Aza Nitrogen Research Articles

Template Synthesis, Spectroscopic Characterization and Antimicrobial Studies of Oxovanadium(IV) Tetraazamacrocyclic Complexes Derived from 1, 8-Diaminonaphthalene and Substituted β-Diketones

A new series of oxovanadium (IV) tetraazamacrocyclic complexes of type [VO(mac)]SO4, have been synthesized via in situ reaction of 1,8-diaminonaphthalene and substituted β-diketones (1-phenyl-3-(phenyl/4-chlorophenyl/4-hydroxyphenyl/4- methoxyphenyl/4-nitrophenyl)-diketone) in the presence of oxovanadium(IV) sulphate in methanol. Attempts to synthesize the corresponding metal free macrocyclic ligands did not prove successful. Complexes were characterized on the basis of elemental analyses, conductance measurements, magnetic properties and spectral (electronic, IR, and EPR) data. The magnetic moments of the complexes lie in the range 1.70-1.78BM suitable for complexes with one unpaired electron and paramagnetic nature. The infrared spectra indicates that the ligands coordinate through four aza nitrogen atoms to vanadium(IV). The fluid solution EPR spectra show an eight-line pattern typical for a mononuclear VO(IV) complexes. The spectral studies support a square pyramidal geometry for the oxovanadium(IV) complexes. The oxovanadium complexes were tested for their antibacterial activity against bacteria Bacillus subtilis, Staphylococcus aureus, Escherichia coli and antifungal activity against fungi Aspergillus niger and Candida albicans. The complexes show moderate to good activity as compared to standard antibiotic tetracycline and the antifungal drug fluconazole.

Precise construction of lithiophilic sites by diyne-linked phthalocyanine polymer for suppressing metallic lithium dendrite.

Uncontrolled growth of lithium dendrite is the key challenge that impedes the practical application of Li anodes in high-energy-density Li-metal batteries. Precisely constructing lithiophilic active sites on the anode surface is expected to be an effective strategy for promoting the anode interfacial properties and alleviating the dendrite growth of lithium. Herein, a diyne-linked phthalocyanine polymer (PcEP) with precise lithiophilic active sites is designed and constructed in a bottom-up manner in situ on the surface of the copper foil via the coupling reaction of tetraethynylphthalocyanine. The lithiophilic electron-rich pyrrolic nitrogen and aza nitrogen in the Pc structure, and the sp-hybridized carbon in the diyne linkage (-CC-CC-) in PcEP can conduct the homogeneous nucleation and deposition processes of lithium, and thus suppress the dendrite growth. This dendrite-free metallic lithium anode exhibits reduced overpotential, high coulombic efficiency (98.6%), and prolonged lifespan (200% longer than that of a Cu anode). These impressive achievements demonstrate that the advanced phthalocyanine polymer might be a promising material for addressing the critical interfacial issues related to the next-generation high-energy-density Li-metal-based storage devices.

Synthesis and Characterization of New Zinc Phthalocyanine - Dodecenyl Succinic Anhydride Benzoic Groups.

The phthalocyanines a series of compounds involves four iso-indole units linked by aza nitrogen atoms bonded with metal atoms that are normally located in the center a phthalocyanines ring. Some of the central metal-phthalocyanines can be excited by ultraviolet light and emit a fluorescence in far-red region. To synthesize a derivative of phthalocyanines namely 4,4',4' '-tri-(dodecenyl succinic anhydride)- 4' ' '-(5-amino salicylic acid) zinc phthalocyanine with a zinc central metal. The reaction of 4- nitro Phthalonitrile and 4- amino Phthalonitrile with ZnCl2 in the presence of dimethyl amino ethanol afforded 4,4',4' '-triamino-4' ' '-nitro zinc phthalocyanine. This product reacted with 5-amino salicylic acid to yield tetra-(5-amino salicylic acid) zinc phthalocyanine. A dodecenyl succinic anhydride was added on the amine group of benzoic rings to afford 4,4',4' '-tri-(dodecenyl succinic anhydride)-4' ' '-(5-amino salicylic acid) zinc phthalocyanine(I), the target compound. Compound I is successfully synthesized with a yield of 72% from tetra-(5-amino salicylic acid) zinc phthalocyanine with dodecenyl succinic anhydride. The newly synthesized molecule of 4,4',4' '-tri-(dodecenyl succinic anhydride)-4' ' '-(5-amino salicylic acid) zinc phthalocyanine (I), tetra-(5-amino salicylic acid) zinc phthalocyanine(E) and 4,4',4' '- triamino-4' ' '-nitro zinc phthalocyanine (S). The reaction of 4- nitro Phthalonitrile and 4- amino and the structure of compound I is confirmed and its formation was proven.

