Zipper assembly of vectorial rigid-rod π-stack architectures with red and blue naphthalenediimides: Toward supramolecular cascade n/p-heterojunctions

Article abstract of DOI:10.1002/anie.200800203

(Chemical Equation Presented) Zipped up: Supramolecular 3D organization on gold with interdigitating intra- and interlayer recognition motifs (see picure, black p-oligophenyl rods; red, blue naphthalenediimide (NDI) stacks) is designed to access supramole

Full text of DOI:10.1002/anie.200800203

Angewandte
Chemie
DOI: 10.1002/anie.200800203
Artificial Photosynthesis
Zipper Assembly of Vectorial Rigid-Rod p-Stack Architectures with
Red and Blue Naphthalenediimides: Toward Supramolecular Cascade
n/p-Heterojunctions**
Adam L. Sisson, Naomi Sakai, Natalie Banerji, Alexandre Fürstenberg, Eric Vauthey,*and
Stefan Matile*
In organic optoelectronics and beyond, control of supra-
molecular organization is essential to create significant
functions.[1–6] For this purpose, we have introduced the
zipper assembly of rigid-rod p-stack architectures composed
of p-oligophenyl (POP) rods and blue naphthalenediimide
(NDI) stacks on conducting surfaces.[4] The fact that the
observed photocurrent was much higher than that with more
conventional layer-by-layer assemblies, and the possibility of
terminating the growth of the layers with the component of
matching length, demonstrated the existence and the func-
tional significance of the ordered structure in zipper assem-
blies. These positive results encouraged us to apply this
approach and tackle more challenging objectives such as
creation of nanoscale n/p-bulk heterojunctions with wide
absorption spectrum. As the first example of multicomponent
zipper assembly, we herein report vectorial POP/NDI archi-
tectures with p-stacks composed of red and blue NDIs that
promise access to unique, supramolecular[1] cascade[2] n/p-
heterojunctions, in which redox gradients along coaxial n- and
p-semiconductors mediate directional flow of electrons on the
molecular level (Figure 1). This approach thus expands the
previously reported concept of supramolecular n/p-hetero-
junctions[1a,b] toward directional cascade architectures and
functional nanodevices.
planarity and p acidity enables the formation of n-semi-
conducting face-to-face p stacks next to strings of p-semi-
conducting POP rods.[4–6] The expected perpendicular orien-
tation of the resulting stack/rod cascade n/p-heterojunctions
with respect to the surface appeared ideal to secure the
elusive high photocurrents in thick chromophore layers
needed for efficient absorption[1,2] while maintaining the fill
factor (FF) by keeping the resistance low.[8]
To assemble multicomponent zippers, such as Au-1-2-3-4,
the POP 2 with eight red 2(3)-alkylamino-6(7)-chloro-1,4,5,8-
NDIs (rNDIs) was synthesized and characterized using
procedures developed for blue bNDI POPs 1, 3, and 4
(Figure 1, and Figure S1 and Scheme S1 in the Supporting
Information).[4–6] The frontier-orbital energy levels of all
zipper components were determined by steady-state fluores-
cence spectroscopy and cyclic voltammetry (e.g., rNDI
monomer: E1/2 (X/X+) = + 1.21 V vs Fc/Fc+, E1/2 (X/XÀ) =
À1.10 V vs Fc/Fc+,[7b] Figure 2a, and Figure S2 and Table S1
in the Supporting Information). To identify the absorption of
the rNDICÀ radical anion, the transient absorption spectra of
monomeric Alloc-rNDI 2a (Alloc = allyloxycarbonyl) mea-
sured in the absence and the presence of excess N,N-
dimethylaniline (DMA) were compared (Figure 2c, C vs B).
Broad bands at 400–510 nm and 550–700 nm separated by the
ground state bleach were attributed to rNDICÀ. Although
complicated by contributions from the S1 state (Figure 2c, C),
the presence of these rNDICÀ bands (Figure 2c, B) in the
transient spectra of 2 demonstrated the occurrence of photo-
induced charge separation (PCS, Figure 2c, A). The decay of
this rNDICÀ revealed that PCS with 2 is more than 15 times
longer-lived than that with 4 (Figure 2d).[5] Lifetime compo-
nents reaching beyond 1 ns were compatible with the
stabilization of PCS by intramolecular charge transfer from
POPs to rNDIs in 2 (Figure 2a, red arrow), although
contributions from other origins such as location in the
Marcus inverted region remain possible. The observed
fluorescence quenching of rNDIs but not bNDIs with p-
quateranisole or hexamethylbenzene confirmed that photo-
induced electron transfer from POP donors is possible to the
rNDIs in 2 but not to the bNDIs in 4 (Figure 2a and b).
