Deconstruction as a strategy for the design of libraries of self-assembling dendrons

Article abstract of DOI:10.1002/anie.201002514

Bringing down the dendron: The deconstruction of self-assembling dendrons represents a new strategy for the rational design of libraries of self-assembling dendrons with unprecedented primary structure (see picture). In this strategy, molecular targets ar

Full text of DOI:10.1002/anie.201002514

Communications
DOI: 10.1002/anie.201002514
Dendrimers
Deconstruction as a Strategy for the Design of Libraries of Self-
Assembling Dendrons**
Brad M. Rosen, Mihai Peterca, Chenghong Huang, Xiangbing Zeng, Goran Ungar, and
Virgil Percec*
Dendrons and dendrimers are perfectly branched molecules
prepared through convergent or divergent iterative synthe-
sis.[1] Self-assembling dendrons[2] serve as building blocks for
self-organizable complex architectures that mimic important
biological structures. The study of these supramolecular
assemblies has expanded the frontiers of nanoscience by
interfacing chemistry, biology, and physics.
through the strategies of generational[3a–c,e] or hybrid dendron
libraries.[3d,f–g] Aside from the generational and combined
generational hybrid strategies, no other rational method to
the design of libraries required to access dendrons with novel
primary structures is available.
Herein we report the deconstruction of self-assembling
dendrons as a fundamentally different strategy for the design
of new libraries of self-assembling dendrons. Starting from
any self-assembling dendron or dendrimer, branches are
systematically and sequentially removed to provide an array
of novel deconstructed structures, which can be synthesized
by the same iterative methods that provided the parent
molecule. We demonstrate the deconstruction design strategy
by using the (3,4BpPr-(3,4,5BpPr)2)12G3-X[3e] (6a: X =
CO2CH3, 6b: X = CH2OH) dendron as the parent compound
(Figure 1). Detailed synthesis of all deconstructed dendrons
and their structural and retrostructural analysis are given in
the Supporting Information.
The current design of self-assembling dendron libraries
involves a generational strategy in which first generation
dendrons are sequentially added to constitutional isomeric
ABn (n ꢀ 2) building blocks to produce higher-generation
dendrons.[2,3a–d] The synthesis, structural, and retrostructural
analysis of generational libraries of self-assembling den-
drons,[3a–c,e] as well as of hybrid ABn-(AB)y dendrons,[3d,f]
enabled the discovery of diverse self-organizable supramolec-
ular dendrimers.[4] Despite differences in building-block
structure, homologous connective topologies of dendrons
from different generational libraries provide a conserved and
therefore predictable[3e] assembly of the supramolecular
dendrimers that is programmed by their primary structure.
This predictability leads to a “nanoperiodic” table[3e,5] of self-
assembling dendrons, and enables the rational design of
complex supramolecular systems. Unfortunately, the predict-
ability reduces the likelihood of finding new supramolecular
structures through the synthesis of additional generational
libraries. The discovery of new supramolecular architectures
from self-assembling dendrons requires the synthesis of
dendritic structures that were not previously accessible
The design of a dendron library through the deconstruc-
tion of 6a,b provides novel dendritic topologies that result in
a variety of previously unencountered 3D phases (Figure 2).
Deconstructed dendrons that provide routinely encountered
structures are discussed in the Supporting Information.
¯
The parent dendrons 6a,b self-organize in Pm3n cubic
phases (Cub, Figure 2). Deconstruction of 6a,b by sequential
removal of two (3,4BpPr-3,4,5BpPr)12G2 subunits from their
periphery provides 7a,b and 8a,b. Compounds 7a,b and 8a
form Cub phases. Compound 8b self-assembles into spherical
¯
dendrimers that self-organize into a body-centered Im3m
cubic lattice (body-centered cubic (BCC), Figure 2).[6] The
BCC lattice is rarely encountered for self-organizing den-
drons, though it has been observed for dendronized poly-
(oxazolines),[7] dendritic metal carboxylates,[6] and benzyl
ether dendrons.[3a]
[*] Dr. B. M. Rosen, Dr. M. Peterca, C. Huang, Prof. V. Percec
Roy & Diana Vagelos Laboratories, Department of Chemistry
University of Pennsylvania
Philadelphia, PA 19104-6323 (USA)
Fax: (+1)215-573-7888
E-mail: percec@sas.upenn.edu
Removal of four (3,4BpPr)12G1 subunits from the
periphery of 7a,b results in novel AB2-AB-AB2 dendritic
architectures, 15a,b. At elevated temperatures, 15a self-
organizes in a vesicular Cub phase, observed previously for
(4BpPr-3,4BpPr-3,5BpPr)12G2-CO2CH3.[3e] The supramolec-
ular spheres assembled from 15a by a mechanism similar to
that previously reported[3e] are composed of 933 quasi-
equivalent dendrons that have a molecular weight of 1.70 ꢀ
106 gmolÀ1 and a diameter of 171.9 ꢁ. These two structures
represent the largest monodisperse supramolecular objects
self-assembled from dendrons reported to date and are
comparable in size to the most complex biological structures
such as the ribosome.[8]
Homepage: http://perceco2.chem.upenn.edu/~percec/
Prof. X. Zeng, Prof. G. Ungar
Department of Engineering Materials
University of Sheffield, Sheffield S1 3JD (UK)
[**] Financial support by the National Science Foundation (DMR-
0548559 and DMR-0520020) and the P. Roy Vagelos Chair at the
University of Pennsylvania are gratefully acknowledged. Prof.
Stephen Z. D. Cheng (Akron) is gratefully acknowledged for per-
forming density measurements. B.M.R. gratefully acknowledges
funding from a NSF Graduate Research Fellowship and an ACS
