Evaluation of Anisole-Substituted Boron Difluoride Formazanate Complexes for Fluorescence Cell Imaging

Evaluation of three subclasses of boron difluoride formazanate complexes bearing o-, m-, and p-anisole N-aryl substituents (Ar) as readily accessible alternatives to boron dipyrromethene (BODIPY) dyes for cell imaging applications is described. While the wavelengths of maximum absorption ((max)) and emission ((em)) observed for each subclass of complexes, which differed by their carbon-bound substituents (R), were similar, the emission quantum yields for 7a-c (R=cyano) were enhanced relative to 8a-c (R=nitro) and 9a-c (R=phenyl). Complexes 7a-c and 8a-c were also significantly easier to reduce electrochemically to their radical anion and dianion forms compared to 9a-c. Within each subclass, the o-substituted derivatives were more difficult to reduce, had shorter (max) and (em), and lower emission quantum yields than the p-substituted analogues as a result of sterically driven twisting of the N-aryl substituents and a decrease in the degree of -conjugation. The m-substituted complexes were the least difficult to reduce and possessed intermediate (max), (em), and quantum yields. The complexes studied also exhibited large Stokes shifts (82-152nm, 2143-5483cm(-1)). Finally, the utility of complex 7c (Ar=p-anisole, R=cyano), which can be prepared for just a few dollars per gram, for fluorescence cell imaging was demonstrated. The use of 7c and 4,6-diamino-2-phenylindole (DAPI) allowed for simultaneous imaging of the cytoplasm and nucleus of mouse fibroblast cells.