The Journal of
Physical Chemistry
0 Copyright, 1983, by the American Chemical Society
VOLUME 87, NUMBER 13
JUNE 23, 1983
LETTERS Chemistry of Singlet Oxygen. 39.+ 9,lO-Dicyanoanthracene Sensitized Formation of Singlet Oxygen Diane C. Dobrowolskl, Peter R. Ogllby, and Christopher S. Foote' Department of Chemlstty and Blochemlstty, Unlverslty of Callfornia. Los Angeles, Callfornla 90024 (Recelved: August 19, 1982; In Fhal Form: May 4, 1983)
Singlet oxygen is the reactive intermediate in some 9,lO-dicyanoanthracene(DCA) sensitized photooxygenations. The limiting quantum yield of singlet oxygen production approaches 2 in benzene and acetonitrile. Singlet oxygen formation was characterized by its emission at 1270 nm and by its reaction with acceptors. There is an increase in '02luminescence intensity under oxygen relative to air. The high yield of IO2 and the oxygen concentration dependence are consistent with energy transfer from 'DCA to 302 to give '02and 3DCA,which subsequently produces a second molecule of IO2.
Much work has been done in the past few years on cyanoaromatic-sensitized photooxygenation reactions.'-8 I t has been demonstrated that phenyl-substituted ethylenes, which are nearly inert to reaction with singlet oxygen, undergo photooxygenation in acetonitrile and other extremely polar solvents via an electron-transfer On the other hand, cyanoaromatic-sensitized photooxygenation of several singlet oxygen-reactive aliphatic olefins, in both polar and nonpolar solvents, yields characteristic product distributions similar to those observed in singlet oxygen reaction^.^ Solvent isotope effects on the rate of photooxygenation of these olefins also suggest the intermediacy of singlet oxygen.6 Previously, Steichen and Foote suggested 9,lO-dicyanoanthracene (DCA) sensitized oxidation of substituted styrenes involved IO2.'Recently, Manring and Foote showed an inefficient route to IO2 involved olefin-enhanced intersystem crossing of DCA.'O 'Paper No. 38 in this series: Gu, C.-L.; Foote, C. S. J . Am. Chem. SOC.1982, 104,6060. 0022-3654/83/2087-2261$01.50/0
We now confirm that singlet oxygen is the reactive intermediate in the DCA-sensitized photooxygenations of at least some 102-reactive olefins, demonstrate that the major pathway for its formation is energy transfer from lDCA to oxygen, and report that the limiting quantum yield of singlet oxygen production approaches 2 in benzene and acetonitrile. Potential reaction and decay routes for lDCA and derived species in the presence of oxygen and substrate (A) (1) Erikson, J.; Foote, C. S.; Parker, T. L. J.Am. Chem. SOC.1977,99, 6455. (2) Eriksen, J.; Foote, C. S. J . Phys. Chem. 1978, 82, 2659. (3) Eriksen, J.; Foote, C. S. J. Am. Chem. SOC.1980, 102, 6083. (4) Spada, L. T.; Foote, C. S. J. Am. Chem. SOC.1980, 102, 391. (5) Goyne, T. E.; Lee, K. J.; Araki,Y.; Jiang, 2.Q., unpublished results. (6) Goyne, T. E.; Araki, Y.; Jiang, Z. Q., unpublished results. (7) Steichen. D. S.: Foote. C. S. J . Am. Chem. SOC.1981. 103. 1855. (8) Schaap, A.P.; iaklika, K. A.; Kaskar, B.; Fung, L. M. J.'Am.Chem. SOC.1980,102, 389. (9) Tsubomura, H.; Mulliken, R. S. J.Am. Chem. SOC.1960,82,5966. (IO) Manring, L. E.; Gu, C.-L.; Foote, C. S. J. Phys. Chem. 1983,87, 40.
