Ortho-substituted Research Articles

An Oligomer Approach for Blue Thermally Activated Delayed Fluorescent Emitters Based on Twisted Donor-Acceptor Units.

The development of efficient blue donor-acceptor thermally activated delayed fluorescence (TADF) emitters remains a challenge. To enhance the efficiency of TADF-related processes of the emitter, we targeted a molecular design that would introduce a large number of intermediate triplet states between the lowest energy excited triplet (T1) and singlet (S1) excited states. Here, we introduce an oligomer approach using repetitive donor-acceptor units to gradually increase the number of quasi-degenerate states. In our design, benzonitrile (BN) moieties were selected as acceptors that are connected together via the amine donors, acting as bridges to adjacent BN acceptors. To preserve the photoluminescence emission wavelength across the series, we employed a design based on an ortho substitution pattern of the donors about the BN acceptor that induces a highly twisted conformation of the emitters, limiting the conjugation. Via a systematic photophysical study, we show that increasing the oligomer size allows for enhancement of the intersystem crossing and reverse intersystem crossing rates. We attribute the increasing intersystem crossing rate to the increasing number of intermediate triplet states along the series, confirmed by the time-dependent density functional theory. Overall, we report an approach to enhance the efficiency of TADF-related processes without changing the blue photoluminescence color.

Tumor-Targeting Near-Infrared Dimeric Heptamethine Cyanine Photosensitizers with an Aromatic Diphenol Linker for Imaging-Guided Cancer Phototherapy.

Phototherapy is considered a promising alternative to conventional tumor treatments due to its noninvasive modality and effective therapeutic effect. However, designing a photosensitizer with satisfactory therapeutic effect and high security remains a considerable challenge. Herein, a series of dimeric heptamethine cyanine photosensitizers with an aromatic diphenol linker at the meso position is developed to improve the photothermal conversion efficiency (PCE). Thanks to the extended conjugate system and high steric hindrance, the screened 26NA-NIR and 44BP-NIR exhibit high PCE (≈35%), bright near-infrared (NIR) fluorescence, excellent reactiveoxygenspecies(ROS) generation capability, and improved photostability. Furthermore, their outstanding performance on imaging-guided PDT-PTT synergistic therapy is demonstrated by in vivo and in vitro experiments. In conclusion, this study designs a series of dimeric heptamethine cyanine photosensitizers and presents two compounds for potential clinical applications. The strategy provides a new method to design NIR photosensitizers for imaging-guided cancer treatment.

Combined Omics Approach Reveals Key Differences between Aerobic and Microaerobic Xylene-Degrading Enrichment Bacterial Communities: Rhodoferax─A Hitherto Unknown Player Emerges from the Microbial Dark Matter.

Among monoaromatic hydrocarbons, xylenes, especially the ortho and para isomers, are the least biodegradable compounds in oxygen-limited subsurface environments. Although much knowledge has been gained regarding the anaerobic degradation of xylene isomers in the past 2 decades, the diversity of those bacteria which are able to degrade them under microaerobic conditions is still unknown. To overcome this limitation, aerobic and microaerobic xylene-degrading enrichment cultures were established using groundwater taken from a xylene-contaminated site, and the associated bacterial communities were investigated using a polyphasic approach. Our results show that the xylene-degrading bacterial communities were distinctly different between aerobic and microaerobic enrichment conditions. Although members of the genus Pseudomonas were the most dominant in both types of enrichments, the Rhodoferax and Azovibrio lineages were only abundant under microaerobic conditions, while Sphingobium entirely replaced them under aerobic conditions. Analysis of a metagenome-assembled genome of a Rhodoferax-related bacterium revealed aromatic hydrocarbon-degrading ability by identifying two catechol 2,3-dioxygenases in the genome. Moreover, phylogenetic analysis indicated that both enzymes belonged to a newly defined subfamily of type I.2 extradiol dioxygenases (EDOs). Aerobic and microaerobic xylene-degradation experiments were conducted on strains Sphingobium sp. AS12 and Pseudomonas sp. MAP12, isolated from the aerobic and microaerobic enrichments, respectively. The obtained results, together with the whole-genome sequence data of the strains, confirmed the observation that members of the genus Sphingobium are excellent aromatic hydrocarbon degraders but effective only under clear aerobic conditions. Overall, it was concluded that the observed differences between the bacterial communities of aerobic and microaerobic xylene-degrading enrichments were driven primarily by (i) the method of aromatic ring activation (monooxygenation vs dioxygenation), (ii) the type of EDO enzymes, and (iii) the ability of degraders to respire utilizing nitrate.

