3987
J. Phys. Chem. 1986, 90, 3987-3993 rhodium(II1) in benzene.I0 From the comparison in photochemistry between CH3Rh%EP and C2H51nn1TPP,we concluded that no wavelength dependence of the quantum yields for the photocomposition of C2H51n1"TPP implies that electronic relaxation from the Soret to the Q band occurs efficiently in case of the excited molecules of C2H51n"'TPP. Laser photolysis studies show that the photoexcited triplet state of C2H51n"'TPP has a T-T absorption spectrum with peak maxima around 480 and 800 nm. Ferrocene is found to quench the triplet state. The quenching rate constant by ferrocene is obtained as 1.0 X lo9 M-' s-I. Since the triplet energy level of the ligand in metallotetraphenylporphyrins" is known to range from 12.5 to 14 X lo3 cm-I and that of ferrocene is ca. 15.0 X lo3cm-I,l2 C2H51n"'TPP is expected to have a ?M* triplet energy slightly lower than that of ferrocene. Herkstroeter et a1.I2 have studied the quenching rate constants by ferrocene with the variation of the triplet energy levels of the sensitizers. According to their results, when the triplet energy-level ranges from 12.5 to 14 X lo3 cm-I, the quenching rate constant is expected to be 5 X 108-2 X lo9 M-' s-'. This value is rather in good accord with the rate constant for quenching of the triplet C2H5In"'TPP by ferrocene. We, therefore, consider that the quenching mechanism by ferrocene, Fer, is expressed by triplet energy transfer 3[C2H51n111TPP] Fer C2H51n111TPP 3Fer
The quantum yields for the photodecomposition of C2H5InII'TPP are decreased with the addition of ferrocene. The triplet state of C2H5In"'TPP is concluded to be responsible for the photoreaction on the basis of the quantum yield and the triplet decay rate measurements in the presence of ferrocene. This conclusion is further supported by the quenching experiment performed with the use of T N F as a quencher: TNF quenches the photoexcited triplet state of C2H51n"'TPP but enhances the quantum yields for photodecomposition of C2H51n"'TPP. The variation in the quantum yields at various concentration of T N F can be explained by assuming the following reaction:
(10) Hoshino, M.; Yasufuku, K. Chem. Phys. Lett. 1985, 259-262. (1 1) Hopf, F. R.; Whitten, D. G. In Porphyrins and Metalloporphyrins; Smith, K. M., Ed.; Elsevier: Amsterdam, 1975; p 667. (12) Herkstroeter, W. G. J. Am. Chem. Soc. 1975, 97, 4161-4167.
(13) Gouterman, M.; Stevenson, P. E. J . Chem. Phys. 1962, 37, 2266-2269. (14) Barry, C. D.; Hill, H. A. 0.;Mann, B. E.: Sadler, P. J.; Williams, R. P. J. J . Am. Chem. SOC.1973, 95, 4545-4551.
+
-
+
+ + TNF
3[C2H51n111TPP]
-
C2H5
+ (1n"TPP) + T N F
The m*triplet state interacts with TNF, resulting in the increase in the quantum yields for the photodecomposition of C2H51dr1TPP. Since the electron affinity of TNF is considered to be large, it is expected that T N F interacts with the porphyrin plane of the triplet state as in the case of the charge-transfer complexes of metalloporphyrins and trinitr~benzene.'~,'~ Consequently the electron density on the central In atom diminishes, and therefore, the strength of the carbon-indium bond tends to decrease, resulting in the facile dissociation of the carbon-indium bond. Registry No. TNF, 129-79-3; C2H5In"'TPP, 63036-65-7; ferrocene, 102-54-5.
Nanosecond Transient Processes in the Triethylamine Quenching of Benzophenone Triplets in Aqueous Alkaline Media. Substituent Effect, Ketyl Radical Deprotonation, and Secondary Photoreduction Kinetics' K. Bhattacharyya and P. K. Das* Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556 (Received: November 25, 1985)
In the course of benzophenone triplet quenching by triethylamine (TEA) at high concentrations in alkaline aqueous acetonitrile, two temporally distinct processes are observed for ketyl radical anion formation. The fast component occurs on a nanosecond time scale, has kinetics sensitive to basicity and water content of the medium, and i s ascribed to the deprotonation of the diphenylhydroxymethyl radical initially produced as a result of subnanosecond intra-ion-pair proton transfer. The slow process occurs on a microsecond time scale and is characterized by pseudo-first-order rate constants linearly dependent on ketone ground-state concentration; this is assigned to the one-electron reduction of the ketone by the methyl(diethylamino)methyl radical (derived from TEA). Substituent effects on the kinetics of the two processes follow trends expected from those of the acidity of diarylhydroxymethyl radicals and of the behavior of diaryl ketones as oxidants. Neither of the two processes is observed with N,N-dimethylaniline (DMA) and 1,4-diazabicyclo[2.2.2]octane(DABCO) as quenchers. The electron or hydrogen transfer yields in the course of diaryl ketone triplet quenching by the three amines are all close to unity, suggesting that the back electron transfer in the triplet ion pairs is relatively unimportant.
Introduction
SCHEME 1
For nearly two decades, the electron-transfer quenching of aromatic carbonyl triplets by amines has been an active area of
>c-d
photochemical
(T,)
+
>CH-NR,
On the basis of photoreduction
(1) The work described herein was supported by the Office of Basic Energy Sciences, Department of Energy. This is Document No. NDRL-2789 from the Notre Dame Radiation Laboratory. (2) Cohen, S.G.: Parola, A. H.; Parsons, Jr., G. H. Chem. Reu. 1973,73, 141-161 and earlier references therein. (3) Scaiano, J. C. J . Photochem. 1973/74, 2, 81-118. (4) Turro, N.J. Modern Molecular Photochemistry; Benjamin/Cummings: Menlo Park, CA, 1978; Chapter 10.
h
/'ilk
2 [>C -O..