0 1983 American Chemical Society
2262
Letters
The Journal of Physical Chemistty, Vol. 87, No. 13, 1983
are shown in Scheme I. There are five possible pathways
Scheme I
hu
DCA 'DCA
'DCA
DCA
4
kf
+~
(1) v F
DCA
(3)
kic
A
kS
3DCA
+A
A
-DCA+A
-
&,A
3DCA
+ 302
kll
A
DCA + lo2
A + '02 7 A02 lo2 k h 302
-
(2)
(9) (10) (11) (12)
(13)
by which singlet oxygen could be generated from DCA. In a spin-forbidden process, singlet DCA could be trapped directly (kSF) by 302 to yield '02and ground-state DCA (reaction 5 ) ; forbidden processes of this sort have often been suggested to be promoted by charge t r a n ~ f e r .Sec~ ond, singlet DCA could undergo direct intersystem crossing (kdto the triplet (reaction 4); this reaction is not a major pathway since the quantum yield of intersystem crossing is only 0.017,'O consistent with the large fluorescence quantum yield of DCA." Alternately, ground-state oxygen might induce intersystem crossing to the triplet, with QCA subsequently sensitizing formation of 1022-14 (reaction 11). Oxygen-induced intersystem crossing of 'DCA could occur either with (km) or without (k'.,) energy transfer to oxygen. Energy transfer can only occur if the singlet-triplet energy gap exceeds 7880 cm-', the energy of singlet oxygen; this process has been shown to occur with several anthracene derivatives.'"20 In the case of energy transfer from singlet DCA to oxygen (reaction 6), a maximum of two molecules of singlet oxygen per molecule of 'DCA can
tum yield of singlet oxygen formation is 1.0. Finally, olefin-enhanced intersystem crossing (ks)to 3DCA (reaction 9) can generate singlet oxygen. As shown by Manring and Foote, a charge transfer interaction of this type, though inefficient, can be an important source of singlet oxygen at low oxygen and high olefin concentrations in polar solvents.1° It is not, however, an important route in this study where conditions were chosen to minimize it.21 In order to determine the limiting quantum yield of singlet oxygen formation from 9,10-dicyanoanthracene,we performed two separate series of experiments. In the first series, the quantum yield of DCA-sensitized photooxygenation of 2-methyl-2-pentene (2M2P)22was determined under air and oxygen atmospheres in benzene and acetonitrile. Solutions of olefin and DCA were irradiated with a Hanovia 1200-W medium-pressure Hg lamp,23the emission of which was measured by Reinecke salt actinom e t r ~ .The ~ ~ reduced 2M2P "ene" products were determined by GLPC with an internal standard. In the second series, solutions of 9,lO-dicyanoanthracene in C6D6and CD3CNunder air and oxygen atmospheres were irradiated at 355 nm with a Nd:YAG laser; '02luminescence was detected at 1270 nmE and was quantified by comparison with that produced from diacenaptho[l,2-b:1',2'4]thiophene (DNT), a sensitizer with a known quantum yield of singlet oxygen production.26 With both DCA and DNT, the 1270-nm luminescence was quenched by addition of (21) The maximum efficiency of olefin-enhancedintersystem crossing was shown in ref 10 to be 4.7% and confirmed in an independent experiment in which solutions of varying substrate concentrations were photolyzed and the oxygenated products determined by GLPC. (22) (a) B a d on the Weller equation,nb electron transfer from 2M2P to DCA is exothermic in acetonitrile. The calculatednb electron-transfer quenching rate, 3 X los M-l s-l, is consistent with the Stern-Volmer derived rate constant for 2M2P quenching of 'DCA, 7 X loBM-l s-1.22a Experimental evidence indicates, however, that electron-transfer auenchinn of 'DCA is not a maior source of oxvgenated Droduct.m 2M2P does not iuench 'DCA appreciably in benze&ym (b) Rehm, D.; Weller, A. Zsr. J. Chem. 1970, 8, 259.
