Highly Strained, Radially π-Conjugated Porphyrinylene Nanohoops
2019
Small
π conjugated nanohoops are difficult to prepare, but offer an excellent
platform for studying the interplay between strain and optoelectronic
properties and increasingly, these shape persistent macrocycles find uses in
host guest chemistry and self assembly. We report the synthesis of a new family
of radially π conjugated porphyrinylene/phenylene nanohoops. The strain energy
in the smallest nanohoop [2]CPT is approx-imately 54 kcal mol-1,
which results in a narrowed HOMO LUMO gap and a red shift in the visible part
of the absorption spectrum. Due to its high degree of preorganization and a
diameter of ca. 13 A, [2]CPT was found to accommodate C60 with a
binding affinity exceeding 108 M-1 despite the fullerene
not fully entering the cavity of the host (X Ray crystallography). Moreover,
the π extended nanohoops [2]CPTN , [3]CPTN and [3]CPTA (N for 1,4 naphthyl; A
for 9,10 anthracenyl) have been prepared using the same strategy, and [2]CPTN
has been shown to bind C70 five times more strongly than [2]CPT. Our
failed synthesis of [2]CPTA highlights a limitation of the experimental
approach most commonly used to prepare strained nanohoops, because in this
particular case the sum of aromatization energies no longer outweighs the
buildup of ring strain in the final reaction step (DFT calculations). These
results indicate that forcing ring strain onto organic semiconductors is a
viable strategy to fundamentally influence both optoelectronic and
supramolecular properties.
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