118
(CD) spectroscopy. The information observed in the latter is directly correlated with
the information resulting from the CPL transitions. The CPL and CD depend mainly
on the electronic transitions of the molecules. However, the CD reveals the information of the electronic ground state in thermal equilibrium, while the CPL represents
the information on the vibronic relaxation of the excited states of a system. The CPL
has attracted significant consideration from researchers for its potential utility in 3D
displays, biocoding, optical data storage, photoelectric devices, quantum information storage, etc. (Han et al. 2018). Conventionally, CPL is obtained from linearlypolarized light by using a quarter-wave plate and polarizer which can consume up
to 50% of the light emitted (Kim et al. 2014). It is thus necessary for the time to
produce CPL emission directly to achieve high efficiency and tunable luminescence. The main challenges associated with CPL are to meet: (1) high luminescence
dissymmetry factor (g lum ), (2) high quantum yield (Φ F ) and (3) solution processable
defect-free polymeric luminophores.
So far, many types of chiral luminous materials have been studied, such as conjugated polymers, lanthanide complexes, liquid crystals, small organic molecules
and supra-molecules (Han et al. 2018). The initial reports on CPL used lanthanide
complex due to magnetic dipole transitions from their internal configuration (Morita
et al. 2004; Do et al. 2008). However, they suffer from relatively low Φ F and limited
abundance of lanthanide elements of the earth crust. The small organic molecules
with their numerous possibilities of chiral molecular structure are promising materials. Although these materials generally exhibit high g lum and Φ F , but their CPL signals are sensitive to the exterior surroundings, such as acidity-basicity, aggregation,
temperature, etc. Similar to π-conjugated small organic molecules, π-conjugated
polymers are an emerging class of the luminophores bearing advantages such as
ease of processing the thin film solution, high g lum and linear amplification of CPL
signals. Various research groups have employed several strategies in order to
amplify the g lum and Φ F in polymer systems, such as doping with chiral molecules,
incorporation of π-conjugated chiral molecules in the main chains, polymeric liquid
crystals, the introduction of chiral pendant side chains, transfer of chirality from
chiral small molecules to achiral polymer chains, etc (Sect. 6.3). In the recent past,
there has been unprecedented progress in the field of polymer exhibiting CPL but,
to date, no systematic account is available in the literature to our knowledge. Thus,
a review of a variety of polymer-based systems and associated unique strategies for
obtaining high g lum and Φ F of CPL is provided in this chapter.
6.2 Brief Theory of CPL
The theoretical development of CPL spectroscopy was based on the existing theoretical bases related to CD and optical rotation dispersion (ORD). First, Emeis and
Oosterhoff (1967, 1971) validated that the orientation distribution of the emitters is
isotropic and that there is a close similarity between the theoretical basis of the CD
and CPL signals. In this line, for a given electronic transition to and from the state
P. Puneet et al.
(CD) spectroscopy. The information observed in the latter is directly correlated with
the information resulting from the CPL transitions. The CPL and CD depend mainly
on the electronic transitions of the molecules. However, the CD reveals the information of the electronic ground state in thermal equilibrium, while the CPL represents
the information on the vibronic relaxation of the excited states of a system. The CPL
has attracted significant consideration from researchers for its potential utility in 3D
displays, biocoding, optical data storage, photoelectric devices, quantum information storage, etc. (Han et al. 2018). Conventionally, CPL is obtained from linearlypolarized light by using a quarter-wave plate and polarizer which can consume up
to 50% of the light emitted (Kim et al. 2014). It is thus necessary for the time to
produce CPL emission directly to achieve high efficiency and tunable luminescence. The main challenges associated with CPL are to meet: (1) high luminescence
dissymmetry factor (g lum ), (2) high quantum yield (Φ F ) and (3) solution processable
defect-free polymeric luminophores.
So far, many types of chiral luminous materials have been studied, such as conjugated polymers, lanthanide complexes, liquid crystals, small organic molecules
and supra-molecules (Han et al. 2018). The initial reports on CPL used lanthanide
complex due to magnetic dipole transitions from their internal configuration (Morita
et al. 2004; Do et al. 2008). However, they suffer from relatively low Φ F and limited
abundance of lanthanide elements of the earth crust. The small organic molecules
with their numerous possibilities of chiral molecular structure are promising materials. Although these materials generally exhibit high g lum and Φ F , but their CPL signals are sensitive to the exterior surroundings, such as acidity-basicity, aggregation,
temperature, etc. Similar to π-conjugated small organic molecules, π-conjugated
polymers are an emerging class of the luminophores bearing advantages such as
ease of processing the thin film solution, high g lum and linear amplification of CPL
signals. Various research groups have employed several strategies in order to
amplify the g lum and Φ F in polymer systems, such as doping with chiral molecules,
incorporation of π-conjugated chiral molecules in the main chains, polymeric liquid
crystals, the introduction of chiral pendant side chains, transfer of chirality from
chiral small molecules to achiral polymer chains, etc (Sect. 6.3). In the recent past,
there has been unprecedented progress in the field of polymer exhibiting CPL but,
to date, no systematic account is available in the literature to our knowledge. Thus,
a review of a variety of polymer-based systems and associated unique strategies for
obtaining high g lum and Φ F of CPL is provided in this chapter.
6.2 Brief Theory of CPL
The theoretical development of CPL spectroscopy was based on the existing theoretical bases related to CD and optical rotation dispersion (ORD). First, Emeis and
Oosterhoff (1967, 1971) validated that the orientation distribution of the emitters is
isotropic and that there is a close similarity between the theoretical basis of the CD
and CPL signals. In this line, for a given electronic transition to and from the state
P. Puneet et al.