J. CHEM. SOC. FARADAY TRANS., 1994, 90(2), 287-291 Electron Transfer from Aromatic Compounds to Phenyliodinium and Diphenylsulfinium Radical Cations Y. Yagcit and W. Schnabel* Hahn-Meitner-lnstitut Berlin GmbH, Bereich S, Glienicker Str. 100,D-14 109 Berlin, Germany A. Wilpert and J. Bendig Humboldt-Universitat zu Berlin, Fachbereich Chemie, Hessische Str. 1-2,D-10115 Berlin, Germany Phenyliodinium (I*+)and diphenylsulfinium radical cations (Il'+)have been generated by flash photolysis (Ainc = 347 nm) of diphenyliodonium ions (I+)and diphenyl(4-phenylthiophenyl)sulfonium ions (II+)in acetonitrile solu- tions at room temperature. I*+and II'+were found to undergo electron-transfer reactions with benzene deriv- atives resulting in the formation of radical cations of the aromatic compounds.A study involving 25 compounds including various methyl- and methoxy-benzenes, biphenyl, phenol and cresols revealed that electron transfer is independent of the ionization energy Ei provided that the rates are encounter-controlled. This applies to cases where Ei does not exceed a critical value: Ei,crit x 820 kJ mol-' (I*+)and 780 kJ mol-' (ll*+).Bimolecular rate constants decrease with increasing Ei in the case of aromatic compounds having ionization energies exceeding the critical values. A Marcus-inverted region was not detected. This paper reports a study concerning the reaction of phenyl- + +iodinium (I' ) and diphenylsulfinium (11' ) radical cations with various aromatic compounds (see Table 2, later) in ace- tonitrile solution.1-II '+ I" and II'+ are formed in the photolysis of diphenyliodonium ions (I+) and diphenyl(4-phenylthio-pheny1)sulfonium ions (11') according to reactions (1) and (2).1-8 r rr In the present study I" and 11" were generated by flash photolysis of acetonitrile solutions of I+ and II+ (Ainc = 347 nm). The reactivity of the radical cations I*+ and 11" towards aromatic compounds is of importance for the elucidation of the mechanism of photo-crosslinking of polymers bearing aromatic pendant groups. According to Crivellog various polystyrene derivatives containing diaryliodonium or tri-phenylsulfonium salts act as negative tone photoresists. It was suggested' that insolubilization, i.e. intermolecular cross- linking of the polymers occurs uia coupling through aromatic nuclei, as is depicted in Scheme 1.-I +2H+ Scheme 1 Photo-coupling of polystyrene derivatives after the attack of aryliodinium radical cations Experimental Materials Diphenyliodonium hexafluorophosphate (I) and diphenyl(4- phenylthiopheny1)sulfonium hexafluoroarsenate (11) were pre- pared according to procedures described in the literature.' The aromatic compounds were commercial products. They were purified by distillation or recrystallization from solu- tions in appropriate solvents. Acetonitrile was refluxed over P,O, and distilled. Laser Flash Photolysis Solutions containing an onium salt and an aromatic com- pound were freed from oxygen by bubbling with purified argon prior to irradiation with 20 ns flashes of 347 nm light.This light was produced with the aid of a ruby laser (Korad, model K1 QS2) operated in conjunction with an ADP fre- quency doubler. Actinometry was performed with a benzene solution containing both benzophenone and naphthalene as described earlier." Dabs,the dose absorbed per flash by the solution was in the order of 6 x lo-' einstein dmP3. Determination of Ionization Energies t On leave from Istanbul Technical University, Department of Ionization energies, Ei , of the aromatic compounds D were Chemistry, Maslak, TR-80626 Istanbul, Turkey. determined according to the method described by Foster' ' 288 and by Zweig et al." in the following way: Optical absorp- tion spectra of charge-transfer complexes