Absorption Spectra Of Transitions Research Articles

Arylgold nanoclusters: Phenyl-stabilized Au44 with thermal-controlled NIR single/dual-channel phosphorescence.

Arylation of gold holds paramount importance in the domain of organometallic chemistry; however, the exploration of arylgold nanoclusters remains in its infancy primarily due to the synthetic challenge. Here, we present a facile and effective arylation strategy to directly synthesize two arylgold nanoclusters (Au44a and Au44b), by using tetraarylborates, capable of transferring aryl fragments to metal centers. X-ray crystallography reveals that both Au44 nanoclusters contain an Au44 kernel co-protected by six aryl groups, two tetrahydrothiophene, and 16 alkynyl-ether ligands, the latter is generated in situ through Williamson ether reaction during the assembly processes. Notably, Au44 nanoclusters exhibit near-infrared (NIR) phosphorescence (λmax = 958 nm) and microsecond radiative relaxation at ambient condition, which is a thermal-controlled single/dual-channel phosphorescent emission revealed by temperature-dependent NIR, time-resolved emission, and femtosecond/nanosecond transition absorption spectra. This work represents a breakthrough in using aryl as protective ligands for the construction of gold nanoclusters, which is poised to have a transformative impact on organometallic nanoclusters.

Ab-initio study of electronic, magnetic, and optical properties of Ni mono-doped and (I, Ni) co-doped CdS nanowires

Diluted magnetic semiconductors typically display induced ferromagnetism and spin-dependent interactions at high Curie temperatures, which is advantageous for application in magneto-electronic devices. The causes of ferromagnetism and optical emission dynamics, which have complex microstructural and compositional properties, are still not fully understood. Here, using first-principles calculations, the impact of Ni(II) ion substitution on the electronic, optical, and magnetic properties of CdS NW has been studied for the first time. The results show that when the size of the nanowire increases, the bandgap and binding energy decrease. The most favorable site for Ni doping was found to be at the surface of the nanowire. The most stable configuration with Ni ions on the surface of the nanowire has AFM as its ground state, which may be described using the super-exchange mechanism. The ground state configuration shifts from AFM to FM due to the addition of the extra electron provided by iodine (I) co-doping, and the estimated Curie temperature is expected to be higher than the ambient temperature. The fundamental bandgap of CdS nanowire is redshifted with Ni(II) doping, and the observed d-d intra-band transition in the absorption spectra at the low energy side of the optical bandgap is well consistent with the experimental results. Furthermore, the optical bandgap and d-d intra-band transitions in the optical absorption spectra are correlated with the magnetic coupling between Ni(II) ions, and we find a red-shift/blue-shift of d-d intra-band transition peaks and bandgap in the FM/AFM coupled ions system, supporting the experimental observations. The enhanced optoelectronic and magnetic properties of (Ni, I) co-doped CdS nanowires suggest their possible application in spintronic and optoelectronic devices, such as remote sensing and photovoltaics.

Analytical fitting of temperature-dependent spin-flip transitions in absorption spectra of Cr3+-doped silicate glasses

Temperature-dependent optical absorption spectra have been recorded for Cr3+ doped silicate glasses between 10 K and 773 K. Due to the intermediate ligand field strength of the Cr3+ coordination environment, overlap of the lowest-energy spin-allowed 4A2 → 4T2 band and the two spin-forbidden transitions to the 2E and 2T1 states at approximately 650 nm results in interference dips. An analytical model equation is fitted to the spectra and electronic structure parameters extracted. This represents the first time that variations of these parameters with temperature are obtained over a large temperature range, revealing characteristics of all three overlapping d-d transitions.

Photo-induced absorption spectra of a poly(p-phenylenevinylene) polymer with fluorinated double bonds

We report on the optical and photo-physical properties of a fluorinated poly(p-phenylenevinylene) derivative, namely poly(2-methoxy-5-[(2-ethylhexyl)oxy]-1,4-phenylenedifluorovinylene) (MEH-PPDFV) and we compare them with those of the well-known non fluorinated reference system, poly(2-methoxy-5-[(2-ethylhexyl)oxy]-1,4-phenylenevinylene) (MEH-PPV) synthesized with the same procedure.Remarkable blue-shifts of absorption and emission spectra of MEH-PPDFV with respect to MEH-PPV are observed, due to the larger distortion from planarity of the fluorinated species. In addition, the blue-shift for optical transitions in absorption spectra is more pronounced than in emission ones, since the molecule conformation in the excited states is more planar than in the ground one.Microsecond photo-induced absorption spectra of MEH-PPDFV reveal three bands possessing spectral properties and/or kinetics different from those observed for MEH-PPV. The assignment of these transitions is provided in terms of free and trapped polarons as well of triplet excitons. Free-polarons have an intrinsic decay constant of 3 μs, while the trapping dynamics has a longer kinetics of 40 μs. The triplet exciton, instead, shows a monoexponetial decay behavior of constant about 10 μs.

