40
J. Phys. Chem. 1983, 87, 40-44
iltonian and which has been reduced with respect to the background modeseg The resulting stochastic Liouville e q ~ a t i o ncontains ~ ~ , ~ ~matrix elements other than the single one (r, = rWrg) introduced here, including elements corresponding to longitudinal (T,)relaxation. The role of the structure of r in determining resonance networks and pathways is just beginning to be understood. (5) Linear Response Theory. In 1971, van KampenZ7 made a case against the physical significance of linear
response theory by asserting that the effects of random motions of the microscopic variables are not adequately accounted for. His argument made use of the analogy with the Galton board. The point to be made here is that adiabatic modulation in the stochastic limit provides the "Galton board" effects.2s It is true that earlier formulations of linear response theory (along with van Kampen) have tacitly ignored this feature, but this is not an intrinsic shortcoming of the theory itself.
(25)(a) Kubo. R.Adu. Chem. Phvs. 1969.16.101. (b) J. Phvs. SOC. Jpn., Suppl. 1969,26,1. (26)(a) Grigolini, P. J. Chem. Phys. 1981,74, 1517. (b) Grigolini, P. In "Proceedings of the European Conference on the Dynamics of Excited States"; in press. (27)van Kampen, N.G.Phys. Noru. 1971,5,279.A discussion of this paper is given by ter Haw, D. In 'Lectures on Selected Topics in Statistical Mechanics"; Pergamon: New York, 1977.
Acknowledgment. This work was supported in part by Contract No. De-AS05-78EV05784 between the Division of Biomedical and Environmental Research of the Department of Energy and Florida State University. (28) Rhodes, W., submitted for publication.
ARTICLES Chemistry of Singlet Oxygen. 40. Enhanced '0, Formation in Cyanoaromatic-Sensitized Photooxldations by Substrate-Enhanced Intersystem Crossing Lewis E. Manring, Chee-liang Gu, and C. S. Foote" Department of Chemistry and 6lochemistry, University of Californla, Los Angeles, Callfornie 90024 (Received: August 17, 1982)
The interaction of a variety of substrates with the singlet excited states of 9-cyanoanthracene, 'CNA, and 9,10-dicyanoanthracene, 'DCA, leads to enhanced intersystem crossing (isc), as demonstrated by transient triplet-triplet (T-T) absorption spectra. The T-T spectra showed maximum absorptions at 427 and 440 nm for 3CNA and 3DCA, respectively. The relative amount of intersystem crossing, determined by the intensity of the transient, is substrate dependent. The enhanced intersystem crossing leads to increased formation of IO2 and is a major source of '02in many cyanoaromatic-sensitized oxidations.
Introduction Previous investigations have shown that cyanoaromatic-sensitized photooxidations of organic substrates can proceed via electron transfer from substrate (SI to singlet excited state sensitizer (lD).' The substrate radical cation can either add 3021c or combine with 02-* (formed to form by oxidation of the sensitizer radical anion by 302), oxidized products (Scheme I). More recently, it has become apparent that '02reactions can be important in both 9,lO-dicyanoanthracene (DCA) and 9-cyanoanthracene (CNA) sensitized oxidations even when almost all of the 'D is trapped by S.ldt2 A t first glance, the most probable source of IO2in these systems is energy transfer from 3D to 302(Scheme 11, step 7 ) . (1)(a) Erikson, J.; Foote,C. S. J.Am. Chem. SOC. 1980,102,6083.(b) Spada, L. T.; Foote,C. S. Ibid. 1980,102,391.(c) Manring, L.E.; Eriksen, J.; Foote, C. S. Ibid. 1980,102,4275.(d) Steichen, D. S.; Foote,C. S. Ibid. 1981,103,1855. (2)Foote, C.S.; Goyne, T. E.; Araki, Y.; Lee, K. J.; Jiang, Z.-Q., unpublished results. 0022-3654l83f 2087-0040$0 1.5010
Scheme I
Triplet energy transfer is known to give lo2efficiently, usually with a quantum yield of 1. Possible sources of 3D are shown in Scheme 11, steps 3, 4, and 6. Formation of 3D by direct intersystem crossing, step 3, is an inefficient process because both 'DCA and 'CNA have high fluorescence quantum yields (3,(DCA) = 0.9,3 aF(CNA) = 0.g4) and kf (step 2) >> kisc (step 3). Formation of 3D by 302 trapping of 'D, step 4, is an important process5 but less so in CH3CN than C6H6. Recently we have reported that the interaction of 302 with 'DCA ultimately produces 2 mol of 102.5 Again, when k,[S] >> k 4 [ 0 2 ]step , 4 will also form 3D inefficiently. In this paper, we present evidence that step 6 can form 'D and that this step can lead to significant '02production.
