Singlet and triplet excitations in chemically functionalized single-walled carbon nanotubes

Abstract

We investigate the lowest-energy singlet (S₁) and triplet (T₁) excitons in covalently functionalized (11,0) single-walled carbon nanotubes (SWCNTs) using spin-polarized constrained - occupation density functional theory (CO-DFT) under periodic boundary conditions. Our study focuses on how the singlet–triplet energy splitting (ΔE_ST) and exciton localization are influenced by the position of the sp³-defects formed by aryl-based adducts with varying electron-donating and electron-withdrawing abilities. We find that the PBE functional within CO-DFT reliably predicts the optical redshift of defect-related singlet excitons (E^∗₁₁) compared to the main-band E₁₁ of the pristine nanotube, matching experimental values better than B3LYP. Across all studied defect configurations, the T₁ state consistently lies below S₁, and we observe a strong correlation between ΔE_ST and the optical redshift: defect configurations inducing larger redshifts also result in greater ΔE_ST, ranging from 6 to 36 meV. Although the molecular adduct has a minor effect on the ΔE_ST energy, aryl with strong electron-withdrawing groups, such as NO₂ significantly enhances the charge-transfer character of both S₁ and T₁ by partially localizing electron density on the molecule, while the hole is located around the defect site. These adducts also reduce geometric and electronic reorganization between S₁ and T₁ excitons. Both properties are the key factors that are known to support thermally activated delayed fluorescence (TADF) provided a smaller value of ΔE_ST. Altogether, our results predict electron-withdrawing functional groups as an effective strategy for engineering TADF in SWCNTs, positioning these systems as promising near-infrared emitters for quantum light sources, spin-based devices, and energy-efficient optoelectronic applications.