Photoinduced transformations of stiff-stilbene-based discrete metallacycles to metallosupramolecular polymers

Abstract

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 reintroduces Z-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.

Keywords: dynamic materials; metal coordination; photoirradiation; reversibility; supramolecular coordination complex.

Fig. 1.

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.

Fig. 2.

NMR characterization of the formation of discrete metallacycles. Partial (A−D) ³¹P and (E−I) ¹H NMR spectra (CD₂Cl₂, 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).

Fig. 3.

Partial ¹H NMR spectra (CD₂Cl₂, 293 K, 500 MHz): (A) discrete metallacycle 4, (B) metallosupramolecular polymer 6, (C) discrete metallacycle 5, and (D) metallosupramolecular polymer 7. c = 5.00 mM.

Fig. 4.

Spectral characterization of the transformation from DMCs to MSPs. (A) UV-visible spectra of DMCs before and after irradiation at 387 nm (c = 20.0 μM). (B) Fluorescence spectra of DMCs before and after irradiation at 387 nm (λ_ex = 365 nm; c = 20.0 μM). (C) Diffusion coefficient D (CD₂Cl₂, 293 K, 500 MHz) of DMCs 4 and 5 and MSPs 7 and 8 (c = 5.00 mM). (D) Size distributions of DMCs before and after irradiation at 387 nm (c = 1.00 mM).

Fig. 5.

Cartoon representation of the photoinduced reversible transformations between MSPs and cyclic oligomers.

Fig. 6.

Fluorescence characterization of the reversible transformations between MSPs and cyclic oligomers. Fluorescence spectra of MSPs 6 (A) and 7 (C) before and after irradiation at 360 nm (λ_ex = 365 nm; c = 10.0 μM). Cyclic changes in the emission intensity of MSPs 6 (B) and 7 (D) upon alternative irradiation at 360 nm and 387 nm.

Fig. 7.

(A and B) TEM images of nanoparticles formed by self-assembly of DMCs 4 and 5 in CH₂Cl₂. (C and D) TEM images of nanofibers formed by irradiation of the solutions of DMCs 4 and 5 in CH₂Cl₂. c = 1.00 × 10⁻⁴ M.