12.2 Pyrene-Based Systems
A privileged candidate for excimer formation is pyrene. The little overlap between
the main S 0 ! S 2 absorption transition with the emission spectrum minimizes the
chance of fluorescence resonance energy transfer (FRET), thus ensuring a low
excimer dissociation constant [10]. This often results in an intense excimer emission
with the maximum around 480–500 nm. Tailored systems with defined geometries
bringing pyrene moieties in close proximity to each other thus allow very intense
excimer emission. If the scaffold on which the fluorophores are mounted is scalemic,
or more generally the environment surrounding the targeted system, then excimer
CPL can be observed depending on the geometry described by the mutual arrangement of the pyrene moieties in the excimer state.
The first observation of excimer CPL on pyrene was carried out by Kano and
Sisido’s groups in 1985 from a self-assembly of a pyrene dimer in the chiral cavity of
a γ-cyclodextrin (γ-CDx) [11]. In this study, the authors were able to measure a strong
CPL signal centered around 490 nm with a g lum of |1.2 Â 10
À2 |, thanks to the
asymmetrically twisted configuration of the two pyrene molecules, acquired in the
excimer state inside the γ-CDx chiral cavity. Interestingly, the dissymmetry factor
measured in absorption for the first Cotton effect in the ECD spectrum was |6 Â 10
À5 |,
indicating very weak asymmetry of the arrangement of the two pyrenes in the ground
state.
Recently, Inouye’s group has taken advantage of the same concept [12]. Two
substituted pyrenes bearing PEGylated chains to ensure an overall water solubility
were included into a γ-CDx. Then, through Sonogashira couplings, 3,5-diaryl
substituted phenyls were linked to the pyrenes at their extremities obtaining compound pyr-1 (Fig. 12.2). By proceeding in this manner, the two pyrenes were locked
physically inside the γ-CDx. A clear CPL associated to excimer emission was
recorded (Fig. 12.2), with a g lum of |1.5 Â 10
À2 |, similar to that measured in the
first experiment by Kano and Sisido (see above) [11].
12.2.1 Poly- and Oligopeptides Bearing Pyrene Units
In another context, that of synthetic polypeptides, pyrene CPL excimers were also
studied to elucidate the changes of secondary/tertiary structures upon varying
conditions, such as temperature and solvent. The chosen strategy was to
functionalize different polypeptides with pyrene moieties and to exploit intramolecular formation of excimers to obtain information on the folding in different conditions. Thanks to the chirality of the polypeptidic backbone, excimer CPL could be
induced and measured. Importantly, as chiroptical signals are extremely sensitive to
the surrounding environment, even minor changes in the secondary structure caused
relevant changes in the CPL response.
276
F. Zinna et al.
A privileged candidate for excimer formation is pyrene. The little overlap between
the main S 0 ! S 2 absorption transition with the emission spectrum minimizes the
chance of fluorescence resonance energy transfer (FRET), thus ensuring a low
excimer dissociation constant [10]. This often results in an intense excimer emission
with the maximum around 480–500 nm. Tailored systems with defined geometries
bringing pyrene moieties in close proximity to each other thus allow very intense
excimer emission. If the scaffold on which the fluorophores are mounted is scalemic,
or more generally the environment surrounding the targeted system, then excimer
CPL can be observed depending on the geometry described by the mutual arrangement of the pyrene moieties in the excimer state.
The first observation of excimer CPL on pyrene was carried out by Kano and
Sisido’s groups in 1985 from a self-assembly of a pyrene dimer in the chiral cavity of
a γ-cyclodextrin (γ-CDx) [11]. In this study, the authors were able to measure a strong
CPL signal centered around 490 nm with a g lum of |1.2 Â 10
À2 |, thanks to the
asymmetrically twisted configuration of the two pyrene molecules, acquired in the
excimer state inside the γ-CDx chiral cavity. Interestingly, the dissymmetry factor
measured in absorption for the first Cotton effect in the ECD spectrum was |6 Â 10
À5 |,
indicating very weak asymmetry of the arrangement of the two pyrenes in the ground
state.
Recently, Inouye’s group has taken advantage of the same concept [12]. Two
substituted pyrenes bearing PEGylated chains to ensure an overall water solubility
were included into a γ-CDx. Then, through Sonogashira couplings, 3,5-diaryl
substituted phenyls were linked to the pyrenes at their extremities obtaining compound pyr-1 (Fig. 12.2). By proceeding in this manner, the two pyrenes were locked
physically inside the γ-CDx. A clear CPL associated to excimer emission was
recorded (Fig. 12.2), with a g lum of |1.5 Â 10
À2 |, similar to that measured in the
first experiment by Kano and Sisido (see above) [11].
12.2.1 Poly- and Oligopeptides Bearing Pyrene Units
In another context, that of synthetic polypeptides, pyrene CPL excimers were also
studied to elucidate the changes of secondary/tertiary structures upon varying
conditions, such as temperature and solvent. The chosen strategy was to
functionalize different polypeptides with pyrene moieties and to exploit intramolecular formation of excimers to obtain information on the folding in different conditions. Thanks to the chirality of the polypeptidic backbone, excimer CPL could be
induced and measured. Importantly, as chiroptical signals are extremely sensitive to
the surrounding environment, even minor changes in the secondary structure caused
relevant changes in the CPL response.
276
F. Zinna et al.