UV Absorption Band Research Articles

Synthesis, luminescence and excited state absorption properties of conjugated D-π-A and D-π-D phenothiazine compounds

Four D-π-A or D-π-D phenothiazine compounds bearing different substituents on central π-bridge (R = -H (PTZ-1), -Br (PTZ-2), -CN (PTZ-3) and -OC12H25 (PTZ-4)) were synthesized and characterized. The influence of the substituents on the photophysics of these compounds is systematically investigated by spectroscopic methods and simulated by density functional theory (DFT) calculations. All compounds exhibit intense absorption bands in UV and visible region, which are influenced by the different substituents on the central π-bridge. Both electron-withdrawing and electron-donating substituents cause bathochromic shifts of the low energy 1π-π*/1ICT (intramolecular charge transfer) absorption bands. Meanwhile, all compounds are emissive in solution at room temperature (λem = 400–800 nm, Φem = 0.01–0.74). Their fluorescent quantum yields decrease when the electron-withdrawing substituents attached on the central π-bridge, but increase when electron-donating substituents were introduced. In addition, PTZ-1–PTZ-4 all exhibit strong triplet transient absorption (TA) in visible region. When the electron-donating substituents attached, the triplet excited-state molar extinction coefficient (190200 L mol−1 cm−1) and triplet excited-state lifetime (53.2 μs) of PTZ-4 were significantly enhanced. Their luminescent and excited state absorption properties could provide us a basis for elucidating the structure-property correlations of phenothiazine compounds and developing novel optical functional materials.

Synthesis, thermal stability and unknown properties of Fe1–xAlxVO4 solid solution

A continuous solid solution Fe1-xAlxVO4 was synthesized by conventional ceramic method and characterized by DTA, XRD, IR, UV–vis/DRS and Mössbauer spectroscopy. Fe1-xAlxVO4 crystallizes in a triclinic system and is isostructural with FeVO4 and AlVO4. With increasing content of Al3+ ions substituted for Fe3+ in the matrix structure of FeVO4 a contraction of crystal lattice was observed, accompanied by shifts of IR absorption bands towards higher wavenumbers and shifts of UV absorption bands towards shorter wavelength. On the basis of the results of UV–vis investigations, the band gap energies were calculated. The solid solution sample of composition Fe0.67Al0.33VO4 was found to have the highest melting point from among all examined samples, which equals to 870 °C. Moreover, the solid product of incongruent melting of Fe1-xAlxVO4 for 0.75 > x ≥ 0.5 is Fe1-xAlxVO4 solid solution, richer in iron. On the grounds of Mössbauer investigation, the partition of iron and aluminum ions over three accessible crystallographic sites was established. Fe0.5Al0.5VO4 sample was successfully used as a reactant for Fe4Al4V10W16O85 preparation.

Optical and infrared spectral investigations of cadmium zinc phosphate glasses doped with WO3 or MoO3 before and after subjecting to gamma irradiation

Optical and FTIR spectral investigations of prepared undoped and WO3-or MoO3-doped glasses from ZnO-CdO-P2O5 glassy system were carried out before and after irradiation with a dose of 8 Mrad (= 8 × 104 Gy). The optical spectrum of the undoped glass reveals strong UV absorption band due to trace ferric ions present as trace impurity within the chemicals. The WO3 or MoO3-doped glasses show additional UV–visible bands due to the dominance presence of the low valence states of the two transition metal (TM) ions (M3+, M4+, M5+). Gamma irradiation extends and increases the UV absorption due to suggested photochemical reactions of ferrous iron present as impurity to react with positive holes during the irradiation process to transfer to additional ferric ions possessing their absorption in the UV region. The TM-doped glasses show some shielding behavior in almost the persistence of the visible spectra due to the TM ions. FTIR spectral bands are related to phosphate groups (mainly Q2 and Q3 units) in accordance with the high content of P2O5 (70 mol%). The effects of dopant ions on the IR spectra are limited due to the low percent (0.25 to 0.75 g/100 g) and their presence in modifying positions. Gamma irradiation decreases the intensity of some IR bands due to OH, water vibrations present mainly in terminal sites.

