Directional stack exchange along oriented oligothiophene stacks

Article abstract of DOI:10.1039/c2cc35773a

Directional growth of π-basic oligothiophene stacks on solid substrates is achieved by self-organizing, surface-initiated disulfide exchange polymerization. Successful addition of co-axial π-acidic stacks by templated hydrazone exchange provides general a

Full text of DOI:10.1039/c2cc35773a

View Online / Journal Homepage
Dynamic Article Links
ChemComm
Cite this: DOI: 10.1039/c2cc35773a
www.rsc.org/chemcomm
COMMUNICATION
Directional stack exchange along oriented oligothiophene stacksw
Jetsuda Areephong, Edvinas Orentas, Naomi Sakai and Stefan Matile*
Received 9th August 2012, Accepted 11th September 2012
DOI: 10.1039/c2cc35773a
Directional growth of p-basic oligothiophene stacks on solid
substrates is achieved by self-organizing, surface-initiated disulfide
exchange polymerization. Successful addition of co-axial p-acidic
stacks by templated hydrazone exchange provides general access
to multicomponent architectures of unique complexity.
Biological functional systems operate with supramolecular
architectures of highest sophistication. One of the longstanding
questions with organic functional materials concerns what we
would get if similar standards of molecular precision and
complexity could be applied.1,2 Contributions from this group
to tackle this challenge have focused on the development of
strategies for directional growth of multicomponent architec-
Fig. 1 Initiator 1 and propagator 2 for SOSIP-TSE with oligothiophenes
tures directly on solid surfaces.3 This directionality is difficult
on indium tin oxide (ITO). The grey background stands for an ITO
to achieve otherwise but unavoidable for photosystems that
surface, dashed grey lines from diphosphonates for covalent anchoring
operate, like their biological counterparts, with oriented redox
gradients.3,4
on this surface, dashed grey lines between 1 and 2 for self-organizing
interactions, and blue arrows for polymerization after deprotection
The emerging method of interest is self-organizing surface-
initiated polymerization (SOSIP).3 This approach was con-
and deprotonation of the thiols on the surface.
for post-SOSIP stack exchange (templation has been shown to
ceived to provide facile access to oriented multicomponent
be essential for successful stack exchange).3c Four terminal
architectures: experimental evidence for templated self-sorting,
diphosphonates are available to bind to the ITO surface,
self-repair, surface reactivation and templated stack exchange
(TSE) exists.3 However, SOSIP beyond the most favorable
and protected thiolates to initiate the polymerization after
deprotection.3 Propagator 2 contains benzaldehyde hydra-
p-acidic naphthalenediimide (NDI) stacks is nearly unexplored.3
zones to template the site for stack exchange and strained
In the following, we demonstrate the compatibility of SOSIP-
disulfides for ring-opening disulfide exchange polymerization.6
TSE with the less favorable p-basic oligothiophenes. Oligo-
thiophenes are some of the most ubiquitous components of
The synthesis of initiator 1 and propagator 2 is outlined in
Scheme 1, detailed procedures can be found in the ESI.w To
prepare for SOSIP, ITO electrodes were dipped into a solution
of 1 in DMSO (1 mM, 48 h). The coated surfaces were
annealed for 1 h at 120 1C. Oligothiophene and NDI absor-
bance, surface coverage estimated from cyclic voltammetry
and inhibition of ferricyanide reduction were consistent with
the presence of a compact monolayer on ITO (Fig. S2 and S3,
ESIw). To deprotect the thiols directly on the surface, the
electrodes were incubated in aqueous DTT (20 mM, 1 h).
For successful SOSIP, careful optimization of solvent, base
and, most importantly, propagator concentration is essential.
Below cSOSIP, SOSIP does not occur. Above cSOSIP, polymeri-
zation occurs also without an initiator. For SOSIP on mono-
layer 3, a solution of propagator 2 at cSOSIP = 4–6 mM in
chloroform–methanol 3 : 1 with 100 mM Hunig base was best
(Fig. S4, ESIw). The growth of photosystem 4 on ITO was
followed by absorption spectroscopy (Fig. 2, dashed; Fig. S4,
ESIw). The thickness of the obtained films could be estimated
from the absorbance of the oligothiophenes.
molecular electronics, photonics and optoelectronics, including
organic photovoltaics and sensors.1,2,5 However, no other method
is available that would provide access to similarly sophisticated
architectures,4 that is, directional growth of oligothiophene stacks
and directional, NDI-templated stack exchange along the
obtained oligothiophene stacks.