Spectroscopic Characterization of the Divalent Metal Docking Motif to Isolated Cyanobenzoate: Direct Observation of Tridentate Binding to ortho-Cyanobenzoate and Implications for the CN Response

Cryogenic ion vibrational spectra of D2-tagged cyanobenzoate (CBA) derivatives are obtained and analyzed to characterize the intrinsic spectroscopic responses of the -CO2- headgroup to its location on the ring in both the isolated anions and the cationic complexes with divalent metal ions, M2+ (M = Mg, Ca, Sr). The benzonitrile functionality establishes the different ring isomers (para, meta, ortho) according to the location of the carboxylate and provides an additional reporter on the molecular response to the proximal charge center. The aromatic carboxylates display shifts slightly smaller than those observed for a related aliphatic system upon metal ion complexation. Although the CBA anions display very similar band patterns for all three ring positions, upon complexation with metal ions, the ortho isomer yields dramatically different spectral responses in both the -CO2- moiety and the CN group. This behavior is traced to the emergence of a tridentate binding motif unique to the ortho isomer in which the metal ions bind to both the oxygen atoms of the carboxylate group and the N atom of the cyano group. In that configuration, the -CO2- moiety is oriented perpendicular to the phenyl ring, and the CN stretching fundamental is both strong and red-shifted relative to its behavior in the isolated neutral. The behaviors of the metal-bound ortho complexes occur in contrast to the usual blue shifts associated with "Lewis" type binding of metal ions end-on to -CN. The origins of these spectroscopic features are analyzed with the aid of electronic structure calculations, which also explore differences expected for complexation of monovalent cations to the ortho carboxylate. The resulting insights have implications for understanding the balance between electrostatic and steric interactions at metal binding sites in chemical and biological systems.

Making and Breaking—Insight into the Symmetry of Salen Analogues

This study focuses on selected members of the general salen-analogues family possessing two O-H⋯N hydrogen bonds, namely three isomers of N,N’-bis(salicylidene)-X-phenylenediamine, denoted as ortho, meta and para. Two of the isomers are not planar in the published crystal structures. The current study tackles the problem of symmetry and interactions within the molecules, as well as in the crystal lattice. The aromaticity of the phenyl rings is evaluated using the Harmonic Oscillator Model of Aromaticity (HOMA) index. Intra- and inter-molecular non-covalent interactions are studied via Hirshfeld surface analysis, Independent Gradient Model (IGM), Quantum Theory of Atoms in Molecules (QTAIM), Non-Covalent Interaction (NCI) index, Electron Localisation Function (ELF), Core-Valence Bifurcation (CVB) index and Symmetry-Adapted Perturbation Theory (SAPT). Density Functional Theory (DFT) simulations were carried out in vacuo and with solvent reaction field based on Polarisable Continuum Model (IEF-PCM formulation) at the ωB97XD/6-311+G(2d,2p) level. Crystal structure analyses were performed for the data reported previously in the literature. The obtained results demonstrate that the three isomers differ greatly in their structural properties (molecular symmetry is broken for the ortho and meta isomers in the solid state) and ability to form intermolecular interactions, while retaining overall similar physico-chemical characteristics, e.g., aromaticity of the phenyl rings. It was found that the presence of the polar solvent does not significantly affect the structure of the studied compounds. An application of the Hirshfeld surface analysis revealed the nature of the non-covalent interactions present in the investigated crystals. The SAPT results showed that the stability of the dimers extracted from the crystals of the Schiff base derivatives arises from electrostatics and dispersion.

Synthesis and Studies of Dibenzofuran Embedded Bis‐(Calix)‐Pentaphyrins

AbstractCondensation of dibenzofuran based tripyrrane with dipyrroethene diol under acid catalyzed porphyrin forming conditions resulted in the formation of dibenzofuran embedded calix pentaphyrins with two meso‐sp3 carbons instead of expected fully conjugated dibenzofuran (DBF) embedded pentaphyrins. Three examples of DBF embedded calix pentaphyrins have been synthesized by adopting the [3+2] condensation methodology and characterized by various spectroscopic and theoretical methods. The NMR studies that include COSY, NOESY, HMBC and HSQC clearly confirmed the molecular structures of DBF embedded bis‐(calix)‐pentaphyrins. The optimized structures showed that the DBF embedded bis‐(calix)‐pentaphyrin macrocycles adopted figure eight conformation due to presence of two sp3 carbon atoms (C1 and C22) at meso positions which obstructed the conjugation throughout the macrocycle. The absorption spectra of macrocycles showed non‐aromatic features with a sharp strong band at 480 nm and a broad band in the region of 600–800 nm which were significantly red shifted with distinct colour change in protonated derivatives of DBF embedded bis‐(calix)‐pentaphyrins.

