Phase Transition in Single Crystals of
Hiroaki Tachibana1,2 *, Noriko Hosaka3, Tadayuki Osaki3, Reiji Kumai1, Mutsuyoshi Matsumoto2, and Yoshinori Tokura1,3
1Joint Research Center for Atom Technology (JRCAT)
2National Institute of Materials and Chemical Research
3University of Tokyo
This is an abstract
for a presentation given at the
Foresight Conference on Molecular Nanotechnology.
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Diacetylenes are known to polymerize in the solid state on irradiation of UV light or g-ray. Two spectroscopically distinct forms, A(blue) and B(red) forms, have been observed for polydiacetylenes (PDA) in single crystals, Langmuir-Blodgett films, vacuum-deposited films, cast films, and bilayers dispersed in water. Reversible thermochromism and photo-induced phase transitions between the two forms have been reported in single crystal of polydiacetylenes with urethane substituents.
In this paper, we synthesized a series of urethane- substituted diacetylenes having side groups of R = (CH2)mOCONHR1 with various lengths of alkyl chain (m = 3, 4, 5). The corresponding monomer diacetylenes with R1 = phenyl and ethyl group used in this study are abbreviated as DA-mUPh and DA-mUEt, respectively. The single crystal of DA-mUPh and DA-mUEt was grown by slow evaporation of the solvent. Polymerization of the crystals was carried out using 60Co g-radiation. The polymerized species are abbreviated as PDA-mUPh and PDA-mUEt.
The effect of alkyl chain length on the crystal structure was investigated by X-ray diffraction measurements of single crystals of DA-mUPh. From the analyses of the obtained data, we estimated the stacking distance (d) between diacetylene moieties along the stacking axis and the inclination angle (q) between the diacetylene rod and the stacking axis. The polymerization of diacetylene in single crystal has been known to depend strongly on the above two parameters. Crystals of all the DA-mUPh were monoclinic with the space group of P21 and different lattice constants. The two parameters, d and q, depended on the alkyl chain length (m): d = 3.5 Å and q = 43 ° for DA-4UPh and DA-6UPh and d = 5.2 Å and q = 69 ° for DA-5UPh. Actually, the crystal of DA-5UPh was markedly less reactive to g-ray than those of DA-4UPh and DA-6UPh.
We measured temperature dependence of reflectance spectra of single crystal of PDA-mUEt. After polymerization with irradiation of g-ray, the exciton peak assigned to the A form was observed around 1.9 eV in all the crystals. Upon heating, the crystals underwent phase transition into the B form and the peak energy shifted to 2.3 eV, irrespective of alkyl chain length (m). The critical temperature for the A-to-B transition is in the range of 400-440 K. Further, odd-even effect of alkyl chain length was observed in the B-to-A transition upon cooling. In the PDA-5UEt crystal, the B form reverted to the initial A form at 370 K. On the other hand, the PDA-4UEt and PDA-6UEt crystals remained in the B form even when cooled down to room temperature. These crystals changed into the A form slowly when kept at room temperature. Photo-induced phase transition between the A and B forms was also studied for PDA-mUn crystals. The results will be described in detail.
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