138
Y. Kubota
(Lu et al. 2012) also prevented intermolecular π–π interactions and contributed the
expression of fluorescence in the solid-state.
For strategy (2), careful choice and combination of suitable substituents are
required to achieve the formation of emissive BODIPY J-aggregates. The combination of an electron-withdrawing meso-substituent, such as a trifluoromethyl or
methoxycarbonyl group, and methyl groups at the β
position induced emissive
J-aggregates (Kim et al. 2015).
For strategy (3), the incorporation of BODIPY dyes into ZIF-8 was achieved
upon ion- and liquid-assisted grinding (ILAG) or accelerated ageing of 1:2 stoichiometric mixtures of ZnO and 2-methylimidazole (HMeIm) in the presence of BODIPY
dye (Glembockyte et al. 2018). The resulting BODIPY@ZIF-8 showed solid-state
fluorescence and a remarkable enhancement in photostability.
Enhancement of water solubility
High water solubility and suppression of the formation of nonfluorescent aggregates in water are required for biological and medical applications of BODIPY dyes.
Incorporation of ionic substituents including sulfonate (Li et al. 2008b), trimethyl(propargyl)ammonium (Poirel et al. 2014), and sulfobetaine (Sutter et al. 2018)
groups into the BODIPY core is a good method to enhance water solubility. For
instance, the red-emitting water-soluble ionic BODIPY dye having two trimethyl(propargyl)ammonium groups on the boron atom has been reported (Poirel et al.
2014) (Fig. 5.21b). Neutral water-soluble BODIPY dyes, which have advantages
over ionic dyes in that they avoid potential nonspecific electrostatic interactions
between BODIPY dyes and biomolecules, have also been synthesized by the introduction of branched oligo(ethylene glycol)methyl ether substituents to the BODIPY
core (Zhu et al. 2011).
Stokes shift
In terms of a Stokes shift (SS), a small Stokes shift leads to self-quenching and
measurement error from excitation light and scattered light, which reduces detection
sensitivity. BODIPY dyes generally have very small Stokes shifts (5–20 nm, in most
cases) due to the rigidified fluorophore which causes limited geometric relaxation
between the Franck–Condon excited state and the equilibrium excited state upon
photoexcitation. In order to expand the Stokes shift, the following strategies have
been reported: (1) energy transfer cassettes (Goze et al. 2007; Qu et al. 2012), (2)
twisted intramolecular charge transfer (TICT) (Hu et al. 2009), (3) non-symmetrical
annulation (Yang et al. 2016; Ren et al. 2018), (4) excited state intramolecular proton
transfer (ESIPT) (Fei et al. 2017), (5) formation of BODIPY oligomers (Hayashi et al.
2011a; Nepomnyashchii et al. 2011), and (6) introduction of susceptible groups (Zhu
et al. 2019).
Through strategy (1), large pseudo-Stokes shifts can be obtained by excitation
energy transfer (EET) from donor (high energy absorbing chromophore) to acceptor
(lower energy emitting fluorophore). The EET process can be classified as fluorescence resonance energy transfer (FRET) or through-bond energy transfer (TBET).
While donor and acceptor are linked together by a non-conjugated spacer in the FRET
Y. Kubota
(Lu et al. 2012) also prevented intermolecular π–π interactions and contributed the
expression of fluorescence in the solid-state.
For strategy (2), careful choice and combination of suitable substituents are
required to achieve the formation of emissive BODIPY J-aggregates. The combination of an electron-withdrawing meso-substituent, such as a trifluoromethyl or
methoxycarbonyl group, and methyl groups at the β
position induced emissive
J-aggregates (Kim et al. 2015).
For strategy (3), the incorporation of BODIPY dyes into ZIF-8 was achieved
upon ion- and liquid-assisted grinding (ILAG) or accelerated ageing of 1:2 stoichiometric mixtures of ZnO and 2-methylimidazole (HMeIm) in the presence of BODIPY
dye (Glembockyte et al. 2018). The resulting BODIPY@ZIF-8 showed solid-state
fluorescence and a remarkable enhancement in photostability.
Enhancement of water solubility
High water solubility and suppression of the formation of nonfluorescent aggregates in water are required for biological and medical applications of BODIPY dyes.
Incorporation of ionic substituents including sulfonate (Li et al. 2008b), trimethyl(propargyl)ammonium (Poirel et al. 2014), and sulfobetaine (Sutter et al. 2018)
groups into the BODIPY core is a good method to enhance water solubility. For
instance, the red-emitting water-soluble ionic BODIPY dye having two trimethyl(propargyl)ammonium groups on the boron atom has been reported (Poirel et al.
2014) (Fig. 5.21b). Neutral water-soluble BODIPY dyes, which have advantages
over ionic dyes in that they avoid potential nonspecific electrostatic interactions
between BODIPY dyes and biomolecules, have also been synthesized by the introduction of branched oligo(ethylene glycol)methyl ether substituents to the BODIPY
core (Zhu et al. 2011).
Stokes shift
In terms of a Stokes shift (SS), a small Stokes shift leads to self-quenching and
measurement error from excitation light and scattered light, which reduces detection
sensitivity. BODIPY dyes generally have very small Stokes shifts (5–20 nm, in most
cases) due to the rigidified fluorophore which causes limited geometric relaxation
between the Franck–Condon excited state and the equilibrium excited state upon
photoexcitation. In order to expand the Stokes shift, the following strategies have
been reported: (1) energy transfer cassettes (Goze et al. 2007; Qu et al. 2012), (2)
twisted intramolecular charge transfer (TICT) (Hu et al. 2009), (3) non-symmetrical
annulation (Yang et al. 2016; Ren et al. 2018), (4) excited state intramolecular proton
transfer (ESIPT) (Fei et al. 2017), (5) formation of BODIPY oligomers (Hayashi et al.
2011a; Nepomnyashchii et al. 2011), and (6) introduction of susceptible groups (Zhu
et al. 2019).
Through strategy (1), large pseudo-Stokes shifts can be obtained by excitation
energy transfer (EET) from donor (high energy absorbing chromophore) to acceptor
(lower energy emitting fluorophore). The EET process can be classified as fluorescence resonance energy transfer (FRET) or through-bond energy transfer (TBET).
While donor and acceptor are linked together by a non-conjugated spacer in the FRET