Yogita Pareek

Smaragdyrins: Emeralds of Expanded Porphyrin Family

Porphyrins are tetrapyrrolic 18 π electron conjugated macrocycles with wide applications that range from materials to medicine. Expanded porphyrins, synthetic analogues of porphyrins that contain more than 18 π electrons in the conjugated pathway, have an increased number of pyrroles or other heterocyles or multiple meso-carbon bridges. The expanded porphyrins have attracted tremendous attention because of unique features such as anion binding or transport that are not present in porphyrins. Expanded porphyrins exhibit wide applications that include their use in the coordination of large metal ions, as contrasting agents in magnetic resonance imaging (MRI), as sensitizers for photodynamic therapy (PDT) and as materials for nonlinear optical (NLO) studies. Pentaphyrin 1, sapphyrin 2, and smaragdyrin 3 are expanded porphyrins that include five pyrroles or heterocyclic rings. They differ from each other in the number of bridging carbons and direct bonds that connect the five heterocyclic rings. Sapphyrins were the first stable expanded porphyrins reported in the literature and remain one of the most extensively studied macrocycles. The strategies used to synthesize sapphyrins are well established, and these macrocycles are versatile anion binding agents. They possess rich porphyrin-like coordination chemistry and have been used in diverse applications. This Account reviews developments in smaragdyrin chemistry. Although smaragdyrins were discovered at the same time as sapphyrins, the chemistry of smaragdyrins remained underdeveloped because of synthetic difficulties and their comparative instability. Earlier efforts resulted in the isolation of stable β-substituted smaragdyrins and meso-aryl isosmaragdyrins. Recently, researchers have synthesized stable meso-aryl smaragdyrins by [3 + 2] oxidative coupling reactions. These results have stimulated renewed research interest in the exploration of these compounds for anion and cation binding, energy transfer, fluorescent sensors, and their NLO properties. Recently reported results on smaragdyrin macrocycles have set the stage for further synthetic studies to produce stable meso-aryl smaragdyrins with different inner cores to study their properties and potential for various applications.

Multiporphyrin Arrays on Cyclophosphazene Scaffolds: Synthesis and Studies

The stable and robust cyclotriphosphazene and cyclotetraphosphazene rings were used as scaffolds to prepare hexa- and octaporphyrin arrays by treating N(3)P(3)Cl(6) and N(4)P(4)Cl(8), respectively, with 5-(4-hydroxyphenyl)-10,15,20-tri(p-tolyl)porphyrin (N(4) core) or with its thiaporphyrin analogues (N(3)S and N(2)S(2) cores) in THF in the presence of Cs(2)CO(3) under simple reaction conditions. Thiaporphyrins were examined in addition to the normal porphyrin to tune the electronic properties of the resultant arrays. Observation of the molecular ion peaks in the mass spectra confirmed the molecular structures of the arrays. 1D and 2D NMR techniques were employed to characterize the multiporphyrin arrays in detail. The (1)H NMR spectra of the multiporphyrin arrays each show a systematic set of signals, indicating that the porphyrin units are arranged in a symmetrical fashion around the cyclophosphazene rings. All signals in the (1)H NMR spectra were assigned with the aid of COSY and NOESY experiments. The protons of each porphyrin unit are subject to upfield and downfield shifts because of the ring-current effects of neighboring porphyrin units. Optical, electrochemical, and fluorescence studies of the arrays indicated that the porphyrin units retain their independent ground- and excited-state characteristics. Cu(II) and Ni(II) derivatives of hexaporphyrin and octaporphyrin arrays containing N(4) porphyrin units and N(3)S porphyrin units were synthesized, and complete metalation of the arrays was confirmed by their mass spectra and by detailed NMR characterization of the Ni(II) derivatives of hexa- and octaporphyrin arrays containing N(4) porphyrin units. Electrochemical studies indicated that Cu(II) and Ni(II) ions present in the thiaporphyrin units of the arrays can be stabilized in the +1 oxidation state, which is not possible with arrays containing normal porphyrin units.

Thiaporphyrins: from building blocks to multiporphyrin arrays

21-Thiaporphyrins and 21,23-dithiaporphyrins resulting from the replacement of one and two pyrrole rings respectively, of tetrapyrrolic porphyrins possess singlet state energy levels which are lower than porphyrins and metalloporphyrins. This specific feature of thiaporphyrins is advantageous when thiaporphyrins are connected either covalently or non-covalently to porphyrins and metalloporphyrins for unidirectional flow of energy transfer/electron transfer. In recent times, the synthetic routes were developed for the synthesis of functionalized thiaporphyrin building blocks which were not accessible earlier. The functionalized thiaporphyrins have been used to synthesize several unsymmetrical thiaporphyrin-based arrays containing two or more different types of porphyrin subunits whose singlet state energy levels are arranged in a favourable way for the energy transfer/electron transfer. In this review, we describe different synthetic approaches being employed for the synthesis of functionalized thiaporphyrin building blocks and their use in the construction of several covalently and non-covalently linked thiaporphyrin-based multiporphyrin arrays ranging from dyads to octads. The photophysical studies are also described to show the possibility of singlet–singlet energy transfer from one porphyrin unit to another in some selected thiaporphyrin-based multiporphyrin arrays.

