1276 Volume 51, Number 9, 1997 APPLIED SPECTROSCOPY0003-7028 / 97 / 5109-1276$2.00 / 0q 1997 Society for Applied Spectroscopy

High-Temperature, Oxygen-Resistant MolecularFluorescence Thermometers

FENGLIAN BAI and LYNN A. MELTON*Department of Chemistry, The University of Texas at Dallas, Richardson, Texas 75083-0688

Four strongly emitting ¯ uorescent molecules have been identi® ed aseffective liquid-phase thermometers for use at temperatures as high as175 8 C under oxygen-saturated conditions. In each case, the thermom-etry is based on shifts and/or changes in the shape of the ¯ uorescenceemission with temperature. The four molecules and the temperatureranges tested were POPOP [1,4-di(2-(5-phenyloxazoyl)benzene], 25±150 8 C; NPO [2-(1-naphthyl)-5-phenyloxazole], 25± 173 8 C; BTBP [N,N-bis(2,5-di-tert-butylphenyl)-3,4,9,10-perylene dicarboximide], 25± 1738 C; and BPB [benzo(ghi)perylene], 25± 171 8 C. Six other laser dyes weretested and were found to be unsatisfactory.

Index Headings: Fluorescence; Temperature; Thermometer ; Oxygenquenching.

INTRODUCTION

Temperature is a variable of great importance in manycommercial, industrial, and research applications. It is theprimary parameter in determining vaporization rates, ratesof chemical reactions, and, in some cases, the dominantreaction mechanism and products in a system. Fluo-rescence-based liquid-phase thermometers (FBLPTs), whichmake use of the temperature-dependent ¯ uorescence of mo-lecular probes, have been reported in recent years.1± 5 These¯ uorescent probes make possible monitoring of microscopicor hostile environments (high pressure or high temperature),where conventional thermometry, based on thermocouples,is impractical or impossible, and can provide two-dimen-sional imaging of the liquid-phase temperature ® eld.Schrum et al. reported that the blue shift with increasingtemperature of the ¯ uorescence of the molecule BTBP[N,N-bis(2,5-di-tert-butylphenyl)-3,4,9,10-perylene dicar-boximide] could serve as a ¯ uorescence-based thermometerover the temperature range 20± 80 8 C.1 In this work, wereport extensions of their work to higher temperatures andto other molecules, with the intent of providing ¯ uores-cence-based liquid-phase thermometers for a variety ofcombustion-related applications.

Development of potential FBLPT systems for precom-bustion in-cylinder monitoring of the liquid-phase temper-ature, whether it be for droplets in a spray or for fuel or oilon the cylinder wall, requires understanding of and responseto the constraints imposed by the experiment to be carriedout. The measurement of absolute intensities is quite dif® -cult in such a dynamic environment, in which beam steeringand attenuation may occur. The liquid-phase temperaturecan be expected to vary from room temperature to the liq-uid-phase boiling point at cylinder pressure, which may beas high as 250 8 C. The precombustion pressure in the cyl-inder may reach 10 or more atmospheres of air. For thisreason, ¯ uorescent diagnostics are needed, for which (1) the

Received 8 April 1996; accepted 18 February 1997.* Author to whom correspondence should be sent.

temperature may be inferred from signi® cant changes in theposition or shape of the ¯ uorescence spectrum of the probemolecule rather than from changes in the absolute intensity;(2) the total ¯ uorescence quantum yield varies minimallyfrom room temperature to 250 8 C; and (3) the ¯ uorescenceis not quenched by oxygen even at high temperatures. Thispaper describes tests of 10 strongly ¯ uorescent molecules,four of which generally meet these criteria and are recom-mended for FBLPT precombustion in-cylinder applications.

POPOP [1,4-di(2-(5-phenyloxazoyl)benzene], NPO [2-(1-naphthyl)-5-phenyloxazole], BTBP, and BPE [ben-zo(ghi)perylene] are likely to be good FBLPT probes forhydrocarbon solutions. Six other laser dyesÐ COU-2[coumarin-2], COU-120 [coumarin-120], Rho-B [rhoda-mine-B], PPO [2,5-diphenyl-oxazole], PBD[2-(4-biphen-ylyl)-5-phenyl-1,3,4-oxadiazole], and BBOT [2,5-bis(5-tert-butyl-2-benzolxazolyl)]Ð have also been studied. Fora variety of reasons (no shift of ¯ uorescence with tem-perature, strong decrease of ¯ uorescence intensity withincreasing temperature, lack of reversibility of system),they are not appropriate as FBLPT systems.

