Hidden Lewis acidity: Studies on the medium and structure dependent fluorescence of zinc(II) complexes

Three new zinc(II) coordination units [Zn(1–3)] based on planar-directing tetradentate Schiff base-like ligands H2(1–3) were synthesized. Their solid-state structures were investigated by single crystal X-ray diffraction, showing the tendency to overcome the square-planar coordination sphere by axial ligation. Affinity in solution towards axial ligation has been tested by extended spectroscopic studies, both in the absorption and emission mode. The electronic spectrum of the pyridine complex [Zn(1)(py)] has been characterized by multiconfiguration pair-DFT to validate the results of extended TD-DFT studies. Green emission of fluorescence-silent solutions of [Zn(1–3)] in chloroform could be switched on in the presence of potent Lewis-bases. While interpretation in terms of an equilibrium of stacked/non-fluorescent and destacked/fluorescent species is in line with precedents from literature, the sensitivity of [Zn(1–3)] was greatly reduced. Results of a computation-based structure search allow to trace the hidden Lewis acidity of [Zn(1–3)] to a new stacking motif, resulting in a strongly enhanced stability of the dimers.