BODIPY dyes, which are boron difluoride compounds supported by dipyrrinato ligands, have gained recognition as being one of the more versatile fluorophores due to their superior photophysical properties.[1,2] BODIPY derivatives are used as stable functional dyes in several fields such as light harvesters, laser dyes, fluorescent switches, and biomolecular labels.[3-6] They gained popularity as biological probes due to the easy modification of the ligand framework, extension of the chromophore, and substitution of the fluorine atoms. Figure 1 shows some commercially available BODIPY dyes used as biological probes.
For this blog entry we are going to focus on the NMR properties of these compounds, and we are going to use compound 1 as an example. The proton spectrum is not particularly interesting, so instead we’re going to focus on the far more interesting boron and fluorine spectra. These are very important to characterize these types of compounds. Figure 2 shows the 11B NMR spectrum acquired in our NMReady-60PRO. There are two NMR active nuclei for boron, 10B and 11B. The latter nucleus was chosen for NMR measurements due to its greater natural abundance, its lower quadrupole moment, and because its NMR spectra are sharper. The 11B NMR spectrum shows a 1:2:1 triplet due to the coupling with two equivalent fluorine atoms, with the resonance peak centered at 0.8 ppm and a one bond 11B-19F coupling constant of 32 Hz.
As expected, the fluorine atoms are coupled to boron, and the 19F NMR spectrum shows one resonance signal with four peaks (1JB-F = 32 Hz). Why do we see a quartet with a 1:1:1:1 pattern and not the common 1:3:3:1 quartet? The rule for predicting the number of lines observed is giving by the following formula: 2nI + 1, where n is the number of equivalent nuclei, and I is the nuclear spin of the coupling nucleus. In this particular case n is 1 (there is only one boron center) and I is 3/2, which gives a 1:1:1:1 quartet.
Interested in heteronuclear NMR or have any questions about incorporating 60 MHz NMR spectrometers into your research program, please contact us to find if the NMReady benchtop NMR spectrometer will be suitable for your chemistry!
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