Fabrication of a “turn-on”-type enantioselective fluorescence sensor via a modified achiral MOF: applications for synchronous detection of phenylalaninol enantiomers

Abstract

Homochiral metal–organic frameworks (HMOFs) have garnered considerable attention due to their extrachiral properties and broad application for chiral recognition. However, assembling a pair of high-quality chiral MOFs for sensing enantiomers precisely is a formidable challenge because of the complicated chiral environment and uncontrollable coordinated conditions. Herein, one pair of homochiral UiO-66 analogues, S-1 (L-AP@UiO-66-(COOH)₂) and R-1 (D-AP@UiO-66-(COOH)₂), are reported for chiral recognition. They were fabricated via a condensation reaction between the carboxyl groups of UiO-66-(COOH)₂ and amino groups of L/D-amino propanol (L/D-AP). These novel fluorescent probes exhibited highly enantioselective fluorescence enhancement towards L/D-phenylalaninol (L/D-PA). For example, when S-1 and R-1 were treated with L-PA or D-PA, they displayed different fluorescence responses: the enantiomeric fluorescence enhancement ratio (ef) was 2.51 and 0.41 for S-1 and R-1, respectively. Hence, a visible difference in fluorescence enhancement for L-PA and D-PA and excellent enantioselective behavior between S-1 and L-PA (or R-1 and D-PA) was displayed. Measurements of fluorescence lifetime, powder X-ray diffraction, molecular-dynamic simulations and Benesi–Hildebrand plots were employed to determine the observed high enantioselectivity for L/D-PA. In brief, we found that two post-modified HMOFs, S-1 and R-1, were outstanding enantioselective sensors for detecting L-PA and D-PA. They had a prominent difference in ef and remarkable enantioselectivity factor α and ΔΔG based on steric hindrance and stereochemical difference.