A bright application…

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.^[6] Figure 1 shows some commercially available BODIPY dyes used as biological probes.

Figure 1. Commercially available BODIPY dyes.^[7] Bottom Right: Selective staining of the Golgi apparatus using BODIPY derivatives.^[8]

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 relatively featureless, so instead we’re going to focus on the far more interesting boron and fluorine spectra. These nuclei are very useful for the characterization of these types of compounds. Figure 2 shows the ¹¹B NMR spectrum acquired in our 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.^[9] 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.

Figure 2. Left: ¹¹B NMR spectrum of 1. Right: ¹⁹F NMR spectrum of 1.

As expected, the fluorine atoms are coupled to boron, and the ¹⁹F NMR spectrum shows one resonance signal with four peaks (¹J_B-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.

If you are 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!

^[1] Lakowicz, J. R. Principles of Fluorescence Spectroscopy; 4th ed.; Springer, 2006
^[2] Loudet, A.; Burgess, K. Chem. Rev. 2007, 107, 4891
^[3] Li, F.; Yang, S. I.; Ciringh, Y.; Seth, J.; Martin, C. H.; Singh, D. L.; Kim, D.; Birge, R. R.; Bocian, D. F.; Holten, D.; Lindsey, J. S. J. Am. Chem. Soc. 1998, 120, 10001.
^[4] Sathyamoorthi, G.; Wolford, L. T.; Haag, A. M.; Boyer, J. H. Heteroatom Chemistry 1994, 5, 245
^[5] Golovkova, T. A.; Kozlov, D. V.; Neckers, D. C. J. Org. Chem. 2005, 70, 5545.
^[6] Haugland, R. P. Handbook of Molecular Probes and Research Products; 9th ed.; Molecular Probes: OR, 2002.
^[7] Molecular Probes™ Handbook A Guide to Fluorescent Probes and Labeling Technologies 11th ed, 2010.
^[8] Pagano, R. E; Martin, O. C.; Kang H. C.; Haugland, R. P. J. Cell. Bio. 1991, 113, 1267
^[9] Smith, W. L. J. Chem. Edu. 1977, 54, 469.