2452
M. V. George, Ch. V. Kurnar, and J. C. Scaiano
The Journal of Physical Chemistry, Vol. 83, No. 19, 1979
Strahlenchemie, MPI fur Kohlenforschung, Mulheim/Ruhr, West Germany. (32) Asada, K.; Kanematsu, S. Agric. Bo/. Chem., 1976, 40, 1891-1892. (33) Bors. W.: Michel. C.: Saran. M.: Lenafelder, E. 2. Naturforsch. C 1978, 33, 891-896. (34) Le Berre, A,; Berguer, Y. C. R. Acad. Sci. 1965, 260, 1995-1998.
(28) Habersbergerova, A.; Janovsky, I.; Kourim, P. Radiat. Res. Rev. 1972, 4, 123-231. (29) Adams, G. E.; Michael, B. D. Trans. Faraday SOC. 1967, 63,
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1171-1180. (30) Neta, P.; Fessenden, R. W. J . Phys. Chem. 1974, 78, 523-529. (31) The experiments were performed by S. Steenken at the Institut fur
Photochemistry and Photooxidation of Tetraphenyl-p-dioxin M. V. George,' Ch. Vijaya Kumar,lb and J. C. Scaiano*la Radiation Laboratory, ",* University of Notre Dame, Notre Dame, Indiana 46556 (Received April 2, 1979) Publication costs assisted by the U.S. Department of Energy
Laser flash photolysis studies of tetraphenyl-p-dioxin have led to the characterization of its triplet state. The T-T absorption spectra shows maxima at 350 and 545 nm; the triplet has a lifetime of 535 ns in methanol and can be quenched by di-tert-butyl nitroxide, paraquat dications, oxygen, and di-tert-butyl selenoketone. The interaction of the triplet with oxygen leads to the formation of singlet oxygen which in turn reacts with the title compound to yield benzil. Introduction In our earlier studies3on the thermal and photochemical transformation of tetraphenyl-p-dioxin ( l ) , we have suggested that the transformations of 1 involve the biradical intermediate 2, which then leads to benzil (3), and minor amounts of tolan (4), Scheme I. We have now examined the photochemistry of 1 in the absence and presence of oxygen by using a combination of quantum yield and nanosecond flash photolysis studies. Combined, these techniques have allowed us to establish the mechanism and kinetics of the photoprocesses involved. We have also been able to characterize the triplet state of 1 from its phosphorescence and T-T absorption spectra. We find that 1 is quite photostable under anaerobic conditions, and that the generation of benzil takes place via the intermediacy of singlet oxygen.
Results This section has been divided according to the experimental technique employed. Laser Flash Photolysis. Excitation of solutions of 1 with the pulses (337.1 nm, -3 mJ, 8 ns) from a nitrogen laser leads to transient absorptions which decay with clean first-order kinetics and a lifetime of 535 ns in methanol and 630 ns in benzene. The lifetimes are concentration independent (0.004-0.0004 M range), indicating that self-quenching is not important, The transient spectrum, observed 10 ns after the laser pulse, is almost identical in both solvents and is shown in Figure 1. Figure 2 shows a typical decay trace and the first-order fitting of the data in the two solvents used. We attribute the observed transient absorptions to the triplet state of 1, on the basis of its behavior toward a variety of molecules (see below). A. and A are the transient absorptions due to triplet 1 immediately after the laser pulse and at time t, respectively. The signals observed can be quenched by di-tert-butyl nitroxide, oxygen, di-tert-butyl selenoketone, and paraquat dications; they cannot be quenched by moderate concentrations of 2,3-dimethyl-2-butene or 2,5-dimethyl2,4-hexadiene. Figure 3 shows the corresponding kinetic plots according to eq 1,where kexptis the first-order rate (1) kexpt - h'expt = k,[Q1 constant associated with the decay of the triplet state, 0022-3654/79/2083-2452$0 1.OO/O
1
0
3
5
TABLE I : Kinetics of Triplet Quenching b y Various Substrates at Room Temperature substrate
solvent
2,5-dimethyl-2,4-hexadiene methanol 2,3-dimethy1-2-butene benzene di-tert-butyl nitroxide benzene di-tert-butyl selenoketone benzene paraquat dications methanol benzene oxygen
~ Q / M - s'- '
< 1 X lo6
< 5 X lo4 5 X lo7 1.8 X l o 8 4.9 x lo9 1 . 7 X lo9
koexpt,the same parameter in the absence of quencher (Le., KOex = q - l ) , and k,, the rate constant for quenching by Q. kinetic data are summarized in Table I. In the case of paraquat dications, 6, the reaction (eq 2) 1*
t H 3 c - t N i ~ - C H a
-
6
with the triplet state results in the formation of the radical 0 1979 Arnerlcan Chemical Society
Photochemistry of Tetraphenyl-pdioxin i
I
The Journal of Physical Chemistry, Vol. 83,
I
I
I
I
500
600
.
