Switching Pathways for Reversible Ligand Photodissociation in Ru(II) Polypyridyl Complexes with Steric Effects

The effect of a minor difference in ligand structure is shown to have a large effect on the photochemical pathways followed by two ruthenium(II) polypyridyl based complexes [Ru(CH₃CN) (LL)]²⁺, 1 and 2, where LL is MeN4Py (1,1-di(pyridin-2-yl)-N,N-bis(pyridin-2-yl-methyl)ethan-1-amine) or N4Py (1,1-di(pyridin-2-yl)-N,N-bis(pyridin-2-yl-methyl)methanamine), respectively. In our earlier report we demonstrated near completely reversible two-way photochromism of 1, in which a pyridyl ring dissociated on irradiation with visible light to form the thermally stable 1P, [Ru(CH₃CN)₂(MeN4Py)]²⁺. Complex 1 was recovered upon irradiation in the near-UV. Here, we show that the methyl group in the ligand backbone is critical to the reversibility by impeding the dissociation of one of the two sets of pyridyl rings. Irradiation of 2, which does not bear the methyl group, with visible light results in formation of two thermally stable isomers 2a and 2b, which are characterized by UV-vis absorption, FTIR, ¹H NMR spectroscopy, ESI mass spectrometry, and X-ray crystallography. In contrast to 1P, in both 2a and 2b, a different pyridyl moiety is dissociated. Whereas UV irradiation returns 2a to its original state (2), the overall reversibility is limited by the relative stability of 2b. The changes to the structure of 2 made possible by the increased freedom for all four pyridyl moieties to dissociate allows access to coordination modes that are not accessible thermally opening opportunities toward new catalysts for oxidation chemistry, photochromism and photoswitching.