The compound according to claim 25, wherein R1 is an aryl group selected from the group consisting of phenyl, naphthyl, indanyl and tetrahydronaphthalenyl, wherein each of the aryl groups is independently unsubstituted or substituted with one to five independently selected R20 moieties which can be the same or different, each R20 moiety being independently selected from the group of R20 moieties.