400
I
1
2500
I
I
I
1
4
I
I
8
6
I
2453
1 , l l
IO
[TME] / [dioxin]
Figure 4. Plot, according to eq 7, for 1 in benzene at r m m temperature.
Figure 1. Transient spectrum observed by laser irradiation of 1 in methanol at room temperature.
250
I
2
1
X,nm
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I
No. 79, 1979
1000 .
750
1250
1500 0
TIME ( n s ) Figure 2. First-order plot for the decay of triplet 1 in benzene (A) and in methanol (0). The insert shows the typical decay trace in methanol, as monitored at 420 nm.
A and A , are the transient absorbances due to 7 (monitored at 603 nm) at time t and after the completion of the reaction (usually 2-3 p s ) , respectively. A few exploratory experiments were also carried out with the sulfur analogue of 1, tetraphenyl-p-dithiin. Weak transient absorptions (presumably from the triplet state) were detected a t 475 nm and decayed with a lifetime of 75 ns. The signals were too weak to allow a detailed study. Luminescence. At 77 K, 1phosphoresces rather strongly in both solvents, benzene and methanol. In the latter, where the spectrum is better resolved, we measured X(0,O) = 516 nm, which corresponds to a triplet energy of 55 kcal mol-'. The corresponding triplet lifetimes are 2.1 and 4.5 ms in benzene and methanol, respectively. At room temperature in benzene we observe weak luminescence ,A( = 391 nm), which presumably is due to fluorescence. Quantum Yields and Product Studies. Solutions of 1 ~ temperature are quite photostable under anaerobic at room conditions (quantum yield of consumption I0.0004) while efficient photodecomposition takes place in the presence of oxygen. The photooxidation of 1 leads to the formation of benzil; for example, a 0.01 M solution of 1 in oxygensaturated benzene produces benzil with a quantum yield of 0.27. The reaction can be entirely quenched by 2,3dimethyl-2-butene (TME). For example, no benzil is generated in an oxygen-saturated solution of 1 in 50:50 benzene:TME. Laser photolysis experiments show that this concentration of TME does not have any significant effect on the triplet lifetime. The effect can be easily understood if we assume that the reactive species is singlet oxygen, produced in the triplet quenching process, i.e.
-+ + + 1*
+ 02
'02 1 IO2
1
102
products (benzil)
TME
dioxetane
(4) (5) (6)
Kinetic analysis of reactions 4-6 leads to eq 7 (assuming [Conc] ,mM
Figure 3. Plots according to eq 1 for various triplet quenchers: (0) paraquat dications; (+) oxygen; (A)di-tert-butyl selenoketone; (V) di-tert-butyl nitroxide; and ( H ) 2,5-dimethyl-2,4-hexadiene.
