The Journal of Physical Chemistry, Vol. 82, No. 4, 1978 459
Organic Scintillators with Large Stokes’ Shifts
(32) S. J. Sheng and D. M. Hanson, Chem. Phys., 10,51 (1975). (33) S.Dym and R. M. Hochstrasser, J . Chem. Phys., 60,368 (1974). (34) (a) L. Goodman and M. Koyanagi, Mol. Photochem., 4,369 (1972);
(38) Y. Tanimoto, T. Azumi, and S.Nagakura, Bull. Chem. SOC.Jpn., 46, 136 (1975). (39) R. Li, Y. H. Li, and E. C. Lim, J . Chem. Phys., 53, 2443 (1970). (40) H. Hayashi and S. Nagakura, Mol. Phys., 24, 801 (1972). (41)C. R. Jones, D. R. Kearns, and R. M. Wing, J . Chem. Phys., 58, 1370 (1973). (42) (a) N. Hirota, and C. A. Hutchison, J. Chem. Phys., 42,2869 (1965); (b) N. Hirota, ibid., 43, 3354 (1965). (43) M. D. Fayer and C. 8.Harris, Phys. Rev. 6, 9,748 (1974). (44) E. T. Harrigan and N. Hirota, J . Chem. Phys., 49, 2301 (1968).
(b) M. Koyanagi, R. J. Zwarich, and L. Goodman, J. Chem. Phys., 56, 3044 (1972). (35) M. L. Bardet, G. Fleury, and M. C. Sablayrolls, J . Mol. Struct., 3,
141 (1969). (36) For example, R. M. Hochstrasser, ”Molecular Aspects of Symmetry”, W. A. Benjamin, New York, N.Y., 1966,Chapter 9. (37) E. T. Harrigan and N. Hirota, Mol. Phys., 31, 663 (1976).
Organic Scintillators with Unusually Large Stokes’ Shifts H. Gusten,” P. Schuster, and W. Seitz KernforschungszentrumKarlsruhe, Instltut fur Radiochemie, 7500 Karlsruhe, Postfach 3640, Federal Republic of Germany (Received July 11, 1977) Publication costs assisted by KernforschungszentrumKarlsruhe
A series of methyl and/or methoxy substituted 1,3-diphenyl-2-pyrazolines were synthesized for evaluation as solutes in liquid scintillation counting systems. The electronic absorption spectra, absolute fluorescence spectra, fluorescence quantum yield, fluorescence decay times, as well as the relative pulse height of 15 1,3-diphenyl-2-pyrazolineswere measured in benzene at room temperature. Unusually large Stokes’ shifts with high fluorescence quantum yields of about 0.90 are obtained when bulky methyl and/or methoxy substituents are attached in ortho, ortho‘ positions of the phenyl rings in 1 and/or 3 positions of the 2-pyrazoline ring. The sterically hindered 1,3-diphenyl-2-pyrazolines have a promising potential as scintillators for liquid scintillation counting. Due to the large Stokes’ shift the highly soluble sterically hindered 1,3-diphenyl-2-pyrazolines can be used as primary solutes without requiring a wavelength shifter.
(1)1,3-Diphenyl-2-pyrazoline, mp 151 “C,lo A, 355 nm, 2.05 X lo4 (isooctane); (2) 1,3,5-triphenyl-2-pyrazoline, mp 136 “C,ll A,, 357 nm, t 2.03 X lo4 (isooctane); (3) l-phenyl-3-mesityl-2-pyrazoline, mp 96 “C, ,A, 292.5 nm, E 1.40 X lo4 (isooctane); (4) l-phenyl-3-(2’,4’,6’-trimethoxyphenyl)-Zpyrazoline,mp 150 “C, ,A, 297.5 nm, E 1.58 X lo4 (isooctane); (5) l-phenyl-3-(2’,6’-dimethoxyphenyl)-2-pyrazoline, mp 131 OC, ,A, 295 nm, E 1.34 X lo4 (isooctane); (6) 1-(2,6-dimethylphenyl)-3-(2’,5’-dimethoxyphenyl)-2-pyrazoline, mp 82 “C, ,A, 339.5 nm, t 1.73 X lo4 (isooctane); (7) 1-(2,6-dimethylphenyl)-3-(2’,6’-dimethoxyphenyl)-2-pyrazoline,mp 117 “C, A,, 285 nm, 6 0.95 X l o 4 (isooctane); (8) l-phenyl-3-(4’-methoxyphenyl)-2-pyrazoline,mp 142 “C,12A,, 358 nm, E 2.03 X lo4 (benzene); (9) l-phenyl-3-(3’-methoxyphenyl)-2pyrazoline, mp 73 “C, ,A, 364 nm, 6 1.99 X lo4 (benzene); Experimental Section (10) l-phenyl-3-(2’-methoxyphenyl)-2-pyrazoline, mp 110 Substances. The methyl and methoxy substituted 361 nm, t 1.65 X lo4 (benzene); (11) l-phenyl“C, A, 1,3-diphenyl-2-pyrazolineswere prepared by standard 3-(2’,4’-dimethoxyphenyl)-2-pyrazoline, mp 106 “C, A, procedure^.