Aggregation of self-assembling branched [n]rotaxanes

Abstract

The so-called slippage methodology has been employed to self-assemble novel [2]-, [3]-, and [4]-rotaxanes incorporating, respectively, one, two, and three bis-p-phenylene-34-crown-10 macrocyclic components and a branched 'dumbbell' component, consisting of three arms containing bipyridinium units attached covalently to a 1,3,5-trisubstituted benzene central core and each bearing at its other end a substituted tetraarylmethane stopper. The absorption spectra, luminescence properties, and electrochemical behaviour of the branched component and its [2]-, [3]-, and [4]-rotaxanes have been investigated and discussed on the basis of the properties of their chromophoric and electroactive units. Charge- and energy-transfer processes between specific chromophoric units and the correlations between the unusual redox patterns of the various compounds have been evidenced and interpreted. The 1H-NMR spectroscopic investigation of the 'free' triply-branched hexacationic core, containing three bipyridinium units, one in each arm and terminated by bulky hydrophobic tetraarylmethane-based stoppers revealed, in chloroform solution, the formation of aggregates - a phenomenon which has been modeled using force field calculations. In addition, the formation of a gel was observed after the slow liquid-liquid diffusion of hexane into a chloroform solution of the triply-branched compound. Field-emission scanning electron microscopic investigation of this gel revealed the presence of domains of regular size. Surface-pressure-area measurements demonstrated the formation of stable monolayers by the 'free' backbone and the rotaxanes at an air-water interface: two distinct aggregates are formed by each compound. Interestingly, for the rotaxanes, the measured limiting area per molecule of both aggregates increases with the number of macrocyclic components which are incorporated within the rotaxane molecule. Atomic force microscopic analyses of the monolayers transferred onto mica revealed significant differences in their shapes when the two distinct aggregates formed by the same compound at different pressures were compared. In particular, the section analyses of the monolayers showed nanosized domains possessing diameters ranging from approximately 10 to 56 nm.