beta.-Halogenated-pyrrole porphyrins. Molecular structures of 2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetramesitylporphyrin, nickel(II) 2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetramesitylporphyrin, and nickel(II) 2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetrakis(pentafluorophenyl)porphyrin

Synthesis and Self-Assembly of β-Octa[(4-Diethoxyphosphoryl)phenyl]porphyrins

The β-substituted porphyrinoids commonly used to form functional assembled systems in nature yet are still scarcely used in material chemistry probably due to the laborious synthesis of these compounds. In this work, β-octa[(4-diethoxyphosphoryl)phenyl]porphyrin (2HOPPP) and its metal (Zn(II), Cd(II), Cu(II), and Ni(II)) complexes were prepared in good yields. These highly soluble chromophores were characterized in solution using spectroscopic (NMR, UV-vis, fluorescence), electrochemical, and spectroelectrochemical methods. Attachment of the electron-deficient residue (ArP(O)(OEt)₂) to the porphyrin macrocycle leads to easier reductions and harder oxidations of the macrocycle for all complexes studied as compared to corresponding meso-tetra[4-(diethoxyphosphoryl)phenyl]porphyrin derivatives reported previously. We demonstrated that the strong electron-deficient character of the MOPPP porphyrins results principally from the increase in the number of electron-withdrawing groups at the periphery of the tetrapyrrolic macrocycle. Electron-deficient porphyrins are highly required in supramolecular and material chemistry in part due to their ability to form supramolecular assemblies via the coordination of axial ligands to the central metal atom. According to single-crystal X-ray data, ZnOPPP forms in the crystalline phase dimers in which each of the two tetrapyrrolic macrocycles is connected through an unusual combination of hydrogen bonding of two phosphoryl groups and the water molecules axially coordinated to the zinc atom of the partner molecule. The involvement of water molecules in porphyrin binding allows for an increase of distance between two porphyrin mean N₄ planes, up to 4.478 Å. The offset of phosphoryl groups attached to the macrocycle through a 1,4-phenylene spacer withdraws the whole porphyrin macrocycle of one molecule from spatial overlap with the macrocycle of a partner molecule and increases the Zn-Zn distance up to 10.372 Å. This still unknown type of porphyrin dimers allows one to get deeper insights into the organization of naturally occurring tetrapyrrolic macrocycles. ZnOPPP also forms a labile dimeric complex in 5.3 × 10^-7-5.8 × 10^-5 M chloroform solutions. In contrast, other complexes prepared in this work exist as monomeric species under these experimental conditions. The self-association constant of ZnOPPP has been determined by electronic absorption spectroscopy.

Indane-1,3-Dione: From Synthetic Strategies to Applications

Indane-1,3-dione is a versatile building block used in numerous applications ranging from biosensing, bioactivity, bioimaging to electronics or photopolymerization. In this review, an overview of the different chemical reactions enabling access to this scaffold but also to the most common derivatives of indane-1,3-dione are presented. Parallel to this, the different applications in which indane-1,3-dione-based structures have been used are also presented, evidencing the versatility of this structure.

Nonplanar porphyrins: synthesis, properties, and unique functionalities

Porphyrins are variously substituted tetrapyrrolic macrocycles, with wide-ranging biological and chemical applications derived from metal chelation in the core and the 18π aromatic surface. Under suitable conditions, the porphyrin framework can deform significantly from regular planar shape, owing to steric overload on the porphyrin periphery or steric repulsion in the core, among other structure modulation strategies. Adopting this nonplanar porphyrin architecture allows guest molecules to interact directly with an exposed core, with guest-responsive and photoactive electronic states of the porphyrin allowing energy, information, atom and electron transfer within and between these species. This functionality can be incorporated and tuned by decoration of functional groups and electronic modifications, with individual deformation profiles adapted to specific key sensing and catalysis applications. Nonplanar porphyrins are assisting breakthroughs in molecular recognition, organo- and photoredox catalysis; simultaneously bio-inspired and distinctly synthetic, these molecules offer a new dimension in shape-responsive host-guest chemistry. In this review, we have summarized the synthetic methods and design aspects of nonplanar porphyrin formation, key properties, structure and functionality of the nonplanar aromatic framework, and the scope and utility of this emerging class towards outstanding scientific, industrial and environmental issues.

