Preparation and Photochemistry of Single Wall Carbon Nanotubes Having Covalently Anchored Viologen Units

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Abstract
An asymmetrically substituted viologen (V) has been covalently anchored to single wall carbon nanotube (SWNT) through an ester linkage by reacting chlorinated purified SWNT with N-methyl-N‘-(6-hydroxyhexyl)-4,4‘-bipyridine. Spectroscopic evidence for the covalent bond of viologen in V−SWNT comes from the chemical shift of the −CH2−O−CO− methylene group in 1H NMR and from the variations of the 1590 and 1380 cm-1 bands in the Raman spectrum of the V−SWNT with respect to SWNT. The fact that the estimated quenching constant of the SWNT emission by viologen is about 2 orders of magnitude higher than the diffusion coefficient indicates the occurrence of a static quenching arising from the formation of a nonemissive viologen−SWNT complex. Laser flash photolysis shows the formation of viologen radical cation upon direct excitation of V−SWNT. The viologen moiety of V−SWNT is able to form a charge-transfer complex with 2,6-dimethoxynaphthalene (DMN) as evidenced by optical spectroscopy and, upon selective photoexcitation in the charge-transfer band, this V−DMN complex anchored to SWNT gives rise to the corresponding charge separated state decaying in the submillisecond time scales.

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