Compounds of formula I are disclosed:wherein X1, X2, X3, X4 are independently H, F, Cl, Br, I, CN, NO2, OR1, SR1, NR1R2, COR1, COOR1, CONR1R2, PO3R1R2, SO2R1, SO3R1 or R3; R1 and R2 are, e.g., H,
alkyl or
aryl or optionally a ring; R3 is, e.g.,
alkyl, alkenyl, alkynyl,
aryl or cycloalkyl; Y is OR1, NR1R2, or NR1R3; Q is O, S, SO2, NR, C(R3)2, Si(R3)2, Ge(R3)2, P(═O)R3 or P(═O)OR3; Q and X1 can optionally form part of a ring; L and M are independently OR1, SR1, NR1R2 and R3; L and M can optionally form part of a ring; Z is O, S, NR1, CR1R3 or
aryl; and Z and X4 can optionally form part of a ring.