12.1 Introduction
According to IUPAC Gold Book an excimer is “an electronically excited dimer,
‘non-bonding’ in the ground state. For example, a complex formed by the interaction of an excited molecular entity with a ground state partner of the same structure
[1].” Typically, in the realm of organic molecules, an excimer is formed between a
pair of aromatic moieties (M) loosely or non-interacting in the ground state. Upon
excitation, a strong π–π interaction takes place between the excited fluorophore (MÃ)
and the one in ground state, generating thus an excited molecular complex, denoted
as (MM)Ã (Eq. 12.1) [2]. Emission from such species is called excimer emission.
Such bands are red-shifted with respect to monomer emission and are broad and
structureless, since the ground state is dissociative (Fig. 12.1) [3]. As for other chiral
luminescent systems [4], excimer emission can be circularly polarized when using
enantioenriched molecules or in non-racemic environments.
M þ M ! M Ã þM ! MM
ð
ÞÃ
ð12:1Þ
When dealing with purely organic fluorescent compounds, circularly polarized
luminescence (CPL) activity is usually observed for intrinsically chiral and chirally
perturbed fluorophores. In these cases, most of the time, a single CPL band is
observed, if Kasha’s rule applies. This band corresponds to the fluorescence stemming from the lowest singlet excited state. If the geometry of the emitting excited
state is not significantly different from that of the ground state, such a band has the
same sign of the most red-shifted Cotton effect (which corresponds to the ECD of the
same transition observed in absorption). Thus the g lum factor is similar to the
absorption dissymmetry factor (g abs ) of the corresponding ECD transitions
(in terms of order of magnitude and sign) [5]. On the other hand and by definition,
an excimer is a state existing only in the excited state. As a consequence, CPL
Fig. 12.1 General scheme
of electronic states involved
in excimer emission.
Reproduced from reference
[3] with permission from
The Royal Society of
Chemistry
274
F. Zinna et al.
According to IUPAC Gold Book an excimer is “an electronically excited dimer,
‘non-bonding’ in the ground state. For example, a complex formed by the interaction of an excited molecular entity with a ground state partner of the same structure
[1].” Typically, in the realm of organic molecules, an excimer is formed between a
pair of aromatic moieties (M) loosely or non-interacting in the ground state. Upon
excitation, a strong π–π interaction takes place between the excited fluorophore (MÃ)
and the one in ground state, generating thus an excited molecular complex, denoted
as (MM)Ã (Eq. 12.1) [2]. Emission from such species is called excimer emission.
Such bands are red-shifted with respect to monomer emission and are broad and
structureless, since the ground state is dissociative (Fig. 12.1) [3]. As for other chiral
luminescent systems [4], excimer emission can be circularly polarized when using
enantioenriched molecules or in non-racemic environments.
M þ M ! M Ã þM ! MM
ð
ÞÃ
ð12:1Þ
When dealing with purely organic fluorescent compounds, circularly polarized
luminescence (CPL) activity is usually observed for intrinsically chiral and chirally
perturbed fluorophores. In these cases, most of the time, a single CPL band is
observed, if Kasha’s rule applies. This band corresponds to the fluorescence stemming from the lowest singlet excited state. If the geometry of the emitting excited
state is not significantly different from that of the ground state, such a band has the
same sign of the most red-shifted Cotton effect (which corresponds to the ECD of the
same transition observed in absorption). Thus the g lum factor is similar to the
absorption dissymmetry factor (g abs ) of the corresponding ECD transitions
(in terms of order of magnitude and sign) [5]. On the other hand and by definition,
an excimer is a state existing only in the excited state. As a consequence, CPL
Fig. 12.1 General scheme
of electronic states involved
in excimer emission.
Reproduced from reference
[3] with permission from
The Royal Society of
Chemistry
274
F. Zinna et al.