S 0 to S 1 transition is involved (Fig. 1.2). Emission in condensed phase, however,
generally occurs from the vibrationally relaxed state, which is different from the
Frank-Condon state. In other words, depending on the nature of the excited-state
potential surface of molecules under study, the degree of relaxation (or structural
change) is different to a certain degree. Nevertheless, the absorption and emission
spectra are usually mirror-imaged each other with a small energy gap called the
Stokes shift, particularly in the rigid π-systems (for which most of the chapters
focusing on in this book). In such systems, the potential energy surfaces and the
vibrational frequency and spacing are comparable between the S 1 and S 0 states. As a
result, the same sign and comparable magnitudes of dissymmetry factors are anticipated in the CPL and CD spectra. Still, the measurement of CPL spectrum, in
comparison with CD spectrum, characterizes the valuable structural differences
between the ground and electronically excited states of the chiral molecules. Readers
are directed to further discussion on this issue in “Perspective” section.
1.3 Definition
When chiral molecules or molecules in chiral environment are excited with an
unpolarized light in a typical CPL spectroscopy, the observed intensity of light
emission is oscillated between (I + ΔI) and (I À ΔI) by a polarizing modulator.
Thus, total intensity
I ¼ I L þ I R
and the difference
Fig. 1.2 Potential energy surfaces in the ground (S 0 ) and excited (S 1 ) states with negligible (left)
and typical (right) geometry change in a rigid aromatic system, exemplified by the transverse S 1
surface displacement. Absorption (CD) and emission (CPL) processes are indicated by dashed and
solid arrows, respectively
4
T. Mori
generally occurs from the vibrationally relaxed state, which is different from the
Frank-Condon state. In other words, depending on the nature of the excited-state
potential surface of molecules under study, the degree of relaxation (or structural
change) is different to a certain degree. Nevertheless, the absorption and emission
spectra are usually mirror-imaged each other with a small energy gap called the
Stokes shift, particularly in the rigid π-systems (for which most of the chapters
focusing on in this book). In such systems, the potential energy surfaces and the
vibrational frequency and spacing are comparable between the S 1 and S 0 states. As a
result, the same sign and comparable magnitudes of dissymmetry factors are anticipated in the CPL and CD spectra. Still, the measurement of CPL spectrum, in
comparison with CD spectrum, characterizes the valuable structural differences
between the ground and electronically excited states of the chiral molecules. Readers
are directed to further discussion on this issue in “Perspective” section.
1.3 Definition
When chiral molecules or molecules in chiral environment are excited with an
unpolarized light in a typical CPL spectroscopy, the observed intensity of light
emission is oscillated between (I + ΔI) and (I À ΔI) by a polarizing modulator.
Thus, total intensity
I ¼ I L þ I R
and the difference
Fig. 1.2 Potential energy surfaces in the ground (S 0 ) and excited (S 1 ) states with negligible (left)
and typical (right) geometry change in a rigid aromatic system, exemplified by the transverse S 1
surface displacement. Absorption (CD) and emission (CPL) processes are indicated by dashed and
solid arrows, respectively
4
T. Mori