Santo Di Bella

Second-order nonlinear optical properties of transition metal complexes

This contribution presents an overview of the principal classes of second-order nonlinear optical (NLO) transition metal (organometallic and coordination) complexes and illustrates how organometallic and coordination chemistry can offer a very large variety of NLO structures in relation to the metal nd configuration, oxidation state, spin state, etc. Moreover, metal complexes can satisfy the very different demands of second-order NLO materials such as switchable, tuneable, and multidimensional NLO properties, depending on the subtle interplay of structure–property relationships. Recent advances and perspectives in metal complexes as NLO materials are also presented.

Synthesis and second-order nonlinear optical properties of bis(salicylaldiminato)M(II) metalloorganic materials

Abstract A new synthetic strategy to second-order nonlinear optical (NLO) metalloorganic chromophores has been developed. These materials exhibit a sizable nonlinearity, an excellent thermal stability, and can be easily incorporated in thermally stable main-chain polymers for poled polymer NLO applications. In addition, substituted donor–acceptor derivatives exhibit two-dimensional NLO characteristics. The effect of the metal center in such complexes is manifold; it templates the formation of acentric molecular structures, imparts high thermal stability to the chelate ring, and both ‘switches on’ and enhance NLO response. This paper briefly reviews recent research developed in our laboratory on this new class of NLO materials.

Supramolecular aggregation/deaggregation in amphiphilic dipolar Schiff-base zinc(II) complexes.

The synthesis, characterization, optical, and fluorescent properties of an amphiphilic Schiff-base bis(salicylaldiminato)zinc(II) complex are reported. Detailed (1)H nuclear magnetic resonance (NMR), diffusion-ordered spectroscopy (DOSY) NMR, optical absorption, and fluorescence spectroscopy studies indicate the existence of aggregate species in noncoordinating solvents. The degree and type of aggregation are related to the concentration and the polarity of the noncoordinating solvent. Dilute solutions are likely characterized by the presence of defined dimers, whereas larger oligomeric aggregates are conceivably formed at higher concentrations. The concentration needed to observe the formation of larger species depends upon solvent polarity. In coordinating solvents or in the presence of coordinating species, a complete deaggregation of the system occurs, because of the axial coordination to the Zn(II) ion, accompanied by considerable changes of (1)H NMR and optical absorption spectra. A dramatic enhancement of fluorescence emission is observed in dichloromethane solutions upon deaggregation with a coordinating agent. The formation of a defined 2:1 supramolecular structure is demonstrated in the case of a ditopic ligand as coordinating species. Overall, these complexes are promising systems for the development of new supramolecular architectures and supramolecular fluorescent probes.

Dipolar donor-acceptor-substituted schiff base complexes with large off-diagonal second-order nonlinear optical tensor components.

The synthesis, characterization, and two-dimensional second-order nonlinear optical (NLO) response of a dipolar NiII donor- acceptor Schiff base complex and the related ligand are reported. Electric-field-induced second-harmonic generation and harmonic light (hyper-Rayleigh) scattering techniques, in combination with INDO/SCI-SOS theoretical calculations, were used to investigate the vector part of the hyperpolarizability tensor and the two-dimensional character of the molecular nonlinearity, respectively. Off-diagonal hyperpolarizability tensors can be related to charge-transfer transitions that are polarized perpendicular to the molecular dipolar axis, while parallel transitions account for the diagonal hyperpolarizability tensor. The role of the metal center in enhancing the two-dimensional NLO response of such molecules is twofold since it acts both as the donor and the bridging moiety of the planar donor-(pi-conjugate-bridge)-acceptor system. These dipolar two-dimensional molecules are interesting candidates from the perspective of polarization-independent NLO materials.

Coordination and Organometallic Complexes as Second-Order Nonlinear Optical Molecular Materials

Coordination and organometallic complexes with second-order nonlinear optical (NLO) properties have attracted increasing attention as potential molecular building block materials for optical communications, optical data processing and storage, or electrooptical devices. In particular, they can offer additional flexibility, when compared to organic chromophores, due to the presence of metal–ligand charge-transfer transitions, usually at relatively low-energy and of high intensity, tunable by virtue of the nature, oxidation state, and coordination sphere of the metal center. This chapter presents an overview of the main classes of second-order NLO coordination and organometallic complexes with various ligands such as substituted amines, pyridines, stilbazoles, chelating ligands (bipyridines, phenanthrolines, terpyridines, Schiff bases), alkynyl, vinylidene, and cyclometallated ligands, macrocyclic ligands (porphyrins and phthalocyanines), metallocene derivatives, and chromophores with two metal centers. The coverage, mainly from 2000 up to now, is focused on NLO properties measured at molecular level from solution studies, as well as on NLO properties of bulk materials.

