MECHANISMB OF PHOTOREACTIONS IN SOLUTION
3'747
Mechanisms of Photoreactions in Solution. XX1.l
Qwnching of
Excited Singlet States of Benzophenone
by Robert P. Foss, Dwaine 0.Cowan, and George S,, Hammond Contribution N o . 5080 from the Gates and Crellin Laboratories of Chemistry, California Institute of Technology, Pasadena, California (Received February 14, 1964)
Tris(dibenzoylmethanato)iron(III) and tris(dibenzoylmethanato)chromium(III) show exceptionally high reactivity a$$quenchers in the photoreduction of benzophenone by beinzhydrol. The dependence of the effect on the concentrations of the quenchers suggests that, in addition to benzophenone triplets, excited singlet states of the ketone are deaotivated by the chelates. Study of the effects of the two chelates on the benzophenoiiesensitized isomerization confirms the singlet quenching. Singlet quenching occurs a t lower concentrations than would have been predicted by approximate theories.
introduction Transfer of singlet electronic excitation in solution has been detected by observation of induced fluorescence.2 Dexter3 and Forster4have formulated a theo,ry to account for the variation of transfer efficiency with concentration of acceptors. The theory has been criticized by Robinson and F r o ~ c hwho , ~ have developed a theory that is more exact in principle but not readily amenable to quantitative calculations of an ab initio nature. The work reported in this paper originated as part of a program to evaluate the relative reactivities of metal chelates as acceptors of triplet excitation.' The reactivity of tris(dibenzoylmethanato)iron(III) and tris(dibenzoylmethanato)chroiiiiuni(III) appeared to be anomalously high; more detailed study indicated that the chelates are capable of intercepting excited singlets of benzophenone before they undergo intersystem crossing.
Results The mechanism foir the photoreduction reaction was presented in the accompanying paper.l The same rate constants are used in this report. Measurements niadie in the presence of the iron(II1) and chromium(II1[) chelates of dibenzoylmethane, 1 and 2, respectively, showed that these compounds are remarkably efficient quenchers of the photoreduction reaction. The data are summarized in Table I.
1
2
Tris(dibenzoylmethanat0)iron( I11 )
Tris(dibenzoylmethanat0)chromium( 111)
Attempts to fit the data to eq. 18 of the previous paper gave widely scattered results as is shown by Fig. 1. The straight lines drawn arbitrarily in the vicinity of the various experimental points have slopes sevei:al times larger than any encountered in other studies of quenchers for excited states of benzophenone. The values of k , / k , calculated on the basis of the best slopes are 3400 for Fe(DBM)a and 1520 for Cr(DBNJ3. A large number of organic quenchers6 have values falling between 580 and 650. This level of activity has
-
(1) Part XX is G. S. Hammond and R. P. Foss, J . Phys. Chem., 168, 3739 (1964). (2) See ref. 4 for leading references on this subject. (3) D. L. Dexter, J . Chem. Phys., 21, 836 (1953). (4) Th. Forster, Discussions Faraday SOC.,27, 7 (1959). (5) G . W. Robinson and It. P. Frosch, J. Chem. Phys., 38, 1187 (1963). (6) More than 70 compounds having quenching constants falling between these limits have been studied.' Only azulene has a higher value, -1000. Note that re-evaluation of data for the quantum yields in the absence of quenchers leads to slightly lower values than were previously reported.* (7) T. D. Walsh, unpulblished results. (8) G. S. Hammond and P. A. Leermakers, J. Phys. Chem., 66, 1144 (1962).
Volume 68, Number 12 December, 1064
R. P. FOSS,D. 0. COWAX,AND @. S. HAMMOND
3748
4.0
-
3.0
2.5
3.0
-p -
2 .o
-
F d Figure 2. Quenching by Fe(DBM),: e, 5.54 X 10-6 M ; 8,2.72 x io-; 3,1.81 x 10-5 M .
Figure 1. Quenching of the photoreduction of benzophenone by benzhydrol in benzene solution: 0, Fe(DBM)g,; 3,Cr(DBM)s.
fw;
been associated with diffusion-controlled quenching. The iniplication is strong that the two chelates in question either quench benzophenone triplets froni positions niore remote than nearest neighbor sites in solution or that some other deactivation mechanism is operative. Although the former possibility would be interesting, we favor the latter alternative.
Table I: Quenching of the Photoreaction of Benzophenone with Benzhydrol by Dibenzoylmethanates of
I
0
Iron(II1) and Chromium(II1)" Run no.
