The present invention relates to methods of attaching a coupling molecule (e.g. a peptide, an amino acid, ...) to a substrate comprising a plurality of carboxylic acid groups for linking to a coupling molecule; contacting said substrate with a carboxylic acid activating compound formulation comprising a carbodiimide precursor (i.e. a tetrazole thione), wherein said carbodiimide precursor converts to a carbodiimide upon exposure to electromagnetic radiation at a defined wavelength; selectively exposing said carboxylic acid activating compound formulation to the electromagnetic radiation, thereby generating said carbodiimide and activating the carboxylic acid groups to form carbonyl groups on said substrate at a selectively exposed area; contacting the substrate with a coupling formulation comprising a coupling molecule and a solvent; coupling the coupling molecule to at least one of said plurality of carbonyl groups at said selectively exposed area. The invention also relates to corresponding carboxylic acid activating formulations.