J . Phys. Chem. 1990,94, 4540-4549
4540
OH source was placed about 20 cm upstream from the OH + SO2
reaction zone in order to allow ample time for the vibrationally excited O H to relax. The killHe determined at 298 K by using this source is within experimental uncertainties of those obtained by using the H NO2 OH + NO reaction. However, the effect on k1'102due to excited OH radicals cannot be positively ruled out. Although H 0 2 reacts with 0, to form O H H 0 2 + 0, OH + 2 0 2 (17)
+
-
-
the slow rate constant k17 = 1 . 1 X exp[(500!:$')/T] cm3 molecule-' S-I suggests that OH regeneration from reaction 17 is unimportant.28 The relative rates of the reactions O H + SO2 + M HOS02 + M (slow) (1)
-
-+
H O S 0 2 + O2
SO,+ H 0 2 (rapid)
(2)
suggest that the rate constant observed under our experimental conditions was mostly due to k l because k2 = 1.34 X exp(-330/T) cm3 molecule-' s-I is much larger than k,[M]," making (29) Charters, P.
E.;Macdonald, R.G.;Polanyi, J. C. Appl. Opt. 1971,
10, 1747, and references therein.
reaction 1 rate-determining. Our estimate (1.0-1.8)X cm6 molecule-2 s-I is smaller than that reported by Leu, (2.46f 0.32) X IO-,' cm6 molecule-2 s-l, using H + NO2 as an OH source. Although the efficiencies for M = N, and O2in many termolecular reactions have been reported to be approximately equal, there are cases in which the efficiency for M = O2is as low as 70% of that for M = N2.,0 Considering the large uncertainties in these measurements, the observed rate coefficient for M = O2 is not beyond this limit. The low estimated value suggests that further study on this reaction in O2 is called for. In conclusion, the temperature dependence of kl1lHeand kI1lSO2 for the title reaction at low pressure has been accurately determined, and the results are in reasonable agreements with the only previous study by Leu. Our experiments also provided the study of killN2 and k11102 at 298 K. The latter was determined without the complication from OH regeneration due to secondary reactions. Acknowledgment. We thank the National Science Council of the Republic of China for support of this research. (30) Atkinson, R.;Baulch, D. L.; Cox, R.A.; Hampson, R. F., Jr.; Kerr, J. A.; Troe, J. J . Phys. Chem. Re/. Data 1989, 18, 881.
Picosecond and Nanosecond Studies of the Photoreduction of Benzophenone by 1,4-Diazabicyclo[ 2.2.2loctane: Characterization of the Transient C. Devadoss and Richard W. Fessenden* Radiation Laboratory and Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556 (Received: October 13, 1989; In Final Form: January 23, 1990)
The transient that is produced in the quenching of triplet benzophenone by 1,4-diazabicyclo[2.2.2]octane (DABCO) has been examined by use of nano- and picosecond laser photolysis. The initial step in all solvents, both polar and nonpolar, is electron transfer to form a triplet contact ion pair. In nonpolar solvents, the ion pair remains in this form until it decays. For polar solvents, the spectra change somewhat over the first 100 ps showing that the solvation changes and the ion pair becomes solvent separated. The lifetime of the ion pair varies greatly with the solvent. In saturated hydrocarbons it is about 80 ps. Nonpolar solvents with either A electrons or a lone pair of electrons stabilize the ion pair on the nanosecond to microsecond time scale. A small amount of alcohol in benzene also stabilizes the ion pair by hydrogen bonding. A shift in the peak position with time toward the blue accompanies the formation of hydrogen bonds in this case. Quenching of the ion pair by oxygen and the dependence of the ion-pair lifetime on temperature show different behavior for the two types of solvent and correlate with the form of the ion pair. The lifetime of the triplet ion pair is controlled alternatively by the rates of intersystem crossing, intrapair proton transfer, or the energetics of back electron transfer.
1. Introduction For a full understanding of a chemical reaction it is essential to understand the properties of any transient reaction intermediates. This statement is particularly true for a photochemical reaction like the quenching of benzophenone triplet by amines which involves electron transfer and an intermediate ion pair. A thorough knowledge of the transient will not only help elucidate the reaction mechanism but will also indicate the means to control the energy-wasting step of back electron transfer to regenerate the starting materials. For the past 20 years the photoreduction of aromatic ketones by amines'-27 has been an active area of ( I ) Cohen, S . G.;Cohen, J. 1. J . Phys. Chem. 1968, 72, 3782. (2) Cohen, S. G . ; Stein, N. J . Am. Chem. SOC.1969, 91. 3690. (3) Cohen, S. G . ; Parola, A.; Parson, G. H. Chem. Reu. 1973, 73, 141. (4) Parola, A. H.; Rose, A. W.; Cohen, S . G. J . Am. Chem. Soc. 1975,97, 6202. (5) Parola, A. H.; Cohen, S. G . J. Photochem. 1980, 12, 41. (6) Inbar, S . ; Linschitz, H.; Cohen, S . G. J . Am. Chem. SOC.1980, 102, 1419. (7) Inbar, S.; Linschitz, H.; Cohen, S . G . J . Am. Chem. SOC.1981, 103, 1048. (8) Davidson, R. S.; Lambeth, P. F. Chem. Commun. 1968, 511. (9) Bartholomew, R.F.;Davidson, R.S.; Lambeth, P. F.; McKeller, J. F.; Turner, P. H. J. Chem. Sot.. Perkin Trans. 2 1972, 577.
0022-3654/90/2094-4.540$02.50/0
research. In addition to optical absorption spectroscopy, techniques like CIDNP28+29 and CIDEP30-31 have been employed to study the (10) Peters, K. S.; Freilich, S.C.; Schaeffer, C. G. J. Am. Chem. SOC. 1980, 102, 5701.
Schaefer, C. G.; Peters, K. S . J . A m . Chem. SOC.1980, 102, 7566. Simon, J. D.; Peters, K. S . J . Am. Chem. SOC.1981, 103, 6403. Simon, J. D.; Peters, K. S . J . Am. Chem. SOC.1982, 104, 6542. Simon, J. D.; Peters, K. S . J . Phys. Chem. 1983, 87, 4855. ( 1 5 ) Scaiano, J. C. J. Photochem. 1973, 2. 81. (16) Griller, D.; Howard, J. A,; Marriott, P. R.;Scaiano, J. C. J . Am. Chem. SOC.1981, 103. 619. (17) Scaiano, J. C.; Stewart, L. C.; Livant, P.; Majors, A. W. Can. J . Chem. 1984, 62, 1339. (18) Bhattacharyya, K.; Das, P. K. J . Phys. Chem. 1986, 90, 3987. (19) Hoshino, M.; Shizuka, H. J . Phys. Chem. 1987, 91, 714. (20) Hoshino, M.; Shizuka, H. In Photoinduced Electron Transfer; Fox, M. A., Chanon, M., Eds.; Elsevier: Amsterdam, 1988; Part C, p 313. (21) Ohtani, H.; Kobayashi, T.; Suzuki, K.; Nagakura, S. Nippon Kagaku Kaushi ~~~. ... 1984. . - - , IO. - - .1479. - .(22) Arimitsu, S . ; Masuhara, H.; Mataga, N.; Tsubomura, H. J . Phys. Chem. 1975, 79, 1255. (23) Levin, P. P.; Kuzmin, V. A. Bull. Acad. Sci. USSR, Diu. Chem. Sci. (11) (12) (13) (14)
19811. ..- -, 37. - . , 807. - - ..
(24) Haselbach, E.;Vauthey, E.; Suppan, P. Tefrahedron 1988,44,7335. (25) Borisevich, N. A.; Lysak, N. A.; Mel'nichuk, S. V.; Tikhomirov, S. A.: Tolstorozhev, G. B. J . Appl. Spectrosc. 1988, 49, 1077.
0 I990 American Chemical Society
Photoreduction of Benzophenone by DABCO SCHEME I
The Journal of Physical Chemistry, Vol. 94, No. 11, 1990 4541 SCHEME 11
+
\C=O*(T*)
/
'BP
'CH-NR2
/
ACN
,
. ..
