Langmuir 1993,9, 1001-1008
1001
Recombination Kinetics of Triplet Radical Ion Pairs Adsorbed onto Microcrystalline Cellulose Studied by Diffuse-Reflectance Laser Flash Photolysis Peter P. Levin,??*L. F. Vieira Ferreira,s and Silvia M. B. Costa',? Centro de Qutmica Estrutural, Complexo 1, Instituto Superior Tgcnico, 1096 Lisbon Codex, Portugal, and Centro de Qulmica m i c a Molecular, Complexo 1, Instituto Superior Tgcnico, 1096 Lisbon Codex, Portugal Received August 11,1992. In Final Form: November 16,1992 Recombinationkinetics of radical ion pairs formed by electron transfer from triphenylamineto triplet benzophenone, 9,lO-anthraquinone, duroquinone, and 2,6-diphenyl-l,4-benzoquinone coadsorbed onto porous microcrystalline cellulose has been studied by using the diffuse-reflectancelaser flash photolysis technique. The kinetics is nonexponentialextending over at least 4orders of magnitude in time scale from -0.1 gs up to 1 ms. The initial part of the kinetic curves (about 75% of conversion from the initial maximum absorption) can be described by a Gaussian distribution of the logarithm of the rate constant at various adsorption sites, while the final part of the decay curves follows the diffusion-like behavior-concentration vs t-1/2. Application of an external magnetic field as well as oxygen has no observableeffect on the radical ion pair decay. The roles of predominant static reaction pathways as well as those of dynamic reaction ones are discussed. 1. Introduction
Investigationson the photochemistry and photophysics of molecules as well as on the behavior of free radicals adsorbed or incorporated onto microcrystalline cellulose reveal interesting possibilitiesof affecting and controlling the kinetics of a variety of mono- and bimolecular prbce~ses.l-~ Cellulosecan be used as a medium for studies of photoinduced electron3v4or energy transfer5p6and also for triplet radical pair (RP) re~ombination.~ Since the development of the diffuse reflectance laser flash technique8there has been a number of direct kinetic observations of adsorbed free radicals and radical ions in opaque samples,Q-l6 particularly in microcrystalline c e l l u l o ~ e . Short ~ ~ lived radical ion pairs (RIP) with lifetimes varying between 0.1 and 9 ps were observed by applying this technique to 2e0lites.l~
* To whom correspondence should be addressed. Centro de Qulmica Estrutural. Institute of Chemical Physics, Russian Academy of Sciences, ul. Kosygina 4, 117334 Moscow, Russia. Centro de Qulmica Flsica Molecular. (1) Schulman, E. M.; Walling, C. J . Phys. Chem. 1973, 77,902. (2) Kaneko, M.; Yamada, A. Macromol. Chem. 1981,182, 1111. (3) Miloeavljevic, B. H.; Thomas, J. K. J . Phys. Chem. 1983,87,3368 1985,89, 183; Int. J . Radiat. Phys. Chem. 1984,23,237; J. Am. Chem. SOC.1986,108,2513. (4) Murtagh, J.; Thomas, J. K. Chem. Phys. Lett. 1988, 148,445. (5) Wilkinson, F.; Vieira Ferreira, L. F. J. Lumin. 1988, 40/41, 704. (6) Wilkineon. F.: Leicester. P. A.: Vieira Ferreira, L. F.: Freire, V. M. M. R. Photocherk. PhotobioL'1991.54. 599. (7) Levin, P. P.; Vieira Ferreira,'L. F.; Costa, S. M. B. Chem. Phys. Lett. 1990, 173, 277. (8) Keesler, R. W.; Wilkinson, F. J. Chem. SOC.,Faraday Trans. 1 1981,77,309. (9) Beck, G.; Thomas, J. K. Chem. Phys. Lett. 1983,94, 553. (10) Wilkinson. F.: Willsher. C. J. Chem. Phvs. Lett. 1983. 104. 272. ill) Kelly, G.; 'Willsher, C. J.; Wilkinson, F:; Netto-Ferreira, J. C.; Olea, A.; Weir, D.; Johnston, L. J.; Scaiano,J. C. Can. J . Chem. 1990,68, 812. (12) Oelkrug, D.; Reich, S.; Wilkinson, F.; Leicester, P. A. J . Phys. Chem. 1991, 95,269. (13) Kazanis, S.;Azarani, A.; Johnston, L. J. J . Phys. Chem. 1991,95, 4430. (14) Oelkrug, D.; Krabichler, G.; Honnen, W.; Wilkinson, F.; Willsher, C. J. J. Phys. Chem. 1988,92, 3589. (15) Pankasem, S.; Thomas, J. K. J . Phys. Chem. 1991, 95,6990. (16) Johnston, L. J.; Scaiano,J. C.; Shi, J. L.; Siebrand, W.; Zerbetto, F. J . Phys. Chem. 1991, 95, 10018. +
* Permanent address:
Nonexponential decay kinetics extending over several orders of magnitude in time scale, from microseconds up to seconds, are quite typical not only of surfaces but also of other solids like glasses and rigid polymers, where the matrix controlsthe chemical process, accountingfor a very wide distribution in reactivity (see for example refs 18 and 19 and references therein). Radical decay kinetics on nonreactive solids is still not fully understood. So far no good model has been good enough to adequately interpret the polydispersivekinetic behavior of free radicals in nonreactive solidsor on surfaces. Detailed models are available for a Gaussian distribution of exponential decays18120and they have been used for a dispersive kinetic analysis of radical decay in glasses18and on ~ u r f a c e s . ~ ~This J ~ Janalysis ~ implies in particular that plots of concentration vs log time give information on parameters associated with the distribution of rate constants. The plota of log concentration vs log time have also been used for triplet radical pairs recombination in rigid polymer films and indicate a linear dependence in a wide time scale.lQ Another approach used to analyze the geminate recombination on surfaces at long times is the linear dependence of concentration on t-1/2 which proved to be adequate to describethe motion of oppositely charged little-spaced particles moving toward one another in one dimension.12 The recombination kinetics of diphenylmethyl radicals generated in the cavities of Na-X zeolites was assumed to be a second-order process and interpreted in terms of a random walk model.16 Using the diffuse reflectance laser flash photolysis technique, we have recently reported a study of geminate recombination kinetics of triplet radical ion pairs (RIP) generated from an electron transfer quenching process of triplet electron acceptors (A) such as benzophenone (BPI, 9,lO-anthraquinone (AQ), duroquinone (DQ), and 2,6diphenyl-1,4-benzoquinone(PQ) by an electron donor (17) Sankararaman, S.; Yoon, K. B.; Yabe, T.; Kochi, J. K. J . Am.
Chem. SOC.1991,113,1419.
(18) Siebrand, W.; Wildman, T. A. Acc. Chem. Res. 1986,19,238. (19) Levin, P. P.; Kuzmin, V. A,; Ivanov, V. B.; Selikhov, V. V. Bull. Acad. Sci. USSR, Diu. Chem. Sci. 1988, 37, 1552. (20) Albery, W. J.; Bartlet, P. N.; Wilds, C. P.; Darwent, J. R. J . Am. Chem. SOC.1985, 107,1854.
0743-7463/93/2409-1001$04.00/00 1993 American Chemical Society
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1002 Langmuir, Vol. 9, No. 4, 1993 triphenylamine (TPA) both adsorbed on porous silica.z1 The RIP decay kinetics in this medium introduces new features, but it also bears some resemblance to those observed in homogeneous solution^^^-^^ and on the surface of transparent silicate porous g1ass,z5 studied with the conventional laser flash photolysis. Namely, in all cases the RIP decay kinetics, under an external magnetic field, showed evidence of some contribution of the magnetosensitive mechanisms of the RIP spin evolution which, however, varies in each medium. Similar studies of geminate recombination of pairs of neutral radicals (RP) of ketyl-phenoxy1 type were undertaken in viscous homogeneous media,z6in micellarz7 and cyclodextrine solutions,z8on porous glass,z5and in rigid polymers.'g Our reported results on the H-atom transfer from 2,4,64rimethylphenol (POH) to BP7 show that the decay kinetics of the RP generated on microcrystalline cellulose is quite different from that in liquid mediaor on silicatesurface,bearing instead somesimilarity to the RP decay kinetics in rigid polymer films. The study reported in this paper extends the formation of RIP to the same systems BP, AQ, DQ, and PQ (acceptors)and TPA (donor) now coadsorbed onto porous microcrystallinecellulose in order to understand how the substrate affects the recombination decay kinetics of the RIPS generated "in situ". 2. Experimental Section 2.1. Apparatus. The absorption spectra and decay kinetics of the intermediates were recorded by laser photolysis using the third harmonic (355 nm, 1 5 mJ, 25 ns) of a Nd-glass laser (J. K. Lasers Ltd., System 2000)as an excitation source? The kinetic spectrophotometer (25-ns resolution) includes an averaging system consisting of a Tektronix 2430A digital oscilloscope coupled to a PDP 11/73microcomputer. Kinetic curves were averaged over 16 laser pulses. In the magnetic field experiments, the sample was placed between two pole pieces of a permanent magnet ( ~ 0 . 0 5T). All measurements were conducted at 20 f 1 OC.
Ground-state absorption spectra of powdered solid samples were obtained using an Olis 14 UV/VIS/NIR spectroscopy operating system with a diffuse reflectance attachment (90mm diameter integrating sphere, internally coated with MgO). Experimental details to obtain accurate ground-state absorption spectra are given elsewhere.29 2.2. Data Analysis. Data are reported as AJt/Jo = 1 - Jt/Jo, where JOand Jl are the light reflection from the sample before and at time t after the laser pulse. The transient signal intensities (120% ) increased proportionally with increasing laser intensity without changes in the decay kinetics, demonstrating that a saturated plug of ground state totally converted into transient is not produced and indicating the validity of this treatment to analyze the monomolecular decay of RIP and RP rather than that of Kubelka-Munk.ll-RoJ1 (21) Levin, P. P.; Vieira Ferreira, L. F.; Costa, S. M. B.; Katalnikov, I. V. Chem. Phys. Lett. 1992, 193, 461. (22) Levin, P. P.; Pluzhnikov, P. F.; Kuzmin, V. A. Chem. Phys. Lett.
