Turn-on fluorescence sensors based on dynamic intramolecular N→B-coordination

Abstract

A series of ten aryl-triazole-functionalized boranes bearing BMes₂-groups and capable of forming intramolecular five-membered N→B-coordinated heterocycles, has been prepared by 1,3-dipolar cycloaddition. Electronically diverse substituents ranging from 4-NMe₂-phenyl (4-NMe₂) and 4-OMe-phenyl (4-OMe) to 4-cyanophenyl (4-CN), 4-nitrophenyl (4-NO₂) and pentafluorophenyl (C₆F₅) have been introduced in the periphery of the compounds to systematically vary their electronic properties and the strength of the N→B-coordination. Experimental and computational studies show that the boranes exist in dynamic equilibrium between a closed N→B-coordinated conformation and two types of non-coordinated conformations (open^syn and open^anti). Their structural, optical and electrochemical properties have been characterized, and their binding affinities for nucleophilic anions has been tested. Individual boranes exhibit either fluorescence quenching (4-Me, 4-OMe) or fluorescence turn-on (4-CN, 4-CF₃, C₆F₅, 3,5-(CF₃)₂) upon coordination of anions. Further analyses show that the binding affinity to cyanide can be tailored through variation of the peripheral substituent, ranging from 5.79 (±0.11) for 4-Me to 6.53 (±0.15) for the 4-CN-substituted borane. These experimental data correlate with computed anion binding affinities, which indicate that the effective Lewis acidity at boron covers a range reaching from the parent compound PhBMes₂ to the much more Lewis acidic Ph₃B.