A Ru(bipyridine)3[PF6]2 Complex with a Rhodamine Unit – Synthesis, Photophysical Properties, and Application in Acid‐Controllable Triplet–Triplet Annihilation Upconversion

Abstract

A ruthenium(II) tris(bipyridine)–rhodamine triad was prepared to control the triplet‐state properties with external stimuli (acid). The rhodamine moiety is an acid‐responsive module, and the RuII coordination center is responsible for triplet‐state formation upon photoexcitation. Through steady‐state UV/Vis absorption and luminescence spectroscopy, electrochemical characterization, and nanosecond/femtosecond transient absorption spectroscopy, we found that the dyad gives a quenched triplet metal‐to‐ligand charge‐transfer (3MLCT) excited state (lifetime τT = 103.6 ns vs. τT = 1.58 µs) in the absence of acid, owing to a photoinduced electron transfer (PET) process. In the presence of acid, the rhodamine unit transforms reversibly from the spirolactam structure into the open‐amide structure; thus, the PET is inhibited, the triplet‐state lifetime (5.70 µs) is prolonged, and the T1 state is relocated to the rhodamine unit. This is the first time that the fundamental triplet‐excited‐state properties of a RuII complex, that is, the triplet lifetime and localization, have been controlled simultaneously. Intramolecular singlet–triplet energy transfer and triplet–triplet energy transfer (TTET, 3MLCT→3IL) were observed for the triad in the femtosecond and nanosecond transient absorption spectra, respectively. The dyad was used for acid‐controllable triplet–triplet annihilation upconversion.