Redox and Photochemistry of Bis(terpyridine)ruthenium(II) Amino Acids and Their Amide Conjugates - from Understanding to Applications

The push‐pull‐substituted bis(terpyridine)ruthenium(II) amino acid [Ru(4′‐tpy‐COOH)(4′‐tpy‐NH2)]2+ ([5]2+; tpy = 2,2′;6′,2″‐terpyridine) with carboxylic acid and amino substituents features exceptional chemical and photophysical properties. Its interaction with photons, electrons, and/or protons results in room‐temperature phosphorescence, reversible oxidative and reductive redox chemistry, reversible acid/base chemistry, proton‐coupled electron transfer, photoinduced reductive and oxidative electron transfer, excited‐state proton transfer and energy transfer reactions. These properties can be fine‐tuned by variations of the bis(terpyridine) amino acid motif, namely extension of the π system and expansion of the chelate ring. Furthermore, the chemically orthogonal functional groups enable the incorporation of this metallo amino acid into peptide architectures in a highly selective manner, even by solid‐phase peptide synthesis protocols. Amide‐linked conjugates with other metal complexes [(terpyridine)ruthenium(II), ferrocene, (bipyridine)rhenium(I), (bipyridine)platinum(II)], organic chromophores, or ZnO nanoparticles underscore the versatile synthetic, redox, and photochemistry of this building block. First real‐world applications of [5]2+ and its derivatives include light‐emitting electrochemical cells (LECs) and dye‐sensitized solar cells (DSSCs). Ruthenium(II) amino acid [Ru(4′‐tpy‐COOH)(4′‐tpy‐NH2)]2+ interacts with photons, electrons, and/or protons, to lead to phosphorescence, oxidative and reductive chemistry, acid/base chemistry, proton‐coupled electron transfer, photoinduced reductive and oxidative electron transfer, excited‐state proton transfer, and energy transfer. Applications include light‐emitting electrochemical and dye‐sensitized solar cells.