Engineering molecular chains in carbon nanotubes† Thomas W. Chamberlain, a Rudolf Pfeiffer, b Jonathan Howells, a Herwig Peterlik, * b Hans Kuzmany, b Bernhard Kr€ autler, c Tatiana Da Ros, d Manuel Melle-Franco, e Francesco Zerbetto, f Dragana Mili  c g and Andrei N. Khlobystov * a Received 3rd September 2012, Accepted 20th September 2012 DOI: 10.1039/c2nr32571c A range of mono- and bis-functionalised fullerenes have been synthesised and inserted into single- walled carbon nanotubes. The effect of the size and shape of the functional groups of the fullerenes on the resultant 1D arrays formed within the nanotubes was investigated by high resolution transmission electron microscopy and X-ray diffraction. The addition of non-planar, sterically bulky chains to the fullerene cage results in highly ordered 1D structures in which the fullerenes are evenly spaced along the internal nanotube cavity. Theoretical calculations reveal that the functional groups interact with neighbouring fullerene cages to space the fullerenes evenly within the confines of the nanotube. The addition of two functional groups to opposite sides of the fullerene cages results in a further increase in the separation of the fullerene cages within the nanotubes at the cost of lower nanotube filling rates. Introduction Filling single-walled carbon nanotubes (SWNTs) has become a hot topic in the field of nanomaterial science over recent years. The insertion of molecules and ions into SWNTs is one of the most powerful methods available for organising matter into quasi-1D chains at the nanoscopic level. The spatial confinement imposed by the nanotube can result in dramatic changes in the structural and dynamic properties of the guest species, facili- tating the formation of otherwise unobtainable packing motifs 1 and significantly altering both molecular translation 2,3 and rotation. 3 The interior of the nanotube can also be utilised to alter the reactivity of the encapsulated molecules, 4 and as a template, leading to the formation of new nanostructures pos- sessing exciting properties. 5 By far the most common species inserted into carbon nano- tubes is fullerenes, where the commensurate match of the internal wall of the nanotube (concave) and the spherical fullerenes (convex) results in efficient van der Waals interactions (up to 288 kJ mol 1 for C 60 ) 6 which act as the driving force for the irreversible encapsulation of the fullerene molecules. The rich and varied reactivity of fullerenes has allowed a wide variety of functional groups to be chemically bound to the carbon cage and the resultant functionalised fullerenes inserted into SWNTs. This methodology has been utilised to introduce a host of organic functional groups, 5,7–9 transition metal centres, 10–12 and electron spin-active groups 13–17 into carbon nanotubes. Such encapsulated molecules can be regarded as dopants, which can locally perturb and modulate the intrinsic properties of the nanotubes. 18–22 This offers a methodology for tuning the functional properties of nanotubes, such as the electronic band gap, and the concentration and mobility of charge carriers. 23–25 The utilisation of nanotubes as containers for magnetic guest- molecules is particularly important for several applications such as spintronics and quantum information processing. As spin active molecules can be viewed as potential carriers of quantum information, 22 the ability to control the spin concentration and separation between the spins in carbon nanotubes is crucial to the successful application of such nanostructures. Previous attempts to engineer the density of magnetic guest-species within SWNTs have involved the insertion of a mixture of spin-carrying endohedral and spin-silent empty fullerene cages, so that the ratio of these molecules defines the average distance between spin-carrying molecules. 15,17,26 Due to the random nature of the nanotube filling process, the local structure in such molecular chains will be disordered with the distance between neighbouring spin carriers varying statistically from them being located next to each other to a situation in which they are separated by numerous spin-silent species. An alternative method of control- ling the intermolecular separation is to use fullerene molecules a Department of Chemistry, University of Nottingham, Nottingham, UK. E-mail: andrei.khlobystov@nottingham.ac.uk; Fax: +44 (0)115 951 3563; Tel: +44 (0)115 951 3917 b Faculty of Physics, University of Vienna, Austria. E-mail: herwig. peterlik@univie.ac.at; Fax: +43 1 4277 9729; Tel: +43 1 4277 51350 c Institute of Organic Chemistry, University of Innsbruck, Austria d Department of Chemical and Pharmaceutical Sciences, University of Trieste, Italy e CCTC, Department of Informatics, University of Minho, 4710-057, Braga, Portugal f Dipartimento di Chimica ‘‘G. Ciamician’’, Universit  a di Bologna, V. F. Selmi 2, 40126 Bologna, Italy g Faculty of Chemistry, University of Belgrade, Serbia † Electronic supplementary information (ESI) available: HRTEM images of 4@SWNT, space filling models of 1–6@SWNT structures and crystal packing considerations of 6. See DOI: 10.1039/c2nr32571c 7540 | Nanoscale, 2012, 4, 7540–7548 This journal is ª The Royal Society of Chemistry 2012 Dynamic Article Links C < Nanoscale Cite this: Nanoscale, 2012, 4, 7540 www.rsc.org/nanoscale PAPER