EXPERIMENTAL

Reagents. The laser dyes POPOP, NPO, PPO, PBD,BBOT, COU-2, COU-120, and Rho-B were obtainedfrom Eastman Chemical Company and were used as re-ceived. BTBP, BPE, hexadecane, decane, and heptanolwere purchased from Aldrich and were used as received.No interfering ¯ uorescence was observed from the threesolvents. Industrial nitrogen gas (99.995%) was pur-chased from Air Liquid Company.

Experimental Apparatus and Procedures. POPOP andNPO were dissolved in decane or hexadecane directly.BTBP and BPE were ® rst dissolved in heptanol; then thissolution was diluted with hexadecane and/or decane.

The ¯ uorescence spectra were taken on a Spex Fluo-rolog spectro¯ uorimeter with a DM1B data system. Acuvette, which could be sealed with a Te¯ ont vacuumstopcock, was inserted into an electrically heated insu-lated assembly and placed in the sample compartment ofthe spectro¯ uorimeter. The temperature was measured byusing a type-K thermocouple/digital thermometer (Ome-ga HH82) located just outside the cuvette in the heatedzone. For the nitrogen-purged solutions, pure nitrogengas was bubbled into the solution for 20 min, and thecuvette was then sealed. For air-saturated solutions, thecuvette was sealed under normal atmosphere.

Each set of data consisted of spectra collected at sev-eral temperatures varying between room temperature anda maximum temperature. Tables I and II list the solvents,probe concentrations, excitation wavelengths, and otherexperimental parameters and results.

APPLIED SPECTROSCOPY 1277

TABLE I. Summary of ¯ uorescence measurements of recommended dyes.a

Dye/solvent

Concentra-tion (M)/

absorbance l ex(nm) l B(nm) l R(nm) l TOT(nm) HT(8 C)N2 purged rev

IHT/IRT

Air saturatedrev IHT/IRT Q(RT)

POPOP/C16

NPO/C16

BTBP/C16 1 C10 1 HPOH

BPe/C16 1 HPOH

5.0E± 60.25

1.9E± 50.23

1.9E± 60.10

1.15E± 50.09

340

320

480

355

350± 390

330± 370

490± 522

370± 415

485± 550

460± 550

570± 650

416± 550

365± 565

360± 560

490± 650

370± 550

250

250

173

171

1.06

0.78

0.78

1.32

Y

Y

Y

Y

1.00

0.79

0.98

0.98

Y

Y

Y

P

0.99

1.06

0.95

0.33

a See Appendix for de® nitions of terms.

TABLE II. Summary of ¯ uorescence measurements of rejected dyes.

Dye/solventConcentration

(M)/absorbanceAtmo-sphere l ex(nm) l TOT(nm) HT(8 C)

IHT/IRT Rev

Reason forrejection

BBOT/C16

PBD/C16

PPO/C16

Coumarin 120/C16 1 HPOH

Coumarin 2/C16 1 HPOH

Rhodamine B/C16 1 HPOH

1.4E± 60.15

3.8E± 60.06

5.0E± 51.30

1.28E± 50.21

Air

Air

Air

Air

Air

Air

313

313

313

355

355

532

370 ± 570

330 ± 530

330 ± 530

370 ± 570

370 ± 570

540± 680

208

200

200

166

140

100

1.18

0.64

0.39

0.05

0.04

0.16

Y

P

N

P

P

P

No shift with T

No shift with T partly rev

Intensity drop not rev

Intensity drop partly rev

Intensity drop partly rev

Intensity drop partly rev

RESULTS AND DISCUSSION

All 10 dyes were screened against the FBLPT probecriteria stated previously: (1) the temperature may be in-ferred from signi® cant changes in the position or shapeof the ¯ uorescence spectrum of the probe molecule ratherthan from changes in the absolute intensity; (2) the total¯ uorescence quantum yield varies minimally from roomtemperature to 250 8 C; and (3) the ¯ uorescence is notsigni® cantly quenched by oxygen even at high tempera-tures. Not all tests were carried out for all 10 dye mol-ecules, since failure on one criterion caused the potentialFBLPT probe molecule to be rejected. Complete testingwas carried out on the four best FBLPT probes: POPOP,NPO, BTBP, and BPE. These are strongly ¯ uorescentmolecules; the quantum yields of ¯ orescence for POPOP,6