cation, 7, and, presumably, the radical cation of 1 as well.* In this case the experimental pseudo-first-order rate constant, k , t, was determined from an analysis of the time profile for tke buildup of 7, according to eq 3. In
A, A, - A
___ =
hex&
(3)
k6
[TME]
k5
[I1
@BO -=1+-----
@B
(7)
that the nonreactive decay of can be ignored), where @Bo and @B are the quantum yields of benzil formation in the absence and presence of TME, respectively. The corresponding Stern-Volmer plot is shown in Figure 4. Taking k6 from the literature5 as -4 X lo7 M-ls-l, we estimate k5 = 1.5 X lo7 M-I s-l. In experiments in the absence of TME, we have examined the effect of oxygen concentration on the yield of
2454
The Journal of Physical Chemistry, Vol. 83, No. 19, 1979
M. V.
George, Ch. V. Kumar, and J. C. Scaiano
benzil. Equation 8 shows the expected Stern-Volmer
behavior, based on relative yields, where a is an arbitrary constant (from the absolute value of aBfor [I] = 0.01 M, we estimate a = 2.15). The corresponding plot is shown in Figure 5 and leads to k4rT 960 M-I, in good agreement with the values of TT and k4 [k4 = k , (for oxygen)], obtained by laser photolysis, which confirms the assignment of the triplet state as the reactive species. Finally, prolonged irradiation of oxygen-saturated solutions of l in benzene leads to the formation of minor amounts (14%of the amount of benzil) of tolan (isolated in earlier studies3) and phenyl benzoate, characterized by gas chromatography-mass spectrometry.6
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Discussion The fact that TME can quench the photooxidation 1 when it cannot quench the transient species of Figure 1 strongly suggests the involvement of singlet oxygen in the reaction, i.e., quenching of the photooxidation is the result of the well-documented5 reactivity of IO2toward TME.7 The transient of Figure 1 is then the precursor of singlet oxygen and is assigned to the triplet state of I, 1". Dienes do not quench 1" because its low triplet energy, derived from the phosphorescence spectrum, makes the process energetically unfavorable. Di-tert-butyl selenoketone is a rather efficient quencher (see Table I) as a result of its low triplet energy, which has been suggested to be as low as 40 kcal mol-1.8 Di-tert-butyl nitroxide is an efficient triplet q u e n ~ h e r . ~ J ~ The relatively low rate of interaction observed presumably reflects the low triplet energy; similar effects have been observed in other systems.1° Paraquat dications are good electron acceptors and have received wide use as biradical traps.4 There is evidence that excited triplet states, e.g., carbonyls4and olefins,i1will also transfer an electron to yield the reduced radical cation. The fact that the species trapped by paraquat and the one shown in Figure 1 are one and the same is supported by the fact that the data derived from the buildup of 7 extrapolate well to the point expected from the lifetime of l*. The mechanism that we propose for the photooxidation of 1 is shown in Scheme 11. The total yields of benzi! at high conversion never reach a 2:l molar ratio with respect to the initial amount of 1, though they systematically exceed a 1:l ratio.3 The difference can be largely accounted for by the formation of phenyl benzoate via the oxidation of benzil in benzene medium. It may be mentioned in this connection that the photolysis of benzil in different solvents such as cyclohexane, 2-propanal, and cumene has been studied earlier by Bunburry and co-workers.12l3 They have observed that a variety of products are formed in these reactions. Finally, all our attempts to measure the kinetics for the formation of the triplet state of 1 from ita singlet state were unsuccessful which, given the time resolution of the instrument used, means that the singlet state of 1 must live less than 8 ns. In conclusion, UV irradiation of 1 in solution leads to the formation of a relatively short-lived triplet state (71' = 630 ns in benzene) which under anaerobic conditions undergoes nonradiative decay. The triplet state of 1 can be quenched by a variety of substrates; in the case of oxygen, the interaction leads to the formation of singlet oxygen which in turn reacts with I to yield benzil.
Figure 5. Effect of oxygen concentration on the ybkl of benzil, accwding to eq 8.
