Siegrun Helm

Photochemistry and photophysics of donor-acceptor-polyenes. I: all-trans-4-dimethylamino-4′-cyano-1,4-diphenylbutadiene (DCB)

Dimethylamino-cyano-diphenylbutadiene (DCB) has been investigated using photochemical and photophysical techniques. HPLC analysis enabled the separation of two photoisomers. Their absorption spectra and extinction coefficients were determined by combining the diode array spectra with the isosbestic points observed in photolysis. The forward and backward quantum yields of photoisomerization were determined. The quantum-chemical modelling of these spectra allowed for a tentative assignment of the photoisomers to the mono-cis isomers ct and tc. Fluorescence lifetimes and quantum yields allowed the conclusion of a negative solvatokinetic behaviour of the nonradiative decay. Catalyzed thermal isomerization of DCB was observed on reversed-phase chromatographic material.

(Coumarin-4-yl)Methyl Ester of cGMP and 8-Br-cGMP: Photochemical Fluorescence Enhancement

We investigated the spectroscopic, photophysical, and photochemical properties of novel caged cyclic nucleotides, the axial and equatorial (7-methoxycoumarin-4-yl)methyl esters of cGMP and of 8-Br-cGMP. The competition between fluorescence and photochemical ester cleavage results in a strong quenching of the coumarin fluorescence. This fluorescence quenching is enhanced additionally by the donor–acceptor interaction between the coumarin unit and the nucleic base, pronounced in the axial configuration. On photochemical decaging the fluorescence intensity increases up to 48-fold and more. This photochemical fluorescence enhancement can be used for the description of the temporal and local distribution of biologically active cGMP derivatives.