Topochemical polymerizations for the solid-state synthesis of organic polymers

Programmable Solid-State [2 + 2] Photocycloadditions of Dienes Directed by Structural Control and Wavelength Selection

Small differences in molecular or solid-state structure can afford significant differences in properties. Here, a diene derivative, 1,3-bis((E)-2-bromostyryl)benzene (2Brm), is synthesized and crystallized into two unique solid-state forms, each exhibiting a different π-π stacking geometry, which imparts distinct reactivity and photoresponsivity. Upon exposure of the two solids to UV-Vis light, a [2 + 2] photocycloaddition occurs to afford regioisomeric products due to the difference in the stacking geometries of the dienes. From a single molecular precursor, we further demonstrate that under different wavelengths of light, the chemical functionality can be programmed into discrete and distinct products containing one, two, or three cyclobutane rings as well as oligomeric/polymeric products. Moreover, the two initial solid forms exhibit wavelength-dependent photomechanical behaviors (i.e., photosalience). This work demonstrates a rare, template-free, self-assembly-based strategy that enables access to a suite of discrete and oligomeric/polymeric products via regiocontrolled solid-state photocycloadditions and further presents potential routes toward the design of photoactuating materials.

Head-to-Tail Packing to Facilitate [2+2] Cycloaddition for Green Synthesis of Cyclobutane Derivatives in Specific Configuration

Topological [2+2] cycloaddition is known to provide a convenient synthetic route for cyclobutane derivatives from favorably dispositioned dienes. In this study, new (2Z,4E)-2-(2,4-difluorophenyl)-5-phenylpenta-2,4-dienenitrile (HDE), (2Z,4E)-2-(2,4-difluorophenyl)-5-(p-tolyl)penta-2,4-dienenitrile (MeDE), (2Z,4E)-5-(4-chlorophenyl)-2-(2,4-difluorophenyl)penta-2,4-dienenitrile (ClDE), (2Z,4E)-5-(4-bromophenyl)-2-(2,4-difluorophenyl)penta-2,4-dienenitrile (BrDE), (2Z,4E)-2-(2,4-difluorophenyl)-5-(4-methoxyphenyl) penta-2,4-dienenitrile (MeODE), and (2Z,4E)-2-(2,4-difluorophenyl)-5-(4-(dimethylamino)phenyl)penta-2,4-dienenitrile (MeNDE) were synthesized, and their reactivity and selectivity were investigated in relation to their molecular packing in the respective crystals. HDE and MeDE, with head-to-tail (HT) arrangement, yielded only one type of photodimer. On the contrary, ClDE and BrDE, with head-to-head (HH) packing, and where the "olefin pairsα,β-α,β" and "olefin pairsγ,δ-γ,δ" satisfy Schimdt's criteria, reacted to a mixture of photoproducts. Kinetics analysis suggests that the reaction rates of HDE and MeDE are higher than those of ClDE and BrDE. This observation may be due to the strong non-covalent interactions between the potentially reactive olefin pairs as suggested by energy decomposition analysis. Furthermore, the reaction activation energies for photodimerization of the HT-packed olefin pairs are indeed lower than those of the HH-arranged ones. The HT packing of the diphenyldienes not only enhances the reactivity in the topological [2+2] cycloaddition but also contributes chemospecificity, regiospecifity, and stereospecificity, all of which are essential for the preparation of specific cyclobutanes derivatives based on photodimerization.

Manipulating Aromaticity to Redirect Topochemical Polymerization Pathways

Topochemical polymerization (TCP) represents an essential route to create regio- and stereoregular polymers through solid-state transformations. Herein, we present an innovative strategy for controlling topochemical polymerization pathways by tailoring the terminal group aromaticity in the para-azaquinodimethane (AQM) ring system. Substituting phenyl groups with less aromatic furyl units extends significant spin density delocalization across the conjugated core upon thermal activation, inducing significant diradicaloid characters at furyl positions and enabling unconventional reactivities in both solution and solid states. Thermal treatment in toluene yields a unique cyclophane dimer formed via furyl-methine C-C coupling, confirmed by X-ray crystallography, while solid-state reactions produce polymers formed via both intercolumnar furyl-methine coupling and intracolumnar methine-methine coupling. The spin-center-directed mechanism underlying these transformations is validated through theoretical modeling and isotopic labeling experiments. This study highlights the prowess of aromaticity modulation in functional pro-aromatic systems, which enables the synthesis of polymers with main chain structures that are otherwise difficult to access.

High-fidelity topochemical polymerization in single crystals, polycrystals, and solution aggregates

Topochemical polymerization (TCP) emerges as a leading approach for synthesizing single crystalline polymers, but is traditionally restricted to transformations in solid-medium. The complexity in achieving single-crystal-to-single-crystal (SCSC) transformations due to lattice disparities and the untapped potential of performing TCP in a liquid medium with solid-state structural fidelity present unsolved challenges. Herein, by using X-rays as the primary means to overcome crystal disintegration, we reveal the details of SCSC transformation during the TCP of chiral azaquinodimethane (AQM) monomers through in situ crystallographic analysis while spotlighting a rare metastable crystalline phase. Complementary in situ investigations of powders and thin films provide critical insights into the side-chain dependent polymerization kinetics of solid-state reactions. Furthermore, we enable TCP of AQM monomers in a liquid medium via an antisolvent-reinforced aggregated state, yielding polymer nanofibers with high crystallinity akin to that of solid-state. This study testifies high structural precision of TCP performed in different states and media, offering critical insights into the synthesis of processable nanostructured polymers with desired structural integrity.

Synthesis of a biphenylene nanoribbon by compressing biphenylene under extreme conditions

Nonbenzenoid graphene nanoribbons such as biphenylene networks have gained increasing attention owing to their promising electronic and transport properties, but their scalable synthesis is still a huge challenge. Pressure-induced topochemical polymerization is an effective method to assemble molecular units into extended carbon materials, and the structure and properties of the carbon material can be tuned by modifying its molecular precursors. Herein, by directly compressing biphenylene at room temperature, we successfully synthesized crystalline biphenylene nanoribbons in milligram scale. By combining the spectroscopy and single crystal X-ray diffraction methods as well as theoretical calculation, we found that biphenylene experiences a minor phase transition above 3 GPa, and two phenyls in biphenylene undergo sequential para-polymerization along the a-axis to form a ribbon structure at 14 GPa. Our work provides an important reference for the high-pressure reaction of aromatics and the synthesis of complex nanoribbons.

