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This study presents a theoretical investigation of indigo and its halogen-substituted derivatives (F, Cl, Br) as organic semiconductors for molecular rectifier applications. Density functional theory (DFT) and time-dependent DFT were used to explore molecular geometry, electronic structure, and excited-state properties. The introduction of electron-acceptor atoms slightly modifies the electronic features while preserving the stability and ambipolar character of the indigo core. Crystal packing and dimer configurations were analyzed to assess charge transport pathways. Planar dimer arrangements showed enhanced π–π interactions and stronger electronic coupling. Reduced density gradient–non-covalent interaction (RDG–NCI) analysis revealed van der Waals and steric interactions as dominant across all structures. These interactions decreased in strength from Indigo to Indigo-Br, indicating substituent influence on packing behavior. Molecular junctions were modeled using DFT combined with the non-equilibrium Green’s function (NEGF) method. Among all derivatives, Indigo-Br exhibited the lowest contact resistance and the most favorable current–voltage characteristics. Overall, Indigo and Indigo-Br are identified as promising eco-friendly candidates for high-performance molecular rectifier applications.
Naserian et al. (Wed,) studied this question.