Preferred substituents for the 4-position in phenyl and naphth-1-yl and on the 5-position in naphth-2-yl include halogen, especially fluoro and chloro and --SR5 and --SOR5 wherein R3 is preferably a C1-2 alkyl, more preferably methyl; of which the fluoro and chloro is more preferred, and most especially preferred is fluoro.