Exploring Photoswitchable Properties of Two Nitro Nickel(II)Complexes with (N,N,O)-Donor Ligands and Their Copper(II)Analogues

Two photoswitchable nickel(II) nitro coordination com-pounds and their copper(II) analogues are reported. In all these systems, the metal center is chelated by (N,N,O)-donor ligands containing either 2-picolylamine or 8-aminoquinoline fragments. The studied compounds werethoroughly investigated using crystallographic and spectroscopic techniquessupplemented by computational analysis. They are easy to synthesize andstable, and all compounds undergo the nitro group isomerization reaction.Nevertheless, there are significant differences between the copper and nickelsystems regarding their structural and switchable properties. According to thesolid-state IR spectroscopy results, 400-660 nm light irradiation of theground-state (eta 2-O,O ')-Kappa-nitrito copper(II) complexes at 10 K induces arather moderate conversion to a metastable linkage isomer, which is visibleonly up to approximately 60-80 K. In turn, upon visible light irradiation (ca.530 nm excitation wavelength), the ground-state nitro isomers of the examined nickel(II) complexes transform into theendo-nitritoforms. It was possible to achieve about 35% conversion for both nickel(II) systems and to determine the resulting crystal structuresat 160 K in the case of single crystals after 30-45 min of exposure to LED light (crystals decayed with longer irradiation), androughly 95% conversion was achieved for thin-film samples as indicated by the IR spectroscopy results. Traces of theendo-nitritolinkage isomers remained up to 200-220 K, and the isomerization reaction was proven to be fully reversible.

Automated summary

Two photoswitchable nickel and copper coordination compounds were studied, and their structural and switchable properties were compared. The copper complexes showed moderate conversion to a metastable linkage isomer under specific light irradiation, while the nickel complexes transformed into theendo-nitritoforms. The isomerization reaction was proven to be fully reversible.