Kinetics of Intersystem Electron Transfer within Triplet Radical Ion

Chem. , 1995, 99 (4), pp 1267–1275 ... View: PDF ... Langmuir 1999 15 (13), 4641-4647 ... Theoretical and Experimental Chemistry 1997 33 (1), 11-15...
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J. Phys. Chem. 1995, 99, 1267-1275

1267

Kinetics of Intersystem Electron Transfer within Triplet Radical Ion Pairs on Silica Studied by Diffuse-Reflectance Laser Flash Photolysis. Bell-Shaped Energy Gap Dependence on the Surface Peter P. Levin,?$'Silvia M. B. Costa,*jt and L. F. Vieira Ferreiras Centro de Quimica Estrutural and Centro de Quimica Fisica Molecular, Complexo 1, Instituto Superior Tecnico, 1096 Lisbon Codex, Portugal Received: May 30, 1994; In Final Form: September 8, I994@ Photoinduced electron transfer (ET) reactions of quinones (A) and tertiary aromatic amines (D) both adsorbed onto porous silica (14 nm pore size) were studied by diffuse-reflectance laser flash photolysis technique. Both diffusion-controlled dynamic and Pemn type static quenching of 3Aby D were observed. Static quenching results in formation of triplet radical ion pairs (RIPs). RIPS decay via an intersystem back electron transfer (ET). The ET kinetics are discussed in terms of two formalisms: a first-order law with Gaussian distribution on the free energy or by a fractal-like analysis with time dependent first-order rate constant k(t) = k/trh. The heterogeneity constant, h, increases with the increase in average rate constant of reaction in accordance with the theoretical predictions for lower dimensional and fractal media. The back ET is a reaction-controlled process at early times and a diffusion-controlled one at times longer than 0.5 ps. The dependence of the average rate constant of back ET and of k(t) at early times on the ET free energy is bell-shaped. This can be quantitatively described in terms of the single quantum mode model of the nonadiabatic ET theory with a higher value of the reorganization energy of environment (0.9 eV) as compared to that in moderately polar solvents (other parameters being the same). The bell-shaped energy gap dependence demonstrates that adsorbed RIPs appear to experience a strongly polar environment.

Introduction The kinetics of a variety of photochemical electron transfer (ET) processes in organic systems adsorbed on the surface of silica gel and other solid materials have been studied by many authors in the last Time-resolved experiments were carried out to follow the kinetics of both the ET quenching and charge recombination reactions. In most systems immobilization on the surface of even one of the reactants results in the retardation and modification of both processes relative to analogs in homogeneous solutions. An ET bimolecular quenching reaction on the surface may be dynamic-controlled by the molecular d i f f ~ s i o n ~ ~ , ~ ~static ~ ~ ~Perrin ~ ~ ~ ~type ~ ~ electron ,~-b~t tunnelling mechanisms with critical radii from 1.27 up to 2.7 nm are also ~ ~ m m ~ n . ~A~quite , ~ ,typical ~ , ~ feature , ~ ~ ,of~the surface kinetics resides in its complicated polydispersivepattem. Nonexponential decay kinetics covering several orders of magnitude in a time scale from microseconds up to seconds has been detected for backward ET within some radical ion pairs RIPS)^^,^,^,^^,^^,^,^,^ as well as for pairs of neutral radicalsgaJ1 on several porous surfaces of different nature. The complex dispersive ET kinetics has been treated in terms of a Gaussian distribution of reactivity.2d-ff'e%f,gb~10 Random walk diffusion approaches have been used to describe well at least part of the kinetics of back ET and recombination of pairs of neutral radicals on the porous ~ u r f a c e s . ~ On ~ . the ~ ~ other , ~ ~ hand, ~ both theoretical and experimental work have demonstrated the applicability of the fractal formalism to a variety of problems in surface science, in particular surface kinetics.'* Indeed, the ET kinetics can be controlled by the short-range irregularity of the medium and correlated with the fractal dimension of the

* To whom correspondence should be addressed.

' Centro de Quimica Estrutural.

*

Permanent address: Institute of Chemical Physics, Academy of Sciences of Russia, ul. Kosygina 4, 117334 Moscow, Russian Federation. 0 Centro de Quimica Fisica Molecular. @Abstract published in Advance ACS Abstracts, December 15, 1994.

