Synthesis and photophysical studies of new fluorescent indole derivatives obtained from β-Bromodehydroamino acids : interaction with fluoride anions

Abstract

Several new indole derivatives were synthesised from β-brominated dehydroamino acids and arylboronic acids by using a strategy developed in our research group that involves a sequential Suzuki–Miyaura cross-coupling reaction and a metal-assisted C–N intramolecular cyclisation. The cyclised products were obtained either by direct cyclisation or by isomerisation followed by cyclisation. The photophysical properties of these compounds were studied in four solvents of different polarity (cyclohexane, diethyl ether, acetonitrile and ethanol). All these compounds have reasonably high fluorescence quantum yields (between 16 and 85%) and show different solvent sensitivity in their fluorescence emission. These results indicate that the indole derivatives prepared are good candidates for fluorescent probes. The response of the new synthesised compounds towards fluoride ion (F–) was evaluated. It was found that methyl 3-methyl- 1H-dibenzo[e,g]indole-2-carboxylate, methyl 3-(phenanthren-9-yl)-1H-dibenzo[e,g]indole-2-carboxylate and methyl 1-(naphthalen-1-yl)-3H-benzo[e]indole-2-carboxylate showed significant spectral changes in fluorescence emission upon F– addition with a decrease in intensity and the appearance of a new band at longer wavelengths. No detectable emission spectral changes were observed when several other anions were added to these compounds, which indicates their selectivity towards F-.