Electronic Absorption Spectroscopy Research Articles

Synthesis, spectroscopic characterization, DNA binding and DFT/PCM calculations of new Hydrogen-bonded charge transfer complex between 4-dimethylaminopyridine and Chloranilic acid

A charge transfer hydrogen bonded complex between the electron donor 4-dimethylaminopyridine (4-DMAP) and the electron acceptor Chloranilic acid (CLA) has been synthesized. The complex was studied experimentally and theoretically. A chemical reaction developed due to strong H-bonding, followed by proton transfer from the CLA to the 4-DMAP in acetonitrile medium (ACN). The straight line approach was employed to verify the 1:1 M ratio of the synthesised CT complex. The Benesi-Hildebrand equation was employed to compute the association constant (KCT), molar absorptivity (εCT), and other physical parameters. The synthesized complex was characterized spectroscopically. The presence of proton and charge transfers in the resultant complex was supported by 1H NMR, FT-IR and SEM-EDX investigations. The complex DNA binding ability of complex was investigated using electron absorption spectroscopy to reveal that the CT complex binding mechanism is intercalative and the intrinsic binding constant (Kb) value is 6.7 × 104 M−1. Theoretical calculations using DFT at the basis set CAM-B3LYP/6–31 G (d, p) were used to validate the experimental outcomes. The Mullikan atomic charges, geometrical parameters, and molecular electrostatic potential maps (MEPs) were computed and interpreted in conformity with the experimental results. In addition to e-transfer, the presence of an H-bond influences complex stability. There is good agreement between experimental results and DFT calculations.

New Ni(II) and Pd(II) complexes bearing derived sulfa drug ligands: synthesis, characterization, DFT calculations, and in silico and in vitro biological activity studies.

In the present study, the synthesis of six new Ni(II) and Pd(II) complexes with three derived sulfamethoxazole drug ligands is reported. The coordination mode, geometry, and chemical formula of all the synthesized compounds have been determined by elemental analysis, mass spectrometry, emission atomic spectroscopy, conductivity measurements, magnetic susceptibility, FTIR, TGA, 1H-NMR, electronic absorption spectroscopy, SEM-EDX along with DFT calculations. The Schiff Base ligands were found to be bidentate and coordinated to the metal ions through sulfonamidic nitrogen and oxazolic nitrogen atoms leading to a square planar geometry for palladium (II) while a distorted octahedral geometry around Nickel (II) ion was suggested. Biological applications of the new complexes including in vitro antimicrobial, antioxidant and anticancer properties were investigated. The results showed that the new metal (II) compounds exhibit remarkable antibacterial inhibition activity against both Gram-positive and Gram-negative bacteria, in addition to noticeable DPPH free radical scavenging activity. The in vitro cytotoxicity assay of the complexes against cell lines of chronic myelogenous leukaemia (K562) showed promising potential for the application of the coordination compounds in antitumor therapy. Subsequently, to evaluate the pharmaceutical potential of the metal-containing compounds, pharmacokinetics and toxicity were studied by ADMET simulations while interactions between the complexes and bacterial proteins were evaluated by molecular docking.

New Nanosized Systems Doxorubicin-Amphiphilic Copolymers of N-Vinylpyrrolidone and (Di)methacrylates with Antitumor Activity.

Nanosized systems of DOX with antitumor activity on the base of micelle-like particles of amphiphilic thermosensitive copolymers of N-vinylpyrrolidone (VP) with triethylene glycol dimethacrylate (TEGDM), and N-vinylpyrrolidone and methacrylic acid (MAA) with TEGDM were explored. They were investigated in aqueous solutions by electron absorption spectroscopy, dynamic light scattering and cyclic voltammetry. Experimental data and quantum-chemical modeling indicated the formation of a hydrogen bond between oxygen-containing groups of monomer units of the copolymers and H-atoms of OH and NH2 groups of DOX; the energies and H-bond lengths in the considered structures were calculated. A simulation of TDDFT spectra of DOX and its complexes with the VP and TEGDM units was carried out. Electrochemical studies in PBS have demonstrated that the oxidation of encapsulated DOX appeared to be easier than that of the free one, and its reduction was somewhat more difficult. The cytotoxicity of VP-TEGDM copolymer compositions containing 1, 5 and 15 wt% DOX was studied in vitro on HeLa cells, and the values of IC50 doses were determined at 24 and 72 h of exposure. The copolymer compositions containing 5 and 15 wt% DOX accumulated actively in cell nuclei and did not cause visual changes in cell morphology.

