We report herein a simple and efficient approach to the synthesis of a variety of meso-substituted purpurinimides. The reaction of meso-substituted purpurinimide with
N-bromosuccinimide regioselectively introduced a bromo functionality at the 20-position, which on further reaction with a variety of
boronic acids under Suzuki reaction conditions yielded the corresponding meso-substituted analogues. Interestingly, the free base and the metalated analogues showed remarkable differences in photosensitizing efficacy (
PDT) and
tumor-imaging ability. For example, the free-base conjugate showed significant in vitro
PDT efficacy, but limited
tumor avidity in mice bearing
tumors, whereas the corresponding Ni(II) derivative did not produce any cell kill, but showed excellent
tumor-imaging ability at a dose of 0.3 μmol kg(-1) at 24, 48, and 72 h post-injection. The limited
PDT efficacy of the Ni(II) analogue could be due to its inability to produce
singlet oxygen, a key
cytotoxic agent required for cell kill in
PDT. Based on electrochemical and spectroelectrochemical data in
DMSO, the first one-electron oxidation (0.52 V vs. SCE) and the first one-electron reduction (-0.57-0.67 V vs. SCE) of both the free base and the corresponding Ni(II) conjugates are centered on the cyanine
dye, whereas the second one-electron reduction (-0.81 V vs. SCE) of the two conjugates is assigned to the purpurinimide part of the molecule. Reduction of the cyanine
dye unit is facile and occurs prior to reduction of the purpurinimide group, which suggests that the cyanine
dye unit as an
oxidant could be the driving force for quenching of the excited triplet state of the molecules. An interaction between the cyanine
dye and the purpurinimide group is clearly observed in the free-base conjugate, which compares with a negligible interaction between the two functional groups in the Ni(II) conjugate. As a result, the larger HOMO-LUMO gap of the free-base conjugate and the corresponding smaller quenching constant is a reason to decrease the intramolecular quenching process and increase the production of
singlet oxygen to some degree.