Novel Tetrazine Organometallic Complexes for Nonlinear Optical Switching: a Combined Density Functional Theory and Experimental Study

Over the past decade, mono- and bi- and oligonuclear organometallic complexes have been deal of great interest by material chemists and engineers due to their remarkable nonlinear optical (NLO), magnetic and electric properties. [1] They were furthermore envisaged as powerful candidates for quantum computing, molecular devices and more interestingly as molecular switches. The definition of molecular switch implies that a compound should have two interconvertable stable forms that have a significant difference in the magnitude of, at least, one of their physico-chemical properties. [2] In the case of organometallic complexes the most expedite way to achieve such interconversion is by redox means, where one takes advantage of the different oxidations states allowed by a single or multiple metal centers that interact with each other, and with the ligands therein. It is known that the choice of an appropriate π-bridging system between the metal centers is crucial to obtain the adequate electronic environment for efficient metal-to-metal and metal-to-ligand interactions, and hence fine tune the optical, electric and magnetic macroscopic properties. Mixed-valance metal complexes from group VIII, particular Fe(II)/Fe(III) and Ru(II)/Ru(III) systems [3,4], have been successfully synthesized as stable species that possess strong metal-to-ligand charge transfer (MLCT) and inter-valence charge transfer (IVCT) absorption bands in visible and NIR regions, respectively.[5, 6] This is the case of the well known Creutz-Taube ion [(H3N)5Ru(m-pz)Ru(NH3)5]5+ (pz=pyrazine) and related compounds. Substituted 1,2,4,5-tetrazine (tz) strongly enhance both the stability and the MLCT and IVCT phenomenon’s since they act as a strong π-acceptor and can stimulate the metal-to-metal interaction. [6] In this work, we present our ongoing studies in the use of substituted 1,2,4,5-tetrazine (tz) ligands as the π-bridge between two transition metal centers, both experimentally and by means of Density Functional Theory (DFT). Synthesis and characterization by NMR, voltammetric and UV-Vis. spectra are presented. The DFT calculations were used to evaluate the nature of the electronic transitions as well as the switching of the NLO properties. The compounds showed to be great candidates for NLO switching since the magnitude of such property can change upon redox stimulus at the metal centers

Synthesis of organometallic ruthenium(II) complexes with strong activity against several human cancer cell lines

A new family of "RuCp" (Cp=η(5)-C(5)H(5)) derivatives with bidentate N,O and N,N'-heteroaromatic ligands revealed outstanding cytotoxic properties against several human cell lines namely, A2780, A2780CisR, HT29, MCF7, MDAMB231, and PC3. IC(50) values were much lower than those found for cisplatin. Crystal structure of compound 4 was determined by X-ray diffraction studies. Density functional theory (DFT) calculations performed for compound 1 showed electronic flow from the ruthenium center to the coordinated bidentate ligand, in agreement with the electrochemical studies and the existence of a metal-to-ligand charge-transfer (MLCT) band evidenced by spectroscopic data

First Hyperpolarizabilities of η5-Monocyclopentadienylmetal Complexes from DFT Calculations

Organometallic complexes have been studied as potential building blocks for second-order nonlinear optical (SONLO) materials in view of their potential application in the area of integrated optics [1]. Experimental work on 5-monocyclopentadienylmetal complexes with benzene based chromophores showed that the first hyperpolarizability decreases with increasing conjugation length of the chromophores [2]. For similar 5-monocyclopentadienyliron complexes possessing thiophene based conjugated backbone, an inverse trend on experimental first hyperpolarizability with increasing conjugation length was observed [3]. In addition, the effect of the metal center on the first hyperpolarizability on 5-monocyclopentadienylmetal complexes with benzene based chromophores follow the order Co < Ni < Ru < Fe [2]. No experimental results for the similar complexes containing thiophene chromophores have been yet obtained. In order to get a better understanding on the electronic factors that may be responsible for the SONLO behavior of these compounds and their correlation with experimental crystallographic, spectroscopic and electrochemical data, DFT calculations using the GAMESS-US [4] package were made in model complexes [MCp(H2PCH2CH2PH2)(NC{SC4H2}nNO2)]+ (M=Fe, Ru; n=1-2). Spatial localization of electron charge has been performed to gain insight into the nature of the chromophores binding to the metal center. First static hyperpolarizabilities has been calculated and correlated to the HOMO-LUMO gaps

Syntheses and characterization of η5-monocyclopentadienylruthenium(II) complexes with thienyl acetylide chromophores for nonlinear optics

