A compound having Formula I R.sub.1--Z--R.sub.2Formula I wherein R.sub.1
is an optionally substituted phenyl ring; R.sub.2 is or comprises an
optionally substituted aromatic ring; and Z is --X--Y--L-- or --Y--X--L--
wherein either X is selected from --S(.dbd.O)(.dbd.O)-- and
--C(.dbd.O)--; and Y is --NR.sub.3--; or X is selected from
--S(.dbd.O)(.dbd.O)-- and --S--, and Y is --C(R.sub.4)(R.sub.5)--; L is
an optional linker; and R.sub.3, R.sub.4 and R.sub.5 are each
independently selected from H and hydrocarbyl; and wherein when R.sub.2
comprises the following structural moiety ##STR00001## wherein Q is an
atom selected from the group consisting of S, O, N and C; the compound is
selected from compounds of the formulae
R.sub.1--C(.dbd.O)--NR.sub.3--L--R.sub.2;
R.sub.1--S(.dbd.O)(.dbd.O)--C(R.sub.4)(R.sub.5)--L--R.sub.2;
R.sub.1--S--C(R.sub.4)(R.sub.5)--L--R.sub.2;
R.sub.1--NR.sub.3--S(.dbd.O)(.dbd.O)--L--R.sub.2;
R.sub.1--NR.sub.3--C(.dbd.O)--L--R.sub.2;
R.sub.1--C(R.sub.4)(R.sub.5)--S(.dbd.O)(.dbd.O)--L--R.sub.2; and
R.sub.1--C(R.sub.4)(R.sub.5)--S--L--R.sub.2.
