Meso-phenyl Rings Research Articles

4-Carboxyphenyl as efficient donor group in nano Zn-Porphyrin for dye sensitized solar cells

Dye-sensitized solar cells, represent the alternate technology in solar research due to their cost effective, easy fabrication processes, higher efficiencies, and design flexibility. In this research, dual donor group modified zinc porphyrin dyes, have been synthesized for DSSCs. The complexes of zinc porphyrin functioned as acceptor or attaching groups within each mesophenyl ring and carboxylic acid. These complexes exhibited diverse alkyl substituents and sizable electron-donating substituents, contributing to their varied chemical structures and potential applications. The dual Donor-π bridge -Acceptor group sensitizers, Zn[5,15-diphenylcarbazole-10,20-(4-carboxyphenyl) Porphyrin] (KSR-1) and Zn [5,15-thiadiazole-10,20-(4-carboxyphenyl) Porphyrin] (KSR-2) have been synthesized and adopted for DSSCs implementation. The molar absorption coefficients (ε) of KSR-2 and KSR-1 Soret bands were 0.56 x 105 mol/L/cm and 0.47 x 105 mol/L/cm, respectively. The Q bands of the KSR-1 and KSR-2 dyes were 1.10 x 105 mol/L/cm and 1.0 x 105 mol/L/cm, respectively and the molar absorption coefficient of the KSR-1 dye was greater when compared to the KSR-2 dye. The molar absorption coefficient of 0.71 x 105 mol/L/cm was visible in the KSR -1 Q-band. DFT calculations and the electrochemical characteristics of the KSR-1 and KSR-2 dyes have been studied and discussed. The exploration involved in investigating the photophysical properties and photovoltaic performance which were affected by varying the length and number of the donor entities. The wall-plug efficiency of the KSR-1 based solar panel was Voc = 0.68 V, Jsc = 8.94 mA/m2, FF = 56 and Efficiency (μ) = 3.44%. The wall-plug efficiency of the KSR-2 based solar panel was Voc = 0.63 V, Jsc = 5.42 mA/m2, FF = 53 and Efficiency (μ) = 1.83%.

A Highly Sterically Congested Bis‐Zinc‐Porphyrin Containing a Single Buta‐1,3‐diyne Linkage: From a Serendipitous Finding to Supramolecular Encapsulation

AbstractA unique example of a bis‐zinc‐porphyrin chemical system in which both macrocycles are covalently connected with a single, short buta‐1,3‐diyne linkage placed at the ortho sites of the meso phenyl rings is presented. This dimeric compound resulted from an homo‐coupling side‐reaction taking place during a copper‐catalyzed click reaction between an alkyne porphyrin and 2‐azidopyridine derivatives. Its unexpected formation was rationalized by control experiments and an improved synthesis was achieved under copper‐catalyzed Glaser‐Hay coupling reaction conditions. This highly sterically congested bis‐zinc‐porphyrin derivative behaved as a supramolecular host for encapsulating ditopic molecular guests such as 1,4‐diazabicyclo[2.2.2]octane (DABCO) with association constant K1.1 in the order of 106 M−1. This value is comparable to current systems that typically feature several connecting linkages between the two zinc‐porphyrin sites resulting in (supra)molecular cages ensuring a high pre‐organization. As such, the requirements to take benefit from supramolecular encapsulation can be reduced to a highly rigid, minimal covalent linkage of four atoms between zinc‐porphyrins as herein described.

