Efficient Energy Conversion in Photochromic Ruthenium DMSO Complexes

The photochromic compounds trans- and cis-[Ru(tpy)(Mepic)(dmso)](OSO2CF3) (2 and 3, respectively; tpy is 2,2‘:6‘,2‘ ‘-terpyridine; Mepic is 6-methyl-2-pyridinecarboxylate; dmso is dimethyl sulfoxide) and cis-[Ru(tpy)(Brpic)(dmso)](PF6) (4; Brpic is 6-bromo-2-pyridinecarboxylate) were prepared and characterized by single-crystal X-ray
crystallography, electrochemistry, NMR, IR, and UV−vis spectroscopy. The geometry labels refer to the relationship
between the carboxylate oxygen of the picolinate ligand and dmso. Electrochemical studies reveal that only the
trans isomer shows S-to-O isomerization following oxidation of Ru(II) and O-to-S isomerization following reduction
of Ru(III). The cis isomers of both complexes feature reversible one-electron Ru(III/II) couples. All complexes undergo
phototriggered S-to-O isomerization following MLCT (metal-to-ligand charge transfer) excitation with quantum yields
(ΦS→O) of 0.79 (2), 0.011 (3), and 0.014 (4). The methyl group in 2 promotes isomerization by hindering rotation
of the dmso ligand about the Ru−S bond. Computational results support this role for the methyl group. Relative
energy calculations show that the barrier to rotation is approximately 8 kcal mol-1. These results suggest that
rotation is an important vibration for isomerization in photochromic ruthenium−dmso complexes.