'CH-i.fi2
/
CT complex I
1
( BP-
SSlP
( BP-
CIP
'6-0- +
/
Hoshino, M.; Seki, H.; Shizuka, H. Chem. Phys. 1989, 129, 395. Hoshino. M.; Kogure, M. J . Phys. Chem. 1989, 93, 728. Roth, H . D.: Lamola, A. A. J . Am. Chem. Soc. 1974, 96, 6270. Roth, H. D.; Manion, M . L. J . Am. Chem. Soc. 1975, 97, 6886. (30) Miyagawa, K.; Murai, H.; I'Haya, Y. J. Chem. Phys. Lett. 1975,118,
(26) (27) (28) (29) 140.
(31) Kaise, M.; Someno, K. Chem. Letr. 1987, 1295.
h,,,
= 715 nm
... DABCO* ) Xmlx
I
DABCO EtOH
...
( BPDABCOI) CIP h,,, = 690 nm
( BP-
... DABCOI)
690 nm
"7
\ /
photoreduction of aromatic ketones. Inbar et al.6.7 used nanosecond laser flash photolysis to measure the quantum yields of the photoreduction of benzophenone by aliphatic amines and proposed a reaction mechanism. According to this mechanism, the initial step is the formation of a chargetransfer (CT) complex between benzophenone triplet and the amine followed either by the transfer of a proton to give the ketyl radical and an a-aminoalkyl radical or by back electron transfer to regenerate the reactants. A third process, dissociation of the complex into separated ions, has been added to this mechanism.I8 (SeeScheme I.) This mechanism has generally been accepted, and it satisfactorily explains many experimental observations. However, Inbar et aL6v7were unable to examine the CT complex directly because of its short lifetime (except in one instance; see below). Peters and c o - w ~ r k e r s ,in~ their ~ ' ~ pioneering work using picosecond absorption spectroscopy, directly observed the electron transfer from amine to benzophenone in polar solvents by detecting the spectrum of the C T complex at short times. This spectrum resembles that of benzophenone anion radical, but the absorption maximum is sensitive to the exact nature of the complex and so provides a handle for studying any rearrangements of the solvation of the ion pair. They reported that the first transient formed in acetonitrile is the solvent-separated ion pair (SSIP) which collapses to a contact ion pair (CIP) and that proton transfer to form ketyl radical takes place in the CIP. In contrast, they suggest that in ethanol the CIP is formed initially in ethanol with subsequent transformation into a H-bonded ion pair. Based on the positions of the absorption maxima, their reaction mechanism is Scheme 11. However, their study did not extend beyond a few nanoseconds and so did not allow correlation of the short-time behavior with the subsequent fate of the transient ion pair in this photochemical reaction. To fully understand the nature, stability, and mode of decay of this transient, it is important to carry out a combined nanosecond and picosecond study of the photoreduction of benzophenone by amines. The only detailed study of this sort that has appeared so far is that by Nagakura et aL2' on the photoreduction of benzophenone by triethylamine. Several recent papers have reported results obtained with benzophenone and different amines in photolysis experiments with nanosecond time resolut i 0 1 - 1 , and ~ ~ ~one ~ ~ paper on benzophenone has appeared using picosecond spectro~copy.~~
..... DABCO' )
+
R
H-bonded Ion pair Xmsx I620 nm
It is important to recognize that the CT complex is in an electronic triplet state so that the rate of return to starting compounds (process I11 in Scheme I) may be governed by the rate of triplet to singlet intersystem crossing in the complex. These species are often called triplet exciplexes, but we will use the term CT complex or ion pair. The spectrum will be discussed as if it comes from B P since that portion of the ion pair represents the main chromophore. One study has probed the implications of the spin multiplicity by measuring the oxygen quenching of triplet exciplexes from quinoned2 and an internal heavy atom effect has been reported.33 In this paper we present the results of combined nanosecond and picosecond studies of the photoreduction of benzophenone by DABCO in polar and nonpolar solvents. Interest in this particular amine came from the observation, using microwave absorption methods, of a polar intermediate (of 50-11slifetime) in benzene solution.34 The stability, nature, and mode of decay of the transient in different solvents have been examined. The effects of a trace amount of alcohol on the stability of the transient and on the yield of ketyl radical in benzene have also been investigated. The effect of temperature on the transient has been probed. By selectively quenching the benzophenone anion with oxygen, it has been possible to demonstrate unambiguously the absorption spectrum of DABCO cation radical. On the basis of the above observations and spectral shift observed on the picosecond time scale, a modified reaction scheme (Scheme 111; see below) has been proposed for the photoreduction of BP by DABCO in various solvents. Less detailed experiments with other amines have been used to show the unique behavior of DABCO. 11. Experimental Section Benzophenone, BP (Aldrich), was recrystallized from aqueous ethanol. 1,4-Diazabicyclo[2.2.2]octane,DABCO, purchased from Eastman, was recrystallized from a 1:1 benzene-hexane mixture and then sublimed under vacuum. Triethylamine, TEA (Aldrich), and N,N-diethylaniline, DEA (Aldrich), were refluxed over KOH and distilled. All other amines used were of high purity and were used as received. Benzene was passed through an alumina column and stored over molecular sieves. Acetonitrile (Aldrich) was distilled over P205. All other solvents were used as received. A few experiments were done with o-dichlorobenzene which had been dried; no changes were noted. The nanosecond laser flash photolysis apparatus has been described in detail e l ~ e w h e r e . ~Briefly, ~ ? ~ ~ pulses (337.1 nm, 8 ns, (32) Levin, P. P.; Pluzhnikov, P. F.; Kuzmin, V. A. Chem. Phys. Lett. 1988, 152, 409.
(33) Levin, P. P.; Kuzmin, V. A. Dokl. Phys. Chem. 1987, 292, 26. (34) Vaidyanathan, S.; Selvarajan, N.; Fessenden, R. W. Unpublished results. ...-...
(35) Das, P. K.; Encinas, M . V.; Small, Jr., R. D.; Scaiano, J. C. J . Am. Chem. SOL-.1979, 101. 6965.
4542
The Journal of Physical Chemistry, Vol. 94, No. I I, 1990
2-3 mJ) from the Molectron nitrogen laser were used for excitation in a front-face geometry. Rectangular quartz cells with path length 2 or 3 mm were used. The transient absorances in the reaction at a preselected wavelength were monitored by a detection system consisting of a monochromator and a photomultiplier tube (PMT). The signal from the PMT was processed by a 7912AD Tektronix transient digitizer controlled by a Digital Equipment Corporation LSI-11/2 microcomputer. Filters to cut off short wavelengths were used to avoid spurious response from the second order of the monochromator grating. The picosecond flash photolysis instrumentation has been discussed elsewhere.37 Picosecond laser photolysis was carried out with an actively-passively mode-locked Quantel YG-5OlDP Nd:YAG laser. The output of the oscillator is passively mode locked by the nonlinear absorption of saturable absorber Kodak V dye and actively mode locked by an acoustooptic modulator inserted into the resonator. Mode locking generates a train of eight to nine pulses, each with a width of 18 ps (fwhm). A single pulse from the train is extracted by using a Pockels cell and amplified in two stages. The fundamental pulse (IO64 nm) is partly frequency doubled (532 nm) and then tripled (355 nm, 3.5 mJ) with KDP crystals (INRAD). The laser operatres with a repetition rate of I O Hz. The remaining fundamental output is focused into a quartz cell ( I O cm long) containing a mixture of H 2 0 and D 2 0 to generate white light continuum which is used as the probe pulse. The probe pulse has a temporal width similar to the excitation pulse and a spectral width ranging from 400 to 860 nm. The continuum pulse is focused on a quartz fiber (0.2-mm diameter) and then split into two by an optical fiber beam splitter. One part of the beam meets the excitation beam at right angles in the cell, and the other part passes through unirradiated solution to serve as the reference beam. The right-angle geometry ensures the maximum overlap between the excitation and probe beams, thus enhancing the absorbance values. The outputs after the sample are collected separately by two optical fibers (1-mm diameter) which are connected to a spectrograph (HR-300, ISA Instruments), and the light is detected by a dual diode array, DDA (Princeton Instruments), which measures simultaneously the intensities of the reference beam (I,) and sample beam ( I ) . This setup eliminates any errors in the absorbance arising from pulse-to-pulse fluctuations in the energy of the excitation laser pulse which are common when the reference and sample beams are detected individually at different times. To reduce the dark current, the DDA is cooled to -15 "C with a Peltier-effect thermoelectric cooler. The controller for the DDA (ST-120 Princeton Instruments) digitizes the data and transfers it to an IBM AT computer. The change in absorbance AA is calculated by using the expression
Devadoss and Fessenden
lm Time.
