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

A detailed examination was performed on photophysical properties of phosphorescent cyclometalated (C<^>N)Pt-(O<^>O) complexes (ppy)Pt(dam) (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 (E0-0 = 479 nm, 1; E0-0 = 495 nm, 2; E0-0 = 470 nm, 3) with disparate radiative rate constants (k(r) = 1.4 x 10(5) s(-1), 1; k(r) = 0.10 x 10(5) s(-1), 2; k(r) = 2.6 x 10(5) s(-1), 3). Zero-field splittings of the triplet states (Delta EIII-I = 11.5 cm(-1), 1'; Delta EIII-I < 2 cm(-1), 2; Delta 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 pi-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.