Photophysics, photochemistry and electrochemistry of mixed-ligand platinum(II) complexes with 2-phenylpyridine and 2-(2′-thienyl)pyridine as cyclometalating ligands

Abstract The spectroscopic and electrochemical properties of two series of some [Pt(ĈN)XY] z complexes are studied where (ĈN) is cyclometalated 2-phenyl-pyridinate, Ppy − , and 2-(2-thienyl)pyridinate, Tpy − ; X and Y represent various monodentate and bidentate ligands: Cl − , CN − , CO, (Mor) 3 P=trismorpholinophosphine, Bpy=2,2-bipyridine, Phen=1,10-phenanthroline, En=ethylenediamine, Edp=1,2-bis(diphenylphosphino)ethane, Etdp=1,2-bis(diphenylphosphino)ethene, Edt=1,2-bis(phenylthio)ethane, Mnt 2− =1,2dicyano-1,2-ethylenedithiolate. The lowest excited state in the [Pt(Ppy)XY] + complexes with an unsaturated ligand, (NN) being Bipy and Phen, is assigned to a 3 MLCT {d(Pt)−π ∗ (NN)} transition, while for the Tpy − complexes it corresponds to a 3 {d(Pt)−π*(ĈN)} transition. The lowest excited state of the [Pt(ĈN)Mnt] − complexes is assigned to a 3 {p(S)/d(Pt)−π ∗ Mnt)} transition. In the related complexes, in which a saturated chelating ligand replaces the unsaturated one, (NN)En, (SS)Edt, (PP) Edp, and in Pt(ĈN)(P(Mor) 3 )Cl, Pt(ĈN)COCl, [Pt(ĈN)(CN) 2 ] − , [Pt(ĈN)Cl 2 ] − the lowest excited state is assigned to a 3 {d(Pt)−π ∗ (ĈN)} transition. When replacing the cyclometalating ligand Ppy − with Tpy − the 3 MLCT {d(Pt)−π ∗ (ĈN)} absorption and emission bands are red shifted by ca. 3000 cm −1 . The redox potentials, however, are very similar and depend only on the additional ligands X and Y showing the linear relationship relative to Lever's electrochemical parameters. The values of the calculated radiative rate constants are sensitive both to the identity of the X,Y ligands (Edp − ,CN − ) 3 P,Cl − ) − ) − ,Cl − )) and the (ĈN) ligands (Tpy − − ). This effect is suggested to be due to a reduction in the amount of charge transfer to the (ĈN) ligand induced by an increase in π-back-bonding to the X,Y-ligands. The photochemical activity of this class of complexes in electron transfer reactions as well as in ground state quenching has been demonstrated.