^ The condensation of the appropriately 356 nm, E 1.72 X lo4 (benzene); (12) l-phenyl-3-(2’,5’-disubstituted Mannich bases with phenylhydrazine or the methoxyphenyl)-2-pyrazoline, mp 90 “C, A,, 368 nm, t appropriately substituted phenylhydrazine gave methyl 1.73 X lo4 (benzene); (13) l-phenyl-3-(3’,4’-dimethoxyand/or methoxy substituted 1,3-diphenyl-2-pyrazolines phenyl)-2-pyrazoline, mp 124 “C, ,A, 360 nm, t 2.30 X lo4 with yields of 55 to 90%. l-Phenyl-3-mesityl-2-pyrazoline (benzene); ( 14) l-phenyl-3-(3’,5’-dimethoxyp hen yl) -2and 1-phenyl-3-(2’,4’,6’-trimethoxyphenyl)-2-pyrazoline pyrazoline, mp 126 “C, A, 363.5 nm, t 2.11 X lo4 were synthesized by condensation of mesityl vinyl ketone (benzene); (15) l-phenyl-3-(3’,4’,5’-trimethoxyphenyl)-2and 2,4,6-trimethoxyphenyl vinyl ketone with phenylpyrazoline, mp 135 “C, ,A, 363 nm, E 2.17 X lo4 (benzene). hydrazine, respectively. The compounds are purified by The photophysical data of the compounds investigated recrystallization and, if necessary, by column chromawere measured in benzene for fluorescence spectroscopy tography. All new compounds furnished correct data for (Merck Co., Darmstadt) and isooctane for UV spectroscopy elemental analysis and were further characterized by their (Merck Co., Darmstadt). Degassing was performed by the absorption and fluorescence spectra as well as by their freeze-thaw technique a t 5 X lo4 Torr in special cuvettes mass spectroscopic fragmentation. with a view to fluorescence spectroscopy.13
Introduction 1,3,5-Triaryl-2-pyrazolines are essential components of a liquid or plastic s~intillator.’-~Besides in conventional liquid scintillation counting this class of highly fluorescent compounds is used as fluorescent whitening agent^.^^^ Although the synthesis of a large number of these industrial fluorescers has been described, there has been a lack of quantitative photophysical data until r e ~ e n t l y . ~ - ~ In a study on the relation between structure and fluorescence properties of 1,3-diphenyl-2-pyrazolines8 we observed unusually large Stokes’ shifts of the fluorescence in solution. We report here the photophysical data of a number of methyl and methoxy substituted 1,3-diphenyl-2-pyrazolines and their promising potential as scintillators for liquid scintillation counting.
0022-3654/78/2082-0459$0 1.00/0
t
0 1978 American Chemical Societv
460
H. Gusten, P. Schuster, and W. Seitz
The Journal of Physical Chemistry, Vo/. 82, No. 4, 1978
TABLE I: Photophysical Data on Absorption, Fluorescence, and Scintillation of Substituted 1,3-Diphenyl-2-pyrazolines in Benzene at Room Temperature
2-Pyrazoline
~,,ns
T , ns
L,
RPHa
0.75
3.25
2.58
1.26
0.96
0.90
0.74
3.68
2.71
1.35
0.96
0.88
0.70
3.01
2.64
1.14
0.99
1.00
0.16
2.28
1.12
0.96
3.31
2.95
1.12
0.98
0.33
1.87
1.66
1.13
0.91
0.62
1.34 1.44
1.16 1.16
1.16 1.24
1.00
cm-l
cm-’
cm-’
QF,
QF
27.55
22.51
4.98
0.92
27.62
22.47
5.15
33.90
23.53
10.37
33.33
23.25
10.08
33.67
22.83
10.84
0.88
0.75
2.55
29.15
22.52
6.63
0.87
0.70
35.21
23.20
12.01
0.39
32.79 27.62
27.55 23.92
5.24 3.10
0.74 0.93b
H3C CH3 OCH3
0.08
OCH3 OCH3
‘OC H 3
OCH3
H3C‘
PPOa POPOPa a
In toluene.
