Langmuir 1993,9, 1299-1305
1299
Surface Complexes as Precursors of Photoinduced Radical Cations of Biphenyl and Subsequent Hydroxylation on Laponite Yun Mao, Guohong Zhang, and J. Kerry Thomas* Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556 Received December 3,1992. In Final Form: February 8,1993 The photochemical reactions of the model compoundbiphenylon laponitesurfaceshave been investigated. Diffuse reflectance and fluorescence spectroscopic studies indicate that adsorbed biphenyl molecules interact with solid surfaces in two ways, a physical interaction which leads to a bathochromic shift and broadening of absorption bands and a chemical interaction which leads to surface complexes with chargetransfer (CT) character. Photoirradiation of the surface complexes results in the formation of radical cations,Ph2’+,which are observed by diffusereflectance and EPR spectroscopy. Product analysis confirms that biphenyl is photochemically degraded on laponite on irradiation into the CT band and that the mechanism is free radical in nature. The important reaction of radical cations on surfaces is hydrolysis resulting in hydroxy compounds. The present results indicate that layered silicates are more active than silica-alumina gels, in the above processes.
Introduction Clay and clay minerals are important media for photochemical reactions in the environment, with particular emphasis placed on layer silicates which swell. The latter provide restricted interlamellar spaces to accommodate reactant molecule^.^-^ In a study of the chemical behavior of industrial chemicals in the environment, it is desirable to explore the effect of the natural conditions on the lifetime of the materials and the final degradation products. Most photochemical reactions on the Earth’s surface occur with the near-UV and visible wavelengths of solar light, i.e., a t wavelengths greater than 290 nm. However, several molecules with absorption bands in the UV region degrade photochemically in nature, the question posed is that of mechanism. The unusually high chemical stability and electrical resistance of polychlorinated biphenyls (PCBs), together with their low volatility and resistance to degradation at high temperature, have led to a range of industrial applications for this group of chemicals. PCBs are extremely persistent and hazardous chemicals, some of which have highly toxic, mutagenic, and potential carcinogenic Since their first detection in wildlife in 1966,traces of PCBs have been detected in numerous samples collected from North America and Europe to the remote areas of the Arctic and Antarctice7t8The chemical behavior of PCBs in the environment is a subject of immediate concern and is addressed in this study. Several reactions of organic compounds on transitionmetal ion-exchanged clays have been reported.lI”l2 Sur-
* Author to whom correspondence should be addressed. (1) Mortland, M. M.; Pinnavania, T. J. Nature (London),Phys. Sci. 1971, 229, 75. (2) Theng, B. K. G. The Chemistry of Clay-Organic Reactions; John Wiley & Sons: New York, 1974. (3) Ballantine, J. A. In Chemical Reactions in Organic and Inorganic Constrained Systems; Setton, R., Ed.; NATO AS1 C ser. vol. 165, D. Reidel Publishing Co.; 1985; p 197. (4) Hutzinger, 0.;Safe, 5. and Zitko, V. The Chemistry of PCB’s; CRC Press: Cleveland, OH, 1974. (5) Norstrom, R. J. In Hazards, Decontamination, and Replacement of PCB; Crine, J.-P., Ed.; Plenum: New York, 1988; Chapter 5, p 85.
(6)Tanabe, S.; Kannan, N.; Subramanian, An.; Watannabe, S.; Tataakawa, R. Enuiron. Pollut. 1987,47, 147. ( 7 ) Eduljee, G. H. Chem. Br. 1988,24, 241. (8)Jones, K. C.; Burnett, V.; Duarte-Davidson, R.; Waterhouse, K. S. Chem. Br. 1991, May, 435.
0743-7463/93/2409-1299$04.00/0
face properties of sodium laponite have been studied recently usingphotoactivearenes.13-15 Studieson reactions of the upper triplet excited states of biphenyl, chlorobiphenyl, and bromobiphenyl in aqueous micellar systems have shown that these molecules are degraded by a combination of UV and visible photons.16 A study of photochemical reactions of biphenyl on aluminaand silicaaluminasurfaceshas been also re~0rted.l~ In these studies it is projected that the spatial restriction in swelling clay providesan effectivemicroenvironment for photochemical reactions. In this report synthesized hectorite, sodium laponite (henceforth referred to as laponite), is selected as a model system, and biphenyl (Ph2) as a model compound for photochemistry of molecules with absorption X < 290 nm. The present studies clearly show that such stable molecules may photodegrade on dried clay through the formation of surfacecomplexes, which absorb light at X > 290 nm.
