1895
EXCITATION TRANSFER IN PULSE RADIOLYSIS
Excitation Transfer in the Pulse Radiolysis of Naphthalene and Benzophenone Solutions* by R. A. Holroyd,a Atomics International, Canoga Park, California, and Chemistry Department, Broohhaven National Laboratory, Upton, New York 11973
L. M. Theard, and F. C. Peterson Gulf General Atomic Inc., San Diego, California (Received August
4,1969)
The nanosecond pulse radiolysis of solutions of 1,Pbenaanthracenein liquid naphthalene at 100" and of anthracene in benzophenone at 30" was investigated. In both solutions solute molecules are not excited at the end of the pulse. After the pulse, excited solute molecules in singlet and triplet states are formed by excitation transfer from the solvent. Ions are not involved in the transfer. The transfer of triplet state energy from benaophenone solvent to anthracene solute was detected by observing that the rate of decay of the excited benzophenone triplet is equal to the rate of formation of anthracene triplets. The rate constant of triplettriplet transfer in molten benaophenone at 30" is (1.6 & 0.1) X l o 9 M-' sec-1 and in molten naphthalene at 100" is 1.1 X 10'0 M-l sec-1. In both solvents the rate of triplet transfer is diffusion controlled. Singletsinglet transfer was also time resolved for a solution of benaanthracene in naphthalene by observing the fluorescence intensity which rose to a maximum 16 nsec after a 5-nsec pulse. Singlet energy transfer is an order of magnitude faster than triplet-triplet transfer in this solvent.
Introduction I n the radiolysis of organic solutions there are several possible processes including excitation transfer, ionization transfer, charge neutralization, and direct excitation by low-energy electrons by which solutes may become electronically excited. I n aromatic solutions excitation transfer from solvent to solute is believed to be important for many systems. This excitation transfer can be of two types in general: singlet-singlet transfer and triplet-triplet transfer. By employing a pulse radiolysis apparatus with a system response time that is short compared to the excitation transfer lifetime it can be expected that excitation transfer can be observed, since in certain cases singlet excitation is detectable by f l u o r e s c e n ~ eand , ~ ~singletbp6 ~ and triplet excitation are detectable by light absorption. In principle, singlet-singlet excitation transfer may be observable by detection of time-resolved decay of solvent fluorescence, buildup of solute fluorescence, decay of light absorption by solvent singlet states, and buildup of absorption by solute singlet states. Triplet-triplet excitation transfer may be observable by time-resolved decay of light absorption by solvent triplet states and buildup of light absorption by solute triplet states. I n practice, however, a careful choice of solvents, solutes, and concentration is required to detect excitation transfer. I n the present work, both singlet-singlet and triplettriplet excitation transfer were detected by nanosecond pulse radiolysis of solutions of 12-benzanthracene in molten naphthalene a t 100". Further, triplet-triplet
excitation transfer was observed in liquid solutions of anthracene in benzophenone at 30". I n previous pulse-radiolysis studies6*'in which the solvent was benzene it was observed that solute triplets were present within a few nsec after the pulse and there was no subsequent grow-in to demonstrate the occurrence of triplet-triplet energy transfer. I n a steadystate study of the radiolysis of liquid naphthalene containing stilbeneIs kinetic evidence was obtained which suggested that isomerization of the stilbene occurs as a result of triplet-triplet transfer.
