Material Changing Colours Depending on the Exposure Energy and Colours Fixed Reversibly

We developed a material which can fix multiple colours reversibly by changing the exposure energy to UV light in a non-polymeric choresteric liquid crystal containing a small amount of azobenzene derivative. During the exposure to UV light, the colours of a film is changed from red to blue via green. The colours are recorded by maintaining the material to the choresteric temperature 87 �Ž. The material shows different colours depending on the exposure energy, and the colours are fixed immediately after rapid cooling. Once the colours are fixed, they are stable against ultra violet light or visible light. Upon heating up to 130 �Ž, the colours disappear and new recordings shall be possible. New recordings of full colour images and letters will be possible by irradiating UV light through a mask that has an appropriate transmittance. This phenomenon explains that the reversible photochemical response of azobenzene derivatives affects the helical molecule ordering of cholesteric liquid crystal which can be fixed reversibly, and consequently, the light wave length which induces the selective reflection of the crystal liquid has changed.

(Background)

The consumption of massive quantity of papers in offices and households due to the development in computer printers and copying machines has been one of the causes of environmental problems. If a material that enables rewriting images shall be developed and can be applied to a paper in order to obtain a rewritable property, namely, a rewritable paper, such papers can be recycled, and thus the destruction of forests and the waste problems may be solved considerably. This kind of rewritable material shall be ideal if it can reproduce not only black and white information but also full colour images, but thus far, such recording materials which satisfy reversibility, full-colour recording and persistency of information with zero power consumption have not been developed.

(Cholesteric Liquid Crystal)

Material used in this study is cholesteric liquid crystalline compound. Cholesteric liquid crystalline compound shows intense iridescent colours in a liquid crystal state. These colours are based on the reflected light due to the alignment of the cholesteric molecules in a spatially periodic twisted helical structure.(Figure 1) In case of having helical period=P molecule alignment, among the incident lights irradiated parallel to helix axis, only the wavelength lambda = n P (n here is average refractive index of liquid crystal) centered wavelength band delta lambda = Pdelta n(delta n=the birefringence) lights are reflected selectively, and the other wavelength region lights transmit. Hence the periodic helical structure of cholesteric liquid crystal is changed by stimuli like temperature, electric fields and impurities, it is also possible to change the wavelength of reflected light within the range of the entire wavelength region of the visible region. Therefore, if there is a system to enable the periodic helical structure of the cholesteric molecules to be changed and fixed reversibly, it is possible to apply it to the full-colour rewritable recording which shows different colours with single type molecule or single composition. We discovered a non-polymeric cholesteric liquid crystal with medium- molecular weight (approximately 1000) which, controlled in high temperature, rapidly changes the reflected colors when needed, and is able to fix selected reflected colours upon immediate rapid cooling. On the bases of this finding, we are proposing the application of this system for rewritable full-color recording material in thermal mode.

(Photo control of cholesteric liquid crystal colour)

During 1970s, Sackmann reported that the reflected colours of the cholesteric liquid crystal are reversibly changed by photochemical cis-trans isomerization of the dissolved azobenzene. Azobenzene photo- isomerizes from trans to cis upon UV light irradiation and from cis to trans around 440 nm visible light. Due to the differences in the chemical properties between trans azobenzene and cis azobenzene, the different effects were induced as the impurities against cholesteric pitch. Therefore, it is possible to continuously control cholesteric reflection colour by changing the exposure energy to UV light.

(Photo control and fixing of reflection colour of non-polymeric cholesteric liquid crystal with medium molecular-weight)

On the assumption that the photo control and the temporary fixing of the reflection colour of liquid crystal may be possible, we investigated the combination of photochromic compounds such as azobenzene and the above mentioned non-polymeric cholesteric liquid crystal. As a result, we discovered the possibilities of the material for the rewritable full-colour recording in a photon mode, and once the colours are fixed after rapid cooling, they are neither affected by the changes in temperature within normal room temperature region nor by light irradiation. The outline of the result is given in the following.
The mixture of 1 and 2 wt% of azobenzene derivative 2 (didodecyl azobenzene:Scheme 2) to 1 (dicholesteryl ester: Scheme 1) was melted between glass plates and cooled to a cholesteric temperature to obtain a thin film with a thickness of about 10 mu m. While maintaining the liquid crystalline temperature around 90 �Ž, the mixture was exposed to UV light. During the exposure, 2 transformed partially from trans isomer to cis isomer, and the color of the film is changed from red to blue via green. At this stage, the sample was rapidly cooled to 0 �Ž and the colours are fixed maintaining the cholesteric colours obtained. In the solid film after rapid cooling, the peak of UV light region decreased by increasing the irradiation energy, and the peak of visible region shifted to short wave length. Moreover, it became clear that at cholesteric temperature, the helical pitch of cholesteric liquid crystal decreases with the photo isomerization of azobenzene, whereas once the liquid crystal is solidified, cis-2 formed by photo reaction isomerizes simply to trans-2 thermally, and the cholesteric reflection band is not affected by the isomerization of 2 at all. Likewise, after the solidification, the helical pitch of the cholesteric liquid crystal is not influenced by the photo reaction of 2. Namely, around 90 �Ž, which is a cholesteric liquid crystal temperature, the communication gate between photochromic compounds and cholesteric liquid crystal opens, and at the temperature below room temperature after rapid cooling, the gate closes. Moreover, the structure 2 and the molecular ordering of the cholesteric liquid crystal are changed reversibly to the original state by increasing the temperature to 130 �Ž.

(Recording property)

By changing the exposure energy of UV light irradiation to the thin film which is the combination of 1 and 2, red, green, blue, and other colours within the visible region can be fixed.(Figure 2) Furthermore, letters and images with different colours are recorded by irradiating UV light through a mask and then cooling rapidly. At the same time, the detailed parts below 20 mu m are resolved. Upon heating the sample over 130 �Ž, the recorded images disappear and a new image can be recorded repeatedly by exposing the sample through a new mask at a cholesteric temperature.