Phenyl Spacer Modulation of 2,5-Substituted D–A-Type Siloles for Efficient Nondoped OLEDs

Organic
fluorophores with planar structures applied in organic
light-emitting diodes (OLEDs) favor π–π stacking
in aggregates, resulting in low photoluminescence quantum yields (PLQYs).
Incorporation of molecular rotors such as phenyls on the periphery
of fluorophores such as silole can prevent π–π
stacking in aggregates to minimize fluorescence quenching. In this
study, we introduced electron-donor (D) units (9,9-dimethylacridine
and phenoxazine) at the positions 2 and 5 of tetraphenylsilole to
form D–A-type molecules, in which the tetraphenylsilole unit
acts as an electron acceptor (A). Compared to the molecular design
of D–A-type siloles, further modification via the addition
of a phenyl spacer group between D and A units leads to the resulting
D–Ph–A-type silole molecules (Silole-Ph-AC and Silole-Ph-PXZ, where Ph-AC is 10-biphenyl-9,9-dimethylacridine
and Ph-PXZ is N-biphenylphenoxazine) being more twisted
and less prone to π–π stacking molecular structures,
resulting in improved photophysical properties such as a blue-shifted
emission, a higher fluorescence efficiency, and a better external
quantum efficiency (EQE) when they are applied in nondoped OLEDs.
Among them, the Silole-Ph-AC-based OLED exhibits the
best EQEmax of 5.52%.