Theoretical investigation of the structure, detonation properties, and stability of bicyclo[3.2.1]octane derivatives.

A series of nitro group and aza nitrogen atom derivatives, based on bicyclo[3.2.1]octane, were designed and studied by theoretical methods. The geometric structure calculations were performed at B3LYP/6-311G(d,p), B3P86/6-311G(d,p), B3LYP/6-31G(d), and B3PW91/6-31G(d,p) levels. The electrostatic potential analysis, heats of formation, densities, heats of sublimation, detonation performances, bond dissociation energies (BDEs), and impact sensitivities of the designed compounds were calculated by reasonable calculation methods to evaluate their comprehensive properties and establish the relationship between structure and performance. Results show that density and detonation properties always increase with the increasing number of nitro groups and aza nitrogen atoms. BDEs generally decrease with the increasing number of nitro groups. Except for BDEs of A9, B9, and D8, BDEs of all designed compounds are larger than 20 kcal mol-1 and meet the requirement for new high energy density compounds (HEDCs). Two theoretical prediction methods show all the designed compounds have an acceptable impact sensitivity. Detonation velocity and detonation pressure were predicted in the range of 5.24-9.59 km/s and 9.97-43.44 GPa, respectively. Eleven compounds have better detonation properties than HMX, and seven compounds meet the criteria for HEDCs. Taking thermal stability and impact sensitivity into consideration, four compounds (C9, E8, F8, and G7) may be considered as new potential HEDCs, and C9, E8, and F8 may have similar detonation properties to the famous CL-20. Graphical abstractFour new HEDCs with acceptable impact sensitivity (C9, E8 and F8 may have similar detonation properties to the famous CL-20).

Exploring function activated chlorins using MCD spectroscopy and DFT methods: design of a chlorin with a remarkably intense, red Q band.

The electronic structures of three previously synthesized Ni-coordinated chlorins with β-substituents of thioketone, fluorene, and ketone were investigated using magnetic circular dichroism spectroscopy (MCD) and density functional theory (DFT) for potential application as sensitizers for dye-sensitized solar cells (DSSCs). Computational studies on modeled Zn-coordinated chlorins allowed identification of charge transfer and d-d transitions of the Ni2+ coordinated chlorins. Two fictive Zn chlorins, M1 and M2, were designed with thiophene units based on the fluorene substituted chlorin. Substitution with thiophene altered the typical arrangement of the four Gouterman molecular orbitals (MOs) and red-shifted and greatly intensified the lowest energy absorption band (the Q band). The introduction of the thiophene-based MO as the LUMO below the usual Gouterman LUMO is predicted to increase the efficiency of electron transfer from the dye to the conduction band of the semiconductor in DSSCs. The addition of a donor group on the opposite pyrrole (M2) red-shifted the Q band further and introduced a donor-based MO between the typical Gouterman HOMO and HOMO-1. Despite the relatively small ΔHOMO, M1 and M2 exhibited remarkably intense Q bands. M2 would be a possible candidate for application in DSSCs due to its panchromatic absorption, intense and red-shifted Q band, and the presence of the substituent based MO properties. Another indicator of a successful dye is the alignment of the ground state and excited state oxidation potentials (GSOP and ESOP, respectively) with respect to the conduction band of the semiconductor. The GSOP for M2 lies 0.55 eV below the I-/I3- redox potential and the ESOP lies 0.48 eV above the TiO2 conduction band. The impact of the thiophene dominance in the LUMO also supports the prediction of efficient sensitization properties. The remarkably intense Q band of M2 predicted to be at 777 nm with a ΔHOMO of just 1.04 eV provides a synthetic route to tetrapyrroles with extremely intense, red Q bands without the need for aza nitrogens of the phthalocyanines. This study illustrates the value of guided synthesis using MCD spectral analysis and computational methods for optimizing the design of porphyrin dyes.

