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S.Babiak and A. C. Testa
(2) T. W. Leland, J. S.Rowiinson, and G. A. Sather, Trans. Faraday SOC.,64, 1447 (1968): T. W. Leland, J. S.Rowlinson, G. A. Sather, and I. D. Watson, ibid., 65, 2034 (1969). (3) F. B. Sprow and J. M. Prausnitz, Trans. Faraday SOC.,62, 1097 (1966). (4) E. A. Guggenheim, J. Chem. Phys., 13, 253 (1945).
(5) I. Prigogine and L. Saraga, J. Chim. Phys., 49, 399 (1952). (6) F. B. Sprow and J. M. Prausnitz, Trans. faraday Soc., 62, 1105 (1966). (7) J. S. Rowlinson, "Liquids and Liquid Mixtures", 2d ed, Butterworth, London, 1969. (8) E. Dickinson, J. Colloidlnterface Sci., 53,467 (1975).
Fluorescence Lifetime Study of Aminopyridines S. Babiak and A. C. Testa* Department of Chemistry, St. John's University, Jamaica, New York I1439 (Received March 8, 1976)
The fluorescence lifetimes of 2- and 3-aminopyridine, measured in different solvent systems, indicate that the rate constant for fluorescence is approximately an order of magnitude smaller in nonpolar solvents than it is in polar and hydrogen bonding solvents. This effect is attributed to significant n,r* character in the lowest excited singlet, resulting from significant vibronic n,T*--R,r* mixing, in a nonpolar solvent such as cyclohexane. The fluorescence rate constant for 4-aminopyridine indicates an n,n" character in all solvents investigated. The hundredfold increase in the &/@F ratio of 4-aminopyridine relative to the 2 and 3 isomers suggests a significant contribution from charge transfer transitions in the former. @F increases while 7~ decreases with increasing solvent polarity due to increasing values for h~ and hisc. In nonpolar solvents the major radiationless process appears to be internal conversion. A scheme based on our data is presented to account for the lack of luminescence in pyridine. The rate constant for the fluorescence quenching of 3-aminopyridine with hydroxide ion via proton transfer in the excited state was determined to be 6.6 X lo9 M-' S-1.
Introduction A problem of continuing interest in electronically excited states is the elucidation of radiative and radiationless processes. In the case of aromatic heterocyclic molecules the situation is particularly interesting owing to the effects that may be induced by interacting n,T* and T,T* states. The fluorescence yields of 2-, 3-, and 4-aminopyridines in a variety of solvents have been previously rep0rted.l In that study the lowest excited singlet was shown to be more basic than the ground state as is expected for a heterocyclic molecule. Our motivation for this work was prompted by the lack of luminescence in pyridine and the need to better understand the luminescence of nitrogen heterocyclics. With the aim of directly following the fluorescence decay of aminopyridines we have used the photon counting technique to measure fluorescence lifetimes. From knowledge of these values and the corresponding quantum yields we could readily analyze the data for the importance of radiative and radiationless processes in different solvents. Measurements were performed in the following solvents in order of increasing dielectric constant: cyclohexane, ethyl ether, ethanol, acetonitrile, and acidic and basic aqueous solutions. Although the measurement of fluorescence lifetimes is no longer a difficult problem there appears to be a dearth of information regarding the effect of solvent on the fluorescence lifetimes of aromatic heterocyclic molecules. On the other hand, this effect is expected to be unimportant in aromatic hydrocarbons. The effect of solvent polarity on the phosphorescence of aromatic molecules, e.g., aromatic ketones such as acetophenone2 and nitrogen heterocyclics, such as a m i n ~ p y r i d i n e shas ~ been shown to be significant. The Journal of Physical Chemistry, Vol. 80, No. 17, 1976
Experimental Section Materials. The isomeric aminopyridines were obtained from Aldrich Chemical Co. and recrystallized according to procedures given e1sewhere.l Spectroquality cyclohexane, ethyl ether, and acetonitrile obtained from Matheson Coleman and Bell were used as received. Fluorometric grade sulfuric acid was obtained from Hartman-Leddon Co. and ultrapure reagent grade sodium hydroxide was obtained from Brinkman Instruments. Ludox light scattering solution, employed to measure the excitation profile, was obtained from E. I. DuPont & Co. Instrumentation. The photon counting apparatus was comprised of the following nuclear modules, available from E. G. & G., Inc.: Model T R 204A/N dual updating discriminator and Model T H 200 A/N time-to-amplitude converter. A 200-channel Packard Instrument Model 930 multichannel analyzer stored the data in the form of number of counts vs. channel number and received via a digital printer. The data were processed on a computer using the radius of gyration method for deconvolution.4 For all measurements the counting time was selected to ensure that the peak channel contained 2 10 000 counts. An Amperex 56 DUVP/03 photomultiplier was used as the fluorescence detector and the nanosecond lamp was of the free firing type. A 5 0 4 termination in the lamp circuit provided the reference pulse to establish zero time. Measurement times were generally less than 30 min. The lamp, made by sealing two sharpened tungsten electrodes into a quartz tube, was filled with 3 atm of deuterium and exhibits a rise time less than 2 ns and a 2.5-11s pulse width at half-height. Absorption spectra of solutions were measured with a Cary