Thiourea Organocatalysts as Emerging Chiral Pollutants: En Route to Porphyrin-Based (Chir)Optical Sensing

Environmental pollution with chiral organic compounds is an emerging problem requiring innovative sensing methods. Amino-functionalized thioureas, such as 2-(dimethylamino)cyclohexyl-(3,5-bis(trifluoromethyl)phenyl)thiourea (Takemoto’s catalyst), are widely used organocatalysts with virtually unknown environmental safety data. Ecotoxicity studies based on the Vibrio fischeri luminescence inhibition test reveal significant toxicity of Takemoto’s catalyst (EC50 = 7.9 mg/L) and its NH2-substituted analog (EC50 = 7.2–7.4 mg/L). The observed toxic effect was pronounced by the influence of the trifluoromethyl moiety. En route to the porphyrin-based chemosensing of Takemoto-type thioureas, their supramolecular binding to a series of zinc porphyrins was studied with UV-Vis and circular dichroism (CD) spectroscopy, computational analysis and single crystal X-ray diffraction. The association constant values generally increased with the increasing electron-withdrawing properties of the porphyrins and electron-donating ability of the thioureas, a result of the predominant Zn⋯N cation–dipole (Lewis acid–base) interaction. The binding event induced a CD signal in the Soret band region of the porphyrin hosts—a crucial property for chirality sensing of Takemoto-type thioureas.

Mn(iii)–porphyrin catalysts for the cycloaddition of CO2 with epoxides at atmospheric pressure: effects of Lewis acidity and ligand structure

Mn(iii)–porphyrin catalysts with electron-withdrawing substituents were designed to uncover electronic and structural aspects in the cycloaddition of CO₂ with epoxides.

Synthesis, Redox, and Spectroscopic Properties of Pd(II) 10,10-Dimethylisocorrole Complexes Prepared via Bromination of Dimethylbiladiene Oligotetrapyrroles

Two brominated 10,10-dimethylisocorrole (10-DMIC) derivatives containing Pd(II) centers have been prepared and characterized. These compounds were prepared via bromination of 10,10-dimethylbiladiene-based oligotetrapyrroles. Bromination of free base 10,10-dimethylbiladiene (DMBil1) followed by metalation with Pd(OAc)2, as well as bromination of the corresponding Pd(II) dimethylbiladiene complex (Pd[DMBil1]) provide routes to Pd(II) hexabromo-10,10-dimethyl-5,15-bis(pentafluorophenyl)-isocorrole (Pd[10-DMIC-Br6]) and Pd(II) octabromo-10,10-dimethyl-5,15-bis(pentafluorophenyl)-isocorrole (Pd[10-DMIC-Br8]). The solid-state structures of the two brominated isocorrole complexes are presented, as is that for a new decabrominated dimethylbiladiene derivative (DMBil-Br10). The electronic and spectroscopic properties of the brominated biladiene and isocorrole derivatives were probed using a combination of voltammetric methods and steady-state UV-vis absorption and emission experiments. Data obtained from these experiments allow the properties of the brominated biladiene and isocorrole derivatives to be compared to previously studied biladiene derivatives (i.e., DMBil1 and Pd[DMBil1]). CV and DPV experiments demonstrate that Pd[10-DMIC-Br6] and Pd[10-DMIC-Br8] support well-behaved multielectron redox chemistry, similar to that which has been observed for other nonaromatic tetrapyrroles containing sp3-hybridized meso-carbons. Spectroscopic experiments reveal that bromination of the dimethylbiladiene core shifts this system's UV-vis absorption profile to lower energy and that the dimethylisocorrole complexes support panchromatic absorption profiles that extend across the UV-vis and into the near-IR region. Photosensitization experiments demonstrate that unlike previously studied Pd(II) biladiene constructs, DMBil-Br10, Pd[10-DMIC-Br6], and Pd[10-DMIC-Br8] support limited triplet excited state chemistry with O2, indicating that the novel nonaromatic tetrapyrrole derivatives described in this work may be best suited for applications other than singlet oxygen sensitization.