Polydimethylsiloxanes with Grafted Dibenzoylmethanatoboron Difluoride: Synthesis and Properties.

A method for the preparation of polydimethylsiloxanes with grafted methoxy-substituted dibenzoylmethanatoboron difluoride has been described. The structures of prepared polymers were confirmed using NMR, IR spectroscopy and gel permeation chromatography methods. Their thermal properties were investigated using thermal gravimetric analysis, differential scanning calorimetry and thermomechanical analysis. The prepared polymers had good thermal (Td5% up to 393 °C) and thermo-oxidative (Td5% = 413 °C) stability. The polymers started to transit in a viscous flow state at about 40 °C (for 3 a) and at about 20 °C (for 3 b). The viscoelastic characteristics of prepared polymers were determined in the sinusoidal oscillating vibrations mode. It was shown that the studied polymers at low frequencies at room temperature are viscoelastic fluids (G′ < G″). Increasing the frequency led to inversion (crossover) of dependences G′ and G″, which indicated the transition of polymers from viscous to elastomeric behavior characteristics, and the beginning of the formation of a physical network. Optical properties were studied using electron absorption, steady-state and time-resolved fluorescence spectroscopy. It was shown that intramolecular H-dimers exist in the ground state. The polymers studied had a bright fluorescence in the solution and in the solid state, consisting of bands of monomer and excimer emission. Thermally-activated delayed fluorescence was observed in the solution and the solid state. The prepared polymers possess intriguing properties that make them useful as optical materials, sensors or imaging agents.

Development of a Highly Selective Ni(II) Chelator in Aqueous Solution.

The design, synthesis, and characterization of a novel Ni(II) chelator SG-20 is reported. SG-20 is selective in binding to Ni(II) versus other metal ions including Cu(II), Fe(II), Co(II), and Zn(II). At pH = 7.1, SG-20 binds Ni(II) with a Kd = 7.0 ± 0.4 μM. Job analysis indicates that SG-20 binds to both Ni(II) and Cu(II) with a 1:1 stoichiometry. Affinity of SG-20 for Ni(II) is pH dependent and decreases upon lowering to pH 4.0. A green solid was isolated from the reaction of SG-20 with NiCl2·6H2O in MeOH and characterized by X-ray photoelectron spectroscopy (XPS), electronic absorption and infrared (IR) spectroscopies, and mass spectrometry. Collectively, XPS and IR analysis revealed Ni-N and Ni-O interactions and a shift in C-O asymmetric and symmetric stretches consistent with Ni binding. Attempts to crystalize a mononuclear complex were unsuccessful, likely due to the Ni-SG-20 complex being in equilibrium with higher order species in solution. However, reaction of SG-20 with NiCl2·6H2O in water followed by slow evaporation yielded green crystals that were characterized by electronic absorption spectroscopy (λmax = 260 nm) and X-ray crystallography. These analyses revealed that SG-20 supports formation of a complex cluster containing six SG-20 ligands, 15 Ni(II), and three Na(I) centers, with two distinct types of Ni atoms in its outer and inner core. The nine Ni atoms present in the inner core were bound by oxo and carbonate bridges, whereas the six Ni atoms present in its outer shell were bound to N, O, and S donor atoms derived from SG-20. Overall, X-ray crystallographic analysis revealed that two chelator arms of SG-20 bind to one Ni(II) ion with an axial aqua ligand, whereas the third arm is free to interact with Ni ions within the central cluster, supporting the goal of Ni capture.

