Strongly emissive long-lived3IL excited state of coumarins in cyclometalated Ir(iii) complexes used as triplet photosensitizers and application in triplet–triplet annihilation upconversion

Three different coumarin chromophores were used to prepare the Ir(III) complexes, i.e. coumarin 6 (Ir-1), 7-diethylamino coumarin (Ir-2) and x-phenyl-y-diethylamino coumarin (Ir-3). Ir(ppy)2(bpy)[PF6] was used as the reference complex (Ir-0). The coumarin units were connected to the coordination center of the complexes by using a C≡C π-conjugation bond. The photophysical properties of the complexes were studied with steady state and time-resolved absorption and luminescence spectroscopy, low-temperature luminescence (77 K), as well as DFT calculations. All the three new complexes show strong absorption of visible light (molar absorption coefficient ε is up to 42,000 M(-1) cm(-1) at 487 nm) and a long-lived triplet excited state (τT = 65.9 μs), compared to the reference complex Ir(ppy)2(bpy)[PF6], which shows the typical weak visible light-absorption (ε < 5000 M(-1) cm(-1) in the region beyond 400 nm) and a short triplet excited state (τT = 0.3 μs). Interestingly the long-lived triplet excited states are strongly phosphorescent (quantum yield is up to 18.2%, with emission maxima at 607 nm), which is rare for phosphorescent transition metal complexes. With nanosecond time-resolved transient difference absorption spectroscopy we proved that a coumarin-localized triplet excited state ((3)IL) was produced upon photoexcitation. The complexes were used as triplet photosensitizers for triplet-triplet annihilation upconversion and upconversion quantum yields up to 22.8% were observed. Our results are useful for the preparation of visible light-harvesting transition metal complexes, the study of the triplet excited state of organic chromophores, as well as the application of these visible light-harvesting transition metal complexes as efficient triplet photosensitizers.