Inducing a high twisted conformation in the polyimide structure by bulky donor moieties for the development of non-volatile memory Tadanori Kurosawa a,1 , An-Dih Yu b,1 , Tomoya Higashihara a , Wen-Chang Chen b,c , Mitsuru Ueda a,⇑ a Department of Organic and Polymeric Materials, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552, Japan b Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan c Institute of Polymer Science and Engineering, National Taiwan University, Taipei 10617, Taiwan article info Article history: Received 11 January 2013 Received in revised form 5 July 2013 Accepted 8 July 2013 Available online 27 July 2013 Keywords: Polyimide Memory device Non-volatile memory Twisted conformation abstract Two polyimides, PI(DAT-6FDA) and PI(DAPT-6FDA), from N-(2,4-diaminophenyl)-N,N- diphenylamine (DAT) or N-(4-(2 0 ,4 0 -diaminophenoxy)phenyl-N,N-diphenylamine (DAPT) and 4,4 0 -(hexafluoroisopropylidene)diphthalic anhydride (6FDA) were prepared to clarify the structural effect on the resulting memory properties. The memory device based on PI(DAT-6FDA) showed an unstable volatile behavior, while the device based on PI(DAPT- 6FDA) with a more bulky donor (D) unit exhibited a stable non-volatile FLASH type mem- ory characteristic with a long retention time over 10 4 s. The theoretical simulation based on the density functional theory (DFT) suggested that the greater distinct charge separation between the ground and charge transfer (CT) states led to a highly stable memory behavior. Also, it was clarified that PI(DAPT-6FDA) had a highly twisted conformation compared to PI(DAT-6FDA) in the ground state, and a more twisted dihedral angle between the D and acceptor (A) units was induced in the CT state, which led to the non-volatile memory characteristic. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Since the first report about the polyimide (PI) memory [1], many kinds of current memory characteristics, from volatile dynamic random access memory (DRAM) [1–8] and static random access memory (SRAM) [5,6,9,10] to non-volatile FLASH [11,12] and write once read many (WORM) [6–8,12–16] type characteristics, have been reproduced in the PI system. These important studies have elucidated that the field induced charge transfer (CT) be- tween the electron donor (D) and electron acceptor (A) moieties is responsible for the memory switching behavior [17,18], and, more importantly, the key factor in determining the volatility of the resulting memory behav- ior is the stability of the generated CT state, that is, an unstable CT state leads to a volatile memory property and vice versa [19,20]. In 2009, Kang et al. reported that the functional PI (P(BPPO)-PI) consisting of 2,5-bis(4-phenoxy-phenyl)- 1,3,4-oxadiazole (BPPO) and phthalimide as the D and A moieties, respectively, showed a ‘‘remnant’’, yet volatile SRAM property [9]. The mediated phenoxy linkage induced a large torsional angle between the D and A units which generated a potential barrier to delay the back CT process. Our group also clarified that the number of mediating phenoxy linkages between the D and A units obviously contributed to the stabilization of the CT state by affecting the conformation of the PI [5]. The PI(AAPT-6FDA) derived from N-(4-(p-aminophenoxy)phenyl)-N-(p-aminophenyl) aniline (AAPT) and 4,4 0 -(hexafluoroisopropylidene)diphthalic 0014-3057/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.eurpolymj.2013.07.016 ⇑ Corresponding author. Tel./fax: +81 3 5734 2127. E-mail address: ueda.m.ad@m.titech.ac.jp (M. Ueda). 1 These authors contributed equally to this work. European Polymer Journal 49 (2013) 3377–3386 Contents lists available at ScienceDirect European Polymer Journal journal homepage: www.elsevier.com/locate/europolj