A facile route to ionic liquids-functionalized ZnO nanorods for the fluorometric sensing of thiabendazole drug

In this report, we introduce a simple and cost-effective hydrothermal method for the functionalization of ZnO nanorods with three different ionic liquids (ILs (1–3)) to check their sensing response with various drugs including thiabendazole, phenylbutazone, chloramphenicol, neomycin, and theophylline through fluorescence spectroscopy. The rod like-microstructure and hexagonal wurtzite structure of synthesized ZnO nanorods were examined with SEM-EDS and XRD techniques. On the other hand, ionic liquids-functionalized ZnO nanorods (ILs (1–3)/ZnO) exhibited various self-organized (layered-sheet~IL1, distorted nanorods~IL2, peanut beaded surface~IL3/ZnO) structures due to the intercalation of ILs on the surface of ZnO nanorods. Compared to ZnO nanorods, ILs (1–3)/ZnO nanorods showed a blue shift in UV-absorption band which resulted to the variation in optical bandgap values. Ionic liquids-functionalized nanorods samples (ILs (1–3)/ZnO) have been selected for their recognition behaviour with available drug molecules using fluorescence spectroscopic technique. Interestingly, with the addition of thiabendazole drug molecules to two samples (ILs (1,3)/ZnO), a significant response corresponding to the peaks at ~350nm and ~357nm was observed, which led to the development of fluorescent sensors with a detection limit of 12 and 304nM, respectively.

Preparation of TiO2-loaded electrospun fibers of polylactide/poly(vinylpyrrolidone) blends for use as catalysts in epoxidation of unsaturated oils

Nanofibers of polylactide (PLA)/poly(vinylpyrrolidone) (PVP) blends, loaded with TiO2 nanoparticles, have been prepared by an electrospinning method. The electrospun fiber mats were characterized by ATR-FTIR, X-ray diffraction (XRD), SEM, EDX, and UV-visible spectroscopy to examine structures, functional groups, crystallinity, surface morphology, and UV absorptivity. It is clearly observed that TiO2 particles are embedded on the filaments. All PLA-based spun fibers are completely amorphous in nature. The surface morphology of those blended with PVP is smoother and more uniform than the corresponding samples without PVP. Neat PLA fibers show a UV absorption band at around 200 nm, whereas the fibers loaded with TiO2 nanoparticles show an additional absorption band covering the 200–380-nm region. Photo-degradation of the fiber samples are conducted in phosphate buffer solution (PBS) under UVA light. The results indicate that the PVP component dissolves into the PBS solution, and the PLA matrix degrades as a function of time. The fibers are then applied as a catalytic system for epoxidation of unsaturated sunflower oil (SFO), for use as additives or plasticizers for biopolymers, employing a performic acid oxidizing agent. The fibers, especially those containing PVP, can effectively enhance the epoxidation yield of oils with a slow rate of undesirable side reactions, which break ester bonds of triglycerides to generate free fatty acids.

Optical absorption and fluorescence spectroscopy studies of Artepillin C, the major component of green propolis

The bioactivity of propolis against several pathogens is well established, leading to the extensive consumption of that bee product to prevent diseases. Brazilian green propolis, collected by the species Apis mellifera, is one of the most consumed in the world. The chemical composition of green propolis is complex and it has been shown that it displays antioxidant, antimicrobial, anti-inflammatory and antitumor activities, especially due to the high content of Artepillin C. The molecule is a derivative of cinnamic acid with two prenylated groups, responsible for the improvement of the affinity of the compound for lipophilic environment. A carboxylic group (COOH) is also present in the molecule, making it a pH-sensitive compound and the pH-dependent structure of Artepillin C, may modulate its biological activity related to interactions with the cellular membrane of organisms and tissues.Molecular properties of Artepillin C on aqueous solution were examined by optical absorption, steady state and time-resolved fluorescence spectroscopies. Acid-base titration based on the spectral position of the near UV absorption band, resulted in the pKa value of 4.65 for the carboxylic group in Artepillin C. In acidic pH, below the pKa value, an absorption band raised around 350nm at Artepillin C concentration above 50μM, due to aggregation of the molecule. In neutral pH, with excitation at 310nm, Artepillin C presents dual emission at 400 and 450nm. In pH close to the pKa, the optical spectra show contribution from both protonated and deprotonated species. A three-exponential function was necessary to fit the intensity decays at the different pHs, dominated by a very short lifetime component, around 0.060ns. The fast decay resulted in emission before fluorescence depolarization, and in values of fluorescence anisotropy higher than could be expected for monomeric forms of the compound. The results give fundamental knowledge about the protonation-deprotonation state of the molecule, that may be relevant in processes mediated by biological membranes.

