Incident
light
CPL and
Fluorescence
CD and
Transmittance
d1
2d2
where Fluo obs and CPL obs are the actually observed fluorescence and CPL
signals, T is transmittance, ΔA is CD in absorbance units, α accounts for the average
path-length of the emitted light inside the cuvette: it is the ratio between the average
path of the emitted light within the cuvette (namely d 2 ) and the path of the
CD/absorption experiment (namely d 1 ).
Finally, we recall that interest is growing in considering cooperative effects
present in aggregate states [27, 75], particularly the solid state, since supramolecular
chiral structures greatly enhance chiroptical response [76]. In these cases, spectroscopic results should be considered more a property of the whole material rather than
a molecular property: one may speak of local versus long-range ordering effects, or
intrinsic versus extrinsic effects. The second case presents a g factor depending on
sample thickness. Many examples are present in the literature where the enhancement of chiroptical response is favored by cholesteric phases doped with
fluorophores or fluorescent achiral polymers assembled in chiral supramolecular
structures induced by a chiral dopant [76–79].
For this purpose, we studied an interesting material showing high chiroptical
response in various spectroscopic ranges: a chiroptically active syndiotactic polystyrene (s-PS) crystallized after sorption–desorption of chiral agents like limonene or
carvone. This material appears to behave as a metamaterial that influences light
polarization properties to a huge extent. Important manifestations have been
observed by VCD spectroscopy; in Ref. [80] a particularly meaningful observation
was made, that is to say, once the polymer had received the imprinting from a
molecule of chosen chirality, the sign of the observed VCD bands is determined once
and for all. In fact, once the polymer had been prepared by sorbing–desorbing (R)carvone, it exhibited the same VCD apparent spectrum when hosting either (R)carvone or (S)-carvone. This evidences that one is not observing an enhancement of
molecular host VCD signal but a modulation of the molecular host absorption due to
the guest material; in other words s-PS is a medium acting differently on the
transmitted left and right circularly polarized components of light. Concerning the
UV-vis region, the difference in left and right circularly polarized transmissions of
these films is quite large over a wide spectral range (Fig. 10.13a). The existence of a
CD signal does not necessarily imply circular dichroism at the molecular level, rather
one can state that the material shows different transmission for left and right
circularly polarized lights. We noticed that no absorption band is present in this
region (280–600 nm) and we checked that the data are independent from rotation of
10 Structural and Electronic Information Drawn. . .
241
light
CPL and
Fluorescence
CD and
Transmittance
d1
2d2
where Fluo obs and CPL obs are the actually observed fluorescence and CPL
signals, T is transmittance, ΔA is CD in absorbance units, α accounts for the average
path-length of the emitted light inside the cuvette: it is the ratio between the average
path of the emitted light within the cuvette (namely d 2 ) and the path of the
CD/absorption experiment (namely d 1 ).
Finally, we recall that interest is growing in considering cooperative effects
present in aggregate states [27, 75], particularly the solid state, since supramolecular
chiral structures greatly enhance chiroptical response [76]. In these cases, spectroscopic results should be considered more a property of the whole material rather than
a molecular property: one may speak of local versus long-range ordering effects, or
intrinsic versus extrinsic effects. The second case presents a g factor depending on
sample thickness. Many examples are present in the literature where the enhancement of chiroptical response is favored by cholesteric phases doped with
fluorophores or fluorescent achiral polymers assembled in chiral supramolecular
structures induced by a chiral dopant [76–79].
For this purpose, we studied an interesting material showing high chiroptical
response in various spectroscopic ranges: a chiroptically active syndiotactic polystyrene (s-PS) crystallized after sorption–desorption of chiral agents like limonene or
carvone. This material appears to behave as a metamaterial that influences light
polarization properties to a huge extent. Important manifestations have been
observed by VCD spectroscopy; in Ref. [80] a particularly meaningful observation
was made, that is to say, once the polymer had received the imprinting from a
molecule of chosen chirality, the sign of the observed VCD bands is determined once
and for all. In fact, once the polymer had been prepared by sorbing–desorbing (R)carvone, it exhibited the same VCD apparent spectrum when hosting either (R)carvone or (S)-carvone. This evidences that one is not observing an enhancement of
molecular host VCD signal but a modulation of the molecular host absorption due to
the guest material; in other words s-PS is a medium acting differently on the
transmitted left and right circularly polarized components of light. Concerning the
UV-vis region, the difference in left and right circularly polarized transmissions of
these films is quite large over a wide spectral range (Fig. 10.13a). The existence of a
CD signal does not necessarily imply circular dichroism at the molecular level, rather
one can state that the material shows different transmission for left and right
circularly polarized lights. We noticed that no absorption band is present in this
region (280–600 nm) and we checked that the data are independent from rotation of
10 Structural and Electronic Information Drawn. . .
241