Efficient Deep-Blue Fluorescent OLEDs with a High Exciton Utilization Efficiency from a Fully Twisted Phenanthroimidazole–Anthracene Emitter

A novel, efficient,
deep-blue fluorescent emitter mPAC, with a
meta-connected donor–acceptor structure containing phenanthroimidazole
(PPI) as the donor and phenylcarbazole-substituted anthracene (An-CzP)
as the acceptor, was designed and synthesized. The meta-linkage provided
a highly twisted molecular conformation, which efficiently interrupts
the intramolecular π-conjugation, resulting in a deep-blue emission.
The optimized nondoped device based on mPAC displayed a deep-blue
emission with a narrow full width at half-maximum of 56 nm and Commission
Internationale de L’Eclairage coordinates of (0.16, 0.09).
The maximum external quantum efficiency (EQEmax) is 6.76%,
corresponding to a high exciton utilization efficiency (EUE) of 59.3–88.9%.
Experimental results and theoretical analysis indicated that the high
EUE is mainly ascribed to the reverse intersystem crossing (RISC)
from T2 to S1, a “hot exciton”
path in which the large T2–T1 energy
gap (1.45 eV) and small T2–S1 energy
difference (0.18 eV, T2 > S1) hamper the
internal
crossing from T2 to T1 and facilitate the RISC
process. For the hot exciton path, the T2 state can be
feasibly arranged to a high energy level, forming a thermal equilibrium
with S1, even slightly higher than the deep-blue S1 to realize an exergonic RISC process, which is usually difficult
for the thermally activated delayed fluorescence emitters.