Near<b>-</b>IR Broadband-Absorbing <i>trans</i>-Bisphosphine Pt(II) Bisacetylide Complexes: Preparation and Study of the Photophysics

Broadband near-IR absorbing <i>trans</i>-bis­(trialkylphosphine) Pt­(II) bisacetylide binuclear complex (<b>Pt–1</b>) was prepared with boron-dipyrromethene (Bodipy) and styrylBodipy acetylide ligands. <b>Pt–1</b> shows strong absorption bands at 731 and 503 nm. Singlet energy transfer (EnT) and efficient intersystem crossing of the central coordinated Bodipy ligand were proposed to be responsible for the efficient funneling of the excitation energy to the triplet-state manifold. Reference complexes containing only a single Bodipy ligand were prepared for comparison (with styrylBodipy ligand <b>Pt–0</b> or Bodipy ligand <b>Pt–2</b>). The molecular structures were confirmed by single-crystal X-ray diffraction. The photophysical properties were studied with steady-state and time-resolved transient absorption spectroscopies, electrochemical characterization, and density functional theory/time-dependent density functional theory calculations. Dual fluorescence was observed for <b>Pt–1</b>. Singlet EnT in <b>Pt–1</b> was proposed based on the fluorescence quenching/excitation spectra, and femtosecond transient absorption spectra (energy transfer rate constant <i>k</i>_EnT = 2.2 × 10¹⁰ s^–1). With nanosecond transient absorption spectra, intramolecular <i>triplet</i>-state energy transfer in <b>Pt–1</b> was proved. Gibbs free energy changes of charge separation indicate that the photoinduced intramolecular electron transfer in <b>Pt–1</b> is thermodynamically prohibited. Intermolecular triplet transfer between <b>Pt–2</b> and <b>L–1</b> was studied with nanosecond transient absorption spectra; the EnT rate and energy transfer efficiency were determined as 3.6 × 10⁴ s^–1 and 94.5%, respectively. The singlet oxygen (¹O₂) photosensitizing of <b>Pt–1</b> was improved as compared to the complexes containing only a single visible-light-absorbing chromophore