Discovery of the Self-Assembly of Liquid Crystal
Molecules by Spin Coating and a Formation of Light Reflective
Glassy Films by Utilizing this Phenomenon

@@ Our research group discovered that a cholesteric solid film was formed rapidly by spin-coating the solution of non-polymeric (approximately 1000 molecular weight) cholesteral derivatives on a substrate. The solid film has cholesteric liquid crystalline ordering at room temperature where it does not thermally show a liquid crystalline phase in normal condition, and selectively reflects a specific wavelength of light. Moreover, we found that the wavelength of the reflection light could be controlled easily by adjusting the temperature of a substrate to be coated. It is expected that this technique of forming a cholesteric solid film will be applied to a simple technique to form a high density molecular memory device for non-destructive reading using reflective light.

Cholesteric Liquid Crystal

@@ A cholesteric liquid crystal reflects specific wavelength light selectively according to the cholesteric molecular alignments with helical structure (Fig.1). In case it has the helical pitch = P, among the incident lights irradiated parallel to helix axis, only half of the circularly polarized light of the wavelength lamda= nP (here, n is average refractive index of liquid crystal) centered wavelength band delta/lamda= Pdeltan (deltan = birefringence) is reflected selectively, and other wavelength region lights transmit. If the reflected light is within the visible region, cholesteric liquid crystal shows iriolescent color.

Background

@@ Among the liquid crystalline compounds that form glassy solids maintaining liquid crystalline molecular ordering, some turn into glassy solid state with liquid crystalline molecular ordering by coating the solution on a substrate and then removing the solvents. However, because most of the compounds which show this property are liquid-crystal polymer compounds slow in forming molecule ordering, the solvent has to be removed slowly, and it takes long time to form the solid films with stable liquid crystalline alignments. Therefore, the solid film with a liquid crystalline alignment does not form because the velocity of the molecular ordering does not catch up with the pace of rapid solvent removal by the spin-coating technique. As for the low molecular weight liquid crystals with fast molecular orderings, the formation of a glassy solid itself is difficult, so that the application using low molecular weight liquid crystals is limited to a liquid crystalline condition with fluidity.
@@ Recently, our research group found that the non-polymeric cholesteral derivatives (approximately 1000 molecule weight) has both properties of fast molecular ordering at cholesteric liquid crystalline phases and the formation of stable glassy solid films at room temperature. On the basis of this discovery, we have been examining further the properties of these cholesteric liquid crystals and the technique to apply them at the same time.

Research Results

@@ The non-polymeric liquid compound shown in Figure 2 is a thermotropic liquid crystal which shows cholesteric liquid phase around 100��. It also forms glassy solid state retaining the liquid crystalline alignment by cooling rapidly from the liquid crystalline phase. Furthermore, a similar reflective solid film (thickness �` 3 mm) was formed rapidly when the solution of the compound 1 dissolved (10-15 wt %) in methylene chloride (CH₂Cl₂) solvent was coated evenly on a glass substrate with spin coater or wire bar around room temperature, and then the solvent is removed naturally. The colors of the solid films depend on the kind of solvent or substrate temperature, and as for the sold film obtained from methylene chloride, lower the formation temperature, longer the wavelength light reflected. The green (lmax = 530 nm) at 40��, the red (lmax = 610 nm) at 20��; furthermore, when formed at 2��, its wavelength of the reflection shifted to infrared light region (lmax = 830 nm). A photograph of solid films on glass plates obtained by spin-coating is shown in Figure 3 (from left, methylene dichloride solution, solid films formed at about 40�� and 22��).
@@ The cholesteric liquid crystalline phase temperature of 1 in figure 2 falls when the solvent is added. It does not form liquid crystalline phase with the concentration under 72 wt%, and the concentration which shows liquid crystalline phase around room temperature did not exist. However, under the non-equilibrium condition during the rapid removal of the solvent after coating the solution on a substrate, a concentration that shows cholesteric phase at room temperature exists in a highly concentrated region. And even later, it is inferred that, by the rapid removal of the solvent (spin coating), the vitrification occurs with the retention of the cholesteric liquid crystalline ordering.
@@ Spin coating is a very useful and effective technique to form the solid films. This research is the first example to show that the glassy solid reflection films with liquid crystalline ordering can be obtained by spin coating.

Application in molecular memory device which enables high density information recording

@@ One of the major problems of CD-R information memory media is that the reflection films currently used absorb certain parts of light and are optically destructed gradually. The reflection films utilized for CD-R are the films of phthalocyanine or cyanine dyes. One of the advantages of these films is that they can be obtained by an easy method such as spin coating. However, because the films are of dyes that absorb light within the reading light wavelength region, they wear off in room light and under reading light as well. On the other hand, for a long time the dielectric substance mirror has been known as a reflection film that does not absorb light within a reflection wavelength region. But the major problem of this film has been lying on how to control the film to have precise thickness so that the reflection light can produce uniform wavelength.
@@ The application of our present research result is believed to be useful to overcome the problem. The light used for reading can be set up to a wavelength different from absorption wavelength of the compounds, and the method of set up is simply to control the temperature of the substrate on which a film will be formed.
@@ Moreover, at present, the problem regarding realization of the high density information recording of CD-R, etc. is because the smallest unit of recording is restricted by the wavelength of read light that merely one bit recording is possible within a given area. But if the reflection films of different reading wavelength can be easily layered, the scale up of the recording capacity will be possible. Our research result opens the possibility to manufacture a medium for multiple recording. We have already succeeded in producing the two-layer films that reflect different lights and have been investigating on the preparation of the multiple layer films.