Computational study of the structure and properties of bicyclo[3.1.1]heptane derivatives for new high-energy density compounds with low impact sensitivity.

To design new high-energy density compounds (HEDCs), a series of new bicyclo[2.2.1]heptane derivatives containing an aza nitrogen atom and nitro substituent were designed and studied theoretically. The density, heat of sublimation and impact sensitivity were estimated by electrostatic potential analysis of the molecular surface. Based on the designed isodesmic reaction, and the reliable heat of formation (HOF) of the reference compounds, HOFs were calculated and compared at B3LYP/6-311G(d,p) and B3P86/6-311G(d,p), respectively. The detonation performances, bond dissociation energies (BDE) and impact sensitivity were calculated to evaluate the designed compounds. The calculated results show that the number of aza nitrogen atoms and NO2 groups are two important factors for improving HOF, density and detonation properties. Thermal stability generally decreases with increasing nitro groups. And the N-NO2 bond is the trigger bond for all designed compounds except B8, whose trigger bond is C-NO2. Importantly, the BDE values are between 86.95 and 179.71kJmol-1 and meet the requirement for HEDCs. Detonation velocity and detonation pressure were found to be 5.77-9.65kms-1 and 12.30-43.64GPa, respectively. After comprehensive consideration of thermal stability, impact sensitivity and detonation properties, A7, A8, B8, C8, D7, E7, F7 and G6 may be considered as potential HEDCs. Especially, A8, B8, C8, and D7 have better detonation properties than the famous caged nitramine CL-20 (D = 9.40km/s, P = 42.00GPa). Besides, all the designed potential HEDCs have reasonable impact sensitivity. Graphical abstract New high-energy density compounds (HEDCs) with low impact sensitivity (A8, B8, C8 and D7 have better detonation properties than CL-20).

Beryllium subphthalocyanines self-assembling properties

Beryllium subphthalocyanines have been recently shown to be suitable candidates for photochemical devices if combined with appropriate donor systems. The ability of beryllium subphthalocyanines to self-assemble is explored for the first time by means of density functional theory calculations. Free dimers of beryllium subphtalocyanine and their corresponding complexes with water and pyridine are computed at the wB97X-D/6-311+G(d,p) level of theory. In contrast with the behavior reported for beryllium phthalocyanines, for beryllium subphtalocyanines, beryllium–aza–nitrogen intermolecular interactions are observed, suggesting that these species are likely to self-assemble. Aggregates of related structures such as beryllium subporphyrazines with axial groups confirm the importance of hydrogen bonds in the stacking.

Gas-phase basicity of aromatic azines: A short review on structural effects

A large number of experimental and computational gas-phase basicity data for monocyclic and polycyclic aromatic azines, including values taken from the recent literature, were examined to explore how internal (structural) effects influence gas-phase basicity (GB) and/or proton affinity (PA) of aza nitrogen. Substituent electron withdrawing effects on the pyridine ring may induce a decrease in PA as large as 200kJmol−1. Polarizability of alkyl and aryl groups increases experimental PA by up to 100kJmol−1, as in the case of 1- and 2-aza[6]helicene. The many computational studies involving azines, often performed in search of “superbasicity”, were reviewed. Calculations at the DFT level, carried out on chelating azine systems bearing strong electron donor substituents, yielded PAs much higher than the current upper limit of the experimental scale. The structural variety of aromatic azines, from hexaazabenzene to the phosphazeno derivative of azacalix[3](2,6)pyridine, generates aza-nitrogen PAs ranging from ca. 650 up to 1300kJmol−1.

