Anal. Chem. 1987, 59, 1644-1646
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External Heavy-Atom Effect in Room-Temperature Phosphorescence Georg
W.S u t e r , * A l a n J. K a l l i r , a n d Urs P. Wild
Physical Chemistry Laboratory, Swiss Federal Institute of Technology, ETH-Zentrum, CH-8092 Zurich, Switzerland T u a n Vo-Dinh Advanced Monitoring Development Group, Health a n d Safety Research Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 -6101
The effect of external heavy-atom perturbers on the roomtemperature phosphorescence (RTP) spectrum of dibenzo[f,h]quinoxaline (DBO) adsorbed on Whatman 4 fllter paper is reported. The totally symmetric vibrational bands are predominantly enhanced, leading to a spectral change in vibratbnal structure. Inhomogeneity effects of moiecule-perturber interactions are investlgated by time-resolved phosphorimetry and lifetime measurements.
Room-temperature phosphorescence (RTP) is a sensitive and simple analytical method to characterize molecules adsorbed on solid substrates (I). Originally observed from compounds adsorbed onto solids ( 2 , 3 ) ,the RTP phenomenon has been further developed for chemical analysis of organic species in solid substrates (4-7) as well as liquid media (8,9). In particular, it has been successfully applied to the detection of polyaromatic hydrocarbons (PAHs) and N-heterocycles in environmental samples (10-13). Filter paper is the most widely used substrate for RTP, since it is inexpensive and commercially available. The corresponding sample preparation is also simple and rapid: only 2-5 yl of the sample solution is required for a typical assay. The solution is spotted on the paper, and after thorough drying in a stream of hot air or under an IR lamp, the sample is ready for measurement in a spectrometer equipped with a standard phosphoroscope (I). In previous works, we have investigated photophysical aspects related to the interactions of molecules with the substrate: the guest-host interaction (electron-phonon coupling) on a filter paper substrate was studied by measuring laserinduced phosphorescence line narrowing (14). Hydrogenbonding interactions were investigated by using two-dimensional total luminescence spectroscopy (15). In this work we investigate the external heavy-atom effect in R T P and study experimental factors that could affect analytical applications. The sensitivity of R T P may be drastically increased by the external heavy-atom effect (16-19) and limits of detection in the picogram range have been achieved. The filter paper may be impregnated with a heavy-atom solution prior to spotting the analyte solution or the two solutions may be spotted simultaneously. The heavy-atom enhancement factors depend not only on the concentration but also on the particular properties of the heavy atoms used: high enhancement factors have been found with T1+and Pb2+for PAHs and with I- and Hg2+ for N-heterocycles (13). Apart from the overal phosphorescence intensity enhancement, other effects such as spectral shifts and changes in the spectral intensity profiles, due to external heavy-atom perturbers, should also be considered in the interpretation of R T P spectra. Short and long R T P decaying components of several indoles on ion-exchange
paper substrates have been previously reported (20). In a recent work (13) it was demonstrated, that R T P spectra of N-heterocycles are red-shifted-compared to the spectrum obtained with no heavy-atom perturbers-when using HgCl, as the heavy atom salt. No such shift was observed when using a mixture of thallous and lead acetate. However, the spectral intensity profiles were reported to remain essentially unchanged. It has been reported from low-temperature experiments that the external heavy atoms could affect the spectral structure of phosphorescence (21). A typical example of this effect is the drastic increase of the 0-0 band in the phosphorescence spectra of coronene and triphenylene, which has also been observed on filter paper at room temperature (1). This effect is based on the dominant quantum mechanical coupling mechanism responsible for the phosphorescence enhancement, namely, a second-order mixing of the perturber’s singlet states with the lowest triplet state T, of the chromophore being analyzed. Since this coupling involves no vibronic term, the bands belonging to totally symmetric modes of the chromophore are enhanced selectively by the external heavy atoms (22). External heavy atoms, therefore, are expected to change the phosphorescence spectral structure whenever both nontotally and totally symmetric vibronic bands of comparable intensity are present. The low-temperature phosphorescence spectra of p-diazines all contain prominent (nontotally symmetric) out-of-plane modes (23,24). In this work we study the external heavy-atom effect in R T P by using dibenzolf,h]quinoxaline (DBQ) as the model compound; this molecule is ideally suited for such an investigation because its phosphorescence spectrum exhibits vibronic modes that could provide information on the heavy-atom interactions. Compared to DBQ, negligible changes in the spectra of benzoquinoxaline, benzo[a]phenazine, and dibenzo[a,c]phenazine adsorbed on Whatman 4 filter paper have been observed. Comparative studies were conducted to study spectra obtained from samples treated with thallous acetate as the heavy-atom perturber and sodium acetate. Time-resolved phosphorescence spectra and lifetime measurements were performed for DBQ adsorbed on filter paper substrates. A general discussion on how these results could affect analytical applications is given. EXPERIMENTAL SECTION Instrumentation and Procedure. The sample preparation was the same as that described previously (14, 15). In brief, the paper samples were prepared by spotting 5 pL of the sample solution together with an equal amount of either 1 M thallous or sodium acetate. Spots with a diameter of -10 mm were obtained in this manner. The samples were subsequently dried either under an infrared lamp ( 5 min) or in a stream of hot air i l min). The measurements were carried out on a computer-
0003-2700/87/0359-1644$01.50/0 C 1987 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 59, NO. 13, JULY 1, 1987
a)
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Table I. The Approximate Relative Enhancement Factors for DBQ (for the 860-cm-* Vibronic Band) as a Function of Temperature with Thallous and Sodium Acetate intensity” temp, K
T1
Na
300 77
6
1
350
40
“The intensities were normalized such that the band at 860 cm-’ of DBQ measured with sodium acetate treated paper has an intensity value of 1.