77 78
79 80 81 82 83 84 85 86 87 88 89 90
Quencher
Fe( DB1L.I)3 Fe(DBM)s Fe( DBhl)3 Fe( DBM)3 Fe(DBM)3 Fe(DBM)3 Fe( DBM), Cr( DBM)8 Cr(DBM), Cr( DBM), Cr( DBM), Cr( DBM)( Cr( DBM), Cr( DBhl),
I
5
I 10
,
I
I
I
15
20
25
[=,I
[&I x
1 106
1.81 2.72 1.81 2.72 5.54 5.54 1.81 4.52 4.52 2.26 2.26 5.15 5.15 4.52
l/*
2.24 2.75 2.29 2.94 3.53 4.04 2.08 2.55 2.46 1.70 1.83 2.21 2.83 2.31
h
.
IBH21e.v
13.8 13.4 19.2 18.6 10.4 21.5 10.5 13.6 19.0 10.4 14.0 10.6 22.2 10.5
a Initial concentration of benzophenone was 0.10 M; benzene was the solvent. BH, is benzhydrol.
Closer scrutiny of the data suggests that the chelates function by two mechanisms, one that is directly conipetitive with attack on benzohydrol and one that is not. As is shown by Fig. 2 and 3, plots of l/@against 1/ The Journal of Physical Chemistry
Figure 3. Quenching by Cr(DBM)s: 0 , 5.15 X 10-6 M ; M. 8,4.52 X 10-6 M ; 3,2.26 X
[BH2] indicate that the maximum number of benzophenone triplets available for reaction with benzhydrol is an inverse function of the chelate concentrations. This suggests that the chelates are quenching excited singlets as well as triplets. The appropriate equation for description of this mechanism is
Plots of 1/@against [BHz]should yield straight lines, for any given value of [&I, with intercepts equal to 0.95 k,q[&]/ki,.9Jo The slopes of the lines should increase with increasing [&] so no crossing, such as appears in Fig. 3, should occur. The data are obviously insufficient to establish thoroughly the law of eq. 1 but,
+
(9) The intersystem crossing efficiency in the absence of quenchers is 0.95.10 (10) A. A. Lamola and D. 0. Cowan, unpublished results.
MECHANISMS OF PHOTOREACTIONS IN SOLUTION
with the single exception noted, seniiqusntitative agreement is observed. Because the chelates absorb the exciting light rather strongly, it is not possible to carry out experiments using higher concentrations of the chelates.ll Consequently, confirination of the singlet quenching mechanism was sought in another type of experiment. It has been shown that benzophenone acts as a sensitizer for the cis-trans photoisomerization of the st ilbenes.l2>la Benzophenone is among the group of cctrnpounds that are classified as "high-energy" sensitizers. l4 With such a a5ensitizer the inechanisni of isomcrizat ion can be simplified to the following scheme in which T represents the triplet group that decays to groundstate molecules.l3
+ + (CtiH&CO*(a)+ tmns-C14H12--% (CtiH,),CO + T (CtiH6)2CO*(a) cis-C14HI2-% (CeH~)zCO T L1
T +C ~ S - C ~ ~ H X ~ La
T -3 tr~n~-C14Hiz The ratio ol the isomers present a t the photostationary state is given by eq. 2
S o htions i riitially containing trans-s t ilbeue, betwophenone, and various iron(II1) chelates were irradiated until stationary states were established. As is shown by the data in Table 11, none of the chelates has any influence on the composition of the systems in the stationary states. The result clearly demonstrates that the chelates do not influence the decay of stilbene triplets; Le., T is not quenched by t,he chelates.
3749
-
tum yields, The results, which are entered in Table III, show that the quantum yields are lowered by Ye(DBL\[)~and &(DB1\1)3 although tris(acety1acetonato) iro ti (111) anld t ris(dipivaloy1met hanato) iron(I [I) have no significant effect on the rates of conversion.
Table 111: Rates of trans-cis Reaction" --o/o Quencher
None Fe( UPIVT)~ Fe(AA), Fe( L)13M)8 Cr( DBM)3 Benzene solution; phenone] = 0.1 M .
conversion----
400 min.
900 min.