Measurement of Hyperfine Structure and Isotope Shift of Transitions in Gadolinium Using Blue Diode Laser

Laser-induced fluorescence spectroscopy using a single-longitudinal-mode diode laser has been applied to investigating isotope shifts and the hyperfine structures of gadolinium in the wavelength region of 400 nm. Six absorption spectra of transitions from the ground and low-lying metastable levels were measured. The isotope shift per unit mass change varied from 0.6 to 1.0 GHz depending on the transition. The hyperfine splitting constants of 157Gd and 155Gd isotopes were determined for the six upper levels of the transitions. The absorption spectrum of the 999–25661 cm-1 transition showed the narrowest spectral width for odd isotopes.

Nonoxovanadium(IV) and Oxovanadium(V) Complexes with Mixed O, X, O-Donor Ligands (X = S, Se, P, or PO)

Ligating properties of four potentially tridentate bisphenol ligands containing [O, X, O] donor atoms (X = S 1, Se 2, P 3, or P=O 4) toward the vanadium ions in +IV or +V oxidation states have been studied. Each ligand with different heterodonor atoms yields as expected nonoxovanadium(IV) complexes, V(IV)L(2), whose structures have been determined by X-ray diffraction methods as having six-coordinate V(IV), VO(4)X(2), core. Compounds 1-4 have also been studied with electrochemical methods, variable-temperature (2-295 K) magnetic susceptibility measurements, X-band electron paramagnetic resonance (EPR) (2-60 K) spectroscopy, and magnetic circular dichroism (MCD) (5 K) measurements. Electrochemical results suggest metal-centered oxidations to V(V) (i.e., no formation of phenoxyl radicals from the coordinated phenolates). A combination of density functional theory calculations and experimental EPR investigations indicates a dramatic effect of the heteroatoms on the electronic structure of 1-4 with consequent reordering of the energy levels; 1 and 3 possess a trigonal ground state (d(z)()(2))(1), but 4 with the phosphoryl oxygen as the heterodonor atom in contrast exhibits a tetragonal ground state, (d(xy)())(1). On the basis of the intense electronic transitions in absorption spectra, all electronic transitions observed for 4 have been assigned to ligand-to-metal charge-transfer transitions, which have been confirmed by preliminary resonance Raman measurements and C/D ratios obtained from low-temperature MCD spectroscopy. Moreover, diamagnetic complexes 5 and 6 containing mononuclear and dinuclear oxovanadium(V) units have also been synthesized and structurally and spectroscopically ((51)V NMR) characterized.

Nature of π-Electronic Transitions in UV Spectra of Deoxyvasicinone and Its Derivatives

The principal types of electronic transitions in absorption spectra of deoxyvasicinone and its 6-amino- and 6-nitro-derivatives were determined by analyzing the electron-density distributions in excited states calculated using ZINDO/S.

Studies on electronic structures and related properties of complexes M(tap)32+ (M = Fe, Ru, Os) by the DFT method

Studies on a series of complexes M(tap)32+ (M = Fe, Ru, Os; tap = 1,4,5,8-tetraazaphenanthrene) are carried out using the DFT method at the B3LYP/LanL2DZ level. The electronic structures and related chemical properties of complexes M(tap)32+, e.g. the energies and components of some frontier molecular orbitals, the spectral properties, and the net charge populations of some main atoms of the complexes, etc. have been investigated. In addition, computations on the complex Ru(phen)32+(phen = 1,10-phenanthroline) are also performed for comparison. The computational results show some very interesting trends in electronic structures and related properties of complexes M(tap)32+. First, the computational energies of some frontier molecular orbitals of Ru(tap)32+ are all lower than those of Ru(phen)32+. In particular, the energies of LUMOs of the complexes Os(tap)32+ and Ru(tap)32+ are all rather low and their electron affinities are rather high, leading to the prediction that an electron is very easily transferred from DNA guanine base to the LUMO of the complexes. This may be a major reason why these cations are excellent photoreagents for DNA. Second, the components of the HOMO of the four complexes all arise mainly from d orbitals of the central metal ions, and the components of the LUMO arise mainly from p orbitals of C and N atoms in the ligands, so their electronic ground state bands are theoretically all assigned to singlet metal-to-ligand charge-transfer (1MLCT) transitions in absorption spectra, similar to those of Ru(bpy)32+. Third, for M(tap)32+, the most negative charges are populated on N1, with the next populated on N4. Fourth, the chemical stabilities (S) of the complexes are in order SIII > SII > SI, according to coordination energy computations. In addition, the central atom M has little effect on the coordination bond lengths and bond angles of the complexes. The obtained results should be important references for the synthesis of the complexes, mechanistic analysis of their photochemistry, electrochemistry and catalysis chemistry.

