Orientational phase transition in a charge-transfer crystal: Triplet excitons as probes for lattice dynamics

Thaer

1978

Phys. Stat. Sol. (a)

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The Raman spectra of the CT‐crystal anthracene‐TCNB are investigated in the temperature range between 10 and 300 K. The orientational phase transition of the crystal at 206 K is observed from the temperature dependence of the resonance frequencies, intensities, and linewidths. It is shown that the low‐temperature structure is ordered and that a superstructure is formed. An assignment of some modes on the basis of isotropic shift and preresonance enhancement data is presented. Especially the frequencies of those anthracene librations are given, that are involved in the phase transition. The most intense line in the spectrum is attributed to a translational motion along the normal coordinate nearly parallel to the stack axis.

Section: Resultssupporting

confidence: 84%

Raman study of the phase transition in the CT-crystal anthracene-TCNB

Thaer

1978

Phys. Stat. Sol. (a)

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“…The independence of triplet exciton diffusion of the presence of a phase transition was observed for anthracene-TCNB [19] and our data indicate that this also holds for naphthalene-TCNB. This is concluded from the typical finding that between 50 and 80 K the trap signal intensity in Fig.…”

supporting

confidence: 80%

Guest Donor Arrangement in CT Crystals

Physica Status Solidi (b)

Erdle

1981

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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.

“…On the other hand many reports continue to appear on molecular dynamics and phase transitions in these systems [3,4]. These are of importance as examples for phase transitions in quasi-ID-systems induced by a freezing in of large amplitude librational motions.…”

Section: Introductionmentioning

confidence: 98%

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

Erdle

Zeitschrift Für Naturforschung A

1980

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Mixed CT-crystals of composition anthracenecphenanthrene1-c-tetracyanobenzene (AnthcPhen1-c-TCNB, 0 ≦ c ≦ 1) have been investigated using ESR, optical and Raman spectroscopy. The data yield information on structural features and exciton transport as well as on the interrelation between these properties. With respect to their structural properties the mixed crystals can be prepared into two classes: orientations and lacking molecular motion as well as a phase transitions. b) Crystals with excess anthracene (c < 0.5) are comparable to Anth-TCNB: they also undergo a first order phase transition though at a lower transitions temperature Tt (Tt = 155 K for c = 0.75, whereas Tt = 206 K for c = 1). Mini-triplet-excitons confined to the anthracene moiety were found. Their diffusion takes place quasi-onedimensionally and independently of temperature, i.e. especially without being affected by the phase transition. This result holds for the fast component of the diffusion tensor as well as for the slow one.