Well-Defined Porphyrin Architectures From Self-Assembly and Dewetting - Molecular Self-Assembly: Advances and Applications - page 242

evaporation is higher than at other locations in the film, which
results in a flow of solute material (in this case porphyrins) in the
direction of the edges of the pinholes, where their concentration
locally increases and the material finally accumulates.

Figure 8.2 Cartoon showing ring formation according to the 'pinhole

mechanism'. Refer the text for explanation.

The rupture of the solution film to form the pinholes can occur
via two possible mechanisms: by dewetting at a heterogeneous
nucleation site, or by spinodal dewetting [7,8]. Heterogeneous
nucleation of a hole can occur due to a surface irregularity or a
contamination (e.g., a small particle). Such a hole generally forms at
an early stage of evaporation, when the solution film is still relatively
thick, and it can subsequently grow in a continuous fashion, resulting
in the formation of rings with different diameters and which are
located at random positions on the surface. On a homogeneous,
clean surface, spinodal dewetting can occur, generally at a later
stage in the dewetting process when the solution film is thinner. In
that case, dewetting is a result of undulations on the surface, which
have a periodicity that is correlated to the spinodal length scale [9].
When rings are formed via this mechanism, they are generally rather
monodisperse in diameter and located at a certain mean distance
on the surface. The porphyrin rings were rather monodisperse and
had an empty interior, which suggests that they are formed via the
spinodal dewetting mechanism. This hypothesis was confirmed by
confocal fluorescence experiments, which revealed that only the
rings themselves fluoresce but not their interior.

8.2.2 Extended Circular Porphyrin Oligomers

The next generation of porphyrin building blocks was based on a
central benzene core, to which six [10] or twelve [11,12] porphyrins