Reference 16.
Instrumentation and Techniques. The ultraviolet spectra were recorded on a Cary Model 15 spectrometer. T h e absolute fluorescence spectra and fluorescence quantum yield were measured on a self-constructed absolute fluorescence spectrofluorimeter. For the determination of the fluorescence quantum yield according to the method of Parker and Rees,14quinine bisulfate in 0.1 N H2S04was used as a reference standard assuming a quantum yield value of 0.55.16 The fluorescence decay times were determined by a self-constructed fluorimeter13 using the pulse-sampling method. Details about the instrumentation and the measurement techniques have been published earlier.sJ3 Scintillation counting was performed at room temperature using a Packard Tricarb liquid scintillation spectrometer Model 2420. All compounds were tested in M concentrations solvated in [14C]toluene (0.03 pCi). It was found previously that the maximum relative light output of the substituted 1,3,5triaryl-2-pyrazolines occurred a t a concentration of 1-3 X M.l The relative pulse heights of the substituted 1,3-diphenyl-2-pyrazolines were counted by comparison with the internal standard and with M 2,5-diphenyloxazole (PPO) with M 1,4-di-2-(5-phenyl-
oxazoyly1)benzene (POPOP) as the secondary solute in [14C]toluene which was taken as 100%. Scintillation counting was performed in air-saturated [ 14C]toluene.
Results and Discussion Photophysical Properties. The photophysical data, such as the maximum of the electronic absorption u, and fluorescence uf, the Stokes’ shift Aust, the fluorescence quantum yield in the presence, QF, and without oxygen QFo, the fluorescence decay time 7 , as well as the relative pulse height (RPH) of 1,3-diphenyl-Z-pyrazoline, 1,3,5-triphenyl-2-pyrazoline, and five sterically hindered 1,3-diphenyl-2-pyrazolines in benzene have been summarized in Table I. All the methyl and methoxy substituted 1,3-diphenyl2-pyrazolines display an unstructured absorption and fluorescence spectrum without any fine structure (see Figure 1). Substitution of the parent compound 1,3diphenyl-2-pyrazoline (1) by bulky methyl or methoxy groups in the ortho, ortho’ positions of the phenyl rings in the 1 andfor 3 position of the pyrazoline ring, compounds 3-7, results in a large hypsochromic shift of the absorption spectra, while the fluorescence spectra remain
The Journal of Physical Chemistry, Vol. 82, No. 4, 1978 461
Organic Scintillators with Large Stokes' Shifts
-Wave number
9 (103cm-'l
-
Figure 1. Electronic absorption and absolute fluorescence spectrum of 1,3-diphenyl-2-pyrazoline (a, bottom) and l-phenyl-3-mesityl-2-pyrazoline (b, top) in cyclohexane.
nearly unaffected. This is demonstrated in Figure 1 by comparison of the absorption and fluorescence spectra of 1,3-diphenyl-2-pyrazoline (1) and l-phenyl-3-mesityl-2pyrazoline (3). The result of the steric hindrance in the absorption spectrum and the unaffected fluorescence spectrum is an unusually large Stokes' shift. Since there is no fine structure in both spectra, the 0-0 transition is difficult to determine and the energy difference of the absorption and emission maxima is therefore taken as a measure of the Stokes' shift. Obviously, ortho, ortho' substitution in the 3-phenyl ring causes a larger Stokes' shift than substitution in the 1-phenyl ring (Table I). On the other hand, the largest Stokes' shift is obtained when both phenyl rings are substituted with bulky methyl or methoxy groups. As expected the molar extinction coefficient decreases with increasing steric hindrance (see Experimental Section and , ~ fluorescence Figure 1). Contrary to the l i t e r a t ~ r ethe quantum yields of the sterically hindered 1,3-diphenyl2-pyrazolines, with the notable exception of 4, do not decrease. Compounds 3, 5, 6, and 7 show high fluorescence quantum yields comparable to that of the parent compound 1. This has been attributed to a planar and polar