Cardiac myosin velocity and force are dramatically improved with an alternative triphosphate substrate.

Cardiac myosin is the molecular motor that drives the pumping action of the heart; therefore, there is strong interest in gaining control over its function in an effort to develop more effective treatments for various forms of heart disease. We sought to determine if the force and motion generating capacity of β-cardiac myosin could be altered in vitro by replacing ATP with different isomers of an alternative energy source, azobenzene triphosphate (AzoTP). Using the ortho-isomer of AzoTP, cardiac myosin moved actin filaments 62% faster than ATP in a motility assay (p<0.001). Subsequent experiments suggested that the increased velocity may be the result of an accelerated rate of ortho-AzoDP-release from myosin. Cardiac myosin also generated 10% more force when using ortho-AzoTP compared to ATP in a laser trap assay (p<0.001), which was likely due to an increased rate of attachment to actin because the frequency of single actomyosin binding events observed in a three-bead laser trap assay was double that observed for ortho-AzoTP vs. ATP (p<0.001). In contrast to ortho-AzoTP, with the meta isomer of AzoTP cardiac myosin translocated actin filaments 90% slower than with ATP (p<0.001). This finding is likely do to a slowed rate of detachment from actin, based on preliminary single molecule findings. With the para-isomer of AzoTP no movement was detected in the motility assay, suggesting it may not bind to the nucleotide-binding site. Thus, these findings demonstrate that the function of beta-cardiac myosin can be manipulated using structural isomers of an alternative energy substrate (AzoTP). These findings therefore may provide important new insights into how to alter the function of cardiac myosin in vivo, which could help in the development of novel therapies to different forms of heart disease.

Mechanistic Insights into Molecular Crystalline Organometallic Heterogeneous Catalysis through Parahydrogen-Based Nuclear Magnetic Resonance Studies.

The heterogeneous solid-gas reactions of crystals of [Rh(L2)(propene)][BArF4] (1, L2 = tBu2PCH2CH2PtBu2) with H2 and propene, 1-butene, propyne, or 1-butyne are explored by gas-phase nuclear magnetic resonance (NMR) spectroscopy under batch conditions at 25 °C. The temporal evolution of the resulting parahydrogen-induced polarization (PHIP) effects measures catalytic flux and thus interrogates the efficiency of catalytic pairwise para-H2 transfer, speciation changes in the crystalline catalyst at the molecular level, and allows for high-quality single-scan 1H, 13C NMR gas-phase spectra for the products to be obtained, as well as 2D-measurements. Complex 1 reacts with H2 to form dimeric [Rh(L2)(H)(μ-H)]2[BArF4]2 (4), as probed using EXAFS; meanwhile, a single-crystal of 1 equilibrates NMR silent para-H2 with its NMR active ortho isomer, contemporaneously converting into 4, and 1 and 4 each convert para-H2 into ortho-H2 at different rates. Hydrogenation of propene using 1 and para-H2 results in very high initial polarization levels in propane (>85%). Strong PHIP was also detected in the hydrogenation products of 1-butene, propyne, and 1-butyne. With propyne, a competing cyclotrimerization deactivation process occurs to afford [Rh(tBu2PCH2CH2PtBu2)(1,3,4-Me3C6H3)][BArF4], while with 1-butyne, rapid isomerization of 1-butyne occurs to give a butadiene complex, which then reacts with H2 more slowly to form catalytically active 4. Surprisingly, the high PHIP hydrogenation efficiencies allow hyperpolarization effects to be seen when H2 is taken directly from a regular cylinder at 25 °C. Finally, changing the chelating phosphine to Cy2PCH2CH2PCy2 results in initial high polarization efficiencies for propene hydrogenation, but rapid quenching of the catalyst competes to form the zwitterion [Rh(Cy2PCH2CH2PCy2){η6-(CF3)2(C6H3)}BArF3].