Experimental Section Chemicals. Sodium laponite (RDS) was received from Laporte Industries. The composition of the laponite,as supplied by the producer, is as follows: SiOz, 55.6%;MgO, 25.1%;NazO, 3.6%;Liz0,0.7%;K~O,0.2%;TiO~,0.15%;A1~03,0.08%;CaO, 0.06%;Fez03,0.04%. The solid powder was used either directly as received, denoted as nonactivated (T,= 20 “C; T, is the activation temperature of the solid), or pretreated by heat activation. Biphenyl (Phz) from Aldrich (99%) was purified by chromatographic separation (silica gel as the stationary phase and cyclohexane as the mobile phase) and recrystallized from cyclohexane. 2(4)-Phenylphenol,2,2’-biphenol,and 4,4’-biphenol (ChemService)were used as HPLC standardsas received without further treatment. The remaining compoundswere generally of highest purity commercially available. (9) Pinnavaia, T. J.; Mortland, M. M. J. Phys. Chem. 1971, 75,3957. (10) Pinnavaia, T. J.; Hall, P. L.; Cady, S. S.;Mortland, M. M. J.Phys. Chem. 1974, 78,994. (11)Boyd, S. A.; Mortland, M. M. Nature (London) 1986,316, 532. (12) Soma, Y.;Soma, M.; Hiuada, I. J. Phys. Chem. 1985,89, 738. (13) Liu, X.; Thomas, J. K. Langmuir 1991, 7, 2808. (14) Liu, X.; Iu, K.-K.; Thomas, J. K. Langmuir 1992, 8, 539. (15) Iu, K.-K.;Liu,X.;Thomas, J. K. Chem.Phys.Lett. 1991,186,198. (16) Hashimoto.. S.:. Thomas. J. K. Photochem. Photobiol. A: Chem. 1991,55, 377. (17) Mao, Y.;Thomas, J. K. J. Chem. SOC.Faraday Trans. 1992,88 (20), 3079.
0 1993 American Chemical Society
Ma0 et al.
1300 Langmuir, Vol. 9, No. 5, 1993 Instrumentation. Absorption spectra were measured by diffuse reflectance spectrometry utilizing a UV-visible spectrophotometer (Perkin-Elmer 552) equipped with an integrating sphere accessory. Steady-state emission was measured on a spectrofluorometer (Perkin-Elmer, MPF 44-B). EPR spectra were recorded on an electron paramagnetic resonance spectrometer (Varian Associate,E-lines century series) equipped with an X-band klystron and a TEloz type cavity. The g value was determined by comparison with DPPH (diphenylpicrylhydrazyl). Samples for EPR measurements were contained in 5 mm 0.d. quartz tubes, connected to wider glass tubing that was attached directly to a vacuum line. Time-resolved diffuse reflectance spectra were measured by laser flash photolysis, which has been described.ls In brief, a 308-nm laser pulse (energy density, 70 mJ/cm2; fwhm, 10 ns) from a XeCl excimer laser (Lambda Physik, Model EMG 100) was used to excite the samples, and a 450-W xenon lamp was used as the analyzing light source. The diffusely reflected monitoring light from the sample was transferred to a monochromator by an optical fiber, and detected by a photomultiplier tube. The output of the photomultiplier was coupled to a Tektronix 7912AD digitizer, connected to a Zenith data system. Time-resolvedemission experiments were carried out with a NdYAG synchronously pumped, mode-locked and cavity dumped dye laser (Continuum, PY61-10),using the fourth harmonic 266 nm, with an energy of 0.1 mJ/pulse and a pulse width fwhm of 20 pa. The emission from samples was transferred to a monochromator by a lens system, and detected by a microchannel plate. The output of the microchannel plate was presented to a Tektronix 7912AD with 7A29 plug in. In order to remove the interference of the laponite solid matrix the detection system was optimized so that the background of the laponite solid matrix is negligible. Steady-state photoirradiation was carried out in a chemical photoreactor equipped with a merry-go-round and RPR 3000 A lamps (Rayonet, Southern New England). The photoirradiation intensities were measured with the chemical actinometer K3Fe(CzOd3. Sample Preparations. Samples of laponite powder (1.0 g) in quartz crucibles were heated for 20 h under air at desired temperatures, 130or 325 "C. After thermal activation,the powder samples were cooled in a desiccator to room temperature. Immediately after cooling,the powder was mixed with an aliquot of biphenyl in cyclohexane. The samples were used for diffuse reflectance and EPR spectroscopy after drying in a stream of Nz. After photoirradiation, thesamples were extractedwith aerated aqueous acetonitrile solutions. Samples for HPLC were first centrifuged to remove most of the powder and then filtered through a Millipore frit (0.45 pm). Separation and quantitative identificationof chemicalproducts were performed by Waters' high-performance liquid chromatography which was equipped with an adsorbosphere phenyl column (Alltech) and UV-absorption detector. The eluent consisted of aqueous acetonitrile solution.
