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 EQE_max of 5.52%.