International audienc

The synthesis, linear optical and nonlinear optical properties, as well as the electrochemical behavior of a series of pro-ligands containing the 4-(4-N,N-dimethylaminostyryl)-1-methyl pyridinium (DASP(+)) group as a push-pull moiety covalently linked to terpyridine or bipyridine as chelating ligands are reported in this full paper. The corresponding multifunctional Ru(II) and Zn(II) complexes were prepared and investigated. The structural, electronic, and optical properties of the pro-ligands and the ruthenium complexes were investigated using density functional theory (DFT) and time-dependent (TD) DFT calculations. A fairly good agreement was observed between the experimental and the calculated electronic spectra of the pro-ligands and their corresponding ruthenium complexes. A quenching of luminescence was evidenced in all ruthenium complexes compared with the free pro-ligands but even the terpyridine-functionalized metal complexes exhibited detectable luminescence at room temperature. Second order nonlinear optical (NLO) measurements were performed by Harmonic Light Scattering and the contribution of the DASP(+) moieties (and their relative ordering) and the metal-polypyridyl core need to be considered to explain the nonlinear optical properties of the metal complexes.

A current challenge is the development of efficient strategies for the design of switchable nonlinear optical (NLO) materials.[1] As most molecules with large first hyperpolarizability values b comprise p systems that are unsymmetrically endcapped with donor and acceptor moieties, various strategies have been explored to alter the electron-donor (or acceptor) capacity of the end groups using external stimuli such as redox methods [2] and protonation/deprotonation reactions. [3] Another elegant approach to the reversible switching of NLO properties is the use of photochromic compounds. [1b, 4] Among them, dithienylethene (DTE) derivatives are the most promising because of their good fatigue resistance, the remarkable thermal stability of both isomers, and the rapid response time, which are prerequisite conditions for practical applications.[5] Typically, DTE derivatives undergo reversible interconversion between an unconjugated open form and a pconjugated closed form when irradiated in the UV and visible spectral ranges, respectively.In recent years, considerable effort has been devoted to the development of transition-metal complexes as NLO chromophores, and their large second-order nonlinearities have been demonstrated.[6] The photoswitching of their NLO properties has not been reported to date, although recent studies have highlighted the potential of metal-containing DTE ligands for luminescence or electron-transfer photoswitching. [7] We previously described the use of 4,4'-bis(dialkylaminostyryl)-2,2'-bipyridine compounds for the molecular engineering of noncentrosymmetric dipolar [8] (Scheme 1, type I) or octupolar metal complexes. [9] With the aim of photoswitching the NLO properties, we designed a new type of 4,4'-bis(ethenyl)-2,2'-bipyridine ligand functionalized by phenyland dimethylaminophenyl DTE groups. These ligands allowed us to prepare the corresponding photochromic dipolar (bipyridyl)zinc(II) complexes (Scheme 1, type II). Herein, we report the synthesis and photochromic properties of these new compounds, as well as the efficiently phototriggered enhancement of the NLO activity of dipolar complexes. The target bipyridine derivatives 1 a(o) (D = H, (o) indicates open form) and 1 b(o) (D = NMe 2 ) were readily prepared in 55-60 % yield by a double Horner-WadsworthEmmons condensation between the bisphosphonate bipyridine and the aryl-substituted DTE aldehydes 6 (Scheme 2). The latter compounds were obtained by a multistep procedure. The two thienyl fragments were prepared independently and were successively connected to C 5 F 8 (see the Supporting Information for details).For both ligands, the photocyclization process was confirmed by 1 H NMR spectroscopy experiments. The colorless solutions of 1 a(o) and 1 b(o) in CD 2 Cl 2 turn blue and green, respectively, upon UV irradiation. The 1 H NMR spectra of the closed forms 1 a(c) and 1 b(c) show the characteristic upfield shift of the two thiophene protons, while the methyl proton signals are shifted downfield. Integration of the methyl-group reso...