threshold voltage include compounds of the formula ##STR3## wherein R is an alkyl group of 1-20 carbon atoms, X.sub.1 and X.sub.3 can be hydrogen ornitro and X.sub.2 can be hydrogen, --NO.sub.2, ##STR4## or -O-CH.sub.2 CN, with the proviso that at least one of X.sub.1 -X.sub.3 is a radical other than hydrogen.