Photoswitchable Arylazopyrazole-Based Ruthenium(II) Arene Complexes

A new family of donor-functionalized photoswitchable arylazopyrazole-based ligands (3-5) was synthesized and characterized. The new ligands have been employed to prepare a series of novel photoswitchable half-sandwich ruthenium(II) cymene complexes of the type [(η6-p-cymene)Ru(L)Cl]+ (L = 1-(2-methylenepyridyl)-4-(phenyldiazenyl)-3,5-dimethyl-1H-pyrazole (6a), 1-(2-methylenepyridyl)-4-((4-bromophenyl)diazenyl)-3,5-dimethyl-1H-pyrazole (6b), 1-(2-benzothiazolyl)-3,5-dimethyl-4-arylazopyrazole (7)). All of the complexes have been fully characterized by 1H NMR, 13C NMR, and UV-vis spectroscopy and elemental analyses. In addition, the structure of complex 6a was determined by X-ray crystallography. The UV-vis spectroscopic studies show that both the ligands and metal complexes exhibit excellent trans to cis photoisomerization of the arylazopyrazole moiety upon irradiation with 365 nm UV light. The cis isomer of the compounds can be switched back nearly quantitatively to the more stable trans form with 530 nm irradiation. Coordination of the metal ion has no significant influence on the photoswitching properties of the ligands. DFT and TD-DFT calculations were performed for geometry optimization of the ligands and to complement the experimental findings of the electronic transitions and absorption bands observed. The data obtained from these studies were in good agreement with the experimental results. These excellent photoswitchable properties make the new cationic Ru(II) azo compounds described in these studies interesting candidates for their potential application as photoswitchable systems in catalytic and medicinal chemistry.