ARTICLE pubs.acs.org/JPCC
Optical, Magnetic, Electrochemical, and Electrical Properties of 8-Hydroxyquinoline-Based Complexes with Al3þ, Cr3þ, Mn2þ, Co2þ, Ni2þ, Cu2þ, and Zn2þ Lorena M. A. Monzon,* Franklyn Burke, and J. M. D. Coey Physics Department, SNIAMS Building, Trinity College Dublin, Ireland ABSTRACT: Metallic complexes containing the 8-hydroxyquinoline ligand were synthesized to investigate the effect of transition-metal ions on their electronic structure and electrical conductivity. The materials were characterized by UVvisible absorption spectroscopy, cyclic voltammetry, and electrical and magnetic measurements. Our results indicate that the π electrons localized on the ligands do not interact with the d electrons of the metal. The free ligand is electroactive only at very low and very high overpotentials. This trend is maintained when the ligands are part of the complexes. The 3d ions with an open-shell configuration, which are all in a high-spin state, show electron-transfer reactions lying in the middle of the potential window. Nevertheless, once in the solid state, these electroactive ions pin the Fermi level of the injecting electrode and act as charge traps. The mobility of electrons hopping among 3d levels is several orders of magnitude less than that of electrons hopping among the π* orbitals of the complexes with Al or Zn.
1. INTRODUCTION Small organic molecules are an interesting and versatile class of materials.1,24 They may be investigated in solution or in the solid state as polycrstalline powders, single crystals, or thin films prepared by low-temperature thermal evaporation, thanks to their weak intermolecular interactions. In particular, tris(8-hydroxyquinoline) aluminum(III), AlQ3, has been employed for the last few decades in organic light-emitting devices, OLEDs, given its thermal stability, adequate charge carrier mobility, and high luminescence efficiency.510 There is a gap of ∼3 eV between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), and photoluminescence and electroluminescence can be observed. The field of spin electronics has emerged as a central concern in magnetism in recent years.1116 Here, the aim is to exploit the spin-polarized electron currents in new device structures that can serve as sensors or memory or logic elements. The field developed using metallic ferromagnets and metallic or insulating transport layers, such as Cu or amorphous AlOx and crystalline MgO. Recently, there has been an interest in using thin organic layers in spin devices, either as transport layers or tunnel barriers.1721 Organic semiconductors offer weak spinorbit coupling and hyperfine interactions, very long spin lifetimes, and moderate spin diffusion lengths, and they are stable, which makes them good candidates for spin-polarized electron transport.22 One approach for modifying the magnetic properties of various molecular semiconductors is to combine a range of magnetic ions with organic molecules via chemical synthesis. To this end, the ligand present in AlQ3, 8-hydroxyquinoline, QH, is chosen to form organometallic complexes with various ions from the 3d transition series. The phenol moiety in the quinoline molecule has r 2011 American Chemical Society
an acidic proton because the anion formed upon deprotonation is stabilized by resonance. Together with the pyridine nitrogen, these two positions have nonbonding electron pairs that establish coordinate covalent bonds with highly polarizant ions. The formation of these complexes takes place via the displacement of the acidic protons by the metallic ions according to Mzþ þ zHQ f MQ z þ zHþ
ð1Þ
where z is the charge of the M ion and HQ is hydroxyquinoline. Thus, the amount of quinoline molecules present in complexes of this type is determined by the charge of the ion. We have synthesized a series of MQz complexes with transition metals from Mn to Zn to study the effect of the cations on their magnetic and electronic properties.
2. EXPERIMENTAL METHODS Tris(8-hydroxyquinoline) aluminum(III), AlQ3, and bis(8hydroxyquinoline) zinc(II), ZnQ2, were used as-received from Sigma. The other bis or tris(8-hydroxyquinoline) M materials with M = Cr, Mn, Co, Ni, and Cu were synthesized in our laboratory, following standard procedures.2325 Typically, 1 g (6.88 mmol) of 8-hydroxyquinoline, HQ, (Sigma) was dissolved in 150 mL of ethanol; the solution was stirred and heated. A separate solution was prepared with the chosen cation M(II) or M(III) chloride salt in water so that the amount of HQ to metal in the solution is in a 2:1 or 3:1 ratio, respectively. This aqueous Received: January 31, 2011 Revised: March 18, 2011 Published: April 20, 2011 9182
dx.doi.org/10.1021/jp201019c | J. Phys. Chem. C 2011, 115, 9182–9192
The Journal of Physical Chemistry C
ARTICLE
Figure 1. UVvis absorbance spectra of (black line) MQz and HQ in DMSO solutions and (red line) films. Concentrations: HQ, 0.3 mM; MQ2, 0.3 mM; and MQ3, 0.2 mM. The film thickness is 80 nm. The intensity of the main transition is similar for AlQ3 and CrQ3, and among the MQ2 molecules, respectively. The film spectra were normalized for comparison.
solution was added to the hot quinoline solution and left refluxing for 2 hours. The product precipitates as a solid in the reaction mixture. The volume of this mixture was then reduced to two-thirds by allowing evaporation of some ethanol. Water was added to enhance precipitation of the solid, which was separated by filtration and washed with 20 mL of cold ethanol and a copius amount of water. The product was then purified by recrystallization, dissolving the solid in the least possible amount of ethanol, followed by addition of water. This procedure was repeated twice. Finally, after filtration, the solid was dried at 80 °C under vacuum in a rotatory evaporator over 1 day. The yield in all cases was 4852%, as the product is soluble in ethanol. A Siemens XRD system with a Cu KR (λ = 1514.1 pm) X-ray source was used to charaterize the MQz powders and films. UVvisible spectra of the molecules in solution and thin films were obtained in a Shimadzu 2401 PC spectrometer. Electrochemical measurements were performed in a M. Braun 130 glovebox with the O2 level
