Structure, Physical Properties and Phase Transition of a Quasi-One-Dimensional Organic Semiconductor DBA(TCNQ) 2 Hailin Peng, † Chunbo Ran, † Zhongfan Liu,* ,† Yunze Long, ‡ Zheming Wang, § Zhengqiang Yu, § Haoling Sun, § Yongge Wei, § Song Gao, § Zhaojia Chen, ‡ and Er-Qiang Chen § Center for Nanoscale Science and Technology (CNST), Beijing National Laboratory for Molecular Sciences (BNLMS), State Key Laboratory for Structural Chemistry of Unstable and Stable Species, College of Chemistry and Molecular Engineering, Peking UniVersity, Beijing 100871, P. R. China, Key Laboratory of Extreme Conditions Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100080, P. R. China, and College of Chemistry and Molecular Engineering, Peking UniVersity, Beijing 100871, P. R. China ReceiVed: March 5, 2008; ReVised Manuscript ReceiVed: April 28, 2008 The present article investigates a reversible structural phase transition, and related physical properties change in a new quasi-one-dimensional (1D) organic semiconductor crystal, DBA(TCNQ) 2 [DBA ) dibutylammonium, TCNQ ) 7,7,8,8-tetracyanoquinodimethane]. Differential scanning calorimetry traces reveal the DBA(TCNQ) 2 single crystal undergoes a first-order reversible phase transition at around 260-270 K, which is accompanied by a dramatic change in both conductivity and magnetic susceptibility. A direct correlation of physical properties with crystal structure is established. Detailed X-ray structure analyses indicate a reversible structural change related to dimer-tetramer transition along the TCNQ chain and disruption of the hydrogen-bonding chains as the temperature decreases from 270 to 253 K, which is the origin of the electronic and magnetic transition. 1. Introduction Organic charge-transfer (CT) complexes have been the subject of extensive investigations for the last four decades, beginning with the first reports of conducting salts of the powerful electron acceptor, 7,7,8,8-tetracyanoquinodimethane (TCNQ). 1,2 Planar TCNQ molecules are stacked face-to-face to form a quasi one- dimensional (1D) conductive chain structures in the CT complexes, which results in interesting electronic, optical, and magnetic properties. 3–5 The characteristic 1D molecular stacking arrangement plays a crucial role in their physical properties, such as spin/Peierls transitions, Mott-Hubbard transitions, and superconductivity. Recently, multifunctional CT materials ex- hibiting conducting properties as well as electric switching of two (or more) stable/metastable states using different external stimuli, such as an electric field, 6 current injection, 7,8 temper- ature, 9 pressure, 10 and so on, have received considerable attention because of their promising applications in organic light- emitting diode device and ultrahigh density memory. For example, the CT complex films of metals with TCNQ, such as Cu-TCNQ and Ag-TCNQ, are well-known to demonstrate electrically bistable switching and memory effects by applying an electric field or optical excitation. 11–15 In our recent work, a new quasi-one-dimensional CT complex semiconductor, DBA(TC- NQ) 2 [DBA ) dibutylammonium], was synthesized as a memory material, 16,17 and showed a thermochemical hole burning (THB) effect for ultrahigh density storage. 16–22 In this paper, we focus our attention on the structural phase transition, and related physical properties change of the single crystal of DBA(TCNQ) 2 . We report a first-order reversible phase change occurs at ca. 260-270 K, indicated by differential scanning calorimetry (DSC) traces. The phase transition is accompanied by a steep change in both conductivity and magnetic susceptibility. The temperature-dependent conductivity measurements exhibit an unusually inverted Peierls-like transi- tion near 267 K on cooling and 270 K on heating, whereas the low-temperature semiconducting phase shows an increase in conductivity. 23 The temperature dependence of magnetic sus- ceptibility clearly reveals a magnetic phase transition near 270 K on heating. To correlate the structural transition and the physical properties for quasi-one-dimensional electron system, we determine X-ray crystal structures at 270, 253, 240, and 290 K, respectively, which reveal the reversible structural change related to dimer-tetramer transition along the 1D TCNQ stacks and disruption of the hydrogen-bonding chains as the temper- ature decreases from 270 to 253 K. 2. Experimental Section Preparation. A single crystal of DBA(TCNQ) 2 was synthe- sized from dibutylammonium iodide and TCNQ according to the method described previously, 16 typically exhibiting a needle shape with the size of several centimeters long, millimeters wide, and thick. DSC Measurements. The DSC measurements for single crystals were performed on a Seiko Instruments EXSTAR 6000 model DSC6200 instruments. A scan was done by first cooling the sample to 233 K in dry nitrogen atmosphere and then taking the data while heating to 314 at 10 K min -1 . This was followed by a cooling scan to 233 at 10 K min -1 . A second heating and cooling scans in the 233-314 K range with a 10 K min -1 rate was carried out and produced similar data. Electrical Transport Measurements. The standard four- probe method was used to measure the conductivity of the crystal along the length (a-axis) and two-probe method for the conductivities along the width and thickness (b- and c-axes), * To whom correspondence should be addressed. Tel & Fax: 00-86-10- 6275-7157. E-mail: zfliu@pku.edu.cn. † Center for Nanoscale Science and Technology (CNST), Beijing National Laboratory for Molecular Sciences (BNLMS), State Key Laboratory for Structural Chemistry of Unstable and Stable Species, College of Chemistry and Molecular Engineering, Peking University. ‡ Chinese Academy of Sciences. § College of Chemistry and Molecular Engineering, Peking University. J. Phys. Chem. C 2008, 112, 11001–11006 11001 10.1021/jp801928y CCC: $40.75  2008 American Chemical Society Published on Web 06/28/2008