Synthesis and molecular structure of perhalogenated rhenium-oxo corroles

As part of our efforts to develop rhenium-oxo corroles as photosensitizers for oxygen sensing and photodynamic therapy, we investigated the potential β-perhalogenation of five ReO meso-tris(para-X-phenyl)corroles, Re[TpXPC](O) (X = CF₃, H, F, CH₃, and OCH₃), with elemental chlorine and bromine. With Cl₂, β-octachlorinated products Re[Cl₈TpXPC](O) were rapidly obtained for X = CF₃, H, and CH₃, but X = OCH₃ resulted in overchlorination on the meso-aryl groups. Full β-octabromination proved slower relative to Cu and Ir corroles, but the desired Re[Br₈TpXPC](O) products were finally obtained for X = H and F after a week at room temperature. For X = CH₃ and OCH₃, these conditions led to undecabrominated products Re[Br₁₁TpXPC](O). Compared to the β-unsubstituted starting materials, the β-octahalogenated products were found to exhibit sharp ¹H NMR signals at room temperature, indicating that the aryl groups are locked in place by the β-halogens, and substantially redshifted Soret and Q bands. Single-crystal X-ray structures of Re[Cl₈TpCF₃PC](O), Re[Cl₈TpCH₃PC](O), and Re[Br₈TpFPC](O) revealed mild saddling for one Cl₈ structure and the Br₈ structure. These structural variations, however, appear too insignificant to explain the slowness of the β-octabromination protocols, which seems best attributed to the deactivating influence of the high-valent Re center.

Crystal structures of 2,3,7,8,12,13,17,18-octa-bromo-5,10,15,20-tetra-kis-(penta-fluoro-phen-yl)porphyrin as the chloro-form monosolvate and tetra-hydro-furan monosolvate

The crystal structures of the title compounds, two solvates (CHCl3 and THF) of a symmetric and highly substituted porphyrin, C44H2Br8F20N4 or OBrTPFPP, are described. These structures each feature a non-planar porphyrin ring, exhibiting a similar conformation of the strained ring independent of solvent identity. These distorted porphyrins are able to form hydrogen bonds and sub-van der Waals halogen inter-actions with enclathrated solvent; supra-molecular inter-actions of proximal macrocycles are additionally affected by solvent choice. The crystal studied for compound 1·CHCl3 was refined as an inversion twin. One penta-fluoro-phenyl group was modelled as disordered over two sites [occupancy ratio = 0.462 (7):0.538 (7)]. The chloro-form solvate was also modelled as disordered over two orientations [occupancy ratio = 0.882 (7): 0.118 (7).

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.

Conformational control of nonplanar free base porphyrins: towards bifunctional catalysts of tunable basicity

No metal needed: nonplanar free base porphyrins act as bifunctional organocatalysts revealing a new mode of action for porphyrins.

Synthesis and structural, photophysical, electrochemical redox and axial ligation properties of highly electron deficient perchlorometalloporphyrins and selective CN− sensing by Co(ii) complexes

Synthesis and characterization of MTPP(NO₂)Cl₇ have been reported. Zn^II porphyrins exhibited higher Lewis acidity as compared to perbromoporphyrins. Co^II porphyrins were utilized for the selective rapid visual detection of cyanide ions.

