Efficient Quenching of Singlet Oxygen via Energy Transfer to Semiconducting Single-Walled Carbon Nanotubes

Abstract

Singlet oxygen, ¹O₂(a¹Δ_g), is efficiently deactivated by single-walled carbon nanotubes (SWNTs) having diameters in the range <i>d</i> ≈ 1<b>−</b>1.6 nm. This is evidenced by quenching of the near-infrared emission of photosensitized ¹O₂ in water−surfactant dispersions of SWNTs. The observed quenching rate is close to the diffusion-limited value. The smaller diameter SWNTs are found to be comparatively less active, despite the presence of metallic tube types. We therefore attribute the quenching to energy transfer from ¹O₂ to primarily semiconducting SWNTs having a sufficiently small band gap (<i>d</i> ≥ 1 nm). Remarkably, photogeneration and quenching of up to ∼10⁹ singlet oxygen molecules per nanotube (ultimately limited by degradation of the rose bengal sensitizer) does not affect the photoluminescence and absorption spectra of SWNTs. This indicates that dispersed SWNTs are highly chemically inert toward ¹O₂ and provides additional support for the proposed physical (energy transfer) quenching mechanism.