Solvent-controlled metal ion binding selectivity and anion interaction of the acridinedione-based heteroditopic host

Acridinedione-based heteroditopic hosts 1a-c, which contain a flexible oxyethylene moiety as a metal ion binding site and amino hydrogen as an anion receptor unit, were synthesized and characterized. Metal ion and anion binding studies were carried out in different solvents to understand the solvent-induced selectivity of the metal ion binding and anion interactions. In acetonitrile, Ca(2+) alone shows a binding with the oxyethylene unit and -OCH(3) group, which results in a fluorescence enhancement without any spectral shift due to the suppression of the photoinduced electron transfer (PET) process. In chloroform, in addition to Ca(2+), Na(+) also shows a fluorescence enhancement with a 14 nm red shift in the emission maximum. (1)H NMR titration studies confirm that the addition of Na(+) does not involve binding at the oxyethylene moiety, while it shows a binding at the acridinedione carbonyl group and -OCH(3) group, which results in the red shift and fluorescence enhancement, respectively. In the anion interaction studies, F(-) shows a neat deprotonation in polar aprotic solvents, whereas in chloroform, it shows a H-bond complex due to the lower anion desolvation energy. The present study clearly signifies the role of solvent in metal ion selectivity, which is an often unnoticed parameter in metal ion binding.