3468
B. STEVENS AND B. E. ALGAR
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carbon reaction, a direct comparison between the present results and the C2H4 system cannot be made. The thermal decomposition of c-C4Hs has been investigated by Walters and coworkers17-1a and Pritchard and coworkers.20 The first-order Arrhenius rate constant, ko, for the decomposition of c-C4Fs is reported to have a frequency factor of 1015-71 sec-' and an activation energy of 62.8 kcal/mol.l4 Butler4 has reported that the frequency factor and activation energy of the unimolecular rate constant for the decomposition of cC4FS are 10l6J sec-l and 74.3 kcal/mol, respectively. The two unimolecular frequency factors are in good agreement. However, the activation energy for the perfluoro compound is 11.5 kcal/mol higher than the
analogous hydrocarbon. Since the c-CIF~ is more stable than c-C4Hs,this result is reasonable. Acknowledgment. This research was sponsored by the Air Force Office of Scientific Research, Office of Aerospace Research, United States Air Force, under Grant No. AF-AFOSR-1144-67. (17) Fa Kern and W. D. Walters, Proo. Nat. Acad. Sei. U.S., 43, 937 (1952). (18) C. T . Genaux and W. D. Walters, J . Amer. Chem. SOC.,73, 4497 (1951). (19) C. T. Genaux and W. D. Walters, ibid., 75, 6196 (1953). (20) H. Pritchard, R. Sowden, and A. Trotman-Dickenson, Proc. Roy. SOC.,A218,416 (1953).
The Photoperoxidation of Unsaturated Organic Molecules. 11. The Autoperoxidation of Aromatic Hydrocarbons by B. Stevens1 and B. E. Algar Department of Chemistry, Shefield University, Shefield, England
(Received March 86, 1068)
The quantum yields of autoperoxidation of 9,10-dimethylanthracene19,10-dimethyl-l,2-benzanthracene, naphthacene, and rubrene have been measured as a function of the dissolved oxygen concentration down to 2.6 X M in benzene at 25". An analysis of the data in terms of the participation of an 0 # A g ) intermediate provides estimates of the triplet-state formation yields of the substrates and leads to the conclusion that Oz(1A,) is produced solely by oxygen quenching of the aromatic hydrocarbon triplet state, which is a yield-limiting process at very low oxygen concentrations.
Introduction The over-all photosensitized peroxidation2 of an unsaturated organic molecule XI in the presence of molecular oxygen may be represented by the process
where the sensitizer S absorbs the incident radiation but remains chemically unchanged, and the peroxide or hydroperoxide NO2 is the sole product. Three types of substrate, 31, subject to this process are: (a) those molecules containing an isolated double bond with a @-hydrogen atom,3 e.g.
CHa
CHs
CH3
CH3
(b) cyclic conjugated he~adienes,~ of which a-terpinene is a classic example5
I
CH3 CH3
CH3 CH,
(c) the larger catacondensed aromatic hydrocarbons, (1) Department of Chemistry, University of South Florida, Tampa, Fla. 33620. (2) The term peroxidation is used here to distinguish the addition of molecular oxygen from oxidative electron and H atom transfer processes. (3) G. 0. Schenck, Angew. Chem., 69, 579 (1957). (4) R. N. Moore and R. V. Lawrence, J . Amer. Chem. Soc., 80, 1438 (1958). (5) G. 0. Schenck and K. Ziegler, Naturwissenschaften, 32, 157 (1944).