0 1983 American Chemical Society
Chemistry of Singlet Oxygen
The Journal of Physical Chemistry, Vol. 87,No. 7, 1983
-
Scheme I1 D kf
'D
+
'D
ID
(1)
+ hu
-
-
+ 302
k4
'D
D
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3D
t
1
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(2)
(3)
+ (lo2or 302)
0.25
-
+S 3D + 302 'D
k6
0.15
-
0.10
-
0.05
-
(5)
+S
(6)
D
+ '02
(7)
k7
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II
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I
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I
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1
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I
41
\
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Experimental Section Chemicals. CH3CN (Mallinckrodt Spectar) was distilled from P2O5. DCA (Eastman) and CNA (Pfaltz and Bauer) were recrystallized from toluene. Anisole, thioanisole (TA), p-dimethyliodobenzene (DMIB, l-iodo-2,5-dimethylbenzene (Eastman)) and 1,4-oxathiane (Aldrich) were distilled prior to use. Diphenyl sulfide (DPS (Aldrich)) was refluxed over CaH2 and then distilled. 2-Methyl-2pentene (2M2P (Aldrich)) was passed through basic alumina prior to use. trans-Stilbene (TS (Eastman)) and p-dimethoxybenzene (DMB (MCB)) were recrystallized from 2-propanol and n-hexane, respectively. Triphenylethylene (TPE) and 2-iodopropane (Aldrich) were used without further purification. 1,5-Dithiacyclooctane (DTCO) and p,p '-dimethoxydiphenyl sulfide (MDPS) were prepared by C.-L. G u , and ~ p-methoxythioanisole (MTA) was prepared by M. L. Kacher.' Sample Preparation for Laser Experiments. The desired amount of substrate was added to a 5-mL volumetric flask and the flask filled to the mark with CH3CN. Part of this solution (3 mL) was added to a test tube containing solid DCA or CNA and the solution stirred in the dark until the sensitizer had dissolved. (The desired amount of solid DCA or CNA had been added to the test tubes as a solution in CH2C12via micropipet and the CH2C12removed in vacuo prior to the addition of sample.) The sample solution was added to a quartz cuvette, sealed with a rubber septum, and saturated with either He or 02.For determination of [SI effects on 3D formation, all samples were prepared from stock solutions by using micropipets and volumetric glassware. Product Analysis. The appearance of the '02products from 2-methyl-2-pentene was monitored (after (C6H5)3P reduction) by GLC (3?% Carbowax column, isoamyl alcohol added as internal standard). The solutions were photolyzed with a 1200-W medium-pressure Hanovia lamp in a water-cooled immersion well. Two l-cm path length filter solutions, the first consisting of 4.4 g of CuS04.5H20 with 200 mL of concentrated NH,OH/(1000 mL of H20), the second consisting of 75 g of NaN02/ (loo0 mL of H20), served to isolate the Hg lines a t 405 and 436 nm.8 (3) Kemp, T. J.; Roberta, J. P. Trans. Faraday Sac. 1969, 65, 725. (4) Meyer, Y.; Astier, R.; J. LeCleray J. Chem. Phys. 1972, 56, 801. (5) Dobrowolski, D.; Ogilby, P.; Foote, C. S.,submitted to J. Phys. Chem. (6) Gu, C.-L. Ph.D. Dissertation, University of California, Los Angeles, 1981. (7) Kacher, M. L. Ph.D. Dissertation, University of California, 1977. (8) Calvert, J. G.; Pitta, J. N., Jr. "Photochemistry"; Wiley New York, 1966; p 737. (9) Shida, T.; Hamill, W. A. J. Chem. Phys. 1966, 44, 2375. (10) Bonifacis, M.; Muckee, H.; Bahnemann, D.; Asmus, K. D. J. Chem. Sac., Perkin Trans. 2 1975 675. (11) Dunckt, E. V.; Barthels, M. R.; Delestinne, A. J. Photochem. 1972173, I , 429.