Experimental Section The naphthalene and benzophenone were purified by zone refining prior to use. Anthracene and 1,Zbenzanthracene (Eastman White Label) were used as received. The solutions were thoroughly degassed prior to sealing in high-purity quartz absorption cells. The (1) This research was supported in part by the Research Division of the U. S. Atomic Energy Commission. (2) Present affiliation, Department of Chemistry, Brookhaven National Laboratory, Upton, New York 11973; previous work at Atomics International, Canoga Park, Calif. (3) M. A. Dillon and M. Burton, "Pulse Radiolysis," Academic Press, Inc., Ltd., London, 1965, p 259 ff. (4) S. Lipsky, "Physical Processes in Radiation Biology," Academic Press, Inc., New York, N. Y., p 215 ff. (5) L. M. Theard, F. C. Peterson, and R. A. Holroyd, J . Chem. Phya., 51, 4126 (1969). (6) R. Cooper and J. K. Thomas, ibid., 48, 5097 (1968). (7) J. W. Hunt and J. K. Thomas, ibid., 46, 2954 (1967). (8) 0.G. Malan, H. Gusten, and D. Schulte-Frohlinde, J. Phya. Chem., 7 2 , 1457 (1968). Volume 74, Number 9 April 90, 1970
R. A. HOLROYD, L. M. THEARD, AND F. C. PETERSON
1896 benzophenone, either pure or containing anthracene, could readily be kept liquid at 30' for long periods of time. Details of the pulse-radiolysis apparatus have been published elsewhere.6 For dosimetry the yield of electrons from air-saturated water was measured and was G(e,,-) was assumed to be 2.65 at lo-* sec and taken to be 1.06 X lo4 M-l cm-l.*
A
Results Solutions of Anthracene in Benzophenone. I n the pulse radiolysis of pure benzophenone a transient absorbing in 540) is observed. There is no fluothe visible ,,X,( rescence to interfere with absorption measurements. The observed absorption spectrum is very similar to that reported for the benzophenone triplet statelowhich has an absorption maximum at 532 mp. The decay of the triplet determined from plots of log (OD - OD,) vs. time is first order and the half-life at 30" is 0.46 psec. There is a 10% residual absorption at 540 mp remaining after decay of the triplet which is attributed to ions. The yield of triplets based on 90% of the initial absorption at 540 mp and a value of E = 10,30010M-1cm-' isG(aBP*) = 2.2 f 0.1. With anthracene present as the solute, triplet-state anthracene molecules are formed after the pulse. I n solutions containing 20 mM or less anthracene, triplet benzophenone is present at the end of the pulse (Figure 1) as in pure benzophenone. However, this transient decays more rapidly than in the absence of anthracene (Figure 2a) and concurrently a new transient builds in (Figure 2b) which absorbs with a peak at 435 mp. The new transient was identified as the triplet state of anthracene on the basis of its absorption spectrum C
0
Q
O(
i I
0. 10
200 nsec
,
t
l
-1
l
l
l
l
1-100 nsec
PULSE Figure 2. Oscilloscope traces showing time-resolved changes in absorption for 1.1 mM anthracene in benzophenone: A, decay of absorption a t 540 mp; B, grow-in of absorption a t 435 mp.
(Figure 1) which compares well with the published absorption spectrum for anthracene.1° The time dependence of triplet-triplet transfer from benzophenone to anthracene was investigated as a function of the concentration of anthracene. Plots of log (OD - OD,) vs. time for the data a t 540 and 435 mp were found to be linear (Figure 3). At each concentration the half-life of the benzophenone triplet (determined from data a t 540 mp) is approximately the same as the half-life of the buildup of the anthracene triplet (determined from the grow-in of absorption at 435 mp). The half-life of these processes decreases
5 nsec
Table I : Results for Solutions of Anthracene in Benzophenone (30") Irradiated with 5-nsec Pulses Anthracene -Half-life in nsec ofDecay at Growth a t concn,
I
450
1 2 ; i I
+
a
0
O.(
-0-
t 29.0% ABS
500
550
c
mM
640 mp
436 mp
0 1.1 10 20 50
460 233 44 21 12b
184 39 19 lob
...
U(nBP*) at 10 nseo
2.2 2.2 1.9
... .* .
lo-%l,
M U(sA*)a
-1
sec-1
...
*..
0.95 1.7 1.3 1.45
1.7 1.5 1.65 1.2b
a Plateau value measured a t 0.1 to 1.0 psec. mate since response time is 5 nsec.
