Analysis of the low temperature fluorescence and phosphorescence spectra of p-difluorobenzene

The low temperature fluorescence spectra (4.2 K) of solid solutions of p-difluorobenzene-h₄ (pDFB-h₄) and -d₄ have been recorded and analyzed. The absence of fluorescence from vibrationally excited states at 4.2 K eliminates the sequence and “hot band” structure which complicate the analysis of the vapor fluorescence spectrum. On the basis of a comparison of our vibrational analysis with those published for the vapor phase fluorescence, several incorrect assignments in the latter have been identified. The high resolution and lack of spectral congestion obtained in the low temperature matrix isolated fluorescence spectra also allowed additional vibrational assignments to be made. These results are of added importance because of the extensive use which has been made of pDFB for radiationless relaxation studies in the vapor phase. The phosphorescence spectrum of crystalline pDFB-h₄ was observed with sufficient intensity to locate the electronic origin to make several vibrational assignments. Additionally, the zero-field splitting of the first triplet excited state was measured by optically detected magnetic resonance techniques.     

Effect of hydrostatic pressure on the fluorescence of indole derivatives

Effects of hydrostatic pressure on the fluorescence emission of L-tryptophan, N-acetyl-L-trytophanamide and indole were investigated. An increase in pressure ranging from 1 bar to 2.4 kbar results in reversible red-shifts of the emission of the three fluorophores. The pressure-induced redshift amounts to about 170 cm^–1 at 2.4 kbar, and appears related to changes in Stokes shift of the fluorophores caused by pressure effects on the dielectric constant and/or refractive index of the medium. As the pressure range investigated here is the range commonly used in studies of protein subunit association and/or folding, these observations raise the need for caution in interpreting pressure-induced spectral shifts. The significance of these observations to pressure studies of proteins is illustrated by investigation of pressure effects on human Cu,Zn Superoxide dismutase (SOD) and azurin fromPseudomonas aeruginosa. A reversible 170 cm^–1 red-shift of the emission of SOD was observed upon pressurization to 2.4 kbar. This might be interpreted as pressure-induced conformational changes of the protein. However, further studies using SOD that had been fully unfolded by guanidine hydrochloride, and fluorescence anisotropy measurements indicated that the observed red-shift was likely due to a direct effect of pressure on the fluorescence of the single tryptophan residue of SOD. Similar pressure-induced red-shifts were also observed for the buried tryptophan residue of azurin or for azurin that had been previously denatured by guanidine hydrochloride. These observations further suggest that the effective dielectric constant of the protein matrix is affected by pressure similarly to water.     

Vapor-phase fluorescence spectra from the second excited singlet state of pyrene and its derivatives

Vapor-phase fluorescence spectra have been measured for pyrene and its simple derivatives, i.e., pyrene-d₁₀, 1-methylpyrene, and 4-methylpyrene. Each of these derivatives shows a weak fluorescence emission which is similar to the fluorescence from the second excited singlet state (S₂) of pyrene and is to be assigned to the S₂-fluorescence. The methyl substitution causes frequency shifts (Δν) in both absorption and fluorescence, and the Δν values for absorption and fluorescence transitions that are associated with the same excited state, i.e., the first excited singlet state (S₁) or the second excited singlet state (S₂), are approximately equal to each other. When the excitation energy increases, the S₂-fluorescence shifts gradually to the red in almost the same way as the S₁-fluorescence. The S₂-fluorescence spectrum has a sort of mirror-image relation to the S₂-absorption. A comparison of S₂-emissions of pyrene and pyrene-d₁₀ suggests that the ratio between the quantum yields of S₂- and S₁-fluorescence may be related to a ratio $\text{?}{}_2\text{?}{}_1$, where ?₁ and ?₂ are the densities of vibrational states in S₁ and S₂ at the energy of excitation.     

Calculation and interpretation of the absorption and fluorescence spectra of indole in the isolated state and aqueous solution

We have calculated the vibronic absorption and fluorescence spectra of the first (¹ L _b ) and second (¹ L _a ) electronic transitions of indole in the isolated state and aqueous solution. The vibrational structure of the absorption and fluorescence spectra has been interpreted. The influence of the aqueous solution on the vibronic spectra has been shown.     

