Sensitive and selective PET-based diimidazole luminophore for Zn(II) ions: a structure-activity correlation.

3-(3-Ethoxymethyl-1H-imidazol-2-yl)-3-(1-ethoxymethyl-1H-imidazol-2-yl)-3H-benzo[de]isochromen-1-one, 4, is a novel photoinduced electron transfer (PET) chemosensor that becomess fluorescent upon binding metal ions and shows a strong preference toward ZnII ions. The new bisimidazol PET sensor and its zinc complex were prepared and characterized in terms of their crystal structures, absorption and emission spectra, and orbital energy diagrams. Free 4 is a weakly luminescent species. On the basis of detailed DFT calculations, we suggest that the poor luminescence yield of free 4 originates from its orbital structure in which two π-orbitals of the two imidazole rings, HOMO and HOMO−1, are situated between two π-orbitals of the isochromene-one system, HOMO−2 and LUMO. The absorption and emission processes occur between the two π-orbitals of the isochromene-one system, HOMO−2 and LUMO, and the two π-imidazole orbitals serve as quenchers for the excited state of the molecule through nonradiative processes. Upon...