Unprecedented visible light-initiated topochemical [2 + 2] cycloaddition in a functionalized bimane dye

Bimanes, a class of molecules based on the 1H,7H-pyrazolo[1,2-a]pyrazole-1,7-dione scaffold, were first introduced by E. M. Kosower in 1978. In this study, we report the topochemical cycloaddition of diethyl 2,6-dichloro-1,7-dioxo-1H,7H-pyrazolo[1,2-a]pyrazole-3,5-dicarboxylate (Cl 2 B), initiated by visible light. Crystal structure analysis confirmed that the reactive double bonds are parallel and coplanar, in line with the Schmidt criteria for topochemical cycloaddition. Additionally, two other bimane derivatives with different substitution patterns were synthesized and investigated. Our findings suggest that functionalizing bimanes to redshift their absorption maxima into the visible-light spectrum provides a promising strategy for synthesizing substituted cyclobutanes without the need for ultraviolet irradiation.

Light-Induced Transformation of a Supramolecular Gel to a Stronger Covalent Polymeric Gel

A polymerizable diacetylene gelator, containing urea and urethane groups, that congeals various non-polar solvents was synthesized. The gelator molecules self-assemble forming non-covalent polymers through intermolecular hydrogen bonding, as evidenced from FT-IR and concentration-dependent 1H NMR spectroscopy. The self-assembly positions the diyne units of adjacent molecules at proximity and in a geometry suitable for their topochemical polymerization. UV irradiation of the gel resulted in topochemical polymerization, transforming the non-covalent polymer to a covalent polymer, in situ, in the gel state. The polymerization was confirmed by characterizing the polydiacetylene (PDA) using UV-Vis and Raman spectroscopy. Time-dependent rheological studies revealed gradual strengthening of the gel with the duration of irradiation, suggesting that the degree of polymerization increases with the duration of irradiation. The PDA formed is a semiconductor, which might be useful for various applications.

Crystallization-Induced Dimerization and Solution-Phase Bond Dissociation of Stable Dibenzoolympicenyl Radicals

Crystallization of organic materials can lead to different assembly structure with different reactivity, but this phenomenon is rarely observed for delocalized hydrocarbon radicals. This report introduces a crystallization-induced radical-radical coupling reaction, which employs a series of stable nonplanar organic π-radicals as reactants. Six stable radical congeners are synthesized, resulting in radical-radical coupling at the allenyl radical site during crystallization to produce close-shell dimers. This coupling reaction is absent in the solution phase, which highlights the importance of preorganization in the lattice. Remarkably, the attempts of cocrystallization of different congeners yielded homocoupling products instead of cross-coupling products. In specific cases, two distinct polymorphs are observed and their reactivity is different according to the distance of the reaction sites. Theoretical calculations indicate that the transition from a metastable preorganized monomer to a dimer is barrierless and spontaneous. The dimer could regenerate free radicals by heating or photoirradiation in the solution phase. This discovery may lead to controllable molecular switches.

Photo-Assisted Bottom-Up Synthesis of Orange Phosphorus

A tubular strand of phosphorus composed of vectorially aligned pentagons has been theoretically predicted as a new allotrope of phosphorus with a polar structure, expecting potential applications. However, it has not been successfully synthesized yet due to the difficulty of creating isolated strands to avoid interchain bonding. Here, such an allotrope named "orange phosphorus" was successfully produced using a photo-assisted synthesis from an amorphous film of solution-processable Na2P16 precursors. A green laser irradiation initiated the phase transition of precursors, inducing chemical reactions like topochemical polymerization and rearrangement, creating a 1D chain of orange phosphorus. 3D electron diffraction crystallography showed that the molecular structure of orange phosphorus consists of one-dimensional polar pentagonal-tubes made up of [P8]P2[ repeat units. Orange phosphorus demonstrates excellent piezoresistivity due to its high strain-sensitive 1D chain structure, showing strain-induced Raman shifts. Its gauge factor exceeds those of 2D materials such as black phosphorus and transition metal dichalcogenides. These findings indicate that orange phosphorus has great potential for use in strain sensor applications.

Toward Sustainable Polydienes

The sustainable management of polydiene waste represents a formidable challenge in the realm of polymer chemistry, given the extensive industrial utilization of polydienes due to their superior elastomeric properties. This comprehensive Perspective addresses the multifaceted obstacles hindering efficient recycling of polydienes, encompassing environmental concerns, technical limitations, and economic disincentives. We systematically dissect the influence of polydienes' chemical structures on their recyclability, tracing the evolution of polydiene utilization and disposal practices while assessing the current landscape of waste management strategies. Our investigation reveals the primary technical challenges associated with polydiene recycling, notably the energy-intensive nature of modification processes and the environmental detriments of prevailing disposal techniques. Furthermore, we critically evaluate existing recycling methodologies─including mechanical recycling, energy recovery, and chemical recycling─highlighting their respective merits, constraints, and environmental implications. Pioneering advancements in recycling technology, such as topochemical polymerization and computational prediction models, are spotlighted for their potential to revolutionize polydiene recycling. Looking forward, we delineate an optimistic trajectory for polydiene waste management, advocating for innovative polymerization methods, the exploration of milder recycling conditions, and the adoption of interdisciplinary approaches to bolster recycling efficiency. The Perspective culminates in a discussion on the pivotal role of policy frameworks, life cycle assessments, and economic analyses in shaping the future of polydiene recycling. Through this scholarly examination, we aim to catalyze further research and development efforts aimed at mitigating the environmental impact of polydiene waste, thereby contributing to the broader objective of sustainable chemistry.

Mechanically interlocked two-dimensional polymers

Mechanical bonds arise between molecules that contain interlocked subunits, such as one macrocycle threaded through another. Within polymers, these linkages will confer distinctive mechanical properties and other emergent behaviors, but polymerizations that form mechanical bonds efficiently and use simple monomeric building blocks are rare. In this work, we introduce a solid-state polymerization in which one monomer infiltrates crystals of another to form a macrocycle and mechanical bond at each repeat unit of a two-dimensional (2D) polymer. This mechanically interlocked 2D polymer is formed as a layered solid that is readily exfoliated in common organic solvents, enabling spectroscopic characterization and atomic-resolution imaging using advanced electron microscopy techniques. The 2D mechanically interlocked polymer is easily prepared on multigram scales, which, along with its solution processibility, enables the facile fabrication of composite fibers with Ultem that exhibit enhanced stiffness and strength.

