2662
The Journal of Physical Chemistry, Vol. 82, No. 25, 1978
Shold, and J. K. McVey, J . Am. Chem. SOC.,97, 5004 (1975). (7) N. J. Turro, “Molecular Photochemistry”, W. A. Benjamin, New York, N.Y., 1967, p 94. (8) Measured by single photon counting, see Experimental Section. (9) D. Rehm and A. Weller, Isr. J . Chem., 8, 259 (1970). The authors used a value of e t / a t = 0.06 eV as an average value in acetonitrile for a large variety of quenchers. (10) Haif-peak potentials vs. SCE in acetonitrile, see Experimental Section. (1 1) Determined from the absorption and fluorescence spectra in acetonitrile. (12) H. Beens, H. Knibbe, and A. Weller, J. Chem. Phys., 47, 1183 (1967).
R. D. Small and J. C. Scalano (13) (14) (15) (16) (17)
F. D. Lewis and T.4. Ho, J . Am. Chem. SOC.,99, 7991 (1977). J. Eriksen and C. S. Foote, to be published. E. A. Chandross and J. Ferguson, J . Chem. Phys., 47, 2557 (1967).
According to manufacturer’s specifications. W. R. Ware in “Creation and Detection of the Excited State”, A. A. Lamoia, Ed., Vol. 1, Part A, Marcel Dekker, New York, N.Y., 1971, Chapter 5. (18) H. E. Zimmerman, D. P. Werthemann, and K. S. Kamm, J. Am. Chem. Soc., 96, 439 (1974); L. J. Cline Love and L. A. Shaver, Anal. Chem., 48, 365A (1976); J. Eriksen, Ph.0. Thesis, New York University, New York, N.Y., 1976.
Electron Transfer Reactions of the Biradicals Produced in the Norrish Type I1 Process R. D. Small, Jr., and J. C. Scaiano” Radiation Laboratory,‘ University of Nofre Dame, Notre Dame, Indiana 46556 (Received June 29, 1978) Publication costs assisted by the Department of Energy
The biradicals generated in the Norrish type I1 reaction are efficient electron donors. They are shown to react with a series of bipyridilium cations, pyridine derivatives, and aromatic nitro compounds. The rates of reaction follow the same trends as those observed for monoradicals. For example, the biradical from y-methylvalerophenone reacts with benzyl viologen with k 3 = 3.9 X lo9 M-’ and with 2,5-dinitrobenzoic acid with k 3 = 7.6 X lo8 M-l
Introduction The biradicals produced in the photochemical Norrish type I1 process (reaction 1) are good electron donors, as Ph
Ph
J
O
H
which formally makes the generation of biradicals an “instantaneous” process and largely simplifies the treatment of the dataa8In all cases the experiments were carried out at room temperature in acetonitrile containing 20% water (v/v), which is a convenient solvent for ymethylvalerophenone and all the electron acceptors used. The samples were excited using the pulses (3 mJ, -8 ns, 337.1 nm) from a nitrogen laser and the reaction was followed by monitoring the time profile for either the decay of B (at 415 or 500 nm) or the buildup of A-. produced in reaction 3. Both approaches yield the same pseudoPh
Ph
I
B
R
MlN@@)Le
eM N 2-
1
I
+ A -
2
indicated, for example, by the rate and efficiency with which they transfer an electron to paraquat (1,l’-dimethyL4,4’-bipyridiliurn, 1) dications, reaction 2,2,3where the asterisk in reaction 1 indicates a triplet state. Our earlier experiments were designed in order to examine biradical properties, in particular their lifetime,2,4,5 using reaction 2 as a monitor and applying nanosecond laser flash photolysis techniques. Reaction 2 can be expected to be a member of a larger family of electron transfer processes in biradicals. This paper reports the results of a study aimed at establishing how general electron transfer processes in biradicals are, and how the rates correlate with those of other electron donors and with redox potentials.
Results and Discussion All the experiments described herein were carried out using y-methylvalerophenone as the biradical precursor. The choice of substrate reflects the short triplet lifetime2,69 0022-3654/78/2082-2662$01 .OO/O
’’
Po+
A-.
