Band 221, Heft 7

Steady-state fluorescence and fluorescence quenching measurements have been carried out to investigate the aggregation behavior for four novel alkylbenzene sulfonate gemini surfactants Ib, Ic, Id, and Ie in aqueous solutions at 298.15 K. Some important micellar parameters critical micelle concentration (cmc), aggregation number ( N ) and micropolarity are obtained. It is found that the cmc values (cmc c ) for the studied gemini surfactants determined by fluorescence quenching measurement are in agreement with the results reported previously by the surface tension measurement or the method using pinacyanol. The aggregation number of surfactant Ie is smaller than those of Ib, Ic and Id. It is explained by the formation of premicellar aggregates inhibiting the formation of micelles due to the increasing hydrophobic portion of the gemini surfactants. Furthermore, the values of I 1 / I 3 for four gemini surfactants are determined. It is shown that the decreases of I 1 / I 3 stretch over a range of concentrations of more than one decade. And it is given the information that the micropolarity in the four gemini surfactant micelles sensed by pyrene is relatively high.

From extraction experiments and γ -activity measurements, the exchange extraction constants corresponding to the general equilibrium M + (aq) + NaL + (nb) ⇔ ML + (nb) + Na + (aq) taking place in the two-phase water–nitrobenzene system (L = tetraethyl p - tert -butylcalix[4]arene tetraacetate; aq = aqueous phase, nb = nitrobenzene phase) were evaluated. Further, the stability constants of the ML + complexes in water saturated nitrobenzene were calculated; they were found to increase in the cation order Cs + < Rb + < K + < Li + < Na + .

Nanoparticles of spinel ferrites Mn x Zn 1- x Fe 2 O 4 with x = 0.0 to 1.0 were prepared by co-precipitation method. The ferrite samples were pre-sintered at 200°C at the rate of 200°C/h for 15h. Finally, one set of samples was sintered at 400°C and the other set at 500°C at the rate of 200°C/h for another 15h. The spinel structure was confirmed by X-ray diffraction (XRD) pattern. The particle size was calculated by using Debye–Scherrer's equation for Lorentzian peak. The calculated average particle size lies between 19–53 nm. The particle size was also investigated by using scanning electron microscopy (SEM). Both the results were found to be in good agreement. The resistivity data was taken by two probe method and resistivity, activation energy and porosity were calculated. There was an increase in the resistivity and activation energy when the ferrite samples were sintered at a higher temperature. Possible models, theories and mechanisms contributing to these processes have been discussed.

An unpublished absorption band of the fluorosulphate radical, FSO 3 , based on earlier experiments of Schumacher and co-workers (Z. Phys. Chem. NF 42 (1964) 174) and detailed calculations of the whole FSO 3 spectra based on the time-dependent (TD) generalization of the density functional theory (DFT), TD-DFT, are reported. Part of this band, that increases in intensity at wavelengths below 275 nm, has been extracted from an absorption spectrum taken from equilibrated mixtures of FSO 3 with the dimer FS(O 2 )OO(O 2 )SF at 403 K. A set of thirteen exchange-correlation hybrid functionals combined with the 6-311+G(3df) basis set predict an intense absorption band centred at 256 ± 10nm in very good agreement with the experimental observations. Another significant band peaked at 184 ± 10nm was also found. The calculations reproduce accurately the position of the maximum and the experimental oscillator strength of the well-known visible FSO 3 band system.

This work reports on theoretical and spectroelectrochemical studies of a thermally and chemically stable fluoranthenopyracylene ladder type molecule 7,10,17,20 tetra- p -dodecylphenyl(bisfluorantheno[8,9-a,c]) pyracylene. Optimal geometries were evaluated for neutral and positively/negatively charged states performing density functional theory (DFT) calculations. Absorption spectral line frequencies as well as oscillator strengths were calculated using the time-dependent DFT and collective electronic oscillator (CEO) methods. The calculated optical spectra are in good agreement with the experimental measurements in solution. The difference of charged states produced by reduction and/or oxidation versus the electronic neutral forms on the vertical excitation energies and CEO electronic density matrix distribution has been characterized and compared to the geometrical changes of the molecular fragments. The localization of excitation in the optical transition of the charged states shows that the excitation is less separated and shows larger electronic coherence in comparison to the neutral form.

The fluorescence quenching of excited singlet state of 2,4,6-triphenylpyrylium tetrafluoroborate (TPPBF 4 or TPP + ), a very good electron acceptor by amines were investigated in a acetonitrile solution using steady state technique. The bimolecular quenching rate constants lie in the range 2.11–10.26 × 10 10 M -1 s -1 . The driving force (ΔG et ) for electron transfer process was calculated from the oxidation potential of amines and the reduction potential of TPP + . The observed k q values correlated well with the driving force for the electron transfer reactions. Aromatic amines show higher k q than aliphatic amines. From the oxidation potential of amines and the quenching rate constant values, a mechanism involving photoinduced electron transfer from amines to excited state TPP is suggested.

The fluorescence quenching of xanthene dyes by TiO 2 has been investigated using steady state technique. The dyes used are eosin, rhodamine B, rhodamine 6G. The quenching was found to obey Stern-Volmer equation and the corresponding Stern-Volmer plots were linear in the range of quencher concentration used [0–5 × 10 --5 ] M. The quenching rate constant ( k q ) is in the range of 0.77–8.30 × 10 12 M -1 s -1 . This study reveals effective quenching of xanthene dyes with TiO 2 . The quenching of xanthene dyes followed electron transfer mechanism.

Extraction of microamounts of strontium by a nitrobenzene solution of rubidium dicarbollylcobaltate (Rb + B - ) in the presence of 15-crown-5 (15C5,L) has been investigated. The equilibrium data have been explained assuming that the complexes RbL + , RbL 2 + and SrL 2 2+ are extracted into the organic phase. The values of extraction and stability constants of the complex species in nitrobenzene saturated with water have been determined.