Photosensitive thermally developed compositions

DETAILED DESCRIPTION INVENTION Leuco Dye Oxidizing Acid Salts Leuco dye oxidizing acid salts as defined herein consist of a salt or mixed salt of an oxidatively triggerable leuco dye and one or more oxidizing acids.
Oxidatively triggerable leuco dyes are well known.
These are colorless compounds which when subjected to an oxidation reaction form colored dyes.
These leuco dyes are well described in the art (e.
g.
, U.
S.
Pat.
No.
3,974,147, The Theory of Photographic Process, 3rd Ed.
; Mees, C.
E.
K.
; James, R.
, Eds.
; MacMillan: New York, 1966, 283-284, 390-391; and Kosar, J.
Light-Sensitive Systems; John Wiley and Sons: New York, 1965; pp.
367, 370-380, 406.
Only those leuco dyes which can be converted to colored dyes by oxidation are useful in the practice of the present invention.
Preferred leuco dyes include acylated leuco azine, phenoxazine, and phenothiazine dyes, examples of which are disclosed in U.
S.
Pat.
Nos.
4,460,677, 4,647,525 and G.
B.
Pat.
No.
1,271,289.
Oxidizing acids are well known in the art and include, but are not limited to nitric, nitrous, peroxonitric, hyponitrous, perchloric, periodic, peroxophosphoric, chromic, permanganic, oxalic, peroxosulfuric, and sulfurous acids as well as organic peracids such as monopermaleic acid.
For the purposes of this invention the term "stable oxidizing acid" is defined as an oxidizing acid which: 1) must be derived from a Group V, VI or VII element, (Barrow, C.
General Chemistry: Wadsworth, Belmont, Calif.
1972, p 162), and 2) forms a stable salt with the leuco dye at ambient temperature, and 3) the oxidizing acid must not react with dialkyl ether or cyclic ethers at 30.
degree.
C.
Preferably, the oxidizing acid is nitric or perchloric acid which generally satisfy the aforementioned conditions.
More preferably the oxidizing acid is nitric acid.
Acid or base sensitive dyes such as phenolphthalein and other indicator dyes are not useful in the present invention.
Further, only those leuco dyes which have basic functionality capable of forming an acid base salt with an oxidizing acid are useful in the practice of the present invention