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, λmax (nm) 369
(ε 36 400), 416 (49 700), 488 (71 200), 562 (16 500), 650 (15 700), 744 (42 300); 2, λmax 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.