Using single atom substitution strategy for constructing a novel Se-substituted hemicyanine dye with hydroxyl group as an ideal activatable scaffold

Photodynamic therapy (PDT) is an effective method to treat diseases, especially for tumor treatment, with remarkable characteristics including good therapeutic results and low drug resistance, which has attracted the attention of researchers. The performances of photosensitizer dye largely determine the tumor treatment results. However, most of the reported photosensitive dyes belong to the "always-on" photosensitive property, which cannot avoid the photodamage caused to normal tissues during tumor PDT. By contrast, activatable photosensitizers possess the desirable characteristic of controllable phototoxicity, ensuring tumor safety treatment. Until now, ideal photosensitizer dye scaffold is still lacking. Therefore, constructing a universal photosensitive dye scaffold is a vital issue to be solved urgently. In this work, inspired by the single atom substitution strategy, the oxygen atom in classical hemicyanine dye has been replaced by selenium (Se) atom to obtain a novel Sesubstituted hemicyanine dye with hydroxy group, HCySe-OH, as a novel dye platform for constructing activatable photosensitizers. We found that HCySe-OH had high singlet oxygen (1O2) generation ability and longer absorption and emission in contrast to the classical O-substituted hemicyanine, endowing its better biomedical application. Additionally, HCySe-OH was successfully utilized to kill cancer cells and bacteria under 760 nm laser irradiation. More importantly, we believe that numerous smart activatable photosensitizers will be developed based on this platform, HCySe-OH.