Dibenzothiophene-Containing Thiacarbaporphyrinoids.

A new type of expanded dibenzothiophene-containing thiacarbaporphyrinoid containing one dibenzothiophene, two p-phenylene, one thiophene, and two pyrrole rings connected via four meso carbons and two direct bonds was synthesized over a sequence of six steps starting with commercially available dibenzothiophene. The X-ray structure obtained for one of the macrocycles revealed that the macrocycle was very nonplanar and the two p-phenylene rings and dibenzothiophene unit were deviated from the mean plane, whereas the thiophene ring attained inverted conformation and was facing toward outside the macrocyclic core. The X-ray structure also revealed that the dibenzothiophene unit of the macrocycle maintained its π-electron delocalization and did not extend the π-electron delocalization with the rest of the macrocycle, and the effective π-conjugation was present only in the lower part of the macrocyclic core, i.e., the thiatripyrrin moiety. The spectral and theoretical studies support the nonaromatic nature of the macrocycles. The macrocycles exhibit one strong absorption band at ∼430 nm and one broad absorption band in the 500-800 nm region, which were bathochromically shifted in diprotonated derivatives which absorb strongly in the visible-NIR region. The electrochemical studies indicated the electron-rich nature of the macrocycles and the theoretical studies corroborated the experimental observations.

Synthesis and Studies of Di(p-Benzi)Decaphyrin and Di(9,10-Anthracenyl)Decaphyrin.

Two novel macrocycles, di(p-benzi)decaphyrin and di(9,10-anthracenyl)decaphyrin have been synthesized by condensing appropriate p-phenylene based pentapyrrane or 9,10-anthracenyl based pentapyrrane with pentafluorobenzaldehyde under acid catalyzed conditions. The pentapyrrane precursors were synthesized over sequence of steps starting with commercially available 1,4-dibromobenzene and 9,10-dibromoanthracene. The decaphyrin macrocycles were freely soluble in common organic solvents and characterized and studied by HR-MS, 1D and 2D NMR, absorption, cyclic voltammetry and computational techniques. The 1 H NMR spectra of both decaphyrins were almost similar with very few resonances, indicating their symmetrical nature in solution and the position of chemical shifts of various protons supports nonaromatic nature of decaphyrins. The DFT optimized structures revealed that both decaphyrins showed a doubly twisted figure of eight conformation and the p-phenylene rings in di(p-benzi)decaphyrin and anthracenyl rings in di(9,10-anthracenyl)decaphyrin did not participate in π-delocalization with rest of the respective decaphyrin macrocycle. Both decaphyrins showed sharp intense band in the region of 400-500 nm and a broad band in the region of 600-900 nm. The absorption bands of di(p-benzi)decaphyrin were significantly red shifted compared to di(9,10-anthracenyl)decaphyrin. The protonated derivatives of decaphyrins generated by addition of TFA to the toluene solution of decaphyrins showed distinct changes in colour and absorption spectral bands. The redox studies indicated that both decaphyrins are electron rich and undergo easier oxidations. The electrochemical and computational studies revealed that HOMO-LUMO energy gap was less in di(p-benzi)decaphyrin compared to di(9,10-anthracenyl)decaphyrin supporting the bathochromic shifts of absorption bands in di(p-benzi)decaphyrin. TD-DFT studies were in agreement with the experimental observations.

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.

Synthesis and Structural Properties of NIR-Absorbing Pyridine-Containing Heptaphyrins.

Four examples of stable nonaromatic pyridine containing heteroheptaphyrins (pyrithiaheptaphyrins) 2-5 were synthesized in 8-13% yields by [5+2] condensation of newly synthesized pyridine-based pentapyrrane 8 and bithiophene diol 9 a-d. The X-ray crystallographic analysis of macrocycle 2 proved that the macrocycle assumes a highly planar structure with two inverted thiophene rings. The heteroheptaphyrins 2-5 are asymmetric and showed a greater number of resonances in 1 H NMR spectra compared to our previously reported symmetric heterohexaphyrin (pyrithiahexaphyrin) 1 c. Most of the macrocyclic core protons in pyrithiahepaphyrins 2-5 experienced upfield/downfield shifts compared to pyrithiahexaphyrin 1 c indicating the alteration of π-conjugation in the macrocycles. The absorption bands were significantly red-shifted and located in the NIR region in macrocycles 2-5 compared to 1 c supporting the increase of π-delocalization. The theoretical studies support the experimental findings and NICS(0) value supports the non-aromaticity of the macrocycles.

Synthesis and Studies of Stable Nonaromatic Dithia Pyribenzihexaphyrins.

We report here one of the rare examples of expanded hexaphyrins named as dithia pyribenzihexaphyrin macrocycles containing six-membered rings such as pyridine and p-phenylene along with five-membered heterocycles such as pyrrole and thiophene as a part of a macrocyclic frame. Trifluoroacetic acid catalyzed [3 + 3] condensation of equimolar mixture of [10,10'-bis(p-tert-butyl phenyl)hydroxymethyl]-1,3-bis(2-thienyl)pyridine diol (2,6-pyri diol) and 1,4-bis(phenyl(1H-pyrrol-2-yl)methyl)benzene (p-benzidipyrrane) in CH2Cl2 followed by oxidation with DDQ afforded stable nonaromatic dithia 2,6-pyri-para-benzihexapyrins 1 and 2 in 6-8% yields. The macrocycles were characterized by high-resolution mass spectroscopy and 1D and 2D NMR spectroscopy. NMR studies revealed the nonaromatic nature of dithia 2,6-pyri-p-benzihexaphyrins and indicated that the para-phenylene ring prefers to be in quininoid form rather than in benzenoid form. The macrocycles displayed sharp absorption bands in the region of ∼380-500 nm and a broad band at ∼700 nm, reflecting their nonaromatic nature. Upon protonation, these macrocycles showed NIR absorption properties. The redox studies of macrocycles indicated their electron-deficient nature. The DFT/TD-DFT studies are in line with the experimental observations.