Solid state emissive azo-Schiff base ligands and their Zn(II) complexes: acidochromism and photoswitching behaviour

The synthesis of Zn(ii) complexes (C1-C2) based on azo-Schiff base ligands 2-((E)-(phenyl-imino)methyl)-4-((E)-(2-(phenylthio)phenyl)diazenyl)phenol (L1) and 4-((E)-(2-(phenyl-thio)phenyl)diazenyl)-2-((E)-(p-tolylimino)methyl)phenol (L2) has been described. These have been thoroughly characterized by spectroscopic studies (IR, H-1, C-13, ESI-MS, electronic absorption, emission) and the structures of C1 and C2 have been determined by X-ray single crystal analyses. The ligands L1 and L2 exhibit reversible acid/base induced "ON-OFF-ON" switching in solution and solid state. Upon exposure to UV light (lambda, 365 nm) C1 and C2 display cis-trans photoisomerisation and after removal of light they transform to more stable trans-form. Electronic absorption and H-1 NMR studies on C1 and C2 revealed rather rapid (1.27 x 10(-1) s(-1)) photoisomerization for C2 relative to C1 (2.7 x 10(-2) s(-1)) which has also been supported by theoretical studies (DFT). The rather fast photoisomerization for C2 compared to C1 may be related to a small energy gap between HOMO and LUMO levels for the respective isomers.