Meso-phenyl Substituents Research Articles

Tailoring the cobalt porphyrin for minimal overpotential in the electrochemical oxygen evolution/reduction reactions: A density functional study

Oxygen reduction and evolution reactions (ORR/OER) have attracted tremendous attention as the means to provide sustainable energy solutions. Bifunctionality for ORR and OER is extremely crucial and relevant for regenerative fuel cell technology. Both reactions use precious metal-based electrocatalysts that are limited and expensive making a strong case to develop low-cost, active and stable electrocatalysts. We propose tailoring of Cobalt Porphyrin (Co–Pr) framework by introducing meso phenyl substituents in the Pr ligand and further substitution of phenyl group by ortho/para-anilino groups (CoTPP, CoTPP-o-NMe3, CoTPP-p-NMe3) to design an efficient and selective bifunctional catalyst. Electrocatalysis by CoTPP-o-NMe3 (ηORR/ηOER = 0.45/0.28 V) outperforms Ir/Ru and Pt based benchmark catalysts. Energy profile, scaling relationship, electrode potential, energy descriptor and FMO analysis performed in the DFT framework rationalize its excellent catalytic activity and selectivity for 4e− reduction of O2 to H2O which is vital for hydrogen production and fuel cell applications.

Periphery-Fused Chiral A2B-Type Subporphyrin.

Despite significant interest, the chiroptical properties of subporphyrins have rarely been investigated because chiral subporphyrins are elusive. Here, inherently chiral subporphyrins are elaborated by forming a fused pyran ring at the periphery of an A2B-type meso-aryl-substituted subporphyrin. Their circular dichroism (CD) properties are largely affected by the peripheral substituents and the dihedral angles between the meso-aryl substituents and the subporphyrin core: the β-perbromo subporphyrin with an orthogonal arrangement of the meso-phenyl substituents to the subporphyrin core exhibits weak CD signals corresponding to the Q bands, whereas the unsubstituted species with smaller dihedral angles shows relatively intense CD signals. A detailed structure–property relationship of these chiral subporphyrins was elucidated by time-dependent (TD) DFT calculations. This study reveals that the CD properties of chiral subporphyrins can be controlled by peripheral substitution and meso-aryl substituents.

Catalytic activity of bis p-nitro A2B (oxo)Mn(V) corroles towards oxygen transfer reaction to sulphides

The 5 A2B Mn(III) corroles have been synthesized, fully characterized and used for oxygen atom transfer (OAT) from isolated (oxo)manganese(V)corrole to sulfide. Oxidation of Mn(III) were carried out using ozone, a clean oxidant, which resulted in the isolation of the 5 red- coloured (oxo)manganese(V) corroles. The stoichiometric OAT to a different p-thioanisoles gave a mechanistic insight. The electron withdrawing and electron donating group containing phenyl group substituents on C10meso carbon were synthesized to gain effect of substituents on the rate constant. The rate constant suggests the disproportionation reaction for OAT from (oxo)manganese(V) corrole to (oxo)manganese(IV) and (oxo)manganese(VI) corroles. Newly synthesized free bases were characterised by 1H NMR, UV–visible spectroscopy. Manganese(III) corrole, (oxo)manganese(V) corrole were characterized by UV–visible spectroscopy. Self-decay and oxygen atom transfer to sulfide rate constant were determined using UV–visible spectroscopy. Electrochemistry of these Mn(III) corroles shows the effect of meso phenyl substituents on the oxidation potential of Mn(III) corroles. DFT calculation of these synthesized Mn(III) corroles have been carried out and HOMO-LUMO gap varies with the substituents on meso carbons.

Photophysical Properties and Electronic Structure of Zinc(II) Porphyrins Bearing 0-4 meso-Phenyl Substituents: Zinc Porphine to Zinc Tetraphenylporphyrin (ZnTPP).