0022-365419512099- 1267$09.00/0

surface.4b Both fractal approach and Gaussian distribution were successfully used to fit ET kinetics of triplet RIPs on a silica surface.6fJ0 A quantitative assessment of the surface phenomena in ET kinetics requires understanding of the effects imposed by the complex chemical and geometrical features associated with most surfaces. The mobility and the accessibility of the reactant molecules at various surface sites should also be considered. The back ET rate within the photogenerated singlet RIPs can be reduced by 6 orders of magnitude on the surface as compared to solution.7a The effect of retardation of the back ET is often attributed to the immobilization of radicals or electrons by trapping or by electron hopping on different adsorption sites of the inhomogeneous s u r f a ~ e . ~On. ~the ~ ~other ~ hand relatively fast ET kinetics, similar to that in solution, was observed in microsecond time scale within adsorbed geminate triplet RIPs with only a minor contribution of long-lived RIPS.^^,^,^^ While a significant number of photochemical ET kinetic studies on the surface have been described, there is still a lack of data conceming fundamental aspects like the energy gap dependence and values of reorganization energy on the surface, in order to enable the evaluation of related parameters, using existing ET theories.13 This is due in part to the difficulty of studying the kinetics of fast ET reactions occurring on opaque materials. Redox couples of triplet quinones- tertiary aromatic amines, which are well studied as model systems for bell-shaped energy gap dependence of return intersystem ET within triplet RIPs in liquid solution^,^^ are also suitable ones to investigate The formation fundamental features of ET on the and decay of those triplet RIPs were observed on optically transparent Vycor silicate porous glass by conventional laser flash photolysis8 as well as by using diffuse-reflectance laser flash photolysis technique on silica and c e l l u l ~ s eopening ,~~~~ up the possibility to study the energy gap dependence of ET with the aim of estimating the reorganization energy.15 0 1995 American Chemical Society

Levin et al.

1268 J. Phys. Chem., Vol. 99, No. 4, 1995

In this paper, we present a quantification of the energy gap dependence for intersystem ET within triplet RIPs on silica surfaces. Toward this end, diffuse-reflectance laser flash photolysis was used to follow the recombination kinetics of RIPs formed by quenching of triplet excited state electron acceptors (A), 9,lO-anthraquinone (AQ), duroquinone (DQ), or 2,6diphenyl- 1,4-benzoquinone (PQ), by electron donors (D), triphenylamine (TPA), p-methoxy-N',N'-dimethylaniline(MDA), and N,N,N',N'-tetramethyl-p-phenylenediamine(TMPD), with both A and D coadsorbed onto porous silica. The mechanism of RIP formation was also investigated. The set of redox couples (A,D) covered a large energy gap from 0.7 up to 2 eV in order to observe both the normal and inverted regions of ET. The kinetic treatment followed throughout this work was used as an attempt to quantify specific surface kinetic effects, namely, to separate the temporal limit where RIP geminate recombination on silica includes a contribution from a pathway with reversible formation of distance-separated RIPs (with diffusion on the surface) to one where ET is the rate-determining step.

Experimental Section Materials. The commercially available compounds A and D were purified by sublimation or by recrystallization from ethanol. Benzene (Aldrich, spectrophotometricgrade) was used as solvent for the adsorbents. Samples of commercially available silica gel with controlled 14 nm pore size (Aldrich grade 62, 60-200 mesh, BET surface area 300 m2/g, pore volume 1.15 cm3/g) were dried in a vacuum oven at *lo0 "C for at least 48 h and then stored in a desiccator. Sample Preparation. Benzene solutions (2-3 mL) of A (5-10pM) and D (1-75 pM) were added to the silica samples (0.5 g). For all the samples the absorption of A at 353 nm was more than 10 times greater than that of D. Thus, the laser excitation light (353 nm) was only absorbed by A molecules.1° The suspension was stirred and allowed to evaporate slowly. Torr and thermostated Samples were evacuated up to 1 x with the accuracy & l o in a water bath. In the experiments with wet silica 0.1 mL of water was present in the separated bulb within the evacuated system containing a cell with silica. Instrumentation. Ground state absorption spectra of solid samples were obtained using an Olis 14 UV/vis/near-IR spectroscopy operating system with a diffuse-reflectance attachment (90 mm diameter integrating sphere, internally coated with MgO). Experimental details to obtain accurate ground state absorption spectra are given elsewhere.I6 The absorption spectra and decay kinetics of the intermediates on the surface were recorded by laser photolysis in diffuse reflectance setup described e l s e ~ h e r eusing ~ ~ ~the ~ third harmonic (353 nm, 550 d,7 ns) of a Nd:YAG laser (SpectraPhysics, Quanta-Ray GCR-3) as an excitation source. The kinetic spectrophotometer(10 ns resolution) included an averaging system consisting of a Tektronix 2430A digital oscilloscope coupled to a PDP 11/73 microcomputer. 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 (%0.05 T). The photochemical reactions were sufficiently reversible to allow measuring without a flow system. Data Analysis. Data are reported as AJIJo = 1 - J&, where JO and Jt are the light reflection from the sample before and at time t after the laser pulse. The transient signal intensities ( 520%) increased proportionally with increasing laser intensity without changes in the decay kinetics, demonstrating that a saturated plug of ground state totally converted into transient

