J. Phys. Chem. 1993, 97, 8630-8637
8630
Nanosecond Laser Flash Photolysis Study of the Kinetics and Mechanism of Photoreduction of Decafluorobenzophenone Lian C. T. Shoute' and Jai P. Mittal' Chemistry Division, Bhabha Atomic Research Centre, Bombay 400 085, India Received: April 14, 1993
Laser pulse excitation of decafluorobenzophenone (DFB) in acetonitrile yields the triplet 3DFB after rapid intersystem crossing from the singlet 1DFB. The triplet has absorption maxima at 310 and 490 nm. 3DFB abstracts hydrogen from donors such as alkanes and alcohols in a radical-like mechanism to yield ketyl radical. The rate constant kq is 1 0 4 0 times higher than that of the triplet benzophenone 3BP. Ketyl radical (DFBH) has absorption maxima at 320 and 530 nm. In aqueous solution, the ketyl radical has a pKa = 6.9, and the anion has maxima at 335 and 630 nm. The kq for reaction between 3DFB and alkenes depend on their redox potentials. This can be interpreted as due to the formation of a triplet charge-transfer complex (CT) or an exciplex. A Rehm-Weller correlation plot log kq vs AG reveals that the exciplex has a weak C T contribution. For alkenes with allylic hydrogen, ketyl radical was detected after the decay of 3DFB. Aliphatic amines form ground-state C T complex with DFB. Amine quenches 3DFB with diffusion-controlled rate constant. The transients formed depend on the amine oxidation potential and solvent polarity. In the case of 1,Cdiazabicyclo[2.2.2]octane (DABCO) = 0.68 V), only dissociated free ions are observed. However, with triethylamine (TEA) = 0.97 V) both ketyl radical and free ions are observed. The ketyl radical yield decreases with solvent polarity and at a longer delay time the ketyl radical ionizes to yield the anion. The photoreduction of DFB by amine can be interpreted on the basis of initial formation of triplet ion-pair (31P) which subsequently undergoes proton transfer to yield ketyl radical or dissociates to yield free ions.
(q72
(q2
formation followed by proton transfer. This has been confirmed by the detection of dissociated free ion in flash photolysis.lsPeters Electron, hydrogen atom, and proton transfer reactions are et a1.I6 were the first to observe the initial ion pair using laser the most fundamental reactions in chemistry and biology. The flash photolysis with picosecond time resolution. photophysics and the photochemistry of benzophenone (BP) have Elaboratestudies of BP photoreduction by amine were reported been extensively studied in the past three decades to understand by Mataga et al.17J8 using femtosecond-picosecond laser flash the dynamics and the mechanism of these reaction^.^-'^ Triplet photolysis. They observed the real time evolution of BP singlet BP abstracts hydrogen from donors such as alkanes and alcohols to the triplet and the subsequent formation of the ion pair which in a radical-like mechanism to yield ketyl radical in polar as well undergoes proton transfer to yield ketyl radical and/or dissociates as nonpolar medium.'-3 to yield free ions. They reported the formation of three kinds of The photocycloaddition reaction of an olefin to a ketone, the ion pairs, viz., '(BP-..*AH+),,m, '(BP-.-AH+),,,, and Patern+Buchi reaction, has been the subject of numerous 3(BP-.AH+),,, formed by excitation of the ground-state CT investigation owing to the importance of its synthetic ~ t i l i t y . ~ , ~ complex and by quenching of 'BP and the 3BP,respectively. The Mechanistic investigations suggest the involvement of 1,4three types of ion pairs decay by different mechanisms: biradical intermediate for the product formation.6 However, it '(BP-..AH+),, decays via charge recombination and ionic has not been possible to determine whether 1,4-biradicalis formed dissociation and yields no ketyl radical; '(BP-.AH+),,, decays by