The anion impact on the self-assembly of naphthalene diimide diimidazolium salts

Naphthalene diimide diimidazolium salts differing for the anion nature were synthesized and their properties as well as their self-assembly behaviour were investigated. In particular, we took into consideration the N,N’-bis-(1-octyl-3- propylimidazolium)-naphthalene diimide cation and anions differing in size, shape and coordination abilities like [I-], [BF4 -]and [NTf2-]. After determination of thermal behaviour, using differential scanning calorimetry and thermal gravimetric analysis, the electrochemical stability and redox properties were assessed by cyclic voltammetry. The self-assembly behaviour of the salts was investigated by concentration and temperature-dependent spectroscopic studies (UV-vis and fluorescence). Data collected demonstrate the ability of salts used to aggregate through an isodesmic pathway. Interestingly, the aggregation is featured by an enhanced emission event. Properties of aggregates were also investigated in solid state by fluorescence measurements and scanning electron microscopy (SEM). On the whole, data collected show that properties of salts and of aggregates that they are able to form, are heavily dependent on the anion nature. Salts used join the properties of a semiconductor, like the naphthalene diimide core, with the ones of imidazolium salts and could have interesting applications as materials for optoelectronic and conservation of energy.