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 (λmax ) and emission (λem ) observed for each subclass of complexes, which differed by their carbon-bound substituents (R), were similar, the emission quantum yields for 7 a-c (R=cyano) were enhanced relative to 8 a-c (R=nitro) and 9 a-c (R=phenyl). Complexes 7 a-c and 8 a-c were also significantly easier to reduce electrochemically to their radical anion and dianion forms compared to 9 a-c. Within each subclass, the o-substituted derivatives were more difficult to reduce, had shorter λmax and λem , 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 π-conjugation. The m-substituted complexes were the least difficult to reduce and possessed intermediate λmax , λem , and quantum yields. The complexes studied also exhibited large Stokes shifts (82-152 nm, 2143-5483 cm(-1) ). Finally, the utility of complex 7 c (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 7 c and 4',6-diamino-2-phenylindole (DAPI) allowed for simultaneous imaging of the cytoplasm and nucleus of mouse fibroblast cells.