Fluorescent BF2 Complexes Inspired by Ageladine A

The pH-dependent UV/Vis absorption and fluorescence of several analogs of the marine natural product ageladine A were investigated. Besides ageladine A itself, its BF2 complex and the corresponding debrominated analogs were included, as were the chloro analog of the debromoageladine-BF2 complex, an imidazolone analog, and a triazole derivative, which lack the free amino group. Importantly, the pH-dependence of the fluorescence of the chloro analog closely resembles that of the natural product ageladine A, but with about 25-fold higher fluorescence intensity and a maximum between pH 3 and 6. The imidazolone derivative behaves in a complementary manner and shows fluorescence only above pH 6, but with less intensity than ageladine A. The calculated Stokes shifts agree well with the experiment, which underlines the accuracy of the DFT calculation method (omega B97XD/TApr-cc-pVDZ, PCM). The conformational fixation of arylated imidazo[4,5-c]pyridines by BF2 units seems to be promising with regard to the development of novel, pH-dependent fluorescent probes.