J. Phys. Chem. 1981, 85, 3431-3435
900-W Xe lamp (or 1000-W Hg-Xe lamp) combined with a 0.5-m focal length Bausch and Lomb monochromator fitted with a 1200 line/mm grating blazed at 3000 A (dispersion ca. 13 A/mm) provided with a scanning accessory. The light was chopped at 480 Hz. The photometric system consisted of an EM1 Model 9659 QA photomultiplier (S-20 photocathode) and a phase-sensitive lock-in amplifier (PAR Model HR8). The samples were placed in 4-mm 0.d. quartz test tubes maintained in a special copper holder providing a vacuum-tight seaLZa Cooling was carried out with a liquid helium flow cryostat (Air Products Model LT-3-110). Laser excitation (at 10 Hz) was carried out with a nitrogen laser (Molectron Model . UV 400) or with a dye laser (Molectron DL 200 tunable dye laser using dye No 70350) pumped with the UV 400 nitrogen laser. Compound I was prepared according to the
3431
1iterature.l' Compounds I1 and 1111*were provided by Professor R. H. Martin, Universit4 Libre de Brwelles. The C5-C12 hydrocarbon solvents were purified by repeated chromatography over activated alumina. Solutions used were lo4 M. No phosphorescence could be detected in the present samples.
-
Acknowledgment. We are very much indebted to Professors E. Fischer and R. s.Becker for valuable discussions, to Dr. R. Korenstein for his participation in the development of the initial experimental setup, and to Mrs. M. Kazes for her technical assistance. ~
~~
(17) Siegrist, A. E.; Liechti, P.; Meyer, H. R.; Weber, K. Helu. Chim. Acta 1969,52, 2521. (18) Martin, R. H. Angew. Chem., Int. Ed. Engl. 1974,13,649. See also: Muszkat, K. A,,; Sharafi-Ozeri, S. Chem. Phys. Lett. 1976,42, 99.
Raman Study of Complexes between Sulfur Dioxide and Some Pyridine Derivatives 2. Pawelka,+ G. Maes,Sand Th. Zeegers-Huyskens" Deparfment of Chemistry. University of Wroclaw, ul. Joliot-Curie 14, 50-383 Wroclaw, Poland and Department of Chemistry, Unlverslty of Leuven, Celestijnenlaan200F, 3030 Heverlee, Belgium (Received: &rch 3 1, 198 1; In Flnal Form: July 13, 198 1)
The three normal vibrations of SOz complexed with some pyridine derivatives in carbon tetrachloride solution have been studied by Raman spectrometry. The fraction of the dative structure, computed from the stretching force constant, varies from 0.04 (3-chloropyridine)to 0.14 (3,4-dimethylpyridine). The intensity of the v,(SOz) band is increased by complex formation. Comparison with charge-transfer complexes of iodine and bromine suggests that the variation of the bond polarizability derivative is smaller when the principal quantum number of the orbital involved in the charge transfer is higher. The asymmetry of the v1 band is discussed and explained by a dipolar interaction between the species present in solution, all being characterized by a strong dipolar character. The electrostatic nature of the complexes between nitrogen bases and SOz is discussed.
