Efficient Photoswitching of the Nonlinear Optical Properties of Dipolar Photochromic Zinc(II) Complexes

Abstract

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...

A trip in the nonlinear optical properties of iridium complexes

“…In both complexes the ILCT transition of the coordinated terpyridine is red-shifted because of an intensification of the π-acceptor capacity of its π * orbitals, and there is a large increase (enhancement factor of ca 4, Table 2) of the permanent dipole moment [50]. The β value of the terpyridine upon coordination to 3d 10 zinc(II) increases (β 1.34 (EFISH) goes from 22 to 67 × 10 -30 esu) and remains positive, similar to observations for various zinc(II) compounds with nitrogen donor ligands [53][54][55][56]. Remarkably, upon coordination to a 5d 6 iridium(III) metal center, there is a large increase of the absolute value of β EFISH but its sign is negative (Table 2, 7a and 7a') [50,51].…”

“…Whereas efficient second-order NLO switches have been reported in the case of ruthenium [17,18,99], zinc [40,55,100], platinum [82,83,[101][102][103], nickel [102], and palladium [102] complexes, to the best of our knowledge no example has been reported involving iridium complexes. Nevertheless, iridium complexes, which have abundant valence states and rich electrochemical properties, could be excellent second-order NLO redox-switch molecular materials, as suggested by recent theoretical investigations [104][105][106].…”

Theoretical Modelling of Photoswitching of Hyperpolarisabilities in Ruthenium Complexes

“…Light-induced (photochromic) molecular rearrangements have been explored relatively widely. 3 , 7 However, the speed of switching accompanying such structural changes is often quite limited, and their effects on macroscopic structures may be substantial. Populating transient electronic excited states should allow much faster modulation effects without significant structural changes.…”

Two‐Way Molecular Switches with Large Nonlinear Optical Contrast

“…[6] Zinc(II) complexes that contain a pair of dithienylethene moieties have also demonstrated photoswitchable activity associated with a variation of the mb contrast by up to a factor of 20. [7] A large contrast of the first hyperpolarizability has also been combined with other mechanisms of commutation, in particular, those based on redox or protonation/deprotonation reactions. Reversible redox-switching NLO behaviors have been investigated in detail in Ru II -and Fe II -based chromophores, [8] as well as in substituted helicenes [9] and in stacks of tetrathiafulvalene.…”

“…In fact, spiropyran‐ and spirooxazine‐derivatives have already been incorporated in poly(methyl methacrylate) (PMMA) films to obtain materials that exhibit both photochromism and photoswitchable NLO responses 6. Zinc(II) complexes that contain a pair of dithienylethene moieties have also demonstrated photoswitchable activity associated with a variation of the μβ contrast by up to a factor of 20 7…”