Photon Upconversion with a Low Threshold Excitation Intensity in Plain Water.

A triplet-triplet annihilation-based photon upconversion (TTA-UC) system with a low threshold excitation intensity () in plain water was developed. Water-soluble anionic porphyrin (PdTPPS) and diphenylanthracene (DCDPA) derivatives were used as light absorbers and emitter molecules, respectively, and no additives such as surfactants were required. The phosphorescence emission from PdTPPS under an excitation wavelength of 528 nm was quenched by DCDPA, resulting in triplet energy transfer, whereas fluorescence from DCDPA was observed in a short wavelength region (400-500 nm). Three independent emission studies utilizing different excitation light sources validated the TTA-UC process in a simple aqueous solution. TTA occurred after the triplet energy transfer, according to the time profiles of phosphorescence and fluorescence detected following pulse laser excitation. The for TTA-UC was estimated to be lower than 6 mW cm, although it could not be exactly determined due to the sensitivity limit of the experimental setup. The upper limit of for the aqueous solution of DCDPA and PdTPPS is the smallest value obtained to date for aqueous systems and comparable to that of high-performance TTA-UC systems in organic solutions.