Altered metal ion selectivity in signalling with heterocyclic tripodal receptor appended rhodamine-B derivatives

Abstract

The rhodamine derivatives (2–7) were shown to exhibit metal ion complexation induced photophysical ‘turn-on’ signalling responses, where the selectivity in signalling were driven by stereo-electronic environment of the receptor units that structurally differ as derivatization of different heterocyclic units. The chromogenic and fluorogenic spectral enhancements of two order higher in logarithmic scale were observed in these tripodal receptor incorporated amino-ethyl-rhodamine probes, where substitution of a 2-[2-[(thiophen-2-yl-methyl)-(2-pyridinylmethyl)-amino]-ethyl]- (as in 3) has shown selectivity for Hg(II) ion, a 2-[2-[(furan-2-yl-methyl)-(2-pyridinyl methyl)-amino]-ethyl]- (as in 5) for Al(III) ion and a 2-[2-[(phen-2-ylmethyl)-(2-pyridinylmethyl)-amino]-ethyl]- (as in 7) for Fe(III) ion respectively. The higher order of association constants (log Ka > 9.0) of complexation, high sensitivity of detection(LOD < 2 nM) and higher rate of interaction between probe and metal ion (k ~ 0.001 s−1) have showed potential of these probes in selective detection of corresponding metal ions. In general, the investigations on complexation induced photophysical signalling in these probes in terms of selectivity and sensitivity of detection, association constants of complex formation, reversibility in signalling, execution at different pH range, kinetics of spiro-ring opening and theoretical structure elucidation have therefore put insights on their coordination mediated stereo-electronic environment to establish a rationality in structure-function correlation.