Zipper assembly of photoactive rigid-rod naphthalenediimide π-stack architectures on gold nanoparticles and gold electrodes

Article abstract of DOI:10.1021/ja077099v

We introduce zipper assembly as a simple and general concept to create complex functional architectures on conducting surfaces. Rigid-rod π-stack architecture composed of p-oligophenyl rods and blue naphthalenediimide (NDI) stacks is selected as an exampl

Full text of DOI:10.1021/ja077099v

Published on Web 11/30/2007
Zipper Assembly of Photoactive Rigid-Rod Naphthalenediimide π-Stack
Architectures on Gold Nanoparticles and Gold Electrodes
Naomi Sakai,† Adam L. Sisson,† Thomas Bu¨rgi,‡ and Stefan Matile*,†
Department of Organic Chemistry, UniVersity of GeneVa, GeneVa, Switzerland, and UniVersity of Neuchaˆtel,
Institute of Microtechnology, Neuchaˆtel, Switzerland
Received September 13, 2007; E-mail: stefan.matile@chiorg.unige.ch
In this report, we introduce zipper assembly as a simple and
general approach to complex functional architectures on conducting
surfaces (Figure 1). In domains such as organic optoelectronics,
reliable access to sophisticated synthetic architectures is often
essential to create significant function.1-3 Zipper assembly was
conceived on the basis of early findings4 related to the Vernier
assembly,5 where mismatches of rod length led to higher-order
assembly of mixed rigid-rod molecules. This characteristic implied
that rigid-rod molecules with “sticky-ends” could serve as powerful
modules to create complex architectures on conducting surfaces.
Rigid-rod π-stack architectures composed of p-oligophenyl rods
and naphthalenediimide (NDI)6,7 stacks3 were selected to explore
zipper assembly. The design was as follows. To initiate zipper
assembly, short p-quaterphenyls 1 with four anionic NDIs are
deposited on gold (Figure 1, Supporting Information Figure S1).
For propagation, double-length p-octiphenyls 2 with eight cationic
NDIs are added. Directed by flanking hydrogen-bonded chains
(Figure S2)3 and interstack ion pairing (Figure S3), one-half of these
NDIs was expected to form π-stacks with initiator 1. The free half
of the cationic propagator 2 remains available as unbendable sticky-
ends on the surface to zip up with the anionic propagator 3 added
next. The resulting anionic sticky-ends can zip up with cationic
propagator 2 and so on. This zipping up of n-semiconducting NDI
stacks along p-semiconducting8 p-oligoanisol rods could produce
supramolecular n/p-heterojunctions.1 Importantly, the photo- and
electrochemical variability of NDIs without global structural
changes9 promised access to multicomponent zippers where the
direction of electron flow is controlled and light is absorbed at
various wavelength.
Figure 1. The concept of zipper assembly on gold. All shown suprastruc-
tures are simplified, in part speculative representations that are, however,
consistent with experimental data on function (below) and molecular
models.3
For zipper assembly, the p-quaterphenyl initiator 1 with anionic
NDIs and a disulfide anchor was synthesized (Scheme S1).10
Moreover, the already available p-octiphenyl 2 with cationic NDIs3
was complemented with the anionic propagator 3 (Scheme S2).
The blue, red-fluorescent p-octiphenyl 2 has been shown to exhibit
ultrafast (<2 ps), quantitative (>97%), and reasonably long-lived
(61 ps) photoinduced charge separation (EHOMO ) -5.4 eV, ELUMO
) -3.5 eV, Table S1).3,10
Figure 2. Zipper assembly on gold nanoparticles. (A) Change in absorption
during assembly of a zipper (sequence Au-1-2-3-2-3-2) with increasing
absorption at 630 nm on gold nanoparticles (solid, (a) Au-1-2; (b) Au-1-
2-3; (c) Au-1-2-3-2). (B) Same for the sequence (a) Au-1-2, (b) Au-1-2-1
and (c) Au-1-2-1 + 2 (c, all in 50% aqueous TFE).10
Zipper assembly was initially evaluated on gold nanoparticles
(Figures 1, 2).11 Freshly prepared, citrate-stabilized gold nanopar-
ticles (d ≈ 13 nm) were coated with initiator 1, and then repeatedly
exposed to cationic and anionic propagators 2 and 3. Zipper growth
was evidenced in absorption spectra of the resulting coated
nanoparticles not only by the expected increase in NDI absorption
around 630 nm but also by the bathochromic shift of the surface
plasmon resonance (SPR) band near 520 nm, providing an accurate
measure for the increase in layer thickness during zipper assembly
(Figures 2A and S4).10 Moreover, the appearance of a hypsochromic
maximum around 600 nm indicated significant face-to-face π-stack-
ing of the NDIs in the zipper.3
Zipper assembly with cationic ends (Au-1-2) could be effectively
terminated using anionic p-quaterphenyls 1. The addition of
terminator 1 to an Au-1-2 zipper caused the expected 50% increase
in NDI absorption of Au-1-2-1 compared to Au-1-2-3 (Figure 2,
curves b). Addition of propagator 2 to the terminated Au-1-2-1 did
not cause any increase in absorption, whereas addition to Au-1-
2-3 resulted in further increase of absorption, proving the formation
of Au-1-2-3-2 (Figure 2, curves c). This complete inhibition of
zipper assembly with terminator 1 demonstrated the existence of
sticky-ends in zipper assembly. Moreover, the found rod-length
† University of Geneva.
‡ University of Neuchaˆtel.
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J. AM. CHEM. SOC. 2007, 129, 15758-15759
10.1021/ja077099v CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
various surface characterization studies are ongoing to gain ad-
ditional insights on structure, the here reported studies provide the
ultimately relevant functional evidence and promise attractive
perspectives. Preliminary results on increasing zipper complexity
with differently colored NDIs to address more challenging functions
are very promising.
Acknowledgment. We thank S. Bhosale and D.-H. Tran for
contributions to synthesis; D. Jeannerat, A. Pinto, and S. Grass for
NMR measurements; P. Perrottet and the group of F. Gu¨lac¸ar for
ESI MS; N. Oudry and G. Hopfgartner for MALDI MS; H.-R.
Hagemann for access to a Xe lamp; F. Wu¨rthner, S. Fukuzumi, A.
Ikeda, S. Kimura, and Y. Kobuke for advise; and the Swiss NSF
and JSPS for financial support.
Figure 3. Zipper assembly on gold electrodes. (A) Change in photocurrent
during assembly of a blue zipper (sequence Au-1-2-3-2-3-2-3-2, solid) and
a capped control zipper (Au-1-2-1 + 2 + 3 + 2 + 3 + 2, dotted) on gold
electrodes (Conditions: 2, 3 (∼10 µM) in TFE/H2O (0.5 mM NanH3-nPO4,
0.5 M NaCl, pH 7) 1:1, room temp, 14 h; rinsed with H2O and EtOH). (B)
Dependence of photocurrent on number of oligo-NDI layers of the blue
zipper (b) compared to blue LBLs (Au-1-pK-3-pK-3-pK-3-pK-3-pK-3, pK
) polylysine, +) and the capped Au-1-2-1 + 2 + 3 + 2 + 3 + 2 (O).10
Supporting Information Available: Experimental details. This
material is available free of charge via the Internet at http://pubs.acs.org.
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