Understanding the Role of H‑Bonding in Aqueous
Self-Assembly of Two Naphthalene Diimide (NDI)-Conjugated Amphiphiles
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Abstract
Supramolecular
architectures with the synchronized combination
of various directional noncovalent forces are ubiquitous in biological
systems. However, reports of such abiotic synthetic systems involving
H-bonding in aqueous medium are rare due to the challenge faced in
the formation of such structures by overcoming the competition from
the water molecules. In this paper we have studied self-assembly of
two structurally related naphthalene-diimide (NDI) conjugated bola-amphiphiles
(NDI-1 and NDI-2) in water with an aim to realize the specific role
of H-bonding among the hydrazide units present in one of the two building
blocks (NDI-2) on the self-assembly. Both chromophores showed vesicular
assembly in aqueous solution driven primarily by π-stacking
among the NDI chromophores, which could be probed by UV–vis
absorption spectra. Contrary to common belief, the lack of an H-bonding
group in NDI-1 was found to be a boon in disguise in terms of the
stability of the aggregates. Whereas NDI-2 aggregates showed LCST
around 65–70 °C owing to the breaking of the H-bonds with
increased temperature, the NDI-1 aggregates were found to be structurally
intact until 90 °C, which may be attributed to the increased
hydrophobicity introduced by the absence of the polar hydrazide group.
Further concentration- and solvent-dependent UV–vis studies
showed that NDI-1 formed assembled structure at greatly dilute solution
and also in a solvent such as THF, confirming greater propensity for
its self-assembly. As both bola-amphiphiles contain an electron-deficient
NDI chromophore, interaction of their vesicles was studied with an
externally added electron-rich pyrene derivative. Surprisingly, NDI-1
did not show any charge-transfer interaction with the donor, whereas
NDI-2 could effectively intercalate, leading to a functional membrane
with tunable surface functionalities. This was attributed to the additional
stability of the intercalated state by H-bonding among the hydrazide
units