1988,147, 283; 1988, 152, 409; Chem. Phys. 1989, 137, 331; Khim. Fiz. 1989,8, 752. (23) Levin, P. P.; Raghavan, P. K. N.; Kuzmin, V. A. Chem. Phys. Lett. 1990, 167, 67. (24) Levin, P. P.; Raghavan, P. K. N. Chem. Phys. Lett. 1991,182,663. (25) Levin, P. P.; Katalnikov, I. V.; Kuzmin, V. A. Bull. Akad. Sci. USSR, Diu.Chem. Sci. 1988,38,1095; Khim. Fiz. 1988,8,1604; Chem. Phys. Lett. 1990, 167,73; Chem. Phys. 1991,154,449. (26) Levin, P. P.; Khudyakov, I. V.; Kuzmin, V. A. J. Phys. Chem. 1989,93,208; Bull. Akad. Sci. USSR,Diu. Chem. Sci. 1987,36,918; 1988, 37, 432. (27) Levin, P. P.; Kuzmin, V. A. Chem. Phys. Lett. 1990, 165, 302; Dokl. Phys. Chem. 1987,292,26; Bull. Akad. Sci. USSR,Diu.Chem. Sci. 1986,35,430; 1987,36,993; 1988,37, 224. (28) Levin, P. P.; Malkin, Ya. N.; Kuzmin, V. A. Chem. Phys. Lett. 1990, 175, 74. (29) Vieira Ferreira, L. F.; Freixo, M. R.; Garcia, A. R.; Wilkinson, F. J. Chem. SOC.,Faraday Trans. 1992,88, 15.
Each decay curve was recorded at 1024 points at 10,100,400, and lo00 ns per point. A total of 2700 points for the corresponding time interval was used to analyze the experimental kinetic curves by resorting to different decay functions. The fitting procedure was a nonlinear least-squares method using the Marquardt a l g ~ r i t h m .The ~ ~ quality of the fittingwas judged by the residuals and autocorrelation as well as by the standard deviation (S)and thestatistical parameter Durbin Watson (DW). The data are presented as an average of results based on calculations performed with at least five kinetic curves for each system. 2.3. Materials. Microcrystalline cellulose (Fluka DSO), 50 pm average particle size, was used. The product was dried in a vacuum oven at =70 O C for at least 24 h before using it for sample preparation. Microcrystalline cellulose is a mechanically disintegrated cellulose resulting from hydrolysisof purified cellulose after 15 min in 2.5 N HCl at 105 f 1 "C. The product obtained is a pure form of cellulose free from organic and inorganic contaminants and with a high degree of crystallinity. The microcrystalline aggregates are rodlike or lamellar in shape. The specific internal area of cellulose varies from 10 to 200 m2/g depending on the particle size.33 Porous particles of the order of 1 pm contain a very large number of pores ranging from 1 to 10 nm and have a total surface area of 200 m2/g.33934 The compounds were purified by sublimation or by recrystallization from ethanol. Ethanol (or acetonitrile) solutions (10 mL) of A (1 X to 0.05 mol/L) and TPA (0.0014.1mol/L) were added to the previously ethanol-covered cellulose samples (0.58). The relative amounts of A and TPA were chosen in such a way that the absorption of A at 355 nm exceeded that of TPA. The suspension was stirred for 1 h and allowed to evaporate slowly. The final traces of the solvent were removed at 60 O C in a vacuum oven. In most samplesonly less than 10% of the porous surface area was covered by A and TPA.
3. Results
3.1. Phatogeneration of Radical Ion Pairs on Cellulose. Ground-state diffuse reflectance measurements of A-TPA systems on cellulose have been carried out in the present investigation. The comparison of the absorption of A and TPA alone on the surface with that of A in the presence of TPA shows that no new chargetransfer absorptionbands are formed. However,excitation at 355 nm can excite both A and TPA depending on the amount of each in the sample (Figure 1). Photoexcitationof A adsorbed alone on microcrystalline cellulose is followed by an efficient production of 3A, in the microsecondtime domain and its transient absorption spectra exhibit characteristic maxima near 530 nm for 370 nm for 3AQ,363490 nm for 3DQ,35937and 3BP15*6,35 around 600 nm for 3PQ.38 In the case of 3BP the timeresolved absorption spectra showed at longer times the formation of the ketyl radical6 in good agreement with data previously reported.39 In all cases, 3A is efficiently formed and relatively long lived. Photoexcitation of TPA adsorbed alone on cellulose at 355 nm produces a long lived transient with very low yield (30) Oelkrug, D.; Honnen, W.; Wilkinson, F.; Willsher, C. J. J. Chem. SOC.,Faraday Trans. 1987,83, 2081. (31) Drake, J. M.; Levitz, P.; Turro, N. J.; Nitsche, K. S.; Caesidy, K. F. J. Phys. Chem. 1988,92, 4680. (32) Marquardt, D. W. J. SOC.Ind. Appl. Math. 1963,11, 431. (33) Battista, 0. A. In Encyclopedia of Polymer Science and Technology; Mark, H. F., Gaylord, N. G., Bikales, N., Eds.; Wiley: New York, 1965; Vol. 3, p 285. (34) Krassig, H. In Ullman's Encyclopedia of Industrial Chemistry; VCH Verlagsgesellshaft Weinheim, Germany, 1986; Vol. 5, p 375. (35) Carmichael, I.; Hug, G. L. J. Phys. Chem. Ref. Data 1986, 15, 1. (36) Hulme, B. E.;Land, E. J.; Phyllips, G. 0. J. Chem. SOC.Faraday Trans. 1 1972, 68, 2003. (37) Kemp, D. R.; Porter, G. R o c . R. SOC.London, A 1971,326, 117. (38) Kuzmin, V. A,; Darmanyan, A. P.; Levin, P. P. Chem. Phys. Lett. 1979,63, 509.