Introduction Experimental Section As pointed out by Tramer; the presence of water in the The charge-transfer complexes between nitrogen bases solutions may change the intermolecular behavior of these (aliphatic amines, aniline derivatives) and sulfur dioxide systems. We have taken careful precautions to avoid any have been studied by infrared ~pectrometry,l-~ but very moisture in the solutions. All preparations and manipufew Raman data are available for these systems. Tramer lations were carried out in a dry nitrogen atmosphere so however has described the Raman spectrum of the pyrithat only clear and colorless solutions were used. The dine-S02 complex in binary mixture and reported some pyridines (Aldrich Europe) were distilled and stored over variations in the vibrational frequencies and intensities of NaOH and K2C03. Carbon tetrachloride (Riedel de Haen, both component^.^ Spectranal) was dried on molecular sieves (4A). SO2 was As suggested by the high values of the equilibrium Matheson (99.98%). constants and of the enthalpies of complex f o r m a t i ~ n , ~ ~from ~ The concentrations of SOz were determined by titration the complexes between nitrogen bases and SO2 may be of the solutions with an excess of NaOH (1M) and with considered as rather strong. This fact is also consistent HCl(1 M). The SOzconcentrations used to determine the with the dielectric properties of the systemss and with the Raman intensity of the ul band varied between and ab initio calculations carried out on the NH3-S02 system.' In previous w0rks,8,~we have determined the frequency, (1) W. E. Byrd, Inorg. Chem., 1, 762 (1961). the intensity, and the depolarization ratio of the Y ~ (X- ~ (2) R. C. Cipolla, Ph.D. Thesis, University of Rhode Island, Kingston, RT 1971 = Br, I) Raman band in charge-transfer complexes between -__".-. (3) I. C. Hisatsune and J. Heicklin, Can. J. Chem., 53, 2643 (1975). bromine or iodine with several pyridine derivatives. The (4) A. Tramer, Bull. Acad. Pol. Sci. CZ. III,4,355 (1956); 5, 501,509 present work extends these studies to the SOz molecule (1957). complexed with the same bases. 90, 2239 (5) J. Grundnes and S. D. Christian, J. Am. Chem. SOC., +University of Wroclaw. University of Leuven and Research Associate of the National Fund for Scientific Research (Belgium). * Address correspondence to this author a t the University of Leuven.
*
0022-3654/81/2085-3431$01.25/0
(1968). (6) J. A. Moede and C. Curran, J. Am. Chem. SOC., 71, 852 (1949). (7) R. R. Lucchese, K. Haber, and H. F. Schaefer, 111,J. Am. Chem. SOC..98. 7617 (1976). (8) G: Maes~and'Th. Zeegers-Huyskens, J. Phys. Chem., 82, 2391 (1978). (9) C. F. Merlevede and G. Maes, Adu. Mol. Relaxation Interact. Processes, 16, 111 (1980).
0 1981 American Chemical Society
The Journal of Physical Chemistry, Vol. 85, No. 23, 1981
3432 61
I
Pawelka et al.
I
IR
L
l
i
l
l
l
l
l
0.2
04
0.6
Figure 1. Relatlve intensity (I,) of the u1(S02)band as a function of the SO2 concentratlon: (A) free SO2; (8)3,5-dImethylpyridine-SO2 complex.
Flgure 2. u l , up, and ug bands of SO2 complexed with pyridine. C , = 3 M (S = CCI,). (a and c) Cso = 1.6 M; laser power = 700 mW; slit = 400 pm. (b) Cso, = 1 M; faser power = 600 mW; slit = 350
w.
1.5 M; the pyridine derivatives were in excess, at concentrations ranging from 0.5 to 3 M-l so that no free band at 1142 cm-’ could be observed. Laser Raman spectra were recorded by using a Coderg Spectrometerwith a Spectra Physics 164 Ar’ laser emitting a maximum power of -2 W at 514.5 nm. A rotating cell was used; the initial and final temperatures, measured with a thermistor immersed in the cell, were 296 and 297 K, respectively. The depolarization ratio of the ul band of SO2was determined by a previously described method.l0 The relative intensity of this band was measured with a planimeter using the ul band (888 cm-’) of cyclopentane (0.3 M) as an internal standard. A plot of the ratio of the relative intensities (IR)as a function of the SO2 concentration is linear, and the slope of the straight line gives the relative An example of molar intensity of the SO2molecule (IRo). this determination is given in Figure 1 for free SO2 molecule (line A) and for the 3,5-dimethylpyridine-SO2complex (line B). Conversion of relative intensities into standard and absolute intensities has been described in previous works.8J1
Results and Discussion Frequency Shift of the SO2 Vibrations. The v1 (symmetric stretching), u2 (deformation), and u3 (asymmetric stretching) vibrations of the free SO2molecule dissolved in CC14 are observed at 1142,518, and 1338 cm-’, respectively; these values differ slightly from those reported in the literature.12 When SO2 is complexed with a pyridine derivative, the v3 and u1 bands are shifted to lower wavenumbers, and the u2 band goes tk higher wavenumbers. An example of a Raman spectrum is given in Figure 2. From this figure, it can be seen that the ul band is asymmetric; this asymmetry remains for all of the concentrations and was also observed by Tramer4 for binary solutions of (10)G.Mae8 and Th. Zeegers-Huyskens, Chem. Phys. Lett., 44,135 (1976). (11) G.Maes and Th. Zeegers-Huyskens, J. Raman. Spectrosc., 7,325 (1978). (12) J. C.David and H. E. Hallam, Spectrochim.Acta, Part A, 23,593 (1967).