direct addition of triplet ketone to the olefin in a radical-like predominantly via charge recombination and ionic dissociation process or it has a charge-transfer (CT) precursor. Several with proton transfer as a minor route; 3(BP--AH+),,, decays via workers7-12 have suggestedthe involvementof triplet CT complex proton transfer and ionic dissociation. They observed that the or exciplex as the primary process in the interaction. However, formation of ketyl radical from IP depends on the oxidation a complete electron-transfer process yielding the radical ions has potential of the amine. The ketyl radical yield increases with the been observed less frequently;examples are thereduction of triplet increase in the amine oxidation potential. BP by methoxybenzenes,lZ1,4-dioxene," tetraethoxyethene, and The photophysics and the photochemistry of decafluoroben1,3-dioxoles.1I Triplet exciplexes are elusive species, and conzophenone (DFB) have been studied by several workers.s25 3DFB sequently their presence in a particular photochemical reaction abstracts hydrogen from donors with rate constants k, 10 times is often inferred on the basis of indirect evidence. Although there or more faster than that of 3BP and reacts with benzene 3 orders is general agreement that there is some sort of CT intervention of magnitude faster.20.21Boate et al.zOhave recently reported the in these reaction, there are recent reports on direct spectroscopic rateconstants of 3DFB with several substrates and its dependence detection of 1,Cdiradical as the primary species.13 on solvent polarity. They obtained a linear correlation between log k, vs The mechanism of photoreduction of BP by amines has been the oxidation potentials of alkenes and aromatextensively investigated.14-'9 Quenching of BP by amine usually ics. Unlike the ketyl radical derived from BP which dimerizes to has high yield of ketyl radical and the reaction rate is rather close to that of diffusion-controlledreactions, while the rate for hydrogen form pinacol, the ketyl radical derived from DFB disproportionates (in polar solvent) to yield decafluorobenzhydrol. It was reported abstraction from alcohols or alkanes is about 10 times smaller than those from amines. On the basis of these results, Cohen et that the stable products produced in the photolysis of DFB are al.14proposed the reaction mechanism for hydrogen abstraction dominated by cross radical reactionz1whereas, for BP, pinacol of 3BP from amines, which assumed CT complex or ion-pair is the main product.z6
Introduction
$;,,
0022-3654/93/2097-8630$04.00/0
0 1993 American Chemical Society
Photoreduction of Decafluorobenzophenone Although a number of workers have reported studies on the photochemistry of DFB, the studies are by no means exhaustive. For example, only a few rate constants have been reported for alkanes, alkenes, alcohols, and amines. In fact, for amines no attempt has been made to study the transients involved. For fully fluorinated compounds the acid dissociation constants are generally quite different from the unfluorinated compounds.27 Extensive data on the rate constants and information on the transients involved in the reactions are essential to understand the mechanism of any photochemical reactions. The photochemistry of DFB will also be interesting in connection with the perfluoro effect.28-30 Since molecular photophysics and photochemistry of BP and its derivatives in fluid organic solvents represent a keystone in the development and history of mechanistic organic photochemistry?l a detailed study of the photochemistry of DFB is quite relevant. In the present paper, we report a detailed study on the kinetics and the transients involved in the photoreduction of DFB to understand the mechanism of photoreduction using nanosecond laser flash photolysis.
