Tetraphenylethylene-incorporated Squaraine Dyes: Structural and Theoretical Insight into the Diverse Emission Behaviors in Solution and Solid State
Endowing organic dyes desired emissive behaviors in diverse state is of great importance for broad practical applications but remains a huge challenge. In this contribution, based on the incorporation of tetraphenylethylene (TPE) into squaraine dyes we successfully synthesized 2TPE-SQ and TPE-SQ, both emitting in dual-state with emission efficiency (ΦPL) up to 0.33 and 0.75, respectively. The intense emission in crystalline state can be ascribed to highly resisted π-π stacking, while their different degrees of intramolecular phenyl motions in 2TPE-SQ and TPE-SQ resulted into weak and moderate emission in solution. Combining density functional theory (DFT) calculations, local excited (LE)/twisted intramolecular charge transfer (TICT) and excited-state intramolecular proton transfer (ESIPT) mechanism accounted well for their very different solution and crystalline emission wavelength. Besides, other emission behaviors such as aggregation-caused quenching (ACQ), crystalline-induced emission enhancement (CIEE) and thermo-/mechanochromism of these two derivatives were also verified in detail, and all these results would bring a new insight into the investigation of organic luminogens in solution and solid state.