excited singlet state of the 1,3-diphenyl-2-pyrazolines which is the result of an intramolecular charge transfer after T-T* excitation.8 The fluorescence decay time obviously decreases with increasing steric hindrance. This results in a lower sensitivity to oxygen quenching as shown by the L, values, the quotient of the fluorescence decay time in degassed and air-saturated solutions of the solute.16 The photophysical data of sterically hindered 1,3-diphenyl-2-pyrazolines have promising potential as organic scintillators for liquid scintillation counting. Scintillation Properties. Several factors must be considered when using a fluorescent organic compound as solute for liquid scintillation counting.17J8 The fluorescence or photon yield of the scintillator must be high and the fluorescence decay time should be short. Spectral matching of the scintillator emission spectrum with the spectral sensitivity of the photocathode of the multiplier must be sufficient. These requirements are a prerequisite because of the very poor absolute scintillation efficiency of organic scintillators of the order of 1.5-2.5% .19 Besides these photophysical properties, the solubility in the solvent used should be high and, for practical applications, the scintillator should be as little affected by quenching agents as possible. It has been common practice t o use the
462
The Journal of Physical Chemistry, Vol. 82, No. 4, 1978
H. Gusten, P. Schuster, and W. Seitz
TABLE 11: Photophysical Data on Absorption and Fluorescence and Relative Pulse Height of Methoxy Substituted 1,3-Diphenyl-2-pyrazolines in Benzene at Room Temperature
R =OCH3
N
103 cm-l
va,
27.93
11
a
28.09
T ~ 103 ,
~ i j 103 ~ ~ ,
cm-l
cm-
QFO
QF
L,
RPHa
22.88
5.05
0.93
0.76
1.23
0.97
4.85
0.91
0.75
1.21
5.33
0.90
0.74
1.22
5.62
0.90
0.74
1.21
0.97
4.95
0.91
0.75
1.22
0.93
22.47
13
27.78
22.88
4.90
0.90
0.74
1.22
0.97
14
27.51
22.47
5.04
0.91
0.75
1.22
0.96
4.82
0.91
0.76
1.20
0.96
In toluene.
relative pulse height (RPH) of the widely used organic scintillator 2,5-diphenyloxazole(PPO) in 3 g/L as an index for the light yield of a given scintillator. Since, however, the substituted 1,3-diphenyl-2-pyrazolinesdisplay fluorescence in the region of 430-450 nm, we compared the R P H values with those of the combination of PPO with the secondary solute POPOP to match the short wavelength fluorescence of PPO with the same spectral region of the photocathode. For comparison, the photophysical data of both well-known organic scintillators are included in Table I. With the exception of 4, the substituted 1,3-diphenyl-2-pyrazolinesexhibit relative pulse heights comparable to that of PPO + POPOP. Although it has been reported on the basis of qualitative fluorescence experiments20 t h a t 1,3,5-triphenyl-2pyrazoline (2) is a better fluorescer than 1 the fluorescence quantum yield as well as the RPH values in Table I confirm that the phenyl ring in 5 position has no influence on the ~-2p,-a conjugation and the excited state behavior of the 1,3-diphenyl-2-pyrazoline system.8 Due to the unusually large Stokes’ shift the sterically hindered 1,3diphenyl-2-pyrazolines in Table I can be used as organic scintillators without a secondary solute. The secondary scintillator is used as wavelength shifter in common scintillation counting17 in order to match the emission spectra of the primary scintillator with the spectral sensitivity of the photocathode of common photomultipliers. The secondary scintillator absorbs the wavelength of light emitted by the primary solute and emits it a t a higher wavelength.18 Another advantage of the sterically hindered 1,3-diphenyl-2-pyrazolines as organic scintillators is the low
self-absorption of the scintillation light due to the large Stokes’ shift. This can be of great importance in largevolume scintillation counting, e.g., in whole body counting,17 since irreversible reabsorption leads to light attenuation. The counting efficiency of 0-emitting nuclides in liquid scintillators is known to be an exponential function of the concentration of a chemical quencher added.21 While the addition of 0.1 mL of C C 4 , a representative quenching agent, reduces by 50% the counting efficiency of PPO + POPOP, the counting efficiency of the substituted 1,3-diphenyl-2-pyrazolines in Table I is reduced by 90% on the average. This is due to the fact that 1,3-diphenyl-2-pyrazoline does not fluoresce in CC14.22In other unpolar and aprotic solvents, however, 1 has exactly the same high fluorescence quantum yield as in