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We performed a combined experimental and theoretical study of the conjugates obtained from single-walled carbon nanotubes and anticancer antibiotic doxorubicin (DOX). Atomic force microscopy (AFM) imaging at lower magnification revealed, extended regions of single-walled carbon nanotubes (SWNTS) fully covered with DOX adsorbed molecules, along with some bare parts without the adsorbed drug, thus suggesting that the DOX adsorption is a cooperative process. Ambient atmosphere scanning tunneling microscopy (STM) at higher resolutions found that individual SWNTs-DOX conjugates exhibit a periodic texture, whose most important morphological feature is alternating depressions and protrusions along the nanotube. Based on the images and profiles measured, we suggest that doxorubicin molecules self-assemble on SWNTs sidewall according to a helical pattern, in which their tetracyclic fragments are turned with respect to the nanotube axis by about 50[Formula: see text]. To provide an additional insight into the structure of noncovalent SWNTs-DOX conjugates, we employed density functional theory (DFT) calculations with three long-range corrected functionals: M05-2X, wB97X-D and LCBLYP, of which M05-2X yielded the most realistic results in terms of geometries and energies.

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We performed density functional theory (DFT) calculations of noncovalently bonded 1:1 complex of meso-tetraphenylporphine H2TPP with fullerene C60. The functionals used were PW91, PBE and BLYP of general gradient approximation (GGA), as well as PWC and VWN of local density approximation (LDA) as implemented in the DMol3 module of Materials Studio package from Accelrys. The computed geometries were compared to the experimental X-ray diffraction data obtained elsewhere for rhombohedral and monoclinic H2TPP + C60 crystalline complexes. If the correlation coefficient between the calculated and experimental data is applied, the covalent bond lengths and angles within H2TPP unit are best reproduced by BLYP functional, whereas PWC and VWN are least precise. On the other hand, PWC and VWN are the best functionals in reproducing the separations between H2TPP and C60 found from X-ray diffraction analysis: the LDA-calculated N(H2TPP) ... C(C60) distances are of about 2.9-3.0 angstroms, whereas the corresponding experimental values are of ca. 3.0-3.1 angstroms. Next are PW91 and PBE functionals, giving N(H2TPP) ... C(C60) distances of ca. 3.5-3.6 angstroms. BLYP produced the separations of around 4.0-4.1 angstroms, which are inconsistent with both X-ray data and the results produced other functionals. We also analyzed functional-dependent variations in formation energies, electrostatic potential, HOMO, LUMO and charge transfer. We concluded that of DFT functionals incorporated into DMol3 module and tested in this study, PWC and VWN are the most adequate ones, and BLYP is the least recommended one for the studies of noncovalent interactions of porphyrins with carbon nanoclusters.

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The noncovalent dyad of tetraphenylporphine and C60 fullerene (H2TPP···C60) and the tetraphenylporphine dimer (H2TPP···H2TPP) were studied by density functional theory (DFT), using functionals that incorporate empirical dispersion correction (DFT-D), functionals that use a long-range correction (LC) scheme, a hybrid functional (B3LYP) and a highly parametrized empirical exchange-correlation functional (M05-2X). The results were compared to X-ray structures and interaction energies reported in previous experimental and theoretical studies. It was found that B3LYP and CAM-B3LYP functionals fail to reproduce the X-ray structures and binding energies of the TPP···C60 system. DFT-D functionals overestimated the π···π energy interactions for both systems, however, the optimized structures agree well with those observed experimentally. The LC-BLYP functional predicts geometries similar to X-ray structures; nevertheless, due to the lack of correction in the dispersion energy, the predicted energies for both model systems are low. On the other hand, the M05-2X functional exhibited the best performance. Both the structures and binding energies calculated with M05-2X are consistent with experimental and theoretical evidence reported by other authors, as well as with our experimental results obtained by means of atomic force microscopy on H2TPP thin films grown on the HOPG/C60 substrate by physical vapor deposition.

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