Abstract
The influence of intra- and intermolecular hydrogen bond formation upon photophysical properties is discussed for various heteroazaaromatics. In the former case, exemplified by porphycenes, a double proton transfer is detected in the first excited singlet state. The phototautomerization rate is a function of the distance between the nitrogen atoms participating in the hydrogen bonds. The rapid movement of internal protons leads to fluorescence depolarization. For such systems, in which the equilibrium between two formally identical tautomeric forms is established in the excited state, emission anisotropy values may be used to determine the electronic transition moment directions. A complementary technique, transient linear dichroism was also discussed. It is capable of studying non-emissive samples as well as the reactions occuring in the triplet state. In the other case, represented by 2-(2′-pyridyl)indoles, efficient fluorescence quenching is observed upon formation of intermolecular hydrogen bonds. We postulate the formation of cyclic, doubly hydrogen-bonded complexes with alcohols as the rate-limiting step of fluorescence quenching. Possible role of electron transfer induced by hydrogen bond and by proton shift is discussed for complexes with alcohols and pyridine.
Published Version
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