Abstract: A compound of formula I wherein Ar1 is a bicyclic aryl or heteroaryl, which may be optionally substituted; X and Y are each independently a group of formula II: -L1-Ar2-L2-Ar3-Q-(CH2)n-NR1R2 L1 and L2 are each independently selected from NR3, C2H2, CH2, —O—, —S— and a bond; Ar2 and Ar3 are independently optionally substituted C5 or C6 aryl or heteroaryl; Q is selected from NH(C?O), NR3, S, O; n is an integer from 1 to 5; R1 and R2 are optionally substituted and are independently hydrogen, C1-7 alkyl, C3-20 heterocyclyl, or C5-20 aryl, or R1 and R2, taken together with the nitrogen atom to which they are attached, form a heterocyclic ring having from 3 to 8 ring atoms; R3 is H or C1-7 alkyl.