β-fused oligoporphyrins: A novel approach to a new type of extended aromatic system

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A novel trimeric porphyrin array in which the macrocycles are directly fused through their β-pyrrolic carbons has been prepared and investigated. These molecules feature a 7,8,17,18-tetraethylporphyrin moiety flanked on opposite sides by two tetraphenylporphyrin (TPP) moieties. The 2,3 and 12,13 β-carbon positions of the tetraethylporphyrin substructure also function as the β-carbons in the 2 and 3 positions of the two TPP macrocycles. This framework was prepared via the reaction of 2,5-bis[(N,N,N-trimethylammonium)methyl]-3,4-diethylpyrrole diiodide with the nickel(II) complex of pyrrolo[3,4-b]-5,10,15,20-tetraphenylporphyrin, which afforded a 62-π-electron 72-atom macrocycle (2) with a central free-base tetraethylporphyrin and two terminal nickel(II) TPP functionalities. The tri-free-base complex (1) was obtained by treatment of the dinickel(II) complex with sulfuric acid followed by neutralization. Crystallographic characterization of 1 (as its tetracation salt) and 2 (as its dication salt) revealed that this type of molecule bears a considerable degree of macrocyclic flexibility. Luminescence spectra of 1 displayed an intense band around 800 nm, making these types of macrocycles promising candidates as chromophores for labels and sensors in biological media. Both 1 and 2 exhibited complex optical spectra, each of which displayed an intensely red shifted Q-band [1, λ(max) (nm) 369 (ε 36 400), 416 (49 700), 488 (71 200), 562 (16 500), 650 (15 700), 744 (42 300); 2, λ(max) nm 361 (ε 87 200), 408 (107 000), 486 (189 000), 558 (27 700), 650 (28 200), 682 (31 100), 716 (175 500)]. Selective protonation of 1 with TFA afforded a green tetracationic species [(H4-H2-H4)4+·1] with an even more red-shifted Q-band (848 nm) while addition of excess TFA yielded a red hexaprotonated species. [(H4-H4-H4)6+·1]. Optical analyses of 1, using the INDO/SCI and orbital localization techniques, were performed to obtain information with regard to the degree of macrocyclic π-electron delocalization. These studies showed that the optical properties of 1 cannot be described within the excitonic model of weakly interacting macrocycles (α > 60%), and that π-electron delocalization over the 72-atom macrocycle is not complete. Even though resonance structures for the 72-atom macrocycle imply a fully conjugated aromatic system, our data indicated that the three constituent porphyrin macrocycles behave somewhat more like discrete aromatic systems.

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Journal of the American Chemical Society

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