J. Phys. Chem. 1995,99, 9909-9917 9909

Formation of Methylviologen Radical Monopositive Cations and Ensuing Reactions with Polychloroalkanes on Silica Gel Surfaces

Yun Mao, Nancy E. Breen, and J. Kerry Thomas* Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556

Received: November 23, 1994; In Final Form: February 27, 1995@

Methylviologen ions (MV*+) readily adsorb on silica gel surfaces, while the chloride counterions (Cl-) remain in the solution phase. Under aerated conditions, photoirradiation of this system induces an oxidative degradation of MV2+, which is monitored via emission studies of the products. Under degassed conditions, the photoirradiation produces the monopositive radical cation (MV"), which is confirmed by its characteristic diffuse reflectance, Raman, and electron paramagnetic resonance spectra. The Raman spectrum of M V " on silica gel shows that perturbation due to the adsorbed state is not significant, while EPR studies show common features of the adsorbed radical cation on a surface. The coadsorption of sodium chloride affects the formation and increases yield of M V " in a complex way. The photogenerated M V " reacts with carbon tetrachloride, chloroform, dichloromethane, and 1 ,Zdichloroethane, which are studied by steady-state and time-resolved methods, and the corresponding rate constants have been determined. Compared to solution, the rigid nature of the silica gel surface stabilizes the initial CT pair (MV'+CP) by several orders of magnitude and increases electron transfer between MV'+ and CCL by an order of magnitude.

Introduction

Methylviologen dichloride (l,l'-methyl-4,4'-bipyridinium dichloride, hereafter referred to as MV2+C12) has been used as a herbicide to interfere with the reduction of ferredoxin in photosystem I. Due to its favorable redox properties and potential application in the solar-energy storage and electro- chromic display devices, the solution chemistry of MV2+ has been extensively studied.'-6 The photolysis of MV2+ on semiconductor and polymer surfaces has been also

In the present studies the silica gel surface, which is mainly composed of siloxane (=Si-0-Si=) and silanol (=SOH) groups, is used as a host system. It is believed that the principal sites responsible for the adsorption of organic compounds on silica gel are silanol groups. The effect that surface adsorption has on probe molecules has been studied using UV-visible absorption and fluorescence spectroscopy,'0 as well as IR spectroscopy.' ' However, far less is known about photoinduced chemical reactions of aromatic compounds on a silica gel surface. I *

In early work, models to describe nonexponential decay of fluorescence, quenching reactions of excited states, movement of adsorbed molecules, and formation of radical cations on silica-based surface have been reported.l3-l* Low-temperature studies have shown that charge transfer (CT) interactions between electron-rich aromatics and silica gel surfaces leads to the formation of organic radical cations and trapped e1ectr0ns.I~ This report will concentrate on photoinduced reactions of MV2+ on silica gel surfaces to explore the effects of the surface on photolysis reactions. It is shown that photoirradiation induces a degradation of MV2+ under aerated conditions, which is conveniently monitored by emission studies. Under degassed conditions, photoirradiation initiates electron transfer leading to the formation of radical monopositive cations of methylvi- ologen, MV+. The copresence of sodium chloride (NaCl) increases the formation yield of MV+, indicating that C1- ions also act as electron donors. On the other hand, the coadsorption of NaCl also induces a rapid decay of MV+. Raman and EPR spectra are used to characterize various features of the adsorbed

* To whom correspondence should be addressed. @ Abstract published in Advance ACS Abstracts, May 15, 1995.

0022-365419512099-9909$09.00/0

M V + on the silica gel surface. The M V + formed reacts with chloroalkanes, and the corresponding rate constants have been determined and compared with those in methanol solution.

