Elsevier

Polyhedron

Volume 69, 17 February 2014, Pages 31-39
Polyhedron

Enhancement of smectic C mesophase stability by using branched alkyl chains in the auxiliary ligands of luminescent Pt(II) and Pd(II) complexes

https://doi.org/10.1016/j.poly.2013.11.015Get rights and content

Abstract

A novel series of Pd(II) and Pt(II) complexes based on cyclometallated imine ligands and N-benzoylthiourea (BTU) derivatives as auxiliary ligands has been prepared and their liquid crystalline properties as well as photophysical properties have been investigated. The crystal structure of one cyclometallated Pt(II) complex with N-(p-F-phenyl)-N′-benzoylthiourea as a co-ligand has been solved. The liquid crystalline properties have been investigated by a combination of DSC, POM and variable temperature powder X-ray diffraction. These new metallomesogens display either a monotropic SmC phase or both SmA and SmC phases, depending on the number of alkoxy groups attached to the imine ligand, alkyl chain length or the use of branched alkoxy terminal groups. We found that the introduction of branched alkoxy terminal groups lead to lower transition temperatures and stabilization of the SmC phase in both the Pd(II) and Pt(II) complexes. While the Pd(II) complexes display no emission, the Pt(II) complexes show good emission properties in solution, in the solid state and as a PMMA film at room temperature, and their investigation is reported.

Graphical abstract

Novel Pt(II) complexes based on cyclometallated imine ligands and N-benzoylthiourea (BTU) derivatives display smectic A and C phases as well as red emission.

  1. Download : Download full-size image

Introduction

Materials with liquid crystalline properties have found various applications, ranging from the manufacturing of LCD to different molecular sensors and detectors, optical switches, spatial light modulator, etc. [1]. The introduction of metal ions adds unique magnetic, optical and electric properties to the mesophase, besides providing an interesting structural role when compared to purely organic liquid crystals [2], [3]. Light-emitting metallomesogens have been intensively studied in recent years [4]. Mesomorphic materials based on cyclometallated Pd(II) and Pt(II) complexes are of great interest due to various choices for tuning the mesogenic properties of these compounds, as well as other physico-chemical properties, such as their promising luminescent properties. It is worth mentioning here that room-temperature emissive Pd(II) complexes are much rarer compared to their Pt(II) analogues due to the presence of low-lying metal-centered excited states which lead to a strong tendency of non-radiative deactivation.

So far, the different mesomorphic behavior and, in some cases, the emission properties of Pt(II) metallomesogens have been reported by several groups [5], including both mono- and dinuclear compounds. The majority of these reported complexes have one cyclometallated ligand derived from the 2-ppy unit and an auxiliary acetylacetonate derivative, and only a few are based on imine compounds.

We would like to show that comparable emission properties can be achieved in simpler Pt(II) systems when the mesomorphic properties can easily be controlled by a variety of options to modify the chemical structure around the Pt atom. Moreover, careful design can lead to lower transition temperatures, preventing chemical decomposition and ensuring the stability of the mesophases near room temperature, to afford the study of emission properties in the liquid crystalline phase. These complexes are based on cyclometallated imine ligands and N-benzoylthiourea (BTU) derivatives as auxiliary ligands. Recently, we have started to investigate the influence of alkoxy-substituted BTU derivatives on the thermal behavior of square-planar Pd(II) and Pt(II) cyclometallated complexes, as well as their emissive properties [6]. Such BTU derivatives proved to be very good chelating ligands that posses two very strong donor groups (carbonyl and thioamide), giving rise to neutral metal complexes with S,O-coordination [7], [8], [9]. We have shown previously that the use of simple N-benzoylthiourea derivatives as co-ligands together with the ortho-metallated imine fragment destabilizes the mesogenic behavior of these complexes, and monotropic nematic or smectic A phases were observed [10]. By proper design of the alkoxy-substituted BTU derivatives, it is possible to obtain different types of liquid crystals and several studies dealing with purely organic liquid crystals based on the N-benzoylthiourea moiety have been reported [11], [12], [13]. In this work we present our results concerning the use of branched-alkoxy-substituted BTU derivatives as co-ligands to generate mononuclear Pd(II) and Pt(II) cyclometallated complexes with Schiff bases. It is well known that branching of the terminal alkoxy chains has a significant influence on the mesomorphic behavior of a liquid crystalline material.

Apart from the possibility to introduce chirality in the molecule, the effect of the branch in the terminal chain is to reduce considerably the melting points and thus the liquid crystal phase stability. Moreover, by introduction of branched terminal chains in the molecule, significant changes of both transition temperatures and the mesophase type of the material can be seen [14], [15], [16], [17]. Materials displaying the smectic C phase and, in particular, the chiral phase SmC have been widely studied because of the big technological potential in fast-switching displays and other optoelectronic devices. For this reason we were interested in studying the potential influence of terminal alkyloxy chain branching on the thermal stability of the smectic C phase of these complexes and we have successfully prepared a Pd(II) complex with the SmC phase stable down to room temperature. Also, the photophysical properties of the Pt(II) complexes have been investigated.

Section snippets

Results and discussion

The preparation of mononuclear Pt(II) and Pd(II) complexes, along with the numbering scheme, is presented in Scheme 1. All the new products were characterized by elemental analysis, IR, 1H and 13C NMR spectroscopy, while the liquid crystal properties were investigated by a combination of differential scanning calorimetry (DSC), polarizing optical microscopy (POM) and powder X-ray diffraction.