Fluorescence decay kinetics indicated that PCS is complex,
ultrafast, and almost quantitative. These findings demon-
strated that stack/rod PCS, i.e., the formation of n/p-hetero-
junctions in zipper assembly, is possible with rNDI 2 but not
with bNDI 4.[9]
NDIs[3–7] are perfect modules for zipper assembly because
their photo- and electrochemical properties can be easily
tuned without global structural changes.[7] Moreover, their
[*] N. Banerji, Dr. A. Fürstenberg, Prof. E. Vauthey
Department of Physical Chemistry
University of Geneva
Geneva (Switzerland)
Fax: (+41)22-379-6518
E-mail: eric.vauthey@chiphy.unige.ch
Dr. A. L. Sisson, Dr. N. Sakai, Prof. S. Matile
Department of Organic Chemistry
University of Geneva
Geneva (Switzerland)
Fax: (+41)22-379-3215
E-mail: stefan.matile@chiorg.unige.ch
Homepage: www.unige.ch/sciences/chiorg/matile/
[**] We thank S. Bhosale and D.-H. Tran for contributions to synthesis,
D. Jeannerat, A. Pintoand S. Grass for NMR measurements, P.
Perrottet and the group of F. Gülaçar, N. Oudry, and G. Hopfgartner
for MS, one referee for helpful comments, and the Swiss NSF for
financial support.
Zipper assembly of Au-1-2-3-4 was initiated by deposition
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
of p-quaterphenyl initiators 1 on gold electrodes (Figure 1).[4]
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The Au-1 obtained was dipped into an
aqueous solution of p-octiphenyl propa-
gator 2. Driven by p–p stacking, intrastack
hydrogen bonding,[6] and interstack elec-
trostatic attraction,[4] the lower half of the
rNDIs of 2 was expected to p stack with
the bNDIs of 1, whereas the upper half
remained available to zip up with anionic
3, which in turn can zip up with cationic 4.
The all-blue Au-1-4-3-4, structural isomer
Au-1-4-3-2, and the doubly red Au-1-2-3-2
were prepared analogously (Figure 3).
Photocurrents were generated by irra-
diation with light in the presence of
triethanolamine (TEOA) as sacrificial
electron donor, a gold electrode as elec-
tron acceptor, and a platinum electrode as
cathode. Current–voltage (I–V) relation-
ships revealed short circuit current ISC,
open circuit potential VOC, and the fill
factor FF (Figure 3a). The I–V profile of
the all-blue Au-1-4-3-4 revealed an
already good FF of 0.43 (Figure 3a, D).
Incorporation of 2 strongly increased the
ISC. This increase was independent of the
directionality of the zipper cascade archi-
tecture (Au-1-2-3-4 (B) versus isomer Au-
1-4-3-2 (C)). However, positioning of
rNDI acceptor 2 against the flow of
electrons in Au-1-4-3-2 did reduce the FF
to 0.31 (Figure 3C). In clear contrast, the
constructive directionality in Au-1-2-3-4
with the rNDI acceptor 2 in perfect
position to accept electrons for transfer
to the gold electrode increased the FF up
to 0.55 (Figure 3a, B). Taking FF = 0.25 of
linear I–V correlations and FF = 0.61 of
Figure 1. The concept of zipper assembly on gold. All rNDIs are mixtures of 2,6- and 3,7-
regioisomers. All suprastructures are speculative representations consistent with experimental
results[4–6] and molecular models,[5,6] and are shown with the only intention to illustrate the
concept. See text for details. 2a, 4a: A=Alloc (allyloxycarbonyl).
optimized organic solar cells[1c] as standard values (0 and
100%, respectively), the significant difference in FFs between
Au-1-4-3-2 and Au-1-2-3-4 is clearly demonstrated, with 17%
for the former (FF = 0.31) and 90% for the latter (FF = 0.55).
Incorporation of another rNDI acceptor 2 in Au-1-2-3-2
further increased the ISC because of the increased number of
rNDI sensitizers (Figure 3a, A). The FF in Au-1-2-3-2
however decreased to 0.48 because of the weaker redox
gradient compared to Au-1-2-3-4. This result confirmed that
the adaptable directionality of multicomponent cascade
zipper architectures provides access to the rational, molec-
ular-level control of the FF, one of the key factors to generate
power with light (Figure 3a). The action spectra of these
model zippers readily revealed the higher photoactivity of red
compared to blue NDIs (Figure 3b). This finding was in
agreement with the optoelectronic properties of rNDI POP 2
as described above and supported the formation of supra-
molecular cascade stack/rod n/p-heterojunctions.
Figure 2. a) Zipper energetics with b) evidence for photoinduced
stack/rod electron transfer, and c, d) long-lived photoinduced charge
separation with 2. a) Frontier orbital energy levels (HOMO, solid;
LUMO, dashed) of bNDIs, rNDIs, and POPs. Solid arrows=allowed
electron transfer (gray: less likely because of energy transfer), dashed
arrows=allowed absorption of light (hn). b) Stern–Volmer plot for
*
*
quenching of 2a ( ) but not 4a ( ) with hexamethylbenzene in
acetonitrile. c) Transient absorption spectra of 2 (A) and 2a (B, 3 ps;
C, 50 ps after a 50 fs laser pulse at 530 nm) in MeOH with (B) or
In conclusion, focusing on functional significance, two
central, interconnected, but otherwise often elusive key
challenges can be addressed with zipper assembly, i.e.,
supramolecular n/p-heterojunctions and multicomponent
without DMA (A,C). d) Intensity-normalized time profiles of the
[5]
*
*
transient absorption of 4 ( ) at 511 nm and 2 at 470 nm ( ).