- A&,,)) = 23.06((1.91)22c- (-0.98)22d- 0.06 - 2.8gZzd) -1.38
AG (kcal/mol) = 23.06((E(D/D*) - E(A-/A) - e?/ae
(c) 2M2P half-wave oxidation potential measured vs. SCE. (d) Reference 2. (e) This work. (f) Araki, Y.; Jiang, Z. Q.,unpublished results. (23) The lamp was contained in a water-cooled immersion well surrounded by 1-cm CuSO, solution (30 g of CuS04.5H20 in 1 L of H2O); the 334- and 366-nm lines were isolated with a Coming 7-37 glass filter. Solutions (6.0 X lob M DCA or 2.0 X loa M DNT in benzene; 9.6 X M DCA or 3.9 X lob M DNT in CH3CN 0.0492 M 2M2P), saturated with air or oxygen, were photolyzed for 1-4 h in Bausch and Lomb Spectronic 20 test tubes. Oxygenated products were reduced with PPh3 and determined by GLPC with an internal standard. FA = k~[Al/(k,[A] + kh) was calculated from the following in benzene: kA = 7.2 X 106 M'' TO
S-lmn
= l / k h = 29.6 p P b
in acetonitrile:
be formed. In the case of induced intersystem crossing
kA = 1.2 X lo6 M-I s-128n
without energy transfer (reaction 7), the maximum quanT~
(11) Ware, W. R.; Rothman, W. Chem. Phys. Lett. 1976, 39, 449. (12) Stevens, B.; Ors, J. A. J.Phys. Chem. 1976,80, 2164. (13) Stevens, B. Acc. Chem. Res. 1973,6,90. 1972,94, 7244. (14) Merkel, P. B.: Kearns, D. R. J. Am. Chem. SOC. (15) (a) Darmanyan, A. P. Chem. Phys. Lett. 1982,86,405. (b) Ibid. 1982, 91, 396. (16) Gurinovich, G. P.; Salokhiddinov, K. I. Chem. Phys. Lett. 1982, 85, 9. (17) Wu, K. C.; Trozzolo, A. M. J. Phys. Chem. 1979,83, 2823. (18) Stevens, B.; Marsh,K. L.; Barltrop, J. A. J. Phys. Chem. 1981, 85, 3079. (19) Stevens, B.; Small, R. Chem. Phys. Lett. 1979,61, 233. (20) Brauer, H. D.; Wagener, H. Mol. Photochem. 1976, 7 , 441.
oxygen solubilities
(X
= l / k h = 65 psZeb
M): Cd-4
oxygen air
6.8lgC 1.4529d
CH,CN
8.129e 1.719*
(24) Wegner, E. E.; Adamson, A. W. J. Am. Chem. SOC. 1966,88,394. 1982, 104, 2069. (25) Ogilby, P. R.; Foote, C. S. J. Am. Chem. SOC. (26) The quantum yields of '02production from DNT, determined in the same manner as the DCA-sensitized singlet oxygen quantum yields in the 2M2P experiments, are 0.98 & 0.03 in C6HBand 0.62 0.07 in CH&N.
The Journal of Physical Chemistry, Vol. 87, No. 13, 1983
Letters
TABLE 11: DCA-Sensitized Singlet-Oxygen Quantum Yields from '0, Luminescence Intensity at 1270 nmalb
TABLE I: DCA-Sensitized Singlet Oxygen Quantum Yields from 2-Methyl-2-pentenePhotooxygenation
ro,i,a solvent iO-3M C,H, C,H, CH,CN CH,CN
6.8 1.45 8.1 1.7
solvent
FA^
C6H6
0.52 0.52 0.79 0.79
0.425 i 0.052d 0.148 i 0.013d 0.196 i 0.015e 0.0455 i 0.004ge
1.66 f 0.35 2.03
i
0.37
a Reference 23. Observed quantum yield of ene products. ' D o = (2kET kfbc)/kqox, the limiting quantum
+
yield of ' 0 , at infinite oxygen concentration. determinations. e Five determinations.
Three
2,3-dimethyl-2-butene, further confirming its origin in singlet oxygen. In the presence of substrate, the quantum yield of oxygenated product, @I!:, is related to the quantum yield of singlet oxygen formation, @6bsd,by the equations in Scheme I, provided the substrate reacts with lo2but does not quench it.n Defining 1/7('DCA) = kF + kic + kk, k Ox =k k'k ,:k kqmb= k k/, and F A = k ~ [ A ] / ( k ~ [ i ] k z (where k? and k are the Stern-Volmer lDCA quenching constants for 9O2 and A, and F A is the fraction of '02trapped by the substrate concentration) yields = FA@^^^^ =
+
+
+
2263
+
C6H6
CH,CN CH,CN
r0,i.c
10-3 M 6.8 1.45 8.1 1.7
@ On bsd
1.05 i 0.01 0.464 i 0.014 1.56 * 0.11 0.90 * 0.22
@n
1.58 i 0.08 2.00
* 0.08
a DCA-sensitized luminescence intensity quantified via comparison with DNT-sensitized luminescence.26 Range of two determinations. Reference 23. Observed quantum yield of ' 0 , formation.
