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(PBu 3 ) 2 (4-ES) 2 and cis- Pt(dppe)(4-ES) 2 ( 2 and 3, respectively, where PBu 3 = 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‘-CO 2 Et-bpy)(4-ES) 2 ( 3 and 4, respectively, where t -Bu-bpy = 4,4‘-di- tert -butyl-2,2‘-bipyridine and 4,4‘-CO 2 Et-bpy = 4,4‘-bis(carboethoxy)-2,2‘-bipyridine). The crystal structure of 2 is reported. The complex features a square planar PtP 2 C 2 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 2 C 2 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 3 π,π* excited state that is localized on one of the 4-ES ligands. At low temperatures, the 3 π,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 3 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.