Unusual red-orange emission from rhodamine-derived polynorbornene for selective binding to Fe3+ ions in an aqueous environment

Abstract

Norbornene-based rhodamine derivative (NR) was observed as an “off–on” probe for Fe³⁺ ions both colorimetrically and fluorimetrically in an aqueous environment. NR and its homopolymer (PNR) were capable of detecting Fe³⁺ ions with high selectivity and sensitivity in an aqueous environment. For NR and PNR the limit of detection (LOD) towards Fe³⁺ ions was found to be 49 nM and 19 nM, respectively, making these materials highly efficient. Most interestingly, PNR has more efficacy towards the detection of Fe³⁺ ions than NR, which is expected due to the favorable side-chain interaction in the presence of multiple sensing motifs between the polymer chains. The sensing behavior was thoroughly studied via spectroscopic techniques. We hypothesized that the Fe³⁺ ion was expected to induce the spirolactam ring-opening of the rhodamine unit due to its Lewis acid nature and preferable interactions with the N and O atoms present in NR as well as PNR.