Supra-Amphiphiles for Controlled Self-Assembly and Disassembly - Molecular Self-Assembly: Advances and Applications - page 214

6.4.1 Responsive One-Dimensional Nanostructure

Generally, the fabrication of one-dimensional nanostructures needs
much tedious chemical synthesis due to the requirement of highly
directional interactions, such as multiple hydrogen bonding and
strong interactions [27-33]. The construction of responsive

one-dimensional nanostructures is even more difficult. However,
using the concept of supra-amphiphile, we are able to realize the
easy fabrication of responsive one-dimensional nanostructures. One
good example is to employ a water-soluble charge transfer complex
to fabricate supra-amphiphile for assembling pH-responsive one-
dimensional nanostructures (Fig. 6.6). An amphiphile containing
electron-deficient viologen group (RV) is chosen as one building block.
The other building block is 8-hydroxypyrene-1,3,6-trisulfonic acid
(HPTS), a pH-responsive electron-rich molecule [17]. Driven by the
charge transfer interactions and electrostatic interactions between
viologen groups and HPTS molecules, RV and HPTS are able to form
a supra-amphiphile when mixed in water in a molar ratio of 1:1. The
supra-amphiphile can self-assemble into ultra-long nanofibers. The
diameter of the nanofiber is about 10 nm, while the length reaches
as long as tens of micrometers. Interestingly, the straightness of the
nanofiber can be tunable reversibly upon pH changes. At the same
time, the physical properties such as conductivity of the nanofibers
change accordingly. The NMR and single-crystal XRD data indicate
that the formation of one-dimensional nanostructure is driven by the
face-to-face packing mode of the charge transfer complex between
viologen and HPTS. The driving force for the one-dimensional packing
of the charge transfer complex is confirmed to be the cooperation of
charge transfer interaction and electrostatic interaction.
When the building block of HPTS is replaced by 6,8-
dihydroxypyrene-1,3-disulfonic acid disodium (DHPS), an
additional hydrogen bonding perpendicular to the axis of the one-
dimensional packing is introduced. This additional interaction
is expected to induce the lateral packing of the one-dimensional
nanostructures (Fig. 6.7). At a higher pH, the hydroxyl groups
are all deprotonated and hydrogen bonds are damaged, then the
lateral packing is disassociated. Indeed, upon pH changes, the self-
assembling structures can be reversibly tuned between nanofibers
and nanoribbons. High-resolution TEM and AFM have proved that
the nanoribbons are resulted from the hierarchical self-assembly
of the nanofibers [34]. At lower pH, the self-assembled nanofibers
can pack laterally with each other to form nanoribbons. When pH is
increased, the nanoribbons gradually disassemble into nanofibers.

π−π