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.