Experimental Section

Silica gel was supplied by Aldrich Chemical (David, 60 A). The average surface area of silica gel measured by the BET method (Brunauer-Emmett-Teller) is -460 m2/g. The main impurities of the commercial silica gel are as follows: C1-, 1 ppm; Nos-, 5 ppm; Sod2-, 600 ppm; and Na+, 100 ppm, which were determined by a Waters high-performance ion chromato- graph (HpIC).20 These impurities can be washed out with acidic solution and Milli-Q water. The residuals were lower than the detection limit: C1-, < 0.2 ppm; S04*-, < 10 ppm; Na+, < 2 ppm. The washed silica gel was preheated at a desired temperature of Ta (activation temperature) in air prior to use. Methylviologen dichloride (MV2+, Aldrich Chemical) was recrystallized at least twice in methanol. The remaining compounds were of the highest purity available.

Steady-state diffuse reflectance spectra of solid samples were measured on a UV-visible spectrophotometer (Perkin-Elmer 552) equipped with an integrating sphere. The spectra were recorded in -log RT, where RT is relative diffuse reflectance of samples versus silica gel. Luminescence spectra were measured on a spectrofluorometer (SLM Aminco, SPF 500C). Raman spectra were recorded using a Coherent krypton ion laser operating at 406.7 nm and a Spex (Model 1403 0.85) double monochromator equipped with a photomultiplier tube (RCA 1P28). The Raman scattered light was collected at 135" in a backscattering geometry. The samples were rotated by placing them in sealed capillary tubes, which were attached to the shaft of a small motor. Electron paramagnetic resonance (EPR) spectra were recorded on a Varian Associate spectrometer (E-line Century Series) equipped with an X-band klystron and a cavity of T E I o ~ type.

Time-resolved diffuse reflectance spectra were measured on a laser photolysis detection system. A XeCl excimer laser with an energy density of 70 mTlcm2 at 308 nm and a fwhm of 10 ns (Lambda Physik, Model EMG 100) was used for excitation of MV2+/silica gel. A 450-W xenon lamp was employed as

0 1995 American Chemical Society

9910 J. Phys. Chem., Vol. 99, No. 24, 1995 Mao et al.

2500 I c

o.8 t 1500

0 1 . 1 . 1 . 1 .

0 2 4 6 8 1 0 1 2

Ifc, l/mM Figure 1. Adsorption isotherm of MV2+ on a silica gel surface in aqueous solution at room temperature. 0 is the coverage on surface, the ratio between the adsorbed MV2+ and the saturated amount of MVZ+ on the surface. and c is the initial concentration of MV2+.

0 200 300 400

Wavelength, nm Figure 2. Steady-state diffuse reflectance spectrum of a MV*+/silica gel sample. The MV2+ loading was 3 x moUg. For comparison, the absorption spectrum of MV2+ in methanol solution is presented (dotted line).

the monitor beam. The diffusely reflected analyzing light from the sample was transferred to a monochromator by an optical fiber and detected by a photomultiplier tube (Hamamatsu, R928). The output of the photomultiplier was fed to a digitizer (Tektronix 7912AD) connected to a Zenith data system. For monitoring of reaction kinetics of MV'+ with polychloroalkane, long time scales (100 ms-10 s) were used. The output of the photomultiplier was fed to an oscilloscope coupled with a digitizer camera (Tektronix DCSO1).

Photoirradiation was carried out in a photochemical reactor equipped with fluorescent lamp RPR 30WA (Rayonet, Southern New England Co.). The chemical actinometer, K3Fe(C204)3, was used to determine the incident photon intensities on the samples.

Solid samples were prepared by mixing silica gel and MV2+ aqueous solution. In most case the loading concentration of

MV2+ was (1 -2) x moYg. After incubation for 4 h, the supernatants were separated by centrifugation. To estimate the effect of C1- on electron transfer, samples were washed with water until a C1- test of the supematants was negative (Cl- less than 5 x moYg), and then dried at 55 "C. Samples were also prepared by mixing a desired amount of sodium chloride or sulfate followed by drying at 55 "C. The sample powder was filled into either a 5 mm 0.d. quartz tube or a thin quartz cuvette (1 mm) and degassed on a vacuum line (< TOK). The concentration of MV2+ and C1- in the supematants were determined by UV-visible spectrophotometer and HPIC, respectively. A Waters high-performance ion chromatograph (HPIC), equipped with an anion column (IC-Pak, Waters) and

