Covalent assembly of molecular ladders

Article abstract of DOI:10.1021/ja0690013

[n]-Rung molecular ladders (n = 3-6) have been prepared by reacting discrete, complimentary m-phenylene ethynylene oligomers using imine formation/exchange. The nanostructures, which in the largest case measure approximately 1.6 × 6.2 nm, have been charac

Full text of DOI:10.1021/ja0690013

Published on Web 03/27/2007
Covalent Assembly of Molecular Ladders
C. Scott Hartley, Erin L. Elliott, and Jeffrey S. Moore*
Departments of Chemistry and Materials Science & Engineering, and the Beckman Institute for AdVanced Science
and Technology, UniVersity of Illinois at UrbanasChampaign, Urbana, Illinois 61801
Received December 15, 2006; E-mail: jsmoore@uiuc.edu
A current challenge for self-assembly is the synthesis of shape-
persistent nanostructures with a high degree of structural control.
These artificial macromolecules could ultimately approach the
sophistication of biomolecules,1 allowing atomic-level spatial
control over multi-nanometer length scales in two and three
dimensions, and the construction of molecular objects of size
commensurate with top-down nanofabrication. Much like the current
use of DNA as a structural element for nanotechnology,2
a
Figure 1. [6]-Rung ladder 16 (OTg ) O(CH2CH2O)3CH3).
multitopic oligomer-based approach allows, in principle, the forma-
tion of complex nonsymmetrical structures by incorporating instruc-
tions for self-assembly into sequence. However, in contrast to small-
molecule building blocks, the use of linear oligomers as components
for nanostructure construction is relatively undeveloped, with a few
exceptions.3-8 Here we show that discrete oligomers are viable
building blocks for fully covalent shape-persistent molecular grids.
We have found that, using dynamic covalent chemistry (DCC),9
complimentary m-phenylene ethynylene (mPE) oligomers will self-
assemble into [n]-rung molecular ladders (1n, n ) 3-6). In the
largest case (16, Figure 1), the core region is approximately 6.2 ×
1.6 nm. In all cases, the desired ladder structure is the only product
observed by MALDI mass spectrometry, although gel permeation
chromatography (GPC) indicates an increasing fraction of high
molecular weight byproducts with increasing n. These ladders
represent the first step toward larger grids that will be developed
as high precision nanofiltration membranes,10 or as “smart-matrix”
grids to position units for solar energy harvesting.11
Scheme 1
Structures 13-16 are similar to other reported finite, solution-
phase molecular ladders based on zinc porphyrin5 and aromatic
amide6 oligomers. However, to our knowledge, these are the first
examples of molecular ladders directly assembled using DCC.12
We chose imine formation and exchange reactions for rung
construction, as they are well established in the synthesis of organic
nanostructures13 and in particular have been used by us to prepare
mPE macrocycles.14 These reactions are fast, and the products can
be trapped by irreversible reduction. Furthermore, like a hydrogen
bond, the imine bond is directional, which prevents homodimer-
ization and could be of future use as a means to larger and more
complicated structures.8
In addition, the shorter ladders 13 and 14 were isolated by
preparatory GPC (57 and 37% yield, respectively) and characterized
1
by H NMR.19
Oligomers 2n and 3n were prepared using a previously reported
solid-phase method developed in our laboratory.15 Ladder formation
was carried out at room temperature by stirring a 3 mM chloroform
solution of aldehyde-substituted 3n and a slight (5%) excess of
amino-substituted 2n with scandium(III) triflate16 (Scheme 1). At
no point during the reaction did we observe a precipitate or
cloudiness in the reaction mixtures, suggesting that these large
ladder structures have good solubility, and that there is no formation
of insoluble polymer network byproducts. After 5 h,17 the reaction
mixtures were quenched with sodium triacetoxyborohydride18 to
reduce the imines and facilitate analysis. After workup, the reaction
products were analyzed by MALDI mass spectrometry and GPC.
MALDI spectra of the crude products are shown in Figure 2.
Remarkably, in each case, the spectra are dominated by peaks
assigned to 13-16 (both M+ and M + Na+ ions are observed). The
desired ladder structures are the only reasonable products corre-
sponding to the observed molecular weights (the instrument is
accurate to 0.05-0.1%). We do not observe any signals corre-
sponding to out-of-register 2n‚3n adducts (which would occur at
+18 au for each unreacted NH2/CHO pair), or higher molecular
weight polymers (i.e., (2n)x(3n)y, with x + y > 2). The small features
at twice the desired molecular weight are assigned to gas phase
aggregate dimers of 1n (i.e., [1n ‚Na+]), which have been observed
2
for other shape-persistent macrocycles.20
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J. AM. CHEM. SOC. 2007, 129, 4512-4513
10.1021/ja0690013 CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
be prone to misassembly, the products of which are kinetically
trapped by multiple imine bonds. The GPC trace was essentially
unchanged when the formation of 15 was carried out at elevated
temperature (75 °C, in a sealed tube).19 This suggests that the system
