3725
LIQUIDBENZENE LUMINESCENCE QUENCHING BY CCL
Liquid Benzene Luminescence Quenching by Carbon Tetrachloride. Consideration of the Presence of Solvent Excimers in the Interpretation of the Observed Rates of Quenching by P. K. Ludwig and C. D. Amata Department of Chemistry and the Radiation Laboratory, 1 Uniwrsity of Notre Dame, Notre Dame, Indiana (Received M a y 7 , 1968)
46666
The luminescence behavior of liquid benzene seems in general t o follow a simple reaction scheme in which excited benzene monomers only are considered. However, it is found that the experimentally observed benzene luminescence decay time as a function of the aromatic concentration in n-nonane deviates from the expected linear relation. This finding together with the spectral evidence for the presence of excited species other than excited benzene monomers in the pure liquid suggests the necessity of using a more complex reaction scheme in describing the benzene fluorescence. The experimental data are discussed in the light of such an extended scheme. ‘The hypothesis of excitation energy migration in liquid benzene is reconsidered. An alternative model is discussed to explain certain “anomalies” of the specific rate of benzene luminescence quenching.
The specific rates of excitation energy transfer from liquid benzene to suitable additives has been found by several workers to be larger than expected on theoretical grounds and furthermore to depend on the benzene concentration.2 -7 The theoretically expected values of these specific rates have been derived from two considerations. If the acceptor has optical properties such that resonance transfer from benzene to it is possible, Forster’s theory of resonance transfer* has been applied and a critical transfer distance Ro derived. I n the case that no long-range interaction between benzene and the acceptor exists, upper limits of the specific rates of energy transfer (quenching) have been estimated from the theory of diFf usion-controlled bimolecular processes. The observation that specific rates of energy transfer from benzene to a variety of acceptors exceed the values calculated on this basis has been taken as evidence for the occurrence of energy migration through the sol~ e n t . ~ - ’ ’ 9Because of the great theoretical interest in such phenomena in a liquid, it is worthwhile to consider critically the assumptions which are made in interpreting the experimental data and deriving the conclusions regarding energy migration. For example, in the case that resonance energy transfer from benzene to the acceptor is possible, Povinellilo has recently shown that some of the reported high specific rates of energy transfer can be the result of an incorrect application of Forster’s theory to a liquid of low viscosity because the theory is derived, strictly speaking, for solid systems only. If however, diffusive motion of acceptor and donor is properly taken into account, calculated values of specific rates of energy transfer agree fairly well with those observed. On the other hand, the mathematical difficulties in extending Forster’s theory to liquids of
low viscosity do not make systems where resonance transfer is possible particularly suitable for studies of energy migration effects. Attention in this paper is focused therefore on processes which presumably involve collisional interactions between donor and acceptor only, that is, short-range interactions only. Specifically, the benzene luminescence behavior in the presence and absence of CC1, is studied under conditions of steady-state and nonsteady-state uv excitation and, significantly, as a function of benzene concentration in an optically inert solvent. I n this case, the theory of diff usion-controlled processes should be applicable. The main objective of this study is to determine whether in benzene systems solvent excimers may play a role in the energy transfer process and whether they may perhaps account for experimental observations that have been ascribed to energy migration. Almost all (1) The Radiation Laboratory of the University of Notre Dame is operated under contract with the U. S. Atomic Energy Commission. This is AEC Document No. COO-38-586. (2) S. Lipsky, W. P. Helman, and J. F. Merklin, “Luminescence of Organic and Inorganic Materials,” H. P. Kallmann and G. M. Spruch, Ed., John Wiley & Sons, Inc., New York, N. Y . , 1962, p 83. (3) C. R. Mullin, M. A. Dillon, and M. Burton, J. Chem. Phys., 40, 3053 (1964). (4) M. A. Dillon and M. Burton, “Pulse Radiolysis,” iM.Ebert, J. P.
Keene, A. J. Swallow, and J. H. Baxendale, Ed., Academic Press, London, 1965, p 259. (5) S. Lipsky and M. Burton, J.Chem. Phys., 31,221 (1959). (6) R. Volts, G. Laustriat, and A. Coche, Compt. R e n d . , 257, 1473 (1963).