Solvent-driven self-assembly of two novel di- and tetra-nuclear Cu(II) bis(salamo)-based complexes

Two novel di- and tetra-nuclear Cu(II) complexes, [Cu2(HL)Cl2(MeOH)]·MeOH (1) and [Cu4(HL)2Cl2(μ2-Cl)2]·4EtOH·H2O (2) (where H3L= bis(salamo)-based ligand) have been synthesized, and structurally characterized by elemental analysis, infrared and electronic absorption spectroscopy. The molecular structures of complexes 1 and 2 were defined by using single crystal X-ray diffraction experiment. The structural results indicated that complexes 1 and 2 have a close comparable [Cu2(HL)Cl2] unit in the unit cell where the ligand acts as a two-compartmental species chelating the metal ions Cu1 and Cu2. However, in complex 2, the [Cu2(HL)Cl2] complexes are paired in a centrosymmetric fashion to form a tetra-nuclear arrangement, connected by μ2-bridging chlorides. The observed different nuclearity is likely induced by the different solvent molecules used in the syntheses, methanol and ethanol in complexes 1 and 2, respectively. A methanol molecule participates in coordination of complex 1, while ethanol species in complex 2 is rather distant from one of the Cu(II) ions (ca 2.82 Å). The molecular frontier orbitals HOMO and LUMO of the complexes were analyzed by DFT calculations, and the results showed that the energy gaps of complexes 1 and 2 are particularly small compared to H3L, which may be caused by the combination of the orbitals of the central atoms and the donor atoms. The reaction sites of the ligand H3L with the metal Cu(II) were analyzed by ESP studies, and the results showed that the experiments were in agreement with the theory. Finally, Hirshfeld surfaces analyses were performed to quantitatively show various weak interactions of complexes 1 and 2.

The Charge Transfer Complex between 2, 3-diamino-5-bromopyridine and Chloranilic acid: Preparation, Spectroscopic Characterization, DNA binding, and DFT/PCM analysis

A charge transfer hydrogen bonded complex was prepared and experimentally explored in an acetonitrile (ACN) medium between the proton acceptor (electron donor) 2, 3-Diamino-5-bromopyridine and the proton donor (electron acceptor) chloranilic acid. The stoichiometry of the charge transfer complex is 1:1. The Benesi-Hildebrand equation is used to calculate the molar absorptivity (εCT), association constant (KCT) and other spectroscopic physical characteristics. The solid compound was synthesized and studied using several spectroscopic methods. The presence of charge and proton transfers in the resultant complex was supported by 1H NMR, FT-IR and SEM-EDX investigations. The complex DNA binding ability was investigated using electron absorption spectroscopy, and the CT complex binding mechanism is intercalative. The intrinsic binding constant (Kb) value is 5.2 × 106M−1. The good binding affinity of the CT complex makes it potentially suitable for usage as a pharmaceutical in the future. Molecular docking calculations have been performed between CT complex and DNA (ID = 1BNA) to study the CT-DNA interaction theoretically. To corroborate the experimental findings, calculations based on DFT were carried out in the gas and PCM analysis where the existence of charge and hydrogen transfers. Finally, good agreement between experimental and theoretical computations was observed confirming that the basis set used is appropriate for the system under examination.

Direct structural observation of ultrafast photoisomerization dynamics in sinapate esters

Sinapate esters have been extensively studied for their potential application in ‘nature-inspired’ photoprotection. There is general consensus that the relaxation mechanism of sinapate esters following photoexcitation with ultraviolet radiation is mediated by geometric isomerization. This has been largely inferred through indirect studies involving transient electronic absorption spectroscopy in conjunction with steady-state spectroscopies. However, to-date, there is no direct experimental evidence tracking the formation of the photoisomer in real-time. Using transient vibrational absorption spectroscopy, we report on the direct structural changes that occur upon photoexcitation, resulting in the photoisomer formation. Our mechanistic analysis predicts that, from the photoprepared ππ* state, internal conversion takes place through a conical intersection (CI) near the geometry of the initial isomer. Our calculations suggest that different CI topographies at relevant points on the seam of intersection may influence the isomerization yield. Altogether, we provide compelling evidence suggesting that a sinapate ester’s geometric isomerization can be a more complex dynamical process than originally thought.

