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 (λ_em,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 (τ_TADF: 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.