Highly Efficient Red Multi-Resonant Thermally Activated Delayed Fluorescence Emitters as Bioimaging Reagents

Multi-resonant thermally activated delayed fluorescence (MR-TADF) emitters have attracted strong interest for organic electroluminescent devices due to their high photoluminescence quantum yield (ΦPL) and superior narrowband emission, resulting in high color purity output in the device. These properties are also crucial for high-performance biological probes, especially red emitters. Orange and red MR-TADF emitters, PhDPA-DiKTa and MeODPA-DiKTa, were designed by decorating the DiKTa core with di([1,1’-biphenyl]-4-yl)amine (PhDPA) and bis(4-methoxyphenyl)amine (MeODPA). Both compounds emit at long wavelengths, with PL of 592 nm (full-width at half-maximum, FWHM = 45 nm) for PhDPA-DiKTa and 633 nm (FWHM= 72 nm) for MeODPA-DiKTa in toluene. As 5 wt% doped films in mCP, PhDPA-DiKTa emits at PL of 617 nm, while MeODPA-DiKTa emits at PL of 655 nm. Both show delayed fluorescence, with delayed lifetimes, td, of 658.4 and 249.2 s, respectively. Water-dispersible glassy organic dots (g-Odots) based on these materials were 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 ΦPL compared to the corresponding 5 wt% doped films in mCP (PL = 618 nm, PL = 77% for PhDPA-DiKTa g-Odots, PL = 663 nm, PL = 38% for MeODPA-DiKTa g-Odots). The TADF character of the emitters was conserved in the g-ODots, with d of 203.9 s for PhDPA-DiKTa g-Odots and 131.6 s for MeODPA-DiKTa g-Odots. These MR-TADF g-Odots were successfully demonstrated as biological imaging probes of HeLa cells.