Quadruple electron storage using visible light with nitrogen-heterocycles under metal-free conditions

A byproduct of the synthesis of 9,11,20,22-tetra-aza-tetrapyrido[3,2-a:2′3′-c:3″,2″-l:2′′′,3′′′]-pentacene (tatpp) is the symmetrical dimer ditatpp, which is linked by a carbon-carbon bond along the central benzene ring. The structure of the dimer has been determined by X-ray crystallography and reveals a dihedral angle of 73° between the two tatpp units, which is likely due lone pair repulsion on the adjacent aza nitrogens on each tatpp unit. Because of this non-planar geometry, this ditatpp dimer is freely soluble in organic solvents, such as ethanol, very much unlike the tatpp ligand, which is sparingly soluble in all common solvents. Photolysis of ditatpp with visible light in the presence of sacrificial donors, such as triethylamine, results in multi-electron reduction on each of the tatpp units, as determined by absorption spectroscopy. As the tatpp units appear to function independently of one another, and each tatpp unit is reduced by two electrons and is doubly protonated in the final compound, the photoreduction results in a net storage of 4 electrons in the fully reduced species.

Comparative theoretical studies of high energetic cyclic nitramines

Density functional theory studies on cyclic nitramines were performed at B3LYP/6‐311G(d,p) level. The crystal structures were obtained by molecular packing calculations. Heats of formation (HOFs) were predicted through designed isodesmic reactions. Results indicate that the value of HOF relates to the number of =N–NO2 group and aza nitrogen atom and increases with the augment of the number of =N–NO2 group and aza nitrogen atom for cyclic nitramines. Detonation performance was evaluated by using the Kamlet–Jacobs equations based on the calculated densities and HOFs. All the cyclic nitramines exhibit better detonation performance than 1,3,5‐trinitro‐1,3,5‐triazacyclohexane and 1,3,5,7‐tetranitro‐1,3,5,7‐tetraazacyclooctane. The stability of cyclic nitramines was investigated by the bond dissociation energies. The result shows that the increase of =N−NO2 group or aza nitrogen atom reduces the stability of the title compounds. These results provide basic information for molecular design of novel high energetic density materials. Copyright © 2013 John Wiley & Sons, Ltd.

Mechanism of Degradation and Improvement of Stability on Mesogenic-Phthalocyanine-Based Bulk Heterojunction Solar Cell

The stability of bulk heterojunction organic solar cells utilizing the phthalocyanine derivative 1,4,8,11,15,18,22,25-octahexylphthalocyanine (C6PcH2) and the fullerene derivative 1-(3-methoxy-carbonyl)-propyl-1-1-phenyl-(6,6)C61 has been studied. In the same environment, C6PcH2-based cells have shown higher stability than the cells fabricated using the conventional donor material poly(3-hexylthiophene). Furthermore, the degradation mechanisms of these solar cells have been investigated. It was found that the chemical bonds of two pyrrole aza nitrogens as well as the four mesobridging aza nitrogens with neighboring carbons in the C6PcH2 molecule were broken after irradiation with a solar simulator, which affected the device lifetime. To improve the stability, various buffer layers have been deposited between the counterelectrode and the active layer. Obviously, the C60 thin film and the oxidized layer play an important role as blocking layers that prevent the diffusion of metal atoms into the active layer, resulting in the higher stability.

