Dynamic Electron Polarization Lasting More Than 10 μs by Hybridizing Porphyrin and TEMPO with Flexible Linkers

Abstract

Dynamic electron polarization (DEP), induced by quenching of photoexcited species by stable radicals, can hyperpolarize electron spins in solution at room temperature. Recently, development of technologies based on electron spin polarization such as dynamic nuclear polarization (DNP) has been progressing, where it is important to design molecules that achieve long-lasting DEP in addition to high DEP. Hybridization by linking dyes and radicals is a promising approach for efficient DEP, but strong interactions between neighboring dyes and radicals often result in the rapid decay of DEP. In this study, we introduce a flexible linker into the hybrid system of porphyrin and TEMPO to achieve both efficient DEP and long-lasting DEP. The structural flexibility of the linker switches the interaction between the radical and the triplet, which promotes the DEP process by bringing the radical and the triplet into close proximity, while avoiding abrupt relaxation due to strong interactions. As a result, the new hybridized system exhibits a larger DEP than the unlinked system, while at the same time achieving a DEP lasting more than 10 μs.

Generation and Transfer of Triplet Electron Spin Polarization at Solid-Liquid Interface

“…Various mechanisms for electron spin polarization in solution have been reported, including the radical triplet pair mechanism (RTPM), [22][23][24][25] the revere quartet mechanism (RQM), 26,27 and electron spin polarization transfer (ESPT). 28,29 Although highly polarized electron spins can be generated in solids, 3,9,[30][31][32][33] the rotational motion of molecules in solution causes severe electron spin relaxation (Figure 1a).…”

“…Since the net polarization of the free-base porphyrin is known to be negative, 28 the observation of negative polarization of TEMPO also supports the ESPT mechanism. Other possible mechanisms include the radical triplet pair mechanism (RTPM) [22][23][24][25] and the reversed quartet mechanism (RQM). 26,27 In RTPM, collisions and dissociations between triplets and radicals generate radical polarization.…”

Zero-Field Splitting Tensor of the Triplet Excited States of Aromatic Molecules: A Valence Full-π Complete Active Space Self-Consistent Field Study

“…This non-Boltzmann distributed polarization can be exploited in dynamic nuclear polarization (DNP) − and MASER − operating under mild conditions. Also, in systems where a triplet chromophore encounters a radical species, electron spin polarization in the ground doublet state can be generated through the ZFS-driven mixing of the states. − The energy gaps between electron spin sublevels, which are described by so-called ZFS parameters, affect the efficiency of polarization transfer in the DNP experiment, and the operating frequency of MASER. NV-center can maintain spin coherence at room temperature and is expected to be utilized as a qubit and quantum sensor .…”

Generation and Transfer of Triplet Electron Spin Polarization at the Solid–Liquid Interface

“…Although many works on Overhauser DNP using electron spins in thermal equilibrium have been reported, − polarized electron spins are expected to provide even better DNP performance. Chemically induced dynamic electron polarization (CIDEP) using photoexcited states and radicals generates polarization of electron and nuclear spins in solution; − however, high concentrations of excited states could not be utilized due to deactivation of the excited states between diffusing dyes. Polarized electrons generated in solids can show long spin–lattice relaxation times (microsecond order); ,,− however, in the liquid state, molecular tumbling leads to much shorter electron spin–lattice relaxation times (nanosecond order, Figure a) .…”

“…Other possible mechanisms include the radical triplet pair mechanism (RTPM) − and the reversed quartet mechanism (RQM). , In RTPM, collisions and dissociations between triplets and radicals generate radical polarization. Since the triplet polarization is not necessary for RTPM, the radical polarization signal is observed not during the triplet spin–lattice relaxation time, but rather during the lifetime of the triplet excited state .…”