Boron-Methylated Dipyrromethene as a Green Light Activated Type I Photoinitiator for Rapid Radical Polymerizations

Initiation-Confined Holographic Photopolymerization under Visible Light

Holographic photopolymerization provides a robust approach for the advanced manufacturing of structurally ordered devices. However, photogenerated radicals in bright regions are prone to cross-regional diffusion and induce unwanted polymerizations in dark regions, making efficient holographic manufacturing exceedingly challenging. Herein, we report a breakthrough by spatially confining the initiation function, enabling the unprecedentedly rapid formation of predesigned submicron-ordered grating structures within merely 0.75 s under visible light holographic exposure. Theoretical computations attribute the success of this initiation-confined approach to orders of magnitude larger reaction rate constants of initiation than propagation. This approach is highly versatile, facilitating the fabrication of both transmission- and reflection-type volume holographic grating structures starting from various reaction systems, and the refractive index modulation can be as high as 0.10, enabling ultraefficient manufacturing of waveguide combiners for high-fidelity near-eye AR displays.

B ← N Lewis Pair Fusion of N,N-Diaryldihydrophenazines: Effect on Structural, Electronic, and Emissive Properties

Doping of polycyclic aromatic hydrocarbons (PAHs) with boron and/or nitrogen is emerging as a powerful tool to tailor the electronic structure and photophysical properties. As N-doped analogues of anthracene, N,N-dihydrophenazines play important roles as redox mediators, battery materials, luminophores, and photoredox catalysts. Although benzannulation has been used successfully as a structural constraint to control the excited state properties, fusion of the N-aryl groups to the phenazine backbone has rarely been explored. Herein, we report the first examples of dihydrophenazines, in which the N-aryl groups are fused to the phenazine backbone via B←N Lewis pair formation. This results in structural rigidification, locking the molecules in a bent conformation, while also modulating the electronic structure through molecular polarization. B─N fusion in BNPz1-BNPz3 induces a quinoid resonance structure with significant C─N(py) double bond character and reduces the antiaromatic character of the central pyrazine ring. Borylation also lowers the HOMO/LUMO (highest occupied/lowest unoccupied molecular orbital) energies and engenders bathochromic shifts in the emission. Further rigidification in the solid state gives rise to enhanced emission quantum yields, consistent with aggregation-induced emission enhancement (AIEE) observed upon water addition to solutions in tetrahydrofuran (THF). The demonstrated structural control and fine-tuning of optoelectronic properties are of great significance to potential applications as emissive materials and in photocatalysis.

Donor-π-Acceptor Photoinitiators for High-Efficiency Visible LED and Sunlight Polymerization and High-Precision 3D Printing

This study presents the development and evaluation of five dyes with varying conjugated energy levels and donor-π-acceptor (D-π-A) structures as photoinitiators for free radical polymerization. Their photoinitiation efficiencies are systematically assessed under both visible-light LED and sunlight. Notably, the conversions reach up to 81% within just 30 s under sunlight, demonstrating the ultrafast and efficient polymerization capabilities of the dyes. The efficient electron transfer is facilitated by the D-π-A structure, where the conjugation is reduced or interrupted by the high distortion between the electron-withdrawing and the electron-releasing units. This distortion can prevent the overlap of frontier molecular orbitals, decreasing the energy difference between the ground state and the excited state of dyes, thereby enhancing the electron transfer reactivity with additives. Additionally, we propose a chemical mechanism for the electron transfer reaction in the three-component systems. The study also explores the application of naphtho[2,3-d]thiazole-4,9-dione-based dyes as donors in additive manufacturing demonstrating their effectiveness in three different 3D printing technologies, i.e., direct laser writing (DLW), digital light processing (DLP), and liquid crystal display (LCD). These three-component formulations achieve high-precision 3D printed objects, with detailed characterization and comparison of the resulting structures.