Six zinc(II) porphyrins bearing 0-4 meso-phenyl substituents have been examined spectroscopically and theoretically. Comparisons with previously examined free base analogues afford a deep understanding of the electronic and photophysical effects of systematic addition of phenyl groups in porphyrins containing a central zinc(II) ion versus two hydrogen atoms. Trends in the wavelengths and relative intensities of the absorption bands are generally consistent with predictions from time-dependent density functional theory calculations and simulations from Gouterman's four-orbital model. These trends derive from a preferential effect of the meso-phenyl groups to raise the energy of the highest occupied molecular orbital. The calculations reveal additional insights, such as a progressive increase in oscillator strength in the violet-red (B-Q) absorption manifold with increasing number of phenyls. Progressive addition of 0-4 phenyl substituents to the zinc porphyrins in O2-free toluene engenders a reduction in the measured lifetime of the lowest singlet excited state (2.5-2.1 ns), an increase in the S1 → S0 fluorescence yield (0.022-0.030), a decrease in the yield of S1 → T1 intersystem crossing (0.93-0.88), and an increase in the yield of S1 → S0 internal conversion (0.048-0.090). The derived rate constants for S1 decay reveal significant differences in the photophysical properties of the zinc chelates versus free base forms. The unexpected finding of a larger rate constant for internal conversion for zinc chelates versus free bases is particularly exemplary. Collectively, the findings afford fundamental insights into the photophysical properties and electronic structure of meso-phenylporphyrins, which are widely used as benchmarks for tetrapyrrole-based architectures in solar energy and life sciences research.

Synthesis and the Effect of Anions on the Spectroscopy and Electrochemistry of Mono(dimethyl sulfoxide)-Ligated Cobalt Corroles.

A new series of cobalt A3-triarylcorroles were synthesized and the compounds examined as to their electrochemical and spectroscopic properties in CH2Cl2 or dimethyl sulfoxide (DMSO) containing 10 different anions added to the solution in the form of tetrabutylammonium salts. The investigated anions were PF6-, BF4-, HSO4-, ClO4-, Br-, I-, Cl-, OAc-, F-, OTs-, and CN-, all but three of which were found to facilitate reduction of the cobalt corrole in dilute CH2Cl2 solutions, as determined by a combination of UV-vis spectroscopy and spectroelectrochemistry. The synthesized corroles are represented as (Ar)3CorCo(DMSO), where Ar is a meso-phenyl group containing one of 10 different electron-donating or -withdrawing substituents. The axial DMSO ligand was found to dissociate in dilute (10-5 M) CH2Cl2 solutions, but this was not the case at the higher electrochemical concentration of 10-3 M, where the investigated corroles exhibit a rich redox reactivity, undergoing up to five reversible one-electron-transfer reactions under the different solution conditions. The reversible half-wave potentials for generation of the singly oxidized corroles varied by over 1.0 V with a change in the electron-donating or -withdrawing meso-phenyl substituents and type of anion added to the solution, ranging from E1/2 = 0.83 V in one extreme to -0.42 V in the other. Much smaller shifts in the potentials (on the order of ∼210 mV) were observed for the reversible first reduction as a function of changes of the anion and/or corrole substituents, with the only exception being in the case of CN-, where the E1/2 values in CH2Cl2 ranged from +0.08 V in solutions containing 0.1 M TBAClO4 to >-1.8 V upon the addition of CN-.

Through the challenging synthesis of tetraphenylporphyrin derivatives bearing nitroxide moieties

Tetraphenylporphyrin derivatives a synthetic heterocycles with convenient preparation and a richness of properties which make them attractive in broad fields such as energy, life and materials sciences. Thus, in the quest for new radical architectures, tetraphenylporphyrins are prime candidates. To this end, we designed free-base tetraphenylporphyrins bearing nitronyl and imino nitroxide moieties covalently bonded to the para-position of the meso-phenyl substituent. Their detailed synthesis and characterization are reported here.

