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 <i>trans-</i>Pt(PBu<sub>3</sub>)<sub>2</sub>(4-ES)<sub>2</sub> and <i>cis-</i>Pt(dppe)(4-ES)<sub>2</sub> (<b>2 </b>and <b>3</b>, respectively, where PBu<sub>3</sub> = tributylphosphine and dppe = bis-1,2-diphosphinoethane). The other two complexes contain substituted 2,2‘-bipyridine ligands and have the structures Pt(<i>t</i>-Bu-bpy)(4-ES)<sub>2</sub> and Pt(4,4‘-CO<sub>2</sub>Et-bpy)(4-ES)<sub>2</sub> (<b>3</b> and <b>4</b>, respectively, where <i>t</i>-Bu-bpy = 4,4‘-di-<i>tert</i>-butyl-2,2‘-bipyridine and 4,4‘-CO<sub>2</sub>Et-bpy = 4,4‘-bis(carboethoxy)-2,2‘-bipyridine). The crystal structure of <b>2</b> is reported. The complex features a square planar PtP<sub>2</sub>C<sub>2</sub> geometry and the planes of the phenyl groups in the 4-ES ligands are twisted approximately 60° relative to the plane defined by the PtP<sub>2</sub>C<sub>2</sub> 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 <sup>3</sup>π,π* excited state that is localized on one of the 4-ES ligands. At low temperatures, the <sup>3</sup>π,π* state exhibits strong phosphorescence that is very similar to the phosphorescence of <i>trans</i>-stilbene. At temperatures above the glass-to-fluid temperature of the solvent medium, the <sup>3</sup>π,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.