(7) J. B. Birks and J. C. Conte, Proc. Roy. Soc., A303, 85 (1968). (8) T. Forster, “Fluoressenz Organischer Verbindungen,” Vandenhoeck und Ruprecht, Gottingen, 1951. (9) C. Tanielian, Thesis, University of Strasbourg, 1965. (10) R. Povinelli, Thesis, University of Notre Dame, 1967, to be published. Volume 72, Number 11
October 1988
P. K. LUDWIGAND C. D. AMATA
3726 studies of benzene luminescence quenching consider the reactions of excited solvent monomer only despite the fact that the emission spectra of benzene as a function of concentration reveal the presence of excited states other than that of the monomer.
M. 2.24
I
I
10
6.72
4.48 I
I
I
I
8.96 I
11.2 I
I
I
Experimental Section Materials. Benzene (Eastman) was distilled through a 45-theoretical plate spinning-band column and the middle fraction was retained. Prior to distillation, benzene was repeatedly recrystallized. As an optically inert diluent n-nonane (Aldrich) was chosen because of the minimal viscosity change when mixed with benzene. After the hydrocarbon was passed twice through a 4-ft silica gel column, it was distilled in the same manner as benzene. Techniques. Samples. For both steady-state and nonsteady-state measurements, samples were contained in Suprasil quartz cells. All samples were degassed by the freeze-thaw-pumping technique. Steady-State Fluorescence Measurements. All fluorescence spectra and intensity measurements were carried out with a Gary 14 spectrophotometer with front-face illumination attachment. The 253.7-nm line from a low-pressure mercury lamp (Hanovia Lo 73587) used for excitation of the sample was isolated with a Bausch and Lomb monochrometer. To permit determination of relative intensities, the stability of the lamp emission was monitored with a fluorescent standard (p-terphenyl in plastic). Because of the low absorption coefficients of benzene, penetration of the exciting light into the sample cannot be neglected at low concentrations ([B] < 5 ~ 0 1 % ) . To correct for the concentration-dependent penetration, solutions of p-terphenyl with the same optical density as that of the dilute benzene solutions were used to obtain a correction factor for this effect. Because pterphenyl does not exhibit self-quenching, all variation of luminescence intensity for different optical densities can be attributed to penetration effects. Ultraviolet-Induced LuminescenceIntensity-Time (LT) Curves. LT curves at 20" were obtained by using a monophoton technique. l 1 A high-pressure hydrogenmercury lamp described by D'Alessio, Ludwig, and Burton served as a pulsed-excitation source.12 The emission decay time for the lamp was 2.7 nsec with an associated electric pulse of about 0.3-nsec decay. The emission around 253.0 nm, isolated by a Zeiss interference reflection filter, UV-R-250, served as excitation of the sample perpendicular to the direction of observation of luminescence.
Results Steady-State Excitation. The reciprocal of the benzene monomer luminescence intensity as a function of benzene concentration is given in Figure 1. Figure 2 represents Stern-Volmer plots for the quenching of The Journal of Physical Chemistry
~~
0
40 60 Vol % benzene.
20
-
80
100
Figure 1. Reciprocal of benzene monomer luminescence intensity as a function of benzene concentration. (Diluent is n-nonane; T,20'; Xemissian 270 nm.)
o t 0
I
I
I
I
I
1.0
2.0
3.0
4.0
5.0
[CCla], 10- M .
Figure 2. Stern-Volmer plots for benzene luminescence quenching by CClr. (Diluent is n-nonane; T,20'; Xemission 270 nm.) A, 100% vol benzene; 0, 0.1 vol yo benzene.
monomer luminescence by carbon tetrachloride for various benzene concentrations. Monomer emission is observed at 270 nm; that of the excimer at 330 nm. Nonsteady-State Excitation. ,The reciprocal of the benzene luminescence decay time as a function of benzene concentration is given in Figure 3. I n all cases, a single exponential decay of luminescence is observed, which is the same for the monomer and excimer emission. Addition of CC14 shortens the luminescence decay times. Discussion The potential effect of solvent excimers on the experimental observations is inspected by considering two reaction schemes. I n the first one, the commonly made assumption is adopted that excited monomers only are of importance for the discussion of the observations, while in the second one, reactions of excimers in addition to that of excited monomers are included. Simple Reaction Scheme. The luminescence behavior of benzene has frequently been described by the following sequence of processes. (11) C.D.Amata and P. K. Ludwig, J. Chem. Phys., 47,3640 (1967). (12) J. T.D'bleasio, P. K. Ludwig, and M. Burton, Rev. Sci. Instr., 35, 1015 (1964).
