The fluorescence and electronic structure of phenyl-substituted tetraazachlorin molecules

The effect of the addition of phenyl groups to pyrrole rings of tetraazachlorins, a new class of tetrapyrroles, on the photophysical properties and electronic structure of the molecules has been investigated by a complex of experimental and theoretical methods. Characteristics of fluorescence at 293 and 77 K have been determined for phenyl-substituted tetraazachlorins. The objects of this study include unsubstituted tetraazaporphine. The introduction of phenyl groups affords a marked increase in the fluorescence quantum yield. For tetraazaporphine and phenyl-substituted tetraazachlorins, fluorescence buildup occurs as the temperature is decreased from 293 to 77 K, but to a lesser extent than for tetraazachlorins having no phenyl groups, which were earlier studied by the authors. The fluorescence buildup mechanism is discussed. The singlet oxygen generation quantum yield has been determined for the tetrapyrroles examined. This characteristic increases upon tetrapyrrole is phenylation. The electronic structure and absorption spectra of unsubstituted porphine and chlorin, tetraazaporphine, tetraazachlorin, octaphenyltetraazaporphine, and tetramethylhexaphenyltetraazachlorin have been calculated by the INDO/Sm method (original modification of the INDO/S method) with molecular geometry optimization using DFT. The results of the quantum-chemical calculation of the absorption spectra are in good agreement with experimental data for transitions to the lowest excited electronic states Q _x (S ₁) and Q _y (S ₂).     

Intraband absorption and emission of light in quantum wells and quantum dots

High-resolution spectroscopy in the mid-infrared spectral range is used to study electronic transitions between size-quantization subbands in stepped quantum wells under picosecond interband excitation. The contributions from intersubband and intrasubband absorption of light are separated by using the difference in time profiles of the absorption coefficient for these cases. For stepped quantum wells, spontaneous interband luminescence and superluminescence are studied for different excitation levels. For structures with quantum dots, the intraband absorption spectra for n-and p-type structures and the spectra of photoinduced intraband absorption and emission (for polarized radiation) for undoped structures are studied.     

Spectral behavior study of 3-(4-dimethylamino-phenyl)-1-{6-[3-(4-dimethylamino-phenyl)-acryloyl]-pyridin-2-yl}-propanone

The photophysical properties such as singlet absorption, molar absorptivity, fluorescence spectra, fluorescence quantum yield (?_f) and transition dipole moment (μ₁₂) of 3-(4-dimethylamino-phenyl)-1-{6-[3-(4-dimethylamino-phenyl)-acryloyl]-pyridin-2-yl}-propanone (DMAPAPP) have been studied in different media. DMAPAPP exhibits a large red-shift in both absorption and emission spectra as the solvent polarity increases, indicating a large change in dipole moment of molecule upon excitation. The fluorescence quantum yield depends on the nature of the solvent. The absorption and emission spectra of DMAPAPP in dioxane–water mixture are also studied. The effect of different type of surfactants to determine their critical micellar concentration (CMC) and the microemulsion effect on the electronic absorption and emission spectra of DMAPAPP are recorded. The effect of acidity on the electronic absorption and emission spectra of DMAPAPP is studied to determine the pK_a and pK_a^? values.     

The effect of annulation of benzene rings on the photophysics and electronic structure of tetraazachlorin molecules

The photophysics and electronic structure of tribenzotetraazachlorins (H₂, Zn, and Mg), which are novel analogues of phtalocyanines, have been studied experimentally and theoretically. At 293 K, the electronic absorption, fluorescence, and fluorescence excitation spectra are recorded and the fluorescence quantum yield and lifetime, as well as the quantum yield of singlet oxygen generation, are measured; at 77 K, the fluorescence, fluorescence excitation, and fluorescence polarization spectra are recorded and the fluorescence lifetime values are measured. The dependences of the absorption spectra and photophysical parameters on the structure variation are analyzed in detail. Quantum-chemical calculations of the electronic structure and absorption spectra of tribenzotetraazachlorins (H₂, Mg) are performed using the INDO/Sm method (modified INDO/S method) based on molecular-geometry optimization by the DFT PBE/TZVP method. The results of quantum-chemical calculations of the electronic absorption spectra are in very good agreement with the experimental data for the transitions to two lower electronic states.     