Fluorescence Lifetime Study of Aminopyridines
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TABLE I: Fluorescence Lifetime Data for Aminopyridines in Different Solvents (25 "C, Degassed)
dF
10-6hF,b
10-6k&,c
5-1
S-1
ns
4P
(77 K, EPA)d
0.04 0.10 0.18 0.25
2-Aminopyridine 24.2 f 2.5 1.7 10.8 f 0.4 9.3 5.1 f 0.3 35 5.0 f 0.3 50
Cyclohexane Ethyl ether Ethanol Acetonitrile
0.02 0.03 0.14 0.05
3-Aminopyridine 26.5 f 1.7 0.8 7.7 f 0.4 3.9 6.5 f 0.3 22 4.6 f 0.1 11
39 126 132 207
0.38
Ethyl ether Ethanol Acetonitrile
0.0002 0.002 0.0003
4-Aminopyridine 17.5 f 1.7 0.01 10.3 f 0.1 0.2 3.4 f 0.2 0.1
50 100 330
0.54
Cyclohexane (2.0)a Ethyl ether (4.3) Ethanol (26) Acetonitrile (38)
a
TF,
40 84 160
0.07
150
Dielectric constant. b FZF = l/TFo = ~ F / T F .c hds = hi, t hiac. d See ref 3.
Model 14 and optical densities of -0.15 were used in most measurements. Fluorescence spectra were recorded with instrumentation already described,l and quantum yields were normalized to the value of 0.64 for 2-aminopyridine in 0.1 N H z S O ~however, ,~ no correction was made for the variation of wavelength sensitivity of the photomultiplier-monochromator combination. Standard 1cm quartz cuvettes with graded seals were used for all measurements and the cells were flame sealed after vacuum degassing to pressures 3 > 4, while the phosphorescence yields The Journal of Physical Chemistry, Vol. 80, No. 17, 1976
280 260 WAVELENGTH, nm.
240
220
Figure 1. Uv absorption spectrum of 2-,3-, and 4-aminopyridine in cyclohexane.
%'0)
nonpolar
so-
( b ) polar solvent
solvent
Figure 2. Electronic state diagram showing rate constants for radiative and radiationless process of aminopyridines in (a)nonpolar and (b) polar solvents. The symmetry of the states are also given for the point group C,, in order to apply it to pyridine. of the aminopyridines decrease in the order 4 > 3 > 2 . l This ~~ trend is quite general since decreasing fluorescence is replaced by increasing phosphorescence. The ratio $Jp/& increases in the order 2 < 3 < 4 and for the 4 isomer is approximately two orders of magnitude larger than for the 2 and 3 isomers. In view of the data given it is tempting to try to explain: (a) the lack of phosphorescence of these molecules in nonpolar solvents, (b) the presence of phosphorescence in polar solvents, and (c) the lack of luminescence in pyridine. The lack of luminescence from pyridine has remained a perenn'ial problem of interest. Hoover and KashalO have suggested that the lowest excited singlet and triplet states in pyridine may be lAZ(n,ag*) and 3AZ(n,~5*), respectively. Using the rate constants from Table I we can construct the diagram shown in Figure 2. For the purpose of attempting a comparison with pyridine the symmetry of states is also given for point group (22". In the nonpolar solvent the most effective relaxation route is internal conversion; furthermore, IZF and hisc are relatively small which account for weak fluorescence and no phosphorescence. The inefficient intersystem crossing could arise from b,a* 3a,~* In. polar solvents kF and hisc increase, as the data demonstrate, and increased fluorescence and the presence of phosphorescence is observed. The larger
--
Methyl Radical Production in Irradiated Acetate Powders
intersystem crossing rate constant would be due to the more favorable process 'P,T* w-+ 3n,1r*,11 which arises from movement of energy levels with solvent change. It should be mentioned, however, that in the case of 4-aminopyridine charge transfer transitions, which are omitted in the diagram, provide an additional intersystem crossing contribution. The extinction coefficient for 4-aminopyridine (-1.2 X 104) is approximately three times larger than the value for the 2 and 3 isomers.' If substituents are absent, i.e., pyridine, it is tempting to use the scheme in Figure 2a to account for the absence of luminescence in pyridine as well as the unlikely possibility of observing triplet-triplet absorption due to a very small triplet yield. I t is interesting to note that the lack of fluorescence and phosphorescence are still compatible with a lowest triplet state that is R,P*.In the singlet manifold although ln,n* (lB1) is assumed to be the lowest state in pyridine, vibronic coupling could still impart R,T* character and
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thus hinder intersystem crossing. In summary our results indicate that internal conversion is a major relaxation mode for aminopyridines only in nonpolar solvents. An attempt to extend our conclusions to pyridine would suggest a rapid internal conversion competing effectively with fluorescence and intersystem crossing.
