High Performance Near-Infrared (NIR) Photoinitiating Systems Operating under Low Light Intensity and in the Presence of Oxygen

Light-mediated intracellular polymerization

The synthesis of synthetic intracellular polymers offers groundbreaking possibilities in cellular biology and medical research, allowing for novel experiments in drug delivery, bioimaging and targeted cancer therapies. These macromolecules, composed of biocompatible monomers, are pivotal in manipulating cellular functions and pathways due to their bioavailability, cytocompatibility and distinct chemical properties. This protocol details two innovative methods for intracellular polymerization. The first one uses 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylpropiophenone (Irgacure 2959) as a photoinitiator for free radical polymerization under UV light (365 nm, 5 mW/cm2). The second method employs photoinduced electron transfer-reversible addition-fragmentation chain-transfer polymerization with visible light (470 nm, 100 mW/cm2). We further elaborate on isolating these intracellular polymers by streptavidin/biotin interaction or immobilized metal ion affinity chromatography for polymers tagged with biotin or histidine. The entire process, from polymerization to isolation, takes ~48 h. Moreover, the intracellular polymers thus generated demonstrate significant potential in enhancing actin polymerization, in bioimaging applications and as a novel avenue in cancer treatment strategies. The protocol extends to animal models, providing a comprehensive approach from cellular to systemic applications. Users are advised to have a basic understanding of organic synthesis and cell biology techniques.

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.

Design of Dyes Based on the Quinoline or Quinoxaline Skeleton towards Visible Light Photoinitiators

Dyes based on quinoline and quinoxaline skeletons were designed for application as visible light photoinitiators. The obtained compounds absorb electromagnetic radiation on the border between ultraviolet and visible light, which allows the use of dental lamps as light sources during the initiation of the photopolymerization reaction. Their another desirable feature is the ability to create a long-lived excited state, which enables the chain reaction to proceed through the mechanism of intermolecular electron transfer. In two-component photoinitiating systems, in the presence of an electron donor or a hydrogen atom donor, the synthesized compounds show excellent abilities to photoinitiate the polymerization of acrylates. In control tests, the efficiency of photopolymerization using modified quinoline and quinoxaline derivatives is comparable to that obtained using a typical, commercial photoinitiator for dentistry, camphorquinone. Moreover, the use of the tested compounds requires a small amount of photoinitiator (only 0.04% by weight) to initiate the reaction. The research also showed a significant acceleration of the photopolymerization process and shortening of the reaction time. In practice, this means that the new two-component initiating systems can be used in much lower concentrations without slowing down the speed of obtaining polymer materials. It is worth emphasizing that these two features of the new initiating system allow for cost reduction by reducing financial outlays on both materials (photoinitiators) and electricity.

Second Generation of Near-Infrared Cyanine-Based Photocatalysts for Faster Organic Transformations

A second generation of cyanine-based near-infrared photocatalysts has been developed to accelerate organic transformations. Cyanines were prepared and fully characterized prior to evaluation of their photocatalytic activities. Catalyst efficiency was determined by using two model oxidation and reduction reactions. For the aza-Henry reaction, cyanines bearing an amino group on the heptamethine chain led to the best results. For trifluoromethylation, the stability of the photocatalyst was found to be the key parameter for efficient and rapid conversion.

Integrating Hybrid Perovskite Nanocrystals into Metal-Organic Framework as Efficient S-Scheme Heterojunction Photocatalyst for Synergistically Boosting Controlled Radical Photopolymerization under 980 nm NIR Light