Kinetics of Triplet Radical Zon Pairs
400 Wavelength, nm
300
5,
350
Langmuir, Vol. 9, No.4, 1993 1003
I
450
,
Wavelength, nm Figure 2. Transient diffuse-reflectancespectra of the AQ-TPA (m) and BP-TPA (A) systems adsorbed on microcrystalline cellulose obtained immediately after the laser pulse.
2, upper curve) is rather sharp and is strongly blue shifted (450 nm) in comparison with that in solution where it
Wavelength, nm Figure 1. Reflectance (a, top) and remission function values (b,
bottom) for the benzophenone-triphenylamine system adsorbed on .microcrystallinecellulose: (0)microcrystalline cellulose; (1) 2.0 X mol of BP g1of the substrate; (2) 2.0 X 10-5 mol of TPA gl of the substrate; (3) 2.0 X 10-5 mol of PB and 2.0 X 10-5 mol of TPA g1of the substrate; (4) 7.2 X 10-4 mol of BP gl of the substrate;(5) 1.1 X lWRmol of PB and 1.6 X 10-4 mol of TPA g1of the substrate.
regardless of the total TPA concentration. The spectrum is very broad ranging from 400 to 700 nm with a maximum at 650 nm. Photoexcitation of A in the presence of TPA in cellulose initiates the well-known process which includes the very efficient exothermic electron transferz1
-- hu
A
'A
kist
ket
3A
TPA
3(A'-,TPA'+)
(1)
where the parentheses enclose the RIP. In each case the concentrations of A and TPA were matched in such a way that the highest amount of triplet RIP was formed. Immediately after the pulse the wellknown absorption of TPA'+with a characteristic maximum at 650 nm and a smaller one at 550 nmzz-z4~40-4z is observed in A-TPA systems on cellulose (Figure 2). There is also another maximum in the transient absorption spectra of the AQ-, DQ- or PQ-, TPk+at 420-450 nm (Figure 2, upper curve), which may be ascribed to the absorption of A'-.zz-z4,41446 The maximum of A Q - on cellulose (Figure (39) The ketyl radical should be formed withii the laser pulee. However due to differences in the extinction coefficienta of ,'BP and BPH' (in acetonitrile,the ratio C:ILIIiHl'/fi,,llHOH' = 21, (Bensamon,R. V.; Cramain, J. C. J . Chem. SOC.,Faraday Trans. 1980, 76, 1801), it is only possible to see the radical at longer times when most of the triplet has already decayed. (40) Burrows, H. D.; Creatorex, D.; Kemp, T. J. J . Phys. Chem. 1972, 76, 20. (41) Amouyal, E.; Bensasson,R.J. Chem. Soc., Faraday Trans. 1 1977, 73, 1561. (42) Levin, P. P.; Kuzmin, V. A. Bull. Akad. Sei. USSR,Diu. Chem. Sci. 1988, 37, 807.
appears at 530 nm in nonhydroxyl solvents or at 480-500 nm in alcohols.zz~z4~43~44 In the case of the BP-TPA system the absorption of BP'- in the region 600-700 nm41942947 is masked by the more intensive absorptionof TPA'+(Figure 2, lower curve). In order to examine the effect of the relative concentration of TPA versus A, samples with different contents of A were studied. Parts a and b of Figure 3 show the time-resolved spectra of two transients obtained respectively from a system BP:TPA ratio 5:l and ratio 1:l. In the first one, the transient spectra show the formation of 3BP (530 nm) with concomitant formation of 3(BP'-,TPA'+) at 650 nm. These two species decay differently, and at longer times only the RIP lives (Figure 3a). In the sample (ratio 1:l) the 3BP is almost totally quenched and the time-resolved spectra (Figure 3b) essentially show one decaying species (identical with Figure 2, lower curve). The lack of data concerning extinction coefficients of the radical ions on cellulose does not make it possible to ascertain the exact concentrationof A'-and TPk- formed. On the basis of the time-resolved spectra of donor and acceptor coadsorbed in cellulose in equal concentrations, it is possible to say that the electron transfer efficiency is nearly 100%. However, in order to quantify the amount of radical ion pair formed, it would be necessary to know the ground state and the triplet extinction coefficients of benzophenone in cellulose which may be very different from those in a homogeneous solution. Nevertheless, the comparison of the absorption spectra of RIP on cellulose with those obtained in different e n v i r ~ n m e n t a ~ ~ ~ ~ ~ - ~ leads to the assumption that radicals A'-and T P k +are present on the surface in equal concentrations, supporting the RIP formation. 33. Decay Kinetics of RP and RIP on Cellulose. The kinetics of the RIP and RP decay on cellulose are nonexponential and extend over several orders of magnitude in time (see Figure 4a,b). The values of the initial (43) Hamanoue,K.;Nakayama,T.;Yamamoto,Y.;Sawada,K.;Yuhara,
Y.; Terenishi, H. Bull. Chem. SOC.Jpn. 1988,61, 1121. (44) Carlson, S.A.; Hercules, D. M. Photochem. Photobiol. 1973,17,
123. (45) Baxendale, J. H.; Hardy, H. R. Trans. Faraday SOC.1963, 49, 1433. (46) Peters, K. S.; Freilich, S. C.; Schaeffer, C. C. J. Am. Chem. SOC. 1980, 10,5701. (47) Simon, J. D.; Peters, K. S. J . Am. Chem. SOC. 1982,104,6542. (48) Das, P. K.; Encinas,W. V.; Scaiano,J. C. J. Am. Chem. SOC.1981, 103,4154,4161.