400

500

600

700

Waueienqth, nm

Figure 1. Transient absorption spectra of duroquinone alone (1) and in the presence of 0.5 pmoUmz p-methoxy-N,N-dimethylaniline (2), triphenylamine (3), or N,N,N',"-tetramethyl-p-phenylenediamine (4) adsorbed on silica and recorded immediately after pulsed laser excitation at 354 nm.

is not produced, thereby supporting the validity of this treatment to analyze the decay of transients rather than that of KubelkaMunk.I7 Each decay curve was recorded at 1024 points, 5 , 10, 20, or 50 ns/point. A total of 2700 points in the time interval covering nearly 100% of the decay were used for the analysis. The fitting procedure was a nonlinear least squares method using the Marquardt algorithm.18 The quality of the fitting was judged by the residuals and autocorrelation as well as by the standard deviation and the statistical parameter of Durbin-Watson. The kinetic data are presented as an average of results based on calculations performed with at least five kinetic curves for each system. All transient decays were well fitted either to the dispersed kinetic model of Alberylg or to a stretched exponential decay,20 with similar goodness of fit.1° Both functions provide an adequate description of the fast components of transient kinetic curves using only two parameters. In the Albery model of Gaussian distribution analysis of first-order rate constants, the absorption intensity is given by

exp[-k,t exp(yx)l dx + pla (1) where AJo is the initial change in light reflection after the laser pulse, k, is the average first-order rate constant, y is the width of the distribution, and pacharacterizes the contribution of the slow component. The stretched exponential function, which contains the same amount of parameters as function 1 and which is able to give the same good fitting of experimentalkinetic curves in the study, is the relation

MjMo = (1 - Ulf) exp(-(kjd)

+ plf

(2)

Relation 2 is derived from the fractal-like behavior with time dependent first-order rate constant k(t) = = fk/d-'. In terms of fractal models the heterogeneity constant, h, is a parameter theoretically related to the spectral fractal dimension of the reaction domain, ranging between 0 (for classical homogeneous kinetics) and 1.20 Both functions I and 2 describe the experimental kinetic curves with fair accuracy and reproducibility.

Results Transient Absorption Spectra. The transient absorption spectra of A in the absence of D on silica have maxima at 420 nm and low-intensity near 600 nm for AQ, 470 nm for DQ (Figure 1, spectrum l), and 600 nm for PQ. These spectra are

