A compound having formula II wherein X 1 to X 5 are independently of each other C or N, R 1 and R 2 are independently of each other H, Hal, -OR',-NR''R''', C 1 -C 12 alkyl, C 1 -C 12 alkoxy, C 1 -C 12 alkanoyl, C 2 -C 12 alkenyl, C 2 -C 12 alkynyl, (C 1 -C 12 alkoxy) carbonyl, and (C 1 -C 12 alkylamino) carbonyl, wherein R' is H or C 1 -C 6 alkyl, and wherein R'' and R''' are independently of each other selected from H, formyl, straight chain or branched C 1 -C 12 alkyl, which is unsubstituted or substituted by at least one CN, Hal, or NO 2, and wherein one or more of embedded, non-adjacent CH 2 groups may independently be replaced by -O-, -CO-, -CO-O-, -CO-NR'-, -CH=CH-, -C=C-, wherein R' is H or C 1 -C 6 alkyl, R 3, R 4 are independently of each other H, formyl, trifluoroacetyl, iminomethyl, nitroso, straight chain or branched C 1 -C 12 alkyl, which is unsubstituted or substituted by at least one CN, Hal, or NO 2, and wherein one or more of embedded, non-adjacent CH 2 groups may independently be replaced by -O-, -CO-, -CO-O-, -CO-NR'-, -CH=CH-, -C=C-, wherein R' is H or C 1 -C 6 alkyl, or R 3 and R 4 form together a C 1 or C 2 -bridge between X 3 and X 5, R 5 is H, C 1 -C 12 alkyl, C 1 -C 12 alkoxy, C 1 -C 12 alkanoyl, C 2 -C 12 alkenyl, C 2 -C 12 alkynyl, (C 1 -C 12 alkoxy) carbonyl, and (C 1 -C 12 alkylamino) carbonyl, m is 0 or 1, p is 0, 1 or 2, q has a value of 1 to 7 X a, X b are independently of each other C, N, O, S, R a, R b are independently of each other H or straight-chain or branched C 1 -C 12 alkyl, C 3 -C 6 cycloalkyl, C 5 -C 14 aryl or C 5 -C 14 heteroaryl, which independently of each other are unsubstituted or substituted by at least one CN, Hal, or NO 2, and wherein one or more of embedded, non-adjacent CH 2 groups may independently be replaced by -O-, -CO-,-CO-O-, -CO-NR'-, -SO 2 -, -CH=CH-, -C≡C-, wherein R' is H or C 1 -C 6 alkyl; and Z 1, Z 2 are independently of each other H or 18 F, with the proviso that one of Z 1 and Z 2 is 18 F. X 1 to X 5 are independently of each other C or N, R 1 and R 2 are independently of each other H, Hal, -OR',-NR''R''', C 1 -C 12 alkyl, C 1 -C 12 alkoxy, C 1 -C 12 alkanoyl, C 2 -C 12 alkenyl, C 2 -C 12 alkynyl, (C 1 -C 12 alkoxy) carbonyl, and (C 1 -C 12 alkylamino) carbonyl, wherein R' is H or C 1 -C 6 alkyl, and wherein R'' and R''' are independently of each other selected from H, formyl, straight chain or branched C 1 -C 12 alkyl, which is unsubstituted or substituted by at least one CN, Hal, or NO 2, and wherein one or more of embedded, non-adjacent CH 2 groups may independently be replaced by -O-, -CO-, -CO-O-, -CO-NR'-, -CH=CH-, -C=C-, wherein R' is H or C 1 -C 6 alkyl, R 3, R 4 are independently of each other H, formyl, trifluoroacetyl, iminomethyl, nitroso, straight chain or branched C 1 -C 12 alkyl, which is unsubstituted or substituted by at least one CN, Hal, or NO 2, and wherein one or more of embedded, non-adjacent CH 2 groups may independently be replaced by -O-, -CO-, -CO-O-, -CO-NR'-, -CH=CH-, -C=C-, wherein R' is H or C 1 -C 6 alkyl, or R 3 and R 4 form together a C 1 or C 2 -bridge between X 3 and X 5, R 5 is H, C 1 -C 12 alkyl, C 1 -C 12 alkoxy, C 1 -C 12 alkanoyl, C 2 -C 12 alkenyl, C 2 -C 12 alkynyl, (C 1 -C 12 alkoxy) carbonyl, and (C 1 -C 12 alkylamino) carbonyl, m is 0 or 1, p is 0, 1 or 2, q has a value of 1 to 7 X a, X b are independently of each other C, N, O, S, R a, R b are independently of each other H or straight-chain or branched C 1 -C 12 alkyl, C 3 -C 6 cycloalkyl, C 5 -C 14 aryl or C 5 -C 14 heteroaryl, which independently of each other are unsubstituted or substituted by at least one CN, Hal, or NO 2, and wherein one or more of embedded, non-adjacent CH 2 groups may independently be replaced by -O-, -CO-,-CO-O-, -CO-NR'-, -SO 2 -, -CH=CH-, -C≡C-, wherein R' is H or C 1 -C 6 alkyl; and Z 1, Z 2 are independently of each other H or 18 F, with the proviso that one of Z 1 and Z 2 is 18 F.