Self-organizing surface-initiated polymerization: Facile access to complex functional systems

Article abstract of DOI:10.1021/ja203792n

Facile access to complex systems is crucial to generate the functional materials of the future. Herein, we report self-organizing surface-initiated polymerization (SOSIP) as a user-friendly method to create ordered as well as oriented functional systems on transparent oxide surfaces. In SOSIP, self-organization of monomers and ring-opening disulfide exchange polymerization are combined to ensure the controlled growth of the polymer from the surface. This approach provides rapid access to thick films with smooth, reactivatable surfaces and long-range order with few defects and high precision, including panchromatic photosystems with oriented four-component redox gradients. The activity of SOSIP architectures is clearly better than that of disordered controls.

Full text of DOI:10.1021/ja203792n

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Self-Organizing Surface-Initiated Polymerization: Facile Access
to Complex Functional Systems
Naomi Sakai,*,† Marco Lista,† Oksana Kel,‡ Shin-ichiro Sakurai,†,§ Daniel Emery,† Jiri Mareda,† Eric Vauthey,‡
and Stefan Matile*,†
†Department of Organic Chemistry and ‡Department of Physical Chemistry, University of Geneva, Geneva 1211, Switzerland
S Supporting Information
b
For ring-opening surface-initiated polymerization, several
ABSTRACT: Facile access to complex systems is crucial to
generate the functional materials of the future. Herein, we
report self-organizing surface-initiated polymerization
(SOSIP) as a user-friendly method to create ordered as well
as oriented functional systems on transparent oxide surfaces.
In SOSIP, self-organization of monomers and ring-opening
disulfide exchange polymerization are combined to ensure
the controlled growth of the polymer from the surface. This
approach provides rapid access to thick films with smooth,
reactivatable surfaces and long-range order with few defects
and high precision, including panchromatic photosystems
with oriented four-component redox gradients. The activity
of SOSIP architectures is clearly better than that of dis-
ordered controls.
candidates were screened. Reversible disulfide exchange turned
out to be best. This reaction is well known from protein folding
and has been used to tackle other challenges in chemistry and
biology, such as lipid bilayer polymerization, dynamic molecular
recognition, and mechanical bonding.23ꢀ29 More classical
methods such as ATRP14 and ROMP,15 as well as bio-inspired
alternatives such as ring-opening thioester30 or hydrazone exchange,31
were more problematic in our hands. To realize SOSIP with ring-
opening disulfide-exchange polymerization, protected cysteine
and asparagusic acid, a strained cyclic disulfide,27,28 were added
next to the self-organizing subunits in initiator 1 and propagators
such as 2. To bind to indium tin oxide (ITO) surfaces in a well-
defined orientation, initiator 1 was equipped with two diphos-
phonate “feet”.32 From here, SOSIP was designed to occur as
follows: Reduction of the disulfides in 7 with (S,S)-dithiothreitol
(DTT) liberates reactive thiolates on the surface of 8. Recogni-
tion of propagators such as 2 on the surface of 8 by the
topologically matching self-organizing and functional subunits
is expected21ꢀ24 to position the terminal disulfides of 2 on top of
two thiolates on the surface of 8. This self-organization was thus
conceived to facilitate intramolecular ring-opening disulfide ex-
change at both termini of 2 to freeze the desired surface archi-
tecture in a covalent macrocycle.23 At the same time, active thiolates
are reproduced on the surface of intermediate 9 to react with the
next propagator 2, to continue with reversible disulfide exchange
to ultimately yield the desired ladderphane23,33 polymer brush
architecture 10.
he construction of supramolecular functional multicompo-
T
nent architectures on solid surfaces that are both oriented
and ordered is a key challenge in current materials sciences
and beyond.1ꢀ19 Essential processes and motifs such as lateral
self-sorting and oriented antiparallel gradients are intrinsically
inaccessible by solution processing and related current methods.19
We have recently introduced zipper assembly of double-gradient
supramolecular n/p-heterojunction (SHJ) photosystems as a
powerful yet less practical solution.17 “Polymer brushes” have
been around for a while as promising high-speed, low-cost
alternatives.14ꢀ16 However, they seem to stop growing well as
soon as more interesting chemistry concerning organization
or function is addressed. Implementing lessons from nature,
we here report self-organizing surface-initiated polymerization
(SOSIP) as a general method to create ordered and oriented
functional systems on transparent oxide surfaces.