+
H+
(3)
first-order kinetic behavior, with the experimental firstorder rate constant given by2 kexpt
=
TB-’ -I- b [ A I
(4)
where TB is the biradical lifetime (81 ns in this case) and k3 is the second-order rate constant for the trapping reaction. A number of electron acceptors can be used in reaction 3 in addition to paraquat dications (l),and structures 3-10 illustrate those examined in this paper. In the case of the bipyridilium type of acceptors we have prefered to use hydrochlorides in all cases (some are commercially available as hydrobromides) in order to avoid the possibility of heavy atom effects on the value of T B . ~ Figure 1 shows typical growth traces obtained in the case of benzyl viologen, 3 (Figure lA), and diquat, 6 (Figure 1B). They can be regarded as the two limiting cases; in the former the measurement is straightforward while in the case of diquat we observe a “jump” followed by the expected growth. Control experiments showed that this jump (Figure 1B)is due to absorption resulting from direct
0 1978 American Chemical Society
The Journal of Physical Chemistry, Vol. 82,No. 25, 1978
Biradicals Produced in the Norrish Type I1 Process
3
4
5
6
+m+ 'm
C02H
2
I
O . O o 0 ~
7
0.05
-
0.04
-
003 -
:
C02H
I
0.0 O.O2l I
00 .01j-
B 200 300 400' TIME ( n s )
100
0
9
photoexcitation of diquat in addition to some initial absorbance due to the biradical. This jump was not taken into consideration for the evaluation of kexpt. Figure 2 shows representative plots of In ( A , / A , - A ) vs. time from which the values of kexptwere obtained (where the buildup of A-- is monitored) according to In
A, - A
= kexptt
(5)
where A , is the plateau absorbance and A is the absorbance at time t. The values of (k, - TB-~) are then plotted vs. the concentration of acceptor according to eq 4. Typical plots are shown in Figure 3. The results are summarized in Table I; in the case of the bipyridilium dications the one-electron redox potentials are knowdo and have also been included. In general we observe that k3 follows the expected trend with redox potentials,15 as well as with the transfer from other electron d ~ n o r s l l -with ~ ~ the only difference of the value for COz-. with 1 and 3 where the values compared are from two independent studies.11J2 It is clear from our results that electron transfer processes of biradicals are quite general. Indeed, in many cases it is easier to examine these processes in the case of biradicals than it is to carry out similar measurements with monoradicals, because the competing modes of decay are clean first-order processes. Finally, it is noteworthy that electron transfer to paraquat and related ions are of importance from the point of view of their herbicidal properties.16 Further, it has been suggested that they could be of interest from the point of view of solar energy conversion and storage.17 A frequent requirement for a photoinduced electron transfer process to be very efficient is the need for a long-lived excited state, which, as a result becomes a good candidate for unwanted quenching. Electron transfer processes in biradicals are unusual in the sense that they do not have the requirement mentioned above, and in fact the most efficient processes usually involve very short-lived excited states. This results from the fact that the electronic-to-chemical energy conversion occurs at an early stage, leading to the biradical, which effectively "stores" the energy until it reacts with a suitable electron acceptor, or otherwise decays.
Experimental Section Materials. Acetonitrile (Aldrich, Gold Label) was used as received. Water was t r i d e distilled. BiDvridilium ~
111
2663
500
Figure 1. Time profiles for the formation of the radical cations from (A) [3] = 0.001 1 M, monitored at 603 nm; (B)[6] = 0.0041 M at 475
nm. 2.5
--
2.0
1.5
21% 1.0
r
0.5 I
25
I
50
I
75 100 TIME ( n s )
I
125
150
Figure 2. Plots according to eq 5 for (V)[8] = 0.037 M; ( 0 )[5] = 0.00443M; (0) [5] = 0.00177 M; (A) [3] = 0.0011 M; (0) [8] = 0.0037 M; and ( X ) [4] = 0.0017M.
0.01 I
[SUBSTRATE], M 0.02 0.03
0.04
I
[SUBSTRATE]
,M
Flgure 3. Plots according to eq 4 for (0) 8, (V)3, (0) 6, and (+) 9.
compounds (1,3-7) were used as the hydrochloride; where this was not commercially available it was prepared by treating the hydrobromide with concentrated hydrochloric acid followed by heating and precipitation of the corresponding hydrochloride with acetone. All samples were reprecipitated at least once. Tetraquat (7), the only one which is not commercially available, was a generous gift
2664
TABLE I: K i n e t i c s of t h e E l e c t r o n Transfer Reactions of T y p e I1 B i r a d i c a l P accep- m e t h tor odb k,/M-’s-‘
3.2 x l o 9 G 3.9 x 109 G 1.4 x 109 G 4.0 x 109 G,D 2.6 x lo9 D. 9 1 X l o * 8 G,D 7.6 X 10’ 9 G.D 1.7 X lo* 10 D’ ~2 x 10’ 1 3 4 5 6 7
R. D. Small and J. C. Scaiano
The Journal of Physical Chemistry, Vol. 82, No. 25, 1978
G
E/mV krD--./M-l s-’
kM+d
M- s
-447c 1.5 x 1Olod - 3 5 4 ~ 6.7 x 109e -356c -548c
4~
those in the l i t e r a t ~ r e . l l - ~ ~ > ~ ~
Acknowledgment. We are grateful to Dr. P. Neta for many valuable comments. References and Notes The research described herein was supported by the Office of Basic Energy Sciences of the Department of Energy. This is Document No. NDRL-1898 from the Notre Dame Radiation Laboratory. R. D. Small, Jr., and J. C. Scaiano, J. Phys. Chem., 81, 828, 2126
loge
1.9x 109f
3.3 x 109f 1.5 x 1 0 9 8 1.0 x lo@
At room temperature in 20% water-acetonitrile in unb u f f e r e d solutions. G for buildup and D for decay. From r e f 10. From r e f 11. e From r e f 12. f From r e f 13. g From r e f 14.