Endowing single-crystal polymers with circularly polarized luminescence

The preparation of single-crystal polymers with circularly polarized luminesce (CPL) remains a challenging task in chemistry and materials science. Herein, we present the single-crystal-to-single-crystal topochemical photopolymerization of a chiral organic salt to achieve this goal. The in-situ reaction of 1,4-bis((E)-2-(pyridin-4-yl)vinyl)benzene (1) with chiral (+)- or (-)-camphorsulfonic acid (CSA) gives the monomer crystal 1[( + )/( - )-CSA]2 showing yellow CPL with a high luminescent dissymmetry factor |glum| of 0.035 and emission quantum yield Φ of 49.7%. Upon photo-induced topochemical [2 + 2] polymerization, single-crystal polyionic polymers of poly-1[( + )/( - )-CSA]2 are obtained. The single-crystal-to-single-crystal (SCSC) photopolymerization is revealed by in situ powder X-ray diffraction, single-crystal X-ray, optical microscopy, infrared, circular dichroism, and CPL spectroscopic analyzes. Interestingly, the photopolymer crystals show blue and handedness-inverted CPL with |glum| of 0.011 (Φ = 14.2%), with respect to the yellow CPL of the monomer crystal. Furthermore, patterned circularly-polarized photonic heterojunctions with alternate blue and yellow CPL sub-blocks are prepared by a mask-assisted photopolymerization method. Our findings provide a vision for fabricating high-performance CPL-active crystalline polymer materials, paving the way for the further development of photo-response chiral systems.

Oxidation Triggered Supramolecular Chirality

Topochemical reactions normally occurring in the solid and crystalline state exhibit solvent-free and catalyst-free properties, with high atom economy properties, which have been widely applied in materials science and polymer synthesis. Herein, we explore the potential of topochemical reactions for controlling the emergence of supramolecular chirality and the precise fabrication of chiroptical materials. Boronic acid pinacol esters (BPin) were conjugated to naphthalimides containing an inherent chiral cholesteryl group linked by alkyl or benzene spacers. The BPin segments were oxidized by H2O2 to form hydroxyl groups, which enhanced luminescence, reduced steric effects, and increased amphiphilicity. The inherent liposomal aggregates underwent in situ oxidation and transformed into 1D nanoarchitectures, exhibiting macroscopic chirality, active Cotton effects, and circularly polarized luminescence. Oxidation could also initiate an intimate interplay between the building units and the guest molecule, by which the chirality and chiroptical evolution in the multiple component chiral assembly system were realized.

Recent trends on polycaprolactone as sustainable polymer-based drug delivery system in the treatment of cancer: Biomedical applications and nanomedicine

The unique properties-such as biocompatibility, biodegradability, bio-absorbability, low cost, easy fabrication, and high versatility-have made polycaprolactone (PCL) the center of attraction for researchers. The derived introduction in this manuscript gives a pretty detailed overview of PCL, so you can first brush up on it. Discussion on the various PCL-based derivatives involves, but is not limited to, poly(ε-caprolactone-co-lactide) (PCL-co-LA), PCL-g-PEG, PCL-g-PMMA, PCL-g-chitosan, PCL-b-PEO, and PCL-g-PU specific properties and their probable applications in biomedicine. This paper has considered examining the differences in the diverse disease subtypes and the therapeutic value of using PCL. Advanced strategies for PCL in delivery systems are also considered. In addition, this review discusses recently patented products to provide a snapshot of recent updates in this field. Furthermore, the text probes into recent advances in PCL-based DDS, for example, nanoparticles, liposomes, hydrogels, and microparticles, while giving special attention to comparing the esters in the delivery of bioactive compounds such as anticancer drugs. Finally, we review future perspectives on using PCL in biomedical applications and the hurdles of PCL-based drug delivery, including fine-tuning mechanical strength/degradation rate, biocompatibility, and long-term effects in living systems.

A Syndiotactic Polymer via Spontaneous Exoselective Single-Crystal-To-Single-Crystal Topochemical Diels-Alder Cycloaddition Reaction

We designed and synthesized an amide-based monomer decorated with furan as the diene unit and maleimide as the dienophile unit at its termini. Single-crystal X-ray diffraction (SCXRD) analysis of its crystal revealed a head-to-tail arrangement of molecules with furan and maleimide groups of neighboring molecules proximally placed in an arrangement suitable for their topochemical Diels-Alder cycloaddition (TDAC) to form a linear polymer. The monomer underwent a spontaneous single-crystal-to-single-crystal (SCSC) polymerization at room temperature, yielding a linear polymer with oxa-bicyclic linkage. SCXRD analysis revealed that the cycloaddition occurred in an exoselective manner, and the absolute stereochemistry of the oxa-bicyclic linkage alternated in successive repeat units, leading to a syndiotactic linear polymer. The polymerization can be accelerated by heating the powder at 120 °C; the topochemical nature of the high-temperature reaction was established by time dependent differential scanning calorimetry (DSC), time-dependent powder X-ray diffraction (PXRD), and UV-visible spectroscopic analysis; the polymer was characterized using solid-state NMR spectroscopy and MALDI-TOF mass spectrometry.

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.

Fundamentally intertwined: anharmonic intermolecular interactions dictate both thermal expansion and terahertz lattice dynamics in molecular crystals

We investigate the anisotropic thermal expansion behavior of a co-crystalline system composed of 4,4'-azopyridine and trimesic acid (TMA-azo). Using variable-temperature single-crystal X-ray diffraction (SC-XRD), low-frequency Raman spectroscopy, and terahertz time-domain spectroscopy (THz-TDS), we observe significant temperature-induced shifting and broadening of the vibrational absorption features, indicating changes in the intermolecular potential. Our findings reveal that thermal expansion is driven by anharmonic interactions and the potential energy topography, rather than increased molecular dynamics. Density functional theory (DFT) simulations support these results, highlighting significant softening of the potential energy surface (PES) with temperature. This comprehensive approach offers valuable insights into the relationship between structural dynamics and thermal properties, providing a robust framework for designing materials with tailored thermal expansion characteristics.

Truxinates and truxillates: building blocks for architecturally complex polymers and advanced materials

Significant advancements in the syntheses of cyclobutane containing small molecules and polymers are described in the last 15 years. Small molecule cyclobutanes are under investigation for their diverse pharmacological activities, while polymers with cyclobutane backbones are emerging as novel mechanophores, stress-responsive materials, and sustainable plastics. Within these chemistries, [2 + 2] photocycloadditions to yield truxinates and truxillates are highly efficient offering a versatile strategy to access complex scaffolds. This article provides a comprehensive review on the synthetic methodologies, properties, and applications of polymer truxinates and truxillates, providing the background necessary to understand current developments and envision future applications. Additionally, we highlight the links between the development, discoveries, and synthetic methodologies of small molecules and cyclobutane polymers. We emphasize structure property relationships and discuss methods to control composition and structure for desired applications. We begin with a discussion of synthetic techniques for small molecule and polymer cyclobutanes followed by their greater applications, including pharmacological and material properties with examples including sustainable plastics and stimuli-responsive systems.

