Isomerization initiated by photoinduced ligand dissociation in Ru(ii) complexes with the ligand 2-p-tolylpyridinecarboxaldimine

Abstract

The excited state reactivity of Ru(II) complexes continues to be a topic of intense investigation because of their widespread use in applications related to solar energy conversion, such as in photoswitches and for medicinal chemistry. In an effort to gain further understanding of photoinduced ligand exchange and isomerization in Ru(II) complexes, various isomers of the formula [Ru(PTPI)₂(CH₃CN)₂]²⁺ (PTPI = 2-p-tolylpyridinecarboxaldimine) were synthesized and characterized, as well as the tris-heteroleptic complexes cis-[Ru(bpy)(PTPI)(CH₃CN)₂]²⁺ (1, bpy = 2,2′-bipyridine) and cis-[Ru(bpy)(PAP)(CH₃CN)₂]²⁺ (2, PAP = 2-(phenylazo)pyridine). All of the complexes containing the PTPI ligand undergo photoinduced ligand exchange of the CH₃CN ligands with the coordinating solvent. Each isolated di-substituted PTPI complex, however, also undergoes isomerization of the bidentate PTPI ligands upon irradiation in CH₃CN to produce the same ratio of a mixture of 63% ε-[Ru(bpy)(PTPI)(CH₃CN)₂]²⁺ and 37% β-[Ru(bpy)(PTPI)(CH₃CN)₂]²⁺. Experiments reveal that the isomerization of PTPI only occurs after the process of ligand dissociation is initiated by light. Evidence of isomerization following photoinduced ligand dissociation was also observed in 1, but not for the analogous complex 2. Electronic structure calculations, which included the relative overall energies of the isomers, and emission data were used to explain the results. The work presented herein may be useful in the design of new complexes for solar energy conversion or photoswitching applications.