5 BODIPY Dyes and Their Analogues
187
the benzene ring to the a bond in the BODIPY skeleton increases the HOMO
level, while the benzene-fusing at the b bond decreases the LUMO level. Syn-bisbenzoBODIPY 177 has a strong absorption and fluorescence at 775 and 781 nm, and
anti-bis-benzoBODIPY 178 shows more redshifted λ max (848 nm) and F max (868 nm)
(Fig. 5.65a) (Nakamura et al. 2012). Benzo[b]-fused BODIPY dimer 179 shows an
intense absorption at 629 nm along with a broad shoulder band at 753 nm; the central
benzene ring significantly deviates from the aromatic benzene geometry, and the
quinoid character is enhanced in the benzodipyrrole moiety (Fig. 5.65b) (Wakamiya
et al. 2013). The para- and meta-quinodimethane-bridged BODIPY dimers 180 and
181 show good stability towards oxidation in air due to the electron-withdrawing BF 2
unit (Fig. 5.65c) (Ni et al. 2016b). BODIPY dimers 180 and 181 have a small amount
of diradical character and exhibit an intense NIR absorption. Bis(borondifluoride)8-imidazodipyrromethene BOIMPY 182 shows intense and sharp absorption and
fluorescence spectra (Fig. 5.65d) (Patalag et al. 2016). The introduction of arylvinyl
groups into the BOIMPY core leads to a further spectral redshift (182: λ max =
598 nm, 183: λ max = 951 nm). Other multinuclear type boron complexes containing
BODIPY cores have also been reported (Köhler et al. 2004; Sakamoto et al. 2010;
Yokoi et al. 2014; Albrett et al. 2014; Chua et al. 2018; Uno et al. 2018; Tay et al.
2019; Szyszko et al. 2019).
Daltrozzo and Zumbusch et al. firstly reported pyrrolopyrrole–cyanine (PPCy)
dyes (Fig. 5.66) (Fischer et al. 2007). PPCy dyes are excellent NIR chromophores and
fluorophores which are synthesized by the condensation reaction of diketopyrrolopyrrole with heteroarylacetonitrile compounds (Fischer et al. 2007, 2009). BF 2 -PPCy
and BPh 2 -PPCy show strong and narrow NIR absorption and fluorescence and have
only negligible absorption in the visible range; the type of heterocyclic rings (e.g.:
BF 2 -PPCy 1: λ max = 754 nm, BF 2 -PPCy 2: λ max = 684 nm) and the substituent R on
the boron atom (e.g.: BF 2 -PPCy 3: λ max = 789 nm, BPh 2 -PPCy 3: λ max = 864 nm)
strongly affect the λ max (684–864 nm) and F max (708–881 nm) while maintaining
the high ε, Φ f , and photostability (Fig. 5.66a–c) (Fischer et al. 2009). Introduction
of substituents on the PCCy core also affects the optical properties (Fischer et al.
2011a). PCCy dye is applied to NIR fluorescence labelling reagents; the PPCy dye
with a carboxy group is linked to the N-terminus of an Arg 9 peptide which is well
known as a cell penetrating peptide, and internalization of the peptide bound PPCy
is demonstrated using live cell microscopy (Fischer et al. 2010). Introduction of
a sulfonate group to the hydrophobic PPCy core leads to good solubility in pure
water (mM range) (Wiktorowski et al. 2014). Water-soluble PPCy dyes with two
aminophenyl donors have been applied to NIR fluorescent pH indicators for strong
acidity (Wiktorowski et al. 2015).
Wang et al. reported some PPCy dyes to form NIR fluorescent J-aggregates
when they co-precipitate with amphiphilic diblock copolymers to form colloidal
nanoparticles in water (Yang et al. 2017); the packing configuration of PPCy in
NPs is determined by factors including the structure, the concentration of the dye,
and the ultrasonication treatment. The nanoparticles composed of spirofluoreneflanked PPCy dye exhibit effective tumour-targeting and high fluorescence contrast
for in vivo bioimaging of xenografted tumour-model mice (Huang et al. 2018a).