Optical, FT infrared and photoluminescence spectra of CeO2 – doped Na2O – ZnO – B2O3 host glass and effects of gamma irradiation

Collective optical, FTIR and photoluminescence spectra of prepared (1 → 10) CeO2 doped Na2O – ZnO – B2O3 glasses have been examined before and after gamma irradiation doses 2 or 9 × 104 Gy (2 or 9 Mrad). Optical absorption spectrum of undoped glass reveals distinct UV absorption bands which are correlated with trace impurities of ferric ions. Upon gamma irradiation, the UV bands reveal limited changes accompanied with the generation of an induced visible band which is assumed to be due to (BOHC) or (NBOHC). The CeO2 – doped glasses show an additional UV band due to Ce3+ ions while the absorption for Ce4+ ions are assumed to be overlapped by the strong charge transfer absorption due to traces impurities of Fe3+ ions. The irradiated CeO2 – doped glasses reveal no induced visible band due to the efficiency of cerium ions to capture electrons and positive holes during the irradiation. The CeO2 – doped glasses show a distinct luminescence peak centered at 410 nm after excitation at 308 nm and its intensity increases with the CeO2. The optical parameters of the band gap energy and Urbach energy are found to change with the CeO2 content. FTIR spectra of the undoped and CeO2 – doped glasses are almost similar with minor variations in the intensities of some vibrational bands and they show vibrational modes due to both triangular and tetrahedral borate groups in their specific different wavenumbers. The IR spectra of the irradiated glasses reveal stability with minor variations in the intensities of some bands.

Topological, chemical and electro-optical characteristics of riboflavin-doped artificial and natural DNA thin films.

DNA is considered as a useful building bio-material, and it serves as an efficient template to align functionalized nanomaterials. Riboflavin (RF)-doped synthetic double-crossover DNA (DX-DNA) lattices and natural salmon DNA (SDNA) thin films were constructed using substrate-assisted growth and drop-casting methods, respectively, and their topological, chemical and electro-optical characteristics were evaluated. The critical doping concentrations of RF ([RF]C, approx. 5 mM) at given concentrations of DX-DNA and SDNA were obtained by observing the phase transition (from crystalline to amorphous structures) of DX-DNA and precipitation of SDNA in solution above [RF]C. [RF]C are verified by analysing the atomic force microscopy images for DX-DNA and current, absorbance and photoluminescence (PL) for SDNA. We study the physical characteristics of RF-embedded SDNA thin films, using the Fourier transform infrared spectrum to understand the interaction between the RF and DNA molecules, current to evaluate the conductance, absorption to understand the RF binding to the DNA and PL to analyse the energy transfer between the RF and DNA. The current and UV absorption band of SDNA thin films decrease up to [RF]C followed by an increase above [RF]C. By contrast, the PL intensity illustrates the reverse trend, as compared to the current and UV absorption behaviour as a function of the varying [RF]. Owing to the intense PL characteristic of RF, the DNA lattices and thin films with RF might offer immense potential to develop efficient bio-sensors and useful bio-photonic devices.

