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

Abstract

Abstract:The 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 analogs 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152 nm, 21435483 cm 1 ). Finally, the utility of complex 7c (Ar = panisole, 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.

Aggregation-induced emission enhancement in boron difluoride complexes of 3-cyanoformazanates

“…The trends observed for the absorption/emission maxima are consistent with a previous report where the presence of ortho substituents led to twisting of the N-aryl substituents relative to the formazanate backbone and a blue-shift in absorption/emission maxima. 22 A decrease in the  F was also observed for the same series of compounds: 5a ( F = 0.46), 5b ( F = 0.03) and 5c ( F < 0.01) (Figs. S5, S6, and Table S1).…”

“…BF 2 formazanates have been incorporated into polymers, 19 explored as electrochemiluminescent materials, 20 used as precursors to B(I) carbenoid intermediates, 21 and employed as fluorescent cell-imaging agents. 22 Herein, we report the first examples of BF 2 formazanates that demonstrate AIEE. BF 2 complex 5c was prepared in 96% yield according to a well-established synthetic methodology and the synthesis and characterization of 5a and 5b have been reported elsewhere.…”

“…The results were compared to data obtained for 5a (E°r ed1 = −0.68 V, E°r ed2 = −1.82 V) and 5b (E°r ed1 = −0.75 V, E°r ed2 = −1.92 V) and demonstrated that the introduction of ortho methoxy substituents made electrochemical reduction more difficult due to a less planar π system. 16,22 Exploration of the AIEE behaviour of 5a−c in THF-water mixtures with different volume fractions of water (f w ) revealed an interesting trend ( Fig. 1 and Table S1).…”

“…BF 2 complex 5c was prepared in 96% yield according to a well-established synthetic methodology and the synthesis and characterization of 5a and 5b have been reported elsewhere. 16,22 We chose these methoxy-substituted compounds for this study due to their superior emission properties, specifically, their relatively higher fluorescence quantum yields ( F ) relative to other substituted BF 2 formazanate derivatives. 22 Complex 5c…”

“…23 The solid-state structures of 5a−c are comprised of carbon-nitrogen and nitrogen-nitrogen bonds with lengths between those of typical single and double bonds of the respective atoms, indicating a delocalized formazanate backbone consistent with previous reports (Tables S2 and S3). 22 In addition, the dihedral angles between the N-aryl substituents and the formazanate backbone deviate from planarity from 4-methoxyphenyl (5a: 15.3°, 21.4°; 47.7°, 47.7°) ‡ to 2- Fig. 1 Normalized UV-vis absorption spectra of 5a (a), 5b (b), and 5c (c) in pure THF solution (black line) and in a THF-water mixture with water volume fraction (fw) of 95% (red line) ([5a−c] = 250 μM).…”