Influence of Ancillary Ligands and Isomerism on the Luminescence of Bis-cyclometalated Platinum(IV) Complexes

The synthesis, characterization, and photophysical properties of a wide variety of bis-cyclometalated Pt­(IV) complexes featuring a C2-symmetrical or unsymmetrical {Pt­(ppy)2} unit (sym or unsym complexes, respectively; ppy = C-deprotonated 2-phenylpyridine) and different ancillary ligands are reported. Complexes sym-[Pt­(ppy)2X2] (X = OTf–, OAc–) were obtained by chloride abstraction from sym-[Pt­(ppy)2Cl2] using the corresponding AgX salts, and the triflate derivative was employed to obtain homologous complexes with X = F–, Br–, I–, trifluoroacetate (TFA–). Complexes unsym-[Pt­(ppy)2(Me)­X] (X = OTf–, F–) were prepared by reacting unsym-[Pt­(ppy)2(Me)­Cl] with AgOTf or AgF, respectively, and the triflate derivative was employed as precursor for the synthesis of the homologues with X = Br–, I–, or TFA– through its reaction with the appropriate anionic ligands. The previously reported complexes unsym-[Pt­(ppy)2X2] (X = Cl–, Br–, OAc–, TFA–) are included in the photophysical study to assess the influence of the arrangement of the cyclometalated ligands. Density functional theory (DFT) and time-dependent DFT calculations on selected derivatives were performed for a better interpretation of the observed excited-state properties. Complexes sym-[Pt­(ppy)2X2] (except X = I–) exhibit phosphorescent emissions in fluid solutions at 298 K arising from essentially 3LC­(ppy) excited states, which are very similar in shape and energy. However, their efficiencies are heavily dependent on the nature of the ancillary ligands, which affect the energy of deactivating ligand-to-ligand charge transfer (LLCT) or ligand-to-metal charge transfer (LMCT) states. The fluoride derivative sym-[Pt­(ppy)2F2] shows the highest quantum yield of this series (Φ = 0.398), mainly because the relatively high metal-to-ligand charge transfer admixture in its emitting state leads to a high radiative rate constant. Complexes unsym-[Pt­(ppy)2X2] emit from 3LC­(ppy) states in frozen matrices at 77 K, but their emissions are totally quenched in fluid solution at 298 K because of the presence of low-lying, dissociative LMCT excited states, which also cause photoisomerization reactions. Complexes unsym-[Pt­(ppy)2(Me)­X] (X = F–, Cl–, Br–, TFA–) show strong emissions in fluid solutions at 298 K (Φ = 0.52–0.63) because deactivating LMCT states lie at high energies. However, derivative unsym-[Pt­(ppy)2(Me)­I] is only weakly emissive at 298 K because of the presence of low-lying LLCT [p­(I) → π*­(ppy)] states.