Tetraphenylethylene-incorporated squaraine dyes: structural and theoretical insights into the diverse emission behaviors in solution and solid state

• Published in: Journal of materials chemistry. C, Materials for optical and electronic devices Materials for optical and electronic devices, 2020-04, Vol.8 (13), p.4549-4556
• Main Authors: Shao, Qingqing, Liang, Kangli, Ling, Huan, Wang, Yigang, Yan, Zhihong, Xia, Guomin, Wang, Hongming
• Format: Article
• Language: English
• Subjects: Charge transfer; Crystal structure; Crystallinity; Crystallography; Density functional theory; Dyes; Emission; Mathematical analysis; Solid state
• ISSN: 2050-7526; 2050-7534
• DOI: 10.1039/c9tc06406k

Endowing organic dyes with desired emissive behaviors in diverse states is of great importance for broad practical applications, but it remains a huge challenge. In this study, based on the incorporation of tetraphenylethylene (TPE) into squaraine dyes, we successfully synthesized 2TPE-SQ and TPE-SQ and both exhibited dual state emission with emission efficiencies ( Φ PL ) up to 0.33 and 0.75, respectively. The intense emission in the crystalline state can be ascribed to highly restricted π-π stacking, while the different degrees of intramolecular phenyl motions in 2TPE-SQ and TPE-SQ result in weak and moderate emissions in solution. Combining density functional theory (DFT) calculations, local excited (LE)/twisted intramolecular charge transfer (TICT) and excited-state intramolecular proton transfer (ESIPT) mechanisms were proposed and accounted well for their very different emission wavelengths in solution and crystalline state, respectively. Besides, other emission behaviors such as aggregation-caused quenching (ACQ), crystalline-induced emission enhancement (CIEE) and thermo-/mechanochromism of these two derivatives were verified in detail, and all these results would bring a new insight into the investigation of organic luminogens in solution and solid state. We report TPE-incorporated squaraines with dual-state emission and decode their LE/TICT mechanism in solution and ESIPT mechanism in the crystalline state.