1004 Langmuir, Vol. 9, No. 4, 1993
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slopes and those at 50% ' of RIP conversion are presented in Table I. For comparison purposes data from ref 7 concerning RP (PO',BPH') decay generated from the interaction of benzophenone (BP) and trimethylphenol (POH) are also included in Table I. The estimation at RIP conversions higher than 90% gave very low values with slopes down to 1 X lo3 s-le The oscilloscopic traces of RIP (or RP) absorption on cellulose were independent of the wavelength of observation in the time range of 0-1 ms and of laser intensity (on the initial concentration of RIP or RP). Repetitive excitation of the same sample area with more than 100 laser shots showed that the intensities and decays of the transient absorption signals are completelyreproducible. These features indicate that the simultaneous decay of a radical from the acceptor and TPA'+ (or PO') as a RIP (orRP) is mainly due to a unimolecularbackward electron transfer with regeneration of initial A and TPA as shown in eqs 2 and 3. These findings, as well as the order of 3(A'-,TPA'+) 3(BPH',PO')
A + TPA
2 BP + POH
(2)
(3)
magnitude of the time scale for the initial part of the process, are similar to those observed for the geminate
TIME, microseconds rl4 -14
20
be
41
I 0
5
TIME, microseconds Figure 4. (a, top; b, middle) Decays of the transient absorption at 650 nm due to the adsorbed RIP [PQ*-,TPA'+](lower curves) and [BP*-,PA'+] (upper curves) in different time scales. (c, bottom) Decay of transient absorption (75%)at 650 nm due to the adsorbed RIP [BP-,TPA*+]:solid line, approximation by Albery law, using parameters from Table I.
recombination for the same RIPSand RP in ~olutions,2~-~~ on the surface of porous in silica,21and in polymer films.lg It may be definitely concluded that at least the main part of RIP (or RP) decay on cellulose corresponds to unimolecular geminate recombination. In contrast with our studies of the same RIPSin silica,21 imposition of a magnetic field has no effect to within 5% on the RIP (or RP) decay. Introduction of air into previously evacuated samples of cellulose covered by A and TPA (or POH) has also no effect on the decay of the RIP (or RP) under investigation. A similar result was previously found for BP and oxazine coadsorbed on microcrystallinecellulose6and reflects a large decrease in
Langmuir, Vol. 9, No.4, 1993 1005
Kinetics of Triplet Radical Zon Pairs Table I. Slopes at Dilferent Conversions for the Recombination of Radical Ion Pairs 3(A'-,TPA*+)Adsorbed on Microcrystalline Cellulose, Average Rate Constants &, and Width of the Distribution in Terms of the Albery Function (4), Obtained by Fitting of Initial Parts of Kinetic Decay Curves (up to 76% Conversion),and Parameters Obtained by Fitting with Function (8) for the Final Parts of Kinetic Decay Curves (275% conversion).
BP-POH BP-TPA AQ-TPA w-TPA PQ-TPA a
0.29* 1.0 1.5 2.2 2.6
0.15 0.37 0.57 0.83 1.0
0.25 0.58 1.2 2.0 2.4
2.0 2.2 2.2 2.2 2.1
1300 1200 1300 1100 870
0.52 0.51 0.52 0.50 0.40
Estimated errors 20%. Reference 7.
