Hydrosoluble two-photon absorbing materials: A series of sulfonated organic inner salts in biological imaging application

• A novel series of two-photon absorptivity hydrosoluble salts have been synthesized.
• The molecular geometry optimization for calculation results from the structure information.
• The dyes exhibit two-photon absorption in near infrared range.
• SL4 displays 2PA activity when dissolved in water.
• SL4 offers a potential application in two-photon fluorescence imaging.

Designed to achieve high two-photon absorptivity dyes, a novel series of D-π-A type hydrosoluble inner salts (SL1, SL2, SL3 and SL4) based on pyridinium have been synthesized and fully characterized. Photophysical properties including linear absorption, one-photon excited fluorescence and two-photon absorption properties were systematically investigated. These results show that there is a shift in the absorption and emission maxima with variation of solvent polarity. Furthermore, the chromophore SL4 shows the strongest intensity of two-photon excited fluorescence among the four chromophores and the largest two-photon absorption cross-section in the near infrared region (∼960 nm), indicating the steric bulk of the electron-donating terminal group plays an important role in two-photon absorption behavior. In addition, SL4 displays two-photon absorption activity when dissolved in water, which can be effectively used in two-photon bioimaging for its reasonable two-photon absorption cross-section. SL4-labeled cells revealed a distinct vesicular localization, most likely representing lysosomal compartments.