4 -

I

4tY) 4511 S i H l 5 5 0 h(Hl

1:1111 \ \11 l11 w . l v , ~ l l ~ r l ~ ~ l l ~ , 11111

Figure 3. Steady-state luminescence spectra of an aerated MV2+/silica gel sample, excited at 360 f 5 nm. The luminescence intensity increases with irradiation time. The photoirradiation times are, from bottom to top, 0, 3.0, 5.0, and 8.0 min. The irradiation intensity was 4 x einsteinl min cm2. Inset: a time profile of the emission, monitored at 450 nm. A nitrogen laser (337.1 nm) was used for excitation; the MV2+ loading was 3 x mol/g.

Methylviologen Radical Monopositive Cations

a conductivity detector (Waters 430), was used. The eluent for HPIC was borate/gluconate (pH 8.5).20

J. Phys. Chem., Vol. 99, No. 24, 1995 9911

Results and Discussion

1. Adsorption of MV2+ Ions on a Silica Gel Surface. MV2+ ions adsorb strongly onto silica gel surfaces from aqueous solution. A plot of the reciprocal of coverage (l/@ versus the reciprocal of initial concentration of MV2+ is linear, indicating a Langmuir type adsorption (Figure 1). Chemical analysis of the aqueous supernatants shows that more than 95% of the MV2+ ions adsorb on silica gel surfaces within the loading range (1 -5) x moVg, while more than 95% of the chloride ions (Cl-) remain in the solution. An additional wash with water removes residual C1- from the surfaces. The diffuse reflectance spectrum of a MV2+/silica gel sample shows an absorption band which is red shifted by 10 nm from that of aqueous MV2+ (Figure 2). This new absorption band is ascribed to chemical adsorption and assigned to a surface complex between MV2+ and the negative sites on the silica gel surface:

MV2+ + surfaces- - (MV2+-surfaces-) (1)

The negative surface sites on silica gel are well documented and result from the surface texture.21 The nature of the surface complexes may be charge transfer (CT) in character and are thought to be similar to the ion-pair complexes between MV2+ and several anions in s o l ~ t i o n s . ~ ~ ~ ~ ~ ~ ~

2. Photoinduced Reactions of MV2+ on a Silica Gel Surface. Under aerated conditions, photoirradiation of a MV2+/ silica gel sample produces a species which show a luminescence with Amax = 500 nm and a short lifetime of -10 ns (Figure 3). This is in agreement with early report^,^,*.^^ and the emission band on a silica gel surface is very similar to that in solution (450-650 nm). The emission intensity increases with increasing time of irradiation. By contrast, photoirradiation under degassed conditions does not induce a significant change in the lumi- nescence intensity. This indicates that the emission involves photooxidized species. The emission may arise from lumines- cent oxidized derivatives of MV2+, which are determined as previously reported.25 The present luminescence studies show a photoinduced degradation of MV2+ on a silica gel surface. The reaction of 0 2 in the photolysis of MV2+ on silica gel will be described later. The studies show that the trace products formed do not affect the formation of MV+. Here, no attempt was made to identify these trace species.

Under degassed conditions, steady-state photoirradiation induces a visible color- change from white to blue, and new absorption bands are seen at 405 and 610 nm (Figure 4a). These bands are reported to arise from the formation of radical monopositive cations of methylviologen, MV.f.539326

The formation of MV'+ is also observed by time-resolved diffuse reflectance spectroscopy. Figure 4b exhibits typical diffuse reflectance spectra of MV2+/silica gel sample taken 0.10 and 2.0 ms after the laser flash. Two bands at 400 and 610 nm are observed, which are ascribed to MV'+ as in the case of steady-state photolysis. These time resolved experiments show that the formation of MV'+ is a rapid process and faster than the 100 ns time resolution of the detection system used.

