A Universal Concept for Bright, Organic Solid State Emitters - Doping of Small Molecule Ionic Isolation Lattices with FRET Acceptors

Abstract

Brightly fluorescent solid-state materials are highly desirable tools in bioimaging, optoelectronic applications and energy harvesting. However, close contact between π-systems leads to strong electronic coupling and often subse-quent quenching of the emitters in dense solid-state materials. Recently, we developed a method to prohibit strong coupling based on small molecule ionic isolation lattices (SMILES) that efficiently isolate fluorophores while in paral-lel ensuring very high densities of the dyes. Nevertheless, efficient FRET energy migration in such dense systems is inevitable. While attractive for energy harvesting applications, FRET also significantly compromises quantum yields of fluorescent solids by funneling excitation energy to dark trap states like crystal defects. Here we investigate the underlying property of FRET and exploit it to our favor by intentionally introducing fluorescent dopants into SMILES materials, acting as FRET acceptors with favorable photophysical properties. This doping is shown to outcompete en-ergy migration to dark trap states, while also ruling out reabsorption effects in dense SMILES materials, resulting in universal fluorescent solid state materials (thin films, powders, crystals) with superior properties. These include emission quantum yields reaching as high as 50-65 %, programmable fluorescence lifetimes with mono-exponential decay, and independent selection of absorption and emission maxima. The volume normalized brightness of these FRET-based SMILES now reach values up to 32200 M-1 cm-1 nm-3 and can deliver freely tunable spectroscopic properties for the fabrication of super bright advanced optical materials.