Reactivity of a nitrosyl ruthenium complex and its potential impact on the fate of DNA – An in vitro investigation

The role of metal complexes on facing DNA has been a topic of major interest. However, metallonitrosyl compounds have been poorly investigated regarding their reactivities and interaction with DNA. A nitrosyl compound, cis-[Ru(bpy)2(SO3)(NO)](PF6)(A), showed a variety of promising biological activities catching our attention. Here, we carried out a series of studies involving the interaction and damage of DNA mediated by the metal complex A and its final product after NO release, cis-[Ru(bpy)2(SO3)(H2O](B). The fate of DNA with these metal complexes was investigated upon light or chemical stimuli using electrophoresis, electronic absorption spectroscopy, circular dichroism, size-exclusion resin, mass spectrometry, electron spin resonance (ESR) and viscometry. Since many biological disorders involve the production of oxidizing species, it is important to evaluate the reactivity of these compounds under such conditions as well. Indeed, the metal complex B exhibited important reactivity with H2O2 enabling DNA degradation, with detection of an unusual oxygenated intermediate. ESR spectroscopy detected mainly the DMPO-OOH adduct, which only emerges if H2O2 and O2 are present together. This result indicated HOO• as a key radical likely involved in DNA damage as supported by agarose gel electrophoresis. Notably, the nitrosyl ruthenium complex did not show evidence of direct DNA damage. However, its aqua product should be carefully considered as potentially harmful to DNA deserving further in vivo studies to better address any genotoxicity.

Radical in the Peroxide-Produced F-Type Ferryl Form of Bovine Cytochrome c Oxidase.

The reduction of O2 in respiratory cytochrome c oxidases (CcO) is associated with the generation of the transmembrane proton gradient by two mechanisms. In one of them, the proton pumping, two different types of the ferryl intermediates of the catalytic heme a3-CuB center P and F forms, participate. Equivalent ferryl states can be also formed by the reaction of the oxidized CcO (O) with H2O2. Interestingly, in acidic solutions a single molecule of H2O2 can generate from the O an additional F-type ferryl form (F•) that should contain, in contrast to the catalytic F intermediate, a free radical at the heme a3-CuB center. In this work, the formation and the endogenous decay of both the ferryl iron of heme a3 and the radical in F• intermediate were examined by the combination of four experimental approaches, isothermal titration calorimetry, electron paramagnetic resonance, and electronic absorption spectroscopy together with the reduction of this form by the defined number of electrons. The results are consistent with the generation of radicals in F• form. However, the radical at the catalytic center is more rapidly quenched than the accompanying ferryl state of heme a3, very likely by the intrinsic oxidation of the enzyme itself.

Intramolecular H-type dimer and excimer formation in dibenzoylmethanatoboron difluoride dyads connected via diphenylsiloxane linkers

A series of new dibenzoylmethanatoboron difluoride (DBMBF2) and methoxy-substituted DBMBF2 dyads linked via flexible diphenylsiloxane linkers of various length have been synthesized and characterized. The structure and optical properties of these compounds were studied by X-ray single-crystal measurements, electron absorption, and steady-state and time-resolved fluorescence spectroscopy. In the dyads under study, which are in an equilibrium between the open and folded conformations, the formation of intramolecular H-dimers in the ground state are observed. For the dyads in dilute solutions in various solvents, both monomer and excimer (through intramolecular excimer formation) emissions are observed at room temperature. The effect of solvent polarity and linker length on the formation of intramolecular excimers has been studied. The contribution of excimers into the total fluorescence spectrum decreases with increasing the solvent polarity, and the length of the siloxane linker. The studied dyads in hexane and cyclohexane solutions have CIE chromaticity coordinates very close to the white color point. The mechanofluorochromic properties of the synthesized dyads were studied.

