Photochromic Rh(II) complexes based on 1,2-dicyano-1,2-bis(2,4,5-trimethyl-3-thienyl)ethene Jing Han, Atsuhiro Nabei, Yusaku Suenaga, Masahiko Maekawa, Hiromichi Isihara, Takayoshi Kuroda-Sowa, Megumu Munakata * Department of Chemistry, Kinki University, Kowakae, Higashi-Osaka, Osaka 577-8502, Japan Received 12 November 2005; accepted 15 February 2006 Available online 16 March 2006 Abstract Reaction of [Rh 2 (O 2 CC(CH 3 ) 3 ) 4 ] with cis-1,2-dicyano-1,2-bis(2,4,5-trimethyl-3-thienyl)ethene (cis-dbe) or closed-dbe in benzene and dichloromethane yielded three novel Rh(II) complexes. Their structures were characterized and photochromic properties were studied. X-ray crystallographic analyses revealed that the coordination environments of the three metal complexes are quite different. Complex 2 exhibits a 1-D infinite chain structure with two cyano groups of the ligand bridging two metal ions while in complexes 1 and 3 the metal ions have a direct interaction with at least one of the sulfur atoms of the two bisthienylethene molecules. Closed-dbe of complex 3 was transferred to the ring-open form in the crystalline phase upon photoexcitation with 529 nm light. It underwent a photocycloreversion reaction although the metal atom is coordinated to the thienyl group. The irreversible cyclization reaction was presumably attributed to the fixed rotation of thienyl rings, short RhAS distance and steric hindrance of anions. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Bisthienylethene; cis-dbe; Rhodium(II); Photochromism; Crystalline phase 1. Introduction Considerable research efforts have been devoted to organic photochromic compounds that exhibit two differ- ent chemical forms and are reversibly interconverted upon irradiation with light of the appropriate wavelength. One of the most promising classes of photochromic materials is diarylethenes with attached thiophene rings (i.e., bisthie- nylethenes). These bisthienylethene derivatives are promis- ing candidates for optical memory and photoswitching molecules because of their outstanding fatigue resistant and high thermal stabilities [1–14]. cis-1,2-Dicyano-1,2-bis(2,4,5-trimethyl-3-thienyl)ethene (cis-dbe) was first synthesized and studied by Irie [9,15,16]. It undergoes the following photochromism in solution and in the solid phase (Scheme 1). The original yellow open- ring form is transformed into the red closed-ring form when irradiated with 405 nm light, and the red isomer con- verted back the initial color upon exposure to 546 nm light. Various properties, including optical and electrochemical, of cis-dbe have also been investigated, which show its promising application to novel function photomemory and photoswitching devices [17,18]. Recently much interest has been directed to the photoisomerization of cis-dbe molecularly doped in amorphous polymer thin films formed by varied techniques such as casting, spin-coating and the vapor transportation method [19–25]. But studies of cis-dbe in the crystalline phase are still extremely rare [26–28]. We have succeeded in the preparation of Ag(I), Cu(I), Mo(II) and Rh(II) complexes with cis-dbe or closed-dbe and their photochromic behaviors were investigated in the crystalline phase [26–28]. Among these complexes only dirhodium(II) trifluoroacetate complexes did not display reversible photochromic reactions. In a continuation of our work and with the aim of changing the coordination 0277-5387/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2006.02.013 * Corresponding author. Tel.: +81 6 6723 2332/4119; fax: +81 6 6723 2721. E-mail address: munakata@chem.kindai.ac.jp (M. Munakata). www.elsevier.com/locate/poly Polyhedron 25 (2006) 2483–2490