Maximizing Photon-to-Electron Conversion for Atom Efficient Photoredox Catalysis.

Photoredox catalysis is a powerful tool to access challenging and diverse syntheses. Absorption of visible light forms the excited state catalyst (*PC) but photons may be wasted if one of several unproductive pathways occur. Facile dissociation of the charge-separated encounter complex [PC:D], also known as (solvent) cage escape, is required for productive chemistry and directly governs availability of the critical PC intermediate.

Electrochemiluminescence Enhanced by a Non-Emissive Dual Redox Mediator.

A sulfonated tris(1-phenylpyrazolato)iridium(III) complex ([Ir(sppz)]) serves as a proof-of-concept non-emissive enhancer of the widely used ECL detection system of tris(2,2'-bipyridine)ruthenium(II) ([Ru(bpy)]) with tri-n-propylamine (TPrA) co-reactant, acting through electrocatalysis of TPrA oxidation and efficient chemi-excitation of the luminophore. Using self-interference ECL spectroscopy, we show that the enhancer extends diffusion of the required electrogenerated precursors from the electrode surface. Previously reported enhancement through these pathways has been confounded by the inherent ECL of the enhancer, but the increase in [Ru(bpy)] ECL intensity using [Ir(sppz)] was obtained without its concomitant emission.

Chemiluminescence and electrochemiluminescence of water-soluble iridium(III) complexes containing a tetraethylene-glycol functionalised triazolylpyridine ligand.

Background: Iridium(III) complexes, exhibiting high luminescence quantum yields and a wide range of emission colours, are promising alternatives to tris(2,2'-bipyridine)ruthenium(II) for chemiluminescence (CL) and electrochemiluminescence (ECL) detection. This emerging class of reagent, however, is limited by the poor solubility of many iridium(III) complexes in aqueous solution, and lack of understanding of their remarkably variable selectivities towards different analytes.

Results: Seven [Ir(C^N)(pt-TEG)] complexes, exhibiting a wide range of reduction potentials and emission energies, were examined with six model analytes.

Electrochemiluminescence of a First-Row d Transition Metal Complex.

We report the electrochemiluminescence (ECL) of a 3d Cr(0) complex ([Cr(L)]; λ=735 nm) with comparable photophysical properties to those of ECL-active complexes of 4d or 5d precious metal ions. The electrochemical potentials of [Cr(L)] are more negative than those of [Ir(ppy)] and render the [Cr(L)]* excited state inaccessible through conventional co-reactant ECL with tri-n-propylamine or oxalate. ECL can be obtained, however, through the annihilation route in which potentials sufficient to oxidise and reduce the luminophore are alternately applied.

Photochemical Povarov-type Reactions: Electron Donor-Acceptor Photoactivation by Visible Light.

This report investigates the mechanism of photochemical Povarov-type reactions of ,-dialkylanilines and maleimides in polar solvents (DMF or dioxane) in the presence of light. Fundamental aspects of the electron donor-acceptor (EDA) photoactivation pathway proposed to underpin this chemistry are examined through integrated experimental and computational studies. This approach provided evidence supporting the involvement of an EDA complex in facilitating this chemistry via a reaction mechanism that does not involve a triplet manifold.

Photoinitiated Energy Transfer in Porous-Cage-Stabilised Silver Nanoparticles.

We report a new composite material consisting of silver nanoparticles decorated with three-dimensional molecular organic cages based on light-absorbing porphyrins. The porphyrin cages serve to both stabilize the particles and allow diffusion and trapping of small molecules close to the metallic surface. Combining these two photoactive components results in a Fano-resonant interaction between the porphyrin Soret band and the nanoparticle-localised surface-plasmon resonance.

Optimal Geometry for Plasmonic Hot-Carrier Extraction in Metal-Semiconductor Nanocrystals.

Plasmon-induced energy and charge transfer from metal nanostructures hold great potential for harvesting solar energy. Presently, the efficiencies of charge-carrier extraction are still low due to the competitive ultrafast mechanisms of plasmon relaxation. Using single-particle electron energy loss spectroscopy, we correlate the geometrical and compositional details of individual nanostructures to their carrier extraction efficiencies.

Extending Photocatalyst Activity through Choice of Electron Donor.

Sacrificial additives are commonly employed in photoredox catalysis as a convenient source of electrons, but what occurs after electron transfer is often overlooked. Tertiary alkylamines initially form radical cations following electron transfer, which readily deprotonate to form strongly reducing, neutral α-amino radicals. Similarly, the oxalate radical anion (CO) rapidly decomposes to form CO ( ≈ -2.

The forgotten reagent of photoredox catalysis.

Visible light powers an ever-expanding suite of reactions to both make and break chemical bonds under otherwise mild conditions. As a reagent in photochemical synthesis, light is obviously critical for reactivity but rarely optimized other than in light/dark controls. This Frontier Article presents an overview of recent research that investigates the unique ways light may be manipulated, and its unusual interactions with homogeneous transition metal and organic photocatalysts.

