Onium Photocages for Visible-Light-Activated Poly(thiourethane) Synthesis and 3D Printing.

The lack of chemical diversity in light-driven reactions for 3D printing poses challenges in the production of structures with long-term ambient stability, recyclability, and breadth in properties (mechanical, optical, etc.). Herein we expand the scope of photochemistries compatible with 3D printing by introducing onium photocages for the rapid formation of poly(thiourethanes) (PTUs). Efficient nonsensitized visible-light photolysis releases organophosphine and -amine derivatives that catalyze thiol-isocyanate polyaddition reactions with excellent temporal control. Two resin formulations comprising commercial isocyanates and thiols were developed for digital light processing (DLP) 3D printing to showcase the fast production of high-resolution PTU objects with disparate mechanical properties. Onium photocages represent valuable tools to advance light-driven manufacturing of next-generation high-performance sustainable materials.

Assembly and Disassembly of Supramolecular Hypervalent Iodine Macrocycles via Anion Coordination.

This study explores the dynamic self-assembly and disassembly of hypervalent iodine-based macrocycles (HIMs) guided by secondary bonding interactions. The reversible disassembly and reassembly of HIMs are facilitated through anion binding via the addition of tetrabutylammonium (TBA) salts or removal of the anion by the addition of silver nitrate. The association constants for HIM monomers with TBA(Cl) and TBA(Br) are calculated and show a correlation with the strength of the iodine-anion bond. A unique tetracoordinate hypervalent iodine-based compound was identified as the disassembled monomer. Last, the study reveals the dynamic bonding nature of these macrocycles in solution, allowing for rearrangement and participation in dynamic bonding chemistry.

Do The Twist: Efficient Heavy-Atom-Free Visible Light Polymerization Facilitated by Spin-Orbit Charge Transfer Inter-system Crossing.

The use of visible light to drive polymerizations with spatiotemporal control offers a mild alternative to contemporary UV-light-based production of soft materials. In this spectral region, photoredox catalysis represents the most efficient polymerization method, yet it relies on the use of heavy-atoms, such as precious metals or toxic halogens. Herein, spin-orbit charge transfer intersystem crossing from boron dipyrromethene (BODIPY) dyads bearing twisted aromatic groups is shown to enable efficient visible light polymerizations in the absence of heavy-atoms. A ≈5-15× increase in polymerization rate and improved photostability was achieved for twisted BODIPYs relative to controls. Furthermore, monomer polarity had a distinct effect on polymerization rate, which was attributed to charge transfer stabilization based on ultrafast transient absorption and phosphorescence spectroscopies. Finally, rapid and high-resolution 3D printing with a green LED was demonstrated using the present photosystem.

Simultaneous Color- and Dose-Controlled Thiol-Ene Resins for Multimodulus 3D Printing with Programmable Interfacial Gradients.

Drawing inspiration from nature's own intricate designs, synthetic multimaterial structures have the potential to offer properties and functionality that exceed those of the individual components. However, several contemporary hurdles, from a lack of efficient chemistries to processing constraints, preclude the rapid and precise manufacturing of such materials. Herein, the development of a photocurable resin comprising color-selective initiators is reported, triggering disparate polymerization mechanisms between acrylate and thiol functionality. Exposure of the resin to UV light (365 nm) leads to the formation of a rigid, highly crosslinked network via a radical chain-growth mechanism, while violet light (405 nm) forms a soft, lightly crosslinked network via an anionic step-growth mechanism. The efficient photocurable resin is employed in multicolor digital light processing 3D printing to provide structures with moduli spanning over two orders of magnitude. Furthermore, local intensity (i.e., grayscale) control enables the formation of programmable stiffness gradients with ≈150× change in modulus occurring across sharp (≈200 µm) and shallow (≈9 mm) interfaces, mimetic of the human knee entheses and squid beaks, respectively. This study provides composition-processing-property relationships to inform advanced manufacturing of next-generation multimaterial objects having a myriad of applications from healthcare to education.

Record release of tetramethylguanidine using a green light activated photocage for rapid synthesis of soft materials.

Photocages have enabled spatiotemporally governed organic materials synthesis with applications ranging from tissue engineering to soft robotics. However, the reliance on high energy UV light to drive an often inefficient uncaging process limits their utility. These hurdles are particularly evident for more reactive cargo, such as strong organobases, despite their attractive potential to catalyze a range of chemical transformations. Herein, two metal-free boron dipyrromethene (BODIPY) photocages bearing tetramethylguanidine (TMG) cargo are shown to induce rapid and efficient polymerizations upon exposure to a low intensity green LED. A suite of spectroscopic characterization tools were employed to identify the underlying uncaging and polymerization mechanisms, while also determining reaction quantum efficiencies. The results are directly compared to state-of-the-art TMG-bearing ortho-nitrobenzyl and coumainylmethyl photocages, finding that the present BODIPY derivatives enable step-growth polymerizations that are >10× faster than the next best performing photocage. As a final demonstration, the inherent multifunctionality of the present BODIPY platform in releasing radicals from one half of the molecule and TMG from the other is leveraged to prepare polymers with starkly disparate physical properties. The present findings are anticipated to enable new applications of photocages in both small-molecule photochemistry for medicine and advanced manufacturing of next generation soft materials.

Cyclopentannulation and cyclodehydrogenation of isomerically pure 5,11-dibromo-anthradithiophenes leading to contorted aromatics.

Isomerically pure 5,11-dibromo-2,8-dihexylanthra[2,3-b:76-b']dithiophene, a brominated analog of anthracenedithiophene (ADT), was prepared and utilized for a palladium catalyzed cyclopentannulation reaction with 3,3'-dimethoxy-phenylacetylene to give cyclopentannulated ADT (CP-ADTs). A further Scholl cyclodehydrogenation reaction gave contorted aromatics with large splay angles, low optical gaps, and low LUMOs.