Achieving two-photon fluorescence bioimaging and photodynamic therapy for D–A conjugated polymers through manipulating twisted intramolecular charge transfer

Two-photon fluorescence bioimaging holds immense promise for deep tissue imaging. However, its widespread application is hampered by the relatively low fluorescence quantum yields (ϕPL) and small two-photon absorption cross-sections (δ2). Herein, we report the achievement of high ϕPL for donor–acceptor alternating conjugated polymers comprising dibenzothiophene-S,S-dioxide derivatives as electron acceptors and a triphenylamine derivative as an electron donor, which is accomplished by manipulating twisted intramolecular charge transfer (TICT). The resulting polymer PDA5 exhibits intensified green-light emission with a high ϕPL value of 70.7% in toluene. Its corresponding nanoparticles (PDA5 NPs), which are fabricated by the nanoprecipitation method, display a uniform size of around 83 nm, a large two-photon absorption cross-section (δ2) of 5.03 × 105 GM in water upon excitation by an NIR laser (840 nm), and a moderate ϕPL value of 40.7% in aqueous medium. These nanoparticles also present good photostability and outstanding two-photon fluorescence imaging with penetrating bladder blood vessels of 225 μm as well as real-time monitoring. Furthermore, PDA5 nanoparticles with impressive reactive oxygen species generation capability under irradiation of a white light source are implemented for photodynamic therapy in in vitro cancer cells. The observations manifest that efficient conjugated polymers with high ϕPL and large δ2 can be materialized for two-photon fluorescence imaging and photodynamic therapy by manipulating twisted intramolecular charge transfer.