Anal. Chem. 1990, 62,2365-2369 (3) Dirk, C. W.; Inabe, T.; Schoch, K. F.; Marks, T. J. J. Am. Chem. SOC. 1983, 105, 1539. (4) Inabe, T.; Gaudiello, J. G.; Maguel, M. K.; Lyding, J. W.; Burton, R. L.; McCarthy, W. J.; Kammewurf, C. R.; Marks, T. J. J. Am. Chem. SOC. 1986, 108, 7595. (5) Nohr, R. S.;Kuznesof, P. M.; Wynne, K. J.; Kenney, M. E.; Siebernmann, P. G. J. Am. Chem. SOC.1981, 103, 4371. (6) Kolesar, E. S.;Wiseman, J. M. Anal. Chem. 1989, 67,2355. (7) Nieuwenhuizen. M. S.;Barendsz, A. W. Sens. Actuators 1987, 7 7 , 45. (8) Armstrong, N. R.; Lee, P.; Pankow, J.; Danzinger, J.; Nebesny, K. W. I n Photoelectrochemistry and Electrosynthesis on Semiconducting Materials: Giniey, D., Armstrong, N., Nozlk, A., Honda, K.,Fujishima, A., Sakata, T., Eds.; Electrochemical Society Publications: Pennington, NJ, 1987. (9) Simon, J.; A n d 6 J.J. Molecular Semiconductors; Springer-Verlag: New York, 1985; pp 73-148. (IO) Rieke. P. C.; Armstrong, N. R. J. Am. Chem. SOC. 1984, 706, 47. (11) Klofta. T.; Rieke, P. C.; Linkous, C. A.; Buttner. W. J.; Nanthakumar, A.; Mewborn, T. D; Armstrong, N. R. J. Electrochem. SOC. 1985, 132, 2134. (12) Klofta, T. J.; Danziger, J.; Lee, P.; Pankow, J.; Nebesny, K. W.; Armstrong, N. R. J. Phys. Chem. 1987, 9 1 , 5646. (13) Sims, T. D.; Pemberton, J. E.; Lee, P.; Armstrong, N. R . Chem. Mater. 1989, I, 26. (14) Rieke, P. C.;Linkous, C. L.; Armstrong, N. R. J. Phys. Chem. 1984, 88, 1351. (15) Hor, A. M.; Loutfy, R. 0.; Hsiao, C. K. Appl. Phys. Lett. 1983, 4 2 , 165. (16) Lee, P.; Pankow, J.; Danziger, J.; Nebesny, K. W.; Armstrong, N. R. I n Deposition and Growth , Frontiers for Microelectronics; Rubeloff, G. W., Ed.: Amerlcan Institute of Physics: New York, 1988. (17) Bebnger, D; Dodelet, J.-P.; Dao, L. D.; Lombos, E. A. J . phvs. Chem. 1984, 88, 4288. (18) Guay. D.; Cote, R.; Marques, R.; Dodelet. J.-P.; Lawrence, M. F.; Gravel, D.; Langford, C. H I n Photoelectrochemistry and €lectrosynthesis on Semiconducting Materials ; Ginley, D., Armstrong, N., Nozik, A., Honda, K.,Fujishima, A., Sakata, T., Eds. EiectrochemiCal Society Publications: Pennington, NJ, 1987. (19) Martin, M.; A n d 6 J.J.: Simon, J. J . Appl. Phys. 1983, 5 4 ( 5 ) , 2792. (20) Barger, W. R.; Wohltjen, H.; Snow, A. W.; Jarvis, N. L. I n Fundamentals and Applications of Chemical Sensors ; Schuetzle, D.. Hammerie, R., Eds.; American Chemical Society: Washington, DC, 1986. (21) Snow, A. W.; Barger, W. R.; Klusty, M.; Wohltjen, H.; Jarvis. N. L. Langmuir 1986, 2, 513. (22) Ricco, A. J.; Martin, S.J.; Zippeman, T. E. Sens Actuators 1985, 8 , 319. (23) Swalen, J. D.; Ailara, D. L.; Andrade, J. D.; Chandross, E. A,; Garoff, S.:Israelachvili, J.; McCarthy, T. J.; Murrar, R.; Pease, R . F.; Rabok, J.
