Darren G. Hamilton

Photophysical and Electrochemical Characterisation of the Interactions between Components in Neutral π‐Associated [2]Catenanes

The electrochemical and photophysical properties of a variety of neutral pi-associated [2]catenanes have been explored by using cyclic voltammetry, absorption and luminescence spectrophotometry and the measurement of exited-state lifetimes. Several trends that could be correlated with interactions between the mechanically linked components of the structures were revealed from comparative study of catenane precursors, model compounds and the [2]catenanes themselves. Throughout, emphasis is placed on pair-wise comparisons between systems which differ in a single structural feature. Substitution in a catenane of a pyromellitic diimide unit by a naphthalene diimide yields more readily reduced derivatives, whose absorption spectra reveal charge transfer within the catenane to be a lower energy process of reduced intensity. Conversion of the butadiyne links within the diimide macrocycle of the catenanes to saturated chains results in an increase in both the energy and intensity of their charge-transfer bands; electrochemically these derivatives are all harder to reduce than the parent systems. Replacement of one of the electron-donating components of the catenanes with a less effective aromatic donor bearing a carboxy group also decreases the energy and intensity of the charge-transfer feature and is accompanied by a slightly more ready reduction. A sequence of reduction and translational events is proposed to explain the intriguing electrochemical behaviour of a catenane that contains one pyromellitic and one naphthalene diimide. For some systems the photophysical and electrochemical techniques, whilst exploring distinct physical phenomena, are shown to be in good agreement by comparison of shifts of electrochemical reduction waves with those of charge-transfer absorption features.

Reversible synthesis of π-associated [2]catenanes by ring-closing metathesis: towards dynamic combinatorial libraries of catenanes

Ring-closing metathesis of aromatic diimides substituted with olefin-terminated alkyl chains in the presence of a dinaphtho crown ether affords neutral [2]catenanes. Templating π-donor/π-acceptor interactions are exploited to drive mechanical interlocking and favour production of the more thermodynamically stable catenane products. Structure proof was provided by single-crystal X-ray synchrotron diffraction of a representative catenane.

2,4-Bis[4-(N,N-dibutylamino)phenyl] squaraine: X-ray crystal structure of a centrosymmetric dye and the second-order non-linear optical properties of its non-centrosymmetric Langmuir–Blodgett films

Crystals of the title compound exist as a green monoclinic phase [space group P21/c with a= 9.046(1), b= 19.615(2), c= 9.055(1)A, β= 116.107(5)°, Z= 2] and a purple triclinic phase. The chromophore is both planar and centrosymmetric and its dimensions indicate a tendency towards a quinoidal structure with extensive delocalisation. The Langmuir–Blodgett (LB) films show two types of aggregation with absorption maxima at 655 nm and 540 nm. These monolayers also exhibit strong second harmonic generation (SHG) comparable to the intensity from films of hemicyanine dyes. The anomalous non-linear optical properties are attributed to a serendipitous non-centrosymmetric packing arrangement within the films and to an intermolecular charge transfer contribution to the bulk second-order susceptibility.

Synthesis and post-assembly modification of some functionalised, neutral π-associated [2]catenanes

Two diaryl crown ethers bearing methyl ester functionality on one of the aromatic components of the macrocycle have been used as templates for the cyclodimerisation of acetylenic aromatic diimides, thereby achieving assembly of several functionalised neutral π-associated [2]catenanes. Post-assembly synthetic modification of some of these interlocked structures, and of some previously reported [2]catenanes, by hydrogenation of the butadiyne linkers in the diimide macrocycle has also been achieved. In contrast to their parent systems, these modified catenanes give room temperature 1H NMR spectra which are poorly resolved; at elevated temperatures the constituent rings of the catenanes are in fast exchange on the chemical shift timescale leading to simplification of the spectra. The crystal structure of one of the ester-derivatised [2]catenanes is also presented. The ester functionality present in these interlocked systems provides options for potential chemical modification via surface attachment or metal-ion complexation.

