The present invention provides a receptor element for use in thermal transfer imaging. The receptor element includes a coating having a polymeric binder and a biguanide bleaching agent. The biguanide bleaching agent is capable of bleaching an infrared-absorbing dye when the biguanide bleaching agent and the infrared-absorbing dye are in contact. A particularly suitable biguanide bleaching agent is 1-(o-tolyl)biguanide. The invention also provides compositions and methods for manufacturing a receptor element. Also provided by the invention is an imaging system for thermal transfer imaging. The imaging system includes a color-bearing element and a bleaching element, wherein the bleaching element includes a coating having a polymeric binder and a biguanide bleaching agent. The invention further provides methods useful in the production of integral proofs.