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F.; Wynne, K. J.; Yu, H. Langmuir W87, 3, 932. Hor, A. M.; Di Paola-Baranyi, G.; Hsiao, C. K. J. Imglng (24) L O W , R. 0.; S d 1985, 29 (3), 116. (25) Wagner, H. J.; Loutfy, R. 0.; Hsiao, C. K. J. Mater. Sci. 1982, 77, 2781. (26) Mockert, H.; Schmeisser, D.; Gopel, W. Sens. Actuators 1989, 19, 159. (27) Collins, R. A.; Mohammed, K. A. J. Phys. D : Appl. Phys. 1988, 2 1 ,
154.
(28) (29) (30) (31) (32) (33) (34) (35) (36) (37) (38) (39) (40) (41) (42) (43) (44) (45)
Dahlberg, S.C.; Musser, M. E. J. Chem. Phys. 1980, 72(12), 6706. Collins, R. A.; Mohammed, K. A. Thin Solid Fllms 1986, 745, 133. Wilson, A.; Collins, R. A. Phys. Status Solids A 1986, 9 8 , 633. (a) Klofta, T. J.; Sims. T. D.; Pankow, J. W.; Danziger, J.; Nebesny, K. W.; Armstrong, N. R. J. Phys. Chem. 1987, 9 1 . 5651. (b) Klofta, T. K. Ph.D. Dissertation, University of Arizona, 1986. Popovic, 2. D. J. Chem. Phys. 1982, 77, 498. Waite, S.;Pankow, J.; Collins, G.; Lee, P.; Armstrong, N. R. Langmuir 1989, 5, 797. Arbour, C.; Armstrong. N. R.; Brina. R.; Collins, G.; Danziger, J.; Doda let, J.-P.; Lee, P.; Nebesny, K. W.; Pankow. J.; Waite, S. Mol. Cryst. Li9. Cryst. 1990, 783,307. Honeybourne, C. L.; Ewen, R. J. J. Phys. Chem. Solids 1983, 4 4 , 215 and 833. Wohltjen, H.; Barger, W. R.; Snow, A. W.; Jarvis, N. L. I€€€ Trans. Electron Devices 1985, 32, 1170. Wilson, A.; Collins, R. A. Sens. Actuators 1987, 12. 389. Linsky, J. P.; Paul, T. R.; Nohr, R. S.;Kenney, M. E. Inorg. Chem. 1980, 79, 3131. Varmuza, K.; Maresch, G.; Meiler, A. Monatsh. Chem. 1974, 105, 327. Pankow, J.; Arbour, C.; Armstrong, N. R. Unpublished work. Meier, H. Organic Semiconductors; Verlag Chemie: Berlin, 1974; pp 3 17-328. Laurs. H.; Heiland, G. Thin Solid Films 1987, 149, 129. van Ewyk, R. L.; Chadwick, A. V.; Wright, J. D. J. Chem. Soc., Faraday Trans. 1 i981, 77, 73. Rager, A,; Gompf, E.; Durselen. L.; Mockert. H.; Schmeisser, D.; a p e l , W. J. Mol. Electron. 1989, 5 , 227. Hirschfeld, T. E. U.S.P.N. 4,674,320, 1987.
RECEIVED for review April 11, 1990. Accepted July 27, 1990. Support for this work by the National Science Foundation, Motorola (University Partnerships in Research), Burr-Brown, and the Materials Characterization Program-University of Arizona is gratefully acknowledged.
Environmental Factors Affecting Micellar Stabilized Room-Temperature Phosphorescence Lifetimes Haidong Kim1 and Stanley R. Crouch*
Department of Chemistry, Michigan State University, East Lansing, Michigan 48824 Matthew J. Zabik* and Salah A. Selim
Pesticide Research Center, Michigan State University, East Lansing, Michigan 48824
Triplet lifethnes of organic compounds at low temperature are well established compared to the room-temperature phosphorescence (RTP) lifetimes. The RTP lifetimes may vary from system to system dependlng on the experimental condltlons employed. The environmental factors affecting micellar-stabllized RTP IHetImes are investigated. I t was found that solutlon deoxygenation method, temperature, and heavy atoms are the major contributor to the varlations of observed RTP ilfetlmes. A kinetic decay model of trlpiet molecules is proposed to describe the effects of triplet quenchers and external heavy atoms on phosphorescence iifetlmes In micellar solution.