TiO2 Nanoribbons/Carbon Nanotubes Composite with Enhanced Photocatalytic Activity; Fabrication, Characterization, and Application

TiO2 nanoribbons (TiO2 NRs) loaded with FeCo-Al2O3 catalyst were synthesized and used as a precursor in the synthesis of TiO2 nanoribbons/carbon nanotubes (TiO2 NRs/CNTs) composite by a chemical vapor deposition (CVD) method. TiO2 NRs and TiO2 NRs/CNTs composite were characterized by XRD, FT-IR, TEM, SEM, EDX and UV-Vis spectrophotometer. The results revealed the formation of TiO2-B and hydrogen titanate nanoribbon like structures by the hydrothermal treatment. After loading TiO2 NRs by FeCo-Al2O3 catalyst and the CVD growth of carbon nanotubes, the synthetic TiO2 nanoribbons converted entirely to TiO2-B nanoribbons with nanopits structure. The composite composed of tube-like nanostructures forming an interlocked network from CNTs and TiO2-B NRs. The composite shows a relatively red-shifted band gap (3.09 eV), broader and stronger UV absorption band relative to TiO2 NRs. The photocatalytic properties of TiO2 NRs and TiO2 NRs/CNTs composite were studied under sunlight irradiation. The photocatalytic degradation of methylene blue (MB) dye was investigated as a function of contact time, dye concentration, and catalyst dose. The kinetics and mechanisms of degradation were discussed. TiO2 NRs/CNTs composite showed higher stability after six runs and 50% shorter irradiation time than TiO2 NRs photocatalyst.

Sucrose capped gold nanoparticles as a plasmonic chemical sensor based on non-covalent interactions: Application for selective detection of vitamins B1 and B6 in brown and white rice food samples

We report simple and selective method for detection of vitamins B1 and B6 in brown and white rice samples using localized surface plasmon resonance (LSPR) of sucrose capped gold nanoparticles (AuNPs) as a chemical sensor colorimetrically. Here, detection is based on the color change of AuNPs from pink to blue followed by a red shift of LSPR absorption band in UV–vis region with the addition of vitamins B1 and B6 into the NPs solution. A good linear range was observed in the range of 25–1000 ngmL−1 with detection limit of 8 ngmL−1 for B1 and 50–1000 ngmL−1 with detection limit of 15 ngmL−1 for vitamins B6. The employment of AuNPs for detection of B1 and B6 vitamins in rice food samples showed remarkable abilities in terms of the simplicity, low cost, stability, reproducibility and sensitivity.

Heterostructure nano-NiO/BiOCl composites with advanced adsorption and photocatalytic performance for organic dyes

Bismuth oxychloride (BiOCl) micro-particles with the heterostructure of deposited nickel oxide (NiO) nanoparticles were successfully synthesized by a green and facile hydrolysis method. Characterisation results confirmed that a NiO/BiOCl heterostructure was present due to the observation of clear absorption bands in UV–vis DRS attributed to the interaction of NiO and BiOCl. The as-prepared NiO/BiOCl sample (with molar ratio of 0.4:1) exhibited an advanced adsorption capacity and photocatalytic ability for photodegradation of methyl orange under visible light irradiation. The photodegradation of MO showed that approximately 95% of MO molecules were degraded in the presence of NiO/BiOCl after irradiation for 30 min. All the experimental results revealed that NiO nano-heterostructure was not only responsible for high visible light photocatalytic activity, but it also played a vital role in improving separation of the photogenerated holes and electrons.

Gamma Irradiation Effect on Structural and Spectral Properties of CeO2, Nd2O3, Gd2O3 or Dy2O3 – Doped Strontium Borate Glass

