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.