Optical Nature and Binding Energetics of Fluorescent Fluoride Sensor Bis(bora)calix[4]arene and Design Strategies of Its Homologues

We report a theoretical assessment on F^– sensing with bis­(bora)­calix[4]­arene, a known fluorescent sensor. Geometries of bis­(bora)­calix[4]­arene and its fluoride-binding complex are optimized at the ONIOM­(B3LYP/6-31+G­(d):B3LYP/3-21G) level of theory in both the gas phase and the CH₂Cl₂ solution by using the Polarizable Continuum Model (PCM). Decreases in UV absorption and fluorescence of bis­(bora)­calix[4]­arene upon F^– binding are explained by Time-Dependent Density Functional Theory (TD-DFT) calculation. The theoretical calculations indicate that the fluorescence quenching behavior when F^– binds with bis­(bora)­calix[4]­arene is the result of cooperative endo- and exo-bindings, which contradicts the previously reported experiment that suggested only the endo-binding. Furthermore, the observed fluorescence can be understood as an emission from the second and higher excited states via prompt fluorescence. This sensor-anion binding is predicted only with fluoride, but not with chloride or bromide anions. The substitution of boron atoms with group 13 and 15 atoms is also explored for the design of effective fluoride sensors. To predict the binding affinity, we calculate the binding energy of chemosensors with F^–. Some of the substituted homologues studied here are expected to be potential fluoride sensors. In this regard, degrees of pyramidality and parallelity act as useful indicators to predict the binding affinity as well as the structure of both homonuclear and heteronuclear motifs.