A heat-sensitive imaging element includes an IR dye having a structure according to Formula I, wherein at least one of the Rd groups is a group which is transformed by a chemical reaction, induced by exposure to IR-radiation or heat, into a group which is a stronger electron-donor than the Rd; or wherein at least one of the Ra groups is a group which is transformed by a chemical reaction, induced by exposure to IR-radiation or heat, into a group which is a stronger electron-acceptor than the Ra. The imaging element is able to form a visible print-out image with a high contrast directly after image-wise exposure by IR-radiation or heating.