the mobility of oxygen in cellulose, in accordance with the rigid nature of the cellulose matrix. 4. Discussion
where AJo stands for the initial change in light reflection after the laser pulse, k is the average first-order geminate recombination rate constant, and y is the width of the distribution. The corresponding k and 7 values obtained by the use of eq 4 up to 75 % of conversion are summarized in Table I. This function is based on a Gaussian distribution in the free energy of the monomolecular process and has already been used for the description of the decay kinetics of pyrene cation radical on alumina,15 which is similar to the decay kinetics under study. The adequacy of this function for a significant part of kinetic curves may indicate that the dispersive behavior of the recombination is mainly due to the distribution in reactivity. The Albery model makes it possible to evaluate both the average rate constant and the width of the distribution which is a measure of the surface inhomogeneity. However, function 4 cannot describe the overall decay kinetics. The remaining part of the decay at conversions 175% is much slower than that predicted from the initial part of the decay using function 4 (see Figure 4b,c). Another function, which has been used for the description of nonexponentialkinetic behavior in many organized systems (see for example refs 17,49, and 50 and references therein) and which contains the same amount of parameters as function 4 is the relation
Johnston et al.lS have described the diphenylmethyl radical recombination observed in Na-X zeolites as a second-orderprocess. The triplet radical pair once formed could fragment or dissociate and the two moieties (or one) could escape to other cavities and perform a random walk with several jumps. If two of the original radicals met again in the same cavity, ageminate recombination process would occur. The authors favored a random walk mechA J ~ / A J=, exp(-k'tf) (5) anism in view of the observation that their kinetic data show an initial rate of radical decay with an approximately Equation 5 is an empirical relation but is based on linear function of logarithmic time, as predicted by the fractal-like behavior with a time-dependentfirst-orderrate model when the recombination time, 8, is larger than the constant. In terms of fractal model f = d$2, where d, is escape time, 7. However the assumptionis only qualitative the spectral fractal dimensionof the reaction domain and and no fit of this model to the experimental data is k' is a constant related to the time-dependentrate constant presented. ask(t) =fk'tf-l. The recombination decay kinetics of thoae Our data cannot be explained via a second-order RIPS in silica2' is well described by function 5 with an recombination and indeed all the spectroscopicand kinetic offset for a small contribution of a slow decay. However evidence obtained points to an ion pair formation within the RIP decays in cellulose do not quite fit function 5 even for the initial part. For example, by use of (5) for the which the radical ions formed recombine, via a unimoinitial parts of kinetic curves, f values close to 0.5 have lecular process, before leaving the pair. been obtained, but the tail of the process corresponds to In the following discussion the kinetic analysis, the f values ranging from 0.2 to 0.3. spectroscopic evidence, and the conclusions on the mechThe other important empirical approach to describe anism will be presented in separate sections. the kinetical behavior of a pair of two particles involved 4.1. Kinetic Analysis. The simplest kinetic analysis in diffusional process is shown in eq 6 of the initial fast decay for the RIP (or RP) could be made in terms of a monoexponential model. This model gives CdC = 1+ kdt'12 (6) the possibility of assessing the back geminate recombiwhere C and CO are the pair concentrations respectively nation and the dissociation in the RP decay processee by at time t and time zero. In the long time range, relation two first-order rate constants and it has already been used 6 gives the very well-known asymptotic law5l for RIP and RP in s o l ~ t i o nand ~ on ~ ~ porous ~ ~ glass.25 + ~ ~ ~ ~ However, the very pronounced dispersive character of c a t-'12 (7) kinetic curves under consideration does not allow the use for a pair of reactants participating in random wondering of a monoexponential function even as a very rough in a three-dimensional space. At t 0 it gives the relation approximation. It is only reasonable to evaluate the initial -dC/dt a t-'12 valid for a one-dimensional nonstationary slopes and those at different percent of conversion, as diffusion model. This equation has been used for the presented in Table I for an easy comparison between description of the ion radicals decay on silica and alumidifferent pairs. na.11J2 To fit the kinetic curves in the present work, we used a more general form of function 6 Two, three, or even four exponential8 can appropriately fit only some part of the decay with parameters which are AJdAJt * 1 + k,tn (8) strongly dependent upon the time range. The use of severalexponentialfunctions in search for a physical sense Thisfunction does not describe the overall kinetic curves seems to be pointless. in the complete time interval; however a good fitting was Albery function20(eq 4) provides an adequate descrip(49) Blumen, A.; Klafter, J.; Zumofen, G. In Fractals in Physics; tion of the initial part up to 75% conversion of the RIP Elsevier: Amsterdam, 1986; p 399. (or RP) decay on cellulose using only two parameters (see (50)Prasad, J.; Kopelman, R. J . Phys. Chem. 1987,91, 265. (51)Noyes, R. M.J . Am. Chem. Soc. 1966, 78,5486. Figure 4c)
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Figure 5. Plots of AJ(O)/AJ(t)on tl/* for the transient decays of BP-TPA coadsorbed on microcrystalline cellulose. Three traces in different time scales were combined in time range where they overlap.
Figure 7. Plots of AJ(O)/AJ(t)on t1I2 for the transient decays of at 650 nm of triphenylamine adsorbed on microcrystalline cellulose. Three traces in different time scales were combined in time range where they overlap.
