1644
Anal. Chem. 1064, 56, 1044-1647
As shown in this paper, the hollow fiber is also practical
as the cell material for improving the sensitivity with reducing
0
1
2
3
4 (Xlo-’)
Absorbance o f 1 cm c e l l
Figure 7. Calibration curves for molybdenum blue absorptions obtained with 25- and 50-m fiber type LCCs and with an ordinary spectropho-
tometer. The procedure to obtain the curves is the same as that given in Figure 4. In the previous paper, we discussed the linear type LCC. The linear LCC is indispensable for colorimetry in solvents whose refractive indices are lower than Pyrex, such as water. Also, this type LCC is effective in the measurement of the absorption of gas sample; i.e., multiphoton absorption of nitrogen monoxide was measured in the linear capillary of 60 cm length and 0.6 mm i.d. (9). However, the major drawback of the linear LCC is the occupation of large space. Although the present “total reflection” type LCC is achieved for the limited solvents, its potential in the extention of the cell length is obvious as the free shape LCC.
the necessary volume of sample, where the convenient sample introduction system is required. There are some other ways to enhance the small absorbance more than lo4 times such as PAS or thermal lensing colorimetry (6) where the signal intensity is much dependent on the source light intensity: For high sensitivity, a stable CW laser was usually required. At this point the present method seems to be superior in terms of low cost and simple manipulation of the system without any special instrument. The inside diameter of the capillary can be smaller, and the length can be longer in the LCC which is reported here. Under the use of transparent solvent, the improvement in detection for small absorptions could approach infinity. Registry No. P, 7723-14-0;I, 20461-54-5; Cu, 7440-50-8;Hg, 7439-97-6; carbon disulfide, 75-15-0; water, 7732-18-5; acetone, 67-64-1; ethanol, 64-17-5; n-butyl alcohol, 71-36-3;acetylacetone, 123-54-6;carbon tetrachloride, 56-23-5;benzene, 71-43-2; acetophenone, 98-86-2; l-bromonaphthalene, 90-11-9.
LITERATURE CITED Wei, L.; Fujiwara, K.; Fuwa, K. Anal. Chem. 1983, 55, 951-955. Walrafen, G. E.: Stone, J. Appl. Spectrosc. 1972, 28, 585-589. Stone, J. J . Chem. Phys. 1978, 69, 4349-4356. Schaefer, J. C.; Chabay, I. Opt. Lett. 1979, 4 , 227-229. Ross, H. 8.; McCiain, W. M. Appl. Spechosc. 1981, 35, 439-442. Fujiwara, K.; Wei, L.; Uchiki, H.; Shimokoshi, F.; Fuwa, K.; Kobayashi, T. Anal. Chem. 1982, 54, 2026-2029. (7) Meites, L., Ed. “Handbook of Analytical Chemistry”; McGraw-Hili, New York, 1963. (8) Wei, L., MS thesis, University of Tokyo, 1982. (9) Guerra. M. A,; Sanchez, A.; Javan, A. Phys. Rev. Lett. 1977, 38, 482-484. (1) (2) (3) (4) (5) (6)
RECEIVED for review July 5, 1983. Accepted April 9, 1984. This research was supported by Grant in Aid No. 58030041 from the Ministry of Culture, Science, and Education.
Determination of the Fluorescence Quantum Yields of Some 2-Substituted Benzthiazoles G. F. Kirkbright* and D. E. M. Spillane DIAS, University of Manchester Institute of Science and Technology, Manchester M60 1QD, United Kingdom Kevin Anthony, R. G. Brown, J. D. Hepworth, and K. W. Hodgson Chemistry Division, Preston Polytechnic, Preston PR1 2TQ, United Kingdom
M. A. West Dauy-Faraday Laboratory, Royal Institution of Great Britain, 21 Albemarle Street, London W 1 X 4BS, United Kingdom
The fluorescence quantum yields (4 ,) of a number of solld 2-substituted benrthiazoies have been determined by using a conventional comparative optical method and by photoacoustic spectroscopy. I n this paper resuns are compared and the reasons for the difference observed are discussed.
