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The Journal of Physical Chemistry, Vol. 83, No. 24, 1979
Wu and Trozzolo
Production of Singlet Molecular Oxygen from the Oxygen Quenching of the Lowest Excited Singlet State of Aromatic Molecules in n-Hexane Solution K. C. Wu' and A. M. Trozzolo" Radiation Laboratotf and Chemistry Department, University of Notre Dame, Notre Dame, Indiana 46556 (Received September 14, 1978; Revised Manuscript Received July 3, 1979) Publication costs assisted by the U.S. Depatfmenf of Energy
The quantum yield of sensitized photoperoxidation of 2,5-dimethylfuran in oxygen-saturated n-hexane solution has been measured. The sensitizers used were 9-methylanthracene,9-phenylanthracene,9,10-dimethylanthracene, 9,10-diphenylanthracene,perylene, pyrene, 1,6-diphenylhexatriene, and rubrene, and the sensitized oxidation quantum yields were found to be 0.97,1.09, 1.14,1.20,1.27,0.79,1.02,and 1.20, respectively. The experimental results indicate that singlet molecular oxygen can be produced directly from the oxygen quenching of the lowest excited singlet state of many aromatic molecules, Le., the process S1 + 302 T1 + IO2 is not negligible when compared to S1 + 302 T1 + 302.This shows that the former process must also be taken into consideration in the general treatment of oxygen quenching of the fluorescence from aromatic molecules.
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-
Introduction It has been shown experimentally and theoretically that, for aromatic molecules, the quenching of the electronically excited singlet states by molecular oxygen results in the formation of triplet states of the molecules in solvents with low dielectric constant^.^-^ The fate of the quenching oxygen is less certain. The oxygen molecule is a somewhat unique molecule; its ground state is a triplet and the next electronic state is a singlet lying only about 7900 cm-' above the ground state tripletsg For many aromatic molecules, the S1-Tl energy gap is greater than 7900 cm-l. Hence, usually, there are two reaction channels that are available in the oxygen quenching of the excited singlet state of aromatic molecules: S1 + 302 T1 + 302 (1)
fluorescent hydrocarbon, e.g., rubrene, the production of singlet oxygen by process 2 was very important." It was shown in these studies that the quantum yield of singlet oxygen was 2 which is consistent with process 2 followed by the additional quenching of T1 by another oxygen molecule to form So and a singlet molecular oxygen. Recently, this latter finding was criticized by another report.18 In that report, the quantum yield of sensitized photooxidation of 1,3-diphenylisobenzofuran(a very good singlet molecular oxygen acceptor) by rubrene in benzene was measured, and it was concluded that the production of singlet molecular oxygen from the oxygen quenching of the excited singlet state of rubrene was ineffective. In a previous paper,lg we had discussed the shortcomings of the previous experiments, and we again studied the sensitized photooxidation of 1,3-diphenylisobenzofuranby rubrene in various organic solvents. The experimental data s1 3 0 2 T~+ io2 (2) were analyzed more carefully, and it was concluded that Both of these processes are spin-allowed transitions and the production of a singlet oxygen directly from the oxygen quenching of S1 of rubrene was indeed very effective. We the second process involves a transfer of electronic energy from S1to the oxygen molecule. Of course, this can only were able to show that about 1.5 molecules of singlet oxoccur in molecules having Si-T1 separations greater than ygen were produced as a cumulative result of a rubrene 7900 cm-l. S1 state being quenched by oxygen. In this paper, we extend our studies to investigate if the same conclusion There have been a number of previous attempts to evaluate the relative importance of the two proce~ses.