0
UmJ
22000
20000
CCM4]
22000
20000
Flgure 1. (a) Room-temperature phosphorescence spectra of DBQ on Whatman 4 filter paper impregnated with thallous acetate (solid curve) and sodium acetate (dashed curve). (b) Phosphorescence spectra of the same samples at 77 K. (The background signal has been subtracted. Note, all spectra are normalized at 23 600 cm-I.)
controlled high-resolution (spectral bandwidth 1.5 nm) luminescence spectrometer and on a Spex Fluorolog with spectral resolutions of 20 nm in excitation and 5 nm in emission. Both spectrometers were equipped with phosphoroscopes operated at a minimum frequency of 100 Hz. The phosphorescence decay curves and the time-resolved spectra were recorded as follows: After steady-state conditions were reached, the excitation light was cut off and the photomultiplier pulses were accumulated in a recently developed multichannel scaler, interfaced to a P D P 11/23 computer (25). Materials and Reagents. A mixture of ethanol/water 1:l was used as a solvent for all components, and the sample concentration in solution was M. All solvents and reagents used are commercially available (Fluka). DBQ was generously provided by J. Sepiol, Academy of Sciences, Poland.
RESULTS AND DISCUSSION Figure l a shows the RTP spectra of DBQ adsorbed on filter paper impregnated with thallous acetate (solid curve: with heavy atom) and sodium acetate (dashed curve). Sodium acetate was used in order to provide an environment similar to that obtained with thallous acetate. For comparison purposes, the intensity scales of the two spectra were adjusted such that the intensities of the 0-0 bands are similar. Whereas no significant spectra shift of the 0band was apparent, the RTP spectra of DBQ with and without external heavy atom are visibly different (Figure la). The emission maximum of the RTP spectrum is located at 21 300 cm-’ for DBQ with sodium acetate and a t 21 000 cm-’ with the heavy perturber (thallous acetate). Although the effect is not dramatic, the difference in the intensity distribution of the two RTP spectra is reproducible. The apparent spectral differences in maximum peaks could be due to either real spectral shifts of vibronic bands or the selective appearance of additional vibronic components. In order to get more conclusive results on spectral assignments, phosphorescence measurements of the same samples were conducted a t 77 K. The low-temperature phosphorescence (LTP) spectra shown in Figure 1b clearly demonstrate that the apparent spectral profiles and shift mentioned previously are due to the different intensity ratios between the 0band and a prominent vibronic band with a frequency a t -860 cm-’ (indicated by an arrow in Figure lb). Note that the intensity scales of the two spectra in Figure l b are also adjusted such that the 0-0 bands have identical intensities. These intensity scale adjustments were necessary since the external heavy-atom effect induces a significant increase in the RTP and LTP spectra. The relative enhancement factors for these spectra are given in Table I. In order to get a better understanding of the effect of external heavy atoms on the R T P spectra, it is useful to discuss the nature of the main vibronic peaks involved in Figure 1. The spectral position of the 0-0band appears unchanged with the use of heavy-atom perturber
-T
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WE 0
I
1
I
2 ISECl
I
3
4
Flgure 2. Phosphorescencedecays of DBQ at 77 K in ethanoVwater containing 1 M of thallous acetate or sodium acetate, and from paper samples obtained by spotting the same solutions on Whatman 4 filter paper.
