Tuning the photophysical properties of N^NPt(ii) bisacetylide complexes with fluorene moiety and its applications for triplet–triplet-annihilation based upconversion

Abstract

Fluorene-containing aryl acetylide ligands were used to prepare N^NPt(II) bisacetylide complexes, where aryl substituents on the fluorene are phenyl (Pt-1), naphthal (Pt-2), anthranyl (Pt-3), pyrenyl (Pt-4), 4-diphenylaminophenyl (Pt-5) and 9,9-di-n-octylfluorene (Pt-6) (where N^N ligand = 4,4′-di-tert-butyl-2,2′-bipyridine, dbbpy). All the complexes show room temperature (RT) phosphorescence. The emissive T₁ excited states of Pt-1, Pt-5 and Pt-6 were assigned as metal-to-ligand-charge-transfer state (³MLCT), whereas for Pt-2, Pt-3 and Pt-4, the emissive T₁ excited states were identified as the intraligand state (³IL), based on steady state emission spectra, the lifetime of the T₁ state, emission spectra at 77 K, spin density analysis and the time-resolved transient absorption spectroscopy. Exceptionally long lived T₁ excited state was observed for Pt-3 (τ = 66.7 μs) and Pt-4 (τ = 54.7 μs), compared to a model complex dbbpy Pt(II) Bisphenylacetylide (τ = 1.25 μs). RT phosphorescence of anthracene was observed at 780 nm with Pt-3. The critical role of the fluorene is to move the absorption of the complexes to the red-end of the spectra, but at the same time, without compromising the energy level of the T₁ state of the complexes. The advantage of this unique spectral tuning effect and the long-lived T₁ excited states of Pt-4 was demonstrated by the enhanced performance of the complexes as triplet sensitizers for triplet–triplet annihilation (TTA) based upconversion; an upconversion quantum yield (Φ_UC) up to 22.4% was observed with Pt-4 as the sensitizer. Other complexes described herein show negligible upconversion. The high upconversion quantum yield of Pt-4 is attributed to its intense absorption of visible light and long-lived T₁ excited state. Based on the result of Pt-4, we propose that weakly phosphorescent, or non-phosphorescent transition metal complexes can be used as triplet sensitizers for TTA upconversion, compared to the phosphorescent triplet sensitizers currently used for TTA upconversion. Our results will be useful for the design of transition metal complexes to enhance the light-absorption and thereafter the cascade photophysical processes, without decreasing the T₁ excited state energy levels, which are important for the application of the complexes as triplet sensitizers in various photophysical processes.