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 (ΔES–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.