A compound of formula or a pharmaceutically acceptable salt thereof: wherein R 1 and R 2 are independently selected from F, I, Br, and At, m and n are integers independently selected from 0 and 1; Ar 1 and Ar 2 are independently selected from aryl and heteroaryl, Ar 1 and Ar 2 each being optionally substituted by one or more substituents selected, independently for each occurrence, from halogen, hydroxy, nitro, cyano, alkyl, haloalkyl, alkenyl, alkynyl, amino, alkylaryl, alkylamino, alkylamine, alkoxy, aryloxy; -(O-CH 2 -CH 2) o -R 10, wherein R 10 is selected from -H, -OH, -OSO 2 alkyl, -OSO 2 aryl, and -F, and o is an integer from 1 to 4; -NR 11 COOalkyl, -NR 11 COOaryCOalkyl, -NR 11 COaryl; -COOalkyl, -COOaryl, -COalkyl, -COaryl, aryl, cycloalkyl; cycloalkylamino-, -cycloalkylamine, heterocycle, a fluorescent label; and -Z-R 12 substitutions wherein Z is selected from O, NR 13, NH and S; and wherein R 11, R 12 and R 13 are independently selected from H, C n H 2n+1, C n H 2n -hal, with n being an integer of 1 to 3, -CH 2 -CH=CH-hal, and -[CH 2 -CH 2 -O] p -[CH 2 -CH 2] q -hal, wherein hal is selected from F, Cl, I, Br, and At, p is an integer of 1 to 3 and q is 1 or 2; wherein suitable pairs of substituents attached to adjacent ring members of Ar 1 or Ar 2, respectively, can be combined to form a ring fused with Ar 1 or Ar 2; for use in a diagnostic method practiced on the human or animal body wherein a radiolabeled compound of formula or a pharmaceutically acceptable salt thereof, wherein one or more groups selected from R 1, R 2, a substituent attached to Ar 1 and a substituent attached to Ar 2 contain a radioisotope of an element which is present therein, is administered to the human or animal for in vivo diagnosis. wherein R 1 and R 2 are independently selected from F, I, Br, and At, m and n are integers independently selected from 0 and 1; Ar 1 and Ar 2 are independently selected from aryl and heteroaryl, Ar 1 and Ar 2 each being optionally substituted by one or more substituents selected, independently for each occurrence, from halogen, hydroxy, nitro, cyano, alkyl, haloalkyl, alkenyl, alkynyl, amino, alkylaryl, alkylamino, alkylamine, alkoxy, aryloxy; -(O-CH 2 -CH 2) o -R 10, wherein R 10 is selected from -H, -OH, -OSO 2 alkyl, -OSO 2 aryl, and -F, and o is an integer from 1 to 4; -NR 11 COOalkyl, -NR 11 COOaryCOalkyl, -NR 11 COaryl; -COOalkyl, -COOaryl, -COalkyl, -COaryl, aryl, cycloalkyl; cycloalkylamino-, -cycloalkylamine, heterocycle, a fluorescent label; and -Z-R 12 substitutions wherein Z is selected from O, NR 13, NH and S; and wherein R 11, R 12 and R 13 are independently selected from H, C n H 2n+1, C n H 2n -hal, with n being an integer of 1 to 3, -CH 2 -CH=CH-hal, and -[CH 2 -CH 2 -O] p -[CH 2 -CH 2] q -hal, wherein hal is selected from F, Cl, I, Br, and At, p is an integer of 1 to 3 and q is 1 or 2; wherein suitable pairs of substituents attached to adjacent ring members of Ar 1 or Ar 2, respectively, can be combined to form a ring fused with Ar 1 or Ar 2; for use in a diagnostic method practiced on the human or animal body wherein a radiolabeled compound of formula or a pharmaceutically acceptable salt thereof, wherein one or more groups selected from R 1, R 2, a substituent attached to Ar 1 and a substituent attached to Ar 2 contain a radioisotope of an element which is present therein, is administered to the human or animal for in vivo diagnosis.