New Basic Insight into Reductive Functionalization Sequences of Single Walled Carbon Nanotubes (SWCNTs)

The reactivity of reduced single walled carbon nanotubes (SWCNTs) (carbon nanotubides), prepared under strict inert conditions in a glovebox with respect to the covalent functionalization with hexyl iodide and subsequent exposure to ambient conditions (air, moisture), was systematically investigated by Raman, absorption, fluorescence, and IR spectroscopy as well as by TG/MS measurements. We have discovered that the alkylation does not lead to a complete discharging of the tubes since follow-up reactions with moisture still take place leading to mixed functionalized carbon nanotube derivatives containing H- and OH-addends (but no carboxylates) next to the hexyl groups. This was confirmed by the exposure of carbon nanotubides to ambient conditions. The degree of hexylation determined both under strict inert (ic) and ambient (ac) conditions increases with an increasing K:C ratio of the reduced SWCNT starting material. The presence of OH-groups covalently attached to the nanotubes was also confirmed by postfunctionalization reactions with 2-thiophenecarbonyl chloride, leading to the corresponding esters. Control experiments with KO₂ give rise to the formation of the same oxygen functionalities. These combined findings allowed for the suggestions of a plausible reaction mechanism, describing all the observed reactions on the SWCNTs side walls. The amount of subsequent side reactions after the treatment of reduced SWCNTs with electrophiles is strongly influenced by the reduction potential of the electrophile, which is responsible for the extent of reoxidation. Incomplete quenching of negative charges allows stronger oxidants/electrophile (e.g., O₂) to perform follow-up reactions