Electro-oxidation of indole-based squaraine dye: A combined in-situ spectroelectrochemical and theoretical study

Indole-based squaraine (ISQ) is a promising group of electroactive dyes that have been widely used as organic photoelectric materials. However, their chemical structure is stable and will cause environmental pollution after being released into nature. Herein, the electrochemical redox chemistry of ISQ and its degradation mechanism on a Pt polycrystalline electrode was investigated. Many results are obtained from voltammograms, electrochemical in-situ UV–Vis and FTIR spectra and quantum chemical calculations: 1) while the potentials were below 1.2 V, ISQ was first oxidized to the radical anion with C[dbnd]O bond formation whereas oxidation shows fully electrochemical irreversibility. 2) an unexpected IR band of CO₂ was collected at increasing potentials whereas a new absorption of 420 nm appeared in UV–Vis spectra, suggesting that cleavage of the C[dbnd]C double bond had occurred in the molecular structure of ISQ. 3) The oxidation products of ISQ were 1‑methyl‑3,3‑dimethylndolenium‑2‑one and 3‑oxo‑cyclobut‑1‑enol anion. These findings provide useful information for better understanding the reaction mechanism and designing a suitable treatment strategy for dye pollutants.