The Factor beyond Schmidt's Criteria Impacting the Photo-Induced [2+2] Cycloaddition Reactivity and Photoactuation of Molecular Crystals Based on Cyclic Chalcone Analogues

Large Molecular Rotation in Crystal Changes the Course of a Topochemical Diels-Alder Reaction from a Predicted Polymerization to an Unexpected Intramolecular Cyclization

A designed anthracene-based monomer for topochemical Diels-Alder cycloaddition polymerization crystallized with head-to-tail arrangement of molecules, as revealed by single-crystal X-ray diffraction (SCXRD) analysis. The diene and dienophile units of adjacent monomer molecules are aligned at an average distance of 4.6 Å, suggesting a favorable crystalline arrangement for their intermolecular Diels-Alder cycloaddition reaction to form a linear polymer. Surprisingly, heating the monomer crystals at a temperature above 125 °C resulted in the formation of intramolecular Diels-Alder cycloadduct, which could be characterized by various spectroscopy and SCXRD analysis. Various time-dependent studies such as NMR, PXRD, and DSC, studies established that the reaction followed topochemical pathway. Schmidt's topochemical postulates are generally used to predict the topochemical reactivity and product, by analyzing the crystal structure of the reactant. Though the crystal arrangement predicted polymerization, upon heating, the molecule avoided this pathway by undergoing a large rotation to form an intramolecular cycloadduct. Theoretical calculations supported the feasibility of the rotation, exploiting the flexibility of the molecule and voids present. These findings caution that the reliance on Schmidt's criteria for topochemical reactions may sometimes be misleading, especially in heat-induced reactions.

Photocycloaddition of Zero-Dimensional Metal Halide Hybrids with Reversible Photochromism

In this work, two zero-dimensional (0D) metal halide hybrids L2ZnBr4 [1, L = (E)-4-(2-(1H-pyrrol-3-yl)vinyl)-1-methylpyridin-1-ium] and L6Pb3Br12 (2) were prepared, which demonstrated photochromism and photoinduced cracking. Upon irradiation at 450 nm, a single crystal-to-single crystal transformation occurred as a result of the [2 + 2] photocycloaddition of L. Interestingly, compared to the complete photocycloaddition of L in 1, only two-thirds of L monomers could be photodimerized in 2 because of the difference in L orientation. 1 shows reversible photochromic behavior including rapid response time, few cracks, high conversion rate, and good reaction reversibility, while 2 exhibits no significant color change but distinct photoinduced cracking because of the large local lattice strain induced by inhomogeneous and anisotropic deformation. Moreover, the photocycloaddition of L results in the distinct shift of photoluminescence of 1 and 2, attributed to the variation in conjugation of π electrons and distortion of metal halide clusters. As a proof-of-concept, reversible optical writing is demonstrated for 1. These findings provide new insights into the design of stimuli-responsive multifunctional materials.

Scaffold-Hopping Strategies in Aurone Optimization: A Comprehensive Review of Synthetic Procedures and Biological Activities of Nitrogen and Sulfur Analogues

Aurones, particular polyphenolic compounds belonging to the class of minor flavonoids and overlooked for a long time, have gained significative attention in medicinal chemistry in recent years. Indeed, considering their unique and outstanding biological properties, they stand out as an intriguing reservoir of new potential lead compounds in the drug discovery context. Nevertheless, several physicochemical, pharmacokinetic, and pharmacodynamic (P3) issues hinder their progression in more advanced phases of the drug discovery pipeline, making lead optimization campaigns necessary. In this context, scaffold hopping has proven to be a valuable approach in the optimization of natural products. This review provides a comprehensive and updated picture of the scaffold-hopping approaches directed at the optimization of natural and synthetic aurones. In the literature analysis, a particular focus is given to nitrogen and sulfur analogues. For each class presented, general synthetic procedures are summarized, highlighting the key advantages and potential issues. Furthermore, the biological activities of the most representative scaffold-hopped compounds are presented, emphasizing the improvements achieved and the potential for further optimization compared to the aurone class.

Massive Molecular Motion in Crystal Leads to an Unexpected Helical Covalent Polymer in a Solid-state Polymerization

We designed a proline-derived monomer with azide and alkene functional groups to enable topochemical ene-azide cycloaddition (TEAC) polymerization. In its crystal, the monomer forms supramolecular helices along the 'a' axis through various non-covalent interactions. Along the 'c' axis, the molecules arrange themselves head-to-tail in a wave-like pattern, positioning the azide and alkene groups of adjacent molecules in close proximity and anti-parallel orientation, complying with Schmidt's criteria for topochemical reaction. This prearranged configuration was expected to facilitate smooth topochemical polymerization, resulting in a 1,4-triazoline-linked polymer. Upon heating, the monomer underwent TEAC polymerization in a remarkable single-crystal-to-single-crystal fashion, but, to our surprise, it yielded an unexpected covalent helical polymer linked by 1,5-disubstituted triazoline units. Remarkably, the crystal avoids the ready-to-react arrangement for polymerization, but connects monomer molecules within the supramolecular helix through the cycloaddition of azide and alkene groups, even though they are not in close proximity nor in the expected orientation. This unexpected path, involving a substantial 134° rotation of the alkene group, yields hitherto unknown 1,5-disubstituted triazoline product regiospecifically. This study serves as a cautionary reminder that relying solely on topochemical postulates for predicting reactivity can sometimes be misleading.