Electronic structure and optical spectra of semiconducting carbon nanotubes functionalized by diazonium salts

► Density Functional Theory (DFT) is used to study interactions, electronic structures, and optical properties of carbon nanotubes covalently and noncovalently functionalized by aryl diazonium groups. ► Different DFT models, e.g., various functionals, basis sets, and solvent effects are investigated for the proper description of the ground and excited state properties of studied systems. ► Physisorption of aryl diazonium weakly perturbs the optical spectra of nanotubes resulting in their small redshifts, attributed as a signature of the pi-pi nanotube-aryl interactions. ► Covalently bound aryl-nanotube complexes demonstrate significant redshifts and a dramatic increase in the oscillator strength of the lowest exciton that is optically dark in pristine nanotubes.