  The functionalization of single-walled carbon nanotubes (SWCNTs) with
luminescent sp$^{3}$ defects has greatly improved their performance in
applications such as quantum light sources and bioimaging. Here, we report the
covalent functionalization of purified semiconducting SWCNTs with stable
organic radicals (perchlorotriphenylmethyl, PTM) carrying a net spin. This
model system allows us to use the near-infrared photoluminescence arising from
the defect-localized exciton as a highly sensitive probe for the short-range
interaction between the PTM radical and the SWCNT. Our results point toward an
increased triplet exciton population due to radical-enhanced intersystem
crossing, which could provide access to the elusive triplet manifold in SWCNTs.
Furthermore, this simple synthetic route to spin-labeled defects could enable
magnetic resonance studies complementary to in vivo fluorescence imaging with
functionalized SWCNTs and facilitate the scalable fabrication of spintronic
devices with magnetically switchable charge transport.
