Analysis of molecular conjugation influence on color characteristics of indigo compounds

Indigo dyes are historically significant and possess a unique π-conjugated core, making them valuable for both traditional pigments and emerging applications in organic electronics. A fundamental challenge is understanding how subtle molecular modifications, particularly substituent effects, quantitatively influence their conjugation extent and resulting color properties. This study systematically investigates the parent indigo and three N, N'-substituted derivatives (phenyl, ethyl, and vinyl) to elucidate the precise relationship between molecular structure, electronic properties, and visible light absorption. Density functional theory (DFT) and Time-dependent DFT (TD-DFT) calculations reveal how substituents modulate the HOMO-LUMO gap and intramolecular interactions, directly correlating with calculated absorption wavelengths (628 to 807 nm) and predicted colors (medium blue to green-cyan) via complementary color theory. These findings provide a quantitative framework for designing indigo-based dyes with targeted optical properties, including near-infrared absorption.

Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations were performed. Ground-state geometry optimizations and frequency calculations were carried out using the B3LYP-D3(BJ) functional with the def2TZVP basis set. Excited-state calculations employed the CAM-B3LYP-D3(BJ)/def2TZVP level, with the IEFPCM solvation model simulating aqueous conditions. The Multiwfn 3.8 and VMD 1.9.3 software packages were used for interaction region indicator (IRI) analysis, electrostatic potential (ESP) mapping, electron density difference (EDD) analysis, hole-electron analysis, and color prediction based on absorption spectra.