Tuning the isomer composition is a key to overcome the performance limits of commercial benzyltoluene as liquid organic hydrogen carrier

This work is the outcome of a three-year journey to develop a technically feasible way to overcome the limits of the benzyltoluene (H0-BT) and perhydro benzyltoluene (H12-BT) pair, recognized as an attractive liquid organic hydrogen carrier (LOHC) system. In the first attempt to synthesize and evaluate three H0-BT regioisomers, the para isomer showed faster H2 storage and release rates than the ortho and meta counterparts. This was attributed to its favorable adsorption capacity and strength on the catalyst surface. The subsequent evaluation of binary and ternary H0-/H12-BT isomeric mixtures, prepared by physical mixing the synthesized isomers, suggested that abundant amounts of para isomer in the mixture were needs to maximize the LOHC performance. Thus, a H0-BT mixture with 72 % para isomer was successfully synthesized using the zeolite H-Beta under optimized conditions, resulting in a faster kinetics of both H2 storage and release than commercial Marlotherm LH. Cycling tests on this mixture indicated that the formation of methylfluorene as a dehydrogenation byproduct was detrimental to the hydrogenation reaction. By reducing the formation of methylfluorene to a level of 3 % using a sulfur-decorated Pt/Al2O3 catalyst, a gravimetric H2 storage density of 6 wt% could be maintained for 10 cycles. Finally, a cost analysis confirmed that the isomeric composition of H0-BT in the mixture should be tuned to increase the para isomer content, while the raw materials for H0-BT synthesis are cheap. The step-by-step approach reported herein will pave the way for the development of novel aromatic LOHC systems in the near future.

Isomer-Dependent Threshold Photoelectron Spectroscopy and Dissociative Photoionization Mechanism of Anisaldehyde.

We studied the threshold photoionization and dissociative ionization of para-, meta-, and ortho-anisaldehyde by photoelectron photoion coincidence spectroscopy in the 8.20-19.00 eV photon energy range. Vertical ionization energies by equation of motion-ionization potential-coupled cluster singles and doubles (EOM-IP-CCSD) calculations reproduce the photoelectron spectral features in all three isomers. The dissociative photoionization (DPI) pathways of para- and meta-anisaldehyde are similar and differ markedly from those of ortho-anisaldehyde. In the para and meta isomers, the lowest-energy DPI channel corresponds to hydrogen atom loss to form the C8H7O2+ fragment at m/z 135, which undergoes sequential dissociation processes at higher energies, such as carbon monoxide loss to C7H7O+ (m/z 107) and further, sequential CH3, CH2O, and CH2CO losses to produce C6H4O+ (m/z 92), C6H5+ (m/z 77), and C5H5+ (m/z 65), respectively. Carbon monoxide loss from the parent ions, yielding C7H8O+ (m/z 108), is a subordinate dissociation channel parallel to H atom loss. At higher energies, it also gives rise to sequential formaldehyde (CH2O) loss to produce C6H6+ (m/z 78). In the ortho-anisaldehyde cation, the vicinity of the aldehyde and methoxy groups opens up low-energy hydrogen-transfer processes, which allow for seven fragmentation channels to compete effectively with the H- and CO-loss channels. Thus, the fragmentation mechanism changes considerably, thanks to the steric interaction of the substituents. Functional group interactions, in particular H transfer pathways, must therefore be considered when predicting the isomer-specific unimolecular decomposition mechanism of cationic and neutral species, as well as mass spectra for isomers.

Design, synthesis, antitumor activity and ct-DNA binding study of photosensitive drugs based on porphyrin framework

Photodynamic therapy is a promising novel tumor treatment method. In this study, novel porphyrin-chrysin photosensitizer derivatives were synthesized. Most of the compounds showed antitumor activity against human cervical cancer HeLa cells and human lung cancer A549 cells, among which compound 4c had the best photodynamic therapy effect on HeLa cells and A549 cells, with IC50 values of 6.26 μM and 23.37 μM, respectively. Free-base porphyrin-chrysin derivatives bind to DNA through surface self-stacking, and zinc metalloporphyrin-chrysin derivatives bind to ct-DNA through intercalation. Notably, the tightness of compound binding to ct-DNA was positively correlated with its antitumor activity. What's more, three-dimensional quantitative conformation studies have shown that increasing the positive charge of the porphyrin ring and introducing a strong electron-withdrawing group at the meso position of the porphyrin ring at the para-position of the benzene ring or reducing the space volume of the compound can enhance the antitumor activity.