Visible-Light-Fueled Polymerizations for 3D Printing

ConspectusLight-driven polymerizations and their application in 3D printing have revolutionized manufacturing across diverse sectors, from healthcare to fine arts. Despite the popularized notion that with 3D printing "imagination is the only limit", we and others in the scientific community have identified fundamental hurdles that restrict our capabilities in this space. Herein, we describe the ZAP group's efforts in developing photochemical systems that respond to nontraditional colors of light to elicit the rapid, spatiotemporally controlled formation of plastics. Our research addresses key limitations in current photopolymerization methods, such as the reliance on high-energy UV light, oxygen sensitivity, and narrow materials scope. We present a comprehensive overview of our advancements in both light-fueled radical and nonradical chemistry and its implementation in vat photopolymerization 3D printing using panchromatic resins. In radical chemistry, we have developed a class of boron dipyrromethene (BODIPY) dye molecules that act as photoradical generators (PRGs). Upon exposure to visible or near-infrared (NIR) light, these molecules induced efficient polymerization of acrylics. Structural modifications, including the installment of halogens, twisted aromatic groups, nitrogen bridgeheads, and thiophenes, have imbued activity across this wide spectral range. Systematic photophysical characterization of these dyes revealed the presence of long-lived excited (high in energy) states, from which we accredited the enhancements in polymerization efficiency. In turn, curing (converting a liquid to solid) with low intensity visible-to-NIR light was possible in mere seconds; a requirement for many light-based 3D printing technologies. Our efforts in nonradical chemistry have been motivated by the need for new materials with properties and functionality currently inaccessible using radical-based 3D printing approaches (e.g., tough and recyclable), while also providing an avenue toward multimaterial fabrication. We have developed photobase generators (PBGs) - dyes that release basic cargo upon light exposure-to catalyze polymerizations beyond acrylic-only resins. These include coumarinylmethyl- and BODIPY-tetramethylguanidine (TMG) derivatives, as well as onium photocages, which enabled photocuring of thiol-ene and thiol-isocyanate resins. Lastly, we have pioneered rapid, high-resolution visible-to-NIR light-based 3D printing. Our work includes the development of reactive photoredox catalyst systems for speed, additives for oxygen-tolerance, NIR-light reactivity for nanoparticle composites, models for streamlined optimization, and triplet fusion for high resolution. These advancements led to build speeds up to 45 mm/h with features <100 μm, rivaling contemporary UV-based technologies. The impact of our research extends beyond academic interest, offering practical solutions for additive manufacturing of (multi)functional materials. By enabling the use of lower-energy light sources, our work paves the way for environmentally friendly, cost-effective, and versatile 3D printing. It opens new possibilities for printing with previously incompatible materials, including UV-sensitive compounds and high-refractive-index nanocomposites. Nascent developments in multimaterial 3D printing via color- and dose-controlled light exposure are enabling the production of objects with precise placement of materials having disparate composition and properties. As we continue to develop photopolymerizations and light-based 3D printing, we anticipate transformative applications in fields ranging from tissue engineering to advanced electronics manufacturing. This will bring the community one step closer to fulfill the dream of creators only being "limited by imagination".

Synthesis and Solvent Free DLP 3D Printing of Degradable Poly(Allyl Glycidyl Ether Succinate)

Digital light processing (DLP) printing forms solid constructs from fluidic resins by photochemically crosslinking polymeric resins with reactive functional groups. DLP is used widely due to its efficient, high-resolution printing, but its use and translational potential has been limited in some applications as state-of-the-art resins experience unpredictable and anisotropic part shrinkage due to the use of solvent needed to reduce resin viscosity and layer dependent crosslinking. Herein, poly(allyl glycidyl ether succinate) (PAGES), a low viscosity, degradable polyester, was synthesized by ring opening copolymerization and used in combination with degradable thiol crosslinkers to afford a solvent free resin that can be utilized in DLP printing. Varying resin formulations of PAGES polymer are shown to decrease part shrinkage from 14 % to 0.3 %. Photochemically printed parts fabricated from PAGES possess tensile moduli between 0.43 and 6.18 MPa and degradation profiles are shown to vary between 12 and 40 days under accelerated conditions based on degree of polymerization and crosslink ratio.

Biomass-Derived Optically Clear Adhesives for Foldable Displays

Novel acrylate monomers, derived from terpenes are synthesized for use in optically clear adhesives (OCAs) suitable for foldable displays. These OCAs are prepared using visible-light-driven polymerization, an eco-friendly method. Through physical, rheological, and mechanical characterization, the prepared OCAs possess low modulus and exhibit outstanding creep and recovery properties, making them suitable for foldable devices.

