296
T. Ishi-i
fluorescent TPA dye 37 was reported by Massue and Ulrich’s group (Frath et al.
2017). The π-extended boranil dye, 37, showed a moderate TPA cross section of
420 GM and visible to near-IR light emission (Fig. 8.11). Although an interesting
near-IR light emission at 690 nm was shown in PBS, the fluorescence quantum yield
decreased to 0.02. The problem was improved by embedding with the amphiphilic
block copolymer of Cremophor EL, resulting in a good fluorescence quantum yield of
0.33. This embedded dye can be developed for real-time widefield imaging in HeLa
cells. A near-IR fluorescent TPA dye was reported by Zhou, Sun, and Liu’s group
(Zhou et al. 2017). The donor–acceptor–donor type cationic pyran dyes, 38a and
38b, emitted near-IR fluorescence at 725 and 750 nm, respectively, upon two-photon
excitation together with large two-photon action cross sections of 180 and 190 GM,
respectively (Fig. 8.11). In HeLa and A375 cells, dye 38a displayed mitochondriaspecific staining that originated from the positively charged nature. A high spatial
resolution at an imaging depth between 90 and 400 μm can be achieved in rat liver
tissue imaging. In dye 38c, an electron-withdrawing azide group as a quencher moiety
was introduced for H 2 S detection. A turn-on type of fluorescence enhancement was
observed in the presence of H 2 S, which reacted with the azide group in 38c to produce
the corresponding amino derivative, 38a, with intensive fluorescence.
8.3 Two-Photon Absorption Dyes Bearing
Aggregation-Induced Emission Nature
Most fluorescent dyes, including TPA dyes, are hydrophobic, and this facilitates
aggregation in aqueous media. The intermolecular interactions in the aggregate state
usually favor the formation of a nonradiative deactivation channel, resulting in significant quenching of emission (Langhals et al. 1989). Further, the efficiency of the longer
wavelength emission of donor–acceptor-type dyes decreases in polar aqueous media
because a highly polarized excited state that originates from the donor–acceptor characteristic through intramolecular charge transfer or twisted intramolecular charge
transfer increases the formation of a nonradiative deactivation channel (Reichardt
1994). However, recently, unusual light-emitting systems have been discovered that
derive their properties from aggregation-induced emission (AIE) in which the aggregation of nonfluorescent or weakly fluorescent compounds results in an enhancement
of emission efficiency (Fig. 8.12) (Luo et al. 2001; Mei et al. 2015; For details of
AIE, see Chap. 9 in this book). The quenched state in aqueous media changes to
an emissive state owing to the restriction of the intramolecular rotation/vibration
and the polarization that arises from aggregate formation. Thus, enhanced emission
systems have been created by a combination of two-photon-induced emission with
AIE to obtain a larger two-photon action cross section. In this section, two-photon
absorbing AIE dyes with red and near-IR light are summarized as well as their
biological applications.
T. Ishi-i
fluorescent TPA dye 37 was reported by Massue and Ulrich’s group (Frath et al.
2017). The π-extended boranil dye, 37, showed a moderate TPA cross section of
420 GM and visible to near-IR light emission (Fig. 8.11). Although an interesting
near-IR light emission at 690 nm was shown in PBS, the fluorescence quantum yield
decreased to 0.02. The problem was improved by embedding with the amphiphilic
block copolymer of Cremophor EL, resulting in a good fluorescence quantum yield of
0.33. This embedded dye can be developed for real-time widefield imaging in HeLa
cells. A near-IR fluorescent TPA dye was reported by Zhou, Sun, and Liu’s group
(Zhou et al. 2017). The donor–acceptor–donor type cationic pyran dyes, 38a and
38b, emitted near-IR fluorescence at 725 and 750 nm, respectively, upon two-photon
excitation together with large two-photon action cross sections of 180 and 190 GM,
respectively (Fig. 8.11). In HeLa and A375 cells, dye 38a displayed mitochondriaspecific staining that originated from the positively charged nature. A high spatial
resolution at an imaging depth between 90 and 400 μm can be achieved in rat liver
tissue imaging. In dye 38c, an electron-withdrawing azide group as a quencher moiety
was introduced for H 2 S detection. A turn-on type of fluorescence enhancement was
observed in the presence of H 2 S, which reacted with the azide group in 38c to produce
the corresponding amino derivative, 38a, with intensive fluorescence.
8.3 Two-Photon Absorption Dyes Bearing
Aggregation-Induced Emission Nature
Most fluorescent dyes, including TPA dyes, are hydrophobic, and this facilitates
aggregation in aqueous media. The intermolecular interactions in the aggregate state
usually favor the formation of a nonradiative deactivation channel, resulting in significant quenching of emission (Langhals et al. 1989). Further, the efficiency of the longer
wavelength emission of donor–acceptor-type dyes decreases in polar aqueous media
because a highly polarized excited state that originates from the donor–acceptor characteristic through intramolecular charge transfer or twisted intramolecular charge
transfer increases the formation of a nonradiative deactivation channel (Reichardt
1994). However, recently, unusual light-emitting systems have been discovered that
derive their properties from aggregation-induced emission (AIE) in which the aggregation of nonfluorescent or weakly fluorescent compounds results in an enhancement
of emission efficiency (Fig. 8.12) (Luo et al. 2001; Mei et al. 2015; For details of
AIE, see Chap. 9 in this book). The quenched state in aqueous media changes to
an emissive state owing to the restriction of the intramolecular rotation/vibration
and the polarization that arises from aggregate formation. Thus, enhanced emission
systems have been created by a combination of two-photon-induced emission with
AIE to obtain a larger two-photon action cross section. In this section, two-photon
absorbing AIE dyes with red and near-IR light are summarized as well as their
biological applications.