1.70 1.70 1.66 1.58 1.59
3.30 3.29 3.26 2.99 3.02
[hns-slilbenejo = 0.05 M ;
[benzo-
Similar n~easurementswere made with cis-slilbene as the substrate with varying aniounts of Cr(DBh9)3 and tpe(DBi\I)3. The results are summarized in Table I[V in terms of per cent trans-stilbene produced during a fixed period of irradiation chosen to produce a sufficient amount of the trans isomer to be compatible with accurate analysis but still maintain the low conversion approxiination. Since the photos t a tionary states contain 40% trans-stilbene, conversions in the 6-7% range represent -157' of the final conversion so correction for back-reaction can be neglected in a first approximation. The initial rates of conversion can be conveniently expressed in the form of eq. 3 if it is assumed that triplets are deactivated only by energy transfer to cisstilbene. The assumption is reasonable in view of the high concentration of the substrate.la The data are plotted in Fig. 4.
Table 11: Stationary States of Photosensitized Isomerization of trans-Stilbene" Quenoherb
None Fe( L)YM)aC Fe(I)BiM)s Fe( AA)yd
[cisl*/ [tvansl.
1.51 1.50 1.52 1.52
" Benzene solution; [tran.s-stilbene]~= 0.05 M ; [benzophenone] = 0.1 M.
methanate.
[Quencher] == 2.2 X 10-6 M . A h = acetylacetonate.
UPM = dipiva1o:yl-
The relative rates of the trans + cis process were then measured in the presence of several chelates. Rates measured a t lo~wconversion are proportional to quan-
(11) Throughout the study we have maintained concentrations of quenchers low enough to keep internal filtering effects very small. Unless pure monochromatic light is used for excitation, precise correction for competitive absorption cannot be made. Careful measurement of the absorption spectra of solutions of the chelates and benzophenone showed no departure from additivity of absorption. With concentrations of the order of those used in the experirnenl,s, only the long wave length tail of the benzophenone absorption could be studied, but the visible absorption bands of the chelates showed no perturbation at all from the rather high concentration of the chelates. (12) G. S. Hammond and J. Saltiel, J. Am. Chem. SOC.,85, 2515 (1963). (13) G. S. Hammond, et aE., ibid., 86, 3197 (1964). (14) A high-ene1ng.y sensitizer has a triplet excitation energy substantially above that required to excite either of the Isomeric stdbenes to triplet states by Franck-Condon processes.
'Volume 68, Number 19 December, 1064
R. P. FOSS,D. 0. COWAN, AND G. S.HAMMOND
3750
Iab& a=-------
ki
+
k2
where Iabs k the intensity of light absorbed in einsteins L - 1 sec.-l.
[a]
x
to-’ M
Figure 4. Plot of eq. 3.
Table IV: Rates of Sensitized Conversion of &-Stilbene to trans-Stilbene in the Presence of Chelates % Chelate
None Fe(DBM)a Fe(DBM)s Fe(DBM)* Cr(DBM)a Cr( DBM)a Cr(DBM)a
[Chelate] X 10 M
transstilbene
...
6.67 6.36 6.20 5.99 6.41 6.22 6.00
2.0 4.0 8.0 2.0
4.0 8.0
1/R, mole-1 1. sec.
x
10-
3.00 3.14 3.22 3.34 3.12 3.22 3.34
Whether or not eq. 3 is really an appropriate form is a moot question. The data available are too few to serve as an appropriate test of the functional form of the dependence of the quenching effect on the concentrations of the chelates. It is clear that data could not be fitted any better by the equations of Forster4 which require a second-order dependence of the rate of transfer on the concentrations of acceptor a t low concentrations of the latter. The results can only be reasonably accounted for by the hypothesis that Fe(DBM)3and Cr(DBY1)s prevent formation of benzophenone triplets. There is very strong evidence that excitation transfer from benzophenone triplets to trans-stilbene is diffusion controlled. l 3 Since the concentration of trans-stilbene is 2.3 X 103 times as large as the concentration of the chelates, the results would require that quenching rate constants be a t least 100 times greater than the diffusion-controlled rate. This conclusion is rejected as The Journal of Physical Chemistry
clearly untenable. The alternative hypothesis that quenching of excited benzophenone singlets occurs is again much more attractive. Comparison of the two studies provides information that could have been supplied by neither alone. I n the photoreduction reaction, competition for triplets involves quenching and a relatively slow process, hydrogen abstraction from benzhydrol; in the isomerization reaction, triplet quenching is pitted against the diffusion-controlled process of energy transfer. Semiquantitative comparison shows that the inhibitory effect of 2 X lod5M Fe(DBNQa on the two reactions is very similar. The result is clearly incompatible with any mechanism which attributes quenching exclusively to reaction with triplets. If it were, a concentration sufficient to give a measurable effect on the rate of stilbene isomerization would, of necessity, essentially eliminate photoreduction completely. The implication that the quencher must intercept a precursor of the triplets easily accounts for the observations. The only likely candidate for a mechanism seems to be quenching of singlets. Examples of very efficient singlet quenching have been observed before in studies of solutions of fluorescent dyes, and the results have usually been treated by eq. 4,OW Forster equation.
