Liquid Crystals

Chapter 1.2 - Electronic Properties

1.2.   ELECTRONIC PROPERTIES

1.2.1.   Electronic Transitions and Ultraviolet Absorption


The electronic properties and processes occurring in liquid crystals are decided
largely by the electronic properties of the constituent molecules. Since liquid crystal
constituent molecules are quite large, their energy level structures are rather complex.
As a matter of fact, just the process of writing down the Hamiltonian for an isolated
molecule itself can be a very tedious undertaking. To also take into account
interactions among the molecular groups and to account for the difference between
individual molecules’ electronic properties and the actual liquid crystals’ responses
will be a monumental task. It is fair to say that existing theories are still not sufficiently
precise in relating the molecular structures and the liquid crystal responses.
We shall limit ourselves here to stating some of the well-established results, mainly
from molecular theory and experimental observations.

In essence, the basic framework of molecular theory is similar to that described in
Chapter 10, except that much more energy levels, or bands, are involved. In general,

Figure 1.6.  → * electronic transitions in a benzene molecule.


the energy levels are referred to as orbitals. There are π, n, and σ orbitals, with their
excited counterparts labeled as π*, n*, and σ*, respectively. The energy differences
between these electronic states which are connected by dipole transitions give the so-
called resonant frequencies (or, if the levels are so large that bands are formed, give
rise to absorption bands) of the molecule; the dependence of the molecular susceptibility
on the frequency of the probing light gives the dispersion of the optical dielectric
constant (see Chapter 10).

Since most liquid crystals are aromatic compounds, containing one or more aromatic
rings, the energy levels or orbitals of aromatic rings play a major role. In particular,
the π→ π* transitions in a benzene molecule have been extensively studied.
Figure 1.6 shows three possible π→ π* transitions in a benzene molecule.

In general, these transitions correspond to the absorption of light in the near-UV
spectral region (≤ 200 nm). These results for a benzene molecule can also be used to
interpret the absorption of liquid crystals containing phenyl rings. On the other hand,
in a saturated cyclohexane ring or band, usually only σ electrons are involved. The
σ→ σ* transitions correspond to the absorption of light of shorter wavelength
(≤ 180 nm) in comparison to the π→ π* transition mentioned previously.

These electronic properties are also often viewed in terms of the presence or
absence of conjugation (i.e., alternations of single and double bonds, as in the case of
a benzene ring). In such conjugated molecules the π electron’s wave function is delocalized
along the conjugation length, resulting in the absorption of light in a longer
wavelength region compared to, for example, that associated with the σ electron in
compounds that do not possess conjugation. Absorption data and spectral dependence
for a variety of molecular constituents, including phenyl rings, biphenyls,
terphenyls, tolanes, and diphenyl-diacetylenes, may be found in Khoo and Wu.5

 

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