>C-OH+>&NR,
(If)
(rn)
I
- H+
*.
>CH-NR~]
>O6+> c H - ~ R ~>C=O(SJ+>CH-NR~ (nz) (PI
t
quantum yields and phosphorescence quenching studies, Cohen and c o - ~ o r k e r s ~first * ~ *proposed ~ that the initial step in the
0022-3654/86/2090-3987$01.50/00 1986 American Chemical Society
Bhattacharyya and Das
3988 The Journal of Physical Chemistry, Vol. 90, No. 17, 1986
TABLE I: Kinetic Data regarding the Quenching of BP Triplets by TEA (kTJ Reduction of BP Ground States by Methyl(dietby1amino)metbyl Radical (kd), and Deprotonation of Ketyl Radicals ( k h ) ;Solvent: 1:9 H 2 0 M e C N (v/v) Containing 0.02 NaOH
ET,’
substituent@)
kcal mol-’
none (1) 4,4’-dimethoxy (2) 4-methoxy (3) 4-methyl (4) 4-flUOrO (5) 4-chloro (6) 4,4’-dichloro (7) 4-trifluoromethyl (8)
69.2 70.3 69.2 69.2 69.7 68.8 68.0 68.1
-E1/2,red.b
V 1.70 1.78 1.74 I .73 1.71 1.63 1.57 1.51
kT
109 Mq-I 2.2 0.55 1.2 1.5 2.7 2.6 3.3 4.2
s-l
kdeprd 106 s-I
lo8 M-I s-l
8.3 7.4 (0.14) 7.4 (0.14) 7.3 (0.072) 8.5 (0.072) 9.8 (0.072) 2 1 5 (0.036) 2 1 5 (0.036)
1.2 0.052 0.29 0.83 2.1 4.3 14 16
krcd.e
Triplets energies, taken from ref 23; the data are in polar solvents, except 4,4’-dichlorobenzophenonefor which the quoted value corresponds to ~ .(see the Experimental Section). ‘Maximum error, f15%. df20%; molar concentrations of TEA are given in the a nonpolar solvent. b ~ SCE parentheses. SCHEME I1 >C-OH+ ~ C - C H - N R .
quenching process is the transfer of an electron from the amine to the ketone triplet (rate constant, kir)forming a charge-transfer complex (CTC). This is followed either by formation of ketyl and aminoalkyl radicals as a result of hydrogen transfer from the a carbon of the amine cation to the carbonyl oxygen (rate constant, kh) or by spin inversion and return to ground states of reactants (rate constant, ke). These steps are shown in Scheme I. In earlier studies, Cohen and c o - ~ o r k e r explained s ~ ~ ~ ~ ~the observed inefficiency of photoreduction under steady-state irradiation in terms of favorable competition between the hydrogen transfer (kh) and the reversion to ground state (k,). Recent nanosecond laser flash photolysis investigations12 have shown that the primary quantum yields of ketyl radical formation from benzophenone triplet are nearly unity with most amines as quenchers (that is, kh 7> ke). Under this situation, the low steady-state photoreduction yields become ascribableI2 to disproportionation reactions of ketyl and alkylaminyl or aminoalkyl radicals on a longer time scale. The fast electron-transfer step leading to a radical anion followed by proton transfer producing a ketyl radical (the latter in the case of triethylamine as quencher) has been amply substantiated in recent studies of picosecond dynamics by Peters et al.l3-I5 An interesting feature in the amine quenching of carbonyl triplets is the observationI6 of the quantum yield of photoreduction greater than unity (at high ketone concentrations) as is also noted in the case of photoreduction by 2-propanol.” To explain this, Cohen et al.18.’9proposed that the aminoalkyl radical (VI) reduces
( 5 ) Parola, A. H.; Rose, A. W.; Cohen, S. G.J. Am. Chem. SOC.1975,97, 6202-6209. ( 6 ) Parola, A. H.; Cohen, S. G. J . Photochem. 1980,12, 41-50. (7) Bartholomew, R. F.; Davidson, R. S.; Lambeth, P. F.; McKellar, .I.F.; Turner, P. H. J. Chem. SOC.,Perkin Trans. 2 1972, 577-582. (8) Davidson, R. S.; Lambeth, P. H.; Santhanam, M. J . Chem. SOC., Perkin Trans. 2 1972, 2351-2355. Davidson, R. S . ; Santhanam, M. Ibid. 1972, 2355-2359. (9) Roth, H. D.; Lamola, A. A. J. Am. Chem. Sac. 1974,96,6270-6275. (10) (a) Arimitsu, S.; Masuhara, H.; Mataga, N.; Tsubmura, H. J. Phys. Chem. 1975, 79, 1255-1259. (b) Arimitsu, S.; Masuhara, H. Chem. Phys. Lett. 1973, 22, 543-546. (11) Gorman, A. A,; Parekh, C. T.; Rodgers, M. A. J.; Smith, P. G. J . Photochem. 1978, 9, 11-17. (12) (a) Inbar, S.; Linschitz, H.; Cohen, S. G. J. Am. Chem. Sac. 1981, 103, 1048-1054; (b) 1980, 102, 1419-1421. (13) Shaefer, C. G.; Peters, K. S . J. Am. Chem. SOC.1980, 102, 7566-7567. (14) Peters, K. S . ; Freilich, S. C.; Schaeffer, C. G. J . Am. Chem. Sac. 1980, 102, 5701-5702. (15) Simon, J. D.; Peters, K. S . J. Am. Chem. Soc. 1981,I03,6403-6406. (16) Cohen, S. C.; Chao, H. M. J. Am. Chem. Sac. 1968, 90, 165-173. (17) Pitts, Jr., J. N.; Letsinger, R. L.; Taylor, R. P.; Patterson, J. M.; Recktenwald, G.; Martin, R. B. J . Am. Chem. Sac. 1959, 81, 1068-1077.
