Transparent and Nonflammable Ionogel Photon Upconverters and Their Solute Transport Properties.

Photon upconversion based on triplet-triplet annihilation (TTA-UC) is a technology to convert presently wasted sub-bandgap photons to usable higher-energy photons. In this paper, ionogel TTA-UC samples are first developed by gelatinizing ionic liquids containing triplet-sensitizing and light-emitting molecules using an ionic gelator, resulting in transparent and nonflammable ionogel photon upconverters. The photophysical properties of the ionogel samples are then investigated, and the results suggest that the effect of gelation on the diffusion of the solutes is negligibly small. To further examine this suggestion and acquire fundamental insight into the solute transport properties of the samples, the diffusion of charge-neutral solute species over much longer distances than microscopic interpolymer distances is measured by electrochemical potential-step chronoamperometry. The results reveal that the diffusion of solute species is not affected by gelation within the tested gelator concentration range, supporting our interpretation of the initial results of the photophysical investigations. Overall, our results show that the advantage of nonfluidity can be imparted to ionic-liquid-based photon upconverters without sacrificing molecular diffusion, optical transparency, and nonflammability.