(Invited) A General Approach to Induce Ultralong Room Temperature Phosphorescence in Organic Small Molecules

Abstract

Interest in organic small molecules that exhibit second-scale phosphorescence at room temperature has grown immensely in recent years due to their potential applications in sensing, anticounterfeiting, and bioimaging. However, such material systems are rare—requiring second-scale triplet lifetimes, efficient intersystem crossing, and slow rates of nonradiative recombination. This third requirement has been met by isolating phosphors in a rigid matrix, or by aggregating them into densely packed crystals or powders to suppress the molecular vibrations that lead to recombination. While these techniques work well for a small subset of molecules with specific properties, most isolated molecules in a rigid matrix do not phosphoresce, and most macroscopic aggregates experience significant triplet quenching. In this work, we find a middle ground between these extremes by forming microscopic [approximately submicron sized] phosphor aggregates in rigid polymer matrices using a simple drop casting and thermal annealing process. Using this technique, we activate second-scale phosphorescence at room temperature in 20 molecules that do not otherwise phosphoresce using conventional matrix-isolation or crystallization approaches. We find that increased chromophore loading increases aggregate sizes. Excitons are thus able to diffuse further and interact more, and triplet-triplet annihilation dominates. Furthermore, we determine that excimer formation in some aggregates leads to increased rates of triplet generation—complementing the effect of nonradiative recombination suppression and further enhancing phosphorescence. In sum, the simplicity and robustness of this blending approach significantly loosens the design constraints to access second-scale emission with organic phosphors, allowing researchers to choose from a broader catalog of organic materials to match the desired properties for a given application.