Photophysical and electrochemical characterisation of the interactions between components in neutral pi-associated [2]catenanes

The electrochemical and photophysical properties of a variety of neutral pi -associated [2]catenanes have been explored by using cyclic voltammetry, ab sorption 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]c atenanes themselves. Throughout, emphasis is placed on pairwise comparisons between systems which differ in a single structural failure. Substitution in a catenane of a pyromellitic diimide unit by a naphthalene diimide yield s more readily reduced derivatives, whose absorption spectra reveal charge transfer within the catenane to be a lower energy process of reduced intens ity. Conversion of the butadiyne links within the diimide macrocycle of the catenanes to saturated chains results in an increase in both the energy an d intensity of their charge-transfer bands; electrochemically these derivat ives are all harder to reduce than the parent systems. Replacement of one o f the electron-donating components of the catenanes with a less effective a romatic donor bearing a carboxy group also decreases the energy and intensi ty of the charge-transfer feature and is accompanied by a slightly more rea dy reduction. A sequence of reduction and translational events is proposed to explain the intriguing electrochemical behaviour of a catenane that cont ains one pyromellitic and one naphthalene diimide. For some systems the pho tophysical and electrochemical techniques, whilst exploring distinct physic al phenomena, are shown to be in good agreement by comparison of shifts of electrochemical reduction waves with those of charge-transfer absorption fe atures.