Electronically Coupled, Cofacially Linked, Hypervalent Antimony(V) Porphyrin Homodimer: Synthesis, Spectroscopy, and Photochemistry

A covalently linked cofacial homodimer has been synthesized using hypervalent antimony(V) porphyrins. The two porphyrins are connected by an -OCH₂O- bridge, maintaining a distance of 5.85 Å between the Sb centers. Despite the positive charge on the porphyrin entities, the resulting homodimer is structurally stable, with minimal or no repulsive forces between them. Density functional theory (DFT)-optimized structures show that the two porphyrins are nearly coplanar with an angle of ∼15° between the two planes. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of the ground state of the dimer are delocalized over both porphyrins, indicating the existence of exciton coupling between them. Optical studies support this observation, suggesting H-type exciton coupling based on the spectral shift of the Soret band. Similar behavior is observed in electrochemical studies as well. Results from the femtosecond transient absorption studies support the existence of exciton coupling, while the triplet-state lifetimes measured by nanosecond transient absorption studies show only minor differences. Time-resolved electron paramagnetic resonance and DFT studies of the lowest triplet state show that it is localized on a single porphyrin. While the localization of the triplet state is consistent with the expected weak coupling between the triplet states due to their small transition dipoles, the optical data, DFT calculations, and structure of the dimer all indicate that the dipole-dipole model of the excitonic coupling is not capable of adequately describing the excited-state properties. Notably, the role of p-block Sb in cofacial porphyrin dimer promoting exciton coupling, irrespective of its higher oxidation state of +5, is borne out from this study.