Guest Donor Arrangement in CT Crystals

Abstract

ESR experiments on triplet states in donor-doped naphthalene-TCNB crystals are reported. The phase transition of the crystal a t 62 K, due to the freezing-in of a librational motion, is observed from a change in ESR linewidth or signal intensity. Data obtained on the orientation and dynamics of the guest donors pyrene, phenanthrene, and anthracene are compared with corresponding values of the host donor. The donors differ drastically with respect to the orientation of symmetry axes, and to the frequencies and activation energies of librations. Of special importance is the finding that two sites observed in the low-temperature phase differ also in the orientation of the plane normals (by 0 ' to 20'). This necessitates a review of the determination of the CT character from measured zfs parameters.Es werden .ESR-Experimente an Triplettzustlnden in Donator-dotierten Naphtalen-TCNB-Kristallen durchgefiihrt. Der Phaseniibergang des Kristalls bei 62 K, der durch das Einfrieren einer Librationsmode verursacht wird, wird ails einer Bnderung der ESR-Linienbreite oder der Signalintensitat bestimmt. Die zur Orientierung und Dynamik der Gast-Donatoren Pyren, Phenanthren und Anthrazen erhaltenen Daten werden mit entsprechenden Werten der Wirtsdonatoren verglichen. Die Donatoren unterscheiden sich betrachtlich bezuglich der Orientierung der Symmetrieachsen und der Frequenzen und Aktivierungsenergien der Librationen. Von besonderer Bedeutung ist das Ergebnis, daO die beiden Platze, die in der Tieftemperaturphase beobachtet werden, sich auch in der Orientierung der Ebenennormalen (durch 0" bis 20') unterscheiden. Dies erfordert eine erneute Diskussion der Bestimmung des CT-Charakters aus den gemessenen zfs-Parametern.

Mini-Excitons and Lattice Dynamics in Mixed CT-Crystals: An ESR, Optical and Raman Spectroscopical Study

“…Raman as well as ESR data proved that no phase transition of the crystal Phen-TCNB exists between 10 K and 400 K. This is conceivable as it was also shown that in this temperature range no donor motion takes place and as the onset of donor librations is the reason for the occurrence of a phase transition. The non-existence of a donor libration in Phen-TCNB is also of interest in view of the fact that phenanthrene, as a guest in the crystal naphthalene-TCNB, does not librate, whereas other donors like naphthalene, anthracene or pyrene do [20]. Thus it may be the unique sickle-like shape of phenanthrene or its special interaction with TCNB which prevent it from motion.…”