Two-dimensional characteristics of the second-order nonlinear optical response in dipolar donor–acceptor coordination complexes

The two-dimensional (2D) nonlinear optical (NLO) response of a series of dipolar donor–acceptor bis(salicylaldiminato)Ni(II) Schiff base planar coordination complexes was investigated using the INDO/SCI-SOS formalism. The in-plane 2D NLO character of this class of materials can be related to various low-energy charge-transfer (CT) states and is achieved by an accurate design of substitution pattern of the unsubstituted bis(salicylaldiminato) framework. While z-polarised electronic transitions, parallel with respect the molecular dipolar (z) axis, contribute to the diagonal hyperpolarisability tensor component, perpendicularx-polarised transitions contribute to off-diagonal hyperpolarisability tensors. Unlike non-dipolar multidimensional NLO structures, whose nonlinearity within the framework of the sum-over-states formalism is necessarily due only to three-level terms, in this case the 2D nonlinearity is generally dominated by two-level terms. It necessarily requires the existence of a strong, lowest energy, perpendicular CT transition in order to provide a substantial two-level contribution to off-diagonal tensors.

Sensitive Fluorescent Detection and Lewis Basicity of Aliphatic Amines

In this contribution is reported the sensitive properties of the Zn(II) Schiff base complex, 1, in dichloromethane with respect a series of primary, secondary, and tertiary aliphatic amines through the study of fluorescence enhancement upon amine coordination to the Lewis acidic Zn(II) metal center with formation of 1:1 adducts. It is found that complex 1 exhibits selectivity and nanomolar sensitivity for primary and alicyclic amines. A distinct selectivity is also observed along the series of secondary or tertiary amines, paralleling the increasing steric hindrance at the nitrogen atom. The binding interaction can be related to the Lewis basicity of the coordinating amine; thus, complex 1 represents a suitable reference Lewis acid, and estimated binding constants within the investigated amine series can be related to their relative Lewis basicity. A relative order of the Lewis basicity can be established for acyclic amines, primary > secondary > tertiary, while an inverted order, tertiary > secondary ≈ primary (acyclic), is found in the case of alicyclic amines. The present approach represents a simple, suitable method to ranking the relative Lewis basicity of aliphatic amines in low-polarity, nonprotogenic solvents.

Monitoring photoswitching of azobenzene-based self-assembled monolayers on ultrathin platinum films by UV/Vis spectroscopy in the transmission modeElectronic supplementary information (ESI) available: synthesis and characterization of 1 and its photoisomerization in solution. See http://www.rsc.org/suppdata/jm/b3/b314710j/

The reversible trans ↔ cis photoisomerization of azobenzene-based self-assembled monolayers on ultrathin platinum films has been monitored for the first time by UV-Vis transmission spectroscopy with a conventional spectrophotometer.

Equilibrium geometries and harmonic vibrational frequencies of lanthanum trihalides LaX3 (X F, Cl). A relativistic effective core potential ab initio MO study

Abstract The equilibrium geometries and the harmonic vibrational frequencies of lanthanum trihalides, LaF 3 and LaCl 3 , have been studied using ab initio relativistic effective core potential Hartree—Fock, complete active space multiconfigurational self-consistent field and configuration interaction calculations. In accordance with experimental data, present ab initio results indicate a pyramidal (C 3v ) arrangement for LaF 3 . Conversely, the planar (D 3h ) geometry is found to be more stable for LaCl 3 . The electronic factors affecting the equilibrium structures are discussed and compared to results of identical calculations made for the simpler, purely σ-bonded, LaH 3 .

On the determination of the molecular static first hyperpolarisability: how reliable are literature data?

The shortcomings of the two-level model in extrapolating the molecular static first hyperpolarisability, β0, are discussed in the context of literature data. However, except for some spurious β0 data, in many other cases, the reported β0 values can be considered reasonably reliable due to a balancing of effects.