Recent Progress in Photon Upconverting Gels

Recent progress in the development of gels showing triplet-triplet annihilation based photon upconversion (TTA-UC) is reviewed. Among the two families of upconverting gels reported, those display TTA-UC based on molecular diffusion show performances comparable to those in solutions, and the TTA-UC therein are affected by dissolved molecular oxygen. Meanwhile, air-stable TTA-UC is achieved in organogels and hydrogels by suitably accumulating TTA-UC chromophores which are stabilized by hydrogen bonding networks of the gelators. The unique feature of the air-stable upconverting gels is that the self-assembled nanostructures are protected from molecular oxygen dissolved in the microscopically interconnected solution phase. The presence of the bicontinuous structures formed by the upconverting fibrous nanoassemblies and the solution phase is utilized to design photochemical reaction systems induced by TTA-UC. Future challenges include in vivo applications of hydrogels showing near infrared-to-visible TTA-UC.