Laser spectroscopy of NO2 under very high resolution: Fluorescence spectra from selectively excited hfs levels

Absorption spectra of NO₂ with reduced Doppler width and resolved hyperfine structure have been obtained by crossing the beam from a single mode argon laser, tunable around 4880 and 5145 Å, with a well-collimated NO₂ beam at low pressures. With the laser frequency stabilized onto selected NO₂ absorption lines the fluorescence spectra of NO₂ molecules in selectively populated hfs levels of an excited electronic state were examined through a grating monochromator which resolved the different rotational lines. From the fluorescence spectra the quantum numbers N′ and K′ of the emitting levels could be determined. The analysis proved that some emitting levels have K′-values differing from that of the primarily excited level. This implies radiationless transitions between different electronic states in the free molecule, which can change the value of K′ but preserve that of N′.     

Spectral and luminescent study of reduced forms of Pd-octaethylporphin

It is shown by electron absorption spectroscopy methods that the reduction of Pd-octaethylporphin does not touch the unfilled d _x ²-_y ² orbitals of metal. In the reduction products (mono-and dianions), excess electrons are distributed on the lowest vacant molecular e _g (π*) orbital of the ligand. This does not contradict the results of the quantum-chemical calculation of electronic states of neutral Pd-porphyrins by the extended Hückel method that are presented in the literature. Differences in electronic absorption spectra of π-anions of Pd and Zn complexes are attributed to the distortion of the plane geometry of the tetrapyrrole skeleton due to the displacement of the Pd(II) ion from the macrocycle plane. Mono-and dianions of their Pd complex lose the ability to be luminescent. The emissive capacity is reconstructed in the product of π-dianion protonation, which is called phlorin-anion (π-monoanion of Pd-octaethylporphin phlorin). As the temperature is decreased from room temperature to 77 K, the fluorescence spectrum of this product becomes narrower, its peak shifts to the blue region, and the quantum yield of luminescence increases. Specific features of the spectral properties of phlorin-anion are described by conformational changes in the excited state. Phlorin-anion was found to quench luminescence of neutral Pd-octaethylporphin molecules at room temperature.     

Phenyl substituted Mg porphyrazines: The effect of annulation of a chalcogen-containing heterocycle on the spectral-luminescent properties

We have performed complex experimental and theoretical investigations of the spectral-luminescent properties and electronic structure of new phthalocyanine analogs, Mg octaphenylporphyrazine and its derivatives with an annulated thiadiazole or selenadiazole ring instead of two phenyl groups. Fluorescence characteristics have been determined at 293 and 77 K: emission, excitation, and fluorescence polarization spectra; fluorescence quantum yield ?? _F , and lifetime ?? _F . Annulation of a five-membered chalcogen-containing heterocycle leads to splitting of the long-wavelength absorption band Q(0-0) and to the bathochromic shift of its longest wavelength component Q _x (0-0), which increase upon passage from S to Se. At the same time, the fluorescence quantum yield ?? _F and lifetime ?? _F decrease, which is related to the intramolecular heavy-atom effect. The geometric structure of the ground state of the Mg porphyrazine molecules has been determined based on the density functional theory (DFT), and excited electronic states have been calculated with modified parametrization of the INDO/S method, INDO/Sm. Semiquantitatively, the calculated level positions of the lowest Q states and spectral shifts of Mg octaphenylporphyrazine and S-derivative agree with experimental data. For the range of the Soret band, calculated transition energies and their intensity distributions substantially depend on the dihedral angle ?? between a phenyl ring and porphyrazine macrocycle. We show that, based on calculations at the angle ?? = 60°, bands in the observed absorption spectra can be assigned with an accuracy of ??2000 cm^?1.     

Electronic structure and intramolecular photophysical processes of cations of symmetric indopolycarbocyanine dyes

The electronic structure, spectra, and rate constants of radiative and nonradiative processes of cations of the vinylogous series of symmetric indopolycarbocyanine dyes are calculated by the semiempirical method of partial neglect of differential overlap. The S ₀ → S _n absorption spectra are interpreted. The dependences of the efficiency of the deactivation of the fluorescent state on the length of the polymethine chain of cations are analyzed. The quantum yields of the fluorescence from the first and second singlet states are calculated. The calculation results agree satisfactorily with experimental data.     

