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

Directly fused diporphyrins display the extensive pi conjugation as evinced by highly perturbed electronic absorption spectra as well as lowered and l argely split first oxidation potentials. Such diporphyrins prepared include meso-beta doubly linked diporphyrins 7, meso-meso beta-beta beta-beta trip ly linked diporphyrins 8, and meso-meso beta-beta doubly linked diporphyrin s 9. Oxidation of 5,15-diaryl-substituted and 5,10,15-triaryl-substituted N i-II-, Cu-II-, and Pd-II-porphyrins with tris(4-bromophenyl)aminium hexachl oroantimonate (BAHA) in CHCl3 afforded 7, and triply linked Cu-II- diporphy rins 8a and 8g were respectively prepared by the oxidation of meso-meso sin gly linked Cu-II-diporphyrins 5c and 5f with BAHA. Meso-meso beta-beta doub ly linked Ni-II-diporphyrin 9a was isolated along with triply linked Ni-II- diporphyrin 8e from the similar oxidation of meso-meso singly linked N-II-d iporphyrin 5a. Doubly linked diporphyrins 7 and 9a both exhibit significant ly perturbed electronic absorption spectra, in which the Soret-like bands a re largely split at around 405-418 and 500-616 am and the Q-bandlike absorp tion bands are substantially intensified and red-shifted at 748-820 nm, pro bably as a consequence of symmetry lowering. Triply linked diporphyrins 8 d isplay more strongly perturbed electronic absorption spectra with split Sor et-like bands at 408-419 and 567-582 nm and Q-bandlike absorption bands rea ching 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 cr ystallography to be nearly coplanar. Both the triply linked diporphyrins 8g and 8j exhibit very flat structures, whereas the doubly linked diporphyrin s 7b and 7c exhibit ruffled structures. The doubly linked diporphyrin 9a sh ows a helically twisted conformation with larger ruffling toward the opposi te directions and has been actually separated into two enantiomers, which d isplay strong Cotton effects in the CD spectra. The first oxidation potenti als (E-oxl) decrease in the order of 5 > 7 greater than or equal to 9 > 8, indicating lift-up of HOMO orbital in this order, and split potential diffe rences DeltaE = E-ox1 - E-ox2, in turn, increase in the reverse order of 5 < 7 <less than or equal to> 9 < 8. The H-1 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 hav e been concluded to increase in the other of 5 << 7 less than or equal to 9 < 8.