J. Phys. Chem. 1083, 87,297-300
that there is no significant potential dependence of the intensity in HzS04,cf. Table 11, we prefer to attribute this anion effect to interaction of the anions with the CO adlayer. From the foregoing discussion we believe that the CO adlayer on the platinum electrode surface in the presence of an electrolyte closely resembles a CO adlayer established from gas-phase CO under clean ultrahigh vacuum conditions although solvent effects may displace peak wavenumbers and give rise to other complications. This suggests that the origin or origins of the coverage-dependent spectral shifts should be the same in the two systems. To a first approximation, the intensity of the CO band is independent of applied potential. Since the intensity is proportional to the vibrational strength and to the number density of absorbers on the surface, the simplest and most likely way in which the intensity can be independent of potential is for both the vibrational strength and the number density to remain constant as the potential is changed. We thus conclude that the dipole-moment derivative with respect to the CO-stretching mode, d p / A r , is not discernibly dependent on the applied potential. It therefore follows that any dipole-dipole coupling term connecting the CO-stretching vibrations of molecules adsorbed on different metal sites is independent of applied potential since both it and the vibrational strength are controlled by the effective value of ( d p / a r ) z . What then is the origin of the potential-induced frequency shift? Our belief is that, as the electrode is more positively polarized, electrons which have occupied via (11)There appears to be a systematic difference in intensity between HzSOl and HCIOl of about 10%. This is of the same order as our uncertainty in drawing in the baseline and may or may not be real.
297
back-bonding the K* orbitals of adsorbed CO molecules are abstracted, increasing the principal force constant of the CO bond.12 Also, if there is, as Moskovitz13has suggested, an electronic contribution to the interaction force constant between CO molecules adsorbed on neighboring sites, this too might be affected by the applied potential although the direction in which it would be affected is not obvious to us. These results are suggestive that these electronic effects, which are responsible for the potential-induced frequency shifts, should also contribute to the coverage-dependent shift of the CO stretching frequency since one expects that, with increasing coverage, there will be increasing competition for the available metal electrons. In other words, the coverage-dependent shift stems from three effects: first the coverage dependence of the principal CO stretching force constant, second the coverage dependence of an interaction force constant which originates in coupling through the electrons in the metal surface and third, the coverage dependence of the dipole-dipole coupling term. The first two effects can, in principle, be probed by electrochemical experiments similar to those described in this paper; it is our hope that further studies will lead to a quantitative separation of these several effects. Acknowledgment. This research was supported by a grant from the National Science Foundation, DMR8016509 Registry No. CO, 630-08-0;Pt, 7440-06-4;HzSO4,7664-93-9; HC104, 7601-90-3. (12)Blyholder, G.J. Phys. Chem. 1964,68,2772. (13)Moskovitz, M.;Hulse, J. E. Surf. Sci. 1978,78, 394.
Effects of Salts Having Complex Metal Halide Anions on Formation and Decay of Biphenyl Radical Cations Studied by Pulse Radiolysis Soukll Mah, Yuklo Yamamoto,’ and Kolchlro Hayashl The Instltute of Sclentlflc and Industrial Research, Osaka Unlverslty, 8- 1 Mlhogaoka, Ibarakl, Osaka 567, Japan (Received: March 3, 1982)
Formation and decay of biphenyl radical cations produced by pulse irradiation have been studied in di(n-C41-&)4NPF6, C7H7PF6, and C7H7BF4. chloromethanein the presence of salts such as (C6H&IPF6,(C6H5)3SPF6, The decay of biphenyl radical cations was retarded by the addition of these salts. The stabilizing effect of the salts was explained in terms of ion-pair formations of biphenyl radical cations with the complex metal halide anions. Slow formation of biphenyl radical cations was observed in the presence of (C&&IPF6 and attributed to the oxidation of radical species by the salt, followed by the charge transfer from the resulting carbocations to biphenyl.