Over the past 20 years, there has been a growing interest in the design and synthesis of organic and organometallic molecules for the development of novel nonlinear optical (NLO) materials owing to their potential application in the area of integrated optics.1 Ultimately, these materials could be used with enhanced efficiency and versatility in areas such as optical communications, computing and data storage. Fundamental research has been focused in the establishment of detailed structure–activity correlations in view to obtain large intrinsic optical nonlinearities. For second-order nonlinear optics, this lead to the development of typical push-pull systems in which the metal center, bonded to a hyperpolarizable organic chromophore, acts as an electron releasing or withdrawing group. Among the organometallic compounds presenting this structural feature, systematic studies were made on half-sandwich complexes presenting benzene-based chromophores bonded to the metal center via nitrile or acetylide linkages.2,3 The results revealed that the compounds possessing acetylide ligands have better NLO properties than the nitrile analogues. In addition, recent results on half-sandwich complexes presenting thiophene nitrile chromophores showed an enhanced NLO performance when compared to that observed on analogue benzenoid structures.4 These results suggest that combination of acetylide thiophene ligands with appropriate organometallic fragments would maximize the NLO response. Thus, we report herein the preliminary results concerning the synthesis and characterization, together with solvatochromic behavior of new compounds derived from the organometallic fragment RuCpL2 (L=phosphines) possessing different thiophene acetylide based chromophores (an example of target molecules is depicted in figure). Spectroscopic data will be used as a probe on the NLO properties of these compounds

Monocyclopentadienyliron (II) and Ruthenium (II) Complexes for NLO Switching

The limitations on semiconductor-based electronics have inspired many researchers on the development of new photonic molecular materials possessing faster and more accurate responses to external electric or optical stimulus. Non-linear optics (NLO) involves manipulation of laser beams and therefore can be used for electro-optic signaling and data processing. The two most used effects in NLO for these purposes are second harmonic generation (SHG) and frequency doubling, both producing high intensity green laser beams.[1] Due to their electronic proprieties, synthetic tailorability and enhanced stability, monocyclopentadienylmetal complexes of group VIII containing aromatic organic chromophores have proven interesting potentialities for NLO applications, namely SHG. Our recent studies have shown that combining both iron (II) and ruthenium (II) organometallic fragments with acetylide or nitrile thiophene chromophores, the static hyperpolarizability (β0), related to the SHG signal, will be drastically enhanced by: 1) increasing the metal-to-ligand charge transfer (MLCT); 2) lowering the redox potential of the metal center; 3)increasing the electronic delocalization in the organic chromophore.[2] Our ongoing work on the molecular NLO-switching properties of Fe(II) and Ru(II) monocyclopentadienyl compounds, was attracted by benzo[c]thiophene based chromophores, which unique electronic behavior originated by their low HOMO–LUMO gaps, can be potentially on the basis of strong NLO effects.[3] This communication presents the synthesis, the electrochemical and the spectroscopic characterization of monocyclopentadienyliron(II)/ruthenium(II) derivatives containing acetylide and nitrile benzo[c]thiophene organic chromophores. Theoretical studies predicting the NLO and switching properties are presented, by the use of Density Functional Theory (DFT) and Time-Dependent DFT studies. Results will be discussed in order to relate the predicted NLO properties with the spectroscopic and electrochemical data, which can be used, eventually, as an experimental probe

Coumarin Dyes with Triple Bonds as pi-Spacer Units for Dye Sensitized Solar Cells

Among metal free organic dyes studied in dye-solar cells (DSC), coumarin derivatives are a promising sensitizer for TiO2 because of their good photoresponse in the visible region and good thermal stability under one sun soaking. On the basis of concept of Donor-pi-conjugation bridge-acceptor structure a series of coumarin dyes have been synthesized by inserting various numbers of thiophene or methine moieties as pi-bridge between coumarin as electron donor and cyano carboxylic acid as electron acceptor. So far, influence of triple bond as pi spacer unit in coumarin dyes has not been studied. Here we report our recent progress in the design and synthesis of coumarin dyes (C1-LEN and C2-LEN) with triple bond as linkers between the donor and acceptor units. The chemical structures of these dyes are shown in Figure 1. Their absorption spectra and photovoltaic properties were investigated and, the electron distribution with different acceptors was performed using density functional theory (DFT) methods and time dependent DFT calculations. A complete optoelectronic characterization of obtained DSCs cells will be presented and discussed