SOSIP with oligothiophenes was envisioned with initiator 1
and propagator 2 (Fig. 1). Their central quaterthiophene is
embedded in a peptide-like hydrogen-bonded network to
account, together with p–p stacking, for the self-organization
of the system. Initiator 1 contains additional NDIs to template
Department of Organic Chemistry, University of Geneva, Geneva,
Switzerland. E-mail: stefan.matile@unige.ch;
Web: www.unige.ch/sciences/chiorg/matile/; Fax: +41 22 379 5123;
Tel: +41 22 379 6523
w Electronic supplementary information (ESI) available: Detailed
procedures and results for all reported experiments. See DOI:
10.1039/c2cc35773a
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun.

View Online
drilled into in photosystem 5 were filled immediately by
dipping the electrode for 6 h into a solution of aldehyde 6 in
DMSO. Nearly quantitative formation of NDI stacks in
photosystem 7 was indicated by the intensity of their absorp-
tion below 400 nm, easily recognizable by the characteristic
vibrational fine structure (Fig. 2a and Fig. S6, ESIw).7
The compatibility of oligothiophene architectures with post-
SOSIP-TSE was explored with the core-substituted NDIs 8,
the core-expanded NDI 9 and the elongated perylenediimide
(PDI)8 homolog 10 (Scheme 1). The red NDI 8 with an amine
and a bromine in the core showed highest activity in previous
studies.3 The core-expanded NDI 9 has appeared recently in
the literature as exceptionally promising, air-stable n-semicon-
ductor with broad absorption at low energy.9 PDI 10, finally,
was of interest to probe for compatibility with elongated
rather than broadened stacks. Of similar color, the frontier
molecular orbitals (FMOs) of core-substituted NDI 8 and PDI
10 are above the original NDI 6, whereas core-expanded NDI
9 is clearly below (Fig. 3a and Table S1, ESIw). The FMOs of all
NDIs and PDIs are clearly below oligothiophenes, confirming
that their stacks could serve as electron-transporting channels
next to the hole-transporting quaterthiophene stacks.4
NDIs 6 and 8 were prepared as described,3 the new, core-
expanded NDI 9 and the longer PDI 10 were accessible in a few
steps by adapting procedures from the literature (Scheme S3,
ESIw).8,9 Both had to be equipped with branched alkyl tails to
increase their solubility (Scheme 1).
Scheme 1 Synthesis of initiator and propagator, SOSIP with oligo-
thiophenes and TSE with NDIs and PDIs. (a) Pd(PPh3)4, DMF, 80 1C,
12 h, 70%; (b) 1. TFA, CH2Cl2, RT, 2 h; 2. HATU, TEA, DMF, RT,
5 min; 3. H-Lys(Fmoc)-OH, Na2CO3, dioxane–THF–H2O, 55%;
(c) NH2NHBoc, HATU, TEA, DMF, RT, 2 h, 63%; (d) 1. piperidine,
DMF, RT, 1 h; 2. 20, TEA, DMF, RT, 30 min, 45% (2 steps);
(e) TFA, thioanisole, benzaldehyde, RT, 90% (2 steps); (f) 1. TFA,
CH2Cl2, RT, 1 h; 2. Boc-Cys(S-t-Bu)-OH, HOBt, EDCI, 2,4,6-
collidine, DMF, 90 min, 55% (2 steps); 3. TFA, thioanisole, CH2Cl2,
RT, 1 h; 4. 22, EDCI, TEA, DMF–CH2Cl2, RT, 90 min, 62% (2 steps);
(g) 1. TFA, TMSBr, thioanisole, CH2Cl2, 0 1C to RT, 24 h, 66%; 2. 24,
TFA, DMSO, RT, 24 h; 3. ITO, DMSO, RT, 48 h; (h); DTT, RT, 1 h;
(i) 2, i-PrNEt2, CHCl3–MeOH 3:1, RT, 24 h; (j) NH2OH, MeOH–H2O
1 : 1, 24 h; (k) 6, TFA, DMSO, RT, 6–12 h, 87%; (l) same with 8,
89%; (m) same with 9, 30%; (n) the same with 10, 87%. For structures
of 20, 22 and 24, see ESI.w
The absorption spectra of photosystems 11 and 12 suggested
that the infiltration of the core-substituted aldehyde 8 into the
porous, reactive photosystem 5 was nearly quantitative, whereas
stack exchange with the core-expanded NDI 9 occurred with
30% yield only (Fig. 2b and c).7 This failure originated pre-
sumably from the poor solubility of NDI 9, providing access
only to solutions that are too dilute for efficient stack exchange.
Fig. 3 (a) HOMO (bold) and LUMO (dashed) energy levels against
vacuum of oligothiophenes (18), NDIs (6, 8, 9) and PDIs (10), from
cyclic voltammetry with À5.1 eV for Fc/Fc+, (Fig. S7, ESIw).