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obtained for the part of the kinetic curves of 175% of conversion with n closeto 0.5(Table I). The plot according to function 6 is presented in Figure 5. The behavior obtained is in agreementwith (7)and showsthat a diffusion process is involved in the long time slow decay. The asymptotic behavior accordingto law 7can be easily seen from the slopes of nearly linear plota of AJC/AJovs log t at long times on Figure 6. The curves on Figure 6a,b show a sigmoid form, up to 75 % conversion as is predicted from the Gaussian distribution of exponential decays.18v20 They can be divided approximately into two parts, with the slope of the initial part being approximately 5 times as large as that of the final one. The two-component-like
behavior exhibited on Figure 6 may serve as evidence for the change of the nature of the rate-determining step in the course of the geminate recombination. The possibility that the long time behavior may be due to the decay of the radical cation originated from direct excitation of TPA cannot be excluded. However, ita contribution could only account for a small percentage of the long lived transient, given the extremely low amount of the transient species produced in the absence of benzophenone (maximumyield 2-3 7%). Furthermore, no linearity is observed on the dependence of the AJdAJt on t1/2or AJtlAJo vs log t (see Figures 7 and 8 and compare with Figures 5 and 6a,b). 4.2. The Nature of the Cellulose Surface and Absorption Spectra of Radical Ions. Microcrystalline cellulose can form hydrogen bonds, both within ita own structure and also with other molecules of low molecular weight attached to the polymer by localized interacas solvent there is evidence that t i o n ~ . Using ~ ~ * ethanol ~~ the solute molecules are not adsorbed at the surface but go deeper in the cellulose polymer chain."
(52) Wollenweber,P. Organic Absorbents. Cellulose and Derivatives. In Thin-LayerChromatograp~y,2nded.;Stall,E.,Ed.;C.Allen&Unwin/. Springer-Verlag: Berlin, 1969; p 32. (53) Dinh, Vo; Vo-Dinh, Yuan Room Temperature Phosphorimetry for Chemical Analysis; Wiley: New York, 1984.
Langmuir, Vol. 9, No. 4, 1993 1007
Kinetics of Triplet Radical Zon Pairs
The radical ions photogenerated in cellulose contain heteroatoms, oxygen and nitrogen, which can form a hydrogen bond with the OH groups of the cellulose chain. A significant blue shift of the absorption band of the 9,10-anthraquinone radical ion (AQ*-)from solution (even for solvents with high affinity for H-bond formation, e.g. alcohols,where it appears at -480-500 nm) to the cellulose, as well as its sharpness leads to the conclusion that this speciesis stronglybonded to the OH groups of the cellulose polymer chain (Figure 2, upper curve). The position of AQ'- absorption being strongly dependent on the microenvironmentmay serve as a tool for understanding the nature of the bonding of A'- to the cellulose surface in the case of A-TPA systems. Protonation of AQ' to give AQH' seems unlikely. On the other hand, the backbone cellulose chain should assist hydrogen bonding. A similar situation happens in transparent silicate porous glass where the AQ'- appears at ~ 4 3 nm,25 0 whereas in silica gel (60-A pore) the absorption maximum was observed around 480 nm and was very broad.21 The absorption spectrum of BP'- is also very sensitive to the envir~nment;~~ however it is masked by the TPA'+ absorption. On the other hand, the positions of the absorption bands of PQ'-, DQ*-,and TPA'+, due to their weaker basicity, have a weaker hydrogen bonding to the cellulose surface and are not so sensitive as AQ-. 4.3. Mechanism and Rate Limiting Stage of RIP (or RP)Geminate Recombination on Cellulose. A brief overview of the shapes of the kinetic curves of RIP or RP decay on cellulose leads to the conclusion that they may be divided into two components: a fast ( t I5-10 ps) and a slow component (t 1 10 ps) as observed earlier.22-25 In solution it has been assumed that the fast component corresponds to ion radical decay through geminate recombination, and the slow component reflects the ion radical decay in the bulk of the medium. This approach implies a reasonable translational mobility of radicals in RIP (or RP), enabling the escape of radicals from the cage. The fact that no magnetic field effect is observed (inhibition of the fast component associated with the geminate recombination) in the systems currently studied seems to indicate that the cellulose environment considerably restricts the translational mobility of radicals. The mechanism operating here should be an intersystem backward electron transfer in triplet contact radical ion (or radical) pairs. Another important question to address is the assignment of the rate-determining step in geminate recombination of the triplet RP or RIP on the cellulose surface. Two possible pathwayscan be envisaged either the intersystem geminate recombination itself associated with a distribution in the unimolecular rate constants due to the inhomogeneityof the cellulosesurface,thereby explaining the dispersive pattern exhibited on the RTP and RP decay on cellulose, or a nonexponential relaxational process of some kind, resulting in the time dependence of the observable first-order rate constants. The latter situation includes,namely, the case where the rate-determining step is the geometrical rearrangement of pairs due to the rotational diffusion of radicals in a cage at the cellulose surface. In rigid polymerslgthere is experimental evidence which points to the very important role of rotational diffusion in geminate recombinationof triplet RP (BPH'PO'). The rigidity of the environment of adsorbed species on cellulose seems to imply that the excited state quenching process (1)is static. If in general the RIP (or RP) can be (54) Vieira Ferreira, L. F. Unpublished results.