The Journal of Physical Chemistry
3469
THEPHOTOPEROXIDATION OF UNSATURATED ORGANIC MOLECULES e.g., anthracene, naphthacene, 1,2-benxanthracene, and
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their derivatives
which are capable of acting as the sensitizer in a process of autoperoxidation.6 Previous studies of the reaction characteristics have shown that the reciprocal quantum yield of the over-all process is a linear function of the reciprocal substrate concentration71~ and that the rate of reaction is directly proportional to the absorbed light intensity,* which eliminates processes involving more than one electronically excited species. The simplest general scheme consistent with these observations is
S + hv S*+S
+S*
(+hv)
S* + 02 +T
T
+ n4 -+i\4o2+ s T+S+02
(4 (b)
(4 (d)
which leads to the photostationary expression (eq I) for the over-all quantum yield, y ~ o but ~ , which raises
OZ(’Ag)state, is directly involved in the photosensitized addition of molecular oxygen to an unsaturated substrate. This may be formed in a process of electronic energy transfer from either the excited singlet or triplet states of the sensitizer in the following exothermic, spinallowed quenching processes lS*
+ 30z
(V°F
3S*
--f
-
VOP
+
IO2*
(e)
> 8000 em-l)
+ 302
----t
(voP
%*
lS
+ lo2*
(f)
> 8000 cm-l)
under the conditions stated parenthetically, where VOF and vop denote 0”-0’ fluorescence and phosphorescence band frequencies, respectively. Evidence for the multiplicity of the participating electronic state S* of the sensitizer is based largely on the dependence of yMo2 on the concentration of dissolved oxygen. Thus Gollnick and SchenckQfound that for the dyesensitized peroxidation of 2,5-dimethylfuran in methanol dyfiIo,/d[02] = 0 a t concentrations of dissolved & orI with , kb = 1O1O oxygen greater than 2 X M-l sec-l (eq I), the actual lifetime T S * of the electronically excited sensitizer is limited by TS*
= l/ka
>> 1/Kb[O2] =5
X 10-8 sec
(process indicative of a triplet-state precursor of loo2 f ) ; in these systems process e is in any case inoperative, since the necessary condition (VOF - v0p > 8000 ern-’) is not fullfilled for the sensitizers used. On the other hand, Livingston and Raoz2reported a
questions concerning the nature of the transferring species, T, and the multiplicity of its electronically excited precursor, S*. It has been suggested that the intermediate, T, is (6) C. Moureau, C. Duffraisse, and P. M. Dean, Campt. Rend., 182, 1440, 1584 (1926); c f , W. Bergmann, and M. J. McLean, Chem. Rev., either a sensitizer-oxygen complex, possibly of signifi28, 367 (1941). cant charge-transfer character,1° or a singlet excited (7) W-.Koblitz and H. J. Schumacher, Z. Phys. Chem. (Leipzig), state of molecular oxygen” which may be either 0,B35, 11 (1935); B37, 462 (1937). (‘B,+) or O2(lAP)at 13,300 or 8000 cm-l above the (8) E. J. Bowen and D. W. Tanner, Trans. Faraday Sac., 51, 475 (1955). 02(32,-)ground state.12 Although to date no spectro(9) A. Schonberg, Ann., 518, 299 (1935); cf. K. Ciollnick and G. scopic evidence is available for either species in a system 0. Schenck, Pure A p p l . Chem., 9, 507 (1964). undergoing photoperoxidation, identical product^'^^^^ (10) H. Tsubomura and R. S. Mulliken, J. Amer. Chem. Sac., 82, with similar isomeric distributions16 are obtained from 6966 (1960). (11) H. Kautsky and H. de Bruijn, Araturwissenschaften, 19, 1043 reactions of the substrate with O,(lA,) produced by an (1931). electrodeless discharge in gaseous 0 2 1 6 or by the alkaline (12) Cf. G. Herzberg, “Spectra of Diatomic AMolecules,” D. Van H202-NaOC1 reaction1’ in which the presence of 02- Nostrand Co., Ino., New York, N. Y., 1950. (‘Ag) is established in emission. Kinetic studiesl8~lg (13) E. J. Corey and W. C. Taylor, J, A m e r . Chem. Sac., 86, 3881 (1964). of the competitive photoperoxidation of two substrates (14) C. S. Foote and S. Wexler, ibid., 86,3879 (1964) ; E. McKeown have shown that the relative rates of peroxide formation and W. A. Waters, J. Chem. Sac., B , 1040 (1966). are independent of the sensitizer used, contrary t o ex(15) C. 8. Foote, S. Wexler, and W. Ando, Tetrahedron Lett., 46, 411 (1965). pectation for a complex intermediate of varying stabil(16) J. S. Arnold, E. A. Ogryslo, and H. Witzke, J. Chem. Phys., ity, while the intergranular migrationz0 and diffusion 40, 1769 (1964); J. S. Arnold, R. J. Brown, and E. A. Ogryzlo, of the transferring species in ethylcellulose films, 21 Photochem. Phofobiol., 4, 963 (1965). necessary to account for photosensitized peroxidation (17) A. H. Kahn and M.Kasha, J. Chem. Phys., 39, 2105 (1963). under conditions where molecular transport is limited, (18) K. R. Kopecky and H. J. Reich, Can. J. Chem., 43, 2265 (1965). are also inconsistent with the assignment of a sensitizer(19) T. Wilson, J. Amer. Chem. SOC.,88, 2898 (1966). oxygen complex to the role of intermediate. (20) H. Kautsky, Trans. Faraday SOC.,35, 216 (1939). The available evidence strongly supports the sug(21) J. Bourdon and B. Schnuriger, Photochem. Photobid., 5, 607 gestion that a singlet oxygen molecule, particularly the (1966). Volume W 9Number 10 October 1968
3470
B. STEVENS AND B. E. ALGAR
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dependence of Y M O ~on the dissolved oxygen concentration for the photoperoxidation of 9,lO-diphenylanthracene in benzene (YF = 1) but not for anthracene in bromobenzene (YF = 0) and concluded that the intermediate is formed from the singlet or triplet state of the sensitizer depending on whether this has a high or low fluorescence yield, YF.2 3 This article reports an investigation of the autoperoxidation of four aromatic hydrocarbons over a wide range of dissolved oxygen concentrations interpreted in terms of a singlet oxgen intermediate with a view to establishing the multiplicity of the sensitizer electronic state from which it is formed.