The determination of the luminescence quantum yield (LQY) of solid materials has always been more difficult to establish than the corresponding measurements for solution samples. Most optical methods are based on a comparison between a luminescence standard and the test material and 0003-2700/84/0356-1644$0 1.50/0
thus produce only a relative measure. Determination of the absolute value of an LQY by an optical method is, however, an extremely exacting procedure requiring detailed knowledge of the experimental geometry, sample reflectance characteristics, and variation in detector response with respect to wavelength. The precision of such methods is poor and values for solid luminescence standards are generally an agreed mean value based on the results obtained by a number of independent laboratories. Thus optical methods, although widely employed, are not particularly satisfactory as a result of the lack of a suitable primary reference standard. In an effort to improve the accuracy and precision of the measurement, a number of calorimetric methods for deter0 1984 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 56, NO. 9,AUGUST 1984
mination of LQY for solid samples have been developed (1-4). This paper presents a comparison between a comparative optical method and an absolute determination using photoacoustic spectroscopy (PAS) which in effect is one of the calorimetric techniques referred to above (3-9). The photoacoustic technique chosen has been that described by Adams et al. (7). The reader is referred to this paper for the salient details of the analysis; the technique employed assumes that the photoacoustic spectrum of the compound of interest exhibits a region in which the photoacoustic signal is saturated; i.e., the optical absorption length is less than the thermal diffusion length or, stated differently, the photoacoustic signal becomes independent of the sample absorption coefficient and the sample is optically opaque. A graph of the ratio of the photoacoustic signal of the compound under study to that of a nonfluorescent black body absorber (y) against wavelength ( x ) in this saturation region should yield a straight line of negative slope such that
;
;
1645
P A SIQNAL
\
AMPLITUDE
(AR~ITRARY UNITS)
I
c
260
_
_
i
300
360
400
460
WAVELENGTH (nm) A
Figure 1. Photoacoustic spectrum of sample 36a.
m 4J = - T X e m 1.
where 4 is the quantum yield of fluorescence for the sample, m is the gradient of the plot obtained, i is the intercept on the y axis, and A, is the mean emission wavelength for the fluorescent material.
EXPERIMENTAL SECTION A. Optical Measurements. Fluorescence and reflectance measurements were made with a commercially available instrument (Optical Integrating Spectrometer, Applied Photophysics Ltd.). Corrected fluorescence spectra were obtained by calibration of the wavelength response of the detection system with quinine sulfate. Fluorescence quantum yields were determined by comparison of the integrated area shown in the emission spectrum of the unknown (A3 with that of the emission spectrum of the reference fluorescent material (A,). The percentage reflectance of the sample (R,) and reference (R,) materials relative to a plate coated with barium sulfate was measured at the wavelength of maximum excitation (380 nm). The fluorescence quantum yield of the sample (4,) was then calculated from the equation
where 4, is the quantum yield of fluorescence of the reference material. The fluorescence of the samples was measured relative to two different standards. These were (1)tetraphenylbutadiene (TPB) (Gold Label Grade, Aldrich Chemical Co., UK), this was taken to have a quantum yield of 0.90, and (2) sodium salicylate (Gold Label Grade, Aldrich Chemical Co., UK), this was taken to have a quantum yield of 0.55. B. Photoacoustic Measurements. The photoacoustic spectrometer used for this study has been described in detail elsewhere (IO). Radiation from a short, focused xenon arc (Model VIX 300, Varian Associates) was focused through a rotating sector (Model 9479, Brookdeal Electronics, La., UK) onto the entrance slit of an f / 4 grating monochromator (Metrospec,DGO, Ltd., UK). The transmitted radiation was folded down into a commercially available photoacoustic cell (Model OAS 401, EDT Research, Ltd., UK) by use of a front-surfaced concave mirror. Unless otherwise stated, the monochromator was operated with a half-bandwidth of 20 nm and the modulation frequency imposed on the incident radiation intensity by the rotating sector was 28 Hz. The signal from the sample cell was taken to a lock-in amplifier (Model 95053, Brookdeal Electronics, Ltd., UK) together with an internally generated reference signal from the rotating sector. The DC output from the lock-in amplifier was digitized and stored in a scan recorder (Model 4101, Princeton Applied Research Corp.). The spectrum thus acquired could be displayed visually on an oscilloscope (Type RM4DlOA, Scopex, Ltd., UK) or hard copy was obtainable via an X-Y recorder (Model 25000, Bryan’s Southern Instruments, Ltd., UK). The spectrometer was operated
\
P A SIGNAL
AMPLITUDE
(ARBITRARY
UNITS)
I
260
300
350
WAVELENGTH ( n m )
400
460
A
Flgure 2. Photoacoustic spectrum of sample 37a.
in single-beam mode, the spectrum being corrected for variations in source intensity by ratiometry against a prerecorded P A spectrum of carbon black (assumed to be a black body absorber in the region of operation of the spectrometer). Single wavelength measurements were made by use of a digital signal averager (Model 4202, Princeton Applied Research Corp., USA) and the results were displayed on a digital voltmeter (Type 701, Fenlow, UK) . To apply the method of Adams et al. (3,a normalized photoacoustic spectrum of each compound was obtained in order to determine the wavelength in which saturation occurred. After identification of the region, a number of single wavelength measurements were taken. The results were then plotted and the quantum yield was calculated as previously described.
