Strongly emissive long-lived 3IL 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.