Only approxi-
( mp)
Figure 1. Absorption spectra of intermediates in pulse radiolysis of a solution of 10-8 M anthracene in benzophenone: 0, 5 nsec after the pulse, 0, 200 nsec after a 5-nsec pulse. T h e Journal of Phyeical Chemistry
(9) L.M. Dorfman and M. 8. Matheson, Progr. React. Kinet., 3, 237 (1966).
(10) E.J. Land, Proc. Roy. Soc., A305,457 (1968).
EXCITATION TRANSFER IN PULSE RADIOLYSIS
1897 IO
5 >-
k
cn
z w I-
z W
2
5 W
U
2
\
I
0
50
I
I
I
100 150 200 TIME (n sec)
1
I
250
Figure 3. First-order plots of decay a t 540 mp (open points) and grow-in a t 435 mp (solid points) for solutions of anthracene in benzophenone; (circles) for 1 mM; (triangles) 10 mM; and (squares) 20 mM.
from 208 nsec at 1.1 mM anthracene to 20 nsec at 20 mM anthracene (Table I). The yield of anthracene triplets (measured 150 nsec after the pulse) increases with the concentration of anthracene and plateaus above 10 mM. The yield is G(3A*) = 1.5 i 0.2 if the extinction coefficient at the maximum is taken as 57,200.lo Solutions of Benzanthracene in Naphthalene. I n the pulse radiolysis of solutions of 1,Zbenzanthracene in naphthalene, time-resolved changes in both the fluorescence and the absorption spectra were observed. The sechanges indicate, respectively, singlet and triplet energy transfer to the benzanthracene. The time dependence of fluorescence of a solution of benzanthracene in naphthalene does not follow a simple exponential decay as observed for pure naphthalene.6 Instead, at wavelengths about 475 mp, where benzanthracene fluoresces much more strongly than naphthalene, the fluorescence intensity for a 1 mM solution of benzanthracene increases immediately after a 4nsec pulse, rises to a maximum at 16 nsec and then decays exponentially with a half-life of 26 nsec (Figures 4 and 5a). The following facts indicate that the first excited singlet state of benzanthracene is populated in
)
I
I
I
I
I
20
40
60
80
100
n sec Figure 4. Fluorescence response a t 460 It 5 mp for a solution of 1 m M benzanthracene in naphthalene a t 100' as a function of time after a 4-nsec pulse. Circles are experimental points; lines are for various values of k~ (see text); -, kg = 9 X 1010 , ks = 15 x 1011; -----,k6 = 5 x 1010; k7 = 2 x 107 and ka = 2.65 X 107.
.....
tens of nanoseconds after the pulse. The fluorescence is more intense than for pure naphthalene by a factor of 4.2 a t 475 mp. The fluorescence spectrum 100 nsec after the pulse is similar to the known fluorescence spectrum for l12-benzanthracene." The half-life of benzanthracene fluorescence in solution a t room temperature is reported to be 31 nsec,12 which compares well with 26 nsec. The absorption spectrum changes with time after the pulse (Figure 5). The absorption spectrum observed 300 nsec after the pulse shows peaks at 435, 475, and 495 mp of relative intensities: 0.66,0.83, and 1.0. This compares well with the reported triplettriplet absorption s p e ~ t r u m ' ~ of l12-benzanthracene which has bands at 434, 461, and 485 mp of relative intensities: 0.53, 0.84, and 1.0. There is a small red shift because the solvent is molten naphthalene. The rate of formation of the benzanthracene triplet (11) J. B. Birks and L. G. Christophorou, Proc. Rog. Soc., A274, 662 (1963). (12) J. B, Birks, D. J. Dyson, and J. A. King, ibid., A277, 270 (1964). (13) G.Porter and M. Windsor, ibid., A245, 238 (1958). Volume 74, Number 9 April 90, 1970
1898
R. A. HOLROYD, L. M. THEARD, AND F. C. PETERSON
A
c
1.0
I
PULSE
I
I
I
100
I50
4n4sec 01n 0
0
n
0.1
0
L_
I
PULSE
4 4 0 k n sec
Figure 5. Oscilloscope traces for 1 mM benzanthracene in naphthalene. Smooth curves are fluorescence; lower curves are combined fluorescence plus absorption; dashed curves are true absorbance corrected for fluorescence; A, 465 mp; B, 495 mM.
