Bimodality in super-oxidizing and -reducing behavior of Indigo: Dramatic expansion of the redox window by two-electron redox processes

Indigo is an extremely popular molecule in dye industry, however, its use in photochemical transformations is surprisingly scarce. This report explores its photocatalytic activity over an unusually wide excited-state redox window. To illustrate further, the dye molecule exhibits bimodality and proves itself as a simultaneous super-reductant and -oxidant, spanning a massive redox window of 5.98 V. This extreme bimodal behavior in indigo originates from the viability of two electron redox processes on the parent architecture. In comparison, major popular photocatalysts (PC) usually can accept or donate only one electron, limiting the span of its bimodal redox window significantly. In the presence of KOtBu and white light irradiation, indigo is converted to its tetraanionic form, Ind4- which displays its super-reducing power to an extent of -3.6 V vs SCE. Such an in situ-generated species has been separately synthesized and the reductive prowess of the PC has been examined for a wide array of reactions which starts with a reductive cleavage of the C-Cl bonds. On the contrary, the super-oxidant behavior of indigo has been explored in the presence of a mild oxidant and blue light irradiation, generating dehydroindigo. The strong oxidizing capability of the PC has been utilized for ipso-substitution of aryl fluorides with a series of nitrogen-based nucleophile.