Energy level tuning of push-pull porphyrin sensitizer by trifluoromethyl group for dye-sensitized solar cells

We have introduced trifluoromethyl groups into meta-positions of two meso-phenyl rings of a push-pull-type porphyrin dye (ZnP-CF[Formula: see text] to modulate the energy levels of the porphyrin dye for dye-sensitized solar cells (DSSCs). The light-harvesting ability of ZnP-CF[Formula: see text] is almost comparable to those of reference porphyrins where the trifluoromethyl groups are replaced with methyl (ZnP-CH[Formula: see text] or tert-butyl groups (YD2). We revealed that the introduction of the electron-withdrawing trifluoromethyl groups is effective to lower the excited-state oxidation potential of the porphyrin (–0.80 V vs NHE), which is consistent with the theoretical calculation. Meanwhile, DSSCs with ZnP-CF[Formula: see text] exhibited a lower power conversion efficiency ([Formula: see text] of 5.95% than DSSCs with ZnP-CH[Formula: see text] ([Formula: see text] = 7.33%) and YD2 ([Formula: see text] = 8.97%) because of the severe aggregation tendency of ZnP-CF[Formula: see text] arising from the trifluoromethyl groups. In addition, the insufficient steric bulkiness of the trifluoromethyl and methyl groups relative to tert-butyl groups would result in the lower short circuit current and open circuit voltage for ZnP-CF[Formula: see text] and ZnP-CH[Formula: see text] due to fast charge recombination between electrons in the conduction band of TiO2 and I[Formula: see text]in the electrolyte solution. Overall, introducing both trifluoromethyl groups and bulky substituents into a porphyrin core would be necessary to boost cell performance.

Hypercorroles Formed via the Tail that Wagged the Dog: Charge Transfer Interactions from Innocent Corroles to Meso-Nitrophenyl Substituents.

A series of cobalt nitrophenylcorroles were spectrally characterized in CH2Cl2, and under certain solution conditions, several compounds were shown to exhibit hypercorrole spectra resulting from charge transfer interactions from the corrole π-system to the redox-active meso-NO2Ph substituents. The resulting spectral pattern has not previously been reported for metallocorroles and in the case of the cobalt derivatives was shown to depend upon the number and position of the meso-nitrophenyl groups on the macrocycle, the position of the NO2 substituent on the meso-phenyl ring(s) (para or meta), and the electronic structure of the corrole, which can exist in its innocent or noninnocent form depending in large part upon the type and number of axial ligands. Cobalt corroles bearing p-nitrophenyl groups at the 5,15- or 5,10,15-positions of the macrocycle exhibited the most marked hypercorrole spectra under solution conditions where the complex was innocent (i.e., Cor3-CoIII), and a systematic analysis of the spectral data suggests the root of this perturbation to be a corrole-to-aryl interaction (i.e., ligand-to-ligand charge transfer or LLCT). The largest interaction between the π-system and the NO2Ph substituents was seen upon coordination of anionic cyanide (CN-) axial ligands to the Co(III) center of the bis-(CN-)-5,15-dinitrophenyl derivative, resulting in a cobalt hypercorrole spectrum where the broad Q-band was red-shifted even further into the NIR region and located at 795 nm in CH2Cl2 and 827 nm in pyridine. Cyclic voltammetry of the bis-CN- adducts showed that the first electrons are added to the LUMOs of the p-NO2Ph substituents rather than the corrole, while the same orbitals for the mono-CN- adducts are nearly degenerate. This redox behavior contrasts with what is seen for the noninnocent nitrophenyl corroles having "normal" unperturbed UV-vis spectra where the first reduction involves the π-system of the macrocycle, followed by reduction of the p-NO2Ph groups at more negative potentials.

Hypervalent Phosphorus(V) Porphyrins with meso-Methoxyphenyl Substituents: Significance of the Number and Position of Methoxy Groups in Promoting Intramolecular Charge Transfer

