Application Limits of the Ferrioxalate Actinometer**

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. Competitive charge recombination, either inside or outside the solvent cage, may limit the overall efficiency of a photochemical reaction or internal quantum yield (defined as the moles of product formed per mole of photons absorbed by PC). Measuring the cage escape efficiency (ϕCE) typically requires time-resolved spectroscopy; however, we demonstrate how to estimate ϕCE using steady-state techniques that measure the efficiency of PC•- formation (ϕPC). Our results show that choice of electron donor critically impacts ϕPC, which directly correlates to improved synthetic and internal quantum yields. Furthermore, we demonstrate how modest structural differences between photocatalysts may afford a sizable effect on reactivity due to changes in ϕPC, and by extension ϕCE. Optimizing experimental conditions for cage escape provides photochemical reactions with improved atom economy and energy input, paving the way for sustainable design of photocatalytic systems.

Photochromic Nucleosides and Oligonucleotides

Photochromism is a reversible phenomenon wherein a material undergoes a change in color upon exposure to light. In organic photochromes, this effect often results from light-induced isomerization reactions, leading to alterations in either the spatial orientation or electronic properties of the photochrome. The incorporation of photochromic moieties into biomolecules, such as proteins or nucleic acids, has become a prevalent approach to render these biomolecules responsive to light stimuli. Utilizing light as a trigger for the manipulation of biomolecular structure and function offers numerous advantages compared to other stimuli, such as chemical or electrical treatments, due to its non-invasive nature. Consequently, light proves particularly advantageous in cellular and tissue applications. In this review, we emphasize recent advancements in the field of photochromic nucleosides and oligonucleotides. We provide an overview of the design principles of different classes of photochromes, synthetic strategies, critical analytical challenges, as well as structure-property relationships. The applications of photochromic nucleic acid derivatives encompass diverse domains, ranging from the precise photoregulation of gene expression to the controlled modulation of the three-dimensional structures of oligonucleotides and the development of DNA-based fluorescence modulators. Moreover, we present a future perspective on potential modifications and applications.

Optimizing reaction conditions for the light-driven hydrogen evolution in a loop photoreactor

Photocatalytic hydrogen production from water is a promising way to fulfill energy demands and attain carbon emission reduction goals effectively. In this study, a loop photoreactor with a total volume of around 500 mL is presented for the photocatalytic hydrogen evolution using a Pt-loaded polymeric carbon nitride photocatalyst under 365 nm irradiation in the presence of sacrificial reducing agents. The fluid flow pattern of the developed photoreactor was characterized experimentally and the photon flux incident to the loop photoreactor was measured by chemical actinometry. The system displayed exceptional stability, with operation sustained over 70 hours. A design of experiment (DOE) analysis was used to systematically investigate the influence of key parameters - photon flux, photocatalyst loading, stirring speed, and inert gas flow rate - on the hydrogen generation rate. Linear relationships were found between hydrogen evolution rate and photon flux as well as inert gas flow rate. Photocatalyst loading and stirring speed also showed linear correlations, but could not be correctly described by DOE analysis. Instead, linear single parameter correlations could be applied. Notably, the loop photoreactor demonstrated an external photon efficiency up to 17 times higher than reported in literature studies, while scaling the reactor size by a factor of 10.

Exploring Ligand-to-Metal Charge-Transfer States in the Photo-Ferrioxalate System Using Excited-State Specific Optimization

The photo-ferrioxalate system (PFS), [Fe(III)(C2O4)]3-, more than an exact chemical actinometer, has been extensively applied in wastewater and environment treatment. Despite many experimental efforts to improve clarity, important aspects of the mechanism of ferrioxalate photolysis are still under debate. In this paper, we employ the recently developed WΓ-CASSCF to investigate the ligand-to-metal charge-transfer states that are key to ferrioxalate photolysis. This investigation provides a qualitative picture of these states and key potential energy surface features related to the photolysis. Our theoretical results are consistent with the prompt charge-transfer picture seen in recent experiments and clarify some features that are not visible in experiments. Two ligand-to-metal charge-transfer states contribute to the photolysis of ferrioxalate, and the avoided crossing barrier between them is low compared with the initial photoexcitation energy. Our data also clarify that one Fe-O bond cleaves first, followed by the C-C bond and the other Fe-O bond.

Comparative Evaluation of Light-Driven Catalysis: A Framework for Standardized Reporting of Data

Light-driven homogeneous and heterogeneous catalysis require a complex interplay between light absorption, charge separation, charge transfer, and catalytic turnover. Optical and irradiation parameters as well as reaction engineering aspects play major roles in controlling catalytic performance. This multitude of factors makes it difficult to objectively compare light-driven catalysts and provide an unbiased performance assessment. This Scientific Perspective highlights the importance of collecting and reporting experimental data in homogeneous and heterogeneous light-driven catalysis. A critical analysis of the benefits and limitations of the commonly used experimental indicators is provided. Data collection and reporting according to FAIR principles is discussed in the context of future automated data analysis. The authors propose a minimum dataset as a basis for unified collecting and reporting of experimental data in homogeneous and heterogeneous light-driven catalysis. The community is encouraged to support the future development of this parameter list through an open online repository.

Multiple wavelength (365-475 nm) complete actinometric characterization of Corning® Lab Photo Reactor using azobenzene as a highly soluble, cheap and robust chemical actinometer

The characterization of commercially available Corning^® Lab Photo Reactor by actinometry at different wavelengths (365, 385, 405 and 475 nm) using azobenzene E ↔ Z photoisomerization is reported. By comparison with photon fluxes determined externally using a radiometer, this method based on NMR spectroscopy is rapid, cheap, robust, reproducible and can applied to UVA-visible range, compared to previously described chemical actinometric protocols. Recalculation of isatin N²-phenylhydrazone isomerization quantum yield at 405 nm gave almost the same value as the literature data ([Formula: see text] = 0.0013) and confirmed the robustness and applicability of this methodology.