Department of Chemistry

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Author: search.filters.author.Grover, NitikaSubject: search.filters.subject.Pyrroles

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    Asymmetrically Crowded “Push–Pull” Octaphenylporphyrins with Modulated Frontier Orbitals: Syntheses, Photophysical, and Electrochemical Redox Properties
    (ACS, 2015-12) Grover, Nitika
    A new series of β-substituted octaphenylporphyrins were synthesized and the influence of unsymmetrical substitution on the photophysical and electrochemical properties of the compounds is elucidated. The examined compounds are represented as MOPP(R)X2 where OPP = the dianion of octaphenylporphyrin, R = NO2, CHO, or CH2OH, X = Br or CN, and M is Co(II), Cu(II), Ni(II), or Zn(II). Routes to the trifunctionalized β-octaphenylporphyrins begin with the synthesis of MOPP(R) (R = NO2, CHO, and CH2OH) and the conversion of MOPP(NO2) to MOPP(NO2)X2 (X = Br and CN). These “push–pull” octaphenylporphyrins exhibit high dipole moments, tunable redox properties, and red-shifted electronic spectral features due to asymmetric β-substitution. Photophysical data on the series of MOPP(R)X2 compounds also reflect the nonplanar conformation of these porphyrins. Quantum yield and lifetime data are invariably lower than what has been reported in the literature for related β-substituted porphyrins. The spectroscopic properties and electrochemical redox potentials of the porphyrins are influenced by both the peripheral substituents and nature of the core metal ion. A decrease in the HOMO–LUMO gap and increase in Δb1 is observed as the number of electron withdrawing groups on the molecule was increased. In addition, a tuning of the redox potentials could be achieved by introducing both electron donating (CH2OH) and withdrawing (CN, NO2, CHO, and Br) substituents into the MOPP skeleton which led to a “cross-polarized push–pull effect” of the β-substituents and a nonplanarity of the molecule. Metal-centered oxidations were exhibited for all of the Co(II) porphyrins and an M(II)/M(III) process was also observed to occur for NiOPP(R) (R = CH2OH, H, CHO, and NO2) and CuOPP(NO2)(CN)2. These electrode reactions for the latter two series of compounds occur after an initial conversion of the neutral porphyrin to its dicationic form under the electrochemical conditions. Evidence for the site of electron transfer is given in part by comparison with data in the literature for related compounds and in part by theoretical calculations and thin-layer spectroelectrochemical data carried out in the current study.
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    Facile Conversion of Ni(II) Cyclopropylchlorins into Novel β-Substituted Porphyrins through Acid-Catalyzed Ring-Opening Reaction
    (ACS, 2016-12) Grover, Nitika
    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.
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    Synthesis and Electrochemical Characterization of Acetylacetone (acac) and Ethyl Acetate (EA) Appended β-Trisubstituted Push–Pull Porphyrins: Formation of Electronically Communicating Porphyrin Dimers
    (ACS, 2018-10) Grover, Nitika
    Two new families of “push–pull” tetraphenylporphyrins with one acetylacetone (acac) or ethyl acetate (EA) moiety at a β-pyrrole position of the macrocycle and two Br or Ph substituents at the antipodal β-positions were synthesized and structurally, spectroscopically, and electrochemically characterized. The examined porphyrins are represented as MTPP(R)2acac and MTPP(R)2EA (where R = Br or Ph and M = H2, Co, Ni, Cu, or Zn). NiTPP(Br)2acac exhibits an extremely nonplanar conformation (Δ24 = 0.44 Å, ΔCβ = 0.82 Å), while H2TPP(Br)2EA and ZnTPP(Ph)2EA exhibit a quasi-planar conformation. All of the synthesized acac-appended porphyrins show a keto–enol tautomerism in solution, which results in formation of hydrogen bonded dimers as evidenced by 1H NMR and mass spectrometry. Dimers were also detected under the electrochemical conditions for the dibromo derivatives but not the diphenyl substituted porphyrins. A facile stepwise and reversible electrogeneration of the electronically communicating porphyrin dimers is observed for MTPP(Br)2acac where M = CuII, NiII, or ZnII.
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    β-Functionalized Dibenzoporphyrins with Mixed Substituents Pattern: Facile Synthesis, Structural, Spectral, and Electrochemical Redox Properties
    (ACS, 2019-02) Grover, Nitika
    A new series of mixed β-substituted dibenzoporphyrins were synthesized, and the effect of β-substitution on the spectral and electrochemical redox properties of the macrocycle was elucidated. The synthetic route to β-tetrasubstituted dibenzoporphyrins begins with the regioselective bromination of NiTPP(Benzo)2 to afford NiTPP(Benzo)2Br4, followed by Pd catalyzed coupling reaction to access NiTPP(Benzo)2(R)4 (R = phenyl (Ph) and phenylethynyl (PE)). Synthesized benzoporphyrins exhibited red-shifted absorption spectral features with tunable redox properties. These benzoporphyrins displayed pronounced electronic effects of β-substituents on the macrocyclic skeleton. NiTPP(Benzo)2(PE)4 exhibited the lowest HOMO–LUMO gap among the series due to extended π-conjugation. Intrestingly, metal-centered oxidation of Ni(II)/Ni(III) was observed for NiOPP(Benzo)2 and NiOPP(Benzo)Br2 after an initial conversion of the neutral porphyrin to its dicationic form under electrochemical conditions.
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    Structural, Photophysical, and Electrochemical Properties of Doubly Fused Porphyrins and Related Fused Chlorins
    (ACS, 2019-12) Grover, Nitika
    The electrochemical and physicochemical properties of tetraphenylporphyrins and tetraphenylchlorins with two fused indanedione (IND) or malononitrile (MN) groups and two antipodal Br, Ph, or H β-substituents are investigated in nonaqueous media. These compounds were synthesized by oxidative fusion of free-base trans-chlorins, followed by metalation. The corresponding free-base di-fused chlorins were also isolated as intermediates and characterized for comparisons. The examined di-fused porphyrins (DFP) and di-fused chlorins (DFC) are represented as MDFP(Y)2(R)2 and H2DFC(Y)2(R)2, where M = 2H, CuII, NiII, ZnII, and CoII, Y is a fused indanedione (IND) or malononitrile group (MN), and R = H, Br, or Ph. The IND- and MN-appended compounds in both series exhibit the expected two one-electron oxidations but quite different redox behavior is observed upon reduction, where the free-base IND-appended chlorins show four reversible one-electron reductions, compared to only two for the related free-base MN-appended chlorins. Although porphyrin trianions and tetraanions have been recently described for derivatives with highly electron-withdrawing and/or π-extending substituents, this seems not to be the case for the doubly fused IND-chlorins, where the first two one-electron additions are proposed to be located at the conjugated macrocycle and the last two at the fused IND groups, each of which is reduced at a different potential, consistent with the behavior expected for two equivalent and interacting redox centers. Unlike the examined chlorins, which are all stable in their electroreduced forms, the electrogenerated anionic forms of the di-fused porphyrins are all highly reactive and characterized by cyclic voltammograms having reduction peaks not only for the synthesized compounds added to solution but also for one or more new redox active species formed at the electrode surface in homogeneous chemical reactions following electron transfer. Comparisons are made between electrochemical behavior of the structurally related porphyrins and chlorins and the sites of electron transfer assigned on the basis of known electrochemical diagnostic criteria. One of the compounds, ZnDFP(MN)2, was also structurally characterized as having a ruffled and twisted macrocyclic conformation.