Fine Modulation of the Higher-Order Excitonic States toward More Efficient Conversion from Upper-Level Triplet to Singlet

Hot
exciton luminogens capable of harvesting nonemissive triplet
excitons via reverse intersystem crossing from high-order triplet
(hRISC) to singlet have great potential in high-efficiency fluorescent
organic light-emitting diodes (OLEDs). Although spin–orbit
coupling (SOC) is regarded as a key factor affecting the RISC process,
its effects on hot exciton materials are poorly understood. Herein,
we design and synthesize two blue-emitting hot exciton luminogens,
PABP and PAIDO, to study this issue by modulating the excited-state
properties. Theoretical and experimental research contributions demonstrate
that a stronger SOC between energetically close S1 (π–π*)
and Tn (T3, n−π*)
of PAIDO gives rise to faster and more efficient hRISC in comparison
to that of PABP, leading to a higher external quantum efficiency and
a higher exciton utilization efficiency. Crucially, the experimentally
measured hRISC rate (khRISC) of hot exciton
materials is on the order of 107 s–1,
which is much faster than that of the thermally activated delayed
fluorescence materials.