Predicting the Structure of Supramolecular Dendrimers
A R T I C L E S
5
subunits were elaborated by other laboratories. Our laboratory
generated methods for the structural and retrostructural analysis
Through the synthesis as well as structural and retrostructural
4
b
4b
17
17
analysis of libraries of AB
2
,
AB
3
,
AB
4
,
AB
5
,
y
and AB -
4c
of the p6mm hexagonal columnar (Φ
columnar (Φr-s), and the c2mm centered rectangular columnar
r-c) periodic lattices to determine the conformation of the
h
), p2mm simple rectangular
AB
n
self-assembling benzyl ether dendrons, it was discovered
that the primary structure of the dendron determines the tertiary
structure of the resulting supramolecular dendrimers, their
quaternary structure (self-organized periodic lattice or quasi-
periodic array), and the mechanism of self-assembly. The
structural and retrostructural analysis of a library of phenylpropyl
(
Φ
4b
dendrons during self-assembly. Subsequently, the first spheri-
cal supramolecular dendrimers that self-organize into cubic
4a
4b,6
j
j
Pm3n (Cub), cubic Im3m (BCC),
2
tetragonal P4 /mnm (Tet)
1
8
7
8
ether dendrons that are more flexible than the corresponding
lattices and into 12-fold quasi-liquid crystalline (QLC) arrays
were discovered, and methods for their retrostructural analysis
were elaborated. Originally, columnar and spherical supramo-
lecular dendrimers were considered to be micellar. However,
adaptation of Cochran, Crick, and Vand helical diffraction
theory to incorporate tilted groups of atoms revealed features
that demonstrated internal helical order in the supramolecular
columns derived from self-assembling dendrons and dendrim-
ers. Recently, it was discovered that supramolecular spheres can
be chiral or chiral hollow, which brought into question the
micellar structure of spherical dendrimers. In addition to
providing insight into the mechanisms of self-assembly in
biological and synthetic systems, the internal structures of
supramolecular dendrimers generated from aryl ether dendrons
and dendrimers have been exploited for the design of complex
systems such as self-repairing supramolecular electronic materi-
4
benzyl ether dendrons proved that self-assembly is also possible
with dendritic building blocks which access conformations
similar to the trans and gauche conformations of the benzyl
ethers building blocks. Benzyl ether dendrons provided the most
investigated class of self-assembling and self-organizable den-
9
4
drons. However, they exhibit acidic and oxidative instability,
and their corresponding supramolecular dendrimers are limited
in size. In addition, the structural and retrostructural analysis
10
18
of libraries of phenylpropyl and biphenyl-4-methyl ether
1
1
12
19
dendrons demonstrated the tolerance of the self-assembly
process to larger dendritic building blocks and provided access
to larger supramolecular structures. However, phenylpropyl ether
dendrons exhibit low phase transitions, although they are more
stable under acidic conditions than benzyl ether dendrons. In
addition, the phenolates derived from phenylpropyl ether
building blocks are oxidatively less stable than those derived
from benzyl ethers. While the biphenyl-4-methyl ether building
blocks exhibited enhanced stability to oxidation, biphenyl-4-
methyl ether dendrons were less soluble, thereby restricting the
size of the library that could be synthesized and analyzed.
1
3
14
12
als, porous protein mimics, supramolecular containers,
thixotropic gels, and nanomechanical actuators.
1
5
16
(
5) For select examples of other approaches to self-assembling dendrons
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Zubarev, E. R.; Pralle, M. U.; Sone, E. D.; Stupp, S. I. J. Am. Chem.
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analysis of a new class of self-assembling biphenylpropyl (BpPr)
ether dendrons is reported. The BpPr building block was
constructed to synergistically combine the size of phenylpropyl
and biphenyl-4-methyl ether dendrons with the most desirable
combination of solubility and acidic and oxidative stability. New
supramolecular structures discovered during the synthesis of the
libraries of BpPr dendrons are also presented. BpPr dendrons
were designed to facilitate comparison of all previously reported
libraries of self-assembling aryl ether dendrons. This compara-
2
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J. AM. CHEM. SOC. 9 VOL. 131, NO. 47, 2009 17501