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 dicyclohex-ylphosphine-substituted biphenyl framework is reported. The pre-ligands form the respective non-palindromic pincer complexes of Pt-II and PdII via double oxidative addition and subsequent comproportionation or C-H activation. The complexes of Pt-II as well as PdII 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 (S-0 ? T-1) excitation of this phosphorescence in the spectral window between the emission and the major singlet-singlet UV absorption. The S-0 ? T-1 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 S-0 ? S-n ? T-1 were determined for the Pt complexes and found to be different in dependence of the ancillary ligand.