0.01
I
b I
50
I
I
TIME (nsec) is concentration dependent. From the absorption data at 495 mp, plots of log (ODs00 nBeo - OD) ~ 5 .time were made (Figure 6) and found to be linear. The half-lives determined from such plots are given in Table 11. The Table II: Results for Solutions of Benzanthracene in Naphthalene 100' (Benzanthracene) concn, mM
1 3.4 10
496 mp,
IO-%a,"
nsec
M-1 sec-1
14.0 8.8 43.6
lo-@ks,b M-1
sec-I
8.1 8.8 -11.5
a Ignoring contribution of intersystem crossing from benaContribution of ISC from benzanthracene anthracene singlet. singlets taken into account (see Discussion).
rate of formation of solute triplets increases with in-
creasing concentration of benzanthracene. A corresponding decay of naphthalene triplets presumably occurs concurrent to the buildup of naphthalene triplets. Measurements were not made at the wavelengths where the naphthalene triplets absorb because of the strong fluorescence of l,&benzanthracene. The yield of benzanthracene triplets was determined for the maximum absorption at 495 mp. At 300 nsec was 94,000 for a 1 mM solution. Thisvalue G X The Journal of Physical Chemistry
corresponds to G(aBA*) = 3.7 if E is 25,100 M-l cm-1.ll Absorption by ions at this wavelength can be neglected. The benzanthracene negative ion has a strong absorption band a t 422 mp,I4 but absorbs weakly throughout the visible and there is a band at 502 which could inter1800 M-' cm-' for the anion.16 The fere but €495 yield of benzanthracene triplets present at the end of the pulse is estimated to be small. For a 1mM solution, the observed optical density 10 nsec after the pulse is 22% of the final value. About one-third of this initial absorption is observed for pure naphthalene (and thus can be attributed to naphthalene transients) and some is probably due to energy transfer to benzanthracene occurring during the pulse (see Discussion); thus the initial yield of solute triplets is small.
-
Half-life of triplet grow-in at
50 f 2 23 f 2 8=t5
Figure 6. First-order plots of grow-in at 495 mp for solutions of ben~anthracenein naphthalene a t 100": 0, 1 mM; A, 3.4 mM; 0 , l O m M .
Discussion The results show that for the aromatic solutions studied, excitation of the solute to the triplet state occurs after the pulse. The rate of grow-in of the solute triplet is dependent on solute concentration. (14) A. G. Evans and B. T. Tabner, J. Chem. Soc., 5560 (1963). (15) The extinction coefficients for the anion BA- were determined by partially reacting a 0.9 mM solution of 1,2-benaanthracene in THF with sodium. The concentration of BA- was determined by ear. Values for the extinction Coefficients of (486 = 1800 and €422 = 6000 M-1 om-1 were measured.
