AGGREGATION OF SELF-ASSEMBLING BRANCHED [N]ROTAXANES

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 b ipyridinium units attached covalently to a 1,3,5-trisubstituted benzen e central core and each bearing at its other end a substituted tetraar ylmethane stopper. The absorption spectra, luminescence properties, an d 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. Char ge- 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 H-1-NMR spectrosco pic investigation of the 'free' triply-branched hexacationic core, con taining three bipyridinium units, one in each arm and terminated by bu lky hydrophobic tetraarylmethane-based stoppers revealed, in chlorofor m solution, the formation of aggregates-a phenomenon which has been mo deled 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 measurement s demonstrated the formation of stable monolayers by the 'free' backbo ne and the rotaxanes at an air-water interface: two distinct aggregate s are formed by each compound. Interestingly, for the rotaxanes, the m easured limiting area per molecule of both aggregates increases with t he 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 shape s when the two distinct aggregates formed by the same compound at diff erent pressures were compared. In particular, the section analyses of the monolayers showed nanosized domains possessing diameters ranging f rom approximately 10 to 56 nm.