Supramolecular Polymerizations

Supramolecular Polymerizations 523
Table 2. Critical concentration and DP in the isotropic phase
for folic acid (selected data taken from ref. [36] ).
Sample Solvent C IN
%
C NH
%
LISO DPISO a) XISO b)
d(GpG) H2O – – 70 15 2.3
d(GpG) H2O +Na + 2.5 18 – – –
d(GpG) H2O +K + 1.5 15 – – –
folate H2O – 35 2.3 1 0.1
folate H2O +Na + 27 35 2.1 9 0.7
a) L/2.35 Š (4.70 for d(GpG)).
b) L/30 Š.
collected in Table 2. The deoxyguanosine derivatives
generally show larger DPs in isotropic solutions and
lower critical concentrations CIN and CNH than the folates.
The presence of NA + and, particularly, K + ions enhances
the stabilization of the aggregates. In the case of folates
in pure H2O, no nematic phase and small DPs were
observed. Due to the considerable diameter of the cylinders
(D L 30 Š) and the thickness of each disk (L = 2.35
Š), the DP in the isotropic phase is extremely small and
the corresponding axial ratio X suggests that thick disks
rather than columns are present. Figure 15b illustrates the
evolution of DP with concentration covering the range
from the isotropic to the hexagonal phase. The smooth
dependence DP vs C does not evidence cooperative
effects in the case of folates. The largest DP (L30), determined
from a 60% (hexagonal) solution, reveals in fact
columns of very small geometric anisotropy (X L 2.3).
The formation of the mesophase may be promoted by the
large excluded volume effect of disks even in the absence
of soft interactions. In fact, simulation studies evidenced
nematic and columnar phases for solutions of extremely
thin disks (0 a L/D a 0.1). [101]
Disk-like supermolecules based on dimers of ureidotriazines
connected by a 4 H-bond scheme similar (but
not identical) to that of the ureidopyrimidone polymers in
Figure 7c were reported by Meijer and coworkers. [34]
These disks stack in columns with loose positional order
and low DP (Figure 16a). Percec and coworkers [44]
reported tubular polymeric assemblies of disks composed
by six tapered molecules of 12-ABG-15C5 complexed
with triflate salt (Figure 16b). The columns assembled
into a hexagonal mesophase revealed as by means of Xray
diffraction from the undiluted system.
Several of the systems described above present a central
cavity into which a covalent polymer can be hosted
or a flow of ions be achieved (cf. also next section). Of
particular interest are nanotubes (Figure 16c) formed by
stacking cyclic peptides connected by H-bonds along the
columnar axis. [37–41] The chemical design of these flat
ring-like peptides was discussed by De Santis and coworkers.
[37] Tubes assembled in solution and the contact
forces for the dimerization (K L 2.5610 3 m –1 ) are too
Figure 16. (a) Monofunctional ureidotriazine disks capable of
assembling into columns. [34] (b) Assembly of tapered 12-ABG-
12C5 into disks, formation of a column of stacked disk, hexagonal
columnar organization. [44] (c) Nanotubules formed by Hbonded
cyclic peptides. [37] (i Am. Chem. Soc. 1994 and 1996.)
small for a large DP unless cooperative effects occur. [38]
Cyclic b-peptides were also considered. [39] The self
assembly of the nanotubules into ion-selective membranes
was discussed as well. [40]
Unimers of most of the systems considered above were
subsequently connected by flexible covalent spacers, producing
main-chain or side-chain SPs that should be
described more appropriately under class C SPs. The
basic ability of the disks to form columnar assemblies
was preserved, but the covalent segments produced
alterations in the stacking details such as the occurrence
of helicity. For instance, a slowly rising helicoidal stack
was produced when the crown ether receptor in Figure
16b was replaced by a flexible endo-receptor (nEO-
PMA) connected as a side-chain to a poly(methyl acrylate)
chain. [42] When the ureidotriazine disks in Figure
16a were main-chain linked through flexible spacers,
helical columns and large DPs were observed. [34]
Applications. The columnar nematic or hexagonal
packing of disk-like molecules and supermolecules could
be exploited as a precursor step for the assembly of large,
oriented structures modeling natural systems. Electronic
mobility due to the p–p interactions along the columnar
axis may be useful for electronic and photonic
devices. [102] Central cavities could be exploited for the
selective hosting of polymer molecules [44] or for ion-
[40, 41]
selective channels.
5.4 Host/Guest Polymeric Assemblies
Covalent polymers can enter a cavity of a columnar
assembly or of single ring-like structures. The result is a
composite host/guest polymeric assembly exhibiting a