Fluorescent Tin(IV) Complexes with Schiff Base Ligands: Synthesis, Structures, and Fluorescence Lifetime

Eleven Schiff base tin(IV) complexes of the type [SnCl3(L)] (HL: Schiff base) were synthesized and structurally characterized by X-ray crystallography and 1H NMR spectroscopy, and their properties were investigated. The Schiff bases are prepared from aldehydes with a benzene or a naphthalene ring and 8-aminoquinoline or its derivatives. The complexes adopt geometries close to octahedral, with three chloride ions coordinated meridionally. Eight of the eleven tin complexes show fluorescence in DMSO, and the emission range from bluish green to orange (λmax: 492–545 nm). Two solid tin complexes are fluorescent, although they are not in DMSO, and one tin complex is not fluorescent even in the solid state. The fluorescence quantum yields (Φf) of the complexes depend very much on the ligands (Φf = 0.21–0.022). The fluorescence lifetimes (τ = 0.49–1.76 ns) were measured and the results will be discussed. DFT calculation (B3LYP/Sn, LanL2DZ; others, 6-31G**) shows that the order of the magnitudes of HOMO–LUMO gaps is consonant with that of the emission wavelengths (λmax) of the complexes. All the cyclic voltammograms of the Schiff base ligands and the complexes exhibited irreversible waves and these results suggest that the reduction products of the ligands and the complexes were unstable under the time scale in CV measurements (0.1 V s−1). Relatively good correlation was observed between reduction potentials in DMSO and the energies of LUMO.