References and Notes A Weisstuch and A. C. Testa, J. Phys. Chem., 72, 1982 (1968). A. Lamola, J. Chem. Phys., 47, 4810 (1967). S. Hotchandaniand A. C. Testa, J. Chem. Phys., 59, 596 (1973). S.S.Brody, Rev. Sci. Instrum., 28, 1021 (1957). R. Rusakowicz and A. C. Testa, J. Phys. Chem., 72, 793 (1968). J. Lemaire, J. Phys. Chem., 71, 612 (1967). H. Boaz and G. K Roliefson, J. Am. Chem. Soc., 72, 3435 (1950). G. Varsanyi, "Vibrational Spectra of Benzene Derivatives", Academic Press, New York, N.Y., 1969. (9) J. M. Hollas, G. H. Kirby, and R. A. Wright, Mol. Phys., 18, 327 (1970). (IO) R. J. Hoover and M. Kasha, J. Am. Chem. Soc., 91,6508 (1969). (11) M. El Sayed, J. Chem. Phys., 38, 2834 (1963). (1) (2) (3) (4) (5) (6) (7) (8)
An ELDOR Study of Methyl Radical Production at 77 K in Irradiated Acetate Powders as a Function of Metal Cation' Carolyn Mottley, Lowell D. Kispert," and Pu Sen Wang Chemistry Department, The University of Alabama, Tuscaloosa, Alabama 35486 (Received December 8, 1975) Publication costs assisted by The University of Alabama
Paramagnetic relaxation characteristics of the methyl radical a t 77 K in irradiated powders of magnesium acetate tetrahydrate, potassium acetate, sodium acetate trihydrate, calcium acetate monohydrate, zinc acetate dihydrate, and lithium acetate dihydrate were measured by power saturation techniques and ELDOR spectroscopy. For magnesium acetate tetrahydrate the characteristic relaxation time (TleT2e)'/2for the methyl radical is relatively constant with radiation dose a t low doses and decreases a t higher doses. For sodium acetate trihydrate the relaxation time decreases even at low dose. ELDOR measurements of the ratio of the intermolecular relaxation time ( Tpo)between methyl radicals to the spin-lattice relaxation time (TI,) shows an increase from 0.05 for magnesium acetate trihydrate to >1 for sodium acetate trihydrate. In addition, the field-swept ELDOR reduction factors for the r n ~= -3/2 ESR line of the methyl radical utilizing 100-kHz field modulation decreases from 73% in irradiated magnesium acetate tetrahydrate to 2% in irradiated calcium acetate monohydrate. These features suggest that the radiation produced methyl radicals are trapped in clusters in acetates a t low dose forming a nonuniform spatial distribution that is dependent on the cation and decreases in the approximate order Mg2+ > K+ > Na+ > Ca2+.No estimate of the amount of clustering in the irradiated Zn2+ and Li+ salts could be made due to the long TIe%exhibited by the methyl radical.
Introduction Relaxation mechanisms which give rise to electron-electron double resonance (ELDOR) spectra of radicals in irradiated crystals generally involve the modulation of some anisotropic interaction2-5 if intramolecular admixture of nuclear spin states,6 quadrupolar7 and spin diffusions relaxation mechanisms are absent. The ELDOR spectra observed for radicals with 0 and cy protons undergoing rotation or exchange have been interpreted as being due to the modulation of the anisotropic hyperfine interaction between the electron and the proton^.^^^ In such cases, the ELDOR intensity is proportional to Tle/Tln,where TI, and TI, are the electron and nuclear
spin-lattice relaxation times, respectively. Furthermore, as the correlation time ( T ~ for ) the rotation varies from to s, the ELDOR spectral intensity in the absence of intermolecular relaxation changes from a reduced to an enhanced ELDOR spectrum.2b This occurs for the following reason. When the frequency of the motion is of the order of the nuclear Larmor frequency (-los Hz), reduced ELDOR spectra are observed as a result of induced nuclear spin flips. As the frequency of the motion increases to that of the electron Larmor frequency (1010 Hz), enhanced ELDOR spectra are observed, as a result of a relaxation process that flips both electron and nuclear spins simultaneously. Because of this frequency dependence, an enhanced The Journal of Physical Chemistry, Vol. 80, No. 77,1976
