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

Noncovalent functionalisation of single-wall carbon nanotubes (SW CNTs) with porphyrins has attracted much attention for their potential applications in photovoltaic cells, catalytic materials and in biological sensors and has also been shown to be helpful in nanotube debundling and solubilization. (1) In this study we use electron paramagnetic resonance (EPR) spectroscopy in combination with absorption, steady state and time resolved fluorescence spectroscopy to characterize paramagnetic cobalt(II)octaethylporphyrin (CoOEP)/CNT nanohybrids. The EPR spectrum of CoOEP is very sensitive for changes in the axial coordination, because the unpaired electron resides in the dz2-orbital, pointing out of the porphyrin plane. Here we present the first EPR investigation of these porphyrin/CNT nanohybrids, clearly demonstrating that the CNTs act as electron acceptors and that the spin density located on the cobalt ion is redistributed towards the CNTs, in agreement with DFT calculations. (2). While measuring on a bulk sample, two different components can be distinguished in the EPR spectrum, arising from porphyrins interacting with metallic and semiconducting CNTs. From analyzing these two EPR contributions, we can conclude that the metallic CNTs are much stronger  -acceptors for the porphyrins than the semiconducting CNTs. Solubilising the SW CNTs in water using bile salts as surfactans (3), we can also study the interaction of individual CNTs with porphyrins. This water solubilization of the porphyrin/CNT nanohybrids, after applying a thorough washing procedure, yields very stable solutions in which at least 99 % of the porphyrins are adsorbed on the CNTs. Thanks to this purification, we observe the isolated absorption spectrum of the interacting porphyrins, which is strongly red-shifted compared to the free porphyrin absorption. In addition also a quasi complete quenching of the porphyrin fluorescence is observed. 1. D. M. Guldi, Phys. Chem. Chem. Phys., 2007, 9, 1400. 2. E. V. Basiuk et al., J. Nanosci. Nanotechnol., 2007, 7, 1350. 3. W. Wenseleers et al., Adv. Funct. Mater., 2004, 14, 1107

Spectroscopic studies of CNTs; CNTs based composites

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