From Introduction To Dynamic Spin Chemistry: Magnetic Field Effects On Chemical And Biochemical Reactions

13.1 MFEs of Luminescence

In 1967 Johnson et al. [1] discovered that the intensity of triplet-exciton annihilation luminescence in anthracene cryatal at room temperature increased in weak magnetic fields up to a maximum increase of 5 % at 35 mT and that the intensity decreased in higher fields. The intensity was found to level off at 80% of the zero-field value at B > 0.5 T. Their typical result is shown in Fig. 13-1.

Figure 13-1: The influence of a magnetic field on the delayed fluorescence intensity from an anthracene crystal with 15 ms triplet lifetime at room temperature. The magnetic field was applied in the ac plane of the crystal in the direction at ? 17 with respect to the a crystal axis as shown in the insert. The dashed lines in the insert indicate the field direction which produce the greatest diminution in delayed fluorescence intensity. (Reproduced from Ref. [1] by permission from The American Physical Society)

The delayed fluorescence can be produced by the following scheme:

In principle, the magnetic field could be affecting all of the above-mentioned processes. Johnson et al. demonstrated the absence of any magnetic field effect (for fields up to 2 T) on intensity of fluorescence from singlet excitons generated directly with uv light. This eliminated the possibility of MFEs of ? and ?. They also measure the MFEs at 35 and 240 mT with a pulsed-magnet. Because the time dependence of the delayed fluorescence...

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