Experimental Section Materials. The sources and methods of purification of the sensitizers, substrates, and solvent are described in part Quantunz Yields. The quantum yields of peroxidation are obtained from measurements of light transmission, I t , of the substrate as a function of time as follows. A parallel beam of filtered radiation at wavelength ,A, from a stabilized 125-W high-pressure mercury arc (Mazda I\fBL/D) was directed onto a plane face of a cylindrical quartz cell, having the same diameter as the light beam, a volume, V , of 3 ml, and a depth, d, of 1 cm, containing the substrate solution through which an 02-N2 mixture of prearranged composition was continuously bubbled from a narrow tube through the neck of the cell; this provided both stirring of the exposed solution and a constant concentration of dissolved oxygen. The transmitted beam was focusedonto the slit of a small Hilger grating monochromator set at ,A (to eliminate solute fluorescence) and was fitted with a Mazda 27A.13 photomultiplier at the exit slit; the photocurrent was earthed through a 7.5 kohm load resistance across which the potential difference was fed into a Brown potentiometric recorder (0-10 mV) with a variable chart speed to obtain recordings of I , at he, as a function of exposure time. Following the initial measurement of light transmitted by the cell and solvent only, a backing-off voltage was applied to the potentiometer to allow amplification of the change in I,. A computer program was written to obtain the quantum yield of substrate consumption yu from the slopes of the recorded curves given by -d In ( I t P o ) - In ( I J I o ) - In (IttiIo) dt t’ - t -N-
Over the short tirne interval t! - t as a function of substrate concentration expressed as
[MI = (lied) In ( l o / I t ) using the relationship The Journal of Physical Chemistry
Y M = YMO* =
-Io
1
dnM - It dt
where N is Avogadro’s number and E denotes the molar extinction coefficient of the substrate at the monitoring (actinic) wavelength hex. The absolute incident light intensity lo, measured by ferrioxalate actinometry26 before and after each run, was found to vary by less than 2% over a period of 2 weeks. In all cases h e x was chosen to avoid absorption by and possible photodecomposition of the peroxide produced, i.e., ,A, 365 mp for 9,lO-dimethylanthracene (DATA) and 9,10-dimethyl-1,2-benzanthracene (DMBA) and Aex 435.8 mp for rubrene and naphthacene. Concentrations of dissolved oxygen were stabilized by flowing 02-Nz mixtures through the solutions for a period of at least 1 hr prior to and during exposure and were estimated from the partial pressure of 02 in the flowstream, which could be varied from 1 to 0.0004 atm by the use of calibrated flow meter^.^^ Variations in the incident light intensity were effected by placing wire-mesh screens of known transmission characteristics between the light source and the cell.
Results Photodimerization, photodecomposition, and photochemical reactions of the solute with the solvent (benzene) were shown to be of negligible significance at the solute concentrations employed (L10-3 ivr) by exposure of deoxygenated solutions for periods of time in excess of those required for photoperoxidation to approach completion; in the absence of dissolved oxygen, the optical density of the solute remained unchanged. Absorption spectra of all substrate solutions in airsaturated cyclohexane, recorded as a function of exposure time, were found to exhibit isosbesticpoints at the: following wavelengths Aiso.
Xisot
rnp
Rubrene
Naphthaoene
DMA
DMBA
263-264
258-259
233
250
The final spectrum in the case of rubrene corresponded to that reported for rubrene peroxide26with peaks at 242 and 290 mfi. This evidence confirms the linear rela-
(22) R. Livingston and V. S. Rao, J . P h y s . Chem., 63, 794 (1959). (23) Although both groups of investigatorsg,2a interpret their data in terms of a sensitizer-oxygen complex intermediate, this does not
invalidate the conclusions reached, since the complex is kinetically indistinguishable from singlet oxygen in the approximation of processes a-d. (24) B. Stevens and B. E. Algar, J . Phys. Chem., 72, 2582 (1968). (25) C. G. Hatchard and C. A. Parker, Proc. Roy. Soc., A235, 518 (1956). (26) G. M. Badger, R. S. Pearce, H. J. Rodda, and I. S. Walker, J . Chem. Soc., 3151 (1954).
