Recognition-Induced Enhanced Emission of Core-Fluorescent ESIPT-type Macrocycles

Core-fluorescent cavitandsbased on 2-(2′-resorcinol)benzimidazole fluorophores (RBIs)merged with the resorcin[4]arene skeleton were designed and synthesized. The cavitands, due to the presence of intramolecular hydrogen bonds and increased acidity,show excited state intramolecular proton transfer (ESIPT)and readily undergo deprotonation to formdianionic cavitands,capable of strong binding to organic cations. The changes in fluorescence are induced by deprotonation and binding events and involve huge Stokes shifts(due to emission from anionic double keto tautomers)and cation-selective enhancement of emission originating from the restriction of intramolecular motion (RIR) upon recognition in the cavity.Ab initio calculations indicate that the macrocyclic scaffold stabilizes the ground state tautomeric forms of the fluorophores that are not observed for non-macrocyclic analogs. In the excited state, the emitting forms for both macrocyclic scaffolds and non-macrocyclic analogs are anionic double keto tautomers, which are theresult of excited state intramolecular proton transfer (ESIPT)or excited state doubleproton transfer(ESDPT).

National Science Centre (OPUS 2017/25/B/ST5/01011) Wroclaw Centre for Networking and Supercomputing (grant no. 299)