LIQUIDBENZENE LUMINESCENCE QUENCHING BY cc14
ko
B --+- B* B*+B
B*
+ B +2B
+Q
B*
Table I: Experimental y , and k, Values Obtained with Use of E q 2a and 2b for Benzene Luminescene Quenching b y CCld (Diluent is n-Nonane; T,20')
ki
+ h~
B*+B
+B
+Q
ke Concn, vol %
ksq[Bl ~ s M-' ,
kq[QI
k,, 1O1O X h l - l s e c - l
where B, B*, and Q refcr to the ground-state benzene molecule, to its excited state, and to the quencher, respectively. The indices of the rate parameters characterize the corresponding process and are selfexplanatory. The expression for the luminescence intensity in the steady and nonsteady state as a function of benzene and quencher concentration obtained for this scheme are
+ Ysq[BI + + [Bl
Ioo/I = 1 = ke
70-l
ki
(14 (1b)
ksq
for self-quenching and
~ o / =I 1 =
7-l
7o-l
3727
+ yq [Q1 + kq[Q]
(24
yq
ksq/(ke
hi)
1
B* B
1
B*
1
8.96
11.2
4
Y 3.2
I 0
165
0.5
200 0.6
232 0.7
260 1.0
I
20
I
I
1
I
I
40
80 Vol % ' benzene.
I
80
Figure 3. Reciprocal benzene fluorescence decay time
as a function of benzene concentration. (Diluent is n-nonane; Z', 20"; XOmission 270 nm.)
(0)
him
(11
+ B* +B2*
B2*
+Q
+Q
kem
(11)
kt[B]
(111)
h ~ 2
+Q 2B + Q
+B
+
(IV)
kid
+ +B
ked
(VI
k,
(VI)
kqm[Ql kqd[&]
(VW (VIII)
The subscripts m and d serve to distinguish between reactions of excited monomcrs and excimers. Mathematical analysis of this scheme leads to expressions for the steady- and nonsteady-state luminescence intensities as a function of various paramet,ers which are fairly complex and not very conveniently applied to the experimental observations. However, as discussed elsewhere,13from the observed time dependence of benzene luminescence upon excitation by uv pulses of about 1-2 X IOw9sec duration, it is possible to obtain information on the relative importance of some of the rates of the processes considered in the extended reaction scheme. This information leads to the following two possible inequalities
Icf [B] >> k m o ; kr I
ko
+h~l
B
Bz*+B*
t
3.0
155 0.5
B2* +2B
kq.70
6.72
100
B2* + 2B
ksq.700
M. 4.48
80
B* +B
From eq 2a and 2b and the experimental curves given in
3.8
50
B+B*
(2b)
+ = = k q / ( h + ki + hq[BI) = =
20
Figures 2 and 3, the values of yq and kq given in Table I were obtained. Comparison of expressions 1 and 2 with the experimental results (Figures 1-3) reveals that the linear variation of the reciprocal luminescence decay time of benzene as a function of its concentration in the absence of quenching (eq lb) is not observed while the other derived expressions seem to agree with the corresponding observations. Reaction Scheme Including Excimey. The presence of solvent excimers in benzene systems may be taken into account by the following ~ c h e m e . ' ~ , ' ~
for foreign quenching. Here loorefers to the benzene luminescence intensity for low enough concentration so that self-quenching can be ignored. The subscript zero refers to benzene at some concentration in the absence of foreign quencher. 7 is the mean lifetime of luminescence, and yes and yq are defined by ysq
0.1
>> k d 0
(3)
or
I
100
kdo
>> km0,
kt[B], IC,
(4)
(13) P.K. Ludwig and C. D. Amata, J. Chem. Phgs., in press. (14) J. B. Birks and I. €1. Munro, Progr. Reaction Kinetics, 4, 256 (1967).