Cyclotriphosphazene appended porphyrins and fulleropyrrolidine complexes as supramolecular multiple photosynthetic reaction centers: steady and excited states photophysical investigation

New multiple photosynthetic reaction centers were constructed from cyclophosphazene decorated multiporphyrin chromophores and a fulleropyrrolidine having a pyridine ligand (FPY). The excited state electron transfer in the self-assembled donor-acceptor assembly was investigated by using steady state absorption and emission, time-resolved emission spectroscopy and nanosecond laser flash photolysis. The effect of metal (Zn(2+)) coordination to porphyrin units in the multiporphyrin arrays on cyclophosphazine scaffold (P3N3Zn) was studied by comparing with metal free porphyrin assembly on a cyclophosphazene scaffold (P3N3). In P3N3Zn, the decrease of absorption and fluorescence intensity and the lowering of the amplitude of longer fluorescence lifetime with increase of FPY concentration reflect the formation of a ground state complex with an association constant of ∼14,910 M(-1). When compared to the metal-free complex P3N3, the metal-coordinated derivative P3N3Zn exhibited shortening of the singlet and triplet state lifetimes and lowering of the singlet and triplet quantum yields. The cause of the decrease of the triplet quantum yields by insertion of zinc metal is discussed along with the possible non-planarity of the porphyrin ring. From the fluorescence lifetime measurements for the P3N3Zn-FPY mixture, it is proposed that self-assembly of the donor-acceptor complex leads to charge separated species with a rate constant of 7.1 × 10(9) s(-1). The decrease of triplet state intensity and lifetime of the P3N3Zn in the P3N3Zn-FPY complex from the nanosecond transient absorption studies support the occurrence of intermolecular electron transfer in the triplet state.

SnIV Porphyrin Scaffolds for Axially Bonded Multiporphyrin Arrays: Synthesis and Structure Elucidation by NMR Studies

A simple, one-step, supramolecular strategy was adopted to synthesize Sn(IV) -porphyrin-based axially bonded triads and higher oligomers by using meso-pyridyl Sn(IV) porphyrin, meso-hydroxyphenyl-21,23-dithiaporphyrin, and Ru(II) porphyrin as building blocks and employing complementary and non-interfering Sn(IV) O and Ru(II) ⋅⋅⋅N interactions. The multiporphyrin arrays are stable and robust and were purified by column chromatography. (1) H, (1) H-(1) H COSY and NOESY NMR spectroscopic studies were used to unequivocally deduce the molecular structures of Sn(IV) -porphyrin-based triads and higher oligomers. Absorption and electrochemical studies indicated weak interaction among the different porphyrin units in triads and higher oligomers, in support of the supramolecular nature of the arrays. Steady-state fluorescence studies on triads indicated the possibility of energy transfer in the singlet state from the basal Sn(IV) porphyrin to the axial 21,23-dithiaporphyrin. However, the higher oligomers were weakly fluorescent due to the presence of heavy Ru(II) porphyrin unit(s), which quench the fluorescence of the Sn(IV) porphyrin and 21,23-dithiaporphyrin units.

Synthesis and studies of covalently linked BF2-oxasmaragdyrin-BODIPY and BF2-oxasmaragdyrin-ferrocene dyads

Covalently linked BF2-oxasmaragdyrin-BODIPY and BF2-oxasmaragdyrin-ferrocene dyads were synthesized by coupling of meso-triaryl oxasmaragdyrin containing meso-iodophenyl group with meso-(p-ethynylphenyl) borondipyrromethene and α-ethynyl ferrocene respectively under mild Pd(0) coupling conditions. NMR, absorption and electrochemical studies indicated that the two moieties in the dyads retain their individual characteristic features. The fluorescence studies indicated a possibility of photoinduced singlet-singlet energy transfer from BODIPY unit to BF2-oxasmaragdyrin unit in BF2-oxasmaragdyrin-BODIPY dyad and photoinduced electron transfer from ferrocene unit to excited state of BF2-oxasmaragdyrin unit in BF2-oxasmaragdyrin-ferrocene dyad.