indicates the emitting electronically excited state; however, transient CD may show
evidence of any type of chiral transient species, in principle. Hence, the transient CD
approach would be considered as a versatile way to access the chirality of the excited
state. Moreover, transient CPL studies are scarce [7, 8], whereas there have been
several reports of transient CD, initiated by the pioneering work by Kliger and
coworkers [9]. Our group has also recently developed a transient CD apparatus
and reported the CD spectrum of small organic molecules in the photo-excited triplet
state [10, 11].
In this section, we describe the measurement technique for transient circular
dichroism measurement of electronically excited states.
15.2 Road to Transient Circular Dichroism (TRCD)
Measurement
CD may be defined as the difference in the molar extinction coefficient for left and
right circularly polarized light (Δε ¼ ε L À ε R ), and CD measurement is essentially a
method to detect this difference. To achieve sensitivity in CD detection, we use a
phase-sensitive technique, which is actualized by the phase-sensitive detection of the
transmitted intensity of left and right circularly polarized light, transformed from
linearly polarized light with a photo-elastic modulator (PEM), operated at thousands
of cycles per second. Because of the use of phase-sensitive detection, the time
resolution of a usual CD detection depends on the operating frequency of the
PEM. The practical time resolution of the manufactured products is considered to
be on the order of milliseconds. Therefore, several efforts have been made for
implementing CD measurement technology without using PEM.
Time-resolved circular dichroism (TRCD) spectroscopy with broadband and
reasonable time response, applicable to a great number of photochemical events in
the ns to μs time regimes, was initiated by the Kliger group [9]. Their approach relied
on CD detection with elliptically polarized light rather than circular-polarized light.
Their approach has been continuously improved and applied to photo-biological
events [12–15]. Along with these ideas, TRCD measurement is a promising method
for detecting the structural dynamics of chiral molecules and molecular ensembles,
ensuring external impulses such as light excitation.
Notwithstanding recent TRCD developments with sophisticated techniques in
ultrafast events (1–1000 ps), the applications were limited to the biological field, and
the applications of TRCD measurement to the photo-excited state detection of small
inorganic and organic molecules are relatively few [16, 17].
Recently, our group also has developed the steady-state CD apparatus, shown in
Fig. 15.1, which employs elliptically polarized light as a probe light to detect
CD. Our method is conceptually the same as the Kliger method [9]; however, we
found the conditions for easier control of light ellipticity by the precise azimuth
control of the retarder, yielding the high reproducibility of the CD signals in the
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Y. Araki
evidence of any type of chiral transient species, in principle. Hence, the transient CD
approach would be considered as a versatile way to access the chirality of the excited
state. Moreover, transient CPL studies are scarce [7, 8], whereas there have been
several reports of transient CD, initiated by the pioneering work by Kliger and
coworkers [9]. Our group has also recently developed a transient CD apparatus
and reported the CD spectrum of small organic molecules in the photo-excited triplet
state [10, 11].
In this section, we describe the measurement technique for transient circular
dichroism measurement of electronically excited states.
15.2 Road to Transient Circular Dichroism (TRCD)
Measurement
CD may be defined as the difference in the molar extinction coefficient for left and
right circularly polarized light (Δε ¼ ε L À ε R ), and CD measurement is essentially a
method to detect this difference. To achieve sensitivity in CD detection, we use a
phase-sensitive technique, which is actualized by the phase-sensitive detection of the
transmitted intensity of left and right circularly polarized light, transformed from
linearly polarized light with a photo-elastic modulator (PEM), operated at thousands
of cycles per second. Because of the use of phase-sensitive detection, the time
resolution of a usual CD detection depends on the operating frequency of the
PEM. The practical time resolution of the manufactured products is considered to
be on the order of milliseconds. Therefore, several efforts have been made for
implementing CD measurement technology without using PEM.
Time-resolved circular dichroism (TRCD) spectroscopy with broadband and
reasonable time response, applicable to a great number of photochemical events in
the ns to μs time regimes, was initiated by the Kliger group [9]. Their approach relied
on CD detection with elliptically polarized light rather than circular-polarized light.
Their approach has been continuously improved and applied to photo-biological
events [12–15]. Along with these ideas, TRCD measurement is a promising method
for detecting the structural dynamics of chiral molecules and molecular ensembles,
ensuring external impulses such as light excitation.
Notwithstanding recent TRCD developments with sophisticated techniques in
ultrafast events (1–1000 ps), the applications were limited to the biological field, and
the applications of TRCD measurement to the photo-excited state detection of small
inorganic and organic molecules are relatively few [16, 17].
Recently, our group also has developed the steady-state CD apparatus, shown in
Fig. 15.1, which employs elliptically polarized light as a probe light to detect
CD. Our method is conceptually the same as the Kliger method [9]; however, we
found the conditions for easier control of light ellipticity by the precise azimuth
control of the retarder, yielding the high reproducibility of the CD signals in the
328
Y. Araki