On the Limited Stability of BDPA Radicals

Abstract

1,3‐Bis(diphenylene)‐2‐phenylallyl (BDPA)‐based radicals are of interest as polarizing agents for dynamic nuclear polarization (DNP). For this purpose, a BDPA‐nitroxide biradical, employing a phosphodiester linkage, was synthesized. Contrary to what is commonly assumed, BDPA‐derived radicals were observed to have limited stability. Hence, the effects of various factors on the stability of BDPA radicals were investigated. Solvent polarity was found to play a significant role on degradation; a polar BDPA radical was observed to degrade faster in a non‐polar solvent, whereas non‐polar radicals were more unstable in polar solvents. The rate of decomposition was found to increase non‐linearly with increasing radical concentration; a 2‐fold increase in concentration led to a 3‐fold increase in the rate of degradation. Collectively, these results indicate that the dimerization is a significant degradation pathway for BDPA radicals and indeed, a dimer of one BDPA radical was detected by mass spectrometry.

1H Thermal Mixing Dynamic Nuclear Polarization with BDPA as Polarizing Agents

“…This asymmetry in the EPR line shape is the result of strong e–e (dipolar as well as exchange) couplings indicating the presence of clustered BDPA radicals. In fact, the clustering of BDPA was recently confirmed to occur by a combination of EPR and Mass Spectroscopy measurements, where an instability originating from radical clustering/aggregation was observed with concentrated BDPA and its derivatives (tested up to 30 mM) prepared in a series of organic solvents above −80 °C . As previously discussed by Karabanov et al and later confirmed by Equbal et al, the asymmetric EPR line is one of the distinctive features of TM DNP.…”

“…In fact, the clustering of BDPA was recently confirmed to occur by a combination of EPR and Mass Spectroscopy measurements, where an instability originating from radical clustering/aggregation was observed with concentrated BDPA and its derivatives (tested up to 30 mM) prepared in a series of organic solvents above −80 °C. 37 As previously discussed by Karabanov et al 17 and later confirmed by Equbal et al, 42 the asymmetric EPR line is one of the distinctive features of TM DNP. Asymmetry is necessary to maintain a large polarization gradient across the dipolar broadened EPR line under μw irradiation and to provide a mechanistic basis to induce effective triple-flips that constructively add up to DNP enhancements.…”

“…A key piece of missing information is the operational electron spin dynamics of the BDPA radical under experimental conditions that yield OE′. There are several EPR studies of BDPA, but of BDPA solution at ultra low temperatures (<1 K) or lower magnetic fields or of BDPA–benzene microcrystals, , which are all very different from the current DNP operating conditions, and hence less relevant.…”

Electro-optical π-radicals: design advances, applications and future perspectives

“…95,110,123 Addition of water-solubilising carboxylic acid groups in 6 and different amounts of sulfonate groups in 7 (Scheme 5) allowed dynamic nuclear polarization studies in aqueous solutions by Swager et al [124][125][126] The radicals were unaffected by the ionic substituents and their absorption profiles in aqueous solutions closely resembled that of BDPA in dichloromethane solution. 123 BDPA radicals are commonly regarded as air-stable but, recently, Sigurdsson and Mandal 127 revealed that they undergo decomposition under various conditions excluding low temperatures at À80 1C or so. Dimerization was one of the main degradation pathways.…”

“…Dimerization was one of the main degradation pathways. BDPA-nitroxide diradicals have been of interest in the dynamic nuclear polarization field, 124,127 and addition of quaternary ammonium groups into BDPA has improved their persistence and made them water-soluble. 128 Swager et al 129 extended the p-conjugation in a planar truxene-based radical 8.…”