The influence of solvent and temperature upon the fluorescence,absorption and excitation spectra of some indole derivatives

We have studied the changes undergone by the fluorescence, absorption and excitation spectra of some indole and carbazole derivatives in various solvents (nonpolar, polar and mixtures of nonpolar with polar solvents), at various temperatures, especially at low temperatures. On the basis of these modifications it has been established that both the solvents through specific interactions and the temperature can lead to the formation of new molecular species. It has been observed that these species may appear both in the excited state and in the ground state of the solute. It has been established that in the case of the achievement of some favourable configurations either under the action of solvent or under the combined action of both the solvent and temperature the investigated compounds (with one exception) form an exciplex with n-propylamine.     

Substituent effects on the triplet-triplet absorption spectra of coumarin and its derivatives

Preliminary work on the triplet-triplet (T-T) absorption of coumarin and two of its 4-hydroxy substituted derivatives is extended to four additional substituted coumarins in order to investigate substituent effects on T-T absorption in these molecules. A steady cross Illumination technique is used in EPA glass at 77° K. Measurements of the decay of T-T absorption are carried out with a flash technique and a comparison made with phosphorescence lifetimes to verify the triplet nature of the absorption. The observed T-T spectra show a marked dependence on the presence and position of hydroxy substitution. An explanation is developed based on the mixing of $\text{nπ}{}^1$ character into the lowest $\text{ππ}{}^1$ triplet state. The effect of methyl substitution is also investigated and, although it leads to relatively small changes in the T-T spectra, the results suggest that it plays a role in the susceptibility of the molecules to photochemical change.     

Nonequilibrium phonons in SiGe quantum-well nanostructures upon excitation by picosecond laser pulses

The phonon pulses initiated by photoexcitation of structures containing Si_0.8Ge_0.2 double quantum wells under picosecond radiation of a MIRA 900P titanium-sapphire laser (λ = 760 nm) are studied. The propagation of nonequilibrium acoustic phonons is detected with a superconducting bolometer. The recorded bolometer response is found to differ substantially from that observed in photoexcitation of the same structure by nanosecond pulses of a nitrogen laser (λ = 337 nm). The generation of coherent acoustic phonons is suggested as an explanation.     

Electronic absorption spectra of antiviral aminophenol derivatives

Electronic absorption spectra of aminophenol derivatives in solutions have been studied. A general property of the absorption spectra of these compounds, the dependence of the maximum of a long-wavelength absorption band on the solvent polarity, is revealed. As a rule, the absorption band maximum of compounds possessing pharmacological properties shows a greater shift to short wavelength with an increase in the medium polarity than that of inactive compounds. Absorption measurements of solutions of aminophenol derivatives can be used for a tentative estimation of their antiviral activity.     

Absorption,fluorescence, and fluorescence excitation spectra of free molecules of indole and its derivatives

We have obtained and analyzed the absorption, fluorescence, and fluorescence excitation spectra of indole vapor, N-acetyl-L-tryptophan vapor, and 3-indole aldehyde vapor. From analysis of the dependence of the fluorescence spectrum of the free indole molecules on the wavelength of the exciting radiation λ_ex, it follows that emission of fluorescence occurs when the molecules undergo a transition from the one electronically excited state ¹L_b. The fluorescence spectra of the studied compounds are insignificantly different, suggesting a major role for the indole chromophore in formation of the compounds. The absorption spectrum of N-acetyl-L-tryptophan, in which the group of atoms is added to the indole ring through a-C-C bond, is similar to the spectrum of indole, while the spectrum of 3-indole aldehyde is significantly different from the indole spectrum due to the effect of the C=O group conjugated with the indole ring. The fluorescence excitation spectra are considerably different from the absorption spectra. This is associated with the strong dependence of the quantum yield for the free molecules on λ_ex. Qualitatively, they are mirror-symmetric to the fluorescence spectra of the stodied compounds. Analysis of the data obtained provides a basis for assuming that in the case of free molecules of indole and its derivatives, the ¹L_a absorption in the extreme long-wavelength region of the spectrum does not overlap ¹L_b absorption. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 2, pp. 218–222, March–April, 2007.     

UV absorption spectra of some γ-phenylpyridine derivatives

The derivatives have methyl, carbethoxy, and carbmethoxy groups or are higher homologs with unsaturated substituents. The bond orders and electron density per atom are calculated by the LCAO MO method in the Hückel approximation for the parent compound and the methyl derivatives. The spectra are recorded with an EPS-2 spectrophotometer, the solvents being ethanol and isooctane.The compounds were synthesized in the organic chemistry at the Lumumba Friendship University by K.J. Mathew and A.I. Sirotin under the direction of Professor N.S. Prostakov.     

The temperature dependence of the resistive response of superconducting Pb films upon picosecond optical excitation

We report a study of the temperature dependence of the transient resistivity of superconducting Pb thin films excited by short duration (70 ps) laser pulses of varying irradiances. The results are analyzed in terms of a model proposed by Elesin which features the existence of a nonstationary, inhomogeneous, intermediate state.     

Temperature dependence of the radiation-stimulated conductivity of CsI crystals upon picosecond excitation by electron beams

The temperature dependence of the pulse conductivity for CsI crystals upon excitation with an electron beam (0.2 MeV, 50 ps, 400 A/cm²) at a time resolution of 150 ps is investigated. Under experimental conditions, the time of bimolecular recombination of electrons and holes (V _k centers) is directly measured in the temperature range 100–300 K. This made it possible to calculate the temperature dependence of the effective recombination cross section S(T)=7.9×10^?8 T² cm². The temperature dependence of the conductivity σ(T) is interpreted within the model of the separation of genetically bound electron-hole pairs. The activation energy of this process is found to be E _G =0.07 eV.     

Luminescence excitation spectra and characteristics of luminescence centers with overlapping absorption bands

For an ensemble of different types of luminescence centers with overlapping absorption bands, with no restrictions on the optical densities, we have obtained relations describing the luminescence excitation spectra for each type of center. We consider transformations of the relations in some limiting cases. We suggest a procedure for using the equations obtained to determine the characteristics of the luminescence centers. Some of these procedures have been experimentally implemented in study of intrinsic radiation color centers in lithium fluoride crystals. We have determined the ratios of the luminescence quantum yields for F₂ and F₃⁺ color centers, and we have observed that a major role is played by nonradiative transitions in deactivation of the first excited singlet state of F₃⁺ centers. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 3, pp. 365–371, May–June, 2008.