Molecular oxygen plays a pivotal role in aerobic organism as a terminal electron acceptor in electron transport chain in mitochondria. Oxygen deprivation (hypoxia) in organism is closely linked to various hypoxia-related cellular functions and diseases including cancer, cerebral infarction, and chronic kidney disease. The quantitative and real-time detection of oxygen levels in living cells and tissues is therefore of great importance in elucidating fundamental studies of cellular metabolism and hypoxia-related diseases. This review focuses on recent advances in biological oxygen sensing and imaging based on phosphorescence lifetime measurements using phosphorescent probes. Principle of oxygen sensing by means of phosphorescence quenching, recent advances of phosphorescent oxygen probes and their applications to intracellular and in vivo oxygen measurements are described, emphasizing the usefulness of Pd(II)- and Pt(II)-porphyrins and iridium(III) complexes as biological oxygen probes.

Photoinduced electron transfer (PET) is a fundamental photochemical process involving single electron transfer (SET) promoted reduction and oxidation (redox) of excited state materials. In nearly half a century, extensive investigations to elucidate the PET reaction mechanism and to discover a variety of organic redox reactions were conducted. Since the reaffirmation of the synthetic potential of PET reactions promoted by visible-light-absorbing transition metal photoredox catalysts, numerous synthetically useful reactions catalyzed by these metal complexes have been reported. From a green and sustainable perspective, the processes promoted by organic photoredox catalysts are often proposed to be more desirable. Although a variety of organic electron-acceptor photocatalysts have been utilized, their electron-donor counterparts have been less well explored. Brief history of organic electron-donor photocatalysts and recent developments of new photocatalysts and catalytic protocols for carrying out reduction reactions of organic molecules are described in this review.

There are increasing reports on the charge-transfer (CT) excited states of the ligand (π)- and 4f-orbitals of lanthanide complexes, where the latter are shielded by the filled 5s² and 5p⁶ orbitals. To date, the factors that influence CT through the 4f-orbitals remain unknown. Therefore, since this research area is relatively new, we believe that the study of photo-excited π-4f-orbital CT will open up a new field of research in photochemistry based on metal complexes. In this review, we highlight the historical views and research progress in the π-4f-orbital CT excited states of lanthanide complexes.

In this study, we used an X-ray free electron laser (XFEL) to visualize in real time the covalent bond formation process between gold and gold atoms excited by light. The X-ray molecular movies measured by time-resolved X-ray solution scattering showed that it is possible to follow the structural changes induced by photochemical reactions in solution in detail and in real time.

Triplet-triplet annihilation-based photon upconversion (TTA-UC) is a methodology to convert lower energy photons into higher energy photons under low-intensity light. While efficient TTA-UC within the visible range has been achieved, it remains difficult to achieve similar efficiencies in TTA-UC from visible to UV or NIR to visible light. As for visible-to-UV TTA-UC, the discovery of key chromophores has dramatically increased its efficiency, making it possible to generate UV light from sunlight or indoor light. The role of triplet pair intermediates in TTA-UC has also been better understood, and the importance of intermolecular orientation, reverse intersystem crossing, and triplet pair dynamics in achieving high TTA-UC efficiency has been clarified.

The development of synthetic organic nanomaterials, such as nanoparticles and two dimensional polymers, offers an attractive means to preset their unique photochemical properties, as well as potential their use in bioscience, such as cellular imaging and functional regulation. Based on the principals of photochemistry, we developed novel imaging and energy conversion methods, to control their photo-functional properties. The fast part of this topic will focus on the development of noble imaging methods using light. The hard X-ray excited imaging nanomaterials and photo-switchable fluorescence materials are discussed in this topic. In the second part, our focus shifts to two dimensional polymer materials for photocatalytic reaction. These studies establish a starting point for the design of unique photo-functional organic nanomaterials toward biological and materials science.