ns I O ~ S
h
6.01
-
Here le and Pe are intensities of the probe beam obtained with and without excitation beam, respectively, and Ibg is the intensity of background in the absence of both excitation and probe beams. The subscript o denotes reference beam. To get a transient absorption spectrum, signals are averaged over 400 laser shots with the excitation beam present and another 400 laser shots without the excitation beam. The probe beam is delayed in time with respect to the excitation beam by inserting optical fibers of desired length between the beam splitter and the fiber on which the probe beam is focussed. In addition, displacement of the position allows adjustment of time delays by about 50 ps. I n this system, time delays from 0 ps to 36 ns can be achieved. The path length in the fused-silica cell is 1 cm. The point in time (which defines t = 0) at which the peaks of the probe and excitation pulses overlap spatially in the sample was determined by monitoring the buildup of the T, T, absorption of BP in acetonitrile. BP triplet is assumed to be
-
( 3 6 ) Nagarajan, V.; Fessenden, R. W . J . Phys. Chem. 1985, 89, 2330. (37) Ebbesen, T. W . Reu. Sci. Insrrum. 1988, 59. 1307.
0.0 400
I
500
600
700
800
Wavelengih, nm Figure 1. Transient absorption spectra of a benzene solution of BP (10 m M ) and DABCO (20 mM)at different times after laser photolysis: (a) end of pulse; (b) 30 ns; (c) 700 ns. The insert shows a kinetic trace taken at 690 nm.
produced within the pulse. The time corresponding to the center of the excitation pulse corresponds to a point where the absorbance is half of its plateau value at longer times. However, in the present study the point t = 0 was taken to be the time at which the plateau value is reached. This difference is about 25 ps. The difference does not affect the present study since no quantitative treatment of the kinetics at short times is made. All of the spectra obtained in the picoseond study are reported without applying any smoothing process. For both nanosecond and picosecond experiments the concentration of benzophenone solution was 10 mM and the solution were deoxygenated by bubbling high-purity argon through the solution before photolysis. Except where indicated, the sample temperature was about 20 OC. 111. Results A. Nanosecond Laser Photolysis. I . Transient Spectra and
Decay Kinetics. 'BP, produced in benzene by flash excitation with 337.1-nm laser pulses, was quenched by all tertiary amines that were studied-DABCO, TEA, DEA, N,N'-dimethylpiperazine, quinuclidine, N,N,N',N'-tetramethylpropanediamine, and N,N,N',N'-tetramethylbutanediamine-and ketyl radicals were produced. Among all these amines, DABCO seems to be unique in that the CT complex was observed on the nanosecond time scale. Inbar et aL7 have also reported observation of this complex. The CT complex resembles an ion pair whose absorption spectrum is a superposition of the spectra of cation radical and anion radical. In this case, the absorption is mainly that of benzophenone radical anion in the broad range 650-750 nm. Figure 1 shows the transient absorption spectra observed (a) immediately after the pulse and (b) 30 ns after the pulse. The broad band with a peak at 725 nm is characteristic of B P which is formed by the transfer of an electron from DABCO to 3BP. This transient, which is an ion pair, decays with first-order kinetics and a lifetime of 50 ns. Figure I C gives the absorption spectrum of the ketyl radical that is formed by the decay of the transient. In their study, Inbar et al.7 reported a transient with A, at 650 nm, but they did not report its lifetime in neat benzene although lifetimes in different benzene-acetonitrile mixed solvents were reported. We studied the BP-DABCO system in more than 30 solvents, and Table I gives the lifetime of the ion pair in some of the solvents. Also shown in Table I are several solvent parameters, the calculated free energy change for ion-pair formation, and the relative ketyl radical yield after collapse of the ion pair. The latter is measured
rhe Journal of Physical Chemistry, Vol. 94, No. I I, 1990 4543
Photoreduction of Benzophenone by DABCO
r
TABLE I: Lifetime of the Ion Pair and Relative Ketyl Radical Yield in Different Solvents lifetime re1 ketyl
dielectric" Kamlet-Taft" const, c polarity, ll*
solvent benzene 1,4-dioxane diethyl ether chlorobenzene dichloromethane
o-dichloro-
benzene butyronitrile acetone
acetonitrile ferf-butyl alcohol 2-propanol ethanol methanol
cyclohexane
vans-decalin
2.28 2.21 4.34 5.62 8.93 9.93
0.59 0.55 0.27 0.71 0.82 0.80
20.30 20.70 37.50 12.47 19.92
0.71 0.71 0.75 0.41 0.48 0.54 0.60 0.00
24.55 32.70 2.02d 2.17d
AG~lp,b
of ion pair: 50 ns
eV -0.17 -0.16 -0.26 -0.28 -0.31 -0.32
75 ns 30 ns 95 ns 5.1 ps 90 ns
-0.34 -0.35 -0.36 -0.33 -0.34 -0.35 -0.36
2.7 ps 1.9 ps 2.1 ps 85 ns 3.0 ps 2.5 ps 0.65 ps
7
-0.15
-0.16
radical yield 0.80 0.82 0.85 0.45 0.15 0.46 0.22 0.00 0.00 0.00 0.15 0.56 0.60 0.87 0.90
" From:
Marcus, Y. fon Soloafion; Wiley: Chichester, New York, 1985. 6Calculated from Weller's equation (see: Weller, A. Z.Phys. Chem. 1982, 133, 93). C I n all cases the decay was first order. dFrom ref 33.
t
1
't
7*0t
(u
0
%$?$
m s
2 0
400
0
c
3.0
51 2.0 a I .o n
500
600
500
600
700
800
Wavelength, nm Figure 3. Transient absorption spectra of an acetonitrile solution of BP (IO mM) and DABCO (20 mM) at different times: (a) end of pulse: (b) 2 ps; (c) 12 ps. The insert shows a kinetic trace at 690 nm. The decay departs only slightly from a good exponential (see the discussion in section III.A.l). There is a small residual absorbance.
benzene (1% MeOH) butyronitrile 2,2,2-trifluoroethanoI methanol acetonitrile
5.0
0.0 400
0
benzene
0 4.0
f!
e 6
cyclohexane
X
0
T i m e , pa
TABLE 11: Quenching Rate Constants of BP Triplets by DABCO in Different Solvents solvent c kdi&calcd),M-l s-I k., M-' s-I
T i m a . ns
6.0
I
\.
