g lum ¼ 4 Â
R
D
¼ 4 Â
m
j j
μ
j j
cos θ
Thus, the luminescence dissymmetry factor g lum is inversely proportional to the
amplitude of μ and directly proportional to that of m, as well as cosine of their
relative angle (θ).
The rotational and dipole strengths, and thus the dissymmetric factor, can be
directly estimated by the quantum chemical calculations. The time-dependent density functional theory (TD-DFT) has been successfully employed for the evaluation
of the CPL spectra of distorted chiral ketones and some π-systems. Superior theoretical methods such as coupled-cluster type theory have been also applied recently.
The accuracy of predicted values, however, depends on the ansatz used in the
calculations as well as the geometrical features (size, flexibility, etc.) of chiral
systems under study. Compared with the calculation of the CD spectra, theoretical
investigation of CPL counterparts are still in the preliminary stage, which is however
most likely dissolved very soon.
1.4 Measurement
To the best of our knowledge, very first CPL was reported by Samoilov in 1948 for
chiral crystals of sodium uranyl acetate at the liquid helium temperature. In 1967, the
first example of CPL from small organic molecule was reported by Emeis and
Oosterhof, employing cyclic chiral ketone, trans-β-hydrindanone (trans-bicyclo
[4.3.0]nonan-8-one). Then for a while, the target organic molecules for SOMsbased CPL study have been restricted to optically active cyclic ketones, almost
nearly until this century. This is because these intrinsically chiral ketones exhibit
relatively large g lum values due to the magnetically allowed electronically forbidden
n-π
à transition of distorted carbonyl group, which appears as an isolated band at
~300 nm. In all these measurements, custom-built instrument has been employed,
which required in-depth technical attentions in order to avoid any possible artifacts.
Recently, measurements became considerably easier since a recent appearance of
commercial CPL spectrofluorimeter such as JASCO CPL-300. In this decade, a rapid
growth was realized in the CPL chemistry as the CPL spectra of most of chiral
π-systems, typically bearing g lum in an order of 10
À3 or less, can be reliably obtained.
Nevertheless, additional difficulties exist for obtaining reliable and reproducible
CPL due to several experimental limitations, which are briefly described below.
In addition to the common precautions taken care of in the measurement of
UV-vis, CD, fluorescence, and phosphorescence spectra such as medium/solvent
and sample cell, amount of sample, temperature dependence, and light intensity (slit
width), special attention should be required for the reliable CPL measurement. At
first, light beam, polarizer/depolarizer, photoelastic modulator, and monochromator
should be properly aligned and carefully calibrated (even for commercial
1 Frontiers of Circularly Polarized Luminescence Chemistry of Isolated Small. . .
7
R
D
¼ 4 Â
m
j j
μ
j j
cos θ
Thus, the luminescence dissymmetry factor g lum is inversely proportional to the
amplitude of μ and directly proportional to that of m, as well as cosine of their
relative angle (θ).
The rotational and dipole strengths, and thus the dissymmetric factor, can be
directly estimated by the quantum chemical calculations. The time-dependent density functional theory (TD-DFT) has been successfully employed for the evaluation
of the CPL spectra of distorted chiral ketones and some π-systems. Superior theoretical methods such as coupled-cluster type theory have been also applied recently.
The accuracy of predicted values, however, depends on the ansatz used in the
calculations as well as the geometrical features (size, flexibility, etc.) of chiral
systems under study. Compared with the calculation of the CD spectra, theoretical
investigation of CPL counterparts are still in the preliminary stage, which is however
most likely dissolved very soon.
1.4 Measurement
To the best of our knowledge, very first CPL was reported by Samoilov in 1948 for
chiral crystals of sodium uranyl acetate at the liquid helium temperature. In 1967, the
first example of CPL from small organic molecule was reported by Emeis and
Oosterhof, employing cyclic chiral ketone, trans-β-hydrindanone (trans-bicyclo
[4.3.0]nonan-8-one). Then for a while, the target organic molecules for SOMsbased CPL study have been restricted to optically active cyclic ketones, almost
nearly until this century. This is because these intrinsically chiral ketones exhibit
relatively large g lum values due to the magnetically allowed electronically forbidden
n-π
à transition of distorted carbonyl group, which appears as an isolated band at
~300 nm. In all these measurements, custom-built instrument has been employed,
which required in-depth technical attentions in order to avoid any possible artifacts.
Recently, measurements became considerably easier since a recent appearance of
commercial CPL spectrofluorimeter such as JASCO CPL-300. In this decade, a rapid
growth was realized in the CPL chemistry as the CPL spectra of most of chiral
π-systems, typically bearing g lum in an order of 10
À3 or less, can be reliably obtained.
Nevertheless, additional difficulties exist for obtaining reliable and reproducible
CPL due to several experimental limitations, which are briefly described below.
In addition to the common precautions taken care of in the measurement of
UV-vis, CD, fluorescence, and phosphorescence spectra such as medium/solvent
and sample cell, amount of sample, temperature dependence, and light intensity (slit
width), special attention should be required for the reliable CPL measurement. At
first, light beam, polarizer/depolarizer, photoelastic modulator, and monochromator
should be properly aligned and carefully calibrated (even for commercial
1 Frontiers of Circularly Polarized Luminescence Chemistry of Isolated Small. . .
7