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
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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
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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.