700
Wavelength, nm Figure 2. Transient absorption spectra of a methanol solution of BP (IO mM) and DABCO (20 mM) at different times: (a) end of pulse; (b) 0.5 ps; (c) 2.5 ps. The insert shows a kinetic trace at 640 nm. at 545 nm and represents the ratio of the absorption by the ketyl radical after decay of the ion pair to that of the initially formed 3BP. This ratio is uncorrected for differences in absorption coefficients for the two species between the solvents but serves as a useful measure nevertheless. As is seen from the table, the lifetime of the ion pair is long in highly polar solvents and is shorter in less polar solvents. This variation indicates the different nature of the ion pair in these two types of solvents. Figure 2 shows the transient spectra obtained in methanol at different time intervals. The absorption band of B P is blue shifted to 61 5 nm as a result of H bonding. This blue shift in the absorption spectrum of B P was observed in all alcohols. Figure 3 shows the transient absorption spectra in acetonitrile obtained at different time intervals. The ion pair is long-lived in acetonitrile ( 7 = 2.1 ps) and has a peak at 720 nm. It decays with first-order kinetics without producing ketyl radicals. Hence, the only mode of decay in acetonitrile is back electron transfer to regenerate the reactants. An interesting feature that can be inferred from Table I is the absence of the ion pair in aliphatic hydrocarbon solvents. Even though the bulk dielectric constants
2.02 2.28
7.4 x 109 1.1 x 10'0
4.6 x 109 6.5 x 109
20.30 26.64 32.70
1.2 x 1010 4.0 x 109 1.2 x 10'0 1.9 X IO'O
4.6 x 109 1.1 x 109 6.9 x 107 1.2 x 109 1.1 x 1010
37.50
of trans-decalin and 1,Cdioxane are not much different (t = 2.17 and 2.21 respectively), the ion pair is stable in 1,4-dioxane but not in trans-decalin. Another nonpolar solvent in which the ion pair is stabilized is benzene. In all the solvents studied, the decay of the ion pair follows clean first-order kinetics as observed by Inbar and co-workers for acetonitrile and acetonitrile-benzene mixed solvent.' The order of the reaction was confirmed by substantially reducing the laser pulse energy and finding only a slight increase in lifetime. This behavior clearly indicates that, in the BP-DABCO system, the majority of the radical ions stay together as a solvent-separated ion pair or a contact ion pair depending on the nature of the solvent. Recently, Haselbach et al.24 reported the formation of free ions in the BP-DABCO system in acetonitrile from their transient absorption and photocurrent experiments. They claimed to have observed B P even in the absence of DABCO, and they claimed that the absorption maximum of BP- is located at 630 nm. In our study, no absorption band with A, at 630 nm has been observed in any aprotic solvent in either picosecond or nanosecond experiments. Further, their failure to observe any signal due to B P in the BP-dimethylaniline (DMA) system in acetonitrile, which has been studied extensively by both optical12and photocurrent methods2*and in which the presence of B P ions has been confirmed, suggests that there is some problem with those experiments and that impurities might have given rise to the band with A, at 630 nm. Additionally, Kuzmin and cc~workers~~ have observed the first-order decay of the ion pairs formed between quinone triplets and aromatic amines in polar and nonpolar solvents. 2. Quenching Rates. The quenching rate constants (k,) of 3BP by DABCO in various solvents are listed in Table 11. These rate constants are obtained from a plot of the pseudo-first-order decay rate constants for 3BP as a function of DABCO concentration.
4544
Devadoss and Fessenden
The Journal of Physical Chemistry, Vol. 94, No. 1 1 , 1990 r
1
b
.
" 1 . bl el
n U
f
W 0
c
+2 0
400
n U
500
600
700 400
500
600
700
WAVELENGTH, n m I
1
I
I
I
I
Time, ns Figure 4. Kinetic traces monitored at 545 nm for BP (IO mM) and DABCO (20 mM) (a) in benzene, (b) in methanol, and (c) in benzene with 0.8% methanol. The plateau at about 700 ns in (b) represents absorption by both ketyl radical and B P in the ion pair.
The triplet decay was monitored at 520 nm where the extinction coefficient of 3BP is higher than that of the ketyl radical. In highly polar acetonitrile, the rate constant is of the order of 1O1O M-I s-I whereas in equally polar methanol it is an order of magnitude smaller. This lower value indicates that H bonding plays a role in inhibiting the electron transfer. Even when methanol is present in benzene in small amounts ( I %), the quenching rate constant is reduced. The quenching is unusually slow with 2,2,2-trifluoroethanol (TFE). TFE is known to be acidic and to have a stronger H-bonding a b i l i t ~with ~ ~ aromatic ,~~ ketones and amines than unsubstituted alcohols. The DABCO must be strongly affected by the presence of TFE. In this case, the characteristic absorption spectrum of B P was not observed. Proton transfer may take place efficiently from the solvent to B P , thus removing the ion pair as fast as it is formed. However, it was possible to see the spectrum of DABCO' by selective quenching with oxygen (see below). 3. Effect of a Small Amount of Methanol. When a small amount (1-4% v/v) of methanol is added to benzene, the lifetime of the ion pair is doubled to 100 ns.' In addition, the ketyl radical yield is drastically reduced almost to zero (see Figure 4) and the absorption maximum of B P is shifted from 725 to 650 nm. The blue shift in the absorption and the lack of proton transfer in the ion pair to form ketyl radical indicate that methanol prevents the proton transfer by H bonding with B P . Other alcohols like 2-propanol, terr-butyl alcohol, and TFE behave similarly; the extent of the blue shift is in the order TFE (625 nm) > MeOH (650 nm) > 2-PrOH (655 nm) > t-BuOH (670 nm) When the amount of alcohol exceeds IO%, the system behaves as if it is in the neat alcohol. 4. Selective Quenching by 02.The absorption spectrum of DABCO' remained elusive with all the solvents we have studied, (38) Mukherjee, L. M.; Grunwald, E. J . Phys. Chem. 1958, 62, 1311. (39) Figueros, J. J . Am. Chem. SOC.1971, 93, 3255. (40) A similar lengthening of the ion-pair lifetime is seen if the benzene used as solvent has been exposed to the atmosphere so it can absorb moisture.
Figure 5. Transient absorption spectra obtained 1.5 ps after laser photolysis for 1:1 acetonitrile-water solutions: (a and b) BP and DABCO; (c and d) xanthone and DABCO; (e and f) BP and diethylaniline. and spectra b, d, and fare in the Spectra a, c, and e are without 02, presence of O2 (concentration not measured).
even though the B P spectrum was distinctly observed. The absorption is weak and so is obscured by that of B P and ketyl radical. The absorption spectrum of DABCO' has been reported in the photoreduction of 2-cyclohexenoneby DABCO?' and Kaise et aL3' observed DABCO' by ESR in the photoreduction of xanthone in acetonitrilewater mixed solvents. However, no direct observations of DABCO' in the photoreduction of BP by DABCO have been reported. Scaiano et a1.I' have used selective quenching by a small concentration of oxygen to obtain the absorption spectra of mesocyclic diamine cation radicals. This method was applied here and the absorption spectrum of DABCO' could be observed. The spectrum is a broad band from 400 to 550 nm with a peak at 480 nm (Figure 5b). The reaction is presumably to form a peroxyl radical with BP-42 although formation of 02-is also possible. The cation radical clearly does not react with oxygen (see below). One interesting point is that the selective quenching of BP- by O2occurs only in highly polar solvents like acetonitrile, methanol, acetone, butyronitrile, and acetonitrile-H20 mixture. In less polar solvents like benzene, chlorobenzene, or dichlorobenzene the ion pair decays in the presence of O2to regenerate the starting materials without forming ketyl radicals. In both types of solvent the quenching rate constant is high (diffusion controlled). Because the quenching by oxygen in less polar solvents may involve the promotion of intersystem crossing in the ion pair, some experiments were also carried out with di-tert-butyl nitroxide (DTBN) as a quencher. Approximately 2 mM was added and the ion-pair lifetimes determined. A pronounced shortening of the ion-pair lifetime was seen in the nonpolar solvents ( k 3 X IO9 M-' s-I in benzene), but little, if any, effect was seen with the more polar solvents ( k 3 X IO8 M-I s-* in acetonitrile). This effect supports the idea that the structure of the ion pair is different in the two types of solvent and that enhancement of the rate of intersystem crossing shortens the ion-pair lifetime in nonpolar solvents. To make sure that the absorption band with A, at 480 nm corresponds to DABCO', the experiment was repeated with at xanthone instead of BP and an identical spectrum with A,
-
(41) Dunn, 107, 2802.
-
D.A.; Schuster, D.1.; Bonneau, R. J . Am. Chem. SOC.1985,
(42) Pribush, A. G.High Energy Chem. 1981, I S . 39.
Photoreduction of Benzophenone by DABCO
0
A
17.2
-
The Journal of Physical Chemistry, Vol. 94, No. 11, 1990 4545 TABLE IV: Lifetime of Ion Pair and Ketyl Radical Yield in Substituted Benzophenones lifetime of ion pair, ns re1 ketyl radical yield
MeOH ACN DCB(Mo0H)
benzene benzene benzene (1% MeOH) benzene (1% MeOH)
BP substituent none
50
100 94
0.80
0.04
78
0.59 0.47 0.38 0.16
0.05 0.04
14.8Kl 4,4'-difluoro 4,4'-dichloro 4,4'-di bromo 4-trifluoromethyl
14.0
2.8
3.0
3.2
3.4
3.6
65
92 90 108
58 83
0.00 0.06
3.0
I/Tl Figure 6. Temperature dependence of the rate of decay of the BPDABCO ion pair in o-dichlorobenzene ( O ) , o-dichlorobenzene with 1%
MeOH
( O ) , methanol (0),and
acetonitrile (A).
