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Anal. Chem. 1 ~ 0 3 65. , 863-865
Luminescence Rule of Polycyclic Aromatic Hydrocarbons in Micelle-Stabilized Room-Temperature Phosphorescence Jin Weijun and Liu Changsong’ Department of Chemistry, Shanxi University, Taiyuan 030006, People’s Republic of China
Thk paper studies the effect of the triplet energy on miceiiestabilized room-temperature phosphorescence (MS-RTP) of polycyclic aromatic hydrocarbons(PAHs). The resultsof the experknents show that lt is difficult or Impossible to induce MS-RTP of PAHs when the triplet energy is less than about 14 000 cm-l, which may be a critical value for inducing MSRTP of PAHs. I n addltbn, ihk paper also discussesthe effects of ring size and linearity on MS-RTP of PAHs.
Room-temperature phosphorimetry (RTP) has been developed rapidly since 1974. Winefordner and co-workers192 established the general w e of RTP as an analytical technique because of ita simplicity, high sensitivity, and good selectivity.334 Establishment of the MS-RTP method, especially chemical deoxygenation MS-RTP, is one of the major technical advances in the use of RTP.5-8 RTP has been applied to the determination of trace amounta of many organic compounds of biochemical and environmental interest.9 MSRTP is useful for analysis of PAHs in the environment,5J0J1 as is solid-substrate RTP.9 Vo-Dinhg conducted a study of the effect of ring size and molecular linearity on the phosphorescence wavelength of PAHs. In this paper, the effects of the triplet energy and the ring size and linearity of PAHs on the intensity ratio, IP/IF, as well as on RTP and fluorescence intensities, etc. in the micelle system have been studied. The results of these experiments indicate that for MS-RTP the effect of the ring size and linearity on RTF and RTP wavelengths is in accordance with the l i t e r a t ~ r e . ~ On the other hand, our experimenta also show that IP/IFdecreases rapidly with the decrease of the triplet energy of PAHs. That is, fluorescence intensities increase, while phosphorescence intensities decrease, relatively, with a decrease in the triplet energy. Thus, it will most likely be difficult or impossible to induce MSRTP of PAHs when the triplet energy is less than about 14 000 cm-1. EXPERIMENTAL SECTION Instruments. All records of luminescence spectra and measurement of luminescence intensities were carried out with (1)Paynter, R. A.; Wellons, S. L.; Winefordner, J. D. Anal. Chem. 1974. 46. (6). 736. (2) Weilons, S. L.; Paynter, R. A.; Winefordner, J. D. Spectrochim. Acta 1974,30A,2133. (3)Parker, R. T.;Freedlander, R.; Dunlap, R. B. Anal. Chim. Acta 1980,120,l-17. (4)LueYen-Bower, E.;Ward, J. L.; Walden, G.; Windfordner, J. D. Talanta 1980,27,380-382. (5)Cline Love, L.J.; Skrilec, M.; Habarta, J. G . Anal. Chem. 1980,52 (4),754. (6)Skrilec, M.; Cline Love, L. J. Anal. Chem. 1980,52(ll),1559-1564. (7)Diaz Garcia, M. E.; Sanz-Medel, A. Anal. Chem. 1986,58,14361440. (8) Wei, Yansheng; Liu, Changsong; Zheng, SusheFenxi Huarue 1990, 18 (3),228. (9)Vo-Dinh, T.Room Temperature Phosphorimetry for Chemical Analysis; Wiley: New York, 1984. (10) Cline Love, L. J.; Skrilec, M. Am. Lab. 1981,13 (3),103-107. (11)Jin, WeiJun; Liu, ChangSong Proceeding of International 4th Beijing Conference and Exhibition on Instrumental Analysis, C161; Science Press: Bejing & New York, 1991. 0003-2700/93/0385-0863$04.00/0
a MPF-4fluorescence spectrophotometer(HITACHI),equipped with a thermostatic cell holder. The spectrometerused a 150-W xenon arc lamp as the excitation light source and a Rd4aF photomultiplier (Hamamatau Co.) as the detector. Reagents. Anthracene and naphthalene, chemical grades, were supplied by ShanghaiReagent Corp. and recrystalliied from warm ethyl alcohol. Phenanthrene, chrysene, and benz[a]anthracene (purum)were purchased from Fluka. Fluoranthene, l&dimethylnaphthalene, fluorene, benzo[a]ppene, perylene, and pyrene are HPLC grade from Fluka which were obtained from The Institute of Coal Chemical Research,Academia Sinica, Shanxi, P. R. China. Stock solutions of PAHs were prepared by dissolving PAHs in an aqueous 0.5 mol/L sodium dodecyl sulfate (SDS)solution, respectively. SDS, purchased from Shen Yang reagent factory, was twice recrystallized from warm ethyl alcohol (95%, AR). Thallous nitrate, sodium sulfite, etc., Analyzed Reagent Grade, were purchased from Shanghai Reagent Co. Sub-boiling doubly distilled water was used to prepare all solutions. The luminescence intensities shown in Table I and Figure 1 are ‘uncorrectedspectra”. The 12-nmslit was used for excitation, and the 9-nm slit was used for emission.