* To w h o m correspondence should b e addressed. Present address: 204 Pesticide Research Center, M i c h i g a n State University, E a s t Lansing, MI 48824.
INTRODUCTION Micelle stabilized room-temperature phosphorescence (MS-RTP) spectrometry is a convenient and useful analytical technique compared to classical low-temperature phosphorescence spectrometry (1,2). Aqueous surfactant solutions exhibit the phenomenon of self-organization. Above a certain concentration, the critical micelle concentration (cmc), surfactant molecules associate spontanously to build up structural entities of colloidal dimensions called micelles. The protective screening effect of micelles from external quenchers by compartmentalization of solubilized lumiphore molecules greatly reduces collisional quenching of triplet molecules. Thus, by using external heavy atoms which increase the phosphorescence yield, phosphorescence of many polynuclear aromatic hydrocarbon (PAH) compounds can be observed in micellar solutions a t room temperature (3, 4 ) .
0003-2700/90/0362-2365$02.50/00 1990 American Chemical Society
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The decay lifetimes of the excited triplet state are important in studies of kinetic processes associated with the triplet state of compounds ( 5 ) . The lifetime of the excited triplet state is a characteristic property of a molecule and its molecular environment. Phosphorescence lifetimes frequently provide a diagnostic tool since compounds having very similar spectral characteristics may have greatly different triplet lifetimes (6). Although triplet lifetimes of organic compounds have been well established in rigid media at low temperature (77 K), RTP lifetimes in fluid media are far more variable and difficult to use for chemical analysis. In the frozen state, at liquid nitrogen temperature, collisional quenching of the triplet state by molecular oxygen and other impurities is significantly minimized, and the temperature of the sample remains constant. These conditions give reliable and consistent phosphorescence lifetimes that can be used as qualitative descriptors of the luminescent species. However, R T P lifetimes of compounds are extremely sensitive to such environmental conditions as temperature, the presence of heavy atoms, the oxygen content, and the presence of other triplet quenchers. Solid-surface R T P utilizes interactions of lumiphors with solid matrices such as filter paper or ionic salts (7-9).The interactions between lumiphors and solid matrices such as hydrogen bonding provide more rigid molecular environment for lumiphors than micellar solution as evidenced by much longer R T P lifetimes (10, 11). This is attributed to the fact that triplet molecules in micellar solutions are in dynamic equilibrium with various sources of triplet quenchers. Therefore, RTP lifetimes in fluids may vary from system to system. This variability can prevent RTP lifetimes from being used for analytical purposes. In this work, various environmental factors affecting MSR T P lifetimes are investigated. Factors such as temperature of the sample solution, the presence of heavy atoms, the purity of the sample, the oxygen content of the sample, and the presence of other triplet quenchers are considered as the sources of the observed R T P lifetime fluctuations. A kinetic decay model is proposed to use alternative parameters in place of RTP lifetimes, which are less dependent on environmental conditions of the sample solution.
THEORY Studies of micelle dynamics were performed to understand the quenching and decay kinetics of luminescence in micellar solutions. Almgren et al. (12) studied the kinetics of solubilization and the solubilities of neutral arenes in ionic micellar systems using phosphorescence, fluorescence, and steady-state absorption techniques. Infelta et al. (13) proposed a kinetic model describing the kinetics of quenching reactions in micelles. Triplet lifetime measurements with solutions containing micellized cetyltrimethylammonium bromide (CTAB), anthracene, and Cu2+ions show that the rate constant for the exit of anthracence from CTAB micelles into the aqueous bulk phase is as low as 2 X