TABLE 111: Arrhenius Parameters for the Decay of Ion Pair solvent A, s-I E,, kcal/mol
o-dichlorobenzene o-dichlorobenzene(1% MeOH) methanol acetonitrile
1.9 X 9.3 X 3.6 X 2.8 x
IO"
5.84
IO6 IO8 107
0.00 3.10 1.54
480 nm was obtained in acetonitrile-water mixed solvent (Figure 5d). To eliminate the possibility of observing a transient formed between DABCO' and 02,the selective quenching by O2 was carried out with BP-diethylaniline (DEA) in acetonitrile-water mixed solvent. The characteristic absorption spectrum of DEA+, a sharp band with a peak at 470 nm, was observed along with the spectrum of B P in the absence of O2 (Figure 5e). When O2 was introduced and the selective quenching of B P was accomplished, an identical spectrum with A,, at 470 nm was obtained (Figure 50. This undoubtedly shows that the spectrum obtained after quenching of B P by O2 is that of the amine cation radical and is not from a transient formed between the amine cation radical and 02.One notable observation is that the absorption spectrum of DABCO+ was obtained in TFE by selective quenching by O2even though a distinct absorption spectrum of B P was not observed in the degassed solution due to rapid decay of the ion pair through intra-ion-pair proton transfer. 5. Temperature Dependence of the Decay. The effect of temperature on the decay of the ion pair was studied over the range 3-76 OC, for dichlorobenzene (DCB), DCB (1% MeOH), methanol, and acetonitrile. The decay rate of the ion pair increases with increasing temperature in all solvents except DCB(Me0H) in which the ion pair decays with a rate constant of 9.3 X IO6 s-I at all temperatures in the range 23-53 OC. Arrhenius parameters obtained from the data in Figure 6 are listed in Table 111. In methanol and acetonitrile the preexponential factor ( A ) is extremely low and the activation energy (E,) is also small. At all temperatures investigated, the ion pair decays with first-order kinetics, thereby indicating that separation of the ion pair into individual radical ions does not take place even at elevated temperatures. In the case of DCB, the relative ketyl radical yield (measured as in the last column of Table I) increases approximately linearly from 0.20 at 4 OC to 0.91 at 76 OC. The intraion-pair proton transfer is facilitated at higher temperatures. This trend was not observed in methanol and acetonitrile. In the latter solvent, the ketyl radical was not formed at any of the temperatures studied. 6 . Electronic and Heavy-Atom Effects. To study the effect of electron-withdrawing groups on the photoreduction, experiments were conducted with Q(trifluoromethy1)- (TFMBP), 4,4'-difluoro(DFBP), 4,4'-dichloro-, and 4,4'-dibromobenzophenones in benzene and in benzene with 1% (v/v) methanol. The lifetime of the ion pair and the relative ketyl yields are listed in Table IV. The lifetimes of the substituted (BP-DABCO+) ion pairs in benzene are longer than that of the unsubstituted one. The effect of adding 1 % methanol to the benzene is opposite to that for the
0.00""""""' 500 550
600
' ' 1 ' ' ' ' ~ ' ' ' ' ' ' 650 700 750
WAVELENGTHl nm Figure 7. Transient absorption spectra for BP (10 mM) and DABCO (0.1 M) in benzene at different times after the laser pulse.
I 0.00 500
1
550
600
650
700
UPS
750
WAVELENGTH, nm Figure 8. Transient absorption spectra for BP (IO mM) and DABCO (0.22 M) in acetonitrile at different times after the laser pulse.
unsubstituted benzophenone, however. Whereas the lifetime of the (BP-DABCO+) ion pair is doubled in benzene with 1% methanol, with the unsubstituted benzophenones the lifetime is actually decreased except in the case of DFBP. Even with DFBP the increase is only 45%. In the substituted (BP-DABCO+) ion pair, back electron transfer is increased. The substituted compounds also produce less ketyl radical than BP itself, TFMBP yielding the least. The ketyl radicals should be more acidic with the substituted benzophenones thus decreasing the proton transfer. Among the dihalosubstituted compounds, dibromobenzophenone gives the lowest yield of ketyl radical in benzene. A similar study of the internal heavy atom effect on the lifetime of the ion pair formed between quinones and amines in benzenemethanol (20:l) mixed solvent has been reported by Levin and K ~ z m i n . ~ ~ B. Picosecond Laser Photolysis. Using an 18-ps laser pulse (355 nm), the photoreduction of IO mM of BP by DABCO (0.1-0.5 M) was investigated in benzene, acetonitrile, methanol, and benzene with methanol. Transient absorption spectra (500-760 nm) obtained at different time intervals are shown in Figure 7 for benzene (BZ), Figure 8 for acetonitrile (ACN), and Figure 9 for methanol. With the decay of the benzophenone triplet (A, at 525 nm in ACN and methanol and 530 nm in benzene), there is a corresponding formation of B P (A, at 725 nm in BZ, 720 nm in ACN, and 615 nm in MeOH). The rate constants for the two processes agree as shown in Table V . This direct ob-
The Journal of Physical Chemistry, Vol. 94, No. 11, 1990
4546
Devadoss and Fessenden
0.65
9 - 1.5 h 2 I 3
500
600
550
650
700
750
WAVELENGTH, nm Figure 9. Transient absorption spectra for BP (IO mM) and DABCO (0.5 M ) in methanol at different times after the laser pulse.
-
Time __
Xmox
25 ps
7 0 0 nm
50 ps
715 nm
loops
720 nm
a
600
650
700
750
600
850 1
WAVELENGTH, nm
a1
5ns
656 nm
b)
3ns
659 nm
cl
2ns
666 nm
dl
I nr
684 nm
e ) 760 ps
689 nm
f l 500 ps
696 nm
WAVELENGTH, nm Figure 11. Spectra showing a shift in peak position with time after the laser pulse for BP (10 mM) and DABCO (0.25 M) in benzene with 1.5%
MeOH.
So I v e nt
W
0
x,,X
725
z a
0 ) 61
U
C) Ezl2-PrOHl
673
d ) 6zlMeOHI
656
el B z l T F E l
626
b l 6 z l t - 6 u O H l 669
m
0 v)
m
a a
x,,,
time -
a - 25 ps b - IO0 C - 250 d - 500 e - 760 f I ns
500
550
600
650
700
750
WAVELENGTH, nm
Figure 10. Transient absorption spectra for BP (IO mM) and DABCO (1 M ) in acetonitrile at three times after the laser pulse. The peak position of B P in the ion pair shifts with time.
Figure 12. Transient absorption spectra for BP (10 mM) and DABCO (0.25-0.3 M) at 5 ns after the laser pulse for benzene with 1.5% of the alcohols listed (TFE = trifluoroethanol).
