Multi-Photon Excitation in Photoredox Catalysis: Concepts, Applications, Methods

The energy of visible photons and the accessible redox potentials of common photocatalysts set thermodynamic limits to photochemical reactions that can be driven by traditional visible-light irradiation. UV excitation can be damaging and induce side reactions, hence visible or even near-IR light is usually preferable. Thus, photochemistry currently faces two divergent challenges, namely the desire to perform ever more thermodynamically demanding reactions with increasingly lower photon energies. The pooling of two low-energy photons can address both challenges simultaneously, and whilst multi-photon spectroscopy is well established, synthetic photoredox chemistry has only recently started to exploit multi-photon processes on the preparative scale. Herein, we have a critical look at currently developed reactions and mechanistic concepts, discuss pertinent experimental methods, and provide an outlook into possible future developments of this rapidly emerging area.

Laser-Induced Wurtz-Type Syntheses with a Metal-Free Photoredox Catalytic Source of Hydrated Electrons

Upon irradiation with ns laser pulses at 355 nm, 2-aminoanthracene in SDS micelles readily produces hydrated electrons. These "super-reductants" rapidly attack substrates such as chloro-organics and convert them into carbon-centred radicals through dissociative electron transfer. For a catalytic cycle, the aminoanthracene needs to be restored from its photoionization by-product, the radical cation, by a sacrificial donor. The ascorbate monoanion can only achieve this across the micelle-water interface, but the monoanion of ascorbyl palmitate results in a fully micelle-contained regenerative electron source. The shielding by the micelle in the latter case not only increases the life of the catalyst but also strongly suppresses the interception of the carbon-centred radicals by the hydrogen-donating ascorbate moiety; and in conjunction with the high local concentrations effected by the pulsed laser, termination by radical dimerization thus dominates. We have obtained a complete and consistent picture through monitoring the individual steps and the assembled system by flash photolysis on fast and slow timescales, from microseconds to minutes; and in preparative studies on a variety of substrates, we have achieved up to quantitative dimerization with a turnover on the order of 1 mmol per hour.

Quantitative insights into charge-separated states from one- and two-pulse laser experiments relevant for artificial photosynthesis

Charge-separated states (CSSs) are key intermediates in photosynthesis and solar energy conversion. However, the factors governing the formation efficiencies of CSSs are still poorly understood, and light-induced electron-hole recombinations as deactivation pathways competing with desired charge accumulations are largely unexplored. This greatly limits the possibility to perform efficient multi-electron transfer, which is essential for artificial photosynthesis. We present a systematic investigation of two donor-sensitizer-acceptor triads (with different donor-acceptor distances) capable of storing as much as 2.0 eV in their CSSs upon the absorption of a visible photon. Using quantitative one- and two-pulse laser flash photolysis, we provide deep insights into both the CSS formation quantum yield, which can reach up to 80%, and the fate of the CSS upon further (secondary) excitation with green photons. The triad with shorter intramolecular distances shows a remarkable excitation wavelength dependence of the CSS formation quantum yield, and the CSS of this triad undergoes more efficient light-induced charge recombination than the longer equivalent by about one order of magnitude, whilst thermal charge recombination shows the exact opposite behavior. The unexpected results of our detailed photophysical study can be rationalized by detrimental singlet charge transfer states or structural considerations, and could significantly contribute to the future design of CSS precursors for accumulative multi-electron transfer and artificial photosynthesis.

Combined static and dynamic intramicellar fluorescence quenching: effects on stationary and time-resolved Stern–Volmer experiments

It ain’t necessarily so—existing theories of combined quenching in micelles are flawed. We derive a consistent model, analyze its properties, and apply it to obtain information on ground-state complexes between fluorophore F and quencher Q.

Resveratrol Radical Repair by Vitamin C at the Micelle-Water Interface: Unexpected Reaction Rates Explained by Ion-Dipole Interactions

Repair reactions of lipophilic phenoxy radicals by hydrophilic co-antioxidants at model membranes are important for understanding the factors that govern the interactions between radical scavengers in biological systems. By using near-UV photoionization, we have selectively generated the phenoxy radical of the famous antioxidant resveratrol inside anionic (SDS), cationic (DTAC), or neutral (TX-100) micelles, as well as in homogeneous aqueous solution, and have compared its repairs in these media by the water-soluble co-antioxidants ascorbic acid and ascorbate monoanion. With all surfactants, these reactions are dynamic processes at the micelle-water interface. Whereas for the combinations ascorbate monoanion/ ionic micelle the repair rates can be rationalized by the Coulombic interactions, unexpected effects were observed with the neutral ascorbic acid and the charged micelles: for the anionic micelles, this repair is three times faster than in homogeneous solution, and two orders of magnitude faster than for the cationic micelles. Given that the repair by a concerted proton-electron transfer demands a coplanar arrangement of the resveratrol phenoxy centre sticking out into the Stern layer and the co-antioxidant hydroxy moiety approaching from the aqueous bulk, we explain these results by ion-dipole interactions: only at a negatively charged micellar surface does the direction of the large dipole moment of ascorbic acid lead to an orientation favourable for the repair.

Laboratory-scale photoredox catalysis using hydrated electrons sustainably generated with a single green laser

The ruthenium-tris-bipyridyl dication as catalyst combined with the ascorbate dianion as bioavailable sacrificial donor provides the first regenerative source of hydrated electrons for chemical syntheses on millimolar scales. This electron generator is operated simply by illumination with a frequency-doubled Nd:YAG laser (532 nm) running at its normal repetition rate. Much more detailed information than by product studies alone was obtained by photokinetical characterization from submicroseconds (time-resolved laser flash photolysis) up to one hour (preparative photolysis). The experiments on short timescales established a reaction mechanism more complex than previously thought, and proved the catalytic action by unchanged concentration traces of the key transients over a number of flashes so large that the accumulated electron total surpassed the catalyst concentration many times. Preparative photolyses revealed that the sacrificial donor greatly enhances the catalyst stability through quenching the initial metal-to-ligand charge-transfer state before destructive dd states can be populated from it, such that the efficiency of this electron generator is no longer limited by catalyst decomposition but by electron scavenging by the accumulating oxidation products of the ascorbate. Applications covered dechlorinations of selected aliphatic and aromatic chlorides and the reduction of a model ketone. All these substrates are impervious to photoredox catalysts exhibiting lower reducing power than the hydrated electron, but the combination of an extremely negative standard potential and a long unquenched life allowed turnover numbers up to 1400 with our method.

Combined static and dynamic quenching in micellar systems—closed-form integrated rate laws verified using a versatile probe

One probe to rule them all: an extremely long-lived radical cation M˙⁺ generated by green-light photoionization allowed new theories of combined static and dynamic intra- and intermicellar quenching to be tested.

3-Aminoperylene and ascorbate in aqueous SDS, one green laser flash … and action! Sustainably detoxifying a recalcitrant chloro-organic

Green light releases the predator e_aq˙⁻ (the hydrated electron) at near-physiological pH through a metal-free catalytic cycle, consuming only ascorbate AscH⁻ as a sacrificial donor, and decomposing organochlorides R–Cl with high turnover numbers.