Metal Coordination-Assisted Near-Infrared Photochromic Behavior: A Large Perturbation on Absorption Wavelength Properties of N,N-Donor Ligands Containing Diarylethene Derivatives by Coordination to the Rhenium(I) Metal Center Penny Ho-Man Lee, Chi-Chiu Ko, † Nianyong Zhu, and Vivian Wing-Wah Yam* Center for Carbon-Rich Molecular and Nano-Scale Metal-Based Materials Research, Department of Chemistry, and HKU-CAS Joint Laboratory on New Materials, The UniVersity of Hong Kong, Pokfulam Road, Hong Kong, PR China Received October 17, 2006; E-mail: wwyam@hku.hk Photochromic diarylethene materials have drawn much attention in recent years since the first report made by Kellogg et al. 1a and subsequent extensive studies by Irie and co-workers 1b-d on the photochromic properties of diarylethene molecules. Their promising fatigue resistance and thermally irreversible properties make them ideal candidates for potential applications in photoswitching mo- lecular devices and optical memory storage. Recently, different attempts have been made to design photochromic compounds that show absorption and reactivity in the near-infrared (NIR) region in order to enhance the semiconductor diode laser susceptibility for applications in optical memory storage. 1c,d,2 One commonly employed approach to achieve NIR absorption is to increase the extent of π-conjugation of the thiophene moiety so as to shift the absorption maxima to longer wavelength. 3 However, the synthesis is usually tedious and non-trivial, and the wavelength shift would eventually reach its convergence limit. Recently, diarylethene derivatives that function as ligands for incorporation into transition metal complexes have been reported by us and others, in which perturbation of the photochromic properties of the diarylethene moiety upon coordination to the metal center has been observed. 4 We believe that, by the judicious design and suitable choice of the ligands and metal centers, the incorporation of diarylethene derivatives as ligands into transition metal complex systems may have an added advantage of extending the wavelength of maximum absorption to the red through an enhancement of the planarity of the π-conjugated system, thus providing an alternative and versatile route toward the design and synthesis of new classes of NIR photochromic materials. Without the need for tedious synthesis, coordination of relatively simple diarylethene molecules into the transition metal complex not only can cause a shift in the absorption and photochromic properties, but also the photochromic behavior can be sensitized through excitation into the relevant excited state of the metal complex chromophore. Herein, we report the syntheses and photosensitized photochromic properties of diarylethene- containing 1-aryl-substituted 2-(2-pyridyl)imidazole ligand deriva- tives and their rhenium(I) complexes and their NIR photochromic behavior upon rhenium(I) coordination. 4,5-Dithienyl-substituted 2-(2-pyridyl)imidazole ligands L1, L2, and L3 were prepared by the Suzuki coupling reaction 5 of 2,5- dimethyl-3-thienyl boronic acid 6 and 1-aryl-4,5-dibromo-2-(2- pyridyl)imidazole using Pd(PPh 3 ) 4 as catalyst in a mixture of aqueous Cs 2 CO 3 (2 M) and dioxane under reflux conditions (Scheme 1). Subsequent reactions of the respective ligands L1, L2, and L3 with Re(CO) 5 Cl in benzene solution under reflux conditions afforded the target rhenium(I) complexes, [Re(CO) 3 (L)Cl] where L ) L1 (1), L2 (2), and L3 (3). All of the ligands and complexes have been characterized by 1 H NMR, IR, and EI-MS and gave satisfactory elemental analyses. Complex 3 has also been character- ized by X-ray crystallography (Figure 1). 1 H NMR spectroscopy of L1-L3 showed the presence of only one set of 1 H NMR signals for the methyl groups on the thiophene moieties, corresponding to the time-averaged signals for both the antiparallel and parallel configurations, 7 indicating a fast interconversion of the two forms 8 which probably resulted from the lack of steric bulk of the imidazole ring. However, upon coordination to the Re metal complex system, the 1 H NMR signals broadened, probably as a result of a reduction in the interconversion rate between the parallel and antiparallel configurations in the complexes due to the steric hindrance imposed upon coordination to the Re(CO) 3 Cl moiety. The open forms of L1-L3 dissolve in chloroform to give colorless solutions, with an intense absorption band at ca. 320 nm corresponding to π f π* and n f π* transitions of the 1-aryl-2- (2-pyridyl)imidazole moiety, with mixing of π f π* and n f π* transitions of the thiophene moieties. On coordination to the Re complex system, this IL absorption band was shifted slightly to the red at ca. 352 nm. In addition to this intense IL absorption band, an absorption shoulder was observed at ca. 425 nm in the electronic absorption spectra of complexes 1-3, ascribed to a MLCT [dπ(Re) f π*(L)] transition, with some mixing of a metal- perturbed IL (π f π*) transition. Upon UV excitation at λ e 350 nm, the ligands showed two additional absorption bands at ca. 410- 425 and 576-586 nm. These new absorption bands at longer wavelengths were tentatively assigned as absorptions of the closed forms, resulting from the photocyclization of the open forms upon UV light irradiation. Upon excitation of complexes 1 and 2 at λ e 450 nm into either the IL or MLCT bands, three absorption bands were generated at ca. 290, 475, and 712 nm and 288, 480, and 713 nm, respectively. These new sets of absorption bands were assigned as metal-perturbed 1 IL transitions of the closed forms of the complexes, with mixing of 1 MLCT transitions in the longest wavelength absorption band. Figure 2a and 2c depict the repre- sentative UV-vis absorption spectral changes of L1 and complex 2 in solution. Such a large shift of the absorption bands of the closed † Current address: Department of Biology and Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, PR China. Scheme 1 Published on Web 04/25/2007 6058 9 J. AM. CHEM. SOC. 2007, 129, 6058-6059 10.1021/ja067425r CCC: $37.00 © 2007 American Chemical Society