The formation of MV'+ illustrates a photoinduced reduction of MV2+ on the silica gel surface and is consistent with the fact that excited MV2+ is a very strong oxidizer. Early work showed that chloride ions, methanol, and cellulose can be oxidized by excited MV2+.5,9.27

( 2 ) hvlRT

MV2+/surface - (MV2')*/surface

0.420

0.336

A 0 2 5 2

0. I08

0.084

0

. . . . . . . . . . . . t !

270 350 430 510 590 670 7 5 0

Wavelength, nm

2 b c)

0.10

o.om

0.00' I 300 390 400 WO wo 7%

Wavelength. nm

Figure 4. (a, top) Diffuse reflectance spectrum of a degassed MV2+/ silica gel sample after photoirradiation at room temperature (solid line). The irradiation intensity was 1 x einstein/cm2. Dotted line: the spectrum of a MV2+/silica gel sample before irradiation. The dashed line illustrates the spectrum after introduction of CC14 vapor (40 mbar) into the sample. (b, bottom) The time-resolved diffuse reflectance spectra of a MV2+/silica gel sample. The spectra were taken 0.1 and 2.0 ms after a laser flash; the MV2+ loading was 3 x moYg.

(MV*+)/surface - MV" + h'/surface (3) Here h+/surface denotes the positive charge or its derivatives on the surface. A possible hole derivative is GSiO', with MV'+ in close proximity, however, the absorption of =Si@ is concealed by the MV2+ spectrum. In methanol solution M V " reacts with its parent molecule leading to the formation of a dimer (MV'+/MV2+), which has absorption bands at 355 and 508 nm.26 No such species is observed on the silica gel surface, suggesting that surface sites where M V " and MV2+ are adsorbed inhibit the dimer formation.

3. Effect of Coadsorption of NaCl on the Formation of MV+. Previous studies have shown that photoirradiation of MV2+(Cl-)2 in aqueous solution produces the short-lived radical pair MV'+ C1'-2.5 Recently, Kevan and co-worker have reported

9912 J. Phys. Chem., Vol. 99, No. 24, 1995

O O 2.5 5.0 7.5 10

Mao et al.

12.5

Loading of NaCl. FmoVg

1 .oo

0.75

)r

v) C Q, C '- 0.50 Q, > a a, E

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.- c -

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TIME ( m i c r o s e c o n d s )

1 .Ox1 0-4 mol/g NaCl

1 1 I

Methylviologen Radical Monopositive Cations J. Phys. Chem., Vol. 99, No. 24, 1995 9913

I

600.00 800.00 1000.00 1200.00 1400.00 1600.00 1800.00

Raman Shift (cm-1)

Figure 6. Resonance Raman spectrum of MV'+ on silica gel recorded at room temperature (upper line). The excitation wavelength was 406.7 nm, and a slit width of 200 p m was used, which corresponds to a resolution of 7 cm-I. For comparison, the resonance Raman spectrum of MV'+ in aqueous methanol is also presented (bottom line). The Raman bands of solvents, water and methanol, are denoted.

TABLE 1: Resonance Raman Band Positions and the Relative Intensities of MV'+ on a Silica Gel Surface and in Aqueous Methanol (Band Postions of MVZ' and Tentative Assignments Also Given)

Raman shift, cm-] re1 intensity MV'+/silica MV"/H20 MV2+C1f MV'+/silica MV'+/H20 assignment

681 683 660 0.42 0.38 y (C-N+-CH,);b v (CC) 22, y (CCC)46, y (CCH)12' 815 816 84 1 1.60 1.60 v (N+-CH3)+, Y(C-N+-CH~);~ v (CC)57, y (CCC)17'

1211 121 1 1195 0.76 0.98 v (N+-CH3);b v (CC)19, y (CCH)68' 1246 1250 1234 0.33 0.27 y (CCH)21C

1532 1535 1539 1 1 v (CC)40, y (CCH)4Y 1661 1661 1655 1.70 1.70 v (CC)66, v (CC),,11, y (CCH)29'

