Tetrathienylethene based red aggregation-enhanced emission probes: super red-shifted mechanochromic behavior and highly photostable cell membrane imaging

Jin-Liang Wang, Jun-Jie Liu, Juliang Yang, Zheng-Feng Chang, Bo Li, Wen-Ting Song, Zujin Zhao, Xiaoding Lou, Jun Dai, Fan Xia

Index: 10.1039/C8QM00008E

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Abstract

A series of branched π-conjugated small molecular red fluorescent probes (TTE-4TPA, TTE-4DPT and TTE-4DTPA) with aggregation-enhanced emission (AEE) characteristics have been synthesized for mechanochromic applications and cell imaging. These red fluorescent probes possess the same 1,1,2,2-tetra(thiophen-2-yl)ethene (TTE) core unit, but different branched terminal groups (triphenylamine (TPA), N,N-diphenylthiophen-2-amine (DPT), and N,N-diphenyl-4-vinylaniline (DTPA)). The photophysical properties, mechanochromic properties, and cell imaging behaviour based on these three red fluorescent probes are investigated. Absorption and emission spectra of TTE-4DPT and TTE-4DTPA showed obvious red-shifts by replacing TPA units with DTPA and DPT terminal groups owing to the enhanced π-conjugation of the molecule and more extended skeletons in comparison to TTE-4TPA. However, TTE-4TPA exhibited the highest fluorescence quantum efficiency of ca. 11 % with the highest αAIE in solid state owing to the most crowded skeletons. TTE-4TPA showed dramatic mechanochromic behaviour (from yellow to deep red powder) with extraordinary red shifts of fluorescence peaks (ca.110 nm) after grinding. In contrast, TTE-4DPT exhibited weak mechanochromic response with slight red shift after grinding. The powder X-ray diffraction (XRD) and differential scanning calorimetry (DSC) experiments indicated that mechanochromic mechanism is associated with morphology change from the more loose crystalline state of TTE-4TPA and TTE-4DPT to the more compact amorphous state. Interestingly, the amorphous state of TTE-4DTPA is a promising cell membrane specific staining AEE probe with low cytotoxicity and excellent photostability. Rainbow imaging and three-dimensional imaging experiments were carried out to further prove the cell membrane specific staining property of TTE-4DTPA. These results demonstrated that the strategy for construction of red fluorescent probes with TTE core and branched TPA analogue terminal groups not only has achieved AEE characteristics with high fluorescence quantum efficiency, but also has obtained and excellent mechanochromic probe and highly photostable cell membrane specific staining probe.