Energetics of Saddling versus Ruffling in Metalloporphyrins: Unusual Ruffled Dodecasubstituted Porphyrins

Presented herein is a first major density functional theory (BP86/D3/STO-TZ2P) survey of the energetics of saddling versus ruffling for a wide range of dodecasubstituted metalloporphyrins with M = Ni, Cu, Zn, Pd, and Pt. For the majority of X₈TPP (i.e., β-octasubstituted-meso-tetraphenylporphyrin), the calculations indicated a clear preference for the saddled conformation, consistent with a large body of experimental data. The preference for the saddled conformation relative to the ruffled conformation was found to vary from about ∼0.3-0.4 eV for Me₈TPP derivatives up to 1 eV for I₈TPP and (CF₃)₈TPP derivatives. For X = Ph, that is, dodecaphenylporphyrins, the saddled and the ruffled conformation are almost equienergetic, with even a slight preference for the ruffled conformation in some cases. This finding provides a satisfactory explanation for the X-ray crystallographic observation of both saddled and ruffled conformations for dodecaphenylporphyrin complexes as well as for spectroscopic evidence for conformational mobility of these complexes in solution. The calculations also indicate near-equienergetic saddled and ruffled conformations for meso-tetraacetylenyltetrabenzoporphyrins, again consonant with key crystallographic findings. By and large, both the energetics and nonplanar distortions of the metalloporphyrin derivatives correlated well with the Charton and Sterimol B₁ steric parameters of the peripheral substituents.

Conformational control of cofactors in nature - the influence of protein-induced macrocycle distortion on the biological function of tetrapyrroles

Tetrapyrrole-containing proteins are one of the most fundamental classes of enzymes in nature and it remains an open question to give a chemical rationale for the multitude of biological reactions that can be catalyzed by these pigment-protein complexes. There are many fundamental processes where the same (i.e., chemically identical) porphyrin cofactor is involved in chemically quite distinct reactions. For example, heme is the active cofactor for oxygen transport and storage (hemoglobin, myoglobin) and for the incorporation of molecular oxygen in organic substrates (cytochrome P450). It is involved in the terminal oxidation (cytochrome c oxidase) and the metabolism of H2O2 (catalases and peroxidases) and catalyzes various electron transfer reactions in cytochromes. Likewise, in photosynthesis the same chlorophyll cofactor may function as a reaction center pigment (charge separation) or as an accessory pigment (exciton transfer) in light harvesting complexes (e.g., chlorophyll a). Whilst differences in the apoprotein sequences alone cannot explain the often drastic differences in physicochemical properties encountered for the same cofactor in diverse protein complexes, a critical factor for all biological functions must be the close structural interplay between bound cofactors and the respective apoprotein in addition to factors such as hydrogen bonding or electronic effects. Here, we explore how nature can use the same chemical molecule as a cofactor for chemically distinct reactions using the concept of conformational flexibility of tetrapyrroles. The multifaceted roles of tetrapyrroles are discussed in the context of the current knowledge on distorted porphyrins. Contemporary analytical methods now allow a more quantitative look at cofactors in protein complexes and the development of the field is illustrated by case studies on hemeproteins and photosynthetic complexes. Specific tetrapyrrole conformations are now used to prepare bioengineered designer proteins with specific catalytic or photochemical properties.

Saddle-shaped porphyrins for dye-sensitized solar cells: new insight into the relationship between nonplanarity and photovoltaic properties

We report on the theoretical and experimental studies of the new dye-sensitized solar cells functionalized with 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin zinc(ii) complexes bearing 2- and 8-bromo substituents at the β positions.

