Europium beta-diketonate temperature sensors

REVIEW OF SCIENTIFIC INSTRUMENTS

VOLUME 75, NUMBER 1

JANUARY 2004

Europium beta-diketonate temperature sensors: Effects of ligands, matrix, and concentration Gamal E. Khalil,a) Kimberly Lau, Gregory D. Phelan, Brenden Carlson, Martin Gouterman, James B. Callis, and Larry R. Dalton Department of Chemistry, The University of Washington, Seattle, Washington 98195

共Received 30 June 2003; accepted 3 October 2003兲 Europium beta diketonates are easily synthesized highly luminescent complexes with high temperature sensitivity. We report on the temperature dependence of the luminescence of recently synthesized europium complexes originally prepared for use as light emitting diodes. It has been discovered that when incorporated in a polymer matrix, their decay lifetime can provide accurate measurement of temperature. Their lifetime as a function of temperature depends on three factors: 共i兲 the type and number of ligands in the complex, 共ii兲 the particular polymer used for the matrix, and 共iii兲 the europium chelate to polymer matrix concentration ratio. Various tris and tetrakis europium chelates are used to study ligand effects, while the polymers FIB, polycarbonate, and Teflon© are used to analyze matrix effects. In all cases studied, higher concentrations give rise to shorter lifetimes and higher temperature sensitivities, with sensitivity defined as ⌬I/(I ref⌬T). We propose to explain this phenomenon by using the following equation: 1/␶ obs⬅K total⫽k r ⫹k nr (T) ⫹k c ( 关 Eu兴 ). Here K total is the observed decay rate, which is the inverse of the observed lifetime, while k r and k nr (T) are the radiative and nonradiative decay rates, respectively. As well as being dependent on temperature, k nr (T) for these complexes is very dependent on the environment, i.e., solvent or polymer, and can be considered as k en(T). The rate k c ( 关 Eu兴 ) is the quenching term dependent on the concentration of the europium complex. © 2004 American Institute of Physics. 关DOI: 10.1063/1.1632997兴

I. INTRODUCTION

A. Luminescent inorganic phosphors

Chromium phosphors (alexandrite): One of the most successful optical temperature sensor is based on the temperature dependent luminescent lifetime of chromium 共III兲 inorganic crystal. The temperature dependency is attributed to a thermal Boltzmann distribution between the two emitting states.7 The luminescence of alexandrite, Cr共III兲 doped crystal, is known to be temperature sensitive where its lifetime varies from 220 ␮s to 300 ␮s for a temperature change between 45 °C to 15 °C.8 Unfortunately, the alexandrite crystal cannot be effectively incorporated in sensor film. Also, using chromium phosphors in powder form does not provide sufficient luminescent emission intensity and would require highly sensitive detection systems. Microcrystalline phosphors: The literature on microcrystalline phosphor-based temperature sensing is quite large.9 Discussion will be limited to two examples of known temperature sensitive phosphors. Zinc sulfide as a temperature sensitive phosphor dates back to 1949, when the temperature response curves for several zinc sulfide 共ZnS兲 based phosphors were extensively studied.10 ZnS phosphors have no temperature sensitivity from 0 °C to 25 °C, but exhibit a continuous and strong intensity change from 25 °C to 50 °C. The europium doped lanthanum oxysulfide crystal (La2 O2 S:Eu) is one of the many rare earth based phosphors that have been used as a luminescent temperature sensing material in a variety of applications.11 The phosphor La2 O2 S:Eu has two luminescent lines, a weak 514 nm line and a strong 620 nm

Most optical methods for measuring temperature can be grouped into two categories. The first is based on the temperature dependence of the absorption or the reflection of certain materials. Examples of this method include thermochromic materials,1 gas band edge absorptions that shift with temperature,2 Fabry-Perot interferometers,3 and changes in refractive indexes.4 The second category is based on luminescence temperature dependence, such as changes in wavelength, intensity, or decay lifetime. There are two types of common luminescent temperature sensors: 共i兲 inorganic phosphors, that include chromium phosphors 共i.e., alexandrite兲 and lanthanum oxysulfides 共i.e., La2 O2 S:Eu) and 共ii兲 organic molecules 共e.g., rhodamine B base dyes兲. Europium chelates are considered organic, luminescent-based temperature sensors, thus discussion will be focused on the latter category. In particular, this paper concerns luminescentbased sensors for which decay lifetime methods have proven to be reliable and accurate measurements of temperature. Measuring the decay rate rather than the intensity of these sensors produces a more robust and stable sensor because lifetime measurements are not affected by variations in light source intensity or sensor film thickness, which may affect the observed signal strength but not the time course of the decay.5,6 a兲

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Rev. Sci. Instrum., Vol. 75, No. 1, January 2004

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line. The 514 nm line is reported to undergo a lifetime change of an order of magnitude, as temperature changes from 0 °C to 100 °C. One of the main drawbacks of using inorganic phosphors as temperature sensors is the difficulty of incorporating the crystals into a polymer film while retaining uniform distribution, thus making them unsuitable for temperature sensitive paints. B. Luminescent organic molecules

