MECHANISMS OF PHOTOCHEMICAL REACTIONS IN SOLUTION computed for the same parameter in benzene solution; this could be a consequence of a lower encounter probability for oxygen quenching of the triplet than of the singlet state which apparently proceed by different mechanisms.
Conclusions The oxygen concentration dependence of the quantum yield of DMBA autoperoxidation in nylon and polyethylene films is similar to that observed for the same system in benzene solution and provides additional
3797
support for the suggestion that an excited singlet state of oxygen, probably 0 2 A g , is the reactive intermediate. Similar conclusions have been reached by Bourdon and Schnuriger,’ who observe high quantum yields (0.20.3) for the erythrosin-sensitized photooxidation of 4-methoxynaphthol in ethyl cellulose under conditions where>he average separation of sensitizer and substrate is 80 A. (7) J. Bourdon and B. Schnuriger, Photochem. Photobiol., 5 , 507 (1966).
Mechanisms of Photochemical Reactions in Solution. LVI.1 A Singlet-Sensitized Reaction by Steven Murov2 and George S. Hammond Contribution No. 8678 from the Gates and Crellin Laboratories of Chemistry, California Institute of Technology, Pasadena, California 91109 (Received M a y 7 , 1968)
Both norbornadiene, 1, and quadricyclene, 2, quench the fluorescence of many aromatic hydrocarbons. Surprisingly, 2 is considerably more effective than 1 as a quencher. Quenching by 2 leads to isomerization of the quencher to 1, but no 2 is produced when 1 acts as a quencher.
We have recently reported that conjugated dienes are remarkably effective in quenching the fluorescence of many aromatic hydrocarbon^.^ The overall process amounts to catalysis of nonradiative decay of fluorescent species. The following formal mechanism provides a basis for discussion.
A h’, A*(’) A*(’) +A hv’
+
(1) (2)
ka
A*(’) + Q If (A. &)*(I) k- 8
(A,&)*(l)kr_ A*(v) + Q*(u)
(3)
controlled, we infer that reaction 4 is often rate limiting and that k-8 >> kq. Under these circumstances the rate law becomes
R,
K3k4
Quenching reactivity is controlled by two factors: the stability of the complex, measured by K3, and the radiationless decay rate, measured by kq. We are inclined to view the binding energy of the excited complex as arising from exciton and charge-transfer interactions.
A*.& t-f A*&* (4)
vibrationally excited groundstate molecules
The excited complexes are visualized as being loosely bound and we would expect that they would probably be formed on nearly every encounter in solution. Since quenching rates in many cases are well below diffusion
[A*(’)J [QJ
A+&-
t--3 A-Q+
Our principal approach to construction of a more detailed model for quenching has been variation of the structures of both quenchers and quenchees. We hope that consistent patterns of structure-reactivity relationships will emerge. I n this spirit we investigated (1) Part LV: R. 8. Cooke andG. S. Hammond, J. Amer. Chsm. Soc,, 90, 2958 (1968). (2) National Institutes of Health Postdoctoral Fellow, 1967-1968. (3) L. M. Stephenson, D. G. Whitten, and G. S. Hammond, “The
Chemistry of Ionisation and Excitation,” Taylor and Francis Ltd., London, 1967,p 35, and references cited therein.
Volume 72, Number 11 October 1068
3798
STEVEN MUROVAND GEORGE S. HAMMOND
the behavior of compound 1 (norbornadiene, 2.2.1bi~yclohepta-2~5-diene).Although the diene does not have a classical conjugated structure, interaction between the two double bonds does lower the energies of the lowest, electronically excited states below those of simple olefins. The first absorption maximum is found a t 2216 A.4 The diene also has low-lying excited triplet states since isomerization to quadricyclene, 2, can be accomplished using benzophenone as a photosensitizer.s Wei and Kuppermanne have found a transition a t 4.0 eV using electron-scattering spectrometry; the excited state is believed to be the lowest triplet. Compound 1 quenches naphthalene fluorescence with a rate constant of 1.3 X lo7 1. mol-' sec-l. This reactivity is significant, being about the same as that of isoprene (1.4 X lo7 1. mol-' sec-'), one of the less reactive conjugated dienes. As a part of the investigation, we examined solutions after irradiation to determine whether or not any isomerization of 1 to 2 had occurred. No trace of 2 could be found. However, experiments intended to be routine controls showed that 2 quenches naphthalene fluorescence f u r more eflciently than 1. Furthermore, extensive isomerization of 2 to 1 accompanies the quenching action. Both observations seemed extraordinary to us so the scope of the phenomena has been investigated using a number of aromatic hydrocarbons as sensitizers. Quenching efficiencies have been measured both by measuring the decrease in fluorescence intensity and by determination of the decrease in fluorescence lifetimes in the presence of quenchers. Results obtained by the two methods are in excellent agreement. Quantum yields for isomerization of 2 to 1 were determined in solutions containing sufficient quadricyclene to quench at least 99% of the fluorescence of the sensitizers. The mechanism outlined above can be expanded to include chemical reaction of the quencher. k6
&*(u)
----f
1
Io I 2
1
(6)
(7)
The Stern-Volmer relationships used to test the form of the mechanism are Fluorescence = intensity I
+
kqTo[~l
Fluorescence 20 = lifetime T
+
kqTO[&l
I
I
I
I
I
I
,002 ,004 .006 ,008 .010 ,012
1
.014
[OUADRICYCLENE] [moles/P)
Figure 1. Quenching of naphthalene fluorescence b y quadricyclene; relative intensity method; emission spectra shown in the inset.
01
Q
ki Q*(u)+ Q'
5
!
0
I
I
I
0.I 0.2 0.3 [OUADRICYCLENE] (moles/P)
4
Figure 2. Quenching of 2,6-dimethylnaphthalene fluorescence b y quadricyclene; lifetime method.
obtained. Equation 9 may be rearranged to give eq 11 for purposes of direct comparison of quenching and isomerization. (9)
T
-=TO
-
T
1 kq~oIQ1
(11)
Figure 4 shows a comparison of data for quantum
Io and r0 = fluorescence intensities and lifetimes in absence of quencher cp0 = maximum quantum yield Figures 1, 2, and 3 show representative plots of data The Journal of Physical Chemistry
(4) M. E. Robin and N. A. Kuebler, J. Chem. Phys., 44, 2664 (1966). (5) G.S. Hammond, N. J. Turro, and A. Fischer, J . Amer. Chem. Sac., 83, 4674 (1961). (6) P. S. Wei, Ph.D. Thesis, California Institute of Technology, 1968.
MECHANISMS OF PHOTOCHEMICAL REACTIONS IN SOLUTION
3799
Table I : Rate Constants for Singlet Quenching of Aromatic Hydrocarbons by Quadricyclene kn ( X lo-*),@
Compound
9,lO-Dichloroanthracene 2-Chloronaphthalene Naphthalene Anthracene 2,3-Benzfluorene 1,2,3,4-Dibenzanthracene Anthanthrene 2,6-Dimethylnaphthalene Pyrene
Octahydro-1,1,4,4,7,7,10,10octamethylnaphthacene Triphenylene 9,lO-Diphenylanthracene Perylene 1,2-Benzanthracene 3,CBenzpyrene 3,4,8,9-Dibenzpyrene 9,lO-Dimethylanthracene Chrysene
1,2,5,6-Dibenzanthracene 1,2,7,8-Dibenzanthracene Tetracene
l./mol sec
113 (70) 32 31 18 10 (7)
A, eVb
I2
1.8 (1.8) (1.7) (1.6) (1.6) (1.2) (1.0) (0.9) (0.3) (0.08) (
