Azobenzene and its derivatives can exist in two forms that differ in the isomerization state of the azo group (-N=N-, Figure 1 . 1). 1 One is the trans-form, which has the stable rod-like form in shape. The other is the cis-form, which has the relatively unstable bent form. The stable trans-form isomerizes into the unstable cis-form when the azobenezenes absorb light at a certain wavelength. Cis to trans back isomerization can also take place thermally and/or photochemically. These

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... ation motions of the azobenzenes induce a drastic change in physical and optical properties, and thus, the azobenzenes are very useful as a molecular-scale switching device. Classification of Azobenzens Azobenzenes have been classified into three groups by spectroscopic analysis: azobenzene-type molecules, aminoazobenzene-type molecules and pseudo-stilbene-type molecules (Figure 1. 2) . 1, 4 Azobenzene-type molecules have two absorption bands due to π-π * and n-π * transition with relatively poor overlap between the two bands. The lifetime of the cis isomer is relatively long up to hours and more. Aminoazobenzene-type molecules, where 4-position of azobenzene ring is substituted with amino group, show significant overlap of the two absorption bands and the lifetime of the cis isomer is shorter than azobenzene-type molecules. Pseudo-stilbene-type molecules, where 4-and 4'-positions of two azobenzene rings are substituted with electron-donating and electron-withdrawing groups, has the two bands overlapped on around the same energy level and the lifetime of the cis isomer is shortest up to second range. Since the π-π * and n-π * bands are practically superimposed, light for photoactivation of the trans-cis isomerization will also photoactivate the cis-trans isomerization, which will significantly speed up the whole process. The process can be repeated under illumination, and thus, pseudo-stilbene-type molecules are the best molecules for inducing drastic changes to their surrounding matrix.