Order Parameter and Dichroic Ratio of Dyes

The difference between 6 and (j) is one of the reasons for the difference in the order parameters between the dye and the liquid crystal host. The main criterion for dichroic dye efficiency is the order parameter of the transition moment of the dye absorption 5, which is responsible for the color and absorption of the dye [14, 16, 46]. It must be clearly understood that the value of Sj-may differ from the order parameter of the long molecular axis of the dye Sj. This is the case when the transition moment of the dye makes an angle with its long molecular axis. If the transition moment makes an angle (not shown in Fig. 2) with the director of the liquid crystal, then by analogy with 

If we assume that the direction of the transition moment, T, of the dye deviates from its long molecular axis by an angle P, the absorbance A of the incident polarized light, the electric vector P of which makes an angle y/ with the director n, is given as [14, 43] 

The order parameter Sj of the transition moment is determined experimentally as  

From Eq. (6) it is clear that decreases significantly as j8 increases, and hence P should be as low as possible in order to increase the dichroic ratio. One can also see that for P=0°, 5j=5d, while for p=90°, 5t=-5d/2, which indicates that for the neg- 

It is worth mentioning that Osman et al. [74] found theoretically that, in planar dyes, such as anthraquinone, the order tensor deviates strongly from cylindrical symmetry. The direction of the optical transition moment does not, in general, coincide with one of the principal axes of the order tensor. Based on their calculations they found that only those anthraquinone dyes that have a small angle between the transition moment and the molecular axis show a good dichroic ratio [74]. 

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