Results 1. Interaction between Biphenyl Molecules and Surface Sites. Figure 1 shows diffuse reflectance spectra of biphenyl on laponite surfaces (T,= 130 and 325 "C). On 325 "C activated laponite samples, a band at 280 nm with a shoulder at 320 nm is observed. A comparison of the band at 280 nm to that in solution (dotted line) shows that this band is that of adsorbed biphenyl molecules with a bathochromic shift of ca. 20 nm and with some band broadening. The extent of the shift and the broadening increases with the activation temperature of the laponite. The bathochromic shift and the broadening of the band result from interaction of the molecules with the solid surface, due to physisorption. An extreme case of solutesurface interaction produces charge-transfer complexes and is discussed below. (18)(a) Pankasem, S.; Thomas, J. K. J. Phys. Chem. 1991,95,6990. (b) Pankasem, S.; Thomas, J. K. J. Phys. Chem. 1991,95, 7385.
0.18
0.15
-3
-.
0.12
-0 .)
3
0.09
0.06
0.03
0 230
310
390
470
550
630
710
Wavelength, nm
Figure 1. Diffuse reflectance spectra of biphenyl on laponite surfaces measured at room temperature. Laponite samples were activated at 130 and 325 OC (dashed and solid line), respectively. The biphenyl loading concentration was 4 X 10-8 mol/g. For comparison, an absorption spectrum of biphenyl in cyclohexane is included (dotted line).
The band at 320 nm is assigned to a charge-transfer (CT)complex,where the biphenyl moleculesact as electron donorsand laponite surfacesites act as electron acceptors. Ph,
+ surface thermal = (Ph,+'-surface4)
(1) Supporting evidence for this assignment is that photoirradiation into the CT band generates radical species (vide infra). Formation of adsorbatesurface complexes in several heterogeneous systems has been reported and is characterized by new broad absorption bands near th.e red edge of the parent compo~nds.~~~J9~20 Under conditions of low adsorbate loading (- 1 X lo-' mol/g) the relative intensity of the 280-nm band becomes weak, indicating that most biphenyl molecules are converted into CT complexes. The interactionbetween excited biphenyl moleculesand surface sites may be characterized by emission spectroscopy. The steady-state emission spectra of biphenyl/ laponite samples are presented in Figure 2 and exhibit a broad fluorescence band at 315 nm; in solution biphenyl shows structured bands at 304 and 315 nm (dotted line in Figure 2). On surfaces, the biphenyl band corresponding to 304 nm in solution is significantly diminished, and the band corresponding to 314 nm in solution is enhanced. In addition, the biphenyUlaponite samplealso shows a broad band over the wavelength range of 350-550 nm, which appears as a tail at the red edge of the main fluorescence band. This band matches the CT absorption band which occurs at the red edge of the biphenyl absorption band (Figure1). Phosphorescence of biphenyulaponite samples, measured at -196 "C are less well structured, compared (19) (a) Oelkrug, D.; Erbse, H.; Plauschinat, M. 2. Phys. Chem. (Munich) 1976,96,283. (b)Oelkrug, D.; Plauschinat,M.; Keclsler, R. W. J.Lumin. 1976, 18119,434. (20) (a) Oelkrug, D.; Krabichler, G.; Honnen, W.; Wilkhon, F.; Willsher, C. J. J. Phys. Chem. 1988,92,3589. (b) Oelkrug, D.; Fleming, W.; Fuellerman, R.; Guenther, R.; Homer, W.: Krabichler, G.: Schafer. M.; Uhl, S. Pure Appl. Chem. 1986, 58, 1207.
