Thermodynamics of the self-assembly of N-annulated perylene bisimides in water. Disentangling the enthalpic and entropic contributions

We report on the self-assembling features of the amphiphilic N-annulated perylene bisimides (N-PBIs) 1-4 in water. Their self-assembly is investigated both experimentally and theoretically and is shown to be entropically driven in all cases. Importantly, the hydrophilic/hydrophobic ratio, determined by the number and flexibility of the oligo(ethylene) glycol (OEG) chains present in monomers 1-4, plays a relevant role in the exothermic/endothermic nature of the self-assembly enthalpy. Thus, the process is enthalpically favoured in compounds 1 and 2, which are end-capped with two flexible OEG chains, but enthalpically unfavoured in compounds 3 and 4, which are end-capped with bulkier and rigid phenyl groups decorated with three OEG chains. Molecular dynamic simulations including water molecules show the influence of the initial arrangement of the hydrophilic side chains and the number of water-side chain interactions on the enthalpy of the self-assembly. Thus, the pi-stacking of 3 to form the aggregated species is accompanied by the rupture of a larger number of stabilizing water-side chain interactions than that computed for compound 1. The loss of these interactions determines the sign of the enthalpy contribution and, therefore, the global stability of the aggregated species in aqueous media.