BODIPY‐Based Profluorescent Probes Containing Meso‐ and β‐Substituted Isoindoline Nitroxides

Abstract

BODIPY is a highly versatile fluorophore for biological imaging with a tunable fluorescence emission (500–800 nm) that overlaps the optically transparent window for tissue (600–1300 nm). Herein, we describe the synthesis of optically distinct BODIPY‐based profluorescent probes bearing meso‐ and β‐substituted isoindoline nitroxides and their corresponding methoxyamine derivatives. These profluorescent nitroxide probes possess strongly suppressed fluorescence, which can be revealed upon reduction or reaction with other radicals. Examination of the pentafluorophenylhydrazine reduction of the prepared probes by using tandem electron paramagnetic resonance (EPR) and fluorescence spectroscopy demonstrated that the asymmetric bis‐β‐substituted probe 9 (λem = 603 nm) was reduced the fastest; however, the greatest difference in fluorescence emission between the nitroxide and its reduced hydroxylamine analogue was observed for probe 7 (λem = 570 nm). The significant difference in fluorescence output between the nitroxides and their corresponding diamagnetic derivatives makes these probes ideal tools for imaging reactive oxygen species in biological systems.

Profluorescent Fluoroquinolone-Nitroxides for Investigating Antibiotic–Bacterial Interactions

“…Extinction coefficients were calculated from the obtained absorbance spectra. This is a standardised method and our values are consistent with the values reported for other profluorescent nitroxides using the same method [17,30,48].…”

Unexpected Nucleophilic Substitution Reaction of BODIPY: Preparation of the BODIPY–TEMPO Triad Showing Radical‐Enhanced Intersystem Crossing

“…However, triplet‐state formation with BODIPY based on mechanisms other than the heavy‐atom effect has rarely been reported , , , . Previously, TEMPO (2,2,6,6‐tetramethyl‐1‐piperidinyloxyl) was attached to BODIPY or other chromophores to tune the fluorescence properties for fluorescent molecular probe applications, but triplet formation was not studied . The verdazyl radical has been attached to BODIPY chromophores, but again the triplet states of the BODIPY chromophores were not reported .…”

Synthesis, Structure, and Fluorescence Behavior of Profluorescent 8‐Amino BODIPY Nitroxides

“…The on/off fluorescence performance of the dyes examined here is comparable with previously synthesized tetramethyl substituted BODIPY profluorescent nitroxides where C‐C linkages are used. [13a], [13b], The ϕ N ‐alkoxyamine /ϕ NO · for meso substituted profluorescent nitroxides 1 – 3 is also on par with that found for isoindoline BODIPY nitroxides with C‐C linkages . The performance of these new dyes is significantly better than previously synthesized BODIPY TEMPO conjugates using phenyl “spacers,” utilizing alkyne–azide cycloaddition, ester or amide linkages to attach the BODIPY and TEMPO moieties.…”

“…The strength of this fluorescence quenching is influenced by proximity of the fluorophore to the nitroxide radical, and is enhanced in cases where the nitroxide and fluorophore are π conjugated and/or attached by rigid C–C bonds rather than flexible linkages . However, synthetic routes to such compounds can be cumbersome, thus synthesis involving accessible 2,2,6,6‐tetramethyl‐piperidine‐1‐oxyl (TEMPO) derivatives is appealing for probe design. Relatively little is understood about the precise geometric parameters important in designing profluorescent nitroxides with high quenching efficiencies …”

“…To better understand the molecular and environmental properties that lead to efficient fluorescence quenching, the syntheses of profluorescent 8‐amino BODIPY nitroxides 1 , 2 , and 3 and their fluorescent N ‐ethoxyamine analogs 4 , 5 , and 6 (Figure ) was undertaken based on methodology developed by Bañuelos . In general, BODIPY dyes have high photo‐ and chemical stability, “tunable” excitation and emission wavelengths ( λ em / λ ex ), large extinction coefficients ( ε ) and high fluorescence quantum yields (ϕ), making them attractive for a wide variety of sensing applications , , . Profluorescent nitroxides 1 , 2 , and 3 were selected for their differences in predicted geometries and λ em / λ ex , to explore the effects of variable bond angles and distances on spectroscopic properties.…”