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

A series of branched p-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 p-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 a_AIE in the 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 a weak mechanochromic response with slight red shift after grinding. The powder X-ray diffraction (XRD) and differential scanning calorimetry (DSC) experiments indicated that the mechanochromic mechanism is associated with a 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 a TTE core and branched TPA analogue terminal groups not only has achieved AEE characteristics with high fluorescence quantum efficiency, but also has resulted in an excellent mechanochromic probe and a highly photostable cell membrane specific staining probe.