Near-Infrared-Emitting Meso-Substituted Heptamethine Cyanine Dyes: From the Synthesis and Photophysics to Their Use in Bioimaging

Heptamethine cyanine dyes were synthesized in good yields by the reaction between quaternary indoles and a pentamethinic salt, under mild reaction conditions minimizing photooxidation. These compounds were used as precursors to prepare meso-substituted derivatives. The cyanine dye precursors presented UV-Vis absorption, related to fully allowed electronic transitions and fluorescence emission in the NIR region, without any evidence of aggregation in both ground and excited states. The substitution at the meso position showed a fundamental role in their photophysics, with the main absorption in the green-orange region related to the monomeric species. Moreover, the excited state photophysics presented emission profiles dependent on the excitation wavelengths, complicating the correlation of spectroscopy and structure. Density Functional Theory and OO-SCS-MP2 calculations under different solvation conditions revealed the heavy impact of conjugation effects on ground and excited states’ geometries and electronic configurations of these compounds. Finally, the observed photophysical features of the meso-substituted heptamethine cyanine dyes were successfully used to explore their application as fluorescent probes in biological media, allowing stable staining in live and fixed cells.

Dipyrromethane-diphosphine: the effect of meso substituents on the formation of nickel complexes and on their performance in the transfer hydrogenation of ketones.

Three dipyrromethane-diphosphine ligands containing phenyl (L1H2), ethyl (L2H2) and cyclohexyl (L3H2) groups at their meso positions and their nickel complexes were synthesized and structurally characterized. Treatment of Ph2C(C4H3N)2-1,9-(CH2PPh2)2 (L1H2) with [NiCl2(DME)] gave complex [NiCl2(κ2-P,P-L1H2)] 2a. Conversely, the analogous reactions of L2H2 and L3H2 with [NiCl2(DME)] showed a mixture of products containing both a pyrrolide nitrogen coordinated complex of type [Ni(κ4-P,N,N,P-L)] 3 without an exogenous base and a chelated complex of type 2a. In addition, all three ligands react with [NiCl2(DME)] in the presence of a strong base to give a complex of type 3. Furthermore, a novel binuclear Ni(0) complex bearing L1H2 was characterized by X-ray crystallography. Both complexes 2a and 3 (0.5 mol% of loading) catalyze the transfer hydrogenation of a series of aromatic and aliphatic ketones (20 substrates) to their corresponding secondary alcohols using iPrOH as a hydrogen source in the presence of KOH at 100 °C in 6 h. The kinetic trace of the catalytic reaction shows that the meso-phenyl substituted diphosphine coordinated nickel complexes perform better than the other two ligand coordinated nickel complexes.

Synthesis, structure and spectroscopic properties of BODIPY dyes incorporating the pentafluorosulfanylphenyl group

Six compounds (BD1–BD6) are discussed based on the borondipyrromethene (BODIPY) core and functionalised in the meso position or the 3,5-positions with pentafluorosulfanylaryl subunits.

Introducing proton/electron mediators enhances the catalytic ability of an iron porphyrin complex for photochemical CO2 reduction.

A novel iron porphyrin complex with hydroquinone moieties as proton/electron mediators at meso positions was designed and synthesised. The complex serves as an efficient catalyst for photochemical CO2 reduction, and its turnover frequency (TOF = 1.3 × 104 h-1) was the highest among those of comparable systems with sufficient durability.

Synthesis, aggregation, and photophysical properties of amphiphilic BODIPY-Pt(II) metallacycles

The construction of metallo-organic macrocycles using transition metals and optoelectronically active donor ligands is a very active area of research due to the potential applicability of these structures as sensors or as drugs for combined photodynamic/chemotherapy treatments. Herein, we report on the synthesis, photophysical properties, and aggregation behavior of a triangular Pt(II)-assembled metallo-supramolecular complex built by the coordination-driven assembly of a BODIPY functionalized with two pyridines at the 2,6 positions and an (R, R)-phenylalanine dipeptide moiety in the meso position. This hydrophilic group provides the metallacycle with water-solubility and self-assembly capabilities, which result in the formation of robust nanoparticles in aqueous solutions with a 50 nm average size.

Effects of functional groups in iron porphyrin on the mechanism and activity of oxygen reduction reaction.

