Selective and Random Syntheses of [n]Cycloparaphenylenes (n = 8–13) and Size Dependence of Their Electronic Properties

Abstract

[n]Cycloparaphenylenes (n = 8-13, CPPs) were synthesized, and their physical properties were systematically investigated. [8] and [12]CPPs were selectively prepared from the reaction of 4,4'-bis(trimethylstannyl)biphenyl and 4,4''-bis(trimethylstannyl)terphenyl, respectively, with Pt(cod)Cl(2) (cod = 1,5-cyclooctadiene) through square-shaped tetranuclear platinum intermediates. A mixture of [8]-[13]CPPs was prepared in good combined yields by mixing biphenyl and terphenyl precursors with platinum sources. Products were easily separated and purified by using gel permeation chromatography. In (1)H NMR spectra, the proton of the CPPs shifts to a lower field as n increased due to an anisotropic effect from the nearby PP moieties. Although the UV-vis spectra were rather insensitive to the size of the CPPs, the fluorescence spectra changed significantly in relation to their size. A larger Stokes shift was observed for the smaller CPPs. Redox properties of the CPPs were measured for the first time by using cyclic voltammetry, and the smaller CPPs had lower oxidation potentials. The results are consistent with the HOMO energies of CPPs, of which the smaller CPPs had higher energies.

Intra‐ and Intermolecular Dispersion Interactions in [n]Cycloparaphenylenes: Do They Influence Their Structural and Electronic Properties?

“…The first one is the diameter of every [n]CPP, defined as the distance between the opposite ipso-carbon atoms for the even members, or as the distance between the ipso-carbon atom and the center of mass of the opposite benzene ring for the odd members. As it would be expected, the influence of dispersion corrections, at the B3LYP-D3(BJ)/6-31+G* level, is to slightly decrease the diameter in all cases with respect to previously calculated (B3LYP/6-31G*) values [11]. The diameter is found to evolve with system size (n) following the relationship d = 1.379n − 0.011.…”

“…This closed configuration induces a large set of new and interesting properties with respect to linear forms [7], such as the diameter of the nanohoop, the strain energy arising from closing these para-phenylenes into a ring, or their necessarily modified electronic and absorption properties. These achievements have attracted recently the interests of several groups [8][9][10][11][12][13][14][15][16][17] in an attempt to understand how these properties evolve with system size (n, where n refers to the number of benzene units connected in para position) and how they manifest when real samples are obtained in solution or forming solid-state phases. Note that the first cycloparaphenylene was synthesized in 2008 [19], and that there is still a lot of opportunities concerning the rich chemistry of these compounds.…”

“…As a matter of example, we summarize next some interesting and highly challenging achievements (in the supramolecular scale) recently accomplished in the field: (i) the encapsulation of C 60 by [10]-cycloparaphenylene ( [10]CPP), forming a host-guest complex stabilized roughly by 40 kcal/mol in toluene [20] and with both molecules separated by a distance of 3.35Å, which equals the interlayer separation in graphite; actually, not all the [n]CPP nanohoops (n = 8 − 12) are able to encapsulate C 60 [21], but only [10]CPP, which might arise from the interplay between attractive and repulsive intraand inter-molecular interactions; (ii) the association of C 70 or the metallofullerene La@C 82 with [11]CPP was also recently studied [22,23], and shows how both molecules are labile enough to accommodate each other, with [11]CPP adopting an ellipsoidal shape in the former case; (iii) the chirality in SWNTs might be induced by appropriately substituted cycloparaphenylenenaphthalenes [24] or cycloparaphenylene-naphthylenes [25] compounds, constituting thus the first step towards the controlled synthesis of chiroptical SWNTs, which might create a diversity of forms and release the strain energy of unsubstituted cycloparaphenylenes; and (iv) the dimerization of [n]CPP nanohoops, thanks to covalently-linked structures at only one specific position along the macrocycle, which might pave the way towards increasingly longer supramolecular architectures with customized nanochannels [26].…”

Expanding the Chemical Space of Biocompatible Fluorophores: Nanohoops in Cells

“…[30] Additionally, due to Laporte forbidden HOMOàLUMO transitions, all nanohoops share a common absorption maxima (labs = 340 nm) with high absorption coefficients (e) and large effective Stokes shifts ranging from 100-200 nm depending on size. [31][32][33][34] Taken together, the nanohoop scaffold offers the possibility of multiplexed imaging using a single excitation source. Moreover, the non-planarity of the benzene rings in the nanohoop also leads to better solubility when compared to planar aromatic systems.…”

Synthesis and physical properties of a ball-like three-dimensional π-conjugated molecule

“…Our method relies on the formation of a square-shaped tetranuclear aryl platinum complex by the transmetallation of arylmetal species and a platinum dihalide (Fig. 1a) 12,[18][19][20] . The process of the complex formation can be regarded as a covalent bonding version of the self-assembly of structurally related coordination complexes of palladium and platinum macrocycles with, for example 4,4 0 -bipyridyl, in supramolecular chemistry (Fig.…”