Syntheses, structural characterizations, and optical and electrochemical properties of directly fused diporphyrins

Directly fused diporphyrins display the extensive π conjugation as evinced by highly perturbed electronic absorption spectra as well as lowered and largely split first oxidation potentials. Such diporphyrins prepared include meso-β doubly linked diporphyrins 7, meso-meso β-β β-β triply linked diporphyrins 8, and meso-meso β-β doubly linked diporphyrins 9. Oxidation of 5,15-diaryl-substituted and 5,10,15-triaryl-substituted Ni^II-, Cu^II-, and Pd^II-porphyrins with tris(4-bromophenyl)aminium hexachloroantimonate (BAHA) in CHCl₃ afforded 7, and triply linked Cu^II- diporphyrins 8a and 8g were respectively prepared by the oxidation of meso-meso singly linked Cu^II-diporphyrins 5c and 5f with BAHA. Meso-meso β-βdoubly linked Ni^II-diporphyrin 9a was isolated along with triply linked Ni^II-diporphyrin 8e from the similar oxidation of meso-meso singly linked Ni^II-diporphyrin 5a. Doubly linked diporphyrins 7 and 9a both exhibit significantly perturbed electronic absorption spectra, in which the Soret-like bands are largely split at around 405-418 and 500-616 nm and the Q-bandlike absorption bands are substantially intensified and red-shifted at 748-820 nm, probably as a consequence of symmetry lowering. Triply linked diporphyrins 8 display more strongly perturbed electronic absorption spectra with split Soret-like bands at 408-419 and 567-582 nm and Q-bandlike absorption bands reaching far-infrared region. Structures of three types of fused diporphyrins 7b and 7c, 8g and 8j, and 9a have been unambiguously determined by X-ray crystallography to be nearly coplanar. Both the triply linked diporphyrins 8g and 8j exhibit very flat structures, whereas the doubly linked diporphyrins 7b and 7c exhibit ruffled structures. The doubly linked diporphyrin 9a shows a helically twisted conformation with larger ruffling toward the opposite directions and has been actually separated into two enantiomers, which display strong Cotton effects in the CD spectra. The first oxidation potentials (E_OX1) decrease in the order of 5 > 7 ≥ 9 > 8, indicating lift-up of HOMO orbital in this order, and split potential differences ΔE = E_OX1 - E_OX2, in turn, increase in the reverse order of 5 < 7 ≤ 9 < 8. The ¹H NMR spectra have indicated that the aromatic porphyrin ring current becomes weakened in the order of 5 > 7 > 8. Collectively, the electronic interactions between the diporphyrins have been concluded to increase in the other of 5 ≪ 7 ≤ 9 < 8.