A compound of formula IV: and salts and solvates thereof, wherein: R 2 and R 2' are H; R 6 and R 9 are independently selected from H, R, OH, OR, SH, SR, NH 2, NHR, NRR', nitro, Me 3 Sn and halo; where R and R' are independently selected from optionally substituted C 1-12 alkyl, C 3-20 heterocyclyl and aromatic ring systems comprising 5-20 ring atoms; either (a) R 7 is selected from H, R, OH, OR, SH, SR, NH 2, NHR, NRR', nitro, Me 3 Sn and halo; R 7' is selected from H, R, OH, OR, SH, SR, NH 2, NHR, NRR', nitro, Me 3 Sn and halo; or (b) R 7 and R 7' together form a group which is: (i) -O-(CH 2) n -O-, where n is from 7 to 16; or (ii) -O-(CH 2 CH 2 O) m -, where m is 2 to 5; R" is a C 3-12 alkylene group, which chain may be interrupted by one or more heteroatoms, selected from O, S, NR N2 where R N2 is H or C 1-4 alkyl, and/or by aromatic rings, selected from benzene or pyridine; Y and Y' are selected from O, S, or NH; R 6' and R 9' are selected from the same groups as R 6, and R 9 respectively; either (i-a) R 10 and R 11 together form a double bond between the N and C atoms to which they are bound; or (i-b) R 10 is H and R 11 is selected from OH and OR A, where R A is C 1-4 alkyl; or (i-c) R 10 and R 11 are both H; either (ii-a) R 20 and R 21 together form a double bond between the N and C atoms to which they are bound; or (ii-b) R 20 is H and R 21 is selected from OH and OR B, where R B is C 1-4 alkyl; or (ii-c) R 20 and R 21 are both H. R 2 and R 2' are H; R 6 and R 9 are independently selected from H, R, OH, OR, SH, SR, NH 2, NHR, NRR', nitro, Me 3 Sn and halo; where R and R' are independently selected from optionally substituted C 1-12 alkyl, C 3-20 heterocyclyl and aromatic ring systems comprising 5-20 ring atoms; either (a) R 7 is selected from H, R, OH, OR, SH, SR, NH 2, NHR, NRR', nitro, Me 3 Sn and halo; R 7' is selected from H, R, OH, OR, SH, SR, NH 2, NHR, NRR', nitro, Me 3 Sn and halo; or (b) R 7 and R 7' together form a group which is: (i) -O-(CH 2) n -O-, where n is from 7 to 16; or (ii) -O-(CH 2 CH 2 O) m -, where m is 2 to 5; (a) R 7 is selected from H, R, OH, OR, SH, SR, NH 2, NHR, NRR', nitro, Me 3 Sn and halo; R 7' is selected from H, R, OH, OR, SH, SR, NH 2, NHR, NRR', nitro, Me 3 Sn and halo; or (b) R 7 and R 7' together form a group which is: (i) -O-(CH 2) n -O-, where n is from 7 to 16; or (ii) -O-(CH 2 CH 2 O) m -, where m is 2 to 5; R" is a C 3-12 alkylene group, which chain may be interrupted by one or more heteroatoms, selected from O, S, NR N2 where R N2 is H or C 1-4 alkyl, and/or by aromatic rings, selected from benzene or pyridine; Y and Y' are selected from O, S, or NH; R 6' and R 9' are selected from the same groups as R 6, and R 9 respectively; either (i-a) R 10 and R 11 together form a double bond between the N and C atoms to which they are bound; or (i-b) R 10 is H and R 11 is selected from OH and OR A, where R A is C 1-4 alkyl; or (i-c) R 10 and R 11 are both H; (i-a) R 10 and R 11 together form a double bond between the N and C atoms to which they are bound; or (i-b) R 10 is H and R 11 is selected from OH and OR A, where R A is C 1-4 alkyl; or (i-c) R 10 and R 11 are both H; either (ii-a) R 20 and R 21 together form a double bond between the N and C atoms to which they are bound; or (ii-b) R 20 is H and R 21 is selected from OH and OR B, where R B is C 1-4 alkyl; or (ii-c) R 20 and R 21 are both H. (ii-a) R 20 and R 21 together form a double bond between the N and C atoms to which they are bound; or (ii-b) R 20 is H and R 21 is selected from OH and OR B, where R B is C 1-4 alkyl; or (ii-c) R 20 and R 21 are both H.