Compounds of the formulae Ia and Ib: ##STR00001## wherein: A is CH.sub.2, or a single bond; R.sub.2 is selected from: R, OH, OR, CO.sub.2H, CO.sub.2R, COH, COR, SO.sub.2R, CN; R.sub.6, R.sub.7 and R.sub.9 are independently selected from H, R, OH, OR, halo, amino, NHR, nitro, Me.sub.3Sn; and R.sub.8 is selected from H, R, OH, OR, halo, amino, NHR, nitro, Me.sub.3Sn, 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 R.sub.8 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 R.sub.2 is not CH.dbd.CH(CONH.sub.2) or CH.dbd.CH(CONMe.sub.2). Other related compounds are also disclosed.