The structure of the new cyclometallated mononuclear complexes can be confirmed readily by IR and 1H NMR

Crystal structure description

The attempts to prepare single-crystals of the Pt(II) and Pd(II) complexes with branched alkyl-substituted BTU derivatives failed. To confirm the formation of such mononuclear complexes with BTU derivatives as co-ligands we prepared the Pt(II) complex with N-(p-F-phenyl)-N′-benzoylthiourea. Indeed, slow crystallization from a mixture of acetone and methanol afforded red crystals that were subjected to X-ray investigation.

The crystallographic data are presented in Table 1 along with selected

Liquid crystals properties

The liquid crystalline properties of the palladium and platinum complexes were investigated by a combination of hot stage polarizing optical microscopy (POM), differential scanning calorimetry (DSC) and powder X-ray diffraction. The thermal data are presented in Table 3. The assignment of liquid crystals phases was done based on their optical texture and X-ray measurements [27]. For instance, the SmA phase was assigned based on its characteristic focal fan shape texture with several homeotropic

Photophysical properties

The photophysical properties of the Pt(II) complexes in the solid state, in dichloromethane solution and as PMMA films have been investigated and the results are summarized in Table 5. All six Pd(II) complexes show no emission at room temperature and this is not surprising as it is known that emissive Pd(II) complexes at room temperature are much rarer as compared to Pt(II) complexes [28], [29], [30], [31], [32], [33].

The UV–Vis absorption spectra of the Pt(II) complexes recorded in CH2Cl2

Experimental

All the chemicals were used as supplied. C, H and N analyses were carried out with a Perkin Elmer instrument. IR spectra were recorded on a Bruker spectrophotometer using the ATR technique. UV–Vis absorption spectra were recorded by using a Jasco V-660 spectrophotometer. 1H and 13C NMR spectra were recorded on a Varian Gemini 300 BB spectrometer operating at 300 MHz, using CDCl3 as the solvent. 1H chemical shifts were referenced to the solvent peak position, δ 7.26 ppm. The phase assignments and

Conclusions

A novel series of Pd(II) and Pt(II) complexes based on cyclometallated imine ligands and N-benzoylthiourea (BTU) derivatives as auxiliary ligands has been prepared and their liquid crystalline properties as well as photophysical properties have been investigated. We were able to show that comparable emission properties with other systems based on Schiff bases reported in the literature can be achieved in simpler Pt(II) systems, making them interesting candidates for possible applications in

Acknowledgments

This work was supported by a grant from the Romanian Authority for Scientific Research, CNCS-UEFISCDI, project number PN-II-ID-PCE-2011-3-0384.

References (38)

  • B. Donnio et al.
  • A. Capape et al.

    Dalton Trans.

    (2007)
  • M. Crespo et al.

    J. Organomet. Chem.

    (2006)
  • M. Dolores Santana et al.

    Dalton Trans.

    (2011)
  • Y. Scaffidi-Domianello et al.

    Inorg. Chem.

    (2007)
  • A. Altomare et al.

    J. Appl. Crystallogr.

    (1999)
  • G.M. Sheldrick, SHELX97 – Programs for Crystal Structure Analysis (Release 97–2), Institüt für Anorganische Chemie der...
  • S. Laschat et al.

    Angew. Chem. Int. Ed.

    (2007)
  • B. Donnio et al.
  • K. Binnemans

    J. Mater. Chem.

    (2009)
  • V.N. Kozhevnikov et al.

    Angew. Chem. Int. Ed.

    (2008)
    A. Santoro et al.

    Chem. Mater.

    (2009)
    C. Damm et al.

    J. Mater. Chem.

    (2006)
    A.S. Mocanu M. Ilis et al.

    Inorg. Chim. Acta

    (2010)
    V. Cîrcu et al.

    Liq. Cryst.

    (2007)
    J. Buey et al.

    Chem. Mater.

    (1996)
    L. Diez et al.

    J. Mater. Chem.

    (2002)
    Y. Wang et al.

    Dalton Trans.

    (2011)
    Y. Wang et al.

    J. Phys. Chem. C

    (2012)
    K. Venkatesan et al.

    J. Mater. Chem.

    (2008)
    B. Bilgin-Eran et al.

    Liq. Cryst.

    (2012)
    K. Praefcke et al.

    Chem. Ber.

    (1994)
  • V. Cîrcu et al.

    Liq. Cryst.

    (2009)
  • L. Beyer et al.

    Z. Chem.

    (1981)
  • P. Muhl et al.

    Z. Chem.

    (1986)
  • K.R. Koch

    Coord. Chem. Rev.

    (2001)
  • V. Cîrcu et al.

    J. Optoelectron. Adv. Mater.

    (2008)
    A.C. Tenchiu et al.

    Polyhedron

    (2008)
  • T. Seshadri et al.

    J. Mater. Chem.

    (1998)
  • A. Deleanu et al.

    Rev. Chim. (Bucharest)

    (2006)
  • T. Seshadri et al.

    Liq. Cryst.

    (2007)
  • Cited by (26)

    • Organometallic Mesogens

      2022, Comprehensive Organometallic Chemistry IV: Volume 1-15
    View all citing articles on Scopus
    View full text