The activity of the oxygen reduction reaction (ORR) on the cathode is one of the dominant factors in the performance of proton exchange membrane fuel cells. Iron porphyrin has low cost, environmental benignity, and maximum efficiency of metal usage. Therefore, this material can be a promising single-atomic metal dispersion catalyst for fuel cell cathodes. The variation of functional groups was proven to effectively modify the activity of the ORR on the iron porphyrin. However, the influences of functional groups on the mechanisms of the ORR remain ambiguous. This work paid attention to the substitution of carboxyl (-COOH), methyl (-CH3), and amino (-NH2) functional groups at the meso positions of the porphyrin ring. By using van der Waals density functional theory (vdW-DF) calculations, we found that the ORR mechanisms can follow the associative and dissociative pathways, respectively. The Gibbs free energy diagrams revealed that the rate-limiting step occurs at the second hydrogenation step for the first pathway and the O2 dissociation step for the second pathway for all considered functional groups. The thermodynamic energy barrier at the rate-limiting step was found to be in the following order: porphyrin-(CH3)4 < porphyrin-(NH2)4 < original porphyrin < porphyrin-(COOH)4 for the associative mechanism and porphyrin-(NH2)4 < porphyrin-(CH3)4 < porphyrin-(COOH)4 < original porphyrin for the dissociative pathway. The findings suggested that porphyrin-(CH3)4 and porphyrin-(NH2)4 should be the best choices among the considered substrates for the oxygen reduction reaction. Furthermore, the interaction between the ORR intermediates and the substrates was attributed to the resonance of the d z 2 , d xz , and d yz components of the Fe d orbital and the C and N p orbitals of the substrates with the p orbitals of the oxygen atoms in the intermediates. Finally, the nature of the interaction between the adsorbent and adsorbate was charge transfer.

Halogen-free photosensitizers based on meso-enamine-BODIPYs for bioimaging and photodynamic therapy.

The search for efficient heavy atom free photosensitizers (PSs) for photodynamic therapy (PDT) is a very active field. We describe herein a simple and easily accessible molecular design based on the attachment of an enamine group as an electron-donor moiety at the meso position of the BODIPY core with different alkylation patterns. The effect of the alkylation degree and solvent polarity on the photophysical properties in terms of splitting absorption bands, fluorescence efficiencies and singlet oxygen production is analyzed in depth experimentally using spectroscopic techniques, including femtosecond and nanosecond transient absorption (fs- and ns-TA) and using computational simulations based on time-dependent density functional theory. The correlation between the theoretical/experimental results permits the rationalization of the observed photophysical behavior exhibited by meso-enamine-BODIPY compounds and the determination of mechanistic details, which rule the population of the triplet state manifold. The potential applicability as a theragnostic agent for the most promising compound is demonstrated through in vitro assays in HeLa cells by analyzing the internalization, localization and phototoxic action.

Pentafluorosulfanyl-functionalised BODIPY push-pull dyes for p-type dye-sensitized solar cells.

We report a push-pull BODIPY-based dye functionalised with an electronegative SF5 group at the meso position for applications in photocathodes in tandem dye-sensitized solar cells (DSSCs). The push-pull character enhances charge-transfer from the mesoporous NiO cathode surface towards the redox mediator. A Knoevenagel condensation reaction was used to introduce the carboxylic acid to anchor the dye to the oxide surface, via a styryl linker which increases the conjugation in the molecule and shifts the absorption to the red. The room-temperature synthesis and high yields, make the dye promising for manufacture on a large scale. The dye was applied in p-DSSCs giving a power conversion efficiency (0.066%), a short circuit photocurrent (J SC) of 3.84 mA cm-2, open circuit voltage (V OC) of 58 mV and fill factor of 30%.

Solid state electrochemical behaviour of porphine in aqueous media

The electrochemical behaviour of porphine was studied by cyclic, differential pulse and square-wave voltammetry, in aqueous media, in the pH range between 3.4 and 9.2. The studies were carried out by solid state voltammetry with the insoluble porphine mechanically transferred onto the glassy carbon electrode. The porphine redox mechanism was established by comparison with the redox behaviour of pyrrole, 1,4-dimethylpyridinium p-toluenesulfonate, and two meso-substituted porphyrins, 5,10,15,20-tetraphenyl-21H,23H-porphine (TPP) and 5,10,15,20-tetra(4-pyridyl)-21H,23H-porphine (TPyP). The porphine oxidation is a two-step, irreversible and pH-dependent process, each oxidation step involving the loss of one electron from the lone pairs in the nitrogen atoms. The porphine reduction is a one-step, irreversible and pH-dependent process, involving the acceptance of one electron and one proton. A second reduction step at more negative potentials is probable, but cannot be detected due to the limitation of the GCE potential range. The presence of phenyl and pyridyl substituents at meso position disturbs the electron density and the planarity of the porphine ring, which results in a shift of the porphine redox potentials to more positive values, consistent with a more difficult oxidation and a facilitated reduction, as observed for TPP and TPyP.