RESULTS A N D DISCUSSION Relationship between the Triplet Energy and I p / I p In the previous paper,ll we mentioned that in the presence of thallous nitrate, the fluorescence of phenanthrene and 1,5dimethylnaphthalene is almost quenched, but the fluorescence of fluoranthene, pyrene, and benz[alanthracene is still very intense. In addition, the literature’ reported that perylene does not display MS-RTP. In our experiment, perylene and anthracene do not display MS-RTP either. Generally, anthracene, which is a very weak phosphor, can only display weak RTP in a very rigid environment, for example, in &CDpaper substrate.12 We try to explain why these phenomena appear from the triplet-singlet energy differences hE(T-S). The triplet energies of PAHs studied and the IP/IF ratios corresponding to [T1+] = 0.025 and 0.035 mol/L are listed in Table I, and their interrelation is given in Figure 1. From Figure 1,we can see thatlp/IF decreases rapidlywith a decrease in the triplet energy and tends toward zero when the triplet energy is less than 14000 cm-l. There is an exception for fluorene. Although ita I p and IFare very strong under the experimental conditions, the I p / I is ~ small. Theoretically, the nonradiative rate constant corresponding to the intersystem crossing (ISC)process Tl-So strongly depends on the magnitude of the energy gap M(Tl-So), that is, the rate constant increases exponentially with decreasing hE(T1S0).9J3J4 The intersystem crossing rate is so great that radiative transition cannot occur when the triplet energy is less than about 14 000 cm-l. The phosphorescence wavelengths of anthracene reported in the literature1*are 674 and 690 nm. The former responds to maximum phosphorescence intensity. The triplet energy converted by 674 nm is 14 928 (12)Vo-Dinh, T.;Alak, A. M. Appl. Spectros. 1987,14 (6),963. (13)Rohatgi-mukherjee, K.K.Fundamentals ofPhotochemistry; John Wiley & Sons: New York, 1978. (14)Siebrand, W. J. Chem. Phys. 1967,47(7),2411-2422. 0 1993 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 7, APRIL 1, 1993
Table I. Triplet Energy of Some PAHs a n d t h e ZP/ZF Ratio8 I p i l ~ratio
no. of benzene rings
compd fluorene phenanthrene naphthalene 1,5-dimethylnaphthalene chrysene fluoranthene pyrene benzo[al anthracene benzo[al pyrene anthracene perylene naphthacene
linearityb
[T1+]= 0.025 mol/L 0.32 4.5 2.7 2.2 1.6 0.77 0.88 0.40 0.0048
triplet energy, cm-l 21 888 21 014 20 980 20 385 19 510 18 287 16 748 16 573 14 510 15 035c 12 600' 10 14OC
3 2 2 4 4 4 5 3 5 4
-
[T1+]= 0.035 mol/L
Ex
EFM
E ~ M
0.52 6.5 4.1 3.0 2.3 0.95 1.2 0.58 0.0075
276 294 284 297 324 361 335 360 381
312 365 328 338 364 442 390 389 408
457 476 477 491 513 547 597 603 689
Temperature: 25-26 "C. [SDSI: 0.05 or 0.03 mol/L. [Na2S031: 0.06 mol/L. pH 7. [PAHsl: 5 x From ref 14. EFM= fluorescence wavelength, E ~ =Mphosphorescence wavelength.
wavelength, nmd
mol/L. L = linearity, B = bent.
