Room-Temperature Phosphorescence from Pd(II) and Pt(II) Complexes as Supramolecular Luminophores: The Role of Self-Assembly, Metal–Metal Interactions, Spin–Orbit Coupling, and Ligand-Field Splitting

Abstract

The synthesis as well as the structural and photophysical characterization of two isoleptic bis-cyclometalated Pt(II) and Pd(II) complexes, namely [PtL] and [PdL], bearing a tailored dianionic tetradentate ligand (L 2− ) are reported. The isostructural character and intermolecular interactions of [PtL] and [PdL] were assessed by NMR spectroscopy and X-ray diffraction analysis. Both complexes show fully ligand-controlled aggregation, demonstrating that a judicious molecular design can tune the photophysical properties. In fact, by introduction of fluorine atoms on defined positions and methoxy groups on complementary sites, metal−metal interactions can be forced by a head-to-tail stacking. Hence, [PtL] shows luminescence from metal-perturbed ligandcentered or from metal-metal-to-ligand charge-transfer triplet states in diluted solutions, in frozen glasses and in crystals, with high photoluminescence quantum yields and long lifetimes in the microsecond range. At room temperature (RT) in concentrated fluid solutions, the palladium analogue [PdL] surprisingly emits luminescence from aggregated species involving supramolecular interactions. Time-resolved photoluminescence and transient absorption spectroscopies demonstrated that ultrafast intersystem crossing occurs for both metals, which outruns any competitive relaxation pathway from the photoexcited singlet state. Furthermore, we demonstrate that the radiationless deactivation can be suppressed in frozen glassy matrices at 77 K and by intermolecular interactions in fluid solutions at RT. In both cases and as indicated by density functional theory calculations, the lowest emissive state acts as an energy trap from which the thermal population of dissociative states with formal occupation of an antibonding Pdcentered 4d x 2 −y 2 orbital is suppressed. This occurs as the energy gap between the emissive and the dark states surpasses kT.

Enhanced luminescence properties through heavy ancillary ligands in [Pt(C^N^C)(L)] complexes, L = AsPh3 and SbPh3

“…The ISC from singlet excited states into the triplet manifold is an ultrafast process for Pt( ii ) complexes, with reported ISC time constants for organometallic Pt( ii ) complexes being typically <5 ps and often even <1 ps. 83–86…”

Catalyzing Singlet Fission by Transition Metals: Second versus Third Row Effects

“…It is noted, however, that the signals of pyridyl protons in M­(L ref ) 2 Cl 2 (M = Pt, δ 8.78; and M = Pd, δ 8.70) and M­(L pc ) 2 Cl 2 (M = Pt, δ 9.03; and M = Pd, δ 8.95) are shifted downfield by 0.24–0.28 and 0.16–0.20 ppm for the Pt and Pd complexes (Figure S13), respectively, compared to the analogous protons of the free ligands L ref and L pc (δ 8.54 and δ 8.75, respectively) . The observed trends in 1 H shifts for the pyridyl groups are consistent with literature reports, which indicate a decreased electron density upon metal coordination of a pyridyl ligand. − The greater deshielding of these protons when bound to Pt­(II) in comparison to Pd­(II) corresponds to the lower spin–orbit coupling for Pd due to its nuclear charge. , …”

“…51−55 The greater deshielding of these protons when bound to Pt(II) in comparison to Pd(II) corresponds to the lower spin−orbit coupling for Pd due to its nuclear charge. 48,49 The molecular structures of monomer Pt(L pc )(L ref )Cl 2 and reference compounds Pt(L ref ) 2 Cl 2 and Pd(L ref ) 2 Cl 2 have been unambiguously established by X-ray crystallography (Scheme 1 and Figure S20). The structures confirm the trans orientation of the ligands as well as the expected square-planar geometry (full details are provided in the Supporting Information).…”

“…As second- and third-row metals, Pd and Pt are congeners, and both dimers Pd­(L pc ) 2 Cl 2 and Pd­(L pc ) 2 Cl 2 feature metals in the M­(II) oxidation state. Thus, while the valence electron structure is the same, the HAE is reduced in Pd compared to Pt as the effective nuclear charge of Pd is lower than Pt. − In comparison to suitable model compounds that cannot undergo iSF, the electronic influence of the heavy atoms and their impact on the mechanism of SF can be carefully studied. In the presence of Pt in dimer Pt­(L pc ) 2 Cl 2 , analyses show that 1 (T 1 T 1 ) forms directly from iSF.…”

Solid state luminescence of phosphine-EWO ligands with fluorinated chalcone skeletons and their PdX2 complexes: metal-promoted phosphorescence enhancement

“…In fact, these happen not to be experimentally detectable. While many examples of either MLCT (MLCT = Metal to Ligand Charge Transfer) or ILCT (IL = Intra-Ligand Charge Transfer) emitting metal compounds can be found in the literature; in our system, only M L M CT processes ( L M = Metal-perturbed Ligand-centered LUMO) 22 lead to observable phosphorescence.…”