Photoreactions in Vesicle Membranes

Light-induced charge separation was treated in the porphyrin section 4.4, activities of entrapped dyes in section 4.5. In the following, we discuss the effect of vesicle viscosity on photoreactions and photochemical labelling. 

Vesicles are ordered fluids or liquid crystals, a fact which reflects well in those photoreactivities that are particular to vesicle solutions. Fatty acid derivatives 24 and 25, for example, show low quantum yields of fluorescence ((j)f) and high quantum yields of cis-trans isomerization ( )c) as well as short fluorescence life times (tf) in both methylcyclohexane and micellar SDS solutions (Table 1). In DPPC vesicles, on the other hand, cis-trans isomerizations are cumbersome and much slower, and the fluorescence yield and lifetime rise considerably (Table 1). For those stilbene derivatives which are embedded in the middle of a fatty acid backbone, isomerization is virtually eliminated in the low-temperature or gel phase of the bilayer. The vesicle thus plays the role of stabilizing trans configurations which fit into the frozen oligomethylene chain matrix. 

Azobenzene and its derivatives are photoisomerized from the trans to the cis form by UV light (313 nm). Heat or visible light (420 nm) regenerate the trans isomer and this isomerization changes the molecular form, the UV/VIS spectrum and the dipole moment of the chromophore °. The trans form in vesicle membranes is also particularly stable below its transition temperature. 

Photoaddition reactions of entrapped bolaamphiphiles should occur regioselectively if both head groups are locked on the inner and outer surface of a vesicle membrane. A detailed study of such a reaction confirmed this pre- 

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