(Figure lb). The significant feature revealed in the L T P spectra is the appearance of the vibronic band at 860 cm-’ with sodium acetate substrate (arrow on dashed curve, Figure lb). This vibronic coupling between n r * and m*states of polycyclic azines and has been investigated in previous studies (24, 25). The phosphorescent state of DBQ is of rr* (czvsymmetry) and polarized perpendicularly to the molecular plane. The 860-cm-* band is a nontotally symmetric out-of-plane vibronic mode (b2 symmetry) involving a folding of the DBQ molecule along the N-N axis. In the absence of heavy-atom perturber, this type of nontotally symmetric vibration is known to be important in inducing phosphorescence emissions through vibronic coupling. When the heavy-atom perturber is used (solid curve, Figure lb), the 860 cm-’ is masked by the prominence of the band at 1350-1450 cm-’. This band was assigned to two vibrational modes at 1330 and 1420 cm-’, which represent in-plane hydrogen rocking and ringstretching modes (24). The results provided by the LTP spectra with and without heavy atom therefore, confirm the spectral changes observed in the RTP spectra and provide a better insight into the external heavy-atom effect in RTP. Heavy-atom perturbation modifies the RTP spectral structure; the effect is related to the selective enhancement of bands belonging to totally symmetric vibrations and, therefore, should be observable whenever a phosphorescence spectrum contains both totally and nontotally symmetric vibrations with comparable intensity. The limited resolution achievable a t room temperature often prevents a detailed discussion of the nature and changes in the vibronic structure. Time-resolved phosphorimetry is an analytical technique with great potential for improving the specificity of RTP. This analytical tool, however, has not yet been extensively exploited in RTP. It is essential to investigate in detail the behavior of spectra under time-resolved conditions in order to avoid potential spectroscopic artifacts and to optimize experimental conditions. In a previous study, it was demonstrated that the phosphorescence decays of chromophores adsorbed onto solid substrates such as filter paper are not represented by a single exponential, as in solutions (15). Figure 2 shows a logarithmic plot of the phosphorescence decay of DBQ a t 77 K in ethanol/water (v/v, 1/1) and on Whatman 4 filter paper with the addition of 1 M sodium
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 13, JULY 1, 1987
CONCLUSION The results obtained in this study are of importance not only to researchers involved in the fundamental investigation of the RTP process but also to analytical chemists dealing with practical applications of the RTP technique. The external heavy-atom effect is an important tool for improving the sensitivity of RTP. Knowledge of subtle but visible spectral changes induced by the heavy-atom effect would lead to improved spectral assignments and more accurate identification of individual components in complex mixtures. The results obtained with time-resolved measurements are also of analytical interest. The data demonstrate that the inhomogeneity in molecule-perturber interactions could lead to substantial differences in the phosphorescence spectra of one single chemical species. Knowledge of this effect is helpful in avoiding possible misinterpretation of RTP spectra in timeresolved phosphorescence measurements.
AB --
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k v)
W
I-
-
---23000
LITERATURE CITED
zziioo
I
/
21000
[CM'']
Figure 3. Time-resolved phosphorescence spectra of DBQ on Whatman 4 filter paper treated with thallous acetate.
or thallous acetate. The time windows A-E referring to the time-resolved spectra of Figure 3 are also indicated. The lifetime of DBQ in ethanol at 77 K is 1.29 s (15). With thallium the decay shortens and clearly deviates from a single exponential function. This effect is much more pronounced on the paper substrate than in solution, which is readily understood since the phosphorescence lifetime of a given chromophore decreases with decreasing distance from the perturbing heavy atom and increasing concentration of neighboring heavy atom (21,27). The overall phosphorescence decay of a heavy-atom sample may, therefore, be understood as the superposition of many exponential decays originating from chromophores with different lifetimes, i.e., different interactions with the heavy atoms. A mathematical analysis of such complex multiexponential decays is, however, rather complicated and inconclusive (28). It is interesting to note that the asymptotic behavior of the overall decay for long time periods is the same with thallium and sodium, which reflects the presence of a small amount of DBQ molecules on the thallium sample with no or very weak coupling to heavy atoms (21, 27). The phosphorescence decays of the samples with heavy atoms were found to be wavelength dependent, thus revealing that different parts of the spectra exhibited different lifetimes. In order to investigate this effect, we measured, time-resolved phosphorescence spectra using different time windows following the excitation cut off (window A, 0-31 ms; B, 31-62 ms; C, 93-124 ms; D, 217-248 ms; E, 2914-3100 ms). Figure 3 depicts the spectra from a filter paper sample of DBQ with thallium measured with the time windows A-E. These spectra are normalized to equal the amplitude at 22800 cm-'. These results show that the long-lived spectra (obtained with time window E) are similar to the spectra in absence of the heavy-atom perturber; Le., they show more intense nontotally symmetric vibronic bands than the steady-state spectrum of the thallous sample. In particular, the relative intensity of the 860-cm-' vibrational band increases drastically. This feature underscores again the relationship between external heavy-atom perturbation, phosphorescence lifetime, and spectral structure.
