Polymorphism-Mediated Regulation of Thermally Activated Delayed Fluorescence and Room-Temperature Phosphorescence via Intra- and Intermolecular Charge Transfer

Even though there is a vast library of self-assembled organic molecules offering practically infinite possibilities to produce thermally activated delayed fluorescence (TADF) and room-temperature phosphorescence (RTP), obtaining both from a single candidate is a formidable challenge due to the difficulty in controlling the excited-state dynamics. Here, we demonstrate how a single polymorphic molecule with a donor-acceptor-donor (D-A-D) architecture can be used to regulate both TADF and RTP with the help of polymorph engineering. Thermodynamically controlled macrocrystals show TADF due to intermolecular charge transfer (inter-CT), which reduces the singlet-triplet energy gap (Delta ES-T), enhances reverse intersystem crossing (RISC), and boosts the delayed singlet radiative decay. The energy gap in kinetically controlled self-assembled microcrystals is greater due to larger intermolecular distance and hence weaker inter-CT. On the other hand, the presence of stronger intramolecular charge transfer (intra-CT), in addition to restricted RISC in the microcrystals, stabilizes the triplet excitons to favor RTP.