Development of Mechanically Stable Long-Lived Room Temperature Phosphorescence Network by Controlling Intramicellar and Intermicellar Interaction

Room temperature phosphorescence (RTP) is one of the widely researched luminescent phenomena these days. This involves continued emission of light from the triplet excitons at room temperature for more than 0.1s even after the excitation source has been removed. In particular, organic RTP materials attract interest due to their low cost and easy molecular modification, exhibiting potential use in the fields of information security, wearable devices, and lasers. A concern of the organic RTP materials is their short lifespan and low light intensity. Recent strategies to achieve long-lived organic RTP (LRTP) include crystallization, matrix rigidification, and host-guest systems to restrict molecular motion and prevent quenching by external quenchers such as oxygen. However, these strategies severely hinder the practical use of LRTP materials because of their brittle, fragile nature and low mechanical properties. Therefore, a new approach to satisfy both mechanical properties and LRTP is needed.<br/>In this work, we synthesized a phosphorescent amphiphilic random copolymer that forms micelles in water, and controlled the intramicellar and intermicellar interactions within micelles to optimize the mechanical properties and phosphorescent performance. For synthesizing an amphiphilic random copolymer with LRTP properties, we introduced a phosphorescent chromophore based on organic quaternary phosphonium salts into an acrylamide and phenylboronic acid backbone with hydrogen bonds to restrict molecular motions of the phosphor. It showed naked-eye observable, blue-colored light emission for over 5s duration after 365 nm UV excitation. We confirmed that the phosphorescent polymer could be self-assembled into micelles with a hydrophobic core and a hydrophilic shell structure.<br/>Afterward, we introduced a doping material, bPEI (branched polyethyleneimine) into the polymer to control intermicellar interactions. With doping bPEI at room temperature, the composite solution showed no gelation. However, the composite solution containing over 30 wt% bPEI underwent gelation with shrinking by syneresis upon heating above 65°C. This meant that a higher content of bPEI could accelerate micelle gelation kinetic. We demonstrated aggregation under external heat and bPEI linker by enhancing physical entanglement and hydrogen-bonding interaction in intermicellar domain. By analyzing the effect of bPEI content on mechanical properties and LRTP, we confirmed that higher bPEI led to enhanced mechanical properties but lower LRTP due to the high mobility of bPEI molecule. Furthermore, intramicellar chemical crosslinking of the inner core of the micelle was achieved with heating in the drying process of the composite. When dried at a temperature of 65°C, the boronic acid structure in the polymer backbone was maintained without any reaction. But when dried above 90°C, the boronic acids formed chemical crosslinking in boroxine structure by dehydration, leading to shrinking of the inner core of the micelle and the composite material itself. By analyzing the effect of drying temperature, it was verified that high temperature enabled the composite to possess improved LRTP.<br/>Lastly, we employed the suggested LRTP system in films, patterns, and coatings for various substrates from its eco-friendly water-processibility. Such a stable LRTP material with easy preparation and broad-range applications highlights its practical use.