TABLE V Electron-Transfer Rate in the BP (10 mM)-DABCO (0.25 M) System )BP decay B P - growth solvent c rate, s-l rate, s-I benzene 2.28 3.5 x 109 3.4 x 109 benzene ( 1% ' MeOH) 1.6 x 109 methanol 32.70 5.2 X IO* 5.5 X IO8 acetonitrile 37.50 4.5 x 109 4.6 x 109
time in the presence of a small amount of methanol. The spectral maximum at 500 ps after photolysis is 696 nm, and that at 5 ns is 656 nm. This gradual shift in spectral maximum must be the result of the formation of hydrogen bonds between BP- and the methanol molecule. When methanol is replaced by other alcohols, similar spectral shifts are observed but the extent of the shift is dependent on the nature of the alcohol. As seen in Figure 12, the spectral peak at 5 ns after photolysis is for tert-butyl alcohol, 689 nm; for 2-propanol, 673 nm; for methanol, 656 nm; and for TFE, 626 nm. This pattern clearly indicates that the stronger the H bonding between the alcohol and B P ion the larger is the spectral shift. Also seen in Figure 12a is an absorption band with A, around 550 nm for benzene solution. This band is from the formation of ketyl radicals that are not produced in the presence of a small amount of the alcohols. This observation agrees with our nanosecond photolysis results in which the ketyl radical yield in the presence of alcohol was found to be negligible. If the rate of spectral shift could be separated from the electron-transfer rate, it would throw more light on the actual processes affecting the ion pair (either H bonding or solvent reorientation). This analysis is difficult as all three transients involved, the 3BP, non-H-bonded B P ion, and H-bonded B P ion, absorb at these wavelengths. Unless the extinction coefficients of these species are known exactly it is not possible to know the rate of the spectral shift accurately. In the nanosecond experiment, the absorption band of B P could not be observed in cyclohexane and other aliphatic hydrocarbon solvents. Photoreduction of BP by DABCO was carried out with picosecond laser photolysis in cyclohexane and 2-methylpentane. Since the solubility of DABCO is very low in these aliphatic hydrocarbon solvents, a saturated solution of DABCO (-0.15 M) was prepared. In the picosecond experiment the electron transfer to 3BP from DABCO was observed by the appearance of a broad band in the spectral range 650-750 nm in both solvents. Figure 13 shows the transient absorption spectra obtained at different time intervals in cyclohexane. As can be seen from the figure, the ion pair starts decaying within 760 ps to give ketyl radical and at 5 ns after photolysis the amount of BP- left is negligible. The observed behavior can be explained if the ion-pair
servation of electron transfer from DABCO to 3BP to form B P has also been reported by Peters et al.10J3 At the concentrations used for the experiments shown in Figures 7-9, very nice isosbestic points are obtained. Thus the spectrum of the ion pair (and triplet) remains the same during the quenching. This result appears to contradict the earlier observ a t i o n ~which '~ reported a shift in peak position over the first few 100 ps for ACN and ethanol solutions (see Scheme 11). However, the formation of the ion pairs occurs over times somewhat longer than the reported time for the wavelength shift so that little of the initial form is present at a given time. With higher concentrations of DABCO (1 .O M), different results are indeed obtained. In ACN, a red shift from 700 to 720 nm is observed between 25 and 100 ps. Beyond 100 ps, there is no change in the spectral maximum. Figure 10 shows the time-dependent shift in the absorption maximum of B P . This shift is in the direction opposite to that reported earlierk3but is quite repeatable. In methanol, the exact peak position at early times is hard to determine because the quenching rate is slow and the band is weak. However, a shift from 670 to 615 nm was seen much like the 690 to 625 nm shift reported earlier for ethanol. No spectral shift was seen for benzene at high concentrations of DABCO. In benzene with 1.5% (v/v) methanol, the appearance of the spectrum of B P is slower than that in neat benzene. This confirms our nanosecond photolysis study in which the quenching rate constant k , was found to be smaller in BZ(Me0H) than in BZ. The interesting feature in this case is the spectral shift with time up to 5 ns. Figure 11 shows how the absorption peak for B P in benzene with 1.5% methanol shifts with time. The A,, for B P in neat benzene is 725 nm. The peak shifts from this value with
.
-.
*
. - . .._-
1 -
The Journal of Physical Chemistry, Vol. 94, No. 11, 1990 4547
Photoreduction ot tlenzopnenone ~y UABLU I n6
''"-1
I
a,
w
0
z
a
U
b c d e f
m K
0
v)
m U
0.00
500
550
600
650
700
-
-
25ps 100 175 760 5ns
- 20
750
WAVELENGTH, nm Figure 13. Transient absorption spectra for BP (IO mM) and DABCO (-0. I5 M) in cyclohexane at different times after the laser pulse.
lifetime is shorter than the decay period of 'BP. A pseudo steady state was assumed between the formation of the ion pair and its decay. On this basis, the lifetime of the ion pair in cyclohexane was calculated to be about 80 ps. (The calculation took the long-wavelength absorption by 'BP into account.) This extremely short lifetime explains the undetectability of the ion pair in the nanosecond laser photolysis experiment with cyclohexane.
IV. Discussion A . Soluent Effect. In the BP-DABCO system the ion pair has been observed to have a lifetime of at least tens of nanoseconds in all solvents except aliphatic hydrocarbons. Two striking features elicited from the lifetime of the ion pair in different solvents (Table I) are the unusual stability of the ion pair in the nonpolar solvents benzene and 1,Cdioxane and the very long lifetime (5.1 ps) of the ion pair in the only moderately polar dichloromethane (e = 8.93). Clearly, the bulk dielectric constant is not the controlling factor as the 1 ,Cdioxane and trans-decalin pair shows. The ion pair is stable on this time scale in dioxane but not in trans-decalin. The most obvious difference between these two solvents is the presence of the lone pairs of electrons in dioxane. The other nonpolar solvent that stabilizes the ion pair is benzene, which has a a-electron cloud. In a recent report, Smit et aL4' in their study of giant dipoles observed that benzene and dioxane have anomalous solvating power due to specific ion-solvent interactions. The behavior found here is quite in accord with this idea in that the ion pair is stable in benzene and dioxane but not in aliphatic hydrocarbons . The unusually long lifetime (5.1 ps) of the ion pair in dichloromethane (t = 8.93) when compared to that in highly polar acetonitrile (T = 2.1 ps, e = 37.5) is puzzling at first sight. However, according to Newbound et aI.@ chlorocarbon solvents like dichloromethane are known to coordinate to cations through the lone pairs of the chlorine atoms. Recently Minto and Das45 studied the photolysis of 9-phenylxanthen-9-01 and observed that the heterolytic fission to form 9-phenylxanthenium cation occurs more readily in 1,2-dichloroethane (e = 10.36) than in acetonitrile (e = 37.5) due to a greater stabilization of the cation by the chlorine lone pairs. Thus the long lifetime of the (BP-DABCO') ion pair in dichloromethane is understandable if the stabilizing power of the lone pairs of the chlorine atoms is taken into account. Among the alcohols, those with greater H-bonding ability would be expected to stabilize the ion pair more. The partial donation of a proton by the hydrogen bond reduces the effective charge on the oxygen and therefore reduces the energy gain from transfer of a proton. The rate of proton transfer should be correspondingly reduced. In the series methanol, ethanol, and 2-propanol such a trend in ion-pair lifetime is seen (Table I). However, in trifluoroethanol (TFE), which is monomeric and has a stronger H-bonding ability than unsubstituted alcohols, BP and DABCO ~~
(43) Smit, K. J.; Warman, J. M.; De Haas, M. P.; Paddon-Row, M. N.; Oliver, A. M. Chem. Phys. Left. 1988, 152, 177. (44) Newbound, T. D.; Colsman, M. R.; Miller, M. M.; Wulfsberg, G. P.; Anderson, 0. P.; Straws, S. H. J . Am. Chem. Soc. 1989, I l l , 3762. (45) Minto, R. E.; Das, P. K. J . Am. Chem. SOC.1989, 1 1 1 . 8858.