Introduction Recently, the use of diaryliodonium, triarylsulfonium, and triarylselenonium salts, which have complex metal halide anions, as photoinitiators for cationic polymerization has been extensively studied.l When irradiated with (1)(a) J. V. Crivello and J. H. W. Lam, J.Polym. Sci., Polym. Symp., 56,383 (1976);(b) J. V.Crivello and J. H. W.Lam, Macromolecules, 10, 1307 (1977);(c) J. V. Crivello and J. H. W. Lam, J.Polym. Sci., Polym. Chem. Ed., 16,2441 (1978);17, 977,1047,1059 (1979);18, 2677,2697 (1980). 0022-365418312087-0297$0 1 .SO10
ultraviolet light, the salts undergo photodecomposition to yield Bronsted acids capable of initiating the cationic polymerization. I t has also been reported that the salts can initiate cationic po1yme:izations by oxidizing free radicals to corresponding carbocations in thermal and photochemical polymerizations initiated by radical initiators such as azobisisobutyronitrile and benzoyl peroxide.* In our preceding s t u d i e ~ the , ~ salts such as diphenyl(2)(a) A. Ledwith, Polymer, 19,1217(1978);(b) A. M.A b d u l - b o d , A. Ledwith, and Y. Yagci, ibid., 19,1219 (1978).
0 1983 American Chemical Society
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iodonium and triphenylsulfonium hexafluorophosphates were found to be effective for the promotion of radiation-induced cationic polymerization of styrene derivatives. The contribution of the decomposition products of the salts to the polymerization was ruled out. The pulse radiolysis study revealed that the cationic species involved in the polymerization are stabilized toward Cl- and basic impurities by ion-pair formations with the complex metal halide anions. Such a stabilizing effect of nonnucleophilic anions is expected to be important in radiation-induced cationic reactions where the cationic species are in free-ion states and have higher reactivities than those in ion-pair states. The present pulse radiolysis study is concerned with the formation and decay of biphenyl radical cations in dichloromethane solutions containing various salts, which have complex metal halide anions. In our earlier it was found that diphenyliodonium hexafluorophosphate added to a dichloromethane solution of biphenyl promotes the formation of biphenyl radical cations and retards their decay. In the present paper, we have extended the study to other kinds of salts and compared the dynamic behaviors of the biphenyl radical cations in the free-ion and ion-paired states.
Experimental Section Dichloromethane, obtained from Wako Pure Chemical Industrial Co., was washed 3 times with an aqueous solution of sodium hydroxide and water and then distilled over calcium hydride. The middle fraction was stored under vacuum over calcium hydride. Biphenyl, an extrapure reagent, was obtained from Nakarai Chemicals and used without further purification. Diphenyliodonium and triphenylsulfonium hexafluorophosphates were provided by 3M Co. Tetra-n-butylammonium hexafluorophosphate was prepared from tetra-n-butylammonium bromide and potassium hexafluorophosphate and twice recrystallized from a methanol-water mixture. Diphenyliodonium and tetra-n-butylammonium chlorides were obtained from Aldrich Chemical Co. and Tokyo Kasei Kogyo, respectively. Tropylium hexafluorophosphate and tropylium tetrafluoroborate were obtained from Aldrich Chemical Co. All the salts were dried under vacuum by heating with warm water (about 50 "C) for 30 min. The samples were prepared by using the vacuum line and sealed into Suprasil cells of 10-mm optical path length. The L-band linear accelerator of Osaka University operating at 28 MeV with a 10-ns pulse of approximately 8 A was used for the pulse radiolysis. The electron beam had a diameter of 4 mm. An analyzing light beam from a 450-W xenon pulse lamp (OPG-450, Osram) passed through the cell and then to a monochromator (Nikon G-250). It was monitored by a photomultiplier tube (Hamamatsu-TV R928). The signal was displayed on an oscilloscope (Tektronix 7834), and its trace was photographically recorded on 3000 ASA Polaroid film.
