Designing pillar-layered metal-organic frameworks with photo-induced electron transfer interactions between ligands for enhanced photodynamic sterilization and photocatalytic degradation of dyes and antibiotics.

Pollution caused by antibiotics, bacteria, and organic dyes presents global public health challenges, posing serious risks to human health. Consequently, new, efficient, fast, and simple photocatalytic systems are urgently required. To this end, 2,7-di(pyridin-4-yl)benzo[lmn][3,8]phenanthroline-1,3,6,8(2H,7H)-tetraone (NDI)-an electron acceptor-is introduced as a connecting column into a porphyrin-based metal-organic layer (2DTcpp) with excellent photocatalytic activity; this modification yields a three-dimensional pillar-layered metal-organic framework (MOF, 3DNDITcpp) with superior photocatalytic reactive oxygen species (ROS) generation capability. Introducing NDI enlarges the pore cavity of 3DNDITcpp creating active sites and boosting type II ROS production. The orderly arrangement of the electron donor (porphyrin layer) and acceptor (NDI) within 3DNDITcpp promotes photo-induced electron transfer (PET) interactions-as confirmed by density functional theory calculations-substantially boosting type I ROS production. Specifically, the energy levels of the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) of the porphyrin derivative ligand are -0.122252 and -0.185307 eV, respectively. The energy levels of the LUMO and HOMO of the NDI ligand are -0.15977 and -0.221199 eV, respectively. The HOMO energy level of the porphyrin ligand is between the HOMO and LUMO of NDI, and higher than the HOMO orbital energy level of NDI, proving that the porphyrin derivative ligand can act as an electron donor and carry out an efficient PET process with the electron acceptor NDI. Various ROS indicators demonstrate the superior ROS generation ability of 3DNDITcpp under light irradiation. Using activated 2,7-dichlorodihydrofluorescein diacetate (DCFH-DA) as an indicator of total ROS, the fluorescence enhancement factors of 2DTcpp, 3DPyTcpp, and 3DNDITcpp were 42.13, 48.24 and 94.21 times, respectively. Both the degradation curve and degradation rate of 9,10-anthracenediyl-bis(methylene)dimalonic acid (ABDA) demonstrated that the order of O production ability was 3DNDITcpp (rate up to 0.312 min) > 3DpyTcpp (0.158 min) ≈ 2DTcpp (0.155 min). In addition, dihydrorhodamine 123 (DHR 123) and hydroxyphenyl fluorescein (HPF) were used as specific indicators of O and OH to monitor the generation of type I ROS of 2DTcpp, 3DPyTcpp, and 3DNDITcpp, respectively. The fluorescence enhancement factors of DHR 123 and HPF aqueous solutions containing 3DNDITcpp were as high as 47.70 and 192.19 times, respectively. The fluorescence enhancement factors of DHR 123 and HPF containing 2DTcpp and 3DPyTcpp were 19.65/63.07 (2DTcpp) and 27.97/134.19 times (3DPyTcpp), respectively. Photocurrent response (3DNDITcpp is 1.2 and 2.7 times better than 3DPyTcpp and 2DTcpp, respectively) and electrochemical impedance (3DNDITcpp is 1.9 and 2.9 times smaller than 3DPyTcpp and 2DTcpp, respectively) measurements confirming its excellent type I ROS production capability. Under low-power light irradiation (60 mW·cm, 5 min), ROS generated by 3DNDITcpp effectively inactivates Escherichia coli and Staphylococcus aureus, with an inhibition zone diameter of approximately 4.00 cm. Furthermore, 3DNDITcpp rapidly degrades various colored dyes and antibiotics within 30 min, achieving degradation rates as high as 0.095 and 0.054 min, outperforming most traditional photosensitizers (PSs). To our knowledge, this is the first instance when differences in the electron clouds of mixed ligands are leveraged to induce PET interactions within pillar-layered MOFs, yielding excellent porous PSs. Overall, our study offers a new approach for developing porous PSs with enhanced ROS generation capacity and advances MOFs crystal engineering based on mixed ligands.