400
410
420
430
440
450
460
470
WAVELENGTH nm Flgure 1. Triplet-triplet absorption spectrum of CNA. Intersystem crossing of 'CNA promoted by 0.1 M thioanisole.
Laser Apparatus. Most of the laser apparatus was built by Dr. L. T. Spada.lbJ2 The sensitizer, D, was excited by a Quanta-Ray DCR-1 Nd:YAG laser and amplifier at 355 nm, 10-ns pulse duration. A xenon flash lamp (manufactured a t UCLA) was used as the probe for early experiments. Later experiments with signal averaging employed either a 1OOO-W Hg arc lamp (Hanovia) or a 300-W high-pressure xenon arc lamp (Varian) chopped by a Vincent Associates Uniblitz 225 electromechanical shutter. In the initial experiments transient absorptions were detected by a photodiode detector designed and built by K. Stabe, UCLA Chemistry Department Electronics Shop. The diode was a United Technology, Inc., PIN silicon photodiode (PIN-020A). The diode had a 13-V bias and its signal was amplified by a National Semiconductor high-speed op-amp (LH0032). This signal was amplified further by a second op-amp (overall gain > 10-V output/microwatt of incident light). The diode is sensitive from 400 to 1050 nm. In later experiments the diode was gated so that no current could flow until 500 ns after the laser flash. The gated photodiode was designed and built by R. J. Ponce, UCLA Chemistry Department Electronics Shop (see paragraph a t end of text regarding supplementary material). Absorption spectra were recorded with an uncalibrated Bausch and Lomb grating monochromator (catalog no. 33-86-82). For the initial experiments, the output from the detector was displayed on a Tektronix 7904 oscilloscope with a 7B92 time base and a 7A24 vertical amplifier. For absorption spectra, the oscilloscope traces were recorded on a Tektronix C-51 oscilloscope camera and analyzed by hand, point by point. In later experiments, where the gated detector was used, the output of the diode was fed into a BIOMATION 805 waveform digitizer, averaged by using a computer built by Drs. J. V. V. Kasper and G. Gust, and analyzed on PDP 11/45 and PDP 11/23 computers. Fluorescence Quenching Experiments. The rate of 'DCA quenching by p-dimethyliodobenzene (DMIB) was determined from the Stern-Volmer equation (eq 11) by using least-squares analysis. Fluorescence quenching was determined at room temperature by using a Spex Fluorolog Instrument equipped with a photon-counting detector. Solutions of the fluorescer (2.7 X M) and quencher in 1-cm quartz cuvettes were sealed with a septum and degassed by bubbling helium through the solutions for 3 min. (12) Spada, L. T. Ph.D. Dissertation, University of California, Los Angeles, 1980.
42
Manring et at.
The Journal of Physical Chemistty, Vol. 87, No. 1, 1983 0. 2n
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Figure 2. Triplet-Triplet absorption spectrum of DCA. Intersystem crossing of 'DCA promoted by 4.4 X lo-' M DMIB.
