Why HS− and CN− can be detected by different chemosensors with similar structures: a quantum mechanics and molecular dynamics study

Abstract

Understanding the sensing mechanism is important for evaluating and developing effective chromogenic anion chemosensors. A challenging question in this area is how do you explain the selectivity of one chemosensor to anions that are usually difficult to distinguish. To this end, the sensing mechanisms of two chemosensors (COUMC & HCHI) with similar structures are theoretically investigated by means of quantum mechanics and molecular dynamics simulations. The selectivity of each chemosensor to three common anions that can possibly influence the detection of each other, namely CN⁻, HS⁻ and F⁻, is thereby explained. The result of the quantum mechanics calculation reveals that COUMC_HS⁻, COUMC_CN⁻ and HCHI_CN⁻, are favored both kinetically and thermodynamically. A further analysis based on molecular dynamics simulations reveals that the affinity of anions to the reaction sites of COUMC follow the order of CN⁻ > HS⁻ > F⁻. On the other side, F⁻ and HS⁻ anions are unable to approach the reaction sites of HCHI. These different affinities are explained subsequently by the different strengths of water–anion complexes and different surface electric potentials between COUMC and HCHI. The photochemistry properties indicate excitation and emission on large conjugated planar structures for a single COUMC/HCHI. For the anion-bonded chemosensors, only localized excitation is observed and no obvious differences on absorption and emission spectra are found when adding different anions. Based on our results, we conclude that the reaction selectivity is determined from both the reaction energy and the affinity of anions to the reaction sites. The different selectivity between COUMC and HCHI is attributed to spatial effects and surface electrostatic potential changes caused by the benzyl substituent in COUMC.