from Dr. P. Neta. 8, 9, and 10 were all Aldrich products and were used as received. Laser Flash Photolysis. The samples were contained in 3 X 7 mm Suprasil cells and were deaerated by bubbling argon or by three freeze-pump-thaw cycles to a residual pressure of torr or less. Transient absorptions were monitored using the system described in previous papers,2J8which used a Molectron UV-400 nitrogen laser for excitation and a He/Ne laser, or a pulsed 150-W Xe lamp as monitoring source. This system has been recently interphased with a PDP11/55 computer and now makes use of a R7912 Tektronix transient digitizer to receive and digitize the traces. Further details on the computer control of the laser system will be reported e1~ewhere.l~ Typically, between 5 and 20 traces were averaged in each measurement. The buildup of the radical ions was typically monitored st one of the absorption maxima, e.g., 603 nm for paraquat. However, in all cases a complete spectroscopic study was carried out in order to establish conclusively the identity of the product. Due to the large number of shots required in these experiments, they were carried out using a flow system. In all cases the spectra obtained agreed well with
I I
,
(1977). R. D. Small, Jr., and J. C. Scaiano, J. Photochem., 6, 453 (1976/77). R. D. Small, Jr., and J. C. Scaiano, J . Am. Chem. Soc., 99, 7713 (1977). M. V. Encinas and J. C. Scaiano, J . Am. Chem. Soc., submitted for publication. P. J. Wagner. Acc. Chem. Res.. 4. 168 (1971). J. Groteioid, D. Soria, C. M. Previtali, and J. C.’Scaiano, J . Photochem., 1, 471 (1972/73). When the triplet lifetime approaches the biradical lifetime the kinetic expressions contain 77, the triplet lifetime.* When 77 becomes fairly long, e.g., acetophenone, there is a possibility of producing photoproducts in triplet state reactions. We have shown that this is the , ~ the case for paraquat (1) where the final yields are m ~ d e r a t ebut transient yields of 2 are comparable with those observed in biradical processes. R. D. Small, Jr., and J. C. Scaiano, Chem. Phys. Lett., submitted for publication. E. Stekhan and T. Kuwana, Ber. Bunsenges. Phys. Chem., 78, 253 (1974). J. A. Farrington, M. Ebert, E. J. Land, and K. Fletcher, Biochim. Biophys. Acta, 314, 372 (1973). n. F. Anderson, Ber. Bunsenges. Phys. Chem., 80, 969 (1976). P. Neta, M. G. Simic, and M. 2. Hoffman, J. Phys. Chem., 80, 2018 (1976). P. Neta and L. K. Patterson, J . Phys. Chem., 78, 2211 (1974). The redox potentials given in Table I correspond to water as a solvent. They could be somewhat different in the solvent used in this work, but one can expect the same trends to prevail. IE. M. Kosower, “Pyridinyl Radicals in Biology”, in “Free Radicals in Biology”, W. A. Pryor, Ed., Vol. 2, Academic Press, New York, 1976, pp 1-53; G. C. Mees, Ann. Appl. Biol., 48, 601 (1960);A. D. Dodge, Endeavour, 30, 130 (1971);T. C. Stancliffe and A. Pirie, FEBS Left., 17, 297 (1971);J. S. Bus, S.D. Aust, and J. E. Gibson, Biochem. Biophys. Res. Commun., 58, 749 (1974). I. S. Shchegoleva, Abstracts of the International Conference on Photochemical Conversion and Storage of Solar Energy, London, Canada, 1976, B l l . R. D. Small, Jr., and J. C. Scaiano, J. Phys. Chem., 82, 2064 (1978). L. K. Patterson and J. C. Scaiano, to be published. L. K. Patterson, R. D. Small, Jr., and J. C. Scaiano, Radiat. Res., 72, 216 (1977).
