Effects of alkyl chain length on Cu(II)-selective green fluorescence of rhodamine–diacetic acid conjugates

Fluorescence properties of rhodamine–diacetic acid conjugates (1a–e), with different alkyl chain length (C2–C6) between the rhodamine and the diacetic acid moieties, have been studied in the presence of Cu2+ and Hg2+. Without cations or with other cations, these probes show almost no fluorescence; however, addition of Cu2+ creates strong green fluorescence at around 530 nm, while Hg2+ addition shows weak orange fluorescence at around 580 nm. The Cu2+-induced green fluorescence originates from self-assembled aggregates of the probes formed by coordination association with multiple Cu2+ ions. Coordination association of the probes with Cu2+ is weakened with an increase in the chain length of the probes due to longer distance between the amide oxygen and the diacetic acid binding sites. In contrast, the green fluorescence intensity is not related to the coordination association strength; some probes show high fluorescence intensity. This is probably because alkyl chains sterically affect the alignment of the probes during aggregation interaction and, hence, produces aggregates with different fluorescence property.