Theoretical study of the photoisomerization of 1,2-bispyrazinyl-ethylene and the halogen ion salts of 1-Pyrazinyl-2-(4'-methylpyrazinyl)ethylene.

CONTEXT: It has been reported that photoexcitation of azastilbene compounds like E-1,2-bispyrazinyl-ethylene (bpe) can undergo E → Z photoisomerization of its quaternary salts via the excited triplet state. However, experimentally it is possible to get low fluorescence and photoisomerisation quantum yields in a state with higher internal conversion than intersystem crossing. We modelled bpe and its methylated derivative (bpeMe), as well as its quaternary halogen salts (bpeMeX with X = F-, Cl-, Br- and I-) to study levels of fluorescence, phosphorescence and excited state potential energy surfaces (PES). Results support experimental observations of molecules where the anion of a salt is an efficient electron donor, that molecules with weak electron-donating anions like Cl- to give increased fluorescence and photoisomerization, as compared to molecules with stronger electron-donating anions like I-, which are dominated by competing electron transfer. The fluorescence of bpeMeF and bpeMeCl was found to be stronger than bpeMeBr and bpeMeI. A deep well in the triplet excited state of bpeMeI is considered responsible for the decreased photoisomerization, compared to what was experimentally observed for bpeMeCl. Uniquely, the bpeMeI molecule is characterised by near-zero splitting of the s1 and t1 excited states that can enhance charge transfer. The quaternary salt of bpeMe with stronger electron-donating Br- anion was observed to undergo fluorescence and phosphorescence at much lower energy compared to those with weak electron-donating F- and Cl- anions. This research shows how to control the excited state fluorescence, phosphorescence and isomerization of quaternary halogen salts of methyl derivatives of 1,2-bispyrazinyl-ethylene, which aids experimental design where excited state isomerization is considered. METHODS: Geometry optimization, molecular electrostatic potential (MESP), and time dependent density functional theory (TDDFT) calculations were conducted utilizing Gaussian 16 with the B3LYP functional and the 6-31 + G(d,p) basis set. The minimum energy path (MEP) for the E to Z isomerization of the molecules was established employing the Nudged-Elastic-Band (NEB) method, implemented in Orca 4.2. Precise energies of the E → Z isomerization reaction path were determined employing CASSCF and a more accurate multireference method, NEVPT2.

Source apportionment of fine atmospheric particles in Bloemfontein, South Africa, using positive matrix factorization.

Air pollution is of major health and environmental concern globally and in South Africa. Studies on the sources of PM2.5 air pollution in low- and middle-income countries such as South Africa are limited. This study aimed to identify local and distant sources of PM2.5 pollution in Bloemfontein. PM2.5 samples were collected from June 16, 2020 to August 18, 2021. Trace element concentrations were determined by EDXRF spectroscopy. By use of the US EPA PMF 5.0 program, local sources were determined to be combustion/wood burning (49%), industry (22%), soil dust (10%), base metal/pyrometallurgical and traffic (9.6%) and water treatment/industry (9.4%). The HYSPLIT program was applied to determine distant PM2.5 source areas and the following clusters were identified: Mpumalanga province (52%), Northern Cape province (35%), Indian Ocean (8%) and Atlantic Ocean (6%). The majority of the air was found to come from the Mpumalanga province in the north-east, while the majority of local sources are ascribed to combustion/wood burning. Results from this study can be used to develop an Air Quality Management Plan for Bloemfontein.

Nanocomposites with ZrO2@S-Doped g-C3N4 as an Enhanced Binder-Free Sensor: Synthesis and Characterization.