~O-~~can be made with other singlet oxygen acceptors. I n i t i a l l ~ , l ' -the ~ ~ self-sensitized photoperoxidations of unThe new singlet oxygen acceptor used in this study is 2,5-dimethylfuran. It was chosen as the new singlet oxygen saturated organic molecules were studied as a function of acceptor because (i) it is very reactive toward singlet mooxygen concentration. It was established that, for aromatic lecular oxygen, (ii) the extent of sensitized photopermolecules with large quantum yields of intersystem oxidation can be followed easily by gas chromatograph,20 crossing, @PT, e.g., naphthacene, process 2 was rather (iii) it is transparent for light with wavelength longer than ineffective compared to process 1. On the other hand, no clear-cut conclusion could be drawn for molecules with a 300 nm, and (iv) for the sensitizers pyreneZ0and phenanthrene,21 the photoperoxidation yields measured in nsmall aT. However, the assumption of a common hexane are about the same as measured in methanol where quenching mechanism for molecules with large and small its reaction with singlet oxygen is well c h a r a c t e r i ~ e d . ~ ~ ~ ~ ~ aTled to the statement that singlet oxygen was produced Furthermore, we would also like to extend our studies to solely by oxygen quenching of the aromatic hydrocarbon other sensitizers beside rubrene. Since triplet states, TI, triplet state." It should be pointed out that the above of organic molecules will also give singlet oxygen on common quenching mechanism assumption is a postulate quenching and it is difficult to obtain accurate values @T, only and has no experimental support. In fact, earlier the intersystem crossing quantum yield (-f15%), we shall experiments with photoperoxidation of rubrene and 9,lOonly limit ourselves to compounds with high to moderate diphenylanthracene have suggested that the interaction fluorescence yields (i.e., small @T) and whose fluorescent of the emitting S1 with oxygen may be different for molemissions are substantially quenched in oxygenated soluecules with high and low fluorescence yields.15J6 tions. We would like to investigate if singlet oxygen proLater, a further investigation of the photoperoxidation duction from S1quenching is limited to one or two moleof aromatic molecules showed that for the strongly
+
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0022-3654/79/2083-3 180$0 1 .OO/O @ 1979 American Chemical Society
Oxygen Quenching of the Singlet State of Aromatic Molecules
cules only. This study would be interesting because it may give new information concerning the oxygen quenching of fluorescent emissions. Experimental Section Materials. 9-Methylanthracene, 9,lO-dimethylanthracene, 9,10-diphenylanthracene,perylene,, 1,6-diphenylhexatriene, and rubrene were obtained from Aldrich Chemical Co. and were used without further purification. Aldrich 9-phenylanthracene was recrystallized once from ethanol before use. Pyrene in cyclohexane solution was purified by passing through activated silica gel. The solvent cyclohexane was then removed to recover the pure pyrene. Aldrich 2,5-dimethylfuran was purified by passing through activated alumina twice before use. Fischer certified-grade n-hexane and Phillips research-grade n-nonane were used directly as obtained. Apparatus. A medium-pressure Bausch & Lomb SP200 mercury lamp was used as the light source in the experiment. After passing through the appropriate light filters, the excitation light beam was focussed onto the front surface of a cylindrical Suprasil cell. The optical setup and the photolysis cell have been described in detail previ0us1y.’~The mercury 313- and 366-nm lines were chosen as the excitation wavelengths. For the mercury 313-nm line, the filters used consisted of a 1-cm path length of basic potassium chromate solution (K2Cr04,0.27 g/L, + Na2C03,1 g/L) and a 2 cm path length of cobalt sulfate solution (100 g/L). The intensity of the 313-nm radiation falling on the photolysis cell was determined by potassium ferrioxalate actinometry to be about 5.6 X einstein/ min. For the 366-nm mercury line, the light filters used consisted of a 1-cm path length of cobalt sulfate solution (100 g/L) and a Baird-Atomic 3650 A interference filter. The intensity of the 366-nm light at the photolysis cell was about 2.7 X 10 einstein/min. A Beckman GC-5 gas chromatograph was used to monitor the disappearance of 2,Ei-dimethylfuranwith n-nonane as an internal standard. Fluorescence emissions of the sensitizers were measured with an Aminco-Bowman spectrophotofluorometer. Absorption spectra were recorded on a Cary 14 spectrometer. Photooxidation S t u d i e s n-Hexane was used as the solvent throughout the experiment. The concentration of the sensitizer was chosen so that more than 99% of the excitation light was absorbed in the 3-cm photolysis cell. This varied from 1.5 X low3 M for 1,lO-dimethylanthracene to about 8 X M for rubrene. Generally, the excitation light used was the 313-nm line