I. All errors are 95% confidence intervals. Alternately, when known numerical values are used in eq I11 and @ois calculated directly, the limiting quantum yield of singlet oxygen formation from 'DCA is 1.8 f 0.2 in benzene and 2.1 f 0.2 in acetonitrile. In the absence of substrate, eq I11 reduces to eq IV, relating the observed quantum yield of singlet oxygen formation determined from 1270-nm '02luminescence intensity, @bbSd,to the limiting quantum yield of singlet oxygen formation at infinite oxygen concentration, a0.
@ibe
In benzene, cPk I 0.13%and 2M2P does not quench DCA = 0.017 and fluorescence appreciably;22in acetonitrile, the limiting efficiency of substrate-induced intersystem crossing and subsequent singlet oxygen formation is 14.7%.1° Therefore, at least 87% of the oxygenated product in benzene and 93% in acetonitrile results from oxygen trapping of 'DCA, i.e., (2kET + k'i,c)[302] >> hisc ks[A]. The quantum yield of oxygenated product then becomes
+
I
( ~ K E T + 12 f3O21 k,0x[302]+ kqSUb[A] l/#DCA)
+
J
(11)
Rearranging and setting 90 = ( 2 k + ~ k'iSc)/k,"" ~ gives
Plotting FA/@f$:,determined under air and oxygen, vs the reciprocal of the oxygen concentration yields an intercept which is the reciprocal of the limiting quantum yield of singlet oxygen formation from lDCA at infinite oxygen concentration, a0. A summary of DCA-sensitized singlet oxygen quantum yields from 2M2P photooxygenation is presented in Table (27) 2M2P reacts with, rather than quenches, singlet oxygen. (a) Foote, C. S.;Ching, T.-Y. J.Am. Chem. SOC. 1975,97,6209. (b) Manring, L. E.; Foote, C. S. Ibid.,in press. (c) Gollnick, K.; Kuhn, H. J. In 'Singlet Oxygen"; Wasserman, H. H., Murray, R. W., Ed.; Academic Press: New York, 1979; p 288. (28) DCA fluorescence quantum yield is 0.87 in cyclohexane;" in nonpolar benzene the DCA fluorescence quantum yield is probably sim5 0.13. ilar to 0.87. Therefore, (29) (a) Stem-Volmer quenching experiments. Manring, L. E.; Foote, C. S., J. Am. Chem. SOC.,submitted for publication. (b) Lifetimes determined in this work are slightly longer than those reviously reported,% but agree with other more recent determinations.ga (c) "International Critical Tables"; McGraw-Hill: New York, 1928; pp 254-63. (d) Calculated from oxygen solubility according to Henry's law. (e) Achord, J. M.; Hussey, C. L. Anal. Chem. 1980,52, 601.
A summary of DCA-sensitized singlet oxygen quantum yields determined from plots of eq IV are presented in Table 11. Additional evidence supporting the results of the 2M2P photooxygenations and '02luminescence studies is provided by the relative rate of oxygenated and aerated 2M2P photooxygenation and the relative '02luminescence intensities. For DNT and DCA, the oxygen concentration dependences in each solvent determined by the two methods agreed well and are consistent with the theoretical oxygen concentration effects calculated from observed Stern-Volmer oxygen quenching constants. The results presented here are consistent with the behavior of other anthracene derivatives with a singlet-triplet energy gap greater than 7880 cm-' for which production of singlet oxygen by energy transfer from the singlet state has been demonstrated.lk20 They establish that singlet oxygen can be formed by energy transfer from 'DCA in significant yield. A smaller yield of '02is observed under air relative to oxygen because there is less quenching of 'DCA at the lower concentration of oxygen. When substrate and singlet DCA trapping efficiencies are normalized to loo%, the extrapolated quantum yield of singlet oxygen formation is 1.6 in benzene and 2.0 in acetonitrile. The large quantum yields indicate oxygen quenching of 'DCA occurs almost exclusively with energy transfer, i.e., k'&