Some selected rare – earth (RE) doped strontium borate glasses were prepared by a melting / annealing technique. Optical, FTIR and photoluminescence (PL) spectra were measured before and after gamma irradiation with a dose of 10 Mrad (10 × 104 Gy). The optical absorption spectrum of undoped glass reveals a strong UV absorption band at 231 nm and with gamma irradiation the intensity of the UV peak increases with the generation of an additional induced visible broad band centered at 570 nm. Rare – earth – doped glasses show the appearance of additional small visible peaks in the Nd2O3 and Dy2O3 – doped glasses while the CeO2 and Gd2O3 glasses reveal only the UV absorption peak as for the undoped sample. Gamma irradiation produces an additional induced visible band in the Gd2O3 and Dy2O3 samples only, while CeO2 – doped glass shows obvious shielding resistance to gamma irradiation and its optical spectrum remains unchanged. Infrared absorption spectral measurements indicate that both triangular and tetrahedral borate units (BO3, BO4) are distinctly vibrating at their specific wavenumbers; the BO4 groups within the range 800 – 1200 cm− 1 while the BO3 groups are within the range 1200 – 1450 cm− 1. Gamma irradiation or the addition of rare – earth dopants causes no distinct variations in the number or position of the main IR vibrational bands due to the two coordination states of the borates. Photoluminescence spectra of rare – earth doped glasses show variable responses after excitation depending on the type of RE – dopant. The doped glasses after gamma irradiation (10 Mrad) exhibit darkness or deeping of their colors and show no distinct emission spectra.

Cerium/aluminum correlation in aluminosilicate glasses and optical silica fiber preforms

Optical, structural properties and redox were studied for Ce-bearing silicate and aluminosilicate glasses and Ce-activated silica (SiO2) fiber preforms, by X-ray Absorption, Raman and Photoluminescence Spectroscopy.The introduction of Al2O3 in Ce-bearing silicate glass strongly stabilizes reduced species in agreement with the optical basicity concept and additionally induces modifications in the Ce3+ surrounding. In Al-free silica preforms, depending on the collapsing conditions, the Ce3+/ΣCe ratio has the highest variability (Ce3+/ΣCe=0.7–0.85±0.07), whereas in Al-bearing fiber preforms, Al codoping induces a strong stabilization of Ce3+ species, despite the collapsing atmosphere.Al presence induces modifications on Ce local environment, and thus variations of the distance between Ce3+ and its ligands, and/or to variations of the covalency of the bonds. Absorption bands in the UV region, ascribed to Charge-transfer (CT) bands, in our samples, occur only for low amount of Ce4+ (Ce3+/ΣCe>0.85±0.07). Indeed, in silicate and aluminosilicate glasses the higher amount of Ce4+ does not give rise to UV absorption bands ascribed to CT processes, whereas, in fiber preforms, Ce3+ species can induce the trapping of hole centers, and enhance the photoluminescence efficiency. More generally, these results enhance our understanding of REE structural and chemical roles in amorphous materials and increase our knowledge for technical applications.

Synthesis, characterization, thermal properties, conductivity and sensor application study of polyaniline/cerium-doped titanium dioxide nanocomposites

This work focuses on the in situ chemical oxidative polymerization of aniline with different contents of cerium-doped titanium dioxide (Ce–TiO2) nanoparticles for the synthesis of polyaniline (PANI)/metal-doped TiO2 nanocomposites. The interaction of nanoparticles with PANI segments was characterized by FT-IR, UV, XRD, SEM, DSC, TGA and conductivity studies. The sensitivity of ammonia gas through the fabricated nanocomposite was also evaluated with respect to different contents of nanoparticles. The shift in FT-IR peak and UV absorption bands of nanocomposite with respect to pure PANI was clearly observed from the spectroscopic studies. Results from XRD studies showed the uniform arrangement of nanoparticles within the polymer matrix. SEM analysis showed the uniform structure of nanocomposites with spherically shaped dispersion of metal oxide nanoparticles. DSC studies revealed the increased glass transition and melting temperature of nanocomposites with an increase in the concentration of nanoparticles indicating the strong intermolecular interaction between nanoparticles and the polymer chain. TGA showed a higher thermal stability of nanocomposites than that of pure PANI, and thermal stability increases with the increase in the concentration of nanoparticles. Electrical properties such as AC conductivity, dielectric constant and dielectric loss tangent of nanocomposites were greater than pure PANI, and the magnitude of these properties increased with the loading of nanoparticles. Like AC conductivity, the DC conductivity of nanocomposite increased with the loading of nanoparticles. The maximum electrical conductivity was noted for 7 wt% composite. The ammonia sensing properties of composite were higher than the pristine PANI.