Organic dyes: Compounds such as Coumarin 485, pyrene, 4-pyrazolinyllnaphthalic anhydride, and rhodamine B have been shown to exhibit luminescent intensity temperature sensitivity of sufficient slope and intensity12 for practical applications. For example, the luminescent intensity of the 580 nm band of rhodamine B was used to monitor temperature distributions for medical applications.13 The temperature dependence of the fluorescence of rhodamine B arises from increased vibrational dissipation of energy with increase in temperature. More specifically, the non-radiative deactivation of the excited state is attributed to the hindered rotation of the diethylamino side groups. From a linear least squares fit of the data between 10 °C and 45 °C, Gouterman and coworkers measured the temperature sensitivity of a rhodamine B dye in a thin polymer film to be ⫺2.30%/°C, but these have very short decay lifetimes and are not suitable for lifetime measurements of temperature.14,15 Recently Lisa Kelly of the University of Maryland16 developed an intensity ratio-metric temperature sensor film by encapsulating perylene in a copolymer containing styrene and N-allyl-N-methylaniline. Upon illumination, perylene emits a blue fluorescence and also forms an exciplex with the aromatic amine, which has a green fluorescence. The ratio of the monomer blue fluorescence of perylene to the green exciplex fluorescence was found to be temperature sensitive. Since it is based on an intensity ratio, the temperature sensor should be independent of the intensity of illumination source and is estimated to produce an accuracy of ⫾1 °C over the 25 °C to 85 °C temperature range. Eu beta-diketonate chelates: Since the classic work of Weissman17 and of Crosby,18 rare earth complexes have attracted many researchers. In particular, europium complexes have been investigated as laser materials, electroluminescent devices, biological indicators, and temperature sensors. As temperature sensors, these europium chelates can correct for temperature problems in pressure sensitive paints, monitor excessive friction over a surface, detect a boundary between laminar and turbulent flow, allow for early measurement of structural failure, and can be used in biological sensors, micro air vehicles, wind tunnel, and insect flight mechanism studies. In general, the organic ligands of the europium chelates absorb light and transfer the energy to the europium ion, Eu3⫹ , through the triplet state of the ligand. Crosby18 elegantly reviewed the process of energy transfer from the triplet manifold of the complex to the rare earth ion levels in 1966 and the weight of the literature to date accepts this plausible mechanism. Emission then occurs from the metal ion as an f – f transition. Essentially all emission originates

FIG. 1. Ligands of the europium chelates absorb light to level S 1 , and effectively transfer the energy through the triplet state, T, to the rare-earth ion levels 5 D 1 or 5 D 0 . The lowest triplet state energy level of the complex must be nearly equal to or must lie above the resonance energy level of the rare earth ion. Emission then occurs from the metal ion as an f – f transition, with thermal quenching occurring as a nonradiative process.

from the 5 D 0 energy level of Eu3⫹ ion with the strongest emission 5 D 0 → 7 F 2 transition at approximately 615 nm. Bhaumik and co-workers19 using time resolved spectroscopy showed that following the 1 S 1 ← 1 S 0 absorption by the ligand, the 5 D 1 state forms within 0.2 ␮s which then depopulates in a time of few microseconds through either radiationless transition to 5 D 0 state or radiative transition to the 7 F manifold. Radiative and nonradiative 5 D 0 → 7 F 2 transitions occur in a time order of hundreds of microseconds. Figure 1 displays the energy levels involved in the transfer of energy from the ligand to rare earth ion; only the energy levels included in this transfer are shown. In emission, the state most likely to emit is the 5 D 0 , thus 5 the D 0 → 7 F 2 transition becomes allowed by the electric dipole effects due to the surrounding ligand fields. According to our calculations, at thermal equilibrium, the 5 D 1 : 5 D 0 population level ratio is approximately 7⫻10⫺4 , meaning that energy from the triplet state is generally transferred to only the 5 D 0 level. The mechanism of the temperature sensitivity of the europium beta diketonate is based on thermal quenching; the major contribution is from the thermal deactivation of 5 D 1 and 5 D 0 europium energy levels. Deactivation is attributed to a nonradiative process due to the coupling of the electronic energy level to the environment through molecular vibration energy levels, which dominate the deactivation processes.20,17 Thermal deactivation of the 5 D 1 to the 5 D 0 level can also play an important role. The thermal deactivation of ligand energy levels prior to transferring the energy to the europium states is believed to have a minor contribution. Several research groups21,17 suggested that tris ligand chelates show larger temperature dependence than tetrakis chelates. It is believed that the addition of a fourth ligand or a Lewis base minimizes the radiationless deactivation process and produces temperature insensitive chelates. They also observed that the tris chelate has a shorter lifetime than the tetrakis chelate. It should also be mentioned that tris chelate europium complexes are very hard to obtain in a pure form,22 mostly due to the highly favored formation of a tetrakis form. An often-studied complex is the europium共III兲 thenoyl-


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FIG. 2. 共a兲 Structure of the often-studied europium tris beta-diketonate chelate, europium共III兲 thenoyltrifluoroacetonate, abbreviated as TTA. TTA has a luminescent lifetime of approximately 200 ␮s at room temperature, with both its quantum yield and lifetime dependent on temperature. 共b兲 Structure of 1,1,1-trimethyl-5,5,6,6,7,7,7-heptafluoro-2,4-heptanedioine, abbreviated as F7. Both F7 and TTA are commercially available europium chelates.

trifluoroacetonate 共TTA兲, shown in Fig. 2共a兲. It has been used successfully as a thin film temperature sensor in a poly共methyl methacrylate兲 polymer matrix for determining heating patterns on integrated circuits.19 The luminescent lifetime of this complex is approximately 200 ␮s at room temperature and it shows very little quenching by oxygen. Gallery14 explored this complex as a temperature sensor for pressure sensor correction, and it was further developed by Sullivan.23 It should be noted that both the quantum yield and lifetime of TTA are temperature dependent, and TTA was found to have an intensity temperature sensitivity of ⫺2.1%/°C.

