Zinc(II) complexes with Schiff bases derived from ethylenediamine and salicylaldehyde: the synthesis and photoluminescent properties

The effect of the nature of organic ligands and complex formation on the photoluminescent characteristics (relative quantum yield, excited-state lifetime) and thermal stability of tetradentate Schiff bases (H2L), derivatives of salicylaldehyde (H2(SAL)1, H2(SAL)2), o-vanillin (H2(MO)1, H2(MO)2) with ethylenediamine and o-phenylenediamine, and their zinc(II) complexes was studied. Zinc(II) complexes were synthesized by the reaction of H2L with Zn(AcO)2·2H2O in MeOH at room temperature or under reflux. In the case of H2L = H2(SAL)2, H2(MO)1, H2(MO)2, complexes of the composition ZnL·H2O were isolated irrespective of the temperature. For H2L = H2(SAL)1, the reaction results in Zn(SAL)1·H2O at room temperature and in anhydrous dimeric complex [Zn(SAL)1]2 under reflux. Density functional calculations of H2L and ZnL confirmed that (1) luminescence of these compounds is due to the π-π* transition between orbitals of the organic ligand and (2) enhancement of conjugation of the chain and introduction of electron-donating substituents lead to a decrease of the energy gap and, there-fore, to a bathochromic shift of the emission maximum.