Читать книгу Liquid Crystals - Iam-Choon Khoo - Страница 13

Since liquid crystal constituent molecules are quite large, their energy level structures are rather complex. In essence, the basic quantum mechanical theory is similar to the one described in Chapter 10 for a multilevel molecule. Generally, the energy levels are referred to as orbitals: π, n, and σ orbitals for the ground and low‐lying levels and π ^*, n ^*, and σ ^* for their excited counterparts. 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) [2]. These results for a benzene molecule can also be used for interpreting 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 absorption of light of shorter wavelength (≤180 nm) in comparison to the π → π ^* transition mentioned previously.

These optical properties are also related to 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 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 [2].

Figure 1.6. The π → π^* electronic transitions in a benzene molecule.