This study describes new electrocatalyst materials that can detect and reduce environmental pollutants. The synthesis and characterization of semiconductor nanocomposites (NCs) made from active ZrO2@S-doped g-C3N4 is presented. Electrochemical impedance spectroscopy (EIS) and Mott-Schottky (M-S) measurements were used to examine electron transfer characteristics of the synthesized samples. Using X-ray diffraction (XRD) and high-resolution scanning electron microscopy (HR-SEM) techniques, inclusion of monoclinic ZrO2 on flower-shaped S-doped-g-C3N4 was visualized. High-resolution X-ray photoelectron spectroscopy (XPS) revealed successful doping of ZrO2 into the lattice of S-doped g-C3N4. The electron transport mechanism between the electrolyte and the fluorine tin-oxide electrode (FTOE) was enhanced by the synergistic interaction between ZrO2 and S-doped g-C3N4 as co-modifiers. Development of a platform with improved conductivity based on an FTOE modified with ZrO2@S-doped g-C3N4 NCs resulted in an ideal platform for the detection of 4-nitrophenol (4-NP) in water. The electrocatalytic activity of the modified electrode was evaluated through determination of 4-NP by cyclic voltammetry (CV) and differential pulse voltammetry (DPV) under optimum conditions (pH 5). ZrO2@S-doped g-C3N4 (20%)/FTOE exhibited good electrocatalytic activity with a linear range from 10 to 100 µM and a low limit of detection (LOD) of 6.65 µM. Typical p-type semiconductor ZrO2@S-doped g-C3N4 NCs significantly impact the superior detection of 4-NP due to its size, shape, optical properties, specific surface area and effective separation of electron-hole pairs. We conclude that the superior electrochemical sensor behavior of the ZrO2@S-doped g-C3N4 (20%)/FTOE surfaces results from the synergistic interaction between S-doped g-C3N4 and ZrO2 surfaces that produce an active NC interface.

Synthesis and characterization of CuO@S-doped g-C3N4 based nanocomposites for binder-free sensor applications.

This study presents the simultaneous exfoliation and modification of heterostructured copper oxide incorporated sulfur doped graphitic carbon nitride (CuO@S-doped g-C3N4) nanocomposites (NCs) synthesized via chemical precipitation and pyrolysis techniques. The results revealed that the approach is feasible and highly efficient in producing 2-dimensional CuO@S-doped g-C3N4 NCs. The findings also showed a promising technique for enhancing the optical and electrical properties of bulk g-C3N4 by combining CuO nanoparticles (NPs) with S-doped g-C3N4. The crystallite and the average size of the NCs were validated using X-ray diffraction (XRD) studies. Incorporation of the cubical structured CuO on flower shaped S-doped-g-C3N4 was visualized and characterized through XRD, HR-SEM/EDS/SED, FT-IR, BET, UV-Vis/DRS, PL, XPS and impedance spectroscopy. The agglomerated NCs had various pore sizes, shapes and nanosized crystals, while being photo-active in the UV-vis range. The synergistic effect of CuO and S-doped g-C3N4 as co-modifiers greatly facilitates the electron transfer process between the electrolyte and the bare glassy carbon electrode. Specific surface areas of the NCs clearly revealed modification of bulk S-doped g-C3N4 when CuO NPs are incorporated with S-doped g-C3N4, providing a suitable environment for the binder-free decorated electrode with sensing behavior for hazardous pollutants. This was tested for the preparation of a 4-nitrophenol sensor.

Polypyridine OsII complexes electrochemical data.

The data presented in this article are relevant to the research article, "Electrochemistry of Os bipyridyl and phenanthroline complexes, comparison with Ru and Fe" (van der Westhuizen, 2020). Cyclic voltammograms illustrating OsII/III oxidations of eight osmium(II) complexes are presented in this article. The data were obtained under similar experimental conditions, at scan rates with magnitudes ranging from 0.05 V.s-1 to 10.00 V.s-1, in acetonitrile as solvent and tetrabutylammonium hexafluorophosphate as supporting electrolyte. Potentials are reported versus the iron(II) redox couple in ferrocene.

Cyclic voltammograms and electrochemical data of FeII polypyridine complexes.

Data in this article is associated with our research article, Electronic Properties of Fe Charge Transfer Complexes - a Combined Experimental and Theoretical Approach [1]. The oxidation and reduction potentials of fourteen FeII complexes are presented here, as extracted from the redox data obtained from its associated cyclic voltammograms, which were measured at scan rates varying from 0.05 V.s-1 to 5.00 V.s-1, under similar experimental conditions. Acetonitrile was used as solvent, and tetrabutylammonium hexafluorophosphate as supporting electrolyte. All data are reported versus the FeII redox couple in ferrocene.

