On the Nature of the Out-of-Plane Distortions in Subporphyrins

The field of subporphyrins has garnered great interest in recent years owing to its unique structure and associated properties. They exhibit spectroscopic features similar to porphyrins and find applications in various optoelectronic devices, photodynamic therapy etc. Most of the synthesized subporphyrins have boron coordination with an axial ligand and exhibits a bowl-shaped geometry. The first isolation of a stable free-base subporphyrin is achieved recently with mesityl groups at two of the meso positions and anthracene at the other. X-ray studies reveal a markedly non-planar structure different from the bowl shape and is attributed to the steric hindrance of the inner N-H bonds. Herein, we report a systematic quantum chemical investigation assisted by symmetry principles on molecular models to characterize the out-of-plane (OOP) distortions observed so far in subporphyrins and unveil the electronic reasons. Correlation of the frontier molecular orbital (FMO) landscape between the D_4h porphyrin and D_3h subporphyrin gives insight into their electronic structure relative to one another and the nature of OOP distortions. Further the effect of a π-donor and σ-acceptor substituent at the meso/β positions of the subporphyrin ring as well as the impact of boron incorporation in the central cavity on the OOP distortions are also discussed.