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Abstract: Hybrid lead halide perovskites exhibit a delicate interplay between average crystallographic symmetry, local structural disorder and A-site orientational dynamics, giving rise to unusual vibrational and electronic behavior. Here, we combine large-scale molecular dynamics with a density-functional-theory-accurate machine learning force field to resolve the structural dynamics of perovskites across mesoscopic length scales. In formamidinium lead iodide (FAPbI3), we identify a high-temperature α phase with dynamic local order and correlated tilt nanodomains, an ordered γ phase with long-range a+a+a+ tilt coherence, and, below ∼100 K, a history-dependent γ′ state consisting of locally γ-like nanoscale regions separated by sharp twin-like boundaries. This low-temperature disordered state is not a distinct bulk polymorph, but a kinetically arrested metastable twin-domain network selected by the interplay between shallow tilt energetics and slowing FA reorientation. This picture is supported by our low-temperature X-ray diffuse scattering measurements and accounts for the broadened low-energy vibrational response found in the simulations. Furthermore, this unique structural landscape imprints a spatially varying electronic disorder with implications for macroscopic optoelectronic properties, reflected in substantial band-edge broadening retained at low temperature. Our results reconcile the debated low-temperature behavior of FAPbI3 in terms of competition between ordered and arrested structural states, and more broadly identify molecular reorientation as a kinetic selector of metastable framework topology in soft molecular crystals, placing thermal history on equal footing with composition as a determinant of structural and optoelectronic properties.

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Abstract: Small molecule synchrotron serial crystallography (smSSX) is used to monitor irreversible photodimerization in α‑trans-cinnamic acid, enabling structure determination of intermediate states and quantification of batch‑level reaction heterogeneity. The method provides statistically robust ensemble insight alongside complete 3D structural information, establishing smSSX as a powerful platform to study heterogeneous microcrystalline photoreactions.

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Abstract: The `photometric selection' approach as a high throughput, sample tolerant, low cost and highly automatable method of carrying out serial crystallography is presented. Crystalline samples are loaded and distributed onto a simple transparent substrate and an in-line camera identifies crystals using image recognition algorithms from the computer vision project OpenCV. In contrast to established serial techniques, which generally require that crystal samples be refined with narrow size distributions and defined habits, the sample requirements when using photometric selection are shown to be minimal. We demonstrate how broadly effective photometric selection can be by collecting high-quality datasets from three exemplar systems: a small-mol­ecule organometallic, a small-mol­ecule organic and a metal–organic framework system. In contrast to previously established grid-scanning techniques, data collection using photometric selection can be up to six times faster.

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Abstract: Knotting and weaving, at both macroscopic and molecular scales, play an essential role in determining the physical properties of materials. While classical crossing points in molecular knots have been extensively studied, synthetic bifurcated knots, containing characteristic junctions where a strand divides into two branches, represent an underexplored class of topologically complex molecular knots. Although reports of such structures are rare due to synthetic challenges, natural bifurcated knots have been identified in proteins and linked to potentially enhancing protein robustness. Here we report the synthesis of a 524-atom bifurcated knot featuring 3 classical crossings and 20 bifurcated junctions via 2 sequential, selective, imine condensations. First, ZnII ions are used to template the subcomponent self-assembly of a ZnII8L6 architecture, with three unreacted aldehyde groups protruding from each of its eight vertices. Second, a geometry-matching tris-aniline condenses with these terminal aldehydes, yielding a covalently linked bifurcated knot with enhanced robustness, as quantified using collision-induced dissociation mass spectrometry. We anticipate that this approach will facilitate the design of new mechanically interlocked molecules and highly entangled molecular materials with increased robustness.

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Abstract: Artemisinin (ART) is mainly used for the treatment of malaria and exhibits polymorphism with two known crystalline forms. In this study, the high-pressure behaviour of these two polymorphs was investigated to evaluate their compressibility and identify if any pressure-induced phase transitions occur with a view to assessing the impact of manufacturing pressure on the active pharmaceutical ingredient. Form (I), the orthorhombic polymorph, is found to be the most compressible of the three. Form (II), a triclinic phase, undergoes a phase transition to a new polymorph that is observed at different pressures depending on the pressure-transmitting medium (PTM) used. The transition to form (III) occurs at 0.75 GPa when compressed in petroleum ether, however, this transition is delayed to 2.02 GPa in silicone oil. This highlights the influence of the PTM on the stability of the crystal structure. The newly characterized form (III) shares structural similarities with form (II) but differs in symmetry where a pseudo-21 screw axis in form (II) becomes a formal 21 screw axis in form (III), resulting in a change from triclinic to monoclinic and a reduction of the asymmetric unit from Z′ = 4 to Z′ = 2. These findings contribute to a deeper understanding of pressure-induced polymorphism in ART and underscores the importance of external factors such as PTM in influencing solid-state transitions relevant to pharmaceutical processing and formulation.