viewed on the surface structure as a "rigid supermolecule" in a wide variety of microenvironments with a resulting distribution in the corresponding interactions (e.g. spinorbital), then it should be valid to assume the static approach for the distribution in the first-order rate Constants for RIP (or RP). The comparison of the results obtained by fitting function 4 to different RIP decays leads to the conclusion that the width of the distribution is independent of the nature of RIP. In the case of RIPS (or RP) adsorbed onto cellulose, this parameter (Table I) has a rather high value when compared with data obtained from the other heterogeneous environments.18 On the other hand, on alumina the values of y = 4.5 have been obtained for the decay of pyrene radical cation.15 The value is not sensitive even to the nature of the process (electron or hydrogen atom transfer) or to the existence of a Coulombic attraction between radicals. Thus, the nature of cellulose determines to a great extent the most important features of reactivityof adsorbedreactants. This feature seems to be a general one for the reactions in rigid polymer matrices.55956 The data obtained in this work (k values and initial slopes, Table I) show a slight difference in the series PQ, DQ, AQ, BP, which is smaller than the one expectedshould the electron transfer process be controlled by the redox potentials. However, the rate constants obtained in this work are smaller than those determined for the rate constants of intersystem electron transfer induced by spinorbit coupling in contact RIP in other en~ironments,~~.~5 of intersystem recombination of RP in glycerol,26of biradi~als,5~*58 of contact triplet RP in the cyclodextrine and of RP on the surface of silicate glass.25The initial slope for the decay of ketyl-phenoxy1 RP on cellulose is an order of magnitude smallerthan that for the geminate recombinationof the same RP in rigid poly(viny1chloride) films.lg Mean rate constant values in the range 107-108 s-l are typical of intersystem recombination or electron transfer induced by spin-orbit coupling of contact aromatic RPs and Thus cellulose structure results in some stabilization of adsorbed RP. The absence of oxygen effect on the decay kinetics of triplet RIP on cellulose may be a further evidence for an isolated and rigid nature of localized sites of RIP on a cellulose surface. This behavior is in contrast to that reported for triplet RIP in solutions, on the surface of porous glass, or on silica, where gas-phase 02 can easily reach the moleculesadsorbed on the ~ u r f a c e The . ~ ~same ~~~ phenomenon of protection of excited adsorbedspecies from deactivation by interaction with molecular oxygen was previously observed for other triplet states and radicals as weii.4-7 The definite sharp change in the curvature of the dependence of the concentration of RIP on logarithm of time (Figure 6) seems to be the consequence of the change in nature of the rate-determining step in the process of geminate recombination on the cellulose surface. If this is true, then the t-112 long time behavior shows the diffusion-like geminate recombination due to some kind of mobility of radicals on the cellulose surface (e.g. some restricted rotational diffusion) or some relaxational processes of diffusion-like nature with reorganization of (55) Emanuel, N. M.; Buchachenko, A. L. Chemical Physics of Aging and Stabilization of Polymers; Nauka: Moscow, 1982; pp 44, 215. (56) Shlyapintokh, V. Ya.Photochemical Transformations and Stabilization of Polymers; Khimiya: Moscow, 1979; p 27. (57) Doubleday,C., Jr.;Turro, N. J.;Wang, J.-F.Acc. Chem.Res. 1989, 22,199.
(58) Scaiano, J. C.; McGimpsey, W. G.;Leigh, W. J.;Jakobs, S.J . Org. Chem. 1987,52,4540.
1008 Langmuir, Vol. 9, No. 4, 1993
mutual geometrical arrangementof RIP and of the distance between the radicals of RIP. In agreement with this proposal there is practically no dependenceof parameters of function 8 on the structure of radicals nor is there any Coulombicattraction between them (Table I). The slopes of plots for the different systemsin Figure 6 are practically the same at long times. It should be pointed out that an exact analysis of the law controlling the shape of the tail of geminate recombination is difficult for an obvious reason-the experimental error increases nonproportionally. Nevertheless, the use of functions which have been derived in terms of different physical models seems to be very useful due to the concrete physical meaning of fitting parameters obtained. The analysis of the values of those parameters provides the opportunity to elucidate the physical background of the process under discussion. 5. Conclusions The use of cellulose as a porous adsorbent enables the photogeneration of relatively long-lived triplet radical ion pairs with high efficiency. The kinetics of geminate
Levin et al.
recombination of radical ion pairs adsorbed on cellulose is nonexponentialwith a wide distribution of reactivity in time due to a strong effect of environmenton the distance and mutual spatial orientation of radical-ions in a pair and hence on spin-orbit interaction in the pair of radicals. The rigid nature of cellulose surface results in several unique features of the behavior of adsorbed radical pairs. In contrast with silica where the geminate recombination of identical radical ion pairs is diffusion-controlled,2l in cellulose the interaction is predominantly static. Some relaxational, e.g., diffusional processes, seem, however, to be involved at long times, determining a complex nature of the recombination process and changing the ratedetermining step during the reaction. Acknowledgment. This work was supported by I.N.I.C. ResearchgrantstoP.P.L. throughI.N.1.C. (Project CQE-IV) are gratefully acknowledged. The authors thank Dr. I. Khemelenski for carrying out some time-resolved spectra and Mr. P. Coutinho for helpful assistance in the data analysis programs.
