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Metal oxides are versatile substrates for the design of a wide range of SAM-based organic−inorganic materials among which ZnO nanostructures modified with phosphonic SAM are promising semiconducting systems for applications in technological fields such as biosensing, photonics, and field-effect transistors (FET). Despite previous studies reported on various successful grafting approaches, issues regarding preferred anchoring modes of phosphonic acids and the role of a second reactive group (i.e., a carboxylic group) are still a matter of controversial interpretations. This paper reports on an experimental and theoretical study on the functionalization of ZnO nanorods with monofunctional alkylphosphonic and bifunctional carboxyalkylphosphonic acids. X-ray photoelectron and infrared spectroscopies have been combined with DFT modeling to explain and understand the interactions that drive the surface anchoring of phosphonic acids on ZnO surface. It was found that both monofunctional and bifunctional acids anchor on ZnO through a multidentate bonding which involves both PO and P−O moieties of the phosphonic group. Moreover, anchored bifunctional acids bend to the surface, promoting a further interaction between surface hydroxyl groups and carboxylic terminations. This secondary interaction can be limited by increasing the surface density of the anchored molecules. ■ INTRODUCTIONFunctionalization of metal oxides with self-assembled monolayers (SAMs) is nowadays a field of great interest since it is a powerful and low-cost method to form stable and flexible surfaces with controlled properties. 1−3 Possible applications range from protective coatings which enhance mechanical properties (such as adhesion, friction, and corrosion resistance) 4−6 to functional layers in specific electronics devices such as field-effect transistors, 7,8 sensors, 9−12 or dye-sensitized solar cells (DSSCs). 13 Therefore, the present decade has seen a surge of interest in the modification of metal oxides (TiO 2 , AgO, Al 2 O 3 , ZrO, ZnO, ITO) with SAM having various anchoring groups, among which phosphonic moieties were proven to be an efficient alternative to the more often adopted carboxylic and siloxane tethering functionalities. 14−17 Among mentioned metal oxides, nanostructured ZnO, a wide-band-gap semiconductor (E g = 3.37 eV at room temperature) 18 used in sensors, 19 electronic devices, 20,21 and solar cells, 22−24 is a promising substrate for the design of SAM-based organic− inorganic materials of technological relevance. 25−32 Examples of SAM-functionalized ZnO nanostructures include nanorods modified either with carboxyalkylphosphonic acids (HOOC-(CH 2 ) n P(O)(OH) 2 (n = 2, 9)) for biosensing 25 or with C60-functionalized phosphonic linkers for photonic devices 26 and nanowire-based ZnO field-effect transistors (FET) which use long-chain alkylphosphonic acids as gate dielectrics. 27 Despite various studies reported on a wide range of successful anchoring approaches, issues regarding preferred anchoring modes (monodenta...

The second-order nonlinear optical (NLO) properties of [Ln(hfac)(3)(diglyme)] (hfac = hexafluoroacetylacetonate; diglyme = bis(2-methoxyethyl)ether; Ln = La, Ce, Pr, Sm, Eu, Gd, Er, Lu) complexes have been investigated by a combination of electric-field second harmonic generation (EFISH) and harmonic light scattering (HLS) techniques, providing evidence for the relevant role of f electrons in tuning the second-order NLO response dominated by the octupolar contribution. These lanthanide NLO chromophores allow a clean valuation of the influence of f electrons on the quadratic hyperpolarizability and on its dipolar and octupolar contributions. Molecular quadratic hyperpolarizability values measured by the EFISH method, beta(EFISH), initially increase rapidly with the number of f electrons, the value for the Gd complex being 11 times that of the La complex, whereas this increase is much lower for the last seven f electrons, the beta(EFISH) value of the Lu complex being only 1.2 times that of the Gd complex. The increase of beta(HLS), which is dominated by an octupolar contribution, is much lower along the Ln series. Remarkably, the good beta(HLS) values of these simple systems, well known for their luminescence properties, are reached at no cost of transparency.

Self-organization is the driving force that led to the evolution of life. Rationalization of the spontaneous self-assembly paradigm will offer tremendous potentialities to obtain a wide variety of complex systems, having specific functionality and properties. Herein, we will propose an overview of the developments in non-covalent syntheses of multi-porphyrin supramolecular species in aqueous solution. This work took inspiration from the pioneering studies aimed at rationalizing the spontaneous aggregation processes, governed by conventional solution properties (i.e. pH, ionic strength, and concentration). The more recent chemical strategies, to hierarchically manipulate the cooperative nature of weak interactions to design and synthesize supramolecular entities having pre-determined structure and properties, demonstrate the feasibility to attain, in a reproducible manner, molecular organization to supramolecular levels. In particular, calixarene-porphyrin species represents concrete evidence of a quantitative complexation, governed by precise hierarchical rules, which together with a rational functionalization of the molecular components leads to supramolecular entities of well-defined and tunable stoichiometry. These systems, thus, represent fertile ground to envisage and implement controlled self-organization strategies as bottom up methodologies to obtain supramolecular nanostructures and smart nanomachines.