(including organic chemistry) and to systematically attain the structure-property
relationship. As a consequence, the development of new CPL emitters becomes an
active field of research, particularly for the last 5 years, by rationally designing
materials with improved dissymmetry factors. Amongst the CPL active compounds,
lanthanide complexes, especially those based on europium or terbium, are normally
reported for greater CPL responses. Recently, self-assembled materials, liquid crystals, and polymers provide more efficient CPL responses, thanks to a supramolecular
chirality, typically with at least one order of magnitude improved luminescence
dissymmetry factors than simple SOMs. Nevertheless, the study on the CPL
responses of SOMs becomes more and more attractive, probably because SOMs
have some intrinsic advantage over the other materials. Hypothetically, their fluorescence efficiency, absorption and emission wavelengths and the bandwidths, and
degree of dissymmetry can be rationally controlled by logical and relatively facile
structural modifications. Additional advantages may be biological compatibility for
in vivo sensing and ease of fabrication and manufacturing for electronic and
photonic devices. You will soon find a variety of such efforts on SOMs and related
systems in the following chapters, by focusing on the nature of respective structural
motifs. In recent CPL research on SOMs, special emphases are placed on the π-π
Ã
transition of extended aromatic systems.
In addition to the materials chemistry standpoint, the CPL spectroscopy is
valuable to elucidate the configurational and/or conformational information of
optical active molecules in their emissive excited states, although the molecules
under investigation must be reasonably luminescent. On the contrary, the information obtained through the CD spectroscopy is based on the thermally equilibrated
electronic ground state. In principle, CD and CPL are mutually complementary
probes for the structural features of chiral molecules in the different electronic states.
When the structural differences and the vibrational contributions can be negligible,
the structural information provided by the two chiroptical methods becomes similar.
In practice, combination of CD and CPL spectroscopies provides a wealth of
information concerning the structural differences in their ground and excited states.
The CPL spectroscopy also provides information concerning the excited-state
dynamics and energetics along the photophysical consequences from the initial
absorption to the emission event. A necessity of fluorophore seems disadvantage
at a glance, but can be beneficial for its selectivity and specificity. The CPL active
emissive state may be accessible either by direct excitation, by indirect energy
transfer process, or by relaxation from the higher excited state, which is in sharp
contrast to the CD spectral technique, where the band overlap or small absorption
may disturb the accurate analysis.
As stated by the Frank-Condon principle, electronic transition (both absorption
and emission) always occurs instantly without accompanying geometrical change,
represented as the vertical transition. According to the Kasha’s rule, luminescence
occurs from the lowest electronically excited state, typically in the S 1 state (or T 1
state in the triplet manifold). Therefore, information obtained by the CPL is responsible to S 1 to S 0 electronic transition, parallels to that of the lowest-energy CD, where
1 Frontiers of Circularly Polarized Luminescence Chemistry of Isolated Small. . .
3
relationship. As a consequence, the development of new CPL emitters becomes an
active field of research, particularly for the last 5 years, by rationally designing
materials with improved dissymmetry factors. Amongst the CPL active compounds,
lanthanide complexes, especially those based on europium or terbium, are normally
reported for greater CPL responses. Recently, self-assembled materials, liquid crystals, and polymers provide more efficient CPL responses, thanks to a supramolecular
chirality, typically with at least one order of magnitude improved luminescence
dissymmetry factors than simple SOMs. Nevertheless, the study on the CPL
responses of SOMs becomes more and more attractive, probably because SOMs
have some intrinsic advantage over the other materials. Hypothetically, their fluorescence efficiency, absorption and emission wavelengths and the bandwidths, and
degree of dissymmetry can be rationally controlled by logical and relatively facile
structural modifications. Additional advantages may be biological compatibility for
in vivo sensing and ease of fabrication and manufacturing for electronic and
photonic devices. You will soon find a variety of such efforts on SOMs and related
systems in the following chapters, by focusing on the nature of respective structural
motifs. In recent CPL research on SOMs, special emphases are placed on the π-π
Ã
transition of extended aromatic systems.
In addition to the materials chemistry standpoint, the CPL spectroscopy is
valuable to elucidate the configurational and/or conformational information of
optical active molecules in their emissive excited states, although the molecules
under investigation must be reasonably luminescent. On the contrary, the information obtained through the CD spectroscopy is based on the thermally equilibrated
electronic ground state. In principle, CD and CPL are mutually complementary
probes for the structural features of chiral molecules in the different electronic states.
When the structural differences and the vibrational contributions can be negligible,
the structural information provided by the two chiroptical methods becomes similar.
In practice, combination of CD and CPL spectroscopies provides a wealth of
information concerning the structural differences in their ground and excited states.
The CPL spectroscopy also provides information concerning the excited-state
dynamics and energetics along the photophysical consequences from the initial
absorption to the emission event. A necessity of fluorophore seems disadvantage
at a glance, but can be beneficial for its selectivity and specificity. The CPL active
emissive state may be accessible either by direct excitation, by indirect energy
transfer process, or by relaxation from the higher excited state, which is in sharp
contrast to the CD spectral technique, where the band overlap or small absorption
may disturb the accurate analysis.
As stated by the Frank-Condon principle, electronic transition (both absorption
and emission) always occurs instantly without accompanying geometrical change,
represented as the vertical transition. According to the Kasha’s rule, luminescence
occurs from the lowest electronically excited state, typically in the S 1 state (or T 1
state in the triplet manifold). Therefore, information obtained by the CPL is responsible to S 1 to S 0 electronic transition, parallels to that of the lowest-energy CD, where
1 Frontiers of Circularly Polarized Luminescence Chemistry of Isolated Small. . .
3