Synthesis and Properties of Nonaromatic Meta-Benziheptaphyrins and Aromatic Para-Benziheptaphyrins.

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.

Synthesis of Pyridine-Containing Crowned Fused Expanded Porphyrins.

New examples of nonaromatic fused expanded porphyrins containing both pyridine and crown ether moiety as a part of macrocyclic framework were synthesized by condensing pyridine based pentapyrrane with polyether-based diol in CH2 Cl2 in the presence of one equivalent of BF3 ⋅ OEt2 under inert conditions followed by oxidation with DDQ in open air. The condensation was expected to form pyridine-containing crowned expanded porphyrins but resulted in the formation of fused crowned expanded porphyrins due to intramolecular fusion of two pyrrole nitrogens with two adjacent inverted thiophene carbons as revealed by X-ray crystallography obtained for one of the macrocycle. HRMS and NMR studies supported the formation of fused crowned pyridine containing expanded porphyrins, and the macrocycles showed simple, well-resolved NMR spectra where all resonances were identified easily by 2D NMR spectroscopy. The macrocycles exhibited typical nonaromatic absorption features and showed one broad band with peak maxima at 535 nm and one or two shoulder bands in the higher energy region. The protonation studies resulted in clear colour change from purple to blue and absorption bands experienced bathochromic shifts with a broad band at 662 nm which was extended up to 800 nm. The electrochemical studies revealed that the macrocycles were easier to oxidize but difficult to reduce. DFT studies indicated that the macrocycle attains a very puckered and distorted 'U' shaped structure owing to the flexibility of the crown ether chain and TD-DFT studies corroborated experimental results. The preliminary studies indicated that the macrocycles could be used as colorimetric optical sensor for the detection of Cu2+ ion.

Crowned Macrocycles Containing Two Pyrrolo[1,2-a] Indoles Created By Intramolecular Fusion.

Heterocyclic fused-ring systems are of utmost importance because of their presence in many natural products with biological activity. Pyrroloindoles are tricyclic heterocycles that are present in various bioactive and medicinally valuable compounds. Herein, we report the synthesis of phenylene-bridged bis-pyrrolo[1,2-a]indole crowned macrocycles 1-3 in which the pyrrolo[1,2-a]indole moieties were formed via intramolecular fusion. The macrocycles were thoroughly characterized by 1D and 2D NMR, HRMS and X-ray crystallographic studies. The X-ray structure revealed that the two pyrrolo[1,2-a]indole moieties were parallel to each other, and one pyrrolo[1,2-a]indole unit was deviated by an angle of 9.54° while the other pyrrolo[1,2-a]indole unit was deviated by an angle of 12.0° from the mean plane defined by 28 core atoms. The macrocycles 1-3 absorb in the visible region and readily undergo oxidations because of their electron rich nature. The macrocycles 1-3 upon treatment with trifluoroacetic acid (TFA) generates the corresponding cation radicals 1-3.+ which were stable in the open air for a week. The cation radicals 1-3.+ absorb strongly in the NIR region and the experimental observations on crowned macrocycles 1-3 were corroborated by DFT and TD-DFT studies.