Electrochemical data of polypyridine complexes of Ru(II).

The data here-in presented is associated to the research article, Electrochemistry and spectroscopy of substituted [Ru(phen)3]2+ and [Ru(bpy)3]2+ complexes [1]. Redox data obtained from cyclic voltammetry experiments of the oxidation of Ru(II) to Ru(III) of thirteen Ru(II)-polypyridine complexes is presented in this data in brief article. Data is obtained from the cyclic voltammograms at scan rates of two orders of magnitude (0.05-5.00 Vs-1) under similar experimental conditions, namely in acetonitrile as solvent and tetrabutylammonium hexafluorophosphate as supporting electrolyte, and reported versus the redox couple of Fe(II) of ferrocene.

Seven Chromisms Associated with Dithizone.

Variations in the electromagnetic wavelengths of absorption and reflection of molecules do not only make life colorful but also are often of central importance in solar energy conversion and optoelectronic information processing and transfer. Dithizone and its derivatives and complexes are, to our knowledge, probably the most "colorful" compound, being responsive to no less than seven different external stimuli that effect color change. Apart from a more detailed discussion of the cyclic voltammetry and observed electrochromism in the SCH3-substituted free ligand and mercury complex, concentratochromism, solvatochromism, halochromism, thermochromism, and chronochromism in the ligand, and photochromism in the carboxy-functionalized phenylmercury(II) complex are also presented here. Color changes are either associated with proton transfer or loss, or isomerization.

Synthesis and kinetics of sterically altered photochromic dithizonatomercury complexes.

Following a previous study where 12 electronically altered dithizones were synthesized, here we report on attempts to synthesize 26 dithizones. The purpose was to explore the boundaries within which dithizones may be synthesized, explore spectral tuning possibilities, and investigate steric effects on the photochromic reaction of its mercury complexes. Contrary to expectation, large substituents like phenoxy groups increased the rate of the photochromic back-reaction. In the series H-, 2-CH3-, 4-CH3-, 3,4-(CH3)2-, 2-OC6H5-, and 4-OC6H5-dithizonatophenylmercury(II), the lowest rate of 0.0004 s(-1) was measured for the 2-CH3 complex, while the rate for the 2-OC6H5 derivative was 20 times higher. A solvent study revealed a direct relationship between dipole moment and the rate of the back-reaction, while the relationship between temperature and rate is exponential, with t1/2 = 2 min 8 s for the 4-phenoxy complex. The crystal structures of two dithizone precursors, 2-phenoxy- and 4-phenoxynitroformazan, are reported.

Femtosecond laser spectroscopy and DFT studies of photochromic dithizonatomercury complexes.

The ultrafast dynamics of the photochromic reaction of dithizonatophenylmercury(II) was recently reported. For purpose of investigating the effect of electronically different substituents (X = o-F, m-F, p-F, p-Cl, o-CH3, m-CH3, p-CH3, m,p-diCH3, p-OCH3, o-SCH3, and p-SCH3) on this reaction, a series of phenyl-substituted dithizones were synthesized and complexed with phenylmercury(II). A variation of more than 3 ps in ground state repopulation times was observed, with the o-methyl derivative absorbing both at shortest wavelength and having the fastest repopulation time, while the p-S-methyl derivative lies at the opposite extremity. An increase in both decay times and λmax values is generally reflected by an increase in electron density in the chromophore. Ultrafast rates also proved to be dependent on solvent polarity, while a profound solvatochromic effect was observed in the transition state absorbance. Density functional theory realistically simulated isomer stabilities, electronic spectra and molecular orbitals. Increased electron density enhances stability in the photoexcited blue isomer relative to the orange resting state, as seen from a comparison between orange and blue isomer total bonding energies. A linear trend between computed HOMO energies and experimental λmax of related aliphatic substituted derivatives was found.

