Molecular dynamics and the phase transition in the naphthalene–tetracyanobenzene charge-transfer complex as studied by 1H NMR and triplet state EPR

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Abstract
1H NMR second moments and static and rotating frame relaxation time measurements were performed on the crystalline naphthalene–tetracyanobenzene charge-transfer complex between ∼50 and 150 K. The results are consistent with a model where the random jumping of naphthalene molecules between equivalent orientations becomes nonrandom below the phase transition at 72±1 K. Below the transition the two orientations must be inequivalent, with the molecules settling in the one of lower enery to yield a phase which ideally is completely ordered at 0 K. The barrier between the two orientations is 8.0 kJ/mol above Tc and 6.1 kJ/mol below Tc. The statistical average energy difference between the orientations increases with decreasing temperature from near zero at Tc and is 7.2 kJ/mol at 62 K. The EPR spectrum of the triplet naphthalene trap becomes weaker at higher temperature and disappears by 90 K. It shows no evidence of the phase transition. The loss of hfs at 55 K reveals a process, presumably thermal detrapping, which is 50 times faster than the naphthalene reorientation. Anthracene traps reorient some five times faster than naphthalene molecules at low temperatures but the two rates become comparable at higher temperatures. Naphthalene triplet excitons show processes only about four times as fast as the naphthalene reorientation. Evidently the triplet EPR spectra do not directly probe the molecular dynamics of the host lattice or the phase transition.

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