instrument). This task is not trivial as the instrumental standard is not established
thus far, although some suggestions are available. In practice, precautions are needed
when the fluorescence and CPL is close to the absorption wavelength. In such cases,
the observed CPL signals should be properly corrected, as a part of the emitted
radiation may be re-absorbed in different degrees for left and right circularly
polarized light. Concentration of samples may also matter in several ways. Aggregation leads to the supramolecular effect for CPL measurement, which gives signals
typically at least one order of magnitude stronger in g lum than the isolated molecule.
Depending on the conditions, an artifact may be also obtained through the
photoselection and birefringent. When scattering phenomena become important,
chiral scattering may manifest itself as apparent CPL. Last but not the least, the
degradation of the sample may occur during the measurement, due to an inevitable
exposure to a relatively strong excitation light during the CPL measurement. It is
thus important to verify the stability (and recovery) of the sample at the end of the
measurement. Unfortunately, above precautions do not always appear to have been
completed in the literature examples. More serious artifact can appear for the CPL
spectra of solid-state samples. One should consult the literature to understand the
possible sources of artifacts before attempting to measure the CPL in the solid state
or in any anisotropic conditions.
1.5 Perspective
Progressively large numbers of successful examples of SOMs-based CPL-active
materials have been appeared in the literature. Nevertheless, total amount of study is
still limited. Therefore, it is natural that a solid structure-property (i.e., CPL)
relationship has not yet been established. Recent investigations are mostly focusing
on the π-π
à transition of rigid aromatic systems, as their fluorescence quantum yields
are normally moderate to high. The ideal CPL-active materials are expected to
simultaneously have high luminescence quantum yield and high (absolute) g lum
value, and if possible that also function with some desired stimuli. It is certainly a
great advantage that rational modification on the structure of SOMs, particularly of
aromatic systems, can effectively modulate the various physical parameters in the
CPL-responsible materials, while the luminescence dissymmetry factors for SOMs
still remain unsatisfactory; further progresses are certainly desired.
We have recently examined quantitative relationship between the dissymmetric
factors of CPL (g lum ) and CD (g abs ) for such π-π
à transitions of extended aromatic
systems. It was the first systematic investigation to experimentally elucidate the
relationship between the dissymmetry factors in order to strategically design the
CPL response through the rational molecular modification on the aromatic systems.
Although the number of the available data was still limited to around 100 examples,
they unexpectedly afforded a good empirical linear correlation as a global fit:
|g lum | ¼ 0.81 Â |g abs | (r
2
¼ 0.60) (Fig. 1.3). It is also to highlight that all the CPL
data used in the study have been published in 2011 or later, except for two cases,
8
T. Mori
thus far, although some suggestions are available. In practice, precautions are needed
when the fluorescence and CPL is close to the absorption wavelength. In such cases,
the observed CPL signals should be properly corrected, as a part of the emitted
radiation may be re-absorbed in different degrees for left and right circularly
polarized light. Concentration of samples may also matter in several ways. Aggregation leads to the supramolecular effect for CPL measurement, which gives signals
typically at least one order of magnitude stronger in g lum than the isolated molecule.
Depending on the conditions, an artifact may be also obtained through the
photoselection and birefringent. When scattering phenomena become important,
chiral scattering may manifest itself as apparent CPL. Last but not the least, the
degradation of the sample may occur during the measurement, due to an inevitable
exposure to a relatively strong excitation light during the CPL measurement. It is
thus important to verify the stability (and recovery) of the sample at the end of the
measurement. Unfortunately, above precautions do not always appear to have been
completed in the literature examples. More serious artifact can appear for the CPL
spectra of solid-state samples. One should consult the literature to understand the
possible sources of artifacts before attempting to measure the CPL in the solid state
or in any anisotropic conditions.
1.5 Perspective
Progressively large numbers of successful examples of SOMs-based CPL-active
materials have been appeared in the literature. Nevertheless, total amount of study is
still limited. Therefore, it is natural that a solid structure-property (i.e., CPL)
relationship has not yet been established. Recent investigations are mostly focusing
on the π-π
à transition of rigid aromatic systems, as their fluorescence quantum yields
are normally moderate to high. The ideal CPL-active materials are expected to
simultaneously have high luminescence quantum yield and high (absolute) g lum
value, and if possible that also function with some desired stimuli. It is certainly a
great advantage that rational modification on the structure of SOMs, particularly of
aromatic systems, can effectively modulate the various physical parameters in the
CPL-responsible materials, while the luminescence dissymmetry factors for SOMs
still remain unsatisfactory; further progresses are certainly desired.
We have recently examined quantitative relationship between the dissymmetric
factors of CPL (g lum ) and CD (g abs ) for such π-π
à transitions of extended aromatic
systems. It was the first systematic investigation to experimentally elucidate the
relationship between the dissymmetry factors in order to strategically design the
CPL response through the rational molecular modification on the aromatic systems.
Although the number of the available data was still limited to around 100 examples,
they unexpectedly afforded a good empirical linear correlation as a global fit:
|g lum | ¼ 0.81 Â |g abs | (r
2
¼ 0.60) (Fig. 1.3). It is also to highlight that all the CPL
data used in the study have been published in 2011 or later, except for two cases,
8
T. Mori