III. EXPERIMENT A. Materials

Europium chelates are commonly synthesized by heating the europium salt together with the desired ligand in a solvent in which both are soluble. Some of these complexes are commercially available. The often studied europium共III兲 thenoyltrifluoroacetonate 共TTA兲 was purchased from Gelest, Tullytown, PA. The FIB polymer was purchased from ISSI, Dayton, Ohio. B. Synthesis of europium chelates

II. THE STUDY OF NOVEL Eu COMPLEXES

Recently Dalton et al. developed new families of europium chelate complexes for producing highly efficient light emitting diodes. Exploratory tests of these complexes by Gamal Khalil exhibited temperature sensitivities of up to 6%/°C over the range 5– 45 °C. These properties make for very useful luminescent temperature sensors. Motivated by these results, we embarked on investigating the critical factors that affect the temperature sensitivity of europium chelates sensor films. In our approach we employed the structure of the chelates and their environment to design the experimental studies. In this paper we evaluate the temperature sensitivities of europium chelate sensor films as a function of the following variables: 共i兲 type and number of ligands, 共ii兲 the carrier polymers, and 共iii兲 concentration ratio of europium chelate to polymer. We use intensity, photodegradation, and lifetime measurements for our studies, focusing on lifetime results. Selected europium chelate complexes were synthesized using different ligands of both tetrakis and tris forms as shown in Figs. 3 and 4. Special attention was given to the tris forms such that only chelates with three ligand molecules per atom of europium were synthesized. We also believe that the relative position of ligand triplet level to that of the 5 D 1 and 5 D 0 europium states is critical to thermal deactivation because using ligands with different conjugations can vary the energy levels.

1. Preparation of 1,10-phenathroline-tris[1phenanthren-3-yl-3-phenanthren-9-yl-propane1,3-dione] europium (Ref. 24) [D2P (see Fig. 3)]

9-acetyl phenanthrene and europium共III兲 chloride hexahydrate were purchased from Aldrich. 1,10phenanthroline was purchased from GFS chemicals. Methyl phenanthrene-9-carboxylate was prepared according to known procedures.25 1-phenanthren-3-yl-3-phenanthren-9-yl-propane-1,3dione 1. A mixture of 9-acetyl-phenanthrene 共2.0 g, 9.1 mmol兲, methyl phenanthrene-9-carboxylate 共2.1 g, 9.1 mmol兲, sodium ethoxide 共0.74 g, 10.8 mmol兲 and anhydrous THF were allowed to mix under nitrogen at ambient temperature for 48 hours. The solution was then acidified with hydrochloric acid and extracted using methylene chloride. After work up, the solid residue was purified by chromatography and recrystallization using ethyl acetate. 1,10phenathroline-tris[1-phenanthren-3-yl-3-phenanthren-9-ylpropane-1,3-dione] europium. To a solution of 1 共0.5 g, 1.2 mmol兲 and 1-10-phenanthroline monohydrate 共0.079 g, 0.4 mmol兲 in 8 mL of THF, 1N NaOH 共0.048 g, 1.2 mmol兲 was added dropwise. Europium共III兲 chloride hexahydrate 共0.146 g, 0.4 mmol兲 was dissolved in 2 mL of distilled water and added dropwise to the THF solution. The solution was allowed to mix at room temperature for 2 hours. A fine yellow precipitate was collected upon the addition of ethanol. The precipitate was purified with benzene and hexane.




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FIG. 3. Selected europium chelates synthesized using the 1-phenanthren-3-yl-3-phenanthren-9-yl-propane-1,3-dione ligand, abbreviated as D2. Tetrakis europium chelates, using 3 of the D2 ligands and a phenanthroline or bathophenanthroline as the fourth ligand, complexed to an europium ion, were synthesized at the University of Washington.

FIG. 4. Selected europium chelates synthesized using the 4,4,5,5,6,6,6-heptafluoro-1-phenanthren-3-yl-hexane-1,3-dione, abbreviated as 3P-HFB. Tris chelates were synthesized by complexing 3 of the 3P-HFB ligands to one europium ion. Tetrakis chelates, using 3 of the 3P-HFB ligands and a phenanthroline or bathophenanthroline as the fourth ligand, complexed to an europium ion, were synthesized at the University of Washington.


