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, 13C NMR, 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. This journal is © the Partner Organisations 2014.