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.

Red-Light-Driven Bifunctionalization of Styrene Derivatives

A red-light-activated phthalocyanine ruthenium complex has been designed as a catalyst for the bifunctionalization of styrene derivatives. The combination of a trifluoromethylation agent resistant to nucleophiles and various nucleophiles facilitates the concurrent incorporation of a trifluoromethyl group and various functional groups onto the double bond of the substrate. This reaction demonstrates the utility of mild, low-energy, and highly transmissive long-wavelength light for intricate molecular transformations in a one-pot procedure.

Near-Infrared Light-Driven Photocatalysis with an Emphasis on Two-Photon Excitation: Concepts, Materials, and Applications

Efficient utilization of sunlight in photocatalysis is widely recognized as a promising solution for addressing the growing energy demand and environmental issues resulting from fossil fuel consumption. Recently, there have been significant developments in various near-infrared (NIR) light-harvesting systems for artificial photosynthesis and photocatalytic environmental remediation. This review provides an overview of the most recent advancements in the utilization of NIR light through the creation of novel nanostructured materials and molecular photosensitizers, as well as modulating strategies to enhance the photocatalytic processes. A special focus is given to the emerging two-photon excitation NIR photocatalysis. The unique features and limitations of different systems are critically evaluated. In particular, it highlights the advantages of utilizing NIR light and two-photon excitation compared to UV-visible irradiation and one-photon excitation. Ongoing challenges and potential solutions for the future exploration of NIR light-responsive materials are also discussed.

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.

Near-Infrared Light and Acid/Base Dual-Regulated Polymerization Utilizing Imidazole-Anion-Fused Perylene Diimides as Photocatalysts

This work presents the first example of acid/base-responsive and near-infrared (NIR)-absorbing photocatalysts based on imidazole-anion-fused perylene diimide chromophores. The photocatalysts were in situ generated by deprotonation of imidazole-fused perylene diimide under an alkaline environment. NIR (λ = 730 nm, 128 mW/cm2) photoinduced atom transfer radical polymerization (ATRP) was implemented, exhibiting high efficiency and excellent livingness under ppm level of photocatalysts (15 ppm relative to monomer) and Cu(II) complex (10 ppm relative to monomer) concentrations. The method showed capabilities to polymerize behind opaque barriers (i.e., paper and pig skin) and under aerobic condition. Notably, this work demonstrated a dual temporal control of polymerization by adding weak base/acid and switching NIR light on/off. The polymerization can even be halted by bubbling CO2 and was then fully recovered by adding triethylamine. The NIR photoATRP of acrylamide monomers in aqueous solution was also performed, which can be regulated by the change of pH.

Unlocking the reactivity of diazo compounds in red light with the use of photochemical tools

Structurally diversified diazoalkanes can be activated under red light irradiation relying on direct photolysis, photosensitization or photoredox catalysis.

Electronic Structures and Photoredox Chemistry of Tungsten(0) Arylisocyanides

ConspectusThe high energy barriers associated with the reaction chemistry of inert substrates can be overcome by employing redox-active photocatalysts. Research in this area has grown exponentially over the past decade, as transition metal photosensitizers have been shown to mediate challenging organic transformations. Critical for the advancement of photoredox catalysis is the discovery, development, and study of complexes based on earth-abundant metals that can replace and/or complement established noble-metal-based photosensitizers.Recent work has focused on redox-active complexes of 3d metals, as photosensitizers containing these metals most likely would be scalable. Although low lying spin doublet ("spin flip") excited states of chromium(III) and metal-to-ligand charge transfer (MLCT) excited states of copper(I) have relatively long lifetimes, the electronic excited states of many other 3d metal complexes fall on dissociative potential energy surfaces, owing to the population of highly energetic σ-antibonding orbitals. Indeed, we and other investigators have shown that low lying spin singlet and triplet excited states of robust closed-shell metal complexes are too short-lived at room temperature to engage in bimolecular reactions in solutions. In principle, this problem could be overcome by designing and constructing 3d metal complexes containing strong field π-acceptor ligands, where thermally equilibrated MLCT or intraligand charge transfer excited states might fall well below the upper surfaces of dissociative 3d-3d states. Notably, such design elements have been exploited by investigators in very recent work on redox-active iron(II) systems. Another approach, one we have actively pursued, is to design and construct closed-shell complexes of earth-abundant 5d metals containing very strong π-acceptor ligands, where vertical excitation of 5d-5d excited states at the ground state geometry would require energies far above minima in the potential surfaces of MLCT excited states. As this requirement is met by tungsten(0) arylisocyanides, these complexes have been the focus of our work aimed at the development of robust redox-active photosensitizers.In the following Account, we review recent work on homoleptic tungsten(0) arylisocyanides. Originally reported by our group 45 years ago, W(CNAr)₆ complexes have exceptionally large one- and two-photon absorption cross-sections. One- or two-photon excitation produces relatively long-lived (hundreds of nanoseconds to microsecond) MLCT excited states in high yields. These MLCT excited states, which are very strong reductants with E°(W⁺/*W⁰) = -2.2 to -3.0 V vs Fc^[+/0], mediate photocatalysis of organic reactions with both visible and near-infrared (NIR) light. Here, we highlight design principles that led to the development of three generations of W(CNAr)₆ photosensitizers; and we discuss likely steps in the mechanism of a prototypal W(CNAr)₆-catalyzed base-promoted homolytic aromatic substitution reaction. Among the many potential applications of these very bright luminophores, two-photon imaging and two-photon-initiated polymerization are ones we plan to pursue.