formed by the aro- matic compounds and tetracyanoethene (TCNE) were recorded in dichloromethane solution at [TCNE] = mol dmP3 and [D] = 10-2-1.0 mol dm-3.From the fre- quency, vct, of the maximum of the charge-transfer band at 25 "C,Ei values were calculated with the aid of eqn. (I).13 c1 = 6.06 eV and c2 = 0.32 (eV)2.14 Results Transient Absorption Spectra formed by Irradiation of I and I1 Upon irradiation of I+PF, and II+AsF, in acetonitrile solu- tion with 20 ns flashes of 347 nm light the transient optical absorption spectra shown in Fig. 1 were formed during the flash. These spectra which possess strong maxima at 660 nm (in the case of I) and at 750 nm (in the case of 11)are attrib-uted to the radical cations I.+ and II.', In the presence of aromatic compounds the decay of the absorp- tion of the radical cations was accompanied by the develop- ment of new absorption spectra indicating the formation of new species.Typical kinetic traces demonstrating the decay of the absorption of phenyliodinium radical cations at i= 660 nm and the simultaneous formation of the absorp- tion of the radical cation of p-methoxytoluene at A = 430 nm are shown in Fig. 2. At relatively high concentrations of the aromatic additive the spectra of the new species had already formed during the flash and the spectra of the radical cations I" and 11" were no longer detectable. Typical optical absorption spectra of the new species are presented in Fig.3. They are attributed to radical cations of the aromatic com- 400 600 800 A/nm Fig. 1 Irradiation of (a) I+PF, (2 x rnol dmP3) and (b) II'AsF; (2.3 x loP4 rnol dm-j) in Ar-saturated acetonitrile solu- tion at room temperature. Transient absorption spectra recorded at the end of the 20 ns flash. Ainc = 347 nm. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 AA = YO2/= flash I Fig. 2 Irradiation of p-methoxytoluene (1.5 x rnol dm-3) in acetonitrile solution containing I+PF, (5 x mol dmP3) at I = 347 nm. Kinetic traces demonstrating changes in the absorbance (A)at 660 nm (a)and at 430 nm (b) during and after the laser flash. pounds, Actually, the absorption spectra of the radical cations reported in the literature closely resemble the absorp- tion spectra shown in Fig.3. This can be seen from Table 1, where the wavelengths of the maxima of the absorption spectra of the radical cations are listed. t 400 480 560 A/nm Fig. 3 Optical absorption spectra of radical cations of various aromatic compounds (c = lo-' mol dm-3) recorded in acetonitrile solution containing I+PF, (c = 2.5 x rnol dm-3) at the end of the flash (Ainc = 347 nm, Dabs= 5.8 x lo-' einstein dm-3). (a) Toluene, (b) 1,2,4,5-tetramethylbenzene,(c) pentamethylbenzene, (d) hexamethylbenzene, (e) 1,2,3-trirnethoxybenzene, (f) 1,2,4-trimethoxybenzene, (9) 1,3,5-trimethoxybenzene,(h) 1,2-dimethoxy-benzene, (i) 1,4dimethoxybenzene, (j) 1,3-dimethoxybenzene. (Since the spectra are not comparable with respect to the amplitude the ordinates have not been labelled.) J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 289 Table 1 Maxima of absorption spectra of radical cations of various do not absorb light at A 2350 nm. Cyclohexadienyl-type aromatic compounds formed by electron transfer to phenyliodinium radicals have an absorption maximum at about 320 nm.22 radical cations (I") Actually, changes in the optical absorption at 45 350 nm could not be measured because of the strong absorption of ~~ the onium salts in this wavelength range. compound this worku other work* ~ ~ ~~~~ ~ ~~~~ ~~ methoxybenzene 435 m 430,' 435,6*' 445/ Determination of Rate Constants of the Reaction of I*+and 1,4dimethoxybenzene 450 m 430-4W II'+ with Aromatic Compounds 1,2-dimethoxybenzene 410 m rn 1,3-dimethoxybenzene 470 m 4W The reactivity of