A benzofuranone of the Formula (1): ##STR1## wherein: R.sup.1 and R.sup.2 are each independently --H or optionally substituted C.sub.1-20 -alkyl; R.sup.3 and R.sup.4 are each independently --H or optionally substituted C.sub.1-6 -alkyl; R.sup.5 is selected from --H, --CN, --COOR.sup.10 and --COR.sup.10, in which R.sup.10 is --H or C.sub.1-6 -alkyl; R.sup.6, R.sup.7, R.sup.8 and R.sup.9 are each independently selected from --H, --NR.sup.1 R.sup.2, --NO.sub.2, halogen, optionally substituted C.sub.1-6 -alkyl and optionally substituted C.sub.1-6 -alkoxy.