Reinterpreting the Fate of Iridium(III) Photocatalysts─Screening a Combinatorial Library to Explore Light-Driven Side-Reactions.

Photoredox catalysts are primarily selected based on ground and excited state properties, but their activity is also intrinsically tied to the nature of their reduced (or oxidized) intermediates. Catalyst reactivity often necessitates an inherent instability, thus these intermediates represent a mechanistic turning point that affords either product formation or side-reactions. In this work, we explore the scope of a previously demonstrated side-reaction that partially saturates one pyridine ring of the ancillary ligand in heteroleptic iridium(III) complexes.

Understanding Ir(III) Photocatalyst Structure-Activity Relationships: A Highly Parallelized Study of Light-Driven Metal Reduction Processes.

High-throughput synthesis and screening methods were used to measure the photochemical activity of 1440 distinct heteroleptic [Ir(C^N)(N^N)] complexes for the photoreduction of Sn(II) and Zn(II) cations to their corresponding neutral metals. Kinetic data collection was carried out using home-built photoreactors and measured initial rates, obtained through an automated fitting algorithm, spanned between 0-120 μM/s for Sn(0) deposition and 0-90 μM/s for Zn(0) deposition. Photochemical reactivity was compared to photophysical properties previously measured such as deaerated excited state lifetime and emission spectral data for these same complexes; however, no clear correlations among these features were observed.

High-Throughput Screening and Automated Data-Driven Analysis of the Triplet Photophysical Properties of Structurally Diverse, Heteroleptic Iridium(III) Complexes.

Steady state emission spectra and excited state lifetimes were measured for 1440 distinct heteroleptic [Ir(C^N)(N^N)] complexes prepared via combinatorial parallelized synthesis; 72% of the complexes were found to be luminescent, and the emission maxima of the library spanned the visible spectrum (652-459 nm). Spectral profiles ranged from broad structureless bands to narrow emissions exhibiting vibrational substructure. Measured excited state lifetimes ranged between ∼0.

High-Throughput Screening of Earth-Abundant Water Reduction Catalysts toward Photocatalytic Hydrogen Evolution.

Noble-metal photosensitizers and water reduction co-catalysts (WRCs) still present the highest activity in homogeneous photocatalytic hydrogen production. The search for earth-abundant alternatives is usually limited by the time required to screen new catalyst combinations; however, here, we utilize newly designed and developed high-throughput photoreactors for the parallel synthesis of novel WRCs and colorimetric screening of hydrogen evolution. This unique approach allowed rapid optimization of photocatalytic water reduction using the organic photosensitizer Eosin Y and the archetypal cobaloxime WRC [Co()pyCl], where is dimethylglyoxime and py is pyridine.

Self-Assembly of Plasmonic Near-Perfect Absorbers of Light: The Effect of Particle Size.

Structures capable of perfect light absorption promise technological advancements in varied applications, including sensing, optoelectronics, and photocatalysis. While it is possible to realize such structures by placing a monolayer of metal nanostructures above a reflecting surface, there remains limited studies on what effect particle size plays on their capacity to absorb light. Here, we fabricate near-perfect absorbers using colloidal Au nanoparticles, via their electrostatic self-assembly on a TiO film supported by a gold mirror.

Tandem Photoredox Catalysis: Enabling Carbonylative Amidation of Aryl and Alkylhalides.

We report a new visible-light-mediated carbonylative amidation of aryl, heteroaryl, and alkyl halides. A tandem catalytic cycle of [Ir(ppy) (dtb-bpy)] generates a potent iridium photoreductant through a second catalytic cycle in the presence of DIPEA, which productively engages aryl bromides, iodides, and even chlorides as well as primary, secondary, and tertiary alkyl iodides. The versatile in situ generated catalyst is compatible with aliphatic and aromatic amines, shows high functional-group tolerance, and enables the late-stage amidation of complex natural products.

Photoexcited Pd(ii) auxiliaries enable light-induced control in C(sp)-H bond functionalisation.

Herein we report the photophysical and photochemical properties of palladacycle complexes derived from 8-aminoquinoline ligands, commonly used auxiliaries in C-H activation. Spectroscopic, electrochemical and computational studies reveal that visible light irradiation induces a mixed LLCT/MLCT charge transfer providing access to synthetically relevant Pd(iii)/Pd(iv) redox couples. The Pd(ii) complex undergoes photoinduced electron transfer with alkyl halides generating C(sp)-H halogenation products rather than C-C bond adducts.

A conceptual framework for the development of iridium(iii) complex-based electrogenerated chemiluminescence labels.

