Dalton Transactions PAPER Cite this: Dalton Trans., 2014, 43, 6365 Received 23rd January 2014, Accepted 4th February 2014 DOI: 10.1039/c4dt00248b www.rsc.org/dalton Swift photoswitching in a binuclear Zn(II) metallacycle relative to a salen-type ligand† Amit Kumar, a Rampal Pandey, a,c Rakesh Kumar Gupta, a Veenu Mishra, b Shaikh M. Mobin b and Daya Shankar Pandey* a 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 (H 2 L) and a binuclear zinc(II) metallacycle [{Zn(L)} 2 ·2H 2 O] (1) have been described. Both H 2 L and 1 have been characterized by satis- factory elemental analyses, spectral (FT-IR, 1 H, 13 C NMR, ESI-MS, electronic absorption, emission) and electrochemical studies. Crystal structures of both H 2 L 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, 1 H NMR and cyclic voltammetric studies revealed that trans–cis isomerization in metallacycle 1 is rather rapid (∼5.0 s) relative to H 2 L (∼25 s) which has been supported by theoretical studies (DFT). Relatively fast photoisomerization in 1 compared to H 2 L is facilitated by a small energy gap between HOMO levels of the trans- and cis-isomers. The percentage trans–cis conversion ratio for both H 2 L and 1 has been evaluated (55–45, H 2 L; 60–40%, 1) by 1 H NMR studies. Introduction Azobenzene and azophenol derivatives as well as complexes based on it have drawn enormous research interest owing to their fascinating characteristics. 1 Organic materials containing a diazo core capable of displaying photoswitching find poten- tial applications in pharmaceuticals, non-linear optics, optical data storage, dye-sensitized solar cells and photoswitching devices, etc. 2–7 The exciting properties of the azo dyes make them extremely useful in textile dyeing as coloring agents and in biological systems. 8 Photoswitching is essentially a pheno- menon that involves reversible chemical changes in photo- active compounds 9–11 and influences their physicochemical properties like dielectric constant, refractive index, and redox potential. 11–13 Further, photoswitching is very common in organic diazo compounds and plenty of reports are available in the literature. 12 Azobenzene derivatives and related compounds usually exist as their stable trans-isomer which changes to cis-form upon photoirradiation accompanied by structural changes; however, the process is reversible. 7 The trans–cis photoisomerization effectively takes place due to mech- anical stress 13 or electrostatic stimulation in the structure, 14 whereas the thermal stability of the trans-isomer endorses spon- taneous reverse cis–trans isomerization under visible light, in the dark, or upon heating. 15,16 The UV/vis induced reversible trans– cis and cis–trans isomerization in the azobenzene based systems involve π–π* and n–π* transitions. 17,18 The salen type ligands invariably form transition metal complexes through strongly coordinating N,O-donor sites. Such derivatives based on substituted phenylene-1,3-diamine and salicyladehyde form stable binuclear metallacycles 19 which find wide applications in asymmetric catalysis, 20 metal ion sensing 19,21 and DNA cleavage. 22 On the other hand, incor- poration of a metal center through a diazo moiety makes the photoswitching interesting as complexation leads to signifi- cant modulation in the physical and chemical properties of the ligands. 11,23 To understand the photodynamics of the diazo core a comparison of the photoswitching behavior of the diazo based ligands and their complexes is a must as com- plexation is followed by drift of electron density from the diazo core to the metal center leading to a change in dipole moment of the diazo moiety. 24 However, in most of the known com- plexes photoisomerization of the diazo ligands is inhibited by its chelation to the metal center through one nitrogen of the –NvN– core and the carbon/nitrogen/oxygen donor atom of the other moiety. 7,25 † Electronic supplementary information (ESI) available: 1 H, 13 C NMR spectra, ESI-MS, UV/vis spectra, weak interactions in crystal system and frontier orbitals by DFT calculation. CCDC 941801 (H 2 L) and 941802 (1). For ESI and crystallo- graphic data in CIF or other electronic format see DOI: 10.1039/c4dt00248b a Department of Chemistry, Faculty of Science, Banaras Hindu University, Varanasi 221 005, India. E-mail: dspbhu@bhu.ac.in; Fax: +91 542 2368174; Tel: +91 542 6702480 b Discipline of Chemistry, Indian Institute of Technology, Indore 452 017, India c Department of Chemistry, Dr. Hari Singh Gour University, Sagar (M.P.) - 470 003, India This journal is © The Royal Society of Chemistry 2014 Dalton Trans. , 2014, 43, 6365–6376 | 6365 Published on 06 March 2014. Downloaded by Banaras Hindu University on 11/04/2014 09:28:22. View Article Online View Journal | View Issue