Complex Phase Transitions\nof Fully Rigid Sphere–Rod\nAmphiphiles Induced by Solvent Polarity in Dilute Solutions

We report complex macrophase and microphase transitions\nof rigid\namphiphiles with spherical Keggin molecular clusters as the solvophilic\nblock and rod-like rigid oligofluorene (OF) as the solvophobic block\nin mixed solvents of water and polar organic solvent. By properly\nadjusting the solvent polarity, the amphiphiles are found to respond\naccordingly by self-assembling into multilayered incomplete onion-like\nstructures (10–25 vol % THF), single-layered vesicular structures\n(60 vol % THF), and an unexpected macrophase separation in the middle\n(40–50 vol % THF), which is due to the anomalous trends in\nKeggin solubility as a result of the nature of TBA<sup>+</sup> counterions.\nThe rigidity of the OF block prevents the amphiphile from assembling\nby following the rule of packing parameters; instead, interdigitation\namong different rods leads to the formation of the solvophobic domain\nto achieve self-assembly. The incomplete onion structures are controlled\nby the interdigitation of rigid rods for the number of layers and\nthe electrostatic interaction among Keggin head groups for the interlayer\ndistance. When the degree of interdigitation becomes lower, the self-assembly\nprocess shows a trend that can be explained by the traditional rule\nof packing parameter. This study demonstrates the formation of different\nself-assembled structures by rigid amphiphiles and their transitions\ninduced by solvent composition. The self-assembly (microphase separation)\nof rigid amphiphiles in a dilute solution could indeed represent a\nbroad area containing complicated, uncharted rules.