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.

Sensitizer-controlled photochemical reactivity via upconversion of red light

By combining the energy input from two red photons, chemical reactions that would normally require blue or ultraviolet irradiation become accessible. Key advantages of this biphotonic excitation strategy are that red light usually penetrates deeper into complex reaction mixtures and causes less photo-damage than direct illumination in the blue or ultraviolet. Here, we demonstrate that the primary light-absorber of a dual photocatalytic system comprised of a transition metal-based photosensitizer and an organic co-catalyst can completely alter the reaction outcome. Photochemical reductions are achieved with a copper(i) complex in the presence of a sacrificial electron donor, whereas oxidative substrate activation occurs with an osmium(ii) photosensitizer. Based on time-resolved laser spectroscopy, this changeover in photochemical reactivity is due to different underlying biphotonic mechanisms. Following triplet energy transfer from the osmium(ii) photosensitizer to 9,10-dicyanoanthracene (DCA) and subsequent triplet-triplet annihilation upconversion, the fluorescent singlet excited state of DCA triggers oxidative substrate activation, which initiates the cis to trans isomerization of an olefin, a [2 + 2] cycloaddition, an aryl ether to ester rearrangement, and a Newman-Kwart rearrangement. This oxidative substrate activation stands in contrast to the reactivity with a copper(i) photosensitizer, where photoinduced electron transfer generates the DCA radical anion, which upon further excitation triggers reductive dehalogenations and detosylations. Our study provides the proof-of-concept for controlling the outcome of a red-light driven biphotonic reaction by altering the photosensitizer, and this seems relevant in the greater context of tailoring photochemical reactivities.

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.

Visible-Light-Induced Cascade Cyclization of 3-(2-(Ethynyl)phenyl)quinazolinones to Phosphorylated Quinolino[2,1-b]quinazolinones

3-(2-(Ethynyl)phenyl)quinazolinones were designed and synthesized as a class of novel and efficient skeletons for phosphorylation/cyclization reactions. Under visible light irradiation, a series of phosphorylated quinolino[2,1-b]quinazolinones (35 examples, up to 87% yield) were first synthesized from 3-(2-(ethynyl)phenyl)quinazolinones and diarylphosphine oxides by using 4CzIPN as a photocatalyst under mild conditions. This reaction was also applicable under sunlight irradiation. Moreover, the reaction efficiency could be significantly improved under continuous-flow conditions.

Recent Advances in the Photoreactions Triggered by Porphyrin-Based Triplet–Triplet Annihilation Upconversion Systems: Molecular Innovations and Nanoarchitectonics

Triplet–triplet annihilation upconversion (TTA-UC) is a very promising technology that could be used to convert low-energy photons to high-energy ones and has been proven to be of great value in various areas. Porphyrins have the characteristics of high molar absorbance, can form a complex with different metal ions and a high proportion of triplet states as well as tunable structures, and thus they are important sensitizers for TTA-UC. Porphyrin-based TTA-UC plays a pivotal role in the TTA-UC systems and has been widely used in many fields such as solar cells, sensing and circularly polarized luminescence. In recent years, applications of porphyrin-based TTA-UC systems for photoinduced reactions have emerged, but have been paid little attention. As a consequence, this review paid close attention to the recent advances in the photoreactions triggered by porphyrin-based TTA-UC systems. First of all, the photochemistry of porphyrin-based TTA-UC for chemical transformations, such as photoisomerization, photocatalytic synthesis, photopolymerization, photodegradation and photochemical/photoelectrochemical water splitting, was discussed in detail, which revealed the different mechanisms of TTA-UC and methods with which to carry out reasonable molecular innovations and nanoarchitectonics to solve the existing problems in practical application. Subsequently, photoreactions driven by porphyrin-based TTA-UC for biomedical applications were demonstrated. Finally, the future developments of porphyrin-based TTA-UC systems for photoreactions were briefly discussed.

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.

