Tunable two-photon absorption near-infrared materials containing different electron-donors and a π-bridge center with applications in bioimaging in live cells

A series of asymmetrical D-pi-D type thiophene-based chromophores with small sizes and different electron-donating groups were designed and synthesized in high yield. Single-crystal X-ray diffraction analysis and theoretical calculations were carried out to further explore the electronic structural features of the chromophores. It was observed that multiple C-H...pi interactions and C-H...O hydrogen bonds played an important role in crystal stacking. Systematic investigations, including fluorescence quantum yields, fluorescence lifetime, and two-photon absorption (2PA) cross sections, revealed that the photophysical properties of the thiophene-based chromophores can be tunable by the modulation of electron-donating terminal units. Furthermore, by introducing a carbonyl group in the p-bridge center, one of the chromophores with a small size exhibited significantly enhanced two-photon absorption with a 36-fold increase of the 2PA cross section compared with that of the corresponding chromophore. Finally, due to their superior 2PA character in the near-infrared region (700-900 nm), two-photon excited bioimaging applications were carried out for the chromophores. The results of live cell imaging experiments showed that the chromophores can be effectively penetrated into the cytosol of HepG2 cells and their physiological activity can be detected in vitro using two-photon fluorescence microscopy.