Phenanthrene triplet level

The only materials that have been examined for this purpose to date have been dienes and other polyenes.12 The major liability of these substances is their chemical reactivity. The finding that 1,3,5-trans-hexatriene quenches the phosphorescent emission from phenanthrene but does not have a significant effect on the fluorescent emission from 1,3,5,7-tetraphenylisobenzofuran indicates that chemical reactivity may not be too severe a problem, and that triplet quenchers may be effective tools for mechanistic work in ECL. Here too, further study is indicated. Unfortunately, or perhaps significantly, the compounds that produce the brightest emission on redox stimulation—1,3,4,7-tetra-phenylisobenzofuran and rubrene—have no detectable phosphorescence. Studies with sensitizers and quenchers have failed to produce any evidence regarding their triplet levels.12 Thus the mechanism of anni-hilative emission remains uncertain. 

This phenomenon is possible in molecules like naphthalene, anthracene and Pyrene, each of which happens to have the lowest singlet energy level about twice hat of the respective triplet. For the generation of these triplets in high concentrations (T — T) type energy transfer from a suitable donor is necessary. Delayed fluorescence in naphthalene has been sensitized by phenanthrene. according to the following scheme ... 

As was pointed out in the introduction, optical detection of the Zeeman transitions of the triplet state preceded the optical detection in zf. Since these former experiments resemble those in ionic crystals, researchers in this field called this technique MODR (microwave-optical double resonance). The assignment of the zf transitions as well as the relative order of the zf levels could be concluded also from the MODR techniques as in the PMDR technique. The first reported MODR experiment was made by Sharnoff (15), in which the Am = 2 transition of the C10D8 tr Plet state in a biphenyl host, using amplitude modulation of the microwave power. A few months later Kwiram reported the optical detection of the Am = + 1 for phenanthrene in biphenyl (16). The experiments were... 

Figure 8. Energy level scheme for 33 and 34. Excitation with 226 nm light populates BP, which undergoes efficient energy transfer to PE and NP. Subsequent energy transfer and intersystem crossing populate the triplet state of the phenanthrene, PE. Excitation to the PE state with 488 nm light results in stepwise energy transfer to BP and NP, the latter of which acts as an energy well.

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