To study the photophysical and redox properties as a function of meso-aryl units, a series of hypervalent phosphorus(V) porphyrins, PP(OMe)2·PF6, PMP(OMe)2·PF6, PDMP(OMe)2·PF6, P345TMP(OMe)2·PF6, and P246TMP(OMe)2·PF6, with phenyl (P), 4-methoxyphenyl (MP), 3,5-dimethoxyphenyl (DMP), 3,4,5-trimethoxyphenyl (345TMP), and 2,4,6-trimethoxyphenyl (246TMP) units, respectively, have been synthesized. The P(+5) in the cavity makes the porphyrin ring electron-poor, whereas the methoxy groups make the meso-phenyl rings electron-rich. The presence of electron-rich and electron-poor portions within the porphyrin molecule promoted an intramolecular charge transfer (ICT). Also, the study suggests that the ICT depends on the number and position of the methoxy groups. The ICT is more prominent in m-methoxy-substituted phosphorus(V) porphyrins (PDMP(OMe)2.PF6, P345TMP(OMe)2·PF6) and almost no ICT was found in no-methoxy, o-methoxy, and/or p-methoxy phosphorus(V) porphyrins (PP(OMe)2·PF6, PMP(OMe)2·PF6, P246TMP(OMe)2·PF6). Transient absorption studies indicate that the ICT takes place on the picosecond time scale. The most striking results come from P246TMP(OMe)2·PF6, where each phenyl ring carries three methoxy units, like the P345TMP(OMe)2·PF6, but it failed to induce the ICT process. Electrochemical studies and time-dependent density functional theory (TD-DFT) calculations were used to support the experimental results. This study extensively explores why and how slight variations in meso-aryl substitutions lead to intricate changes in the photophysical and redox properties of phosphorus(V) porphyrins.

Synthesis and study of fluorine-functionalized ZnTPPs

Five fluorinated zinc tetraphenyl porphyrins derivatives, fluorinated either on all ortho positions of the meso phenyl rings (Zn(II)-5,10,15,20-tetrakis(2,6-difluorophenyl)porphyrin) or in [Formula: see text]-positions with two, four, six or eight fluorine groups (2,3-difluoro-5,10,15,20-tetraphenylporphyrin, 7, 8,17,18-tetrafluoro-5,10,15,20-tetraphenylporphyrin, 7,8,12,13,17,18-hexafluoro-5,10,15,20-tetraphenyl- porphyrin and 2,3,7,8,12,13,17,18-octafluoro-5,10,15,20-tetraphenylporphyrin, respectively), were studied to probe the effect of fluorine in on-surface synthesis approaches. Additionally, bulkier substituents have been used in 7,8,17,18-tetrabromo-5,10,15,20-tetraphenylporphyrin and 7,8,17,18-tetramethyl-5,10,15,20-tetraphenylporphyrin, for a direct comparison with the corresponding fluorinated compounds. Reported are the synthesis and electronic structure characterization of the compounds, using UV-vis absorption and fluorescence spectroscopies as well as electronic structure calculations of the ground state molecular properties. Steric and electronic effects of fluorination are explored. For all molecules studied, substitution with fluorine maintains a planar tetrapyrrole macrocycle and shifts in the absorption spectra were consistent with calculated changes in the HOMOs and LUMOs energies of the molecules. In contrast, the bromo and methyl-substituted derivatives exhibit chemical instability and spectral shifts, compared to parent compound ZnTPP, consistent with structural distortions and accounted for calculations. The site-specific effects of fluorination on the electronic structure are discussed in detail.

Electrocatalytic hydrogen production by CN– substituted cobalt triaryl corroles

Four cobalt corrole complexes bearing 0–3 cyano groups on thepara-position of the threemeso-phenyl rings of the macrocycle were synthesized, characterized and applied for electrocatalytic H2production in both organic solvent and neutral aqueous media.

