Thermally Activated Delayed Fluorescence (TADF) and Self-Healing Ultra-Stable Luminophores for Bioimaging

• Applicant: Dr. Rafal Czerwieniec
• Subject Area: Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry ; Inorganic Molecular Chemistry - Synthesis and Characterisation ; Biomaterials ; Synthesis and Properties of Functional Materials ; Organic Molecular Chemistry - Synthesis and Characterisation
• Term: from 2017 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 389797483

It is aimed to introduce the mechanism of the thermally activated delayed fluorescence (TADF) as a mechanism of fast depopulation of excited triplet states of fluorescent bioprobes and labels. Population of the long-lived, non-emissive (dark) and often reactive triplet states leads to photobleaching and photoblinking of the fluorophores and causes photodegradation. These effects strongly limit the time scale of possible biological fluorescence microscopy experiments. Therefore, an efficient and fast relaxation of triplet excited fluorophore molecules to the ground state is necessary for a high bioprobe photostability. Current strategies for a facilitated relaxation (self-healing) of fluorophore molecules captured in the triplet state involve charge-transfer (redox) and proton transfer reactions between the electronically excited chromophore fragment of the probe molecule and other groups of the molecule. Thus, the synthesis of highly sophisticated (and large) conjugates comprizing different functional units is necessary. In effect, the applicability of these strategies is strongly restricted. Thermally activated delayed fluorescence (TADF) is a luminescence mechanism different from the prompt fluorescence (spin allowed) and phosphorescence (spin forbidden) that involves both singlet and triplet excited states. In a molecule characterized by the small energy difference between the lowest-energy singlet and triplet excited states the higher-energy singlet state can be thermally populated from the lower-energy triplet state. In effect, the emission stems from the singlet state and decays slower than the prompt fluorescence but much faster than the phosphorescence. Applied to luminescent bioprobes, TADF offers a purely physical self-healing mechanism, without a need for any excited-state chemical reactions. Therefore, the material design of future TADF-based bioprobes can be drastically simplified. Novel highly photostable materials may also allow for a distinctly wider range of applications in biology and medicine than it is possible with traditional fluorophores. Moreover, biolabels based on the TADF-emitting compounds may replace phosphorescent (photostable) luminophors based on complexes of heavy metal ions, such as iridium, platinum, or rhenium. This project aims at elucidation of molecular design principles allowing for efficient thermal relaxation of the triplet excited molecules, via singlet excited state, into the electronic ground state. A significant part of the investigations will be devoted to the electrostatic and mechanic (rigidity, sterics) interactions between the luminophore molecule and its local environment that are essential for the TADF behavior. Thus, it is planned to investigate water- and lipid-soluble compounds exhibiting TADF as well as a number of conjugates with biologically relevant groups, for instance for docking into a DNA double helix or for covalent binding to proteins.

• DFG Programme: Research Grants