| Popis: |
The temperature dependence of the EPR spectrum of triplet excitons in single crystals of the charge-transfer complex between anthracene and 1,2,4,5-tetracyanobenzene is reported. There is a phase transition at T c ≈ 202 K. Above T f the EPR spectrum shows the existence of a single magnetic species, while below T c there are two magnetically inequivalent sites related by symmetry. The EPR lines are narrow lorentizens in both phases and are attributed to triplet excitons. Analysis of the linewidths indicates two types of exciton motion. The faster motion is within one of the two sublattices of magnetically equivalent species ( v 11 ≳ 6.4 × 10 8 s −1 ) and shows no indication of change with temperature. The slower motion is between magnetically inequivalent sites and depends on temprature ( v 2 = 1.1 × 10 8 s −1 200 K) with an activation energy ≈ 700 cm −1 . It is suggested that, although the triple excitation is almost entirely localized on the anthracene Joners, the motion may proceed effectively along the linear stacks. These results are discussed with particular reference to earlier work. |
