properties of such transitions cannot thus be inferred from the ECD bands. For
example, the sign observed for the first ECD and CPL bands are generally not
related, indicating that the excimer geometry can be totally different to that of the
ground state. In this way CPL gives new information which is complementary to that
extracted from absorption spectroscopies.
To the best of our knowledge, in all the cases reported, the g lum factor measured
for excimer transitions is higher than g abs by one or two orders of magnitude and
typically falls in the range of 10
À2 . A possible explanation for such high g lum can be
the fact that an excimer is a single, extended intrinsically chiral fluorophore. As a
comparison, we note that most of chirally perturbed chromophores/fluorophores or
exciton systems display g lum in the 10
À4
–10
À3 range. The n–πà transition from
carbonyl groups in chiral molecules has typically a g lum factor around 10
À2 , as it is
magnetically allowed but electrically forbidden, but for the same reason very low
quantum yields are observed in these cases [4]. In order to fully compare the overall
polarization efficiency, it is convenient to take into account other relevant
photophysical parameters beside dissymmetry factor. To this purpose, circular
polarization brightness (B CP ) was proposed (Eq. 12.2) in which ε λ is the extinction
coefficient at the excitation wavelength, Ф is the emission quantum yield, and B is
the resulting brightness [6].
B CP ¼ ε λ Á Φ Á
j g lum j
2
¼ B Á
j g lum j
2
ð12:2Þ
Considering, as a prototypical example, the circularly polarized excimer emission
from pyrene, it is possible to elaborate the following numbers: ε λ ~4 Â 10
4
M
À1 cm
À1 , quantum yield ~0.3, g lum ~10
À2 . These numbers yield a B CP around
60 M
À1 cm
À1 , which is around one order of magnitude higher than the values typical
for most non-aggregated organic systems, and it approaches the figures of some
lanthanide chiral complexes [7, 8].
Typical intermolecular excimer formation is a diffusion-controlled process. For
this reason, excimer allied CPL is often observed in solid state or in aggregates [9] of
non-racemic fluorophoric small molecules or oligomers and polymers. An effective
strategy to alleviate this dependence is to allow intramolecular excimer formation by
linking two or more fluorophores to a chiral molecular scaffold through suitable
chains. In such cases, if the scaffold or the chains connecting the fluorophore units
are scalemic, CPL can be observed with the typical g lum factors discussed above.
In this chapter, we will review examples of CPL from intramolecularly formed
excimers only, in different molecular systems and different contexts. A focus will be
given on systems based on pyrene, perylene, and perylene bisimide and
1,8-naphthalene monoimide (NMI) moieties.
12 Circularly Polarized Luminescence from Intramolecular Excimers
275
example, the sign observed for the first ECD and CPL bands are generally not
related, indicating that the excimer geometry can be totally different to that of the
ground state. In this way CPL gives new information which is complementary to that
extracted from absorption spectroscopies.
To the best of our knowledge, in all the cases reported, the g lum factor measured
for excimer transitions is higher than g abs by one or two orders of magnitude and
typically falls in the range of 10
À2 . A possible explanation for such high g lum can be
the fact that an excimer is a single, extended intrinsically chiral fluorophore. As a
comparison, we note that most of chirally perturbed chromophores/fluorophores or
exciton systems display g lum in the 10
À4
–10
À3 range. The n–πà transition from
carbonyl groups in chiral molecules has typically a g lum factor around 10
À2 , as it is
magnetically allowed but electrically forbidden, but for the same reason very low
quantum yields are observed in these cases [4]. In order to fully compare the overall
polarization efficiency, it is convenient to take into account other relevant
photophysical parameters beside dissymmetry factor. To this purpose, circular
polarization brightness (B CP ) was proposed (Eq. 12.2) in which ε λ is the extinction
coefficient at the excitation wavelength, Ф is the emission quantum yield, and B is
the resulting brightness [6].
B CP ¼ ε λ Á Φ Á
j g lum j
2
¼ B Á
j g lum j
2
ð12:2Þ
Considering, as a prototypical example, the circularly polarized excimer emission
from pyrene, it is possible to elaborate the following numbers: ε λ ~4 Â 10
4
M
À1 cm
À1 , quantum yield ~0.3, g lum ~10
À2 . These numbers yield a B CP around
60 M
À1 cm
À1 , which is around one order of magnitude higher than the values typical
for most non-aggregated organic systems, and it approaches the figures of some
lanthanide chiral complexes [7, 8].
Typical intermolecular excimer formation is a diffusion-controlled process. For
this reason, excimer allied CPL is often observed in solid state or in aggregates [9] of
non-racemic fluorophoric small molecules or oligomers and polymers. An effective
strategy to alleviate this dependence is to allow intramolecular excimer formation by
linking two or more fluorophores to a chiral molecular scaffold through suitable
chains. In such cases, if the scaffold or the chains connecting the fluorophore units
are scalemic, CPL can be observed with the typical g lum factors discussed above.
In this chapter, we will review examples of CPL from intramolecularly formed
excimers only, in different molecular systems and different contexts. A focus will be
given on systems based on pyrene, perylene, and perylene bisimide and
1,8-naphthalene monoimide (NMI) moieties.
12 Circularly Polarized Luminescence from Intramolecular Excimers
275