Exceedingly Large Stokes Shift Induced by Low‐Barrier H‐Bond‐Assisted Internal Charge Transfer

Abstract By only replacing phenyl group with N ‐methylpyridinium in a 2‐(2‐hydroxyphenyl)benzothiazole (HBT) derivative HBT‐s‐Ph, which is an excited state intramolecular proton transfer (ESIPT)‐active dye, the conjugated HBT– N ‐methylpyridinium hybrid HBT‐Py , emits high‐efficiency fluorescence ( Φ f : 29 %) with an exceedingly large Stokes shift (213 nm) even in aqueous solution. Emission spectroscopy and computational calculations indicate that this typical photophysical property owes to low‐barrier H‐bond (LBHB)‐assisted internal charge transfer (ICT), not ESIPT.