Identification of Superoxide O2− during Thermal Decomposition of Molten KNO3-NaNO2-NaNO3 Salt by Electron Paramagnetic Resonance and UV-Vis Absorption Spectroscopy

On account of excellent thermal physical properties, molten nitrates/nitrites salt has been widely employed in heat transfer and thermal storage industry, especially in concentrated solar power system. The thermal stability study of molten nitrate/nitrite salt is of great importance for this system, and the decomposition mechanism is the most complicated part of it. The oxide species O22− and O2− were considered as intermediates in molten KNO3-NaNO3 while hard to been detected in high temperature molten salt due to their trace concentration and low stability. In this work, the homemade in situ high temperature UV-Vis instrument and a commercial electron paramagnetic resonance were utilized to supply evidence for the formation of superoxide during a slow decomposition process of heat transfer salt (HTS, 53 wt% KNO3/40 wt% NaNO2/7 wt% NaNO3). It is found that the superoxide is more easily generated from molten NaNO2 compared to NaNO3, and it has an absorption band at 420–440 nm in HTS which red shifts as temperature increases. The band is assigned to charge-transfer transition in NaO2 or KO2, responsible for the yellow color of the molten nitrate/nitrite salt. Furthermore, the UV absorption bands of molten NaNO2 and NaNO3 are also obtained and compared with that of HTS.

Effect of morphology control on optical properties of PMMA/boehmite nano-hybrid prepared through facile one-pot process

The morphology of PMMA/boehmite hybrid nanoparticles was controlled through the facile aqueous one-pot process starting directly from primary precursors. The effect of morphology control on optical properties of PMMA/boehmite nanocomposites was examined. Boehmite content varied from 0 to 5% and some physical properties such as density, band gap, visible transparency (400–800 nm) and UV absorption were studied. Transparent PMMA/boehmite hybrid nanoparticles (400–800 nm) with controlled morphologies were prepared. It was shown that the position of the absorption band in UV–Vis spectra of PMMA/boehmite nanocomposite depends on morphology. Morphology control leads to 13.7% increase of band gap without changing the composition. The band gap also increased with increasing boehmite in both morphologies. The results suggest these polymeric nanocomposites have good potential for application in the field of UV filters due to their high absorption in the range of 400–200 nm and high transparency in the visible region.

New Insights into the Wavelength Dependence of MALDI Mass Spectrometry.

The interplay between the wavelength of the laser and the absorption profile of the matrix constitutes a crucial factor in matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS). Numerous studies have shown that typically best analytical results are obtained if the laser wavelength matches the UV absorption band of the matrix in the solid state well. However, many powerful matrices exhibit peak absorptions which differ notably from the standard MALDI laser wavelengths of 337, 349, and 355 nm, respectively. Here we used two wavelength-tunable lasers to investigate the MALDI wavelength dependence with a selected set of such matrices. We studied 3-hydroxypicolinic acid (3-HPA), 2,4,6-trihydroxyacetophenon (THAP), dithranol (1,8-dihydroxy-10H-anthracen-9-on), 2-(4'-hydroxybenzeneazo)benzoic acid (HABA), and 6-aza-2-thiothymine (ATT). For analyte systems we investigated DNA oligomers (3-HPA), phospholipids (dithranol, THAP, HABA), and non-covalent peptide-peptide and protein-peptide complexes (ATT). We recorded analyte ion and total ion counts as a function of wavelength and laser fluence between 213 and 600 nm. Although the so-generated comprehensive heat maps generally corroborated the previously made findings, several fine features became notable. For example, despite a still high optical absorption exhibited by some of the matrices in the visible wavelength range, ion yields generally dropped strongly, indicating a change in ionization mechanism. Moreover, the non-covalent complexes were optimally detected at wavelengths corresponding to a relatively low optical absorptivity of the ATT matrix, presumably because of ejection of a particular cold MALDI plume. Our comprehensive data shed useful light into the MALDI mechanisms and could assist in further methodological advancement of the technique.

