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

A detailed examination was performed on photophysical properties of phosphorescent cyclometalated (CN)Pt(OO) complexes (ppy)Pt(dpm) (1), (ppy)Pt(acac) (1?), and (bzq)Pt(dpm) (2) and newly synthesized (dbq)Pt(dpm) (3) (CN = 2-phenylpyridine (ppy), benzo[h]quinoline (bzq), dibenzo[f,h]quinoline (dbq); OO = 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 CN ligand precursors. The compounds emit from metal-perturbed, ligand-centered triplet states (E0-0 = 479 nm, 1; E0-0 = 495 nm, 2; E0-0 = 470 nm, 3) with disparate radiative rate constants (kr = 1.4 × 105 s-1, 1; kr = 0.10 × 105 s-1, 2; kr = 2.6 × 105 s-1, 3). Zero-field splittings of the triplet states (?EIII-I = 11.5 cm-1, 1?; ?EIII-I < 2 cm-1, 2; ?EIII-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 ?-conjugation in the aromatic CN 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 CN ligands in the triplet state alter the spin-orbit coupling in the complexes.