Kenneth D. Legg
J. Phys. Chem. 1970.74:2114-2118. Downloaded from pubs.acs.org by UNIV OF WINNIPEG on 01/23/19. For personal use only.
2114
the doses of 150 to 200 rads used in the present experiments a concentration of ^ 5 X 10~7 M Cl would be formed if all OH were converted into Cl atoms. At 2 X 10-6 M dye and 10-1 M Cl- the lifetime of Cl atoms would be very short due to its reactions with Cl- and dye, leaving little chance for formation of Cl*- via reaction of Cl atoms with eaq~. Therefore energy transfer from Cl*- to the dye cannot be involved in the present emission enhancement. In fact irradiated NaCl crystals on dissolution probably give rise to the same species as obtained on irradiating NaCl solutions. Therefore the emissions obtained on dissolving irradiated NaCl in dye solutions are probably formed via a similar mechanism to the one now proposed. (5) Effect of CNS~. From the experimental results it could be deduced that, like iodide, the corresponding reactions of CNS-dye products with eaq- would explain the enhanced emissions. The dotted lines of Figure 5b thus calculated using the appropriate rate constants given in Table I lead to the optimum relative values Fit Fs, and Fio given in Table II. (6) Conclusions. The enhancement of the emission from irradiated acriflavin solutions by addition of various halides or pseudo halides seems in all cases to be associated with the formation of dye products, via attack of halide intermediates X and X2-, which then react with eaq~ in chemiluminescent reactions.
and
David M. Hercules
Neither the emission nor the absorption measurements carried out led to a definite decision as to whether the semioxidized dye and/or the OH, X, and X2adducts of the dye participate in the emission-forming step. The relative reactivities of the various X and X2- intermediates toward acriflavin follow the order I CNS > OH > Cl2Br2- > (CNS)2- » I2-. This is similar to the order of oxidizing power: OH > Cl2~ > Br2- » I2- found by Langmuir and Hayon23 from the measured reactivities of these species with various solutes, mainly alcohols. Advances in this system are possible by measurements of the absolute yields of emissions and by further absorption studies using both pulse radiolysis and flash photolysis in order to identify the spectrum of semioxidized acriflavin. ~
~
Acknowledgments. The authors wish to thank the late Professor K. Sommermeyer for stimulating this work, Dr. L. G, Lajtha and Dr. M. Ebert for encouragement and support, and Dr. J. P. Keene and Mr. B. W. Hodgson for constant supervision of the accelerator and pulse radiolysis apparatus. Financial support to W. P. was given by the Deutsche Forschungsgemeinschaft. (23)
.
E. Langmuir and E. Hayon, J. Phys. Chem., 71, 3808
(1967).
Quenching of Lucigenin Fluorescence by Kenneth D. Leggla Department of Chemistry and Laboratory for Nuclear Science, Massachusetts Institute of Technology, Cambridge, Massachusetts
and David M. Herculeslb Department of Chemistry, University of Georgia, Athens, Georgia
80601
(Received December 9, 1969)
A study is reported for the quenching of lucigenin (dimethylbis(acridinium) nitrate) fluorescence by various anions and amines. A linear relationship is found between quenching efficiency and ionization potential. Efficient quenchers such as chloride, cyanide, sulfite, thiocyanate, and sulfide ions have diffusion-controlled rate constants. The quenching of lucigenin fluorescence has been shown to proceed by formation of a transient charge-transfer complex. In the absence of quenchers, lucigenin undergoes a photochemical reaction which probably occurs from the lowest excited singlet state.
Introduction Weber2 has reported the quenching of lucigenin (dimethylbis (acridinium) nitrate) fluorescence by chloride, bromide, iodide, and thiocyanate ions. He noted that the quenching by iodide and bromide appeared to be due to the “heavy-atom” effect,3 while The Journal of Physical Chemistry
quenching by chloride and thiocyanate appeared to their oxidation potentials. Leonhardt
be related to
(1) (a) NIH Predoctoral Fellow, 1965-1968. (b) Address all correspondence to this author. (2) K. Weber, Z. Phys. Chem., B50, 100 (1941). (3) J. G. Calvert and J. W. Pitts, Jr., “Photochemistry,” John Wiley and Sons, Inc., New York, N. Y., 1967.
