Preparation and photochemistry of single wall carbon nanotubes having covalently anchored viologen units

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.