Design Guidelines to Elongate Spin–Lattice Relaxation Times of Porphyrins with Large Triplet Electron Polarization

Abstract

The spin-polarized triplet state generated by light irradiation has potential for applications such as triplet dynamic nuclear polarization (triplet-DNP). Recently, we have reported free-base porphyrins as versatile and biocompatible polarizing agents for triplet-DNP. However, the electron polarization of free-base porphyrins is not very high, and the dilemma is that the high polarization of metalloporphyrins is accompanied by a too short spin–lattice relaxation time to be used for triplet-DNP. We report here that the introduction of electron-withdrawing fluorine groups into Zn porphyrins enables a long enough spin–lattice relaxation time (>1 μs) while maintaining a high polarization (P x:P y:P z = 0:0:1.0) at room temperature. Interestingly, the spin–lattice relaxation time of Zn porphyrin becomes much longer by introducing fluorine substituents, whereas the spin–lattice relaxation time of free-base porphyrin becomes shorter by the fluorine substitution. Theoretical calculations suggest that this is because the introduction of the electron-withdrawing fluorine substituents reduces the spin density on Zn atoms and weakens the spin–orbit interaction.

Triplet Dynamic Nuclear Polarization of Guest Molecules through Induced Fit in a Flexible Metal–Organic Framework**

“…Despite many recent efforts, [16–21] only a few studies have achieved triplet‐DNP of biologically relevant molecules, and these systems often require low temperatures (≈100 K) [16, 19, 22] . The reason for this, despite the temperature‐independent polarization source being a key feature for triplet‐DNP, is mainly that the polarization source and target molecules need to be dispersed in the low‐temperature glasses to stop the molecular motions for the effective polarization transfer.…”

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

“…36,37 On the other hand, absorption spectra of TCPP neat solid showed significant broadening due to excitonic interactions between aggregated TCPP. 37,38 Interestingly, TCPP/ASG particles in toluene showed absorption peaks similar to TCPP methanol solution, suggesting the good dispersibility of TCPP on the ASG pore walls.…”

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

“…Porphyrin derivatives can be excited by visible light, which is less harmful to biomolecules than ultraviolet light, and they have high stability. Furthermore, the lower energy level of the porphyrin triplet (∼1.5 eV) 59 than the TEMPO doublet (∼2.6 eV) 40 suppresses triplet deactivation by energy transfer.…”

“…Porphyrin derivatives can be excited by visible light, which is less harmful to biomolecules than ultraviolet light, and they have high stability. Furthermore, the lower energy level of the porphyrin triplet (∼1.5 eV) than the TEMPO doublet (∼2.6 eV) suppresses triplet deactivation by energy transfer. Examination of different porphyrin derivatives and linker lengths provides insight into the influence of molecular structure and dynamics on intramolecular RTPM.…”