Table 11. Triplet Energies a n d Tl+/Na+ Ratios Corresponding to Maximum R T P Intensitiesa PAHs
triplet energy, cm-l
Tl+/Na+ mole ratio, %
5.FL UOR A NTHE NE 6.PYRENE
phenanthrene fluoranthene benzo[a]anthracene
21 740 18 510 16 520
31.3 38.5 46.7
8.BEN 20 (0)PY RENE
0
- 7.BENZO(O)ANTHRACENE 2 -
6 I
I
10490
17480
i400o
I
ziooo
I
24480
Edcm-1)
Flgure 1. Interrelation between the triplet energy (ET)and the &/IF ratio.
cm-l. The triplet energy of anthracene reported in the literaturel5 is 43 kcal/mol, i.e., 15 035 cm-l. Therefore anthracene is a weak phosphor. The triplet energies of perylene and naphthacene reported in the literature15are 36 and 29 kcalimol, i.e., 12 600 and 10 140 cm-l, respectively. Thus it is very difficult or impossible to induce their MSRTP. Actually, the phosphorescence or triplet yields of PAHs and their susceptibility to increase by heavy-atom (HA) perturbation are the result of a complex interplay of radiative and radiationless transition rates. The three main radiationless processes that compete with the luminescence or radiation process are S1 So, SI- TI, and TI-. SO. The nonradiative rate constants of these processes are closely related to the individual energy separation between the electronic states corresponding to the p r o c e ~ s . ~ J ~The J~J~ rate constants of internal conversion (SI SO)and intersystem crossing (SI TI, T1 So) are very important factors in determining the RTP yield. However,T, states may intervene between S1 and TI, but none may intervene between T1 and SO. Thus the phosphorescence and intersystem crossing are directly interrelated by a common electronic transition, T1 SO. If the effects of the three processes above on phosphorescence were considered simultaneously, it would
-
-
- -
-
~
+ 2[Na~S031;Tl+/Na+= [Tl+I/([Na+l+ [Tl+I).
be more difficult to interpret them clearly. But it is not appropriate to attribute completely the factors that affect the phosphorescence yield to T1 SOprocesses. The other aspects still require further study. Relationship between Triplet Energy and the Tl+/ Na+mole ratio. Generally, PAHs show intense fluorescence, and no phosphorescence can be detected in the SDS system. Only external HA perturbation, via thallous ions, is added to the SDS system; fluorescence is quenched and phosphorescence is enhanced. Thallous ions, as SDS counterions, partly replace Na+ on the micelle surface and further form thallous dodecyl sulfate (TlDS). However, TlDS is easily precipitated at lower temperature and the Tl+/Na+mole ratio is less than 30% in the ~ y s t e m .Nugara ~ et al.17 reported that the TI+/ Na+ mole ratio can reach 50% without precipitation with a mixed micelle system. Maximum phosphorescenceintensities of naphthalene are obtained when the Tl+/Na+ratio is 30%. In our experiments, the Tl+/Na+ratio can be close to 50% because of adopting a higher temperature, 25-26 "C. For a given PAH, I p / l increased ~ with an increase in T1+ concentration or reached a maximum value and then decreased with a further increase in T1+ concentration. Generally, phosphorescence intensities show a steady increase when the Tl+ concentration ranges from 0 to 0.025 or 0.035 mol/L, but phosphorescence and fluorescence intensities decrease simultaneously when the T1+concentration is more than 0.025 or 0.035 mol/L. The experiments show that the Tl+/Na+ratio is not the same in the SDS system for various PAHs when phosphorescence intensities reach maximum. It may also be concerned with the triplet energy of the PAHs. The triplet energies and Tl+/Na+ ratios of PAHs corresponding to maximum RTP intensities are shown in Table 11. Because it is difficult to induce RTP at lower triplet energies, a higher T1+ ion concentration and an intense magnetic field would be required. However, the TI+ ion concentration cannot be increased indefinitely because of the solubility of TlDS, the quenching of Tl+ion at high concentration, and other action."
