Propagative Sidewall Alkylcarboxylation that Induces Red-Shifted Near-IR Photoluminescence in Single-Walled Carbon Nanotubes

Semiconducting single-walled carbon nanotubes (SWCNTs) are direct band gap materials in which exciton photoluminescence (PL) occurs at the same wavelength as excitation. Here, we show that propagative sidewall alkylation can induce a new PL peak in (6,5) SWCNTs red-shifted from the E11 near-infrared exciton excitation and emission by ∼140 meV. The magnitude of the red-shift is weakly dependent on the terminal functional group. This new emission peak is relatively bright even after a high degree of functionalization because the reaction occurs by propagating outward from initial defects, creating bands of functional groups while maintaining the number of effective defect sites. Density functional theory computations suggest that the covalently attached alkyl functional groups introduce a new, optically allowed, low-lying state from which this new emission may arise. This method of shifting nanotube PL away from the bare nanotube excitation may find applications in near-infrared (IR) fluorescence imaging by allowing both excitation and emission to occur in the optically transparent window for biological tissues.