wherein X is OH, C1-C6 alkoxyl; Y and Z are independently selected from a C1-C6 alkyl, C1-C6 alkoxyl, halogen, un-substituted or C1-C6 substituted amine, 18F, 19F, or H; and n is 1, 2, or 3; and wherein for structure (I), if n are both 1 and Y and Z are both H and X is OH.