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Are you asking about the mechanical or chemical mechanism? If chemical, which type, the heat sensitive paper or the ribbon type? Or, are you asking about the electronic circuit?
A thermo printer doesn't use ink. It uses hot needles (pins) instead.
The hot needle punches heat sensitive paper which turns black at the impact point.
Thermo print paper has short life only and is not allowed to print documents. After five to ten years of storage (even in dark and dry rooms) the printed text or image deminishes completely.
The first generations of telefax machines used thermo prints and I sometimes had problems reading customer's information after 1/2 year.
The mechanical construction is very similar to a pin printer (which uses needles to punch a carbon tape - the first generation had cotton or nylon tapes like typewriters).
The simplest ink is made of a phenothiazine dye, like methylene blue. The dye is reduced to a colorless compound (e.g., leucomethylene blue), which is then converted to a heat-labile amide or carbamate) at the thiazine nitrogen. The amides/carbamates are almost colorless and are incorporated into the paper.
When spots on the paper are heated as mentioned by Boncuk, the amide/carbamate decomposes, leaving the parent reduced phenothiazine intact, which very rapidly oxidizes in air to produce color. By changing the side groups attached to the phenothiazine, the color can be controlled to an extent. However, the dark blue of almost black ink is most common.
The other thermal printer mechanism uses a ribbon that has a colored wax on it. That type of thermal printer uses heat to transfer the wax to the paper, somewhat like using carbon paper. Some inexpensive, plain-paper fax machines use that method, rather than laser printers.
1) It appears to be simply a thermo-labile toner in a modified laser printer.
2) The term"leuco," meaning white or colorless, is usually used to refer to the reduced form of the dye, as I mentioned with respect to methylene blue. However, it is not necessarily the reduced form.
3) As a wild guess, look up formazan chemistry. That group of compounds is unusual, almost unique, in that the leuco form is the more oxidized form.
4) The details are probably closely guarded by Toshiba. That process has had no impact in the USA. Bulk reprocessing of used paper is probably more cost effective and certainly much more secure than reprocessing single sheets, from which latent images might be recovered.
5) You might also be interested in the chemistry of Crayola magical markers that are invisible until developed.
Still, why are you interested in this arcane subject?
1) Basic details of mechanism, particularly the chemistry, are not likely to be in any manual.
2) Maybe the machine was a commercial flop for the reasons I mentioned.
3) If you are interested in that particular area of chemistry, follow the leads I gave you.
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