Thermal imaging method

Claim 1

A substantially colorless di- or triarylmethane compound of the formula ##STR54## wherein ring B represents a carbocyclic aryl ring or a heterocyclic aryl ring; C 1 represents the meso carbon atom of said di- or triarylmethane compound; X represents ##STR55## or --CH 2 -- and completes a moiety ring-closed on said meso carbon, said moiety including the nitrogen atom bonded directly to said meso carbon; Y represents ##STR56## wherein L is a leaving group that departs upon thermal fragmentation to unmask --N=C=O for effecting intramolecular acylation of said nitrogen atom to open the N-containing ring and form a new group in the ortho position of ring B that cannot bond to said meso carbon atom; E is hydrogen, an electron-donating group, an electron-withdrawing group or a group, either an electron-donating group or an electron-neutral group, that undergoes fragmentation upon heating to liberate an electron-withdrawing group; s is 0 or 1 and Z and Z' taken individually represent the moieties to complete the auxochromophoric system of a diarylmethane or triarylmethane dye when said N-containing ring is open and Z and Z' taken together represent the bridged moieties to complete the auxochromophoric system of a bridged triarylmethane dye when said N-containing ring is open.