Constructing Sandwich-Type Rare Earth Double-Decker Complexes with N-Confused Porphyrinato and Phthalocyaninato Ligands

Reaction of the half-sandwich complexes M(III)(Pc)(acac) (M = La, Eu, Y, Lu; Pc = phthalocyaninate; acac = acetylacetonate) with the metal-free N-confused 5,10,15,20-tetrakis[(4-tert-butyl)phenyl]porphyrin (H(2)NTBPP) or its N2-position methylated analogue H(CH(3))NTBPP in refluxing 1,2,4-trichlorobenzene (TCB) led to the isolation of M(III)(Pc)(HNTBPP) (M = La, Eu, Y, Lu) or Y(III)(Pc)[(CH(3))NTBPP] in 8-15% yield. These represent the first examples of sandwich-type rare earth complexes with N-confused porphyrinato ligands. The complexes were characterized with various spectroscopic methods and elemental analysis. The molecular structures of four of these double-decker complexes were also determined by single-crystal X-ray diffraction analysis. In each of these complexes, the metal center is octa-coordinated by four isoindole nitrogen atoms of the Pc ligand, three pyrrole nitrogen atoms, and the inverted pyrrole carbon atom of the HNTBPP or (CH(3))NTBPP ligand, forming a distorted coordination square antiprism. For Eu(III)(Pc)(HNTBPP), the two macrocyclic rings are further bound to a CH(3)OH molecule through two hydrogen bonds formed between the hydroxyl group of CH(3)OH and an aza nitrogen atom of the Pc ring or the inverted pyrrole nitrogen atom of the HNTBPP ring, respectively. The location of the acidic proton at the inverted pyrrole nitrogen atom (N2) of the protonated double-deckers was revealed by (1)H NMR spectroscopy.

A coumarin-based fluorescent PET sensor utilizing supramolecular pKa shifts

A new coumarin derivative containing benzothiazole and piperazine substituents was synthesized. Preferential inclusion of the benzothiazole group, over the coumarin and piperazine groups, inside the cavity of the molecular container cucurbit[7]uril (CB7) was evidenced by using optical and NMR techniques. The binding constant of the new complex with CB7 is higher in its protonated forms (e.g., K=2.8×106M−1) than in its neutral forms, which led to an increase in the pKa value associated with protonation of the aza nitrogen on the benzothiazole ring of ca. 2.5units. Such CB7-induced protonation disabled the photoinduced electron transfer (PET) in the included molecule, enhancing its coumarin fluorescence up to ca. 45-fold (pH 3.5, 410nm). The results are discussed in the context of designing sensitive analytical tools for reversible monitoring of optically inactive analytes by competitive displacement experiments.

Chemical Reactivity of the Imidazole: A Semblance of Pyridine and Pyrrole?

It has been suggested that pyridine and pyrrole could be patterns for imidazole reactivity studies due to the amine (-NH-) and aza (-N═) nitrogen atoms. The analyses of the local and global electronic indexes prove and quantify that imidazole has an intermediate analogy between pyrrole and pyridine.

Influences on the rotated structure of diiron dithiolate complexes: electronic asymmetry vs. secondary coordination sphere interaction

In the pursuit of a "rotated" structure, and exploration of the influence of the aza nitrogen lone pair, the Fe(I)Fe(I) model complexes wherein two Fe(CO)(3-x)P(x) moieties are significantly twisted from the ideal configuration (torsion angle >30°) are reported. [Fe(2)(μ-S(CH(2))(2)N(i)Pr(X)(CH(2))(2)S)(CO)(4)(κ(2)-dppe)](2)(2+) (X = H, 4; Me, 5) prepared from protonation and methylation, respectively, of [Fe(2)(μ-S(CH(2))(2)N(i)Pr(CH(2))(2)S)(CO)(4)(κ(2)-dppe)](2), 1, possess Φ angles of 34.1 and 35.4° (av.), respectively. Such dramatic twist is attributed to asymmetric substitution within the Fe(2) unit in which a dppe ligand is coordinated to one Fe site in the κ(2)-mode. In the presence of the N···C(CO(ap)) interaction, the torsion angle is decreased to 10.8°, suggesting availability of lone pairs of the aza nitrogen sites within 1 is in control of the twist. Backbones of the bridging diphosphine ligands also affect distortion. For a shorter ligand, the more compact structure of [Fe(2)(μ-S(CH(2))(2)N(i)Pr(CH(2))(2)S)(μ-dppm)(CO)(4)](2), 7, is formed. Dppm in a bridging manner allows achievement of the nearly eclipsed configuration. In contrast, dppe in [Fe(2)(μ-S(CH(2))(2)N(i)Pr(CH(2))(2)S)(μ-dppe)(CO)(4)](2), 6, could twist the Fe(CO)(3-x)L(x) fragment to adopt the least strained structure. In addition, the NC(CO(ap)) interaction would direct the twist towards a specific direction for the closer contact. In return, the shorter N···C(CO(ap)) distance of 3.721(7) Å and larger Φ angle of 26.5° are obtained in 6. For comparison, 3.987(7) Å and 3.9° of the corresponding parameters are observed in 7. Conversion of (μ-dppe)[Fe(2)(μ-S(CH(2))(2)N(i)Pr(CH(2))(2)S)(CO)(5)](2), 2, to complex 1 via an associative mechanism is studied.