1345 1356 1301 0.90 0.76 v (C-CIlr;bV (CC)12, v (CC),, 43, y (CCC)lOC

a The band positions cited are from a Raman spectrum of M M V 2 C1-2 in aqueous methanol excited at 406.7 nm. See ref 30. v = stretching; y = in-plane bending; ir = inter-ring. See ref 46, potential energy distribution of biphenyl in percent, only values t 10% were noted.

i

t = -10 s, which is much longer than that of MV'+C12'- in solution (t = -10 For comparison, the decay of the photogenerated MV'+ in solution is shown as an inset of Figure 5b. The effect of the surface rigidity on the stability of the CT pairs in zeolites has been reported.29 The stability of MV" on the surface may be also caused by the secondary reaction of Cl' with the silica gel framework,28 forming =Sio'/Cl- or ESiOCl-. These data show that surface sites participate in the photoreaction, acting as electron donors, while coadsorption of C1- produces CT pairs of MV2+ and C1-.

4. Observation by Raman and EPR Spectroscopy of MV'+ Adsorbed on a Silica Gel Surface. Figure 6 shows the resonance Raman spectrum obtained from a MV'+/silica gel sample, as well as the spectrum of MV'+ in an aqueous methanol solution. Table 1 summarizes the observed band positions and their relative intensities. The sloping background towards higher wavenumber for the MV'+/silica gel sample is residual lumi- nescence background, most of which has been subtracted out in the data reduction process. The band positions of MV'+ in solution are in good agreement with those reported by Forster30 and do not exhibit any significant shift upon adsorption on to a silica gel surface. The relative intensities of the bands are not perturbed by surface adsorption, as seen in Table 1. The peak intensities are normalized to the band at 1532 cm-I which is thought to be a mixture of C-C stretching and C-C-H in- plane bending.30 These results are somewhat surprising, in that previous studies of neutral benzene molecules adsorbed on silica gel s ~ r f a c e ~ ~ , ~ ~ and ruthenium bis(bipyridine) cations on clay

I

DPPH

Figure 7. First-derivative EPR spectrum of a photoirradiated MV2+/ silica gel sample, recorded at room temperature. The vertical arrow indicates g value of 2.0037 from DPPH. The EPR instrument settings are as follows: microwave power 2 mW; microwave frequency -9.3 GHz; modulation frequency 100 kHz; modulation amplitude 1 G; receiver gain factor 1.25 x lo4.

9914 J. Phys. Chem., Vol. 99, No. 24, 1995

1.00

0.75

x v) C al c '- 0.50 al > a 0, [r

c .-

c

.- c -

0.25

Mao et al.

-

-

-

-

h 0 mbar CCI,

I I

3 mbar CCI, 0.501 I 0.15 t \ I ' mbar cc14

0 2.5

1 .o

0.75

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Q) > a al [r

c .-

c

'- 0.50 .- c -

0.25

0

5.0 7.5 10.0 12.5

Time (seconds)

I X f O . 6 mol/g NaCl

1 Xl O-5 mollg NaCl \ 0 2.5 5.0 7.5 10.0 12.5

Time (seconds) Figure 8. (a, top) Time profiles of laser-irradiated MV*+/silica gel samples under various pressures of CC14. The CC14 pressures are, from top to bottom, 0, 1, 2, and 3 mbar at room temperature. The diffuse reflectance was monitored at 600 nm. Inset: an example of the Gaussian fitting of the decay profile. (b, bottom) The influence of NaCl loading on the reaction of MV'+ with cc14. MVZ+ loading was mol/g, and under a 3 mbar c c l 4 at room temperature. Monitored at 600 nm in diffuse reflectance mode.

surfaces,33 reported changes in the relative intensity of the Raman bands on surface adsorption.