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2. Preparation of 1,10-phenanthrolinetris[4,4,5,5,6,6,6-heptafluoro-1-phenanthren3-yl-hexane-1,3-dione] europium (Ref. 27) [3P-HFB (see Fig. 4)]

3-acetylphenanthrene and ethyl heptafluorobutyrate butane were purchased from Aldrich and used without further purification. 1,10-phenanthroline and 4,7-diphenyl-1,10phenanthroline were purchased from GFS chemicals and used without further purification. 4,4,5,5,6,6,6-heptafluoro-1-phenanthren-3-yl-hexane-1, 3-dione 2. A mixture of 3-acetyl-phenanthrene 共2.0 g, 9 mmol兲, ethyl heptafluorobutyrate 共1.57 mL, 9.0 mmol兲, sodium methoxide 共0.51 g, 9.1 mmol兲, and anhydrous THF 共35 mL兲 were mixed under nitrogen at ambient temperature for 90 h. The solution was then acidified with HCl and extracted with CHCl3 . After work-up, the solid residue was purified by chromatography on a silica gel column followed by recrystallization yielding 3.70 g of needlelike yellow powder 共98% yield兲 1 H NMR: 7.36共s, 1H兲, 7.73– 8.34 共m, 7H兲, 8.97– 8.99 共d, 1H兲, 9.60共s, 1H兲. UV ␭ max : 250 nm, 370 nm. When irradiated at 360 nm (␭ max in excitation scan兲, fluorescence was observed at 460 nm. Elemental analysis calculation for C20H11F7 O2 : C, 57.70%; H, 2.66%. Found C, 57.39%; H, 2.51%. 1,10-phenanthroline - tris [4, 4, 5, 5, 6, 6, 6-heptafluoro -1 phenanthren-3-yl-hexane-1,3-dione]-europium: To a solution of 2 共0.100 g, 0.24 mmol兲 in 4 mL THF, 1N solution of NaOH 共0.010 g, 0.24 mmol兲 was added followed by the addition of a aqueous solution of EuCl3 6H2 O 共0.029 g, 0.08 mmol兲. The reaction mixture was heated at 60 °C for 2 hours. The solution was cooled to room temperature. A precipitate was obtained by the addition of cold ethanol to the mixture. It was then purified by dissolving in benzene and precipitated with hexane. Fast atom bombardment mass spectrometry was used to confirm expected molecular mass. UV ␭ max : When irradiated at 365 nm (␭ max in excitation scan兲, fluorescence was observed at 610 nm. Elemental analysis calculation for C72H38EuN2 O6 : C, 54.80%; H, 2.43%; N, 1.78. Found C, 53.49%; H, 2.31%; N, 1.62%. C. Sensor paint formulation

Polymer stock solutions of 5% polycarbonate-silicone copolymer in dichloromethane and 5% FIB in trifluorotoluene were prepared for each dilution series. Dilutions of dye to polymer were made at 1:10, 1:50, 1:200, and 1:600 ratios, by weight. Sensor films were fabricated by spraying or spin coating samples onto a surface of aluminum or glass. A Paasche, single action/external use airbrush was used to apply approximately 5 ml of solution to a 1⫻1 inch square of aluminum. Thin sensor films were also prepared using a Single Wafer Spin Processor WS-400A-6NPP/LITE by Laurell Technologies Corporation 共North Wales, PA兲. Approximately 2 ml of the solution was pipetted onto an aluminum or glass coupon surface, which was placed over the vacuum seal of the machine. The apparatus was programmed to spin at 1000 rpm for 20 seconds. The resulting films produced a uniform thickness of ⬃1 ␮m. An American ScientificProducts DS-44 oven at 70 °C was used to dry and anneal the paints to the aluminum surface for a duration of 30 minutes.

D. Instrumentation

Absorbance was measured with a Hewlett Packard 8452A Diode Array UV–Vis spectrophotometer. Emission was measured using a Perkin Elmer L55013 Fluorimeter. Samples were excited at the wavelength of the Soret band absorbance. Emission spectra were scanned from 500–750 nm. Excitation scans were run on all emission peaks and agreed with the peaks of their respective absorbance spectra. The temperature slopes ( ␦ ln(I/Imax)/␦T)P at constant pressures were determined using a locally built Photomultiplier Tube 共PMT兲 Survey Apparatus designed by Sheldon Danielson. The sample chamber is made of aluminum and fitted with a quartz window. It connects to a mechanical vacuum pump through a set of valves to bring about pressure changes within the chamber where the sample is located. Sample temperature is computer controlled using a thermoelectric module 共TEM兲 that can add or subtract heat and set temperature to the desired value. A TEM is a small solidstate device which consists of two different conductors connected electrically in series and positioned between two ceramic plates. When current flows through the module in one direction heat moves to one side of the TEM, creating a hot side and a cold side. If the current is reversed, then heat moves to the opposite side of the TEM. Temperature is measured by this integrated circuit with a 0.1 °C resolution. Measurements are made from 5–50 °C in 5 °C increments. The excitation light source for the PMT Survey Apparatus is a tungsten-halogen lamp, connected to a stabilized DC power supply. Variable light intensities can be selected since the 0–20 V power supply voltage is manually controlled. After the light source passes an appropriate excitation filter, it reaches the sample and the emitted light at the desired wavelength is detected by the PMT. In these experiments, a 360 nm band-pass excitation filter and a 600 nm band-pass emission filter were used to measure the temperature sensitivities of the majority of the different paints. The experiment is visibly monitored by computer display. The PMT Survey Apparatus also is used to measure photodegradation. While under typical illumination intensity, sensor film intensity is monitored at 1 atm and 25 °C under constant illumination intensity for a period of at least 1 hour. Sample heating by the exciting light is not offset. Luminescence lifetimes were measured on a homemade instrument 共the Lifetime Survey Apparatus兲 described earlier.26 Samples of pressure and temperature sensitive point 共PSP and TSP兲 on a rigid substrate 共up to 2.5 cm⫻2.5 cm兲 are placed in a temperature and pressure regulated chamber. The substrate is held to a copper plate using the cohesive force of thermal grease. An Analog Devices AD590 temperature sensing integrated circuit 共IC兲 is affixed to the copper mounting plate to provide measurement of the PSP or TSP sample temperature. The pressure inside the chamber is set using a manifold of four Red Hat Gas solenoids that are under computer control. The copper plate is thermally regulated using a solidstate thermoelectric heat pump that is controlled by an integrating proportional temperature controller. This controller




