Upconversion‐Mediated Near‐Infrared Singlet Oxygen Generation Using a Chlorophyll‐Derived Photosensitizer

Near‐infrared (NIR)‐driven photochemistry is attractive for biomedical applications owing to the relatively deep tissue penetration and reduced photodamage associated with NIR irradiation. However, most conventional molecular photosensitizers require visible‐light excitation. Herein, we demonstrate NIR‐induced singlet oxygen ( 1 O 2 ) generation by combining lanthanide‐based upconversion particles with methyl pyropheophorbide‐ a (MPPa), a chlorophyll‐derived photosensitizer prepared from Spirulina . Hexagonal β ‐NaYF 4 :Yb 3+ ,Er 3+ upconversion particles convert 980 nm NIR light into visible emission, including a red emission band that overlaps with the Qy‐absorption band of MPPa. Photoluminescence lifetime measurements, together with the observed spectral changes, are consistent with radiative reabsorption of upconverted photons by MPPa rather than dominant nonradiative energy transfer. Under direct red‐light excitation, MPPa exhibited an estimated 1 O 2 quantum yield of 23.6%, whereas the upconversion‐mediated system exhibited an estimated overall 1 O 2 quantum yield of 0.42% under 980 nm irradiation, based on absorbed‐photon correction and an indirect DPBF assay. Furthermore, incorporation of MPPa and upconversion particles into a poly( ε ‐caprolactone) matrix enabled NIR‐induced photosensitized oxidation in the solid state. These findings demonstrate the potential of coupling lanthanide‐based upconversion with naturally derived photosensitizers to develop NIR‐responsive photochemical materials in solution and polymer films.