The Effect of Monodentate Co-Ligands on the Properties of Pt(II) Complexes Bearing a Tridentate C^N*N-Luminophore.

In this study, the insertion of different monodentate co-ligands on Pt(II) complexes bearing a monoanionic C^N*N luminophore as a tridentate chelator was achieved beyond the previously reported chlorido- ([ PtCl(L) ]) and cyanido-decorated ([ PtCN(L) ]) analogues. To investigate the impact of the auxiliary ligand on the photophysical properties, we introduced a neutral carbonyl-ligand and observed a lower photoluminescence quantum yield ( Φ _L ) than with a cyanido moiety. However, the direct substitution of the chlorido co-ligand by a NO-related derivative was not successful. Interestingly, the attempted reduction of the successfully inserted nitrito- N -ligand in [ PtNO ₂ (L) ] resulted in the oxidation of the Pt(II)-center to Pt(IV), as demonstrated by X-ray diffractometry. For comparison, the trifluoroacetato Pt(II) and chlorido Pt(IV) complexes ([ PtTFA(L) ] and [ PtCl ₃ (L) ], respectively) were also synthesized. The photophysical characterization revealed similar photoluminescence profiles for all complexes, indicating a weak effect of the co-ligand on the excited state; in fact, all complexes display emission from metal-perturbed ligand-centered states (even the Pt(IV) species). Nonetheless, longer excited state lifetimes ( τ _av ) suggest a reduced thermally-activated radiationless deactivation via metal-centered states upon exchange of the chlorido units for other monodentate entities, yet without significantly improving the overall Φ _L at room temperature. The irreversible oxidation waves (measured via cyclic voltammetry) mostly stem from the Pt(II)-center; the co-ligand-related drop of these potentials correlates with the increasing σ -donating capacities of the ancillary ligand. In summary, an enhanced π -acceptor capacity does not necessarily improve the Φ _L and can even impair radiative rates by compromising the perturbative participation of the metal center on the emissive triplet state; in addition, strong σ -donor abilities improve the phosphorescence efficiencies by hampering the thermal population of dissociative electronic configurations related to the participation of antibonding d *-orbitals at the metal center.