a second molecule of the ketone in the ground state, see Scheme 11. This mechanism gained support from the oxidative cleavage of amines in the course of ketone photoreduction in aqueous media. The products isolated are explained in terms of hydrolysis of the enamine (VII) or the immonium ion (VIII). We have carried out a nanosecond laser flash photolysis study of the kinetics of the evolution of ketyl radical anions in the course of the quenching of benzophenone triplets by triethylamine (TEA) in basic, aqueous media. We have observed that, in spite of the high pH, the transient species at the beginning of the nanosecond time domain is the ketyl radical (11) which undergoes deprotonation over 50-600 ns. This result fits well into the electron-transfer schemes delineated by Cohen, Peters, and their co-workers. Over a longer time scale (microseconds), a second process has been noted for radical anion generation; this is ascribed to the secondary photoreduction of the ketone ground state by a-aminoalkyl radical. Although chemical evidence has been gathered’6.18s19 in favor of the secondary photoreduction, no detailed study of the dynamics of this interesting process has been performed (except for a brief mention in a recent report% from our group). In the present paper, we not only describe the kinetic aspects of the electron transfer to the ground and triplet excited states of benzophenone and several of its 4-substituted derivatives, but also present quantitative data regarding transient absorption spectra of radical anions and efficiency of electron transfer in the course of triplet quenching by TEA, diazabicyclo[2.2.2]octane (DABCO), and N,N-dimethylaniline (DMA).
Experimental Section Benzophenone, BP, and substituted benzophenones were obtained as best grades commercially available and were recrystallized from aqueous ethanol. Triethylamine, TEA (Eastman), and N,N-dimethylaniline, DMA (Aldrich), were distilled over KOH and stored under argon at 0 OC. 1,4-Diazabicyclo[2.2.2]octane, DABCO, purchased from Aldrich, was recrystallized from a 1:1 benzenex-hexane mixture and then sublimed under vacuum. Acetonitrile (Aldrich) was distilled over P20,. Water was purified by a Millipore Milli-Q system. For all laser flash photolysis experiments, use was made of nitrogen laser pulses (337.1 nm, 2-3 mJ, -8 ns) from a Molectron UV-400 system. The laser excitation was carried out in a front-face geometry, the angle between the directions of the laser a n d the monitoring light being 15-20’. Rectangular quartz cells with path lengths 1-3 mm were used. For experiments in which transient absorbance changes were compared with one another for the purpose of actinometry or extinction coefficient determination, precision cells with optically flat surfaces were chosen. Details of the kinetic spectrophotometer and data collection system are available in previous from this laboratory. (18) Cohen, S . G.; Chao, H. M.; Stein, N. J. Am. Chem. SOC.1969, 90, 521-522. (19) Cohen, S. G.; Stein, N. J. Chem. SOC.1969, 91, 3690-3691. (20) Bhattacharyya, S . N.; Das, P. K. J. Chem. SOC.,Faraday Trans. 2 1984, 80, 1107-11 16. (21) Das, P. K.; Encinas, M. V.;Small, Jr., R. D.; Scaiano, J. C. J. Am. Chem. SOC.1979. 101, 6965-6970.
Quenching of Benzophenone Triplets
The Journal of Physical Chemistry, Vol. 90, No. 17. 1986 3989
Deoxygenation of solutions was effected by bubbling high-purity argon. The reduction potentials of the benzophenones were measured in 1:9 H20:MeCN containing 0.01 M NaOH and -0.1 tetran-butylammonium perchlorate. The cyclovoltammograms were obtained in a BAS- 100 electrochemical analyzer, using glassy carbon (working) and SSCE (reference) electrodes. The E1/2,rcd values in Table I correspond to SCE as the reference electrode.
Results ( a ) Rate Constants for Carbonyl Triplet Quenching by TEA. In aqueous acetonitrile made basic by addition of N a O H (0.01-0.04 M), diary1 ketone triplets decay with lifetimes (5-10 ~ s longer ) than in benzene. The triplets, produced by flash excitation with 337.1-nm laser pulses (attenuated to 6 1 mJ/pulse), are conveniently monitored by their absorption at or near the long-wavelength maxima (520-545 r ~ m ) . * The ~ rate constants (kTObd)for enhanced decay of the triplets were measured at varying TEA concentrations ( 0 - 5 mM). From the slopes of the linear plots of kTowvs. [TEA], the bimolecular rate constants (kTJ for triplet quenching by TEA were obtained (see Table I). As expected, the decay of triplets in the presence of TEA under alkaline conditions led to residual, long-lived transient absorptions due to BP radical anions, and hence, the integrated equation for the kinetics of triplet decay was appropriately modified25 to take into account the concomitant, underlying growing-in of the photoproduct absorption. As it will be shown shortly, the initial product of the ketone triplet triplet quenching is, in fact, the ketyl radical (11) which becomes deprotonated over a short time scale, 50-600 ns (see Scheme I). Fortunately, in the spectral region near triplet absorption maxima, wavelengths can be found where the ketyl radical/radical anion pairs have isosbestic points. It was a t these wavelengths that the ketone triplets were monitored so that no serious corrections were necessary for the fast deprotonation kinetics of ketyl radical (that is, photoproduct absorptions appeared as “permanent” at the time scale of enhanced triplet decay). ( b ) Ketyl Radical Deprotonation Kinetics. At high TEA concentrations (>0.05 M) when the triplet quenching becomes complete within a short time ( 6 0 ns) from the laser pulse, relatively slow growth kinetics is observed for the formation of radical anions when the latter are monitored at long wavelengths (600-700 nm). This is illustrated for BP by the kinetic trace in the inset of Figure 1. As shown in Figure 1, the transient spectra at the initial stage of the growth process are due to the ketyl radical (Arnx = 545 nm) along with a small contribution from the ketone triplet (Arnx = 520 nm). On the other hand, the spectra following the completion of the growth process are due to the ketone radical anion (A, = 625 nm). The kinetics of the growth process is essentially independent of the ketone ground-state concentration (0.5-5 mM), but becomes progressively enhanced with increasing water content and basicity of the medium. All these results strongly suggest that we are observing the deprotonation of the ketyl radical initially produced as a result of ketone triplet quenching by TEA. Measurements of the pseudo-first-order rate constant (kdcp) for the radical anion formation as a function of [H20], [TEA], and [NaOH] show sublinear dependence, suggesting that the activities of these species participating in the deprotonation process are not properly represented by the concentrations used. That the deprotonation of the diphenylhydroxymethyl radical occurs rather slowly on the nanosecond time scale is confirmed by the laser flash photolysis of BP in alkaline aqueous acetonitrile (22) Nagarajan, V.; Fessenden, R. W. J . Phys. Chem. 1985, 89, 2330-2335. (23) Murov, S.L. Handbook of Photochemistry; Marcel Dekker: New York, 1973. (24) Baral-Tosh, S.;Chattopadhyay, S. K.; Das, P. K. J . Phys. Chem. 1984, 26, 1404-1408. (25) The integrated equation is of the form In [ ( A - A , ) / ( & - A , ) ] = -k,,,, where A,,, A , and A , are the absorbance changes at time 0, 1, and infinity, respectively.
