Ruthenium (II) polypyridyl complexes as two-photon absorbers and sensitizers: Design, structure-properties relationships and applications

This review addresses recent advancements in understanding multiphoton excitation of ruthenium polypyridyl complexes, highlighting their relevance in the fields of molecular materials and bio-medical applications. Emphasis is placed on two-photon absorption and two-photon excited photoluminescence for which these complexes meet several requirements as two-photon sensitizers. Alongside their appealing one-photon properties, these complexes are photostable under two-photon excitation and can exhibit noticeable two-photon absorption cross-sections in the near-infrared (NIR) region. Moreover, the presence of the ruthenium atom favors spin-orbit coupling, facilitating rapid and efficient population of triplet excited states, thereby extending emission lifetimes, or uncaging reactions, according to their structural characteristics. Indeed, ruthenium polypyridyl complexes offer great versatility in terms of functionalization, enabling property fine-tuning. This review also outlines forthcoming challenges and prospects in the field through representative examples in the area of materials, bio-imaging and phototherapies.