Supramolecular Structure-Dependent Thermally-Activated Delayed Fluorescence (TADF) Properties of Organic Polymorphs

The
increasing demand for high-performance organic light-emitting devices
(OLEDs) based on thermally-activated delayed fluorescence (TADF) principle
urgently requires to establish an efficient preparation strategy of
high-performance TADF materials. Although considerable progress has
been made in molecular design approaches for TADF materials, it still
remains an unaddressed issue how molecular aggregated states or supramolecular
structures determine the TADF property of organic solids. Herein,
we present an organic molecule 3-(10H-phenoxazin-10-yl)-9H-xanthen-9-one (3-PXZ-XO) with TADF and polymorph characteristics.
Three kinds of 3-PXZ-XO based crystals A, B, and C with different TADF properties were obtained.
The three crystals display obviously different emission maxima (λem,max: 535 nm for A, 555 nm for B, and 576 nm for C), photoluminescence (PL) quantum
yields (Φ: 51% for A, 28% for B, and
39% for C), and delayed lifetimes of excited states (τTADF: 914 ns for A, 774 ns for B,
and 994 ns for C). Single-crystal X-ray diffraction analyses
revealed that in A, B, and C there are different intermolecular π···π
stacking interaction modes between the adjacent donor planes or acceptor
planes. The different TADF properties of the three polymorphs are
mainly attributed to their different supramolecular structures. Appropriate
donor···donor and acceptor···acceptor
stacking interactions inducing aggregation structures can strongly
enhance TADF property of organic solids.