Excited State Intramolecular Proton Transfer in Electron-Rich and Electron-Poor Derivatives of 10-Hydroxybenzo[<i>h</i>]quinoline

Eight previously inaccessible derivatives of 10-hydroxybenzo­[<i>h</i>]­quinoline were prepared via a straightforward strategy comprising formation of the benzo­[<i>h</i>]­quinoline skeleton followed by C–H acetoxylation at position 10. The occurrence of excited state intramolecular proton transfer (ESIPT) was detected in all cases since emission was observed only from the excited keto-tautomer. Studies on derivatives bearing both electron-donating and electron-withdrawing groups adjacent to the pyridine ring allowed us to identify some design patterns giving rise to NIR emission and large Stokes shifts. For a derivative of 10-hydroxybenzo­[<i>c</i>]­acridine, emission at 745 nm was observed, one of the lowest energy fluorescence ever reported for ESIPT system. On the basis of time-resolved measurements, proton transfer was found to be extremely fast with time constants in the range (0.08–0.45 ps)