Visible light-harvesting trans bis(alkylphosphine) platinum(ii)-alkynyl complexes showing long-lived triplet excited states as triplet photosensitizers for triplet–triplet annihilation upconversion

Abstract

Symmetric and asymmetric linear trans-bis(tributylphosphine) Pt(II) bis(acetylide) complexes with functionalized aryl alkynyl ligands (coumarin, naphthalimide and phenyl acetylides) were prepared, which show enhanced absorption in the visible region (molar absorption coefficients up to 76 800 M⁻¹ cm⁻¹ at 459 nm) and long-lived triplet excited states (up to 139.9 μs). At room temperature, the naphthalimide acetylide–phenyl acetylide complex (Pt-4) shows dual emission (fluorescence–phosphorescence), whereas other complexes show only fluorescence emission. The triplet excited states of the complexes were studied with nanosecond time-resolved transient difference absorption spectroscopy and DFT calculations on the spin density surface. The complexes were used as triplet photosensitizers for ratiometric O₂ sensing, as well as triplet–triplet annihilation (TTA) upconversion (upconversion quantum yield up to 27.2%). The TTA upconversion of the complexes requires triplet acceptors with different T₁ state energy levels and was studied with nanosecond time-resolved emission spectroscopy. Our results are useful for designing new Pt(II) complexes that show strong absorption of visible light and long-lived triplet excited states, as well as for the application of these complexes as triplet photosensitizers for O₂ sensing, photocatalysis and TTA upconversion.