In general, it is preferable to use a base in a heat-developable light-sensitive material for the purpose of accelerating the heat development of said material, and it is necessary to incorporate said base in said light-sensitive material in the form of a base precursor for increasing the stability of said material. In order to use such base precursor practically, which means herein a compound capable of being decomposed under heat to be able to release a basic component, it is necessary that said base precursor must have both stability at normal temperature and rapid decomposability under heat.
Various kinds of conventional base precursors have heretofore been known including, for example, ureas as described in U.S. Pat. No. 2,732,299 and Belgian Patent No. 625,554; urea or ammonium salts of urea and weak acid as described in Japanese Patent Publication No. 1699/65; hexamethylene-tetramines or semicarbazides as described in U.S. Pat. No. 3,157,503; combinations of triazine compound and carboxylic acid as described in U.S. Pat. No. 3,493,374; dicyan-diamide derivatives as described in U.S. Pat. No. 3,271,155; N-sulfonylureas as described in U.S. Pat. No. 3,420,665; amine-imides as described in "Research Disclosure" (1977), RD No. 15776; and salts of decomposable acids such as trichloroacetic acid as described in British Patent No. 998,949.
However, image forming materials containing such conventional base precursors have serious defects. In particular, such conventional base precursors do not fully satisfy the properties desired both with respect to high stability during preservation at normal temperature (e.g., 0.degree.-30.degree. C.) and rapid decomposability during development treatment under heat. Therefore, a high and sufficient image density cannot be obtained, or the S/N (signal/noise) value of the formed image extremely decreases, as the base component tends to be released from said base precursor during preservation thereof, and these phenomena are extremely serious problems.