Irreversible Bleaching of Donor–Acceptor Stenhouse Adducts on the Surfaces of Magnetite Nanoparticles

Two novel donor–acceptor Stenhouse adducts (DASAs) featuring the catechol moiety were synthesized and characterized. Both compounds bind strongly to the surfaces of magnetite nanoparticles. An adrenaline-derived DASA renders the particles insoluble in all common solvents, likely because of poor solvation of the zwitterionic isomer generated on the nanoparticle surfaces. Well-soluble nanoparticles were successfully obtained using dopamine-derived DASA equipped with a long alkyl chain. Upon its attachment to nanoparticles, this DASA undergoes an irreversible decoloration reaction owing to the formation of the zwitterionic form. The reaction follows first-order kinetics and proceeds more rapidly on large nanoparticles. Interestingly, decoloration can be suppressed in the presence of free DASA molecules in solution or at high nanoparticle concentrations.