Synthesis of crown ether appended 25-Oxasmaragdyrins and their BF2-Complexes

Three examples of crown ether appended 25-oxasmaragdyrins were synthesized in 15–20% yields by [3 + 2] oxidative condensation of 16-oxatripyrrane and crown ether appended meso-aryl dipyrromethane in CH2Cl2 in the presence of catalytic amount TFA under inert atmosphere followed by oxidation with DDQ in open air. The crown ether appended 25-oxasmaragdyrins were treated with BF3.OEt2 in CH2Cl2/TEA on hot water bath with 50 °C for 30 min to afford crown ether appended BF2-oxasmaragdyrins in 60–70% yields. The identities of crown ether appended 25-oxasmaragdyrins and their BF2 complexes were confirmed by molecular ion peak in mass spectra and detailed 1D & 2D NMR, absorption, fluorescence and DFT/TD-DFT techniques were used to study the macrocycles. DFT studies revealed that the crown ether appended 25-oxasmaragdyrin and its BF2 complex were almost planar with the meso-phenyl ring containing the crown ether moiety deviated by an angle of 54.86◦ with respect to mean plane defined by three meso carbon atoms. The absorption studies indicated that crown ether appended smaragdyrins and their BF2 complexes showed one or two strong Soret bands in the region of 440–480 nm and four weak Q-bands in 500–750 nm region. The crown ether appended 25-oxasmaragdyrins and their BF2 complexes were weakly fluorescent.

Structure and Luminescence Properties of Lutetium(Iii) Complexes with 5,10,15,20-Tetraphenylporphine and its Derivatives

The spectral and luminescent properties of new complexes of lutetium(III)–chloride with 5,10,15,20-tetraphenylporphine and its mono-nitrophenyl derivative with a para-nitro group on one of the meso-phenyl rings were studied. The complexes showed weak fluorescence (φfl = 5·10–4) and moderate phosphorescence (φphosph = 3·10–2) at 77 K and phosphorescence at room temperature. The type of extra ligand (monodentate chloride and bidentate acetylacetonate) and the presence of an electron-withdrawing NO2 group on the meso-phenyl ring did not have an appreciable effect on the photophysics of the Lu(III)-porphyrins. The phosphorescence spectra of the complexes were shifted to the long-wavelength region. The phosphorescence lifetimes were longer (2800–3100 μs) than those of the known oxygen sensors Pt(II)- or Pd(II)-porphyrins. The oxygen determination limit in solution was estimated from quenching of the Lu(III)-porphyrin phosphorescence.

Synthesis of Zn(II) porphyrin dyes and revealing an influence of their alkyl substituents on performance of dye-sensitized solar cells

In this study, the series of A3B-type zinc porphyrin dyes have been synthesized for dye-sensitized solar cells (DSSC). All the received zinc-porphyrin complexes bear different alkyl substituents (none, tert-butyl-, methoxy-, hexyloxy-, dodecyloxy-, a pair of hexyloxy-), bulky electron-donating substituent (diphenylamino) in each of three meso-phenyl rings and carboxylic acid as acceptor/anchoring group. The influence of the number and length of alkyl chains on the photophysical properties and DSSCs photovoltaic performance was investigated, as well as quantum-chemical modeling was carried out. Our results reveal that the alkyl chains length and number affect cell performance. We found that DSSC power conversion efficiency increases with an increase in the length and number of alkyl substituents in the porphyrins. A maximum efficiency of 1.29 % was observed in solar cells with dodecyloxy-substituted zinc porphyrin.

Highly efficient electrocatalytic hydrogen evolution from neutral aqueous solution by water soluble copper (II) porphyrin

A water soluble Cu (II) porphyrin bearing para-sulfonic group at mesophenyl rings is reported as molecular electrocatalyst for H2 generation from neutral aqueous solution. Electrocatalysis study in buffer solution of pH 7.2 showed that the complex is highly active, efficient and stable toward H2 evolution. It features an observed rate constant (kobs) of 8.6 × 103 s−1 and Faradaic efficiency of 98.3% at an overpotential of 470 mV. The role of functional group on catalytic activity is also investigated by conducting heterogeneous catalysis involving Cu-porphyrin with (CuTPPS) and without (CuTPP) para-sulfonic groups. Hence differential pulse voltammetry, controlled-potential electrolysis and electrochemical impedance spectroscopic measurements show that, CuTPPS-modified electrode perform much better than that of CuTPP-modified. This may be attributed to an increase in the amount of catalyst at the vicinity of electrode surface because of the ability of para-sulfonic functional group to anchor to electrode surface. Moreover, its electron withdrawing behaviour and ability to donate proton to the solution in turn thermodynamically favour H2 evolution.

