Phosphorescence
versus Thermally Activated Delayed
Fluorescence. Controlling Singlet–Triplet Splitting in Brightly
Emitting and Sublimable Cu(I) Compounds
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Abstract
Photophysical properties of two highly
emissive three-coordinate
Cu(I) complexes, (IPr)Cu(py<sub>2</sub>-BMe<sub>2</sub>) (<b>1</b>) and (Bzl-3,5Me)Cu(py<sub>2</sub>-BMe<sub>2</sub>) (<b>2</b>), with two different N-heterocyclic (NHC) ligands were investigated
in detail (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene;
Bzl-3,5Me = 1,3-bis(3,5-dimethylphenyl)-1<i>H</i>-benzo[<i>d</i>]imidazol-2-ylidene; py<sub>2</sub>-BMe<sub>2</sub> = di(2-pyridyl)dimethylborate).
The compounds exhibit remarkably high emission quantum yields of more
than 70% in the powder phase. Despite similar chemical structures
of both complexes, only compound <b>1</b> exhibits thermally
activated delayed blue fluorescence (TADF), whereas compound <b>2</b> shows a pure, yellow phosphorescence. This behavior is related
to the torsion angles between the two ligands. Changing this angle
has a huge impact on the energy splitting between the first excited
singlet state S<sub>1</sub> and triplet state T<sub>1</sub> and therefore
on the TADF properties. In addition, it was found that, in both compounds,
spin–orbit coupling (SOC) is particularly effective compared
to other Cu(I) complexes. This is reflected in short emission decay
times of the triplet states of only 34 μs (<b>1</b>) and
21 μs (<b>2</b>), respectively, as well as in the zero-field
splittings of the triplet states amounting to 4 cm<sup>–1</sup> (0.5 meV) for <b>1</b> and 5 cm<sup>–1</sup> (0.6 meV)
for <b>2</b>. Accordingly, at ambient temperature, compound <b>1</b> exhibits <i>two</i> radiative decay paths which
are thermally equilibrated: one via the S<sub>1</sub> state as TADF
path (62%) and one via the T<sub>1</sub> state as phosphorescence
path (38%). Thus, if this material is applied in an organic light-emitting
diode, the generated excitons are harvested mainly in the singlet
state, but to a significant portion also in the triplet state. This
novel mechanism based on two separate radiative decay paths reduces
the overall emission decay time distinctly
