Preface
Circularly polarized luminescence (CPL) is one of the optical properties of chiral
materials that measures the intensity difference between the left and right circularly
polarized spontaneous emission from an intrinsically chiral fluorophore or
fluorophore in a chiral environment. Therefore, CPL can be considered as an
emission analog of circular dichroisms (CD). While tremendous efforts have been
devoted to characterize the observed CD based on the structure of the molecule,
successfully generating the so-called structure–chiroptical property relationships,
such study on the CPL has been hitherto rather limited, due primarily to the
instrumental limitations. As the CPL measurement becomes more accessible, the
related study has been more concerned, particularly in this decade. Because the CPL
responses are fundamentally based on the (emissive) excited state of the fluorophore,
this property can be also used to effectively probe the configurational and/or
conformational features of a chiral molecule in its electronic excited states. The
degree of circular polarization is usually quantified by the dissymmetry factor, i.e.,
the relative intensity difference of left and right circularly polarized absorption
(in CD) or emission (in CPL), respectively. Theoretically, the degrees of these are
characterized by the intensity and orientation of electric and magnetic transition
dipole moments of the relevant transition. These features are not easily deduced from
the molecular structure alone, but the quantum chemical calculations, especially with
the ansatz equivalent or superior to the cost-efficient time-dependent density functional theory, are ordinarily respectable for accurately reproducing these values and
better understanding of the relationship, although a reliable yet accessible general
calculation is still in the developing stage. The differences between CPL and CD
(of the emissive state) are often small, as the structural difference between the
thermally equilibrated ground state and the excited state, particularly of fluorescent
π-systems, is relatively minor, and can also be significant in some cases, where
nature of emission state, their structural relaxation in the excited state, effect of
forbidden state, vibrational coupling, as well as excitonic coupling may play a
substantial role. A complete understanding of all these and other factors are definitely required in order to fully understand the CPL characteristics of the chiral
v
Circularly polarized luminescence (CPL) is one of the optical properties of chiral
materials that measures the intensity difference between the left and right circularly
polarized spontaneous emission from an intrinsically chiral fluorophore or
fluorophore in a chiral environment. Therefore, CPL can be considered as an
emission analog of circular dichroisms (CD). While tremendous efforts have been
devoted to characterize the observed CD based on the structure of the molecule,
successfully generating the so-called structure–chiroptical property relationships,
such study on the CPL has been hitherto rather limited, due primarily to the
instrumental limitations. As the CPL measurement becomes more accessible, the
related study has been more concerned, particularly in this decade. Because the CPL
responses are fundamentally based on the (emissive) excited state of the fluorophore,
this property can be also used to effectively probe the configurational and/or
conformational features of a chiral molecule in its electronic excited states. The
degree of circular polarization is usually quantified by the dissymmetry factor, i.e.,
the relative intensity difference of left and right circularly polarized absorption
(in CD) or emission (in CPL), respectively. Theoretically, the degrees of these are
characterized by the intensity and orientation of electric and magnetic transition
dipole moments of the relevant transition. These features are not easily deduced from
the molecular structure alone, but the quantum chemical calculations, especially with
the ansatz equivalent or superior to the cost-efficient time-dependent density functional theory, are ordinarily respectable for accurately reproducing these values and
better understanding of the relationship, although a reliable yet accessible general
calculation is still in the developing stage. The differences between CPL and CD
(of the emissive state) are often small, as the structural difference between the
thermally equilibrated ground state and the excited state, particularly of fluorescent
π-systems, is relatively minor, and can also be significant in some cases, where
nature of emission state, their structural relaxation in the excited state, effect of
forbidden state, vibrational coupling, as well as excitonic coupling may play a
substantial role. A complete understanding of all these and other factors are definitely required in order to fully understand the CPL characteristics of the chiral
v