Non-Palindromic C∧C∧P Platinum and Palladium Pincer Complexes Showing Intense Phosphorescence via Direct Spin-Forbidden S₀ → T₁ Excitation

The synthesis of C∧C∧P pre-ligands based on a dicyclohexylphosphine-substituted biphenyl framework is reported. The pre-ligands form the respective non-palindromic pincer complexes of Pt^II and Pd^II via double oxidative addition and subsequent comproportionation or C-H activation. The complexes of Pt^II as well as Pd^II emit similar green phosphorescence efficiently in the solid state, the former also in solution albeit with less intensity. The most fascinating photophysical feature, however, is a direct singlet-triplet (S₀ → T₁) excitation of this phosphorescence in the spectral window between the emission and the major singlet-singlet UV absorption. The S₀ → T₁ excitation spectra show a rich vibronic pattern, which is especially pronounced for the solid samples at cryogenic temperatures. The molar extinction of the lowest-energy singlet-triplet absorption band of the homologous Pt and Pd complexes as well as that of the Pt complex with a different (NHC) ancillary ligand were determined in tetrahydrofuran solutions. Quantum efficiencies of triplet formation (by intersystem crossing) via the "standard" excitation pathway S₀ → S → T₁ were determined for the Pt complexes and found to be different in dependence of the ancillary ligand.