DICHROIC DYES: developments and applications

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by Ivan M. Obushenko, Ph.D.

The term "dichroic" may be interpreted from Grecian as "two-colored" and in respect to the dyes sounds at least contradictory. Indeed how a dye can be simulteneously black and white or pink and yellow ?
Historically the phenomena of "dichroism" (two-coloring) was discovered on natural transparent gems (Alexandrite, for example), which appeared two colored from different points (angles) of view. It was described the observation of even three colors (pleochroism) on some synthetic crystals possessing with lower degree of symmetry.
The general cause of dichroism is anisotropy (non-uniformity) of the media containing the coloring sources - some atoms (ions) or molecules.
Liquid crystals (LC) is a fruitfull media for some sort of dyes to show their dichroism. However it will be usefull to remember that all applications of liquid crystals are available due to the development of means for artificial alignement of LC molecules either in parallel (horisontally, homogeneously) or in perpendicular (vertically, homeothropically) in respect to the flat transparent surface (glass). It is possible to say so because the corresponding molecules have clearly expressed elongated shape (stick-like).
The ability of some dyes to show a change of their hue in the oriented liquid crystals (between two parallel pieces of glass) either at different point of view or by applying some voltage (through the pair of transparent electrodes) was discovered by Heilmeier and Zanoni at R.C.A. laboratories (U.S.A.) in 1968 [1].
That first dye was Methyl Red (it must be noticed that the Methyl Red produced very little dichroism - as I assured in my own experiments - and we should be notably grateful to the sharp observation of Heilmeier and Zanoni who did not miss the effect, which they had called "guest-host" effect:dye molecules were guest in the anisothropic liquid crystal host media).

(to be continued)

[1] Cox R.J. Liquid Crystal Guest-Host Systems.- Mol.Cryst.Liq.Cryst.,1979,vol.55,pp.1-32.

Here you are a few useful links for more info :

1. Evaluation Technique
2. Materials used in Liquid Crystal Displays
3. Example of commercial dichroic display
4.Ultra-Fast Guest-Host Display
5.Applications for Light Polarization
6. Non-linear optics and holographic applications
7. The Latest Monograph on Subject



The following dichroic dyes have been selected for their superior properties you might purchase (#10 however is a so-called "black mixture" currently available from alternative source and presented for comparison purpose; one can noted that "clear" state in #10 definitely better than for presented dyes, while the "dark" state is about the same. This is because in #10 cell the vertically alligned orientation film was applied in differ of the rest of dyes; and the "dark" state was caused by application of voltage, while for others LC-cells the only cells' rotation toward light polarization was applied) :

Section for Buyers of "know-how".

We would like to stress here on the one old problem - namely dimming mirrors for automotive application.
Untill 1996 there was a niche for LCD's here: a cholesteric dichroic mixture provided attenuation in "Off" state (no voltage applied) and made rear view mirrors bright in "On" state (about 15-20 Volts were applied).
After introducing of legislation required that rear view mirrors must be bright in "Off" state the LCD's approach was failed.
Only the elctro-chromic mirrors are on the market now in some luxury cars. There were never lack of researches, articles or patents on the subject but another legislation requirement - at least 70% reflectance in "Off" state - made all LCD's approaches to be failed because of necessity of using at least one sheet of polarizer having transmittance only 40% about in one direction ...
Here is the answer on the question : "Why there is no LCD-type dimming mirror on the market ?"
We claim that our group possesses with "know-how" to make LCD approach to be applicable for the rear view mirrors - a big spot on the automotive market and possibly for alfa-numerical displays providing them a quite distinctive bright appearance.
The details' disclosure is available for those who will be able to arrange viable contract (some demos however are available under non-disclosure agreement).
The main advantages of the LCDs approach over the electro-chromic one are:

- much higher speed of switching between "On" and "Off" states (by factor of at least 20);
- much lower power consumption, which is typical for all LCDs don't required of lighting and which promises the wireless mirror in differ of the electro-chromic one;
- possible lower cost and zero capital investments - some current plants surplusing information display (lap-top) market might be just successfully re-oriented.

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