Experimental Section The laser kinetic spectrophotometer was supplied by Applied Photophysics Ltd. UK (K-347). The excitation sources were either an Excimer laser (KrF, 248 nm, 12 ns, 100mJ) or a Photon Technology (PL-2300) nitrogen laser (337 nm, 600 ps, 1.5 mJ). The detection system consisted of a 250-W pulse xenon arc lamp (IREM, Italy), a 1 cm optical path length irradiation cell, a monochromator (APP Ltd.), a R-955 photomultiplier, appropriate shutters, lenses, and optical filters. The signals were digitized with a Gould Biomation 4500 transient oscilloscope and the signal were analyzed by a PC. The samples for laser flash photolysis were freshly prepared and deaerated by bubbling high-purity nitrogen (IOLAR grade from Indian Oxygen Ltd.). The pH of the solutions for pKa measurement were adjusted by using AnalaR grade HClO,, KH2PO1, Na2HP04, Na~B407,and KOH. Water was purified by a Millipore Super Q system. Optically thin solution was used for determination of the extinction coefficients and the quantum yields. Decafluorobenzophenone (Aldrich) was used as received. Acetonitrile, benzene, n-hexane, cyclohexane, methanol, ethanol, and 2-propanol were spectroscopic grade obtained from Spectrochem India Ltd. Analytical grade tert-butyl alcohol was from SISCO Research Lab. Haxafluorobenzene and 2,3,4,5,6-pentafluorostyrene was from PCR research chemicals. Methacrylonitrile and vinyl acetate (both from Fluka) were passed through activated neutral alumina column. Cyclohexene (Koch Light Lab.) was vacuum distilled. 1,3-Cyclohexadiene (Merck) and 1,4-~yclohexadiene(Merck), 3-methylpent-1 -ene (Fluka), diphenylamine (Aldrich), and 1,4-diazabicyclo[2.2.2]octane (Fluka) were used as received. Spectrograde carbon tetrachloride (Spectrochem) and 3-methylpentane (Fluka) were distilled. 2-Methyltetrahydrofuran (Fluka) was treated with NaOH and distilled with metallic sodium. Allyl alcohol (BDH) was vacuum distilled. N,N-Dimethylaniline, N,N-dimethyl-p-toluidine, and triethylamine (all Fluka) were vacuum distilled with NaOH.
Results and Discussion The triplet energy of DFB (ET = 68.5 kcal/mol) in both polar and nonpolar solvents is similar to that of BP.203 Both have unit intersystem crossing efficiency from the singlet to the triplet (@ISC = 1) and the triplet state has n,r* configuration. Hence, their photochemistry can be expected to be similar in general, although differences may arise due to the difference in the ionization potential, electron affinity,32and strengthening of the u bond due to perfluoro effect.28-30
The Journal of Physical Chemistry, Vol. 97, No. 33, 1993 8631
t 0.04
n 0 a
4
0.02
t
0.00 200
300
400
500
600
700
wavelength/nm Figure 1. Transient absorptionspectra for 5 X 1tSM DFB in acetonitrile obtained at different times after excitation with a 124s pulse of 248-nm KrF laser: ( 0 )0.1 ps and (A) 8 p s . 0.12
0.08
n 0 a 0.04
0.00
200
300
400
500
600
700
wavelength/nm Figure 2. Transient absorption spectra of the ketyl radical of DFB for 5 X 1 t SM DFB in 2-propanol at different times after excitation with
a 12-11spulse of 248-nm KrF laser:
( 0 )0.1 ps
and (A) 28 ps.
Transients in Organic Solvents. Figure 1 shows transient spectrum observed on laser flash excitation at 248 nm of acetonitrile solution of 5 X 10-5 M DFB. The spectrum is similar to the triplet spectrum reported earlierZoand has maxima at 310 and 490 nm. Similar spectrum was obtained in non-hydrogendonating solvents such as carbon tetrachloride and in the inert solvents perfluorocyclohexane and perfluoro-2-butyltetrahydrofuran on excitation a t 337 nm. This shows that the spectrum is due to the triplet state of DFB formed by rapid intersystem crossing from thesinglet state produced on laser excitation. In acetonitrile, the triplet (3DFB) decay essentially by first-order kinetics with a lifetime of 3 ps. The lifetime remains essentially unchanged when the laser intensity was reduced a 100 times on excitation with wavelength of 337 nm. The triplet lifetimes in acetonitrile, carbon tetrachloride, perfluorocyclohexane, and perfluoro-2butyltetrahydrofuran are 3,2,4.5, and 4.5 ps, respectively. The shorter triplet lifetime*22 in our system could be due to impurities, but no attempt was made to further purify the samples as this does not affect our results. The decay of 3DFB leaves a small residual spectrum which can be attributed to the ketyl radical in acetonitrile but not in inert solvents. Figure 2 shows the spectrum of the ketyl radical obtained on laser excitation a t 248 nm of 5 X 10-5 M DFB in 2-propanol. The spectrum has bands a t 320 and 530 nm similar to the reported spectrum in literature.20 Ketyl radical was also obtained on laser
Shoute and Mittal
8632 The Journal of Physical Chemistry, Vol. 97, No. 33, 1993
1 0.08
0'15 0.10
n
-
n 0.06 I
0
0
a
a 0.04
-
0.02
, , , , , , , , , , ,
+,, 200
600
400
800
wavelength/nm Figure 3. Transient absorption spectra for 5 X M DFB in aqueous 2-propanol (1:l H20: i-PrOH) solution on excitation with a 12-nepulse of 248-nm KrF laser. (A) Ketyl radical absorption at delays ( 0 )0.1 and (A)70 ps in acidic solution (2 X M HCIO,). (B) Anion absorption at delays (D) 0.1 and (0) 70 ps in alkaline solution (1k2M KOH).