26
a
h
WAVELENGTH ,NM Figure 1. Transient absorption spectra on a short time scale (nanoseconds) following laser flash photolysis of BP in the presence of 0.072 M TEA in 1:9 H,O:MeCN (v/v) 0.02 M NaOH. The times after the laser flash are A, 20; B, 30; C, 40; and D, 400 ns. Inset: a kinetic trace showing the fast growth of BP radical anion at 625 nm.
+
saturated with diphenylmethanol and containing 2-5 mM 2,5dimethyl-2,4-hexadiene. The purpose of adding the diene is to render the ketone lifetime short (via energy-transfer quenching) so that the ketyl radical deprotonation can be kinetically distinguished from the triplet decay. The diphenylhydroxymethyl radical formed as a result of the partial quenching of BP triplet by diphenylmethanol (via hydrogen abstraction) is found to transform to the anion over a relatively long time scale. The first-order rate constant (kdep) for the anion formation is 3.4 X lo6 s-I in 9:l MeCN:H20 containing 0.02 M NaOH and excess of diphenylmethanol. This rate constant is slower than that measured under TEA quenching (kdep= 8.3 X lo6 s-l), understandably because of the higher viscosity of the solution under saturation with diphenylmethanol. kdep data for BP and 4-substituted BP‘s in 1:9 H20:MeCN in the presence of 0.02 M NaOH are given in Table I. As expected under a given set of conditions, kdeisare smaller for BP’s containing electron-donating substituents (that is, ketyl radicals with higher pK,’s). For 4-trifluoromethyl- and 4,4’-dichlorobenzophenones, very fast components of radical anion formation are observed within nanoseconds after the laser pulse; this may very well represent the direct dissociation of the triplet ion-pair (Le., charge-transfer complex). ( c ) Secondary Reduction of Ketone Ground State. The monitoring of the ketyl radical anion on a longer time scale (1-100 ks) shows a second process for its formation (see the kinetic trace in the inset of Figure 2). There is no difference in the transient spectra a t the beginning and at the end of this slow process, both corresponding to the ketyl radical anion (Figure 2). Unlike the relatively fast deprotonation of ketyl radical (see above), the kinetics of the secondary radical anion formation process depend linearly on the ketone ground-state concentration. This establishes that the slow process is due to electron transfer to the ketone from a reducing species produced in the course of ketone triplet quenching by TEA. The most plausible candidate for this species is the methyl(diethy1amino)methyl radical, (C2H5)2N-CH-CH3, derived from TEA (through loss of a hydrogen atom to the ketone triplet). As expected, at relatively high [BPI, the magnitude of radical anion absorbance due to the slow process approaches that
0.14rl
Bhattacharyya and Das
3990 The Journal of Physical Chemistry, Vol. 90, No. 17, 1986
[KETONE], mM
/.7f'"'
0.05
05
10
1
I
15
I
0.12
-
20
25
30
I '
"/
W
0 2
a
I V
w V
z a m Lz
s:m d
WAVELENGTH, NM
Figure 2. Transient absorption spectra on a long time scale (microseconds) following laser flash photolysis of BP in the presence of 0.072 M TEA in 1:9 H20:MeCN (v/v) 0.02 M NaOH. The spectra A and B correspond tb 1.0 and 7.0 CIS, respectively, following the laser flash. Inset: a kinetic trace showing the slow growth of BP radical anion at 625 nm.
+
I TABLE II: Pseudo-First-Order Rate Constants for the Growth of Transient Absorption of BP Radical Anion as a Result of Ground-State Reduction by (C3H&N-&-CHp in Acetonitrile-Water Mixtures Containing 0.02 M NaOH and 1 mM BP % H 2 0 in MeCN (v/v)
10 20 30 40
kobsdra
lo6 s-' 0.2 1 0.35 0.44 0.75
5% H20in MeCN (v/v)
lo6 s-'
50 60 70 80
0.82 1.2 1.6 1.5
kobsd?
I
2
I
4
I
6
I
8
1
0
[KETONE] , m M
Figure 3. Plots of observed, pseudo-first-order rate constant (k&d) for the slow growth of ketone radical anion vs. ketone concentration: (A) 4,4'-dimethoxy-, (B) 4-methoxy-, (C) 4-methyl-, and (D) 4-flUOrObenzophenone.
