33 Turn-on Mode Photoswitchable Fluorescent Diarylethenes …
569
the substituents at 6- and 6
-positions, which affect the coplanarity and the πconjugation length of the closed-ring isomer. In other words, the cycloreversion
quantum yield can be controlled by the dihedral angle between the phenyl ring at
6-position (or 6
-position) and the benzothiophene ring. To increase the cycloreversion quantum yield for the application to RESOLFT microscopy, diarylethenes 5–7
having ortho-substituted phenyl groups at 6- and 6
-positions of the benzothiophene
1,1-dioxide rings were synthesized [37]. The absorption maxima of the photogenerated closed-ring isomers are located at 456 nm (3b), 445 nm (5b), 441 nm (6b), and
430 nm (7b) (Table 33.1). The closed-ring isomers having ortho-substituted phenyl
groups, 5b–7b, exhibit a small but significant hypsochromic shift as compared to
3b having unsubstituted phenyl groups, indicating that 5b–7b have less extended
π-conjugation.
Table 33.1 shows cyclization and cycloreversion quantum yields of 5–7 having
ortho-substituted phenyl groups. The cyclization quantum yields of 5a–7a are similar
to that of 3a. On the other hand, the cycloreversion quantum yield varies strongly
depending on the ortho-substitution of the phenyl groups. The yield of 3b is as low
as 5.9 × 10
−4 , while the yield increases to 2.8 × 10
−3 (5b), 3.2 × 10
−3 (6b), and
1.8 × 10
−2 (7b) by introducing fluoro or methyl group(s) at ortho-position(s). By
replacing ortho-hydrogen with fluorine, the yield increased by 5 times. Introduction
of the ortho-dimethylphenyl groups increased the yield by as large as 30 times. The
ortho-substitution is an effective way to increase the cycloreversion quantum yield up
to 10
−2 –10
−3 . The fluorescence quantum yields of 3b, 5b, 6b, and 7b in 1,4-dioxane
were measured to be 0.87, 0.79, 0.84, and 0.79, respectively, and are very similar to
each other. The ortho-substitution scarcely affects the fluorescent quantum yields.
The ortho-substitution of the phenyl groups is considered to decrease the coplanarity of the closed-ring isomers. The dihedral angles between the phenyl and
benzothiophene rings were measured by single-crystal X-ray analysis. Replacement
of ortho-hydrogen with fluorine increases the dihedral angle from 30.2° (3b) to
35.9° (5b). The angle is further increased up to 61.0° (6b) or 65.7° (7b) by introducing the ortho-methyl group(s). The increase in the dihedral angle lowers the coplanarity of the closed-ring isomer and reduces the π-conjugation length, leading to the
hypsochromic shift of the absorption spectrum and the increase in the cycloreversion
quantum yield.
Figure 33.4 shows the relationship between the absorption maxima of the closedring isomers and the cyclization/cycloreversion quantum yields in less polar 1,4dioxane and polar ethanol. The fluorescence quantum yields of the closed-ring
isomers are also shown. The cyclization quantum yields are higher than 0.58 in 1,4dioxane and the closed-ring isomers exhibit brilliant green fluorescence, the quantum
yield of which is higher than 0.79. The ortho-substitution scarcely affects the fluorescence quantum yields but significantly increases the cycloreversion quantum yields
by as large as 30 times. The cycloreversion quantum yields of 5b–7b are in the
range of 2.8 × 10
−3 to 1.8 × 10
−2 . These molecules fulfill the requirements for the
application to RESOLFT microscopy.
Another strategy to increase the cycloreversion quantum yield is to introduce
isobutyl groups at 2- and 2
-positions (reactive carbon atoms) of the benzothiophene
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the substituents at 6- and 6
-positions, which affect the coplanarity and the πconjugation length of the closed-ring isomer. In other words, the cycloreversion
quantum yield can be controlled by the dihedral angle between the phenyl ring at
6-position (or 6
-position) and the benzothiophene ring. To increase the cycloreversion quantum yield for the application to RESOLFT microscopy, diarylethenes 5–7
having ortho-substituted phenyl groups at 6- and 6
-positions of the benzothiophene
1,1-dioxide rings were synthesized [37]. The absorption maxima of the photogenerated closed-ring isomers are located at 456 nm (3b), 445 nm (5b), 441 nm (6b), and
430 nm (7b) (Table 33.1). The closed-ring isomers having ortho-substituted phenyl
groups, 5b–7b, exhibit a small but significant hypsochromic shift as compared to
3b having unsubstituted phenyl groups, indicating that 5b–7b have less extended
π-conjugation.
Table 33.1 shows cyclization and cycloreversion quantum yields of 5–7 having
ortho-substituted phenyl groups. The cyclization quantum yields of 5a–7a are similar
to that of 3a. On the other hand, the cycloreversion quantum yield varies strongly
depending on the ortho-substitution of the phenyl groups. The yield of 3b is as low
as 5.9 × 10
−4 , while the yield increases to 2.8 × 10
−3 (5b), 3.2 × 10
−3 (6b), and
1.8 × 10
−2 (7b) by introducing fluoro or methyl group(s) at ortho-position(s). By
replacing ortho-hydrogen with fluorine, the yield increased by 5 times. Introduction
of the ortho-dimethylphenyl groups increased the yield by as large as 30 times. The
ortho-substitution is an effective way to increase the cycloreversion quantum yield up
to 10
−2 –10
−3 . The fluorescence quantum yields of 3b, 5b, 6b, and 7b in 1,4-dioxane
were measured to be 0.87, 0.79, 0.84, and 0.79, respectively, and are very similar to
each other. The ortho-substitution scarcely affects the fluorescent quantum yields.
The ortho-substitution of the phenyl groups is considered to decrease the coplanarity of the closed-ring isomers. The dihedral angles between the phenyl and
benzothiophene rings were measured by single-crystal X-ray analysis. Replacement
of ortho-hydrogen with fluorine increases the dihedral angle from 30.2° (3b) to
35.9° (5b). The angle is further increased up to 61.0° (6b) or 65.7° (7b) by introducing the ortho-methyl group(s). The increase in the dihedral angle lowers the coplanarity of the closed-ring isomer and reduces the π-conjugation length, leading to the
hypsochromic shift of the absorption spectrum and the increase in the cycloreversion
quantum yield.
Figure 33.4 shows the relationship between the absorption maxima of the closedring isomers and the cyclization/cycloreversion quantum yields in less polar 1,4dioxane and polar ethanol. The fluorescence quantum yields of the closed-ring
isomers are also shown. The cyclization quantum yields are higher than 0.58 in 1,4dioxane and the closed-ring isomers exhibit brilliant green fluorescence, the quantum
yield of which is higher than 0.79. The ortho-substitution scarcely affects the fluorescence quantum yields but significantly increases the cycloreversion quantum yields
by as large as 30 times. The cycloreversion quantum yields of 5b–7b are in the
range of 2.8 × 10
−3 to 1.8 × 10
−2 . These molecules fulfill the requirements for the
application to RESOLFT microscopy.
Another strategy to increase the cycloreversion quantum yield is to introduce
isobutyl groups at 2- and 2
-positions (reactive carbon atoms) of the benzothiophene