Abstract: Leuco dyes are provided which comprise the coupling product of a N-acyl substituted aromatic amino color developer and a dye-forming coupler moiety substituted at the coupling carbon with a thermally removable leaving group. Thermal imaging systems employing these leuco dyes have the advantage of reduced bubble formation relative to thermal imaging systems employing prior art leuco dyes containing a group which thermally fragments into one or more gases.

Abstract: Squarylium compounds of the formula: ##STR1## wherein Q.sup.1 and Q.sup.2 are each independently a pyrylium, thiopyrylium, selenopyrylium, benzpyrylium, benzthiopyrylium or benzselenopyrylium nucleus, and R.sup.1 and R.sup.2 are each independently an aliphatic or cycloaliphatic group, can be prepared by reacting a squaric acid derivative of the formula: ##STR2## with a compound of the formula Q.sup.2 CH.sub.2 R.sup.2 in the presence of a base. The derivatives of Formula II may be prepared by condensing a 2,3,4,4-tetrahalocyclobut-2-en-1-one with a compound of the formula Q.sup.1 CH.sub.2 R.sup.1 in the presence of a base to produce a compound of the formula: ##STR3## wherein Q.sup.1 and R.sup.1 are as defined above, and X represents chlorine or bromine, and hydrolyzing the compound of Formula III. Alternatively, the derivatives of Formula II may be prepared by reacting a diester, monoacid chloride monoester or diacid chloride of squaric acid with a compound of the formula Q.sup.1 CH.sub.2 R.sup.