A Neutral Donor-Acceptor p-Stack: Solid-State Structures of 1 : 1 Pyromellitic Diimide-Dialkoxynaphthalene Cocrystals

Pyromellitic diimide forms orange-coloured cocrystals of 1 : 1 stoichiometry with dialkoxynaphthalene derivatives. The solid-state structures of two examples are presented. The cocrystal formed with 2,6-dimethoxynaphthalene presents vertical stacks of alternating π-rich and π-deficient subunits with the long axes of the respective components approximately parallel. Investigation of the packing in the cocrystal also reveals a stabilizing array of hydrogen bonds between the components of adjacent stacks. Cocrystallization with 1,5-[2-(2-hydroxyethoxy)ethoxy]naphthalene, a derivative bearing hydroxy terminated ethyleneoxy chains, gives rise to an altered structural arrangement. Alternating donor- acceptor stacks once again dominate the structure but adopt a geometry where the long axes of the constituents are essentially perpendicular. Hydrogen-bonding interactions result in the formation of continuous non-covalently linked columns of donor and acceptor subunits by linking the terminal hydroxy functions of the naphthalene component to the imide protons. The structural preferences revealed by these solid-state analyses indicate that these complexes are useful prototypes of more complex neutral supramolecular assemblies.

Neutral [2]catenanes from oxidative coupling of π-stackedcomponents

Oxidative dimerisation of bis-acetylenic precursors in the presence of a dinaphtho-crown ether affords neutral [2]catenanes in good yields; the interaction of the complementary donor–acceptor π-systems ensures assembly in a geometry favouring catenane formation.

An approach to thermodynamically controlled supramolecular assembly possessing an integral locking mechanism.

A general approach for thermodynamically controlled molecular assembly employing carbonyl addition chemistry is exemplified with a prototypical rotaxane synthesis. Recognition-driven assembly of the rotaxane proceeds under conditions of mild base catalysis, while subsequent treatment with catalytic acid triggers a dehydrative locking mechanism and fixes the evolved molecular architecture in place.

Synthesis of macrocycles and an unusually asymmetric [2]catenane via templated acetylenic couplings

An unusually asymmetric [2]catenane has been prepared by interlocking a hybrid crown macrocycle (containing one polyether and one butadiyne linker) with a diimide-derived macrocycle itself constructed with a pair of rigid butadiyne links. This system of complementary building blocks and connectors has proved a versatile vehicle for demonstration of a number of templating effects. Synthesis of the hybrid crown monomer by intramolecular cyclisation is promoted by the presence of a benzene diimide, acting as a positive template, but a sufficiently different effect is exerted by a naphthalene diimide derivative that intermolecular dimerisation becomes significant. Templating the hybrid crown synthesis with an ‘active’ benzene diimide, itself bearing acetylene functions, allows the first tandem synthesis of a catenane comprising two different rings: the diimide first acts as a template for the formation of the hybrid crown, which itself in turn acts as a template for cyclodimerisation of the acetylenic diimide. Remarkably, the yield of catenane is essentially independent of the initial cyclisation state of the crown component. No significant production of covalently linked donor–acceptor species is observed from these reactions, supporting the presence of an intermediate complex pre-organised to catenane formation. In contrast, oxidative coupling of an alkyl solubilised diimide gave no interlocked products and yielded only small amounts of cyclic poly-imide macrocyles.

Acylation of alkyl ferrocenecarboxylates: An approach to unsymmetrical 1,1′-disubstituted ferrocene derivatives and bridged metallocene receptors

Abstract Acylation of benzyl ferrocenecarboxylate 3 with benzoyl chloride affords the known ferrocene keto-acid 5 in one step and provides a convenient entry to a variety of unsymmetrical 1,1′-substituted ferrocene derivatives. Similar acylation procedures have been employed to bridge two ferrocene units leading to a series of rigid bis-ferrocene derivatives, which are precursors to more elaborate receptor architectures.

Critical re-evaluation of FABMS analysis of ligand–cation interactions

Whilst the use of FABMS in the analysis of ligand-cation interactions has become widespread, there are a number of experimental variables associated with the use of this technique, the effects of which have not previously been addressed. The interactions of 12-crown-4, 15-crown-5 and 18-crown-6 with the alkali metal cations reveal two important observations: (i) that the matrix employed for the FAB studies has a dramatic effect on the observed binding behaviour and (ii) that the observed relative intensities of protonated ions ([M + H]+) and metallated ions ([M + Metal]+) differ enormously. Determination of the ratios of these signals would allow quantitative arguments to be applied to experiments of this type. Accordingly, previous reports of the use of FABMS for the quantitative study of ligand–cation interactions must be viewed with caution.