Tamal Chatterjee

Heteroatom-Containing Porphyrin Analogues.

The heteroatom-containing porphyrin analogues or core-modified porphyrins that resulted from the replacement of one or two pyrrole rings with other five-membered heterocycles such as furan, thiophene, selenophene, tellurophene, indene, phosphole, and silole are highly promising macrocycles and exhibit quite different physicochemical properties compared to regular azaporphyrins. The properties of heteroporphyrins depend on the nature and number of different heterocycle(s) present in place of pyrrole ring(s). The heteroporphyrins provide unique and unprecedented coordination environments for metals. Unlike regular porphyrins, the monoheteroporphyrins are known to stabilize metals in unusual oxidation states such as Cu and Ni in +1 oxidation states. The diheteroporphyrins, which are neutral macrocycles without ionizable protons, also showed interesting coordination chemistry. Thus, significant progress has been made in last few decades on core-modified porphyrins in terms of their synthesis, their use in building multiporphyrin arrays for light-harvesting applications, their use as ligands to form interesting metal complexes, and also their use for several other studies. The synthetic methods available in the literature allow one to prepare mono- and diheteroporphyrins and their functionalized derivatives, which were used extensively to prepare several covalent and noncovalent heteroporphyrin-based multiporphyrin arrays. The methods are also developed to synthesize different hetero analogues of porphyrin derivatives such as heterocorroles, heterochlorins, heterocarbaporphyrinoids, heteroatom-substituted confused porphyrins, and so on. This Review summarizes the key developments that have occurred in heteroporphyrin chemistry over the last four decades.

Smaragdyrins and Sapphyrins Analogues

Porphyrins and expanded porphyrins have attracted the attention of chemists for a long time in view of their diverse applications in catalysis; as anion, cation, and neutral substrate receptors; as ligands to coordinate large metal ions; as nonlinear optical materials, MRI contrasting agents, and sensitizers for photodynamic therapy (PDT); and more recently as models for aromaticity (both Hückel and Möbius). A diverse range of synthetic expanded porphyrins containing up to 96π electrons have been reported, and their properties have been exploited for various applications. The present Review is only confined to 22π electron expanded porphyrins containing five pyrrole/heterocyclic rings such as sapphyrins and smaragdyrins. Even though these two macrocycles contain 22π electrons and five pyrrole/heterocyclic rings, they are structurally different. In sapphyrins, the five pyrrole/heterocyclic rings are connected through four meso-carbon bridges and one direct pyrrole-pyrrole bond, whereas in smaragdyrins, the five pyrrole/heterocyclic rings are connected through three meso-carbon bridges and two direct pyrrole-pyrrole bonds. The chemistry of sapphyrins has been well-established in recent years due to the availability of easy and efficient synthetic methods. On the other hand, smaragdyrins are not explored significantly because of their unstable nature. However, recently it was shown that smaragdyrins can be stabilized if one of the pyrrole rings is replaced with a furan ring to afford stable oxasmaragdyrin. The availability of oxasmaragdyrin allowed the exploration of smaragdyrin in recent years. Thus, an attempt has been made in this Review to describe the chemistry of both sapphyrins and smaragdyrins in terms of their synthesis, characterization, metal ion coordination, and anion-recognition properties.

Directly Connected AzaBODIPY–BODIPY Dyad: Synthesis, Crystal Structure, and Ground- and Excited-State Interactions

Directly connected, strongly interacting sensitizer donor-acceptor dyads mimic light-induced photochemical events of photosynthesis. Here, we devised a dyad composed of BF2-chelated dipyrromethene (BODIPY) directly linked to BF2-chelated tetraarylazadipyrromethene (azaBODIPY) through the β-pyrrole position of azaBODIPY. Structural integrity of the dyad was arrived from two-dimensional NMR spectral studies, while single-crystal X-ray structure of the dyad provided the relative orientation of the two macrocycles to be ∼62°. Because of direct linking of the two entities, ultrafast energy transfer from the (1)BODIPY* to azaBODIPY was witnessed. A good agreement between the theoretically estimated Förster energy transfer rate and experimentally determined rate was observed, and this rate was found to be higher than that reported for BODIPY-azaBODIPY analogues connected with spacer units. In agreement with the free-energy calculations, the product of energy transfer, (1)azaBODIPY* revealed additional photochemical events such as electron transfer leading to the creation of BODIPY(•+)-azaBODIPY(•-) radical ion pair, more so in polar benzonitrile than in nonpolar toluene, as evidenced by femtosecond transient spectroscopic studies. Additionally, the spectral, electrochemical, and photochemical studies of the precursor compound azaBODIPY-dipyrromethane also revealed occurrence of excited-state events. In this case, electron transfer from the (1)azaBODIPY* to dipyrromethane (DPM) yielded DPM(•+)-azaBODIPY(•-) charge-separated state. The study described here stresses the role of close association of the donor and acceptor entities to promote ultrafast photochemical events, applicable of building fast-response optoelectronic and energy-harvesting devices.

Unusual formation of 21-oxacorrole from 21-oxaporphyrin induced by phosphoryl chloride.

An unusual formation of 21-oxacorrole from 21-oxaporphyrin by concomitant elimination of a meso-aryl group and ring contraction is reported.