Gamma rays Interactions with Bismuth Phosphate Glasses Doped with 3d Transition Metal Oxides

Bismuth phosphate glasses of the basic composition (Bi2O3 30 mol%-P2O5 70 mol%) with additional dopants 3d TM oxides (0.2 wt%) were prepared by the melting and annealing technique. Combined optical (UV/vis.) and FT infrared absorption spectra were measured for the prepared samples before and after gamma irradiation with a dose of 8 Mrad (8×104 Gy). Optical spectra reveal strong UV absorption bands due to trace iron impurity together with an additional absorption band due to Bi3+ beside characteristic absorption related to specific 3d TM ions with preference for the lower valences due to the reducing effect of phosphate host glass. FTIR spectra show vibrational bands due to phosphate chains with the sharing of absorption bands due to Bi-O vibrations. Gamma irradiation causes limited changes due to the presence of heavy metal Bi3+ ions which show some shielding behavior towards gamma irradiation as revealed by optical and FT infrared absorption measurements. Some suggested photochemical reactions are forward to interpret the changes in the UV spectra beside the formation of an induced phosphorus oxygen hole center (POHC) in the visible region.

High-energy exciton transitions in quasi-two-dimensional cadmium chalcogenide nanoplatelets

Semiconductor nanoparticles of cadmium chalcogenides are known to exhibit pronounced thickness-dependent $E_0$ series of exciton transitions at the $\Gamma$ point of the Brillouin zone (BZ). In this work, we report an experimental evidence for high-energy series of exciton transitions, which originates from BZ points different from the $\Gamma$ point, in the family of cadmium chalcogenide quasi-2D nanoplatelets (NPLs). Intensive UV absorption bands demonstrating a pronounced size effect are observed for CdTe, CdSe, and CdS NPLs in addition to the $E_0$ exciton bands in the visible region. These new bands are attributed to transitions analogous to the $E_1$, $E_1+\Delta_1$, and $E_2$ series observed in bulk crystals. First-principles DFT calculations of the electronic structure and absorption spectra support this explanation and show that the main contribution to these optical transitions comes from $X$ and $M$ points of the 2D BZ, which originate from $L$ and $X$ points of the 3D BZ. At the same time, the $E_0$ series of transitions at the $\Gamma$ point is well described by the multiband effective-mass model. The observation of the UV exciton bands reveals tunable optical properties of cadmium chalcogenide NPLs in UV spectral region, which may be interesting for practical applications.

Structural and spectroscopic characterization of five coordinate iron and cobalt bis(dithiolene)-trimethylphosphine complexes

Heteroleptic bis(dithiolene)-phosphine iron and cobalt complexes [Fe(adt)2(PMe3)] (1) and [Co(adt)2(PMe3)] (2) (adt = para-anisyldithiolene) have been synthesized from corresponding bis(dithiolene) dimers [M2(adt)4]2 (M = Fe, Co) by reacting with excess PMe3 in dichloromethane. Solid-state structures of 1 and 2 have been determined by single crystal X-ray crystallography, and the dithiolene ligands CS (≈1.72 Å) and CC (≈1.37 Å) bond distances reveal the coordination of π-radical monoanionic dithiolene (adt•1−) ligands to metal centers. Intense low energy ligand-to-ligand-charge transfer (LLCT) absorption band (743 nm for 1; 905 nm for 2) in UV–vis spectra and characteristic ν(CS•) (1168 cm−1 for 1; 1170 cm−1 for 2) in IR spectra affirms coordination of π-radical monoanionic dithiolenes to divalent metal ions. The cyclic voltammogram of 1 and 2 shows reversible oxidation and reduction waves attributed to MII → MIII + e− (+0.52 V for 1; +0.29 V for 2) and adt•1− + e− → adt2− (−0.63 V for 1; −0.46 V for 2) redox process respectively. Comprehensive structural and spectroscopic investigations conclude, [M2III(adt2−)2(adt•1-)2] → 2 [MII(adt•1-)2(PMe3)] intramolecular redox interplay during phosphine coordination induced cleavage of homoleptic bis(dithiolene) dimers to produce square pyramidal complexes.