Trans A2B2 Porphyrins: Synthesis, Crystal Structure Determinations and Hirshfeld Surface Analysis

AbstractTrans‐A2B2 porphyrins containing p‐methoxyphenyl (A)/p‐methylphenyl (B) (P4) and p‐chlorophenyl (A)/p‐methylphenyl (B) (P5) at the four meso (−CH=) positions of the tetrapyrrolic macrocycles have been synthesized by employing the two steps modified Adler's methodology. Remarkably, the present synthetic scheme afforded us the needle shaped single crystals of (P4) and (P5) that helped us investigate the three dimensional structural architectures of the prepared Trans‐A2B2 porphyrins. Hirschfeld surface analysis (HSs) was used to visualize the fidelity of the crystal structures of (P4) and (P5). This method permitted the identification of individual types of intermolecular contacts and their impact on crystal packings. Molecules are linked by a combination of H⋅ ⋅ ⋅H and C⋅ ⋅ ⋅H contacts, which have clear signatures in the corresponding fingerprint plots. The effect of incorporating two different non sterically demanding functionalities at the para positions of meso phenyl rings on the optical and thermal properties of symmetric (A4) porphyrins i. e. Tetra‐para‐methoxy phenyl porphyrin P1, Tetra‐para‐methyl phenyl porphyrin P2 and Tetra‐para‐chloro phenyl porphyrin P3, have been investigated and the results have been discussed in detail.

N‐Confused Porphyrin – A Unique “Turn‐On” Chemosensor for CN− and F− ions and “Turn‐Off” Sensor for ClO4− ions

N-Confused meso-tetrakis(4-carbomethoxyphenyl)porphyrin (1) and its Ni(II) complex (1 a) have been synthesized and utilized for anion sensing studies, and the results are compared with N-confused meso-tetraphenylporphyrin (NCTPP). Anion susceptibilities of 1 and 1 a were investigated using spectroscopic, electrochemical, and DFT studies. Porphyrins 1 and 1 a were able to detect CN- , F- , and ClO4 - ions selectively over the tested set of anions even at ppm level. Interestingly, the addition of ClO4 - ions resulted in fluorescence quenching (turn off) whereas the addition of F- or CN- resulted in fluorescence enhancement (turn on). Notably, the TFA addition resulted in fluorescence quenching, whereas the fluorescence enhancement was observed while adding TBAOH. The higher association constant (Ka ) values with anions, lower detection limit, and shifts in redox potentials are due to the electron-withdrawing effect of the -COOCH3 group at the para-position of the meso-phenyl ring. This electron-withdrawing nature is crucial for the higher affinity towards anions. The anion sensing description in this article may not only unveil the built-in nature of N-confused porphyrins, but may also provide a general proposal for the development of novel anion sensors based on porphyrinoids. The electron-deficient porphyrin framework, large polarisable π-system, and anion binding through the outer NH or a combination of the above factors serve as a foundation for N-confused porphyrin to act as an anion sensor.

Electrochemical Properties of Mono- and Bis-CN Ligated Cobalt Corroles

Corroles have attracted a great deal of attention in terms of application and understanding their electronic configuration, especially the innocence or non-innocence of the macrocyclic ligand. These properties are known to be strongly influenced by the metal ion and degree of axial coordination, namely 4, 5 and 6-coordinate. In addition, these properties are also dependent on the solvent and possibly peripheral substituents. We have previously examined bis-pyridine and mono-DMSO cobalt corroles and in this work describe the electrochemical and spectroscopic properties of newly synthesized series of anionic axially ligated mono-CN and in-situ generated bis-CN cobalt A3-triarylcorroles with a tetrabutylammonium counterion. Examples of the synthesized corroles are shown in the figure and represented as [(Ar)3CorCo(CN)]- where (Ar)3Cor is the corrole macrocycle and Ar is one of several meso-phenyl rings containing electron-withdrawing or electron-donating substituents. Figure 1

