Photophysical Properties of Cyclometalated Pt(II) Complexes: Counterintuitive Blue Shift in Emission with an Expanded Ligand π System

Abstract

A detailed examination was performed on photophysical properties of phosphorescent cyclometalated (C(^)N)Pt(O(^)O) complexes (ppy)Pt(dpm) (1), (ppy)Pt(acac) (1'), and (bzq)Pt(dpm) (2) and newly synthesized (dbq)Pt(dpm) (3) (C(^)N = 2-phenylpyridine (ppy), benzo[h]quinoline (bzq), dibenzo[f,h]quinoline (dbq); O(^)O = dipivolylmethanoate (dpm), acetylacetonate (acac)). Compounds 1, 1', 2, and 3 were further characterized by single crystal X-ray diffraction. Structural changes brought about by cyclometalation were determined by comparison with X-ray data from model C(^)N ligand precursors. The compounds emit from metal-perturbed, ligand-centered triplet states (E(0-0) = 479 nm, 1; E(0-0) = 495 nm, 2; E(0-0) = 470 nm, 3) with disparate radiative rate constants (kr = 1.4 × 10(5) s(-1), 1; kr = 0.10 × 10(5) s(-1), 2; kr = 2.6 × 10(5) s(-1), 3). Zero-field splittings of the triplet states (ΔE(III-I) = 11.5 cm(-1), 1'; ΔE(III-I) < 2 cm(-1), 2; ΔE(III-I) = 46.5 cm(-1), 3) were determined using high resolution spectra recorded in Shpol'skii matrices. The fact that the E0-0 energies do not correspond to the extent of π-conjugation in the aromatic C(^)N ligand is rationalized on the basis of structural distortions that occur upon cyclometalation using data from single crystal X-ray analyses of the complexes and ligand precursors along with the triplet state properties evaluated using theoretical calculations. The wide variation in the radiative rate constants and zero-field splittings is also explained on the basis of how changes in the electronic spin density in the C(^)N ligands in the triplet state alter the spin-orbit coupling in the complexes.

Phosphorescence versus Thermally Activated Delayed Fluorescence. Controlling Singlet–Triplet Splitting in Brightly Emitting and Sublimable Cu(I) Compounds

“…A similar behavior is well-known from other transition metal compounds, such as Ir(III) and Pt(II) compounds, [5][6][7][8]11,47,48 and can be related to the energy splitting of the triplet state into three substates. This so-called ZFS is a consequence of SOC.…”

Cyclometallated platinum(ii) complexes containing NHC ligands: synthesis, characterization, photophysics and their application as emitters in OLEDs

“…The former value fits well with the vibrational frequencies of phenylpyridine and clearly points to the strong involvement of the aromatic system into the emissive excited state, whereas the latter might be explained by the difference in vibrational frequencies of the benzoquinoline aromatic systems involved in the emissive transitions. Analogous to the previously studied Pt(II) complexes 3,4,30 this emission may be mostly determined by the mixed 3 IL(π → π*)/ 3 MLCT(dπ → π*) transitions with a substantial prevalence of the former one. It is also worth noting that the shape of the emission bands as well as the excitation spectra for the complexes containing phenylpyridine (1-3, 5, 6), see Fig.…”

“…3,4,30 Photoexcitation of the complexes 1-6 in a degassed CH 2 Cl 2 solution gives green emission with the quantum yield ranging from 0.3 to 8%. Large Stokes shift together with the lifetime values in the microsecond domain are indicative of emission origin from the triplet manifold.…”

Phosphorescent, Cyclometalated Cinchophen-Derived Platinum Complexes: Syntheses, Structures, and Electronic Properties

“…The Pt−C bond length was 1.960(4) Å (compared to [Pt(ppy)(acac)] reported at 1.948 Å). 37 The structure of [Pt(ppy)(acac)] has reported metallophilic interactions with a Pt−Pt distance of ∼3.669 Å (well within the van der Waals radius for Pt), 38 (Table 1). Despite this, and unlike [PtCl(L1)(DMSO)], a longer Pt−Pt distance was observed at ∼4.3413(5) Å, which is at the limit of interactivity based on the van der Waal's radius calculated for Pt by Alvarez.…”