A compound having a structure represented by formula I: wherein each R1 is independently H, alkynyl or substituted alkynyl; each R2 is independently H, alkynyl or substituted alkynyl; each R3 is independently alkyl or halogen; each R4 is independently H, alkyl, or halogen; each R5 is independently H or halogen; n is 0 or 1; and m is 0 or 1, provided that if n is 0 then m is 0, and if n is 1 then m is 1.