Generation of long-lived electron spin polarization in chromophore-nitroxide molecular rulers for sensitivity enhancement of magnetic resonance techniques

We report a new hyperpolarization mechanism in weakly-coupled triplet-radical spin systems where the chromophore and the nitroxide radical are connected by a peptide bridge.1 In this regime, the hyperpolarization of the stable paramagnetic centre can have a major impact in the enhancement of the sensitivity of EPR and NMR techniques, as demonstrated in the case of light-induced Pulsed Dipolar Spectroscopy.2,3
In these molecular rulers the nitroxide position is progressively changed along the peptide alpha helix in a range from 1.4 nm to 4.2 nm from the chromophore. The chromophores employed are either a Zn-tetraphenylporphyrin, the corresponding metal free base, 2,6-diiodo-1,3,5,7-tetramethylbodipy or erythrosin B tethered at the extremity of the amino acid sequence. A helicene-based bridge has also been investigated.
Upon photoexcitation of the triplet state,4 a spin-polarized radical is also detected by time-resolved EPR spectroscopy. The signal intensity is shown to scale with the chromophore-radical distance as 𝑟−3, in agreement with the behavior expected in the weak-coupling regime. Importantly, in the chromophore-nitroxide conjugates long-distance transfer of long-lived electron spin polarization can be achieved, which is an important requisite for polarizing agents.

References:
[1] M.G. Dal Farra et al., Phys. Chem. Chem. Phys. 2020, 22, 19982-19991.
[2] M. Di Valentin et al., J. Am. Chem. Soc. 2014, 136, 6582-6585.
[3] A. Bertran et al., J. Am. Chem. Soc. 2023, 145, 22859–22865.
[4] D. Panariti et al., J. Chem. Phys. 2025, 162, 114201-114212.