Influence of Mixed Substituents on the Macrocyclic Ring Distortions of Free Base Porphyrins and Their Metal Complexes

Effect of solvent on the electronic absorption spectral properties of mixed β-octasubstituted free base tetraphenylporphyrins

A near planar macrocycle containing 2,3,12,13-tetraphenylethynyl-5,10,15,20-tetrakis-(4[Formula: see text]-[Formula: see text]-butylphenyl)porphyrin, H2T(4[Formula: see text]-[Formula: see text]-Bu Ph)P(PE)4 and a series of antipodally mixed substituted nonplanar porphyrins, 2,3,5,10,12,13,15,20-octaphenyl-7,8,17,18-tetra(2[Formula: see text]-thienyl/phenylethynyl, PE)porphyrin, H2OPP(2[Formula: see text]-Th/PE)4 and 2,3,12,13-tetramethyl-7,8,17,18-tetra(2[Formula: see text]-thienyl/PE)-5,10,15,20-tetraphenylporphyrin, H2TPP(CH[Formula: see text](2[Formula: see text]-Th/PE)4were examined by electronic absorption spectroscopy in twenty different solvents. The presence of push-pull substituents at the antipodal [Formula: see text]-pyrrole positions of the nonplanar macrocycle induces varying degrees of orthogonal dipole moments to the porphyrin ring. The influence of different solvents on the degree of nonplanarity and electronic nature of the macrocycle on their electronic absorption spectral properties were examined. Generally, free base porphyrins showed dramatic solvent dependent absorption spectral band shifts and follow the order: H2OPP(PE)4 > H2OPP(2[Formula: see text]-Th)[Formula: see text] H2TPP(CH[Formula: see text](2[Formula: see text]-Th)4 >[Formula: see text]H2TPP(CH[Formula: see text](PE)4 > H2T(4[Formula: see text]-[Formula: see text]Bu Ph)P(PE)4. Absorption spectral data in different solvents was analyzed using selected solvatochromic parameters, ([Formula: see text] – 1)/(2[Formula: see text] + 1), [Formula: see text] (30),[Formula: see text], and [Formula: see text]*. The enhanced red-shift of the absorption bands of the mixed substituted porphyrins in polar solvents was influenced by solvent-core (porphyrin) interaction and is reflected from the 1H NMR chemical shift of the core imino-hydrogens in polar solvents relative that observed in less polar solvents. The magnitude of the difference in chemical shift ([Formula: see text], ppm) of imino-hydrogens in DMSO-d6 relative to that in CDCl3 follow the order: H2TPP(CH[Formula: see text](2[Formula: see text]-Th)4 (1.37 ppm) > H2OPP(PE)4 (1.17 ppm) > H2TPP(CH[Formula: see text](PE)4 (0.57 ppm). The large red-shift in B and Q bands in polar solvents relative to apolar (or less polar) solvents has been possible due to the combined effect of the electronic nature of the macrocycle, its nonplanarity, and solvent-porphyrin core interactions.

Facile Supramolecular Engineering of Porphyrin cis Tautomers: The Case of β-Octabromo-meso-tetraarylporphyrins

A porphyrin cis tautomer, where the two central NH protons are on adjacent pyrrole rings, has long been invoked as an intermediate in porphyrin tautomerism. Only recently, however, has such a species been isolated and structurally characterized. Thus, single-crystal X-ray structure determinations of two highly saddled free-base porphyrins, β-heptakis(trifluoromethyl)-meso-tetrakis(p-fluorophenyl)porphyrin, H₂[(CF₃)₇TFPP], and β-octaiodo-5,10,15,20-tetrakis(4'-trifluoromethylphenyl)porphyrin, H₂[I₈TCF₃PP], unambiguously revealed cis tautomeric structures, each stabilized as a termolecular complex with a pair of ROH (R = CH₃ or H) molecules that form hydrogen-bonded N-H···O-H···N straps connecting the central NH groups with the antipodal unprotonated nitrogens. The unusual substitution patterns of these two porphyrins, however, have left open the question how readily such supramolecular assemblies might be engineered, which prompted us to examine the much more synthetically accessible β-octabromo-meso-tetraphenylporphyrins. Herein, single-crystal X-ray structures were obtained for two such compounds, 2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetrakis(4'-trifluoromethylphenyl)porphyrin, H₂[Br₈TCF₃PP], and 2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetrakis(4'-fluorophenyl)porphyrin, H₂[Br₈TFPP], and although the central hydrogens could not all be located unambiguously, the electron density could be convincingly modeled as porphyrin cis tautomers, existing in each case as a bis-methanol adduct. In addition, a perusal of the Cambridge Structural Database suggests that there may well be additional examples of porphyrin cis tautomers that have not been recognized as such. We are therefore increasingly confident that porphyrin cis tautomers are readily accessible via supramolecular engineering, involving the simple stratagem of crystallizing a strongly saddled porphyrin from a solvent system containing an amphiprotic species such as water or an alcohol.

