According to the present invention, a coloring property-improved novel leuco dye which can develop color by heat even with single molecule without requiring excess developer can be obtained, and an image recording medium having high sensitivity and excellent resolution effected by including the dye and a high sensitivity image recording medium further combined with an acid generator can also be obtained. Particularly, it relates to a leuco dye represented by a formula (1) (LD--(A--P.sup.1).sub.n) and to an image recording medium which contains the leuco dye (wherein LD represents an acid-sensitive leuco dye mother nucleus, --A represents the residue of an acidic group (--AH) selected from --SO.sub.3 H, --CO.sub.2 H and --P(.dbd.O)(OH).sub.x R.sub.y, R represents a hydrogen atom or a substituent group, x and y are x=1 and y=1 or x=2 and y=0, P.sup.1 represents a protecting group which can be removed by the action of light, heat or acid, and n is an integer of 1 or more).

Disclosed is a leuco dye in which a hydrogen atom capable of transferring by isomerization in a dye represented by formula (1) or a tautomer thereof is substituted by a substituent group P which can be removed by the action of heat or acid ##STR1## wherein (A) represents an atomic group necessary for forming a nitrogen-containing hetero ring; (B) represents an atomic group necessary for forming a six-membered hetero ring; and X represents an oxygen atom, a sulfur atom or a nitrogen atom, which may have a hydrogen atom or a possible substituent group.

Pyrrolo[3,4-c]pyrroles of formula ##STR1## wherein D and E are each independently of the other a group of formula ##STR2## The symbols Q, R.sub.8, R.sub.9, R.sub.10, R.sub.11, X, Y, Z, m and n are as defined in claim 1. These pyrrolo[3,4-c]pyrroles are distinguished by outstanding solid state fluorescence in the UV range as well as by the ease with which they can be converted to pyrrolopyrrole pigments even in the substrate in which they are incorporated.

Compounds of formula wherein x is an integer from 1 to 4, A is the radical of a chromophore of the quinacridone, anthraquinone, perylene, indigo, quinophthalone, isoindolinone, isoindoline, dioxazine, phthalocyanine or azo series, which radical contains x N-atoms attached to B, preferably with at least one directly adjacent or conjugated carbonyl group, B is a group of formula ##STR1## and, if x is 2, 3 or 4, may also be one, two or three hydrogen atoms. The symbols Q, R.sub.1, R.sub.2, R.sub.3, R.sub.4, X, Y, Z, m und n, are as defined in claim 1. These carbamate group-containing soluble chromophores are distinguished by outstanding solid state fluorescence in the UV range as well as the by ease with which they can be converted to the corresponding pigments in the substrate in which they are incorporated.

A thermal recording medium comprising a ultraviolet absorber precursor represented by formula (1-A) or an image forming compound changing hue and an acid, which is high in thermal sensitivity, recordable with such a low output laser that no ablation takes place even when a thermal heat mode image recording system using a laser is utilized, requiring no different receiving sheet, and excellent in keeping quality: ##STR1## wherein P represents a protecting group for a hydroxyl group which is deblocked by heating to 250.degree. C. or less in the presence of an acid; R.sup.1 and R.sup.2, which may be the same or different, each represents a substitutable group; and 1 and m each represents an integer of 0 to 4.