J. Phys. Chem., Vol. 99, No. 4, 1995 1269

Triplet Radical Ion Pairs on Silica

similar to the triplet-triplet absorption of A in solutions.21We therefore assign these transient species as being adsorbed 3A. The assignment of diffuse reflectance transients identity based on the analogy with solution spectra is not straightforward. I However, this analogy has been successfully applied for many m systems (for example, see ref 6e and references therein). Differences between transient absorption spectra measured by 'D transmission and diffuse reflectance may occur if there is a specific interaction between the transient species and environment. The 3AQ absorption bands on the silica surface are noticeably shifted with respect to solutions. This is probably due to the strong hydrogen bonding to the surface, as was found for other compounds with basic properties on the surfaces, which ~-- **- - t- - A-- - -4 are capable of hydrogen bond formation.8-10s22 The transient absorption spectra of A in the presence of 20.2 pmoVm2 D on silica obtained immediately after the laser pulse are very similar to the equimolar superposition of the wellknown spectra of A- with maxima at 480 nm and a low-intensity band in the region 2600 nm for AQ,10,21"23 435 nm for DQ,l0v2le and 460 nm for PQIM and D+ with a characteristic maximum at 650 nm and a smaller one at 550 nm for TPA,21e,23b 470 and 510 nm for MDA,23cand 570 and 615 nm for TMPD23dradical 0' cations (see spectra 2-4 in Figure 1). These superpositions 00 0 1 0 2 0 3 0 4 0 5 may be easily observed under the photoexcitation of the given [TPA], u m o l / m L systems in polar solvents where the RIPs dissociate to give the Figure 2. Plots of the rate constants ( k ) of transient absorption decay corresponding radical ion in a bulk solvent or in nonpolar as a function of triphenylamine concentration on the silica surface for solvents where triplet RIPs are formed (for details see ref 14 the duroquinone-triphenylamine system. (A) Average rate constant and references therein). k, at 480 nm (1) and 650 nm (2) without magnetic field and that at 650 nm under a 0.05 T magnetic field (3). (B, C) Time dependent The position of the pronounced absorption band of AQ- near rate constant k ( t ) at t = 20 ns (l), 100 ns (2), and 1 ps (3) observed at 500 nm, being very sensitive to the presence of hydroxy groups 480 nm (B) and 650 nm (C). in the molecular environment, is blue-shifted to 490 nm in alcoholic solutions compared to nonhydroxylic solvents, in (0.8-1.1) x lo7 L/(mol s) if the reaction volume is defined by which it appears at 530-550 nm.14e,21bThis absorption band the product of silica BET surface area and the sum of the van on silica has the maximum at 480 nm and is broader than that der Waals radii of the reactants,24which is assumed to be = in solutions.1° The reasons for a broadening of AQ- absorption 0.6 nm. In solution the quenching of 3A by D is diffusionon silica have been discussed in terms of a distribution of RIPs controlled in most systems. l4 T h e r e f ~ r e , ~the ~ ~correspond~~~.*~ in a variety of different microenvironments.'O Its position near ing slopes observed on the surface seem to be the diffusion480 nm clearly demonstrates the effective participation of radical controlled limit for the given compounds on the silica surface. anions in hydrogen bonding with the silanol groups on the Comparison with the similar quenching data for other systems surface. on silica surfaces shows that the mutual mobility of 3A and D Dynamic and Static Quenching of Quinone Triplets by is significantly higher than that of acridine22or pyreneZ4but Aromatic Amines on the Silica Surface. Formation of RIPs. lower than that of na~htha1ene.l~~ The lifetimes of 3AQ, 3DQ, and 3PQ on the silica surface in the Figure 2B shows the concentration dependence of k(t) for absence of D are equal to 15, 10, and 5 ps, respectively, in the DQ-TPA system calculated at different times from kinetic agreement with the usual observations that lifetimes of triplet curves measured at maximum 3DQ absorption and fitted with states of carbonyl and related compounds on Si02 surfaces are eq 2. The corresponding initial slope decreases with time reflect comparable or even longer than those in inert solvents.8~11b~17b~2*~~ the decrease of dynamic time dependent quenching rate constant The decay of 3DQ is very close to a single exponential, which with time. The fitting of the time dependent quenching rate indicates that any heterogeneity of adsorption sites available to constant in terms of a fractal model shows, at low donor DQ on the silica surface is not reflected in DQ photophysics.6f concentration, a related heterogeneity constant h = 0.25. On the other hand, the decays of 3AQ and 3PQ are not single We did not succeed in observing the kinetics of RIP formation exponential (they follow dispersive kinetics with y values of simultaneously with 3A decay kinetics at any concentration of 1.5 and 1.0, respectively), in accordance with the common D. The estimations have definitely shown that the contribution observation of nonexponential decays of excited states on the of the usual dynamic mechanism of RIP formation similar to surfaces.' This may be due to the operation of some chemical that in solution does not exceed 10% on the given silica surface. quenching mechanisms2' (e.g. reduction) in the case of 3AQ On the other hand, the transient absorption spectra clearly a n d 3PQ, which are more reactive than 3DQ.14g demonstrate that photoexcitation of A in the presence of 20.02 pmoVm2 D on the surface initiates the well-known series of In the presence of D on silica surface, 3A decays become processes, which are similar to those in solutions. They include faster and do not follow in all cases single-exponential kinetics. the efficient ET and formation of RIPS.^,^^,'^.'^ At relatively small amounts of D ( S O . 1 pmoVm2) the dynamic quenching of 3A is observed with close to linear dependences of k, on D concentration (Figure 2A). Initial slopes of the concentration dependences of k, measured at the wavelength where the brackets enclose the RIP. Besides the decrease of of 3A absorption maxima (see plot 1 in Figure 2A) are in the 3A lifetimes, the increase of D concentration on the surface range (1.3-1.8) x l O I 3 m2/(mol s) for all systems studied or