"&lo%.
due to the initial fast component. It should be noted that when a phenolate ion is used as a quencher instead of TEA (other conditions remaining the same), the radical anion formation takes place in only one step, commensurate with the decay of the ketone triplet. Figure 3 shows some representative plots of pseudo-first-order rate constant (kobsd)for the slow process against ground-state ketone concentrations. The slopes of these plots gave the bimolecular rate con.stants ( k , d ) for one-electron reduction of ketones by (C2H5)2N-CH-CH3. As the data in Table I show, the electron-withdrawing or -releasing nature of the substituent in the ketone has a pronounced effect on the magnitude of krd. Experiments with BP at 1 mM in alkaline acetonitrile-water mixtures of varying compositions (lo-80% water, each containing 0.02 M NaOH) show a monotonous increase in koM with increasing water content (Table 11). The data indicate a tenfold increase in krcd on going from 1:9 H20:MeCN to 7:3 H20:MeCN, underlining the importance of solvent polarity on the electron-transfer kinetics (see Scheme 11). (4 Triplet Quenching by DMA and DABCO. For comparison, several experiments were done with DMA and DABCO as quenchers for the triplets of BP and its 4-methoxy and 4-trifluoromethyl derivatives. The rate constants (kT,) for ketone triplet quenching by DMA and DABCO in acetonitrile are (9-12) X lo9 and (3-5) X lo9 M-' s-' respectively. On going from acetonitrile to acetonitrile + 10% water, it is found that kT,'s remain essentially unchanged for DMA, but drop by 25-40% for DABCO. In the presence of relatively high concentrations of DABCO (0.1-0.2 M) or DMA (0.05-0.10 M) in alkaline aqueous acetonitrile (10% H20,0.02 M NaOH), the transient absorption spectra at the end of BP triplet decay correspond primarily to BP radical anion, BY- (Figure 4). With DMA as the quencher, additional transient absorption due to the quencher cation (Amax = 460 nm)26
W
a z a I u W
u 2 a m a
s:m a
092 0.0 .. 300
400
500
600
700
__ WAVELENGTH, NM Figure 4. Transient absorption spectra at (A, B) 1.O and (A', B') 13 ws following the laser flash photolysis of benzophenone in the presence of (A, A') 0.05 M DABCO and (B, B') 0.03 M DMA in 1:9 H20:MeCN (v/v) 0.02 M NaOH. Insets: (a) kinetic trace for decay of BP'- (at 625 nm) when generated from triplet quenching by DABCO and (b) fit of the data into second-order, equal-concentration kinetics.
+
is noticeable at 450-480 nm (Figure 4B). In either case, the transient behavim on short time scales (100-500 ns) at long wavelengths near the maximum of BP- absorption (620-640 nm) shows no fast formation process attributable to the deprotonation ~~
(26) Shida, T.; Hamill, W. H. J. Chem. Phys. 1966, 44, 2369-2374. Shida, T.; Nosaka, Y.; Kato, T. J. Phys. G e m . 1978,82, 695-698. Land, E. J.; Porter, G. Tram. Faraday SOC.1963, 59, 2027-2037. Yates, S. F.; Schuster, G. B. J. Org. Chem. 1984, 49, 3349-3356.
The Journal of Physical Chemistry, Vol. 90, No. 17, 1986 3991
Quenching of Benzophenone Triplets
TABLE 111: Absorption Spectral Data for BP Radical Anions and Efficiencies of Electron Transfer in BP Triplet Quenching by Amines in 1:9 H,OMeCN (v/v) Containing 0.02 M NaOH ~m,,a
substituents
nm
none
4,4'-dimethoxy 4-methoxy 4-methyl 4-fluor0 4-chloro 4,4'-dichloro 4-trifluoromethyl
300
400
500
600
700
WAVELENGTH ,NM
Figure 5. Absorption spectra of radical anions of (A) 4,4'-dimethoxybenzophenone and (B) 4,4'-dichlorobenzophenone, produced from triplet quenching by TEA in 1:9 H20:MeCN (v/v) + 0.02 M NaOH. of an initially produced ketyl radical. The persistence of BP'following electron transfer from DABCO even in a neutral medium has been noted in the picosecond dynamical studies by Peters and ~ e w o r k e r s .The ~ ~ difficulty of proton transfer in BP-DABCO'+ charge-transfer complex has been explained by Inbar et ale1%in terms of the high stability*' of DABCOO+ and the decreased overlap between the n-orbital of N and a-C-H orbitals in the bicyclic structure of DABCO (being responsible for less stabilization of the corresponding a-aminoalkyl radical). On the other hand, with DMA as the quencher, the results of a picosecond study by Simon and Peters15 have shown the intra-ion-pair proton transfer to be facile; e.g., Kq = (ketyl radical/radical anion) = 2.7 at 1 M DMA in acetonitrile. More concerning this will be considered in the Discussion section. On longer time scales (50-100 ps), the decay of BP radical anions produced from triplet quenching by DMA and DABCO wcurs more rapidly than when they are produced from triplet quenching by TEA. A kinetic trace showing the radical anion decay with DABCO as the quencher is presented in the inset a of Figure 4. The decay in this case can be fitted very well into second-order equal-concentration kinetics (see inset b of Figure 4), suggesting that the diffusional back electron transfer between BP'- and DABCO" constitutes the predominant mode of the decay. Based on a value of 6.0 X lo3 M-' for the extinction coefficient of BP'- at 625 nm (see later), we have measured 8.7 X lo9 M-' cm-' as the second-order rate constant for the back electron transfer. Essentially similar values, namely, 1.O X 1Olo and 8.1 X lo9 M-' s-l, have been obtained in the case of 4-trifluoromethyl and 4-methoxybenzophenone, respectively. With DMA as the quencher, the situation is complex as evident from the decay of DMA" (monitored at 460 nm) being more rapid than that of BY- (monitored at 620-660 nm). Possible modes of decay of DMA'+ under the conditions of our experiments would be deprotonation to the a-aminoalkyl radical, C6H5(CH3)NCH2, and hydroxylation at the aromatic nucleus (in addition to back electron transfer to BP-). With either DMA or DABCO, there was no evidence for slow, secondary reduction of the ketone ground state as a result of electron transfer from a reducing radical. The quenching of diaryl ketone triplets by DABCO in acetonitrile gives ketyl radical anions that decay by clean first-order kinetics. The rate constants (kd) for decay are in the range (1.5-1.9) X lo6 s-' and show practically no dependence on the nature of substituents. The anions are quenched by water giving rise to ketyl radicals. The rate constants for the quenching, (2.6-2.9) X lo6 M-' s-' , a re also essentially independent of the substituents in the diaryl ketone radical anions.
