Question about structure and mechanism of action of sensitizing dyes.

[mghis]

Hello.

Most of the spectral sensitizing dyes have, as the main chromophore group, an unsaturated conjugated carbon skeleton, often bonded with nitrogen heterocycles. I was wondering why only dyes of these classes work and which is the mechanism of the sensitization.

Thank you in advance for any response.

 

[Gerald C Koch]

Simply put the length of the carbon skeleton determines the absorption frequency for a particular dye. The rest is merely solid state chemistry. I remember a lab exercise in graduate school where we calculated the absorption frequency from the chain length and compared it to the measured frequency. For calculations of this type based on average chemical bond lengths and Hooke's Law the results were quit good. The following article may be helpful.

https://en.wikipedia.org/wiki/Particle_in_a_box

Actually dyes of this type are not the only sensitizing one. Older sensitizers like eosin do no fit this description. The dye family that you mention is very expensive because the dyes are particularly difficult to synthesize. Fortunately they are used in very small amounts.

 

[mghis]

Thank you for your response.

Indeed the structure is related to the absorption frequencies. That's how IR spectroscopy works, for instance. I am more interested in the way in which this and a few others types of dye can influence the absorption frequencies of silver halide. I assume the dye coordinates in some way with the metal to reduce it. Why, for instance, can these dyes sensitize silver and azo dyes cannot?

 

[Gerald C Koch]

A sensitizing dye binds to a silver halide grain. (It is well known that eosin is capable of doing this. Eosin is used as an indicator in the volumetric analysis for chloride ion using silver nitrate. The dye specifically binds to silver chloride causing it to change from white to pink at the end point.) When a photon of the correct wavelength strikes the dye molecule its energy is eventually transferred to the silver halide grain. This creates an activation site enabling light of the correct wavelength to create an image. Unsensitized emulsion is sensitive to only blue light. By selecting one or more dyes the entire visible spectrum can be imaged. A small number of dyes can also carry this out into the infra-red region of the spectrum.

As I mentioned before not all sensitizing dyes have to be of the type you specify. Besides eosin chlorophyll has been used in home brew emulsions. It sensitizes for the yellow-orange region. Useful candidates would have to bind with the silver halide.

Search the web for references to Hermann Wilhelm Vogel, the father of dye sensitization. The notes to these articles are often very useful.

https://en.wikipedia.org/wiki/Hermann_Wilhelm_Vogel

 

[mghis]

Thank you again, especially for the reference to Vogel, whose experiments and publications were unknown to me.