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FIG. 5. Spectrum of observed absorption and emission intensity for three europium chelates in dilute solution, taken at room temperature. Although absorption intensities vary for different europium chelates, they all exhibit a sharp, intense emission band at 615 nm.

series of europium chelates are shown in Table I. While a high extinction coefficient is desirable for larger transfer of energy from the ligand to the metal center of a light emitting complex, the energy overlap between the two energies is also very important.27 Figure 5 also shows the emission spectra of europium complex sensor films all exhibiting a very sharp intense band at 615 nm at room temperature.

seeks to maintain the temperature of the mounting plate at a computer or manually set temperature. The sample is then excited with a nitrogen-pumped laser at 337.1 nm, and the emission received at the photomultiplier tube detector is filtered with an appropriate long wavelength band-pass filter. The detector signal is fed into a preamplifier and then to a computer and digitized every 0.1 ␮s. Lifetimes were extracted from the intensity of the phosphorescence band and lifetime decay curves were fit to a dual exponential model. The preamplifier and system decay time were measured using MgTFPP, which has a 650 nm fluorescent band with 10 ns lifetime. Our instrument produced a MgTFPP lifetime of 0.1 ␮s. Deconvolution of the data with the system response was attempted but did not yield improved residual error due to uncertainty in the start of emission relative to the time of the first data point. Using La2 O2 S:Eu lifetime measurements which include nine lifetime measurements taken between 5–50 °C, the average lifetime was calculated as 346.7 ␮s and the standard deviation was calculated as ⫾2.7 ␮s. From dividing the standard deviation by the average lifetime, the coefficient of variation 共CV兲 of the Lifetime Survey Apparatus was estimated as 0.8%. The error in a single lifetime measurement can then be calculated by determining the upper and lower bounds 共⫾0.8%兲 of any series of lifetime measurements.

The europium chelate sensors show no dependence on the amount of oxygen above the surface layer, even at 100% oxygen as shown in Fig. 6. The europium complexes are not quenched by oxygen partly because the transfer of energy from the organic triplet state of the ligand to the emitting level of the europium ion is an intramolecular process believed to be less than 0.2 ␮s,28 and partly due to the shielding effects of f – f transitions. Because these TSP europium chelates show no oxygen sensitivity, one can be incorporated along with a PSP into a single polymer matrix, providing a lifetime based dual pressure and temperature sensor. In this dual system, both luminophors have absorption bands within the 370– 400 nm region, but nonoverlapping emission spectra upon excitation with a broadband illumination at 400 nm.29

IV. RESULTS AND DISCUSSION

TABLE I. Spectral characterization of 10⫺4 dilutions of various Eu chelates.

B. Oxygen sensitivity

A. Spectral characterization

Figure 5 shows the absorption spectra of several europium chelates in dilute solution at room temperature. The absorbance of these chelates is seen to depend on the type and number of ligands present. The compounds show a main absorbance in the range of 350 nm to 400 nm, depending on the ligand. The molar extinction coefficients at the peaks of a

Excitation wavelength ⑀ Area under the curve ⌽a a

Eu(3P-HFB) 3

Eu(2P-HFB) 3

347 62 900 15.0⫻103 1.00

333 339 75 400 30 900 9.75⫻103 3.73⫻103 0.54 0.51

Eu(9P-HFB) 3

All quantum yields were relative to that of Eu(3P-HFB) 3 which was set to 1.


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FIG. 6. 共a兲 Intensity vs time for the pressure sensitive paint 共PSP兲, PtTFPL in FIB for various pressures at room temperature, ␭ em⫽740 nm. 共b兲 Intensity vs time for the temperature sensitive paint 共TSP兲, D2 in FIB for various pressures at room temperature, ␭ em⫽615 nm.

C. Ligand effects

The ligand effects of various europium chelates were studied using tris and tetrakis europium chelates. Eu共III兲 thenoyltrifluoroacetonate 共TTA兲 and 1,1,1-trimethyl5,5,6,6,7,7,7-heptafluoro-2,4-heptanedioine 共F7兲 were commercially available through Gelest, while C105H71EuN2 O6 (D2P), (3P-HFB) 3 Eu, (3P-HFB) 3 EuP, (3P-HFB) 3 EuBP, and F10 were synthesized at the University of Washington 共see Figs. 2, 3, and 4兲. The first experiments studied the temperature behavior of four europium chelates based on commonly available ligands in two polymers, which were screened for temperature sensitivity, photodegradation, and lifetime decay 共Table II兲. The second experiments studied the same effects using three novel temperature sensors (3P-HFB) 3 Eu, (3P-HFB) 3 TABLE II. Temperature dependence of various europium chelates as a function of ligand.