3471
THEPHOTOPEROXIDATION OF UNSATURATED ORGANIC MOLECULES
..
0
D ?
0
Q
Q
Q&@O
0
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10
00
0
0
10
1
t7 If
4
8
12 x Id
Figure 1. Dependence of the autoperoxidation quantum yield on the substrate concentration a t various incident light intensities, 10: A, naphthacene; B, rubrene; C, DMA; D, DMBA; 0, lo = 1.01; 0, 10= 0.751; 0,1o = 0.501; 0, 10= 0.251. [OZ] = 3.65 X loW3 M . The solvent is benzene a t 25'.
Figure 3. Dependence of the autoperoxidation quantum yield on the concentration of dissolved oxygen in benzene a t 25' for 10-4 M DMBA (0)and 4 x M naphthacene ( 0 ) : solid curves, drawn according t o eq V with tabulated values of rate parameters; dashed lines, extrapolation of low oxygen concentration data in Figure 4.
40 r
30
Figure 2. Dependence of the autoperoxidation quantum yield on the substrate concentration in the absence (0)and presence ( 0 )of added peroxide a t 5 X 10-4 M : A, naphthacene; B, rubrene; C, DMA; D, DMBA. [O,] = 3.65 X M. The solvent is benzene a t 25'.
-
I
1
I
1
0
10
20
30
xrdt
Figure 4. Dependence of the autoperoxidation quantum yield on the concentration of dissolved oxygen in benzene a t 25" for lo-' M DMBA ( 0 )and 4 x M naphthacene ( 0 ) : solid curves, drawn in accordance with eq V.
the incident (or absorbed) light intenshy, as previously reported,s and of the product (peroxide) concentration, confirming that the peroxide decomposition is unimportant during the observation period. Moreover, in d [MOz]/dt = -d [M]/dt agreement with published ~ / Y M O is ~ a linear and the estimation of the quantum yield ( 7 ~of ~per~ ) function of the reciprocal substrate concentration, as oxidation is based. As shown in Figures 1 and 2, Y M O * is independent of (27) M. D. Cohen and E. Fisoher, J. Chem. Soc., 3044 (1962). tionship between the reactant and the product concentrationZ7on which the rate of peroxide formation, given by
Volume 72, Number 10 October 1968
3472
B. STEVENSAND B. E. ALGAR 50
r
substrate from Fo to F in the presence of dissolved oxygen at the same concentration.
0
Fo/F = (ki
+ kz + ks + k4[0z] +
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k~[Oz])/(ki
+ kz + ha)
(111)
Higher Oxygen Concentrations. At concentrations of dissolved oxygen where fluorescence quenching is experimentally significant, it is assumed that k6
Figure 5. Dependence of the autoperoxidation quantum yield of rubrene (0)and of DMA ( 0 )on dissolved oxygen concentration: solid lines, drawn from eq VI11 with kz = 0; dashed line, drawn according to eq VI11 with an intersystem-crossing yield of 0.02 for rubrene.
[o,~ -0 d [(Fo/F) - 1I/d(rMOaFO/F) = both energy-transfer processes 5 and 7 and colliYISP/(a! P) sion-induced intersystem-crossing processes 4 and 8 are collected in Table I together with valuesz4for the w * -+n4 hVF (1) corresponding fluorescence yield, Y F , in the same dein/r* -+ 3119* (2) oxygenated solvent. An examination of the experimental findings subject 1M* -+ A I (3) to the condition 1M* 3 0 z + (4) [YIsP/(a PI1 ['YISa/(a P)1 ?'IC YF = 1 ini* so2-+ 3ni* ioz* (5) shows that in the case of (a) DMBA 3n4* --f M (6) [rxsa/(. P)] YIC = 0.00 0.09 w* 3 0 2 -+na io2* (7) L e . , (i) the internal conversion efficiency defined as 30z +M 302 (8)
+
+
+
+
+ +
+ +
+ +
+ +
n/r + 'Oz* +MOZ
+ + +
+ +
+ +
')'IC
(9)
=
k3/(kl
*
+ + k2
k3)
(10)
is zero within the combined limits of error, and (ii) a o ( c ) naphthacene TIC
.+ y1sa/(a + p) = 0.20
f
0.09
providing the alternative interpretations: (i) a