Cyclic Voltammetry Studies of η5-Monocyclopentadienyliron(II) Complexes

Recently, organometallic complexes have emerged as potential building blocks for second-order nonlinear optical (SONLO) materials in view of their potential application in the area of integrated optics [1]. Organometallic complexes can possess low energy, sometimes intense, electronic charge transfer excitations that are responsible for the NLO response. Most of these electronic transitions are MLCT or LMCT excitations which energy can be related to the HOMO-LUMO gaps. Experimental work on 5-monocyclopentadienylmetal complexes with p-substituted benzonitrile chromophores showed the fundamental role played by the organometallic fragment, namely their electronic richness, in the second-order nonlinear optical response [2]. In addition, the first hyperpolarizability was found to decrease with increasing conjugation length of the chromophores, being the larger values associated to the lower HOMO-LUMO gaps [3,4]. For similar 5-monocyclopentadienyliron complexes possessing thiophene based conjugated backbone, an inverse trend on experimental first hyperpolarizability with increasing conjugation length was observed [5]. In order to obtain a deeper insight into the electronic richness of the organometallic moiety and to get an evaluation of the corresponding HOMO-LUMO gaps and the connection with NLO properties, the electrochemical behavior of the compounds [FeCp(P_P)(NC{SC4H2}nNO2)]+ (P_P=DPPE, (+)-DIOP; n=1-3) were studied by cyclic voltammetry in dichloromethane and acetonitrile

Syntheses of half-sandwich nickel and iron complexes presenting hyperpolarizable thiophene based ligands

The search for new organometallic materials with second-order nonlinear optical (SONLO) properties is currently the subject of considerable interest due to their potential technologic applications in photonic devices for telecommunications and optical computing.1 The main studies have been made in push-pull systems in which the metal center, bonded to a hyperpolarizable organic conjugated backbone, acts as an electron releasing or withdrawing group. In spite of the research in this field, an improved understanding of the relationship between the molecular structure and the microscopic optical nonlinearities is crucial for a broadening of reflection and incorporation of novel molecules. Experimental work on h5-monocyclopentadienylmetal complexes with benzene-based nitrile chromophores showed the fundamental role played by the organometallic fragment on the SONLO response.2 Also, recent results seem to demonstrate that NMR spectroscopic data can be used as a probe on the SONLO properties of these complexes, since a quantitative correlation was found.3 However, it is important to check if such correlation can be used in compounds with different chromophores, namely thiophene derivatives. Thiophene based chromophores were already recognized to have real merits in NLO properties. Thus, we report herein the preliminary results concerning the synthesis and spectroscopic characterization of new compounds derived from FeCpDPPE (DPPE=1,2-bis(diphenylphosphino)ethane) and NiCpPPh3 organometallic fragments possessing different thiophene based chromophores. (examples of target molecules are depicted in figure). Spectroscopic data will be analyzed in order to give an insight on the NLO properties of these compounds

Polymerization of ɛ-caprolactone using Ru(II) Catalysts: mechanistic insights

Ring-opening polymerization of cyclic esters is an important field in polymerization chemistry due to the biocompatibility and biodegradability of the obtained materials. Even if the literature concerning the ROP of lactones is vast, there is not detailed research for Group 8 transition metal based catalysts or initiators. It was only in 2006 that was reported the first example regarding the use of ruthenium derivatives in ROP of lactones. In this contribution we present the results of the polymerization of ε-caprolactone by [Ru(η5-C5H5)(η6-substituted arene)][PF6] complexes in the presence of isopropyl alcohol. By in situ NMR techniques it was found that this polymerization proceeds via an activated monomer mechanism. A DFT study of the polymerization initiation step will also be presented in order to corroborate this mechanism

Production of Anticentauro Events in Ultra-Relativistic Heavy Ion Collisions

We propose a novel method for studying the production of anticentauro events in high energy heavy ion collisions utilizing Chebyshev expansion coefficients. These coefficients have proved to be very efficient in investigating the pattern of fluctuations in neutral pion fraction. For the anticentauro like events, the magnitude of first few coefficients is strongly enhanced (~3 times) as compared to those of normal HIJING events. Various characteristics of Chebyshev coefficients are studied in detail and the probability of formation of exotic events is calculated from the simulated events.Comment: Proceedings of Poster Session, 19th International Conference on Ultra-Relativistic Nucleus-Nucleus Collisions (Quark Matter 2006), November 14-20, 2006, Shanghai, Chin