Phosphorescence- and delayed-fluorescence-detected magnetic resonance at zero-field on mobile and localized triplet states in anthracene

Optically-detected magnetic resonance (ODMR)-experiments at zero field were performed on the lowest triplet state of anthracene single crystals monitoring the delayed fluorescence (DF) as well as the phosphorescence (P) at 1.2 K. At low RF-power levels the P-ODMR and DF-ODMR signals are almost identical and can be attributed to at leat two X-traps with the following ZFS-parameters: D₁ = 0.06967(3), |E₁| = 0.00793(3), D₂ = 0.06940(3) and |E₂| = 0.00808(3). At RF-levels exceeding the threshold of the trap signals by about 5 orders of magnitude, DF-ODMR signals from excitons are observed. Their ZFS-parameters show clearly, that the motion is mainly between translationally non-equivalent oriented anthracene molecules D^* = ?0.004256(1) cm ^?1 and E^* = 0.03311(1) cm ^?1]. ODMR-linewidth analysis for optical excitation directly into one of the two Davydov components of the first triplet exciton band indicate, that radiationless interbranch transitions between both components can populate k ≠ 0 states in the lower band with band energies higher than kT. The thermalization time within this band can be estimated to be about 10 ^?7 s.