Photophysical
Properties of Cyclometalated Pt(II)
Complexes: Counterintuitive Blue Shift in Emission with an Expanded
Ligand π System
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Abstract
A detailed
examination was performed on photophysical properties
of phosphorescent cyclometalated (C<sup>∧</sup>N)Pt(O<sup>∧</sup>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<sup>∧</sup>N = 2-phenylpyridine
(ppy), benzo[<i>h</i>]quinoline (bzq), dibenzo[<i>f</i>,<i>h</i>]quinoline (dbq); O<sup>∧</sup>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<sup>∧</sup>N ligand precursors. The compounds emit
from metal-perturbed, ligand-centered triplet states (<i>E</i><sub>0–0</sub> = 479 nm, <b>1</b>; <i>E</i><sub>0–0</sub> = 495 nm, <b>2</b>; <i>E</i><sub>0–0</sub> = 470 nm, <b>3</b>) with disparate radiative
rate constants (<i>k</i><sub>r</sub> = 1.4 × 10<sup>5</sup> s<sup>–1</sup>, <b>1</b>; <i>k</i><sub>r</sub> = 0.10 × 10<sup>5</sup> s<sup>–1</sup>, <b>2</b>; <i>k</i><sub>r</sub> = 2.6 × 10<sup>5</sup> s<sup>–1</sup>, <b>3</b>). Zero-field splittings of
the triplet states (Δ<i>E</i><sub>III–I</sub> = 11.5 cm<sup>–1</sup>, <b>1</b>′; Δ<i>E</i><sub>III–I</sub> < 2 cm<sup>–1</sup>, <b>2</b>; Δ<i>E</i><sub>III–I</sub> = 46.5
cm<sup>–1</sup>, <b>3</b>) were determined using high
resolution spectra recorded in Shpol’skii matrices. The fact
that the <i>E</i><sub>0–0</sub> energies do not correspond
to the extent of π-conjugation in the aromatic C<sup>∧</sup>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<sup>∧</sup>N ligands in the triplet state alter the
spin–orbit coupling in the complexes