Ligand D2P TTA F10 F7 a

EuP, and (3P-HFB) 3 EuBP 共Table III兲 based on ligands recently synthesized at the University of Washington. While conserving the concentration, varying the types and amounts of ligands actually affects temperature sensitivity, photodegradation, and mean lifetime 共see Fig. 7兲. The second study using the three novel chelates shows that although photodegradation was lower with the tris complex, with this family of chelates, the tetrakis complexes displayed higher intensities and longer mean lifetimes 共see Fig. 8兲. Because these europium chelates are ligand type dependent as well as number dependent, we cannot simply state that either tris or tetrakis chelates are always better than the other. Temperature dependency will vary between different ligand types and families as well as on the type of fourth ligand used, if any. TABLE III. Temperature dependence of three novel europium chelates as function of ligand.

Temperature sensitivity Degradation Lifetime sensitivity 共% Intensity/°C兲共% Intensity lost/h兲共␮s/°C兲 ⫺4.42 ⫺4.28 ⫺1.08 ⫺0.9

All 1:600 dilution dye:polymer in FIB.

⫺12.6 ⫺5.4 ⫺1.2 ⫺1.8

⫺4.28 ⫺6.43 ⫺0.54 NA

Ligand (3P-HFB) 3 Eu (3P-HFB) 3 EuP (3P-HFB) 3 EuBP a

Temperature sensitivity 共% Intensity/°C兲

Degradation 共% Intensity lost/h兲

Lifetime sensitivity 共␮s/°C兲

⫺1.41 ⫺2.40 ⫺2.27

⫺7.20 ⫺16.20 ⫺15

⫺3.77 ⫺6.20 ⫺6.37

All 1:600 dilution dye:polymer in FIB.




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FIG. 7. Temperature sensitivity of various europium chelates in FIB polymer at a 1:600 dilution, as affected by only the type and number of ligands chelated to the europium ion. 共a兲 Temperature sensitivity is seen over a range of 5–50 °C. 共b兲 The photodegradation of the same europium chelates is shown to be ligand dependent over a range of 1 hour. 共c兲 With the same samples, change in mean lifetime 共␮s/degree兲 is observed.

D. Polymer effects

Three polymers were tested in the experiments done throughout this study. Poly共bisphenol A carbonate兲共polycarbonate兲, a high molecular weight polymer averaging 64 000 g/mol 共GPC兲, can be synthesized by solid-state polymerization. The fluoroacrylic polymer 共FIB兲 can be synthesized by copolymerization, occurring in one step with a peroxide inhibitor. Polymers like FIB are hypothesized to have a reduced induction effect, fast response times, low photodegradation, and low temperature dependence. Finally, Teflon©

was used as a quick and easy matrix for some of these studies. The largest changes in temperature sensitivity and mean lifetimes between the same samples at different dilutions are highest with the FIB polymer, followed by polycarbonate, with Teflon© showing the least amount of change 共see Fig. 9兲. E. Concentration effects

The first concentration study was done using 1:10, 1:50, 1:200, and 1:600 dye to polymer dilutions 共gram ratio兲 of the


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FIG. 8. Temperature sensitivity of three novel temperature sensors, (3P-HFB) 3 Eu, (3P-HFB) 3 EuP, and (3P-HFB) 3 EuBP in FIB polymer at a 1:600 dilution, as affected by only the type and number of ligands chelated to the europium ion. 共a兲 Temperature sensitivity is seen over a range of 5–50 °C. 共b兲 The photodegradation of the same europium chelates is shown to be ligand dependent over a range of 1 hour. 共c兲 With the same samples, change in mean lifetime 共␮s/degree兲 is observed. This comparison of tris and tetrakis europium chelates of the same ligand family shows that the tetrakis chelates display higher temperature dependence 共␮s/degree兲 than the tris chelate.

tetrakis europium chelate, D2P, in the polymers FIB 共see Fig. 10兲, polycarbonate, and Teflon©. The second concentration study used the tris europium chelate, TTA, in the polymers FIB 共see Fig. 11兲 and polycarbonate, with the same dilutions under the same conditions. For both D2P and TTA chelates, temperature sensitivity results show an increase in intensity with an increase in concentration, as expected. However, the slope of the lifetime (⌬ ␶ /⌬T) actually increases with decreasing concentrations. Overall results show that for TTA and D2P as a function of concentration, the tris complex 共TTA兲 is a better temperature sensor than the tetrakis 共D2P兲. In all dilution conditions, pho-

todegradation was less and the change in intensity temperature sensitivity 共% intensity/°C兲 as well as decay lifetime 共␮s/°C兲 was larger in the tris complex in comparison to the tetrakis 共see Table IV兲. These derivatives determine how accurately TSP lifetimes can measure temperature. These trends remain constant in different polymer matrices. V. MODEL

As stated, the nonradiative relaxation of europium complexes may occur by the interaction between the 5 D 1 and 5 D 0 electronic levels and the vibrational modes of the envi-




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FIG. 9. Mean lifetimes of the TSP, D2, in three different polymer matrices at a 1:600 共w/w兲 dilution showing the polymer effect on the temperature sensitivity of europium chelates. The three polymers are fluoroacrylic polymer 共FIB兲, poly共bisphenol A carbonate兲共polycarbonate兲, and Teflon©.

