This is sometimes referred to as the “face-to-face arrangement.” Figure
5.22 illustrates the difference between a J-aggregate and an H-aggregate.
One of the most characteristic properties of J-type aggregation is that such
materials absorb higher wavelength light with respect to the monomer
absorption. We say that it is red-shifted in the absorption spectrum with
respect to the monomer absorption (see Chapter 6). The absorption
wavelength of the H-aggregate is a little lower (or blue-shifted) with
respect to the monomer absorption wavelength. The energy shift of the
absorption wavelengths of the aggregates has been explained by exciton
theory. This theory will not be covered in detail in this text, but we can
apply our understanding of conjugation length and the particle-in-a-box
models to help understand this observation.
First assume that the electron motion is along the dipole only. A
J-aggregate can be considered a line along which free electrons can move.
Since the dipoles are arranged end-to-end, the electron motion can be
considered as delocalized along the entire length of the aggregate.
According to Equation 5.31, the length a is much greater than the length
of an individual monomer. Therefore, ΔE is small and is inversely proportional to wavelength (Equation 5.30), and the aggregate absorbs light
energy at a higher wavelength compared to a monomer. Conversely, an
H-aggregate will have a short conjugation length and a larger ΔE value
corresponding to the absorption of light of a smaller wavelength. The
unaggregated monomer can be considered intermediate between these
two extremes, and so the ΔE value corresponding to the excitation lies
between the values for the H- and J-aggregate.
5.4.5 π–π stacking interactions
We end this chapter with a brief discussion of a very weak electron
coupling interaction leading to aggregation known as the π–π stacking
interaction. These interactions occur due to the presence of p-orbitals in
conjugated ring systems such as benzene. The net effect of such an
interaction is face stacking of planar rings as shown in Figure 5.23a for
naphthalene. Although the effect is actually insignificant in small systems
such as benzene, the interaction becomes stronger as the number of πelectrons increases. It should be pointed out, however, that even in these
systems, electrostatic forces usually overcome π–π stacking interactions.
Nonetheless, the interaction is particularly strong in planar polycyclic
CHAPTER 5: Intermolecular Interactions and Self-Assembly
176
5.22 illustrates the difference between a J-aggregate and an H-aggregate.
One of the most characteristic properties of J-type aggregation is that such
materials absorb higher wavelength light with respect to the monomer
absorption. We say that it is red-shifted in the absorption spectrum with
respect to the monomer absorption (see Chapter 6). The absorption
wavelength of the H-aggregate is a little lower (or blue-shifted) with
respect to the monomer absorption wavelength. The energy shift of the
absorption wavelengths of the aggregates has been explained by exciton
theory. This theory will not be covered in detail in this text, but we can
apply our understanding of conjugation length and the particle-in-a-box
models to help understand this observation.
First assume that the electron motion is along the dipole only. A
J-aggregate can be considered a line along which free electrons can move.
Since the dipoles are arranged end-to-end, the electron motion can be
considered as delocalized along the entire length of the aggregate.
According to Equation 5.31, the length a is much greater than the length
of an individual monomer. Therefore, ΔE is small and is inversely proportional to wavelength (Equation 5.30), and the aggregate absorbs light
energy at a higher wavelength compared to a monomer. Conversely, an
H-aggregate will have a short conjugation length and a larger ΔE value
corresponding to the absorption of light of a smaller wavelength. The
unaggregated monomer can be considered intermediate between these
two extremes, and so the ΔE value corresponding to the excitation lies
between the values for the H- and J-aggregate.
5.4.5 π–π stacking interactions
We end this chapter with a brief discussion of a very weak electron
coupling interaction leading to aggregation known as the π–π stacking
interaction. These interactions occur due to the presence of p-orbitals in
conjugated ring systems such as benzene. The net effect of such an
interaction is face stacking of planar rings as shown in Figure 5.23a for
naphthalene. Although the effect is actually insignificant in small systems
such as benzene, the interaction becomes stronger as the number of πelectrons increases. It should be pointed out, however, that even in these
systems, electrostatic forces usually overcome π–π stacking interactions.
Nonetheless, the interaction is particularly strong in planar polycyclic
CHAPTER 5: Intermolecular Interactions and Self-Assembly
176