Luminescent Properties of Lanthanide Dibenzoylmethide Triethylammonium Compounds

Fontenot et al. Journal of Theoretical and Applied Physics 2013, 7:30 http://www.jtaphys.com/content/7/1/30 RESEARCH Open Access Luminescent properties of lanthanide dibenzoylmethide triethylammonium compounds Ross S Fontenot1*, William A Hollerman2, Kamala N Bhat1, Stephen W Allison3 and Mohan D Aggarwal1 Abstract Triboluminescence (TL) is defined as the emission of cold light based on mechanical action. In 1999, Sage and Geddes used this property to design a sensor capable of discerning the location of impacts. By coating a structure with various triboluminescent materials, impacts to structures could be monitored with simple light detectors. However, the intensity of most materials is very low. Of the thousands of known triboluminescent materials, only a few can emit enough light to be seen in daylight. One of these materials is europium dibenzoylmethide triethylammonium (EuD4TEA). This material shows 206% of the TL yield compared to the more commonly known manganese-doped zinc sulfide. Due to the high TL yield of EuD4TEA, exploration of the lanthanide series compounds was attempted for different emission wavelengths. This will help to monitor the locations of impacts on structures. This paper will investigate the TL yields, TL decay times, and the spectra of various lanthanide dibenzoylmethide triethylammonium compounds. Keywords: Triboluminescence, Rare earth luminescence, Lanthanide luminescence, Mechanoluminescence, Europium tetrakis, EuD4TEA, Lanthanide tetrakis, Emission, Excitation PACS: 78.60.Mq, 78.55.Bq, 71.20.Eh Background fully understood, but it is believed that TL is associated The generalized process of ‘cold light’ emission is called lu- with asymmetric crystal structure. Crystal bonds are minescence, and it refers to the absorption of energy by a broken along planes with opposing charge, and when material with the subsequent emission of photons [1]. It is they re-connect, light is emitted as the charges pass a phenomenon distinct from blackbody radiation, incandes- through the separations created from the fracture. A cence, or other such effects that cause materials to glow at classic example of TL light is found in the crystals used high temperatures [1-4]. There are various forms of lumi- for real wintergreen-flavored Lifesavers® [6,7]. The nescence, usually denoted by the method from which the green/blue sparks seen when chewing the candy is TL electrons were excited. For example, electroluminescence light being emitted from the crystal breakage in the happens when an electrostatic field excites the electrons. sucrose [6,7]. TL can also be produced by the peeling of Photoluminescence is the excitation of electrons due tape in a vacuum [8] and as a result of plate stresses to electromagnetic radiation. Likewise, triboluminescence during and just prior to an earthquake [4,9,10]. (TL) is light produced by the fracture of crystals [4,5]. When a TL crystal is fractured, electrons are torn away The word triboluminescence comes from the Greek from their parent atoms, resulting in a static discharge tribein, meaning ‘to rub,’ and the Latin prefix lumen, across the gap of the fracture [11,12]. The light is emitted meaning ‘light.’ TL is a specific form of mechano- by several distinct, material-dependent mechanisms. The luminescence, which is the production of non-thermal emission spectrum for sugar indicates that the light comes light from any type of mechanical action or application from the atmospheric nitrogen that fills the gap during of stress [4,5]. The mechanisms of TL light are still not fracture. This is the same source of light as that from lightning or touching a doorknob on a winter day. Spectra * Correspondence: [email protected] for other samples show emission characteristic of the 1Department of Physics, Chemistry, and Mathematics, Alabama A&M material as well as nitrogen lines. Such spectra suggest a University, P.O. Box 1268, Normal, AL 35762, USA secondary energy process [11,12]. Other substances Full list of author information is available at the end of the article © 2013 Fontenot et al.; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Fontenot et al. Journal of Theoretical and Applied Physics 2013, 7:30 Page 2 of 10 http://www.jtaphys.com/content/7/1/30 exhibit a spectrum characteristic of the material alone. Lanthanide (Ln) compounds have distinctive optical While abrupt charge separation is the same in all cases, properties, which include long luminescence lifetimes emission mechanisms depend on the material [11,12]. that range from microseconds to milliseconds and sharp In 1999, Sage and Geddes used the property of TL to