Results Detection of 3CNA. Flash photolysis of He-saturated 0.1 M solutions of thioanisole (TA), trans-stilbene (TS), diphenyl sulfide (DPS), p-dimethoxybenzene (DMB), triphenylethylene (TPE), p-methoxythioanisole (MTA), 1,5-dithiocyclooctane (DTCO), or p,p'-dimethoxyphenyl sulfide (MDPS) with 3.4 X M CNA gave rise to a transient within 1pus after the flash with, , A a t 425-427 nm (Figure 1). The sulfides (TA, DTCO, DPS, MTA, and MDPS) also gave rise to a transient with a very broad absorbance extending from 460 to >560 nm; TS also showed a transient with a sharp, , A at 473 nm known to be TS+;9 T P E showed a second absorption at 500 nm, probably TPE+;gDMB gave no other transients. The broad absorbance for the sulfides is possibly due to dimeric species, since simple alkyl sulfide radical cations are known to react rapidly with neutral sulfides to form (R2S),+.'0 In all of the above cases, the transient with, , A at 425 nm disappeared completely after O2saturation of the solution. The other transients were mostly unaffected. We assign the transient with, , A at 425 nm to 3CNA. Consistent with this assignment is its sensitivity to oxygen, its similarity to the reported, ,A for 3CNA (430 nm),3311 and the fact that common spectra are obtained with various types of substrates. The other possible 0,-sensitive species that could be common to all the systems studied is CNA-. CNA- is reported to have a ,A, a t 580 nm;', however, a transient at this wavelength was not common with all of the compounds which showed the 425-nm species. Furthermore, addition of nitrobenzene, 5 X M, to the solution had little effect on the lifetime of the transient produced from TA and 'CNA a t 425 nm, even though nitrobenzene would have oxidized CNA- at a diffusion-controlled rate ( E , ,(C6H5No2)= -1.15 V,13 (CNA) = -1.58 V14 vs. SC!E). Detection of 3DCA. The detection of 3DCA was less common, occurring only with TA, 1,4-oxathiane, anisole, and dimethyliodobenzene (DMIB). The oxygen-sensitive at 440 nm and a shoulder at 425 nm transient had a ,A, (Figure 2). The 3DCA spectrum has been reported previously to be broad, 420-440 nm, with a, , A a t 426 nm in heptane.15 Gated Detector. Because the intense fluorescence of 'CNA and 'DCA overlaps their triplet absorbance spectra (Figures 1 and 2), it was not possible to observe the triplets (13) Mann, C. K.; Barnes, K. K. 'Electrochemical Reactions in Nonaqueous Systems"; Marcel Dekker: New York, 1970. (14) Eriksen, J.; Foote, C. S. J.Phys. Chem. 1980, 82, 2659. (15) Soboleva, I. V.; Saovskii, N. A.; Kuz'min, M. G. Dokl. Akad. Nauk SSSR 1978. 238, 400.
In
plot o f 'DCA.
droay
5 0
10
2n
TIME
30
40
50
60
(microseconds)
Flgure 4. First-order plot for 3DGA decay.