Red Light-Based Dual Photoredox Strategy Resembling the Z-Scheme of Natural Photosynthesis

Photoredox catalysis typically relies on the use of single chromophores, whereas strategies, in which two different light absorbers are combined, are rare. In photosystems I and II of green plants, the two separate chromophores P₆₈₀ and P₇₀₀ both absorb light independently of one another, and then their excitation energy is combined in the so-called Z-scheme, to drive an overall reaction that is thermodynamically very demanding. Here, we adapt this concept to perform photoredox reactions on organic substrates with the combined energy input of two red photons instead of blue or UV light. Specifically, a Cu^I bis(α-diimine) complex in combination with in situ formed 9,10-dicyanoanthracenyl radical anion in the presence of excess diisopropylethylamine catalyzes ca. 50 dehalogenation and detosylation reactions. This dual photoredox approach seems useful because red light is less damaging and has a greater penetration depth than blue or UV radiation. UV-vis transient absorption spectroscopy reveals that the subtle change in solvent from acetonitrile to acetone induces a changeover in the reaction mechanism, involving either a dominant photoinduced electron transfer or a dominant triplet-triplet energy transfer pathway. Our study illustrates the mechanistic complexity in systems operating under multiphotonic excitation conditions, and it provides insights into how the competition between desirable and unwanted reaction steps can become more controllable.

Direct Utilization of Near-Infrared Light for Photooxidation with a Metal-Free Photocatalyst

Near-infrared (NIR) light-triggered photoredox catalysis is highly desirable because NIR light occupies almost 50% of solar energy and possesses excellent penetrating power in various media. Herein we utilize a metal-free boron dipyrromethene (BODIPY) derivative as the photocatalyst to achieve NIR light (720 nm LED)–driven oxidation of benzylamine derivatives, sulfides, and aryl boronic acids. Compared to blue light–driven photooxidation using Ru(bpy)₃Cl₂ as a photocatalyst, NIR light–driven photooxidation exhibited solvent independence and superior performance in large-volume (20 mL) reaction, presumably thanks to the neutral structure of a BODIPY photocatalyst and the deeper penetration depth of NIR light. We further demonstrate the application of this metal-free NIR photooxidation to prodrug activation and combination with Cu-catalysis for cross coupling reaction, exhibiting the potential of metal-free NIR photooxidation as a toolbox for organic synthesis and drug development.

Bioinspired Supercharging of Photoredox Catalysis for Applications in Energy and Chemical Manufacturing

ConspectusFor more than a decade, photoredox catalysis has been demonstrating that when photoactive catalysts are irradiated with visible light, reactions occur under milder, cheaper, and environmentally friendlier conditions. Furthermore, this methodology allows for the activation of abundant chemicals into valuable products through novel mechanisms that are otherwise inaccessible. The photoredox approach, however, has been primarily used for pharmaceutical applications, where its implementation has been highly effective, but typically with a more rudimentary understanding of the mechanisms involved in these transformations. From a global perspective, the manufacture of everyday chemicals by the chemical industry as a whole currently accounts for 10% of total global energy consumption and generates 7% of the world's greenhouse gases annually. In this context, the Bio-Inspired Light-Escalated Chemistry (BioLEC) Energy Frontier Research Center (EFRC) was founded to supercharge the photoredox approach for applications in chemical manufacturing aimed at reducing its energy consumption and emissions burden, by using bioinspired schemes to harvest multiple electrons to drive endothermically uphill chemical reactions. The Center comprises a diverse group of researchers with expertise that includes synthetic chemistry, biophysics, physical chemistry, and engineering. The team works together to gain a deeper understanding of the mechanistic details of photoredox reactions while amplifying the applications of these light-driven methodologies.In this Account, we review some of the major advances in understanding, approach, and applicability made possible by this collaborative Center. Combining sophisticated spectroscopic tools and photophysics tactics with enhanced photoredox reactions has led to the development of novel techniques and reactivities that greatly expand the field and its capabilities. The Account is intended to highlight how the interplay between disciplines can have a major impact and facilitate the advance of the field. For example, techniques such as time-resolved dielectric loss (TRDL) and pulse radiolysis are providing mechanistic insights not previously available. Hypothesis-driven photocatalyst design thus led to broadening of the scope of several existing transformations. Moreover, bioconjugation approaches and the implementation of triplet-triplet annihilation mechanisms created new avenues for the exploration of reactivities. Lastly, our multidisciplinary approach to tackling real-world problems has inspired the development of efficient methods for the depolymerization of lignin and artificial polymers.

State of knowledge in photoredox-catalysed direct difluoromethylation

The combination of visible light photoredox catalysis with direct difluoromethylation has allowed the synthesis of a large choice of CF2H-containing value-added molecules under very mild reaction conditions.

Exploiting the upconversion luminescence, Lewis acid catalytic and photothermal properties of lanthanide-based nanomaterials for chemical and polymerization reactions

Lanthanide-based nanocrystals possess three unique physical properties that make them attractive for facilitating photoreactions, namely photon upconversion, Lewis acid catalytic activity and photothermal effect. When co-doped with suitable sensitizer and...