The photoinduced and surface-mediated MV'+ is observed by EPR spectroscopy, showing two distinct signals superim- posed on each other, a broad line with a width of 62.5 G and a g value of 2.0037, and a multiline signal which is resolved up to 16 components with a splitting of 1.75 G (Figure 7). The multiline is assigned to MV", because it is similar to that of M V + in ethanol solution.34 However, the corresponding hyperfine structure which occurs in the solution sample is not

present in the solid sample. This is a common feature of EPR spectra on solid samples. The broad line may be assigned to paramagnetic species derived from holes on the silica gel surface or their derivatives. In the y-radiolysis studies EPR spectra of holes on silica-alumina gel surfaces were stated to be poorly resolved and resulted in a broad line.35

A part of MV'+ species is stable for several months in the absence of 0 2 . As soon as oxygenlair is introduced into the sample, the blue color, and the corresponding spectral absorption bands and the EPR signal disappear, as MV'+ reacts with

Methylviologen Radical Monopositive Cations

molecular oxygen:

J. Phys. Chem., Vol. 99, No. 24, 1995 9915

MV'+/surface + 0, - MV2+ + 0,'-/surface (4)

The reaction of photogenerated MV'+ with 0 2 on a silica gel surface has been studied in both steady-state and time- resolved mode, where the formation of superoxide anion (02.7 was observed by EPR spectroscopy and the corresponding reaction rate constant was determined as 2.5 x lo5 M-' s-1.36

5. The Reaction of MVf with Polychloroalkanes. The reactions of the photogenerated MV'+ with carbon tetrachloride, chloroform, dichloromethane, and 1,2-dichloroethane have been studied. These compounds are selected as models for poly- chloroalkanes. Carbon tetrachloride (CC4) will be studied in some detail. The introduction of CC4 vapor into a photoirra- diated MV2+/silica gel sample results in the disappearance of the characteristic blue color, and a broad absorption band at 300-550 nm appears as a tail of the MV2+ absorption (Figure 4a). After extraction of the sample with water, HPIC studies show that chloride ions (Cl-) are formed as a reaction product (vide infra) and indicate that the reaction of MV'+ with CCh is electron transfer in nature. The whole reaction may be described in two steps: the MV'+ is first generated by photoirradiation (eqs 2 and 3) and then reacts with cc14:

MV*+/surface + CC1, - MV2+/surface + C1- + product

( 5 )

Figure 8a shows time profiles of pulse-laser irradiated MV2+/ silica gel samples under various pressures of CCL, monitored at 600 nm in diffuse reflectance mode. With increasing CCld vapor pressure over the samples, MV'+ decays more rapidly. The time profiles of diffuse reflectance of MV'+ on silica gel surface fit well by the Gaussian distribution m ~ d e l . ~ ' - ~ ~ The model is developed on the basis that the heterogeneity on solid surfaces induces discrete rate constant ( k ) of transient species, and a dimensionless variable x is introduced, which obeys a Gaussian distribution:

1 k x = - l n =

Y k Here y is the width of the distribution, and it is the average

of rate constant. The Kubelka-Munk function F(&) is used to quantitatively describe MV'+ on the finely divided opaque material:40*41

RT is the relative reflectance intensity again blank sample, yo is a proportional constant, and c is the concentration of the species interested. The decay of the transient species can be described as

FJFo = n-"2j-+-exp(x-2) m exp[-kt exp(yx)] dx (8)

Here Ft and FO are the Kubelka-Munk function at time t and t = 0, respectively. As an example, a typical Gaussian fitting is presented in the inset of Figure 8a.

With increasing NaCl loading the reaction of MV'+ with CC4 becomes more rapid (Figure 8b), showing a medium-dependent process, and a coadsorption of Na2S04 also induces acceleration of the reaction. Both salts indicate that the reaction is dependent on the ionic and polar nature of the medium. This is reminiscent of the fact that the reaction of MV'+ with 0 2 is medium dependent, for example, the overall rate constant decreases from 3 x lo8 M-' s-' i n water to lo6 M-' s-I i n pure methanoL4*

L 10.0

0 0 2.5 5.0 7.5 10.0 1 2 . 5

Time (seconds)

Figure 9. Time profile of the MV'+ decay in the pumping experiments, monitored at 600 nm in a diffuse reflectance mode; the pumping time, from bottom to top: 0, 10, 30, 100 s. With increasing pumping time, the decay becomes slower. The initial CCL, vapor pressure was 4 mbar.

