Tailoring Chain‐Length in Through‐Space Charge‐Transfer Donor‐σ‐Acceptor Multifunctional Emitters for Efficient Long‐Term Bioimaging

Abstract This work presents a simple strategy to develop a new series of through‐space charge‐transfer (TSCT) donor–acceptor luminogens with varying alkyl chain lengths as spacer groups (D‐σ‐A), CznDPy [5‐(4‐(9H‐carbazol‐9‐yl) alkyl)‐5H‐pyrazino[2,3‐b] indole‐2,3‐dicarbonitrile] ( n = 2, 3, 4, and 5) displaying rare multifunctional optical behaviors of aggregation‐induced emission (AIE), thermally activated delayed fluorescence (TADF), aggregation‐induced delayed fluorescence (AIDF), mechanoluminescence (ML), and mechanochromism (MCL) properties. Fine‐tuning the flexible chain lengths between donor and acceptor results in variable intermolecular interactions and charge transfer (CT) behavior, thereby modulating their respective optical properties in aggregated/solid states. The integrated emitters exhibit a maximum total quantum yield of 85% with a prolonged lifetime of 344 µs. Considering the challenges existing for developing AIDF molecules with longer lifetimes, enhanced fluorescence efficiency, and tunable emission characteristics, their utility is emphasized for efficient long‐term bioimaging of human breast cancer cells up to 7 days. These molecules constitute the first examples of multifunctional organic TSCT AIDF small molecules with such long lifetimes, tunable emission colors, and higher fluorescence efficiency used for the diagnostic imaging of cancer cells. This study also sheds light on the structure‐property relationships and offers valuable insights into the new class of tailored optical materials for various applications.