Emerging solid-state cycloaddition chemistry for molecular solar thermal energy storage

Recently discovered designs of solid-state molecular solar thermal energy storage systems are illustrated, including alkenes, imines, and anthracenes that undergo reversible [2 + 2] and [4 + 4] photocycloadditions for photon energy storage and release. The energy storage densities of various molecular designs, from 6 kJ mol-1 to 146 kJ mol-1 (or up to 318 J g-1), are compared and summarized, along with effective strategies for engineering their crystal packing structures that facilitate solid-state reactions. Many promising molecular scaffolds introduced here highlight the potential for achieving successful solid-state solar energy storage, guiding further discoveries and the development of new molecular systems for applications in solid-state solar thermal batteries.

Photodimerization-Triggered Photopolymerization of Triene Coordination Polymers Enables Macroscopic Photomechanical Movements

Controlling the packing of olefinic molecules in crystals is essential for triggering solid-state [2 + 2] photocycloaddition reactions and the synthesis of photocontrolled smart materials. Herein, we report the stepwise photodimerization-triggered photopolymerization of two triene coordination polymers (CPs), {[Zn(2-BBA)2(tpeb)]·0.5CH3CN}n (1, 2-HBBA = 2-bromobenzoic acid, tpeb = 1,3,5-tri-4-pyridyl-1,2-ethenylbenzene) and {[Zn(3-BBA)2(tpeb)]·CH3CN)}n (2, 3-HBBA = 3-bromobenzoic acid). Upon irradiation with 420 nm light, each pair of closely packed and parallel olefinic bonds in 1 undergoes a [2 + 2] cycloaddition reaction, which connects two adjacent Z-shaped chains into a ladder-like coordination chain [Zn(2-BBA)2(bpbdpvpcb)0.5]n (1a, bpbdpvpcb = 1,3-bis(4-pyridyl)-2,4-bis(3,5-di(2-(4-pyridyl)vinyl)phenyl]cyclobutene) through single-crystal to single-crystal (SCSC) transformation. After photodimerization from 1 to 1a has occurred, the olefinic bonds that were initially distant are brought in close enough proximity to meet the requirements for a subsequent [2 + 2] cycloaddition reaction. Upon further light irradiation, the neighboring bpbdpvpcb ligands in 1a experience a SCSC photopolymerization based on [2 + 2] photocycloaddition and transform into poly-3b,4,5,5a,8b,9,10a-octahydro-4,5,9,10-tetrapyridyl-2,7-di(2-(4-pyridyl)vinyl)dicyclobuta[e,l]-pyren (poly-otpdpvdcbp). 2 showed similar structural changes under UV light illumination. Under light exposure, single crystals of 1 and 2 with different morphologies exhibit bending, cracking, and jumping photomechanical motions. The composite film (1-PVA) engineered by dispersing crystalline particles of 1 in poly(vinyl alcohol) (PVA) displays interesting light-wavelength-dependent photomechanical motions and can perform photodriven swimming on a liquid surface. This work provides a useful and promising approach to enable photodimerization of those photoinactive olefin pairs embedded in CPs and opens a new route to synthesize organic polymers by using olefinic CP platforms.

Topochemical Polymerization at Diacetylene Metal-Organic Framework Thin Films for Tuning Nonlinear Optics

Developing the topochemical polymerization of metal-organic frameworks (MOFs) is of pronounced significance for expanding their functionalities but is still a challenge on third-order nonlinear optics (NLO). Here, we report diacetylene MOF (CAS-1-3) films prepared using a stepwise deposition method and film structural transformation approach, featuring dynamic structural diversity. The MOF structures were determined by the three-dimensional electron diffraction (3D ED) method from nanocrystals collected from the films, which provides a reliable strategy for determining the precise structure of unknown MOF films. We demonstrate the well-aligned diacetylene groups in the MOFs can promote topological polymerization to produce a highly conjugated system under thermal stimulation. As a result, the three MOF films have distinct NLO properties: the CAS-1 film exhibits saturable absorption (SA) while CAS-2 and CAS-3 films exhibit reverse saturable absorption (RSA). Interestingly, due to the topochemical polymerization of the MOF films, a transition from SA to RSA response was observed with increasing temperatures, and the optical limiting effect was significantly enhanced (∼46 times). This study provides a new strategy for preparing NLO materials and thermally regulation of nonlinear optics.

Volumetric Additive Manufacturing of Dicyclopentadiene by Solid-State Photopolymerization

Polymerization in the solid state is generally infeasible due to restrictions on mobility. However, in this work, the solid-state photopolymerization of crystalline dicyclopentadiene is demonstrated via photoinitiated ring-opening metathesis polymerization. The source of mobility in the solid state is attributed to the plastic crystal nature of dicyclopentadiene, which yields local short-range mobility due to orientational degrees of freedom. Polymerization in the solid state enables photopatterning, volumetric additive manufacturing of free-standing structures, and fabrication with embedded components. Solid-state photopolymerization of dicyclopentadiene offers a new paradigm for advanced and freeform fabrication of high-performance thermosets.

Hierarchical single-crystal-to-single-crystal transformations of a monomer to a 1D-polymer and then to a 2D-polymer

Designing and synthesizing flawless two-dimensional polymers (2D-Ps) via meticulous molecular preorganization presents an intriguing yet challenging frontier in research. We report here the single-crystal-to-single-crystal (SCSC) synthesis of a 2D-P via thermally induced topochemical azide-alkyne cycloaddition (TAAC) reaction. A designed monomer incorporating two azide and two alkyne units is synthesized. The azide and alkyne groups are preorganized in the monomer crystal in reactive geometries for polymerizations in two orthogonal directions. On heating, the polymerizations proceed in a hierarchical manner; at first, the monomer reacts regiospecifically in a SCSC fashion to form a 1,5-triazolyl-linked 1D polymer (1D-P), which upon further heating undergoes another SCSC polymerization to a 2D-P through a second regiospecific TAAC reaction forming 1,4-triazolyl-linkages. Two different linkages in orthogonal directions make this an architecturally attractive 2D-P, as determined, at atomic resolution, by single-crystal X-ray diffraction. The 2D-P reported here is thermally stable in view of the robust triazole-linkages and can be exfoliated as 2D-sheets.

