A Porphyrin-Based Metal–Organic Framework as a Photosensitizer for Singlet Oxygen Generation and Photocatalytic Water Treatment

A porphyrin-based metal–organic framework, 3D-[Zn2(TPyP)­(NO2)2]n, (ZnTPyP = 5,10,15,20-tetrapyridylporphyrinzinc­(II), node = Zn­(NO2)2) UDS-2, which exhibits distinct photophysical properties and efficient singlet oxygen photosensitization, has been synthesized and characterized via single-crystal X-ray diffraction. This MOF features an interpenetrated 3D-structure with two different zinc coordination environments, Zn1 (square pyramidal; porphyrin) and Zn2 (quasi-octahedral; Zn­(NO2)2). Computational analysis reveals an uncommon electronic structure in UDS-2 among porphyrin-based frameworks, where the O atoms of Zn­(NO2)2 dominate the HOMO contribution leading to a low-energy charge transfer, CT, transition of quasi-nil oscillator strength leading to an optically silent CT state. Conversely, the other frontier MOs are those from the Gouterman model for the porphyrin as expected. UDS-2 demonstrates superior singlet oxygen photosensitization compared to other common porphyrin-based CPs and MOFs such as the 3D-[ZnTPyP]n framework, PCN-222, and PCN-224. Furthermore, its structural model enables efficient singlet–singlet energy transfer (surface x → core) and exciton migration, which improves singlet oxygen photosensitization. The photocatalytic efficiency of UDS-2 is demonstrated through the degradation of Rhodamine 6G dye and several antibiotics (trimethoprim, ciprofloxacin, ofloxacin, and amoxicillin) in water, where UDS-2 achieves a significant pollutant removal efficiency under visible light at environmentally relevant concentrations (100 ppb). These properties establish UDS-2 as an efficient photosensitizer, generating singlet oxygen in both the solid and aqueous phases.