except for perylene whose solubility in nhexane was so small that the mercury 366-nm line had to be used as the excitation source to ensure sufficient absorption. For all the sensitizers used, the concentration employed was small enough that there was no complication in the photochemistry owing to excimer formation as illustrated from the emission spectra.24 The concentrations of n-nonane and 2,5-dimethylfuran used were 6.5 x and 1.3 X M, respectively. This concentration of the acceptor 2,5-dimethylfuran was sufficient to capture more than 99% of the singlet oxygen produced in the solution.20 In a typical experiment, 100 mL of the solution containing the sensitizer, n-nonane, and 2,5-dimethylfuran was placed into the photolysis cell. Oxygen, presaturated with nhexane, was passed through the well-stirred solution. After about 30 min, two 10-mL portions of the oxygen-saturated solution were withdrawn from the photolysis cell. The remaining 80 mL solution was irradiated. The time of exposure of the solution was such that about 12-18% of the 2,fI-dimethylfuran was consumed. The amount of
The Journal of Physical Chemistry, Vol. 83,
No. 24, 1979 3181
2,5-dimethylfuran consumed was determined from gas chromatography of the unirradiated and irradiated solutions. Light intensity was determined by potassium ferrioxalate actinometry at the end of each set of experiments. Results The quantum yields, yo, of sensitized photoperoxidation of 2,5-dimethylfuran in n-hexane by the sensitizers were determined and tabulated in Table I. The yo value for each sensitizer reported in Table I is the mean value of four to five determinations. The reported uncertainty in yo was the extreme deviation of the four to five determinations from the mean value. In some instances, e.g., perylene and rubrene, the yields were found to be greater than unity. The degradation of the sensitizer in the experiment was negligible. In fact, it was estimated that some sensitizers, e.g., pyrene and rubrene, were able to sensitize the oxidation of about 50 times their quantity of the acceptor without any noticeable degradation themselves. Table I also contains the value of PT, the probability of the oxygen quenching of the sensitizer fluorescence in an oxygen-saturated n-hexane solution. It was also found that, for the concentration of 2,5-dimethylfuran used in the experiment, no quenching effect on the fluorescence emission of the sensitizer could be observed. In a similar experiment where the solution was saturated with nitrogen instead of oxygen, no detectable disappearance of 2,5-dimethylfuran could be observed. Discussion Since singlet oxygen is also produced from the oxygen quenching of the excited triplet states of aromatic molecules, the intersystem crossing yields have to be taken into account in the quantitative treatment of production of singlet oxygen. The overall mechanism by which singlet oxygen is produced from the excited states of aromatic molecules is presented as follows:
ki
SI + 302
S1
-
TI + 302
s1+ so22 T~+ io2 So (radiative and nonradiative)
k4
TI + 302
kS
+ 302 So + IO2 So
(3a)
(4)
T1 + 302 (5) In the above mechanism, it is implicitly assumed that (a) the quenching of S1 by oxygen leads to TI3-*and (b) the oxygen quenching of TI in oxygen-saturated n-hexane solution is the most important deactivation channel of TI. From the above mechanism, the quantum yield, Qo, of production of singlet molecular oxygen from the fluorescent solution is &o = CXPT €@TI (6)
+
where cy = k , / ( k l + k 2 ) , = k 5 / ( k 4 + k5), PT is the probability of fluorescence quenching by oxygen, and @T‘ is the quantum yield of the triplet state of the sensitizer in the presence of oxygen. Then @T’ = [ P T + @T(1 - PT)] (7) where @T is the intrinsic quantum yield of the triplet state of the sensitizer in the absence of oxygen perturbation. Then, from (6) and (7) &O
= “PT
+ 6[PT + @T(1 - PT)]
(8)
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The Journal of Physical Chemistry, Vol. 83, No.
24, 1979
Wu and Trozzolo
TABLE I: Quantum Yields of Sensitized Photooxidation of 2,5-Dimethylfuran in n-Hexane
PT sensitizer 9-methylanthracene 9-phenylanthracene 9,lO-dimethylanthracene 9,lO-diphenylanthracene perylene pyrene 1,6-diphenylhexatriene rubrene
@Td
PT
0.48 0.37 0.03 0.12 0.02 0.38 0.35 -0
0.70 0.69 0.80 0.80 0.77 0.98 0.81 0.78
a Uncertainties of ~ 0 . 0 2 in PT and ~ 1 5 % in (1 - P T ) Q T ] . References 25 and 26.