l
l
l
l
i
l
0.LO
Flgure 3. Il(SO,) as a function of in CCI, solutions. 0
l
0.35
0.30
(E
- 1)/(2~+ 1) for pyrldlne-SOp
TABLE I: Wavenumber of the u l , u,, and v 3 Bands of SO, Complexed with Some Pyridine Derivatives (S = CCl,) pyridine derivative dissolved free SO, 3-chloropyridine pyridine 3-methylpyridine 4-methylpyridine 3,5-dimethylpyridine 3,4-dimethylpyridine a
VI
VZ
V3
1142 1141.5 1139.6 1137.8 1137.5 1136.5 11 34
518 528 532 a 534 534 a
1338 1324 1300 1296 1293 1291 1288
Overlapping with a solvent band.
pyridine and SO2. The origin of this asymmetry will be further discussed. The frequency of the u1 vibration is very sensitive to the SO2concentration; for the pyridine-SO2 complex, for example, u1 varies from 1138 to 1135 cm-’ when the SO2 concentration varies from 0.020 to 1.2 M. We attribute this variation to a change of the polarity of the medium. The dielectric constants, E, of the solutions of picoline, SO2, and CCll were calculated by an Onsager-Frohlich equation,13J4taking a dipole moment of 6.2 D for the pyridine-SO2 complex and of 2.3 D for the free pyridine m01ecule.l~ The equation used, which is valid for an excess of base with respect to SO2, is t
= n2+ (n2 + 2)2 (2 + n2/t)
[ ;oi~y] (4
X
~PL,,mp12Fso2 + PpyYFpr - FSOJ
+ PS2FS)
FSOFpy,and Fs are the formal concentrations in mol drn”
of db2,pyridine, and solvent. n2, the square of the refractive index, was taken equal to 2.132. The square of the apparent moment (hs2)of the solvent (which results from the difference between the “internal” refractive index of the solvent and the value above) is equal to 0.137 D2. Figure 3 shows that there is a linear relationship between the wavenumber of the symmetric vibration of SO2 and (E - 1)/(2t + l),suggesting that the frequency lowering can be attributed to an enhanced polarity of the medium. The u2 and u3 bands do not show this concentration dependence. Table I lists the wavenumbers of the ul, u2, and us bands. The u1 values quoted in this table have been extrapolated to zero SO2 concentration. As can be seen from the results of this table, complex formation of SO2 with pyridine derivatives brings about a moderate variation of the frequencies of the three normal vibrational modes of SO2. The frequency lowering of the vibrations of the acceptor molecule can be considered as a measure of the fraction of the dative structure (b2 + abS), and most of the data reported in the literature for com(13)L.Onsager, J. Am. Chem. SOC., 68,1486 (1936). (14)R. Nouwen and P. Huyskens, J. Mol. Struct., 16, 459 (1973). (15)A.L.McClellan, “Tables of Experimental Dipole Moments”, W. H. Freeman, San Francisco, CA, 1963.