Unsymmetrical nonplanar 'push-pull' β-octasubstituted porphyrins: facile synthesis, structural, photophysical and electrochemical redox properties

Mixed substitution at the β-position of porphyrins influences their photophysical and electrochemical redox properties. Two new series of asymmetrically mixed β-octasubstituted porphyrins viz. MTPP(Ph)2Br5X (X = NO2 or Br and M = 2H, Co(ii), Ni(ii), Cu(ii), and Zn(ii)) have been synthesized and characterized by various spectroscopic techniques. The single crystal X-ray structure of H2TPP(NO2)(Ph)2Br5 showed a nonplanar saddle shape conformation of the macrocyclic core. Furthermore, the fully optimized geometries confirmed the saddle shape conformation of H2TPP(Ph)2Br5X (X = NO2 or Br). Electronic spectra revealed a significant bathochromic shift by appending both electron donor and acceptor substituents at the β-position of the meso-tetraphenylporphyrin skeleton, which reflects the following order H2TPP < H2TPP(NO2) < H2TPP(NO2)(Ph)2 < H2TPP(Ph)2Br6 < H2TPP(NO2)(Ph)2Br5. H2TPP(Ph)2Br5X (X = NO2 or Br) exhibited a significant bathochromic shift (Δλmax = 53-61 nm) in the Soret band and (Δλmax = 90-95 nm) in the longest wavelength Qx(0,0) band as compared to H2TPP. Nonplanar conformations and electron withdrawing β-substituents induce higher protonation and deprotonation constants for H2TPP(NO2)(Ph)2Br5 and H2TPP(Ph)2Br6 as compared to precursor porphyrins viz. H2TPP, H2TPP(NO2) and H2TPP(NO2)(Ph)2. The electronic spectral properties and redox potentials of MTPP(Ph)2Br5X (X = NO2 or Br and M = 2H, Co, Ni, Cu and Zn) are affected by β-substituents at the periphery of the porphyrin core. Redox tunability was achieved by appending push-pull substituents at the β-position of the MTPP (M = 2H, CoII, NiII, CuII, and ZnII) skeleton of the macrocycle. CuTPP(Ph)2Br6 and CuTPP(NO2)(Ph)2Br5 exhibited a dramatically reduced HOMO-LUMO gap with a difference of 0.55 V and 0.62 V, respectively as compared to CuTPP due to the push-pull effect of β-substituents and nonplanarity of the porphyrin core.

Effect of solvent on the electronic absorption spectral properties of some mixed β-octasubstituted Zn(II)-tetraphenylporphyrins

A series of mixed β-octasubstituted Zn(II)-porphyrins, 2,3,12,13-tetra(chloro/cyano/methyl)-5,7,8,10,15,17,18,20-octaphenylporphinato zinc(II), ZnTPP(Ph)₄X₄ (X=CN, Cl and CH₃) have been examined by electronic absorption spectroscopy in various solvents. These Zn(II)-porphyrins exhibited varying degree of red-shift of absorption bands as high as 20-30nm in 'B' band and 50-60nm in longest wavelength band, 'Q(0,0)' band in polar solvents relative to that found in nonpolar solvents. The red-shift of B and Q(0,0) bands showed an unusual trend, ZnTPP(Ph)₄(CN)₄>ZnTPP(Ph)₄(CH₃)₄>ZnTPP(Ph)₄Cl₄ but fails to follow an anticipated anodic shift in first porphyrin ring oxidation (vs Ag/AgCl) potential: ZnTPP(Ph)₄(CN)₄ (1.02V)>ZnTPP(Ph)₄Cl₄ (0.74V)>ZnTPP(Ph)₄(CH₃)₄ (0.38V). Such a trend suggests the combined effect of non-planarity of the macrocycle and electronic effect of the peripheral substituents. The equilibrium constants for the binding of nitrogenous bases with the Zn(II)-porphyrins showed as high as twenty fold increase for ZnTPP(Ph)₄X₄ (X=Br and CN) relative to ZnTPP(Ph)₄(CH₃)₄ and follow the order: ZnTPP(Ph)₄(CN)₄>ZnTPP(Ph)₄Br₄>ZnTPP(Ph)₄(CH₃)₄≤ZnTPP which is approximately in line with an increase in anodic shift of their first ring redox potentials (ZnTPP(Ph)₄(CN)₄ (1.02V)>ZnTPP(Ph)₄Br₄ (0.72V)>ZnTPP (0.84V)>ZnTPP(Ph)₄(CH₃)₄) (0.38V).