15L' 1
2 1 0
"
400
500 Wavelength,
600 nm
Figure 1. Translent absorption spectra of 5 X lo-' M biphenyl solution (1) at the end in CH,CI, in the presence of 5 X lo3 M (n-C,H,),NPF,: of pulse; (2) at 100 ns after the pulse; and (3) at 500 ns after the pulse. ( 100
. in
n
a,
0 100 nsec
4 k 100 nsec
100
. m
n Q
100 nsec
-4k
100 nsec
- - X F
100 nsec
Figure 2. Oscilloscope traces of transient absorption at 680 nm. Additive: (A) none; (B) 5 X M (C&i5)$PF6; (C) 5 X M (C&),IPF,; (D) 5 X IO3 M (nC,H&,NFF,; (E) 1 X IO3 M (CsH,),ICI; and (F) 1 X M (n-C,H&,NCI.
Results The pulse radiolysis experiments were carried out at a biphenyl concentration of 5 X loT2M in CHzClzat room temperature. The pulse irradiation of biphenyl in CH2C1, was accompanied by light absorption in the 600-720-nm region and below 400 nm. The transient absorption spectrum was almost identical with that obtained in 1,2-
dichloroethane which has been earlier published: and the absorption is attributable to biphenyl radical cations. No particular change in the absorption spectrum was observed when various kinds of salts were added to the solution. The transient absorption spectra for the pulse-irradiated M ( ~ L - C ~ H ~ )are ~ NillusPF~ solution containing 5 X trated in Figure 1. Comparison of the spectra at different times after the pulse shows that the absorption below 400 nm is not due to single transient species. As has been r e p ~ r t e d ,there ~ is a small contribution of long-lived species, the first triplet excited state of biphenyl, which has an absorption band at around 360 nm.5 On the other
(3) (a) S.Mah, Y.Yamamoto,and K.Hayashi, J. Polym. Sei., Polym. Chem. Ed., 20,1709 (1982);(b) S. Mah, Y.Yamamoto,and K.Hayashi, ibid., in press; (c) S. Mah, Y. Y m m o t o , and K. Hayashi, Macronolecules, submitted.
(4) S. Arai, H. Ueda, R. F. Firestone, and L. M. Dorfman, J. Chem. Phys., 50, 1072 (1969). (5) G. Porter and M. W. Windsor, Proc. R . SOC.London, Ser. A, 245, 238 (1958).
The Journal of Physical Chemistry, Vol. 87, No. 2, 1983
Formation and Decay of Biphenyl Radical Cations
299
m -0.8
0
I
1
I
1
I
100
0
Time
I
p u l s e , nsec
Flgure 3. First- and secondorder kinetic plots for the decay at 680 nm In the absence of salt.
Figure 5. Second-order klnetic plots for the decay at 680 nm in the presence of (0)5 X lo-, and (0)1 X lo3 M (C6HS),IPF6 compared with that in the absence of salt (dotted line). I
I
12-
iI 0
200 Time
400
c
3
T i m e a f t e r p u l s e , psec
300
200 after
2
1
I
I
/’
I
I
I
I
-
//
, -
2I
I
I
I
1
I
I
600
a f t e r p u l s e , nsec
Flgure 4. Second-order kinetic plot for the decay at 680 nm in the presence of 5 X lo3 M (C6H,),SPF, compared with that in the absence of salt (dotted line).