S-scheme heterojunction photocatalyst MAPbI3@PCN-222 with light absorption extending to the NIR region is constructed by embedding organic-inorganic hybrid perovskite (MAPbI3) into porphyrinic Zr-MOF (PCN-222). Both in situ X-ray photoelectron spectroscopy, ultraviolet photoelectron spectral characterization, and photocatalytic polymerization experiment prove the formation of S-scheme heterojunction. MAPbI3@PCN-222 with a low dosage (90 ppm) displays an impressive photocatalytic ability for 980 nm light-mediated photoinduced electron/energy-transfer-reversible addition-fragmentation chain-transfer (PET-RAFT) polymerization in air. The well-defined controllable-molecular weight polymers including block copolymers and ultrahigh-molecular weight polymers can be achieved with narrow distributions (Mw/Mn < 1.20) via rapid photopolymerization. The industrial application potential of the photocatalyst also has been proved by scale-up synthesis of polymers with low polydispersity under NIR light-induced photopolymerization in a large-volume reaction system (200 mL) with high monomer conversion up to 99%. The penetration photopolymerization through the 5 mm polytetrafluoroethylene plate and excellent photocontrollable behavior illustrate the existence of long-term photogenerated electron transfer of heterojunction and abundant free radicals in photopolymerization. The photocatalyst still retains high catalytic activity after 10 cycles of photopolymerization in air. It is revealed for the first time that the special PET-RAFT polymerization pathway is initiated by the aldehyde-bearing α-aminoalkyl radical derived from the oxidization of triethanolamine (TEOA) by the heterojunction photocatalyst. This research offers a new insight into understanding the NIR-light-activated PET-RAFT polymerization mechanism in the presence of TEOA.

Recent Advances in Monocomponent Visible Light Photoinitiating Systems Based on Sulfonium Salts

During the last decades, multicomponent photoinitiating systems have been the focus of intense research efforts, especially for the design of visible light photoinitiating systems. Although highly reactive three-component and even four-component photoinitiating systems have been designed, the complexity to elaborate such mixtures has incited researchers to design monocomponent Type II photoinitiators. Using this approach, the photosensitizer and the radical/cation generator can be combined within a unique molecule, greatly simplifying the elaboration of the photocurable resins. In this field, sulfonium salts are remarkable photoinitiators but these structures lack absorption in the visible range. Over the years, various structural modifications have been carried out in order to redshift their absorptions in the visible region. In this work, an overview of the different sulfonium salts activable under visible light and reported to date is proposed.

Trends in Photopolymerizable Bioinks for 3D Bioprinting of Tumor Models

Three-dimensional (3D) bioprinting technologies involving photopolymerizable bioinks (PBs) have attracted enormous attention in recent times owing to their ability to recreate complex structures with high resolution, mechanical stability, and favorable printing conditions that are suited for encapsulating cells. 3D bioprinted tissue constructs involving PBs can offer better insights into the tumor microenvironment and offer platforms for drug screening to advance cancer research. These bioinks enable the incorporation of physiologically relevant cell densities, tissue-mimetic stiffness, and vascularized channels and biochemical gradients in the 3D tumor models, unlike conventional two-dimensional (2D) cultures or other 3D scaffold fabrication technologies. In this perspective, we present the emerging techniques of 3D bioprinting using PBs in the context of cancer research, with a specific focus on the efforts to recapitulate the complexity of the tumor microenvironment. We describe printing approaches and various PB formulations compatible with these techniques along with recent attempts to bioprint 3D tumor models for studying migration and metastasis, cell-cell interactions, cell-extracellular matrix interactions, and drug screening relevant to cancer. We discuss the limitations and identify unexplored opportunities in this field for clinical and commercial translation of these emerging technologies.

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

Unimolecular (Type I) radical photoinitiators (PIs) have transformed the chemical manufacturing industry by enabling (stereo)lithography for microelectronics and emergent 3D printing technologies. However, the reliance on high energy UV-violet light (≤420 nm) restricts the end-use applications. Herein, boron-methylated dipyrromethene (methylated-BODIPY) is shown to act as a highly efficient Type I radical PI upon irradiation with low energy green light. Using a low intensity (∼4 mW/cm2) light emitting diode centered at 530 nm and a low PI concentration (0.3 mol %), acrylic-based resins were polymerized to maximum conversion in ∼10 s. Under equivalent conditions (wavelength, intensity, and PI concentration), state-of-the-art visible light PIs Ivocerin and Irgacure 784 show no appreciable polymerization. Spectroscopic characterization suggests that homolytic β-scission at the boron-carbon bond results in radical formation, which is further facilitated by accessing long-lived triplet excited states through installment of bromine. Alkylated-BODIPYs represent a new modular visible light PI platform with exciting potential to enable next generation manufacturing and biomedical applications where a spectrally discrete, low energy, and thus benign light source is required.

