Synthesis and Photoelectrochemical Characterisation of Indole-Based Organic Dyes

Two novel donor–acceptor (D–A)-type indole-based organic dyes, In-C8-BA and In-C8-TBA, were synthesized and thoroughly characterized for advanced photoactive applications. These dyes integrate indole as a robust electron-donating moiety with either barbituric acid (BA) or thiobarbituric acid (TBA) as strong electron-withdrawing acceptors, facilitating pronounced intramolecular charge transfer (ICT). Comprehensive structural validation was performed using ¹H/¹³C NMR, FT-IR, and mass spectrometry, confirming successful molecular construction. Optical studies revealed broad and intense absorption bands in the visible region, with In-C8-TBA exhibiting a red-shifted profile due to the enhanced electron affinity of the sulfur-containing acceptor. Thermal gravimetric analysis (TGA) demonstrated high thermal stability, crucial for device integration. Cyclic voltammetry established well-aligned HOMO–LUMO energy levels suitable for both p- and n-type semiconductor interfaces, enabling versatile photosensitization capabilities. Photoelectrochemical measurements in protic electrolytes (e.g., aqueous phosphate buffer) yielded stable, reproducible photocurrents, suggesting active participation in proton-coupled electron transfer (PCET)—a key mechanism in solar fuel generation. Density functional theory (DFT) computations and molecular electrostatic potential (MESP) maps corroborated experimental data, revealing asymmetric charge distribution favoring directional ICT from indole to the acceptor. These combined attributes underscore In-C8-BA and In-C8-TBA as promising candidates for dye-sensitized solar cells, organic photodetectors, and photocatalytic systems, where efficient light harvesting and interfacial charge dynamics are paramount.