Designing Highly Luminescent Molecular Aggregates via Bottom-Up Nanoscale Engineering

Abstract

Coupling of excitations between organic fluorophores in J-aggregates leads to coherent delocalization of excitons across multiple molecules, resulting in materials with high extinction coefficients, long-range exciton transport, and, in particular, short radiative lifetimes. Despite these favorable optical properties, uses of J-aggregates as high-speed light sources have been hindered by their low photoluminescence (PL) quantum yields (QYs). Here, we take a bottom-up approach to design a novel J-aggregate system with a large extinction coefficient, a high QY, and a short lifetime. To achieve this goal, we first select a J-aggregating cyanine chromophore and reduce its nonradiative pathways by rigidifying the backbone of the cyanine dye. The resulting conformationally restrained cyanine dye exhibits strong absorbance at 530 nm and fluorescence at 550 nm with 90% QY and 2.3 ns lifetime. We develop optimal conditions for the self-assembly of highly emissive J-aggregates. Cryogenic transmission electron microscopy (cryo-TEM) and dynamic light scattering (DLS) reveal micron-scale extended structures with two-dimensional (2D) sheetlike morphology, indicating a long-range structural order. These novel J-aggregates have strong, red-shifted absorption at 600 nm, resonant fluorescence with no Stokes shift, 50% QY, and 220 ps lifetime at room temperature. We further stabilize these aggregates in a glassy sugar matrix and study their excitonic behavior using temperature-dependent absorption and fluorescence spectroscopy. These temperature-dependent studies confirm J-type excitonic coupling and superradiance. Our results have implications for the development of a new generation of organic fluorophores that combine high speed, high QY, and solution processing.