Microcontact printing (μCP) was used to prove the occur-
rence of SOSIP.34 An ITO surface was patterned with initiator 1
(Figure 2A), activatedwithDTT, andincubatedwith2and iPr2NEt
as a base catalyst under optimized conditions (see below). AFM
images demonstrated that polymerization occurred exclusively
on activated surfaces (Figure 2B).
SOSIP was strongly dependent on the nature and concentra-
tion of propagators 2ꢀ6 and the base catalyst (Figures S5 and
S6). The dependence on propagator concentration was char-
acterized by cSOSIP, the critical propagator concentration needed
to achieve significant SOSIP. Above cSOSIP, SOSIP showed a
steep nonlinear increase with propagator concentration (Figure
S5). This behavior, characteristic for polymerizations, hampered
quantitative reproducibility, whereas the qualitative reproduci-
bility of SOSIP was perfect. The onset of competing polymerization
in solution occurred at higher concentration, cSOL. Whereasabsolute
We thought that reversible surface-initiated polymerization of
functional monomers equipped with functional subunits embedded
within self-organizing subunits would afford highly ordered and
oriented architectures. To test the hypothesis, initiator 1 and
propagators 2ꢀ6 were designed and synthesized (Figure 1A, also
see Supporting Information). As functional subunits, naphtha-
lenediimides (NDIs) were selected. Their π-stacks can be used
to transport electrons efficiently, and their color and redox
properties can be changed without global structural changes.20
As self-organizing subunits in 1 and 2, lysine-derived diamides
were envisioned to organize SOSIP architectures with networks
of hydrogen bonds17ꢀ19,21,22 around the equally organizing NDI
π-stacks.22ꢀ24
Received: April 25, 2011
Published: June 16, 2011
r
2011 American Chemical Society
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Figure 2. Comparing SOSIP architectures 10 with disorganized con-
trols 100 obtained by polymerization in solution. Both were made with
propagator 2. (A) AFM height image after μCP of 1 on ITO; the printed
areas are not visible at the AFM detection limits. (B) μCP becomes
visible by AFM after treatment with DTT and then propagator 2 (z = 0
(black)ꢀ100 nm (white)). (C) AFM height images of 10. (D) Same for
100 (z = 0 (black)ꢀ50 nm (white)). (E) AFM phase-contrast image of
10 (corresponding to panel C). (F) Zoom-in of panel E. (G) Transient
absorption at 550 nm of 10 (green) and 100 (blue) after excitation at
400 nm with a 100-fs laser pulse. (H) Transmittance-normalized
photocurrent generated by 10 (—) and 100 (---).
Figure 1. Design and model of SOSIP architectures with structures of
initiator 1 and propagators 2ꢀ6. (A) Synthesis of 10 by (i) deposition of
initiator 1 on ITO, (ii) activation with DTT, self-organizing recognition
of propagator 2 and ring-opening disulfide exchange (iii), for continuing
SOSIP (iv). For full chemical structures, see Figure S2. Polymerization
mechanisms and polymer structures are shown only to explain design
strategies. They can be considered as mostly speculative but are consistent
with all reported experimental data. (B) General Amber Force Field
(GAFF) models of 10 (NDIs, yellow; H-bond donors, red; H-bond
acceptors, blue; polymer backbone, silver).
synthesis at c > cSOL in solution. Comparison of the obtained
architectures 100 with SOSIP architectures 10 was ideal to charac-
terize the latter, using propagator 2 in both cases (Figure 2).
According to AFM images, the surface of SOSIP architectures 10
(Figure 2C, roughness Ra = 5.1 nm) was clearly smoother than
that of solution-polymerized 100 (Figure 2D, Ra = 87 nm) and not
much rougher than the bare ITO surfaces used. Phase-contrast
images of SOSIP architectures 10 revealed long-range order with
very few defects (Figure 2E) and exceptionally high resolution
(Figure 2F).
values for cSOSIP and cSOL varied from case to case, a significant
SOSIP window cSOSIP < c < cSOL was reproducibly found for all
bis-asparagusyl propagators. Very weak polymerization found
with mono-asparagusyl propagator 6 supported the importance
of surface-bound and preorganized dithiolates to template the
polymerization (Figure S5D).35
SOSIP with propagator 2, performed in the window between
cSOSIP = 7 mM and cSOL = 11 mM, exhibited saturation behavior
with t50 = 6 h and a maximal NDI absorption of ∼0.15 at 470 nm
(Figure S4). This inactive surface could be reactivated with DTT
to continue with polymerization g23 times (Figures S7ꢀS9).
