Photophysical properties of Meso-tetra(4-N,N,N-trimethylanilinium) porphyrin encapsulated within the MOM-11 metal organic framework

Qaisar Maqbool

University of South Florida

The development of new photocatalytic materials with tunable properties is critically important in light harvesting applications including photovoltaics, solar photochemistry, and renewable energy. An important class of photoactive materials are the metal organic frameworks (MOFs) that contain photoactive guests within the pores of the framework. Previous studies have demonstrated that encapsulation of porphyrins within the cavities of various MOFs can modulate their photophysical properties.  One system demonstrating such modulation is the MOF MOM-11 containing the cationic porphyrin tetra(N-methlypyridyl) porphyrin (TMPyP) encapsulated within the large pores of the MOF. The TMPyP is crystallographically resolved allowing for the determination of the porphyrin conformation. Photophysical studies of this system reveal system alterations in the porphyrin photophysics that correlate with the orientation of the peripheral pyridinium groups. These perturbations were attributed to a change in the coupling between the TMPyP S₁ and charge transfer (CT) excited states. Here the excited state properties of another cationic porphyrin, tetra(4-N,N,N-trimethylanilinium) porphine (4TANP), are examined upon encapsulation within MOM-11. The 4TANP does not possess a charge transfer excited state as the anilinium groups do not delocalize significant charge onto the peripheral phenyl groups of the porphyrin. The peripheral anilinium groups also contain bulkier substituents which can affect the overall porphyrin conformation within MOF cages. The results presented here demonstrate successful encapsulation of 4TANP within the cavities of MOM-11 with crystallographically resolved porphyrin. The 4TANP contains a Cd²⁺ ion in a ‘sit-atop’ conformation similar to the TMPyP@MOM-11 system. The results further demonstrate significant photophysical changes associated with 4TANP upon encapsulation. These changes are attributed to the trimethyl anilino groups modulating the relative orientation of the associated phenyl groups which alters the coupling to the porphyrin p-system. These results further demonstrate the ability of MOF cavities to modulate guest photophysics through conformational distortions.

Qaisar Maqbool is the presenter and I am working with Dr Randy Larsen from university of south Florida.