NH Tautomerism of a Quadruply Fused Porphyrin: Rigid Fused Structure Delays the Proton Transfer

We report herein NH tautomerism of a freebase derivative of a quadruply fused porphyrin (H₂QFP-Mes, 3), which has one mesityl group at one of the β-positions of the nonfused pyrroles to lower the structural symmetry, allowing us to observe the NH tautomerism with ¹H NMR spectroscopy. Compound 3 was revealed to have the two inner NH protons on the two nonfused pyrroles, and the NH tautomerism of 3 was evidenced by variable-temperature (VT) ¹H NMR experiments in various deuterated solvents. The VT-NMR studies revealed that the activation barrier for the NH tautomerism of 3 was larger than that of tetraphenylporphyrin. The positive activation entropy (ΔS^‡ = 89 J mol^-1 K^-1), determined for the NH tautomerism, can be explained by dissociation of the π-π stacked dimer structure of 3 in the ground state, as evidenced by the crystal structure and NMR measurements. On the basis of the kinetic studies and density functional theory calculations, the stability of intermediates in the NH tautomerism of 3 and the transition states have been discussed in detail.

An insight into the communication between β-olefin/phenyl olefin-mediated acceptors and porphyrin π-system: a way to establish porphyrin based chemodosimeters and chemosensors

Porphyrins bearing electron acceptors attached to β-carbon through olefin or phenyl–olefin bridges act as chemodosimeters for CN⁻ as well as chemosensors for F⁻, CN⁻ and OAc⁻ ions depends on acceptor strength and position.

Mono- and tri-β-substituted unsymmetrical metalloporphyrins: synthesis, structural, spectral and electrochemical properties

Mono-/tri-β-substituted metalloporphyrins have been synthesized and characterized. Dramatic reduction in the HOMO–LUMO gap with tunable electronic, spectral and electrochemical redox potentials were observed as the number of electron withdrawing groups increased.

Solvothermal synthesis, crystal structure and properties of a novel zinc porphyrin

A novel zinc porphyrin, Zn[(tcpp)(thf)2] (1) (tcpp = meso-tetra(4-carboxyphenyl)porphyrin; thf = tetrahydrofuran), has been prepared via a solvothermal reaction and structurally characterized by single crystal X-ray diffraction. The zinc ion coordinates to four nitrogen and two oxygen atoms. The substance crystallizes orthorhombically in the space group Cmca with a = 30.8367(6), b = 8.7099(5), c = 15.3526(2) Å, V = 4123.5(3) Å3, C56H44N4O10Zn, Z = 4, Mr = 998.34, Dc = 1.608 g/cm3, S = 0.991, μ(MoKα ) = 0.673 mm–1, F(000) = 2072, R = 0.0555 and wR = 0.1338. Photoluminescent investigation shows that complex 1 exhibits an emission in the blue region. The spectral data of FT–IR were also reported.

β-Tetrabromo-meso-tetrakis(4′-substituted phenyl)porphyrins: synthesis and electrochemical redox properties

A series of 2,3,12,13-tetrabromo-5,10,15,20-tetrakis(4′-substituted phenyl)porphyrins, H2 T(4′-R Ph) PBr4 ( R = OCH3 , t-butyl, H and CO2CH3 ) and their metal ( Co(II) , Cu(II) , and Zn(II) ) complexes were synthesized and their electrochemical redox properties were explored. The plot of E1/2vs. the Hammett parameter (σp) of the substituents (R) follow a fairly linear relationship for the ring centered redox potentials.