New Application of Multiresonance Organic Delayed Fluorescence Dyes: High-Performance Photoinitiating Systems for Acrylate and Epoxy Photopolymerization and Photoluminescent Pattern Preparation

The primary focus of photopolymerization research is to advance highly efficient visible photoinitiating systems (PISs) as alternatives to conventional ultraviolet (UV) photoinitiators. We developed four multiresonance emitters (BIC-pCz, BNO1, BO-DICz, and TPABO-DICz) to sensitize iodonium salt (Iod) and initiate free-radical and cationic photopolymerization under visible light for the first time. The TPABO-DICz/Iod system achieved a double-bond conversion of over 70% within just 4 s of exposure to green light (520 nm), while the BNO1/Iod system achieved a double-bond conversion exceeding 50% with 10 s of exposure to red light (630 nm). The photochemical properties were studied through thermodynamic research, steady-state photolysis, and electron spin resonance. Photolithography techniques were employed to fabricate photoluminescent films and micrometer-scale patterns utilizing the blue-emitting BIC-pCz dye, showcasing the potential of photolithography in the production of photoluminescent pixels. Additionally, the BIC-pCz/Iod and TPABO-DICz/Iod systems have been employed to rapidly fabricate photoluminescent polymer patterns using a digital-light-processing 3D printer with a low-intensity light (3.2 mW cm-2). These multiresonance emitters show exceptional photosensitizing effects and can act as fluorescent dyes in photoluminescent patterns, highlighting the potential of utilizing photopolymerization for OLED applications.

Near-infrared photocatalysis with cyanines: synthesis, applications and perspectives

Cyanines are organic dyes bearing two aza-heterocycles linked by a polymethine chain. Excited states, fluorescence, redox activity, and energy transfer are interesting properties of cyanines which have been used by chemists. Moreover, they are easily accessible and highly tunable. For all these reasons, cyanines are often selected for applications like fluorescent probes, phototherapy and photovoltaics. However, considering cyanines as photocatalysts is a new field of investigation and has been sparsely reported in the literature. This field of research has been launched on the basis of near-infrared light photocatalysis. With a deeper NIR light penetration, the irradiation is compatible with biological tissues. Due to the longer wavelengths that are involved, the safety of the operator can be guaranteed. In this perspective review, the photophysical/redox properties of cyanines are reported as well as their preparations and applications in modern synthetic approaches. Finally, recent examples of cyanine-based NIR-photocatalysis are discussed including photopolymerization and organic synthesis.

Near-infrared Light-Induced Polymerizations: Mechanisms and Applications

Photopolymerizations have garnered significant attention in polymer science due to their low polymerization temperature, high production efficiency, environmental friendliness, and spatial controllability. Despite these merits, the poor penetration and severe chemical damage from ultraviolet/visible (UV/Vis) light resources pose significant barriers to their success in conventional photopolymerizations. A recent breakthrough involving the utilization of near-infrared (NIR) laser with long wavelength has been exploited for diverse applications. With the combination of a NIR photosensitizer (PS), NIR-induced photopolymerizations have been successfully developed to alleviate the challenges in conventional methods. The enhancement of penetration depth and safety of NIR-induced photopolymerizations can contribute significantly to improving the efficiency of polymerization for production of intricate structures across various scales. In this concept, the typical types of PSs and polymerization mechanisms (PMs) within the NIR-induced photopolymerization systems have been classified in detail. Additionally, the applications of various polymers achieved by NIR-induced photopolymerizations are summarized. Furthermore, research directions and future challenges of this field are also discussed comprehensively.

Upconversion particle-assisted NIR polymerization enables microdomain gradient photopolymerization at inter-particulate length scale

High crosslinking and low shrinkage stress are difficult to reconcile in the preparation of performance-enhancing photopolymer materials. Here we report the unique mechanism of upconversion particles-assisted NIR polymerization (UCAP) in reducing shrinkage stress and enhancing mechanical properties of cured materials. The excited upconversion particle emit UV-vis light with gradient intensity to the surroundings, forming a domain-limited gradient photopolymerization centered on the particle, and the photopolymer grows within this domain. The curing system remains fluid until the percolated photopolymer network is formed and starts gelation at high functional group conversion, with most of the shrinkage stresses generated by the crosslinking reaction having been released prior to gelation. Longer exposures after gelation contribute to a homogeneous solidification of cured material, and polymer materials cured by UCAP exhibit high gel point conversion, low shrinkage stress and strong mechanical properties than those cured by conventional UV polymerization techniques.

"Invisible" Digital Light Processing 3D Printing with Near Infrared Light

The ability to 3D print structures with low-intensity, long-wavelength light will broaden the materials scope to facilitate inclusion of biological components and nanoparticles. Current materials limitations arise from the pervasive absorption, scattering, and/or degradation that occurs upon exposure to high-intensity, short-wavelength (ultraviolet) light, which is the present-day standard used in light-based 3D printers. State-of-the-art techniques have recently extended printability to orange/red light. However, as the wavelength of light increases, so do the inherent challenges to match the speed and resolution of traditional UV light-induced solidification processes (i.e., photocuring). Herein, a photosystem is demonstrated to enable low-intensity (<5 mW/cm2), long-wavelength (∼850 nm) near-infrared (NIR) light-driven 3D printing, "invisible" to the human eye. The combination of a NIR absorbing cyanine dye with electron-rich and -deficient redox pairs was required for rapid photocuring in a catalytic manner. The rate of polymerization and time to solidification upon exposure to NIR light were characterized via in situ spectroscopic and rheological monitoring. Translation to NIR digital light processing (projection-based) 3D printing was accomplished through rigorous optimization of resin composition and printing parameters to balance the speed (<60 s/layer) and resolution (<300 μm features). As a proof-of-concept, composite 3D printing with nanoparticle-infused resins was accomplished. Preliminary analysis showed improved feature fidelity for structures produced with NIR relative to UV light. The present report provides key insight that will inform next-generation light-based photocuring technology, such as wavelength-selective multimaterial 3D bio- and composite-printing.

NIR-sensitized hybrid radical and cationic photopolymerization of several cyanines in combination with diaryliodonium bis(trifluoromethyl)sulfonyl imide

A series of cyanines exhibiting absorption between 750 and 930 nm reacted after NIR excitation with the bis(t-butylphenyl) iodonium cation comprising the [(CF3SO2)2N]− anion (NTf2)−, resulting in the generation of free radicals and conjugate acids.

Upconversion nanoparticle-assisted cationic and radical/cationic hybrid photopolymerization using sulfonium salts

Several sulfonium salts as unimolecular photoinitiators can efficiently initiate UCNP-assisted direct cationic and radial/cationic photopolymerization under 980 nm NIR light irradiation.