Division of Organic Chemistry Graduate Fellowship (Roche). G.U.
acknowledges support from the WCU program through the
National Research Foundation of Korea funded by the Ministry of
Education, Science and Technology (R31-10013).
The dendritic alcohol 15b forms a hexagonal columnar Fh
phase at low temperatures. At elevated temperatures, this
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201002514.
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Figure 1. The deconstruction of 6a,b. In each step of the deconstruction (solid arrows), the fragment highlighted by the wedge of the
corresponding color (red or gray) is removed. Dotted arrows indicate construction.
phase transforms into a new columnar simple orthorhombic
lattice with Pmna symmetry (Figure 3). The recon-
F
À
s
o
structed electron densities suggest that the columns of the Fh
phase produce shorter segments with comparable diameters
upon the F –F transition (Figure 3d).
À
s o
h
Deconstruction of 8a,b can also proceed via removal of
(3,4BpPr)12G1 to provide 9a,b. Oriented-fiber XRD of
structures assembled from 9b show a Fh phase at low
3D-SL
temperatures and a hexagonal columnar superlattice Fh
at high temperatures (Figure 4). Fh3D-SL phases were observed
only in the coassembly of twin-tapered dendritic benzamides
with three[9a] and four[9b] cylindrical bundle dendronized
3D-SL
polymers. The Fh
phases of 5b and 9a,b are the first
reported examples of these phases generated by self-assembly
rather than coassembly. The wide-angle X-ray (WAXS) fiber
patterns collected in the Fh3D-SL of 5b and of 9a,b show only
diffuse scattering that corresponds to alkyl chain–chain
Figure 2. Novel structures from deconstructed dendritic esters (blue
arrows) and alcohols (purple dotted arrows).
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Communications
Figure 3. a) Oriented-fiber, b) powder XRD diffractograms, and
c) reconstructed 3D electron density and model of the F phase with
À
s
o
Pmna symmetry generated from 15b. d) The 2D map of the z=1/2
plane is shown at scale with the corresponding map from the lower
temperature Fh phase.
3D-SL
Figure 5. a) Model of the Fh–Fh
transition and b) its potential role
as a supramolecular switch.
correlations around 4.7 ꢁ. This result demonstrates that the
columns posses a soft, liquid-like alkyl jacket. Therefore, the
presence of 3D column-to-column correlations at high
temperatures can be explained only by significant variations
of the column profiles. This hypothesis is supported by the
relative electron density distributions (Figure 4). The 3D
correlations are generated by the periodic coupling of
“undulated” columns. This periodicity doubles the size of
Cubtri phase has been observed in a few phasmidic mole-
cules,[10] but never before in self-assembling dendrons.
Deconstruction of 9 by the removal of (3,4BpPr)12G1
provided 10. The analogous structures 12a,b can be derived
from the deconstruction of (3,4,5BpPr)312G3-X, or by
addition of one C12H25 chain to 10. At elevated temperatures,
3D-SL
¯
the low-temperature unit cell, hence generating the Fh
compounds 12a,b self-organize into bicontinous Ia3d cubic
3D-SL
phase (Figure 5a). The Fh–Fh
transition of these assem-
phases (Cubbi, Figure 6). The Cubbi phase has also been
blies can be utilized as a supramolecular switch for molecular
observed for phasmidic molecules[10b] and dendrons with
electronics (Figure 5b).
3D-SL
Interestingly, above the Fh
of 9a, a triply continuous
¯
Im3m(I) cubic phase (Cubtri) is observed (Figure 2). This
Figure 4. Oriented-fiber XRD and corresponding reconstructed relative
3D-SL
electron density distributions of the Fh and Fh
from 9b.
phases generated
Figure 6. a) The molecular model of 12a, b) 2D, and c) 3D recon-
structed relative electron density distributions of the Cubbi phase.
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semifluorinated periphery,[11] but has not been found in
generational libraries of self-assembling dendrons[3a–c,e] or
self-assembling hybrid dendrons.[3d,f] The Cubbi phase gener-
ated from 12a,b can be approximated by interconnected
distorted columnar segments with a diameter of approxi-
mately 70 ꢁ (Figure 6). Reconstructed electron density dis-
tributions indicate a continuous aromatic domain surrounded
by a continuous aliphatic insulating jacket (Figure 6b,c).
The deconstruction of self-assembling dendrons provides
access to novel dendron primary structures, thereby over-
coming the predictability of the generational approach.[3e]
This library of deconstructed dendrons generated self-assem-
bled structures that were not found in previous generational
libraries; such structures include triple continuous and
bicontinous cubic lattices, hexagonal columnar superlattices,
and a novel columnar simple orthorhombic phase. The co-
assembly of dendronized donors, dendronized acceptors, or
donor and acceptor polymers provided access to self-repair-
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continuous and bicontinuous structures discovered here may
be used in the design of 3D electronic materials that do not
require alignment in order to bridge the space between two
electrodes.[13,14] The deconstruction strategy can be applied to
any dendron and dendrimer primary structure, regardless of
whether or not it has been previously synthesized, thus
providing nearly limitless inspiration for new rationally
designed structures.
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Received: April 27, 2010
Published online: August 16, 2010
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Keywords: dendrimers · liquid crystals · nanostructures ·
self-assembly · supramolecular chemistry
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