IO2 s-l. Cline Love et al. (14) studied the influence of analyte-heavy atom micelle dynamics on RTP lifetimes and spectra. They proposed limiting rates determining MS-RTP lifetimes of analyte for long-lived, intermediate, and short-lived cases. Although all the previous kinetic decay models adequately treat quenchers, they all neglect external heavy atoms simply because heavy a@ms are not triplet quenchers. The triplet decay model proposed below focuses on the influence of external heavy atoms on MS-RTP lifetimes. The main difference between this model and previous kinetic models is the incorporation of heavy atom terms. The proposed triplet decay model is shown schematically in Figure 1. T o successfully apply this proposed model, the following assumptions are made: (i) The detectable RTP comes mainly from triplet molecules
UP t h3
P t b3
P
YP
Figure 1. Kinetic model of micellar-stabilized RTP: M, micelle; P, triplet probe; Q, quencher; H, heavy atom. MP' represents excited triplet
probe located inside micelle. located inside the micelles. This assumption is valid because R T P cannot be observed in the absence of micelles. (ii) Quenching of the triplet state occurs mainly at the micellar surface and in the aqueous phase. It has been observed that R T P lifetimes of lumiphors in micellar solution decrease with increasing quencher concentration until a plateau level is reached (15). If quenching occurs mainly inside or outside the micelle, the triplet lifetime must decrease continuously as the quencher concentration increases. With a fixed micelle concentration, the micellar surface area is also fixed to a limiting value for triplet quenching. This supports the assumption of micellar surface quenching. (iii) The probe molecules must be far more soluble in the micellar phase than in the aqueous phase. This condition is necessary for the application of steady-state approximation. (iv) The concentration of the phosphorescent probe is so low that self-quenching of the excited state is unimportant. The partitioning of a phosphorescent probe P* between an aqueous phase and micelle M is given by k-
+ P*
M P * F= M k+
where MP* and P* denote the triplet probe located inside and outside the micelle, respectively. The triplet emission mainly comes from the phosphorescent probe located inside the micelle MP*
,k
MP
+ hv
(2) Triplet quenching by quencher Q, and reaction of heavy atom H with excited triplet probes occur at the micellar surface and in the aqueous phase MP*
__+
+H
Kmh
MP
+H
+H P+H MP* + Q -!z MP + Q P* + Q .LP + Q P*
(3)
(4) (5)
(6) As a result, the rates of disappearance of MP* and P* are given by -d[MP*]/dt = k-[MP*] - k+[MJ[P*] + k,,[MP*] f kmh[MP*l [HI k q [ M P * l [&I ( 7 ) -d[P*]/dt = k+[Ml[P*l + k,[P*I[Hl + kq[P*I[QI - k-[MP*l (8) If the phosphorescent probe P* is far more soluble in the micellar solution than in aqueous phase, such that [MP*] >> [P*], the steady-state approximation can be applied -d[P*]/dt = 0 (9) From eqs 7 , 8, and 9 we obtain -d[MP*]/dt = [MP*][k- + k,, + k d [ H ] + kmq[Q](k+k-[Ml/(k+[Ml + kh[Hl + kq[QI)Jl (10)
ANALYTICAL CHEMISTRY, VOL. 62, NO. 21, NOVEMBER 1, 1990 2367
iH1 Flgure 2. Decay rate constants of triplet probe with different heavy
atom concentrations in ideal conditions. k, is reciprocal of the observed MS-RTP lifetime and [HI, is the saturated heavy atom concentration at which k, reaches steady state. Therefore, the observed disappearance rate constant of MP*, k,, which is the reciprocal of lifetime T is given by
k, = 1 / ~ = k- + k m p + kmdH1 + kmq[QIP+k-[Ml/(k+[Ml + kdH1 + kq[QI)J (11) This equation shows that at a fixed concentration of micelle, the observed lifetime decreases as the heavy atom and quencher concentrations increase. In a given system, if the concentration of quencher remains constant, then eq 11becomes
k, = 1 / r = k- + k,
+ k,h[H] + k’-
Ik+k-[M]/(k+[Ml + h [ H I + k’?J (12)
where k’= kmq[Q]and k ” = kq[Q], respectively. If [HIis zero and k+[M] >> k”, the k , = k , + k’. Also, if heavy atoms react with the triplet probe mainly at the micellar surface, the observed R T P lifetime will decrease continuously until [HI reaches a saturated value [HI, as shown in Figure 2.