@T
a
References 1 7 and 25.
E,, cm-' 18 900 23 000 25 100 25 500 26 900 26 900 26 200 25 900
Calculated by @ T t
(1 - PT)'
0.84 i 0.04 0.80 i 0.04 0.81 i 0.02 0.82 * 0.03 0.77 c 0.02 0.98 * 0.02 0.88 k 0.02 0.78 i 0.02
0.97 i 0.03 1.09 i 0.05 1.14 i 0.05 1.20 c 0.06 1.27 t 0.06b 0.79 i 0.04 1.02 i: 0.05 1.20 i: 0.02
0.13 k 0.07 0.29 i 0.09 0.33 i 0.07 0.38 i 0.09 0.50 _t: 0.08
he, =
- ET
9 000 10 400 11 100 11 200
1 6 900
Z
AC
366 nm.
0.14 0.42
k i.
0.07 0.04
A = yo
-
[PTt
-
E T , cm-' 1 0 000 1 2 600 1 4 000 1 4 300
@f
Y I1
Ta
are assumed in the calculation.
TABLE 11: Spectroscopic Parametersa of the Sensitizers sensitizer ru brene perylene 9,lO-dimethylanthracene 9,lO-diphenylanthracene 1,6-diphenylhexatriene pyrene 9-phenylanthracene 9-methylanthracene
-t
1 0 000 -11 000 -11 000
@
@T
f
-1 -1 0.89 0.81 0.65 0.58 0.57 0.51
-0 -0 0.03 0.12 0.35b 0.38 0.37 0.48
A /pT
0.54 0.65 0.41 0.48 0.17 0.42 0.19
1,
The singlet oxygen produced in this process either reacts with the acceptor 2,5-dimethylfuran, M, or is deactivated in the solution as follows:
a singlet oxygen molecule and the triplet state of the sensitizer are produced as a result of a S1 excited state quenching by oxygen. The last column of Table I is the value of A, the difference between yo and [PT (1- PT)@T]which is the total quantum yield of triplet state of the sensitizer in the presence of oxygen. It can be noted that except for pyrene, A is positive for the other sensitizers. Since t = k , / ( k , k5) 5 1,then ~ [ P T (1-PT)@T] 5 [PT+ (1-P'~)@T].From the positive values of A, it can be concluded that singlet molecular oxygen is produced effectively by process 2 for the sensitizer 9-phenylanthracene, 9,lO-dimethylanthracene, 9,10-diphenylanthracene,perylene, and rubrene in n-hexane solutions. For 9-methylanthracene and 1,6-diphenylhexatriene,the A are too close to zero to give any positive conclusion. Theory has predicted that t is close to ~ n i t y In . ~this ~ ~experiment, ~ we can say that t I0.8 for pyrene and it has been observed elsewhere that t 5 0.8-0.9.23,27aWith N 0.8-0.9, it can be argued that for 9-methylanthracene and 1,6-diphenylhexatriene, it is probable that singlet oxygen is also produced by process 2 in these two compounds. From (12) and 6 I 1, it can be shown that aPT I. A
+
The probability, P R , by which the singlet oxygen reacts with the acceptor 2,5-dimethylfuran is given by
PR= [Ml/([Ml
+ P)
(11)
where P = k l o / k g . With p N MZ7and [MI N 1.3 X M in the experiment, PRN 1,Le., nearly all the singlet oxygen is intercepted by the acceptor. Hence the quantum yield, yo, of the sensitized photooxidation of 2,5-dimethylfuran under this condition is equal to the yield of production of singlet oxygen. Hence from (8) 70
=
ffPT
+ C[PT + @T(1
-
PT)]
(12)
Table I gives the values of yo for the aromatic sensitizers in n-hexane solutions with pyrene as the sensitizer and 2,5-dimethylfuran as the singlet oxygen acceptor, yo = (0.79 f 0.04) which is in good agreement with the literature value of yo = (0.83 f O.O5).'O Furthermore, yo = (1.20 f 0.02) for the sensitizer rubrene and this compares favorably with yo = (1.14 f 0.06) in our previous studylg with 1,3diphenylisobenzofwan as the singlet oxygen acceptor. This shows that probably both the acceptor 1,3-diphenylisobenzofuran and 2,5-dimethylfuran have relatively little physical quenching of the singlet oxygen.28 This agreement confirms the previous conclusion that singlet oxygen can be produced directly from the oxygen quenching of the rubrene S1excited state. It is interesting to note that, for the sensitizers rubrene, perylene, and 9,10-diphenylanthracene, yo is significantly greater than unity. This result is obtained directly from the experimental data without any correction or extrapolation. This may be the first report of a directly observed sensitized photoperoxidation yield greater than one. A yield greater than unity indicates that singlet oxygen cannot be produced solely from the triplet state of the sensitizer and some must originate from process 2 where