are so strongly H bonded in the ground state that the rate of quenching of 'BPs by DABCO is drastically reduced (Table 11). The short lifetime of the ion pair in this case may involve protonation of B P by the acidic alcohol. In 2-propano1, which is a good H-atom donor to 'BP and forms ketyl radicals with a primary quantum yield of 1, the ketyl radical yield is smaller in the presence of DABCO. This is due to the H bonding of the alcohol to BP- and to the poor proton-donating ability of 2propanol. In tert-butyl alcohol, which is bulkier and has weak H-bonding ability, the lifetime of the ion pair is small compared to other alcohols and the ketyl radical yield is nil. This indicates the different nature of the ion pair in this alcohol (see below). In this study, a mention of polarity parameters would be in order. A quick perusal of Table I indicates that solvents with positive Kamlet-Taft parameter, rI*, stabilizes the ion pair on the nanosecond time scale. In solvents like cyclohexane (n*= 0.00) and n-hexane (n*= -0.08) the ion pair could not be observed. In diethyl ether, which has a low rI* value of 0.27, the ion pair is stable. The Kamlet-Taft polarity parameter seems to be a better indicator of the stability of the (BP-DABCO') ion pair than the dielectric constant. However, there is no simple relationship between either of the parameters and the lifetime of the ion pair. The main reason for the absence of such a correlation is the proton-transfer reaction in the ion pair which depends on more factors than just polarity. Even though the ion pair in cyclohexane and other aliphatic hydrocarbon solvents cannot be seen on the nanosecond time scale, there were indications in our nanosecond photolysis study that a very short lived ion pair was an intermediate in the reaction. The quenching rate constant of 'BP by DABCO in cyclohexane (4.6 X lo9 M-I s-l) is an order of magnitude higher than the hydrogen abstraction rate constant. Also, in contrast to the situation in 2-propano1, the yield of ketyl radicals in the presence of DABCO is higher than in its absence. Both observations point to electron transfer from DABCO to 3BP to form the ( B P DABCO+) ion pair in cyclohexane. In addition, the calculated AG value is energetically favorable for electron transfer. This conclusion was proved to be correct by the picosecond photolysis experiments. It was possible to monitor the decay of 'BP and the formation of B P by electron transfer in cyclohexane. However, unlike in other solvents, the ion pair decayed with a lifetime of 80 ps. The behavior was similar in 2-methylpentane. This observation clearly proves that electron transfer from DABCO to 'BP is the initial step in the photoreduction of BP, irrespective of the nature of the solvent. B. Nature of the Ion Pair. Based on the results presented here, the nature of the ion pair in the photoreduction of BP by DABCO needs further discussion. Reaction Scheme 11, with either CIP or SSIP depending on the solvent, was developed by Simon and to account for the shifts in absorption maxima with time. In the present study, with low concentrations (0.1-0.5 M) of the quencher DABCO, no spectral shift in the absorption spectrum of B P has been observed but, with higher concentrations (1 M), a red shift in ACN and a blue shift in MeOH have been seen. This suggests that the transformation of one type of ion pair into another occurs at a high rate. Only when the quenching rate is increased by increasing the DABCO concentration does this transformation become resolvable. In acetonitrile, the spectral maximum shifts from 700 to 720 nm, which is in the direction opposite to that reported by Simon and Peters." They proposed that the initial species is the SSIP (A,, = 715 nm) which transforms to a CIP (A- = 690 nm). But energetically the SSIP is more stable than the CIP because of solvation. Therefore, the transformation of CIP to SSIP is more plausible. It is well-known that the CIP absorbs at a shorter wavelength than the SSIP.46 Hence the spectral shift from 700 to 720 nm must correspond to the transformation of CIP to SSIP. Further, the transient absorption spectrum for B P obtained in the nanosecond experiments on the BP-DABCO system in ACN has a peak at 720 nm which is at the identical position as the peak after the spectral shift in (46) Szwarc, M. Ace. Chem. Res. 1969, 2, 81.
4548
The Journal of Physical Chemistry, Vol. 94, No. 11, 1990
our picosecond study. This correspondence between picosecond and nanosecond experimental results confirms the red shift rather than the blue shift. As regards the BP-DABCO system in MeOH, our results are completely in accordance with the previous observation.I3 The blue shift from 670 to 615 nm indicates the transformation of the CIP to an H-bonded ion pair. In benzene, the ion pair should be a CIP since the nonpolar benzene cannot stabilize the SSIP and no spectral shift is seen. We do not know the reason for the disagreement between our result and that reported by Simon and PetersI3 for the direction of the spectral shift in acetonitrile. The newer picosecond spectrometer used in the present work gives better signal to noise ratio, so finding the position of the absorption is much easier. Also, their reported position for longer times (690 nm) does not agree with our nanosecond result and is shifted in the direction that would be expected if some H-bonding compound were present. Some water introduced with either the solvent or DABCO might explain the observation. Although we hesitate to suggest that effect for the different earlier data, considerable care was taken here to avoid the presence of water in the samples. The present results were found to be reproducible. The nanosecond study reported here indicates that there are two types of ion pairs in the BP-DABCO system depending on the nature of the solvent. In highly polar solvents ( t > IO) the lifetime of the ion pair is on the microsecond scale and the relative ketyl radical is low (the one exception being tert-butyl alcohol, T = 85 ns), whereas in the less polar solvents ( t < lo), the lifetime of the ion pair is less than 100 ns and the ketyl radical yield is high (again with the one exception of dichloromethane, T = 5.1 ~ s ) .To differentiate between these two types of ion pairs, the temperature dependence of the decay of the ion pair was investigated in ACN, MeOH, and o-dichlorobenzene (DCB) and two interesting things were observed. First, the ketyl radical yield in DCB increased with temperature while no such trend was seen in ACN and MeOH. Second, both Arrhenius parameters are very low in ACN and MeOH as compared with the values in DCB (Table 111). This result clearly shows that the nature of the ion pair in DCB is different from that in ACN and MeOH. Since ketyl radicals are formed in DCB, the cation and anion radicals must be close to each other to facilitate the proton transfer in the ion pair. Therefore, the transient in DCB should be the contact ion pair (CIP). With increasing temperature, proton transfer within the ion pair is increased, leading to an increased ketyl radical yield. In ACN, in contrast, there is no ketyl radical formation. Thus the cation and anion radicals should be farther apart, implying a solvent-separated ion pair (SSIP). In polar solvents like ACN, the formation of the SSIP by separation of the ions of the CIP is energetically favorable since the solvation energy is sufficient to overcome the attractive Coulomb energy. In this case, as the temperature increases, the back electron transfer increases, leading to the regeneration of the starting materials. In MeOH, the transient should also be the SSIP as the lifetime of the ion pair is long and there is no dependence of ketyl radical yield on temperature. Other evidence for differing types of ion pairs in polar and less polar solvents comes from the oxygen-quenching studies. In less polar solvents, the ion pair must undergo back electron transfer as no product is evident while in more polar solvents, B P is removed leaving the amine cation. Oxygen can quench a transient in four different way^:'^,^^ energy transfer, electron transfer, enhanced intersystem crossing, and chemical reaction. No evidence was found for quenching of the intermediate by energy transfer to the diamagnetic quenchers perylene (ET = 35 kcal/mol) and trans-stilbene (ET = 50 kcal/mol), but energy transfer to oxygen ( E T = 22.3 kcal/mol) may be possible. The simple quenching, which leaves no observable product in nonpolar solvents with both oxygen and DTBN, rules out reaction and electron transfer to the quencher and strongly suggests enhanced intersystem crossing as the route for those two quenchers. The behavior (47) Timpson, C. J.; Carter, C.C.; Olmsted, 111, J . J . Phys. Chem. 1989, 93. 41 16.