Applications of red light photoredox catalysis in organic synthesis

Photoredox catalysis has emerged as an efficient and versatile approach for developing novel synthetic methodologies. Particularly, red light photocatalysis has attracted more attention due to its intrinsic advantages of low energy, few health risks, few side reactions, and high penetration depth through various media. Impressive progress has been made in this field. In this review, we outline the applications of different photoredox catalysts in a wide range of red light-mediated reactions including direct red light photoredox catalysis, red light photoredox catalysis through upconversion, and dual red light photoredox catalysis. Due to the similarities between near-infrared (NIR) and red light, an overview of NIR-induced reactions is also presented. Lastly, current evidence showing the advantages of red light and NIR photoredox catalysis is also described.

Recent Advances on Furan-Based Visible Light Photoinitiators of Polymerization

Photopolymerization is an active research field enabling to polymerize in greener conditions than that performed with traditional thermal polymerization. At present, a great deal of effort is devoted to developing visible light photoinitiating systems. Indeed, the traditional UV photoinitiating systems are currently the focus of numerous safety concerns so alternatives to UV light are being actively researched. However, visible light photons are less energetic than UV photons so the reactivity of the photoinitiating systems should be improved to address this issue. In this field, furane constitutes an interesting candidate for the design of photocatalysts of polymerization due to its low cost and its easy chemical modification. In this review, an overview concerning the design of furane-based photoinitiators is provided. Comparisons with reference systems are also established to demonstrate evidence of the interest of these photoinitiators in innovative structures.

Squaraines as near-infrared photocatalysts for organic reactions

Herein, unprecedented uses of squaraine derivatives as new organic near-infrared photocatalysts are reported. These efficient molecular tools are able to promote oxidation and reduction for organic transformations through photocatalytic conditions. A mechanistic investigation is performed to distinguish between competitive Single Electron Transfer and Energy Transfer pathways.

Efficacy analysis of new copper complex for visible light (455, 530 nm) radical/cationic photopolymerization: The synergic effects and catalytic cycle

The kinetics and the conversion features of two 3-component systems (A/B/N), based on the proposed new kinetic schemes of Mokbel and Mau et al, in which a visible LED is used to excite a copper complex to its excited triplet state (G*). The coupling of G* with iodonium salt and ethyl 4-(dimethylamino)benzoate (EDB) produces both free radical polymerization (FRP) of acrylates and the free radical promoted cationic polymerization (CP) of epoxides using various new copper complex as the initiator. Higher FRP and CP conversion can be achieved by co-additive of [B] and N, via the dual function of (i) regeneration [A], and (ii) generation of extra radicals. The interpenetrated polymer network (IPN) capable of initiating both FRP and CP in a blend of TMPTA and EPOX. The synergic effects due to CP include: (i) CP can increase viscosity limiting the diffusional oxygen replenishment; (ii) the cation also acts as a diluting agent for the IPN network, and (iii) the exothermic property of the CP. The catalytic cycle, synergic effects, and the oxygen inhibition are theoretically confirmed to support the experimental hypothesis. The measured results of Mokbel and Mau et al are well analyzed and matching the predicted features of our modeling.

Photocatalyzed decarboxylation of oxamic acids under near-infrared conditions

Photocatalyzed oxidative decarboxylation of oxamic acids under near-infrared irradiation using Os(bptpy)₂(PF₆)₂ as catalyst is reported. The reaction was applied to the synthesis of urethanes and heterocyclic amides. Mechanistic studies and comparative penetration depths between the NIR and the visible light mediated processes are discussed.

