Indolo‐Fused MR‐TADF Materials and their Application in Organelle‐Targeted Bioimaging

Organic multiresonance thermally activated delayed fluorescence (MR‐TADF) emitters offer a promising platform for achieving narrowband emission with high color purity, however, tuning emission color into the red region while maintaining high efficiency remains a key challenge. Herein, we report a series of indolo‐fused quinolino[3,2,1‐de]acridine‐5,9‐dione (QAO)‐based MR‐TADF emitters mN‐QAO, mN‐SQAO, pN‐QAO, and pN‐SQAO to systematically investigate the impact of fusion position and sulfur locking on their photophysical properties. Positional variation of the indole unit enables efficient color tuning from blue (λ max = 455 nm) to green emission (λ max = 516 nm), while preserving narrowband emission (FWHM = 21–24 nm). Incorporation of a sulfur lock further modulates the electronic structure, promoting enhanced charge‐transfer (CT) and red‐shifted emission with moderate photoluminescence quantum yields (∼48%). Combined experimental and theoretical analyses reveal that sulfur locking enhances HOMO delocalization and introduces a balance between short‐ and long‐range CT, which governs the observed emission behavior. Furthermore, the red‐emitting pN‐SQAO was employed to fabricate water‐dispersible glassy organic dots (g‐Odots), which retain MR‐TADF characteristics in aqueous media and exhibit efficient cellular uptake. Notably, these nanostructures demonstrate lysosomal localization, while further functionalization with a triphenylphosphonium moiety (Mito‐pN‐SQAO) enables selective mitochondrial targeting. This represents the first example of g‐Odots achieving targeted mitochondrial localization. This study establishes a versatile molecular design strategy for tuning MR‐TADF properties and expands their applications into targeted bioimaging.