1-(2-Meth-oxy-phen-yl)-2-{[2-(2-meth-oxy-phen-yl)hydrazinyl-idene](nitro)-meth-yl}diazene.

In the title compound, C(15)H(15)N(5)O(4), a nitro-formazan derivative, the formazan unit is essentially planar with an r.m.s. deviation of 0.0204 (6) Šand adopts a closed syn,s-cis configuration with an intra-molecular N-H⋯N hydrogen bond. The formazan plane makes dihedral angles of 4.32 (5) and 24.35 (5)° with the benzene rings. The dihedral angle between the formazan plane and the nitro group is 12.58 (8)°. In the crystal, C-H⋯O inter-actions connect the mol-ecules into an inversion dimer.

1,5-Bis(2-methyl-phen-yl)-3-nitro-formazan.

In the title compound, C(15)H(15)N(5)O(2), the nitro O atoms are disordered over two sets of sites with an occupancy ratio of 0.75 (4):0.25 (4). Amine-imine tautomerism is observed in the formazan group. This was evident from the similar C-N bond distances in the formazan [1.319 (2) and 1.332 (3) Å], as well as the distribution of the imine proton in the Fourier difference map which refined to a 0.53 (3):0.47 (3) ratio. C-H⋯O and π-π inter-actions [centroid-centroid distances = 3.4813 (1) and 3.3976 (1) Å] are observed in the crystal packing.

(E)-1-[2-(Methyl-sulfan-yl)phen-yl]-2-({(E)-2-[2-(methyl-sulfan-yl)phen-yl]hydrazinyl-idene}(nitro)-meth-yl)diazene.

In the title compound, C(15)H(15)N(5)O(2)S(2), the phenyl rings make dihedral angles of 4.03 (4) and 9.77 (5)° with the plane defined by the central N-N-C-N-N atoms (r.m.s. deviation = 0.010 Å). The C-S-C-C torsion angles of the methyl-sulfanyl groups with their respective phenyl rings are -7.47 (13) and -72.07 (13)°. The shortest centroid-centroid distance of 3.707 Šoccurs between the two π-systems N-N-C-N-N and the benzene ring in the diazene 1-position. The H atom bound to the N atom is involved in intra-molecular N-H⋯N and N-H⋯S contacts, while the nitro O atoms are involved in inter-molecular C-H⋯O contacts.

(1,5-Diphenyl-thio-carbazonato-κS)trimethyl-tin(IV).

In the title compound, [Sn(C13H11N4S)(CH3)3], the Sn(IV) atom is coordinated by an S atom from the 1,5-diphenyl-thio-carbazonato (L) ligand [Sn-S 2.4710 (6) Å] and by three methyl groups [Sn-C 2.123 (3)-2.130 (2) Å] in a distorted tetra-hedral geometry. The aromatic rings of the L ligand form a dihedral angle of 2.1 (1)°.

[1,5-Bis(4-fluoro-phen-yl)thio-carbazo-nato-κN,S]phenyl-mercury(II) dichloro-methane hemisolvate.

In the title compound, [Hg(C(6)H(5))(C(13)H(9)F(2)N(4)S)]·0.5CH(2)Cl(2), the Hg(C(6)H(5)) units are twisted out of the planes of the thio-carbazo-nate ligands by 61.49 (10) and 67.79 (11)° in the two complex mol-ecules comprising the asymmetric unit. Important geometrical parameters include Hg-C = 2.079 (4) and 2.087 (4) Å, Hg-S = 2.3869 (10) and 2.3889 (11) Å, and C-Hg-S = 166.42 (12) and 168.09 (13)°. Weak intramolecular Hg-N bonding inter-actions of 2.589 (4) and 2.626 (4) Šare observed. In the crystal, C-H⋯Cl, C-H⋯F, C-H⋯N, C-H⋯π and π-π [centroid-centroid distances = 3.648 (3) and 3.641 (3) Å] inter-actions, create parallel planes along [101].

Dithizone and its oxidation products: a DFT, spectroscopic, and X-ray structural study.

