Category Archives: photoinduced

Effect of heavy atoms on photochemically induced dynamic nuclear polarization in liquids

Okuno, Y. and S. Cavagnero, Effect of heavy atoms on photochemically induced dynamic nuclear polarization in liquids. Journal of Magnetic Resonance, 2018. 286: p. 172-187.

http://www.sciencedirect.com/science/article/pii/S1090780717302938

Given its short hyperpolarization time (∼10−6 s) and mostly non-perturbative nature, photo-chemically induced dynamic nuclear polarization (photo-CIDNP) is a powerful tool for sensitivity enhancement in nuclear magnetic resonance. In this study, we explore the extent of 1H-detected 13C nuclear hyperpolarization that can be gained via photo-CIDNP in the presence of small-molecule additives containing a heavy atom. The underlying rationale for this methodology is the well-known external-heavy-atom (EHA) effect, which leads to significant enhancements in the intersystem-crossing rate of selected photosensitizer dyes from photoexcited singlet to triplet. We exploited the EHA effect upon addition of moderate amounts of halogen-atom-containing cosolutes. The resulting increase in the transient triplet-state population of the photo-CIDNP sensitizer fluorescein resulted in a significant increase in the nuclear hyperpolarization achievable via photo-CIDNP in liquids. We also explored the internal-heavy-atom (IHA) effect, which is mediated by halogen atoms covalently incorporated into the photosensitizer dye. Widely different outcomes were achieved in the case of EHA and IHA, with EHA being largely preferable in terms of net hyperpolarization.

Liquid-State 13C Polarization of 30% through Photo-Induced Non-Persistent Radicals #DNPNMR

Capozzi, A., et al., Liquid-State 13C Polarization of 30% through Photo-Induced Non-Persistent Radicals. The Journal of Physical Chemistry C, 2018.

https://doi.org/10.1021/acs.jpcc.8b01482

Hyperpolarization via dissolution Dynamic Nuclear Polarization (dDNP) is crucial to significantly increase the magnetic resonance imaging (MRI) sensitivity, opening up for in vivo real-time MRI using in particular 13C-labelled substrates. The range of applications is however limited by the relatively fast decay of the nuclear spin polarization together with the constraint of having to polarize the spins near the MRI magnet. As recently demonstrated, the employment of UV-induced non-persistent radicals represents an elegant solution to tackle these drawbacks. Nevertheless, since its introduction, the spread of the technique has been prevented by the relatively low achievable polarization, slow buildup time and time-consuming sample preparation. In the present work, thanks to a thorough investigation of the radical generation step, we provide a robust protocol to enhance the efficiency and performance of the UV-radical technique. Under optimal conditions, it was possible to produce up to 60 mM radical in less than 5 min, and reach maximum DNP enhancement with a buildup time constant of approx. 25 min, at 6.7 T and 1 K, resulting in 30% 13C liquid-state polarization.

Photo-induced radical polarization and liquid-state dynamic nuclear polarization using fullerene nitroxide derivatives #DNPNMR

Liu, G., et al., Photo-induced radical polarization and liquid-state dynamic nuclear polarization using fullerene nitroxide derivatives. Phys. Chem. Chem. Phys., 2017. 19(47): p. 31823-31829.

https://www.ncbi.nlm.nih.gov/pubmed/29171613

We report on radical polarization and optically-driven liquid DNP using nitroxide radicals functionalized by photoexcitable fullerene derivatives. Pulse laser excitation of the fullerene moiety leads to transient nitroxide radical polarization that is one order of magnitude larger than that at the Boltzmann equilibrium. The life time of the radical polarization increases with the size of the fullerene derivative and is correlated with the electronic spin-lattice relaxation time T1e. Overhauser NMR signal enhancements of toluene solvent protons were observed under steady-state illumination, which replaced microwave irradiation.

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