Synthesis and Spectral and Coordination Properties of meso-Tetraarylporphyrins

5,10,15,20-Tetrakis(3,5-dibromophenyl)porphyrin and 5,10,15,20-tetrakis(3-bromo-4-methoxy-phenyl)porphyrin have been synthesized, and their complexing properties with respect to Pd2+ ion in acetonitrile at 308–328 K have been studied by spectrophotometry. The synthesized compounds were characterized by UV-Vis, 1H NMR, and mass spectra. The effect of substitueras in the meso-phenyl rings of the ligands on their spectral and coordination properties was estimated in comparison to 5,10,15,20-tetraphenylporphyrin.

Axial coordination reactions with nitrogenous bases and determination of equilibrium constants for zinc tetraarylporphyrins containing four β,β′-fused butano and benzo groups in nonaqueous media

The axial coordination properties of six zinc tetraarylporphyrins with seven different nitrogenous bases were examined in CH2Cl2 for derivatives containing four [Formula: see text],[Formula: see text]-fused butano or benzo groups and the equilibrium constants (log[Formula: see text] determined using spectral titration methods. The examined compounds are represented as butano(YPh)4PorZn and benzo(YPh)4PorZn, where Por is the porphyrin dianion and Y is a CH3, H or Cl substituent on the para-position of each meso-phenyl ring of the macrocycle. The initial four-coordinate butano- and benzoporphyrins will axially bind one nitrogenous base to form five-coordinate derivatives in CH2Cl2 and this leads to a 4–22 nm red-shift of the Soret and Q bands. The log[Formula: see text] values range from 1.98 to 4.69 for butano(YPh)4PorZn and from 3.42 to 5.36 for benzo(YPh)4PorZn, with the exact value depending upon the meso and [Formula: see text]-substituents of the porphyrin and the conjugate acid dissociation constants (p[Formula: see text] of the nitrogenous base.

Synthesis, electrochemistry, protonation and X-ray analysis of meso-aryl substituted open-chain pentapyrroles

Five meso-tetraaryl open-chain pentapyrroles were synthesized and characterized as to their electrochemistry and protonation reactions in nonaqueous media. The investigated compounds are represented as (Ar)4PPyH3 where Ar [Formula: see text],[Formula: see text]-F2Ph, [Formula: see text]-BrPh, Ph, [Formula: see text],[Formula: see text],[Formula: see text]-(OMe)3Ph or [Formula: see text]-MePh and were characterized by UV-vis and 1H NMR spectroscopy, mass spectrometry and electrochemistry. Cyclic voltammetry was used to measure redox potentials, while protonation involving the conversion of (Ar)4PPyH3 to [(Ar)4PPyH5][Formula: see text] was monitored by UV-vis absorption spectroscopy. Equilibrium constants for proton addition were calculated using the Hill equation. One of the pentapyrroles was also structurally characterized. The electrochemical data, protonation constants and crystal structure were then compared with data for previously examined pentapyrroles and analyzed as a function of the solvent properties and nature of substituents on the meso-phenyl rings of the macrocycle.