RAFT Polymerization of Semifluorinated Monomers Mediated by a NIR Fluorinated Photocatalyst

Near-infrared (NIR) light plays an increasingly important role in the field of photoinduced electron/energy transfer-reversible addition-fragmentation chain transfer (PET-RAFT) polymerization due to its unique properties. Yet, the NIR photocatalyst with good stability for PET-RAFT polymerization remains promising. Here, a strategy of NIR PET-RAFT polymerization of semifluorinated monomers using fluorophenyl bacteriochlorin as a photocatalyst with strong absorption at the NIR light region (710-780 nm) is reported. In which, the F atoms are used to modify reduced tetraphenylporphyrin structure with enhanced photostability of photocatalyst. Under the irradiation of NIR light (λ_max = 740 nm), the PET-RAFT polymerization of semifluorinated methylacrylic monomers presents living/control characteristics and temporal modulation. By the PET-RAFT polymerization-induced self-assembly (PISA) strategy, stable fluorine-containing micelles are constructed in various solvents. In addition, the fluorinated hydrophobic surface is fabricated via a surface-initiated PET-RAFT (SI-PET-RAFT) polymerization using silicon wafer bearing RAFT agents with tunable surface hydrophobicity. This strategy not only enlightens the application of further modified compounds based on porphyrin structure in photopolymerization, but also shows promising potential for the construction of well-defined functional fluoropolymers.

"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.

Bio-based porphyrins pyropheophorbide a and its Zn-complex as performing visible-light photosensitizers for free-radical photopolymerization

A chlorophyll a derivative, namely pyropheophorbide a (Pyro), and the corresponding zinc (II) complex (Zn-Pyro) were used for the first time as performing visible-light photosensitizers (PS) for free-radical photopolymerization (FRP)...

Rational Selection of Cyanines to Generate Conjugate Acid and Free Radicals for Photopolymerization upon Exposure at 860 nm

Different cyanines absorbing in the NIR between 750 and 930 nm were applied to study the efficiency of both radical and cationic polymerization in combination with diaryliodonium salt. Variation of the connecting methine chain and structure of the terminal indolium moiety provided a deeper insight in the structure of the cyanine NIR‐sensitizer and the efficiency to generate initiating radicals and conjugate acid. Photophysical studies were pursued by fluorescence spectroscopy providing a deeper understanding regarding the lifetime of the excited state and contribution of nonradiative deactivation resulting in generation of additional heat in the polymerization process. Furthermore, electrochemical experiments demonstrated connection to oxidation and reduction capability as influenced by the structural pattern of the sensitizer. LC–MS measurements provided a deeper pattern about the photoproducts formed. A nonamethine‐based cyanine showed the best performance regarding bleaching in combination with an iodonium salt at 860 nm.

Azacalixphyrins as an innovative alternative for the free-radical photopolymerization under visible and NIR irradiation without the need of co-initiators

Azacalixphyrins are unique aromatic macrocycles featuring strong absorption from the visible to the near-infrared (NIR) spectral ranges. This work demonstrates through EPR spin-trapping experiments that the N-alkyl tetrasubstituted azacalixphyrin (ACP) can lead to the formation of carbon-centered radicals initiating for the free-radical photopolymerization (FRP) of bio-based acrylate monomer upon the irradiation of several light emitting diodes, which emissions range from 455 to 660 nm. Compared to other previously reported systems, the tremendous advantage of the ACP photoinitiating system is its ability to promote photopolymerization on its own, avoiding the introduction of co-initiators. A new potential application of this promising photoinitiator is highlighted through the fabrication of well-defined microstructures under NIR laser diode irradiation at λ = 800 nm.

