Aggregation enhanced two-photon fluorescence of organic nanoparticles

• We synthesized and investigated the photophysical properties of a new donor–π–acceptor molecule BNSN.
• BNSN exhibits a red-shifted emission and an unconventional dramatic increase of emission efficiency in polar solvent.
• The fluorescence intensity and two-photon absorption cross-section all enhanced when the molecules aggregated.
• The nanoparticles of BNSN exhibited aggregation-enhanced two-photon fluorescence in aqueous media.
• Cell imaging capability in living cancer cells was examined with low toxicity and high photostability.

Two-photon excited fluorescence bio-probes have obtained a great deal of interests in biological imaging and sensing applications. We synthesized a new donor–π–acceptor molecular (BNSN) with strong intramolecular charge-transfer (ICT). Interestingly, different from conventional ICT compounds, BNSN molecules present an unusual ICT phenomenon, leading to a red-shifted emission and an increased fluorescence quantum yield (Φ) with increasing the polarity of solvents. Moreover, the fluorescence intensity and two-photon absorption cross-section all enhanced when the molecules aggregated. The nanoparticles of BNSN exhibit strong two-photon excited fluorescence in aqueous media, exhibiting the property of aggregation-enhanced two-photon fluorescence (AE-TPF). Our results demonstrate that the BNSN molecules with AE-TPF effect provide a new molecular type to construct efficient two-photon excited fluorescence nano-probes for bio-imaging.

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