Quenching
Lucigenin Fluorescence
of
2115
and Weller* observed charge-transfer quenching of perylene fluorescence by electron donors, notably amines. They confirmed what others5 surmised, namely, that quenching proceeds through a charge**4
transfer intermediate
F* + Q
—
(F-Q+)
F + Q
—
In the
case of perylene quenching by amines, they were able to observe the monoanion radical of perylene in polar solvents. They were also able to show a relationship between the ionization potential of the amine and its quenching efficiency. These observations were in accord with the mechanism
P* + Q—> (P'-Q'+) (P -Q +) ·
·
P “solvQ
-
·
(P--Q-+)—*P +
1
(1)
added, the lifetime of the excited state in this absence of quenching, and 7q the intensity at quencher concentration, (Q). These rate constants give a good indication of relative quenching efficiencies of the various ions. To a first approximation the quenching efficiency appears to be related to the ionization potential, although this is far from an exact correlation. In certain cases a general correlation is complicated by rapid photoreaction between lucigenin and the anion. Fluorescence lifetime measurements gave quenching constants comparable to those in Table I.
Table I:
(3)
Q
Added salt0
not able to observe the charge-transfer They complex (P-Q+) due to its short lifetime (CIO-7 sec). As a part of our studies6 on the chemiluminescence of lucigenin, we felt it important to study the quenching of lucigenin fluoresence as well as the nature of the excited state responsible for its photochemical reaction. The results of these studies are reported here.
Experimental Section Chemicals. Lucigenin (dimethylbis (acridinium) nitrate) was obtained from Columbia Organic Chemicals and was recrystallized twice from 1:1 methanol-ethanol. All other organic and inorganic chemicals were reagent grade and were used without purification. Solvents. Absolute ethanol (U. S. Industrial Chemicals Co.), dimethyl sulfoxide (DMSO), dimethylformamide (DMF) (Matheson Coleman and Bell, spectroscopic grade), and acetonitrile (AN) (Eastman, spectroscopic grade) were used as obtained. Apparatus. Absorption spectra were obtained on a Cary Model 14 spectrophotometer. Fluorescence studies used a Turner Model 210 absolute spectrofluorometer. Fluorescence lifetimes were measured using a TRW Model 31A nanosecond fluorometry system.7 The flash photolysis system was constructed from components manufactured by Xenon Corp. (Medford, Mass.) and has been described in detail by Bailey, et al.s
Results and Discussion The effect of a series of anions on the fluorescence intensity of lucigenin is shown in Table I. Iodide and bromide were excluded from this study because of the possibility of quenching by the “heavy-atom” effect. The rate constants for quenching, fcq, were calculated from the Stern-Volmer relationship
where
70
=
1
+
MQ)
is the fluorescence intensity
(I) with
on
Lucigenin Fluorescence in Water
(2)
+solv
None
were
f°
Effect of Anions
no
quencher
KF NaClOi Na2S04 NaC2Hs02
100 100 100 60 31
NaHS03 NaOH
8.5
KCN
5.5 5.1
KC1 Na2S03
NaSCN Na2S
3.4 1.4 0.3
Ionization potential
Quenching const, M~1 sec-1
Fluorescence intens6
of anion,6 eV
17.4
7.1 X 2.3 X 1.1 X 1.1 X 1.8 X 1.9 X 4.4 X 3.0 X 7.4 X
10s
10.35
10»
10“ 10 “d
13.2 13.7 13.0 13.0
10“ 10“ 10“d 10“ 10“
10.5
e
“
6
Concentration of all quenchers 5.00 X 10-2 M. Measured relative to a value of 100 with no quencher added. Concentration of lucigenin in all solutions was 1.00 X 10 “4 M. c R. W. Kiser, U. S. Atomic Energy Commission Report TID-6142, d From fluorescence lifetime measure1960, Washington, D. C. e ments. Photoreacts. A calculated k gave 3.2 X 1011 A/-1 sec-1.