Towards new NIR dyes for free radical photopolymerization processes

The use of cheap and safe near-infrared (NIR) light is still the subject of intense research efforts but remains a huge challenge due to the associated low photon energy (wavelength from 0.78 to 2.5 µm). In this study, a series of 17 NIR dyes mainly based on a well-established cyanine scaffold is proposed. Remarkably, 11 of them were never synthesized before. Markedly, noncharged structures, negatively charged cyanine bearing Na⁺ as counter cation, and positively charged cyanines bearing (B(Ph)₄⁻) or (I⁻) as counter anions were examined as promising NIR light photoinitiating systems. Excellent photoinitiating abilities were found for some reported dyes when used in combination with iodonium salt and amine. Markedly, photothermal effects with a huge heater behavior were also observed for different NIR dye structures. Interestingly, the synthesis of interpenetrating polymer networks (IPNs, e.g., for the polymerization of acrylate/epoxy monomer blends) can also be carried out upon NIR light with the proposed systems.

A Critical Review for Synergic Kinetics and Strategies for Enhanced Photopolymerizations for 3D-Printing and Additive Manufacturing

The synergic features and enhancing strategies for various photopolymerization systems are reviewed by kinetic schemes and the associated measurements. The important topics include (i) photo crosslinking of corneas for the treatment of corneal diseases using UVA-light (365 nm) light and riboflavin as the photosensitizer; (ii) synergic effects by a dual-function enhancer in a three-initiator system; (iii) synergic effects by a three-initiator C/B/A system, with electron-transfer and oxygen-mediated energy-transfer pathways; (iv) copper-complex (G1) photoredox catalyst in G1/Iod/NVK systems for free radical (FRP) and cationic photopolymerization (CP); (v) radical-mediated thiol-ene (TE) photopolymerizations; (vi) superbase photogenerator based-catalyzed thiol-acrylate Michael (TM) addition reaction; and the combined system of TE and TM using dual wavelength; (vii) dual-wavelength (UV and blue) controlled photopolymerization confinement (PC); (viii) dual-wavelength (UV and red) selectively controlled 3D printing; and (ix) three-wavelength selectively controlled in 3D printing and additive manufacturing (AM). With minimum mathematics, we present (for the first time) the synergic features and enhancing strategies for various systems of multi-components, initiators, monomers, and under one-, two-, and three-wavelength light. Therefore, this review provides not only the bridging between modeling and measurements, but also guidance for further experimental studies and new applications in 3D printings and additive manufacturing (AM), based on the innovative concepts (kinetics/schemes).

The Rise of Silicon Phthalocyanine: From Organic Photovoltaics to Organic Thin Film Transistors

Silicon phthalocyanines are emerging n-type semiconductors for use in organic photovoltaics (OPVs) and organic thin-film transistors (OTFTs). Their low synthetic complexity paired with their versatile axial group facilitates the fine-tuning of their chemical properties, solution properties and processing characteristics without significantly affecting their frontier orbital levels or their absorption properties. The crystal engineering and film forming characteristics of silicon phthalocyanine semiconductors can be tuned through appropriate axial group functionalization, therefore facilitating their integration into both OTFTs and OPVs by solution processing or vapor deposition. This Spotlight on Applications will discuss recent advances in the integration of this exciting class of phthalocyanine into OTFTs and OPVs and highlights their promising future.

Recent Advances in bis-Chalcone-Based Photoinitiators of Polymerization: From Mechanistic Investigations to Applications

Over the past several decades, photopolymerization has become an active research field, and the ongoing efforts to develop new photoinitiating systems are supported by the different applications in which this polymerization technique is involved-including dentistry, 3D and 4D printing, adhesives, and laser writing. In the search for new structures, bis-chalcones that combine two chalcones' moieties within a unique structure were determined as being promising photosensitizers to initiate both the free-radical polymerization of acrylates and the cationic polymerization of epoxides. In this review, an overview of the different bis-chalcones reported to date is provided. Parallel to the mechanistic investigations aiming at elucidating the polymerization mechanisms, bis-chalcones-based photoinitiating systems were used for different applications, which are detailed in this review.

Cyanine-based near infra-red organic photoredox catalysis

Direct metal-free near infra-red photoredox catalysis is applied to organic oxidation, photosensitization and reduction, involving cyanines as photocatalysts. This photocatalyst is competitive with conventional reactions catalyzed under visible light. Kinetic and quenching experiments are also reported. Interestingly, these systems are compatible with water media, opening perspective for various applications.