Cyclic 2,12-Porphyrinylene Nanorings as a Porphyrin Analogue of Cycloparaphenylenes

β-to-β Directly linked
cyclic Ni­(II) porphyrin trimer,
tetramer, and pentamer ([3]­CP, [4]­CP, and [5]­CP) have been synthesized by reaction of a 2,12-diborylated
Ni­(II) porphyrin with Pt­(cod)­Cl2 followed by reductive
elimination. The structures of these cyclic porphyrin arrays have
been revealed by X-ray diffraction analysis. The strain energies of
these cyclic oligomers are calculated to be 77, 57, and 47 kcal/mol
for [3]­CP, [4]­CP, and [5]­CP, respectively. Intramolecular excitation energy hopping was observed
between the 3(d,d) states of the Ni­(II) porphyrins with
rates of 3.0, 4.4, and 4.6 ps for [3]­CP, [4]­CP, and [5]­CP, respectively, reflecting the close proximity
of the Ni­(II) centers.