BIII 5-Arylsubporphyrins and BIII Subporphine.

Boron(III) 5-arylsubporphyrins and BIII subporphine are promising precursors for functional BIII subporphyrins bearing asymmetric meso-substituents. Herein, we report the first synthesis of these molecules. Among many aryl acid chlorides examined, 4-nitrobenzoyl chloride gave BIII 5-(4-nitrophenyl)subporphyrin in 10 % yield in condensation with triethylamine-tri-N-tripyrromethene-borane. The nitro group of this BIII subporphyrin was reduced with NaBH4 to prepare BIII 5-(aminophenyl)subporphyrin, which was converted into BIII 5-phenylsubporphyrin via the corresponding diazonium salt. BIII subporphine was synthesized by condensation of triethyl orthoformate with triethylamine-tri-N-tripyrromethene-borane. Progressive removal of meso-phenyl substituents leads to continuous changes in the optical properties, whereas the BIII subporphine deviates from this trend in some properties.

Charge Stabilization in High-Potential Zinc Porphyrin-Fullerene via Axial Ligation of Tetrathiafulvalene

Extending the lifetime of the charge-separated states generated during photoinduced electron transfer in a covalently linked high-potential zinc porphyrin-fullerene dyad, (F15P)Zn–C60, was accomplished by metal–ligand axial coordination of pyridine-functionalized tetrathiafulvalene (TTF) via a dual-electron-transfer/hole migration mechanism. The meso-aryl positions of the zinc porphyrin carried three penta-fluorophenyl substituents that made the zinc porphyrin ring harder to oxidize by 0.43 V compared with zinc porphyrin with meso-phenyl substituents. Two TTF derivatives, a first with a pyridine directly linked to TTF (Py-TTF) and a second with a phenyl spacer between the pyridine and TTF (Py-phTTF), were employed to vary the distance between the primary photosensitizer/electron donor, zinc porphyrin, and the secondary electron donor, TTF. Both Py-TTF and Py-phTTF coordinated via the pyridine entity to the Zn center with 1:1 molecular stoichiometry and moderate binding constants. The supramolecular triads were characterized by optical absorption and emission, electrochemistry, and computational studies. An energy-level diagram was established to realize the different photochemical events in the triads. Using femtosecond transient absorption spectroscopy, it was possible to show that the coordinated TTF participated in electron transfer from the 1(F15P)Zn* in the case of the C60-(F15P)Zn:TTF triads to produce C60-(F15P)Zn•-:TTF•+ charge-separated state competitively with the electron transfer from the 1(F15P)Zn* to covalently linked C60 to produce C60•--(F15P)Zn•+:TTF charge-separated state. The two charge-separated states, C60-(F15P)Zn•-:TTF•+ and C60•--(F15P)Zn•+:TTF, were further involved in electron migration in the former case and hole transfer in the latter case to produce the C60•--(F15P)Zn:TTF•+ charge-separated state as the ultimate electron-transfer product. Due to distal separation of the positive and negative radical ions, long-lived charge-separated states persistent for about 0.35 μs was possible to accomplish, as shown by nanosecond transient absorption spectral studies.

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.

Temperature dependent steric hindrance effects in triplet state relaxation of meso-phenyl-substituted Pd-octaethylporphyrins

The analysis of spectral-kinetic results of transient absorption and phosphorescence experiments has been carried out in a wide temperature range (80–293K) for a series of Pd-octaethylporphyrin derivatives (PdOEP) with increasing number of bulky meso-phenyl substituents. In order to elucidate the influence of steric hindrance interactions, as well as porphyrin macrocycle non-planarity on photophysical characteristics and relaxation pathways of triplet states, the following parameters were compared: i) T-state positions on the energy scale, ii) phosphorescence lifetimes and T1−Tn transient absorption spectra, iii) phosphorescence quantum efficiencies, iv) activation energies of temperature-dependent phosphorescence rate constants for planar PdOEP and non-planar PdOEP-meso(Ph)n molecules.