Pitfalls in bromination reactions of zinc porphyrins: two-sided ring opening of the porphyrin macrocycle

Reaction of [Zn(II)(TTP)] (1) (TTP = dianion of 5,10,15,20-meso-tetrakis(p-tolyl)porphyrin) with 16 equiv of N-bromosuccinamide (NBS) in methanol at reflux led to the unexpected two-sidedd open-ring brominated product [Zn(II)(C26H20N2O2Br5)2] (2). Similar observations have been made with other meso-substituted zinc porphyrins as well [Zn(II)(por)] {por = dianion of 5,10,15,20-meso-tetrakis(aryl)porphyrin; aryl = phenyl (TPP), p-(t)Bu-phenyl (TBPP), m-Cl-phenyl (TClPP)}. The respective products [Zn(II)(C24H16N2O2Br5)2] (3), [Zn(II)(C32H32N2O2Br5)2] (4), and [Zn(II)(C24H14N2O2Cl2Br5)2] (5) have been isolated in good to moderate yields and characterized by elemental analysis and UV-vis, (1)H NMR, and mass spectrometry. Additional bromination reaction of 1 with 8 equiv of NBS in a chloroform/methanol mixture led (after the two-sided ring opening) to nonmetalated brominated bi(pyrrole) product, C36H34N2O4Br4 (6). The detailed structures of complexes 1, 2, 3, and 6, available in a single crystal form, have been confirmed by X-ray diffraction analysis.

Kinetics and Mechanism of the Reaction of Mercury(II) with a Water-soluble Octabromoporphyrin

The reaction of mercury(II) hydroxide with 2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin, [Formula: see text]( H 2P4−), to form the mercury(II) porphyrin [( TPPSBr 8) Hg ]4−( HgP 4−) was investigated in the pH range 6.2-8.5. The observed rate constant was first-order with respect to the mercury(II) concentration and decreased with increasing pH from pH 6.2 to 7.5 and then increased from pH 7.5 to 8.5. The rate expression was written as d [ HgP 4−]/dt = (kHPK−1[ H +]−1 + k H 2 P + k H 3P K 1[ H +])(1 + K1[ H +] + K−1[ H +]−1)−1[ Hg ( OH )2][ H 2 P 4−]. The kHP, kH2P and kH3P values were found to be (1.33 ± 0.02) × 108, (5.50 ± 0.08) × 106 and (1.40 ± 0.08) × 108 M −1 s−1 respectively, with K1 = [ H 3 P 3−][ H 2 P 4−]−1[ H +]−1 = 104.83 ± 0.04 and K−1 = [ HP 5−][ H +][ H 2 P 4−]−1 = 1010.02 ± 0.02. The activation parameters were [Formula: see text] and ΔS‡ HP = 226 ± 22 J K −1 mol −1 for the k HP path, [Formula: see text] and [Formula: see text] for the k H 2 P path and [Formula: see text] and [Formula: see text] for the k H 3 P path. The kinetic results show the high reactivity of mercury(II) hydroxide towards the protonated porphyrin.

Electrochemical and Spectroelectrochemical Characterization of Water-soluble, β-Pyrrolebrominated Cobalt Porphyrins

The electrochemical and spectroelectrochemical characterization of two water-soluble porphyrins, namely the tetrachloro salt of cobalt(II) tetrakis-(N-methyl pyridyl)-β-octabromoporphyrin, [( Br 8 TMPyP ) Co II] Cl 4, and the tetrasodium salt of cobalt(II) tetrakis-(4-sulfonatophenyl)-β-octabromoporphyrin, Na 4[( Br 8 TPPS ) Co II], under different pH conditions is reported. The investigated porphyrins are highly non-planar and electron-deficient owing to the presence of eight bromides at the β-pyrrole positions and the four meso-aryl substituents. The redox potentials corresponding to the first oxidation and first reduction are shifted positively compared with the redox potentials of the respective unbrominated porphyrin derivatives. Spectroelectrochemical studies reveal the formation of a cobalt(III) complex during the first oxidation of both cobalt porphyrins. The first reduction of Br 8 TPPS ) Co II]4− results in a cobalt(I) complex, while involvement of the peripheral N-methyl pyridyl groups in the case of Br 8 TMPyP ) Co II]4+ is suggested. The peak potentials of the first oxidation corresponding to a Co II/ Co III redox couple of both investigated compounds are found to be pH-dependent. Debromination of the β-pyrrole bromo substituents of Br 8 TMPyP ) Co II]4+ at more negative potentials is observed. The UV-vis spectrum obtained after bulk electrolysis of Br 8 TMPyP ) Co II]4+ at −1.0 V vs Ag / AgCl followed by reoxidation at 0.2 V indicates complete elimination of the bromo substituents of the porphyrin periphery.