Volume 72, Number 1I
October 1968
P. K. LUDWIG AND C. D. AMATA
3728 Inequality 3 corresponds to a rapid establishment of quasiequilibrium between excited monomer and excimer which may be represented by
+ B*
B
kfP1
Bz* kI
{kdO
bmOJ
B
2B
The second condition implies that any excimer that is formed is rapidly deactivated by processes other than dissociation into the excited monomer. A clear decision which of the two conditions (3) and (4) applies is presently not possible. By analogy to other excimer system^,'^ we assume validity of (3), that is, the establishment of rapid equilibrium between excited monomers and excimers. With this assumption, the relevant theoretical expressions take the simpler form
1
Imoo/Im =
=
[km'
[B]/kmokr)
(54
+ (kmoklc,/kdokf[B]) (5b) + kd'(kf/kr) [B]/[1 + (kf/kr) [B]] (5C)
Idm/ID = r0-1
(kd'kf
1
+
for the self-quenching of benzene. Here k m o = l ~ , , ~ h i m ; kd' = ked k i d ; IM'' refers to the monomer luminescence intensity in the absence of self-quenching and ID^ to that of the excimer extrapolated to infinite benzene concentration. The expressions for foreign quenching are
+
Imo/Im IdO/Id
+ rm[QI = 1 + [Q] =
1
(64 (6b)
r d
and I d o represent the monomer and excimer where Im0 luminescence intensity, respectively, for a particular benzene concentration.
rm=
rd
=
(kqm
+
kqd(kf/h)
[BI)/ (km'
and kmo
km
=
kd
= kd'
+ +
+
kqm
kqd
kdo(hf/kr)
IB I)
[QI
[& 1
+
with kmo = kern him, kdo = ked k i d . For the reCipr0cal luminescence decay time as a function of quencher concentration, one obtains 7-l
=
7o-l
+ K[Q]
with
K
= (kqm
+
kqd(kf/kr)
[B1)/1
(kf/kr) EBI) ( 6 ~ )
Comparison of eq 5 and 6 with the corresponding expressions derived from the simple reaction scheme (eq 1 to 2) shows agreement to the extent that linear relations between the observables and the various The Journal of Physical Chemistry
parameters are found except for the reciprocal lifetimebenzene concentration dependence (eq 5c). Despite the similarity of the linear relations 5a, 5b, and 6a-c with those obtained from the simple reaction scheme, the slopes of the corresponding experimental curves have, however, quite different meanings. For example, the slope of a Stern-Volmer plot of fluorescence quenching is according to eq 6a no longer related in a simple manner to the specific rate of quenching as given by (2a). If, therefore, the extended reaction scheme applies, specific rates of quenching cannot be extracted from Stern-Volmer plots in the usual fashion. I n other words, such values do not represent meaningful quantities because they are complex functions of other specific rates and the benzene concentration. The potential role of excimers in energy transfer processes in benzene systems seems t o have been largely ignored because the steady-state luminescence behavior appears to be completely describable by the simple reaction scheme. The fact that the emission spectra of liquid benzene solutions suggest the presence of excited species other than monomers does not necessarily invalidate such simplified treatment of the benzene system because excimer formation may be considered as a form of self-quenching assuming that the excimers, once formed, possess a mode of deactivation independent of that of the excited monomers. From the data presented here on the time dependence of benzene luminescence, however, it appears that excited monomer and excimer are closely connected with one another. As shown in Figure 3, the variation of benzene luminescence decay time with benzene concentration does not conform with the linear relationship l b derived from the simple reaction scheme. On the other hand, eq 5c reflects the character of the curve of Figure 3. It is, therefore, thought that the more comprehensive reaction scheme must be considered in discussing the luminescence behavior of benzene. I n other words, in evaluating self-quenching and foreign quenching experiments, relations 5-6 must be applied rather than eq 1-2. With regard to the question of energy migration in liquid benzene solutions, it is of particular interest to determine the values of lcqm and k q d , the specific rates of quenching of the luminescence of excited monomer and excimer in view of this conclusion. This can be done in the following manner. Determination of k q , and kqd. Let Imo' denote the monomer luminescence intensity at a given quencher concentration [Q] for so low a benzene concentration that self-quenching is negligible ([B] < 0.1 vol 70). One then obtains13
I, denotes the monomer luminescence intensity at concentration
[B] and [Q]. Combination of this
LIQUIDBENZENE LUMINESCENCE QUENCHING BY
cc14