Synthesis, X-ray structure, spectral and electrochemical properties of a β-meso covalently linked BODIPY–Ru(II) dipyrrin complex

We synthesized the BODIPY–Ru(II) dipyrrin complex in decent yield by reacting β-dipyrromethanyl substituted BODIPY with [{(η6-C10H14)RuCl(μ-Cl)}2] under simple reaction conditions. The BODIPY–Ru(II) dipyrrin complex was characterized by 1D, 2D, 13C, 11B, 19F NMR, HR-MS, UV-vis, fluorescence and electrochemical techniques. The BODIPY–Ru(II) dipyrrin complex was further characterized by X-ray structure analysis, which showed that the Ru(II)-dipyrrin complex is in perpendicular orientation to the BODIPY plane in the BODIPY–Ru(II) dipyrrin complex. The ruthenium metal ion in the BODIPY–Ru(II) dipyrrin complex is bonded to dipyrrin nitrogens, one chloro group and arene ring (arene = C10H14) in an η6 manner resulting in a hexa coordination geometry around the Ru(II) ion. The absorption and steady-state fluorescence spectra of the BODIPY–Ru(II) dipyrrin complex showed a broad and hypsochromically shifted absorption and emission bands compared to that of β-dipyrromethanyl substituted BODIPY. The electrochemical studies indicated that the BODIPY–Ru(II) dipyrrin complex was easy to reduce compared to β-dipyrromethanyl substituted BODIPY supporting the electron deficient nature of the BODIPY–Ru(II) dipyrrin complex.

Effects of five membered aromatic heterocycles at the meso-position on the electronic properties of 3-pyrrolyl BODIPY

The first examples of 3-pyrrolyl boron dipyrromethenes (3-pyrrolyl BODIPY) containing five membered aromatic heterocycles such as pyrrole, thiophene and furan in place of a six membered aryl group at the meso-position were synthesized under simple reaction conditions. The presence of a five membered aromatic heterocycle in place of a six membered aryl group at the meso-position significantly alters the electronic properties of 3-pyrrolyl BODIPY as reflected in their spectral, photophysical and electrochemical properties. The X-ray structure solved for meso-furyl 3-pyrrolyl BODIPY showed that the appended pyrrole is more deviated from the mean plane of the BODIPY core compared to meso-aryl 3-pyrrolyl BODIPY. However, the structure also revealed that the meso-furyl group is more in plane with the BODIPY core and participates in π-delocalization unlike meso-aryl 3-pyrrolyl BODIPY in which the meso-aryl group is almost in perpendicular orientation with the BODIPY core. The absorption studies indicated that the meso-heterocyclic 3-pyrrolyl BODIPYs showed bathochromic shifts with maximum shifts noted for meso-furyl 3-pyrrolyl BODIPY compared to meso-aryl BODIPY. The meso-heterocyclic 3-pyrrolyl BODIPYs were weakly fluorescent and stable under electrochemical reduction conditions.

Stable core-modified calixsmaragdyrins: synthesis, structure and specific sensing of the hydrogen sulfate ion

Stable calixoxa- and calixthiasmaragdyrins containing three methine bridges and two direct bonds connecting the five pyrrole/heterocycle rings were synthesized by [3 + 2] condensation of dipyrromethane with 16-oxatripyrrane and 16-thiatripyrrane respectively under mild acid-catalyzed conditions. The compounds were characterized by HR-MS, 1D & 2D NMR, absorption and electrochemical techniques and the structure of calixoxasmaragdyrin was solved by X-ray crystallography. The crystal structure analysis indicated that the calixoxasmaragdyrin macrocycle was highly distorted due to the flexibility introduced by one sp(3)meso-carbon. The compounds show ill-defined absorption bands and irreversible oxidation and reduction waves which were attributed to the disruption of conjugation of the macrocycle by incorporation of one sp(3)meso-carbon. The anion binding studies indicated that the calixoxasmaragdyrin exhibited specific sensing ability for the HSO4(-) ion over other anions whereas calixthiasmaragdyrins did not even show an ability to bind anions.

High singlet oxygen production and negative solvatochromism of octabrominated 3-pyrrolyl boron dipyrromethenes

The newly synthesized and structurally characterized octabrominated 3-pyrrolyl boron dipyrromethenes are shown to reveal negative solvatochromism and quantitative generation of singlet oxygen; properties relevant to develop a new class of reagents and photodynamic therapy agents.

Synthesis, structure, and catalytic activity of Pd(II) complex of calixoxasmaragdyrin.

The palladium(II) complex of calixoxasmaragdyrin was prepared in 80% yield by treating the free base calixoxasmaragdyrin with PdCl2 in CH3CN at reflux temperature. The crystal structure solved for Pd(II) calixoxasmaragdyrin indicates that the calixoxasmaragdyrin macrocycle is highly distorted and attained a boat shaped structure. The Pd(II) ion is coordinated to four pyrrolic nitrogens in square planar fashion, and it is placed at ~0.138 Å above from the four coordinating pyrrole nitrogens plane (N1N2N3N4). The Pd-N bond lengths are inequivalent, and the Pd(II) ion is positioned more toward the dipyrromethane moiety of calixoxasmaragdyrin. The complex shows one broad absorption band at 477 nm and is not very stable under redox conditions. The Pd(II) calixoxasmaragdyrin showed good catalytic activity in the Suzuki-Miyaura cross coupling reactions.

Synthesis and properties of Oxasmaragdyrins containing one Five-membered Heterocycle at Meso-position

AbstractThe 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 five-membered 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 meso-position 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. Graphical AbstractSynthesis and properties of mono meso-heterocyclic substituted Oxasmaragdyrins are reported.