illustrated, among the others, in Refs. [62-64]. As stated by Steinberg and Ehrenberg
in 1974 [62] “if Brownian motion is not fast enough during the lifetime of the excited
state, the polarization properties of the light emitted from the excited molecules will
depend on polarization characteristics of the excitation beam.” The use of a highly
fluorescent achiral molecule like fluorescein in solvents of different viscosity shows
the presence of CPL signals which are the leaking of linearly polarized components
induced by photoselection and observed in CPL measurements due to nonideal
optics of the system [63].
In case of different concomitant optical phenomena, detailed analysis of each
contribution can be conducted through the analysis of Mueller matrices [65–68];
recently such analysis has been undertaken also for the development of innovative
experimental settings working also at the microscopic level [69, 70]. A review of
Mueller analysis is beyond the scope of this chapter, so we just draw the attention
here to common (sometimes overlooked) effects.
An often disregarded problem is the possibility of auto-absorption: in such a
situation, it may be possible to observe a circularly polarized signal that is difficult to
attribute to absorption (CD) or emission (CPL). The results can be particularly tricky
when several species are present with differences in absorption wavelength and in
emissive properties. An observed dependence of the shape of the signal on concentration or on path-length and inversion of the expected CPL signal (compared to CD)
difficult to explain are important symptoms to analyze.
This problem is often encountered when aggregation phenomena occur in solution, for example, induced by appropriate solvent mixture or temperature variations,
a situation often studied because it may originate enhancement of chiroptical properties [71, 72]. In several cases, genuine CPL is obtained. One of the first example is
given by a chiral polyalkylthiophene by Meijer et al. [73]; in other cases absorption
and emission effects are co-present with either possible cancellation or reinforcement: the emitted light (with circularly polarized components: CPL) can be
reabsorbed differently in its right and left circularly polarized components (CD) so
that it can be difficult to discriminate which phenomenon one is observing. We met
with a clear example considering another chiral polyalkylthiophene in Ref. [74]:
upon increasing the aggregated component, by changing solvent mixture, the system
presents absorption at longer wavelength that interferes with the emission from the
nonaggregated specie, giving rise to a sort of CPL-detected CD with sign opposite to
that of the lowest energy recorded CD band. A simple analysis can be conducted
using both transmittance and CD data, together with CPL and fluorescence measurements. In this way, fluorescence and CPL data can be corrected with the
following expression:
Fluo corr ¼
Fluo obs
T
CPL corr ¼
1
T
Fluo obs α ∙ ln 10
ð Þ∙ ΔA
À
Á þ
1
T
CPL obs
240
G. Longhi and S. Abbate
in 1974 [62] “if Brownian motion is not fast enough during the lifetime of the excited
state, the polarization properties of the light emitted from the excited molecules will
depend on polarization characteristics of the excitation beam.” The use of a highly
fluorescent achiral molecule like fluorescein in solvents of different viscosity shows
the presence of CPL signals which are the leaking of linearly polarized components
induced by photoselection and observed in CPL measurements due to nonideal
optics of the system [63].
In case of different concomitant optical phenomena, detailed analysis of each
contribution can be conducted through the analysis of Mueller matrices [65–68];
recently such analysis has been undertaken also for the development of innovative
experimental settings working also at the microscopic level [69, 70]. A review of
Mueller analysis is beyond the scope of this chapter, so we just draw the attention
here to common (sometimes overlooked) effects.
An often disregarded problem is the possibility of auto-absorption: in such a
situation, it may be possible to observe a circularly polarized signal that is difficult to
attribute to absorption (CD) or emission (CPL). The results can be particularly tricky
when several species are present with differences in absorption wavelength and in
emissive properties. An observed dependence of the shape of the signal on concentration or on path-length and inversion of the expected CPL signal (compared to CD)
difficult to explain are important symptoms to analyze.
This problem is often encountered when aggregation phenomena occur in solution, for example, induced by appropriate solvent mixture or temperature variations,
a situation often studied because it may originate enhancement of chiroptical properties [71, 72]. In several cases, genuine CPL is obtained. One of the first example is
given by a chiral polyalkylthiophene by Meijer et al. [73]; in other cases absorption
and emission effects are co-present with either possible cancellation or reinforcement: the emitted light (with circularly polarized components: CPL) can be
reabsorbed differently in its right and left circularly polarized components (CD) so
that it can be difficult to discriminate which phenomenon one is observing. We met
with a clear example considering another chiral polyalkylthiophene in Ref. [74]:
upon increasing the aggregated component, by changing solvent mixture, the system
presents absorption at longer wavelength that interferes with the emission from the
nonaggregated specie, giving rise to a sort of CPL-detected CD with sign opposite to
that of the lowest energy recorded CD band. A simple analysis can be conducted
using both transmittance and CD data, together with CPL and fluorescence measurements. In this way, fluorescence and CPL data can be corrected with the
following expression:
Fluo corr ¼
Fluo obs
T
CPL corr ¼
1
T
Fluo obs α ∙ ln 10
ð Þ∙ ΔA
À
Á þ
1
T
CPL obs
240
G. Longhi and S. Abbate