Structure-Photoreactivity Studies of BODIPY Photocages: Limitations of the Activation Barrier for Optimizing Photoreactions

BODIPY photocages are photoreactive chromophores that release covalently linked cargo upon absorption of visible light. Here, we used computations of the T1 photoheterolysis barrier to ascertain whether a computational approach could assist in a priori structure design by identifying new structures with higher quantum yields of photorelease. The electronic structure-photoreactivity relationships were elucidated for boron-substituted and core-functionalized 2-substituted BODIPY photocages as well as aryl substitutions at the meso-methyl position. Although there is a clear trend for the 2-substituted derivatives, with donor-substituted derivatives featuring both lower computed barriers and higher experimental quantum yields, no trend in the quantum yield with the computed activation barrier is found for the meso-methyl-substituted or boron-substituted derivatives. The lack of a correlation between the experimental quantum yield with the computed barrier in the latter two substitution cases is attributed to the substituents having larger effects on the rates of competing channels (internal conversion and competitive photoreactions) than on the rate of the photoheterolysis channel. Thus, although in some cases computed photoreaction barriers can aid in identifying structures with higher quantum yields, the ignored impacts of how changing the structure affects the rates of competing photophysical/photochemical channels limit the effectiveness of this single-parameter approach.

Visible-Light Activation of Diorganyl Bis(pyridylimino) Isoindolide Aluminum(III) Complexes and Their Organometallic Radical Reactivity

We report on the synthesis and characterization of a series of (mostly) air-stable diorganyl bis(pyridylimino) isoindolide (BPI) aluminum complexes and their chemistry upon visible-light excitation. The redox non-innocent BPI pincer ligand allows for efficient charge transfer homolytic processes of the title compounds. This makes them a universal platform for the generation of carbon-centered radicals. The photo-induced homolytic cleavage of the Al-C bonds was investigated by means of stationary and transient UV/Vis spectroscopy, spin trapping experiments, as well as EPR and NMR spectroscopy. The experimental findings were supported by quantum chemical calculations. Reactivity studies enabled the utilization of the aluminum complexes as reactants in tin-free Giese-type reactions and carbonyl alkylations under ambient conditions, which both indicated radical-polar crossover behavior. A deeper understanding of the physical fundamentals and photochemical process was provided, furnishing in turn a new strategy to control the reactivity of bench-stable aluminum organometallics.

Ultraviolet light blocking optically clear adhesives for foldable displays via highly efficient visible-light curing

In developing an organic light-emitting diode (OLED) panel for a foldable smartphone (specifically, a color filter on encapsulation) aimed at reducing power consumption, the use of a new optically clear adhesive (OCA) that blocks UV light was crucial. However, the incorporation of a UV-blocking agent within the OCA presented a challenge, as it restricted the traditional UV-curing methods commonly used in the manufacturing process. Although a visible-light curing technique for producing UV-blocking OCA was proposed, its slow curing speed posed a barrier to commercialization. Our study introduces a highly efficient photo-initiating system (PIS) for the rapid production of UV-blocking OCAs utilizing visible light. We have carefully selected the photocatalyst (PC) to minimize electron and energy transfer to UV-blocking agents and have chosen co-initiators that allow for faster electron transfer and more rapid PC regeneration compared to previously established amine-based co-initiators. This advancement enabled a tenfold increase in the production speed of UV-blocking OCAs, while maintaining their essential protective, transparent, and flexible properties. When applied to OLED devices, this OCA demonstrated UV protection, suggesting its potential for broader application in the safeguarding of various smart devices.

Giving the Green Light to Photochemical Uncaging of Large Biomolecules in High Vacuum

The isolation of biomolecules in a high vacuum enables experiments on fragile species in the absence of a perturbing environment. Since many molecular properties are influenced by local electric fields, here we seek to gain control over the number of charges on a biopolymer by photochemical uncaging. We present the design, modeling, and synthesis of photoactive molecular tags, their labeling to peptides and proteins as well as their photochemical validation in solution and in the gas phase. The tailored tags can be selectively cleaved off at a well-defined time and without the need for any external charge-transferring agents. The energy of a single or two green photons can already trigger the process, and it is soft enough to ensure the integrity of the released biomolecular cargo. We exploit differences in the cleavage pathways in solution and in vacuum and observe a surprising robustness in upscaling the approach from a model system to genuine proteins. The interaction wavelength of 532 nm is compatible with various biomolecular entities, such as oligonucleotides or oligosaccharides.

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.