(b) Photocurrent generated by photosystems 4, 12, 7, 13 and 11 (left
to right), irradiated with a solar simulator (66 mW cmÀ2). (c) Action
spectra of photosystems 4 (x), 12 (m), 7 (n), 13 (J) and 11 (K),
Y = IPCE/(1 À T420), IPCE = incident photon-to-current efficiency,
T420 = transmittance at 420 nm. (d) Comparison of oligothiophene
maxima in action (K) and absorption spectra before (4, dashed) and
after TSE (11, solid, Fig. 2b). (e) The same for NDI maxima.
Fig. 2 Absorption spectra of SOSIP photosystems before (4, dashed)
and after TSE for (a) 7, (b) 11, (c) 12 and (d) 13 (solid).
For stack exchange, the benzaldehydes in photosystem 4
were removed with 1.0 M hydroxylamine in H2O–MeOH 1 : 1
(HPLC kinetics: Fig. S5, ESIw). The hydrazide-rich pores
c
Chem. Commun.
This journal is The Royal Society of Chemistry 2012

View Online
In contrast, TSE with PDI 10 occurred in 90% yield (Fig. 2d).
The obtained photosystem 13 showed the characteristic,
strongly hypochromic absorptions of PDI stacks at 501 and
545 nm (monomer: 528 nm).3d,8 The blue shift to 501 nm is
characteristic of face-to-face H-aggregates, the red shift to
545 nm of rotational displacement between neighboring PDIs
in the helical stacks.
We thank S. Kassem and D.-H. Tran for contributions to
synthesis, D. Jeannerat, A. Pinto and S. Grass for NMR
measurements, the Sciences Mass Spectrometry (SMS) plat-
form 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. J. A. acknowledges a Curie Fellowship, E. O. is a
Sciex Fellow.
The photocurrents generated by photosystems 4, 7 and
11–13 were measured under routine assay conditions.3 In brief,
the photosystems were used as a working electrode together
with a Pt counter electrode, a Ag/AgCl reference electrode and
triethanolamine (TEOA) as a mobile carrier.10 Irradiated with
a solar simulator, the oligothiophene photosystem 4 generated
comparably3 little photocurrent (Fig. 3b).11 The addition of
co-axial channels for electron transport improved the situation
slightly with photosystem 7 and dramatically with photo-
system 11. Stacks with the newly introduced, core-expanded
NDI 9 in photosystem 12 generated clearly less photocurrent.
Incomplete TSE rather than intrinsic properties of the chromo-
phore is likely to account for this poor performance, although
exceptional activity of the core-expanded NDIs should be
observable also in this less than perfect photosystem. Compared
to the 64-fold increase achieved with core-substituted NDIs in
photosystem 11, the 21-fold increase obtained with PDI stacks
in photosystem 13 was mildly disappointing.
Notes and references
1 (a) J. Roncali, Acc. Chem. Res., 2009, 42, 1719–1730; (b) R. Fitzner,
E. Reinold, A. Mishra, E. Mena-Osteritz, H. Ziehlke, C. Korner,
K. Leo, M. Riede, M. Weil, O. Tsaryova, A. Weiß, C. Uhrich,
M. Pfeiffer and P. Bauerle, Adv. Funct. Mater., 2011, 21, 897–910;
(c) D. M. Bassani, L. Jonusauskaite, A. Lavie-Cambot, N. D.
McClenaghan, J.-L. Pozzo, D. Ray and G. Vives, Coord. Chem.
Rev., 2010, 254, 2429–2445; (d) R. J. Kumar, J. M. MacDonald,
T. B. Singh, L. J. Waddington and A. B. Holmes, J. Am. Chem.
Soc., 2011, 133, 8564–8573; (e) K. Sugiyasu, S. Kawano, N. Fujita
and S. Shinkai, Chem. Mater., 2008, 20, 2863–2865.
2 (a) T. Aida, E. W. Meijer and S. I. Stupp, Science, 2012, 335,
813–817; (b) J. L. Delgado, P.-A. Bouit, S. Filippone, M. A.
Herranz and N. Martın, Chem. Commun., 2010, 46, 4853–4865;
(c) M. V. Martınez-Dıaz, G. de la Torre and T. Torres, Chem.
Commun., 2010, 46, 7090–7108; (d) D. M. Guldi, I. Zilbermann,
G. Anderson, A. Li, D. Balbinot, N. Jux, M. Hatzimarinaki,
A. Hirsch and M. Prato, Chem. Commun., 2004, 726–727;
(e) A. Kira, T. Umeyama, Y. Matano, K. Yoshida, S. Isoda,
J. K. Park, D. Kim and H. Imahori, J. Am. Chem. Soc., 2009, 131,
3198–3200; (f) R. Bhosale, J. Mısek, N. Sakai and S. Matile, Chem.
