Photo-induced electron transfer in a diamino-substituted Ru(bpy)3[PF6]2 complex and its application as a triplet photosensitizer for nitric oxide (NO)-activated triplet–triplet annihilation upconversion

Abstract

A system demonstrating Nitric Oxide (NO) activated Triplet–Triplet Annihilation (TTA) upconversion has been devised, based on a substituted [Ru^II(bpy)₃](PF₆)₂ complex (bpy = 2,2′-dipyridine) bearing a single 1,2-diaminophenyl moiety as an NO activatable triplet photosensitizer (Ru-1), and 9,10-diphenylanthracene (DPA) as a triplet acceptor/emitter. The excited triplet state of Ru-1 is significantly quenched (Φ_T ∼ 22%) by a Photoinduced Electron Transfer (PET) reaction, as confirmed by steady state phosphorescence and transient absorption spectroscopy, and hence Ru-1 does not function as a TTA upconversion sensitizer. However, in the presence of NO/O₂, the 1,2-diaminophenyl group of Ru-1 is transformed into a benzotriazole. This inhibits PET, and the triplet state quantum yield is increased to ca. 85%, switching on the TTA upconversion process which increases by 10-fold. These processes were studied using a combination of steady state and time-resolved luminescence together with transient absorption spectroscopy on the nanosecond and femtosecond timescales. The energy level of the charge transfer state (CTS) for Ru-1 was also obtained electrochemically, supporting the PET mechanism of triplet state quenching and hence the lack of TTA upconversion with Ru-1.