FIG. 10. 共a兲 The mean lifetimes of the TSP tetrakis europium chelate, D2, in FIB polymer at 1:10, 1:50, 1:200, and 1:600 共w/w兲 dilutions showing the concentration effect on their temperature sensitivity. For each concentration, sensitivity is monitored by the change in lifetime 共␮s/°C兲 as a function of temperature. In all cases, we observe an increase in sensitivity, with a decrease in concentration. 共b兲 The total rate constant, K 共the inverse of the mean lifetime兲, is plotted as a function of temperature in degrees Kelvin.


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FIG. 11. 共a兲 The mean lifetimes of the TSP tris europium chelate, TTA, in FIB polymer at 1:10, 1:50, 1:200, and 1:600 共w/w兲 dilutions showing the concentration effect on their temperature sensitivity. For each concentration, sensitivity is monitored by the change in lifetime 共␮s/°C兲 as a function of temperature. In all cases, we observe an increase in sensitivity, with a decrease in concentration. 共b兲 The total rate constant, K 共the inverse of the mean lifetime兲, is plotted as a function of temperature in degrees Kelvin.

ronment. It is hypothesized that the efficiency of this process is dependent on the environment 共i.e., the polymer matrix, concentration兲 and on the temperature. We propose to describe quenching by the following equation: 1/␶ obs⬅K total⫽k r ⫹k nr 共 T 兲 ⫹k c 共关 Eu兴兲 .

共1兲

Here K total is the observed decay rate, which is the inverse of the observed lifetime, k r and k nr (T) are the radiative and nonradiative decay rates, respectively. As well as being dependent on temperature, k nr (T) for these complexes is very dependent on the environment, i.e., solvent or polymer, and henceforth will be denoted k en(T). The rate k c ( 关 Eu兴 ) is the

quenching term dependent on the concentration of the europium complex, here abbreviated as 关Eu兴, that is assumed to be linear as in Eq. 共1兲. From this energy level diagram 共Fig. 12兲, we were able to determine the rate equations applicable to the transfer of energy from the ligands to europium ion, d关 S0兴 ⫽⫺ 共 K ex 兲关 S 0 兴 ⫹ 共 k f ⫹k 1 兲关 S 1 兴 ⫹ 共 k p ⫹k 3 兲关 T 兴 , dt d关 S1兴 ⫽ 共 Kex 兲关 S 0 兴 ⫺ 共 k f ⫹k 1 ⫹k 2 兲关 S 1 兴 , dt

TABLE IV. Table of results for TTA and D2P at various dilutions in FIB.

Dilution 1 1 1 1 1 1 1 1

to to to to to to to to

10 10 50 50 200 200 600 600

Complex

# Ligands

Temperature sensitivity 共% Intensity/°C兲

TTA D2P TTA D2P TTA D2P TTA D2P

3 4 3 4 3 4 3 4

⫺5.21 ⫺3.70 ⫺4.48 ⫺3.57 ⫺3.97 ⫺2.16 ⫺3.25 ⫺2.67

Degradation 共% Intensity lost/h兲

Lifetime sensitivity 共␮s/°C兲

0.60 4.20 0.54 7.80 0.42 3.00 1.80 3.00

⫺3.59 ⫺0.41 ⫺4.4 ⫺0.54 ⫺6.57 ⫺2.07 ⫺6.72 ⫺4.28




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FIG. 12. Energy level diagram depicting the transfer of energy from ligand to europium metal. We describe the quenching of the temperature sensors by the equation: 1/␶ obs⫽K total⫽k r ⫹k nr (T)⫹k c 关 Eu兴 .

d关T兴 ⫽ 共 k 2 兲关 S 1 兴 ⫺ 共 k p ⫹k 3 ⫹k 4 ⫹k 4⬘ 兲关 T 兴 , dt 共2兲 d 关 5D 1兴 ⫽k ⬘4 关 T 兴 ⫺ 共 k D1 ⫹k 5 ⫹k 6 ⫹k en⫹k C 关 Eu 兴兲关 5 D 1 兴 , dt d 关 5D 0兴 ⫽k 4 关 T 兴 ⫹k 5 关 5 D 1 兴 ⫺ 共 k D0 ⫹k 7 ⫹k en⫹k C 关 Eu 兴兲 dt ⫻ 关 5D 0兴 , d 关 7F 2兴 ⫽ 共 k D1 ⫹k 6 兲关 5 D 1 兴 ⫹ 共 k D0 ⫹k 7 兲关 5 D 1 兴 . dt Based on our observations, we assume that at low temperatures and dilute dye concentrations, k en(T) and k c ( 关 Eu兴 ) goes to zero. At any concentration, one can determine the rate attributed to k en from the difference k en共 T 兲 ⫽K total⫺K total 共as T→0).

共3兲

The concentration quenching constant, k c can be determined from k c 共关 Eu兴兲 ⫽K total⫺K total 共as 关Eu]→0).

共4兲

Thus, by our hypothesis, both k en(T) and k c ( 关 Eu兴 ) can be determined at multiple temperatures, while activation energies and coefficients can be determined from Arrhenius plots. The luminescent decay of these europium chelates can be described by the equation I 共 t 兲 ⫽I 0 ⫹I 1 e ⫺t/ ␶ 1 ⫹I 2 e ⫺t/ ␶ 2 .

