192
Y. Kubota
from the reaction of pyrrole-2-carboxaldehyde with 2-hydrazinylpyridine or 2hydrazinylbenzo[d]thiazole and the subsequent boron-complexation (Fig. 5.69b) (Yu
et al. 2018). BOPPY dyes are highly fluorescent both in solution and solid powder
states. BOPPY dyes show good two-photon absorption cross sections of more than
900 GM in the NIR region and are applied to one-photon microscopy (OPM) and
two-photon microscopy (TPM) imaging of living cells. Indolo[3,2-b]carbazole-based
bisboron complex 197 shows NIR absorption (Fig. 5.69c) (Mula et al. 2018).
An expanded ring size (seven-membered ring) distorts the perfect tetrahedral
structure of the boron bridge, making it more labile to a fluoride attack (Fig. 5.69d).
Because of the relatively easy B ← N bond dissociation character, 198 is applied
to chromogenic sensors for fluoride and cyanide ions (Guliyev et al. 2012). The
gradual addition of tetrabutylammonium fluorides into the chloroform solution of
198 decreases the broad absorption at 465 nm with a concomitant increase in new
absorptions at 565 and 600 nm. NMR and MS spectrometric analyses suggest that
at lower concentrations of fluoride ions, initially, the BF 2 unit bridging two pyrroles
is detached. Furthermore, addition of excess amounts of fluoride ion results in the
appearance of absorption at around 520 nm with a concomitant disappearance of
absorptions at 565 and 600 nm. Higher concentrations result in the removal of the
BF 2 unit bridging two oxygen atoms in the form of BF 4
− .
Examples of NˆO type bisboron complexes are shown in Figs. 5.70, 5.71 and
5.72. Pyrimidine-based diboron complexes bearing β-iminoenolate ligands show
redshifted λ max (199: 489 nm) compared with that of the corresponding monoboron
complex (397 nm) (Fig. 5.70a) (Kubota et al. 2016). The diboron complexes with D–
π–A structures, that is, the dimethylamino derivatives (R
1
= NMe 2 ), show redshifted
λ max values (λ max = 531–541 nm) compared with that of the parent compound 199
due to the ICT character. In the dimethylamino derivatives, the ε values decrease with
increasing ICT character (R
2
= OMe: ε = 119,600, R
2
= CN: ε = 73,600). The F max
and Φ f values are redshifted and decrease with increasing ICT character (R
2
= OMe:
F max = 583 nm, Φ f = 0.84, R
2
= CN: F max = 639 nm, Φ f = 0.06) in DCM. The
dimethylamino derivatives exhibit positive fluorescence solvatochromism; the cyano
derivative (R
2
= CN) exhibits the most dramatic redshift of F max with increasing
solvent polarity (from 551 nm in hexane to 710 nm in acetonitrile). Pyrimidine-based
diboron complexes show fluorescence in the crystal state (F max = 578–706 nm, Φ f
= 0.06–0.27). In the case of trifluoromethyl derivative 200, toluene inclusion crystal
and ethyl acetate-inclusion crystals are obtained via recrystallization. The colours of
the crystals are obviously different. The toluene inclusion crystal (F max = 668 nm,
Φ f = 0.16) exhibits a blueshifted F max and higher Φ f compared with those of the
original trifluoromethyl derivative (F max = 694 nm, Φ f = 0.08) in the crystal state. In
contrast, the F max (709 nm) and Φ f (0.04) values of the ethyl acetate-inclusion crystal
are redshifted and lower, respectively. The X-ray crystallographic results indicate that
although the lower Φ f of the ethyl acetate-inclusion crystal is due to the formation of
consecutive π–π interactions between fluorophores, consecutive π–π interactions
between the fluorophores and toluene molecules in the toluene inclusion crystal have
Y. Kubota
from the reaction of pyrrole-2-carboxaldehyde with 2-hydrazinylpyridine or 2hydrazinylbenzo[d]thiazole and the subsequent boron-complexation (Fig. 5.69b) (Yu
et al. 2018). BOPPY dyes are highly fluorescent both in solution and solid powder
states. BOPPY dyes show good two-photon absorption cross sections of more than
900 GM in the NIR region and are applied to one-photon microscopy (OPM) and
two-photon microscopy (TPM) imaging of living cells. Indolo[3,2-b]carbazole-based
bisboron complex 197 shows NIR absorption (Fig. 5.69c) (Mula et al. 2018).
An expanded ring size (seven-membered ring) distorts the perfect tetrahedral
structure of the boron bridge, making it more labile to a fluoride attack (Fig. 5.69d).
Because of the relatively easy B ← N bond dissociation character, 198 is applied
to chromogenic sensors for fluoride and cyanide ions (Guliyev et al. 2012). The
gradual addition of tetrabutylammonium fluorides into the chloroform solution of
198 decreases the broad absorption at 465 nm with a concomitant increase in new
absorptions at 565 and 600 nm. NMR and MS spectrometric analyses suggest that
at lower concentrations of fluoride ions, initially, the BF 2 unit bridging two pyrroles
is detached. Furthermore, addition of excess amounts of fluoride ion results in the
appearance of absorption at around 520 nm with a concomitant disappearance of
absorptions at 565 and 600 nm. Higher concentrations result in the removal of the
BF 2 unit bridging two oxygen atoms in the form of BF 4
− .
Examples of NˆO type bisboron complexes are shown in Figs. 5.70, 5.71 and
5.72. Pyrimidine-based diboron complexes bearing β-iminoenolate ligands show
redshifted λ max (199: 489 nm) compared with that of the corresponding monoboron
complex (397 nm) (Fig. 5.70a) (Kubota et al. 2016). The diboron complexes with D–
π–A structures, that is, the dimethylamino derivatives (R
1
= NMe 2 ), show redshifted
λ max values (λ max = 531–541 nm) compared with that of the parent compound 199
due to the ICT character. In the dimethylamino derivatives, the ε values decrease with
increasing ICT character (R
2
= OMe: ε = 119,600, R
2
= CN: ε = 73,600). The F max
and Φ f values are redshifted and decrease with increasing ICT character (R
2
= OMe:
F max = 583 nm, Φ f = 0.84, R
2
= CN: F max = 639 nm, Φ f = 0.06) in DCM. The
dimethylamino derivatives exhibit positive fluorescence solvatochromism; the cyano
derivative (R
2
= CN) exhibits the most dramatic redshift of F max with increasing
solvent polarity (from 551 nm in hexane to 710 nm in acetonitrile). Pyrimidine-based
diboron complexes show fluorescence in the crystal state (F max = 578–706 nm, Φ f
= 0.06–0.27). In the case of trifluoromethyl derivative 200, toluene inclusion crystal
and ethyl acetate-inclusion crystals are obtained via recrystallization. The colours of
the crystals are obviously different. The toluene inclusion crystal (F max = 668 nm,
Φ f = 0.16) exhibits a blueshifted F max and higher Φ f compared with those of the
original trifluoromethyl derivative (F max = 694 nm, Φ f = 0.08) in the crystal state. In
contrast, the F max (709 nm) and Φ f (0.04) values of the ethyl acetate-inclusion crystal
are redshifted and lower, respectively. The X-ray crystallographic results indicate that
although the lower Φ f of the ethyl acetate-inclusion crystal is due to the formation of
consecutive π–π interactions between fluorophores, consecutive π–π interactions
between the fluorophores and toluene molecules in the toluene inclusion crystal have