RESULTS AND DISCUSSION Detailed comment on the results obtained by fluorescence measurements has been reported elsewhere (11)together with a description of the synthesis of the test materials and data on the fluorescent lifetimes and rate constants for radiative and nonradiative decay processes. The spectroscopic results obtained by photoacoustic spectroscopy are illustrated in Figures 1-3. Figure 1shows the photoacoustic spectrum of an efficient fluorescer (4 = 0.48 by optical methods, 4 = 0.53 by PAS measurement). The saturated portion of the spectrum lies between ca. 300 nm and ca. 400 nm. It is evident that the absorption profile in this wavelength region is approximately linear and of negative slope. Figure 2 shows the spectrum obtained for a compound of low quantum efficiency of an inefficient fluorescer (4 = 0.01 by spectrofluorimetry, 4 = 0 by PAS). The saturation region again occurs between ca. 300 nm and 400 nm; the absorption
1646
ANALYTICAL CHEMISTRY, VOL. 56, NO. 9, AUGUST 1984
Table I. Compounds Examined, Average Emission Wavelengths, and the Quantum Yield of Fluorescence Determined by Optical and Photoacoustic Measurements
\
's Me
R R
36a, R = H b, R = Br
1-34 35
37a, R = H b, R = Br fluores-
compd no. 1
2 3 4 5 6 7 8
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34
RZ OH OCH3 H H OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH
R3 H H OH H H H H H Br H H Br
c1
H H
c1
H H H OH H
H H I OCH3 H
R4 H H H OH H H H H H Br H H H c1 H H NEtz NMez H H OH H H H H OCH, H H CH3 H H H H H
R6
H H H H 2"
NHCOCH3 NHCOPh NCHPh H H Br Br H H c1
c1 H H F
H H OH H I
H
35 36a 36b 378 37b
profile, however, is effectively flat over this region. Figure 3 illustrates the photoacoustic spectrum of a nonfluorescent material; although the absorption profiie shows a region which is apparently linear and of negative slope this arises from an absorption centered at ca. 390 nm. It can be seen from the results shown in Figures 1-3 that some care must be taken when attempting to identify the region of photoacoustic saturation as a similar effect can be produced by a genuine absorption feature. Table I lists the compounds employed in this study together with the mean emission wavelength and the fluorescence quantum yield measured by the spectrofluorimetric and photoacoustic techniques for each compound. Results are presented for 40 compounds; of these three were found to be nonfluorescent and 12 exhibit a very weak fluorescence so that reliable results in the determination of their luminescence
R6 H H H H H H H H H H H H H H H H H H H H H H OH H H H H H H H H H H H
cence
fluorescence
quantum
quantum
mean emission wavelength (M,nm
yield
yield
(optical determination)
(PAS determination)
520
0.38
0.45
660 540 550 600 530 520 535 560 525 520 540 550 480 480 550 570 520 550 525 535 550 500 600 540 525 560 550 530 520 525
0.04 0.18 0.25 0.16 0.25 0.21 0.27 0.32 0.47 0.33 0.25 0.37 0.04 0.05 0.28 0.07 0.34 0.25 0.02 0.04 0.22 0.46 0.07 0.36 0.39 0.35 0.23 0.05 0.02
515 520 520 530 530
0.39 0.48 0.20
0.28 0.43 0.19 0.35 0.30 0.26 0.41 0.54 0.40 0.34 0.51 0.37 0.40
0.27 0.20 0.53 0.60 0.46 0.42 0.28
0.01
0.67 0.53 0.39
0.01 0.01
quantum efficiency by photoacoustic determination could not be obtained. Twenty five (25) samples gave results for which the fluorescence quantum yield could be measured by both optical and photoacoustic techniques and compared. It will be noted that only one value is quoted for & measured optically, as the agreement between the measurements using T P B and sodium salicylate as references was very good. A comparison between the results obtained by spectrofluorimetric and photoacoustic measurements leads to a number of interesting observations: (1)The quantum yields measured by PAS are consistently higher than those measured optically; this is observed for 23 of the 25 samples for which a comparison may be made. (2) The graphical method adopted for the PAS determinations is not suitable for estimation of luminescence quantum efficiency of weakly fluorescent compounds where & < 0.1. (3)Although the photoa-
ANALYTICAL CHEMISTRY, VOL. 56, NO. 9, AUGUST 1984
250
300
350 WAVELENQTH (nm)
400
1647
absorption in the cell windows. Either of these phenomena can be expected to provide a positive offset in the observed linear relationship between the ratioed photoacoustic signal and wavelength. This then increases the magnitude of i in eq 1 and results in a decrease in the computed value for the fluorescence quantum yield. (2) The samples may not be in saturation over the measurement range used. This would invalidate the analysis of Adams et al. (7) and produce incorrect values for C#Ip (3) The emission profile of the sample is not constant with respect to wavelength. This again invalidates the analysis. It is possible, from examination of the observed values that the reported photoacoustic values are systematically high and therefore errors of the type considered above in (1) seem unlikely. 450
Flgure 3. Photoacoustic spectrum of Sample 2.
