Room temperature long-lived triplet excited state of fluorescein in N^N Pt(II) bisacetylide complex and its applications for triplet–triplet annihilation based upconversions

The room temperature (RT) long-lived triplet excited state of fluorescein was observed in a dbbpy Pt(II) bisfluorescein acetylide complex (Pt-1, where dbbpy = 4,4′-di(tert-butyl)-2,2′-bipyridine). The complex shows strong absorption of visible light (ε = 24,600 M−1 cm−1 at 457 nm). Interestingly, the phosphorescence of the coordination centre is completely quenched and only the fluorescence of ligand was observed for Pt-1 at RT. At 77 K, a phosphorescence band at 667 nm was observed. Spin density analysis and nanosecond time-resolved transient difference absorption spectroscopy proved that the lowest-lying triplet excited state of the complex Pt-1 is localized on the fluorescein acetylide ligand (3IL state), for which the lifetime was determined as 16.4 μs (RT, by the time-resolved transient absorption spectra). The quenching of the 3MLCT phosphorescence in Pt-1 is due to the intramolecular energy transfer of 3MLCT → 3IL. The complex Pt-1 was used as the triplet sensitizer for the triplet–triplet annihilation (TTA) based upconversion. Our results will be useful for preparation of transition metal complexes that show intense absorption of visible light and long-lived triplet excited states, these photophysical properties are useful for applications in photovoltaics, photocatalysis or upconversions, etc.