This paper show a 42% of nuclear spin polarization on phenanthrene doped in fluorene 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.