For the avoidance of doubt, the phrase R1 and R2 are each independently selected from the group consisting of: H and a substituted covers the following cases in which (1) R1 is H and R2 is a substituent, (2) R2 is H and R1 is a substituent, (3) R1 is H and R2 is H and (4) R1 is a substituent and R2 is a substituent.