emission bands with a full width half maximum that patent a design for a sensor capable of discerning the lo- rarely exceeds 10 nm. These properties make lan- cations of impact [13-15]. Their design involved coating thanides useful for organic light-emitting diodes, laser a material with a triboluminescent crystal or creating a materials, and sensors [25]. In order to increase lumines- composite triboluminescent object [13-15]. A sensor cence excitation, the Ln ion is usually coordinated to the would then be embedded within the structure or ligands of β-diketone and aromatic amine derivatives. mounted on its surface [14]. Impacts to the structure This typically increases the quantum efficiency through would produce light which would be recorded and ana- ‘synergistic effects’ and prevents the coordination of lyzed to determine the location [14]. In addition, Sage solvent molecules that quench the emissions [25]. et al. proposed that several different triboluminescent Besides their outstanding photoluminescent proper- materials could be used and arranged at various loca- ties, lanthanide β-diketone compounds containing chiral tions [13-15]. The advantage is that when an impact groups have additional properties that include second takes place, its location could be determined by the harmonic generation, ferroelectricity, TL, chirality sens- wavelength emitted [14]. For example, by placing two ing, circularly polarized luminescence, symmetric cata- different triboluminescent materials at known distances lysis, and enantiomer selective synthesis [25]. In this from the detector, it is possible to determine the ap- paper, we investigate the luminescent properties of proximate location of the impact by measuring the emit- dibenzoylmethide triethylammonium compounds syn- ted wavelength. thesized using a selection of Ln nitrates. However, the triboluminescent emission yield for most materials is small, and the resulting light can be Methods difficult to detect. For the last few years, Fontenot Synthesizing lanthanide-based dibenzoylmethide and Hollerman et al. have been working with euro- triethylammonium compounds pium tetrakis dibenzoylmethide triethylammonium The synthesis of each lanthanide dibenzoylmethide (EuD4TEA) [16-18]. This material was found to have triethylammonium compound was based on the proce- 206% of the triboluminescent emission yield compared dures and methods used for EuD4TEA as shown in [17]. to ZnS:Mn when subjected to low-energy impacts The synthesis began by dissolving 4 mmol of the lan- [17,19]. Both EuD4TEA and ZnS:Mn emit strong TL when thanide nitrate in a hot solution of 35 mL anhydrous their crystals are crushed, scratched, or struck [20]. denatured ethyl alcohol. Then, 13 mmol of 1,3-diphenyl- The first practical EuD4TEA material was synthesized 1,3-propanedione also known as dibenzoylmethane by Hurt et al. [18]. In 1987, Sweeting et al. studied the (DBM) and 14 mmol of triethylamine (TEA) was added crystal form for EuD4TEA [21] using the synthesis de- to the hot solution. The solution was then kept aside to scribed by Hurt et al. Purification by recrystallization cool at ambient temperature. Each Ln compound that was accomplished by room temperature evaporation of formed was filtered and air-dried at room temperature. methanol and dichloromethane [21]. After testing both After formation, the synthesized compounds were materials for triboluminescence by grinding each of the shown to be very stable with no degradation due to hu- crystals with a glass rod or steel spatula, it was disco- midity. Table 1 shows the color of the resulting solution vered that the methanol-based material exhibited TL or product at each step of the synthesis process. Table 1 while the dichloromethane lacked TL [21]. In order to also shows the color of the photoluminescent emission understand the structure of EuD4TEA, the final products when the synthesized crystallites were exposed to ultra- were analyzed using X-ray diffraction. From these re- violet light. The Ho, Ce, and La compounds do not emit sults, Sweeting et al. determined that the material made fluorescence when excited with ultraviolet irradiation. with methanol showed no evidence of solvent incorpo- All lanthanide elements form trivalent cations whose ration, was monoclinic, and had a centric symmetry be- chemistry is largely determined by the ionic radius, longing to the I2/a space group [21]. However, in 2001, which decreases steadily from lanthanum to lutetium. Cotton et al. showed that EuD4TEA does not belong to For this reason, the notation LnD4TEA can be used to the I2/a space group, but instead to the non-centric (and represent a generic lanthanide dibenzoylmethide

Load more