J. Am. Chem. SOC. 1992, 114, 682-688
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Sensitizer-Substrate Interactions in the 9,lO-Dicyanoanthracene-SensitizedPhotooxygenation of trans-Stilbene Richard C. Kanner and Christopher S. Foote* Contribution from the Department of Chemistry and Biochemistry, University of California, Los Angeles. California 90024. Received April 1 , 1991. Revised Manuscript Received September 6, 1991
Abstract: The quantum yield of singlet oxygen (IO,)production from 9,lO-dicyanoanthracene (DCA) in the presence of trans-stilbene (TS) was determined by measurement of its 1268-nm emission. The formation of IO2is quenched by TS in acetonitrile but enhanced in benzene. Exciplex emission is observed in both solvents. These observations suggest that exciplex formation in benzene leads to enhanced intersystem crossing in DCA, but that ion-pair separation is too rapid to allow this process in acetonitrile.
Introduction Previous studies indicate that both Type I (electron transfer) and Type I1 (singlet oxygen) mechanisms are possible for dicyanoanthracene-sensitized photooxygenation reaction^.^" The electron-transfer pathway has been shown for the dicyanoanthracene (DCA)/trans-stilbene (TS) system,",5whereas the IO2 pathway has been demonstrated for the oxidation of other olefins.2s637 Spectroscopic evidence has been presented for both mechanisms in the DCA-sensitized photooxygenation of TS. Foote and Spada observed both the trans-stilbene radical cation (TS'+) and the DCA radical anion (DCN-) by transient absorption spectroscopy? DCA- has also been observed by ESR.8,9 The yield of free radical ions in the DCA/TS system was determined by Farid and Lewiss*Io and by transient conductivity.ll Direct production of IO2 by energy transfer from both singlet and triplet DCA has been demonstrated by 1268-nm luminescence (see accompanying paThe mechanisms of DCA-sensitized TS photooxygenation and IO2 production are shown in Scheme I. Manring et al. showed that there is an additional mechanism for the production of IO2 involving interaction of the substrate (S) with singlet cyanoaromatic sensitizers in acetonitrile.]) This mechanism leads to enhanced intersystem crossing for DCA and 9-cyanoanthracene (CA) in acetonitrile, probably via an exciplex, leading to increased triplet production. Heavy-atom-containing and electron-rich substrates enhance IO2 formation by this mechanism with both sensitizers; TS also causes this process with CA but not with DCA in acetonitrile. SantamariaI4proposed that the principal mechanism for IO2 production from DCA is not energy transfer but a substrate interaction similar to that shown ~~~
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(1) Foote, C. S. Tetrahedron 1985, 41, 2221-2227. (2) Araki, Y . ;Dobrowolski, D. C.; Goyne, T.; Hanson, D. C.; Jiang, Z. Q.; Lee K. J.; Foote, C. S.J . Am. Chem. SOC.1984, 106, 4570-4575. (3) Kanner, R. C.; Foote, C. S. J . Am. Chem. Soc.,preceding paper in this issue. (4) Eriksen, J.; Foote, C. S. J . Am. Chem. SOC.1980, 102, 6083-6088. ( 5 ) Lewis, F. D.; Bedell, A. M.; Dykstra, R. E.; Elbert, J. E.; Gould, I. R.; Farid, S.J . Am. Chem. SOC.1990, 112, 8055-8064. (6) Steichen, D. S.;Foote, C. S. Tetrahedron Lett. 1979, 4363-4366. (7) Cao, Y . ;Zhang, B. W.; Ming, Y . F.; Chen, J. X . J . Photochem. 1987, 38, 131-144. ( 8 ) Spada, L. T.; Foote, C. S.J . Am. Chem. SOC.1980, 102, 391-393. (9) Schaap, A. P.; Zaklika, K. A,; Kaskar, B.; Fung, W.-M. J . Am. Chem. SOC.1980. 102.
389-391.
(10) Lewis, F. D.; Dykstra, R. E.; Gould, I. R.; Farid, S.J . Phys. Chem.
1988. 92. 7042-7043
(1'l)Hoyec W.; Foote, C. S. Manuscript in preparation. (12) Dobrowolski, D. C.; Ogilby, P. R.; Foote, C. S. J . Phys. Chem. 1983, 87. - , 2261-2263. - - - -- - - (13) Manring, L. E.; Gu, C.-L.; Foote, C. S . J . Phys. Chem. 1983, 87, 40-44. (14) Truong, T. B.; Santamaria, J. J . Chem. SOC.,Perkin Trans. 2 1987, 20, 1-5. ~
0002-1863/92/15 14-682$03.00/0
Scheme I
TS
'DCA
DCA"
+
TS"
302
ODCA
+
-
lo2
Scheme I1 'DCA
S
'[DCA ...SI
I
ionic products
S
1
5[DCA...S]
so2
1 slngiet oxygen products
01-
by Manring et al.,') which leads to the [DCA'---S'+] radical ion pair. This ion pair can separate, leading to electron-transfer products, or can collapse to yield the triplet exciplex 3[DCA--S] or triplet DCA (3DCA), one of which produces IO2 as shown in Scheme 11. The reaction products of TS have been established for both the electron-transfer and IO2 pathways and are indicative of the active p a t h ~ a y . ~In~ ~the~ 'IO2 ~ reaction, the major initial product is the diendoperoxide; however, the rate of reaction of IO2 with TS is very S ~ O W . ~ The ~ ' ~ electron-transfer pathway yields mainly benzaldehyde, with variable amounts of trans-stilbene oxide and benzil. In this paper, we use the 1268-nm luminescence of IO2 as a function of TS in benzene and acetonitrile as a direct probe of some of these steps. Experimental Section IO2Quantum Yields. The equipment and procedures used are described in the accompanying paper.3 All samples were irradiated at 355 nm. Singlet oxygen luminescence intensities at time zero were corrected to 100% sensitizer absorbance (from 0.8 at 355 nm) and compared to the (15) Kwon, B. M.; Foote, C. S.;Khan, S. I. J . Org. Chem. 1989, 54, 3378-3382.
