Coordination-driven self-assembly is a synthetic method used to generate complex structures based on the simple formation of metal–ligand bonds between molecular building blocks. These building blocks can be functionalized in a variety of ways, endowing the process with a wide range of modularity and tunability. Here, a building block is used that changes its structure when it absorbs light. Because the geometry of each building block is a critical determinant of overall structure, when the precursor changes its shape the final assembly transforms from a discrete metallacycle to an extended 1D network. This technique demonstrates that coordination-driven self-assembly can be used to obtain dynamic materials that respond to external stimuli.
Control over structural transformations in supramolecular entities by external stimuli is critical for the development of adaptable and functional soft materials. Herein, we have designed and synthesized a dipyridyl donor containing a central Z-configured stiff-stilbene unit that self-assembles in the presence of two 180° di-Pt(II) acceptors to produce size-controllable discrete organoplatinum(II) metallacycles with high efficiency by means of the directional-bonding approach. These discrete metallacycles undergo transformation into extended metallosupramolecular polymers upon the conformational switching of the dipyridyl ligand from Z-configured (0°) to E-configured (180°) when photoirradiated. This transformation is accompanied by interesting morphological changes at nanoscopic length scales. The discrete metallacycles aggregate to spherical nanoparticles that evolve into long nanofibers upon polymer formation. These fibers can be reversibly converted to cyclic oligomers by changing the wavelength of irradiation, which reintroducesZ-configured building blocks owing to the reversible nature of stiff-stilbene photoisomerization. The design strategy defined here represents a novel self-assembly pathway to deliver advanced supramolecular assemblies by means of photocontrol.
PNAS, 2014
Photoisomerization of the ligand and discrete metallacycles.
(A)Reversible photoisomerization of the pyridine-linked stiff-stilbene derivatives Z-1 and E-1.
(B) Cartoon representation of the formation of discrete organoplatinum(II) metallacycles 4 and 5 and infinite metallosupramolecular polymers 6 and 7.
NMR characterization of the formation of discrete metallacycles.Partial (A−D) 31P and (E−I) 1 H NMR spectra (CD2 Cl2, 293 K) of free building blocks Z-1 (G), 2 (A and E), and 3 (C and I) and discrete metallacycles 4 (B and F) and 5 (D and H).
Cartoon representation of the photoinduced reversible transformations between MSPs and cyclic oligomers.
. (A and B) TEM images of nanoparticles formed by self-assembly of DMCs 4 and 5 in CH2 Cl2. (C and D) TEM images of nanofibers formed by irradiation of the solutions of DMCs 4 and 5 in CH2 Cl2. c = 1.00 × 10−4 M.
