for |g lum |/|g abs | show a good correlation coefficient for chiral cyclophanes, while for
the other classes of molecules the values of g lum and g abs span such a large variety
within each class of molecules that the r
2 value is sometimes fairly small. The
authors of the paper then relate such ratio to the conformational relaxation in the
emissive excited state.
In line with the discussion in this subsection, which is about whether it is possible
to define relations between the sign of CPL band and one of the lowest energy CD
band, beyond what is obvious and just discussed above, we wish to present some
exceptions and caveats regarding the rule of monosignate CPL maintaining the sign
of the corresponding CD band.
Care must be taken when comparing the sign of CPL to the sign of the CD band at
lowest energy (red edge), which may present low rotational strengths: in some cases
the first CD band is due to a nearly forbidden transition gaining intensity from
vibronic contributions (see, for example, simple helicenes [30, 31]); in other cases,
some examples can be found in molecules with axial chirality, rotational strength is
strongly dependent on the mutual orientation of the two moieties that can be tuned
by different substituents [32]. Usually the sign is conserved considering S 0 ! S 1 and
S 1 ! S 0 , meaning that S 0 and S 1 geometries are not so different as to cause a sign
change of rotational strength.
Well-known counterexamples can be found in some ketones: not only camphor
[33] (Fig. 10.1) but also (lS,3R)-4-methyleneadamantan-2-one [34] and other similar
examples. In particular the adamantanone derivative, reported by Dekkers and Closs
[35], shows inversion of sign; on the contrary camphor exhibits two oppositely
signed CPL bands. The sign of the strongest CPL band for camphor is opposite to the
sign of CD band at lowest energy, with |g abs | % 0.03 and |g lum | % 2.8 Â 10
–3 so that
|g lum |/|g abs | % 0.09, being the same order of magnitude as the average behavior
commented by Mori et al. for chiral ketones. These examples, belonging to ketone
class, can be explained considering that S 1 (giving rise to CPL) presents two possible
geometries with slightly different energy, while the ground state geometry can be
assumed as the one corresponding to the interconversion barrier between the two. In
some cases (camphor) both excited state structures are observed, in other cases only
one dominates (adamantanone derivative). We notice, by the way, that for camphor
the less stable geometry, with CPL at lower wavelength, has the same octantconfiguration (as defined by Moscowitz [36] and by Lightner and Gurst [37]) as
the ground state geometry and thus its CPL has the same sign as CD. Instead, the
lowest energy one, at higher wavelength, has opposite octant-configuration and thus
its CPL has opposite sign to CD (see Ref. [33] for a discussion).
In general, one of the following circumstances is expected: (1) One dominant
conformer in its ground state S 0 is excited and readjusts to the energy minimum for
S 1 , which usually has a slightly different geometry, such that relative orientations of
electric and magnetic dipole transition moments are not so different as to change
rotational strength sign. (2) One dominant conformer in its ground state S 0 reaches
the first excited state; however, if the ground state geometry is a saddle point for S 1 ,
the geometry of the emissive transition has two possible structures eventually
generating rotational strengths of different signs. That is the case of camphor and
adamantanone derivative. (3) Many conformers are present, each one with its own S 0
222
G. Longhi and S. Abbate
the other classes of molecules the values of g lum and g abs span such a large variety
within each class of molecules that the r
2 value is sometimes fairly small. The
authors of the paper then relate such ratio to the conformational relaxation in the
emissive excited state.
In line with the discussion in this subsection, which is about whether it is possible
to define relations between the sign of CPL band and one of the lowest energy CD
band, beyond what is obvious and just discussed above, we wish to present some
exceptions and caveats regarding the rule of monosignate CPL maintaining the sign
of the corresponding CD band.
Care must be taken when comparing the sign of CPL to the sign of the CD band at
lowest energy (red edge), which may present low rotational strengths: in some cases
the first CD band is due to a nearly forbidden transition gaining intensity from
vibronic contributions (see, for example, simple helicenes [30, 31]); in other cases,
some examples can be found in molecules with axial chirality, rotational strength is
strongly dependent on the mutual orientation of the two moieties that can be tuned
by different substituents [32]. Usually the sign is conserved considering S 0 ! S 1 and
S 1 ! S 0 , meaning that S 0 and S 1 geometries are not so different as to cause a sign
change of rotational strength.
Well-known counterexamples can be found in some ketones: not only camphor
[33] (Fig. 10.1) but also (lS,3R)-4-methyleneadamantan-2-one [34] and other similar
examples. In particular the adamantanone derivative, reported by Dekkers and Closs
[35], shows inversion of sign; on the contrary camphor exhibits two oppositely
signed CPL bands. The sign of the strongest CPL band for camphor is opposite to the
sign of CD band at lowest energy, with |g abs | % 0.03 and |g lum | % 2.8 Â 10
–3 so that
|g lum |/|g abs | % 0.09, being the same order of magnitude as the average behavior
commented by Mori et al. for chiral ketones. These examples, belonging to ketone
class, can be explained considering that S 1 (giving rise to CPL) presents two possible
geometries with slightly different energy, while the ground state geometry can be
assumed as the one corresponding to the interconversion barrier between the two. In
some cases (camphor) both excited state structures are observed, in other cases only
one dominates (adamantanone derivative). We notice, by the way, that for camphor
the less stable geometry, with CPL at lower wavelength, has the same octantconfiguration (as defined by Moscowitz [36] and by Lightner and Gurst [37]) as
the ground state geometry and thus its CPL has the same sign as CD. Instead, the
lowest energy one, at higher wavelength, has opposite octant-configuration and thus
its CPL has opposite sign to CD (see Ref. [33] for a discussion).
In general, one of the following circumstances is expected: (1) One dominant
conformer in its ground state S 0 is excited and readjusts to the energy minimum for
S 1 , which usually has a slightly different geometry, such that relative orientations of
electric and magnetic dipole transition moments are not so different as to change
rotational strength sign. (2) One dominant conformer in its ground state S 0 reaches
the first excited state; however, if the ground state geometry is a saddle point for S 1 ,
the geometry of the emissive transition has two possible structures eventually
generating rotational strengths of different signs. That is the case of camphor and
adamantanone derivative. (3) Many conformers are present, each one with its own S 0
222
G. Longhi and S. Abbate