the radical cations I'+ and II'+ towards the 1,2,3-trimethoxybenzene 400 s, 470 m 4 10-42W aromatic compounds listed in Table 2 was investigated in the 1,2,4trimethoxybenzene 430 s, 450 m 410-45W following way: The rate of decay of the absorption of I" and 1,3,5-trimethoxybenzene 590 m 580" II'+ at 660 and 750 nm, respectively, was measured in the 1,3,5-trimethylbenzene 450 m, 470 s, 485 s 455,' 4758 absence and presence of aromatic compounds.The decay rate 1,2,4,5tetramethylbenzene 455 m 46Y8 was accelerated by the aromatic additives and at sufficiently hexamethylbenzene 490 m, 510 s 495'4 5Wh high additive concentrations the decay followed first-order et h ylbenzene 380 s, 450 m p-chlorotoluene 380 s, 470 s, 490 m kinetics.Therefore, rate constants of the reaction of I*+ and toluene 380 m, 450 s II'+ with donors D could be determined via pseudo-first-1,2-dimethylbenzene 400 s, 440 m, 460 s order kinetic data treatment : 1,3-dimethylbenzene 430 s, 460 s 1 ,Cdimethylbenzene 400 s, 450 m, 470 s k, = k + k,,[D] p-met hoxytoluene 435 m, 450 s where k, is the pseudo-first-order rate constant, k the rate a m, maximum; s, shoulder or second maximum. Absorption constant in the absence of aromatic compound D and k,, the maxima. Ref. 15. Ref. 16. 'Ref. 17. Ref. 18. Ref. 19. Ref. 20. bimolecular rate constant of electron transfer from the donor ' Ref. 21. D to the radical cation. Typical results obtained with the system p-methoxytoluene-1' are presented in Fig.4, which shows a Principally, phenyl and 4-phenylthiophenyl radicals, plot of k, us. [D]. All k,, values determined in this work formed according to reactions (1) and (2), respectively, or (error limit f10%) are compiled in Table 2. This table also reaction products of these species can also give rise to tran- contains the ionization energies of the aromatic compounds sient absorptions. These transient absorptions are considered which have been determined in this work. In Fig. 5, In k,, is not to interfere substantially with the absorption bands at plotted as a function of Ei, the ionization energy of the aro- long wavelengths of the radical cations I*+ and 11". For matic compounds. Obviously, In k,, increases with decreasing instance, phenyl radicals are very likely to abstract hydrogen values of Ei and becomes constant when the values of k,, are from the solvent or to add to phenyl rings of I+ or 11'.The of the order of magnitude of encounter-controlled (diffusion- resulting solvent radicals and cyclohexadienyl-type radicals controlled) reactions. Table 2 Electron transfer from aromatic compounds to I*+ and 11'' : bimolecular rate constants and ionization energies of aromatic com- pounds k,,*/dm mol -s -' compound no. compound E:/eV molecule -I' + 11' + methoxy benzene 8.35 (8.20) 1.1 x 1o'O 6.8 x lo6 1,4-dimethoxybenzene 7.8 9.3 x 109 1,2-dimethoxybenzene 7.96 1.1 x 1O'O 1,3-dimethoxybenzene 8.16 3.9 109 1,2,3-t rimet hox ybenzene 8.3 1.5 x 109 1,2,4-trimethoxybenzene 7.36 1.2 x 1O'O 1.2 x 1o'O 1,3,5-t rimet hox ybenzene 8.11 9.4 109 1,3,5-trimethylbenzene 8.58 (8.39) 7.5 x 109 2.8 x lo6 1,2,4-trirnet hylbenzene 8.62 (8.27) 9.5 109 2.2 105 1,2,4,Stetramethylbenzene 8.32 (8.41) 9.4 x 109 3.5 x 107 pen tamet hy lbenzene 8.31 (7.92) 8.6 x lo9 2.6 x lo8 hexamet h y lbenzene 8.18 (7.85) 1.1 x 1o'O 5.5 x 109 ethylbenzene 8.91 (8.77) 1.4 x 109 p-methoxytoluene 8.09 1.1 x 10'O 2.7 109 p-chlorotoluene 8.73 (8.69) 2.1 x 109 toluene 8.95 (8.82) 1.0 109 1,2*dimethylbenzene 8.77 (8.56) 6.3 109 1,3-dimethylbenzene 8.77 (8.56) 4.5 109 1,4dirnethylbenzene 8.73 (8.44) 4.6 109 cumene 8.92 (8.69) 8.6 x 10' biphenyl 8.39 (8.35) 5.0 x lo6 phenol 8.47 (8.50) 1.1 x 1O'O 3.5 107 o-cresol 8.30 2.2 x 109 rn-cresol 8.39 7.0 x lo8 p-cresol 8.22 9.2 x 109 7.8 x 109 Ionization energy, in brackets: values compiled in ref.23. Error limit lo"?. 