Cyclic porphyrin oligomers have been studied as models
for photosynthetic
light-harvesting antenna complexes and as potential receptors for
supramolecular chemistry. Here, we report the synthesis of unprecedented
β,β-directly linked cyclic zinc porphyrin oligomers, the
trimer (CP3) and tetramer (CP4), by Yamamoto
coupling of a 2,3-dibromoporphyrin precursor. Their three-dimensional
structures were confirmed by nuclear magnetic resonance (NMR) spectroscopy,
mass spectrometry, and single-crystal X-ray diffraction analyses.
The minimum-energy geometries ofCP3andCP4have propeller and saddle shapes, respectively, as calculated using
density functional theory. Their different geometries result in distinct
photophysical and electrochemical properties. The smaller dihedral
angles between the porphyrin units inCP3, compared withCP4, result in stronger π-conjugation, splitting the
ultraviolet–vis absorption bands and shifting them to longer
wavelengths. Analysis of the crystallographic bond lengths indicates
that the central benzene ring of theCP3is partially
aromatic [harmonic oscillator model of aromaticity (HOMA) 0.52], whereas
the central cyclooctatetraene ring of theCP4is non-aromatic
(HOMA –0.02). The saddle-shaped structure ofCP4makes it a ditopic receptor for fullerenes, with affinity constants
of (1.1 ± 0.4) × 105M–1for
C70and (2.2 ± 0.1) × 104M–1for C60, respectively, in toluene solution at 298 K.
The formation of a 1:2 complex with C60is confirmed by
NMR titration and single-crystal X-ray diffraction.