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Item Asymmetrically Crowded “Push–Pull” Octaphenylporphyrins with Modulated Frontier Orbitals: Syntheses, Photophysical, and Electrochemical Redox Properties(ACS, 2015-12) Grover, NitikaA 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.Item Selective Conversion of Planar trans-Chlorins into Highly Twisted Doubly Fused Porphyrins or Chlorins via Oxidative Fusion(ACS, 2018-05) Grover, Nitikaβ-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.Item Nickel-Induced Skeletal Rearrangement of Free Base trans-Chlorins into Monofused NiII-Porphyrins: Synthesis, Structural, Spectral, and Electrochemical Redox Properties(ACS, 2018-09) Grover, NitikaSeveral 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.Item 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, NitikaTwo 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.Item β-Functionalized Dibenzoporphyrins with Mixed Substituents Pattern: Facile Synthesis, Structural, Spectral, and Electrochemical Redox Properties(ACS, 2019-02) Grover, NitikaA 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.