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%.