Design, Synthesis and Characterisation of π-Extended Viologen-Based Molecular and Supramolecular Complexes with Cucurbit[n]urils
Viologens, 4,4'-bipyridinium dications (V$^{2+}$), 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, V$^{2+}$, to radical monocations, V$^{+.}$. % Moreover, the dynamic interactions of V$^{2+}$ 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 V$^{2+}$ based inclusion complexes make these systems highly attractive for supramolecular chemistry and dynamic nanosystems, $\textit{i.e.}$ molecular machines. % This doctoral thesis focuses on design, synthesis and characterisation of $\pi$-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 (MV$^{2+}$) 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 MV$^{2+}$ 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 $\pi$-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 ${\mathrm{CB[8]_2}}\cdot{(\mathrm{EV[X]R})_2}$ 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.
Cucrbit[n]uril, Extended viologen, Extended aryl viologen, Asymmetric viologen, Viologen dimerisation, Excimers
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