β-fused oligoporphyrins: A novel approach to a new type of extended aromatic system
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.
Publication Source (Journal or Book title)
Journal of the American Chemical Society
Paolesse, R., Lugli, P., Smith, K., Jaquinod, L., Della Sala, F., Nurco, D., Prodi, L., Montalti, M., Di Natale, C., D'Amico, A., & Di Carlo, A. (2000). β-fused oligoporphyrins: A novel approach to a new type of extended aromatic system. Journal of the American Chemical Society, 122 (46), 11295-11302. https://doi.org/10.1021/ja001316+