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) (<b>1</b>), (ppy)­Pt­(acac) (<b>1</b>′), and (bzq)­Pt­(dpm) (<b>2</b>) and newly synthesized (dbq)­Pt­(dpm) (<b>3</b>) (C^∧N = 2-phenylpyridine (ppy), benzo­[<i>h</i>]­quinoline (bzq), dibenzo­[<i>f</i>,<i>h</i>]­quinoline (dbq); O^∧O = dipivolylmethanoate (dpm), acetylacetonate (acac)). Compounds <b>1</b>, <b>1</b>′, <b>2</b>, and <b>3</b> 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 (<i>E</i>_0–0 = 479 nm, <b>1</b>; <i>E</i>_0–0 = 495 nm, <b>2</b>; <i>E</i>_0–0 = 470 nm, <b>3</b>) with disparate radiative rate constants (<i>k</i>_r = 1.4 × 10⁵ s^–1, <b>1</b>; <i>k</i>_r = 0.10 × 10⁵ s^–1, <b>2</b>; <i>k</i>_r = 2.6 × 10⁵ s^–1, <b>3</b>). Zero-field splittings of the triplet states (Δ<i>E</i>_III–I = 11.5 cm^–1, <b>1</b>′; Δ<i>E</i>_III–I < 2 cm^–1, <b>2</b>; Δ<i>E</i>_III–I = 46.5 cm^–1, <b>3</b>) were determined using high resolution spectra recorded in Shpol’skii matrices. The fact that the <i>E</i>_0–0 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