Qualitatively, the results of Table I are quite interIt is not surprising that ions like fluoride and perchlorate do not show a quenching effect, nor is it surprising that strong reducers such as sulfide, sulfite, or bisulfite are quenchers. What is interesting, though, is that mild reducing agents such as thiocyanate, chloride, cyanide, and acetate have very large quenching constants. Even sulfate ions show a quenching efesting.
fect. (4) H. Leonhardt and A. Weller in “Luminescence of Organic and Inorganic Materials,” H. D. Kallmann and G. M. Spruch, Ed., John Wiley and Sons, Inc., New York, N. Y., 1962. (5) (a) E. Bauer, Z. Phys. Chem., B16, 465 (1932); (b) J. Weiss and H. Fischgold, ibid., B32, 135 (1938). (6) K. D. Legg and D. M. Hercules, J. Amer. Chem. Soc., 91, 1902
(1969). (7) “TRW Fluorometry Handbook,” TRW Instruments, El Segundo, Calif., 1968. (8) (a) D. N. Bailey, Ph.D. Thesis, Massachusetts Institute of Technology, Cambridge, Mass., 1968; (b) D. N. Bailey, D. K. Roe, and D. M. Hercules, J. Amer. Chem. Soc., 90, 6291 (1968).
Volume 74, Number 10
May 14, 1970
2116
Kenneth D. Lego
and
David M. Hercules
Table II: Chloride Quenching of Lucigenin Fluorescence as a Function of Solvent Solvent
/o//Q°
Water Ethanol
5.4 5.1
Solvent
Dimethyl
/o//q“
5.5
sulfoxide
Acetonitrile
5.3
Ia is the fluorescence intensity measured from a 10-4 solution of lucigenin in the appropriate solvent. 7q is the fluorescence intensity in the same solvent containing 10-í M tetramethylammonium chloride. Relative precision of the rates is ±0.15.
Figure 1. Relationship between the ionization potentials of amines and their efficiency of quenching lucigenin fluorescence. h is the measured fluorescence intensity of a solution containing 10~4 M lucigenin and 2 X 10-z M amine in water. With no amine added, the relative fluorescence intensity was 10.0. Amines used and their ionization potentials:7,8 methylamine (8.97), isopropylamine (8.72), n-butylamine (8.71), dimethylamine (8.24), triethylamine (7.82), and triethylamine (7.56).
Because amines were shown to be efficient chargetransfer quenchers of aromatic hydrocarbons4 and because ionization potential data are readily available for amines,9,10 a study was performed of the quenching of lucigenin by a series of aliphatic amines. A plot of relative fluorescence intensities with quencher present vs. ionization potential of the quencher is shown in Figure 1. The linear dependence of fluorescence quenching efficiency on the ionization potential of the quencher is good evidence that electron transfer is involved in the quenching step. That ionization potential alone is not the only factor important in lucigenin fluorescence quenching can be seen by reference to Figure 1. Quenching by chloride gives an I¡ value of 1.2 on the scale of the figure, even though the ionization potential of chloride is 13.0 eV. This indicates that chloride is a more efficient quencher than any of the amines studied. It is doubtful if these results are due to collisional frequency factors, since chloride and the amines all have essentially diffusioncontrolled rate constants. Rather, the increased efficiency of chloride quenching probably reflects better bonding between the positively and negatively charged species, resulting in greater orbital overlap and a more efficient pathway for electron transfer. Leonhardt and Weller were able to observe an effect of solvent polarity on the quenching efficiency of amines This was ascribed to solon perylene fluorescence.4 ions formed by the of the stabilization radical vent more polar solvents showing charge-transfer step, Similar studies were run for the greater efficiency. chloride quenching of lucigenin as shown in Table II. From these data it is evident there is no solvent effect on the quenching efficiency. The studies described above clearly indicate that quenching of lucigenin fluorescence occurs by a chargeThe Journal of Physical Chemistry
transfer mechanism similar to that proposed by Leonhardt and Weller for quenching of perylene fluorescence by amines.4 On this basis the mechanism of quenching of lucigenin fluorescence by chloride ion may be written
+ hv—> (L2+)* (L2+)* + Cl"—> (L-+C1-) (L· +C1 ·) —> L2+ + GiL2+
lt was not possible to detect the presence of the L- + radical or to determine if the lucigenin triplet is produced by the second electron-transfer step. Lucigenin was found to show serious photodecomposition under flash excitation which would be necessary for such studies. The mechanism is also consistent with the lack of a solvent effect on the efficiency of the chargetransfer reaction. Because the charge-transfer complex (L,+C1·) has essentially the same net charge whether the electron is on the chlorine or the lucigenin, solvent reorientation effects on electron transfer would be expected to be small. One would expect a larger solvent effect on amine quenching, because of the charge separation produced in the complex (L,+A·-). Howthe presence of amines caused serious photodecomposition of lucigenin in nonaqueous solvents, complicating such studies. That anions generally are more efficient quenchers of lucigenin fluorescence than are amines is consistent with the proposed mechanism. A doubly charged cation would have a stronger attraction for an anion than for an amine. This would result in a lower activation energy for formation of the charge-transfer complex and an overall increase in quenching efficiency. The proposed mechanism for dissipation, of energy from the excited singlet state of lucigenin is shown in Figure 2. Although the absolute ground- and excitedstate energies of L2+, L· +, and Cl- are not known, because chloride does not reduce lucigenin, its highest occupied orbital must lie below the lowest unfilled orbital of lucigenin. In Figure 2, part 1 is the state diaever
(9) K. Watanabe, J. Chem. Phys., 26, 542 (1957). (10) K. Watanabe, ibid., 26, 1773 (1957).
