOSIP7 demonstrate that the reversibility10 of disulfide
exchange polymerization is compatible with self-repair. However,
it will be difficult to fully confirm the validity of this interpreta-
tion. Possibly, the blue stacks simply go on growing selectively in
the absence of yellow propagators and collapse into the void over
the terminated yellow stacks only once they are long enough.
During self-sorting by co-SOSIP, the propagator concentra-
tions have to be kept constant at the critical SOSIP concentration,
Supporting Information. Details on experimental pro-
b
cedures. This material is available free of charge via the Internet at
http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
stefan.matile@unige.ch
’ ACKNOWLEDGMENT
We thank F. Stellacci (EPF Lausanne) for helpful suggestions,
D. Jeannerat, A. Pinto, and S. Grass for NMR measurements, the
Sciences Mass Spectrometry (SMS) platform for mass spectro-
metry services, P. Maroni and M. Borkovec for access to and
assistance with surface analytics equipment, D.-H. Tran for
contributions to synthesis, 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. J.A. acknowl-
edges a Marie Curie Fellowship.
c
SOSIP. Below cSOSIP, polymerization does not occur; above cSOSIP,
polymerization occurs also in solution (Figures S1 and S2).7 To
vary the composition of self-sorted SOSIP architectures in a
controlled manner, surface templation was explored. Namely, the
blue initiator 11 and the original initiator 10 were deposited on
the surface at various mole fractions x (Figure S5). Their different
alkyl tails were selected to hopefully template for uniform lateral
self-sorting into the straightforward, non-cooperative macro-
domain photosystems psFÀH. The absorption of blue NDI in
psFÀH obtained by co-SOSIP with blue and yellow propagators
7 and 1 at their respective cSOSIP increased with increasing mole
fractions of blue initiators 11 (Figure 3B, O). The activity of
photosystems correspondingly decreased with the increase of the
inactive blue macrodomains (Figure 3B, Â). Clearly, blue initiators
11 templated SOSIP of blue propagators 7, whereas the original
initiators 10 templated SOSIP of yellow propagators 1. These
results confirmed the fundamental importance of initiators for
SOSIP and identified surface templation as an attractive method
to grow multicomponent architectures on solid surfaces.
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