Structure-Performance Relationships in Coupling Reagents: A Review of DFT- and QSPR-Guided Design of Azo Chromogens for Drug Determination
Abstract
Diazotization followed by azo coupling is still one of the most heavily used derivatization routes in pharmaceutical UV-visible spectrophotometry, and yet the selection of the coupling reagent is very often made by trial and error, or simply by copying whatever the previous paper in the same journal happened to use. This review asks a narrower question than the usual survey of applications: what structural features of a coupler actually control the analytical figures of merit, and how far can density functional theory (DFT) and quantitative structure-property relationships (QSPR) replace that guesswork with prediction? We revisit the coupling step as an electrophilic aromatic substitution whose rate, regiochemistry and pH window are set by the ionization state of the coupler and by the electrophilicity of the arenediazonium ion generated from the drug. We then connect those variables to the quantities an analyst actually cares about: absorption maximum, molar absorptivity and colour stability. Conceptual-DFT indices, in particular the electrophilicity index, the nucleophilicity index and condensed Fukui functions, are shown to rationalize reagent behaviour that is otherwise reported as an empirical observation. Time-dependent DFT reproduces azo absorption maxima to within roughly 6-12 nm when the functional and the solvation model are chosen with care, while recent QSPR and machine-learning models predict λmax across hundreds of azo dyes with external coefficients of determination approaching 0.9. A structure-performance map of the common couplers is assembled, recurring errors in present computational practice are identified, and a minimum reporting set is proposed.
Keywords:
Azo coupling, coupling reagent, density functional theory, QSPR, spectrophotometry, drug determination, azo-hydrazone tautomerism, green analytical chemistry
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Najem, W. M. (2026). Structure-Performance Relationships in Coupling Reagents: A Review of DFT- and QSPR-Guided Design of Azo Chromogens for Drug Determination. Journal of Integral Sciences, 9(2), 46-62. https://doi.org/10.37022/jis.v9i2.150
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