A compound with the formula I, wherein R2 is of formula ΙI, where A is a C5-7 aryl group, X is selected from the group consisting of : OH, SH, CO2H, COH, N=C=0, NHNH2, CONHNH2, (III), (IV), and NHRN, wherein RN is selected from H and C1-4 alkyl, and either : (i) Q1 is a single bond, and Q2 is selected from a single bond and -Z-(CH2)n-, where Z is selected from a single bond, O, S and NH and n is from 1 to 3; or (ii) Q1 is -CH=CH-, and Q2 is a single bond; and its conjugates.