Dramatic expansion of bimodal redox window of indigo by two-electron redox processes

Article https://doi.org/10.1038/s41467-025-66186-w Dramatic expansion of bimodal redox window of indigo by two-electron redox processes Received: 18 December 2024 Check for updates 1234567890():,; 1234567890():,; Accepted: 29 October 2025 Monojit Roy1, Shyamali Maji1, Vikramjeet Singh1, Dhananjay Dey Debashis Adhikari 1 1,2 & 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 KOtBu and white light irradiation, IndH2 is converted to its tetraanionic form Ind4- 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. Visible light photoredox catalysis has likely seen a renaissance in recent times and has become a powerful tool for solving challenging chemical transformations that are sometimes even impossible to achieve by thermal pathways1–5. The utility of these catalysis processes further lies in the selective activation of specific functional groups by appropriate photon energy, keeping other vulnerable groups intact during those transformations. Earlier developments in photocatalysis heavily utilized ruthenium6,7, iridium8–10, palladiumbased11,12 catalysts, while there is a clear shift in momentum to discover more organic photocatalysts as they could be more sustainable and environmentally benign. A broad interest in developing new organic photocatalysts (PC) or unraveling a new facet of existing photoactive molecules propels the recent photochemical research13,14. Typically, a PC absorbs a photon and reaches its electronic excited state which facilitates either energy or electron transfer to different substrate molecules. Different strategies have also been further devised to harness the reducing or oxidizing power of a photocatalyst at its excited state14–19. Tandem electrophotochemical approach20–22, sensitization-enhanced electron transfer17,23, consecutive photoinduced electron transfer (con-PET) are a few of the popular techniques in this direction3,5,18,24–27. In the context of the excited state reducing/oxidizing power of a PC molecule, its ability to operate in both directions is an important attribute to the PC (Fig. 1a)28. Such accessibility of bimodal operation ensures that each oxidative or reductive transformation does not require a tailor-made PC. Some of the widely used transition metalbased photocatalysts exhibit bimodality, however, over a rather narrow range. For example, [Ru(bpy)3]2+ displays the excited state oxidation and reduction potentials at −0.81 V and + 0.77 V (vs SCE), respectively28,29, spanning an excited state redox window of 1.58 eV. Similarly, fac-Ir(ppy)3 photocatalyst displays the same potentials at −1.73 V and +0.31 V, respectively, covering a potential range of 2.04 eV (Fig. 1b)28,29. Organic photocatalysts, often owing to their high excitation energy (Eo,o), afford a sizeable redox window; for example 4CzIPN spans a window of 2.61 eV. However, the excited state oxidation potential for the aforementioned PC, −1.18 V is barely adequate to break a difficult-to-reduce aryl chloride bond (Ered = −2.8 V vs SCE). By the same token, the excited state reduction 1 Department of Chemical Sciences, IISER Mohali, Mohali, Punjab, India. 2Department of Chemistry, C. S. J. M. University, Kanpur, India. e-mail: Nature Communications | (2025)16:11387 1 Article https://doi.org/10.1038/s41467-025-66186-w Fig. 1 | Selective C–C and C–N bond formation via indigo dye as a photocatalyst. a Conceptual framework to expand the bimodal redox window. b Comparison of bimodality with popular PCs and salient feature of indigo. c Model reactions to demonstrate the oxidative and reductive power of indigo. potential of the same catalyst, +1.43 V, is nowhere close to oxidize the reluctant arenes, since such substrate molecules demand an oxidation potential of + 2 V or even higher30. Henceforth, a single PC molecule that can afford a very large excited state redox window, eliciting the simultaneous super-reductant and super-oxidant trait, is virtually unknown. In a closer scrutiny, the bottleneck directly hints at the operational mode of these PC. Almost invariably, the PC in its excited state releases one electron to reductively cleave a target bond, or it accepts an electron to conduct oxidative transformation to the substrate molecule. So, the span of its excited state potential encompassing both oxidative and reductive ends is limited by single electron redox event. This limitation primarily stems from the inherent nature of the PC, where two-electron redox events are not accessible. Intuitively, if a PC undergoes two-electron redox processes to generate an active catalyst along both directions without any detrimental bond cleavage, there is a strong possibility that it will virtually double its operational redox window. However, the chemical transformation will be dictated by one-electron redox processes. To illustrate, a PC molecule can act simultaneously as both superreductant and super-oxidant once it can manage two-electron redox transformations onto it. We identify a popular dye, indigo that possesses a cross-conjugated p-quinone and a p-phenylene diamine-type architecture31,32. We posit such redox motifs will allow to perform two-electron oxidation and two-electron reduction discretely so that the excited state window can reach a massively large value. Herein, it is further demonstrated that simple in situ chemical modification of indigo by an extremely mild reductant and a mild oxidant can prepare the highly reducing and oxidizing states, respectively so that Nature Communications | (2025)16:11387 their super-redox trait can be harnessed over an unusually wide potential window of 5.98 eV (Fig. 1a). Results and discussion Photophysical study of catalytic intermediates Indigo’s prolific use as a dye element to cloths, fabrics are age old33–35. The deep blue color and immense photostability of the indigo under sunlight are perhaps the reasons behind its enormous popularity as a dyeing agent. Its tremendous photostability even sparked research interest to find means to decompose such a molecule in the presence of a catalyst. Surprisingly, such a robust photocatalyst has not been examined widely for steering photochemical reactions. Paren (...truncated)