8 Red and Near-IR Fluorescent Two-Photon Absorption Dyes
285
charge transfer by using model compounds of bis(styryl)benzene derivatives that have
electron donor, electron acceptor, and π-spacer moieties. After that breakthrough,
a number of organic compounds including donor–acceptor, donor–acceptor–donor,
acceptor–donor–acceptor, donor–π–acceptor, and donor–π–donor molecules were
synthesized, and the relationship between the molecular structure and the TPA property was investigated to provide guidelines for the development of materials with large
TPA cross sections (He et al. 2008; Pawlicki et al. 2009; Kim and Cho 2009; Terenziani et al. 2008). In addition to the structure–property relationship, TPA applications
for different fields have been comprehensively reviewed. Here, we mainly focus on
fluorescent TPA dyes emitting longer wavelength red and near-IR light. Additionally, we summarize the recent advances made in their development for applications
in biological systems. In the last section, the unique features of TPA in aggregate
systems are summarized.
8.2 Fluorescent Two-Photon Absorption Dyes
8.2.1 History and Demands
Fluorescent TPA dyes have attracted the attention of chemists as well as biologists because of their applications in biological imaging. Microscopic observation
combined with near-IR two-photon excitation provides versatile advantages such as
increased imaging depth and reduced photodamage and photobleaching. Since twophoton microscopy was introduced by Denk and Webb (Denk et al. 1990; Zipfel et al.
2003a), many light-emitting TPA dyes have been designed and prepared. Currently,
various types of TPA dyes that exhibit large TPA cross sections and good fluorescence
quantum yields are accessible (Kim and Cho 2015; Ventelon et al. 2001; Werts et al.
2004; Kim et al. 2008; Wang et al. 2010; Zhang et al. 2011; Heo et al. 2016; Tang et al.
2016; Maeda et al. 2016). The fluorescent color emitted from TPA dyes has mostly
been restricted to the shorter wavelength blue and green regions (Ventelon et al.
2001; Werts et al. 2004; Kim et al. 2008; Wang et al. 2010; Zhang et al. 2011; Heo
et al. 2016; Tang et al. 2016; Maeda et al. 2016), which competes with the autofluorescence from intrinsic biomolecules, such as nicotinamide adenine, riboflavin, and
flavoproteins, aromatic amino acids even at the low level of the TPA cross section
(Zipfel et al. 2003b). To achieve efficient two-photon microscopic imaging, the emission of red and near-IR light is required upon two-photon excitation because it is
within the biological optical window (Weissleder 2001). However, the emission of
longer wavelength red and near-IR light with two-photon excitation is rare compared
to blue and green wavelengths because an elongated π-system and/or an increased
intramolecular charge transfer in fluorescent TPA dyes are required. In this section,
we focus on fluorescent TPA dyes that have a red or near-IR light-emitting nature.
285
charge transfer by using model compounds of bis(styryl)benzene derivatives that have
electron donor, electron acceptor, and π-spacer moieties. After that breakthrough,
a number of organic compounds including donor–acceptor, donor–acceptor–donor,
acceptor–donor–acceptor, donor–π–acceptor, and donor–π–donor molecules were
synthesized, and the relationship between the molecular structure and the TPA property was investigated to provide guidelines for the development of materials with large
TPA cross sections (He et al. 2008; Pawlicki et al. 2009; Kim and Cho 2009; Terenziani et al. 2008). In addition to the structure–property relationship, TPA applications
for different fields have been comprehensively reviewed. Here, we mainly focus on
fluorescent TPA dyes emitting longer wavelength red and near-IR light. Additionally, we summarize the recent advances made in their development for applications
in biological systems. In the last section, the unique features of TPA in aggregate
systems are summarized.
8.2 Fluorescent Two-Photon Absorption Dyes
8.2.1 History and Demands
Fluorescent TPA dyes have attracted the attention of chemists as well as biologists because of their applications in biological imaging. Microscopic observation
combined with near-IR two-photon excitation provides versatile advantages such as
increased imaging depth and reduced photodamage and photobleaching. Since twophoton microscopy was introduced by Denk and Webb (Denk et al. 1990; Zipfel et al.
2003a), many light-emitting TPA dyes have been designed and prepared. Currently,
various types of TPA dyes that exhibit large TPA cross sections and good fluorescence
quantum yields are accessible (Kim and Cho 2015; Ventelon et al. 2001; Werts et al.
2004; Kim et al. 2008; Wang et al. 2010; Zhang et al. 2011; Heo et al. 2016; Tang et al.
2016; Maeda et al. 2016). The fluorescent color emitted from TPA dyes has mostly
been restricted to the shorter wavelength blue and green regions (Ventelon et al.
2001; Werts et al. 2004; Kim et al. 2008; Wang et al. 2010; Zhang et al. 2011; Heo
et al. 2016; Tang et al. 2016; Maeda et al. 2016), which competes with the autofluorescence from intrinsic biomolecules, such as nicotinamide adenine, riboflavin, and
flavoproteins, aromatic amino acids even at the low level of the TPA cross section
(Zipfel et al. 2003b). To achieve efficient two-photon microscopic imaging, the emission of red and near-IR light is required upon two-photon excitation because it is
within the biological optical window (Weissleder 2001). However, the emission of
longer wavelength red and near-IR light with two-photon excitation is rare compared
to blue and green wavelengths because an elongated π-system and/or an increased
intramolecular charge transfer in fluorescent TPA dyes are required. In this section,
we focus on fluorescent TPA dyes that have a red or near-IR light-emitting nature.