High-valent iron porphyrins

The term ‘high-valent’ refers to iron complexes of porphyrins and related macrocycles in which the oxidation state of the iron center exceeds III. High-valent iron porphyrins and chlorins are important biological transients whose intermediacy has been demonstrated in numerous peroxidase and catalase enzymes. Two species, compounds I and II, are spectroscopically detectable upon stoichiometric addition of monooxygen donors to resting ferric enzymes. Compounds I and II are formally two and one oxidizing equivalents respectively above the ferric state. In compound II the oxidizing equivalent has been shown by spectroscopic studies to be located on iron as an oxoiron(IV) unit. The spectroscopic and magnetic properties of compound I support the structural assignment of an S = 1 oxoiron(IV) unit magnetically coupled to a heme π-cation radical (S = 1/2). Studies on model hemes have contributed much to the understanding of protein chemistry. Much work has been accomplished with meso-tetaarylporphyrins and, more recently, with physiologically congruent meso-unsubstituted pyrrole β-substituted complexes. Compounds I of both proteins and synthetic models have been characterized by a wide array of spectroscopic methods, including UV-vis, NMR, resonance Raman, EPR, variable-temperature/variable-field magnetic Mössbauer, magnetic circular dichroism and extended X-ray absorption fine structure spectroscopy. Results of these studies are summarized. Recent developments, which promise to yield a detailed picture of electronic structure, are variable-temperature magnetic circular dichroism, studies in the pre-K-edge region and L-edge X-ray absorption spectroscopy. Time-resolved X-ray diffraction techniques have been applied to obtain the first structural data on the protein forms of compound I.

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.

Effect of bromination on the electrochemistry, frontier orbitals, and spectroscopy of metallocorroles

A series of fully β-pyrrole brominated triarylcorrole metal complexes has been prepared for investigating the changes in visible spectra and redox potentials relative to the non-brominated derivatives, as well as for comparing the effect of bromination in corroles and porphyrins. The results reveal that bromination has a much larger effect on the electrochemistry of metallocorroles relative to metalloporphyrins, for both macrocycle- and metal-centered redox processes. The HOMO–LUMO gap energy of the triarylcorrole post-transition metal complexes decreases upon bromination because the effect on the LUMO is about twice as large of as on the HOMO; and both the HOMO and the LUMO are more affected in corroles than in porphyrins. Spectroscopic examinations of the transition metal complexes reveal that the synthetic access to divalent metallocorroles becomes feasible for the brominated derivatives.

Perhalogenated porphyrins as a sink of excitation energy in porphyrin heterodimers

The photophysics of a new family of free-base and zinc derivatives of meso- tetraphenylporphyrin heterodimers have been studied by UV-vis absorption, fluorescence and nanosecond flash photolysis techniques. An almost complete (≈99%) and directionally controlled transfer of excitation energy from a donor porphyrin moiety was obtained by multiple bromination (four and eight Br substituents in the two series of compounds investigated) on the β-pyrrole positions of the acceptor porphyrin molecule. The covalently linked porphyrin dimers populate almost exclusively low energy triplet states because of the extremely efficient intramolecular singlet-to-triplet inter system crossing (ISC) process which is enhanced by the multiple heavy atoms substitutions. The nature of the electronic interactions determining the actual relaxation pathway followed by the porphyrin donor-acceptor pair is discussed.

