Abstract: Compounds according to general formulae (Ia to Ie) wherein: X?O or S; Y is O, S or CH2, CHR, CRR, where R is C1-7 alkyl; Z is O or S; R1 is H or C1-7 alkyl; R2 is H or C1-7 alkyl; R4 is H or C1-7 alkyl at any vacant position on the aromatic ring; R3 is C1-7 alkyl-L1-R5-L2-R6—COOH, C3-10 cycloalkyl-L1-R5-L2-R6—COOH or Ar—C0-7 alkyl-L1-R5-L2-R6—COOH; each of L1 and L2 is absent or a suitable linker such as an amide CONH; or an ether —O—, or a thioether —S— or a sulphone —SO2—; R5 is C1-7 alkyl, C3-10 cycloalkyl or Ar—C0-7 alkyl each of which is substituted with either NR8R9, where the nitrogen atom is capable of being protonated in solution to give N+HR8R9; or a quaternary nitrogen atom N+R8R9R10, such that R5 contains a positive charge; each of R8, R9 and R10 is independently C1-7 alkyl, C3-10 cycloalkyl or Ar—C0-7 alkyl, or any two or more of R8, R9 and R10 together form an alicyclic or arylalicyclic ring system; R6 is C1-7 alkyl, C3-10 cycloalkyl or Ar—C0-7 alkyl; and their salts, hydrates, solvates, complex