Red-light excitable fluorescent platinum(ii) bis(aryleneethynylene) bis(trialkylphosphine) complexes showing long-lived triplet excited states as triplet photosensitizers for triplet–triplet annihilation upconversion

Abstract

Symmetric and asymmetric platinum(II) bis(phosphine) bis(aryleneethynylene) complexes that show strong absorption of visible light and long-lived triplet excited states with boron dipyrromethane (Bodipy) chromophore visible light-harvesting antennae attached to the Pt(II) centres were prepared for the first time. The bisnuclear complexes Pt-2 and Pt-3, with two Pt(II) coordination centres connected to the π-core of the Bodipy ligands, show red-shifted absorption (e.g.Pt-2, λ_abs = 643 nm, ε = 42 300 M⁻¹ cm⁻¹) compared to the mononuclear Pt(II) complexes (Pt-1 and Pt-4), in which only one Pt(II) coordination centre is connected to the Bodipy chromophore (e.g.Pt-1, λ_abs = 570 nm, ε = 38 300 M⁻¹ cm⁻¹). The complexes are excitable with red-light, which is rare for transition metal complexes. All the complexes show fluorescence at room temperature (627–671 nm, Φ_F = 1.4–6.7%), and weak phosphorescence. Long-lived Bodipy ligand-localized triplet excited states are observed for all the complexes (τ_T = 57.9–72.4 μs) with nanosecond transient absorption spectra, which is supported by spin density analysis. The platinum(II) bis(phosphine) bis(aryleneethynylene) complexes are used as triplet photosensitizers for the first time for red-light excited triplet–triplet annihilation (TTA) based upconversion and upconversion quantum yields of up to 19.0% are observed, and the anti-Stokes shift is up to 0.82 eV. The effects of different triplet energy transfer driving forces on the TTA upconversion with perylene and perylenebisimide as triplet acceptors are investigated. Our results are useful for the preparation of visible light-harvesting linear platinum(II) phosphine alkynyl complexes and for their applications in photocatalysis, non-linear optics and TTA upconversions.