EXCITATION TRANSFER IN PULSE RADIOLYSIS
1899
Further, for solutions of anthracene in benzophenone, the triplet state of benzophenone is formed initially and the rate of its decayis identicalwith the rate of growin of the anthracene triplet a t each concentration. For solutions of benzanthracene in naphthalene the fluorescence peaks 16 nsec after the pulse. This fluorescence is associated mainly with the excited benzanthracene singlet. These facts indicate that solute molecules are not excited initially in these solvents but become excited subsequently as a result of energy transfer. Other mechanisms of triplet excitation can be shown not to occur or to be unimportant. The recombination of ions is expected to lead to triplet excitation but the lifetimes of most of the ions in aromatic solvents is much shorters-6 than the lifetimes of triplet grow-in observed in this work. Triplets can also be formed by intersystem crossing from the excited singlet state of the solute. This mechanism is unimportant for anthracene in benzophenone because the excited singlets are short-lived (lifetime < 5 nsec)16 and no initial yield of anthracene triplet was observed. For the solutions of benzanthracene in naphthalene, intersystem crossing may occur since benzanthracene singlets are formed, as shown by the fluorescence results. However, triplet-triplet transfer is shown to be a principal mode of solute excitation in this case also. Triplet-Triplet Transfer. The data for solutions of anthracene in benzophenone can be interpreted entirely in terms of a mechanism in which excitation of the anthracene occurs by triplet-triplet transfer, reaction 1. Benzophenone triplets are also removed by the first-order process, reaction 2, which includes 3benzophenone*
+ anthracene
--f
3anthracene*
+ benzophenone
(1)
self-quenching and phosphorescence. Thus the life3benzophenone* +benzophenone time
r
(2)
of the benzophenone triplet is given by eq I. r =
(kz
+ kl[anthracene])-l
(1)
The value of kz is determined from the triplet lifetime in the absence of solute (Table I) and is 1.5 X lo6 M-l sec-l. Values of kl, the rate constant for energy transfer, were calculated from the lifetime of triplettriplet transfer at each concentration and the value is kl = (1.6 0.1) X lo9 M-l sec-l. The rate of diffusional encounters in molten benzophenone a t 30" can be estimated from the relationship k~ = 8RT/3000~.'? Since the viscosity is 0.136 poise, at 250,18 ICD is -0.6 X lo0. Thus the experimental rate constant is comparable to the diff usion-controlled rate, considering the uncertainty in the estimated value of k ~ , . The observed yield of anthracene triplets a t high anthracene concentrations is less than the yield of
*
benzophenone triplets observed for pure benzophenone. The explanation for this discrepancy may be that the absorption spectra of the benzophenone and anthracene triplets are broadened to different extents,lg and the extinction coefficients used may not apply in this solvent. The actual yield of triplets is probably greater than either of the yields reported. I n the solutions containing benzanthracene in naphthalene the half-life of formation of benzanthracene triplets also decreases with increasing concentration of benzanthracene. This fact requires that the solute triplets are formed, as in benzophenone, by triplet-triplet transfer, reaction 3. Values of k3 calanaphthalene*
+ benzanthracene + abenzanthracene* + naphthalene
(3)
culated from the observed half-lives are shown in Table I1 (column 3) and k3 is 1.0 X 10'0 M-l sec-' if a contribution by intersystem crossing is neglected. However, a contribution by intersystem crossing should be considered since excited benzanthracene singlets are formed. A fraction equal to 0.5520of the singlets is expected to intersystem cross to the excited triplet state, reaction 4. The occurrence of two firstorder processes can often be detected in plots of log lbenzanthracene* +3benzanthracene*
(4)
