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

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 similar to 530% for SFX-2,7-DDPA (similar to 410% for SFX-2,7-DCz) and similar to 58% for SFX-2-DPA (similar to 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 (similar to 57%) as a whispering-gallery-mode (WGM) microcavity, reaching a laser threshold of 154.5 mW/cm(2). 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.

Automated summary

The research focuses on the properties of spirofluorenexanthanes as organic semiconductor materials, highlighting the impact of structural factors on aggregate behaviors and the enhancement of excitonic photophysics through nanocrystallization, providing insights for high-performance material design.