Simultaneous and in situ syntheses of an enantiomeric pair of homochiral polymers as their perfect stereocomplex in a crystal

Circumventing the issues of conventional stereocomplexation of preformed polymers, herein, we synthesize two enantiopure polymers of opposite chirality simultaneously and in situ as their 1:1 stereocomplex via topochemical polymerization. We design and synthesize an inositol-based achiral monomer for topochemical ene-azide cycloaddition (TEAC) polymerization. In the crystal, the monomer exhibits conformational enantiomerism, and its conformational enantiomers are self-sorted in an arrangement for TEAC polymerization to yield two enantiopure polymers of opposite chirality. Upon heating the monomer crystals, each self-sorted set of conformational enantiomers undergoes regio- and stereospecific polymerization in a single-crystal-to-single-crystal fashion, generating two 1, 4-triazolinyl-linked polymers of opposite chirality simultaneously. The new chiral carbons in all the triazoline rings of a particular polymer chain have the same absolute configuration. These homochiral polymer strands align parallelly, forming a layer, and such enantiopure layers of opposite chirality stack alternately, forming a perfect 1:1 stereocomplex, which we confirmed using single-crystal XRD analysis.

Topochemical cross-linking of diacetylene in a highly interpenetrated three-dimensional covalent organic framework

Topochemical polymerization is a highly promising and effective method for constructing complex three-dimensional structures and functionalized polymers. Herein, we present for the first time a topochemical reaction of diacetylene within a meticulously designed covalent organic framework (ZNUC-1). Our experimental results revealed that ZNUC-1 underwent a 1,4-addition reaction under thermal induction, forming an ene-yne-conjugated structure.

Visible Light [2 + 2] Cycloadditions of Thermoresponsive Dendronized Styryltriazines To Exhibit Tunable Microconfinement

Visible light-triggered photochemical reactions in aqueous media are highly valuable to tailor molecular structures and properties in an ecofriendly manner. Here we report visible light-induced catalyst-free [2 + 2] cycloadditions of thermoresponsive dendronized styryltriazines, which show tunable microconfinement to guest dyes in aqueous media. These dendronized styryltriazines are constituted of conjugated mono- or tristyryltriazines, which carry hydrophilic dendritic oligoethylene glycol (OEG) pendants. They underwent efficient [2 + 2] cycloadditions to form dendronized cyclobutane dimers or oligomers in water through irradiation with visible light of 400 nm, and their cycloaddition behavior was dominated by dendritic architectures and solvent conditions. Dendronization with dendritic OEGs also afforded them characteristic thermoresponsive properties with tunable phase transition temperatures in the range 36-65 °C, which can be further modulated through photocycloaddition of styryltriazine chromophores. Importantly, dendronized styryltriazines can form tunable microenvironments in aqueous media, which encapsulate hydrophobic solvatochromic Nile red to exhibit variable photophysical properties. The encapsulated guest dye can be simultaneously released through noninvasive visible light-induced [2 + 2] cycloaddition reactions.

Template-Directed In Crystallo Photopolymerization of a Donor-Acceptor Cyclopropane: When Everything Falls into Place!

A single-crystal-to-single-crystal solid-state reaction of vinylogous donor-acceptor cyclopropanes is documented. The enantiospecific synthesis of new products, distinct from those obtained in solution, is achieved for the target compounds. Photopolymerization occurred upon X-ray exposure to the crystals. Notably, in one case, this reactivity exhibits selectivity since an ordered arrangement of polymers and unreacted cocrystallized monomeric conformers has been observed. Structural characterization of the complete transformation monitored through single-crystal X-ray diffraction and supported by molecular dynamics simulations sheds light on the subtle role of crystal packing in the reaction process. Moreover, the X-ray diffraction (XRD)-resolved structure of a donor-acceptor cyclopropane intermediate reveals an elongation in bond length that corroborates the existence of the so-called "push-pull effect".

Mechanoactivated amorphization and photopolymerization of styryldipyryliums

Conventional topochemical photopolymerization reactions occur exclusively in precisely-engineered photoactive crystalline states, which often produces high-insoluble polymers. To mitigate this, here, we report the mechanoactivation of photostable styryldipyrylium-based monomers, which results in their amorphization-enabled solid-state photopolymerization and produces soluble and processable amorphous polymers. A combination of solid-state nuclear magnetic resonance, X-ray diffraction, and absorption/fluorescence spectroscopy reveals the crucial role of a mechanically-disordered monomer phase in yielding polymers via photo-induced [2 + 2] cycloaddition reaction. Hence, mechanoactivation and amorphization can expand the scope of topochemical polymerization conditions to open up opportunities for generating polymers that are otherwise difficult to synthesize and analyze.

Topochemical-like bandgap regulation engineering: A bismuth thiooxide nanocatalyst for breast cancer phototherapy

The physical property tuning of nanomaterials is of great importance in energy, medicine, environment, catalysis, and other fields. Topochemical synthesis of nanomaterials can achieve precise control of material properties. Here, we synthesized a kind of element-doped bismuth-based nanomaterial (BOS) by topochemical-like synthesis and used it for the phototherapy of tumors. In this study, we employed bismuth fluoride nanoflowers as a template and fabricated element-doped bismuth oxide nanoflowers by reduction conditions. The product is consistent with the precursor in crystal structure and nanomorphology, realizing topochemical-like synthesis under mild conditions. BOS can generate reactive oxygen species, consume glutathione, and perform photothermal conversion under 730 nm light irradiation. In vitro and in vivo studies demonstrate that BOS could suppress tumor growth by inducing apoptosis and ferroptosis through phototherapy. Therefore, this study offers a general regulation method for tuning the physical properties of nanomaterials by using a topochemical-like synthesis strategy.