New 2-thiazol-5-yl-benzimidazoles: synthesis and determining of their potential as chemosensor reagents for polyvalent metal ions

Thiabendazole and its analogues are widely used and reasonably well studied in terms of their biological activity, while their potential as chemosensory compounds with optical detection and sensitivity to metal ions remains unexplored. On the other hand, the introduction of a thiazole ring into the heteroaromatic system of benzimidazole significantly expands the directions of practical application of benzimidazole derivatives. Thus, one of the important structural features of the benzimidazole-thiazole system is the presence of a chelate cavity. It is a potential center of complexation with metal ions, which, with their bright fluorescence, create additional opportunities for their use as luminescent probes and chemosensors. Considering the affinity of the sulfur atom for polyvalent heavy metals, sensors based on these compounds can be used in medically and environmentally oriented analysis of toxic metal compounds. In this publication, we focus on the synthesis of various 2- and 4-substituted 2-thiazol-5-yl-benzimidazoles, as well as the determination of their spectral-fluorescence properties. Therefore, we started from 2,4-dihalogen 5-formyl thiazoles, the methods of synthesis of which are well known. Their modification in 2nd position we have described in previous works. To obtain the target products, the Weidenhagen method was chosen, which consists in the reaction of o-phenylenediamine with an aromatic or aliphatic aldehyde under oxidation conditions. For creating such conditions copper acetate was used. The possibility of coordination of metal ions by the obtained compounds was established using electronic absorption and fluorescence spectroscopy. It was found that these compounds selectively form complexes with Cd2+ and Mg2+ ions. Our qualitative studies allow us to make conclusion that the benzimidazole-thiazole fragment is a promising coordination site for metal ions, which can be used in the design of fluorescent probes for the determination of Cd2+ and Mg2+ ions.

Metal-Support Interactions in Molecular Single-Site Cluster Catalysts.

This study provides atomistic insights into the interface between a single-site catalyst and a transition metal chalcogenide support and reveals that peak catalytic activity occurs when edge/support redox cooperativity is maximized. A molecular platform MCo6Se8(PEt3)4(L)2 (1-M, M = Cr, Mn, Fe, Co, Cu, and Zn) was designed in which the active site (M)/support (Co6Se8) interactions are interrogated by systematically probing the electronic and structural changes that occur as the identity of the metal varies. All 3d transition metal 1-M clusters display remarkable catalytic activity for coupling tosyl azide and tert-butyl isocyanide, with Mn and Co derivatives showing the fastest turnover in the series. Structural, electronic, and magnetic characterization of the clusters was performed using single crystal X-ray diffraction, 1H and 31P nuclear magnetic resonance spectroscopy, electronic absorption spectroscopy, cyclic voltammetry, and computational methods. Distinct metal/support redox regimes can be accessed in 1-M based on the energy of the edge metal's frontier orbitals with respect to those of the cluster support. As the degree of electronic interaction between the edge and the support increases, a cooperative regime is reached wherein the support can deliver electrons to the catalytic site, increasing the reactivity of key metal-nitrenoid intermediates.

Actinide-Oxygen Multiple Bonds from Air: Synthesis and Characterization of a Thorium Oxo Supported by Redox-Active Ligands.

The first non-uranyl, f-element oxo complex synthesized from dioxygen in dry air is presented in this work. The synthesis was accomplished by treating the redox-active thorium amidophenolate complex, [Th(dippap)3][K(15-c-5)2]2 (1-ap crown), with dioxygen in dry air, forming a rare terminal thorium oxo, [O═Th(dippisq)2(dippap)][K(15-c-5)2]2 (2-oxo). Compound 1-ap crown was regenerated by treating 2-oxo with potassium graphite. X-ray crystallography of 2-oxo revealed a comparatively longer bond length for the thorium-oxygen double bond when compared to other thorium oxos. As such, several thorium-oxygen single bonds were synthesized for comparison, including Th(dippisq)2(OSiMe3)2(THF) (4-OSiMe3), Th(OSiMe3)4(bipy)2 (5-OSiMe3), and [Th(OH)2 (dippHap)4][K(15-c-5)2]2 (6-OH). Full spectroscopic and structural characterization of the complexes was performed via 1H NMR spectroscopy, X-ray crystallography, EPR spectroscopy, and electronic absorption spectroscopy as well as SQUID magnetometry, which all confirmed the electronic structure of these complexes.