Synthesis and femtosecond third order nonlinear optical properties of push-pull trans-A2B-corroles

Six novel push-pull trans-A2B free base corroles (1–6) were synthesized and characterized by various spectroscopic techniques. The charge-transfer ability of these trans-A2B-corroles was reflected from the broadness of the UV–visible absorption spectral bands. Corroles 1 and 2 exhibited absorption at lower wavelengths, higher quantum yield and longer singlet state lifetime as compared to 3–6. The ultrafast nonlinear optical (NLO) properties of these corroles were evaluated in visible range (at 800 nm) using the femtosecond Z-scan technique. The investigated corroles possess negative type of nonlinearity and self-defocusing behavior with the magnitude of nonlinear refractive index (n2) in the range of 0.36–2.84 × 10−11 cm2/W, while the two-photon absorption (β) coefficients were in the range of 6.5–68 cm/GW with |χ(3)| in the range of 10−9 e.s.u. The resultant dipole moments of these corroles were calculated theoretically and found to be 1.47–7.26 D depending on the nature of push-pull groups on the meso-phenyl substituents. 1–6 demonstrated excellent NLO activity due to their ‘push-pull’ design. Among the investigated corroles, 1 and 2 possess strong two-photon absorption coefficient and large values of χ(3) suggesting that the nonlinearity of corrole molecules highly depends on the electronic nature and number of meso-substituents. Corroles 1 and 2 possessing two acceptor and one donor moieties depicted superior NLO behavior as compared to corroles (3–6) with one acceptor and two donors due to strong electron withdrawing nature of pentafluorophenyl groups as well as higher singlet excited state lifetime and quantum yield.

Expanded Rosarin: A Versatile Fullerene (C60) Receptor.

An expanded rosarian (P3P6) with a bowl-like conformation has been prepared and characterized in a one-pot procedure that involves condensing a bispyrrole pyridine precursor (P1P2) with benzaldehyde, followed by oxidation. Single crystal X-ray diffraction analysis reveals a bowl-like conformation in the solid state with an upper rim diameter defined by the meso-phenyl substituents of ca. 13.5 Å and a depth of roughly 6.3 Å. P3P6 forms both 1:1 and 2:1 complexes with C60 in the solid state. DFT reveals similar energies for the two binding modes. A 1:1 binding stoichiometry dominates in 1,2-dichlorobenzene-d4 at the millimolar concentrations dictated by solubility consideration. The solution phase interactions between rosarian and C60 were studied using 1H NMR, UV-vis, and femtosecond transient absorption spectroscopies in 1,2-dichlorobenzene-d4 or toluene. To our knowledge, this is the first report of an unfunctionalized porphyrinoid that forms a well-defined complex with C60 in solution as well as in solid state.

Photophysical Properties and Electronic Structure of Porphyrins Bearing Zero to Four meso-Phenyl Substituents: New Insights into Seemingly Well Understood Tetrapyrroles.

