Red multi-resonant thermally activated delayed fluorescence emitters as bioimaging agents

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.