Molecular insights into diazonium salt binding and colour development in wood substrates

Sustainable wood marking and tracking are essential for responsible forest management and the long-term conservation of natural resources. Functional modification of wood surfaces via azo coupling with diazonium salts offers a sustainable approach to coloration and marking. We investigate the substrate specificity and temporal dynamics of this reaction across softwood and hardwood species (oak, beech, and spruce), probing the roles of lignin, extractives, hemicellulose, and cellulose. Phenolic groups from lignin and extractives emerge as the primary reactive sites. While extractive content drives reaction kinetics in raw wood, overall coupling efficiency after their removal is strongly influenced by lignin composition—particularly the guaiacyl-to-syringyl ratio and degree of lignin condensation. Among the diazonium salts tested, Fast Black K (FB) exhibits superior performance, likely due to enhanced molecular planarity, stronger π-π stacking, and Zn 2+ counterion effects that promote robust interactions with wood. Despite variations in salt and wood type, final coloration consistently converges towards reddish hues, as confirmed by CIELab measurements, reflecting lignin's intrinsic chromophoric properties. This study highlights the interactions between wood components, their chemistry and diazonium salt design, that offers opportunities for controlled functionalization of wood surfaces. • Surface coupling with diazonium salts enables direct wood coloration by targeting phenolic groups in lignin and extractives. • Color development and reaction kinetics depend on extractive content and lignin structure and structure of the diazonium salt. • Final hues consistently fall within the reddish-yellow spectrum, indicating selective tuning of lignin chromophores.