460
D. P. Singh et al.
tion spectrum is found to be red-shifted. Similarly, by increasing the π and π
∗ orbitals
(such as in 1-butene, λ max = 176 nm, and 1,3-butadiene, λ max = 292 nm), the absorption spectrum is found to be red-shifted. In the liquid crystalline materials (LCM),
three types of electronic transitions, viz., σ −σ
∗ , n−π
∗ and π −π
∗ , are frequently
encountered. If any LCM contains a cyclohexane ring, only σ −σ
∗ transitions take
place in the vacuum ultraviolet region (λ <180 nm). Therefore, the absorption of
a totally saturated LCM becomes negligible in the visible region. The cyclohexane
rings and unsaturated phenyl rings mainly contribute in the absorption and refractive
index of the LCM. The presence of a heteroatom (such as oxygen or nitrogen) in
chemical design of the LCM favors n−π
∗ transitions. The oscillator strength of the
n−π
∗ transition is not very strong. Thus, its contribution to refractive index is not
significant. The π −π
∗ electronic transitions take place in the compounds which contain phenyl rings; that’s why they commonly occur in the LCM having an absorption
band in the UV or UV-visible region. From the group theory, a benzene molecule
belongs to the D 6h point group, but a phenyl ring belongs to the C 2v group. Evidently,
the degree of symmetry of a phenyl ring in a liquid crystal compound is lower than
that of an unbounded benzene molecule due to the constraints from the connecting
segments. If a liquid crystal molecule contains a phenyl ring, its absorption spectrum would be similar to that of a pure benzene molecule. The degree of similarity
depends on how many conjugated bonds are attached to the phenyl ring. In the threeband model, only the two major π −π
∗ transitions (i.e.,
1 A 1g →
1 E 1u , λ 1 band, and
1 A 1g →
1 B 1u , λ 2 band) and σ −σ
∗ (λ 0 band) transitions are included. From the selection rule,
1 A 1g →
1 E 1u is a spin and symmetry allowed transition. In a pure benzene
molecule, this band has a two-fold degeneracy and large extinction coefficient; but
in a phenyl ring, this degeneracy gradually splits as the conjugation length increases.
The
1 A 1g →
1 B 1u is a spin allowed, symmetry-forbidden, but vibronically allowed
transition. Its extinction coefficient is modest. This transition is a spin-allowed but
symmetry-forbidden transition having very weak extinction coefficient. Most of the
LCMs such as 5CB, 8CB, 8OCB, etc. [28, 29], follow the above-mentioned electronic transitions. The absorption and emission details of various LCMs and their
composites with different dopants are summarized in Table 2.
5.2 FLS for Mesogenic-Nano Composites
In the LCM/nanoparticle composites, fluorescence emission could be excitation
wavelength dependent. Goel et al. [30] have demonstrated the red shift in the violet
emission band of the nano-CeO 2 dispersed ferroelectric liquid crystals i.e., from 368
to 396 nm with increasing excitation wavelength (Fig. 11a), and that is attributed to
the recombination of electrons trapped in the defect band and the deeply trapped
holes in oxygen vacancies. When we excite a fluorescent material, some energy
is dissipated leaving the fluorophore in the lowest vibrational level. This relaxation
D. P. Singh et al.
tion spectrum is found to be red-shifted. Similarly, by increasing the π and π
∗ orbitals
(such as in 1-butene, λ max = 176 nm, and 1,3-butadiene, λ max = 292 nm), the absorption spectrum is found to be red-shifted. In the liquid crystalline materials (LCM),
three types of electronic transitions, viz., σ −σ
∗ , n−π
∗ and π −π
∗ , are frequently
encountered. If any LCM contains a cyclohexane ring, only σ −σ
∗ transitions take
place in the vacuum ultraviolet region (λ <180 nm). Therefore, the absorption of
a totally saturated LCM becomes negligible in the visible region. The cyclohexane
rings and unsaturated phenyl rings mainly contribute in the absorption and refractive
index of the LCM. The presence of a heteroatom (such as oxygen or nitrogen) in
chemical design of the LCM favors n−π
∗ transitions. The oscillator strength of the
n−π
∗ transition is not very strong. Thus, its contribution to refractive index is not
significant. The π −π
∗ electronic transitions take place in the compounds which contain phenyl rings; that’s why they commonly occur in the LCM having an absorption
band in the UV or UV-visible region. From the group theory, a benzene molecule
belongs to the D 6h point group, but a phenyl ring belongs to the C 2v group. Evidently,
the degree of symmetry of a phenyl ring in a liquid crystal compound is lower than
that of an unbounded benzene molecule due to the constraints from the connecting
segments. If a liquid crystal molecule contains a phenyl ring, its absorption spectrum would be similar to that of a pure benzene molecule. The degree of similarity
depends on how many conjugated bonds are attached to the phenyl ring. In the threeband model, only the two major π −π
∗ transitions (i.e.,
1 A 1g →
1 E 1u , λ 1 band, and
1 A 1g →
1 B 1u , λ 2 band) and σ −σ
∗ (λ 0 band) transitions are included. From the selection rule,
1 A 1g →
1 E 1u is a spin and symmetry allowed transition. In a pure benzene
molecule, this band has a two-fold degeneracy and large extinction coefficient; but
in a phenyl ring, this degeneracy gradually splits as the conjugation length increases.
The
1 A 1g →
1 B 1u is a spin allowed, symmetry-forbidden, but vibronically allowed
transition. Its extinction coefficient is modest. This transition is a spin-allowed but
symmetry-forbidden transition having very weak extinction coefficient. Most of the
LCMs such as 5CB, 8CB, 8OCB, etc. [28, 29], follow the above-mentioned electronic transitions. The absorption and emission details of various LCMs and their
composites with different dopants are summarized in Table 2.
5.2 FLS for Mesogenic-Nano Composites
In the LCM/nanoparticle composites, fluorescence emission could be excitation
wavelength dependent. Goel et al. [30] have demonstrated the red shift in the violet
emission band of the nano-CeO 2 dispersed ferroelectric liquid crystals i.e., from 368
to 396 nm with increasing excitation wavelength (Fig. 11a), and that is attributed to
the recombination of electrons trapped in the defect band and the deeply trapped
holes in oxygen vacancies. When we excite a fluorescent material, some energy
is dissipated leaving the fluorophore in the lowest vibrational level. This relaxation