Thermoreactive recording material contains, as developer, a mixture of an acidic-modified polymer of (meth)acrylonitrile and/or methacrylonitrile and a diphenol of the formula ##STR1## in which Y is, for example, a single bond, an alkylene or alkylidene radical, gives, on thermal printing, a dye which has a reduced tendency to sublime.
Compounds of the formula ##STR1## in which R.sup.1 /R.sup.2 are alkyl, cycloalkyl, aralkyl or alkylene, R.sup.3 is alkyl, cycloalkyl or aralkyl, Z is alkylene, X is O or are highly suitable as color formers in recording materials based on acid developers. They give yellow or orange hues which have excellent sublimation and light fastness.
Tetraindolyl-heptamethine derivatives of the isomeric formulae ##STR1## in which A, B, D and E denote ##STR2## and may be identical to or different from one another, and the other symbols have the meaning indicated in the description, are used in pressure-sensitive and heat-sensitive recording materials.
Color-forming agents of the present formula ##STR1## wherein X denotes hydroxyl, alkoxy, alkenyloxy, aralkoxy, cycloalkoxy, aryloxy, acyloxy, alkylamino, dialkylamino, acylamino, aralkylamino or arylamino, R denotes alkyl, alkenyl or aralkyl, R.sub.1 denotes hydrogen, halogen or alkyl, R.sub.2 denotes alkoxy, aralkoxy, aryloxy or a radical of the formula ##STR2## Y.sub.1 and Y.sub.2 independently of one another denote alkyl, aryl, cycloalkyl or aralkyl or R.sub.2, with A in the o-position, forms a heterocyclic ring, a benzene ring can be fused onto the ring A, in the o-/m-position relative to R.sub.2, and the ring A and its fused-on rings and the cyclic and acyclic radicals can carry further non-ionic substitutents customary in dyestuffs chemistry, are used in thermoreactive papers and pressure-sensitive recording materials and for the preparation of highly concentrated solutions in organic solvents.