3 research outputs found
Thermally Activated Delayed Fluorescence (TADF) and Enhancing Photoluminescence Quantum Yields of [Cu<sup>I</sup>(diimine)(diphosphine)]<sup>+</sup> Complexesî—¸Photophysical, Structural, and Computational Studies
The complexes [CuÂ(I)Â(POP)Â(dmbpy)]Â[BF<sub>4</sub>] (<b>1</b>) and [CuÂ(I)Â(POP)Â(tmbpy)]Â[BF<sub>4</sub>]
(<b>2</b>) (dmbpy = 4,4′-dimethyl-2,2′-bipyridyl;
tmbpy = 4,4′,6,6′-tetramethyl-2,2′-bipyridyl;
POP = bisÂ[2-(diphenylphosphino)-phenyl]Âether) have been studied in
a wide temperature range by steady-state and time-resolved emission
spectroscopy in fluid solution, frozen solution, and as solid powders.
Emission quantum yields of up to 74% were observed for <b>2</b> in a rigid matrix (powder), substantially higher than for <b>1</b> of around 9% under the same conditions. Importantly, it
was found that the emission of <b>2</b> at ambient temperature
represents a thermally activated delayed fluorescence (TADF) which
renders the compound to be a good candidate for singlet harvesting
in OLEDs. The role of steric constraints within the complexes, in
particular their influences on the emission quantum yields, were investigated
by hybrid-DFT calculations for the excited triplet state of <b>1</b> and <b>2</b> while manipulating the torsion angle
between the bipyridyl and POP ligands. Both complexes showed similar
flexibility within a ±10° range of the torsion angle; however, <b>2</b> appeared limited to this range, whereas <b>1</b> could
be further twisted with little energy demand. It is concluded that
a restricted flexibility leads to a reduction of nonradiative deactivation
and thus an increase of emission quantum yield
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<sup>∧</sup>N)ÂPtÂ(O<sup>∧</sup>O) complexes (ppy)ÂPtÂ(dpm) (<b>1</b>), (ppy)ÂPtÂ(acac) (<b>1</b>′), and (bzq)ÂPtÂ(dpm) (<b>2</b>) and newly synthesized
(dbq)ÂPtÂ(dpm) (<b>3</b>) (C<sup>∧</sup>N = 2-phenylpyridine
(ppy), benzoÂ[<i>h</i>]Âquinoline (bzq), dibenzoÂ[<i>f</i>,<i>h</i>]Âquinoline (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
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<sup>∧</sup>N)ÂPtÂ(O<sup>∧</sup>O) complexes (ppy)ÂPtÂ(dpm) (<b>1</b>), (ppy)ÂPtÂ(acac) (<b>1</b>′), and (bzq)ÂPtÂ(dpm) (<b>2</b>) and newly synthesized
(dbq)ÂPtÂ(dpm) (<b>3</b>) (C<sup>∧</sup>N = 2-phenylpyridine
(ppy), benzoÂ[<i>h</i>]Âquinoline (bzq), dibenzoÂ[<i>f</i>,<i>h</i>]Âquinoline (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