External Heavy Atom Effects Accelerate Intersystem Crossing in Glassy Organic Dots

Glassy organic dots (g-Odots) are an emerging class of luminescent nanoparticles containing a core of hydrophobic organic glass. These nanoparticles are particularly promising for the encapsulation of materials that show thermally activated delayed fluorescence (TADF), as their glassy matrix preserves the delayed emission of the emitter in aerated aqueous media, enabling time-resolved fluorescence imaging of biological samples. Until now, the only reports of g-Odots utilize 1,3-bis(N-carbazolyl)benzene (mCP) as the host material within the glassy core of the particle. In this work, we explore the use of the halogenated mCP derivatives mCP-Br, mCP-Br3, mCP-Br5, and mCP-I as host materials within g-Odots. We hypothesized that the use of halogenated hosts would improve the performance of a TADF-active dopant through an external heavy atom effect. Each of the host materials, except for mCP-Br5, is found to be suitable for g-Odot fabrication, while mCP-Br5 lacks a glass transition within a suitable temperature range. Photophysical analysis reveals that halogenated hosts accelerate intersystem crossing (ISC) and reverse intersystem crossing (rISC) within the TADF dopant, with >5× acceleration in rISC observed for g-Odots containing mCP-I. The improved TADF performance is expected to benefit applications such as time-resolved fluorescence imaging and luminescence-based data encryption.