Self-Assemblies, Folding, and Properties of Perylene Monomers, Oligomers, and Polymers - Molecular Self-Assembly: Advances and Applications - page 125

Finally, we will summarize the folding and unfolding processes
using single-molecule fluorescence microscopy and spectroscopy.
Interestingly, these studies reveal some electronic and optical aspects
about self-assemblies and folding. Specifically, the folded or self-
assembled nanostructures have very different electronic structures
and hence optical properties both in absorption and emission than
those of free monomers.

5.2 Self-Assemblies of Large Planar Molecules

The formation of molecular self-assemblies is driven by weak
intermolecular interactions. The important weak secondary
interactions include hydrogen bonding, ion-pairing attractions,
hydrophobic effects, and interaction. In particular, self-

organization of perylene tetracarboxylic diimide (PDI) is used in this
chapter to illustrate the self-assembly principles that apply to many
other molecules. In the PDI structure, the weak interactions directing
the self-organization processes come from hydrophobic effects and/
or

π−π

interactions. Considering recent examples wherein such self-
organization can also occur in organic solvents [1

π−π

−

3], we attribute
the self-organization of PDI driving force of perylene stacking in
organic solvents mainly to the

π−π

interactions because hydrophobic
effects are largely eliminated in organic solvents.

The origin of

π−π

interactions has been controversial. Previous

π−π

models for interactions include (1) the solvophobic model [4],

(2) the electron donor and acceptor model [5,6], (3) the atomic
charge model [7], (4) the nature of

π−π

interaction model [8], and
(5) the quadruple interaction model [9]. Each model emphasizes the
particular contribution from an individual effect and their prediction
and interpretation are not always in agreement. Modern approaches
seem to only compute the stabilization energies of the stacked

dimers and to compare them with similar or different structures.
Yet the true nature of

π−π

π−π

interactions remains to be satisfactorily
illustrated. At this stage, a couple of general remarks can be made. In
organic solvent, solvophobic effects appear to be unimportant. Lack
of electron-transfer evidence suggests the unlikeness of interactions
between an electron donor and an electron acceptor. The experimental
facts, however, that large planar

π

-molecules do favor formation of
open-ended cofacial stacking in solution indicate the existence of

π−π

attractive forces or secondary binding interactions.