“Molecular lock” - triggered single-arm twisting strategy to enhance the photothermal conversion ability and its mechanism

Boron dipyrromethene (BODIPY) is a high fluorescent sensitizer with visible light absorption properties (radiative transition: RT). In order to broaden its light absorption range and optimize its excited state performance, we have constructed BODIPY-like sensitizers with a "single-arm" structure using the "molecular lock" strategy. Theoretical calculations showed that the construction of the "single-arm" structure of the regular heterocyclic conjugated system raised the HOMO energy level of the molecule compared to BODIPY monomers, leading to a lower HOMO-LUMO energy gap and a broadened light absorption range in the near-infrared (NIR) region. The lower fluorescence quantum yield (5 %) indicated that the photosensitizer 4 had weak RT processes. TD-DFT calculations demonstrated that the "single-arm" twisting of the regular heterocyclic conjugated system slightly enhanced the intersystem crossing (ISC) abilities of photosensitizer 4, which was further verified by transient absorption spectroscopy and singlet oxygen yield measurements. Under light irradiation, photosensitizer 4 exhibited reliable photothermal conversion efficiency, indicating strong non-radiative transition (NRT) abilities. Therefore, the "molecular lock" strategy can provide inspiration and experience for constructing NIR BODIPY-type photosensitizers with efficient NRT abilities.