A novel perylenediimide derivative which has a bay region hydroxyl function behaves as a pH-sensitive dye. The deprotonation of the aryl-OH yields a green solution with a lambda(max) x of 692 nm and this deprotonated form has strong internal charge transfer characteristics. On reprotonation in acidic solutions, the typical PDI absorbance spectrum is restored. The experimentally determined pK(a) (7.45) is very close to physiological pH, therefore the most significant changes in color take place in the vicinity of this pH value. (c) 2005 Elsevier Ltd. All rights reserved.