625 615 620 625 625 665 680 665
&tb
:e-
lo3 M-* cm-I TEAc DMA DABCO 6.0 1.0 1.1 0.98 5.2 1.0 1.0 0.88 5.2 1.1 1.0 0.99 5.5 1.1 1.0 1.1 0.90 5.9 0.94 0.91 0.73 8.5 0.90 0.78 9.4 0.95 0.96 0.89 7.6 0.91 0.74 0.75
"&5 nm; the absorption maxima at 330-345 nm with extinction coefficients 2-3 times higher than those at the longer wavelength maxima are not included. bMaximum variation, &15%. 'Radical anions arising from the fast deprotonation of initially produced ketyl radicals were monitored.
( e ) Transient Absorption Spectra of Ketone Radical Anions. The transient absorption spectra of radical anions of eight benzophenones were recorded by 337.1-nm laser flash photolysis of solutions in 1:9 H20:MeCN containing 0.05-0.10 M TEA and 0.02 M NaOH. The spectra for 4,4'-dimethoxy- and 4,4'-dichlorobenzophenones are presented in Figure 5 . A definitive substituent effect is noted in the long-wavelength absorption maxima of the radical anions. Thus,while electron-releasing groups (e.g., methoxy) cause small blue shifts in , ,A those of electron-withdrawing nature (e.g., trifluoromethyl) lead to large red shifts (see Table 111). The extinction coefficients of the radical anions were obtained in the following manner. First a solution of the ketone in 10-20% water in MeCN containing 0.02-0.4 M NaOH and 0.01-0.03 M phenolate ion was flash-photolyzed at 337.1 nm and the endof-pulse absorbance change (UM) at the maximum of the ketone radical anion formed as a result of diffusion-controlled electron transfer2*from the phenolate ion was noted. To another portion of the same solution in an identical photolysis cell, 0.2-0.5 mM methylviologen (paraquat, PQ2+) was added and the absorbance change (Urn.+) due to reduced paraquat radical ion (PQ") was measured at 615 nm following 337.1-nm laser flash photolysis. In the latter case, PQ.+ was produced as a result of electron transfer from the radical anion originating from the reaction of the ketone triplet with phenolate ion, the latter being at a much higher concentration than The various steps were as follows:
(u
w+.
>C=O
>C=O* (S,)
+ C&O--.* >e-@+ PQ2+
>c=o*
-
-.*
>C=O* (TI)
>e-@+ C&O' >C=O
+ PQ.'
(2) (3)
Note that under the conditions of our experiment, the decay kinetics of the carbonyl triplet and the radical anion were controlled to the extent of >98% by reactions 2 and 3, respectively. The radical anion extinction coefficients were calculated by using eq 4. For ern.+, a value29of 11.5 X lo3 M-' cm-' (at 615 nm) (4) was used. It should be noted that in the alkaline medium, PQ2+ is slowly reduced by the phenolate ion (by thermal electron transfer) leading to the slow development of a blue color. The laser flash experiments had to be done immediately after the addition of PQ2+. The extinction coefficient data are compiled in Table 111. The value (6.0 X lo3 M-l) obtained for B P - (unsubstituted) is in general agreement with the eml values reported by Hayon et (6.1 X lo3 M-' cm-' at 615 nm) and by Adams (28) Das, P. K.; Bhattacharyya, S. N . J . Phys. Chem. 1981, 85,
1391-1395.
(27) McKinney, T. M.;Gerske, D. H.J. Chem. Soc. 196587,3013-3014.
(1)
(29) Trudinger,
P.A. Anal. Biochem. 1970, 36, 222-225.
Bhattacharyya and Das
3992 The Journal of Physical Chemistry, Vol. 90, No. 17, 1986 and Willson3' (4.8 X IO3 M-' cm-I at 600 nm) based on pulse radiolysis in aqueous solutions. v) Efficiency of Electron Transfer in Ketone Triplet Quenching by Amines. The efficiencies (&.) of electron or hydrogen transfer in the course of the quenching of benzophenone triplets by TEA, DMA, and DABCO were measured in terms of transient absorbances due to radical anions monitored at their maxima at long wavelengths. In a typical experiment, the end-of-pulse absorbance change (AAT) due to a ketone triplet at its maximum in the absence of a quencher was compared with that (Urn) due to the radical anion as the latter was produced under >98% quenching of the triplet by an amine. Equation 5 was used to calculate &, the triplet extinction coefficient (9)data being taken from the l i t e r a t ~ r e . With ~ ~ TEA as the quencher, URA cor-
0.6
-
0.5
-
0.4
-
I
(5)
,
I
-1.8
-I .6
I
-I .4
E, ,red,V 12
responded only to the fast component of the radical anion formation. @et data are given in Table 111.
