Photobase‐Driven Excited‐State Intramolecular Proton Transfer (ESIPT) in a Strapped π‐Electron System

We report a new design strategy for an excited‐state intramolecular proton transfer (ESIPT) fluorophore that can be used in acidic media. A photobasic pyridine‐centered donor‐acceptor‐donor‐type fluorophore is combined with a basic trialkylamine “strap”. In the presence of an acid, protonation occurs predominantly at the amine moiety in the ground state. A single‐crystal X‐ray diffraction analysis confirmed the formation of a pre‐organized intramolecular hydrogen‐bonded structure between the resulting ammonium moiety and the pyridine ring. Upon excitation, the intramolecular charge‐transfer transition increases the basicity of the pyridine moiety in the excited state, resulting in proton transfer from the amine to the pyridine moiety. Consequently, the fluorophore takes on a polymethine‐dye character in the ESIPT state, which gives rise to significantly red‐shifted emission with an increased fluorescence quantum yield. A pyridine‐centered donor‐acceptor‐donor‐type fluorophore acts as a photobase to induce strap‐to‐π excited‐state intramolecular proton transfer (ESIPT) from a strapped ammonium moiety. This new design of an ESIPT fluorophore, which is suitable for use in acidic media, produces a significantly red‐shifted orange emission with an increased fluorescence quantum yield.