0 0 1 2 [PMT]/10-4 mol dm-3 Fig. 4 Reaction of p-methoxytoluene (PMT) with I*+ in Ar-saturated acetonitrile solution. Dependence of the pseudo-first-order rate constant of the decay of the optical absorption at 660 nm on the concentration of p-methoxytoluene. p+PF;] = 2.5 x lo-* mol dm-3, Dabs= 5.8 x einstein dmP3. 24 I-I 23 *;22 C-15' 21 16 I 2o t (a) I 19 7 8 9 EJeV molecule-' I 7 8 9 EJeV molecule-' Fig. 5 Dependence of the rate constant of the reaction of I" (a)and 11" (b) with various aromatic compounds, D, on the ionization energy of D.Plot of In k,, us. Ei. The dotted lines are straight lines of slope rn = -(RT)-l. Discussion The important feature of this paper concerns the high reacti- vity of phenyliodinium (I*+)and diphenylsulfinium radical cations (II'+) towards many aromatic compounds. In all J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 cases studied the radical ions undergo electron-transfer reac- tions as indicated by the formation of radical cations of the aromatic compounds. For example, I*+ reacts with p-methoxytoluene as depicted by reaction (3): Note that the intensity of the spectra of I" and II" decreased with increasing concentration of the aromatic com- pounds. At suficiently high concentrations the absorption spectra of I*+and 11" were not detectable anymore, because the radical cations reacted very quickly with the aromatic compounds. Therefore, it is concluded, that most of the radical cations (1.' and 11") underwent electron-transfer reactions according to the type represented by reaction (3).Inspection of the kinetics of these reactions reveals the fol- lowing: plots of In k,, us. E, of the aromatic compounds (see Fig. 5) demonstrate the independence of k,, of Ei provided that the rates are encounter-controlled. This applies to com- pounds having ionization energies lower than the critical values of E,: Ei,crit = 8.5 eV molecule-' (820 kJ mol-I), in the case of I*+, and Ei,,rit= 8.1 eV molecule-' (782 kJ mol-') in the case of 11". At Ei values exceeding the critical values, In k,, decreases with increasing E,.As can be seen from Fig. 5(a) and (b) the curves representing most experi- mental points merge, as Ei increases, into straight lines with slope m = -(RT)-'. Notably, this kind of dependence corre- sponds with the behaviour described by Rehm and Weller24,25 for electron-transfer reactions if the dependence of AG, the Gibbs energy of the reaction, on the ionization energy according to eqn. (111) is taken into account: AG= Ej -E, + C (111) where E, is the ionization energy of the aromatic compound acting as donor, E, the electron affinity of the radical cation and C is a constant. Actually, k,, should be related to AG according to eqn. (IV) in the case of activation-controlled reactions with a transfer constant a = 1.0,21i.e. at E, > E,,crit: k,, = k, exp( -g) aAGIn k,, = In k, --RT On the basis of eqn.(111) and (IVb) the following expressions are obtained for the dependence of In k,, on E, for the region Ei > Ei,crit: In k,, = In k, + mE,+ n (V) The parameter m corresponds to the slope of the linear por- tions of the curves in Fig. 5(a) and (b). For example, m = -(RT)-', i.e. for T = 298 K, rn = -38.7 molecule eV-' = -0.4 mol kJ-'. Obviously, not all of the experimental points fit the curves in Fig. 5(a) and (b).Although this could be due to experimen- tal errors it also might reflect an influence of the chemical nature of the aromatic compounds on their reactivity towards the radical cations I*+ and 11" which is not prop- erly describable in terms of a dependence of k,, on the ioniza- tion energy or oxidation potential. J.CHEM. SOC. 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