Scanning force microscopy of upright-standing, isolated calixarene-porphyrin heterodimers

A water soluble calixarene [4] arene 1 with four guanidinium substituents on the upper rim and propyl groups below was anchored in the propylamino coating of smooth silica particles, and a tricarboxylate-tripod porphyrin 2 of 2 nm height was attached to these cationic islands. The molecular complex with a height of 3 nm was unequivocally detected on the particles' surface by atomic force microscopy in the tapping mode. Although deposits of 1 (height: 1 nm) and 2 (height: 2 nm) were also evident on the smooth silica particles, 3 nm seems to be the minimal height to identify single objects. The soft surface of the particles not only allowed tight attachment of molecular edge amphiphiles by the hydrophobic effect but also immobilized the particles on the mica surface by amine-silicate interactions.