meso-Pyrrolyl boron dipyrromethene (BODIPY) was prepared under simple reaction conditions by using commercially available chemicals. Prior to this work, the BODIPY compound was prepared in multiple steps by using precursors which were not readily available. The X-ray structure of BODIPY revealed that the meso-pyrrole ring is tilted towards the BF2-dipyrrin moiety with a dihedral angle of 33.94°. The reactivity of the meso-pyrrole ring of BODIPY was tested by subjecting it to bromination and formylation reactions, which afforded (α-bromopyrrolyl) BODIPY and (α-formylpyrrolyl) BODIPY in decent yields. The (α-formylpyrrolyl) BODIPY was used to prepare the pyrrole bridged BODIPY dyad. The pyrrole bridged BODIPY dyad exhibited a 15-20 nm bathochromic shift in the absorption band and was weakly fluorescent compared to meso-pyrrolyl BODIPY. Furthermore, our studies show that the meso-pyrrolyl BODIPY can be used as a specific sensor for F(-) ions because of the presence of meso-pyrrole NH which is involved in interactions with F(-) ions. To prove that meso-pyrrole NH is involved in sensing F(-) ions, we also prepared meso-(N-methylpyrrolyl)-BODIPY and characterized it by various spectroscopic techniques and crystallography. Our studies reveal that meso-(N-methylpyrrolyl)-BODIPY does not sense F(-) ions, supporting the involvement of meso-pyrrole NH in sensing F(-) ions.

This review article presents an overview of the progress in heterocorrole chemistry including their syntheses, key structural aspects and properties.

The oxasmaragdyrins containing one five membered heterocycle such as pyrrole, thiophene and furan in place of one of the meso-phenyl group were synthesized by acid-catalyzed oxidative coupling reaction of meso-heterocycle substituted dipyrromethane with 16-oxatripyrrane in the presence of catalytic amount of trifluoroacetic acid followed by oxidation with DDQ. The smaragdyrin macrocycles containing one fivemembered heterocycle at meso-position were characterized by HR-MS and detailed 1D and 2D NMR studies. The absorption and fluorescence studies revealed that the presence of five membered heterocycle at mesoposition of smaragdyrin resulted in bathochromic shifts in absorption and emission bands with slight reduction in quantum yields compared to smraragdyrin macrocycle containing six membered meso-phenyl groups. The electrochemical studies revealed that the meso-heterocycle smaragdyrins are electron deficient compared to meso-phenyl smaragdyrins.

The acid‐catalyzed [3+2] condensation of appropriate tripyrrane and dipyrromethane yields first examples of two novel ABC type 22‐oxacorroles containing three different types of five‐membered heterocycles such as pyrrole, thiophene, and furan at three meso‐positions in 8–12 % yields. The two 22‐oxacorroles were characterized by HR‐MS, 1D & 2D NMR spectroscopy, absorption, fluorescence, electrochemical and DFT studies. The absorption maxima and quantum yields of the oxacorroles were found to be dependent on the relative positions of meso‐heteroaryl substituents and absorption bands appeared in the region of 410–660 with low quantum yields (0.08–0.14). The electrochemical and DFT studies reveal that the redox properties of 22‐oxacorroles were significantly altered by replacing six‐membered meso‐aryl groups with three different heterocycles. The meso‐heteroaryl substituents such as furyl, pyrrolyl and thienyl groups were aligned with respect to the plane of macrocycles with varying degrees of dihedral angles.

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A series of 22-oxacorroles containing mixed substituents such as either two five-membered heterocycles and one six-membered aryl group or three five-membered heterocycles at three meso positions were synthesized by 3 + 2 oxidative coupling of appropriate 16-oxatripyrrane and meso-substituted dipyrromethane under mild acid catalyzed conditions. The identities of the 22-oxacorroles were confirmed by the respective molecular ion peaks in HR-MS spectra and the structures were deduced by detailed 1D and 2D NMR spectroscopy. NMR studies clearly showed upfield or downfield shifts of core pyrrole and furan protons and inner NH protons depending on the type of substituent present at the meso positions. All 22-oxacorroles absorb in the 410–655 nm region, and the position of the absorption bands depends on the type of meso substituents. The A2B oxacorroles showed one broad and weak fluorescence band in the 600–700 nm region with low fluorescence quantum yields in the 0.05–0.12 range. Electrochemical studies revealed that the A2B-type 22-oxacorroles are easier to oxidize and also easier to reduce than triphenyl 22-oxacorroles. Thus, the electronic properties of 22-oxacorroles were significantly altered when six-membered aryl groups were replaced with five-membered heterocycles as reflected in spectral and electrochemical measurements.

A simple meso-pyrrolyl BODIPY-Schiff base conjugate was synthesized by reacting ([Formula: see text]-formylpyrrolyl) BODIPY with 2-aminophenol in ethanol at reflux followed by recrystallization from CH2Cl2/petroleum ether, affording the conjugate in 72% yield. The conjugate was thoroughly characterized by HR-MS, 1D and 2D NMR and X-ray crystallographic techniques. The X-ray structure of the meso-pyrrolyl BODIPY-Schiff base conjugate revealed that the meso-pyrrole and the phenyl substituents were deviated by an angle of 35° and 49°, respectively, from the plane of the BF2-dipyrrin core. The absorption spectrum of the conjugate was similar to the ([Formula: see text]-formylpyrrolyl) BODIPY with a strong absorption band at 508 nm, whereas the fluorescence of the ([Formula: see text]-formylpyrrolyl) BODIPY was completely quenched in the BODIPY-Schiff base conjugate. Furthermore, cation sensing studies revealed that the conjugate has a specific sensing ability for the Cu(II) ion even in the presence of the other metal ions, as verified by the visual, absorption and mass spectral studies. The DFT optimized structure revealed that the Cu(II) ion was bound to pyrrolic nitrogen, imine nitrogen, phenolic oxygen and two water molecules in a distorted square pyramidal fashion. TD-DFT studies accounted well for the absorption spectra of the BODIPY-Schiff base conjugate and its Cu[Formula: see text] bound complex.

A series of stable fluorescent compounds namely the pyrrolyl bridged bis-(BODIPY) and bis-(22-oxacorrole) dimers were synthesized using pyrrolyl bridged bis-(dipyrromethane) as key precursor. The pyrrolyl bridged bis-(dipyrromethane) was synthesized in two steps by using commercially available compounds and used for the synthesis of pyrrolyl bridged bis-(BODIPY). The chloro groups present at the bridging pyrrole ring of bis-(BODIPY) dimer were replaced with aryl groups by the treatment of two different aryl boronic acids under Suzuki Coupling conditions. In order to show a further use of the pyrrole bridged bis-(dipyrromethane) precursor, it was condensed with 16-oxatripyrrane which rendered the pyrrolyl bridged bis-(22-oxacorrole) dimer as a green fluorescent compound in 8% yield. The dimers were thoroughly characterized by NMR and mass and the properties were investigated by absorption, fluorescence, electrochemical and DFT/TD-DFT studies. DFT studies showed that the bis-(BODIPY) and bis-(22-oxacorrole) dimers were highly symmetric and TD-DFT studies were in agreement with the experimental absorption and redox properties.