expression with that for the reciprocal decay time, eq 512, leads to km
kd =
+ kf/kr[-4])Imo’/Im
= ~-l(1
+ kf/kr[A])(ImO’/Im
~-l(1
-
(7%)
1) X
Im/’Imo’kr/kf
[A] (7b)
The ratio kf/kr has been given in ref 13, while the other terms on the right-hand side of eq 7a and 7b are obtainable from the experimental data. The values of km and kd thus calculated for various benzene concentrations vary linearily with quencher concentration, as expected from the relations ksn = k m o
kd =
kdo
+ [QI + kqd[&] kqm
given earlier. The slopes of these curves, therefore, represent the specific rates of quenching. They are listed in Table 11. Table I1 : Specific Rates of Monomer and Excimer Quenching by CC14 for Benzene
0.1
20 50 80 100
0.5 0.4 0.4 0.4
0.5
0.5 0.7 0.8 1.1
Consideration of Values of Specific Rates of Quenching. The most striking result of the previous section is the invariance of the specific rate of benzene monomer luminescence quenching. This is quite different from the results given in Table I where the specific rates of quenching are listed as obtained from the simple reaction scheme and which show an increase with increasing benzene concentration. It is also observed that the value of kqm is well within the limit of what can be expected from the theory of diffusion-controlled processes. One must, therefore, conclude that these data on monomer luminescence quenching by carbon tetrachloride do not support any hypothesis of energy migration in benzene. While the results for the monomer seem to be quite straightforward, those for the quenching of excimer luminescence require further discussion. Whether the systematic variation of kqd with benzene concentration can be related l o a model of energy migration as suggested by VolteOcannot be easily decided. The fact that the specific rate of monomer quenching does not seem to vary with benzene concentration does not seem to make it plausible that energy migration accounts for the observations on the excimer quenching. I n addi-
3729 tion, the fact that the values of kqd are larger than those of k,, suggests a rather simple explanation. Model of Digusion-Controlled Quenching Process. So far it has been assumed that excimers of benzene, particularly, in the pure solvent are dimers. This picture is a simple adaptation of the model of excimers first proposed by Kasper and Forster to interpret the concentration dependence of the emission spectra of certain dyes.15 Whether this model can be applied to such concentrated solutions as pure benzene cannot really be decided on the basis of the luminescence spectra of benzene solutions. It seems quite possible that more complex excimer may exist. It is, therefore, suggested that in addition to excited monomers and dimers more complex excited benzene aggregates might have to be considered to form a quasiequilibrium. If such is the case, the experimentally observed quenching of benzene luminescence by CC14 seems to be describable by rather conventional theory of diff usion-controlled processes. For example, the specific rate of quenching as introduced in the simplest reaction scheme would actually represent a value averaged over different excited molecular aggregates. The concentration dependence of such a specific rate simply reflects the weights of each particular aggregate to the observed average and these weights depend on the benzene concentration. I n the next approximation, as given in this paper, if an “equilibrium constant” between excited monomer and the aggregates, summarily denoted as excimers, is established, the specific rate of monomer quenching can be extracted from the experimental data. I n principle, one might continue this process if equilibrium constants for the assumed higher complexes could be established. Unfortunately, this is not the case because of the lack of sufficient resolution of the emission spectra of benzene solutions. The model of benzene luminescence quenching here suggested implies that the specific rate increases with increasing size of the excited molecular aggregate. This is not immediately clear because the effect of an increased interaction radius of the aggregates may be compensated by a decreased diffusion coefficient. However, applying in an admittedly simplified manner Smoluchowski’s expression for bimolecular rate constants to the quenching process in question
k,
=
4nN’(r,
+ re)(Dg+ De)
(r, and re are the radius of quencher and excimer, respectively; D, and De are their diffusion coefficients), it can easily be shownlethat k, has a minimum value for re = (15) K. Kasper and T. Forster, 2. Phys. Chem. (Frankfurt am Main), 12, 15 (1951).