Scheme I1
Experimental Section Starting Materials. Tetraphenyl-p-dioxin (l),mp 218 "C, was prepared by a reported p r ~ c e d u r e . ~Di-tert-butyl J~ selenoketone was prepared by the reaction of di-tert-butyl ketone triphenylphospharanylidene hydrazone with selenium metal.15 The di-tert-butyl nitroxide employed was an Eastman product, whereas both 2,5-dimethyl-2,4hexadiene and 2,3-dimethyl-2-butene were Aldrich products. Paraquat hydrochloride was a K & K product and was purified as indicated in earlier report^.^ The solvents used were Aldrich, Gold-Label. Laser Photolysis. The system makes use of a Molectron WV-400 nitrogen laser for excitation. The signals from an RCA-4840 photomultiplier tube were terminated into 93 ohm and into a Tektronix R7912 transient digitizer. The system ha5 been interphased to a PDP 11/55 computer which controls the experiment, averages the information from several traces, and processes the data. Further details will be given elsewhere.16 Phosphorescence Lifetimes. The samples were contained in quartz tubes, immersed in liquid nitrogen in a Dewar with Suprasil windows. The samples were excited with the same laser mentioned above, and the signals from an RCA-1P28 detector were terminated into 1000 ohm and into a Tektronix 7623 storage oscilloscope. The corresponding traces were recorded photographically. Quantum Yield and Product Studies. The samples were contained in matched tubes made of precision bore tubing (0.2500 f 0.0002 in. id., Corning 7740 glass, Lab Crest Scientific). When the samples contained oxygen, the same precautions as in earlier work were taken in order to ensure that the gas-liquid equilibrium was maintained at all times.17 Analysis were carried out by gas chromatography with a Beckman GC-5 instrument, equipped with flame ionization detectors. Depending on the type of analysis, we
H-Bonding of Amine with Phenols
The Journal of Physical Chemistry, Vol. 83, No. 79, 1979 2455
used a SE-30 Silicone oil column or an Apiezon L column. Gas chromatography-mass spectrometric studies were carried out with a DP-1 mass spectrometer system (Du Pont Instruments). Spectroscopy. UV-visible spectra were recorded with a Cary-219 instrument. Emission spectra were obtained with an Aminco-Bowman spectroflurimeter.
based on the original value from Algar, B. E.; Stevens, B. J . Phys. Chem. 1970, 74,3029-3034. (6) The mechanism for the formation of phenyl benzoate is unclear; however, irradiation of authentic samples of benzil in oxygen-saturated benzene leaves no doubt that this is the major product of the reaction. (7) A related molecule, 2,3-diphenyl-p-dioxene, is known to react with singlet oxygen, ultimately leading to carbonyl-containing products: Schaap, A. P.; Thayer, A. L.; Blossey, E. C.; Neckers, D. C. J. Am. Chem. SOC.1975, 97, 3741-3745;Srinivasan, V. S.; Podolski, D.; Westrick, N. J.; Neckers, D. C. J. Am. Chem. SOC. 1978, 100,
Acknowledgment. Thanks are due to Dr. J. Brummer for valuable suggestions.
6513-6515. (8) Scaiano, J. C. J. Am. Chem. SOC. 1977, 99, 1494-1498. (9) Schwerzed, R. E.; Caldweli, R. A. J . Am. Chem. SOC. 1973, 95, 1382-1389;Webs, D. D. J. Photochem. 1976/77, 6 , 301-304; Formosinho, S.J. Mol. Photochem. 1976, 7, 13-39. (10) Watkins, A. R. Chem. Phys. Lett. 1974, 29, 526-528;Gijzeman,
References and Notes (1) (a) Radiation Laboratory, University of Notre Dame. (b) Department of Chemistry, Indian Institute of Technology, Kanpur, India. (2) The research described herein was supported by the Office of Basic Energy Sciences of the Department of Energy. This is Document No. NDRL-1990from the Notre Dame Radiation Laboratory. (3) Lahiri, S.;Dabral, V.; Bhat, V.; Jemmis, E. D.; George, M. V. Proc. Indian Acad. Sci. Sect. A 1977, 86, 1-14. (4) For related references on the technique see: Small, Jr., R. D.; Scaiano, J. C. J . Phys. Chem. 1977, 81, 828-832,2126-2131;1978, 83,
2662-2664. (5) Brummer, J.; Wilkinson, F. J. Phys. Chem. Ref. Data, to be published:
0. L. J.; Kaufman, F.; Porter, G. J. Chem. SOC., Faraday Trans. 11973, 69, 727-737. (11) Caldwell, R. A.; Pac, C . Chem. Phys. Lett. 1979, 64,303-306. (12) Burnburry, D. L.; Wang, C. T. Can. J. Chem. 1968, 46,1437-1479. (13) Burnbuny, D. L.; Chuang, T. T. Can. J. Chem. 1969, 47, 2045-2055. (14) Mladelung, W.; Oberwegner, M. E. Annales 1936, 526, 245. (15) Beck, T. G.; Barton, D. H. R.; Britten-Kelly, M. R.; Guziec, Jr., F. S. J. Chem. SOC., Chem. Commun. 1975, 539. (16) Patterson, L. K.; Scaiano, J. C., to be published. (17) Small, Jr., R. D.; Scaiano, J. C. J. Am. Chem. Sac. 1978, 100,
45 12-45 19.