EXPERIMENTAL SECTION Reagents. Sodium sulfite (J.T. Baker) was recrystallized from warm water (0.5 mL/g). Thallium(1) nitrate (99.99%, Aldrich), sodium bromide (99.9%, J. T. Baker), sodium dodecyl sulfate, SDS (99%, Sigma), and methanol (GR grade, EM Science) were used as received. The analytes used in this research, naphthalene (Eastman), pyrene (J. T. Baker), 2-bromonaphthalene (97%, Aldrich), and biphenyl (J.T. Baker), were all recrystallized twice from absolute ethanol. Distilled-deionized water was used for RTP sample preparation throughout the experiment. Apparatus. RTP spectra and RTP lifetimes were measured by a Perkin-Elmer LS-5B spectrometer. Data were acquired with an IBM-XT compatible personal computer interfaced to the LS-5B. Software written with Turbo Pascal (Borland International) and Macro Assembler (Microsoft) was used to control the instrument, acquire luminescence data, and perform data analysis. RTP lifetimes were calculated by linear least-squares method. The ranges of delay times and gate times for RTP measurements were 0.03-0.05 ms and 1-3 ms, respectively. Sample Preparation and General Procedure. Stock solutions of naphthalene, pyrene, biphenyl, and 2-bromonaphthalene were made by dissolving appropriate amounts of analytes in 10 mL of methanol and diluting to 100 mL with water. For the external heavy atom source, a 0.25 M thallium nitrate stock solution was used. A 0.05 M sodium dodecyl sulfate stock solution
was prepared for the micellar solution, and a 0.4 M sodium sulfite stock solution was used for deoxygenation of the sample solution. An aliquot of analyte stock solution was pipetted into a 10-mL volumetric flask and solvent was evaporated to a minimum volume using nitrogen gas. Sample solutions were made by adding appropriate amounts of stock solutions of thallium nitrate, SDS, and sodium sulfite into the 10-mL volumetric flask containing analyte. These solutions were diluted to a 10 mL final volume with distilled-deionizedwater. After thorough mixing, the solution was transferred to a cuvette covered with a Teflon stopper. The cuvette holder was maintained at constant temperature. The progress of chemical oxygen scavenging was monitored by measuring the RTP intensity development at the emission maximum with the appropriate excitation wavelength on the spectrometer. Uncorrected RTP spectra were obtained after the RTP intensity reached steady state and remained constant for at least 5 min. RESULTS AND DISCUSSION Experimental Conditions for MS-RTP. Deoxygenation of the micellar solution was the most experimentally troublesome of the early MS-RTP studies. The use of an inert gas such as nitrogen to remove dissolved oxygen generate foams. Furthermore, degassing time and quality of inert gas generally affect the observed MS-RTP lifetimes. Chemical deoxygenation method by sulfite ions was used for this work because of its simplicity and reliability (16). T o find the optimum concentration of sodium sulfite, various amounts of sodium sulfite were added to a solution of a fixed amount of naphthalene (2 X 10” M), SDS, and heavy atom (Tl+). The concentration of SDS and T1+were 3.5 X and 1.5 X M, respectively. RTP intensities were measured 15 min after mixing. The optimum concentration range of sodium sulfite was found to be 0.8 X to 1.5 X M. Below this range, the RTP intensity decreased due to incomplete and slow deoxygenation. Above the optimum range, the R T P of naphthalene showed a reduced intensity due to a high concentration of sodium ions, which a t such levels can displace thallium ions from the micellar surface. The reported value of the critical micelle concentration (cmc) of SDS is 8.2 X M ( I 7). Effective screening of the triplet sample molecules by the micelle is expected a t surfactant concentrations above the cmc. T o find the optimum SDS concentration, various amounts of SDS were added to a solution containing fixed amounts of T1+,sodium sulfite, and naphthalene. The maximum RTP intensity was observed when the SDS concentration was in the 1.5 X to 5.0 X M range. The R T P intensity began to decrease at SDS concentrations above 5.0 X M. MS-RTP lifetimes of PAHs are found to increase upon increasing the surfactant concentration up to 0.05 M. At higher surfactant concentrations, the lifetime decreased possibly because of increase in impurities. There are several possible reasons for the reduced R T P intensity a t high SDS concentrations. First, the number of heavy atoms per micelle should decrease with increasing SDS concentration. As a result, the effective concentration of heavy atom will decrease; this results in reduced spin-orbital interactions between the singlet molecules and heavy atoms. Second, the size, shape, and dynamic motion of the micelle will change at high SDS concentration and this could affect the stability of triplet molecules. Third, it has been reported that the removal of dissolved oxygen by sodium sulfite becomes more difficult as the surfactant concentration increases (16). Also, there are more possibilities of R T P quenching by impurities in the SDS a t higher SDS concentration. MS-RTP Lifetimes of Polyaromatic Hydrocarbon Molecules. The MS-RTP lifetimes of PAHs were measured a t room temperature. The MS-RTP lifetimes of selected PAHs under optimum conditions are shown in Table 1. These MS-RTP lifetimes are considerably longer than other reported
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Table I. MS-RTP Lifetimes of Selected Arenes compound
naphthalene 2-bromonaphthalene biphenyl pyrene
wavelength, nm excitation emission lifetime," ms 290 293 280 337
511 527 475 595
1.05 f 0.07 0.82 f 0.08 1.12 f 0.07 7.81 f 0.05
"Al sample concentrations were 5 X M, [SDS] = 0.05 M, [Na2S0,] = 0.02 M, and [TINO,] = 0.025 M. Above lifetimes were calculated with the linear least-squares method and correlation coefficients were above 0.998 in all cases. Table 11. Effect of Temperature on RTP of Pyrene
.14
loo0
- 12
eo-
-
eo400 -
200 -
:I:---;:_--__ 7
10
-8
e 4
- 2
0'
0
Heavy atom (TI') effect on MS-RTP intensity and MS-RTP lifetime of pyrene: [pyrene] = 5 X 10" M, [SDS] = 0.05 M, [SO:-] = 0.02M; excitation 337 nm, emission 595 nm.
