Photophysics and Photochemistry of Stilbene-Containing Platinum Acetylides

Abstract

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-Bubpy ) 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 CdC 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.

Propellers and Planes: Phosphorescent PtII σ‐Acetylides from Polyaromatic Ligands

“…[8] This provided a means by which to systematically compare both cis and trans structural isomers for the first time. [9] Results and Discussion…”

“…[8b, 9,17] Absorption spectra for cyclised complexes C 2 and T 2 are dominated by typical HBC bands but again show slight redshifts on moving from ligand to cis to trans complex [10] (β-band at 363 nm for L 2 , 365 nm for C 2 , and 367 nm for T 2 ; Figure 4). The extra absorption observed for C 2 and T 2 (C 2 : 414 nm; T 2 : 419 nm) compared to L 2 may be a result of some intraligand character with a contribution from an MLCT transition.…”

Switching between Ligand‐to‐Ligand Charge‐Transfer, Intraligand Charge‐Transfer, and Metal‐to‐Ligand Charge‐Transfer Excited States in Platinum(II) Terpyridyl Acetylide Complexes Induced by pH Change and Metal Ions

“…Keywords: alkyne ligands · charge transfer · excited states · platinum such complexes exhibit a low-lying intraligand p!p* (IL) excited state that is derived from transition between p-type orbitals localized on the terpyridyl or acetylide ligand. [9,13,15,37] On the other hand, our group [35] has investigated the photophysical properties of platinum(II) terpyridyl acetylide complexes bearing amino or azacrown moieties on the acetylide ligands, [PtA C H T U N G T R E N N U N G (terpy)(CCC 6 H 4 -R-4)], where terpy = 2,2':6',2''-terpyridyl and R = NA C H T U N G T R E N N U N G (CH 3 ) 2 or N- [15]monoazacrown-5. In neutral or basic solutions, these complexes exhibit a lowest-energy absorption band attributable to an LLCT transition from the amino-substituted acet-A C H T U N G T R E N N U N G ylide to the terpyridyl acceptor.…”

Synthesis of the First Example of Selenium‐Containing Platinum(II)–Alkenylarylalkynyl Complexes

“…The absorption bands are slightly influenced by the presence of different organic spacers and phosphine groups. The lowest-energy band, in each case, was tentatively assigned to a predominantly π(CϵC)Ǟπ*(CϵC) transition by comparison with related systems [1a,19] along with π(C=C)Ǟπ*(C=C) transition on the alkenylaryl moiety, [23] but they can be considered to have some LMCT character resulting from the possible admixture of platinum (n + 1) p orbitals and a ligand π* orbital. [20] The position of the bands moves to longer wavelengths as a result of the coordination of acetylide ligands with alkenylaryl groups containing two phenylseleno moieties on each alkenyl backbone.…”