Biphenyl on Laponite Surfaces
Langmuir, Vol. 9, No.5,1993 1301 and laponite shows that the effects are more prominent in layered 1aponite.l' 2. PhotoinducedChemical Reactions of Biphenyl on Laponite Surfaces. Photoirradiation of biphenyl/ laponite (Ta = 20 OC) samples with fluorescent lamps (A > 285 nm) at room temperature initiates photochemical reaction. As shown in Figure 4, photolysis leads to a broad absorption band over the wavelength range of 31U-650 nm which gradually increases with time of irradiation. The photochemical reaction occurs in both aerated and degassed samples. The broad band at 31U-660 nm is assigned to a mixture of intermediate surface species and chemical products. Later studies show that extraction with water produces products of biphenyl.
-
CT-huIRT
Ph,/surface
310
750
3"
430
470
510
550
S90
Wavelength, nm
Figure 2. Steady-state luminescence spectra of biphenyl: solid line, biphenyl/laponite samples, 4 X 10-6 mol/g; dotted line, biphenyl/isopentane samples, -5 x lo4 M. The fluorescence spectra were recorded at room temperature, with an excitation wavelength of 250 f 10 nm; phosphorescence spectra were recorded at -196 O C , and the excitation source is a 150-Wxenon lamp with UV bandpass filter (260-380nm).
with spectra in isopentane glass (right portion of Figure 2). The phosphorescence spectrum also exhibits a change in the relative band intensities, and a bathochromicshift, indicating an interaction between biphenyl triplet states and surface sites. Figure 3 exhibits time-resolved emission data taken at 325 nm. The decay of excited states of biphenyl on the surface is much faster than that in solution, indicating a quenching effect of the surface states. The time profile of the 325-nm band in solution is monoexponential, while the time profile of the 325-nm band on the surface is fitted well by a Gaussian distribution function (see the inset in Figure 3).21a This treatment has been successfully employed for time-resolved emission studies of pyrene on silica21b~c and alumina.18a The data are fitted with the equation
AJAo = r-'izcmexp(-z2) exp[-kt exp(7x)l dx where k is the average rate constant and 7 is the width of distribution. This treatment indicates that the surface bonded biphenyl molecules lie in a variety of surface sites. It is pertinent to mention that the emission band of a biphenyl/laponifa sample extends up to wavelengths of about 450 nm; in contrast, the emission of solutions of biphenyl does not show any observable signal beyond 380 nm. The emission at longer wavelengths can also be fitted by a Gaussian distribution. The fitting parameters are summarized in Table I. On the basis of the aforementioned results, it is concluded that adsorbedbiphenyl molecules interact with laponite surfacesresulting in marked changes in absorption and emission spectra. The interaction also leads to a quenching of excited states and formation of CT complexes. A comparison of studiea in alumina, silica-alumina,
(Ph,+*...surfaceb)/products
(2)
Photoirradiation into the CT band at -196 OC gives rise to a color change in the sample from white to light blue, and the diffuse reflectance spectrum measured at low temperature (ca.-150 "C) exhibitsstrong absorptionbands at 390 and 685 nm, and a weak band over the wavelength range 450-550 nm (Figure 5). The band at 390 nm also shows structure at 375,385, and 400 nm. The bands at 390 and 685 nm are assigned to biphenyl radical cations (Phz*+),an assignment consistent with early reports of Phf+ in solution^.^^^^ A small shift of the absorption bands results from the interaction between Phz'+ and its microenvironment. Similar phenomena are observed also for other radical cations on solid s u r f a c e ~ . ~ ~The ~O formation of Phz*+via CT band irradiation confirms the existence of CT complex on surfaces and the electron transfer between biphenyl molecules and surface sites.