NPO,6 BTBP,1 and BPE7 are 0.93, 0.94, 0.99, and 0.3±0.4, respectively. The results are summarized in TablesI± III. Tables I and II summarize, respectively, the exper-imental results for the four best ¯ uorescence probes andfor the six nonrecommended probes. Table III shows theestimated thermometry results that can be obtained withPOPOP, NPO, BTBP, and BPE. The abbreviations usedin Tables I± III are de® ned in the Appendix.

By way of example, the experiments and results forPOPOP and BBOT, the ® rst systems listed in Tables Iand II, respectively, will be discussed. In this section, thetemperature and oxygen effects for the four recommend-ed systems are discussed together.

Figure 1 shows the ¯ uorescence spectra of an air-sat-urated solution of 5.0E± 6 M solution of POPOP in hex-adecane as a function of temperature. As the temperatureincreases from room temperature to 250 8 C, the integratedintensity of the ¯ uorescence remains almost constant un-der both nitrogen-purged and air-saturated conditions. As

the temperature increases, the peak position shifts to low-er wavelength; the intensity in the blue range (350± 390nm) increases, and the intensity in the red range (485±550 nm) decreases. In addition, the vibronic structure be-comes less distinct. Table I indicates that, for such a POPOPsolution, the heating process is reversible; i.e., both theinitial spectrum and intensity are recovered for both ni-trogen-purged and air-saturated solutions upon cooling ofthe solution to room temperature. The initial integratedintensity obtained with the air-saturated solution is 0.99of that obtained with the nitrogen-purged solution; i.e.,there is negligible oxygen quenching. Table III indicatesthat, for POPOP in hexadecane, under either air-saturatedor nitrogen-purged conditions, a temperature resolutionof 3± 4 8 C can be expected over a temperature range of25± 150 8 C. Table II shows that, for a 1.4E± 6 M solutionof BBOT in hexadecane under air-saturated conditions,the integrated intensity increased by 18% in going from25 8 C to 208 8 C, that the system was reversible withtemperature, and that BBOT was rejected because thespectrum showed negligible blue shift or change of vi-bronic structure with temperature.

The remainder of the discussion is directed toward thefour recommended systems: POPOP, NPO, BTBP, andBPE. Figures 1± 4 show the emission spectra of POPOP,NPO, BTBP, and BPE at different temperatures in air-saturated solutions. The spectra are virtually the same innitrogen-purged solutions except for changes in the totalintensity.

Temperature Effects on Integrated Intensity (Nitro-gen Purged). Figure 5 shows the integrated intensity ofPOPOP, NPO, BTBP, and BPE ¯ uorescence as a functionof temperature in nitrogen-purged solutions. FBLPT sys-tems do not require that the integrated intensity be constant

1278 Volume 51, Number 9, 1997

TABLE III. Summary of thermometry systems.

Dye/solvent Atmosphere T range ( 8 C) Rn(T)( 8 C)Standard error

of ® tTemperature

resolution (8 C)

POPOP/C16POPOP/C16NPO/C16NPO/C16BTBP/C16 1 C10 1 HPOHBTBP/C16 1 C10 1 HPOHBPe/C16 1 HPOHBPe/C16 1 HPOH

N2

AirN2

AirN2

AirN2

Air

25± 15025± 15025± 15025± 15025± 17325± 17325± 17125± 171

0.9034 1 0.0048T0.9091 1 0.0047T0.9306 1 0.0039T0.9391 1 0.0039T0.7251 1 0.0138T0.7058 1 0.0140T0.8739 1 0.0051T0.8724 1 0.0051T

0.01120.01350.02550.02890.06270.05760.00930.0159

2.32.96.57.44.54.13.93.9

FIG. 2. Fluorescence spectra of NPO in air-saturated solution at dif-ferent temperatures: ( ) 25 8 C; (± ± ± ) 77 8 C; ( ) 115 8 C;(´́ ´́ ´́ ´́ ) 165 8 C.

