Of Acid/Base Controlled Chiroptical Switches And Their Chemical Sensing Properties

Chirality can be observed on various levels in nature, which is one of basic characteristics of living process. The well-known α-helix protein and double-helix DNA possess the right-handed second structure although the former and the latter consist of levorotatory amino acids and dextrorotatory ribose, respectively. Research related to chiral molecules and chiral polymers has been received increasing attention from different scientific fields including chemistry, materials and life science. Especially, chiroptical switches has become one of the most topics due to the potential applications in the chemosensing, enantioselective recognition, molecular device, data storage and so on.A chiral molecular switch is a molecule that can be reversibly shifted between two or more stable states with distinct chiroptical properties. The molecules may be shifted between the states in response to environmental stimuli, such as changes in pH, light, temperature, an electrical current, or in the presence of a ligand. Usually, chiral molecular switches based on the sterically overcrowded olefins, diarylethenes, and helicenes, the reversible and stereochemically controlled transformation of one optically pure diastereomer to another is realized through the cleavage and formation of a π-bond even of σ-bond. For chiral supramolecular aggregates, helical polymers, foldamers and metal complexes, the chiroptical switch is driven by the the cleavage and reorganization hydrogen-bonding, π-π stacking, electrostatic interaction or metal coordination. Although a lot of chiral switch systems have been reported, the development of a chiral molecular switch with structural simplicity, rapid responsiveness, high reversibility, and long lifetime is still challenging.Herein we describe a new approach to construct chiral molecular switches with the features of stimuli-controlled and reversible chiroptical inversion. Its fundamental concept involves an interchange of two conformations presenting diverse chiroptical properties by acid/base-mediated C-C single bond rotation. As a proof of principle, two simple chiral molecular switches have been established using N-salicylidene Schiff base and salicylamide as the peculiar skeleton motifs. Upon addition of base in solution, the circular dichroism (CD) spectra of these chiral compounds displayed unique changes featuring an inversion of the Cotton effect’s signs, and the original CD profiles can be recoverd thoroughly by acidification. Various spectroscopic studies as well as the conformational analysis combining with time-dependent density functional theory (TDDFT) calculations allowed clear elucidation of the chiroptical inversion mechanism. Furthermore, such a chiroptical inversion may be interpreted in terms of the exciton chirality rule. Compared to the existing molecule-based chiral inversion systems, this promising new type of chiroptical switching molecule is relatively unique as it does not involve any covalent bond formation/breakage and thus possesses distinct advantages, such as fast switching rate, high reversibility and fatigue resistance, and the nondestructive readout. Such dynamic chiroptical inversion systems are expected to find potential applications in molecular recognition, chemosensors, or the construction of molecular-scale devices. More importantly, these findings suggest that the use of the conformational transition about a single bond may serve as the basis for designing chiroptical inversion systems.Thus, by using ATRP technique in combination with post-modification the stimuli-responsive chiral salicylidenimino (SA) motif was covalently attached to the pendant group of polymethacrylates to construct a new kind of acid/base-controlled polymeric chiroptical switches. As a chemosensor, the resulting polymer showed high selectivity for F-relative to other anions including Cl-, Br-, CH3CO2, and C6H5CO2in DMSO solution as judged from fluorescence and circular dichroism spectrophotometric titrations. Moreover, this dynamic chiroptical inversion system can be employed as an enantioselective chemosensor for a-hydroxy acids such as mandelic acid.In addition, we designed a new functional monomer containing chiral imine unit, N-{[2-(4-vinylbenzyloxy)-1-naphthyl]-methylene}-(,S)-2-phenylglycinol (VNP). The homopolymer PVNP and copolymer poly(HEMA-co-VNP) were synthesized through RAFT and conventional free radical polymerization, respectively. The two polymers showed a highly selective red-shifted emission and a unique chiroptical response upon HSO4-binding in organic solution. Interestingly, the HSO4--induced CD or fluorescence signal can be totally reversed with addition of base and eventually recovered the initial state, leading to a reproducible molecular switch with two distinguished "on" and "off’ states. As expected, the copolymer has excellent hydrophilicity, flexibility and good film-forming properties. Thus, high-quality film probe could be easily fabricated on quartz plates through spin-casting techniques. The resultant polymeric films can recognize HSO4-ion among a series of common anions in aqueous solution with high selectivity and sensitivity; the detection limit determined based on the S/B criteria is ca.50μM. The promising new film probe for HSO4-has distinct characteristics such as rapid response, enough stability in aqueous media, and practicality. To the best of our knowledge, until now no similar reports on polymer-based fluorescent sensors, particularly polymeric film probe, have been made for the selective detection of bisulfate ions in aqueous media.