Nitika Grover

Facile Conversion of Ni(II) Cyclopropylchlorins into Novel β-Substituted Porphyrins through Acid-Catalyzed Ring-Opening Reaction.

The conversion of cyclopropylchlorins into porphyrins represents a key step in the synthetic manipulation of macrocycles with tunable physical and chemical properties. Herein, we report a facile method for the synthesis of novel β-substituted porphyrins from cyclopropylchlorins. A series of Ni(II) cyclopropylchlorins was converted into the corresponding Ni(II) and free base porphyrins using TFA and H2SO4 under mild reaction conditions in good yields (75-86%). The new chlorins and porphyrins were characterized by various spectroscopic techniques and the single-crystal X-ray diffraction method. The reaction proceeds very fast (<5 min.) with complete conversion of chlorin into porphyrin with distinct color change. Facile conversion, shorter reaction time scale, and good yield (75-86%) without any side products are the significant features of this new protocol. These porphyrinoids exhibited red-shifted electronic spectral features with varying degrees nonplanar conformation, tunable redox properties, and porphyrin core basicity.

Versatile Synthetic Route for β-Functionalized Chlorins and Porphyrins by Varying the Size of Michael Donors: Syntheses, Photophysical, and Electrochemical Redox Properties

One-pot facile synthesis and characterization of novel β-substituted meso-tetraphenylporphyrins and/or chlorins were described. The high regioselective reactivity of active methylene compounds in Michael addition reaction was reported to access β-substituted trans-chlorins. Size-dependent approach was applied for the fine-tuning of product formation from porphyrins to chlorins. Notably, we were able to isolate mono/trisubstituted porphyrin and/or di/tetra-substituted chlorin from one-pot synthesis for the first time in porphyrin chemistry. Single-crystal X-ray diffraction analysis revealed the quasiplanar to moderate nonplanar conformation of chlorins due to trans orientation of β-substituents, whereas porphyrins exhibited higher mean plane deviation from 24-atom core (Δ24) as compared to chlorins. β-Functionalized chlorins exhibited lower protonation constants and much higher deprotonation constants as compared to porphyrins revealing the combined effect of the conformation of macrocyclic core and the electronic nature of β-substituents. Facile synthesis of porphyrins and/or chlorins based on the size of Michael donor employed and in turn resulted in tunable photophysical and electrochemical redox properties are the significant features of the present work.

Spectral investigations of meso-tetraalkylporphyrin-fullerene host–guest complexes

Meso-tetraalkylporphyrins and their Zn(II) complexes were synthesized and characterized by various spectroscopic techniques. Single crystal X-ray structure of meso-tetrapropylporphyrin (3) revealed the orientation of alkyl chains and planar conformation of porphyrin macrocycle. Spectroscopic, photophysical and electrochemical redox properties of self-assembled donor–acceptor dyads formed by meso-tetraalkylporphyrins and fullerene C60 were investigated. These studies revealed 1:1 supramolecular dyad formation between the electron donor porphyrins and the electron acceptor, fullerene entities. The determined association constants (K) follow the order: H2TMeP (1) > H2TEtP (2) > H2TPrP (3) > H2THexP > H2TPP. The effect of alkyl chain length on porphyrin-fullerene complexation was investigated. The redox behavior of self-assembled dyads was investigated in PhCN containing 0.1 M TBAPF6 as supporting electrolyte. The oxidation potentials of dyads are positively shifted (20–100 mV) as compared to corresponding meso-tetraalkylporphyrins indicating the supramolecular interactions between the constituents in the ground state. The geometric and electronic structure of 1:C60 was probed by DFT calculations which suggested the possibility of charge transfer from meso-tetraalkylporphyrin to fullerene C60.

Selective Conversion of Planar trans-Chlorins into Highly Twisted Doubly Fused Porphyrins or Chlorins via Oxidative Fusion

β-to- o-phenyl doubly fused porphyrins (DFPs) or chlorins (DFCs) were selectively synthesized by facile oxidative fusion of trans-chlorins using 2,3-dichloro-5,6-dicyano-1,4- benzoquinone (DDQ) in good-to-excellent yields (70-92%) under mild reaction conditions with high atom economy. The selectivity in product formation (difused porphyrin or chlorin) was controlled by the presence or absence of a Ni(II) ion in the macrocyclic core. Notably, nickel(II) trans-chlorins selectively yielded DFPs, whereas free-base trans-chlorins afforded only DFCs. The synthesized fused porphyrinoids exhibited significantly red-shifted electronic spectral features (Δλmax = 16-53 nm) of the Soret band due to the extended π conjugation and highly twisted macrocyclic conformation (twist angle ∼20-34°). Inner-core NHs of fused chlorins exhibited a tremendous downfield shift (Δδ = 1.71-2.02 ppm) compared to their precursors. The overall protonation constants for indanedione-substituted free-base-difused chlorins (4-6) were profoundly higher (∼20-50-fold) compared to dicyanomethyl-appended free-base-difused chlorins (10-12) because of the combined effect of the electronic nature of the β-substituents and nonplanarity of the macrocyclic core. The first oxidation potential of H2DFC(MN)2Ph2 (12) was 0.54 V cathodically shifted with respect to H2DFC(MN)2 (10) because of the electron-donating nature of the β-phenyl groups, which resulted in extensive destabilization of the highest occupied molecular orbital.

Nickel-Induced Skeletal Rearrangement of Free Base trans-Chlorins into Monofused NiII-Porphyrins: Synthesis, Structural, Spectral, and Electrochemical Redox Properties

Several ruffled β-to- ortho-phenyl monofused metalloporphyrins (MIIMFPs) with 1,3-indanedione functionality have been synthesized by oxidative fusion of free base trans-chlorins via nickel insertion followed by the skeletal rearrangement of macrocycle. The synthesized monofused porphyrins exhibited red-shifted electronic spectral features as compared to precursor, trans-chlorins or structurally related unfused porphyrins (mono/tri-β-substituted porphyrins) due to extended π-conjugation and nonplanar conformation of the macrocyclic core. Four of the synthesized porphyrins were structurally characterized by X-ray diffraction analysis. Ring fusion resulted in twisted macrocyclic conformation, and the twist angles (the angle between 24-atom core and fused part) were found to be in the range of 20.97° to 27.97°. NiMFP(IND)Ph2 (3b) exhibited modestly ruffled conformation of the macrocyclic core which was further confirmed by higher Δ24 (0.362 Å) and ΔCβ (0.279 Å) values. Free base monofused porphyrins have shown upfield shifted (Δδ = 0.27-0.29 ppm) inner core NHs as compared to precursors ( trans-chlorins), whereas much downfield shifted (Δδ = 0.9 ppm) as compared to H2TPP. Electron-rich Ni(II) complexes, that is, NiMFP(IND)R2 (where R = H and Ph) (1b and 3b), exhibited metal centered oxidation (NiII/NiIII) due to extended π-conjugation of macrocyclic core and electron-donating β-substituents. Facile synthesis with good yields (60-90%), unexpected nickel-induced oxidative fusion, and selective conversion of trans-chlorins into monofused NiII-porphyrins are the significant features of the present work.