-
CT-hul-196 ' C
(Ph,+*--surfaceb)
(Ph,'+/surface-)
(3)
Increasing the intensity of the photoirradiation leads to a linear increase in the Phz'+ yield. This is similar to results on silica-alumina surfaces and suggests that the photoionization may be monophotonic in nature.17 This is further confirmed by the observation that aerated samples also show comparable yields of Phz'+. 02removes long-lived intermediates,e.g. triplet stateswhich may enter into two-photon processes. Warming the samples induces a fading of the light blue color, with a concomitantdecrease in the Phz*+absorption bands at 390 and 685 nm. The weak band at 450-550 nm first increases and subsequently decreases during the annealing,showingthe formation of reaction intermediates (seeFigure 5). Finally,the annealed sampleshows a broad absorption band, which is ascribed to a mixture of biphenyl and its products. During the warming period, a thermal chemiluminescence is observed, with a strong band at 480 nm and a weak band at -320 nm, as shown in Figure 6. This luminescenceoriginatesfrom recombination of Phz*+with electrons trapped in the surfaces. The spectrum was recorded under nonisothermal conditions,which may lead to some distortion of the spectrum. However,a comparison
(22) Shida, T.; Hamill, W. H. J. Chem. Phys. 1966, 44, 2378. (23) Dorfman, L. M. Acc. Chem. Res. 1970, 3, 224. (24) Baxendale, J. H.; Wardman, P. Int. J. Radiat. Phys. Chem. 1971, 3, 377. (25) Sehested, K.; Hart, E. J. J. Phys. Chem. 1975, 79,1639. (26) Mao, Y. and Thomas, J. K. Unpublished data, 1992. (27) Weller, A.; Zachariasse, K. J. Chem. Phys. 1967,46,4984. (21) (a) Albery, W. J.; Bartlett, P. N.; Wilde, C. P.; Darwent, J. R. J. (28) Weller, A.; Zachariease, K. Chem. Phys. Lett. 1971,10, 197. Am.Chem.Soc. 1986,107,1854. (b)Kranenansky,R.;Koike,K.;Thomas, (29) Mao, Y.;Pankasem, S.; Thomas,J. K. Langmuir, in press. J. K.J.Phys. &em. 1990,94,4521. (c) Hite,P.;Kransnasky, R.;Thomas, (30) Mao, Y.;Thomas, J. K.Langmuir 1992, 8, 2501. J. K. J.Phys. Chem. 1986,90,5765.
Ma0 et al.
1302 Langmuir, Vol. 9,No. 5, 1993 1.00
-
0
--
-~
ew,
1
>-
k
0.75
H
117
-
lz¶
-
B2.s
-
m
z W I-
2
H 0.50
w
0
>
40 tlr /
W
nr
w
I 100
H F -I
0.25
w
CT
0.00 0
20
TIME
BO
60
40
100
(nanoseconds)
Figure 3. Time-resolved emission measured at room temperature, with a biphenyl loading of 4 X 10-6mol/g, using the fourth harmonic 266 nm from a Nd-YAG synchronously pumped, mode-locked, and cavity dumped dye laser: -, monitored at 325 nm, N2-saturated biphenyl in cyclohexane (5X 1W M);..-,monitored at 325 nm, degassed solid sample; monitored at 400 nm, degassed solid sample. Inset: the Gaussian distribution fitting of the fluorescencedecay, monitored at 325 nm: jagged line, data; smooth line, compute fitting with the equation (see text). The upper part exhibits the residuals of the fitting.
....
Table I. Fitting Data of Time-Resolved Emission Intensity' fitting parameters sample monitoring X (nm) A0 kils-' P hz/cyclohexane 325 93.4 5.16x 107 fitting parameters monitoring at h (nm) y kls-1 sample 325 0.807 1.49 X 108 Phzllaponite 400 1.70 3.48 X 108 Phz/laponite Monoexponentional function is A ( t ) = A0 e-klf;kl is the rate constant. The Gaussian distribution is given in the text.
of the band position and shape with that produced in steady-state emission studies (Figure 2) indicates that the thermal luminescence band at 480 nm is the phosphorescence of biphenyl and the weak band at 320nm is the fluorescence of biphenyl. The intensity of thermal phosphorescence is much stronger than that of fluorescence, the latter being very weak. The recombination of the cations with electrons generates triplet states (3Ph2),and subsequentlytriplet-triplet annihilation may give excited singlet states
-
-
recomb.
(Ph,'+/surface-)
(3Ph,/surface)
(4)
3Ph2/surface
(3Ph2/surface) (Ph,*/surface) (5) Such processes are observed for the neutralization of Wurster's blue cations27928and also other aromatic radical
i. -I
O.'*
-
0.09
-
0.06
-
0.03
-
230
310
3N
470
550
630
710
Wavelength, nm
Figure 4. Diffuse reflectance spectrum of biphenyvlaponite (T. = 130 "C) measured at room temperature. Samples were irradiated with RPR 3000-A lamps (-8 X 1W einstein/min) at room temperature. Irradiation time, from bottom to top: 0,20, 30,and 40 min. A biphenyl loading of 2 X 10-6movg was used.
cations on solid surfaces.29J0 Diffusion of 3Ph2 on the surface is restricted and leads to a low yield of lPh2*. The intensity of the fluorescence also depends on competition between triplet emission and triplet-triplet annihiliation of biphenyl. Fluorescenceand phosphoreecenceproduced from neutralization of surface-bonded Ph2'+ are compa-
Langmuir, Vol. 9, No. 5,1993 1303
Biphenyl on Laponite Surface5 0 ??