Phenylalanine fluorescence and phosphorescence used as a probe of conformation for cod parvalbumin

The fluorescence emission and triplet absorption properties of phenylalanine in cod fish parvalbumin type II, a protein that contains no Trp or Tyr, was examined in the time scale ranging from nanoseconds to microseconds at 25°C in aqueous buffer (pH 7.0). In the presence of Ca(II), the decay of fluorescence gave two lifetimes (5.9 and 53 ns) and the triplet lifetime was 425 s. Upon removal of Ca, the fluorescence intensity decreased to values approaching that for free Phe, while the longest fluorescence decay component was 17 ns. At the same time, the decay of triplet showed complex nonexponential kinetics with decay rates faster than in the presence of Ca. Quenching and denaturation analyses suggest that the Phe's are present in a hydrophobic environment in the Ca-bound protein but that the Ca-free protein is relatively unstructured. It is concluded that Phe luminescence in proteins is sensitive to conformation and that the long lifetime of Phe excited states needs to be considered when studying its photochemistry in proteins.     

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.     

温度、pH对含油污水荧光特性的影响

研究了中石化炼油厂出口处含油污水在温度5～55℃,pH 3～12范围内的荧光光谱特性.结果表明,温度只对含油污水的荧光强度有影响,而不改变荧光峰位置;在酸性条件下荧光峰右移,从413 nm移到426.5 nm;且荧光强度随酸性增强而递增.碱性条件下随着碱性增强荧光强度降低,但荧光峰的位置没有移动,始终保持在398 nm...     

Evidence for the existence of Sarkovskii ordering in a two-dimensional map

We analyse numerically the bifurcation structure of a two-dimensional noninvertible map and show that different periodic cycles are arranged in it exactly in the same order as in the case of the logistic map. We also show that this map satisfies the general criteria for the existence of Sarkovskii ordering, which supports our numerical result. Incidently, this is the first report of the existence of Sarkovskii ordering in a two-dimensional map.     

Effect of the fluorine substitution in ethyl groups of 1,4-distyrylbenzene on the fine structure fluorescence and fluorescence excitation spectra

The fine-structure fluorescence and fluorescence excitation spectra of conjugated chain compounds, 1,4-distyrylbenzene (DSB) and its fluorine-substituted derivative α,ω-1,4-distyrylbenzene, have been obtained by the Shpolskii method in an n-octane matrix at a temperature of 4.2 K. These spectra have been simulated by representing the band of each of the vibronic transitions as the sum of a zero-phonon line and a phonon wing with the corresponding parameters, such as the half-widths of the spectral lines and the Debye-Waller factors. Based on this simulation, the relative intensities of vibronic transitions have been determined and the frequencies of normal vibrations in the S ₀ and S ₁^* states have been refined. It has been found that the energy of the purely electronic transition in the molecule of the fluorine-substituted derivative is higher by 950 cm⁻¹ compared to the unsubstituted DSB. The parameters of the Franck-Condon and Herzberg-Teller interactions have been determined. The observed violation of the mirror symmetry between the conjugated spectra is explained by the interference of intramolecular interactions.     

Evidence for the existence of a metastable self-interstitial configuration in irradiated niobium

By means of electrical-resistivity measurements Petzold has found that the isochronal recovery spectrum of low-temperature electron-irradiated niobium changes in a spectacular manner if the irradiation temperature is lowered from 8 K to 5 K. It is shown that this observation, which does not appear to be explicable in terms of the so-called one-interstitial model, constitutes strong evidence for the existence of a metastable self-interstitial configuration in niobium irradiated at low temperatures.     

Effect of excitation intensity on fluorescence spectra in ZnO nanostructures and its origin

ZnO nanostructures with three kinds of morphologies, namely, tetrapod-, rod-, and sheet-like shape, are synthesized by chemical vapor deposition, conventional solution-phase, and hydrothermal methods, respectively. The fluorescence measurements display that the spectra of these nanostructures exhibit similar unexpected change laws with the altering excitation intensity. It is observed that when the excitation intensity increases, for the green emission band, the peak position shows a small blue-shift, the width turns broader, and the intensity grows first stronger and then weaker; for the UV emission band, the peak position exhibits a significant red-shift, and the width and intensity have the similar behaviors with those of the green band. Additionally, the relative intensity of green emission to UV emission decreases gradually. It is clarified that the origin of this abnormal phenomenon is ascribed to the local laser heating effect and the high sensitivity of nanostructures to this heating effect. Supported by the National Basic Research Program of China (Grant No. 2005CB623603)