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Title: Design, synthesis and characterisation of π-extended viologen-based molecular and supramolecular complexes with cucurbit[n]urils
Author: Olesinska, Magdalena
ISNI:       0000 0004 7962 005X
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2019
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Viologens, 4,4'-bipyridinium dications (V²⁺), have attracted much attention as electron mediators in a variety of photochemical, electrochemical and chemical processes ranging from catalysis, electrochromic devices to solar energy conversion and storage systems. The different applications of viologens are related primarily to their facile one electron reduction of dicationic species, V²⁺, to radical monocations, V⁺. Moreover, the dynamic interactions of V²⁺ derivatives with cucurbit[n]uril (CB[n], for n=7 or 8) macrocycles, the dimerisation of V⁺ radical cations inside the CB[8] molecular cavity as well as redox controlled stoichiometry of V²⁺ based inclusion complexes make these systems highly attractive for supramolecular chemistry and dynamic nanosystems, i.e. molecular machines. This doctoral thesis focuses on design, synthesis and characterisation of π-extended viologen-based molecular and supramolecular systems. Integral to the project was to study assembly processes that involve extended viologen derivatives and CB[n] macrocycles as well as characterisation of the physicochemical properties of resulting host-guest systems. A brief overview of viologen and CB[n] chemistry as well as fundamentals regarding self-assembly processes and how can they influence the optoelectronic properties of materials are discussed in Chapter 1. Chapter 2 describes the synthesis and characterisation of extended methyl viologen (EV[X]MV) derivatives with an aromatic extension between the pyridinium rings (X= phenylene, naphthalene, pyrene, chrysene, thiophene and dithiophene), their interactions with CB[7] and CB[8] macrocycles as well as physicochemical properties of the resultant host-guest systems are studied. Methyl viologen (MV²⁺) is a well-known guest for both CB[7] and CB[8] macrocycles. On account of the different binding modes with CB[n], the optical as well as redox properties of MV²⁺ can be modulated. Here, the effect of the aromatic extension ([X]) on EV[X]MVs before and upon complexation with CB[7] and CB[8] is investigated using nuclear magnetic resonance (NMR), steady-state UV-Vis and photoluminescence (PL) spectroscopies as well as isothermal titration calorimetry (ITC) and cyclic voltammetry (CV). Interestingly, dimerisation of selected extended methyl viologens with CB[8] and the charge accumulative properties of the resultant complexes were observed. In Chapter 3, synthesis and characterisation of a family of extended viologens with aromatic substituents (EV[X]R, further denoted as extended aryl viologens), their interactions with CB[7] and CB[8] macrocycles as well as physiochemical properties of the resultant host-guest systems are studied. The design and synthesis of these new class of π-conjugated viologen derivatives are elaborated involving different aromatic bridging units, as well as rigid aromatic peripheral functionalisation with electron donating and electron withdrawing groups on the pyridinium rings. Upon mixing with CB[8], the extended aryl viologen derivatives functionalised with electron donating groups self-assemble into 2:2 complexes. Moreover, the extended work on EV[X]R molecules functionalised with different aromatic substituents, which are simultaneously encapsulated within CB[8] to form heteroquarternery complexes is discussed. The nature and dynamics of the resulting CB[8]₂·(EV[X]R)₂ homo- and hetero-complexes are studied both in the ground as well as exited states upon interaction with light utilising NMR, steady-state and time-resolved spectroscopies as well as ITC experiments. Finally, in Chapter 4 the synthetic approach towards asymmetric aryl viologen (AV) and extended aryl viologen (EV[X]RR', where [X]= naphthalene) is presented. Firstly, synthetic pathways based on the Zinke reaction for asymmetric aryl substitution of AV and EV are discussed. Secondly, the supramolecular binding of the obtained asymmetric EV[Np]RR' with CB[8] is investigated along with the effect of supramolecular dimer formation on the optical properties. Finally, the application of asymmetric aryl viologens in the formation of a bio-inspired nanofibers is presented.
Supervisor: Scherman, Oren Alexander Sponsor: Marie Curie FP7 SASSYPOL ITN (607602) programme
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: Cucrbit[n]uril ; Extended viologen ; Extended aryl viologen ; Asymmetric viologen ; Viologen dimerisation ; Excimers