may not be kinetically trapped at large n. Further studies to probe
the mechanism are underway, as is the development of strategies
to overcome these limitations.
To summarize, we have demonstrated the self-assembly of
[n]-rung molecular ladders 1n using DCC to cross-link discrete mPE
oligomers. Despite their large aromatic surface, these structures
show good solubility under the reaction conditions. MALDI spectra
of the unpurified products are remarkably clean and provide direct
evidence for the formation of the desired ladder structures 1n.
However, GPC traces suggest that, as the oligomer length increases,
the yield decreases as a greater proportion of higher molecular
weight material is observed. Nevertheless, these structures dem-
onstrate the utility of discrete oligomers as components for two-
dimensional nanostructure assembly.
Figure 2. MALDI spectra of 13-16 (TCNQ matrix). Calculated molecular
weights: [13 + Na+] 2358.7; [14 + Na+] 3110.6; [15 + Na+] 3862.4; [16
+ Na+] 4614.3.
Acknowledgment. This work was supported by the National
Science Foundation (CHE-0642413 and DMI-0328162).
Supporting Information Available: Experimental procedures for
the synthesis of 2n and 3n and the self-assembly of 1n, 1H NMR spectra
of 13 and 14, deconvoluted GPC traces, concentration dependence of
the GPC trace of 15, and formation of 15 at 75 °C. This material is
available free of charge via the Internet at http://pubs.acs.org.
References
(1) Whitesides, G. M.; Mathias, J. P.; Seto, C. T. Science 1991, 254, 1312.
(2) See for example: Rothemund, P. W. K. Nature 2006, 440, 297.
(3) Albrecht, M. Chem. ReV. 2001, 101, 3457.
(4) Ruben, M.; Rojo, J.; Romero-Salguero, F. J.; Uppadine, L. H.; Lehn, J.-
M. Angew. Chem., Int. Ed. 2004, 43, 3644.
(5) Taylor, P. N.; Anderson, H. L. J. Am. Chem. Soc. 1999, 121, 11538.
(6) Bisson, A. P.; Carver, F. J.; Eggleston, D. S.; Haltiwanger, R. C.; Hunter,
C. A.; Livingstone, D. L.; McCabe, J. F.; Rotger, C.; Rowan, A. E. J.
Am. Chem. Soc. 2000, 122, 8856.
(7) Zeng, H.; Miller, R. S.; Flowers, R. A., II; Gong, B. J. Am. Chem. Soc.
2000, 122, 2635.
Figure 3. GPC traces for crude 13-16. The traces are normalized to the
height of the largest peak. The target ladder structures are labeled with the
/ symbol. 13, tr ) 26.6 min, Mn ) 2080, PDI ) 1.03; 14, tr ) 26.3 min, Mn
) 2720, PDI ) 1.03; 15, tr ) 25.8 min, Mn ) 4050, PDI ) 1.02; 16, tr )
25.6 min, Mn ) 4740, PDI ) 1.02 (polystyrene standards).
(8) Hunter, C. A.; Tomas, S. J. Am. Chem. Soc. 2006, 128, 8975.
(9) Corbett, P. T.; Leclaire, J.; Vial, L.; West, K. R.; Wietor, J.-L.; Sanders,
J. K. M.; Otto, S. Chem. ReV. 2006, 106, 3652.
(10) Ulbricht, M. Polymer 2006, 47, 2217.
(11) Lewis, N. S. Basic Research Needs for Solar Energy Utilization; U.S.
Dept. of Energy: http://www.sc.doe.gov/bes/reports/files/SEU_rpt.pdf,
2005.
(12) A Zn-porphyrin ladder complex cross-linked by olefin metathesis post-
assembly has recently been reported: Ishida, T.; Morisaki, Y.; Chujo, Y.
Tetrahedron Lett. 2006, 47, 5265.
(13) For recent examples, see: (a) Liu, X.; Warmuth, R. J. Am. Chem. Soc.
2006, 128, 14120. (b) Chichak, K. S.; Cantrill, S. J.; Pearse, A. R.; Chiu,
S.-H.; Cave, G. W. V.; Atwood, J. L.; Stoddart, J. F. Science 2004, 304,
1308.
(14) Zhao, D.; Moore, J. S. J. Org. Chem. 2002, 67, 3548.
(15) Elliott, E. L.; Ray, C. R.; Kraft, S.; Atkins, J. R.; Moore, J. S. J. Org.
Chem. 2006, 71, 5282.
(16) Giuseppone, N.; Schmitt, J.-L.; Schwartz, E.; Lehn, J.-M. J. Am. Chem.
Soc. 2005, 127, 5528.
(17) No significant changes in the GPC traces or MALDI spectra were observed
after longer reaction times (24 h).
(18) Abdel-Magid, A. F.; Carson, K. G.; Harris, B. D.; Maryanoff, C. A.; Shah,
R. D. J. Org. Chem. 1996, 61, 3849.
(19) See Supporting Information.
In contrast to the clean MALDI spectra, GPC of the product
mixtures suggests the formation of high molecular weight byprod-
ucts (Figure 3). For 13 and 14, the desired molecular ladders are
formed in high yield (71 and 62%, respectively, by deconvolution19
of the GPC traces). However, for the larger 15 and 16, the yields
are much lower (17 and 10%), and broad peaks are observed at
high molecular weight. We believe that these are high molecular
weight species which have widely varying molar mass and thus do
not contribute to a unique MALDI signal.21 It is unlikely that
aggregation of 15 and 16 contributes to these features as the GPC
trace of 15 is concentration independent,19 and the literature22
suggests that aggregates bound by weak intermolecular forces
should yield a single asymmetric (tailing) GPC peak, in contrast
to the distinct product/byproduct peaks that have been observed
here.
(20) Ho¨ger, S.; Spickermann, J.; Morrison, D. L.; Dziezok, P.; Ra¨der, H. J.
Macromolecules 1997, 30, 3110.
(21) Preliminary studies of the analogous [7]-rung molecular ladder 17 have
so far been inconclusive.
There are two possible explanations for the trend toward higher
molecular weight byproducts with increasing n. The equilibrium
distribution may be less populated by the desired [n]-rung ladder
as n increases. Alternatively, the longer oligomers 15 and 16 may
(22) (a) Stevens, F. J. Biophys. J. 1989, 55, 1155. (b) Mathias, J. P.; Seto, C.
T.; Simanek, E. E.; Whitesides, G. M. J. Am. Chem. Soc. 1994, 116, 1725.
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