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(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 (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 (ΔEIII–I = 11.5 cm–1, 1′; ΔEIII–I < 2 cm–1, 2; ΔEIII–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.