0 1992 American Chemical Society
Photooxygenation of trans-Stilbene 4.0
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J . Am. Chem. SOC.,Vol. 114, No. 2, 1992 683
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Figure 1. Typical singlet oxygen decay curve sensitized by DCA in air-saturated benzene with no TS, average of 10 laser shots. Inset: Plot of In ( A - A , ) showing extrapolation to zero time ( A = relative intensity).
t\.
Figure 3. aio2vs [trans-stilbene], sensitized by DCA (1.0 X lo4 M), irradiated at 355 nm in air-saturated C6H6.
3.0ii 25
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Figure 2. Quantum yield (@I%)of IO2vs [trans-stilbene], sensitized by DCA (1.0 X M), irradiated at 355 nm in air-saturated CH3CN.
IO2quantum yield (@io2)from DCA in the absence of TS under air, determined by comparison to acridine.16 Exciplex Emission. Visible exciplex emission spectra were recorded on a SPEX Ruorolog 2 fluorimeter, exciting DCA (1.2 X lW7 M) at 355 nm in methylcyclohexane, diethyl ether, benzene, and 1,2-dichloroethane. Steady-State Near-InfraredSpectra Near-infrared spectra were taken on a steady-state detection system built by Dr. N. Haegel (UCLA Department of Electrical Engineering), which consisted of a germanium diode (grown in-house) mounted in a liquid-nitrogen-cooleddewar, with a preamplifier operating at room temperature. The output signal was fed into a lock-in amplifier, digitized, and collected on an IBM PC. Wavelength selection was performed with a l/rm monochromator (PTI) with a computer-controlled stepper motor (2000-pm slits). Second-order spectra were removed by appropriate cutoff filters. Excitation was carried out with a 300-W Xe lamp (Varian Associates). Triplet Yields. The equipment and procedures used are described in the accompanying paper.-' Samples were irradiated at 355 nm in benzene and deaerated by purging with argon for 10 min. SAcridine, monitored at 440 nm, was used as a standard" and compared to the absorbance of -'DCA,I8 monitored at 440 nm as a function of increasing TS concentration. Materials. DCA (Aldrich Chemical Co.) was recrystallized from toluene. TS (Aldrich) was recrystallized from ethanol. Methylcyclohexene (Aldrich) was used as received. Acridine (Aldrich) was recrystallized from toluene. All solvents were spectroscopic grade (Mallinkrodt) or Gold Label (Aldrich). Deuterated solvents were from Cambridge Isotopes. Results A. IO2 Quantum Yields as a Function of TS. A typical IO2 decay curve, DCA-sensitized and obtained from the time-resolved luminescence apparatus (TRL), is shown in Figure 1. The inset of Figure 1 shows the fitted line extrapolated back to zero time, (16) Redmond, R. W.; Braslavsky, S. E. Chem. Phys. Left. 1988, 148, 523-529. (17) Carmichael, I.; Hug, G. L. J . Phys. Chem. ReJ Data 1986, I S , 1-250. (18) Darmanyan, A. P. Chem. Phys. Lett. 1984, 110, 89-94.
800
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1 )O
Wavelength (nm)
Figure 4. Near-infrared emission from DCA (1.0 X lo4 M) in acetonitrile in air- and helium-purged solutions.
giving the initial intensity at the onset of the decay, which is proportional to the IO2 quantum yield (@lo,) (seethe Experimental Section). The IO2 quantum yield (@lo,) was measured in this way as a function of [TS] in both acetonitrile and benzene. In CH,CN, decreases sharply with increasing [TS] (Figure 2). In contrast, @lO2 increases dramatically with increasing [TS] in benzene (Figure 3). B. IO2 Quenching by TS. In the measurement of aio2,IO2 lifetimes decreased appreciably at high TS concentrations in CH3CN. However, IO2 formed by rose bengal sensitization was not quenched by TS in CD3CN at the highest concentrations used. Solubility limited the concentration of TS, so that an exact quenching rate could not be determined. However, the amount of quenching by TS in CH3CN must be even less than in CD3CN, in which IO2 has a much longer lifetime, and therefore cannot be the cause of the decreased lifetime. The quenching rate constant in benzene-d6 is only 1.15 X 1O4 M-I s-I, too small to cause appreciable quenching at the concentrations used. C. Near-Infrared Emission Spectrum. During the course of measuring @lo, as a function of [TS], an initial spike in the time-resolved luminescence was observed with a decay
