Reticular polarization engineering of covalent organic frameworks for accelerated generation of superoxide anion radicals

The exploration of type I mechanism remains a great challenge in photodynamic therapy (PDT) due to the limited electron transfer from the photosensitizer to ground state oxygen to form superoxide anion radical (O2 center dot- ). In this study, we design three kinds of nanoscale covalent organic frameworks (COFs) with tunable local polarization for accelerating the generation of O2 center dot- . Among three COFs, the tetrakis-(4-formylphenyl)benzenebased COF (TB-COF) exhibits highly aggravated local polarization with 3.29 Debye of the dipole moment, and thus drives the photoinduced e- reduction of O2/O2 center dot-, which matches with the energy level of TB-COF. The elongated charge-separated state of TB-COF is also identified by femtosecond transient absorption spectra and time-dependent density functional theory calculation. Significantly, the inhibited electron-hole recombination of COFs further improves the O2 center dot- production efficiency upon protonation and thus strengthens anti-cancer effect in the acidic microenvironment of tumor cell or in vivo. The reticular polarization engineering of COFs provides a new type I PDT paradigm in biomedical applications.