共5兲

Here I(t) is the observed intensity as a function of time after flash excitation. It is fit with five parameters. I 0 is a constant term, while the decay is fit as a dual exponential decay with parameters I 1 , ␶ 1 , I 2 , ␶ 2 . The weighted average tau mean (¯␶ ) is determined from ¯␶ ⫽

I 1 ␶ 1 ⫹I 2 ␶ 2 1 ⬅ . I 1 ⫹I 2 具k典

共6兲

We describe temperature dependence by the Arrhenius equation with 具 k 典 , the average decay rate,

具 k 典 ⫽k 0 ⫹k 1 e ⫺T * /RT ⬅K total with T * ⫽⌬E 共 cals兲 . 共7兲

This Arrhenius equation of rate stands as our hypothesis for the determination of the total rate constant, K total , derived in Eq. 共1兲. Here k 0 is equivalent to ¯␶ at 77 K and dilute concentrations, thus minimizing the effects of K c and K en . These values were measured for the europium chelates D2P and TTA, as seen in Table V. The constant k 1 is called the frequency factor, or pre-exponential factor, and T * represents the activation energy, determined in the Arrhenius plots in Figs. 13 and 14. The plot of ln(k⫺k0) vs 1/T yields the activation energy from the slope of the curve, while the frequency factor is determined from the y intercept. Temperature dependence can be well measured using lifetime measurements extrapolated from the lifetime decay curves. For these europium chelates in polymers examined in this study, we have found that only at lower temperatures in the range studied 共5–50 °C兲 and at dilute concentrations 共1:600兲, can europium chelate decays be fit by a mono exponential. Otherwise, europium chelates are well fit by a double exponential. Average lifetime measurements are robust for light source intensity changes, sensor thickness variation, which affects only response time, and dye concentration if low. Random residuals of the fit show that at low temperatures and low concentrations, the decay curves fit well to the dual exponential program used to calculate mean lifetimes. However, at high temperatures and high concentrations, the residuals become less randomized, suggesting that higher exponential factors are present 共see Figs. 15 and 16兲. This nonexponential behavior may be attributed to multienvironments present in the film, such as k en and k c . With room for improvement in the dual decay program, lifetime methods have TABLE V. Lifetime sensitivities of 10⫺5 dilutions of D2P and TTA in various solvents.

D2P in ethanol D2P in acetone D2P in benzene TTA in ethanol TTA in acetone TTA in benzene

Lifetime sensitivity at 77 K 共␮s/°C兲

Lifetime sensitivity at RT 共␮s/°C兲

435 556 476 417 588 400

6.93 14.3 33.0 345 588 208


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FIG. 13. Arrhenius plots for decay rate for various dilutions of D2 in FIB, showing the natural log of (K⫺k 0 ) vs (1/T) in units 共Kelvin兲⫺1. Here k0 is equivalent to ¯␶ at 77 K and dilute concentrations, since these conditions minimize the effects of K c and K en , while k 1 is the frequency factor, and T * represents the activation energy. Because the variables 具 k 典共equivalent to K total), k 0 , and T, are known or can be determined, Arrhenius plots above are used to solve for k 1 共y intercept兲 and T * 共activation energy兲.

FIG. 14. Arrhenius plots for decay rate for various dilutions of TTA in FIB, showing the natural log of (K⫺k 0 ) vs (1/T) in units 共Kelvin兲⫺1. Here k 0 is equivalent to ¯␶ at 77 K and dilute concentrations, since these conditions minimize the effects of K c and K en , while k 1 is the frequency factor, and T * represents the activation energy. Because the variables 具 k 典共equivalent to K total), k 0 , and T, are known or can be determined, Arrhenius plots above are used to solve for k 1 共y intercept兲 and T * 共activation energy兲.




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FIG. 15. The plots of the residuals of the fit for the decay curve I(t) show that at lower temperatures, the decay curves fit well to the dual exponential program, giving random residuals. These plots show the europium chelate TTA in FIB polymer at a concentration of 1:600 共w/w sensor to dye兲. The residuals at higher temperatures become less randomized, suggesting a higher order exponential decay is being observed. This may be attributed to multi environments present in the film, such as the proposed environmental rate constant, K en(T).

FIG. 16. The plots of the residuals of the fit for the decay curve I(t) show that at lower concentrations, the decay curves fit well to the dual exponential program, giving random residuals. These plots show the europium chelate TTA in FIB polymer at various concentrations 共w/w sensor to dye兲 at a constant temperature of 25 °C. The residuals at higher concentrations become less randomized, suggesting a higher order exponential decay is being observed. This may be attributed to multienvironments present in the film, such as the proposed concentration quenching rate constant, K c ( 关 Eu兴 ).


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shown to provide reliable and accurate measurements of temperature sensitivity for these europium chelates. ACKNOWLEDGMENTS

The authors would like to thank the Department of Defense Multi-Disciplinary University Research Initiative 共MURI兲 Center on Polymeric Smart Skin Materials through the Air Force Office of Scientific Research Contract No. F49620-01-1-0364 under Professor Larry Dalton and NSF for funding unsteady flow, and fluid dynamics research under Professor James Callis both at the University of Washington. We thank Tim Bencic of NASA-Glenn, who also provided funding. Lafe Purvis, Biniam Zelelow, Christina McGraw, Alvin Chang, and Severin Grenoble provided useful help in the laboratory. 1

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