462
D. P. Singh et al.
takes place in 10
−12 s which is presumably a result of a strong overlap among numerous states of nearly equal energy. Due to this rapid relaxation, emission spectra are
invariant of the excitation wavelength [31]. On the other hand, a slow relaxation
leads to the excitation wavelength dependent fluorescence. When we add any nanomaterials like nanoparticles, nanorods, quantum dots, etc. in the host LCM matrix,
the fluorescence of the host matrix either increases or quenches depending on the
circumstances. Kumar et al. [32] have investigated the composites of a deformed
helix ferroelectric liquid crystal (DHFLC) material and gold nanoparticles (Au NPs)
using FLS and found that the presence of metallic nanoparticles enhances the PL
properties of the DHFLC (Fig. 11b). Most of the metallic nanoparticles like Au,
Ag, Cu, etc., enhance the PL properties of the host LCM via localized surface plasmon resonance [33]. The plasmons of metal nanoparticles couple with the host LC
molecules, resulting in an increase in excitation and emission rate of the LCM in the
localized electromagnetic field. The presence of graphene [34], graphene oxide (GO)
[35, 36] or graphene QDs (GQDs) [37] can manipulate the photophysical properties
of the host LCM. In these cases, usually, the fluorescence property of the host LCM
is found to be enhanced (Fig. 11c, d). Normally, graphitic nanomaterials like GO,
GQDs exhibit an emission at 362 nm attributed to the n−π
∗ transition of the isolated sp
2 domains within the carbon-oxygen sp
3 matrix [38]. When these graphitic
nanostructured materials are dispersed in LCM, the mechanisms such as the change
in optical density [35], electron-phonon coupling [35, 39] and carrier multiplication [37] via breaking of π -bonds take place which offers fluorescence modulations
in the LCM composites. It is also observed that the variation of fluorescence as a
function of graphitic dopant varies nonlinearly which is attributed to the interfacial
dynamics of the LCM molecules and graphitic flakes. Besides this factor, the relative orientation of the LCM molecules and graphitic dopants could be another factor
responsible for this observation. In addition to this, such composites mostly follow the inverse temperature quenching mechanism, i.e., fluorescence increases with
increasing temperature. Contrary to metallic nanoparticles (NPs), semiconducting
NPs, or QDs like Cd 1−x Zn x S/ZnS [40], ZnS:Mn
2+ [41], CdSe/ZnS [42] usually
quench the fluorescence properties of the host LCM (Fig. 11e). It normally happens
because the absorption band of the NPs coincides with the emission band of the LCM.
Besides, the recombination process and traps are other factors for such quenching of
fluorescence. However, the insertion of CdS nanorods could lead to enhanced photoluminescence properties of the antiferroelectric LCM [43]. It is also noticed that the
insulating TiO 2 NPs cause enhancement or quenching of photoluminescence of the
host LCM depending upon the size and concentration of NPs [7, 44]. In the electrochemiluminescence of tetraphenylporphyrin, Zhang et al. [45] demonstrated that the
photophysical spectroscopic characterization along with electrochemical analyses
identified the fact that reducing the symmetry (i.e., symmetry breaking) from D 4h in
ZnTPP to D 2h for H 2 TPP removes the accidental degeneracy of the highest occupied
molecular orbital resulting in significantly augmented photoluminescence (Fig. 11f).
D. P. Singh et al.
takes place in 10
−12 s which is presumably a result of a strong overlap among numerous states of nearly equal energy. Due to this rapid relaxation, emission spectra are
invariant of the excitation wavelength [31]. On the other hand, a slow relaxation
leads to the excitation wavelength dependent fluorescence. When we add any nanomaterials like nanoparticles, nanorods, quantum dots, etc. in the host LCM matrix,
the fluorescence of the host matrix either increases or quenches depending on the
circumstances. Kumar et al. [32] have investigated the composites of a deformed
helix ferroelectric liquid crystal (DHFLC) material and gold nanoparticles (Au NPs)
using FLS and found that the presence of metallic nanoparticles enhances the PL
properties of the DHFLC (Fig. 11b). Most of the metallic nanoparticles like Au,
Ag, Cu, etc., enhance the PL properties of the host LCM via localized surface plasmon resonance [33]. The plasmons of metal nanoparticles couple with the host LC
molecules, resulting in an increase in excitation and emission rate of the LCM in the
localized electromagnetic field. The presence of graphene [34], graphene oxide (GO)
[35, 36] or graphene QDs (GQDs) [37] can manipulate the photophysical properties
of the host LCM. In these cases, usually, the fluorescence property of the host LCM
is found to be enhanced (Fig. 11c, d). Normally, graphitic nanomaterials like GO,
GQDs exhibit an emission at 362 nm attributed to the n−π
∗ transition of the isolated sp
2 domains within the carbon-oxygen sp
3 matrix [38]. When these graphitic
nanostructured materials are dispersed in LCM, the mechanisms such as the change
in optical density [35], electron-phonon coupling [35, 39] and carrier multiplication [37] via breaking of π -bonds take place which offers fluorescence modulations
in the LCM composites. It is also observed that the variation of fluorescence as a
function of graphitic dopant varies nonlinearly which is attributed to the interfacial
dynamics of the LCM molecules and graphitic flakes. Besides this factor, the relative orientation of the LCM molecules and graphitic dopants could be another factor
responsible for this observation. In addition to this, such composites mostly follow the inverse temperature quenching mechanism, i.e., fluorescence increases with
increasing temperature. Contrary to metallic nanoparticles (NPs), semiconducting
NPs, or QDs like Cd 1−x Zn x S/ZnS [40], ZnS:Mn
2+ [41], CdSe/ZnS [42] usually
quench the fluorescence properties of the host LCM (Fig. 11e). It normally happens
because the absorption band of the NPs coincides with the emission band of the LCM.
Besides, the recombination process and traps are other factors for such quenching of
fluorescence. However, the insertion of CdS nanorods could lead to enhanced photoluminescence properties of the antiferroelectric LCM [43]. It is also noticed that the
insulating TiO 2 NPs cause enhancement or quenching of photoluminescence of the
host LCM depending upon the size and concentration of NPs [7, 44]. In the electrochemiluminescence of tetraphenylporphyrin, Zhang et al. [45] demonstrated that the
photophysical spectroscopic characterization along with electrochemical analyses
identified the fact that reducing the symmetry (i.e., symmetry breaking) from D 4h in
ZnTPP to D 2h for H 2 TPP removes the accidental degeneracy of the highest occupied
molecular orbital resulting in significantly augmented photoluminescence (Fig. 11f).