Unusual solvent dependent electronic absorption spectral properties of nickel(II) and copper(II) perhaloporphyrins

Electronic absorption spectra of divalent metal ( Ni (II) and Cu (II)) complexes of 2,3,7,8,12,13,17,18-octa(bromo/chloro)-5,10,15,20-tetraphenylporphyrins (MOXTPP; X = Br , Cl ) were examined in various solvents. M (II) perhaloporphyrins exhibited dramatic shifts in their optical absorption spectral features relative to the corresponding metallotetraphenylporphyrins, MTPPs. Copper(II) perhaloporphyrins show significant red-shifts of the absorption bands in coordinating solvents relative to that observed for nickel(II) perhaloporphyrins. The large red-shift of the electronic absorption bands and the gain in intensity of the longest wavelength band (Q(0,0)), of Cu (II) perhaloporphyrins in certain coordinating solvents is comparable to that found in meso-tetraphenylporphinatozinc(II), ZnTPP. The solvent dependent spectral features of M (II) perhaloporphyrins are attributed to a coordinative interaction of the solvent with the core metal ion induced by the electron deficient porphyrin macrocycle.

β-Pyrrole brominated meso-tetraphenylporphyrins: synthesis, spectral and electrochemical properties

The synthesis, catalytic properties, UV-visible spectra and electrochemistry of β-pyrrole brominated porphyrins are summarized in this brief review. The effect of the Br substituents of the porphyrin ring on the redox behavior, and on axial CO or pyridine binding to the porphyrins is also discussed.

Influence of steric hindrance on the solvent-dependent absorption spectral behavior of highly brominated free base porphyrin and its Zn(II) complex

Electronic absorption spectra of highly brominated free base porphyrin, 2,3,7,8,12,13,17,18-octabromo-tetrakis(2′,6′-dibromo-3′,5′-dimethoxyphenyl)porphyrin, H 2 T (3′,5′-DMP) PBr 16 and its Zn (II) complex were examined in various solvents. These derivatives exhibit red-shifted absorption spectral features in polar solvents relative to that observed in less polar or nonpolar solvents. The extent of red shifts observed in highly brominated porphyrins is less than those reported for the corresponding 2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetraphenylporphyrin, H 2 TPPBr 8 and its Zn (II) complex. The crystal structure of the H 2T(3′,5′-DMP) PBr 16 showed saddle-shaped geometry with steric hindrance of the bromo groups on the opposite phenyl rings which provide limited open access to the core. The decreased red shift of the absorption spectral bands of MT (3′,5′-DMP) PBr 16 ( M = 2 H , Zn (II)) derivatives when compared to unhindered MTPPBr 8 is possibly due to the steric hindrance offered by the ortho-bromo groups rather than the electronic effects of the porphyrin π-system.

Highly brominated porphyrins: synthesis, structure and their properties

This article reports the first perbromination of β-pyrrole and meso-phenyl groups of the 5,10,15,20-tetrakis(3',5'-dimethoxyphenyl)porphinato copper(II) to generate highly brominated porphyrin, 2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetrakis(2',6'-dibromo-3',5'-dimethoxyphenyl)-porphinato copper(II), CuT (3',5'- DMP ) PBr 16 complex. Its crystal structure exhibited unusual five-coordination geometry with saddle shaped conformation of the porphyrin core. H2T(3',5'- DMP ) PBr 16 and its metal complexes exhibited large anodic shift of oxidation potentials with marginal changes in reduction potentials relative to their corresponding octabromotetraphenylporphyrin, MTPPBr 8 derivatives. The enhanced electron deficiency of the ZnT (3',5'-DMP)PBr16 complex was probed by axial ligation of various Lewis bases with differing pK a values. The ZnT (3',5'-DMP)PBr16 complex exhibited decrease in equilibrium constants for the ligation of bases relative to sterically unhindered ZnTPPBr 8. This has been ascribed to the sterics induced by the bulky ortho-bromo phenyl substituents that prevents the facile binding of Lewis bases to the Zn (II)-center.