(16) This result is obtained when the diffusion coefficients in Smoluchowski’s equation are expressed as functions of the radii of the reactants using the Stokes-Einstein formula, D = ICT/Gs?r, and differentiating k, with respect to one of the radii.
Volume ‘79, iVumber 11 October 1968
WERNERF. SCHMIDTAND A. 0. ALLEN
3730 rs. Taking the molecular dimensions of CClA and
benzene monomer as about equal,” a larger aggregate of excited benzene molecules is expected to be quenched more efficiently. A simple test of this model could be made by a study of benzene luminescence quenching by collisional quenchers with possibly much larger molecular dimensions than the benzene molecule. I n this case, a decrease or more likely a reduction of the benzene concentration dependence of IC, is expected.
benzene in the absence and presence of CCl, as quencher and as a function of benzene concentration suggest very strongly that in considerations of benzene luminescence the formation and fate of excimers cannot be ignored. It seems that published data on energy transfer studies in benzene can be understood, at least qualitatively, by inclusion of excimers in the reaction scheme. The data presented here give little support to the existence of energy migration in liquid benzene in the sense of the Voltz model.
Conclusions The experimental results obtained for the steadystate and nonsteady-state luminescence behavior of
(17) R. Daudel, R. Lefebvre, and C. Moser, “Quantum Chemistry,” Interscience Publishers, Inc., New York, N. Y . , 1959, p125.
Yield of Free Ions in Irradiated Liquids; Determination by a Clearing Field’ by Werner F. Schmidt and A. 0. Allen Chemistry Department, Brookhaven hTational Laboratory, Upton, N e w York 11.978 (Received M a y 7, 1.968)
A new method is described for determining the yield of free ions formed by irradiation of a liquid. Yield values are reported for 19 pure liquids a t room temperature, for three liquids over a range of temperatures, and for two mixtures over the complete range of compositions. The results are discussed in terms of the mean distances that electrons, formed by ionization, can penetrate into the liquid before becoming thermalized. The free-ion yields in the various liquids show much more variation than anticipated; thus the yield in neopentane is six times as great, and the penetration distance two and one-half times as great, as in normal pentane. The reason for these variations is far from obvious.
The number of free ions formed by irradiation in liquids is of interest in the study of the behavior of free electrons in condensed systems and the basic mechanisms of radiation effects in liquids generally. It has been apparent since 190fP that most ions formed in the irradiation of a liquid never become free, but rapidly disappear by “initial” recombination. Applied fields of a few thousand volts per centimeter are sufficient to pull all the free ions to the electrodes without appreciable bulk recombination, but fields of this magnitude already interfere considerably in the process of initial recombination. Determination of the number of ions escaping initial recombination with no applied field can be attempted by extrapolating the high-field results to zero field,a but with many liquids this extrapolation is quite uncertain. More accurate in theory but more difficult experimentally is the method of determining the steady-state conductivity under irradiation at essentially zero applied field, together with an estimaThe Journal of Physical Chemistry
tion of the mobilities of the positive and negative ions, either by an approximate theoretical formula4 or by direct experimental d e t e r m i n a t i ~ n . ~This , ~ paper describes a new and less cumbersome method of determining the number of ions escaping recombination in the absence of an applied field and presents values obtained for a number of liquids and liquid mixtures. The method consists of ionizing the liquid by a short pulse of radiation, then immediately applying a clearing field of such magnitude that all the free ions are drawn to the electrodes before any appreciable amount of (1) Research performed under the auspices of the U. S. Atomic Energy Commission. (2) G. Jaffe, Ann. Phys., 25, 257 (1908). (3) G. Jaffe, Le radium, 10, 126 (1913). (4) G. R . Freeman and J. M. Fayadh, J. Chem. Phus., 43, 86 (1965). (5) A. Hummel, A. 0. Allen, and F. H. Watson, Jr., ibid., 44, 3431 (1966). (6) W. F. Schmidt, Z . ivuturforsch., 23b, 126 (1968).
