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Fig. 28 Crystal structure ( left ) and packing orientation ( right ) of [12]CPP [ 41 ]
The availability of [9]CPP and [12]CPP on a more significant scale allowed for
the analysis of their molecular structures by single crystal X-ray diffraction for the
first time (Fig. 28 ). These slightly disordered structures were refined enough to
confirm the benzenoid structure of [12]CPP with ipso-ipso bond lengths of 1.481 ˚
and also hint at the trend towards quinoidal structures in smaller CPPs showing
shortening of the ipso-ipso bond lengths of [9]CPP to 1.468 ˚ , still longer than the
expected region for alkene bonds. In addition, it was shown that the crystal packing
of [12]CPP favors a herringbone pattern with channels clearly visible from the
appropriate angle, but not strictly nanotube-like [ 41 , 42 ].
Yamago also published a synthesis of new CPP sizes in 2011 [ 38 ]. By utilizing
4,4 0 -bis(trimethyl)stannylbiphenyl and 4,4 0 -bis(trimethyl)stannylterphenyl as
starting materials, Yamago showed that [8]CPP and [12]CPP can be selectively
formed by his platinum-mediated synthesis and also that using a “shotgun” mixture
of both provides a mixture of metallocycles which can be converted to the CPPs
then separated by preparatory GPC to offer [8]-[13]CPP (Fig. 29 ).
The formation of [13]CPP is interesting in that it implies the formation of either
a 2 + 2 + 3 + 3 + 3 or a 2 + 2 + 2 + 2 + 2 + 3 macrocycle. With these several
sizes in hand, Yamago determined the optical characteristics by UV/vis and fluo-
rescence spectroscopy and the oxidation potentials of these CPPs by cyclic
voltammetry. The absorbance of all the synthesized CPPs was similar, centered
around 338 nm, and the fluorescence followed the emission-shift trend observed by
Jasti (Fig. 30 )[ 32 ]. It was theorized that the relaxation and relief of strain in the
excited state geometry of these molecules accounts for the observed shift [ 57 ].
The oxidation potentials were shown to be lower with decreasing size. This trend
is indicative of the increasing quinodimethane structure and therefore polyene
character of the smaller CPPs. These CV observations also correlate well with
Yamago's calculated HOMO-LUMO gap, which narrows with decreasing size
(Fig. 31 ). The opposite trend is calculated for OPPs [ 38 ].
In an effort to probe the interesting properties of even smaller carbon nanohoops,
Jasti published the first synthesis of [7]CPP [ 43 ]. This very strained polyphenylene
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