Stabilization of reactive carbon-rich systems by supramolecular encapsulation

Thesis

This thesis explores efforts toward the synthesis and stabilization of reactive π-systems through supramolecular encapsulation. Carbon allotropes composed entirely of sp carbon atoms have intrigued chemists for decades but remain challenging to isolate or characterize due to their extreme reactivity. This work documents the progress made toward isolating both linear and cyclic sp carbon chains and also expands the use of supramolecular encapsulation to stabilize cyanine dyes.

Chapter 1 reviews the history of carbon allotropes, with a focus on the cyclic and linear forms of sp carbon. Synthetic strategies for preparing acetylene-rich molecules, interlocked systems, and stabilization via encapsulation are discussed.

Chapter 2 introduces a novel approach to the synthesis of long polyyne [n]rotaxanes. By coupling a [2]rotaxane bearing temporary bulky MAE stoppers, precursors of polyyne [n]rotaxanes were accessed. Subsequent unmasking of the MAEs successfully yielded polyyne [n]rotaxanes with up to 34 contiguous acetylene units. Stability studies revealed a 16.5-fold increase in solid-state stability for polyynes with 24 contiguous acetylenes upon encapsulation.

Chapter 3 extends the strategy developed for polyyne [n]rotaxanes to the synthesis of cyclocarbon [n]catenanes. Protective macrocycles were redesigned, and the active metal template method using dicobalt-masked precursors was optimized. Unmasking experiments under ambient conditions provided evidence for the generation of encapsulated cyclo[48]carbon and cyclo[64]carbon, as monitored by UV-vis absorption spectroscopy. Furthermore, mass spectrometry confirmed the formation of these species.

Chapter 4 outlines an approach to [Mn]cyclophynes and their catenanes via the dicobalt precursor-based strategy. Unmasking experiments suggested the generation of polyynic species and were assigned to [Mn]cyclophynes. Further studies however were hindered by the limited stability of the generated species.

Chapter 5 describes the synthesis of encapsulated heptamethine cyanine dyes using αcyclodextrin. Photochemical bleaching experiments showed a significant increase in stability, and a slight reduction in fluorescence quantum yield, while the fluorescence brightness of the rotaxanes remains similar to the free dye. Additionally, the chirality of the macrocycle was transferred to the dye, and the induced Cotton effect could be tuned by altering the substituents on the cyanine dye.

Authors: Gupta, P

• DOI: 10.5287/ora-x5voy4rkm
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: cyanines; cyclocarbons; cyclophyne; masked alkyne equivalent; catenanes; rotaxanes; polyynes
• Subjects: Supramolecular chemistry; Chemistry, Organic