The Journal of Physical Chemistry, Vol. 85, No. 23, lQ8l 3433
Complexes between SO2 and Pyridine Derivatives TABLE 11: kC(SO,),e , and b 2 + abS Values for the Complexes of SO, and Some Pyridine Derivatives pyridine derivatives dissolved free SO, 3-chloropyridine pyridine 3-methylpyridine 4-methylpyridine 3,5-dimethyIpyridine 3,4-dimethylpyridine
so,-
a
kC, 0, N m” degree
b Z+ abS
pKa
974 9 61 940 935 933 931 926 63ga
0.037 0.101 0.114 0.122 0.130 0.142
2.84 5.17 5.68 5.98 6.15 6.45
--
118.2 116.3 113.2 112.9 112.5 112.4 112.3 lloa
0.1 5
0.10
0.05
From ref 18.
plexes of halogens with nitrogen bases have been computed from the frequency lowering (or force-constant lowering) of the X-X band.16 If kf, k,, and k-l are the stretching force constants in the free, complexed, and SO, molecules respectively, the values of b2 + abS can be computed by the following equation:
b2 + abS =
0
I
I
Flgure 4. b2
I
+
(16)J. Yarwood, Ed., “Spectroscopy and Structure of Molecular Complexes”, Plenum Press, New York, 1973. (17) H. B. Friedrich and W. B. Person, J. Chem. Phys., 44,216(1966). (18)D.E.Mulligan and M. E. Jacox, J.Chem. Phys., 49,1003(1971). (19)J. N. Linett, Tans. Faraday Soc., 41,223 (1945). (20)G.Herzberg, “Infrared and Raman Spectra”, Part 11, Van Nostrand, New York, 1956,p 170. (21)D.Van der Helen, J. D. Childs, and S. D.Christian, Chem. Commun., 887 (1969).
I
I
I
6.0
5.0
7.0
+ abSas a function of the pK, of the pyrldine derlvatke.
TABLE 111: Relative Molar Intensity (IR’), De olarization Ratio ( p ), and Absolute Intensity (Iafs)of the v 1 (SO,) Band
kf - kC
The three SO2- vibrational fundamentals have been assigned for the Cs+...SOL species in which the interaction between SO2- and the cation was shown to be very small;lg the stretching force constant of the SO2- radical ion, calculated by a valence force field, was 639 N m-l; this value was adopted in this work. The kf and kc values, along with the OS0 angle (6) are calculated by the Linett equation,lg neglecting the bending and interaction force constants. This assumption is justified in this case; the calculated 6 value of 118’ 2’ for the free SO2 molecule is very close to the experimental value (119’ 5 9 , and the stretching force constant of 974 N m-l does not markedly differ from that of 997 N m-l (ref 20) calculated under the assumption of a valence force field. Complex formation brings about an angle decrease; the value of 113’ 2’ calculated for the pyridine-SO2 complex is very close to the experimental value of 114” 8’ found by X-ray diffraction for the (CH3)3N-S02complex.21 It must also be pointed out that the errors on the computed kf and kc values will have only a small effect on their difference in eq 1. Table I1 lists the values of kc, 6, and b2 + abS for the complexes studied in this work. The stretching force constant for the free SO, molecule (k? and for the SO, ion (P)are also indicated in this table. As suggested by the moderate frequency decrease of the two stretching bands, the values of b2 + abS are not very high, taking a maximum value of -0.14 for the more basic pyridine. For the (CH3)3N-S02complex, the contribution of the dative structure was estimated as 20-30% on the basis of the dipole-moment measurement^.^^^ This estimation must be considered as approximate owing to the lack of knowledge of the dipole moment in the pure dative state; but, owing to the greater basicity of the aliphatic amines, the b coefficient must certainly be higher than for complexes involving the pyridine derivatives. As seen by Figure 4, the values of b2 abS are nicely related to the pK, of the pyridine derivatives.
I
I
L.0
3.0
pyridine derivative dissolved free SO, 3-chloropyridine pyridine 3-methylpyridine 4-methylp yridine 3,5-dimethylpyridine 3,4-dimethylpyridine a
IR
M-
;,
3.1 5.4 7.2 9.1 10.8 9.8 12
Pa
10 W s , cm4 arnu-’
0.034 0.047 0.061 0.072 0.073 0.072 0.084
35.9 63 82.7 105.5 123.6 113 137.1
Experimental accuracy = 0.003-0.005.
