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

Brightly fluorescent solid-state materials are highly desirable for bioimaging, optoelectronic applications, and energy harvesting. However, the close contact between pi-systems most often leads to quenching. Recently, we developed small-molecule ionic isolation lattices (SMILES) that efficiently isolate fluorophores while ensuring very high densities of the dyes. migration in such dense systems is inevitable. While attractive for energy harvesting applications, FRET also significantly compromises quantum yields of fluorescent solids by funneling the excitation energy to dark trap states. 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 energy 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, and 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 32,200 M-1 cm-1 nm-3 and can deliver freely tunable spectroscopic properties for the fabrication of super-bright advanced optical materials. It is found that SMILES prohibit PET quenching between donor and acceptor dyes that is observed for non-SMILES mixtures of the same dyes. This allows a very broad selection of donor and acceptor dyes for use in FRET SMILES.

Automated summary

Brightly fluorescent solid-state materials are highly desirable for various applications. However, the close contact between pi-systems often leads to quenching. In this study, small-molecule ionic isolation lattices (SMILES) are developed to efficiently isolate fluorophores and ensure high densities of dyes. By intentionally introducing fluorescent dopants, FRET effects are exploited and superior fluorescent solid-state materials are created with freely tunable spectroscopic properties.