Chiroptical molecular switches:application of inherently dissymmetric alkenes

In dit proefschrift beschrijf ik het ontwerp en de synthese van chirak inherent dissymetrische alkenen als moleculaire schakelingen en hun mogelijke toepassingen in reversibele optische data opslag en controle van de eigenschappen van materialen zoals vloeibare kristallen en polymeren. Wat zijn moleculaire schakelaars? Moleculaire schakelaars zijn opgebouwd uit twee vormen van een molecuul die met behulp van een extern signaal (S) in elkaar omgezet kumen worden. Waarom zijn moleculaire schakelaars zo belangrijk? Schakelaars zijn de belangrijkste componenten van computers. De huidige vraag naar infonnatie opslag en verwerking is zo groot dat de vraag naar kleine, zeer snelle schakelaars enorm is toegenomen. De huidige technologic om steeds meer informatie per oppervlakte eenheid op te slaan, nadert zijn gremn (binaire systemen in chips bijvoorbeeld). Schakelaars op moleculair niveau zijn klein en wanneer zij met licht geschakeld kunnen worden zijn zij ook snel. Een voorbeeld uit de natuur, is bijvoorbeeld de moleculaire lichtschakelaar, het retinal, in het oog, waardoor wij kunnen zien. Wanneer organische moleculen gebruikt worden, kunnen de organische schakelaars synthetisch gemakkelijk gemodificeerd worden zodat verschillende functies op moleculair niveau aangestuurd kunnen worden

Chiroptical molecular switches

Het onderzoek dat beschreven is in dit proefschrift gaat over chiroptische moleculaire schakelaars op basis van sterisch gehinderde alkenen. ... Zie: Samenvatting

Synthesis and 1H-NMR Complexation Studies of Alkalimetal Bithioxanthylidene Crown Ether Complexes

Two synthetic routes to a novel class of sterically overcrowded alkenes, bithioxanthylidene crown ethers 1, are described. The 1H spin-lattice relaxation times (T1) of the crown ethers as well as those of the corresponding complexes with Li+, Na+, K+, Rb+ and Cs+ in CDCl3 were measured and the results were interpreted in terms of complexation affinities.

Bisthioxanthylidene biscrown ethers as potential stereodivergent chiral ligands

The concept of bisthioxanthylidene biscrown ethers as potential stereodivergent chiral ligands in asymmetric synthesis is introduced. Substituted bisthioxanthylidenes may be chiral and can exist as stable enantiomers due to their folded structure. As a result, both a right-handed helix (P) and left-handed helix (M) are present in this type of molecule. This offers the unique possibility to construct two crown ether moieties, attached to the same molecule, of which one exhibits (P)-helicity and the other (M)-helicity. When the crown ether moieties differ in size they can be complexed selectively with a base containing a cation of appropriate diameter. In this manner the (P)-helix and the (M)-helix can be activated selectively to serve as a chiral environment for base catalyzed asymmetric synthesis. Thus, we envisioned the new concept of a single chiral ligand to separately synthesize two enantiomers of a chiral product just by varying the added base. For this purpose, four new bisthioxanthylidene monocrown ethers and two new bisthioxanthylidene biscrown ethers were synthesized. Two biscrowns and two monocrowns were separated into their respective enantiomers (HPLC) and optical data (UV and CD) were collected to ensure stability of enantiomers at ambient temperatures. Ion complexation of one mono- and two biscrown ethers with potassium and sodium cations was investigated.

Switching of Chirality by Light

Optically active photoresponsive molecules are described by which control of chirality is achieved by light. These chiroptical molecular switches are based on inherently dissymmetric overcrowded alkenes and the synthesis, resolution and dynamic stereochemical properties are discussed. Introduction of nitro-acceptor and dimethylamine-donor substituents in a naphto [2,1-b]thiopyran-1’-ylidene-9H-thioxanthene helical shaped alkene resulted in a highly diastereoselective modulation of photostationary states with 70%-M or 90%-P helicity, respectively. The chiroptical switches offer attractive prospects as control elements for (supra)molecular functions and organization. This is demonstrated in dual mode photoswitching of luminescence and photoswitching between liquid crystalline phases.

Sterically Overcrowded Alkenes; Synthesis, Resolution and Circular Dichroism Studies of Substituted Bithioxanthylidenes

Bithioxanthylidenes with small substituents at positions 2 and 2’ have been synthesized and resolved by chiral HPLC. The X-ray molecular structure of 9-(2’-methyl-9’H-thioxanthene-9’-ylidene)-9H-xanthene (17) confirms the folded form of these molecules. UV and CD studies of several bithioxanthylidenes are presented. The CD spectra of these molecules can be described qualitatively by subtracting the CD spectra of both differently sensed helices (M and P) present in the molecule, which can be distinguished by the substituents at positions 2 and 2’.

Mechanism of the Transformation of a Stiff Polymer Lyotropic Nematic Liquid Crystal to the Cholesteric State by Dopant-Mediated Chiral Information Transfer

In this work the lyotropic liquid crystal nematic state of poly(n-hexyl isocyanate) (PHIC) was converted to the cholesteric state by doping with a variety of chiral small molecules and with optically active polyisocyanates. Circular dichroism experiments in dilute isotropic PHIC solutions show that several of the small molecules used as dopants in the liquid crystal work cause an excess of one helical sense in the otherwise helically dynamically racemic PHIC. Both the helical sense and excess of this sense in dilute solution correlate to the cholesteric sense and twisting power of the same dopants in the concentrated PHIC liquid crystal solutions. The experimental results are consistent with a mechanism of cholesteric formation in which the chiral dopants displace slightly the otherwise equal population of the dynamically interconvertable mirror helical senses of the PHIC, which is then further amplified by the liquid crystal matrix. Direct evidence for the amplification of the intrinsic helical sense excess of the polyisocyanate by the liquid crystal state could be demonstrated by comparing the cholesteric properties of systems doped with polyisocyanates in which the ratio of the mirror helical senses of the dopants in two compared experiments are identical but in one experiment the helical senses could interconvert and in a separate experiment could not interconvert. The amplification mechanism is driven by the reduction in population of the mobile kinked helical reversals which likely act as “bad neighbors” to the local liquid crystal organization.