We studied the UV-vis absorption and fluorescence in solution/solid states of [n]cycloparaphenylene ([n]CPP: n = 9, 12, 14, 15, and 16), and conducted theoretical studies to better understand the experimental results. The representative experimental findings include (i) the most intense absorption maxima (λ(abs1)) display remarkably close values (338-339 nm), (ii) the longest-wavelength absorption maxima (λ(abs2)) are blue-shifted with increasing the ring size (395 → 365 nm), (iii) the emission maxima (λ(em)) are blue-shifted with increasing the ring size (494 → 438 nm for longest-wavelength maxima), (iv) the fluorescent quantum yields (Φ(F)) in solution are high (0.73-0.90), (v) the fluorescence lifetimes (τ(s)) of [9]- and [12]CPP are 10.6 and 2.2 ns, respectively, and (vi) the Φ(F) values slightly increase in polymer matrix but significantly decrease in the crystalline state. According to TD-DFT calculations, the longest-wavelength absorption (λ(abs2)) corresponds to a forbidden HOMO → LUMO transition and the most intense absorption (λ(abs1)) corresponds to degenerate HOMO - 1 → LUMO and HOMO → LUMO + 1 transitions with high oscillator strength. The interesting and counterintuitive optical properties of CPPs (constant λ(abs1) and blue shift of λ(abs2)) could be ascribed mainly to the ring-size effect in frontier molecular orbitals (in particular the increase of the HOMO-LUMO gap as the number of benzene rings increases). On the basis of comparative calculations using hypothetical model geometries, we conclude that the unique behavior of HOMO and LUMO of CPPs is due mainly to their lack of a conjugation length dependence in combination with a significant bending effect (particularly to HOMO) and a torsion effect (particularly to LUMO).

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A new strategy for the non-chromatographic extraction of metallofullerenes from solutions of arc-processed raw soot is based on the size-selective complexation with cycloparaphenylene (CPP). [11]CPP has a high affinity for Mx @C82 (x=1, 2); for example, Gd@C82 can be selectively extracted from a fullerene mixture by the addition of [11]CPP. This approach should open new opportunities in metallofullerene chemistry, including for the bulk extraction of metallofullerenes.

Nickel-mediated synthesis of [9]cycloparaphenylene ([9]CPP) is described. L-shaped diphenylcyclohexane monomers underwent cyclotrimerization under the influence of [Ni(cod) 2 ]/bipy. X-ray crystal-structure analysis and VT NMR analysis of the thus-formed cyclic trimer confirmed its triangle structure and interesting cyclohexane-flipping behavior in solution. Cyclic trimer was converted to [9]CPP by acidmediated aromatization. The first X-ray crystal structure of [9]CPP is also described.Highly symmetric macrocyclic structures have fascinated chemists for decades and centuries. Cycloparaphenylenes ([n]CPPs) (Figure 1), simple and beautiful ring-shaped compounds in which n benzene rings are bound to each other at their para positions, have been targeted by synthetic chemists. In an effort to provide CPP in useful quantities by a more concise, cost-effective, and scalable route, we recently developed a nickel-mediated synthesis of [12]CPP.3c One of the key reactions en route to [12]CPP has been the Ni 0 -mediated "shotgun" macrocyclization of cis-1,4-bis(4-halophenyl)cyclohexane monomer 1a or 1b furnishing square-shaped tetramer 2 in moderate yield (2025%, up to 1 g) as shown in Scheme 1. During our investigation trying to identify by-products in the Nimediated reaction of 1, we found that the triangle-shaped trimer 3 could be also generated in the "shotgun" macrocyclization reaction. We herein report the synthesis and structure of cyclic trimer 3 and its conversion to [9]CPP. The ring structure of [9]CPP¢2THF is also confirmed by X-ray crystallography.A single crystal generated from a by-product fraction in the macrocyclization was subjected to X-ray crystallography. Counter to our initial approximation, the compound was found not to be an acyclic oligomeric product but to be the cyclic trimer 3.5 During the crystallization, one EtOAc molecule was incorporated into 3 without any disorder. As shown in Figure 2, all three cyclohexane moieties in 3 adopt chair conformations with included angles of ca. 70°. To gain understanding of the stability of cyclic trimer 3, strain energy was estimated by DFT calculation using B3LYP with a 6-31G(d) basis set. Strain energy of the framework of 3 was estimated to be 8.0 kcal mol ¹1 , which is 6.3 kcal mol ¹1 higher than that of 2 (see Supporting Information for detail 8 ).Having identified the existence of a pathway forming cyclic trimer 3, we re-examined the conditions of Ni-mediated macrocyclization (Table 1). The halogen atom of monomer (I for 1a, Br for 1b) did not affect the ring-size selectivity significantly n -11