Translation of the highly promising electrogenerated chemiluminescence (ECL) properties of Ir(iii) complexes (with tri--propylamine (TPrA) as a co-reactant) into a new generation of ECL labels for ligand binding assays necessitates the introduction of functionality suitable for bioconjugation. Modification of the ligands, however, can affect not only the photophysical and electrochemical properties of the complex, but also the reaction pathways available to generate light. Through a combined theoretical and experimental study, we reveal the limitations of conventional approaches to the design of electrochemiluminophores and introduce a new class of ECL label, [Ir(C^N)(pt-TOxT-Sq)] (where C^N is a range of possible cyclometalating ligands, and pt-TOxT-Sq is a pyridyltriazole ligand with trioxatridecane chain and squarate amide ethyl ester), which outperformed commercial Ir(iii) complex labels in two commonly used assay formats.

The Tandem Photoredox Catalysis Mechanism of [Ir(ppy)(dtb-bpy)] Enabling Access to Energy Demanding Organic Substrates.

We report the discovery of a tandem catalytic process to reduce energy demanding substrates, using the [Ir(ppy)(dtb-bpy)] () photocatalyst. The immediate products of photoinitiated electron transfer (PET) between and triethylamine (TEA) undergo subsequent reactions to generate a previously unknown, highly reducing species (). Formation of occurs via reduction and semisaturation of the ancillary dtb-bpy ligand, where the TEA radical cation serves as an effective hydrogen atom donor, confirmed by nuclear magnetic resonance, mass spectrometry, and deuterium labeling experiments.

Site-Specific Glycation and Chemo-enzymatic Antibody Sortagging for the Retargeting of rAAV6 to Inflamed Endothelium.

Gene therapy holds great potential for conditions such as cardiovascular disease, including atherosclerosis and also vascular cancers, yet available vectors such as the adeno-associated virus (rAAV) transduce the vasculature poorly. To enable retargeting, a single-chain antibody (scFv) that binds to the vascular cell-adhesion molecule (VCAM-1) overexpressed at areas of endothelial inflammation was site specifically and covalently conjugated to the exterior of rAAV6. To achieve conjugation, the scFv was functionalized with an orthogonal click chemistry group.

Mixed annihilation electrogenerated chemiluminescence of iridium(iii) complexes.

Previously reported annihilation ECL of mixtures of metal complexes have generally comprised Ir(ppy)3 or a close analogue as a higher energy donor/emitter (green/blue light) and [Ru(bpy)3]2+ or its derivative as a lower energy acceptor/emitter (red light). In contrast, here we examine Ir(ppy)3 as the lower energy acceptor/emitter, by combining it with a second Ir(iii) complex: [Ir(df-ppy)2(ptb)]+ (where ptb = 1-benzyl-1,2,3-triazol-4-ylpyridine). The application of potentials sufficient to attain the first single-electron oxidation and reduction products can be exploited to detect Ir(ppy)3 at orders of magnitude lower concentration, or enhance its maximum emission intensity at high concentration far beyond that achievable through conventional annihilation ECL of Ir(ppy)3 involving comproportionation.

Photocatalytic and Chemoselective Transfer Hydrogenation of Diarylimines in Batch and Continuous Flow.

A visible-light photocalytic method for the chemoselective transfer hydrogenation of imines in batch and continuous flow is described. The reaction utilizes EtN as both hydrogen source and single-electron donor, enabling the selective reduction of imines derived from diarylketimines containing other reducible functional groups including nitriles, halides, esters, and ketones. The dual role of EtN was confirmed by fluorescence quenching measurements, transient absorption spectroscopy, and deuterium-labeling studies.

Luminescence of a Transition Metal Complex Inside a Metamaterial Nanocavity.

Modification of the local density of optical states using metallic nanostructures leads to enhancement in the number of emitted quanta and photocatalytic turnover of luminescent materials. In this work, the fabrication of a metamaterial is presented that consists of a nanowire separated from a metallic mirror by a polymer thin film doped with a luminescent organometallic iridium(III) complex. The large spin-orbit coupling of the heavy metal atom results in an excited state with significant magnetic-dipole character.

The efficient synthesis and purification of amyloid-β(1-42) using an oligoethylene glycol-containing photocleavable lysine tag.

A short, monodisperse oligoethylene glycol-containing photocleavable lysine tag was developed to facilitate the efficient purification of hydrophobic and fibril-forming peptides. This new tag was used to prepare a modified Aβ peptide with increased solubility and decreased propensity to aggregate in aqueous media. The solubilising tag was readily removed by irradiation with UV light and permitted the preparation and isolation of Aβ in high purity and yield.

Luminescent Iridium(III) Cyclometalated Complexes with 1,2,3-Triazole "Click" Ligands.

A series of cyclometalated iridium(III) complexes with either 4-(2-pyridyl)-1,2,3-triazole or 1-(2-picolyl)-1,2,3-triazole ancillary ligands to give complexes with either 5- or 6-membered chelate rings were synthesized and characterized by a combination of X-ray crystallography, electron spin ionization-high-resolution mass spectroscopy (ESI-HRMS), and nuclear magnetic resonance (NMR) spectroscopy. The electronic properties of the complexes were probed using absorption and emission spectroscopy, as well as cyclic voltammetry. The relative stability of the complexes formed from each ligand class was measured, and their excited-state properties were compared.