18 February 2000 Ž . Chemical Physics Letters 318 2000 75–78 www.elsevier.nlrlocatercplett Can the molecule involved in a CT interaction reorient itself in the crystal lattice? Phase transition and glass transition in trans-stilbene-TCNQ 1 Kazuya Saito ) , Mizuho Okada, Hiroki Akutsu 2 , Michio Sorai Research Center for Molecular Thermodynamics, Graduate School of Science, Osaka UniÕersity, Toyonaka, Osaka 560-0043, Japan Received 29 October 1999; in final form 26 November 1999 Abstract The heat capacity of the title compound has been measured below room temperature by adiabatic calorimetry. A structural phase transition and a glass transition, both probably related to the intramolecular crankshaft motion, were detected Ž . at 273 K and 250 K, respectively. The degree of charge transfer CT was determined from IR spectroscopy as approximately 0.2 e y . Since the central C5C moiety of the stilbene molecule has a significant density in the HOMO, the observation of the phase and glass transitions means that the part involved in the CT interaction reorients itself in the crystal lattice above the glass transition temperature. q 2000 Elsevier Science B.V. All rights reserved. Recently, we have found glass transitions arising from conformational freezing of the terminal six- Ž . membered ring in organic conductors DMET X 2 Ž Ž . DMET s dimethyl ethylenedithio diselenadithiaful- . wx valene, X s BF and ClO 1 and superconductors 4 4 Ž . w Ž . x Ž Ž . k- ET Cu N CN X ET s bis ethylenedithio - 2 2 . w x tetrathia-fulvalene, X s Cl and Br 2,3 . In the case Ž . w Ž . x of k- ET Cu N CN Br, the glass transition signi- 2 2 ficantly affects the conducting properties, including w x the superconducting transition temperature 2–4 . It is widely accepted that the terminal ethylene moiety ) Corresponding author. Tel.: q81-6-6850-5525; fax: q81-6- 6850-5526; e-mail: kazuya@chem.sci.osaka-u.ac.jp 1 Contribution No. 11 from the Research Center for Molecular Thermodynamics. 2 Present address: Department of Materials Science, Himeji Institute of Technology, Kouto 3-2-1, Kamigoori, Hyogo 678- 1297, Japan. in the ET and DMET molecules has only minor p electron density associated with the HOMO. In this respect, the glass transitions we found are just the same as those in usual molecular crystals. One may then ask whether reorientation of the charge-transfer Ž . CT interacting molecule is possible and what phys- Ž . ical electrical, optical etc. properties result from it. This communication presents experimental evidence that the part involved in the CT interaction can reorient itself in the crystal lattice of the charge transfer salt trans-stilbene-TCNQ. Ž . Crystals of trans-stilbene STB and its deriva- tives are known to show some structural peculiarities including unusually short bond lengths and orienta- w x tional disorder 5,6 . In the case of the crystal of neat STB, both peculiarities have been reported and ex- w x tensively studied 5–10 . The structural disorder ob- served at room temperature is dynamical in nature as evidenced by variable temperature structural studies 0009-2614r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. Ž . PII: S0009-2614 99 01454-2