Gabriella Fischer

The fluorescence of the cis and trans forms of naphthyl-ethylenes

Fluorescence spectra and quantum yields of the two 1-phenyl-2-naphthyl ethylenes and the three 1,2-dinaphthyl ethylenes were studied between +25° and -180°, both of the trans and cis isomers. Contrary to recent reports by other authors we find a close similarity between the fluorescence behaviour of the two isomers of stilbene and the above five compounds. The fluorescence spectra of the cis isomers are unstructured and red-shifted in comparison with the trans isomers. The quantum yield of the trans isomers is 0.8–0.9, that of the cis isomers <0.01 at temperatures above -100°, but increases sharply on cooling.

Photovoltaic behavior of bacteriorhodopsin-loaded cation-exchange membranes☆

Abstract The properties of low impedance charged synthetic membranes containing oriented bacteriorhodopsin have been described in previous studies from our laboratory. Two methods of preparation were used: field orientation and surface orientation. Photocurrents measured at short-circuit reached current densities as high as 20 μA/cm 2 . We have since studied the photovoltaie behavior of these membranes over the entire range from short-circuit to open-circuit. The voltage—current relationships are linear, and can be represented by an equivalent circuit consisting of a “light pathway” and a “dark pathway” in parallel. The dark pathway comprises a resistance in series with an asymmetry potential. The latter may be quite high, and decays only very slowly with time. In the presence of light, the parallel light pathway is “switched on”. This pathway has its own characteristic resistance in series with the photovoltaic electromotive force. The behavior of the components of the equivalent circuit as a function of ambient salt concentration is described. The photocurrent generated by the light pathway in the range 10 −1 —10 −3 M KCl is relatively independent of salt concentration. The photovoltage on the other hand reaches maximum values at low salt concentrations (∼10 −3 M KCl). These values are of the order of 50 mV, which is comparable to the values measured in proteoliposomes containing bacteriorhodopsin. In the case of surface-oriented membranes, the output power of the light pathway tends to a maximum of about 75μW/cm 2 at the lowest salt levels. Since the intensity of absorbed illumination in these experiments is about 17 mW/cm 2 , the efficiency of energy conversion is of the order of 0.5%. In the case of field-oriented membranes a somewhat higher efficiency can be achieved.

The thermodynamic equilibrium between cis- and trans-isomers in stilbene and some derivatives

The thermodynamic cis⇌trans equilibrium in stilbene and its derivatives can be established at room temperature only by the use of catalysts such as atomic iodine. The latter has now been used to determine the above equilibrium in a number of cases. The fraction of cis-isomer at equilibrium is ca. 0·002 in stilbene and the para-substituted derivatives investigated, but increases to 0·07 in αβ-difluorostilbene and 2,4,6-trimethylstilbene, and to 0·20 in α-methylstilbene. These values were used to calculate the respective differences in free energy. Catalysed isomerization, in particular if the catalyst is a product of irradiation, may, under certain conditions, occur concurrently with direct or sensitized photoisomerization, and distort the results of the latter reactions. It is therefore important to distinguish between true photoisomerization, involving the excited states of the molecules undergoing isomerization, and photocatalysed isomerization, involving only the ground states of these molecules.

Conformational equilibria in 1,2-diarylethylenes manifested in their emission in solution. Part 5. 2,6-Distyrylnaphthalene and related compounds

The emission spectra of the title compounds vary with the wavelength of the exciting light. These spectra can be described as superpositions, for each compound, of three sets of emission peaks which contribute to the overall spectra to an extent varying with the excitation wavelength. It is suggested that in solutions of each compound three modifications exist in a dynamic equilibrium. They have essentially similar but slightly shifted absorption and emission spectra, so that the fraction of exciting light absorbed by each modification varies with the wavelength of the exciting light. The three modifications are assigned to rotational conformers involving the two quasi-single styryl–naphthyl bonds. In 1,4-distyryl- and 1,5-distyryl-naphthalene, in which only one relatively unhindered conformer can be envisaged, no variation of the emission spectra with the excitation wavelength was observed.

Temperature dependence of radiation induced trans → cis isomerization of stilbene in toluene

Abstract The rate of γ-ray-induced trans → cis isomerization of stilbene in dilute toluene solutions from −95° to −100° shows a maximum at about −25°. The results are interpreted in terms of interplay of opposing reactions, and of the role of triplet toluene.