Rational design of asymmetric red fluorescent probes for live cell imaging with high AIE effects and large two-photon absorption cross sections using tunable terminal groups

In this work, we report the synthesis of a family of donor-acceptor (D-A) π-conjugated aggregation-induced red emission materials (<b>TPABT</b>, <b>DTPABT</b>, <b>TPEBT</b> and <b>DTPEBT</b>) with the same core 2,2-(2,2-diphenylethene-1,1-diyl)dithiophene (DPDT) and different amounts and different strengths of electron-donating terminal moieties. Interestingly, <b>TPABT</b> and <b>TPEBT</b>, which have asymmetric structures, give obviously higher solid fluorescence quantum efficiencies in comparison with those of the corresponding symmetric structures, <b>DTPABT</b> and <b>DTPEBT</b>, respectively. In particular, the thin film of <b>TPEBT</b> exhibited the highest fluorescence quantum efficiency of <i>ca.</i> 38% with the highest <i>α</i>_AIE. Moreover, <b>TPEBT</b> and <b>DTPEBT</b> with TPE groups showed two-photon absorption cross-sections of (<i>δ</i>) 1.75 × 10³ GM and 1.94 × 10³ GM at 780 nm, respectively, which are obviously higher than the other two red fluorescent materials with triphenylamine groups. Then, the one-photon and two-photon fluorescence imaging of MCF-7 breast cancer cells and Hela cells, and cytotoxicity experiments, were carried out with these red fluorescent materials. Intense intracellular red fluorescence was observed for all the molecules using one-photon excitation and for <b>TPABT</b> using two-photon excitation in the cell cytoplasm. Finally, <b>TPEBT</b> is biocompatible and functions well in mouse brain blood vascular visualization. It is indicated that these materials can be used as a specific stain fluorescent probe for live cell imaging.