benzenea8 Like the scintillation solute PPO the 1,3-diphenyl-2pyrazolines are susceptible to acid quenching which is due to protonation of the scintillator solute. Protonation causes a nonemitting excited singlet state.8 Surprisingly, only the compound 4 with methoxy groups in the 2‘, 4’,and 6’ positions shows a low fluorescence quantum yield and, hence, a poor scintillation performance (Table I). Compound 5 , however, with methoxy groups in the 2’ and 6’ positions only, has the tenfold fluorescence quaritum yield as all the other 1,3-diphenyl-2-pyrazolinesinvestigated. This led us to a systematic investigation of the influence of the number and position of the methoxy groups in the 3-phenyl ring of the 2-pyrazolines. The photophysical data and the R P H values have been summarized in Table 11. Within experimental error, all the methoxy substituted 1,3-diphenyl-2-pyrazolinesexhibit spectral shifts, fluorescence quantum yields, and RPH values comparable
14N NMR of Sulfur Containing Compounds
to that of parent compound 1 and t o that of PPO + POPOP. Fluorescence quantum yield as well as pulse height variations among the different methoxy substituted 1,3-diphenyl-2-pyrazolines are too small to indicate any significant correlation with variations in molecular structure. Thus, we do not offer here an explanation for the unusually low fluorescence quantum yield of 4. A similar decrease in the relative fluorescence quantum yield due to a 2,4,6-trimethoxyphenylgroup has been observed by russian authors.23 In summary, the sterically hindered 1,3-diphenyl-2pyrazolines in Table I as well as the methoxy substituted 1,3-diphenyl-2-pyrazolinesin Table I1 exhibit good scintillation properties comparable to that of the widely used PPO and POPOP. Due to the large Stokes’ shift, the sterically hindered 1,3-diphenyl-Z-pyrazolines can be used as a primary solute without requiring a wavelength shifter. By substituting 1,3-diphenyl-2-pyrazoline with methyl and/or methoxy groups the solubility increases significantly, an effect which was previously found by Wirth24i25 on the class of p-oligophenylenes. Thus, the maximum photon yield in liquid scintillation counting can be obtained easily with these new efficient scintillators.
References a n d Notes (1) R. H. Wiley, C. H. Jarboe, F. N. Hayes, E. Hansbury, J. T. Nielsen, P. X. Callahan, and M. C. Sellars, J . Org. Chem., 23, 732 (1958). (2) S. R. Sandler and K. C. Tsou, J. Chem. Phys., 39, 1062 (1963).
The Journal of Physical Chemistry, Vol. 82, No. 4, 1978 463
(3) P. Leggate and D. Owen, Mol. Cryst., 4, 357 (1968). (4) A. Wagner, C. W. Schellhammer, and S. Petersen, Angew. Chem., 78, 769 (1966); Angew. Chem., Int. Edit. Engl., 5, 699 (1966). (5) H. Gold in “The Chemistry of Synthetic Dyes”, Vol. 5, K. Venkataraman, Ed., Academic Press, New York, N.Y., 1971, p 535. (6) I. H. Leaver and D. E. Rivett, Mol. Phofochem., 6, 113 (1974). (7) Z. Raciszewski and J. F Stephen, J. Am. Chem. Soc., 91, 4338 (1969). (8) H. Strahle, W. Seitz, and H. Gusten, Ber. Bunsenges. Phys. Chem., 80, 288 (1976). (9) H. Strahle, W. Seitz, and H. Gusten, Z. Nafurforsch. B , 31, 1248 (1976). (10) B. H. Chase and J. M. Evans, J. Chem. Soc., 4825 (1964). (11) K. v. Auwers and H. Hollmann, Ber. Dfsch. Chem. Ges., 596, 601 (1926). (12) E. Profft, F. Runge, gnd A. Jamur, J. Prakt. Chem., (4) 1, 57 (1954). (13) H. Blume and H. Gusten in ”Ultraviolette Strahlen”, J. Kiefer, Ed., Walter de Gruyter Verlag, Berlin, 1977, Chapter 6. (14) C. A. Parker and W. T. Rees, Analyst, 85, 587 (1960). (15) J. N. Demas and G. A. Crosby, J . Phys. Chem., 75, 991 (1971). (16) I. B. Berlman, “Handbook of Fluorescence Spectra of Aromatic Molecules”, Academic Press, New York, N.Y., 1971. (17) J. B. Birks, “The Theory and Practice of Scintillation Counting”, Pergamon Press, Oxford, 1964. (18) D. L. Horrocks and C. T. Peng, “Organic Scintillators and Liquid Scintillation Counting”, Academic Press, New York, N.Y., 1971. (19) H. Gusten and D. Schulte-Frohlinde, Z. Phys. Chem., 75, 113 (1971). (20) L. Herforth, Wiss. Ann., 5, 744 (1956). (21) V. N. Kerr, F. N. Hayes, and D. G. Ott,Inf. J. Appl. Rad. Isotopes, 1, 284 (1957). (22) H. Gusten, unpublished. The fluorescence quantum yield is