Single-Crystal-to-Single-Crystal Photosynthesis of Supramolecular Organoboron Polymers with Dynamic Effects

The solid-state synthesis of single-crystalline organic polymers, having functional properties, remains an attractive and developing research area in polymer chemistry and materials science. However, light-triggered topochemical synthesis of crystalline polymers comprising an organoboron backbone has not yet been reported. Here, we describe an intriguing example of single-crystal-to-single-crystal (SCSC) rapid photosynthesis (occurs on a seconds-scale) of two structurally different linear organoboron polymers, driven by environmentally sustainable visible/sun light, obtained from the same monomer molecule. A newly designed Lewis acid-base type molecular B ← N organoboron adduct (consisting of an organoboron core and naphthylvinylpyridine ligands) crystallizes in two solid-state forms featuring the same chemical structure but different 3D structural topologies, namely, monomers 1 and 2. The solvate molecule-free crystals of 1 undergo topochemical photopolymerization via an unusual olefin-naphthyl ring [2 + 2] cyclization to yield the single crystalline [3]-ladderane polymer 1P growing along the B ← N linkages, accompanied by instantaneous and violent macroscopic mechanical motions or photosalient effects (such as bending-reshaping and jumping motions). In contrast, visible light-harvesting single crystals of 2 quantitatively polymerize to a B ← N bond-stabilized polymer 2P in a SCSC fashion owing to the rapid [2 + 2] cycloaddition reaction among olefin double bonds. Such olefin bonds in the crystals of 2 are suitably preorganized for photoreaction due to the presence of solvate molecules in the crystal packing. Single crystals of 2 also show photodynamic jumping motions - in response to visible light but in a relatively slower fashion than the crystals of 1. In addition to SCSC topochemical polymerization and dynamic motions, both monomer crystals and their single-crystalline polymers feature green emissive and short-lived room-temperature phosphorescence properties upon excitation with visible-light wavelength.

Topochemistry for Difficult Peptide-Polymer Synthesis: Single-Crystal-to-Single-Crystal Synthesis of an Isoleucine-Based Polymer, a Hydrophobic Coating Material

Polymers of hydrophobic amino acids are predicted to be potential coating materials for the creation of hydrophobic surfaces. The oligopeptides of hydrophobic amino acids are called "difficult peptides"; as the name suggests, it is difficult to synthesize them by conventional methods. We circumvented this synthetic challenge by adopting topochemical azide-alkyne cycloaddition (TAAC) polymerization of a hydrophobic dipeptide monomer. We designed an Ile-based dipeptide, decorated with azide and alkyne, which arrange in the crystal in a head-to-tail fashion with the azide and alkyne of the adjacent molecules in a ready-to-react orientation. The monomer, on mild heating of its crystals, undergoes regiospecific TAAC polymerization to yield a 1,4-disubstituted-triazole-linked polymer in a single-crystal-to-single-crystal fashion. The solid obtained after evaporation of the monomer solution also maintained crystallinity and underwent regiospecific topochemical polymerization as in the case of crystals. This topochemical polymerization could be studied using different techniques such as FTIR, NMR, DSC, GPC, MALDI, PXRD, and SCXRD. Since the polymer is insoluble in common solvents and hence difficult to coat surfaces, the monomer was first sprayed and evaporated on various surfaces and polymerized on the surface. Such polymer-coated surfaces exhibited water contact angles of up to 134°, showing that this Ile-derived polymer is very hydrophobic and can potentially be used as a coating material for various applications.

Selective Activation of Chalcogen Bonding: An Efficient Structuring Tool toward Crystal Engineering Strategies

ConspectusAmong the noncovalent interactions available in the toolbox of crystal engineering, chalcogen bonding (ChB) has recently entered the growing family of σ-hole interactions, following the strong developments based on the halogen bonding (XB) interaction over the last 30 years. The monovalent character of halogens provides halogen bonding directionality and strength. Combined with the extensive organic chemistry of Br and I derivatives, it has led to many applications of XB, in solution (organo-catalysis, anion recognition and transport), in the solid state (cocrystals, conducting materials, fluorescent materials, topochemical reactions, ...), in soft matter (liquid crystals, gels,···), and in biochemistry. The recognition of the presence of two σ-holes on divalent chalcogens and the ability to activate them, as in XB, with electron-withdrawing groups (EWG) has fueled more recent interest in chalcogen bonding. However, despite being identified for many years, ChB still struggles to make a mark due to (i) the underdeveloped synthetic chemistry of heavier Se and Te; (ii) the limited stability of organic chalcogenides, especially tellurides; and (iii) the poor predictability of ChB associated with the presence of two σ-holes. It therefore invites a great deal of attention of molecular chemists to design and develop selected ChB donors, for the scrutiny of fundamentals of ChB and their successful use in different applications. This Account aims to summarize our own contributions in this direction that extend from fundamental studies focused on addressing the lack of directionality/predictability in ChB to a systematic demonstration of its potential, specifically in crystal engineering, and particularly toward anionic networks on the one hand, topochemical reactions on the other hand.In this Account, we share our recent results aimed at recovering with ChB the same degree of strength and predictability found with XB, by focusing on divalent Se and Te systems with two different substituents, one of them with an EWG, to strongly unbalance both σ-holes. For that purpose, we explored this dissymmetrization concept within three chemical families, selenocyanates R-SeCN, alkynyl derivatives R-C≡C-(Se/Te)Me, and o-carborane derivatives. Such compounds were systematically engaged in cocrystals with either halides or neutral bipyridines as ChB acceptors, revealing their strong potential to chelate halides as well as their ability to organize reactive molecules such as alkenes and butadiynes toward [2+2] cycloadditions and polydiacetylene formation, respectively. This selective activation concept is not limited to ChB but can be effectively used on all other σ-hole interactions (pnictogen bond, tetrel bond, etc.) where one needs to control the directionality of the interaction.

A Living Topochemical Ring-Opening Polymerization of Achiral Amino Acid N-Carboxy-Anhydrides in Single Crystals

A living topochemical ring-opening polymerization (ROP) of achiral amino-acid N-carboxyanhydrides (NCAs) is reported. Single crystals of the NCAs of α-aminoisobutyric acid (Aib) and 1-aminocyclohexanecarboxylic acid (ACHC) were grown, allowing a ring-opening polymerization macroscopically induced by amines. The single crystals could be polymerized at temperatures from 25-50 °C after physically contacting the amine-based initiator with the crystals. Topochemical polymerization of the crystals was proven by MALDI-ToF MS and XRD, generating polymers with chain lengths of up to 40 units and a complete affixation of the initiating amine at the polymer's head. Due to the proper alignment of the reacting groups in the crystal, longer polymer chains with improved purities can be reached, as chain-transfer is reduced as compared to solution polymerization. Simple purification of the polymers can be achieved by separation of the unreacted NCA via dispersion in acetonitrile. Overall, this method enables the preparation of polymers with higher chain length and purities at mild conditions, finally demonstrating a crystal-based ring opening polymerization.