Raman Intensity and Depolarization Ratio of the ul Band. As shown by Figure 1, complex formation brings about an increase of the relative molar intensity of the u3 band of SO,; the same considerations also hold for the absolute intensity and the depolarization ratio ( p ) of this band, indicated in Table 111. The absolute intensity (Iab”) of a Raman line is related to the derivative of the isotropic part of the polarizability ad and to the anisotropy derivative yq‘ (both taken with respect to the normal coordinate) by the expression22 I”bS = 45aq‘2 7y,‘2 (2)
+
The depolarization ratio is related to expression ‘
p =
37,‘2/(45aq‘2
a-,‘ and y,‘ by the
+4 7 3
(3)
Further, for a nonlinear symmetric triatomic molecule, the bond polarizability and bond anisotropy derivatives are related to a{ and yd by the following relations: aa/ar = aR’= (MO/2)lJ2ad (4) ay/ar = yR’ = (M,,/2)lJ2y{
(5)
where M, is the mass of the oxygen atom. The aR’ and yR’ values for the free and complexed SO2 are indicated in Table IV; the table also lists their ratio of the free SO2 molecule. These results indicate that the variation of the anisotropy derivative is more important than the variation of the mean value a derivative. Similar results (intensity enhancement, increase of the depolarization ratio, and of the bond polarizability derivatives) have been found for the vx-x Raman band of brominegand iodineg complexed with pyridine bases. These previous results have suggested that the increase of the bond polarizability derivative of (22) R. E. Hester in H. A. Szymansky, Ed. “Raman Spectroscopy. Theory and Practice”, Plenum Press, New York, 1967,p 101.
The Journal of Physical Chemistry, Vol. 85, No. 23, 198 1
3434
Pawelka et al.
TABLE IV: Bond Polarizability and Anisotropy Derivatives and Their Ratio to the Free SO, Molecule 1 O%R’, cmz 2.43 3.17 3.56 3.97 4.30 4.12 4.47
pyridine derivative dissolved free SO, 3-chloropyridine pyridine 3-methylpyridine 4-methylpyridine 3,5-dimethylpyridine 3,4dimethylpyridine
1016yR‘, cm* 1.78 2.74 3.56 4.35 4.74 5.62 5.32
(ffR ’ )
(7R’ I“/ (YR’)
1.30 1.46 1.63 1.77 1.70 1.84
1.54 2 2.44 2.66 3.16 2.99
(ffR’)Y
A 1137.2
Ic
/”
4
I
/
d
5,5crii15,5cfl
-
0
I2
b2 + a b S
1
II
nn 0
0.1
0.2
0.3
LLl
II
0.4
+
Figure 5. (&‘)c/a~)f as a function of b 2 abS for I,, Br,, and SO, complexed with pyridine derivatives. The data for I, and Br, are taken from ref 9 and 8.
the X-X bond could not be explained only by an increase of the X-X distance, appearing as a third power in Long and Plane’s but was also related to a variation of the extent of charge transfer when the X-X bond stretches.17 These two effects will also probably contribute to the Raman intensity enhancement for the SO2 complexes but are difficult to separate owing to the lack of knowledge of the S-0 distance for the systems studied in this work. The charge transfer taking place from the free electron pair of the nitrogen atom to the 3(p7r-dn) antibonding orbital of the s-0 bondz4will probably cause a slight increase of the S-0 distance. It is however interesting to compare the ratio for the iodine, bromine, and SO2 complexes. Figure 5 shows that, for the same b2 + abS values, this ratio follows the order I2 < Br2 < SOz When the charge transfer is 0.15 e, the (&R’)C/(aR’)f ratio is ca. 1.10 for 12,1.30 for Br2, and 1.80 for SO2 complexes. These results suggest that, when the same fraction of a n electron is transferred to a n antibonding orbital, the variation of the bond polarizability derivative will be smaller when the principal quantum number of the orbital is higher. In the iodine and bromine complexes, the charge transfer takes place to the 4a,* and 5a,* antibonding orbitals of the halogen molecule; for the SO2 complexes, a 3(p?r-d7r) antibonding orbital is available for charge transfer. It must also be pointed out that the order found for the (BR’)’/(&R’)~ ratio is precisely the inverse order of the polarizabilities of the molecules which decrease in the order% I2 > Br2 >> SO2 (23) T. V. Long and R. A. Plane, J. Chem. Phys., 43, 457 (1965). (24) J. D. Dunitz, Acta Crystallogr.,9, 579 (1956). (25) Landolt-Bornstein, “Zahlenwerte und Funktionen”, Vol. I. Part 3, Springer, West Berlin, 1951, p 510.
