Abstract
This paper aims at designing chromophores with high aggregation induced emission (AIE) properties for twophoton fluorescence microscopy (2PFM), which is one of the best suited types of microscopy for the imaging of living organism or thick biological tissues. Tetraphenylethylene (TPE) derivatives are common building blocks in the design chromophores with efficient AIE properties. Therefore, in this study, extended TPE AIEgens specifically optimized for twophoton absorption (2PA) are synthetized and the resulting (E/Z) isomers are separated using chromatography on chiral supports. Comparative characterization of the AIE properties is performed on the pure (Z) and (E) isomers and the mixture, allowing us, in combination with powder X-ray diffraction and solid-state NMR to document a profound impact of crystallinity on solid state fluorescence properties. In particular, we show that stereopure AIEgens form aggregates of superior crystallinity, which in turn exhibit a higher fluorescence quantum yield compared to diastereoisomers mixtures. Preparation of stereopure organic nanoparticles affords very bright fluorescent contrast agents, which interest for cellular and intravital two-photon microscopy is illustrated on human breast cancer cell line and on zebrafish embryos.