Design, sintesi e caratterizzazione di nuovi composti organometallici per NLO

Responsabili di progetto

Alessandra Forni, Yurii Chumakov

Accordo

MOLDOVA - ASM-not in force - Academy of Sciences of Moldova

Bando

CNR/ASM 2013-2014

Dipartimento

Progettazione Molecolare

Area tematica

Scienze chimiche e tecnologie dei materiali

Stato del progetto

Nuovo

Proposta di ricerca

The possibility to modulate specific properties of optical beams, by generating other frequencies or shifting  their frequency, is of paramount importance for a variety of applications in optoelectronics, spanning from telecommunications to sensing and medical diagnostics. In particular, nonlinear optical (NLO) materials are nowadays widely used in state of the art telecommunication devices such as optical switches, light modulators, optical information storage devices and optical logic units [1,2].
Research in the field of materials for optoelectronics was in the past mainly focused on inorganic materials and, only subsequently, on molecular materials which soon revealed to be promising candidates owing to their better NLO tunability and solution processability with respect to inorganic materials. Only recently, coordination and organometallic complexes have as well attracted attention because of their unique structural, electronic, magnetic and electrochemical properties, which allow to further extend the potentialities of molecular-based NLO devices [3,4].
To date, many types of metal complexes have been studied for NLO applications. Among them, organometallic complexes with ‘salen-type’ tetradentate Schiff bases possess large thermal and chemical stabilities, making them very appealing for technological applications [5,6]. Moreover, they are characterized by high degree of synthetic variability, owing to the presence of a number of substitution positions (different diamines and carbonyl derivatives). In these complexes, the metal centre has a powerful active role, due to the possibility of changing its oxidation state and therefore d configuration, leading to a different activity of the metal in the electronic system of the complex (the metal can act as donor, acceptor or electronic bridging group lowering charge transfer transitions).
In the present project, new asymmetrical Schiff base complexes of Ni, Cu and Ag will be synthesized by template condensation of thiosemicarbazide derivatives with variously substituted salicylaldehyde or 2-formylpyridine. The complexes will be  characterized by elemental analysis, IR, UV-vis and X-ray diffraction. The possible application of this class of compounds as nonlinear optical (NLO) materials will be then evaluated in relation to (i) the architecture itself of these complexes compared, in particular, with that of more conventional push-pull salen-type complexes [7]; (ii) the different transition-metal centers considered under the same coordinating scheme, and (iii) the adding of electron acceptor groups, having in particular  chromophoric features, onto the starting unsubstituted ligand structure.
The proposed compounds are expected to possess high nonlinearity owing to their similarity with bis(salicylaldiminato) complexes, which are known to possess NLO properties. The novelty of the proposed research lies principally on the design of compounds which do not possess the typical push-pull structure    typical of most molecular materials for NLO applications. Such feature could be particularly useful in view of their technological application, because it should promote, with increased probability with respect to polar molecules, a non-centrosymmetrical aggregation of the compounds, so allowing to obtain enhanced NLO response at macroscopic level. Preliminary investigation of some of the proposed coordination compounds  has in fact shown that some of them crystallized in noncentrosymmetric space groups.
The NLO response of the compounds will be determined through a combined experimental and theoretical study. The molecular quadratic hyperpolarizabilities will be measured in solution by the solution-phase direct current  electric-field-induced second harmonic (EFISH) generation method, which can provide direct information on the intrinsic molecular NLO properties, and in the solid state, through the Kurtz-Perry technique, which provides the NLO response at macroscopic level. The theoretical study will be based on the application of quantum-mechanical computational methods. In particular, ground state calculations will be performed within both the DFT and the MP2 approaches. The TDDFT formalism will be used to simulate the absorption spectra of the complexes, in order to give a definite assignment to the electronic transitions. The static and dynamic second harmonic generation hyperpolarizability tensor will be determined using the Coupled Perturbed Kohn-Sham (CP-KS) approach. Solvent effects will be taken into account through Polarized Continuum Model approaches.
The project needs the presence of multidisciplinary competences encompassing different fields such as computational chemistry, chemical synthesis, material science and analytical chemistry. The Moldavian team will be engaged in synthesis, characterization and X-ray analysis of the designed compounds while the Italian team will perform the second-order NLO response measurements and will tackle the molecular modelling study of the compounds.
The proposing ASM and CNR partners have already established a consolidated collaboration on the design, synthesis and characterization of tetradentate Schiff bases complexes with optimized NLO response [8].
 
[1] G. Stegeman, P. Likamwa, in: A. Miller, K.R. Welford, B.Daino (Eds.), Nonlinear Optical Materials and Devices for Applications in Information Technology, 289, Kluwer, Dordrecht, 1995, p. 285.
[2] J. Zyss, Molecular Nonlinear Optics, Academic Press, Boston, 1994.
[3] E. Cariati, M. Pizzotti, D. Roberto, F. Tessore, R. Ugo, Coord.Chem. Rev. 2006, 250, 1210.
[4] F. Yuksel, Yu. Chumakov, D. Luneau.   Inorg. Chem. Commun., 2008, 11, 749.
[5] P.G. Lacroix, Eur. J. Inorg. Chem. 2001, 339.
[6] S. Di Bella, I. Fragala`, New J. Chem. 2002, 26, 285.  
[7] L. Rigamonti,F. Demartin, A.Forni, S. Righetto, A. Pasini, Inorg.Chem. 2006, 45, 10976.
[8] see e.g.: J. Gradinaru, A. Forni, V. Druta, F. Tessore, S. Zecchin, S. Quici, N. Garbalau, Inorg. Chem. 46, 884, 2007

Obiettivi della ricerca

New asymmetrical coordination compounds with large NLO response will be prepared through the following intermediate objectives:
- Design of compounds with the prefixed architecture by means of molecular modelling techniques. The nature and location of suitable chromophoric groups, such as azomethine, to be inserted in the complexes will be optimized in relation to their NLO response. Calculations will be carried out using the best state of the art techniques. In particular, long-range corrected hybrid functionals will be used for CP-KS calculations, in conjunction with extended basis sets, to get a proper description of the NLO response. While the suitability of such functionals for calculating such properties has been assessed for small reference molecules [1], a benchmark study on d-metal complexes is still lacking;
- Synthesis of the most promising complexes, as individuated in the previous analysis, by template condensation of the designed ligands in the presence of the selected d-metals;
- Characterization of the synthesized compounds by elemental analysis, IR, UV-vis and X-ray diffraction;
- 2nd order NLO measurements in solution (by EFISH generation technique) and on powder (by Kurtz-Perry method) of the synthesized compounds. Comparison of the EFISH measurements with the computed hyperpolarizability will allow to evaluate the possible presence of aggregation effects on the experimental NLO response.
[1] F. Castet, B. Champagne, J. Chem. Theory and Comput. 8, 2044, 2012

Ultimo aggiornamento: 30/04/2025