0.025
I
0 36
0.020
0 30
0
-
--. 3 J
0.01s
C
n O
a
024
9
o.oi0
0
018
0.0050
0 12
0.00
J Wavelength / nm
Figure 7. Time-resolved diffuse reflectance spectrum, at a biphenyl loading of 4 X 10-6mol/g. The spectrum was taken 100 pa after the laser flash. Inset: the decay curves of diffuse reflectance spectrum,monitored at 390 and 680 nm, respectively.
0 06
0 230
310
470
390
S50
630
710
Wavrlcngih. nm
Figure 5. Diffuse reflectancespectrumof biphenyl/laponite (57, = 325 O C ) , measured at ca. -150 O C (solid line), the sample was degassed and irradiated at -196 O C . The dashed line shows the annealing effect; the dotted line shows the final spectrum after 1 h of annealing. 14
I
i
F
e
l2 12 lo 10
-
i
e
8 -
.
e e
25 gauss
e e
e
e
I 300
400
500
600
700
wavelength, nm
Figure 6. Thermal chemiluminescence,the sample biphenyl/ laponite (2'. = 130"C) was irradiated (A 290-340nm) with alight intensity of -3 X 10-3 einstein/min.cm2for 5 min at -196 "C; the chemiluminescencewas recorded during the annealing process at room temperature (see text). rable in yieldsq26This indicates a minor role for any such formation mode of fluorescence in the present studies. 3. Observation of Time-Resolved Diffuse Reflectance Spectra. In the time-resolvedstudies, sampleswere excited by a 308-nm XeCl excimer laser, and the transitory reflectance spectra were recorded. Figure 7a shows a typical time-resolved diffuse reflectance spectrm of a degassed biphenyl/laponite (T,= 130 "C)sample taken 100 ps after the laser pulse. Absorption bands at 390 and 685 nm are observed and, as in steady-state studies, are ascribed to Phz*+. The formation of Phz*+is very rapid, within the resolution time of the detection system, 100 ns. Phz'+ is stable over the time scale of the experiment, i.e., 5 ms (Figure 7b). However, Phz'+ disappears -10 min after the laser flash, which indicates subsequent reactions of Phz'+ on the surface. Ions are produced by low-intensity steady-state illumination and high-intensity laser excitation. The former
-
(2)
Figure 8. First derivative EPR absorption spectra recorded at room temperature. The biphenyl loading was 2 x 10-6 mol/g: (1)thermal sample; (2) photoirradiatedsample, withRPR lamps in the photoreactor. The EPR spectrum was recorded with the following conditions: microwave power, 10 mW; microwave frequency, -9.5 GHz; modulation frequency, 100 kHz; modulation amplitude, 1 G; receiver gain factor, -1.25 X lo*. results indicate that the photoionization process is monophotonic. This is also true for laser intensities up to 70 mJ/cm2 where the yield of cation increases linearly with laser intensity. At higher laser intensities the cation yield increases more steeply indicating that multiphotonic processes also occur. 4. Observationof Photogenerated Surface Species by EPR. Incubation of the laponite powder (Ta = 325 "C)with an aliquot of biphenyl cyclohexane solution, to give an aerated suspension, leads to a weak EPR signal due to the radical species induced thermally. At room temperature, photoirradiation into the CT band generates a strong EPR signal with a width of -25 G (Figure 8).The formation of this paramagnetic species on the laponite surface at room temperature is irreversible, and cessation of photoirradiation does not diminish the EPR signal. The EPR spectrum of biphenyl/laponite (Ta = 325 "C) irradiated at room temperature is poorly defined and is due to an envelope of spectra corresponding to all possible propagating paramagnetic species on the surface. This is confirmed by the following observation at -196 "C. Photoirradiation into the CT band at -196 "C also produces an EPR signal (Figure 9a), which exhibits a distinct septet structure with a splitting of 3.5 G. In-
Ma0 et al.