hand, in the 600-720-nm region, the decay characteristics were the same for all wavelengths measured, indicating that the absorption is to be attributed to single transient species, the biphenyl radical cations. The decay behavior of biphenyl radical cations was appreciably affected by the addition of the salts such as (C6H&SPF6, ( C G H ~ ~ I P( ~F-~C, ~ H ~ ) ~ N(Cd%2Icl, P F G , and ( ~ J - C ~ H ~ ) ~Figure N C ~ 2. shows the oscilloscope traces of absorption at 680 nm for the solutions containing the salts together with that for the solution not containing salt. Comparison of the decay behaviors demonstrates that the decay is retarded by the addition of (C6H5)3SPF6and (nC4H9)4NPF6,and it is accelerated by the addition of the salts having nucleophilic C1- such as (C6H6)21Cland (nC4G)4NCl. A striking feature observed is a slow formation of biphenyl radical cations when (C6H6)2n?F6 is added; i.e., the absorption gradually increases in the early stage and then decreases after reaching a maximum at around 100 ns after the pulse. Figure 3 shows the analysis of first- and second-order kinetics for the decay at 680 nm in the absence of salt. Although it fits neither first- nor second-order kinetics, it appears to be better represented by the second-order kinetics. Figures 4-6 show the second-order kinetic plots for the SOlUtiOnS COnt.hing (C&),SPF6, ( C & S ) ~ P Fand ~, (n-C4H9)4NPF6, respectively, where the plot for the solution not containing salt is also shown for comparison (dotted line). The second-order plots for the decay in the presence of the salts having PF6- give straight lines. In the case of (C6H5)21PF6,the plot is extended to the microsecond time scale because of the slow decay. Its deviation from linearity in the early stage is ascribed to the slow formation of biphenyl radical cations. The slope of
none (C7.H5)21PF6 (C6H5
)3SPF6
(n-C,H,),NPF6
C7H7PF6
C7H7BF4 (C,H,),ICl (n-C4H,),NCl
5x 1x 1x 5x 1x 5x 1x 1x 1x 1x 1x
10+
10-3 10-2
10-3 10-3 10-3 10-3 10-3
10-3 10-3
1 . 2 x lo6 2 . 0 x 106 1 . 2 x 107 1.0 x 107 1.1 x lo7 1.3 x 107 9.7 x 106 1.5 x 107 9.7 x 106
1 1.1
1.0 1.1 1.2
1.1 1.0
1.0 0.91
0.53
the plot is affected by the concentration of added (c6H5),IPF,. On the other hand, in the cases of (C6H5)3SPF6 and (n-C4H9),NPF6,the slopes were almost constant at various salt concentrations examined. The values of the slopes are listed in Table I. The fast decay in the presence of 1 X M (n-C4H9)4NC1 appeared to be of first-order kinetics. The decay in the presence of 1 X M (c6H5)21Clwas also shown to be of a mixed first- and second-order kinetics fitting closely to the fist-order kinetics. For the sake of comparison, the pulse radiolysis experiments were exposed tQ solutions containing C7H7PF6and C7H,BF4. Figure 7 shows the second-order kinetic plots for the decay at 680 nm, which give straight lines. The values of the slopes are also presented in Table I. The optical densities at 680 nm at the end of pulse were compared in experiments of the same run to investigate the effects of salts on the yield of biphenyl radical cations. The relative optical densities are presented in Table I.
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The Journal of Physical Chemistry, Vol. 87, No. 2, 1983
21
I
0
1
1
200
100
300
Time a f t e r
I
400
/ I
500
p u l s e , nsec
Figure 7. Second-order kinetic plots for the decay at 680 nm in the M (0) C,H,PF, and ( 0 )C,H,BF, compared with presence of 1 X that in the absence of salt (dotted line).
There is no appreciable change in the optical density by the addition of the salts except for (n-C4H9),NC1.
Discussion As reported earlier; the irradiation of halogenated-hydrocarbon solutions of biphenyl results in the formation of biphenyl radical cations and halide anions. The formation processes in CH2C12are presented as follows:
-+ -
CHZCl2+.+ e
CHzClz CHZClz+. e
BP
+ CH2Clz
-+
CHzClz C1-
+ BP+.
+ CHzC1.