OD us. time, but when the two processes have comparable half-lives, as is the case here, their sum yields a plot which is linear within experimental error. Thus intersystem crossing, which is assumed2I to have a half-life of 26 nsec, probably contributes to the formation of triplet benzanthracene, although direct evidence for this process was not obtained. Thus the observed half-lives may be averages of the half-lives of intersystem crossing and energy transfer. If intersystem crossing is taken into account then slightly different decay times corresponding to energy transfer are obtained. However, the average value of ka obtained in this way (see Table 11) is the same as the value obtained neglecting intersystem crossing in benzanthracene. The observed rate of triplet-triplet transfer in molten naphthalene is close to the rate of diffusional encounters in naphthalene which is 1.1 X 10'0 M-l sec-l, estimated from viscosity data.'* If triplet excitons22or excimers were present to any significant (16) I. B. Berlman, "Handbook of Fluorescence Spectra of Aromatic Molecules," Academic Press, Inc., New York, N. Y., 1965. (17) P. J. Debye, Trans. Electrochem. Soc., 82, 265 (1942). (18) "International Critical Tables," McGraw-Hill Book Go., New York, N. Y., 1926, Vol. V I I , p 220. (19) The half-width for anthracene is -250 A in Figure 1 and in cyclohexane the half-width is reported to be -100 A (see ref 9). (20) H. Labhart, Helv. Chim. Acta, 47, 2279 (1964). (21) It is assumed here that intersystem crossing has the same lifetime as fluorescence. (22) H. Baessler, J. Chew. Phys., 49, 5198 (1968). Volume 74s Number 9 A p t 4 90,1970
R. A. HOLROYD, L. M. THEARD, AND F. C. PETERSON
1900
(6)
values of kg were tried and the best fit to the data was obtained for a value of ka = 9 X 10'0 (see Figure 4). This value is an order of magnitude greater than the rate of triplet-triplet transfer in this liquid and the latter process is diffusion controlled. Rates of excitation transfer greater than the diffusionlimited rate have been observed for other solutions where solvent molecules are initially excited. For example, for quenching of excited benzene by oxygen in benzene the apparent quenching rate constant is 9 X 10'0 M-I ~ e c - l , ~There are several mechanismsz3 which may account for energy migration at a rate faster than molecular diffusion. Since excimers are present in n a ~ h t h a l e n e rapid ,~ migration via repeated sequences of excimer formation and dissociation may be significant in this case.
(7)
Conclusion
extent, a larger rate constant might have been observed. The observed rate constant is explicable entirely in terms of a diffusion-controlled reaction of excited naphthalene triplets with benzanthracene molecules. Singlet-Singlet Transfer. The observed changes in fluorescence intensity with time (Figure 4) can best be accounted for in terms of singlet-singlet transfer, reaction 5, followed by fluorescence of the benzanthracene, reaction 6. The first-order processes of fluorescence and intersystem crossing, reaction 7, compete 'naphthalene*
+ benzanthracene
----+
+ naphthalene 'benzanthracene* -+ benzanthracene + hv lnaphthalene* naphthalene + hv 'benzanthracene*
---f
(5)
+anaphthalene*
with energy transfer. The time dependence of the fluorescence has been analyzed in terms of the assumed mechanism. The intensity of naphthalene fluorescence at 465 mp is much less than that of benzanthracene and is neglected. The intensity of benzanthracene fluorescence is proportional to the concentration of excited benzanthracene ['B*] which is given as a function of time after the pulse by eq I1 where ['M*]o is the initial concentration of excited naphthalene
singlets, and [B]is the concentration of benzanthracene. All the rate constants in eq I1 are known except k5, the rate constant for singlet energy transfer. The fluorescence half-life of benzanthracene is 26 nsec, thus is 2.6 X 107; k7 is 2 X lo7 M-I sec-l.5 Various
The Journal of Phyaical Chmietru
The results show that radiation-induced excitation of solutes, when present at low concentrations in aromatic solvents, occurs largely by excitation transfer from the solvent. The transfer of triplet-state energy is diffusion controlled while singlet-state energy transfer is much faster than the diffusion-limited rate. Ion recombination does not result in solute excitation, since most of the ions recombine in times shorter than a few nanosecondss and solute triplets are not present a few nanoseconds after the pulse. This further indicates that solute ions are not formed in these solvents as they are in cyclohe~ane.~4J5Apparently charge and electron transfer to the solutes is a minor process in aromatic solvents. (23) J. B. Birks in "Energetics and Mechanisms in Radiation Biologv," Academic Press, Inc., Ltd., London, 1968, p 203 ff. (24) J. K. Thomas, K. Johnson, T. Klippert, and R. Lowers, J. Chem. Phys., 48, 1608 (1968). (25) R.R.Hentz and R. J. Knight, J . Phys. Chem., 7 2 , 1783 (1968).
