ortho-Halogen substituent design for creating near-infrared color centers in single-walled carbon nanotubes.

Near-infrared (NIR) color centers (CCs) formed in chemically functionalized single-walled carbon nanotubes (SWCNTs) are promising for bioimaging/sensing and quantum photonics applications. Controlling defect-binding configurations at CCs is essential for tuning their photoluminescence (PL), yet design principles for the chemical reagents remain limited. Here, we develop aryldiazonium salts bearing ortho-halogen and para-functional substituents to synthesize locally functionalized SWCNTs (lf-SWCNTs). The halogen identity strongly dictates CC PL: the ortho-I substituent produces bright at 1263 nm with >90% selectivity, whereas ortho-F, -Cl, and -Br substituents generate weaker with (∼1150 nm), with red-shifting and selectivity increasing as the halogen size increases. The para substituents further tune the emission wavelengths in correlation with the Hammett substituent constants. The ortho-I-aryl-functionalized lf-SWCNTs are substantially brighter, with up to ∼2.7-fold enhancement relative to unmodified SWCNTs, and exhibit exciton lifetimes >100 ps owing to efficient exciton trapping at the CCs. Simulations indicate that ortho-F, Cl, and Br substituents introduce local strain, leading to PL red-shifts and reduced intensities, whereas the I substituent behaves exceptionally because its anisotropic electron distribution mitigates steric strain at the CC. These findings provide a molecularly guided route to selective and wavelength-tunable NIR CCs in SWCNTs.