FIG. 1. Fluorescence spectra of POPOP in air-saturated hexadecanesolution at different temperatures: ( ) 25 8 C; (± ± ± ) 75 8 C; ( )127 8 C; (´́ ´́ ´́ ´́ ) 178 8 C; (± ± ± ) 225 8 C.

with temperature, but rather that the ¯ uorescence be suf® -ciently strong at room temperature and at high temperatureto ensure that accurate ratios of the integrated intensities,IB(T) and IR(T), in the blue and red ranges of the spectrum,respectively, can be obtained. The variations shown estab-lish that, if the systems are usable near room temperature,they will also be usable at elevated temperatures.

Temperature Effects on Integrated Intensity (Air Sat-urated). Figure 6 shows the integrated intensity of POPOP,NPO, BTBP, and BPE ¯ uorescence as a function of tem-perature in (initially) air-saturated solutions. The small risesin the intensity of the POPOP and NPO ¯ uorescence at 1708 C and 160 8 C, respectively, and the factor of 4 rise in theintensity of the BPE ¯ uorescence at 150 8 C are reproduc-ible. It is thought that these rises are the result of a reductionin the amount of molecular oxygen initially present in thesolution, either through lowered solubility or through re-action with the dye molecule or the solvent, and, conse-quently, decreased quenching of the dye ¯ uorescence bymolecular oxygen.8 Similar effects have recently been ob-served for pyrene/hexadecane solutions in our laboratoryunder conditions in which oxygen release to the vaporphase was not possible.9 Because of the dramatic changesin the its intensity, BPE should be viewed as a marginallyacceptable FBLPT system. However, plots of Rn(T) vs. tem-perature for all four probes [where Rn(T) is the ratio R(T)of the intensity in the blue range of the ¯ uorescent spectrumto that in the red range at temperature T normalized to thevalue R(T) at room temperature] show no evidence of theintensity changes. In particular, for BPE, the system for

which the most signi® cant integrated intensity changes areobserved near 150 8 C, no corresponding change in the slopeis apparent near 150 8 C.

Use as Fluorescence-Based Liquid-Phase Thermom-etry Systems. Rn(T) appears to be a linear function oftemperature, although there is no theoretical reason toexpect it to be linear. This apparent linearity is an asset;in studies of potential systematic errors resulting from theuse of FBLPT systems to determine the average temper-ature of heating and cooling droplets, it was found thatmany of the errors that can result from nonuniform op-tical weighting of the transient inhomogeneous tempera-ture ® eld are minimized when the calibration curve forthe FBLPT system is linear with temperature.10

The slopes of the Rn(T) vs. T lines were determined forall four FBLPT molecules (Table III). For each molecule,there is no statistically signi® cant difference between theslope that was obtained with a nitrogen-purged solution andthat obtained with an air-saturated solution. Thus, these fourFBLPT systems should give results that are independent ofthe liquid-phase oxygen concentration.

In order to estimate the temperature resolution that canbe obtained with these FBLPT systems, we have assumedthat the experiment can be designed so as to determineRn(T) with a reproducibility of 0.01. [For the wavelengthranges chosen in this work Rn(T) typically has a rangefrom 1.0 at room temperature to 2 6 0.2 at 200 8 C.] Thetemperature resolution can then be conservatively esti-mated as the temperature change required to change Rn(T)by 2 3 0.01 5 0.02; i.e., the temperature resolution is

APPLIED SPECTROSCOPY 1279

FIG. 3. Fluorescence spectra of BTBP in air-saturated solution at dif-ferent temperatures: ( ) 25 8 C; (± ± ± ) 79 8 C; ( ) 126 8 C;(´́ ´́ ´́ ´́ ) 175 8 C.

FIG. 5. Integrated intensity of POPOP, NPO, BTBP, and BPE ¯ uores-cence as a function of temperature in nitrogen-purged solutions. (m)POPOP; ( 1 ) NPO; (*) BTBP; (M) BPE.

FIG. 6. Integrated intensity of POPOP, NPO, BTBP, and BPE ¯ uores-cence as a function of temperature in air-saturated solutions: (m) POPOP;( 1 ) NPO; (*) BTBP; (M) BPE.