Spin crossovers in Mott–Hubbard insulators at high pressures

The high-pressure induced phase transitions initiated by electronic transition in 3d ions from the high-spin (HS) to the low-spin (LS) state (HS-LS spin-crossover) are considered. Behavior of the system with d⁶ electronic configuration is investigated in the ground state of zero temperature and critical pressure P_c. Magnetic properties of the Mott–Hubbard insulator (Mg_1−xFe_x)O are studied in the vicinity of the quantum critical point (T=0, P_c). At the critical pressure of spin crossover P_c, the spin gap energy ε_S between HS and LS states is zero. The quantum spins fluctuations HS⇔LS do not require any energy, and the antiferromagnetism is destroyed in the quantum critical point by the first order transition.     

Giant red shift of the absorption spectra due to nonstoichiometry in GdCoO<Subscript>3–δ</Subscript>

The GdCoO_3–δ perovskite is a semiconductor with the energy gap E _g ≈ 0.5 eV from electrical transport measurements. It reveals unusual optical absorption spectra without transparency window expected for semiconductors. Instead we have measured the narrow transmittance peak at the photon energy ε₀ = 0.087 eV. To reconcile the transport and optical data we have studied the effect of oxygen vacancies on the electronic structure of the GdCoO_3–δ. We have found that oxygen vacancies result in the in-gap states inside the charge-transfer energy gap of the GdCoO₃. It is a multielectron effect due to strong electron correlations forming the electronic structure of the GdCoO_3–δ. These in-gap states decrease the transparency window and result in a narrow absorption minimum. The predicted temperature dependence of the absorption spectra has been confirmed by our measurements.     

Electron-phonon interactions and temperature dependence of infrared absorption in the conducting organic salt trimethylbenzimidazol-tetracyanoquinodimethane (TMB-TCNQ)

Infrared spectra of the simple salt TCNQ with trimethylbenzimidazol were studied vs temperature. The analysis of the mechanisms causing the absorption coefficient of the salt TMB-TCNQ to be dependent on temperature shows that three elements should be taken into account. The changes in electronic interactions described by the function W(T), the reduction of occupation of the ground state described by the function n_g(T), and the changes in geometry of the dimer described by the function D(T) are the main mechanisms defining the thermal dependences of the absorption coefficients. The electron-phonon coupling constants for different temperatures were determined.     

The origin of an elastic line in the L 3 x-ray emission spectrum of metallic manganese

A strong elastic line has been found in the L ₃ x-ray emission spectrum of metallic Mn. To unravel its physical nature, the ground-state properties of α-Mn were studied in comparison with those of 3d metals (Cr and Fe in bcc structure), as well as the properties of the L ₃ absorption final states of these three transition metals. To model the electronic structure, LSDA band-structure calculations of Cr, Mn, and Fe were carried out, and L ₃ absorption spectra of these metals were computed in the atomic approximation. A joint analysis of the properties of the ground state and of the final absorption state excited by an x-ray-produced core hole suggests that the elastic line in the α-Mn spectra should be assigned to the specific character of the absorption final-state multiplet.     

Electronic structure of RbCl and Rb2ZnCl4in the solid and the liquid state: Analysis of the local fine structure of x-ray absorption

Experimental absorption spectra were obtained near the absorption K edge of Rb in RbCl and Rb₂ZnCl₄ in the solid state and in a melt. Detailed analysis of the electronic structure of RbCl was conducted by means of the total multiple scattering method, and good agreement with the experimental spectrum was obtained. The fine structure of the unoccupied electronic states was determined in the conduction band of the compound studied. The unoccupied p states of rubidium were shown to be hybridized, for the most part, with the Cl p states in the conduction band. It was proved that the RbCl melts can be described in good approximation on the basis of the cluster model with the Debye-Waller parameter σ²=0.3 Å^?2.     

The effect of atomic ordering on the photoelectron spectra of V3Au

The X-ray photoelectron (XPS) spectra of the valence bands are measured on 3 different modifications of the compound V₃Au, as obtained from the same starting sample. Two of these modifications are of the same crystal structure type (A 15), but have a different LRO parameter, S. An increase of the XPS V3d density of states, in agreement with the observed increase of the electronic specific heat coefficient, γ, and the superconducting transition temperature, T_c.     

