molecular aggregates, of polymers and of solvent dependence of CPL spectra. In this
introductory part, we mention also that Kawai group is carrying on, with great
determination, the issue of setting up the design of a CPL-based instrument for
spatially resolved imaging of emitting bodies [14]. Finally, we mention our own
review [28], where we discuss experimental and theoretical/computational aspects of
the CPL technique and data. At this stage, our feeling is that, notwithstanding the
50-year history we have roughly outlined above, the technique is still rather “young”
and poses questions more than providing answers. In other words, we find that
puzzling behaviors are encountered by analyzing CPL data, even in the sets of
molecules just discussed. For this reason, in the discussion section we will present a
few examples (from Sects. 10.2.1 to 10.2.5) of some issues from CPL spectroscopy,
namely in Sect. 10.2.1, whether CPL simply allows one to measure the geometrical
distortion in the structure of the excited state or to check also finer electronic features
thereof. In Sect. 10.2.2, we will discuss: the use of fluorescence probes, even achiral
ones like thioflavin, to monitor dissymmetric fibril-bundling and self-aggregation of
biomolecules; the switching on/off fluorescence and CPL into ortho-oligo-phenyleneethynylene (o-OPE) molecules interacting with metals; and the dependence of CPL on
pH. In these cases, we will discuss the efficacy of CPL with respect to the concomitant
CD phenomenon; particularly in the case of o-OPEs, we discuss if a ratiometric probe
can be built in the ad hoc designed molecules bearing two distinct CPL transitions. In
Sect. 10.2.3 we will then consider CPL data for molecules useful for material science,
encompassing inherently dissymmetric thiophene oligomers, substituted helicenes,
and chiral peropyrenes, the latter ones mimicking the behavior of short twisted
graphenes. In Sect. 10.2.4, as anticipated above and as amply treated by Crassous
et al. [29], we find it interesting to deal with the association of metals to organic
ligands, or chiral organometallic compounds, related to well-known and amply studied
transition metal (Pt and Ir) complexes. In Sect. 10.2.5, finally, we will touch on how to
handle CPL data of mesomorphic chiral materials.
10.2 Discussion
10.2.1 CPL and CD: A Sometimes Difficult Relationship
In the large majority of cases, the observed CPL spectra consist in a monosignate
band. This stems from Kasha’s rule, which states that fluorescence originates from
the first excited electronic state in its minimum geometry. Consequently, fluorescence and CPL depend on the same electronic transition moments as absorption and
CD, with due account of the different starting geometry. This should allow one to
conclude that the observed sign for the CPL band is the same as for the CD band
observed at the highest wavelength. As recalled above, Mori et al. [25] moved a step
further, by adding information on the absolute values of g lum , namely
|g lum |/|g abs | ¼ 0.14 for chiral ketones, |g lum |/|g abs | ¼ 0.94 for chiral cyclophanes,
|g lum |/|g abs | ¼ 0.93 for axially chiral biaryls, |g lum |/|g abs | ¼ 0.83 for helicenes/
helicenoids, and |g lum |/|g abs | ¼ 1.02 for BODIPY-type compounds. These values
10 Structural and Electronic Information Drawn. . .
221
introductory part, we mention also that Kawai group is carrying on, with great
determination, the issue of setting up the design of a CPL-based instrument for
spatially resolved imaging of emitting bodies [14]. Finally, we mention our own
review [28], where we discuss experimental and theoretical/computational aspects of
the CPL technique and data. At this stage, our feeling is that, notwithstanding the
50-year history we have roughly outlined above, the technique is still rather “young”
and poses questions more than providing answers. In other words, we find that
puzzling behaviors are encountered by analyzing CPL data, even in the sets of
molecules just discussed. For this reason, in the discussion section we will present a
few examples (from Sects. 10.2.1 to 10.2.5) of some issues from CPL spectroscopy,
namely in Sect. 10.2.1, whether CPL simply allows one to measure the geometrical
distortion in the structure of the excited state or to check also finer electronic features
thereof. In Sect. 10.2.2, we will discuss: the use of fluorescence probes, even achiral
ones like thioflavin, to monitor dissymmetric fibril-bundling and self-aggregation of
biomolecules; the switching on/off fluorescence and CPL into ortho-oligo-phenyleneethynylene (o-OPE) molecules interacting with metals; and the dependence of CPL on
pH. In these cases, we will discuss the efficacy of CPL with respect to the concomitant
CD phenomenon; particularly in the case of o-OPEs, we discuss if a ratiometric probe
can be built in the ad hoc designed molecules bearing two distinct CPL transitions. In
Sect. 10.2.3 we will then consider CPL data for molecules useful for material science,
encompassing inherently dissymmetric thiophene oligomers, substituted helicenes,
and chiral peropyrenes, the latter ones mimicking the behavior of short twisted
graphenes. In Sect. 10.2.4, as anticipated above and as amply treated by Crassous
et al. [29], we find it interesting to deal with the association of metals to organic
ligands, or chiral organometallic compounds, related to well-known and amply studied
transition metal (Pt and Ir) complexes. In Sect. 10.2.5, finally, we will touch on how to
handle CPL data of mesomorphic chiral materials.
10.2 Discussion
10.2.1 CPL and CD: A Sometimes Difficult Relationship
In the large majority of cases, the observed CPL spectra consist in a monosignate
band. This stems from Kasha’s rule, which states that fluorescence originates from
the first excited electronic state in its minimum geometry. Consequently, fluorescence and CPL depend on the same electronic transition moments as absorption and
CD, with due account of the different starting geometry. This should allow one to
conclude that the observed sign for the CPL band is the same as for the CD band
observed at the highest wavelength. As recalled above, Mori et al. [25] moved a step
further, by adding information on the absolute values of g lum , namely
|g lum |/|g abs | ¼ 0.14 for chiral ketones, |g lum |/|g abs | ¼ 0.94 for chiral cyclophanes,
|g lum |/|g abs | ¼ 0.93 for axially chiral biaryls, |g lum |/|g abs | ¼ 0.83 for helicenes/
helicenoids, and |g lum |/|g abs | ¼ 1.02 for BODIPY-type compounds. These values
10 Structural and Electronic Information Drawn. . .
221