This paper show a 42% of nuclear spin polarization on phenanthrene $C_{14}D_{10}$  doped in fluorene $C_{13}H_{10}$ by Dynamic Nuclear Polarization (DNP), or more specific, the Integrated Solid Effect (ISE) or Integrated Cross-Polarization (ICP), or Microwave-Induced Optical Nuclear Polarization (MIONP). they use 75GHz microwave at 1.4K.

the paper pointed out that conventional guest molecule is paramagnetic in ground state. That provided a channel for nuclear spin-relaxation and  reduce the polarization. in contrast, this paper use a paramagnetic triplet state and diamagnetic ground state. Thus, when the excitation laser is turned off, the nuclear-spin relaxation can prevented.

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