Photo-Isomerization of the Azo Group
Azobenzene is a well-known photo-responsive chromophore, and its photo-induced and thermal geometric isomerizations have been extensively explored [200]. Azobenzene and its derivatives assume both trans and cis conformations with respect to the azo linkage. Azo π → π * excitation and azo n → π * excitation trigger trans-to-cis and cis-to-trans isomerizations [203–205]. The azo linkage normally exists in the more stable trans form. Also, the trans isomer of azobenzene exhibits an intense absorption around 320 nm due to the π → π *transition, while the cis isomer shows a weak absorption of the n → π*transition, around 430 nm [206]. Reversible isomerizations between cis and trans structures are due to these transitions. Photo-isomerization can proceed almost quantitatively [207]. By comparison, the thermal isomerizations from cis to trans configurations take place due to low activation energy of the cis-to-trans process. Isomerizations of the azo chromophore in compounds are often accompanied by drastic changes in a number of properties such as, for instance, changes in the dipole moments [208]. The isomerization back to the trans configuration can be readily carried out either thermally, or by visible light irradiation. Changes in the molecular structure, such as cis-trans isomerization in polymers, can induce contraction and expansion of the polymeric chains on both microscopic and macroscopic scale. This was demonstrated on a polymers with azo linkages. Exposure from dark to light can result in a contraction of as much as 0.5% [209]:
One example is a copolymer of styrene, where azobenzene structures are attached in the comonomer to the benzene portion through amide linkages [210]:
After 10 min of irradiation with ultraviolet light, the photo-stationary state is reached, consisting of 79%of the cis isomer. Back isomerization to trans of the sample is slow in the dark (less than 10% in 60 min), but is much faster when exposed to visible light [210]. A Japanese patent [211] describes preparation of isophthalic type polyesters that include monomers with pendant azo groups:
where Y is a hydrogen or a lower alkyl group; m = 1–3; n = 2–18. Polyesters based on this monomers are claimed to be useful for optical recording media such as holograms recording with low light absorption or without loss and wide range of working wavelengths. Izumi and coworkers carried out similar preparations of conjugated polymers with azobenzenes in the main chain [212, 213]. Application of various palladium-catalyzed coupling methods such as the Suzuki coupling and the Heck reactions allowed formation of poly(p-phenylene)- and poly (phenyl vinylene)-based polymers:
These isomerization processes are also accompanied by changes in the three-dimensional hydro dynamic volumes of the polymers [213].
