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 (epsilon = 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 ((IL)-I-3 state), for which the lifetime was determined as 16.4 mu s (RT, by the time-resolved transient absorption spectra). The quenching of the (MLCT)-M-3 phosphorescence in Pt-1 is due to the intramolecular energy transfer of (MLCT)-M-3 -> (IL)-I-3. 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. (C) 2012 Elsevier B.V. All rights reserved.