Synthesis, electrochemical and spectroelectrochemical characterization of iron(III) tetraarylporphyrins containing four β,β′-butano and β,β′-benzo fused rings

Six iron(III) tetraarylporphyrins containing four [Formula: see text]-butano or [Formula: see text]-benzo fused rings were synthesized and characterized by electrochemistry and spectroelectrochemistry in nonaqueous media. The examined compounds are represented as butano(TpYPP)FeCl and benzo(TpYPP)FeCl, where TpYPP is a dianion of the meso-substituted porphyrin, Y is a CH[Formula: see text], H or Cl substituent on the para-position of the four meso-phenyl rings and butano and benzo are the [Formula: see text]-substituents on each of the four pyrrole rings of the compound. Up to three reductions are observed for each Fe(III) butano- and benzoporphyrin in CH[Formula: see text]Cl[Formula: see text] or pyridine containing 0.1 M TBAP, the first of which is assigned in each case to a metal-centered electron transfer. The second reduction is also metal-centered in CH[Formula: see text]Cl[Formula: see text] and leads to formation of an Fe(I) porphyrin, but it is porphyrin ring-centered and gives an Fe(II) porphyrin [Formula: see text]-anion radical reduction product when pyridine is used as the solvent. The effects of the solvent and type of fused ring system (butano or benzo) on the UV-vis spectra and electrochemical properties of the Fe(III) porphyrins are discussed and comparisons are made to both the structurally related non-[Formula: see text]-substituted iron porphyrins and earlier described butano- or benzotetraarylporphyrins containing Cu(II) or Co(II) central metal ions.

Electrochemical Hydrogen Evolution by Cobalt (II) Porphyrins: Effects of Ligand Modification on Catalytic Activity, Efficiency and Overpotential

Electrochemical H2 evolution of a series of cobalt(II) porphyrins with electron-withdrawing (EW) and electron-donating (ED) substituents at the para positions of the meso-phenyl rings has been investigated in DMSO using acetic acid as a proton source. Our study showed that the nature of substituents significantly influences catalytic activity, efficiency, and the potential at which catalysis occurs. Faradaic efficiencies (FE) ranging from 44 to 99%, turnover numbers (TONs) from 1.5 to 104 (∼11 h electrolysis), turnover frequencies (TOFs) from 0.23 to 9.1 h−1, and onset overpotentials from 25 to 445 mV were obtained by tuning the porphyrinic substituents. Cobalt porphyrins with -SO3H, -COOH, or -NH2 groups as the substituents showed high activity and efficiency with more positive onset potentials as compared to the parent [Co(TPP)]. Supports also from the low hydrogen generation activities for complexes with -COOMe, -OMe and -OH groups as the substituents suggest that the acidity of the meso-phenyl substituent plays a key role in enhancing the hydrogen evolution activities during the catalytic processes.

Synthesis, Characterization, and Electrochemistry of Open-Chain Pentapyrroles and Sapphyrins with Highly Electron-Withdrawing meso-Tetraaryl Substituents.

A series of open-chain pentapyrroles and sapphyrins with highly electron-withdrawing substituents (i.e., CN, CF3 , or CO2 Me) on the meso-phenyl rings was synthesized and characterized as to the spectral properties, protonation reactions, and electrochemistry in non-aqueous media. The investigated compounds are represented as (Ar)4 PPyH3 and (Ar)4 SapH3 where PPy and Sap correspond to the tri-anion of the open-chain pentapyrrole and sapphyrin, respectively, and Ar=p-CNPh, p-CF3 Ph, or p-CO2 MePh. UV/Vis and 1 H NMR spectroscopy as well as mass spectrometry data are given for the confirmation of the structures for the newly synthesized compounds. An X-ray structure for one of the pentapyrroles, that is, (p-CF3 Ph)4 PPyH3 (2), is also presented. The protonation processes were examined by UV/Vis absorption spectroscopy during the titration of the compounds with trifluoroacetic acid (TFA) in CH2 Cl2 . Equilibrium constants for the protonation reactions were calculated by using both the Hill equation and the mole ratio method. The protonation-initiated conversion of pentapyrroles to sapphyrins upon oxidation was also investigated. Cyclic voltammetry was used to measure the redox potentials in CH2 Cl2 , PhCN, and/or pyridine (Py). Electrochemical properties, protonation constants, and chemical reactions of the six compounds in the two series were then analyzed as a function of the solvent properties and the type of the electron-withdrawing groups on the meso-phenyl rings.