Synthesis of [n]Cyclo-5,15-porphyrinylene-4,4′-biphenylenes Displaying Size-Dependent Excitation-Energy Hopping

A set of 5,15-biphenylene-bridged porphyrin wheels, namely, [n]cyclo-5,15-porphyrinylene-4,4′-biphenylenes [n]CPB, have been synthesized through the platination of 5,15-bis(4-(pinacolboranyl)phenyl) nickel(II) porphyrin and subsequent reductive elimination of Pt^II(cod)-bridged cyclic porphyrin intermediates. The calculated strain energies for [3]CPB, [4]CPB, [5]CPB, and [6]CPB are 49.3, 32.9, 23.5, and 16.0 kcal mol^-1, respectively. UV/Vis absorption spectra and cyclic voltammetry indicated characteristic ring-size-dependent absorption-peak shifts and redox-potential shifts, which presumably reflect the degree of strain in the π-systems. Excitation-energy hopping (EEH) times were determined to be 5.1, 8.0, 8.0, and 9.6 ps for [3]CPB, [4]CPB, [5]CPB, and [6]CPB, respectively, in a pump-power-dependent TA experiment. Molecular wheels: A series of 5,15-biphenylene-bridged cyclic porphyrin arrays (see example; Ar=3,5-tBu₂C₆H₃) were obtained by the platination of a 5,15-bis(4-pinacolateboryl)phenyl Ni^II porphyrin and subsequent reductive elimination of the Pt^II(cod)-bridged cyclic porphyrin intermediates. The ¹H NMR and UV/Vis spectra, redox potentials, and intramolecular excitation-energy-hopping characteristics of the nanorings depend on the ring size.