Thermally Activated Delayed Fluorescence and Aggregation Induced Emission with Through-Space Charge Transfer

Emissive molecules comprising a donor
and an acceptor bridged by
9,9-dimethylxanthene, were studied (XPT, XCT, and XtBuCT). The structures
position the donor and acceptor with cofacial alignment at distances
of 3.3–3.5 Å wherein efficient spatial charge transfer
can occur. The quantum yields were enhanced by excluding molecular
oxygen and thermally activated delayed fluorescence with lifetimes
on the order of microseconds was observed. Although the molecules
displayed low quantum yields in solution, higher quantum yields were
observed in the solid state. Crystal structures revealed π–π
intramolecular interactions between a donor and an acceptor, however,
the dominant intermolecular interactions were CH···π,
which likely restrict the molecular dynamics to create aggregation-induced
enhanced emission. Organic light emitting devices using XPT and XtBuCT
as dopants displayed electroluminescence external quantum efficiencies
as high as 10%.