llL5
1135
1125
1115
Flgure 6. Raman spectrum in the u,(SO,) range of solutions of ypicoline and SO,: (a) binary solution ( C s o = 6.3 M; CplcdllM= 10.3 M); (b) ternary solution in CCI., ( C , = 3.45 M; C,, = 5.15 M; (a and b) laser power = 200 mW; slit = 200 pm. (c) C s o z= 0.78 M; C-_’ 1.07 M; (d) C, = 0.1 M; C= 0.12 M; (c and d) laser power 400 rnW slit = 400 pm. The halves of the half-bandwldths on both sides of the ,8, are indicated by an arrow.
-
This seems to indicate that the charge-transfer effect influences to a lower extent the bond polarizability derivative of the more polarizable molecules. Asymmetry of the u1 Band. In carbon tetrachloride solution, the vl and u3 bands of free SO2 are totally symmetric in shape. As said before, the u l ( S 0 2 ) band shape of complexed SOz is asymmetric; and this asymmetry was observed for all concentrations used for the determination of the intensity of the band. The asymmetry of the u3(S02) band is less pronounced. The origin of this asymmetry has been discussed by Tramer,5who studied Raman spectra of binary solutions of pyridine and SO2and explained them by a Stepanov mechanism%involving successive transitions with u(~...S) = 0, 1, 2, .... In order to explain the possible origin of the asymmetry for the solutions studied in this work, we recorded Raman spectra of solutions whose polarity was strongly different, going from a concentrated binary solution of 4-picoline and SO2 to progressively dilute solutions in carbon tetrachloride (Figure 6). For the first solution (spectrum a), the band is symmetric; for the second solution (spectrumb), the spectrum is characterized (26) B. Stepanov, Zh. Eksp. Theor. Fiz., 15, 435 (1945).
J. Phys. Chem. 1981. 85,3435-3440
by two bands; for the third solution (spectrum c), the band is clearly asymmetric; and for the more dilute solution (spectrum d), the band is nearly symmetric. This behavior strongly suggests a dipolar association between the species present in solution.27 This kind of interaction can be especially important in this case owing to the high dipolar character of the complex C. = 6.2 D) and of the free species (pso = 1.6 D, ppy = 2.3 D). The precise composition of the dipolar interacting species is however difficult to determine from the results of this work. Spectrum d of the more dilute solution probably corresponds to the 1:l complex; spectrum a of the more concentrated binary solution corresponds perhaps to the self-associated species (dipole-dipole or quadrupole interaction). In binary solution of 4-picoline and SOz, at very low SO2 concentration (0.08 M), two well-defined bands are observed at 1136.5 and 1130.5 cm-'. In this case, the first band probably corresponds to the 1:l species; the second band is probably ascribable to a molecular complex formed by one molecule of SO2 and two or more pyridine molecules. This is in accordance with earlier data.28 Further, it has been shown by UV spectrophotometry that, in very dilute carbon tetrachloride solution, the molar ratio of sulfur dioxide and pyridine which participates in complex formation is 2:l; the second SO2 molecule probably would be bonded to the T electron of the heteroaromatic ringsz9 The asymmetry of the vl(S02)band observed in this work cannot be accounted for by this structure because the v1 band of SOz dissolved in pure benzene is also characterized by a weak asymmetry and (27) C. Garrigou-Lagrange, C. de LOG,P. Bacelon, Ph. Combelas, and J. Daeaut. J. Chim. Phvs. Phvs.-Chim. Biol.. 1936 (1971). (28 P.'A. Abdykanmov, B. Zhemazhanova,'and E.' A. Buketov, Vestn. Akad. Nuuk Kuz. SSR,27,32 (1971). (29) M. Matsuda and T. Hirayama, J. Polym. Sci., 5, 2769 (1967).