FIG. 4. Fluorescence spectra of BPE in air-saturated solution at dif-ferent temperatures: ( ) 26 8 C; (± ± ± ) 75 8 C; ( ) 127 8 C;(´́ ´́ ´́ ´́ ) 171 8 C.

0.02/average slope, and for these systems it is about 48 C. With these same assumptions, the reproducibility ofan individual temperature measurement would be 2± 4 8 C.

Relationship to Theory. The results described in thispaper are empirical; the search for additional, more effectiveFBLPT systems would be more ef® cient if a theoreticalbasis existed that would allow the prediction of the pho-tophysical properties that would guarantee satisfaction ofthe selection criteria: (1) insensitivity to oxygen quenching,(2) insensitivity to temperature, (3) reversible behavior(probably best characterized as lack of reaction with oxy-gen), and (4) signi® cant shift (or change) of spectrum withtemperature. Basic models for some of these phenomenaare described in the following paragraphs; however, in gen-eral, these models are not adequate for the prediction ofeffective high-temperature FBLPT systems.

Oxygen is generally assumed to quench the ¯ uores-cence of organic molecules with unit ef® ciency, and thusmolecules with short ¯ uorescence lifetimes are expectedto be less sensitive to quenching by oxygen. The ¯ uo-rescent lifetimes for POPOP, NPO, BTBP, and BPE are

1.8 ns,11 2.8 ns,11 3.8 ns,12 and 80± 100 ns,13 respectively.Quenching by oxygen is expected to be minimal for allbut BPE (see Table I).

The insensitivity of the total ¯ uorescence to tempera-ture is more dif® cult to characterize. Most likely, the con-trolling parameters are the rates of internal conversionand/or intersystem crossing, which may be temperaturedependent and which are dif® cult to predict from thestructure of the molecule. Reactivity with oxygen at hightemperature is similarly dif® cult to predict.

The blue shift of ¯ uorescence spectra with increasingtemperature may be due to the ¯ uorescence analog of thered shift of absorption spectra with increasing temperature.As the temperature increases, higher vibrational levels ofboth the ground and excited electronic states become pop-ulated. In absorption, this leads to increased `̀ red-edge’’absorption, since lower energy photons can cause transi-tions from excited vibrational levels of the ground electron-ic state to the lowest vibrational levels of the excited elec-tronic state. In ¯ uorescence, the same effect can lead toincreased `̀ blue-edge’’ ¯ uorescence, since higher vibration-al levels of the excited electronic state can emit to the low-est vibrational levels of the ground state. However, this ex-planation predicts that all ¯ uorescent molecules shouldshow enhanced blue-edge emission as temperature increas-es. In the work reported here, BBOT and PBD showednegligible shifts with temperature.

1280 Volume 51, Number 9, 1997

The temperature shifts are reversed if solvent relaxa-tion phenomena are signi® cant; an increase in tempera-ture generally results in a red shift, not a blue shift, ofthe ¯ uorescence spectra.14

The disappointing conclusion is that current photo-physical knowledge is probably inadequate to predict thephenomena necessary to guarantee satisfaction of the cri-teria for effective FBLPT systems, and empirical studiessuch as this one must be used to identify FBLPT systems.

Limitations on Use. These compounds should be re-garded as mildly toxic; they are not known to be carci-nogenic. Experiments should be designed with adequateventilation, and chronic contact with the skin should beavoided.

The limited solubility of the more polar ¯ uorescentmolecules, BTBP and BPE, in alkanes can be overcomeby using small amounts of heptanol (approximately 1%v/v) to prepare mixed solutions, which should give resultsthat are virtually identical to those that would have beenobtained with pure alkane solvents.

Users should regard the calibrations given in this workas indicative and should determine calibration curveswith their own apparatus prior to making temperaturemeasurements on unknown systems.

The four recommended FBLPT systems have high mo-lecular weights and boiling points. Consequently, exper-iments in which the solvent (or fuel) evaporates maycause the involatile probe molecule to be left behind atincreasing concentration. No studies of the ef® cacy ofthese probe molecules at higher concentrations have beenmade, and users should re-examine the systems prior touse under higher concentrations. It is expected that nochanges in photophysics will be introduced by dilutionto lower concentrations.

