Dendritic Molecular Switch: Chiral Folding and Helicity Inversion

Abstract

Appropriately designed chemical architectures can fold to adopt well-defined secondary structures without the need for structural motifs of biological origin. We have designed tris(N-salicylideneaniline)-based hyperbranched molecules that spontaneously collapse to compact three-blade propeller geometry of either (P)- or (M)-handedness. For a homologous series of compounds, a direct correlation was established between the absolute screw sense, either (P)- or (M)-, of this helical folding and the absolute configuration, either (R)- or (S)-, of the chiral alcohol groups introducing local asymmetric bias to the conformationally restricted molecular backbone. 1H NMR and CD spectroscopic studies provided significant insights into structural folding and unfolding of these chiral molecules in solution, which proceed via reversible assembly and disassembly of the C3-symmetric hydrogen-bonding network. Notably, solvents profoundly influenced this dynamic process. A strong correlation between the solvent donor number (DN) or solvent basicity (SB) parameters and the change in the Cotton effects pointed toward specific O-H...solvent interactions that drive structural unfolding and eventual refolding to apparently opposite helicity. This unusual chirality inversion process could also be induced by installation of chemical protecting groups that simulate specific solvent-solute interactions. Removal of this covalent mimic of the solvent shell restored the original screw sense of the parent molecule, thus establishing the feasibility of covalently triggered helicity inversion as a new mode of operation for chiroptical molecular switches.

The Chain of Chirality Transfer as Determinant of Brain Functional Laterality. Breaking the Chirality Silence: Search for New Generation of Biomarkers; Relevance to Neurodegenerative Diseases, Cognitive Psychology, and Nutrition Science

“…The diversity of stereospecific (chiral) phenomena was observed in molecular structures including chirality recognition/sensing [95,96], chirality transfer [97], chirality/helicity induction [98][99][100][101], chirality amplification [102][103][104], chirality breaking [61,105], chirality conflict [106,107], helicity inversion [108][109][110], and chiral phase transitions XVII [111][112][113][114]116]. Progress in the studies of molecular chirality transformation is helpful in resolving three questions.…”

Supramolecular Chirality in Dynamic Coordination Chemistry

“…such as small organic molecules [13][14][15][16][17][18][19], polymers [20][21][22][23], metamaterials [24], and assemblies [25][26][27][28], have been developed for chirality induction and switching in response to external stimuli, more sophisticated systems are required to mimic natural systems.…”

Hydrogen‐Bond‐Assisted Helical Folding of Propeller‐Shaped Molecules: Effects of Extended π‐Conjugation on Chiral Selection, Conformational Stability, and Exciton Coupling

“…[20] As shown in Scheme 2, close steric contacts between the three mobile components ("blades") constituting the stereodynamic structure B dictate that their tilting motions proceed in a unidirectional fashion to provide either a right-handed P conformer or a left-handed M conformer. [21,22] Classical examples of such stereodynamic systems include C 3 -symmetric triarylcarbonium and triarylcyclopropenium cations, in which torsional motions occurring at different parts of the molecule are correlated through tight van der Waals contacts.…”

“…[5] Our first-generation molecular prototypes -(S,S) 3 -2 ( Figure 1) and its mirror-image isomer (R,R) 3 -2 -were thus constructed with the use of tris(N-salicylidenamine) [20,[25][26][27] as the C 3 -symmetric core and three 2,6-disubstituted aryl groups as the blades. Through the installation of bulky phenyl groups as part of the chiral secondary alcohol groups at the "wingtips", we wished to enhance repulsive van der Waals interactions in the misfolded conformations so that the correctly folded structure would be further stabilized in the relative energy scale.…”

“…Chemical structure of (S,S) 3 -2 and space-filling models of M and P helical conformations obtained by hydrogen-bondingassisted structural folding. [20] The "misfolded" P chiral conformation suffers from steric congestion between phenyl groups that converge at the molecular core. As a consequence, it is energetically destabilized with respect to the M conformer, which is the "correctly folded" form of the molecule (NOTE: these conformers are not mirror images of one another but are diastereomeric and therefore differ in energy).…”