excitation of DFB in a number of hydrogen-donating solvents/ substrates (RH) such as alkanes, alcohols, and ethers. The n,r* triplet carbonyl has similar electronic configuration to that of an oxygen-centered Hence, 3DFBabstracts hydrogen from R H in a radical-like mechanism as follows:
hv
'DFB 'DFB
-
+ DFB +RH
'DFB
(1)
3DFB DFBH
(2)
+R
(3)
The singlet 'DFB undergoes fast intersystem crossingto the triplet, so it cannot be observed in the nanosecond time scale. The ketyl radical has long lifetime. The absorption coefficientsfor the triplet and the ketyl radical of DFB were determined by comparative method using BP triplet and ketyl radical as standards.17 The extinction coefficients obtained are ~(490)= 3000 M-1 cm-1 for 3DFB relative to ( ( 5 2 5 ) = 6500 M-1 cm-1 for 3BP in acetonitrile and ((530) = 5800 M-l cm-1 for DFBH relative to 4545) = 4600 M-1 cm-1 for BPH in 2-propanol. In the calculation, CPK = 1 at 100 ns is assumed for both DFBH and BPH. Transients in Aqueous Solution. Figure 3 shows the spectrum of the ketyl radical and the radical anion of DFB obtained on laser excitation at 248 nm in aqueous 2-propanol (1:l water:Z propanol). The spectrum of the ketyl radical was obtained in acidic solution (2 X 10-3 M HC104) and the anion spectrum was obtained in alkaline solution ( M KOH). The ketyl radical spectrum has bands at 320 and 530 nm as in organic solvents. The anion has bands at 335 and 630 nm similar to the spectrum of the anion obtained on addition of electron to DFB in alcoholic 3-methylpentane at low temperature reported earlier.34 Similar spectrum was obtained for the anion on pulse radiolysis35 of DFB in aqueous alcohol solution. The rate constants determined by pulse radiolysis for reaction of DFB with hydrated electron and (CH3)zCOH are 1.4 X 10'0 and 9.2 X 107 M-1 6-1, respectively. Despite their high electron affinities, reduction of perfluorinated compounds by organic reducing radicals has been reported to have slow rate c0nstants.3~ The pK, of the ketyl radical was determined by laser excitation at 248 nm of aqueous 2-propanol solution (1:l H2O:iPrOH) of 5 X 10-5 M DFB. pK. = 6.9
DFBH
C=
DFB-+H+
(4)
Figure4. Dependenceof anion absorption at 630 nm on pH of the solution for 5 X 10-5 M DFB in aqueous 2-propanol (1 :1 H2Oi-PrOH). The plot yields a pK, of 6.9.
