Optical Nuclear Polarization in the 1:1 CT‐Crystal Anthracene/Tetracyanobenzene. Consequences of the Phase Transition at 206 K

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Abstract
In the 1:1 CT‐complex anthracene/tetracyanobenzene the optical excitation of triplet states causes a nuclear polarization (ONP), which is two orders of magnitude larger than the Boltzmann magnetization. In single crystals this ONP is investigated as a function of the temperature (1.6 K ≦ ≦ T ≦ 300 K), of the magnetic field (0 < Bp ≦ I T) and of the angle between Bp, and the molecular axes. From the extrema of the ONP curves the identity and orientation of the triplet state inducing the ONP may be determined. In addition from cross‐relaxation experiments a quadrupole coupling constant A = (1.0 ± 0.1) MHz and an asymmetry factor η = 0.08 ± 0.02 for the 14N nuclei of TCNB is found. Below the phase transition temperature at Te = ± 206 K the anthracene molecules occupy two magnetically inequivalent positions. For the ONP process between 205 and 40 K this magnetic inequivalence is released by the delocalisation of the triplet state as an exciton.