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Molecular Aggregations and Anisotropy Photoinduction in Azo Dyes
Gelatin or other gel films dyed by azo dyes are the subject of a keen interest because of their possible application in the information optical recording, nonlinear optics, holography etc, that is based on the phenomena of the optical anisotropy photoinduction.
By Tariel Ebralidze
Feb. 29, 2016

It is obvious that in an application and informative consideration it is important to know what mechanism is used for information recording in the film during the anisotropy photoinduction.

It is known that film transparency with the photoinduction anisotropy is the function of active light exposition (the abovementioned transparency rises according to exposition increase and reaches the saturationafter several times). Thus, when we imagine a film with photoinducted anisotropy as a monocrystal, it is considered that the abovementioned modulation takes place based on the modulation of birefringence value of the film. However, besides, in our opinion this phenomenon can take place in the other way. In particular, anisotropy can be induced not in continuous shape, but in a granular form, and their concentration would be modulated by light exposition.

According to the existing views, photoinduction of anisotropy in azodyes is a result of cis-trans isomerisation process. Thus, to obtain a clear effect, it is necessary that dye concentration in the solution is as high as possible. However, the high amount of dye concentration in the solutions unconditionally leads us to self-induction of the dye molecules in solid films, or leads to such state when aggregates are induced by light stimulation. Thus, decisive importance may have here not a modulation of the birefringence value, but a modulation of the integral area of the photoinduced anisotropy.

In the gelatin film painted by dye “Mordant Pure Yellow” (MPY) anisotropy photoinduction in the “granular” form has been first observed by us as a result of the experiment. The spectral transmissions of the film having photoinduced anisotropy in the crossed polarizers have been studied. We studied the spectral transmission depending on the light exposition which arises anisotropy. We have got the result — during the anisotropy photoinduction the integral area modulation of the photoinduced “granular” anisotropy takes place.

In the recent 2 works, time formation of the molecular aggregates (anisotropy “grains”) for the polyvinyl films dyed by azo dye MPY have been studied for the first time by using video microscopy.

It has been visually shown that during the photoinduced anisotropy, depending on the active light exposition the integral area of the photoinduced anisotropy in the film is modulated. After certain exposition time the film becomes nearly continuously anisotropic. This result corresponds to an experimental fact that the film transparency (with photoinduced anisotropy) in the crossed polarizers increases at first, and then comes to saturation. This circumstance is very difficult to explain in the azo dyes in case of existing mechanism of anisotropy photoinduction (modulation of birefringence value).

The image of the photoinduced molecular aggregates in the crossed polarizers.

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