Devadoss and Fessenden of the two quenchers for highly polar solvents is different. DTBN seems to have only a low rate constant, but oxygen rapidly removed B P , leaving DABCO'. Oxygen is known to interact with B P in aqueous solution to form peroxyl radical42 and hence the quenching of the ion pair in polar solvents by chemical reaction is not surprising. A difference in behavior depending on the solvent polarity is reasonable if the ion pair changes from CIP in less polar solvents to SSIP in more polar ones. With the CIP, back electron transfer is rapid and follows intersystem crossing induced by the paramagnetic species. If the ion pair is solvent separated, specific reaction of BP- with oxygen is possible without back electron transfer. The lower reactivity of DABCO' with O2allows it to remain and be observed. C. Mode of Decay of the Transient. The transient's decay follows first-order kinetics in all solvents. In less polar solvents ( t < 10) the yield of the ketyl radical is large, and in highly polar solvents ( t > I O ) it is low. In solvents like acetonitrile, acetone, and tert-butyl alcohol there is no formation of ketyl radicals. In these solvents, back electron transfer is the only mode of decay. In highly polar solvents, even though dissociation of the ion pair into individual radical ions is reasonable, this process does not Seem to take place. As suggested by Kavarnos and Turro,48 the SSIP must be more stable than the free ions in these solvents. In less polar solvents, where the CIP is present, intra-ion-pair proton transfer takes place to give ketyl radicals. In benzene, the presence of a small amount of alcohol inhibits proton transfer by H bonding between the alcohol and B P leading to back electron transfer as the only mode of decay. tert-Butyl alcohol differs from the other alcohols in that the transient is short-lived in this solvent and appears to be a CIP. However, proton transfer within the ion pair is slowed by H bonding as well as the lack of acidic character of the O H proton, so no ketyl radical is produced. In methanol a small amount of ketyl radical is formed even though the transient is an SSIP. In this case the proton may be transferred from the methanol solvent. The present observations on the effect of paramagnetic quenchers, the effect of small amounts of methanol in benzene, and the temperature-dependence studies all help define the role of intersystem crossing in controlling the ion-pair lifetime and the reaction pathway. In benzene, for example, the ion-pair lifetime is short (50 ns) and the ketyl radical yield relatively large. Therefore, the main pathway is proton transfer and separation into neutral radicals which does not depend on intersystem crossing. When the lifetime is increased to IO0 ns by addition of methanol, the proton transfer is slower and decay is by back electron transfer. From this result, intersystem crossing must be the rate-determining step and it must happen with 100-ns lifetime. The behavior in pure o-dichlorobenzene is similar-the lifetime is 90 ns and the decay pathway is partially proton transfer and partially intersystem crossing followed by back electron transfer. Higher temperatures increase the proton transfer. In DCB with 1 % methanol the proton transfer is inhibited and the lifetime is close to 100 ns. The lifetime is independent of temperature as might be expected for intersystem crossing which is controlled more by electronic factors (hyperfine interactions, exchange coupling, and spin-orbit coupling) than energetics. The low preexponential factor of 9.3 X IO6 s-I in DCB(Me0H) is comparable to values for intersystem crossing in b i r a d i ~ a l s ~ ~ while the value for pure o-dichlorobenzene solvent (1.9 X 10" s-I) is more typical of an ordinary activated process. It is less clear what controls the lifetime of the solvent-separated ion pairs in the polar solvents (which also decay by back electron transfer). The decay of the ion pair in acetonitrile and methanol also shows low preexponential factors. However, only slow quenching by DTBN was found and oxygen reacts chemically rather than to speed up intersystem crossing and back electron transfer. This different quenching behavior can be explained if back electron transfer does (48) Kavarnos, G . J.; Turro, N. J. Chem. Reo. 1986, 86, 401. (49) Wang, J.; Doubleday, C.; Turro, N . J. J . Am. Chem. Soc. 1989, I l l . 3962.
. . ... Photoreduction of Benzophenone by UABLU _.
- . - - A
SCHEME 111 'BP
+
DABCO
back electron transfer
i
Alcohol ( MeOH )
Polar solvent ( ACN )
( BP-
CIP
...DABCO')
imaX = 700nm
BP-...
(
CIP
DABCO') = 670 nm
hmlx
I
( BPSSIP
..... DABCO' j A,..
= 720 nm
back electron transfer
t
(
BP- ... DABCO')
"7
R
H-bonded Ion pair hma. = 615 nm
ketyl radlcal BY(S0)
not immediately follow intersystem crossing in this case. It is likely that the lifetimes of 1 ps and longer are controlled by the distance of separation of the ions in the SSIP and the energetics of the back electron transfer rather than by the intersystem crossing. The low preexponential factor would be consistent with nonadiabatic electron transfer. The above discussion emphasizes that slow proton transfer is the reason for the uniqueness of DABCO among aliphatic amines for forming long-lived ion pairs in solvents such as benzene. The possibility of a stereoelectronic effect involving the orientation of the p orbital of the nitrogen in DABCO+ and the C-H bond that will supply the proton has already been noted.I6 The importance of the rigidity of the amine is illustrated here by the immediate formation of ketyl radical when )BP was quenched by N,N'-dimethylpiperazine, N,N,N',N'-tetramethylpropanediamine, and N,N,N',N'-tetramethylbutanediamine.Similar results were obtained by Scaiano et a1.I' with several mesocyclic diamines. D. Spectral S h q t in the Presence of Small Amount of Alcohols. In the picosecond photolysis experiments, a time-dependent spectral shift of the absorption spectrum of B P to shorter wavelengths was observed in the presence of small amounts of various alcohols in benzene. This blue shift continues for up to 5 ns. This spectral shift may be attributed to either reorganization of the alcohol molecules around the oxygen atom of B P or the formation of H bonding between the alcohol molecules and B P . The time scale of the spectral shift shows that it cannot be a simple solvent reorganization since that process for the alcohols under study occurs on a much shorter time scale (a few tens of picoseconds). The extent of the spectral shift depends on the Hbonding ability of the alcohols. The order of the spectral shift is TFE > MeOH > 2 PrOH > t-BuOH, which parallels the strength of the H bonding or acidity. On this basis, the spectral shift is caused by formation of H bonding between alcohol molecules and B P . A question then arises as to the difference in the H bonding of the ground-state ketone on the one hand and B P on the other. Benzophenone is known to be H-bonded to alcohols in the ground state and therefore B P is expected to be
The Journal of Physical Chemistry, Vol. 94, No. 11 1990 4549 H-bonded when it is formed. How can the spectral shift be attributed to H-bond formation? There are two possible explanations. One is, as Ichikawa et aLso reported, that the site of H bonding changes from the px to the pz orbital of the oxygen atom of B P . The other explanation also involves the detailed electronic structure. The lowest triplet state, T I , of BP has n-r* character and a lower dipole moment than the ground state.51 Thus H bonding in the ground state may be broken in the excited state as a result of the shift in electron density away from the oxygen atom. After the ion pair is produced, the H bond will be formed again at the favored site. The present work does not distinguish between these two types of rearrangements. E. Proposed Mechanism. On the basis of the above experimental observations, we propose a mechanism (Scheme 111) which is a modification of that given by Simon and Peters and also includes the decay mechanism for the ion pair. According to this mechanism, the initial transient formed in the photoreduction is a CIP produced by electron transfer from DABCO to 3BP. In alcohols, the CIP transforms to an H-bonded ion pair which has A,, at a shorter wavelength than the CIP. In polar solvents ( t > lo), the CIP transforms into an SSIP which absorbs at longer wavelength. (The one exception among the solvents studied is tert-butyl alcohol, which is a weak H-bonding alcohol.) As discussed in section IV.D, the rate of decay of the ion pair and the corresponding reaction pathway are affected by a complex interplay among various factors including intersystem crossing (ISC), solvation of the ion pair, the associated energetics, and ease of proton transfer. V. Conclusions The combined nanosecond and picosecond photolysis study described here gives a clearer and more complete picture of the transient in the photoreduction of BP by DABCO than could be obtained from either type of experiment alone. The initial step in this photoreduction, in all solvents polar and nonpolar, is an electron transfer from DABCO to 3BP to form a CIP. Nonpolar solvents with a lone pair of electrons or with A electrons stabilize this (BP--DABCO+) ion pair. The ketyl radical yield is large in less polar solvents and minimal in highly polar solvents. A small amount of alcohol in benzene stabilizes the ion pair by H bonding. However, the energy-wasting step of back electron transfer is facilitated while the ketyl radical yield is drastically reduced due to inhibition of intra-ion-pair proton transfer. A blue shift in the spectrum of B P is observed in the presence of trace amounts of alcohols. The magnitude of the spectral shift depends on the strength of H bonding by the alcohol. It has proved possible to obtain the absorption spectrum of DABCO+ ion by selectively quenching B P by oxygen in highly polar solvents. The pico- and nanosecond absorption spectra combined with the temperaturedependence study and quenching by oxygen indicate that the initial contact ion pair (CIP) remains (on the 100-ps time scale) in less polar solvents and is converted into a solvent-separated ion pair (SSIP) in highly polar solvents. The CIP decays either by proton transfer to form ketyl radical or, if this transfer is inhibited by H bonding, by intersystem crossing that leads to back electron transfer. The SSIP decay rate depends on factors other than intersystem crossing. Contrary to the report by Simon and Pet e r ~ , a' ~shift in the absorption maximum to the red is seen in ACN. This shift supports the idea of a transformation from CIP to SSIP.
Acknowledgment. The research herein was supported by the Office of Basic Energy Sciences of the Department of Energy. This is Document No. NDRL-3239 from the Notre Dame Radiation Laboratory. We thank Drs. P. K. Das, T. Ebbesen, and J.,D. Simon for helpful discussions. (SO) Ichikawa, T.; Ishikawa, Y.; Yoshida. H. J . Phys. Chem. 1988,92, 508. (51) Fessenden, R. W.; Carton, P. M.; Shimamori, H.; Scaiano, J. C. J . Phys. Chem. 1982, 86, 3803.
