Cycloparaphenylene
A cycloparaphenylene is a molecule that consists of several benzene rings connected by covalent bonds in the para positions to form a hoop- or necklace-like structure.
[citation needed] The normal configuration of each phenylene element would be planar, with the bonds in the para position pointing opposite to each other in a straight line.
This trend opposite to that observed in linear polyparaphenylenes where the HOMO-LUMO gap decreases as size increases.
In the initial synthesis, cycloparaphenylenes with n = 9, 12, and 18 have been synthesized starting from macrocycles containing 1,4-syn-dimethoxy-2,5-cyclohexadiene units as masked aromatic rings.
[12] A mixture of [8-13]cycloparaphenylenes can be obtained in good combined yields by mixing biphenyl and terphenyl precursors with the platinum sources.
[12] A third method for the synthesis of cycloparaphenylenes developed in the Wegner group is based on rhodium-catalyzed [2+2+2]cycloadditions of substituted alkynes.
[19][20] Potential applications of cycloparaphenylenes include host–guest chemistry,[10] seeds for carbon nanotube growth, and hybrid nanostructures containing nanohoop-type substituents.
Potential applications of these structures include nanolasers, single electron transistors, spin-qubit arrays for quantum computing, nanopipettes, and data storage devices.
[25][26][27] Specifically, the π-π interactions and the concave interior of the cycloparaphenylenes is expected to bind to π conjugated systems with convex surfaces that can fit inside the ring.
[29] It has been observed that such "ball-in-hoop" interactions are stronger for endohedral metallo-fullerenes, in which a positively charged metal ion is trapped inside a fullerene cage and makes it more electronegative.
[30][22] Specifically, [12]CPP was found to preferentially enclose metallo-fullerenes instead of "empty" fullerenes, reducing their solubility in toluene; which provides a convenient separation method for the two species.
Similar to the original (n,n) cycloparaphenylenes, these chiral nanorings also exhibit unusual optoelectronic properties with excitation energies growing larger as a function of size; however, the (n+3,n+1) chiral nanoring exhibits larger photoinduced transitions compared to the original (n,n) cycloparaphenylenes, resulting in more readily observable optical properties in spectroscopic experiments.
N-methylaza[n]CPP showed that a lowering of the LUMO could be enhanced by decreasing the ring size, while the HOMO energy level remained the same.
