5 BODIPY Dyes and Their Analogues
181
5.2.7 NˆC Monoboron Complex
Four-coordinate organoboron complexes with a NˆC bidentate ligand have been
reported (Fig. 5.61). For
5 NˆC type, in 2006, Yamaguchi et al. reported borylsubstituted thienylthiazoles (Fig. 5.61a) (Wakamiya et al. 2006). Dimesitylborylsubstituted thienylthiazole 158 shows slightly redshifted λ max (333 nm) and lower
Φ f (0.09) compared with those of 2-thienyl-2-thiazle (λ max = 313 nm, Φ f = 0.11),
which is the NˆC bidentate ligand of 158. The boron-complexation of 2-thienyl-2thiazle not only decreases the reduction potential, but also stabilizes the produced
radical anion. 3-Boryl-substituted bithiophene 159 has a significantly twisted structure with a dihedral angle between the two thiophenes of 56.0 (Wakamiya et al.
2007). Although 159 has a relatively lower k f value (k f = 5.5 × 10
7 s
−1 ) which is
consistent with the less-allowed S 0 to S 1 transition (ε = 4,500), the k nr value is also
low (k nr = 2.8 × 10
7 s
−1 ). Due to the lower k nr value, 159 shows a relatively high Φ f
value (Φ f = 0.66). Additionally, 159 shows a relatively large Stokes shift (106 nm)
presumably because of the change from the twisted structure to the planar structure
in the excited state. Because the S 0 to S 1 transition of 159 is essentially assigned to
the charge transfer transition from the bithiophene moiety to the boron moiety, the
extension of π-conjugation and the introduction of an electron-donating group in the
bithiophene moiety leads to the redshift of F max (THF: F max = 510–660 nm, Φ f =
0.38–0.93). The 3-boryl-substituted bithiophene derivatives also show fluorescence
in the solid-state (film: F max = 486–657 nm, Φ f = 0.30–0.87).
In 2007, Kawashima et al. reported 2-borylazobenzenes (Fig. 5.61b) (Yoshino
et al. 2007). Although azobenzene derivatives do not show fluorescence, the boron
complexes, which are 2-borylazobenzenes, show fluorescence (e.g.: 160, F max =
503 nm, Φ f = 0.23) because of the prevention of photoisomerization of the azobenzene moiety, enhancement of the molecular rigidity, and the change in nature of the
transition between the lowest excited state and the ground state from the optically
forbidden n–π* transition to the allowed π–π* transition (Yoshino et al. 2013). In
the 2-borylazobenzenes, the introduction of a strong electron-withdrawing pentafluorophenyl group into the boron atom is important for intense fluorescence. Replacement of the pentafluorophenyl groups to less electron-withdrawing p-fluorophenyl
groups leads to fluorescence quenching because of the change in the S 0 to S 1 transition to a forbidden π–π* transition caused by spatial separation. Boron-substituted
aromatic aldimine 161 shows blueshifted absorption (λ max = 324 nm) and fluorescence (F max = 460 nm) compared with those of 160 (Yoshino et al. 2013). Boronsubstituted aromatic aldimines can be applied to cyanide ion sensing. For instance,
the reaction of 161 with cyanide ions gives cyanide adduct, in which a cyanide ion
binds to the imine carbon atom, subsequently leading to fluorescence quenching.
For
6 NˆC type, benzothiadiazole-based monoboron complexes have been reported
(Fig. 5.61c) (Crossley et al. 2015). BCl 2 derivatives (R = Cl) are synthesized by electrophilic C–H borylation which is the reaction of the corresponding non-borylated
ligand with BCl 3 . Because initial borylation reduces the nucleophilicity of the other
nitrogen atom in benzothiadiazole, the diboron complex is not obtained. Although
181
5.2.7 NˆC Monoboron Complex
Four-coordinate organoboron complexes with a NˆC bidentate ligand have been
reported (Fig. 5.61). For
5 NˆC type, in 2006, Yamaguchi et al. reported borylsubstituted thienylthiazoles (Fig. 5.61a) (Wakamiya et al. 2006). Dimesitylborylsubstituted thienylthiazole 158 shows slightly redshifted λ max (333 nm) and lower
Φ f (0.09) compared with those of 2-thienyl-2-thiazle (λ max = 313 nm, Φ f = 0.11),
which is the NˆC bidentate ligand of 158. The boron-complexation of 2-thienyl-2thiazle not only decreases the reduction potential, but also stabilizes the produced
radical anion. 3-Boryl-substituted bithiophene 159 has a significantly twisted structure with a dihedral angle between the two thiophenes of 56.0 (Wakamiya et al.
2007). Although 159 has a relatively lower k f value (k f = 5.5 × 10
7 s
−1 ) which is
consistent with the less-allowed S 0 to S 1 transition (ε = 4,500), the k nr value is also
low (k nr = 2.8 × 10
7 s
−1 ). Due to the lower k nr value, 159 shows a relatively high Φ f
value (Φ f = 0.66). Additionally, 159 shows a relatively large Stokes shift (106 nm)
presumably because of the change from the twisted structure to the planar structure
in the excited state. Because the S 0 to S 1 transition of 159 is essentially assigned to
the charge transfer transition from the bithiophene moiety to the boron moiety, the
extension of π-conjugation and the introduction of an electron-donating group in the
bithiophene moiety leads to the redshift of F max (THF: F max = 510–660 nm, Φ f =
0.38–0.93). The 3-boryl-substituted bithiophene derivatives also show fluorescence
in the solid-state (film: F max = 486–657 nm, Φ f = 0.30–0.87).
In 2007, Kawashima et al. reported 2-borylazobenzenes (Fig. 5.61b) (Yoshino
et al. 2007). Although azobenzene derivatives do not show fluorescence, the boron
complexes, which are 2-borylazobenzenes, show fluorescence (e.g.: 160, F max =
503 nm, Φ f = 0.23) because of the prevention of photoisomerization of the azobenzene moiety, enhancement of the molecular rigidity, and the change in nature of the
transition between the lowest excited state and the ground state from the optically
forbidden n–π* transition to the allowed π–π* transition (Yoshino et al. 2013). In
the 2-borylazobenzenes, the introduction of a strong electron-withdrawing pentafluorophenyl group into the boron atom is important for intense fluorescence. Replacement of the pentafluorophenyl groups to less electron-withdrawing p-fluorophenyl
groups leads to fluorescence quenching because of the change in the S 0 to S 1 transition to a forbidden π–π* transition caused by spatial separation. Boron-substituted
aromatic aldimine 161 shows blueshifted absorption (λ max = 324 nm) and fluorescence (F max = 460 nm) compared with those of 160 (Yoshino et al. 2013). Boronsubstituted aromatic aldimines can be applied to cyanide ion sensing. For instance,
the reaction of 161 with cyanide ions gives cyanide adduct, in which a cyanide ion
binds to the imine carbon atom, subsequently leading to fluorescence quenching.
For
6 NˆC type, benzothiadiazole-based monoboron complexes have been reported
(Fig. 5.61c) (Crossley et al. 2015). BCl 2 derivatives (R = Cl) are synthesized by electrophilic C–H borylation which is the reaction of the corresponding non-borylated
ligand with BCl 3 . Because initial borylation reduces the nucleophilicity of the other
nitrogen atom in benzothiadiazole, the diboron complex is not obtained. Although