A compound comprising the following formula: wherein R1 is selected from H and a substituted or unsubstituted organic group; R2 may be present or absent and is independently selected from H and a substituted or unsubstituted organic group; R1 and R2 may together form a ring; R3 is independently selected from H and a substituted or unsubstituted organic group; R6 may be present or absent and is independently selected from H and a substituted or unsubstituted organic group; Z1 and Z2 are independently selected from C and N; and L comprises an organic group as defined herein.
A compound comprising the following formula: wherein R1 is selected from H and a substituted or unsubstituted organic group; R2 may be present or absent and is independently selected from H and a substituted or unsubstituted organic group; R1 and R2 may together form a ring; R3 is independently selected from H and a substituted or unsubstituted organic group; R6 may be present or absent and is independently selected from H and a substituted or unsubstituted organic group; Z1 and Z2 are independently selected from C and N; and L comprises an organic group as defined herein.