Planar and Nonplanar Polymorphic Forms of Ni(II)-Porphyrin: Photophysical, Photocatalysis, and DFT Study

Herein, we report the polymorphic forms of Ni(II)-tetra(4-bromo-2,6-difluorophenyl) porphyrin (<b>1</b>), which have applications in the photocatalytic conversion of indoline to indole under blue light. X-ray diffraction studies reveal that one polymorphic form of <b>1</b> (namely, <b>1α</b>) stabilizes in the monoclinic system and has a planar geometry, while the other form (namely, <b>1β</b>) has a nonplanar geometry and crystallizes in the tetragonal system. Due to their different conformations (planar and nonplanar), these polymorphs exhibit distinct solid-state properties. For instance, the solid-state absorption spectrum of <b>1β</b> shows a broad absorption Soret band compared to <b>1α</b>. Complementary theoretical studies show that compound <b>1α</b> is energetically more stable than compound <b>1β</b> by 16.5 kcal mol^-1. In addition, electrostatic potential isosurfaces clearly reveal the presence of σ-holes on the bromine atom, indicative of halogen bond interactions in the crystalline state. The electronic structures of porphyrins directly influence the photophysical properties through two major effects: (a) porphyrin core geometry and (b) heavy atom effect (HAE). The photocatalytic study demonstrates the efficient conversion of indoline to indole using compound <b>1</b> in the presence of blue light.