(1) (2)
(3)
where BP denotes biphenyl. The formation of BP+. is completed within the pulse duration, 10 ns, and the decay due to charge neutralization with C1- is important. The deviation from second-order kinetics in the absence of salt (Figure 3), however, suggests that reactions other than neutralization with C1- take place. Basic impurities in the solution and radiolysis products may contribute to the decay of BP+-,causing the deviation from second-order kinetics, as reported earlier., The retardation of the decay by the addition of the salts having complex metal halide anions can be interpreted in terms of ion-pair formations of BP+. with the anions. A similar stabilizing effect has been observed for dimer radical cations produced from styrene and a-methylstyrene in CH2C1z.3The obedience to second-order kinetics in the presence of the salts demonstrates that the decay of BP+. paired with the complex metal halide anions is mainly due to the neutralization with C1-. The slope of the secondorder kinetic plot corresponds to k 2 1 4 where k2, e, and 1 are the rate constant of the second-order reaction, the molecular extinction coefficient of BP+-, and the optical path length of the pulse radiolysis system, respectively. The values are almost identical for the solutions containing the salts having PF6-regardless of their cationic moieties, except for (C,H,),IPF,. The independence of the slope and the salt concentration in the solutions containing (c6H5)3SPF6and (n-C4H9),NPF6can be explained if we consider that the salts are so dissociated in CH2ClZthat most of BP+. form ion pairs with a large excess of PF6-. The decay of BP+. was accelerated by the addition of (C6H5)21C1and (n-C4Hg),NC1,demonstrating the occurrence of neutralization with C1-. The faster decay in the
presence of (n-C4H9),NC1rather than in the presence of (C6H5)21C1 may be attributed to the higher dissociation tendency of (n-C4Hg),NC1in CH2C12. The remarkable decrease in the yield of BP+. by the addition of (nC,Hg),NC1 suggests that the decay due to neutralization with a large excess of Cl- begins within the pulse duration. The effect of (C6H5)21PF6 is strikingly different from those of other salts having PF6-. It is interesting that the slow formation of BP+. can be observed with (c6H5),IPF6 but not with (C6H5)3SPF6, although both salts have been reported to promote cationic polymerizations by oxidizing free radicals to carbocations.2 Such a slow formation of radical cations has also been observed in the pulse radiolysis of styrene in CH2C12,while it was negligibly small in the case of (C,&)3SPF6.3a9b The promotion of the radical cation formation by (C6H5),IPF6can be explained by the oxidation of radical species by the salt, followed by charge transfer from resulting carbocations to substrate, as has been proposed in the previous paper^.^ Thus, the slow formation of BP+. can be represented by the following equations: Re + (C,&)&F' 6 R+ + PF6- + C&5I + C6H5. (4)
-
Re + BP+* (5) where R- denotes radical species such as CH2C1.and C6H5. having ionization potentials higher than that of BP. It is believed that the promotion of the BP+. formation is not important in the case of (C6H5),SPF6because of its lower electron affinity and the higher ionization potentials of the radical species involved in reactions 4 and 5. It has been reported that (C6H5)21PF6 is 10 times less dissociated than (C&I&$PF, in CH2C12.6 Thus, the dependence of the slope of the second-order kinetic plot can be accounted for by low concentration of the dissociated PFe-. However, the more remarkable retardation of the decay by (C&,)2IPF6 cannot be accounted for only by the ion-pair formation with PFc. The slow formation of BP+. may continue up to the microsecond time scale if the radical species are reproduced by reaction 5 . Its contribution should apparently bring the retardation of the decay depending on the salt concentration. Furthermore, the second-order kinetic plot is based on the assumption that the concentration of BP+.is identical with that of Cl-. This is not the case for the solution containing (C6H5)21PF6 because of the slow formation of BP+.. At this stage we have not confirmed the mechanism to account for the decay kinetics of BP+. in the presence of (C6H5)21PF6.
R+ + BP
-+
Acknowledgment. We are grateful to Central Research Laboratories, 3M Co., for diphenyliodonium and triphenylsulfonium hexafluorophosphates. We are also indebted to Mr. Kunihiko Tsumori, Mr. Norio Kimura, Mr. Tamotsu Yamamoto,and Mr. Toshihiko Hori for help with the pulse radiolysis experiments. figistry NO. (C&j)2IPFs, 58109-40-3;(C&)3SPFs, 5783599-1; ( ~ - C ~ H S ) ~ N3109-63-5; P F ~ , C T H ~ P F29663-54-5; ~, CTH7BF4, 27081-10-3;(C&,)2IC1,1483-72-3;(n-C4Hs),NCl, 1112-67-0;biphenyl, 92-52-4; biphenyl radical cation, 34507-30-7. (6) A. Ledwith, S. Al-Kass, D. C.Shenington,and P.Bonner, Polymer, 22, 143 (1981).
