Photophysics and Photochemistry of Stilbene-Containing Platinum Acetylides

A series of four platinum acetylide complexes that contain 4-ethynylstilbene (4-ES) ligands have been subjected to a detailed photochemical and photophysical investigation. The objective of the work is to understand the structure and reactivity of the lowest excited states in π-conjugated Pt-acetylides. The 4-ES ligand was chosen for this work because the excited-state properties of the stilbene chromophore are well understood. In particular, stilbene features fluorescence and phosphorescence, and also undergoes trans−cis photoisomerization from the triplet excited state. Two of the complexes investigated contain phosphine ligands and have the structures trans-Pt(PBu₃)₂(4-ES)₂ and cis-Pt(dppe)(4-ES)₂ (2 and 3, respectively, where PBu₃ = tributylphosphine and dppe = bis-1,2-diphosphinoethane). The other two complexes contain substituted 2,2‘-bipyridine ligands and have the structures Pt(t-Bu-bpy)(4-ES)₂ and Pt(4,4‘-CO₂Et-bpy)(4-ES)₂ (3 and 4, respectively, where t-Bu-bpy = 4,4‘-di-tert-butyl-2,2‘-bipyridine and 4,4‘-CO₂Et-bpy = 4,4‘-bis(carboethoxy)-2,2‘-bipyridine). The crystal structure of 2 is reported. The complex features a square planar PtP₂C₂ geometry and the planes of the phenyl groups in the 4-ES ligands are twisted approximately 60° relative to the plane defined by the PtP₂C₂ core. The series of complexes was examined by using absorption, variable-temperature photoluminescence, and transient absorption spectroscopy. In addition, the photochemical reactivity of the complexes was explored by UV−visible absorption and NMR spectroscopy. The available experimental data indicate that in all of the complexes excitation leads to high-yield production of a ³π,π* excited state that is localized on one of the 4-ES ligands. At low temperatures, the ³π,π* state exhibits strong phosphorescence that is very similar to the phosphorescence of trans-stilbene. At temperatures above the glass-to-fluid temperature of the solvent medium, the ³π,π* state decays rapidly (τ ≈ 40 ns). The decay pathway is believed to involve rotation around the CC bond of one of the 4-ES moieties. This model is supported by the photochemical results, which show that steady-state photolysis leads to trans−cis isomerization of one of the 4-ES ligands with a quantum efficiency of 0.4.