Compounds of formula (Ia) and (Ib) wherein A is CH₂, or a single bond; R₂ is selected from: R, OH, OR, CO₂H, CO₂R, COH, COR, SO₂R, CN; R₆, R₇ and R₉ are independently selected from H, R, OH, OR, halo, amino, NHR, nitro, Me₃Sn; and R₈ is selected from H, R, OH, OR, halo, amino, NHR, nitro, Me₃Sn, where R is as defined above, or the compound is a dimer with each monomer being the same or different and being of formula (Ia) or (Ib), where the R₈ groups of the monomers form together a bridge having the formula —X—R—X— linking the monomers, where R is an alkylene chain containing from 3 to 12 carbon atoms, which chain may be interrupted by one or more hetero-atoms and/or aromatic rings and may contain one or more carbon—carbon double or triple bonds, and each X is independently selected from O, S, or N; except that in a compound of formula (Ia) when A is a single bond, then R₂ is not CHCH(CONH₂) or CHCH(CONMe₂). Other related compounds are also disclosed ##STR1##