A compound of the formula II: ##STR00001## wherein: R'.sub.2 is CHR''.sub.2, where R''.sub.2 is H. R.sub.6 and R.sub.9 are H and R.sub.7 is OMe and the compound is a dimer with each monomer being the same and being of formula II, where the R.sub.8 groups of the monomers form together a bridge having the formula --O--(CH.sub.2).sub.p--O--, where p is 5, linking the monomers.