1304 Langmuir, Vol. 9, No. 5, 1993
(1)
(2) 2.0 m W
(1) 0.20m W
(2)
(3)
Figure 10. First derivative EPR absorption spectra recorded at -196 "C with different annealing times; (1)0 min; (2)2.0 min; (3)4.0 min. The biphenyl loading was 2 X 10-6 mol/g, and the sample was photoirradiated at -196 "C for 10 min in the photoreactor. The EPR spectrum was recorded as in Figure 9.
(3) 20 m W
a
6.0
v
e
e .
A
.-b
e
.-a
.A
0
A
4 A
e
2t
01 ,001
B -
e
A
A
.g B
e
A
e : A ' ' l * l . l - l
'".*s*I
.01
.1
'"*...*'
*
1
5.0
Y
'''*r.*'
10
*
4.0
3.0
4 100
microwave power, mM Figure 9. (a) First derivative EPR absorption spectra recorded at -196 O C with different microwave power: (1)0.20 mW; (2)2.0 mW; (3)20 mW. The biphenyl loading was 2 X lo4 mol/g, and the sample was photoirradiated at -196 O C for 7 min in the photoreactor. The EPR spectra were recorded with the following conditions; microwave power, 2 mW, microwavefrequency, -9.3 GHz; modulation frequency, 100 kHz; modulation amplitude, 1 G; receiver gain factor, -1.25 X lo4. (b) Microwave power saturation effect on EPR signals.
creasingthe microwave power induces a significant change in the spectral shape, i.e., the multiline spectrum degenerates to a broad single line. Figure 9b shows the power saturation effect on the two different l i e s , indicating that the spectrum is composed of two parts, one a multiline component with a width of 20 G and a g value of 2.0030 and another broad line with a width of 45 G and a g value of 2.0051. In order to assign the EPR signals,the irradiated sample was annealed at room temperature for several minutes. Figure 10shows the annealing effect on the EPRspectrum, where the relative intensity of the multiline component diminishes, while the component with the broad line increases. By comparison with spectral observations of Ph2*+under similar conditions where Ph2*+decreases on warming, the multiline component may be assigned to Ph2*+,and the component with a simple broad line is a propagating radical species derived from Ph2*+. The assignment of the septet spectrum to Ph2*+ is supportsd also by a comparison with EPR spectra of Ph$+ in solution. The EPR spectrum of Ph2*+ in CF&OOH/Hg(CF&00)3- solution shows seven lines, separated by 3.2 G and with a g value of 2.0027.31,32Each
2.0
I .o
0
Time, min
Figure 11. HPLC chromatographic spectrum of the photoinduced chemical products. Biphenyl was adsorbed on laponite activated at 325 O C from cyclohexane solution. The reactant suspensionswere irradiated in the photoreactor with RPR 3000-A lamps. The reactant mixture was then extracted with aqueous methanol (90% vol. methanol); see text. The conditions were adsorbosphere phenyl column (Alltech), isocratic elution with 60 vol % acetonitrile/HzO, and a flow rate of 0.5 mL/min.
of the lines exhibit further structure which consists of five lines. The septet spectrum on the surface is basically consistent with the seven grouped lines of Ph$+ in solution. However,the interactionof surface microenvironment with Ph2'+ obscures the further hyperfine structure and also induces a change of splitting. 5. Hydrolysis of Photogenerated Surface Species. Photoinducedsurface species are only removed by aqueous solution, which indicates that the extraction is a chemical process. Figure 11 shows an HPLC chromatogram of a photoirradiated biphenyl/laponite (T,= 325 "C) sample. The product appears in front of the biphenyl elution, and the retention time of the product is very close to that of 2(4)-phenylphenol and 2,2'(4,4')-biphenol. The short
~~~
(31) Leu, W.; Huffman, J. C.; Kochi, J. K. J. Am. Chem. SOC.1982,104, 5515.
(32) Copmidge, J.L.;Davieaa,A. G.;Clark,T.; Wilhelm,D.J . Chem.
SOC.,Perkrn Trans. 1984, 1197.
Biphenyl on Laponite Surfaces
Langmuir, Vol. 9, No. 6,1993 1306
retention time on the reversed-phase column indicates that the products are more polar than the parent compound. The photoirradiated sample of biphenyl/laponite (Ta 20 "C) gives an identical product spectrum to that of an activated laponite sample, but with a lower yield, indicating similar chemical events lead to the products.
