Selectively Investigating Molecular Configuration Effect on Blue Electrophosphorescent Host Performance through a Series of Hydrocarbon Oligomers

Hydrocarbon
oligomers X9F, including S9F, D9F, and T9F as monomer,
dimer, and trimer, respectively, were designed and prepared on the
basis of indirect linkage and 9,9-diphenylfluorene (S9F) as repeat
unit to form planar, linear, and V-shaped configurations without polarity
variation and function amplification. The identical optical and electrochemical
properties of X9F were achieved because of the effectively blocked
intramolecualr electronic interactions by indirect linkage, including
the same T1 value of 2.98 eV, high enough for hosts in
blue phosphorescent organic light-emitting diodes (PHOLEDs), and the
approximate FMO energy levels, which established the basis for selective
investigation of independent configuration effect on the optoelectronic
performance of host materials. Density function theory simulation
manifested the frontier molecular orbital (FMO) location extension
after oligomerization and the specific T1 locations on
peripheral fluorenyls in X9F, giving rise to their different carrier-transporting
abilities and host-localized triplet–triplet annihilation (TTA)
and triplet–polaron quenching (TPQ) effects. As a result, D9F
with the linear and locally unsymmetrical configuration revealed electron-predominant
characteristics for charge balance, restrained triplet interaction
for TTA suppression, and partially separated FMO and T1 locations for TPQ suppression. Consequently, the low driving voltages
and the favorable maximum efficiencies, such as ∼11% for external
quantum efficiency (EQE), as well as reduced roll-offs less than 8%
for EQE at 1000 cd m–2, were achieved by
D9F-based blue PHOLEDs as the highest performance among X9F, in which
device efficiencies were improved by 50% compared to that of conventional
polarized host mCP. It is conceivable that molecular configuration
has significant effects on electrical properties and quenching effects
of organic semiconductors with remarkable influence on intermolecular
interplay and excited-state
locations.