The one-dimensional coordination polymer [PbBr2(3F-spy)2] (3F-spy = 3-fluoro-4'-styrylpyridine) is a rare single-crystalline material that combines pronounced elastic bendability (bending strain about 1.3%) with light-driven actuation: UV irradiation triggers a topochemical [2+2] photodimerization of the styryl groups, and the macroscopic response - photosalient jumping, splitting or explosion, twisting, bending away from the light, or curling of fiber-like crystals - is selected by crystal cross-section and irradiated face. The mechanism underlying this mode selection is not understood: the fully dimerized photoproduct has no single-crystal structure (crystals lose crystallinity upon complete conversion), the spatial distribution of reacted versus unreacted regions and the anisotropic strain pathway that decides which mechanical mode occurs have never been measured, and it is unknown whether the multiactuation transfers to related compositions (other fluorine positions on the ligand, other halides or pseudohalides in place of PbBr2). The open problem is to establish this quantitative structure-mechanism-actuation relationship - the photoproduct structure and the spatially resolved reaction and strain course with a mechanistic model of the observed mode selection - as the basis for predictive design of multi-actuating photomechanical coordination polymers.

Background

A coordination polymer (CP) is a crystalline solid built from metal ions linked into extended structures by bridging organic ligands; in a one-dimensional CP the metal-ligand connectivity forms chains that pack side by side. Photomechanical crystals convert light into macroscopic motion, and the photosalient effect is the violent jumping, splitting or explosion of a crystal when internal stress generated by a photochemical transformation is suddenly released; it was first observed in a CP in 2014, when a [2+2]-photodimerizing zinc CP crystal popped under UV light (Medishetty et al., Angew. Chem. Int. Ed. 2014). The underlying photochemistry is the solid-state [2+2] photodimerization: two parallel C=C double bonds held close together and suitably aligned in the crystal (the topochemical conditions codified by Schmidt) combine under UV light into a cyclobutane ring; the relative configuration of the four substituents on the ring is denoted by descriptors such as rctt. When such a reaction runs in an intact single crystal without loss of crystallinity - a single-crystal-to-single-crystal (SCSC) transformation - the product structure, including how the unit cell deforms step by step, can be followed directly by single-crystal X-ray diffraction (SCXRD). Rath and Vittal demonstrated exactly this for the photosalient Pb(II) CP [PbI2(5-SPym)(DMF)] (5-SPym = 5-styrylpyrimidine): they solved the 100% dimerized product, three intermediate structures at roughly 52%, 65% and 91% conversion, a linear relation between cell volume and conversion, and heterogeneous photoproduct domains mapped by confocal fluorescence microscopy (JACS 2020). The present problem concerns the sibling compound [PbBr2(3F-spy)2] (Rath and Vittal, Chem. Mater. 2021), which is deliberately different: here the photoreaction is not SCSC. This CP forms slender, highly elastic crystals (monoclinic space group P21/cP2_1/c, with Pb(μ\mu-Br)2_2 chains along the crystallographic aa axis), which can be bent into closed loops (elastic bending strain ε=h/2R≈1.3%\varepsilon = h/2R \approx 1.3\% for crystal thickness hh and loop radius RR) and recover, rationalized by interchain C–H···F and C–H···Br(Pb) contacts, a roughly 41-degree slant between chains, and tensile flexibility of the Pb(μ\mu-Br)2_2 units - though this attribution rests on structural correlation, since no direct probe of the stressed regions existed. Under 360 nm UV light the 3F-spy ligands dimerize to the cyclobutane rctt-3F-ppcb (confirmed by 1^1H NMR: pyridyl signal shift from 8.57 to 8.34 ppm, cyclobutane signal at 4.61 ppm), and the crystals actuate: crystals with cross-section (width × thickness) above roughly 85 μm × 45 μm show photosalient jumping, splitting and explosion (with only about 29% conversion in the fragments); intermediate sizes twist; cross-sections below roughly 65 μm × 25 μm bend away from the light (about 49% conversion in residues after motion); and fiber-like crystals below roughly 18 μm × 10 μm curl and intertwine, performing measurable work (a fiber bundle displaced a glass rod roughly ten times its own mass). Irradiating the (001) face gives bending while irradiating the (100) face gives splitting, so illumination geometry is a control parameter. After 15 minutes the dimerization is complete, yielding a new dimerized CP (compound 2), but the crystals deteriorate, the diffraction pattern can no longer be indexed, and only powder X-ray diffraction (PXRD) evidence - peaks shifting to lower angles, indicating lattice expansion - is available. The size thresholds and face dependence were established only qualitatively from representative videos; the anisotropic strain tensor that governs the differential responses was never measured; the authors proposed spatially resolved probes such as microfocus infrared spectroscopy to inspect stressed bent regions, and proposed substituent variation (other fluorine positions) or replacement of PbBr2 by other halides or pseudohalides as the route to families of actuating CPs, while warning that such chemical changes may produce qualitatively different mechanical responses.

Problem Statement

Establish the quantitative structure-mechanism-actuation relationship for multi-mode photoactuation of the one-dimensional coordination polymer [PbBr2(3F-spy)2], as left open by Rath and Vittal (Chem. Mater. 2021). Concretely, an answer must resolve two linked questions. (1) Structure of the photoproduct: determine the crystal structure of the fully dimerized product 2 (the CP built from PbBr2 units and the rctt-3F-ppcb cyclobutane ligand) at single-crystal-level detail, or by an equivalent local-structure determination, including the local atomic rearrangements accompanying dimerization, overcoming the deterioration of crystals upon complete conversion that defeated conventional SCXRD. (2) Mechanism of mode selection: measure and model, with spatial resolution in the intact crystal, how the [2+2] photodimerization proceeds - the distribution of reacted versus unreacted regions, the anisotropic unit-cell and strain evolution, and how internal stress accumulates and is released - sufficiently to explain how the anisotropic strain pathway, crystal dimensions (approximate width × thickness thresholds near 85 × 45, 65 × 25 and 18 × 10 μm), irradiated face ((001) versus (100)) and conversion extent govern which macroscopic mode occurs (photosalient jumping/splitting/explosion, twisting, bending away from the light, or curling/intertwining of fiber crystals). Spatially resolved probes such as microfocus infrared spectroscopy, microfocus X-ray diffraction, or in-situ photocrystallography (X-ray diffraction data collected while the crystal is under illumination) are means to this end, not separate deliverables. The common objective is a mechanism-based account of how photochemical lattice strain in a coordination polymer is converted into a selected macroscopic mechanical response, with [PbBr2(3F-spy)2] as the reference system. The source authors proposed substituent variation (other fluorine positions) or replacement of PbBr2 by other halides or pseudohalides as a future design route toward families of actuating CPs; such compositional extension is background context for the significance of the problem and not a required component of an answer.

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