Photoionization related phenomena in doped insulators: The role of inversion site symmetry of impurity centers

The light induced ionization of impurity centers and defects in insulating crystals results in many phenomena such as charge transfer optical transitions in absorption spectra, photoconductivity, the photovoltaic effect, optical bleaching, and photochemical persistent spectral hole burning. This paper considers specific effects in photoionization related phenomena which are closely connected with inversion symmetry of impurity centers. These recently observed effects include (a) the drastic dependence of the photoionization energy on impurity sites lacking inversion symmetry (Li2Ge7O15:Cr3+), (b) the direct dependence of the sign of the photovoltaic effect on the polarity of impurity sites in doped pyroelectric crystals (LiNbO3:Cr3+), and (c) the anomalous broadening of photochemical persistent spectral holes in zero-phonon spectra of inversionless impurity centers (CaS:Eu).

Spectral broadening and its effect in Stark spectra of carotenoids

Electrooptical parameters of eight carotenoids with different molecular structures were determined from the absorption and electroabsorption (Stark) spectra in glassy solvents and in poly(methyl methacrylate) films at temperatures 108–115 K. The results point to a limited significance of solvent-induced dipole moments in carotenoids resulting from local electric fields generated by solvent molecules, and indicate that the features in Stark spectra related to apparent dipole moments display a correlation with the bandwidth of individual vibronic transitions in absorption spectra. Model calculations show that it is possible to describe the Stark spectra by assuming a close link between the molecular polarizability and the inhomogeneous spectral shift. It is postulated that the dipole-related features in Stark spectra and the inhomogeneous broadening have a common origin in the distortions of molecular geometry in the form of torsional deformations about the single C–C bonds.

Structures of Rydberg transitions in the absorption spectra of methyl-substituted derivatives of bis(η6-benzene)chromium

The effect of methylation of ligands in bis(η6-benzene)chromium (1) on the structure of Rydberg transitions in absorption spectra has been studied. A detailed analysis and interpretation of all Rydberg elements of the vapor-phase spectra of bis(η6-benzene)chromium (2), bis(η6-o-xylene)chromium (3), bis(η6-m-xylene)chromium (4), and bis(η6-mesitylene)chromium (5) was carried out. The vapor-phase electronic absorption spectrum of bis(η6-p-xylene)chromium (6) was measured, and the assignment of the Rydberg bands was made for the first time. The first ionization potentials of complexes 2–5 were refined. The energy of detachment of the 3dz 2 electron and the parameters of the Rydberg excitations for molecule 6 were determined. The vibronic components of the 3dz 2→R4p x,y transition in the spectra of complexes 2 and 6 were assigned. The differences in the Rydberg structure of the spectra of compounds 2–6 were analyzed in terms of the selection rules for optical transitions in the corresponding symmetry groups. The vapor-phase spectra correspond to conformers with the symmetry groupsC 2v andC 2 for complexes 2–4, with the symmetry groupsD 3h andD 3 for compound 5, and with the symmetry groupD 2d for complex 6.

The nature of electronic transitions in absorption spectra of phenyl-substituted 1,3,4-oxadiazoles and 1,3,4-sym-triazoles

The electronic absorption spectra of 1,3,4-oxadiazole, 1,3,4-sym-triazole and their phenyl derivatives were calculated within the π-electron approximation, by the PPP-Cl procedure. The nature of the electronic transitions was analysed in terms of special quantum-mechanical excitation indices: excitation-localization indices and charge-transfer indices. An orbital analysis was performed within the framework of the structure-orbital approach. The transitions calculated for unsubstituted azoles should be treated as perturbed ones of the models-cys-azabutadiene molecule. In the case of phenyl derivatives one can classify the transitions as being either local or molecular in character. The dependence of their characteristics on the π-system structure has been studied for the latter transitions. The results of the theoretical treatment are in good agreement with experimental data.

Infrared optical properties of lateral superlattices on vicinal planes

In a quantum well on a vicinal plane, the lowest band gap caused by a lateral periodic potential due to artificial structures with the period of the steps is estimated. The absorption spectrum of transitions across the minigap is calculated. Factors leading to possible applications as opto-electronic devices are discussed.

Spectroscopic studies on the mixed Nd 3+-La 3+ and Ho 3+-Gd 3+ acetate systems as an example of the determination of stability constants from electronic spectroscopic data

The previously worked out spectroscopic method of determination of stability constants of lanthanide ions displaying hypersensitive transitions in absorption spectra [ions of f 2– f 6 and f 8– f 12 configurations] has been extended to all the lanthanide ions. The method is based upon measurements of the oscillator strength of hypersensitive transitions of mixed systems of equimolar concentrations containing two different lanthanide ions, in the spectrum one of which a hypersensitive transition is easily observed. The method was proved on the La 3+-acetate and Gd 3+-acetate systems, both the lanthanide ions having no hypersensitive transitions within the generally available UV and VIS measurement range.