10.2.3.2 Helicenes
The second example consists of hexahelicenes and of helicene-based molecules:
through the years, helicenes have been thought of as the prototypes of inherently
dissymmetric chromophores due to the chemical structure of such molecules, which
are at the same time strongly conjugated and dissymmetrically distorted, being
constituted by ortho-fused benzene rings. Experimentally, it has been known for a
long time that helicene molecules possess large specific rotations (OR) at 589 nm, of
the order of thousands, and this fact generated the expectation that all their
chiroptical properties would be large and easy to measure. Indeed for
2-Br-hexahelicene, besides OR, also ECD, VCD, ROA, and CPL spectra were
measured. The last spectra, though, were particularly weak also due to heavy atom
effect quenching fluorescence. In Fig. 10.9 we report three cases of hexahelicenebased molecules, namely carbo-hexahelicene (HEX), 5-aza-hexahelicene (5N-HEX)
[30], and thia-bridged triarylamine-hetero [6]-helicene (THIA-HEX) [53]. The spectra are for both enantiomers of each molecule.
The ECD spectra of HEX and 5N-HEX are overall pretty similar; they differ in
the two tiny bands at ca. 410 nm, which are the lowest frequency ones and are
ca. 30 times as intense in the second case with respect to the first case. Due to
Kasha’s rule, the lowest energy band is the one corresponding to the CPL band,
which indeed has the same sign and is more intense for 5N-HEX. g abs and g lum
approximately triple from HEX to 5-N-HEX, however maintaining the ratio
g lum /g abs % 1.1 in both cases (see Table 10.1). We can conclude that CPL is quite
weak a phenomenon for simple hexahelicene systems, since the CPL, as well as the
ECD lowest energy band, originates from an L b transition that presents not only
weak rotational strength, but also weak dipole strength, being dipole forbidden, and
gains intensity from vibronic contributions [31]. In case of nearly forbidden transitions (hexahelicene and methylhexahelicene), absorption, emission, CD, and CPL
are weak and the sign of the bands is sensitive to substituents as observed on many
CD spectra of this kind of molecules in Ref. [54]. As a further characteristic of these
systems, dipole and rotational strengths are sensitive to heteroatoms: the presence of
nitrogen in 5N-HEX (the geometries of the ground state and excited state of HEX
and 5N-HEX are quite similar) not only increases electric dipole transition moment
but also changes the relative orientation of magnetic and electric dipole moments
giving a higher rotational strength and dissymmetry ratio both in absorption and in
emission. The role of heteroatom is even more important in the case of TRIA-HEX
system (Fig. 10.9): a further increase in both g lum and g abs can be noticed, while the
ratio g lum /g abs is maintained at ~1.1. Comparing the Stokes shift, it increases from
about 15 nm, in HEX and 5N-HEX, to ca. 100 nm, in TRIA-HEX; in this last case,
the CPL spectrum loses evident vibrational features.
234
G. Longhi and S. Abbate
The second example consists of hexahelicenes and of helicene-based molecules:
through the years, helicenes have been thought of as the prototypes of inherently
dissymmetric chromophores due to the chemical structure of such molecules, which
are at the same time strongly conjugated and dissymmetrically distorted, being
constituted by ortho-fused benzene rings. Experimentally, it has been known for a
long time that helicene molecules possess large specific rotations (OR) at 589 nm, of
the order of thousands, and this fact generated the expectation that all their
chiroptical properties would be large and easy to measure. Indeed for
2-Br-hexahelicene, besides OR, also ECD, VCD, ROA, and CPL spectra were
measured. The last spectra, though, were particularly weak also due to heavy atom
effect quenching fluorescence. In Fig. 10.9 we report three cases of hexahelicenebased molecules, namely carbo-hexahelicene (HEX), 5-aza-hexahelicene (5N-HEX)
[30], and thia-bridged triarylamine-hetero [6]-helicene (THIA-HEX) [53]. The spectra are for both enantiomers of each molecule.
The ECD spectra of HEX and 5N-HEX are overall pretty similar; they differ in
the two tiny bands at ca. 410 nm, which are the lowest frequency ones and are
ca. 30 times as intense in the second case with respect to the first case. Due to
Kasha’s rule, the lowest energy band is the one corresponding to the CPL band,
which indeed has the same sign and is more intense for 5N-HEX. g abs and g lum
approximately triple from HEX to 5-N-HEX, however maintaining the ratio
g lum /g abs % 1.1 in both cases (see Table 10.1). We can conclude that CPL is quite
weak a phenomenon for simple hexahelicene systems, since the CPL, as well as the
ECD lowest energy band, originates from an L b transition that presents not only
weak rotational strength, but also weak dipole strength, being dipole forbidden, and
gains intensity from vibronic contributions [31]. In case of nearly forbidden transitions (hexahelicene and methylhexahelicene), absorption, emission, CD, and CPL
are weak and the sign of the bands is sensitive to substituents as observed on many
CD spectra of this kind of molecules in Ref. [54]. As a further characteristic of these
systems, dipole and rotational strengths are sensitive to heteroatoms: the presence of
nitrogen in 5N-HEX (the geometries of the ground state and excited state of HEX
and 5N-HEX are quite similar) not only increases electric dipole transition moment
but also changes the relative orientation of magnetic and electric dipole moments
giving a higher rotational strength and dissymmetry ratio both in absorption and in
emission. The role of heteroatom is even more important in the case of TRIA-HEX
system (Fig. 10.9): a further increase in both g lum and g abs can be noticed, while the
ratio g lum /g abs is maintained at ~1.1. Comparing the Stokes shift, it increases from
about 15 nm, in HEX and 5N-HEX, to ca. 100 nm, in TRIA-HEX; in this last case,
the CPL spectrum loses evident vibrational features.
234
G. Longhi and S. Abbate