Air oxidation of ortho-fluorodithizone resulted in the first X-ray resolved structure of a disulfide of dithizone, validating the last outstanding X-ray structure in the oxidation of dithizone, H(2)Dz, which proceeds via the disulfide, (HDz)(2), to the deprotonated dehydrodithizone tetrazolium salt, Dz. Density functional theory calculations established the energetically favored tautomers along the entire pathway; in gas phase and in polar as well as nonpolar solvent environments. DFT calculations using the classic pure OLYP and PW91, or the newer B3LYP hybrid functional, as well as MP2 calculations, yielded the lowest energy structures in agreement with corresponding experimental X-ray crystallographic results. Atomic charge distribution patterns confirmed the cyclization reaction pathway and crystal packing of Dz. Time dependent DFT for the first time gave satisfactory explanation for the solvatochromic properties of dithizone, pointing to different tautomers that give rise to the observed orange color in methanol and green in dichloromethane. Concentratochromism of H(2)Dz was observed in methanol. Computed orbitals and oscillators are in close agreement with UV-visible spectroscopic experimental results.

Ultrafast photochemistry of dithizonatophenylmercury(II).

The initial photochromic reaction of dithizonatophenylmercury(II) in solution was investigated by femtosecond transient absorption spectroscopy. Ultrafast excitation within less than 100 fs caused a radiationless photoreaction with a time constant of 1.5 ps, which is interpreted as C=N isomerization through a conical intersection. The orthogonally twisted intermediate state was observed through its excited-state absorption. Bifurcation along pathways towards the ground states of the orange cis and blue trans configurations occurs below the funnel of the conical intersection. The photochromism of the title compound in a very polar solvent such as methanol is observed for the first time.

Low-temperature redetermination of 1,3-bis-(penta-fluoro-phen-yl)triazene.

The crystal structure of the title compound, (C(6)F(5))(2)N(3)H, is stabilized by N-H⋯N hydrogen bonding, forming centrosymmetric dimers organized in a herringbone motif. Important geometrical parameters are N-N = 1.272 (2) and 1.330 (2) Šand N-N-N = 112.56 (15)°. The dihedral angle between C(6)F(5) groups is 21.22 (9)°. The room temperature structure was reported by Leman et al. (1993). Inorg. Chem.32, 4324-4336]. In the current determination, the data were collected to a higher θ angle, resulting in higher precision for the C-C bond lengths(0.001-0.005 versus 0.003 Å).

2,3-Bis(3-fluoro-phen-yl)tetra-zolium-5-thiol-ate.

The zwitterionic title compound, C(13)H(8)F(2)N(4)S, is situated on a twofold rotation axis running along the C-S [1.691 (2) Å] single bond. The phenyl-ene ring is twisted out of the tetra-zolium plane by 42.18 (7)°. Relatively short distances [3.7572 (9) and 4.0625 (6) Å] between the centroids of the phenyl-ene and tetra-zolium rings of neighbouring mol-ecules suggest π-π inter-actions. The crystal under investigation was a non-merohedral twin, with a 33% twin component.

A DFT perspective on the structures and electronic spectra of the orange and blue isomers of photochromic dithizonatophenylmercury(II).

The molecular structure and electronic spectra of the orange and blue isomers of the photochromic compound dithizonatophenylmercury(II) were theoretically studied utilizing density functional (DFT) methods. Computed structural results are in agreement with previously reported X-ray crystal data of the orange resting state. The herewith newly proposed geometrical structure of the blue photoexcited state is favored by more than 35 kJ x mol-1 relative to the historically hypothesized geometry of the blue isomeric form. The key difference lies in the position of the backbone amine proton, being situated on the N4 position in the newly proposed structure, rather than on the N2 position as in the previously hypothesized geometry. Time dependent density functional theory as implemented in the Amsterdam Density Functional (ADF) and Gaussian 03 (G03) program systems yielded excitation energies for the blue isomer exhibiting bathochromic shifts, as observed in the experimentally determined UV/visible spectrum. B3LYP calculated excitation energies and oscillator strengths gave the best approximation of the experimentally observed electronic spectra of both isomers.