ABSTRACT:
Methyl Red (MR) is an azo dye widely recognized for its role as a pH indicator and, more recently, as a versatile functional compound in advanced materials engineering. Its conjugated π-electron system, reversible photoisomerization behavior, and amphiphilic molecular architecture have driven its integration into an expanding range of technological platforms, including nanostructured adsorbents, dye-sensitized solar cells, photoresponsive smart coatings, optical sensors, and hybrid sol-gel matrices. This systematic review, conducted in full conformity with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) methodology, critically appraises and synthesizes the scientific literature pertaining to the physicochemical, energetic, and technological applications of methyl red in materials science. A comprehensive search across Scopus, ScienceDirect, Web of Science, SpringerLink, and IEEE Xplore yielded 150 candidate publications, of which 50 peer-reviewed studies published between 2000 and 2024 were retained after rigorous screening. The selected studies demonstrate that MR adsorption follows Langmuir monolayer thermodynamics and pseudo-second-order kinetics; its photoisomerization on TiO₂ nanoparticles is mechanistically enabled by a TiO₂-induced pseudo-conical intersection on the first excited-state energy surface; and its covalent immobilization in silica matrices via GPTMS functionalization reduces UV photodegradation rates by one order of magnitude. Quantum chemical investigations employing DFT, TD-DFT, Monte Carlo simulations, and molecular dynamics further elucidated the HOMO-LUMO electronic landscape and intermolecular binding mechanisms. Collectively, the findings confirm that methyl red has transitioned from a classical analytical reagent into an advanced multifunctional molecular material with considerable prospects for energy conversion, environmental remediation, and next-generation smart device engineering.