Single-Crystal-to-Single-Crystal Topochemical Synthesis of a Collagen-inspired Covalent Helical Polymer

There is much demand for crystalline covalent helical polymers. Inspired by the helical structure of collagen, we synthesized a covalent helical polymer wherein the repeating dipeptide Gly-Pro units are connected by triazole linkages. We synthesized an azide and alkyne-modified dipeptide monomer made up of the repeating amino acid sequence of collagen. In its crystals, the monomer molecules aligned in head-to-tail fashion with proximally placed azide and alkyne forming supramolecular helices. At 60 °C, the monomer underwent single-crystal-to-single-crystal (SCSC) topochemical azide-alkyne cycloaddition polymerization, yielding a covalent helical polymer as confirmed by single-crystal X-ray diffraction (SCXRD) analysis. Compared to the monomer crystals, the polymer single-crystals were very strong and showed three-fold increase in Young's modulus, which is higher than collagen, many synthetic polymers and other materials. The crystals of this covalent helical polymer could bear loads as high as 1.5 million times of their own weight without deformation. These crystals could also withstand high compression force and did not disintegrate even at an applied force of 98 kN. Such light-weight strong materials are in demand for various technological applications.

Single-crystal polymers (SCPs): from 1D to 3D architectures

Single-crystal polymers (SCPs) with unambiguous chemical structures at atomic-level resolutions have attracted great attention. Obtaining precise structural information of these materials is critical as it enables a deeper understanding of the potential driving forces for specific packing and long-range order, secondary interactions, and kinetic and thermodynamic factors. Such information can ultimately lead to success in controlling the synthesis or engineering of their crystal structures for targeted applications, which could have far-reaching impact. Successful synthesis of SCPs with atomic level control of the structures, especially for those with 2D and 3D architectures, is rare. In this review, we summarize the recent progress in the synthesis of SCPs, including 1D, 2D, and 3D architectures. Solution synthesis, topochemical synthesis, and extreme condition synthesis are summarized and compared. Around 70 examples of SCPs with unambiguous structure information are presented, and their synthesis methods and structural analysis are discussed. This review offers critical insights into the structure-property relationships, providing guidance for the future rational design and bottom-up synthesis of a variety of highly ordered polymers with unprecedented functions and properties.

Elucidation of Substituent-Responsive Reactivities via Hierarchical and Asymmetric Assemblies in Crystalline p-Quinodimethane Derivatives

We propose a mechanism for substituent-responsive reactivities of p-quinodimethane derivatives with four ester groups through their hierarchical and asymmetric assembly modes. Four asymmetric 7,8,8-tris(methoxycarbonyl)-p-quinodimethanes with a 7-positioned ethoxycarbonyl (2 a(H)), 2'-fluoroethoxycarbonyl (2 b(F)), 2'-chloroethoxycarbonyl (2 c(Cl)), or 2'-bromoethoxycarbonyl (2 d(Br)) were synthesized and crystallized. 2 a(H), 2 b(F) and 2 d(Br) afforded only one shape crystal, while 2 c(Cl) did two polymorphic 2 c(Cl)-α and 2 c(Cl)-β. UV-irradiation induced topochemical polymerization for 2 a(H), no reactions for 2 b(F) and 2 c(Cl)-α, and [6+6] photocycloaddition dimerization for 2 c(Cl)-β and 2 d(Br). Such substituent-responsive reactivities and crystal structures were compared with those of the known symmetric 7,7,8,8-tetrakis(alkoxycarbonyl)-p-quinodimethanes such as 7,7,8,8-tetrakis(methoxycarbonyl)- (1 a(Me)-α and 1 a(Me)-β), 7,7,8,8-tetrakis(ethoxycarbonyl)- (1 b(Et)), and 7,7,8,8-tetrakis(bromoethoxycarbonyl)- (1 c(BrEt)). The comparative study clarified that the reactivities and crystal structures are classified into four types that link to each other. This linkage is understandable when we analyze the crystal structures through the following hierarchical and asymmetric assemblies; conformers, dimers, one dimensional (1D)-columns, two dimensional (2D)-sheets, and three dimensional (3D)-stacked sheets (3D-crystals). This supramolecular viewpoint is supported by intermolecular interaction energies among neighbored molecules with the density functional theory (DFT) calculation. Such research enables us to elucidate the substituent-responsive reactivities of the crystals, and reminds us of the selection of the right path in a so-called "maze game".

Topochemical Syntheses of Polyarylopeptides Involving Large Molecular Motions: Frustrated Monomer Packing Leads to the Formation of Polymer Blends

We report the topochemical syntheses of three polyarylopeptides, wherein triazolylphenyl group is integrated into the backbone of peptide chains. We synthesized three different monomers having azide and arylacetylene as end-groups from glycine, L-alanine and L-valine. We crystallized these monomers and the crystal structures of two of them were determined by single-crystal X-ray diffractometry. Due to the steric constraints, both of these monomers crystallized with two molecules, viz. conformers A and B, in the asymmetric unit. Consistently, in both cases, the A-conformers are antiparallelly π-stacked and B-conformers are parallelly slip-stacked, exploiting weak interactions. Though the arrangements of molecules in the pristine crystals were unsuitable for topochemical reaction, upon heating, they undergo large motion inside the crystal lattice to reach a transient reactive orientation and thereby the self-sorted conformer stacks react to give a blend of triazole-linked polyarylopeptides having two different linkages. Due to the large molecular motion inside crystals, the product phase loses its crystallinity.

Rational design and topochemical synthesis of polymorphs of a polymer

Packing a polymer in different ways can give polymorphs of the polymer having different properties. β-Turn forming peptides such as 2-aminoisobutyric acid (Aib)-rich peptides adopt several conformations by varying the dihedral angles. Aiming at this, a β-turn-forming peptide monomer would give different polymorphs and these polymorphs upon topochemical polymerization would yield polymorphs of the polymer, we designed an Aib-rich monomer N3-(Aib)3-NHCH2-C[triple bond, length as m-dash]CH. This monomer crystallizes as two polymorphs and one hydrate. In all forms, the peptide adopts β-turn conformations and arranges in a head-to-tail manner with their azide and alkyne units proximally placed in a ready-to-react alignment. On heating, both the polymorphs undergo topochemical azide-alkyne cycloaddition polymerization. Polymorph I polymerized in a single-crystal-to-single-crystal (SCSC) fashion and the single-crystal X-ray diffraction analysis of the polymer revealed its screw-sense reversing helical structure. Polymorph II maintains its crystallinity during polymerization but gradually becomes amorphous upon storage. The hydrate III undergoes a dehydrative transition to polymorph II. Nanoindentation studies revealed that different polymorphs of the monomer and the corresponding polymers exhibited different mechanical properties, in accordance with their crystal packing. This work demonstrates the promising future of the marriage of polymorphism and topochemistry for obtaining polymorphs of polymers.