Quenching
of
(LA)*
Lucigenin Fluorescence
A
Cl"
(1)
—
A
Cl
(2)
2117
-
Cl" 13)
Energy-level diagrams showing the proposed mechanism of energy dissipation in the fluorescence quenching of lucigenin by chloride.
Figure 2.
gram for excited singlet-state lucigenin (L2+)* and for chloride ion. When the charge-transfer complex with chloride is formed, the situation depicted in part 2 of Figure 2 results. Energetically the electron from the chloride will preferably go into the lowest possible level in lucigenin as shown by the arrow in part 1 of Figure 2. When the charge-transfer complex breaks up, the electron transferred back to the chlorine atom will be that from the highest energy level of L·+ which then leaves lucigenin and chloride both in the ground state (Figure 2, part 3). It was observed that oxygen showed little or no quenching effect on lucigenin fluorescence. Although this may seem, strange because of the extensive fluorescence quenching by oxygen in many systems, when forming charge-transfer complexes, oxygen acts as an electron acceptor,n while the effective quenchers reported here act as electron donors. During the course of flash photolysis studies, it was noted that lucigenin photoreacts in water but that when chloride was present, no photoreaction occurred. Figure 3 shows the absorption spectra before and after 10 flashes. When chloride was present at 10_1 M, no absorption change was noted. Three experiments were run to determine the nature of the photoreaction. A solution of lucigenin in water was vacuum degassed by six freeze-pump-thaw cycles and irradiated in a photoreactor for 1 hr using 365-nm radiation. Significant photoreaction occurred. Next, a solution of lucigenin in water was saturated with oxygen and irradiated under the same conditions. Photoreaction was observed amounting to 90% of that of the degassed sample. Third, a solution of lucigenin in water containing 0.1 M KC1 was irradiated deunder the same conditions after being vacuum gassed. No evidence of photoreaction was seen. These three experiments are consistent with photoreaction occurring from the lowest excited singlet state of lucigenin. The small amount of quenching observed for a solution saturated with oxygen is inconsistent with the behavior of most known triplets.3 Assuming that oxygen would quench the lucigenin triplet by a reaction that is nearly diffusion controlled, one can calculate by eq 1 that the triplet lifetime in the
Figure 3. Absorption spectra of lucigenin in water before and after flashing: -, before flashing;.....- -, after 10 flashes, 500 J per flash.
Figure 4. Absorbance changes vs. fluorescence lifetime for is the change in lucigenin solutions containing chloride. absorbance observed after 1 hr of irradiation at 365 nm on a carrousel reactor. Measurements were performed at 450 nm. Solutions had sufficient chloride added to adjust the fluorescence lifetime of lucigenin in the range of 3-20 nsec.
absence of quenching would be ca. 10-7 to 10-8 sec. Because there is no readily apparent source of extensive spin-orbit coupling in the lucigenin molecule, a triplet having such a short lifetime seems unlikely. Furthermore, a triplet state having such a short lifetime would probably be radiative.12 The slight decrease in photoreactivity when oxygen was present probably resulted from an enhancement of the intersystem-
crossing rate by oxygen perturbation.8 To identify the photoreactive state further as the singlet, solutions of lucigenin containing varying amounts of chloride were photolyzed. This provided a series of solutions of lucigenin with varying quantum efficiencies of fluorescence and fluorescent lifetimes. The lifetimes were measured and the solutions were placed in a carrousel photoreactor for 1 hr. At the end of this time the decrease in absorbance at 450 nm was measured. The change in absorbance, , at (11) H. Tsubomura and R. S. Mulliken, J. Amer. Chem. Soc., 82, 5966 (1960). (12) F. E. Lytle and D. M. Hercules, ibid., 91, 253 (1969).
