We studied the photophysical and electroluminescent (EL) characteristics of a series of azaborine derivatives having a pair of boron and nitrogen aimed at the multi-resonance (MR) effect. The computational study with the STEOM-DLPNO-CCSD method clarified that the combination of a BN ring-fusion and a terminal carbazole enhanced the MR effect and spin-orbit coupling matrix element (SOCME), simultaneously. Also, we clarified that the second triplet excited state (T2) plays an important role in efficient MR-based thermally activated delayed fluorescence (TADF). Furthermore, we obtained a blue–violet OLED with an external EL quantum efficiency (EQE) of 9.1%, implying the presence of a pronounced nonradiative decay path from the lowest triplet excited state (T1).
Because the spatial distribution
of frontier molecular orbitals
(FMOs) regulates the thermally activated delayed fluorescence (TADF)
property, researchers synthesize TADF emitters by designing their
FMO distribution. However, it remains challenging to clarify how the
FMO distribution is altered at molecular interfaces. Thus, visualizing
the FMOs at molecular interfaces helps us to understand the working
behavior of TADF emitters. Using scanning tunneling microscopy (STM),
we investigated the electronic structure of a single TADF emitter,
hexamethylazatriangulene-triazine, at molecule–metal and molecule–insulating
film interfaces. FMOs at the molecule–metal interface were
not spatially confined to the donor–acceptor moieties because
of hybridization. Meanwhile, FMOs at the molecule–insulator
interface exhibited spatially separated filled and empty states confined
to each moiety, similar to the calculated gas-phase FMOs. These observations
illustrate that the molecule–environment interaction alters
the spatial distribution of FMOs, proving that STM is a powerful tool
for studying TADF molecules.
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