A compound having Formula I wherein one of R.sub.1 and R.sub.2 is a group of the formula wherein R.sub.4 is selected from H and hydrocarbyl, R.sub.5 is a hydrocarbyl group and L is an optional linker group, or R.sub.1 and R.sub.2 together form a ring substituted with the group wherein R.sub.3 is H or a substituent, and wherein X is selected from S, O, NR.sub.6 and C(R.sub.7)(R.sub.8), wherein R.sub.6 is selected from H and hydrocarbyl groups, wherein each of R.sub.7 and R.sub.8 are independently selected from H and hydrocarbyl groups.