Solvent-Induced Changes in Photophysics and Photostability of Indole-Naphthyridines.

Molecules that can simultaneously act as hydrogen bond donors and acceptors often exhibit completely different photophysical behavior in protic and aprotic solvents. Formation of multiple hydrogen bonds with, for example, water or alcohols, may lead to enhanced internal conversion; as a result, triplet formation efficiency can be reduced. These changes in photophysical characteristics may influence the photostability. In order to check this hypothesis, we have investigated spectroscopy, photophysics, and changes in photostability caused by interaction with aprotic and protic solvents for 2-(1'H-indol-2'-yl)-[1,5]naphthyridine and 2-(1'H-indol-2'-yl)-[1,8]naphthyridine, molecules with hydrogen bond accepting and donating functionalities. The photostability of these compounds in n-hexane, acetonitrile, and alcohols was studied in the regime of 365 nm irradiation. The photodegradation yield was found to be significantly lower in alcohols. In polar and protic solvents, the presence of two species was detected and attributed to syn and anti rotameric forms; the former are dominant in all environments.