Combined effect of hydrogen bonding interactions and freezing of rotameric equilibrium on the enhancement of photostability

The photophysics and photostability of 12,13-dihydro-5 H -indolo[3,2- c ]acridine ( IA ), a rigid bifunctional indole derivative with proton donor/acceptor functionalities, can be drastically changed by the environment. The formation of hydrogen bonds with alcohols leads to a significant decrease of the triplet formation efficiency and an increase of photostability. The photodegradation yield was found to be about two hundred times lower in methanol and 1-propanol than in n -hexane or acetonitrile. A similar effect has been reported for two indole-naphthyridines, molecules that can exist in syn and anti rotameric forms. We demonstrate that IA , which can exist only in the syn form, is more photostable in alcohols than similar, but non-rigid molecules. This additional photostability enhancement is due to the elimination of a slower channel of excited state deactivation in alcohol complexes, S 0 ← S 1 internal conversion. The dominant, faster channel of S 1 depopulation is the excited state double proton transfer, manifested by the presence of low energy tautomeric fluorescence. Rigid structure and hydrogen-bonding interactions provide a higher photostability of organic chromophores.