Discussion Perhaps the most interesting finding in the present study is the fast, but relatively delayed, formation of BY- on a nanosecond time scale following the quenching of BP triplets by excess of TEA. Such a process is not observed with DMA and DABCO as quenchers under similar conditions. Two assignments are possible for the fast growth of BP-, namely, deprotonation of the ketyl radical initially produced as result of intra-ion-pair proton transfer, and dissociation of a triplet exciplex of BP and TEA into radical ions. On the following grounds, we rule out the second interpretation. First, the transient spectra at the beginning of the nanosecond growth process are very close to those of ketyl radicals. In view of the exothermicity of electron transfer (seelater) between BP triplet and TEA and the high polarity of the medium, it is expected that the triplet exciplex of BP and TEA have a large ionic character. Hence, the transient spectrum of the exciplex should be closer to BP'- than to 3BP+. Second, quantitative intra-ion-pair proton transfer on a picosecond time scale has been convincingly established by the work of Schaefer and Peters.13 Third, the flash photolysis experiments with diphenylmethanol as hydrogen donor clearly shows the deprotonation to be a process on the nanosecond time scale under alkaline conditions. While the lack of observation of an analogous deprotonation behavior with DABCO as the electron donor is understandable in terms of the stability of DABCO'+ against the transfer of proton in the ion pair, the same result in the case of DMA seems slightly surprising. This is particularly so in the light of the picosecond ob~ervation'~ of the dominance of the ketyl radical (K, = 2.7) following the intra-ion-pair processes in neat acetonitrile. It appears that the small increase in dielectric constant (e) on going from acetonitrile (e = 37.5) to 1:9 HzO:acetonitrile ( e = 48) is sufficient for tilting the equilibrium in favor of the radical anion. A strong dielectric dependence of Kq has been noted in the study by Simon and Peters; for example, Kq (at 1 M DMA) increases from 2.7 to 10.8 on going from acetonitrile to hexanonitrile (e = 17.3). That solvents of lower dielectric constants favor the formation of ketyl radical over radical anion has also been documented in related nanosecond studies'O of BP triplet quenching by amines. At the same time, under the given environmental condition, that is, 1:9 H,O:acetonitrile (+0.02 M NaOH) used in the present study, pKa's of amine radical cations and ketyl radicals become important factors in determining the decay modes of BP'--.AM'+ ion pairs (proton transfer vs. dissociation). This is exemplified by the difference in the behaviors of TEA, DMA, and DABCO, as well as by the observation of fast, direct dissociation of BP'--TEA*+ ion pairs in the case of benzophenones containing electron-withdrawing groups (Le., ketyl radicals with low ~ K , ' S ) . ~ ~ (30) Hayon, E.; Ibata, T.; Lichtin, N. N.; Simic, M. J. Phys. Chem. 1972, 76. 2072-2078. (31) Adams, G. E.; Willson, R. L. Trans. Faraday Soc. 1973,69,719-729.
Figure 6. Plot of RT In k,, vs. diaryl ketone ground-state reduction potential. k,, is the diffusionally corrected38 rate constant for electron transfer from (C2H5),NCHCH3to diaryl ketone ground state. For the numbers identifying diaryl ketones, see Table I. For the best-fit straight line, points 2 and 8 were excluded.
There is little rmm for doubt in our interpretation of the slower process for radical anion generation in terms of reduction of ketone ground state by the methyl(diethy1amino)methyl radical. Besides the chemical e v i d e n ~ e ' ~from J ~ the isolation of two-electron-oxidation products from TEA in the course of steady-state photoreduction of related ketones, the linear dependence of the pseudo-first-order formation rate constant (k,M) on [ketone] and the variation of the bimolecular rate constant ( k , d ) with respect to substituent nature (Table I) strongly support the interpretation. In a kinetic study based on nanosecond laser flash photolysis, Scaiano" has shown the reduction of benzil by a-aminoalkyl radicals to be very facile (krd = 1.8 X lo9 M-I s-l in 1:9 H,O:acetonitrile in the presence of 0.003 M NaOH). Also, CIDEP experiments by McLaughlan and c o - ~ o r k e r shave ~~ suggested hydrogen transfer from a-amino radicals to biacetyl to be an efficient process. We should note that the a-diketones are more reducible3s than the monoketone, benzophenone. Thus, the reduction of benzil by (C2H&NCHCH3 is more exothermic than that of benzophenone by 19.4 kcal mol-'; this large difference in free energy change manifests itself in krd being 15-fold higher for benzil (in a very similar solvent system).36 Although the rate constants for TEA quenching (kTq, Table I) follow a trend expected from the substituent nature, the magnitude of the change in them is rather small. This is because the free energy changes for electron transfer from TEA (32) The reported pKa of benzophenone-derivedketyl radical is 9.2-9.3 in aqueous solution^.^^^' It is expected that electron-withdrawing and electron-releasing substituents would render pK,'s lower and higher, respectively. In fact, for affitophenone-derived ketyl radicals, the lowering of pK, by electron-withdrawing groups (Cl, CN, and NO,) has been ~bserved.~' (33) Scaiano, J. C. J . Phys. Chem. 1981,85, 2851-2855. (34) McLaughlan, K. A.; Sealy, R. C. Chem. Phys. Lett. 1976, 39, 31C-311. McLaughlan, K. A.; Sealy, R. C.; Wittmann, J. M. J . Chem. SOC., Faraday Tram. 2 1977,73,926-932. Ibid. Mol. Phys. 1978,35, 51-63; 1978, 36, 1397-1407. (35) Measurements of Ell,,rdfrom the same laboratory" gave values of -1.55, -1.03, and -0.71 V (vs. SCE, in 50% aqueous ethanol at pH 12.65) for benzophenone, biacetyl, and benzil, respectively. (36) In an earlier work,20the similarity of krd for benzophenone in 4:1 H,O:acetonitrile (+0.02M NaOH) and bend1 in 1:9 H,Oacetonitrile (+0.003 NaOH) was taken to mean that the oxidation potential of (C2H5)2NCHCH3 is favorable for exothermic electron transfer to both of the aromatic ketones. The present work, however, shows that the water content in H,OAcetonitrile mixtures has a significant effect on krd of benzophenone (see Table 11). Understandably, it was erroneous to compare krd values measured under different solvent conditions. (37) (a) AGet was calculated by using the equation AGel = -ET + Ei/z,ox+ W,, where ET is the ketone triplet energy, is the reduction potential of the ketone (ground state), is the oxidation potential of the amine quencher, and W is the Coulom6ic work in bringing together the rcduct ions to form the'ion pair. Assuming an interionic distance ( a ) of 8 %, Wp is calculated to be -0.025 V, (using the relationship: W (in V) = -0.30/[0(1 0.423ap'l2)]).(b) AGkl was calculated by using the equation AGk, = El,2,r4- El/2,0r - W,. For explanation of the terms, see above.