ChemInform Abstract: Computational Study of Relative Bond Strengths and Stabilities of a Series of Amine and Nitro Derivatives of Triprismane and Some Azatriprismanes

An ab initio SCF-MO procedure has been used to compute the structures, bond orders, and isodesmic reaction energies of various NH2 and NO2 derivatives of triprismane and some azatriprismanes. Each of these substituents, individually, is found to have a weakening influence upon one or more bonds. The presence of aza nitrogens in the molecular framework enhances this tendency for the NH2 group but essentially eliminates it for NO2. No reinforcement (‘push-pull” effect) is observed when the two substituents are present simultaneously on adjacent carbons. The NH2 group is found to diminish the strain energies of these molecules; however the effect of the NO2 appears to depend upon the number of aza nitrogens.

Biomimetic Model Featuring the NH Proton and Bridging Hydride Related to a Proposed Intermediate in Enzymatic H2 Production by Fe-Only Hydrogenase

Iron azadithiolate phosphine-substituted complex and its protonated species featuring the NH proton and/or bridging Fe hydride, [Fe(2)(mu-S(CH(2))(2)N(n)Pr(H)(m)(CH(2))(2)S)(mu-H)(n)(CO)(4)(PMe(3))(2)](2)((2m+2n)+) (1, m = n = 0; [1-2H(N)](2+), m = 1, n = 0; [1-2H(N)2H(Fe)](4+), m = n = 1), are prepared to mimic the active site of Fe-only hydrogenase. X-ray crystallographic analyses of these three complexes reveal that two diiron subunits are linked by two azadiethylenethiolate bridges to construct a dimer-of-dimer structure. (31)P NMR spectroscopy confirms two trimethylphosphine ligands within the diiron moiety are arranged in the apical/basal configuration, which is consistent with the solid-state structural characterization. Deprotonation of the NH proton in [1-2H(N)](2+) and [1-2H(N)2H(Fe)](4+) occurs in the presence of triethanolamine (TEOA), which generates 1 and [1-2H(Fe)](2+), respectively. Deprotonation of the Fe hydride is accomplished by addition of bistriphenylphosphineimminium chloride ([PPN]Cl). It is observed that the Fe hydride species, [1-2H(Fe)](2+), is a kinetic product which converts to its thermodynamically stable tautomer, [1-2H(N)](2+), in solution, as evidenced by IR and NMR spectroscopy. The pK(a) values of the aza nitrogen and the diiron sites are estimated to be 8.9-15.9 and <8.9, respectively. [1-2H(N)2H(Fe)](4+) has been observed to evolve H(2) electrocatalytically at a mild potential (-1.42 V vs Fc/Fc(+)) in CH(3)CN solution. Catalysis of [1-2H(N)2H(Fe)](4+) is found to be as efficient as that of the related diiron azadithiolate complexes. In the absence of a proton source, [1-2H(N)2H(Fe)](4+) undergoes four irreversible reduction processes at -1.26, -1.42, -1.82, and -2.43 V, which are attributed to the reduction events from [1-2H(N)2H(Fe)](4+), [1-2H(Fe)](2+), [1-2H(N)](2+), and 1, respectively, according to bulk electrolysis and voltammetry in combination of titration experiments with acids.

