Synthesis and study of crown ether-appended boron dipyrrin chemosensors for cation detection

Boron dipyrrin (BDP) bearing crown ethers of varying cavity sizes, namely, 15-crown-5, 18-crown-6, and 21-crown-7, at the meso-position are synthesized and employed as chemosensors for cation detection in solution. In the absence of metal cations, the emission of the BDP moiety is found to be quenched to some extent by an intramolecular charge transfer (ICT) process from the donor oxygen atoms to the acceptor BDP unit. Coordination of metal ions to the oxygen donor atoms in the crown ether cavity inhibits intramolecular charge transfer to the BDP acceptor, leading to cation-induced fluorescence enhancement. The fluorescence enhancement is systematically probed as a function of crown ether cavity and metal ion sizes to achieve metal ion selectivity.