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Benzimidazole as a structural unit in fluorescent chemical sensors: the hidden properties of a multifunctional heterocyclic scaffold

Abstract Benzimidazole (BI) derivatives are structurally similar to naturally occurring nucleotides, a fact that enables their use as scaffolds in biologically active organic synthetic compounds. However, BI can serve as a multifunctional unit in heteroaromatic molecular systems for optoelectronics and non-linear optics, photovoltaics, and in sensing and bioimaging. Its many structure-related properties such as electron accepting ability, π-bridging, chromogenic pH sensitivity/switching and metal-ion chelating properties, also make BI an exceptional structural candidate for optical chemical sensors. The design of novel benzimidazole-based chemosensors is a challenging task. In this review, molecular sensing systems based on intramolecular charge transfer, photoinduced electron transfer and excited-state intramolecular proton transfer mechanisms are systematically examined and the role of the benzimidazole unit is discussed. Novel and emerging developments such as sensors based on aggregation-induced emission phenomena are also included. Conclusions are made on the advantages and disadvantages of existing fluorophores and chemosensors and guidelines are given for further design and development of new sensing systems based on benzimidazole.