Organic Micro-/Nanocrystals of SFX-Based Attractor–Repulsor Molecules with the Feature of Crystal-Induced Luminescence Enhancement

Abstract

Spirofluorenexanthanes (SFXs), as burgeoning second-generation spiro compounds beyond spirobifluorenes (SBFs), can serve as versatile scaffolds for low-cost organic semiconductors in various fluorescent, phosphorescent and delayed fluorescent emitters, hole/electron transport materials, charge-trapping elements, and photovoltaic donor/acceptors. However, the detailed effects of structural factors on the diverse aggregate-dependent behaviors are still required for high-performance material designs to illustrate the mechanisms and advantages that are related to not only the nature of a single molecular state but also the aggregation states. Herein, we focus on attractor–repulsor molecules of four SFX-based hole-transport materials including SFX-2-Cz, SFX-2,7-DCz, SFX-2-DPA, and SFX-2,7-DDPA (where Cz is carbazole and DPA is diphenylamine) to examine the nanocrystallization effect (distinguished from amorphous states) on their excitonic photophysics. Both synthesized via the C–N coupling reaction, SFX-2-DPA can form one-dimensional microrods, whereas SFX-2,7-DDPA can be transformed into two-dimensional nanosheets. Furthermore, the photoluminescence quantum yield (PLQY) increases were observed to be ∼530% for SFX-2,7-DDPA (∼410% for SFX-2,7-DCz) and ∼58% for SFX-2-DPA (∼210% for SFX-2-Cz) in micro-/nanocrystal films compared with that of the amorphous films. In addition, the 2D nanosheets of SFX-2,7-DDPA exhibited high fluorescence quantum yields (∼57%) as a whispering-gallery-mode (WGM) microcavity, reaching a laser threshold of 154.5 mW/cm2. In summary, these results demonstrate that SFXs are the key building blocks for the rational attractor–repulsor design of organic nanocrystalline semiconductors with crystal-induced luminescence or stability enhancement (CLoSE) features.

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