+
Quenching of Benzophenone Triplets
The Journal of Physical Chemistry, Vol. 90, No. 17, 1986 3993
to the ketone triplets under consideration are all negative and vary from -2.4 kcal mol-' for 4,4/-dimethoxybenzophenoneto -6.4 kcal mol-' for 4-trifluoromethylbenzophenone.On the other hand, the change in kd (Table I) is of much larger magnitude. In this case, it is worthwhile to obtain a plot of RT In k,, vs. of ketones, where k,, is the electron transfer rate constant corrected for the initial diffusional process.38 This is shown in Figure 6. Based on the Marcus theory,39 under certain condition^,^^ k,, can be expressed as a function of AG,, as shown in eq 6,where k,,(O)
*"'[
:(]
R T In k,, = RT In k,,(O) - - 1 + 2
(6)
represents the electron-transfer rate constant a t AGet = 0, and X/4 is the barrier for electron transfer arising from reorganization of inner and outer coordination shells. At IAGetl 700 nm.42 Upon pairing with cations, the maximum (38) k,, was computed from the relationship k,;' = krd-' - kdiff-',where kdirtis the rate constant (1.4 X 10" M-' s-I for diffusion as calculated from the Smoluchowski equation. (39) (a) Marcus, R. A. J . Chem. Phys. 1965,43, 679-701. (b) Marcus, R. A,; Sutin, N. Inorg. Chem. 1975, 14, 213-219. (40) For a detailed consideration of the derivation of eq 6, see: Bock, C. R.; Connor, J. A.; Gutierrez, A. R.; Meyer, T. J.; Whitten, D. G.; Sullivan, B. P.; Nagle, J . K. J . Am. Chem. SOC.1979, 101,4815-4824. (41) Siegerman, H. In Techniques of Chemistry; Weinberg, W. L., Ed.; Wiley: New York, 1975; Vol. V, Part 11. (42) Beaumond, D.; Rodgers, M. A. J. Trans. Faraday SOC.1969, 65, 2973-2980.
shifts to shorter wavelengths (600-700 nm).42 The maximum (625 nm) observed for BY-at a relatively short wavelength in aqueous acetonitrile appears to be due to the hydrogen-bonding interaction of the anion with water. It is pertinent to mention in this context that large hypsochromic shifts occur in the absorption maxima of polyenal radical anions on going from nonprotic and non-hydrogen-bonding solvents to protic ones.43 This solvent effect, reminiscent of that on n , r * transitions in closed-shell heteroatom-containing ?r-systems, suggests that in radical anions of ?rsystems the ground state has more uneven charge distribution than the first doublet excited state (that is, the odd electron in the ground state is relatively localized on the carbonyl moiety). Hence, the ground state is subject to greater stabilization by hydrogen bonding and cation pairing; The substituent effect on absorption maxima, that is, red shifts caused by electron-withdrawing groups (Table 111), is compatible with the solvent effect. Like an n,?r* transition, the lowest doublet-doublet transition in BP'-'s may be considered as having a charge-transfer character in the sense that the odd electron in the first excited doublet state is relatively delocalized into the aromatic ring. The energy of such an excited state is expected to be lowered by electron-withdrawing groups and raised by electron-donating groups.
Summary The present time-resolved study based on transient absorption measurements has dealt with several facets of triplet-mediated photoreduction of aromatic ketones by amines. In the basic polar media, deprotonation of the ketyl radical produced via fast intra-ion-pair proton transfer has been observed with TEA as the quencher. Direct kinetic evidence has been obtained for secondary reduction of ketone ground states by TEA-derived aminoalkyl radical; the oxidation potential of the latter has been estimated at -1.6 V (vs. SCE). Quantitative data have been obtained for transient absorption spectra of substituted benzophenone radical anions. These in turn have enabled us to determine the primary yields of electron transfer in the course of quenching by TEA, DABCO, and DMA. The electron-transfer efficiencies in the strongly polar medium are all close to unity and show very little dependence on the nature of amine or substituents on aromatic ketones. Acknowledgment. We are grateful to Dr. P. V. Kamat for assistance in the cyclovoltammetric measurements of reduction potentials. Registry No. DABCO,280-57-9; benzophenone, 1 19-6 1-9; triethylamine, 121-44-8; methyl(diethy1amino)methyl radical, 26374-14-1; 4,4'-dimethoxybenzophenone radical anion, 34493-36-2; 4-methoxybenzophenone radical anion, 60466-08-2; 4-methylbenzophenone radical anion, 59228-14-7; 4-fluorobenzophenone radical anion, 34473-92-2; 4-chlorobenzophenone radical anion, 81439-06-7; 4,4'-dichlorobenzophenone radical anion, 90-98-2; 4-trifluoromethylbenzophenoneradical anion, 728-86-9; benzophenone radical anion, 16592-08-8; N,N-dimethylaniline, 12 1-69-7. (43) (a) Land, E. J.; Lafferty, J.; Sinclaier, R. S.; Truscott, T. G. J . Chem. SOC.,Faraday Trans. 1 1978, 74, 538-545. (b) Lafferty, J.; Roach, A. C.; Sinclair, R. S.; Truscott, T. G.; Land, E. J . J . Chem. SOC.,Faraday Trans. I 1977,73,416-429. (c) Bobrowski, K.; Das, P. K. J . Phys. Chem. 1985,89, 5733-5738.