Volume 74, Number 10
May 14, 1970
2118
W. C. Meyer
nm is a measure of the amount of photoreaction. When the lifetime of lucigenin is plotted vs. , a straight line should result if the reaction proceeds from the singlet state, since chloride quenches the singlet state of lucigenin. Figure 4 shows the plot of vs. the lifetime which indicates that the photoreaction proceeds through the first excited singlet state of lucigenin. It might be argued that chloride quenching of the lowest singlet and triplet states of lucigenin could account for the results of Figure 4. Although this is possible, to have the slope of Figure 4 so close to unity and the reaction proceed mainly from the triplet would require a rather fortuitous combination of excited-state
450
lifetimes and quenching rate constants. Long-term photolysis of lucigenin has yielded a product of yet undetermined structure. Traces of N-methylacridone are present, and it has been shown that the reaction product is not dimethylbiacridine. The work was supported in part through funds provided for by the U. S. Atomic Energy Commission under Contract AT(30-1)-905. We wish to thank Anthony Vaudo for his assistance in the flash photolysis work. We also thank a reviewer for suggesting the explanation for the lack of oxygen quenching. Acknowledgments.
Halogen-Sensitized Photoionization of
, , ', '-Tetramethyl-p-phenylenediamine in Liquid Halogenomethanes by W. C. Meyer Physical Research Laboratory, The Dow Chemical Company, Midland, Michigan
48640
(Received October 3, 1969)
Production of Wurster’s Blue is shown to be a one-photon process independent of the solvent dielectric constant. Sensitization is confined to halogenated electron acceptors. The constancy of quantum yields with excitation energy excludes vibronic photochemistry. Yields in chlorinated solvents are distinctly less than in brominated solvents, which lends credence to the contention that the organic halide undergoes dissociative reduction to stabilize the transient ion pair of the excited donor-acceptor entity.
Introduction Photoionizations of aromatic molecules isolated in rigid solutions have been found to be biphotonic phenomena,1,2 with evidence that the triplet state is an intermediate which absorbs the second quantum to complete the ionization step.2 In fluid solution the two-photon requirement is sustained, but a partially ionized state replaces the triplet state as intermediate, and a single quantum path emerges in solvents of low polarity, thought to be due to the trace presence of oxidizers.3 When certain electron-accepting solutes are deliberately added to aromatic amines in hydrocarbon glasses, photoionization is promoted; in fact in some cases no ionization occurs in their absence.4 (The reaction also assumes a linear dependence on light intensity.) Donor-acceptor complexes, a type of association expected in these systems, can dissociate into ion radicals in polar solvents6 or upon light absorption.6 The single quantum ionization path has received less attention with regard to ionization efficiency, excitation wavelength dependence, etc. The Journal of Physical Chemistry
Because there
was
a
variability in the efficiency of organic halides to
en-
hance photoionization and possible mechanisms other
than dissociative reduction of the halide were not considered,4 a more thorough study of the role of the organic halide in the process was undertaken. How then is sensitization promoted by organic halides? Charge-transfer complexes between aromatic amines and halogenomethanes have been reported.7
What relation, if any, does complex formation have with sensitized photoionization? Is complex formation (1) J. P.
Ray and T. D.
S.
Hamilton, Nature, 206, 1040 (1965). Albrecht, J. Phys. Chem., 72, 929
(2) K. D. Cadogan and A. C.
(1968)
.
(3) R. Potashnik, M.
(1969)
Ottolenghi, and R. Bensasson, ibid., 73, 1912
.
(4) M. Kondo, M. R. Ronayne, J. P. Guarino, and W. H. Hamill, J. Amer. Chem. Soc., 86, 1297 (1964). (5) R. Foster and T. J. Thomson, Trans. Faraday Soc., 58, 860
(1962). (6) C. Lagercrantz and M.
Yhland, Acta Chem. Scand., 16, 1043.
(1962). (7) K. M. C. Davis and M. F. Farmer, J. Chem. Soc., B, 28 (1967)