187
the benzene ring to the a bond in the BODIPY skeleton increases the HOMO
level, while the benzene-fusing at the b bond decreases the LUMO level. Syn-bisbenzoBODIPY 177 has a strong absorption and fluorescence at 775 and 781 nm, and
anti-bis-benzoBODIPY 178 shows more redshifted λ max (848 nm) and F max (868 nm)
(Fig. 5.65a) (Nakamura et al. 2012). Benzo[b]-fused BODIPY dimer 179 shows an
intense absorption at 629 nm along with a broad shoulder band at 753 nm; the central
benzene ring significantly deviates from the aromatic benzene geometry, and the
quinoid character is enhanced in the benzodipyrrole moiety (Fig. 5.65b) (Wakamiya
et al. 2013). The para- and meta-quinodimethane-bridged BODIPY dimers 180 and
181 show good stability towards oxidation in air due to the electron-withdrawing BF 2
unit (Fig. 5.65c) (Ni et al. 2016b). BODIPY dimers 180 and 181 have a small amount
of diradical character and exhibit an intense NIR absorption. Bis(borondifluoride)8-imidazodipyrromethene BOIMPY 182 shows intense and sharp absorption and
fluorescence spectra (Fig. 5.65d) (Patalag et al. 2016). The introduction of arylvinyl
groups into the BOIMPY core leads to a further spectral redshift (182: λ max =
598 nm, 183: λ max = 951 nm). Other multinuclear type boron complexes containing
BODIPY cores have also been reported (Köhler et al. 2004; Sakamoto et al. 2010;
Yokoi et al. 2014; Albrett et al. 2014; Chua et al. 2018; Uno et al. 2018; Tay et al.
2019; Szyszko et al. 2019).
Daltrozzo and Zumbusch et al. firstly reported pyrrolopyrrole–cyanine (PPCy)
dyes (Fig. 5.66) (Fischer et al. 2007). PPCy dyes are excellent NIR chromophores and
fluorophores which are synthesized by the condensation reaction of diketopyrrolopyrrole with heteroarylacetonitrile compounds (Fischer et al. 2007, 2009). BF 2 -PPCy
and BPh 2 -PPCy show strong and narrow NIR absorption and fluorescence and have
only negligible absorption in the visible range; the type of heterocyclic rings (e.g.:
BF 2 -PPCy 1: λ max = 754 nm, BF 2 -PPCy 2: λ max = 684 nm) and the substituent R on
the boron atom (e.g.: BF 2 -PPCy 3: λ max = 789 nm, BPh 2 -PPCy 3: λ max = 864 nm)
strongly affect the λ max (684–864 nm) and F max (708–881 nm) while maintaining
the high ε, Φ f , and photostability (Fig. 5.66a–c) (Fischer et al. 2009). Introduction
of substituents on the PCCy core also affects the optical properties (Fischer et al.
2011a). PCCy dye is applied to NIR fluorescence labelling reagents; the PPCy dye
with a carboxy group is linked to the N-terminus of an Arg 9 peptide which is well
known as a cell penetrating peptide, and internalization of the peptide bound PPCy
is demonstrated using live cell microscopy (Fischer et al. 2010). Introduction of
a sulfonate group to the hydrophobic PPCy core leads to good solubility in pure
water (mM range) (Wiktorowski et al. 2014). Water-soluble PPCy dyes with two
aminophenyl donors have been applied to NIR fluorescent pH indicators for strong
acidity (Wiktorowski et al. 2015).
Wang et al. reported some PPCy dyes to form NIR fluorescent J-aggregates
when they co-precipitate with amphiphilic diblock copolymers to form colloidal
nanoparticles in water (Yang et al. 2017); the packing configuration of PPCy in
NPs is determined by factors including the structure, the concentration of the dye,
and the ultrasonication treatment. The nanoparticles composed of spirofluoreneflanked PPCy dye exhibit effective tumour-targeting and high fluorescence contrast
for in vivo bioimaging of xenografted tumour-model mice (Huang et al. 2018a).