From Isolated Porphyrin Ligands to Periodic Al-PMOF: A Comparative Study of the Optical Properties Using DFT/TDDFT

The use of photosensitizers as organic ligands in metal-organic frameworks (MOFs) is a common practice to engineer their UV/vis optical absorption. For instance, MOFs consisting of porphyrin ligands usually inherit their light-harvesting properties and thus follow the Gouterman model in which the low-lying excitations correspond to pi -> pi* transitions. However, the characterization of the excited states of porphyrin ligands in MOFs requires an appropriate description of the periodic crystal including the metal nodes and intermolecular interactions. Here, we investigate the UV/vis absorption properties of porphyrin-MOF Al-PMOF, its two metalated forms, Zn-Al-PMOF and Co-AlPMOF, and their corresponding isolated porphyrin ligands using density functional theory (DFT)/time-dependent DFT (TDDFT) simulations with Perdew-Burke-Ernzerhof (PBE), PBE0, and CAM-B3LYP functionals. Our results indicate that hybrid functionals are necessary to capture the proper nature of the transitions and the excitonic effects of the optical and fundamental gaps in porphyrin molecules and porphyrin-MOFs that the PBE functional fails to describe. Likewise, the simulations show that a wrong representation of some excitations can be obtained depending on the functional and when the Tamm-Dancoff approximation is used. Finally, our results show that the PBE and PBE0 functionals are not able to capture the gap renormalization when going from the isolated molecules to the periodic crystals. Overall, the nature of the optical transitions, excitonic effects, and gap renormalization are important features to assess in the prediction of optical properties in MOF crystals that require considering proper functionals and approximations to overcome the main failures of DFT/TDDFT calculations.