+
+
or
In Table 11, the values of A/PT and several other spectroscopic parameters of the sensitizers are listed. With a 2 0.5 for 9-phenylanthracene, 9,10-diphenylanthracene, 9,10-dimethylanthracene, perylene, and rubrene, then, for these molecules, a t least 50% of the oxygen quenching of the S1excited state gives rise to a triplet excited state and a singlet molecular oxygen. Furthermore, Table I1 shows that A l p T (an indication of the efficiency of production of singlet oxygen directly from the oxygen quenching of the SI state of the sensitizer) is not correlated entirely to any single spectroscopic parameter of the sensitizer listed in the table. The general trend is that the higher the fluorescence quantum yield of the sensitizer, the greater will be the efficiency of the production of singlet oxygen
The Journal of Physical Chemistry, Vol. 83,
Communications t o the Editor
from SIby process 2. This general trend is not without exception, e.g., pyrene and 9-phenylanthracene. Hence it is certain that the production of singlet molecular oxygen from the oxygen quenching of SIis effective in many aromatic molecules and is not limited to rubrene alone. These studies show that process 2 is as important as process 1 in many molecules. Up to now, process 2 is usually written off as unimportant in the theory of oxygen quenching of aromatic fluorescence. Hence, some refinements have to be made in the theory in order to accommodate the new experimental findings.
No. 24, 1979
3183
B. E. Algar and B. Stevens, J . Phys. Chem., 74, 3029 (1970). 0.L. J. Gijzeman, J. Chem. Soc., Farachy Trans. 2, 70, 1143 (1974). M. Kasha and A. V. Khan, Ann. N . Y . Acad. Sci., 171, 5 (1970). C. S.Parmenter and J. D. Rau, J . Chem. Phys., 51, 2242 (1969). B. Stevens and B. E. Algar, J . Phys. Chem., 72, 3468 (1968). B. Stevens and B. E. Algar, J . Phys. Chem., 73, 171 1 (1969). B. Stevens and B. E. Algar, Ann. N . Y . Acad. Sci., 171, 50 (1970). 6. Stevens and L. E. Mills, Chem. Phys. Lett., 15, 381 (1972). E. J. Bowen and A. H. Williams, Trans. Farachy Soc., 35, 765 (1939). R. Livingston and V. S. Rao, J . Phys. Chem., 63, 794 (1959). B. Stevens and J. A. Ors, J . Phys. Chem., 80, 2164 (1976). P. B. Merkel and W. G. Herkstroeter, Chem. Phys. Lett., 53, 350 (1978). K. C. Wu and A. M. Trozzolo, J . Phys. Chem., 83, 2823 (1979). D. M. Shold, J . Photochem., 8, 39 (1978). K. C. Wu, unpublished results. C. S. Foote, Acc. Chem. Res., 1, 104 (1968). K. Gollnick, T. Franken, G. Schade, and G. Dorhofer, Ann. N . Y . Acad. Sci., 171, 89 (1970). I. B. Berlman "Handbook of Fluorescence Spectra of Aromatic Molecules", Academic Press, New York, 1965. J. B. Birks, "Photophysics of Aromatic Molecules", Wiley-Interscience, New York, 1970, and references described therein. S. L. Murov, "Handbook of Photochemistry", Marcel Dekker, New York, 1975, and references therein, From the rate constants and p in (a) P. B. Merkel and D. R. Kearns, J . Am. Chem. Soc., 94, 7244 (1972); (b) B. Stevens, S. R. Perez, and J. A. Ors, ibid., 96, 6846 (1974). P. B. Merkel and D. R. Kearns, J. Am. Chem. Soc., 97, 462 (1975). D. R. Kearns, Chem. Rev., 71, 395 (1971).
