Y and Z are independently selected from the group consisting of O, S, CH, CH2 and NR8, wherein R8 is selected from the group D consisting of C1 to C6 alkyl, C1 to C6 acyl and hydrogen; R5 is selected from the group A; and R6 and R7 are independently selected from the group consisting of hydrogen and C1 to C6 alkyl, with the proviso that when Y in formula (V) is NR8, Z is a carbon atom; or B and B?