-
1 1
0
[Na+] = [SDS]
~~
(15) Bartrop, J. A.; Coyle, J. D.Principles of Photochemistry, 2nd ed.; 1978. (16) Dreeskamp. H.; Pabst, J. Chem. Phys. Lett. 1979,61 (2), 262-265.
(17) Nugara, N. E.; King, A. D., Jr. Anal. Chem. 1989,61 (13), 14311435.
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ration procedure but also may “pick out’’lB benzo[alpyrene from a complex environmental sample.
CONCLUSION
nrn 35700
27800
22730
19230 16670 WAVELENGTH
14700 13890
Figure 2. Excltatlon and emlsslon spectra of benro[a]pyrene. Condltlons: [SDS], 0.05 mol/L; [thallousnltrate], 0.035 mol/L; [benzomol/L. Key: F = fluorescence (I = 3300); P [elpyrene], 2 X = phosphorescence ( I = 25).
Effect of Ring Size and Linearity on the RTP. Our results on the effect of ring size and linearity on the RTP wavelength are in accordance with the literature: as shown in Table I. On the other hand, PAHs which possess linearity and large ring sizes fluoresce easily but, relatively, bent PAHs phosphoresce easily. For example, anthracene is an intense fluorescence compound but naphthalene and phenanthrene are intense phosphors. Analytical Considerations. Phosphorescencespectra are usually located in the red regions. This is an aspect of RTP selectivity. As shown in Figure 2, for benzo[alpyrene, the excitation and emission wavelengths are 381 and 689 nm, respectively; little emission occurs at 689 nm. Therefore, although benzo[al pyrene has a very low Ipll~ratio, analytical selectivityof MS-RTP using the feature of its longer excitation and emission wavelengths is higher than for fluorometry. Therefore, MS-RTP not only may reduce a complex sepa(18)Vo-Dinh, T.;Hooyman, J. R. Anal. Chem. 1979,51 (E), 19151921.
The relationship between the triplet energy and the IPIIF and Tl+/Na+ ratios has been demonstrated. The effect of the ring size and linearity of PAHs on MS-RTP is shown. Analytically, sensitivity and selectivity should be considered simultaneously to decide whether MS-RTP or fluorometry should be chosen for PAHs analysis. Several primary beneficial predictions can be made on the basis of our results which may shorten the experimental procedure; for example, for analysts, they can judge by the ET value if a certain PAH is of intense MS-RTP. This is helpful for selecting the proper analytical methods (fluorometry or phosphorimetry) for this compound. The effect of ET on the Tl+/Na+ ratio and phosphorescence yields is only one of various factors. Other factors stillneed further study. We believe that the conclusion of this experiment is also significant for use with other RTP methods for PAH analysis, for example, solid-substrate RTP.
ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China and Shanxi Provincial Natural Science Foundation. RECEIVED for review May 11, 1992. Accepted September 18, 1992. Registry No. Fluorene, 86-73-7; phenanthrene, 85-01-8; naphthalene, 91-20-3; 1,5-dimethylnaphthalene, 571-61-9; chrysene, 218-01-9;fluoranthene, 206-44-0; pyrene, 129-00-0;benzo[alanthracene,56-55-3;benzo[a]pyrene, 50-32-8;anthracene, 12012-7; perylene, 198-55-0;naphthacene, 92-24-0; thallous nitrate, 10102-45-1;sodium sulfite, 7757-83-7.