TABLE I: Rate Constants for Quenching of 3DFB by Hydrogen Donors in Acetonitrile quencher IO-%,(M-I s-I) cyclohexane 13.3 1.9 n-hexane 5.8 3-methylpcntane 0.1 fert-butyl alcohol 4.9 methanol ethanol
20.0 49.1 334.0
2-propanol 2-methyltetrahydrofuran
The dependence of the OD at 630 nm due to the anion was measured as a function of the pH of the solution. As in the case of BPH,97 slow deprotonation of DFBH was observed in the microsecond time scale. Figure 4 shows the plot of OD(630) vs pH. From this plot, a pK, = 6.9 was obtained. This shows that DFBH is much more acidic than that of BPH (pK = 9.2).38 The enhanced acidity of DFBH is as expected for fluorine substituted compo~nds.2~J6 Hydrogen Abstraction Reactions. Triplet ketone abstracts hydrogen from donors (RH) such as alkanes and alcohols in a radical-like mechanism to yield ketyl radical. The rate constant b, i.e., eq 3, for hydrogen abstraction was determined in acetonitrilesolution by following the decay of the triplet absorption at 490 nm. The experimentally measured pseudo-first-orderrate kob, In ((OD - OD,)/(ODo - OD,)) = -kobt), obtained at different concentration of RH, is related to the bimolecular rate constant kq.
+
= k, k,[RH] (5) where ko-l is the lifetime of 3DFB in the absence of solute. The plot kob vs [RH] is linear and the slope yields kq. Table I lists the rate constants for hydrogen abstraction from alkanes and alcohols. For alkanesa good measure of the reactivity depending on the nature of hydrogen, Le., primary, secondary, and tertiary, can be obtained from the ratio of the rate constant to the number of abstractable hydrogens. By comparing these values obtained for cyclohexane, n-hexane, 3-methylpentane, and tert-butyl alcohol, a reasonable estimate on their reactivity can be obtained viz., primary < secondary < tertiary and cyclic are more reactive than acyclic. The same holds true for alcohols. In all these compounds, the quenching of 3DFB results in the formation of ketyl radical. An interesting observation is that the rate constants for hydrogen abstraction for 3DFB is an order of magnitude higher than the corresponding value for 3BP.l-3 Since both 3DFB and
,k
Photoreduction of Decafluorobenzophenone
The Journal of Physical Chemistry, Vol. 97, No. 33, 1993 8633
TABLE II: Rate Constants and Ketyl Radical Yield for Quenching of 3DFB by Alkenes and Amines in Acetonitrile quencher 1 P & , (M-l s-') q,"(V vs SCE) AG (eV) aK lo+, (M-I s-l) l P & , (M-1 methacrylonitrile hexafluorobenzene allyl alcohol benzene vinyl acetate 3-methylpent- 1-ene cyclohexene 1,4-~yclohexadiene 1,3-~yclohexadiene
2,3,4,5,6-pentafluorostyrene triethylamine diphenylamine N,N-dimethylaniline
N,N-dimethyl-p-toluidine
DABCO
0.37 0.008 2.1 0.73 2.6 2.3 19.4 18.7 58 41 96 194 147 29 1 75
3.76 3.21 3.00 2.61 2.55 2.38 2.33 2.21 1.83 0.97 0.96 0.76 0.69 0.68
3BP have the same triplet energy, ET = 68.5 kcal/mol, and configuration n,?r*, these differences can be attributed to two effects, viz., electron affinity32 and perfluoro e f f e ~ t . ~ ~ The -~O higher electron affinity of DFB (1-61 and 0.62 eV for DFB and BP, respectively)32 would make the oxygen of the carbonyl more electrophilic as the electron cloud is more delocalized in the phenyl ring. The perfluoro effect, which stabilizes the u bond, would enhance the strength of 0-H bond formed. Hence, the enhanced reactivity of 3DFB can be accounted by electron affinity and perfluoroeffect. Reaction with Alkenes. The major remaining question in the mechanism of the Paternc-Buchi reaction concerns the means of formation of the biradical:613 is it the result of direct attack of the excited ketone on the olefin, or is it formed after initial charge transfer to generate exciplex? To get an insight into this question for the reaction of 3DFB with alkenes, we have determined the rate constant k, as a function of the free energy change AG.In general, the feasibility of producing radical ions in polar solvents via photoinduced electron transfer can be predicted by the wellknown equation derived by Rehm and Weller.39
