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

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.