Photon upconversion by triplet-triplet annihilation mechanism (TTA-UC) is a promising methodology for advanced utilization of sunlight. Conventionally, TTA-UC has utilized diffusion and collision of molecules in organic solvents, while we have developed molecularly organized TTA-UC by combining the concepts of TTA-UC and molecular self-assembly. We herein describe the development of our research from molecular self-assembly in solution systems to solid-state materials.

Heavy rare-earth ion doped upconversion phosphors can emit blue-green-red visible light with higher energy than the excitation light by multi-photon multi-step excitation by irradiating near-infrared light around 980 nm. Yb³⁺ is used to absorb near-infrared light at 980 nm, and the energy is transferred to Ho³⁺, Er³⁺, and Tm³⁺, which are responsible for luminescence. The excited states of f-electrons have long lifetimes, and the ions responsible for luminescence are excited again by another energy transfer from Yb³⁺. The radiative relaxation of the f electrons in the luminescent ions, which are multi-photon multi-step excited by these processes, emits visible light with higher energy than the excitation light. This is an extremely unique luminescence phenomena and is used in security inks. The high bio-permeability of infrared light used for excitation has led to research into its application in bioimaging, and it is expected to be used in combination with solar cells to improve power generation efficiency by converting infrared light that cannot be used for power generation into visible light.

Studies on up-conversion (UC) fluorescent materials are closely related to the emergence of materials that exhibit UC fluorescence and the development of their synthetic processes, and is also an opportunity to develop biophotonics using near-infrared light as excitation light, which has been at tracting attention in recent years. The basis of the design of a fluorescent material is how to emit the given excitation energy as light without converting it into heat. The establishment of a method for syn thesizing fluoride nanoparticles had a great influence on the development of UC fluorescent materials for biophotonics, and once a stable method for synthesizing nanoparticles is established, the design and synthesis of organic-inorganic composites using them had broken out. An important guideline in this case is again how to avoid the heat generation. It was inevitable to consider the effect of organic molecules, which have not been discussed so far, on the thermal relaxation of the excited electrons of rare earth ions in ceramic nanoparticles. This paper introduces the development of biophotonics, which originated from the studies on UC fluorescent materials, and presents the concept of the influence of organic molecular systems on electrons of rare earth ions in inorganic nanoparticles.

Turbidity measurement was found to be a simple and effective method to evaluate the solubilization of water in Aerosol OT (AOT) / cosurfactant / mineral oil systems. The systems showed greater ability to solubilize water when the mineral oil was a long chain hydrocarbon with a carbon number between C10 and C12. Because these hydrocarbons do not easily penetrate the monolayer of the reversed micelles, the effective volume of the monolayer does not increase and the micellar size is maintained. The most suitable cosurfactants were sorbitan trioleate (Span 85) and sorbitan monolaurate (Span 20), and each increased the solubilization of water by a different mechanism. It seems that Span 85 acts on the lipophilic groups of the AOT molecules, thickening the monolayer of the reversed micelle to prevent the mineral oil from penetrating it, thereby promoting formation of larger, stabler reversed micelles. Meanwhile, Span 20 may form reversed micelles with the AOT molecules, weakening the electrostatic interaction between the hydrophilic groups to allow formation of larger and stabler reversed micelles.