(1) Vo-Dinh, T. Room Temperature Phosphorimetry for Chemical Analysis; Wiley: New York, 1984. (2) Roth. M. J. Chromafogr. 1967, 30,276. (3) Schulman, E. M.; Walling, C. J . Phys. Chem. 1972, 178, 53. (4) Vo-Dinh, T.; Winefordner, J. D. Appl. Spectrosc. Rev. 1977, 13, 261. (5) Parker, R. T.; Freedlander, R. S . ; Dunlap, R, B. Anal. Chim. Acta 1980, 119, 189. (6) Parker, R. T.; Freedlander. R. S . ; Duniap, R. B. Anal. Chim. Acta 1980, 120, 1. (7) Hurtubise, H. J. Solid Surface Luminescence Analysis; Marcel Dekker: New York, 1981. (8) Donkerbrock, J. J.; Goo'jer, C.; Velthorst, N. H.; Frei, R. W. Talanta 1981, 28, 717. (9) Cline Love, L. J.; Skrilec, M.; Habarta, J. G. Anal. Chem. 1980, 5 2 , 754. (10) Vo-Dinh, T.; Gammage, R. B. Anal. Chem. 1978, 50,2054. (11) Vo-Dinh, T.; Hooyman, J. R. Anal. Chem. 1979, 5 1 , 1915. (12) Vo-Dinh, T.; Gammage, R. B.; Martinez, P. R. Anal. Chim. Acta 1980, 118. 313. (13) Abbott. D. W.; Vo-Dinh, T. Anal. Chem. 1985, 5 7 , 41. (14) Vo-Dinh, T.; Suter, G. W.; Kallir, A. J.; Wild, U. P. J . Phys. Chem. 1985. 89. 3025. (15) Suter, G. W.; Kallir. A. J.; Wild, U. P.; Vo-Dinh. T. J . Phys. Chem. 1986. 90. 4941. (16) White, W.; Seybold, P. G. J . Phys. Chem. 1977, 81, 2035. (17) Vo-Dinh, T.; LueYen, E.; Winefordner, J. D. Anal. Chem. 1976, 4 8 , 1186. (18) Jakovljevic, I. M. Anal. Chem. 1977, 4 9 , 2048. (19) LueYen, Bower E.; Winefordner, J. D. Anal. Chim. Acta 1978, 102, 1. (20) Aaron, J. J.; Andino, M.; Winefordner, J. D. Anal. Chim. Acta 1984, 160. 171. (21) Najbar, J.; Radakiewicz-Nowak, J.; Chodkowska, A. J . Lumin. 1978, 17, 449. (22) Giachino, G. G.; Kearns, D. R. J . Chem. Phys. 1970, 5 3 , 3886. (23) Suter, G. W.; Wild, U. P.; Brenner, K.; Ruziewicz, Z. J . Chem. Phys. 1985, 98,455. (24) Lim, E. C.; Li. I . R.; Li, Y. H. J . Chem. Pbys. 1969, 5 0 , 4925. (25) Tschaggeiar, R.; Forrer, J.; Kaliir, A. J.; Wild, U. P., to be submitted for publication. (26) Brenner, K.; Ruziewicz, Z. J . Lumin. 1977, 15, 235. (27) Najbar, J.; Birks, J. B.; Hamilton, T. D. S.Chem. Phys. 1977, 23. 281. (28) Knight, A. E. W.; Selinger, B. K. Aust. J . Chem. 1973, 26. 1.
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RECEIVED for review October 27, 1986. Accepted March 20, 1987. Financial support from the Swiss National Science Foundation is gratefully acknowledged. T.V.D. acknowledges the U S . Department of Energy, Office of Health and Environmental Research (Contract DE-AC05-840R21400 with Martin Marietta Energy Systems, Inc.) for support in the preparation of this paper.
