Near-Unity Superradiant Emission from Delocalized Frenkel Excitons in a Two-Dimensional Supramolecular Assembly

We demonstrate three general effective strategies to mitigate non-radiative losses in the superradiant emission from supramolecular assemblies. We focus on J-aggregates of 5,5',6,6'-tetrachloro-1,1'-diethyl-3,3'-di(4-sulfobutyl)-benzimidazolocarbocyanine (TDBC) and elucidate the nature of their nonradiative processes. We show that self-annealing at room temperature, photo-brightening, and the purification of the dye monomers all lead to substantial increases in emission quantum yields (QYs) and a concomitant lengthening of the emission lifetime, with purification of the monomers having the largest effect. We use structural and optical measurements to support a microscopic model that emphasizes the deleterious effects of a small number of impurity and defect sites that serve as non-radiative recombination centers. This understanding has yielded a room temperature molecular fluorophore in solution with an unprecedented combination of fast emissive lifetime and high QY. We obtain superradiant emission from J-aggregates of TDBC in solution at room temperature with a QY of 82% coupled with an emissive lifetime of 174 ps. This combination of high QY and fast lifetime at room temperature makes supramolecular assemblies of purified TDBC a model system for the study of fundamental superradiance phenomena. High QY J-aggregates are uniquely suited for the development of applications that require high speed and high brightness fluorophores such as devices for high speed optical communication.