Gibbs ensembles in relativistic classical and quantum mechanics

Classical and quantum Gibbs ensembles are constructed for equilibrium statistical mechanics in the framework of an extension to many-body theory of a relativistic mechanics proposed by Stueckelberg. In addition to the usual chemical potential in the grand canonical ensemble, there is a new potential corresponding to the mass degree of freedom of relativistic systems. It is shown that in the nonrelativistic limit the relativistic ensembles we have obtained reduce to the usual ones, and mass fluctuations for the free-particle gas approach the fluctuations in N. The ultrarelativistic limit of the canonical ensemble for the free-particle gas differs from the corresponding limit of the ensemble proposed by Jüttner and Pauli. Due to the mass degree of freedom, the quantum counting of states is different from that of the nonrelativistic theory. If the mass distribution is sufficiently sharp, the thermodynamical effects of this multiplicity will not be large. There may, however, be detectable effects such as a shift in the Fermi level and the critical temperature for Bose-Einstein condensation, and some change in specific heats.     

New analytic approach to the theory of charge exchange in atom-multiply charged ion collisions

A new method is discussed for the solution of many-level charge-exchange problems. The results provide the distribution of the final electronic states over the angular quantum numbers in analytical form. The obtained Z oscillations (Z is the ion charge) of the cross sections are found to be in good agreement with recent experimental data.     

Quantum effects of potential fluctuations in GaAs δ-doping superlattices

The electronic and optical properties of δ-doping n-i-p-i superlattices are strongly influenced by the random distribution of donors and acceptors within the doping layers. A Monte-Carlo method is applied to investigate the resulting potential fluctuations and local changes of energy levels and wavefunctions. We study disorder effects on the density of states, the capacitance and the intraband absorption coefficient as a function of excitation level. In addition, the luminescence spectra are calculated and compared to electroluminescence measurements. Excellent agreement is achieved without using any fitting parameters, if the local wavefunction shrinkage of the tail states is included. While contributions of different subbands cannot be resolved in the luminescence, the simulation of conduction band (CB) intraband absorption confirms that this is possible using resonant Raman scattering.     

Theoretical and experimental photophysical studies of the tris(4,4,4-trifluoro-1-(1-naphthyl)-1,3-butanedionate) (2,2′-bipiridyl)-europium(III)

The complexes tris(4,4,4-Trifluoro-1-(1-naphthyl)-1,3-butanedionate) (2,2′-bipiridyl) Ln(III), Ln(tan)₃bipy, where Ln(III)=Eu³⁺ and Gd³⁺ have been synthesized, characterized and their photophysical properties (absorption, excitation and luminescence spectra and emission quantum yield) investigated down to 4.2 K. The Eu(tan)₃bipy complex has its molecular structure experimentally determined using X-ray crystallography and theoretically using the SMLC/AM1 method as well as their electronic singlet and triplet states were calculated, using the INDO/S-CI method with a point charge model to represent the Eu³⁺ ion, where two values were adopted, +3.0e and +3.5e, to investigate the imperfect shielding of the 4f shells. The so calculated +3.5e model electronic absorption spectrum and low lying triplet state energies agreed very well with the experimental ones. The emission quantum yield of the Eu³⁺ complex is quite low at room temperature, namely 7%, probably due to the too low lying triplet state, 19,050 cm^-1, and increases by a factor of three when the temperature is lowered to 4.2 K. This strong thermal effect indicates the presence of a channel deactivating the main emitting state, what can be due to a LMCT state possibly lying in the same spectral region, as usually found in Eu³⁺ compounds.     

Third-order nonlinear-optical parameters of gold nanoparticles in different matrices

Third-order nonlinear-optical properties of gold nanoparticles embedded in Al₂O₃, ZnO and SiO₂ have been investigated by the Z-scan method at the wavelength of 532 nm using nanosecond Nd³⁺:YAG laser radiation. The nonlinear refractive index, nonlinear absorption coefficient, and the real and imaginary parts of the third-order nonlinear susceptibility are deduced. The results of the investigation of nonlinear refraction using the off-axis Z-scan configuration are presented and the mechanisms responsible for the nonlinear response are discussed. The prevailing influence of the electronic Kerr effect over the possible thermo-optical contribution is demonstrated.     

Electronic states in mixed Cd1−xPbxF2 crystals

A parameterized tight binding model for the band structures of fluorite-type compounds is used in a CPA study of the electronic structure of substitutional non-isoelectronic alloys Cd_1−xPb_xF₂. The calculated densities of states are used to interpret the experimental UPS spectra and the peculiar concentration development of the absorption edge in these compounds.