Red Multi‐Resonant Thermally Activated Delayed Fluorescence Emitters as Bioimaging Agents

Abstract Multi‐resonant thermally activated delayed fluorescence (MR‐TADF) emitters have attracted strong interest for organic electroluminescent devices due to their high photoluminescence quantum yield (Φ PL ) and narrowband emission. These properties are also crucial for high‐performance biological probes, especially red emitters. Orange and red MR‐TADF emitters, PhDPA‐DiKTa and MeODPA‐DiKTa , are designed by decorating the DiKTa core with di([1,1′‐biphenyl]‐4‐yl)amine and bis(4‐methoxyphenyl)amine, respectively. As 5 wt.% doped films in 1,3‐di(9H‐carbazol‐9‐yl)benzene (mCP), PhDPA‐DiKTa emits at λ PL of 617 nm, while MeODPA‐DiKTa emits at λ PL of 655 nm. Both show delayed fluorescence, with delayed lifetimes, τ d , of 658.4 and 249.2 µs, respectively. Water‐dispersible glassy organic dots (g‐Odots) based on these materials are prepared by encapsulating them and mCP host into an amphiphilic DSPE‐PEG2k polymer. Both families of g‐Odots showed a deeper red emission and enhanced Φ PL compared to the corresponding 5 wt.% doped films in mCP (λ PL = 618 nm, Φ PL = 77% for PhDPA‐DiKTa g‐Odots, λ PL = 663 nm, Φ PL = 38% for MeODPA‐DiKTa g‐Odots). The TADF character of the emitters is conserved in the g‐ODots, with τ d of 203.9 µs for PhDPA‐DiKTa g‐Odots and 131.6 µs for MeODPA‐DiKTa g‐Odots. These MR‐TADF g‐Odots are successfully demonstrated as biological imaging probes of HeLa cells.