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

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 Ru-II 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 ((MLCT)-M-3) excited state (lifetime (T) = 103.6 ns vs. (T) = 1.58 mu s) 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 mu s) is prolonged, and the T-1 state is relocated to the rhodamine unit. This is the first time that the fundamental triplet-excited-state properties of a Ru-II complex, that is, the triplet lifetime and localization, have been controlled simultaneously. Intramolecular singlet-triplet energy transfer and triplet-triplet energy transfer (TTET, (MLCTIL)-M-3-I-3) 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.