3435
the maximum of the band lies at 1141 cm-' (3 cm-' higher than for the very dilute solutions). The weakness of the asymmetry can be explained by the apolar character of the free benzene molecule. It can thus be concluded that, if the v1 band is symmetric for well-defined concentrations of pyridine and SO2, the Stepanov mechanism cannot explain the band profile. The asymmetry of the band can better be explained by an electrostatic interaction between the 1:l complex and the free molecules, all species being characterized by a strong dipolar character. As said in the introduction of this work, the complexes between nitrogen bases and SO2 may be considered as strong, and this is consistent with the high values of the equilibrium constants and of the enthalpies of complex formation. For the complex between trimethylamine and SO2, in heptane, K and -AH are 2550 M-l and 11 kcal/ m01-l.~ The latter is very close to the value obtained for the complex of the same base with iodine (-AH = 12.1 k~al/mol-l).~~ For this last complex, the charge transfer is obviously more pronounced, and so one can conclude that other effects such as the dipole-dipole interaction contribute to the stability of the complex. This is consistent with the above discussion. This is also fully in agreement with the conclusion of Drago and W e n ~ ,who ~l have shown that the interaction between dimethylacetamide and SOz is essentially electrostatic. Acknowledgment. We thank Professor P. Huyskens, who has computed the dielectric constant of the solutions. We are also indebted to the Catholic University of Leuven (KUL) for a fellowship (Z.P.) and to the Belgian government (convention No. 76/81.11.4) for financial support. (30) H. Yada, J. Tanaka, and S. Nagakura, Bull. Chem. SOC.Jpn., 33, 1660 (1960). (31) R. S. Drago and D. A. Wenz, J. Am. Chem. SOC.,84,526 (1962).
Amine Quenching of Fluorescence of Phenylated Anthraceness David G. Llshan,+ George S. Hammond,t and W. Atom Yee" Division of Natural Sciences, Universw of California, Santa Cruz, California 95084 (Received: April 22, 198 1; In Final Form: July 20, 198 1)
Fluorescence quenching of various phenyl-substitutedanthracene analogues by aliphatic and aromatic electron-rich amines in organic nonpolar solvents has been studied. The Stern-Volmer quenching constants obtained were interpreted by using a theory of electron-transferreactions to rationalize trends. The Stern-Volmer quenching constants indicate the electronic and geometric requirements necessary for efficient charge-transfer fluorescence quenching. Phenyl substitution dramatically reduces the quenching rate and three orders of magnitude separate the quenching rates of the compounds studied. Low quenching rates are attributed primarily to shielding of the 9 and 10 positions of the anthracenes from close contact with the quenchers.
Introduction We have reported the efficient quenching of rubrene fluorescence by N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD) in benzene.' Although we obtained no direct evidence for an exciplex in that system, it appears a plausible intermediate in nonpolar solvent to explain the 'Corporate Research & Development, Allied Corporation, P.O. Box 1021R Morristown, N J 07960. *Address correspondence to this author a t the Division of Natural Sciences, State University of New York, Purchase, NY 10577. t D.G.L. and W.A.Y. dedicate this manuscript to George S. Hammond on the occasion of his 60th birthday and in appreciation for his friendship and guidance.
TMPD
rubrene
quenching in direct malogy to systems like anthracene and N,N-dimethylaniline (DMA).6
0022-3654/81/2085-3435$01.25/00 1981 American Chemical Society
