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

Abstract

The orientational phase transition in the charge-transfer (CT) crystal anthracene-TCNB ( s-tetracyanobenze) is investigated by ESR and by Raman spectroscopy. ESR spectra of triplet excitons are observed and analysed with respect to orientational changes during the transition between two different phases. The data yield the mean molecular orientations fx (relative to a crystal fixed axis) as a function of temperature. Besides a gradual orientational change with temperature there is also an abrupt change (Δ fx ≈ 5° within 1 K) at the transition temperature suggesting a first order phase transition. A model is presented that uses exciton dynamics as a probe for lattice dynamics. The size of domains of equally oriented molecules is obtained as a function of temperature. The phase transition is also detected from the appearance of different phonon lines in the Raman spectra. These spectra gain their special value from a comparison with the behaviour of an order parameter fx, characterizing the phase transition.