Six free base porphyrins bearing 0-4 meso substituents have been examined by steady-state and time-resolved absorption and fluorescence spectroscopy in both toluene and N,N-dimethylformamide (DMF). The lifetime of the lowest singlet excited state (S1) decreases with an increase in the number of meso-phenyl groups; the values in toluene are H2P-0 (15.5 ns) > H2P-1 (14.9 ns) > H2P-2c (14.4 ns) > H2P-2t (13.8 ns) ∼ H2P-3 (13.8 ns) > H2P-4 (12.8 ns), where "H2P" refers to the core free base porphyrin, the numerical suffix indicates the number of meso-phenyl groups, and "c" and "t" refer to cis and trans, respectively. The opposite trend is found for the fluorescence quantum yield; the values in toluene are H2P-0 (0.049) < H2P-1 (0.063) ∼ H2P-2c (0.063) < H2P-2t (0.071) < H2P-3 (0.073) < H2P-4 (0.090). Similar trends occur in DMF. All radiative and nonradiative (internal conversion and intersystem crossing) rate constants for S1 decay increase with the increasing number of meso-phenyl groups. The increase in the rate constant for fluorescence parallels an increase in oscillator strength of the S0 → S1 absorption manifold. The trend is reproduced by time-dependent density functional theory calculations. The calculations within the context of the four-orbital model reveal that the enhanced S0 ↔ S1 radiative probabilities derive from a preferential effect of the meso-phenyl groups to raise the energy of the highest occupied molecular orbital, which underpins a parallel bathochromic shift in the S0 → S1 absorption wavelength. Polarizations of the S1 and S2 excited states with respect to molecular structural features (e.g., the central proton axis) are analyzed in the context of historical conventions for porphyrins versus chlorins and bacteriochlorins, where some ambiguity exists, including for porphine, one of the simplest tetrapyrroles. Collectively, the study affords fundamental insights into the photophysical properties and electronic structure of meso-phenylporphyrins that should aid their continued widespread use as benchmarks for tetrapyrrole-based architectures in chemical, solar-energy, and life-sciences research.

Peripherally Hexasulfanylated Subporphyrins.

Peripherally hexachlorinated meso-triphenyl subporphyrin 4 was prepared by chlorination of meso-triphenyl subporphyrin 1 with N-chlorosuccinimide and was effectively transformed to hexasulfanylated subporphyrins 5-8 via nucleophilic aromatic substitution (S(N)Ar) reactions with the corresponding thiols under basic conditions. The structures of 5-8 have been all well characterized by single-crystal X-ray analysis. (1)H NMR studies indicated that the meso-phenyl substituents undergo restricted rotation for 5-8, while the β-sulfanyl substituents are conformationally flexible in 5, 6, and 8, and are strictly regulated to an anti-conformation in 7. Judging from the absorption spectra, the oxidation and reduction potentials, and the DFT calculations, the substituent effects decrease in the order of 5>6>7>8. Subporphyrin 8 effectively captures C60 in a 1:1 manner in [D8 ]toluene solution.

Self-assembling corroles.

trans-A2B-corroles bearing -OCH2CONHR groups at the ortho position of the meso-phenyl substituent undergo self-organization both in the solid state as well as in solution. The lack of additional donor atoms induces sheet formation, but if the pyridine unit is present in the structure, more complex helical forms are formed.

Structural Dynamics and Relaxation Processes with Participation of Excited Singlet and Triplet States in Sterically Hindered Porphyrins and Their Chemical Dimers

steric interactions and porphyrin non-planarity remained to be not studied yet. We found for the first time that mono- and di-meso-phenyl substitution in octaethylporphyrins led to a drastic shortening of T 1 decays (∼1.5 ms → 2–5 µs) in deaerated solutions at 295 K without considerable influence on spectral-kinetic parameters of singlet states. These effects have been systematically studied for porphyrins and their chemical dimers with a controllable structure of meso-phenyl substituents as well as for meso-phenyl-substituted octaarylporphyrins with increasing number (n=1÷4) of meso-phenyl rings. In some cases, quantum-chemical calculations have been used to explain experimental findings. Paper cont ains also some recent results showing what circumstances should be taken into account when using these compounds in various areas (excited state deactivation in multiporphyrin complexes, photoinduced electron transfer, singlet oxygen generation).

Sulphur bridged [22]annulene[2.1.2.1] based organic field-effect transistors: interplay of the steric bulk and charge transport

New, neutral, slightly puckered aromatic meso-substituted tetrathia[22]porphyrin(2.1.2.1) macrocyclic architectures display efficacious p-type semiconductor behaviour with improved on/off ratios.