The resulting A470 g 0.9 corresponds to a thickness of ∼250 nm
or, in an ideal SOSIP architecture, a stack of ∼750 NDIs. Suc-
cessful reactivation of smooth, ordered surfaces (Figure 2C,E)
with DTT supported the expected vertical growth of the polymer
from the surface and confirmed reversibility of the polymeriza-
tion. The latter should be important for self-organization-driven
self-repair23ꢀ29,36 to minimize cross-linking and produce low-
defect ladderphane23,33 polymer brushes.
In ultrafast spectroscopy measurements, SOSIP architectures
10 and the disorganized control 100, both made with propagator
2, had identical properties except for the important background
noise produced by the rough surface of 100 (Figures 2G, S12,
and S18). This finding confirmed that both the long-range
organization and the orientation with respect to the oxide surface
account for the superior performance of SOSIP photosystem 10
compared to solution-polymerized 100, whereas their local struc-
tures are indistinguishable. Transient absorption spectra with SOSIP
architectures (Figure S14) compared to those of photochemi-
cally generated NDIꢀ• 18 and NDI+• (Figure S15) demonstrated
that the yellow NDIs in architectures 10 and 100 can function as
both donors and acceptors to undergo symmetry-breaking photo-
induced charge separation (PCS) (Figure S16).37 Ultrafast non-
radiative deactivation in fluorescence kinetics confirmed that PCS
occurs with an average time constant of about 6 ps (Figure S12).
The decay of the transient absorption of the charge-separated
The polymeric precipitate obtained at c > cSOL was insoluble
in all common solvents. Only small oligomers could be detected
by MALDI-MS (Figure S19). Polymers could adsorb randomly
on initiator-free ITO surfaces when precipitating during their
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in organic solar cells to ultimately build the oriented multicolored
antiparallel redox gradient (OMARG) SHJ architectures19 needed
for long-distance PCS and high efficiencies.35,36 Preliminary
results confirm that double-channel SHJ architectures obtained
by co-SOSIP and self-sorting of NDI propagators already suffice
to obtain at least 40 times higher activity.40
’ ASSOCIATED CONTENT
S
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; naomi.sakai@unige.ch
Figure 3. Panchromatic photosystems with four-component redox
gradients. (A) HOMO and LUMO energies of 1, 2, 4, and 520 with
possible electron- and hole-transfer cascades. (B) SOSIP of 11 by
deposition (i) and activation (ii) of 1 on ITO followed by incubation
with propagators 2 (iii), 4 (iv), and 5 (v). (C) Transmission spectrum of
11. (D) Action spectra of multicolor photosystem 11 (b) compared to
unicolor photosystem 10 (O) in transmittance-normalized IPCE (in %).
Present Addresses
§Institute for Materials Chemistry and Engineering, Kyushu
University, Fukuoka, Japan
’ ACKNOWLEDGMENT
state was found to be biphasic, with lifetimes of 80 ps and >2 ns
(Figures 2G and S18).
We thank D. Jeannerat, A. Pinto, and S. Grass for NMR
measurements, the Sciences Mass Spectrometry (SMS) platform
for mass spectrometry 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,
S.M.), the National Centre of Competence in Research (NCCR)
Chemical Biology (S.M.), the NCCR MUST (E.V.), and the
Swiss NSF for financial support (S.M., E.V.).
Photocurrent generation was determined with a wet setup
analogue to dye-sensitized solar cells,38,39 using SOSIP photo-
systems 10 as working electrode, a Pt wire counter electrode, a
Ag/AgCl reference electrode, and TEOA as mobile electron
carrier (Figure 2H, solid line).17ꢀ19 Compared to disorganized
photosystems 100, SOSIP photosytems produced about 5 times
more photocurrent (Figure 2H, dashed line). Photocurrent
generation by SOSIP 10 compared to control 100 was also much
faster, consistent with decreasing resistivity with increasing
organization (Figure S10).
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