Figure 3.
temp, "C
RTP intensity
RTP lifetime," ms
15 20 25 30 35
656 590 532 502 468
6.02 f 0.03 5.36 f 0.04 5.02 f 0.04 4.93 f 0.06 4.89 f 0.06
Sample solutions were deoxygenated with 0.02 M sodium sulfite; concentrations of SDS and TI+ were 0.05 and 0.035 M, respectively: excitation wavelength 337 nm, emission wavelength 595 nm.
200
160
(I
lifetime values. Cline Love et al. (18) reported MS-RTP lifetimes of selected PAHs. Their reported values for several compounds rarely exceed 1.2 ms. On the other hand, Kalyanasundaram et al. (19) observed much longer MS-RTP lifetimes of several PAHs in various surfactant solutions. Their reported MS-RTP lifetime values lie in the 1.2-20 ms range, which are, in some cases, much longer than our values (20). The differences in MS-RTP lifetimes among different systems indicate that R T P lifetimes are extremely sensitive to environmental conditions. The concentrations of dissolved oxygen and heavy atom, the temperature, and the presence of impurities in the sample solution can directly affect the observed R T P lifetime. The effect of temperature on MS-RTP intensities and lifetimes was investigated. The R T P intensity of pyrene decreased by 2970, and the R T P lifetime also decreased by 19% from 6.02 to 4.89 ms as the temperature increased from 15 to 35 "C as shown in Table 11. Although temperature variations will be under 10 O C in most cases, the changes in R T P intensity and R T P lifetimes of compounds are still significant. Therefore, temperature control of the sample solution seems to be necessary for accurate and consistent R T P data. Heavy Atom Effect on MS-RTP Lifetimes. The use of external heavy atoms to increase phosphorescence yield has become a common practice in RTP. The interactions between the heavy atoms and the sample molecules frequently result in an increase in the phosphorescence quantum yield and a corresponding decrease in the fluorescence yield. Also, the observed phosphorescence lifetimes with heavy atoms are usually shorter than those without heavy atoms. As a result, R T P lifetimes of many PAHs may vary significantly depending on the concentration of heavy atoms used. The effect of the external heavy atom on the RTP lifetime was investigated by using thallium ions as the heavy atom source. The R T P intensity and lifetime of pyrene were measured with various amounts of thallium ion concentrations. The RTP intensity increased up to 0.06 M thallium ion concentration as shown in Figure 3. At concentrations higher than 0.06 M thallium, precipitation occurred in the solution. On the other hand, the R T P lifetimes of pyrene decreased continuously up to 0.05 M thallium concentration. The same trend was observed with naphthalene even though the RTP
DWY
at. Conatant
a,d)
-
100.
