Concentration-dependent thermochromism and supramolecular aggregation in solution of triblock copolymers based on lengthy oligothiophene cores and poly(benzyl ether) dendrons

Self-complexation of triblock copolymers based on undeca- and heptadecathiophene cores attached to generation three Frechet-type poly(benzyl ether) dendrons in solution at low temperature leads to the formation of relatively small but well-defined supramolecular assemblies. The formation of these aggregates is associated with a strong thermochromic effect. Upon cooling dilute solutions, minor concentration-independent changes in the optical absorption due to intrachain planarization precede significant concentration-dependent thermochromic changes that are shown to originate from intermolecular pi-pi stacking interactions. The structurally well-defined nature of the substrates enables the determination of distinct temperature regimes for both intra- and intermolecular thermochromic changes. Furthermore, combining thermodynamic expressions for both the concentration and temperature dependence of aggregation allows a quantitative analysis of the self-assembly phenomenon. The results show that the supramolecular aggregates are relatively small, yet well-defined involving from 5 to 6 molecules in an average aggregate. The apparent size limitation of the aggregates is ascribed to steric constraints imparted by the dendritic wedges. Relatively high exothermicities accompany the aggregation process, increasing with conjugation length from -73.0 kJ/mol with the undecathiophene triblock (G3-T11-G3) to -85.8 kJ/mol for the heptadecathiophene analogue (G3-T17-G3), suggesting that; pi-pi stacking is the driving force for complexation.