Hydrogen Bonding Interactions of Aliphatic Amines with Ortho-Substituted Phenols Louis Farah, George Giles, Donna Wilson, Agnes Ohno,+ and Ronald M. Scott" Department of Chemistty, Eastern Michigan University, Ypsilanti, Michigan 48 197 (Received February 22, 1979)
Hydrogen bond formation between aliphatic amines and ]phenolsis not inhibited by a single ortho substitution, but is prevented when both ortho positions are substituted. Tertiary aliphatic amines are somewhat hindered sterically in forming hydrogen bonds when compared to primary and secondary aliphatic amines.
Introduction Spectrophotometric analysis has been used to study hydrogen bonding of a number of phen01s.l-l~ In one of these studies,13 the hydrogen bonding of p-chlorophenol with a variety of amines was studied in cyclohexane solution. It was found that a linear relationship exists between the log of the equilibrium constant for the formation of the hydrogen bond and the strength as Bronsted bases of primary and secondary aliphatic amines as represented by their aqueous pK,. Tertiary aliphatic amines also display a linear relationship between log K and aqueous pK,, but one whose slope indicates that hydrogen bond formation is less favored. Comparative studies were performed with N-ethylmorpholine and triethylenediamine, tertiary amines in which the nitrogen is a member of a ring so that alkyl groups are held back from interfering sterically with the phenol. N-Ethylmorpholine is more reactive in hydrogen bonding to phenol than its aqueous pK, would predict for a tertiary amine, and triethylenediamine was comparable to primary and secondary amines in reactivity. Steric hinderance was therefore described as the cause of the low equilibrium constants associated with the normal aliphatic tertiary amines. The data reported here are a continuation of the investigation of phenol-amine hydrogen bonding. The experiments run with p-chlorophenol are repeated with three more phenols: o-cresol, o-sec-butylphenol, and 'Department of Biological Chemistry, University of Michigan, Ann Arbor, Mich. 48106. 0022-3654/79/2083-2455$01 .OO/O
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p-cresol. These phenols were selected because they are very similar in acid strength (pK, 10.2) but differ in substitution a t the ortho position. By comparison of results the steric effect of ortho substitution could be assessed.
Experimental Section Sublimation was used to purify o-cresol, p-cresol, 2,6dimethylphenol, and triethylenediamine. The remainder of the reagents and cyclohexane were purified by distillation. Phenol and amine stock solutions were prepared by weighing both solute and solvent. Solutions were prepared by pipetting aliquots into volumetric flasks, and weighing after e,ach addition. The order of addition was cyclohexane, phenol stock, amine stock, and cyclohexane to the mark. In each study the phenol concentration was held constant and amine concentration was varied. Absorption spectra were obtained with a Beckman Model DK-2A spectrophotometer with matched 1-cm silica cells. The temperature of both sample and reference cells was controlled by circulating water from a Lauda K-2/R constant temperature bath through the thermospacers of a brass cell holder similar to that described by Coggeshall and Lang.ll The spectra of the solutions were recorded from 240 to 315 nm. Each run consisted of at least five different temperatures ranging from 15 to 55 "C. All calculations of equilibrium and thermodynamic parameters were performed on the DEC 10 computer of Eastern Michigan University. Programs in Fortran IV
0 1979 American
Chemical Society