The heat of adsorption of CC14 on a silica gel surface was spectroscopically estimated as 6 kcal/m01,~~ indicating that the interaction between C C 4 and the silica gel surface cannot be neglected. Pumping experiments were performed in order to examine whether the bombardment of C C 4 from the gas phase or the migration of cc14 on the surface is the predominant process for the reaction of MV'+ with CC4. First, an equilib- rium between gas-phase CC14 and adsorbed CC4 was reached, and the decay of MV'+ via reaction with C C 4 measured. The gas phase CC14 was then rapidly pumped away (-1 s) and the rate of decay of MV'+ was again monitored. It is seen that it takes at least -5 s for the cc14 to desorb from the surface, as the decay rate of MV'+ remains unchanged. Figure 9 shows that the decay becomes slower with increasing pumping time up to 100 s. This indicates that the reaction of MV'+ with CC4 occurs via adsorbed species, Le., a Langmuir-Hinshelwood type reaction, and not via bombardment of MV'+ by CCld from the gas phase. This is similar to the reaction of excited pyrene with 0 2 on silica gel surface.39

The surface reaction models are well established by Samuel et a1.44 and by Freeman and Doll.45 On the basis of the concept of these models, the reaction between MV'+ and CC14 on the surface can be referred as to two-dimensional reaction. The average rate constant (it), which is derived from the Gaussian fitting, depends on the loading concentration of CC14:

k = k, + k,[CCl,] (9)

Here is the decay rate constant without loading of CC4. From the k values under various cc14 loading concentrations, a bimolecular rate constant (k2) is calculated as 1.10 x lo7 (mol/ m2)-' s-I. From the temperature-dependence of the k, an activation energy (Ea), according to the Arrhenius relationship, is calculated as 1.05 kcal mol-'.

The reaction rate constants of MV'+ with other volatile chloroalkanes, and the corresponding activation energies are also calculated and summarized in Table 2. It is seen that increasing the degree of chlorosubstitution in the polychloroalkane in- creases the reaction rate constant. This may be explained as follows: a higher degree of electron-withdrawing substitution increases the electron affinity of the chloroalkane and subse- quently leads to a more rapid rate of reaction.

Little attention has been paid to the reactions of M V " with polychloroalkanes in solutions, and it was necessary to study the reaction of MV'+ with CC14 in methanol solution. Pho-

9916 J. Phys. Chem., Vol. 99, No. 24, 1995

TABLE 2: Reaction Rate Constants of Photoinduced MV'+ with Model Compounds of Polychloroalkanes on a Silica Gel Surface at Room Temperature

Mao et al.

rate constant k2 4 t activation energy

chloroalkane (mol/m2)-' s - I (moVL)-' s - I E, (kcal mol-I)

carbon tetrachloride 1.10 x lo7 1.15 x 10' 1.05 chloroform 2.90 x lo6 3.02 2.95 dichloromethane 1.30 x lo6 1.36 1,2-dichloroethane 1.08 x lo6 1.13 2.23

a The average rate constants k (s-l) were obtained from a Gaussian fitting within the time range 0-0.10 s after the laser flash. The MV2+ loading was (3-5) x moUg, and the residue of C1- was (3-5) x lo-' mol/g which gives reasonable MV'+ yield and does not induce significant medium effect. The rate constants kz are calculated according to the two-dimensional model, see text. If the loading concentration unit (mol/m2) of CC14 is converted into the mol per liter of silica gel, the unit of corresponding rate constants kz is changed into (mol/L)-] s-l.

toirradiation of a MV2+/methanol sample produces a stable blue color, indicating the formation of MV+:

(MV2+)* + CH30H - MV*++ 'CH,OWCH,O* + H+ (10)

