Abstract
The ability of metal organic frameworks (MOFs) to modulate the photophysical properties of encapsulated guests is of importance for the applications of these materials in light harvesting and photocatalysis. Here, the photophysical properties of tetra (N-methyl-4-pyridyl)-21H,23H porphine (TMPyP) encapsulated within two Cd-based MOFs (MOM-11 and MOM-12) are reported. It is shown that the pores of CdTMPyP@MOM-11 and CdTMPyP@MOM-12 fix the orientation of the porphyrin peripheral pyridinium groups relative to the porphyrin plane. These fixed orientations have a significant effect on the porphyrin S-1-CT coupled excited state leading to differences in steady state emission, emission lifetimes, and absorption properties. In addition, it is observed that the pores of CdTMPyP@MOM-11 and CdTMPyP@MOM-12 also restrict out of plane porphyrin ring distortions associated with sit-atop porphyrins resulting in extended triplet state lifetimes. The results demonstrate that specific pore structures can systematically modulate the excited state properties of TMPyP type porphyrins by regulating accessible guest conformations.