Journal Name Cite this: DOI: 10.1039/c0xx00000x www.rsc.org/xxxxxx Dynamic Article Links ► ARTICLE TYPE This journal is © The Royal Society of Chemistry [year] [journal], [year], [vol], 00–00 | 1 Chameleon-like Behaviour of Cyclo[n]paraphenylenes in Complexes with C 70 . On Their Impressive Electronic and Structural Adaptability as probed By Raman spectroscopy. Juan Casado Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX 5 DOI: 10.1039/b000000x A series of four 1:1 host-guest supramolecular complexes of [n]CPPs and C 70 have been analyzed by Raman spectroscopy in solid state and complemented with the analysis of their spectroscopic responses under mechanical and thermal stresses. By following the frequency behaviour of the G and RBM modes we have found that [10]CPP in the [10]CPP@C 70 complex displays a more “ordered” structure. However, in [11]CPP@C 70 , the nanoring gets ovallized with closer contacts with the C 70 poles and less conformational restriction in the 10 flattened region. By mechanical and thermal stresses we are able to modify the lying conformation of [10]CPP@C 70 towards a standing shape. [11]CPP@C 70 resists pressure changes, although it tends to shift from the standing to the lying orientation by heating. As for the crystal cell, the [n]CPPs occupy the residual empty spaces while the main crystallographic positions are reserved to C 70 . These are new examples of the impressive adaptability of the [n]CPP molecules to different physico-chemical environments, a chameleon-like property which reveals the delicate equilibrium provided by cyclic conjugation and ring strain. 15 A Introduction Cyclo[n]paraphenylenes (abbreviated as [n]CPP, see Figure 1) are a class of conjugated pure-hydrocarbon molecules featured by a cyclic disposition of 1,4 substituted benzenes with the p z carbon orbitals oriented towards the macroring center. Given their 20 aesthetic shape, they attracted the interest of chemists for a long time. 1 It was in 2008 when Jasti and Bertozzi 2 were able to synthesize cyclo[n]paraphenylene compounds, for the first time, [8]CPP, [12]CPP and [18]CPP, paving the way for the preparation of [n]CPP of almost all sizes, [6]- to [18]CPP, in a 25 beautiful competition race mainly between the groups of Itami 3 , Yamago 4 and Jasti 5 . Very recently, the smallest member of the series, [5]CPP, has been successfully prepared almost simultaneously by Yamago and Jasti. 6 30 Figure 1. Left: Chemical structures of the [n]CPP compounds (s=0 for [9]CPP, s=1 for [10]CPP, s=2 for [11]CPP and s=3 for [12]CPP). Middle: lying disposition for [10]CPP@C70 and standing disposition for [11]CPP@C70. The analogy between [n]CPPs and armchair single wall carbon 35 nanotubes (SWCNT) is straightforward: the [n]CPP can be considered as the shortest version of arm-chair SWCNTs and, as such, can be considered as their molecular models. While the chemical undefinition of SWCNTs is a insurmountable obstacle for the full understanding of their properties, the perfect 40 knowledge of the chemical structures of the [n]CPP series allows to evaluate the evolution of their electronic, structural and optical properties as a function of the [n]CPP size thus permitting the first oligomeric approach to the physical properties of SWCNT. But [n]CPPs have been also used as templates for the chemical 45 synthesis of SWCNT via a controlled vertical condensation of [n]CPP, as reported by Itami, 7 which supposed the first successful preparation of nanotubes by using an orthodox synthetic organic approach. On their own way, the [n]CPPs display very interesting optoelectronic properties as well thanks to the co-existence of 50 rather unusual effects such as molecular strain and cyclic conjugation. Cyclic strain increases with the decrease of the number of benzenes 8 and produces in these rings some slight deformations from planarity and their partial benzo- quinonoidization. 9 On the other hand, cyclic conjugation, which 55 is provided by these p z orbitals contained in the circumferential molecular plane, further stabilizes the whole cyclic arrangement. As a result, these appealing molecules contain highly polarisable electronic shapes in the belt which originate other interesting chemical properties such as the ability to form host-guest  60 and van der Waals supramolecular complexes. These complexes are the focus of this article. 10 We have recently reported on the dependence of the structural and electronic properties of the [n]CPPs (from [6]CPP to [12]CPP) with their size, and established robust relationships 65 between quinonoidization, cyclic strain, cyclic conjugation and [n]CPP dimension, using Raman spectroscopy as the revealing tool. 11 Another structural feature of significance is their conformational effect that makes these [n]CPPs to be irregular cylinders with the successive benzenes canted or staggered with 70 opposite angles. 8,9 We have also addressed the structural flexibility of the whole nanohoop which, for the larger