Volume 95, Issue 8

The history of the IUPAC Photochemistry Committee since its creation in 1976 and its transition in 2001 to the IUPAC Photochemistry Sub-Committee are reviewed as well as the connections of Committee and Sub-Committee to the various photochemical associations ( E uropean P hotochemical A ssociation, EPA , I nter- A merican P hotochemical S ociety, I-APS and A sian and Oceanian P hotochemistry A ssociation, APA ). The participants in both the Commission and the Sub-Committee over the years are listed as well as the Recommendations and Technical Reports produced since the creation of the Committee until the present days.

Quantum yields of fluorescence of porphycenes – porphyrin isomers – can vary by orders of magnitude, even for very similar derivatives, such as meso -dimethyl- vs. meso -tetramethylporphycene. In weakly emitting porphycenes the fluorescence intensity strongly depends on viscosity and can be recovered by placing a molecule in a rigid environment. We postulate that the efficient nonradiative deactivation is due to the quantum effect, delocalization of the inner protons. The delocalization, which increases with the strength of intramolecular hydrogen bonds may induce structural changes that lead to distortion from planarity and, as a result, efficient S 0  ← S 1 internal conversion. The effect seems to be general, as indicated by good correlation between the quantum yield of fluorescence and the distance between H-bonded nitrogen atoms, the latter being a reliable measure of hydrogen bonding strength. Based on the available photophysical and X-ray data, such correlation was found so far for over 20 differently substituted porphycenes.

Caged compounds, also called phototriggers are formed by a photo-removable protecting group attached to a molecule of interest, hindering its potential interactions or reaction partners. A particular chemical bond is broken when the phototrigger absorbs light of a given wavelength, yielding a non-interacting “cage” and a free interacting molecule. Numerous organic based caged compounds have been devised, and many of them have broad applications, usually in physiology research. The tunability of these phototriggers is scarce, and the common strategy consists in changing the photoremovable group. Conversely, ruthenium-polypyridine caged compounds are built around a Ru center that can accommodate six coordinated molecules or groups including the photo-releasable molecule. The design of the coordination sphere yields many ways to achieve a desired property, or modulate a property, such as hydrophilicity, redox potential, absorption, 2P capabilities, action cross section, etc. In this work we will show how the tuning of quantum yield of photorelease, absorption wavelength and thermal stability is feasible, and discuss the rationale and the limits of the ligand-tuning technique.

X-ray crystallography and X-ray spectroscopy using X-ray free electron lasers plays an important role in understanding the interplay of structural changes in the protein and the chemical changes at the metal active site of metalloenzymes through their catalytic cycles. As a part of such an effort, we report here our recent development of methods for X-ray absorption spectroscopy (XAS) at XFELs to study dilute biological samples, available in limited volumes. Our prime target is Photosystem II (PS II), a multi subunit membrane protein complex, that catalyzes the light-driven water oxidation reaction at the Mn 4 CaO 5 cluster. This is an ideal system to investigate how to control multi-electron/proton chemistry, using the flexibility of metal redox states, in coordination with the protein and the water network. We describe the method that we have developed to collect XAS data using PS II samples with a Mn concentration of <1 mM, using a drop-on-demand sample delivery method.

Photocatalysis is an emerging area of chemistry that takes advantage of light as the primary source of energy to carry out chemical transformations. In this context, organic photocatalysts appear as an alternative that has proven to be efficient in treating polluted effluents. Although organic photocatalysts are not able to generate hydroxyl radical, their photoactivated excited states generated using visible light can act as strong oxidants in most cases. In fact, pollutant photooxidation can be produced from an initial electron transfer between an excited state of an organic photocatalyst and the contaminant, generating their respective radical anion and cation (Type I mechanism). However, as most of the organic photocatalysts are able to generate singlet oxygen, pollutant degradation can also be initiated from this oxidative species (Type II mechanism). Moreover, the heterogenization of the photocatalysts seems the straightforward step to boost photostability and facilitate recovery after the reaction. In the present review, we chronicle our research progress and how interestingly, it cannot be assumed that the main reaction pathways of a photocatalyst are the same under homogeneous conditions as in heterogeneous media. Herein we have selected Rose Bengal (RB), Riboflavin (RF), and a perylene diimide derivative (PDI) to illustrate the different modes of action of these organic photocatalysts under homogeneous/heterogeneous conditions.

Palladium nanostructures are interesting heterogeneous catalysts because of their high catalytic activity in a vast range of highly relevant reactions such as cross couplings, dehalogenations, and nitro-to-amine reductions. In the latter case, the catalyst Pd@GW (palladium on glass wool) shows exceptional performance and durability in reducing nitrobenzene to aniline under ambient conditions in aqueous solutions. To enhance our understanding, we use a combination of optical and electron microscopy, in-flow single molecule fluorescence, and bench chemistry combined with a fluorogenic system to develop an intimate understanding of Pd@GW in nitro-to-amine reductions. We fully characterize our catalyst in situ using advanced microscopy techniques, providing deep insights into its catalytic performance. We also explore Pd cluster migration on the surface of the support under flow conditions, providing insights into the mechanism of catalysis. We show that even under flow, Pd migration from anchoring sites seems to be minimal over 4 h, with the catalyst stability assisted by APTES anchoring.

Multiphoton absorption and multiple excitation can lead to the formation of highly electronically excited states of molecules. We have been applying these excitation methods to explore specific photochemical reactions, which are rather difficult to attain by normal one-photon absorption processes. In the present review, we will introduce several examples of these photochemical responses specific to highly excited state in the condensed phase, such as two-photon-gated cycloreversion, one-color control of both reactions in photochromic systems and rapid capture of an electron ejected from the higher excited state leading to rapid generation of charge-separated states at the high energy level with a lifetime much longer than microseconds.

Self-assembly, which occurs through noncovalent interactions among molecules, is a ubiquitous phenomenon in the natural world. Light is a particularly attractive stimulus for manipulating self-assembled structures due to its precise and noninvasive nature. Photoresponsive ruthenium (Ru) complexes are emerging as promising candidates for controlling self-assembly due to their unique coordination chemistry and reversible light-triggered behavior. Specifically, Ru complexes can undergo photodissociation of their ligands in aqueous solutions, leading to the formation of Ru-H 2 O species, and this process can be used to control the disassembly of assembled structures upon illumination. Conversely, upon cessation of the light stimulus, some Ru–ligand coordination bonds can be restored, resulting in reassembly of the structures. Herein, we mainly introduce our recent progress in the use of Ru(Ⅱ) complexes to create photocontrolled self-assemblies with applications ranging from cancer therapy to the manipulation of the morphology and properties of nanoscale materials. Finally, we discuss the challenges and future directions of photocontrolled assemblies with Ru complexes.