trans-Bis(alkylphosphine) platinum(ii)-alkynyl complexes showing broadband visible light absorption and long-lived triplet excited states

Abstract

Heteroleptic trans-bis(alkylphosphine) platinum(II) bisacetylide complexes Pt-1 and Pt-2 were prepared to achieve broadband absorption of visible light. Two different ethynylBodipy ligands, 2-ethynylBodipy and 2,6-diethynylBodipy or 8-(4′-ethynylphenyl)Bodipy, were used in each complex. Each Bodipy ligand gives strong absorption in the visible spectral region, but at different wavelengths, thus broadband absorption in the visible spectral region was achieved for the Pt(II) complexes (ε is up to 1.85 × 10⁵ M⁻¹ cm⁻¹ in the region of 450–700 nm). Singlet energy transfer from the peripheral coordinated Bodipy to the central coordinated Bodipy (with 2,6-diethynyl substitution) was confirmed by steady state absorption/luminescence spectroscopy, fluorescence excitation spectroscopy and nanosecond/femtosecond ultrafast time-resolved transient absorption spectroscopy. Long-lived triplet excited states were observed for both complexes (τ_T = 63.13 μs for Pt-1 and τ_T = 94.18 μs for Pt-2). Nanosecond time-resolved transient absorption spectroscopy indicated that the triplet excited state of Pt-1 is distributed on both Bodipy units. For Pt-2, however, the T₁ state is confined to the central coordinated Bodipy ligand. These complexes show high singlet oxygen (¹O₂) quantum yields (Φ_Δ = 76.0%). With nanosecond pulsed laser excitation, delayed fluorescence was observed for the complexes (τ_DF = 43.8 μs for Pt-1 and τ_DF = 111.0 μs for Pt-2), which is rarely reported for transition metal complexes. The complexes were used as efficient multi-wavelength excitable triplet photosensitizers for triplet–triplet annihilation upconversion.