coustic results are consistently higher than those obtained optically, the measurements agree to within 0.1 in the majority of cases (for 19 out of the 25 compounds). (4) In cases where the divergence between the results is large (>0.1), there does not appear to be any correlation with type or position of substituent groups in the molecular structure, the mean emission wavelength, the magnitude of quantum yield, or the phyvical nature of the sample (Le., crystalline state). Thus from the above observations agreement between the results obtained by the two techniques is not invariably close and in some cases a wide divergence of results can be noted. It is further evident that these divergences do not appear to be related to either the physical or chemical properties of the samples. It is reasonable therefore to conclude that the principal cause of the variations in the resulb is measurement error in one or both of the systems employed. The principal potential source of error in the optical measurements may be the accuracy of the values of C#If employed for the reference measurements. As stated earlier, absolute measurement of quantum yield is an exacting measurement to perform and large uncertainty in the c$f values obtained may arise. The quantum yield values used in this study were 0.55 for sodium salicylate and 0.90 for TPB. Literature values of 0.58 (8) and 0.60 (13) have been quoted for sodium salicylate and 0.94 (7,12)for TPB. If these values were used for referencing in the optical method, the agreement between optical and photoacoustic measurements would be considerably improved. Error sources in the photoacoustic system are more difficult to quantify. There are three principal categories of error source in the photoacoustic measurement: (1) Additional photoacoustic signal amplitude may be observed due to either overlap of the absorption and emission bands or absorption of scattered light by the walls of the photoacoustic cell or by
CONCLUSION
Photoacoustic spectroscopy provides a useful technique for rapid determination of the absolute value of the fluorescence quantum yield of solid samples. There are a number of potential error sources inherent in the measurement, however, and if accurate values are required it is necessary to check carefully the validity of the assumptions used to make the measurements; these studies are at present in progress. Registry No. 1, 3411-95-8; 2, 6269-47-2; 3, 25389-28-0; 4, 6265-55-0; 5,30616-387; 6,2842-13-9;7,2533-14-4; 8,6092837-2; 9, 90481-36-0; 10, 90481-37-1; 11, 6344-17-8; 12, 90481-38-2; 13, 90481-39-3; 14, 90481-40-6; 15, 6265-97-0; 16, 30616-42-3; 17, 55489-32-2; 18, 90481-41-7; 19, 90481-42-8; 20, 90481-43-9; 21, 6265-56-1; 22, 24978-47-0; 23, 90481-44-0; 24, 90481-45-1; 25, 6265-93-6; 26, 90481-46-2; 27, 30612-17-0; 28, 56048-53-4; 29, 56048-54-5; 30, 21703-55-9; 31, 90481-47-3; 32, 30616-41-2; 33, 6276-77-3;34,90481-48-4; 35, 90481-49-5;36a, 76995-70-5;36b, 90481-50-8;37a, 25389-29-1;37b, 90481-51-9. LITERATURE CITED (1) Bodo, 2. Acta phvs. Aced. Sci. Hung. 1963, 3 , 23. (2) Callis, J. B. J. Res. Natl. Bur. Stand., Sect. A 1976, 80,413. (3) Murphy, J. C.; Aamodt, L. C. J . Appl. Phys. 1977, 48, 3502. (4) Qulmby, R. S.;Yen, W. M. Opt. Lett. 1976, 3 , 181. (5) Powell, R . C.;Neikirk, D. P.; Serdar, D. J. Opt. SOC.Am. 1980, 7 0 , 406. (6) Ohia, Y.; Mizuta, M.; Kukimota, H. J. Lumin. 1983, 2 8 , 1 1 1 . (7) Adams, M. J.; Hlghfield, J. G.; Kirkbright, G. F. Anal. Chem. 1960, 5 2 , 1260. (8)Adams, M. J.; Highfield, J. G.; Kirkbright, G. F. Analyst (London) 1961, 106, 850. (9) Rosencwaig, A.; Hildum, E. A. Phys. Rev. B 1981, 2 3 , 3301. (IO) Adams, M. J.; Beadle, B. C.; King, A. A.; Kirkbright, G. F. Analyst (London) 1976, 101, 553. (11) Antony, K.; Brown, R. G.; Hepworth, J. D.; Hodgson, K. W.; May, 8.; West, M. A., paper in preparation.
(12) Burton, W. M.; Powell, B. A., Appl. Opt. 1973, 12, 37. (13) Bril, A.; de Jager-Vennis, A. W. J . Nectrochem. SOC. 1976, 123, 396.
RECEIVED for review February 1984.
13,1984. Accepted April 12,