Soc. Rev., 2010, 39, 138–149.
The action spectrum of the most active photosystem 11
demonstrated that both the yellow oligothiophene and the
red NDI stacks generate photocurrent (Fig. 3c, K). Oligo-
thiophenes generated maximal photocurrent around 440 nm,
which is slightly red-shifted compared to the absorption
maximum of the photosystem 11 around 420 nm (Fig. 3d).
With NDIs, absorption and photocurrent maxima coincided
at 540 nm (Fig. 3e). The red-shifted maximum for oligo-
thiophenes in the action spectrum of photosystem 11 could
thus indicate the presence of highly active stacks with planarized
oligothiophenes.5 Other possible contributions to the high activity
of photosystem 11 include absorption of light up to 600 nm,
efficient charge separation in co-axial donor–acceptor stacks,3,4
slow charge recombination (high LUMO of NDI, heavy-atom
effects) as well as high charge mobility in the co-axial hole- and
electron-transporting pathways.3,4
3 (a) N. Sakai, M. Lista, O. Kel, S. Sakurai, D. Emery, J. Mareda,
E. Vauthey and S. Matile, J. Am. Chem. Soc., 2011, 133,
15224–15227; (b) M. Lista, J. Areephong, N. Sakai and
S. Matile, J. Am. Chem. Soc., 2011, 133, 15228–15230; (c) N. Sakai
and S. Matile, J. Am. Chem. Soc., 2011, 133, 18542–18545;
(d) P. Charbonnaz, N. Sakai and S. Matile, Chem. Sci., 2012, 3,
1492–1496; (e) E. Orentas, M. Lista, N.-T. Lin, N. Sakai and
S. Matile, Nat. Chem., 2012, 4, 746–750.
4 Architectures with oriented multicolored antiparallel redox
gradients in co-axial hole- and electron-transporting channels
have been referred to as OMARG-SHJs (SHJ, supramolecular
n/p-heterojunctions).2,3
.
5 (a) A. Mishra, C. Ma and P. Bauerle, Chem. Rev., 2009, 109,
1141–1276; (b) Y. Ie, A. Han, T. Otsubo and Y. Aso, Chem.
Commun., 2009, 3020–3022; (c) I. Osaka and R. D. McCullough,
Acc. Chem. Res., 2008, 41, 1202–1214; (d) M. Zambianchi,
F. Di Maria, A. Cazzato, G. Gigli, M. Piacenza, F. Della Sala
and G. Barbarella, J. Am. Chem. Soc., 2009, 131, 10892–10900;
(e) D. T. McQuade, A. E. Pullen and T. M. Swager, Chem. Rev.,
2002, 100, 2537–2574.
6 E.-K. Bang, M. Lista, G. Sforazzini, N. Sakai and S. Matile, Chem.
Sci., 2012, 3, 1752–1763.
7 To estimate extinction coefficients of SOSIP architectures, they
were, if possible, dissolved in mercaptoethanol to record absorp-
tion spectra in solution.
8 F. Wurthner, Chem. Commun., 2004, 1564–1579.
9 Y. Hu, X. Gao, C. Di, X. Yang, F. Zhang, Y. Liu, H. Li and
D. Zhu, Chem. Mater., 2011, 23, 1204–1215.
10 Carriers other than TEOA gave similar results, including
MDESA.3d
11 Comparisons of results obtained under non-optimized test condi-
tions without any special precautions should be done with caution
and on a relative scale; quantitative comparisons with dedicated
device engineering are not meaningful in this context. Without
alternative synthetic methods to produce comparably sophisticated
architectures, meaningful negative controls were not available
either (increasing activities compared to random deposition have
been confirmed).
In summary, these results demonstrate that SOSIP-TSE is a
general (and so far unique) method to build sophisticated
surface architectures in a directional manner. Introducing
oligothiophenes, SOSIP with the least favorable p-basic stacks
is shown, for the first time, to occur as reliably as with the most
favorable p-acidic stacks. Templated by NDIs on the surface,
post-SOSIP stack exchange along the oligothiophene stacks is
compatible with NDI and PDI stacks as long as the solubility
of the building blocks is sufficient. Ultimately, SOSIP-TSE is
expected to provide general access to multichannel architec-
tures with multiple gradients of freely variable composition.
Oligothiophenes are particularly attractive to build oriented
p-channels with multicomponent gradients because their
HOMO energies are extensively variable with terminal and
lateral substituents as well as with thiophene analogues in their
backbone (e.g., benzothiadiazoles). Efforts to live up to these
high expectations are ongoing and will be reported in due course.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun.