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.

Automated summary

In this study, three kinds of nanoscale covalent organic frameworks (COFs) with tunable local polarization were designed to accelerate the generation of O2 center dot- in type I photodynamic therapy. The tetrakis-(4-formylphenyl)benzene-based COF (TB-COF) exhibited highly aggravated local polarization and matched with the energy level required for the photoinduced e- reduction of O2/O2 center dot-. The elongated charge-separated state of TB-COF was identified and the inhibited electron-hole recombination of COFs improved the O2 center dot- production efficiency in the acidic microenvironment of tumor cells or in vivo, enhancing the anti-cancer effect. The reticular polarization engineering of COFs provides a new paradigm for type I PDT in biomedical applications.