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Dedicated to Professor Gerhard Erker on the occasion of his 60th birthdayMultiphoton absorption is of intense interest for a variety of potential applications, such as optical power limiting, data storage, multiphoton microscopy, microfabrication, and frequency up-conversion lasing.[1] Multiphoton processes possess significant advantages, such as superior resolution, which enables precise spatial control for the various applications, together with the possibility of using infrared lasers at wavelengths that are technologically important in biology, medicine, and telecommunications. Thus far, most studies have considered two-photon absorption (2PA).[2] Enormous strides have been made in our understanding of how molecular structure controls 2PA efficiency by examining a range of compounds with dipolar, quadrupolar, and octupolar geometries. Three-photon absorption (3PA) may be superior to 2PA, because its cubic dependence on incident-light intensity leads to superior spatial confinement of the excitation volume and it also affords the possibility of using a much longer excitation wavelength. Whereas there have been several reports of three-photon processes resulting from 2PA followed by excited-state absorption (ESA), [3] there have been fewer studies of instantaneous 3PA [4] and very few examining the wavelength dependence of 3PA.[5] We report herein the first such study for an organometallic dendrimer and a record 3PA coefficient that highlights the potential of organometallic dendrimers.The synthesis of the dendrimer employed in the present study (1; dppe = 1,2-bis(diphenylphosphanyl)ethane) is described elsewhere.[6] The nonlinear absorption properties of 1 were evaluated by Z-scan experiments in the spectral range 625-1500 nm by employing fs pulses, the latter minimizing contributions from ESA and therefore giving a better prospect of affording the intrinsic nonlinearity; the short wavelength limit was chosen to ensure that all data were collected in the region of optical transparency of the dendrimer. Both closed-aperture and open-aperture Z-scan measurements were undertaken, thus permitting simultaneous evaluation of spectral dependences of both components of the complex hyperpolarizability (g real and g imag ); this is only the second such study for an inorganic compound. [7] The results for the range 625-950 nm are shown in Figure 1. Figure 1 reveals that g real is negative over this spectral range, whereas g imag is positive with a maximum at around [*] J.

Second-harmonic-generation (SHG) efficiency and birefringence are two crucial parameters for nonlinear optical (NLO) crystals. However, the simultaneous optimization of these two key parameters remains a great challenge due to their contrasting microstructure prerequisites. In this paper, the first example of tetravalent rare-earth iodate-sulfate Ce­(IO3)2(SO4) (CISO) was designed by a defluorinated homovalent substitution strategy based on the parent compound CeF2(SO4)·H2O (CFSO) from the centrosymmetric to the noncentrosymmetric structure, which shows a large SHG effect of 3.5 × KDP in metal sulfates and a significant birefringence (0.259 at 546 nm) in hetero-oxyanion tetrahedral-group-based optical crystals. In the structure of CISO, both [IO3] and [SO4] groups are in the cis-position to each other, while [CeO8] polyhedra are highly distorted due to the different coordination environments of two kinds of oxyanions in one structure, which displays optimized optical anisotropy and polarizability attributed to their synergistic effect, leading to the strong SHG efficiency and sufficient birefringence. First-principles simulations for CISO have been employed to rationalize the correlations between the molecular structure and optical properties.

The impending breakdown in Moores law has prompted the search for molecule-based information-processing components such as molecular switches and logic gates. [1] Bistable molecules that afford states possessing distinct linear optical properties (absorbance, fluorescence) and that can be interconverted by chemical, redox, magnetic, or photonic stimuli have attracted considerable interest for Boolean logic operations. [2] In principle, molecular computing could exploit ternary or higher-order digit representations, which would permit smaller device components. "Switchable" molecular properties are also of intense interest for sensor applications. Despite this interest, molecules that can exist in more than two stable and independently addressable states, which could be employed for complex and higher-order logic functions, have been explored significantly less than two-state molecules. One potentially very important procedure to exploit molecular switches is to utilize their cubic nonlinear optical (NLO) properties, [3] and in particular their nonlinear absorption. [4] However, this avenue is also poorly explored and is to date restricted to switching between three states at most, [5,6] although such switching offers 1) the prospect of broadening the available spectral range (and in particular utilizing telecommunications-relevant wavelengths) and 2) the possibility of enhanced spatial control (compared to linear optical switching) because of the higher-order dependence on the intensity of the incident light. Herein we show that a specific binuclear metal alkynyl complex [7] incorporating a functionalized 5,5'-dithienylperfluorocyclopentene (DTE) bridge [8] can afford six stable and switchable states that possess distinct cubic NLO properties. The complex is comprised of independently addressable modules that respond orthogonally to protic (alkynyl ligandQvinylidene ligand), electrochemical (metal-centered redox: Ru II QRu III ), and photochemical (DTE ring-openingQring-closing) stimuli. The six states are interconverted along seven pathways, all of which result in distinct changes to cubic nonlinearity for specific regions of the spectrum. Our results demonstrate that complexes of this type have the potential to be used, among other things, in the construction of multi-input logic gates responding to diverse stimuli across a broad spectral range.The synthesis of the dinuclear ruthenium alkynyl complex oa(II) is depicted in Figure S1 in the Supporting Information. The DTE unit is obtained in the "open" form as its 5,5'diethynyl derivative; the open DTE is thermally stable, and this open form persists through the preparative steps that ultimately afford oa(II). Complete synthetic and characterization details are given in the Supporting Information. Complex oa(II) can be reversibly protonated to the di(vinylidene) complex ov(II), reversibly oxidized to the Ru III complex dication oa(III), and photoisomerized with UV light to the closed alkynyl complex ca(II). Complex ca(II) can be reversibly protonated to cv(II) an...