References and Notes Chemistry Department, University of Western Ontario, London, Canada N6A 567. The research described herein was supported by the Office of Basic Energy Sciences of the U S . Department of Energy. This is Document No. NDRL-1930 from the Notre Dame Radiation Laboratory. G. Porter and M. R. Topp, Proc. R . SOC.London, Ser. A , 315, 163 (1970). J. T. Richards, G. West, and J. K. Thomas, J. Phys. Chem., 74, 4137 I 1970). C . R. Goldschmidt, R. Postashnik, and M. Ottolenghi, Chem. Phys. Lett., 9 , 424 (1971). R. Potashnik, C. R. Goidschmidt, and M. Ottoienghi, J. Phys. Chem., 75, 1025 (1971).
COMMUNICATIONS TO THE EDITOR PhotodissociationStudy of the Electrocyclic Interconversion of 1,3-Cyclohexadiene and 1,3,5-Hexatriene Radical Cations
Sir: The consequences of molecular and orbital symmetry in determining the facility of rearrangement and reaction pathways have been widely explored and emphasized by Woodward and Hoffman among 0thers.l The collision-free environment of gas-phase ions under high vacuum is an area where application of these concepts can be of considerable value, because ions formed by electron impact typically have substantial internal energy which cannot be dissipated by collisions, and the failure of such ions to undergo an available exothermic rearrangement must clearly be associated with a very substantial energy barrier. It is attractive to look to orbital symmetry considerations in the prediction (and rationalization) of such energy barriers. We describe here a case where such a barrier appears to exist, and to be nicely accounted for by the theory, although it will be seen that in this case the conservation of state symmetry provides an even more powerful argument. The ion of interest is C6H8+-,derived from either 1,3cyclohexadiene (I) or 1,3,5-hexatriene (11). The electrocyclic interconversion of I and I1 is a textbook example of the application of orbital symmetry arguments,2but only recently have Shida and co-workers3 examined the corresponding radical cations by optical spectroscopy in lowtemperature rigid matrices. Consistent with the predictions of orbital symmetry conservation, they find distinct, thermally stable I+. and II+. isomers, and photochemical conversion of 1.' to II+.. The orbital correlations are shown in Figure l a for the conrotatory and disrotatory electro0022-3654/79/2083-3183$01 .OO/O
cyclic processes, and it is clear that for the radical cations neither rearrangement path correlates the ground states. Our work described here exploits the capability of photodissociation spectroscopy for characterizing the isomeric C6H8+-gas-phase ions in a completely unambiguous way. As expected for conjugated x-system radical ions,4the isomers of C&&+' show two photodissociation bands: a UV band corresponding to 7~ T* electron promotion, and a visible band corresponding to x x electron promotion within the set of occupied x orbitals. The spectra obtained for the two isomers are shown in Figure 2. The ions are formed by electron impact at 10.0 eV, and are trapped in torr for several seconds. the ICR ion trap a t 3 X Rearrangement of either isomer to the other under these conditions and on this time scale would be reflected in the photodissociation spectra, and the fact that the two spectra obtained are entirely distinct in the visible region is conclusive proof that no such thermal rearrangement takes place. That the ions actually retain the structures of the parent neutrals is strongly suggested by the agreement with photoelectron spectra. The correspondence of the 1,3,5hexatriene peak at 2.0 eV with the PES spectrum has been di~cussed;~ the x ir transition in 1,3-cyclohexadiene ions a t 2.5 eV is in excellent agreement with the corresponding PES peak,6 as well as with theoretical calculation^.^ As can be expected for radical ions, whose electronic ground states are in general not totally symmetric, it is possible to state a symmetry restriction stronger than orbital symmetry for this rearrangement. Figure l b shows the ground and first excited state of the two structures in C,, symmetry. In any rearrangement which preserves either the C2 axis or the uL plane of symmetry, it is easily seen that a state of 2B1symmetry in one structure must
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0 1979
American Chemical Society
