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Thermally Activated Delayed Fluorescence (TADF) and Enhancing Photoluminescence Quantum Yields of [CuI(diimine)(diphosphine)]+ ComplexesPhotophysical, Structural, and Computational Studies
journal contribution
posted on 2014-10-20, 00:00 authored by Charlotte L. Linfoot, Markus J. Leitl, Patricia Richardson, Andreas F. Rausch, Oleg Chepelin, Fraser J. White, Hartmut Yersin, Neil RobertsonThe complexes [Cu(I)(POP)(dmbpy)][BF4] (1) and [Cu(I)(POP)(tmbpy)][BF4]
(2) (dmbpy = 4,4′-dimethyl-2,2′-bipyridyl;
tmbpy = 4,4′,6,6′-tetramethyl-2,2′-bipyridyl;
POP = bis[2-(diphenylphosphino)-phenyl]ether) 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 2 in a rigid matrix (powder), substantially higher than for 1 of around 9% under the same conditions. Importantly, it
was found that the emission of 2 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 1 and 2 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, 2 appeared limited to this range, whereas 1 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.