Anionic infrared-ray absorbing agent, photosensitive composition and planographic printing plate precursor using same

A photosensitive composition for a planographic printing plate that comprises the following components (a) and (b) and becomes soluble in an aqueous alkaline solution by heat generated by irradiation with an infrared laser:

• (a) the anionic, infrared-ray absorbing agent;
• and
• (b) a polymeric compound insoluble in water but soluble in an aqueous alkaline solution, wherein the anionic, infrared-ray absorbing agent includes a chemical structure that substantially absorbs infrared-ray radiation and generates heat, the chemical structure having a negative electric charge and the anionic infrared-ray absorbing agent is a compound represented by the following general formula (I):
• {Ga−—M—Gb}m Xm+ (I)
• wherein, Ga − represents an anionic substituent group which includes an oxygen atom, a sulfur atom or a carbon atom;
• M represents a conjugated chain of carbon atoms;
• Gb represents a neutral substituent group;
• and Xm+ represents a cation including a proton, said cation having a valence of from 1 to m, wherein m represents an integer ranging from 1 to 6.