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Characterisation of cobalt(II)porphyrin/carbon nanotube nanohybrids by electron paramagnetic resonance and optical spectroscopy

Tailoring the properties of single-walled carbon nanotubes (SWNTs) with organic molecules by non-covalent functionalisation yields very promising materials for organic electronics. Recent progress in the characterisation of the opening/closing of SWNTs[1] allows for the quantitative preparation of both inclusion and external adsorption complexes. Using paramagnetic molecules, these nanohybrids can be studied by EPR spectroscopy. Here, we present the first EPR investigation of paramagnetic metalloporphyrins adsorbed on the nanotube walls through >-stacking. This functionalisation of SWNTs with porphyrin molecules has attracted much attention due to their use as donor-acceptor building blocks in photovoltaic cells, for their catalytic properties and for their use in biological sensors.[2] We study multi-component EPR spectra of cobalt(II)porphyrin/SWNT nanohybrids. Their g-anisotropy, hyperfine interaction and g- and hyperfine strains are monitored under different treatments. Our results affirm the electron acceptor behaviour of the SWNTs in these nanohybrids, in agreement with DFT calculations.[3] In the EPR spectra we can distinguish both interacting and non-interacting porphyrin molecules, which in turn allowed us to optimise the purification of the nanohybrids. Solubilising the SWNTs in water using bile salts as surfactants, [4] we can also study the interaction of individual carbon nanotubes with the molecules. The EPR investigation is combined with optical absorption and with steady state and time resolved fluorescence spectroscopy. Large red shifts of the porphyrin absorption bands and a complete quenching of the porphyrin fluorescence indicate strong electronic interactions of the porphyrin system with the SWNTs, as was also evidenced by EPR spectroscopy. [1] W. Wenseleers, S. Cambré, J. Culin, A. Bouwen, E. Goovaerts, Adv.Mater. 2007, 19, 2274 [2] D.M. Guldi, Phys.Chem.Chem.Phys. 2007, 9, 1400 [3] E.V. Basiuk, V.A. Basiuk, P. Santiago, I. Puente-Lee, J.Nanosc.Nanotech. 2007, 7, 1 [4] W. Wenseleers, I.I. Vlasov, E. Goovaerts, E.D. Obraztsova, A.S. Lobach, A. Bouwen, Adv.Funct.Mater. 2004, 14, 1107

single-walled carbon nanotubes; nanohybrids

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