Demonstration of covalent sidewall functionalization of single wall carbon nanotubes by NMR spectroscopy: Side chain length dependence on the observation of the sidewall sp3 carbons

Abstract

Carboxylic acid-functionalized single walled carbon nanotubes (SWNTs) prepared via the reaction of an amino acid, NH2(CH2)(n)CO2H where n = 1 (glycine, GLY), 5 (6-aminohexanoic acid, AHA), 10 (11-aminoundecanoic acid, AUDA), with fluorinated single walled carbon nanotubes (F-SWNTs) have been characterized by MAS C-13 NMR spectroscopy. The ease of observing the aliphatic CH2 groups and the resolution of the signal are dependent on the length of the amino acid's aliphatic chain. We have proposed that where substituent chains are short (making NMR data collection difficult) chemical modification to extend the chain length should alleviate analysis problems. In this regard, we have investigated the esterification of the carboxylic acid termini. The amino acid-functionalized SWNTs were esterified with an appropriate alcohol to ensure parity of the overall substituent length, i.e., GLY-SWNT (C-1) + 1-dodecanol (C-12) = DOD-GLY-SWNT (1), AHA-SWNT (C-5) + 1-octanol (C-8) = OCT-AHA-SWNT (2), and AUDA-SWNT (C-10) + 1-propanol (C-3) = PRO-AUDA-SWNT (3). The C-13 NMR shift for the sp(3) nitrogen-substituted carbon atoms of the SWNT sidewall is observed at delta approximate to 75 ppm. Increasing the length of SWNT sidewall functional groups enhances the ability to observe the sidewall sp3 carbon. The methylene carbon signal intensity is less attenuated in the dipolar dephasing spectrum of the ester-functionalized SWNTs than their associated amino acid derivatives, suggesting more motional freedom of the side chain in the solid state. The confirmation of the dipolar dephasing spectral effects was assisted by the characterization of the ester of AUDA-SWNT with 1,3-propanediol: PPD-AUDA-SWNT (4).