I 0 0
1
2
0
4
6
Thallium Concentration (xlOOY)
Figure 4. Plot of observed decay rate constant of pyrene versus heavy atom (TI') concentration in SDS micellar solution: [pyrene] = 5 X lom5 M, [SDS] = 0.05 M, [SO,*-] = 0.02 M; excitation 337 nm, emission 595 nm.
lifetimes were rather unstable a t lower heavy atom concentrations. The use of silver ion instead of thallium as external heavy atom did not work in this system due to the very low solubility of Ag2SO3,which was formed by the reaction of dissolved silver ions and sulfite ions. It was found that the decrease in RTP lifetime is a function of heavy atom concentration. The plot of reciprocal lifetime, h,, versus thallium ion concentration was linear (Figure 4) as is expected from eq 12. In eq 12, the reciprocal of lifetime is a function of the concentrations of micelle and heavy atom. If the concentration of micelle is fixed in a given system, then rate constant k , mainly depends on k,h[H] and kh[H] terms. It is expected that reaction rate constant kmhof excited triplet molecule with heavy atom at micellar surface should be much larger t h m k , because micelles and heavy atoms have opposite polarity in most cases. Therefore, eq 12 can be simplified as a linear function of k,,[H]. Also, as the concentrations of heavy atom increases, micelle surface will become saturated to give kmh[H],,which will yield a plateau value. From the slope and intercept of the plot, kmh and k,, + k'were found to be 2560 and 65, respectively. SUllite ions are known to form light-absorbing complexes with thallium ions in micellar solutions (21),particularly at wavelengths below 275 nm. It is not clear whether T1+-sulfite complexes quench the triplet state of molecules as well as attenuate the exciting radiation. It is also possible that triplet molecules may have multiple collisions with heavy atoms during their relatively long lifetimes. However, it may not be reasonable to call heavy atoms triplet quenchers since increased phosphorescence yield is observed with heavy atoms. It may be more reasonable to say that heavy atoms affect triplet states by other mechanisms (22-24).
ANALYTICAL CHEMISTRY, VOL. 62, NO. 21, NOVEMBER 1, 1990
It is well-known that heavy atoms reduce phosphorescence lifetimes through spin-orbital interactions (25). The spin and orbital motions of the electrons are not independent, even in atoms with very small nuclear charge. The orbital motion of the electron induces a magnetic field that interacts with its spin magnetic momentum. This spin-orbital interaction leads to a change in the direction of the spin-angular momentum of an electron, thereby changing a singlet state into triplet state, and vice versa. The external heavy atom effect may take place in two ways: one, by a complex formation with the heavy atom, and another, by a long range interaction through a statistical distribution of the heavy atoms around the phosphorescing molecule (26). It has been shown that the rate constants of both S1 TI and T, So processes are increased (27). This is evident from the reduced RTP lifetimes of the probe molecules at high heavy atom concentrations. Furthermore, it has also been demonstrated that the T, So radiationless transition is only slightly enhanced and the observed reduction in the triplet lifetimes is mainly due to an increase in the radiative transition probability (28). While various mechanisms to explain the enhanced spinorbital interaction and the reduced triplet lifetime have been proposed, they are still not well understood (29). The various schemes that have been previously proposed share a common factor in that the singlet state is ultimately responsible for introducing reduced phosphorescence lifetimes. That is, the triplet state of the emitting molecule is mixed with the singlet state of another molecule. The source of the singlet state can be the same molecule (30), the perturber (31),or a charge transfer state of the molecule-perturber complex (32).
-
-
-
CONCLUSIONS The R T P lifetimes of PAHs in micellar solution are a function of various environmental factors. Major contributors to the magnitude of the observed RTP lifetimes are the nature and concentration of the external heavy atoms used. Also, the method of deoxygenation and solution temperature affect observed RTP lifetimes significantly. This results in different RTP lifetimes under different experimental conditions, which is undesirable from an analytical point of view. Therefore, it is necessary to carefully analyze observed R T P lifetimes according to the system employed to correctly interpret RTP lifetime data. The proposed triplet decay model clearly indicates that external heavy atoms along with triplet quenchers affect observed R T P lifetimes significantly.
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I t is recommended that the use of other constants such as k,, or kmh instead of RTP lifetimes will give a more valid criterion in identifying different compounds in MS-RTP. Although k,, cannot be calculated directly, kmhcan be obtained from the slope of the plot of k, versus [HI. The observed MS-RTP lifetimes may vary drastically depending on the experimental conditions employed, but the triplet decay rate constant (k?,) and reaction rate constant ( k d ) with heavy atoms in the micelle should give more consistent values.
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RECEIVED for review May 30, 1990. Accepted July 31, 1990.