(4) where Lo is the pseudo-first-order rate constant for decay of excited singlet donors by energy transfer; T~ is the actual mean lifetime of the excited donor, taking into account all modes of deactivation; Ro is the “critical” distance separating donor and acceptor, a t which the rate of transfer becomes equal to the rates of unimolecular decay of the donor; in the present case, the principal competing process must be intersystem crossing; and R is the average distance between donor and acceptor in the system under observation. Although our data do not fit eq. 4, and probably should not (vide infra), we can make a comparison of the efficiency of the quenching process that we have observed with that studied by induced fluorescence. Equation 4 may be rewritten as eq. 5 for the case in which the principal unimolecular process is intersystem crossing.
The value of Rois easily estimated from a knowledge of the concentration of acceptor sufficient to deactivate
MECHANISM^
OF
PHOTOREACTIONS IN SOLUTION
3751
half the excited donor singlets by energy transfer. At this point ICi, = IC, [ Q ] ,and eq. 5 becomes 1 6 = (6)
11.00
(2)
10.00 9.00
Examination of the intercepts in Fig. 2 indicates that a concentration of Pe(DBM)3 of -3 X M is sufficient to reduce the yield of benzophenone triplets by half. Assuming a random distribution of quencher molecules thsoughoiit the solution, this gives a value of 5.6 X 107 ,?ina as the average volume of the chelate molecules and makes the average distance from a benzophenone yolecule to the nearest quencher molecule about 190 A. Substitution of this value for R in eq. 6 gives a value of Ro of 171 A, Since the valueEi of Roestimated from studies of induced fluorescence are of the order of 50-100 8. as a maximum, the phenomena which we are observing are obviously of relatively high efficiency. Another tentative measure of efficiency may be obtained from the slope of the straight line drawn tentatively in Fig. 4. The indicated value of IC,,/ki, is 1.26 X lo4 1. mole-l. The rate constant, for intersystem crossing is not known, but it must be LOs set.-' or greater since most of the excited singlets of benzophenone become triplets, despite the fact that they are too short-lived to give detectable fluorescence. This leads to an estimated value of >lo" 1. mole-l sec.-l for IC,, a value far in excess of diffusion-controlled bimolecular reactions.
Experimental Tris(dibensoylmethanato)iron(III), F~(DBIvI)~, and tris(dibenzoylmethanato)chromium(III), Cr(DBM)3, were prepared by the same procedure. The metallic chloride hexahydrate was dissolved in varying (not critical) volumes of 50 :50 ethanol-water containing excess sodium acetate. A solution of dibenzoylmethane in the minimum volume of absolute ethanol was then added, and the chelates were precipitated. After a short period of warming on the steam bath, benzene was added and the layers were separated. The organic layer was dried, first with anhydrous calcium chloride and then with anhydrous magnesium sulfate, and the residual mixtures were then evaporated to dryness. The chelates could be recrystallized from petroleum ether. Fe(D13M)3,dark red, m.p. 275'. Anal. Calcd. for Cd6H330BFe:C, 74.47; H, 4.59. Found: C, 73.85; H, 4.50. Cr(DBM)3, yellowish green, m.p. 316'. Anal. Calcd. for CtsHa30&r: C, 74.77; H, 4.60. Found: C, 74.20; H, 4.49. Quenching of Reaction of Benzophenone with Benzhydrol. The apparatus has been described previously The procedure was the same as is described in the ac-
5.00
70
-
7.00
6.00
d .
2 c C 01
.-
C rn
a 50
-E
-
c 0
LL
-
5.00
b
u-
h
.-
4.00
.E a
-
-a e
n
0
3.00
30-
;. 0" L
2.00
1-00 I
3500
3600 3700 Wavelength (%I
3800
Figure 5. Comparison of absorption by Fe(DBM)X and benzophenone with li.ght transmitted by the filter system.
companying paper. The lines in the 3650-A. region of the emission fromL the high-pressure mercury arc were isolated by a combination of Corning glass filters 0-52 and 7-60, The extinction coefficient of Fe(DB&1\I3at 3650 A. is 4.04 X lo3 times as large as that of benzophenone. Consequently, only 4% of the 3650-A. radliation is absorbed by the chelate in a solution containing 0.1 M benzophenone and 10-6 M chelate. The ratio of the extinction coefficients of Cr(DBM)3 and benzophenone is 3.97 x 103 at the same wave length. Although the internal filtering effect due to competitive absorption by the chelates is significant, it is insufficient to account for more than 20% of the measured quenching effects. Since the source is not monochromatic, it was necessary to investigate the possibility that competitive absorption by the chelates at the edges of the filter window could account for the measured effects. Figure 5 shows a comparison of the absorption by benzophenone and Fe(DBM)3with the profile of the exciting beam. A prism spectrograph was placed at the focus of the parabolic mirror which terminates the optical bench. (16) W. M. Moore, G. S. Hammond, and R. P. Foss, Soc., 83, 2789 (1961).
