Theoretical study in donor–acceptor carbon nanohorn-based hybrids Ioannis D. Petsalakis, Georgia Pagona, Nikos Tagmatarchis, Giannoula Theodorakopoulos * Theoretical and Physical Chemistry Institute, National Hellenic Research Foundation, 48 Vass. Constantinou Avenue, Athens 116 35, Greece Received 26 June 2007; in final form 24 September 2007 Available online 29 September 2007 Abstract The binding energy of a fused pyrrolidine ring at different binding sites of a carbon nanohorn (CNH) has been determined by density functional theory (DFT) calculations employing a 217 C-atom model of CNH. In agreement with previous work, significant binding is found at sites closer to the conical tip. Secondly, DFT and time dependent density functional theory (TDDFT) calculations have been carried out on CNH nanohybrids with photo- or electro-active substituents, porphyrin, pyrene and tetrathiafulvalene, of interest for photo-induced charge transfer. It is found that the relevant excitations in the individual substituents retain their characteristics in the nanohybrid systems. Ó 2007 Elsevier B.V. All rights reserved. 1. Introduction Carbon nanohorns (CNH) [1], possessing a conical tip and properties differentiating them significantly from car- bon nanotubes [2,3], are of particular interest for potential applications in nanoscience, including in clean-energy tech- nologies and drug-delivery applications ([4] and references therein). Current research interest on the spectroscopy and photochemistry of nanostructures is quite intense because they are considered to be ideal candidates as the electron- acceptors in hybrid systems suitable for photo-induced transfer of electrons and/or energy and thus of interest for applications in solar cells [5,6]. Related to this, the abil- ity of CNHs to accept electrons and readily diffuse them along the cone main axis has resulted in the formation of donor–acceptor nanosystems consisting of CNHs and pyr- enes [7–9], and more recently, in the formation of a nano- hybrid involving covalent connection of CNHs with porphyrin, where both electron transfer and energy trans- fer processes are found, depending on the polarity of the solvent, from the excited singlet-state of the porphyrin to the CNHs, while the charge-separated states have been observed by transient absorption spectroscopy [4]. Electronic structure calculations on carbon nanostruc- tures, namely fullerenes and nanotubes, are quite common in the literature, mostly by density functional theory (DFT) [10], offering information on the stable geometries and binding energies, and recently by time dependent density functional theory (TDDFT) [11] on excitations in fullerene nanohybrids [12–14]. However such electronic structure calculations on carbon nanohorns are scarce and this fact can be attributed to the greater technical difficulties involved in such calculations on CNHs as compared to fullerenes and nanotubes. The existence of five 5-membered rings for the generation of the conical tip of CNH results in a significant structural variation along the length of a CNH, from the tip to the outer edge, which cannot be reproduced adequately with small chair-type models, often used in calculations on nanotubes [15]. A previous publica- tion by the present authors [16] involved electronic struc- ture calculations by AM1 [17], DFT and ONIOM methods [18], where the DFT calculations involved a lim- ited section of the nanohorn model, with AM1 employed for the remaining part, in an ONIOM approach of calcula- tion. While the ONIOM method has been criticized as not being sufficiently accurate for the study of small molecules at nanotube surfaces [19], the results of the previous study 0009-2614/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2007.09.067 * Corresponding author. Fax: +30 210 7273794. E-mail address: ithe@eie.gr (G. Theodorakopoulos). www.elsevier.com/locate/cplett Available online at www.sciencedirect.com Chemical Physics Letters 448 (2007) 115–120