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

Abstract

Herein, we report the polymorphic forms of Ni(II)-tetra(4-bromo-2,6-difluorophenyl) porphyrin (1), which have applications in the photocatalytic conversion of indoline to indole under blue light. X-ray diffraction studies reveal that one polymorphic form of 1 (namely, 1α) stabilizes in the monoclinic system and has a planar geometry, while the other form (namely, 1β) 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 1β shows a broad absorption Soret band compared to 1α. Complementary theoretical studies show that compound 1α is energetically more stable than compound 1β 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 1 in the presence of blue light.