J. Am. Chsm..
'Volume 68, Number i2 December, 1.964
3752
K. B. YERRICK AND M. E. RUSSELL
The curve shown is a densitometer trace of the photographic plate. The response of the film i s not perfectly linear, but the relative intensities indicated by the trace should be sufficiently accurate to guarantee
that competitive absorption cannot account for the effectof the chelate on the photoreactions. Acknowledgment. This work was supported by the U. S. Atomic Energy Commission.
Kinetic Order Determination in the Thermal Decomposition of Dimethylmercury
by K. B. Yerrick and M. E. Russell Department of Chemistry, Michigan State University, East Lansing, Michigan
(Received February $8,196'4)
A kinetic order determination was made of the thermal decomposition of dimethylmercury in the temperature range from 275 to 330". The rate expression used to fit the experimental data is -d(DMM)/dt = kB(DMM) kb(DMM)2. The temperature dependence of the rate constants is k, = 2.6 X lo9 exp(-39,000/M') sec.-l and k b = 9.5 X exp ( -71,00O/RT) cc. mole-l set.-'.
+
Introduction The kinetics of the thermal decomposition of dimethylmercury has been examined by a number of ~ 0 r k e r s . l - l ~However, the kinetic order of this decomposition in the vicinity of 300' has not been studied in much detail, and the few determinations which have been done conflict with one another. Laurie and Long4 reported first-order kinetics in the temperature range 294 to 333O whereas Yeddanapalli, et u Z . , ~ in the temperature range 305 to 342') found their data were best correlated by using a three-halvesorder expression for the decomposition. The work of Russell and Bernstein8 yielded first-order kinetics for the cyclopentane-inhibited reaction (290 to 375") , but they did not perform an order determination for the uninhibited reaction. Since the mechanisms proposeda,6*8.1lJ4,15for the reaction depend upon the order, it seemed necessary to re-examine the kinetics over a wider concentration range than had been used previously. Accordingly, the initial concentration was varied about 10-fold at each of five temperatures in the range 275 to 330". The results of this study are consistent with neither first- nor three-halves-order kinetics but can be well correlated using a two-term rate expression. The Journal of Phy8kal Chemistry
Experimental A conventional high-vacuum system was used. The decompositions (static) took place in a Vycor vessel whose volume was 870 cc. and whose surface-to-volume ratio was 0.73 cm.-I. I n certain runs the surface(1) J. P. Cunningham and H. s. Taylor, J . Chem. Phys., 6 , 359 (1938). (2) B. G. Gowenlock, J. C. Polanyi, and E. Warhurst, Proc. R o y . SOC. (London), A218, 269 (1953). (3) L. M. Yeddanapalli, R. Srinivasan, and V. J. Paul, J. Sci. Ind. Res. (India), 13B, 232 (1954). (4) C. M. Laurie and L. H. Long, Trans. Faraday SDC., 51, 665 (1955). (5) L. H.Long, ibid., 51, 673 (1955). (6) S. J. W. Price and A. F. Trotman-Dickinson, ibid., 53, 939 (1957). (7) R. Srinivasan, J . Chem. Phys., 28, 895 (1958). (8) M.E. Russell and R. B. Bernstein, ibid., 30, 607, 613 (1959). (9) J. Cattanach and L. H. Long, Trans. Faraday SOC.,56, 1286 (1960). (10) R. Ganesan, J. Sei. Ind. Res. (India), 20B, 228 (1961). (11) R. Ganesan, 2. physik. Chem. (Frankfurt), 31, 328 (1962). (12) R. E. Weston, Jr., and S. Seltzer, J. Phys. Chem., 6 6 , 2192 (1962). (13) M.Xrech and S. J. Price, Can. J . Chem., 41,224 (1963). (14) A. 9.Kallend and J. H. Purnell, Trans. Faraday SOC.,60, 93, 103 (1964). (16) L. H. Long, J . Chem. SOC.,3410 (1956).