Links between surface electrostatic potentials of energetic molecules, impact sensitivities and C–NO2/N–NO2 bond dissociation energies

Earlier work has shown a link—not necessarily a causal relationship—between the impact sensitivities of energetic compounds and the electrostatic potentials on their molecular surfaces. The latter are anomalous in that the positive regions are strikingly dominant, contrary to what is typical of organic molecules. In this work, we show that the presence of several electron-withdrawing NO2 groups and/or aza nitrogens (common features of energetic systems), which is the reason for the anomalously positive surface potentials, also weakens the C–NO2 and/or N–NO2 bonds. Thus, insofar as these are trigger linkages, the rupture of which is a key step in detonation initiation, an approximate correlation between the features of the surface potentials and sensitivity may be expected. A group of eight nitramines is used to demonstrate this. Work is in progress to elucidate the basis for the surface potential–sensitivity link when a non-trigger-linkage mechanism is operative.

Electrochemical and photophysical characterization of non-peripherally-octaalkyl substituted dichlorotin(IV) phthalocyanine and tetrabenzotriazaporphyrin compounds

Three non-peripherally substituted tin(IV) macrocylic compounds, octahexylphthalocyaninato dichlorotin(IV) (3a), octahexyltetrabenzo-5,10,15- triazaporphyrinato dichlorotin(IV) (3b) and octadecylphthalocyaninato dichlorotin(IV) (3c) were synthesized and their photophysical and electrochemical behavior studied. Complex 3b, containing a CH group in place of one of the aza nitrogen atoms of the phthalocyanine core, shows a split Q-band due to its lower symmetry. The triplet state quantum yields were found to be lower than would be expected on the basis of the heavy atom effect of tin as the central metal for phthalocyanine derivatives (3a and 3c). In contrast, 3b shows a triplet quantum yield Φ T = 0.78. The triplet state lifetimes were solvent dependent, and were higher in tetrahydrofuran than in toluene. Cyclic voltammetry and spectroelectrochemistry of the complexes revealed only ring-based redox processes.

Influence of pH on the Photochemical and Electrochemical Reduction of the Dinuclear Ruthenium Complex, [(phen)2Ru(tatpp)Ru(phen)2]Cl4, in Water: Proton‐Coupled Sequential and Concerted Multi‐Electron Reduction

The dinuclear ruthenium complex [(phen)2Ru(tatpp)Ru(phen)2]4+ (P; in which phen is 1,10-phenanthroline and tatpp is 9,11,20,22-tetraaza tetrapyrido[3,2-a:2'3'-c:3'',2''-l:2''',3''']-pentacene) undergoes a photodriven two-electron reduction in aqueous solution, thus storing light energy as chemical potential within its structure. The mechanism of this reduction is strongly influenced by the pH, in that basic conditions favor a sequential process involving two one-electron reductions and neutral or slightly acidic conditions favor a proton-coupled, bielectronic process. In this complex, the central tatpp ligand is the site of electron storage and protonation of the central aza nitrogen atoms in the reduced products is observed as a function of the solution pH. The reduction mechanism and characterization of the rich array of products were determined by using a combination of cyclic and AC voltammetry along with UV-visible reflectance spectroelectrochemistry experiments. Both the reduction and protonation state of P could be followed as a function of pH and potential. From these data, estimates of the various reduced species' pKa values were obtained and the mechanism to form the doubly reduced, doubly protonated complex, [(phen)2Ru(H2tatpp)Ru(phen)2]4+ (H2P) at low pH (< or =7) could be shown to be a two-proton, two-electron process. Importantly, H2P is also formed in the photochemical reaction with sacrificial reducing agents, albeit at reduced yields relative to those at higher pH.