Can conformational flexibility influence the self-assembly behavior and sensing efficacy of fluorogenic amphiphiles? A case study with bisbenzimidazole-based probes

In this work, we have explored the metal ion sensing properties of two bisbenzimidazole-based fluorescent probes, that differ in their conformational flexibility, in an aqueous medium. The compound with a flexible methyl spacer (1) experienced blue shifts in its absorption and emission maxima (along with a turn-off response) upon the addition of Hg²⁺ ions. On the contrary, the compound with a relatively rigid structure (2) showed red shifts in both its absorption and emission maxima (along with a turn-off response) when treated with Hg²⁺ under similar conditions. Detailed mechanistic studies indicated that compound 1 formed a 1 : 2 complex with the Hg²⁺ ions, where both the pyridine nitrogen ends and benzimidazole units are involved in the coordination. However, for compound 2, the binding stoichiometry changed to 1 : 1 and the Hg²⁺ ions remained connected only through the pyridine nitrogen ends. Not only did changes in the molecular level interactions alter the optical response, but they also altered the efficacy of the Hg²⁺ sensing; the degree of response was more prominent with compound 1 than that of compound 2. Also, it was observed that the addition of Hg²⁺ to compound 1 could dissociate the preformed aggregated structures of 1, while in the case of compound 2, the addition of Hg²⁺ promotes aggregate formation. The sensor was further utilized for analysis of real-life water samples and the detection of Hg²⁺ under intracellular conditions (HeLa cells).