Web Release Date: January 23,
Dual Aperture Control on pH- and Anion-Driven Supramolecular Nanoscopic Hybrid Gate-like Ensembles
ñez,*
and
Instituto de Química Molecular Aplicada, Departamento de Química, Universidad Politécnica de
Valencia, Camino de Vera s/n, E-46022 Valencia, Spain, Institut de Ciència del Materials,
Universitat de València, P.O. Box 22085, E-46071 València, Spain, Departament de Química
Inorgànica, Universitat de Barcelona, Diagonal 647, E-08028 Barcelona, Spain, Institut de
Química Te
rica i Computacional and Institut de Nanociència i Nanotecnologia, Universitat de
Barcelona, E-08028 Barcelona, Spain, and Institució Catalana de Recerca i Estudis Avançats,
08003 Barcelona, Spain
Received August 9, 2007
This paper contains enhanced objects
available on the Internet at http://pubs.acs.org.
Abstract:
The development of gate-like systems able to perform certain programmed functions is an
interesting way of taking chemistry to the frontiers of nanoscience. In relation to this field, we report a
complete study of the behavior of a pH-driven and anion-controlled nano-supramolecular gate-like ensemble
obtained by anchoring suitable polyamines on the pore outlets of mesoporous materials of the type MCM-41 (solid N3-S). The release of an entrapped dye (Ru(bipy)32+) from the pore voids into the bulk solution
allows us to study the gating effect. A pH-driven open/close mechanism was observed that arises from the
hydrogen-bonding interaction between amines at neutral pH (open gate) and Coulombic repulsions at acidic
pH between closely located polyammoniums at the pore openings (closed gate). Molecular dynamics
simulations using force field methods have been carried out to explain the pH-driven open/close mechanism.
For this purpose, a mesoporous silica structure was constructed, taking as base the (1
1) plane of the
-crystoballite structure on which large hexagonal nanopores and anchored polyamines were included.
From these calculations, it was observed how completely unprotonated amines display poor coverage of
the pore (fully open gate), whereas completely protonated amines (simulating a pH 2 or lower) result in a
clear reduction of the pore aperture, in agreement with the experimental results. In additional to the pH-driven protocol, opening/closing of the gate-like ensemble can also be modulated via an anion-controlled
mechanism. This study was carried out by monitoring the dye released from the pore voids of the N3-S
solid at a certain pH in the presence of a range of anions with different structural dimensions and charges,
including chloride, sulfate, phosphate, and ATP (Canion = 1 × 10-2 mol dm-3). The choice of a certain
anionic guest results in a different gate-like ensemble behavior, ranging from basically no action (chloride)
to complete (ATP) or partial pore blockage, depending on the pH (sulfate and phosphate). The remarkable
anion-controllable response of the gate-like ensemble can be explained in terms of anion complex formation
with the tethered polyamines. These experimental studies are also in agreement with computational
simulations with fluoride, chloride, iodide, and dihydrogen phosphate anions. In the model, larger anions
push the tethered polyamines toward the pore openings more efficiently, and therefore the pore aperture
decreases. The studies also show that, for anions showing a strong tendency to form hydrogen-bonding
networks (e.g., phosphate), complete pore blockage was observed at acidic pH. Finally, selectivity patterns
have been discussed in terms of kinetic rates of the liberation of the Ru(bipy)32+ dye from the amine-functionalized dye-containing material N3-S.
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