CONCLUSION

These results suggest that the ¯ uorescent molecules, PO-POP, NPO, BTBP, and BPE, are useful as ¯ uorescence-

based liquid-phase thermometry (FBLPT) systems for tem-peratures ranging up to 175 8 C. These compounds havehigh ¯ uorescence quantum yields, nearly constant ¯ uores-cence intensity as a function of temperature, insensitivity toquenching by molecular oxygen, reversible temperature ef-fects, nearly linear temperature calibration curves, and anexpected temperature resolution of approximately 4 8 C.

ACKNOWLEDGMENTS

Funding for this work was provided by Ford Motor Company throughtheir University Research Program and by the U.S. Army ResearchOf® ce through Grant DAAL04-94-G-0033. Mr. Qingzheng Lu and Mr.Yadong Zhao have provided assistance and helpful discussions.

1. F. Schrum, A. M. Williams, S. A. Haerther, and D. Ben-Amotz,Anal. Chem. 66, 2788 (1994).

2. A. M. Murray and L. A. Melton, Appl. Opt. 24, 2783 (1985).3. H. E. Gossage and L. A. Melton, Appl. Opt. 26, 2256 (1987).4. M. R. Wells and L. A. Melton, J. Heat Transfer 112, 1008 (1990).5. T. R. Hanlon and L. A. Melton, J. Heat Transfer 114, 450 (1992).6. T. G. Pavlopoulos and P. R. Hammond, J. Am. Chem. Soc. 96,

6568 (1974).7. R. Waris, W. E. Acree, Jr., and K. W. Street, Jr., Analyst 113, 1465

(1988).8. J. R. Gord, S. W. Buckner, and W. L. Weaver, `̀ Dissolved Oxygen

Quanti® cation in Fuel through Measurement of Dynamically QuenchedFluorescence Lifetimes’ ’ , ICIASF 95 Record [International Congresson Instrumentation in Aerospace Simulation Facilities (U.S.A.F. WrightLaboratory, Flight Dynamics Directorate, Wright-Patterson AFB, Ohio)July 18± 21, 1995], IEEE Publication 95CH3482-7.

9. Y. Zhao, `̀ Tests of Oxygen Independent Exciplex FluorescenceShift Thermometer’ ’ , Apprenticeship Practicum Report, Doctor ofChemistry Program, August 1996, University of Texas at Dallas(unpublished).

10. J. Zhang and L. A. Melton, J. Heat Transfer 115, 325 (1993).11. I. B. Berlman, Handbook of Fluorescence Spectra of Aromatic Mol-

ecules (Academic Press, New York, 1965).12. B. Ben-Amotz and J. M. Drake, J. Chem. Phys. 89, 10109 (1988).13. S. M. Meyerhoffer and L. B. McGown, Anal. Chem. 63, 2082

(1991).14. J. R. Lakowicz, Principles of Fluorescence Spectroscopy (Plenum

Press, New York, 1983), p. 29.

APPENDIXAbbreviations used in Tables I± III.

C16

C10

HPOH

HexadecaneDecaneHeptanol

HTRTRev

Highest temperatureRoom temperatureReversible (Y, yes; N, no), i.e., same spectrum and intensity obtained for initial solution and for heated solution after

return to room temperature; the designation P (partly reversible) is used for systems for which the intensity wassigni® cantly lower after heating and cooling than it was in the initial system

lex

lTOT

lB

lR

I(T)

Excitation wavelengthWavelength range for the entire emission spectrumWavelength range of the blue range of the emission spectrumWavelength range of the red range of the emission spectrumIntegrated intensity of the emission spectrum at temperature T

IB(T)IR(T)P(T) 5 I(T)/I(RT)

Integrated intensity of emission in blue range at temperature TIntegrated intensity of emission in red range at temperature TRatio of integrated intensity at temperature T to that at room temperature; a measure of the effect of temperature on

¯ uorescence intensityR(T) 5 IB(T)/IR(T)Rn(T) 5 R(T)/R(RT)Q(T) 5 I(T)air/I(T)N2

Ratio of integrated intensity in blue range to that in red range at temperature TR(T) normalized to value of R(RT) at room temperatureRatio of integrated intensity under air-saturated conditions to that under nitrogen-purged conditions; a measure of the

effect of oxygen quenching