270
Y. Yokoyama
terminal acetylene, the Ru(I)-catalyzed Huisgen reaction works on the internal acetylene. The reaction product is 1-benzyl-4,5-bis(4-methyl-2-phenyl-1,3-thiazol-5-yl)1,2,3-triazole, which showed thermally reversible photochromism due to the loss of
large aromatic stabilization energy upon photocyclization of the hexatriene moiety.
It is noteworthy that the thermal ring opening reaction rate can be controlled by the
substituents attached to the phenyl rings on the thiazole units [36].
While the triazole unit formed by the Cu(I)-catalyzed Huisgen reaction worked
only as a linker between two independent functional groups, the 1,4,5-trisubstituted
triazole unit formed by the Ru(I)-catalyzed Huisgen reaction works to build highly
integrated functional conglomerates in which the bisaryltriazole array itself works
as the thermally reversible photochromic gear.
15.5 Conclusion
The creation of functional photochromic systems in which multiple molecules work
cooperatively to produce and transmit information other than color change has been
the focus of our most recent research.
At first, we investigated a photochromic spiropyran which generates strong protic
acid by visible light irradiation. The proton thus formed was able to continuously
change the color of a photochromic dye [9]. The proton generated by visible light also
unlocked the photochromic nature of the diarylethene, which otherwise did not show
any photochromism with UV light irradiation. This combination of visible and UV
light irradiation can thus be regarded as a fail-safe mechanism for this photochromic
system [16].
Secondly, we have succeeded in the transfer of chirality with very high selectivity from naturally occurring human serum albumin (HSA) to diarylethenes. It was
achieved by the careful selection of the functional groups attached to the diarylethene
and by meticulous adjustment of the environmental solution composition [29].
Finally, since the synthesis of photochromic molecules with specific and useful
functions requires much effort and preparation, the development of simple methods to
construct the hexatriene system is essential. One of the two ways we have described
here is to conduct the McMurry cross-coupling reaction of an acetyl-substituted
aromatic compound and an enone compound. We used commercially available
compounds and succeeded in preparing a stealth photochromic compound in just
one step [32]. The other way is to use the Ru(I)-catalyzed Huisgen reaction. It is a
type of “click” reaction and is secure, quick, stereochemically selective, facile and
can be widely applied. We prepared three 1,2-bis(thiazolyl)-substituted acetylenes
which were reacted with benzyl azide. The products showed thermally reversible
photochromism, and the thermal back reaction rates were dependent on the nature
of the substituents attached to the phenyl groups on the peripheral of the molecules
[36].
Y. Yokoyama
terminal acetylene, the Ru(I)-catalyzed Huisgen reaction works on the internal acetylene. The reaction product is 1-benzyl-4,5-bis(4-methyl-2-phenyl-1,3-thiazol-5-yl)1,2,3-triazole, which showed thermally reversible photochromism due to the loss of
large aromatic stabilization energy upon photocyclization of the hexatriene moiety.
It is noteworthy that the thermal ring opening reaction rate can be controlled by the
substituents attached to the phenyl rings on the thiazole units [36].
While the triazole unit formed by the Cu(I)-catalyzed Huisgen reaction worked
only as a linker between two independent functional groups, the 1,4,5-trisubstituted
triazole unit formed by the Ru(I)-catalyzed Huisgen reaction works to build highly
integrated functional conglomerates in which the bisaryltriazole array itself works
as the thermally reversible photochromic gear.
15.5 Conclusion
The creation of functional photochromic systems in which multiple molecules work
cooperatively to produce and transmit information other than color change has been
the focus of our most recent research.
At first, we investigated a photochromic spiropyran which generates strong protic
acid by visible light irradiation. The proton thus formed was able to continuously
change the color of a photochromic dye [9]. The proton generated by visible light also
unlocked the photochromic nature of the diarylethene, which otherwise did not show
any photochromism with UV light irradiation. This combination of visible and UV
light irradiation can thus be regarded as a fail-safe mechanism for this photochromic
system [16].
Secondly, we have succeeded in the transfer of chirality with very high selectivity from naturally occurring human serum albumin (HSA) to diarylethenes. It was
achieved by the careful selection of the functional groups attached to the diarylethene
and by meticulous adjustment of the environmental solution composition [29].
Finally, since the synthesis of photochromic molecules with specific and useful
functions requires much effort and preparation, the development of simple methods to
construct the hexatriene system is essential. One of the two ways we have described
here is to conduct the McMurry cross-coupling reaction of an acetyl-substituted
aromatic compound and an enone compound. We used commercially available
compounds and succeeded in preparing a stealth photochromic compound in just
one step [32]. The other way is to use the Ru(I)-catalyzed Huisgen reaction. It is a
type of “click” reaction and is secure, quick, stereochemically selective, facile and
can be widely applied. We prepared three 1,2-bis(thiazolyl)-substituted acetylenes
which were reacted with benzyl azide. The products showed thermally reversible
photochromism, and the thermal back reaction rates were dependent on the nature
of the substituents attached to the phenyl groups on the peripheral of the molecules
[36].