Comparative analysis of the influence of bulky β-alkyl substituents on fluorescent properties of some series of spatially distorted meso-phenyl substituted porphyrins

We present results of our systematic investigation and comparative analysis of the spectral-kinetic properties of absorption and fluorescence of two series of related planar porphyrins in liquid media at 293 K, in which deviations from the planarity are caused by contact steric interactions of eight peripheral β-pyrrole methyl (−CH3) or ethyl (−C2H5) groups with a successively increasing number (from one to four) and varying position of meso-phenyl (Ph) substituents in the porphyrin macrocycle. It is substantiated that considerable differences between the spectral-kinetic properties of absorption and fluorescence of the octaethyl- and octamethylporphyrin molecules, which have the same number of meso-phenyl substituents, are caused by fundamentally different roles played by β-ethyl and β-methyl groups, which differ in volume and which determine differences in the character of steric interactions and the efficiency of the dynamic relaxation of the tetrapyrrole macrocycle. It is revealed that there is a difference between calculated f theor and experimentally determined f exp values of the fluorescence probability, which is caused by conformational rearrangements of spatially distorted porphyrin macrocycles in the S 1 excited state, which reduce the oscillator strength and lower the energy of the long-wavelength transition S 0 → S 1. It is found that the chemical nature, the size, and the number of bulky -CH3 or -C2H5 β-pyrrole groups affect weakly rate constants of fluorescence quenching by molecular oxygen and do not create noticeable steric hindrances for contact interactions of the oxygen molecule with the π-conjugated system of the tetrapyrrole macrocycle.

Manganese porphyrin in solution and heterogenized in different materials mediates oxidation of hydrocarbons by iodosylbenzene

We prepared a free base porphyrin (HP) containing hydroxy and methoxy groups in the mesophenyl substituents of the porphyrin ring. We then inserted Mn(III) ion into the resulting HP, to obtain MnP. We used silica synthesized by the sol–gel process to immobilize the MnP in either acidic or basic medium, which afforded the solids MnPA and MnPB, respectively. We also conducted the solvothermal Mn(III) ion insertion into HP, which furnished the self-structured solid MnPS. Because the MnP was catalytically active in homogeneous medium, we investigated the activity of all the prepared solids as heterogeneous catalysts in the oxidation of various hydrocarbons. MnPA, MnPB, and MnPS presented similar or higher activity than the homogeneous MnP in the oxidation of cyclooctene, cyclohexene, cyclohexane and n-heptane. Concerning alkane oxidation, the heterogeneous catalysts were more selective for the alcohol than the MnP in homogeneous solution. Moreover, the solid catalysts favored the oxidation of linear alkane, with selectivity toward the primary alcohol. We were able to recover all the solids at the end of the reaction and reuse them.

New meso-substituted trans-A2B2 di(4-pyridyl)porphyrins as building blocks for metal-mediated self-assembling of 4 + 4 Re(i)–porphyrin metallacycles

The reaction between 5-(4-pyridyl)dipyrrylmethane and aromatic aldehydes affords meso-arylsubstituted trans-A2B2 di(4-pyridyl)porphyrins which are key building blocks in the metal-mediated self-assembling of supramolecular structures. A careful optimization of the reaction conditions allowed us to obtain 5,15-diphenyl-10,20-di(4-pyridyl)porphyrin (P1), and two analogues bearing on the meso-phenyl substituents two dipropyl- (P4) or dihexyl-alkyl chains (P5), with yields ranging from 53 to 63%. Porphyrin P1 reacts with Re(CO5)Br to give the expected 4 + 4 Re(I)-porphyrin metallacycle which has been fully characterized by means of infrared, NMR and UV-Vis (absorption and emission) spectroscopies and by guest inclusion studies. Unexpectedly the addition of alkyl chains to the porphyrin fragment, which increase the solubility of the porphyrin in organic solvents, has the opposite effect on the adduct with Re(I). Indeed, the reaction between Re(CO5)Br and porphyrins P4,5 gives very insoluble materials, hampering their complete characterization.