Two examples of nonaromatic m-benziheptaphyrins and two examples of aromatic p-benziheptaphyrins were synthesized by [5+2] condensation of appropriate m-benzi pentapyrrane and p-benzi pentapyrrane respectively and bithiophene diol in CH2 Cl2 in the presence of one equivalent of TFA under inert conditions for 30 min followed by oxidation with DDQ in open air for 1 h. The 1 H NMR studies carried out at room temperature as well as at lower temperature indicated the nonaromatic nature of m-benziheptaphyrins with inversion of two thiophene rings and aromatic nature of p-benziheptaphyrins with inversion of one of the thiophene ring. The X-ray structure obtained for one of the p-benziheptaphyrins showed a planar conformation with alignment of one of the thiophene ring away from the macrocyclic inner core but maintained its coplanarity with the mean plane and supported the aromatic nature of the macrocycle. The absorption spectra of m-benziheptaphyrins resembled with the nonaromatic systems and showed two intense bands at 445 nm, 555 nm and a very broad band in the region of 600-1100 nm whereas the p-benziheptaphyrin showed three sharp intense bands at 534 nm, 585 nm and 832 nm due to their aromatic nature. The protonation of m-benziheptaphyrins and p-benziheptaphyrins resulted in significant bathochromic shifts in their absorption maxima and showed strong absorption in NIR region. The electrochemical studies indicated that m- & p-benziheptaphyrins undergo oxidations and reductions easily. DFT and TD-DFT studies were in agreement with the experimental observations.
Keywords: Aromaticity; Core-modified porphyrinoids; Expanded porphyrinoids; Heptaphyrins; NIR absorbing molecules.
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