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Angewandte
Chemie
cannot be excluded despite the availability of a consistent set
of experimental data. As far as applications are concerned,
other conductive substrates will be needed. Because of its
well-characterized surface chemistry, gold is an ideal sub-
strate to determine the importance of multicomponent
cascade architectures for function (expressed, for example,
in fill factors or relative photocurrents). But it is not an ideal
substrate to measure meaningful efficiencies, as evidenced by
the increase of photocurrents by a factor of ca. 280 when
moving from gold to indium tin oxide electrodes.[10] Efforts
toward building zipper assemblies on the other conducting
materials are also ongoing.
Received: January 15, 2008
Revised: February 11, 2008
Published online: March 25, 2008
Keywords: artificial photosynthesis ·multicomponent reactions·
.
photochemistry · solar energy · supramolecular chemistry
Figure 3. a) Current–voltage profiles with indication of short circuit current
(ISC) and open circuit voltage (VOC, both for Au-1-2-3-2), and maximum-
power rectangle and maximum-power point (Au-1-2-3-4 vs Au-1-4-3-2).
The fill factor (FF) is defined as maximum power/(VOC ISC). b) Action
[1] a) F. Würthner, Z. Chen, F. J. M. Hoeben, P. Osswald, C.-C. You,
P. Jonkheijm, J. Herrikhuyzen, A. P. H. J. Schenning, P. P. A. M.
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Chem. Soc. 2002, 124, 4796 – 4803; b) M. Morisue, S. Yamatsu, N.
Haruta, Y. Kobuke, Chem. Eur. J. 2005, 11, 5563 – 5574; c) D. M.
Guldi, J. Phys. Chem. B 2005, 109, 11432 – 11441.
[3] B. A. Jones, A. Facchetti, M. R. Wasielewski, T. J. Marks, J. Am.
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[4] N. Sakai, A. L. Sisson, T. Bürgi, S. Matile, J. Am. Chem. Soc.
2007, 129, 15758 – 15759.
[5] S. Bhosale, A. L. Sisson, P. Talukdar, A. Fürstenberg, N. Banerji,
E. Vauthey, G. Bollot, J. Mareda, C. Rꢀger, F. Würthner, N.
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~
*
^
spectra of Au-1-2-3-2 ( ), Au-1-2-3-4 ( ), and Au-1-4-3-4 ( ) compared to
absorption spectra of monomeric bNDIs, rNDIs, and p-octiphenyls in
MeOH (dotted blue, pink, and gray lines, respectively).
cascade architecture. Compatibility with supramolecular n/p-
heterojunctions is demonstrated with photoinduced stack/rod
electron transfer from POP donors to the new rNDI accept-
ors, and reflected in long-lived PCS and high relative photo-
currents in current–voltage relationships and action spectra.
Functional relevance of the directionality of multicomponent
cascade architecture is demonstrated with rational control of
the fill factors in current–voltage relationships, that is, the
generation of power with light. The perspective to zip in
bromo-, alkyloxy-, alkylsulfido-, hydrogen-, or cyano-substi-
tuted NDIs[3,7] as variable electron/energy donors/acceptors
of variable color in variable patterns promises access to the
complexity associated with interesting function, not to speak
of the enormous structural and functional scope conceivable
with other rods, stacks, and surfaces. With the evidence for
significant function in hand, structural and mechanistic
studies became more attractive. Collaborative efforts on
structural characterization of zipper architectures are ongo-
ing; preliminary results from quartz crystal microbalance and
atomic force microscopy are compatible with design and
function. However, the complexity of the system calls for
caution and suggests that alternative explanations including
the existence of different suprastructural packing motifs
[7] a) F. Würthner, S. Ahmed, C. Thalacker, T. Debaerdemaeker,
Chem. Eur. J. 2002, 8, 4742 – 4750; b) C. Thalacker, C. Rꢀger, F.
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M. Fischer, C. von Hꢁnisch, M. Mayor, Helv. Chim. Acta 2006,
89, 1986 – 2005.
[8] D. W. Sievers, V. Shrotriya, Y. Yang, J. Appl. Phys. 2006, 100,
114509 – 114517.
[9] The photophysics of these systems is complex, and a compre-
hensive analysis will be reported in a specialized journal.
[10] H. Yamada, H. Imahori, Y. Nishimura, I. Yamazaki, T. K. Ahn,
S. K. Kim, D. Kim, S. Fukuzumi, J. Am. Chem. Soc. 2003, 125,
9129 – 9139.
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