The free enthalpy change (AG)for the photoinduced electron transfer is related to the redox potentials of the acceptor @$(A) and of the donor (GY2(D)), the energy of the excited (singlet or triplet) state of the acceptor (Ecxc(A*)), and a term eEcoulr which takes into account the solvent properties far ion separation (eEcOul= -0.055 eV in acetonitrile). TheE&(D) (vs SCE) for alkenes (as donors) were evaluated from their ionization potentials using relation q 2 ( D ) = 0.92(IP) - 5.9.40 The triplet of DFB is quenched by various alkenes in acetonitrile at room temperature. The quenching rate constant k, in acetonitrile was obtained from the slope of the linear plot of the experimentally measured decay rate kob of the triplet at 490 nm as a function of alkene concentration as in eq 5. The rate constants are listed in Table 11. The plot of log k,vs AG,the Rehm-Weller correlation plot, is shown in Figure 5. In the exergonic region AG < 0, k, values for the amines are also plotted to show the plateau region. In the endergonic region AG > 0, the correlation is linear and yields slope -1.1 eV-l for the alkenes. The CT contribution to the excited complex, or the exciplex, can be estimated in different ways. Quenching processes which involve essentially complete CT display linear log k, vs AG with slope approaching -1 7 eV-1,39 whereas processes involving only partial CT show shallower slopes. The slope of the line in Figure 5 correspondsto ca. 6.5% CT contribution in the complex. A second probe for extent of CT is the polar solvent effect. Whereas k,(CH3CN)/k,(CCl4) = 13for full CT, we have found negligible solvent effect kq(CH3CN)/k,(CC14) = 0.6-1.2 for the DFB/alkene system. A higher ratio of 1.1-2 was reported by Boate et al.,O for the aromatics. Small solvent effects have been reported in
1.96 1.41 1.20 0.8 1 0.76 0.58 0.54 0.42 0.03
-
0.0 0.0 0.15 0.0 0.0 0.06 0.09 0.45 0.06
-
s-1)
-
0.31
1.8
-
-
0.14 1.8 8.4 3.5
2.2 17.7 10.3 54.7
-0.85 -0.86 -1.06 -1.13 -1.14
c
0 -2.0
4
-1.0
0.0
1.o
2.0
AG Figure 5. Rehm-Weller correlation plot of log &, vs AG for alkenes and amines. The plot yields slope -1.1 eV-1 for the AG > 0 region which corresponds to 6.5% charge transfer.
many systems believed to involve only fractional CT in the quenching process. Compared to similar correlations (Rehm-Weller) obtained for biacetyl, benzil, and BP, the slope for DFB/alkene system is shallower.11 The plot differs considerably from that of biacetyl and benzil/alkene systems but is quite similar to that of the BP/ alkene system. However, it is interesting to note that the slope for DFB/alkene (-1 .I eV-l) is less than that of BP/alkene (-1.5 eV-1) system. Based on the reduction potential (Ef;dz(A) of -2.16, -1.66, -1.50, and -1.21 V for BP, biacetyl, benzil, and DFB, respectively),llJO the slope for the DFB system can be expected to be more stiff than that of BP and the correlation similar to that of biacetyl and benzil. The slope for biacetyl and benzil is -1.7 eV-1 in the endergonic region and that in the exergonic region is much higher, >-5.8 eV-1. The main difference between DFB and BP on the one hand and biacetyl and benzil on the other is that k, is several orders of magnitude higher for the former in the AG > 0 region than the latter.1-12.20s31 There are reports which treat k, as the sum of the rate constants for hydrogen abstraction and interaction with ?r bond,4I that is, k, = k, + k,, and the quenching does not proceed via a common intermediate CT complex. This apparently could explain smaller slope for DFB compared to BP because k, is higher for DFB in the AG > 0 region. Similar arguments could be given for the biacetyl and benzil system as their k, is much smaller than that of the DFB and BP systems. However, determination of the quantum yield @K of the ketyl radical reveal that this mechanism is not true as k,