Overcoming the Usual Reactivity of β-Nitroenones: Synthesis of Polyfunctionalized Homoallylic Alcohols and Conjugated Nitrotriene Systems

Herein, we report a new application of β-nitroenones as valuable building blocks for the preparation of polyfunctionalized homoallylic alcohols; they can be used as key precursors of conjugated nitrotriene systems. The synthesis of homoallylic alcohols was performed exploiting the chemoselective addition of metal allylating agents to the ketone moiety vs the nitroalkenyl group. The conversion of alcohols into nitrotrienes was achieved under Lewis-acid-promoted conditions. Both classes of compounds were obtained in good to excellent yields.

Recent advances in n-type and ambipolar organic semiconductors and their multi-functional applications

Organic semiconductors have received broad attention and research interest due to their unique integration of semiconducting properties with structural tunability, intrinsic flexibiltiy and low cost. In order to meet the requirements of organic electronic devices and their integrated circuits, p-type, n-type and ambipolar organic semiconductors are all necessary. However, due to the limitation in both material synthesis and device fabrication, the development of n-type and ambipolar materials is quite behind that of p-type materials. Recent development in synthetic methods of organic semiconductors greatly enriches the range of n-type and ambipolar materials. Moreover, the newly developed materials with multiple functions also put forward multi-functional device applications, including some emerging research areas. In this review, we give a timely summary on these impressive advances in n-type and ambipolar organic semiconductors with a special focus on their synthesis methods and advanced materials with enhanced properties of charge carrier mobility, integration of high mobility and strong emission and thermoelectric properties. Finally, multi-functional device applications are further demonstrated as an example of these developed n-type and ambipolar materials.

Exceptionally Long Lifetimes of Strongly Entangled Acyl-Trityl Radical Pairs Photochemically Generated in Crystalline Trityl Ketones

Triplet acyl-alkyl radical pairs generated by pulsed laser excitation within the constraints of their nanocrystalline ketone precursors were recently introduced as a potential platform for the robust and repeated instantiation of spin qubit pairs for applications in quantum information science. Here, we report the transient spectroscopy of a series of nanocrystalline trityl-alkyl and trityl-aryl ketones capable of generating correlated triplet radical pairs with persistent triphenylmethyl radicals forced to remain within bonding distances of highly reactive acyl radicals. Whereas triplet trityl-acyl radical pairs decay by competing product-forming decarbonylation and intersystem crossing, triplet trityl-benzoyl radical pairs have lifetimes of up to ca. 4 ms and exclusively regenerate the starting ketone. We propose that these long lifetimes are the result of the short inter-radical distances and the colinear orientation of the two singly occupied orbitals, which are expected to result in large singlet-triplet energy gaps, large zero-field splitting parameters, and a poor geometry for spin-obit coupling. Ketones generating trityl-benzoyl radical pairs demonstrate promising performance along multiple dimensions that are crucial for quantum information science.

Linear Cyclobutane-Containing Polymer Synthesis via [2 + 2] Photopolymerization in an Unconfined Environment under Visible Light

The [2 + 2] photopolymerization of diolefinic monomers is an appealing approach for the construction of polymeric materials. Herein, we demonstrate that the establishment of an effective donor-acceptor conjugation by introducing electron-donating alkoxy groups at appropriate positions of the benzene ring could activate p-phenylenediacrylate (PDA), thus enabling the development of the first solution [2 + 2] photopolymerization of such monomers under the irradiation of visible light. Variation on the alkoxy groups and the ester parts could allow access to a series of linear cyclobutane-containing polymer products with high molecular weight (up to 140 kDa) and good solubility in common solvents. Further, temporal control and postpolymerization modification with preinstalled pendant C═C bonds via thiol-ene click reaction are also demonstrated with this [2 + 2] photopolymerization system.

Photochemically Mediated Polymerization of Molecular Furan and Pyridine: Synthesis of Nanothreads at Reduced Pressures

Nanothreads are emerging one-dimensional sp³-hybridized materials with high predicted tensile strength and a tunable band gap. They can be synthesized by compressing aromatic or nonaromatic small molecules to pressures ranging from 15-30 GPa. Recently, new avenues are being sought that reduce the pressure required to afford nanothreads; the focus has been placed on the polymerization of molecules with reduced aromaticity, favorable stacking, and/or the use of higher reaction temperatures. Herein, we report the photochemically mediated polymerization of pyridine and furan aromatic precursors, which achieves nanothread formation at reduced pressures. In the case of pyridine, it was found that a combination of slow compression/decompression with broadband UV light exposure yielded a crystalline product featuring a six-fold diffraction pattern with similar interplanar spacings to previously synthesized pyridine-derived nanothreads at a reduced pressure. When furan is compressed to 8 GPa and exposed to broadband UV light, a crystalline solid is recovered that similarly demonstrates X-ray diffraction with an interplanar spacing akin to that of the high-pressure synthesized furan-derived nanothreads. Our method realizes a 1.9-fold reduction in the maximum pressure required to afford furan-derived nanothreads and a 1.4-fold reduction in pressure required for pyridine-derived nanothreads. Density functional theory and multiconfigurational wavefunction-based computations were used to understand the photochemical activation of furan and subsequent cascade thermal cycloadditions. The reduction of the onset pressure is caused by an initial [4+4] cycloaddition followed by increasingly facile thermal [4+2]-cycloadditions during polymerization.

Side‐Chain Control of Topochemical Polymer Single Crystals with Tunable Elastic Modulus

Topochemical polymerizations hold the promise of producing high molecular weight and stereoregular single crystalline polymers by first aligning monomers before polymerization. However, monomer modifications often alter the crystal packing and result in non-reactive polymorphs. Here, we report a systematic study on the side chain functionalization of the bis(indandione) derivative system that can be polymerized under visible light. Precisely engineered side chains help organize the monomer crystals in a one-dimensional fashion to maintain the topochemical reactivity. By optimizing the side chain length and end group of monomers, the elastic modulus of the resulting polymer single crystals can also be greatly enhanced. Lastly, using ultrasonication, insoluble polymer single crystals can be processed into free-standing and robust polymer thin films. This work provides new insights on the molecular design of topochemical reactions and paves the way for future applications of this fascinating family of materials.