Dramatic expansion of bimodal redox window of indigo 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, spanning over 5.98 V. The dye molecule exhibits bimodality and proves itself as a simultaneous super-reductant and -oxidant. This extreme bimodal behavior in indigo (IndH₂) originates from the viability of two electron redox processes on the parent architecture. In comparison, major popular photocatalysts (PC) possess singly oxidized/reduced state, limiting the span of such bimodal redox window significantly. In the presence of KO^tBu and white light irradiation, IndH₂ is converted to its tetraanionic form Ind^4- by two-electron reduction and two successive deprotonation steps, exhibiting its reductive power to -3.6 V vs SCE. On the other hand, two-electron oxidized form of IndH₂ forms dehydroindigo, a superoxidant capable of oxidizing substrates up to +2.38 V vs SCE.