Effects of Al2O3 addition on microstructure and luminescence of transparent germanosilicate glass-ceramics with incorporated spinel Ga-oxide nanocrystals

Ga-oxide spinel nanocrystals are wide band gap systems, which can be incorporated in a glass matrix by phase separation mechanisms. In suitable conditions, this kind of processes can give rise to transparent nanostructured glass-ceramics with UV excitation and luminescence properties potentially interesting in several technological areas. Nanophase size dispersion and volume fraction have been demonstrated to be controllable, at some extent, by suitable thermal treatments for nucleation and nano-crystallization in low-alkali gallium germanosilicate system. Here we report the results on the role of Al2O3 additions on the microstructure and optical response of the glass-ceramics fabricated in this system. Data of differential scanning calorimetry, X-ray diffraction, transmission electron microscopy, absorption and fluorescence spectroscopy show that Al2O3 addition, up to 4.5 mol%, turns out to have a considerable impact on the size and number density of precipitated nanocrystals, which are solid solutions of γ-Ga2-xAlxO3 resulting from the partial incorporation of Al3+ ions into the crystalline phase. We show that the use of Al2O3 as an additive in the composition of gallium germanosilicates facilitates glass melting and leads to glass-ceramics with significantly modified photoluminescence characteristics such as decay lifetime and integrated intensity of light emission. The possible reasons are discussed.

Protonation and Non-Innocent Ligand Behavior in Pyranopterin Dithiolene Molybdenum Complexes.

The complex [TEA][Tp*MoIV(O)(S2BMOPP)] (1) [TEA = tetraethylammonium, Tp* = tris(3,5-dimethylpyrazolyl)hydroborate, and BMOPP = 6-(3-butynyl-2-methyl-2-ol)-2-pivaloyl pterin] is a structural analogue of the molybdenum cofactor common to all pyranopterin molybdenum enzymes because it possesses a pyranopterin-ene-1,2-dithiolate ligand (S2BMOPP) that exists primarily in the ring-closed pyrano structure as a resonance hybrid of ene-dithiolate and thione-thiolate forms. Compound 1, the protonated [Tp*MoIV(O)(S2BMOPP-H)] (1-H) and one-electron-oxidized [Tp*MoV(O)(S2BMOPP)] [1-Mo(5+)] species have been studied using a combination of electrochemistry, electronic absorption, and electron paramagnetic resonance (EPR) spectroscopy. Additional insight into the nature of these molecules has been derived from electronic structure computations. Differences in dithiolene C-S bond lengths correlate with relative contributions from both ene-dithiolate and thione-thiolate resonance structures. Upon protonation of 1 to form 1-H, large spectroscopic changes are observed with transitions assigned as Mo(xy) → pyranopterin metal-to-ligand charge transfer and dithiolene → pyranopterin intraligand charge transfer, respectively, and this underscores a dramatic change in electronic structure between 1 and 1-H. The changes in electronic structure that occur upon protonation of 1 are also reflected in a large >300 mV increase in the Mo(V/IV) redox potential for 1-H, resulting from the greater thione-thiolate resonance contribution and decreased charge donation that stabilize the Mo(IV) state in 1-H with respect to one-electron oxidation. EPR spin Hamiltonian parameters for one-electron-oxidized 1-Mo(5+) and uncyclized [Tp*MoV(O)(S2BDMPP)] [3-Mo(5+)] [BDMPP = 6-(3-butynyl-2,2-dimethyl)-2-pivaloyl pterin] are very similar to each other and to those of [Tp*MoVO(bdt)] (bdt = 1,2-ene-dithiolate). This indicates that the dithiolate form of the ligand dominates at the Mo(V) level, consistent with the demand for greater S → Mo charge donation and a corresponding increase in Mo-S covalency as the oxidation state of the metal is increased. Protonation of 1 represents a simple reaction that models how the transfer of a proton from neighboring acidic amino acid residues to the Mo cofactor at a nitrogen atom within the pyranopterin dithiolene (PDT) ligand in pyranopterin molybdenum enzymes can impact the electronic structure of the Mo-PDT unit. This work also illustrates how pyran ring-chain tautomerization drives changes in resonance contributions to the dithiolene chelate and may adjust the reduction potential of the Mo ion.

Spectroelectrochemistry of Electroactive Polymer Composite Materials.

