Chiroptical Switching in Molecular and Extended Framework Systems

Abstract

Chiroptical switches are stimuli-responsive materials whose properties can be reversibly altered by external stimuli. Chiroptical switches are of great interest for applications in optical displays, optoelectronics, and chiral sensing. This thesis explores the design, synthesis, and characterisation of novel chiroptical switches in both molecular and framework systems. Chapter 3 presents the synthesis of enantiomers (S/R)-1,1′-binaphthalene-2,2′-bis(1,8-naphthalimide), combining an axially chiral binaphthalene scaffold with redox-active 1,8-naphthalimide groups. Their chiroptical and electronic behaviours were investigated via UV-vis, CD, and EPR spectroelectrochemistry, revealing reversible redox-modulated chiroptical switching with excellent sensitivity and stability.

Chapter 4 reports the chiral ferrocene amino acid bioconjugates, L/D-Fc(MeLeu)2. Chirality arises from intramolecular hydrogen bonding that induces an axially chiral conformation. Their chiroptical response was found to be solvent dependent, with more intense signals in nonpolar solvents. Temperature-modulated chiroptical switching was also demonstrated, with CD intensity increasing upon cooling. Redox-modulated switching via reversible oxidation of the ferrocene core was also performed, successfully achieving dual-responsive chiroptical switching. Concentration effects on switching reversibility and stability are discussed.

Chapter 5 details the synthesis of lanthanide-based MOFs, [Ln2(CBA)2(H2O)4]·4H2O (Ln = La, Ce, Pr, Nd), using chiral ligand (4-carboxybenzoyl)-L-aspartic acid (H3CBA). These MOFs were found to be achiral due to ligand racemisation during synthesis. Strategies to prevent this and the underlying causes are examined. A second chiral ligand, N-(4-carboxyphenyl)-L-alanine (H2CPA), successfully yielded a chiral MOF with Cd(NO3)2·4H2O.

This work advances the understanding of stimuli-responsive chiroptical materials and offers key insights into promising areas of future exploration.