In contrast to the case of MV*+/silica gel samples, time- resolved data in methanol show a formation process of MV'+ within a time scale of 10 ps (Figure loa). The formation process of MV" is explained on the basis that the methanol radical species ('CH20WCH30'), which are initially formed according to reaction 10, react with MV2+, leading to the secondary formation of MV'+, and is consistent with early work on the photochemistry of MV2+ in metha1-101.~' Such secondary formation of MV'+ is also observed in poly(methacry1ic acid)- MV2+ system, where methyl-like radicals produce secondary MV'+.2* MV'+ may also react with MV2+ to form dimersz6 The whole reaction mechanism gives rise to complex kinetics. However, in methanol the presence of CC4 leads to the slow disappearance of blue color. Time-resolved studies of a MV2+/ methanol sample exhibit decay of MV'+ in the presence of CC4 (Figure lob), indicating the reaction of M Y + with CC4. These data show that the MV'+ species are initially formed by laser flash over a scale of 10 ps and subsequently decay over a much long time scale, 10 s. An apparent rate constant of MV'+ disappearance is estimated as 2.4 M-' s - I . This is to be compared with the rate constant on the surface of 1 1.5 M-' s-' (Table 2) , which is calculated by using the loading of CC4 moVg and converting to M. However, the actual loading of CC14 on the surface may be higher because of the porosity of the silica gel and if only a portion of the surface is used. No attempt is made to resolve the elementary kinetics because of the complicated kinetic process.

6. MV'+-Mediated Dechlorination of Polychloroalkane. Steady-state studies of MV'+-mediated dechlorination of CC4 have been performed. 02-free MV*+/silica gel samples under 40 mbar of CCl4 were irradiated with RPR 3000 8, lamps. The irradiated samples were extracted with water, and the extracts were analyzed by HPIC. Figure 11 shows that the C1- yield increases with increasing photoirradiation. These data confirm that the reaction between MV" and CC14 is electron transfer in nature.

Summary MV2+ ions adsorb strongly on a silica gel surface and exhibit

a shift of the absorption band which is assigned to surface complexes. Photoirradiation of degassed MV*+/silica gel samples produces MV'+, which is confirmed by diffuse reflectance, Raman and EPR spectroscopy. The co-adsorption

I - o t , I I 4 1 I 1

0 2 4 6 0

Time , 200 m s /division

._ __...,- .C..' , 1 1

0 2 4 6 8 10

Time (microseconds)

Figure 10. (a, top) Time-resolved transient absorption of a MV2+/ CCL in methanol solution after a laser flash. The absorption was monitored at 600 nm. MV2+: 1 x M, CC4: 1 x lo-' M. (b, bottom) The formation process of MV'+ in a nitrogen-saturated MV2+/ methanol sample after a laser flash, the dotted line at bottom is the

0 /

background.

120

100 - /

E, 80 -

i p1 60 - 0 8 (5 40 - /

0 20 - /

e , ,

0 8 .

0 2 4 6 0 IO

Irradiation time, min

Figure 11. Photoinduced and MV"-mediated dechlorination of CC14. The samples were irradiated with an intensity of 4.5 x einsteid cm2 min under 40 mbar of CC14 at room temperature, the MV2+ loading was 3 x mol/g.

of C1- increases the MV+ yield. Studies show that the adsorbed state of MV'+ does not lead to a significant perturbation of the MV'+ Raman bands. Photogenerated MV" reacts with poly- chloroalkane, predominantly via surface migration of chloro- compounds to the MV+' on the surface. On the basis of the concept of the surface reaction models the relevant bimolecular

Methylviologen Radical Monopositive Cations

rate constants (k2) are calculated from the average rate constant E. The surface stabilizes the MV'+Cl' pair to much greater than that of MV'+C12'- obtained in solution. The surface enables us to monitor the MV'+CI' pair, a situation that is not possible in solution. The surface also enhances the rate of the reaction MV'+ with CCL, which is possible due to the rigidity of the surface. These data clearly illustrate the unique effects of the silica gel surface on these two electron transfer reactions.

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Acknowledgment. We acknowledge the financial support by the National Science Foundation. Dr. S . Pankasem is thanked for the writing a data conversion program.

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J. P.; Jackson, P. E.; Jandik, P.; Jones, W. R. Waters Innovative Methods