Large fluorescence response by alcohol from a bis(benzoxazole)-zinc(II) complex: the role of excited state intramolecular proton transfer.

The formation of a bis(HBO) anion is known to turn on the fluorescence to give red emission, via controlling the excited-state intramolecular proton transfer (ESIPT). The poor stability of the formed anion, however, hampered its application. The anion stability is found to be greatly improved by attaching the anion to Zn(2+) cation (i.e., forming zinc complex), whose emission is at λem ≈ 550 and 760 nm. Interestingly, addition of methanol to the zinc complex induces a remarkable red fluorescence (λem ≈ 630 nm, φfl ≈ 0.8). With the aid of spectroscopic studies ((1)H NMR, UV-vis, fluorescence, and mass spectra), the structures of the zinc complexes are characterized. The emission species is identified as a dimer-like structure. The study thus reveals an effective fluorescence switching mechanism that could further advance the application of ESIPT-based sensors.