Swift photoswitching in a binuclear Zn(II) metallacycle relative to a salen-type ligand

Abstract

The synthesis, characterization and photoswitching behavior of a new salen type Schiff base N,N′-bis(2-hydroxy-5-phenylazobenzilidene)-2,4,6-trimethylbenzene-1,3-diamine (H₂L) and a binuclear zinc(II) metallacycle [{Zn(L)}₂·2H₂O] (1) have been described. Both H₂L and 1 have been characterized by satisfactory elemental analyses, spectral (FT-IR, ¹H, ¹³NMR, ESI-MS, electronic absorption, emission) and electrochemical studies. Crystal structures of both H₂L and 1 have been authenticated by single crystal X-ray diffraction analyses. These exhibit trans–cis photoisomerization upon exposure to UV light (365 nm) and get back to a more stable trans-form after withdrawal of the light. Electronic absorption, emission, ¹H NMR and cyclic voltammetric studies revealed that trans–cis isomerization in metallacycle 1 is rather rapid (∼5.0 s) relative to H₂L (∼25 s) which has been supported by theoretical studies (DFT). Relatively fast photoisomerization in 1 compared to H₂L is facilitated by a small energy gap between HOMO levels of the trans- and cis-isomers. The percentage trans–cis conversion ratio for both H₂L and 1 has been evaluated (55–45, H₂L; 60–40%, 1) by ¹H NMR studies.