Well-Defined Porphyrin Architectures From Self-Assembly and Dewetting - Molecular Self-Assembly: Advances and Applications - page 248

the propeller-like conformations of the hexamer shown in Fig. 8.3
(top). As a result of this conformation, the porphyrins in the stacks
are tilted at slight displacement angles with respect to the central
benzene core.
According to the pinhole mechanism, ring formation is governed
by the properties of both the solvent and the surface, which together
influence the dewetting process. The specific interactions between
the solvent and the surface determine the ease by which the solution
flow carries the dissolved molecules to the inner edge of the growing
pinhole. The apolar solvent chloroform, which was used in the
experiments with the porphyrin hexamers, wets a hydrophilic carbon-
coated surface well, but it is also expected to wet the hydrophilic
carbon-coated glass surface, albeit to a lesser extent. The latter
effect implies that the hexamers remain dissolved longer during
the evaporation of the solution. When the concentration increases,
they are allowed to self-assemble in a thermodynamically controlled
process, which eventually results in the deposition of ordered arrays.
When a solvent wets a surface better, the thermodynamic processes
of intermolecular assembly and rearrangement of the molecules in
the aggregates compete with the kinetically favoured adsorption to
the surface, and rings with a less perfect internal order are formed.
Thus, ring formation is governed by a complex interplay of molecular
properties (solubility, stackability, and adsorption) and physical
properties (speed of solvent evaporation and surface wettability).

8.2.3 From Rings to Lines

The cyclic porphyrin oligomers that formed the ring-like

assemblies described in the previous sections all have in common that
they exist in disk-like conformations possessing extended

2 and 3

-surfaces,
which allows them to interact with each other via favourable

π

π−π

stacking interactions. Despite the progressive increase in the number
of these interactions upon going from porphyrin dimers to hexamers
to dodecamers, small angle neutron scattering (SANS) studies
revealed that only relatively small columnar aggregates were formed
in organic solvents, containing less than 10 molecules. With the aim
to strengthen the intermolecular interactions between the porphyrin
disks, we decided to change the linkers between the porphyrins and
the cores from ethers to amides. It is generally known that benzene