in the absence of substrate. When substrate was not trapping >95% of the 'D, the fluorescence saturated the electronics of our detection system,16which required over 150 ps to recover. In order to alleviate this problem, we built a detector that could be turned on only after most of the 'D fluorescence had decayed (see Experimental Section; 7pCA= 15.3 ns, 7fCNA = 17.2 ns).14 This detector allowed observation of the sample 3 ps after the laser flash, even in the absence of any 'D quencher. The apparatus was further improved at this time for signal averaging and computer analysis of the decay data. Results with Gated Detector. A typical decay trace for 3DCA in the presence of 4.4 X M dimethyliodobenzene (DMIB) is shown in Figure 3. This trace was obtained by subtracting the signal produced under He saturation from the signal under O2 saturation. The change in transmittance of the probe light (7')was kept below 25% ( T /To > 0.75) where absorbance is directly proportional to T/To,so that [3DCA]is linear with T/To. A first-order plot of the decay is shown in Figure 4. The decay is clearly first order from 25 to 60 ps; however, it deviates from first order immediately after the flash. Further analysis indicated that the decay was second order a t early decay times, probably due to increased triplet-triplet annihilation when [3DCA]is highest. Deviation from first order was greatest when large amounts of 3DCA were formed and almost disappeared when little 3DCA formed. The firstorder decay rate for 3DCA was independent of [DMIB], being 4.0 X lo4 and 3.8 X lo4 M-' s-' when [DMIB] was 4.4 X and 2.4 X M, respectively. (16) We believe the amplifiers could not handle the intense signal.
Chemistry of Singlet Oxygen 175
1
I
The Journal of Physical Chemistry, Vol. 87, No. 1, 1983 I
I
I
150 - I
8 c 8 6I
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43
,-I
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0.01
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0.02
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0.03
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0.04
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Fo/F>-l
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Flgure 5. Effect of DMIB concentration on amount of 3DCA formed.
Similar results were obtained with 3CNA and thioanisole (TA) as the 'CNA quencher. The first-order decay rate for 3CNA from this experiment was (2 f 0.5) X lo4 M-I s-'. We do not propose that these triplet lifetimes are the limiting values, because our solutions were not rigorously degassed; however, they do indicate that these substrates do not quench the triplets. That the quencher promotes intersystem crossing was shown by determining the effect of quencher concentration on the amount of 3D formed. Figure 5 is a plot of relative [3DCA] formed vs. [DMIB]. The amount of 3DCA increases as [DMIB] increases and then levels off when DMIB is trapping all of the 'DCA. We can calculate the rate of quenching of 'DCA by DMIB from the data in Figure 5. A plot of 3DCAm/(3DCAm - 3DCA) vs. [DMIB], where 3DCAmis the amount of 3DCA formed a t infinite [DMIB] (determined from intercept of 1/3DCA vs. 1/ [DMIB] plot), is the same as plotting F ' / F vs. [DMIB] (Stern-Volmer equation, eq 11) and will give a slope of kq7. Using 7 = 15.2 nsI4 we obtain a k, of 1.05 X 1O1O M-' s-l for DMIB quenching of 'DCA. An independent k, determation, from fluorescence quenching data, gave 8.76 X lo9 M-I s-'. The similarity between the two values of k, further indicates that DMIB is promoting 3DCA formation by trapping 'DCA. Medinger and Wilkinson showed that quenching of singlet anthracenes by compounds containing heavy atoms led to the triplet e x c l ~ s i v e l y . ~ This ~ ~ ' ~result has been confirmed by DeToma and Cowan.lg Wilkinson and Medinger's kinetic analysis yielded eq I, where F' =
fluorescence intensity with no added quencher, F = fluorescence intensity with added quencher, DTo = relative amount of triplet formed without added quencher, and DT = triplet formed with added quencher. If k5 aryl sulfides > aryl ethers > olefins) is also consistent with a heavy-atom effect enhancing the amount of triplet formed. To the best of our knowledge, the quantum yield of intersystem crossing for 'DCA has not been reported previously. Our value of 1.7% is consistent with the high fluorescence quantum yields reported for 'DCA.3 Cyanoanthracene, which also has a high fluorescence quantum yield, is reported to undergo 2.1% intersystem crossing in acetonitrile." These low intersystem crossing yields set an upper limit for 3D formation via step 3 and indicate, as predicted, that direct intersystem crossing of D is not an important source of lo2in these reactions. The experiments where '02production is enhanced by addition of DMIB indicate that 4.7% of the 'DCA evenare present. tually creates '02when only 2M2P and 302 Since 0.1 M 2M2P and air quench 92% of the IDCA, we can assume that