Discussion 1. Surface Effect. The diffuse reflectance spectra of biphenyVlaponite samplesshow significantbathochromic shifts due to interaction of the arene with surface sites (Figure 1). This provides a possibility of initiating photochemical reactions of biphenyl on surfaces under solar irradiation (A > 290 nm), in particular in a natural environment. Emission spectroscopy of a biphenyl/laponite sample shows marked changes in the vibronic structure due to the interaction of the excited state of biphenyl with the surface (Figure 2), indicating that the excited states of the adsorbedbiphenyl have different molecular conformation from free biphenyl molecules. The fact that adsorbed biphenyl shows a shorter fluorescence lifetime indicates that the laponite surface quenches the excited states. The time-resolved diffuse reflectance spectrum shows the formation of Phz'+ on the biphenyl/laponite sample after laser irradiation. These fiidings indicate the electron transfer from excited adsorbed biphenyl to surface sites is a important reaction (eq 7).
--
-
(Ph,'+/surface-) (7) (Ph,+6-*surfaceb)* (Ph,/surface) + hv' (8) Both time-resolved and steady-state emission experimenta show an emission band which extends up to 450 nm for adsorbed biphenyl, while solutions of biphenyl do not show any emission in the wavelength range of 380-450 nm. The longer wavelength emission is assigned to an excited CT complex (eq 8).33r20a Herehu'denotes emission at longer wavelengths (ca. 380-450 nm). The polar surface environment on laponite promotes charge transfer from the excited biphenyl molecules to surface sites, and it is generally recognized that these electron-deficient sites, or Lewis acid sites, are electron a c c e p t o r ~ . ~ At ~ J ~low temperatures the electrons are trapped, and warming the sample releases the electrons to the cation which gives rise to thermoluminescence. The ('Ph,*/surface)
(33)Nakagura, S.InEzcited States; Lim, E. C., Ed.; Academic Press: New York, 1975;Vol. 2, p 322, and references therein.
trapped electrons do not give any observable absorption bands in UV-visible wavelength range and are not detected in our studies. 2. Assignment of Chemical Products and Proposal of Reaction Mechanism. On the basis of the present experimental data and previous work- the photoinduced degradation of biphenyl on a laponite surface is a radical process, Le., electron-transfer to form Ph2+*species which react further via hydrolysis. The degradation products are assigned to hydroxy compounds, similar to reactions on silica-alumina surfaces.17 Addition of water to the cation center with subsequent deprotonation is shown in the following reactions Ph,"
-
+ H20
Ph2(0H2)'+
(9)
+
Ph,(OH2)'+-Ph2(0H)' H+ (10) The PL(0H)' radicals react further with Ph$+ through electron transfer resulting in the formation of hydroxy compounds. Ph,(OH)'
+ Ph," + H,O
-
Ph,
+ Ph,(OH),
(11)
This agrees wtih the half-regeneration mechanism of radical cation^.^^^^^
Conclusion The study confirms an interaction between an aromatic adsorbate and activated surface sites on a model clay system. According to the extent of the interaction, a shift and broadening of the spectral absorption bands takes place, surface complexes and ionization also occur. Photoirradiation into the CT band initiates charge separation, and the radical cations formed on the surface via radical processes are observed by diffuse reflectance and EPR spectrometry. After extraction with aqueous solutions, the surface species convert into products which are separated by HPLC. The present result shows that biphenyl adsorbed on laponite surfaces can be photochemically degraded through surface complexes at wavelengths longer than that of the unperturbed compound. Acknowledgment. We acknowledge research support by the EnvironmentalProtection Agency (EPA-R-81595301-0). Drs. Surapol Pankasem and Kai K. Iu are thanked for helpful discussion. (34)Bard, A. J.; Ledwith, A.; Shine, H. J. Formation, Properties and Reactionsof Cation Radicalsin Solution. In Advances in Physical Organic Chemistry; Gold, V., Bethell, D., Eds.; Academic Press: London, 1976; Vol. 13. (35)Hammrich, O.;Parker, V. D. Kinetics and Mechanismof Reactions of Organic Cation Radicals in Solution. In Advances in Physical Organic Chemistry; Ed. Gold, V., Bethell, D., Eds.; Academic Press: London, 1984;Vol. 20,p 55. (36)Ristagno, C. V.; Shine, H. J. J. Org. Chem. 1971,36, 4050.
