398
Y. Niko and S. Watanabe
fluorescence at around 480 nm. On the other hand, when the pre-formed dimer
was structurally fixed enough to inhibit its relaxation in excited state, relatively blueshifted excimer-like emission, i.e., static excimer could be observed at around 420 nm.
Importantly, the dynamic excimer emission or that formed by pre-associated dimer
could be distinguished from each other by measuring the time-resolved fluorescence
or excitation spectra.
11.4.7 Assembly- or Disassembly-Induced Emission
Typical organic fluorescent dyes are known to be less or non-fluorescent when
they aggregate, which is called aggregation-caused quenching (ACQ) (Hong et al.
2009). ACQ could be explained by several ways, for example, the formation of
non-fluorescent “H-aggregates” through dipole–dipole interaction between two dyes,
and/or consecutive energy transfer within aggregates (HOMO-FRET) that gradually
diminishes the photon output (fluorescence) against the photon input (excitation).
On the other hand, aggregation does not always quench the fluorescence from a dye.
In some cases, molecules existing as H-aggregates in the ground state may still form
fluorescent dimer (e.g., excimer, see Sect. 11.4.6) through structural relaxation in
the excited state. In addition, some J-aggregates are known to exhibit red-shifted
absorption and fluorescence in contrast to the H-aggregates (Fig. 11.17a) (Spano
and Introduction 2009). Furthermore, Tang proposed that dyes with propeller-like
structures such as hexaphenylsilole and tetraphenyl ethene are basically less or nonfluorescent due to efficient thermal deactivation, whereas they become fluorescent
in the aggregated state due to suppressed molecular motion. This phenomenon is
known as “aggregation-induced emission” (AIE) (Fig. 11.17b) (Hong et al. 2009).
In summary, organic dyes vary their fluorescence behaviors in response to the formation of aggregation/disaggregation states, which is a useful characteristic in the design
of fluorescent chemosensors.
An example of fluorescent chemosensors based on aggregation/disaggregation is
shown in Fig. 11.18. Hamachi and co-workers developed a fluorescent chemosensor
8 that forms non-fluorescent aggregates in water (OFF state) (Mizusawa et al. 2012).
This sensor contains the binding site to capture specific proteins on the cell surface
(so this binding site could be called a “ligand”). As a result, some of the dye can leave
the aggregates to bind to the protein, exhibiting a recovery of fluorescence (ON state,
i.e., “turn-on”). The concept of this sensor is very useful for imaging non-enzymatic
proteins with high S/N ratio.
Another sensor based on aggregation/disaggregation was developed by Klymchenko and co-workers (Niko et al. 2014). The sensor 9 has Nile Red as a fluorophore,
and a detergent structure containing long alkyl chain as the hydrophobic part, and
ornithine and quarternalized amine as the hydrophilic part. As shown in Fig. 11.19a,
this molecule is initially fluorescent but becomes quenched upon forming micelles
in water (OFF state). These micelles are stabilized by polymerization using a reagent
Y. Niko and S. Watanabe
fluorescence at around 480 nm. On the other hand, when the pre-formed dimer
was structurally fixed enough to inhibit its relaxation in excited state, relatively blueshifted excimer-like emission, i.e., static excimer could be observed at around 420 nm.
Importantly, the dynamic excimer emission or that formed by pre-associated dimer
could be distinguished from each other by measuring the time-resolved fluorescence
or excitation spectra.
11.4.7 Assembly- or Disassembly-Induced Emission
Typical organic fluorescent dyes are known to be less or non-fluorescent when
they aggregate, which is called aggregation-caused quenching (ACQ) (Hong et al.
2009). ACQ could be explained by several ways, for example, the formation of
non-fluorescent “H-aggregates” through dipole–dipole interaction between two dyes,
and/or consecutive energy transfer within aggregates (HOMO-FRET) that gradually
diminishes the photon output (fluorescence) against the photon input (excitation).
On the other hand, aggregation does not always quench the fluorescence from a dye.
In some cases, molecules existing as H-aggregates in the ground state may still form
fluorescent dimer (e.g., excimer, see Sect. 11.4.6) through structural relaxation in
the excited state. In addition, some J-aggregates are known to exhibit red-shifted
absorption and fluorescence in contrast to the H-aggregates (Fig. 11.17a) (Spano
and Introduction 2009). Furthermore, Tang proposed that dyes with propeller-like
structures such as hexaphenylsilole and tetraphenyl ethene are basically less or nonfluorescent due to efficient thermal deactivation, whereas they become fluorescent
in the aggregated state due to suppressed molecular motion. This phenomenon is
known as “aggregation-induced emission” (AIE) (Fig. 11.17b) (Hong et al. 2009).
In summary, organic dyes vary their fluorescence behaviors in response to the formation of aggregation/disaggregation states, which is a useful characteristic in the design
of fluorescent chemosensors.
An example of fluorescent chemosensors based on aggregation/disaggregation is
shown in Fig. 11.18. Hamachi and co-workers developed a fluorescent chemosensor
8 that forms non-fluorescent aggregates in water (OFF state) (Mizusawa et al. 2012).
This sensor contains the binding site to capture specific proteins on the cell surface
(so this binding site could be called a “ligand”). As a result, some of the dye can leave
the aggregates to bind to the protein, exhibiting a recovery of fluorescence (ON state,
i.e., “turn-on”). The concept of this sensor is very useful for imaging non-enzymatic
proteins with high S/N ratio.
Another sensor based on aggregation/disaggregation was developed by Klymchenko and co-workers (Niko et al. 2014). The sensor 9 has Nile Red as a fluorophore,
and a detergent structure containing long alkyl chain as the hydrophobic part, and
ornithine and quarternalized amine as the hydrophilic part. As shown in Fig. 11.19a,
this molecule is initially fluorescent but becomes quenched upon forming micelles
in water (OFF state). These micelles are stabilized by polymerization using a reagent