Title : Structural engineering of Ni(II)-porphyrins: Exploring polymorphism and photocatalytic behaviour
Abstract:
Polymorphism in porphyrins is a rare but structurally and functionally significant phenomenon, capable of tuning electronic, optical, and catalytic properties. Understanding and controlling these polymorphic forms can provide valuable insights into structure and function relationship in complex macrocyclic systems. This study highlights the structural engineering of Ni(II)-tetra(4-bromo-2,6-difluorophenyl)porphyrin (1), which exists in two polymorphic forms, planar (1α) and nonplanar (1β) (Figure 1). Through strategic crystallization and X-ray analysis, we demonstrate how subtle conformational changes lead to distinct supramolecular assemblies and influence solid-state photophysical properties. Notably, the planar form (1α) exhibits Ni···Br halogen interactions, while 1β lacks such metal–halogen contacts and adopts a saddle-ruffled geometry, as revealed by NSD analysis and DFT calculations. Both polymorphs were evaluated as photocatalysts for the oxidative aromatization of indoline to indole under blue light irradiation, displayed excellent catalytic efficiency, achieving up to 95% yield under optimized conditions. Mechanistic studies suggest a radical pathway involving singlet oxygen formation, with polymorphic distortion and electronic effects contributing to catalytic performance. This work provides key insights into how polymorphism in metallo-porphyrins can be harnessed to modulate their functional behaviour in visible-light-driven catalysis.