In this review, we have summarized the main advantages of the method of spectroelectrochemistry as applied to recent studies on electrosynthesis and redox processes of electroactive polymer composite materials, which have found wide application in designing organic optoelectronic devices, batteries and sensors. These polymer composites include electroactive polymer complexes with large unmovable dopant anions such as polymer electrolytes, organic dyes, cyclodextrins, poly(β-hydroxyethers), as well as polymer-inorganic nanocomposites. The spectroelectrochemical methods reviewed include in situ electron absorption, Raman, infrared and electron spin resonance spectroscopies.

Bis-Alkynyl Complexes of Fe(III) Tetraaza Macrocycles─A Tale of Two Rings.

Reported herein are new Fe bis-alkynyl complexes [FeIII(L)(C2R)2]BPh4 based on tetraimine macrocycle (L = HMTI = meso-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradeca-1,3,8,10-tetraene; 1a-1c; R = C6H5 (a), C10H9 (b), SiMe3 (c)) and tetraamine macrocycle (L = HMC = meso-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane; 2a-2c). These complexes have been characterized using single-crystal X-ray diffraction, electronic absorption spectroscopy, and cyclic and differential pulse voltammetry. Spectroelectrochemical studies of 1a and 2a allowed for investigation of the FeII oxidation state, which revealed a strong dependence on the nature of the macrocycle for both the energies of the FeII to C2Ph metal-to-ligand charge transfer (MLCT) and the ν(C≡C). The ν(C≡C) was further influenced by the oxidation state, though sensitivity to the formal metal oxidation state was much higher in the case of 2a than in 1a. These findings are rationalized on the basis of the relative energies of the formally metal-centered orbitals via density functional theory calculations.

Higher-order derivative electronic absorption spectroscopy of ferricyanide electrogenerated in situ by optically transparent thin layer electrochemical cell

• Higher-order derivative UV-vis spectra obtained in spectroelectrochemical measurements. • Separated spectra of ferricyanide obtained by electrolysis in OTTLE. • The maxima of the potential-dependent fourth-derivative spectra satisfy Nernst’s relation. • Shoulder peaks in spectra of electrolytic products could also be used for reaction analysis. Higher-order differential transformations of electronic absorption spectra obtained by in situ thin-layer spectroelectrochemical cell were performed. The characteristics of the derivative spectra obtained in the electro-oxidation of ferrocyanide were in good agreement with those of ferricyanide in homogeneous solution systems. Several common crossing points (CCP) appearing in the derivative spectra depending on the applied potential of the reversible system made it clear that the reaction was unitary. The maximum peaks of the derivative spectra corresponding to the shoulders of the original spectra and the applied potential satisfy the Nernstian relationship, which allows us to evaluate the n and E o ′ values in the electrode reaction.

Formation of Stilbene Azo-Dimer by Direct Irradiation of p-Azidostilbene†.

Triplet arylnitrenes may provide direct access to aryl azo-dimers, which have broad commercial applicability. Herein, the photolysis of p-azidostilbene (1) in argon-saturated methanol yielded stilbene azo-dimer (2) through the dimerization of triplet p-nitrenostilbene (3 1N). The formation of 3 1N was verified by electron paramagnetic resonance spectroscopy and absorption spectroscopy (λmax ~ 375 nm) in cryogenic 2-methyltetrahydrofuran matrices. At ambient temperature, laser flash photolysis of 1 in methanol formed 3 1N (λmax ~ 370 nm, 2.85 × 107 s-1 ). On shorter timescales, a transient absorption (λmax ~ 390 nm) that decayed with a similar rate constant (3.11 × 107 s-1 ) was assigned to a triplet excited state (T) of 1. Density functional theory calculations yielded three configurations for T of 1, with the unpaired electrons on the azido (TA ) or stilbene moiety (TTw , twisted and TFl , flat). The transient was assigned to TTw based on its calculated spectrum. CASPT2 calculations gave a singlet-triplet energy gap of 16.6 kcal mol-1 for 1 N; thus, intersystem crossing of 1 1N to 3 1N is unlikely at ambient temperature, supporting the formation of 3 1N from T of 1. Thus, sustainable synthetic methods for aryl azo-dimers can be developed using the visible-light irradiation of aryl azides to form triplet arylnitrenes.