-

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Levin et al. 0

1

G

0

I

00

01

0.2

[Electron a o n o r ] , p , r r > o ~ / m ~

Figure 3. Perrin plots for steady state triphenylamine (2) or N,N,”,”tetramethyl-p-phenylenediamine(1 and 3) quenching of duroquinone triplet yield on silica calculated from the absorption measurements immediately after the laser pulse at the triplet absorption maximum at 480 nm only (1 and 2, H) or at both 480 nm and the radical cation absorption maximum at 650 nm (2, 0 ) or 615 nm (3).

results in the decrease of 3A relative yield and in the increase of the yield of RIPs, both observed immediately after the laser pulse. Therefore, the efficient “fast” mechanism of 3A quenching by D and RIP formation occurs on the surface in time scale less than 10 ns. This static mechanism (see below) of RIP formation is the most important one for these systems on the silica surface. The different mechanisms of RIP formation can be easily distinguished due to the fact that there are spectral regions where the absorption of 3A differs significantly from that of RIP. With the content of D on the surface 2 0.2 pmoY m2 the absorption of 3A becomes negligible and only relatively short-lived RIP transient absorption is observed. On the other hand, as was already mentioned (see above), the kinetics of RIP dynamic formation, which should be simultaneous with 3A decay kinetics, was not observed in a time scale 2 10 ns at any concentration of D. The dependences of 3A and RIP initial yields (both measured immediately after the laser pulse) on D concentration have been studied quantitatively for DQ-TPA and DQ-TMPD systems. The maximum of 3DQ absorption corresponds to the minimum of RIP absorption, and 3DQ as well as DQ- has no absorption in the wavelength range 2550 nm, where only strong absorption of TPA+ or TMPD+ is observed (Figure 1). This minor overlap of the spectra of initial and final transients enables the calculation of relative concentrations of both 3DQ and the RIP on the surface in order to estimate the relative efficiency of RIP formation. The relation between the absorptivity of TPA+ at 650 nm or TMPD+ at 615 nm and 3DQ at 480 nm on the surface is assumed to be equal to that between the corresponding extinction coefficients in solutions, which is equal to 1.5 or 1.2, respectively. 14,21,23 Figure 3 definitely shows that the “fast” quenching follows static kinetics of Pemn25 type, in which a plot of ln(ZdT) versus D concentration is linear, where IO and Z are the absorptions of 3DQ measured immediately after the laser pulse in the absence and presence of D, or I is the difference between 10and the absorption of D+ corrected to the relation between the corresponding absorptivities. The small absorption of RIPS at 480 nm has been also taken into account in I calculations from absorption at 480 nm. The standard form of analysis of Perrin kinetics can be used to calculate the effective radius (Ro)of quenching. Using the approximation that the Si02 surface can be treated as twodimensional, Ro = ( S ~ J ~ N A=) 2.5 ~ ’ ~and 2.1 nm, with the slopes

of Penin plots being SO= 12 m2/pmol and SO= 8.3 m2/pmol for DQ-TMPD and DQ-TPA systems, respectively. This range of critical exchange radii is exceptionally large for a Dexter type ET, which proceeds in a time scale smaller than 10 ns. The approach of random adsorption on the twodimensional surface of silica with the BET area is very rough for the given systems. For instance, only half of the BET surface area of silica is accessible to pyrene.2f Our estimations showed that the saturation limit for TPA, MDA, and TMPD is less than 1 pmoVm2, which corresponds to less than 20% covering of the BET surface. Therefore, the value of RO is in fact in the range 0.9-1.1 nm. This seems to be reasonable in view of the ET time scale, which is smaller than 10 ns on one hand but larger than that of very fast intersystem crossing in quinones (130ps21b,27). Both Pemn plots from 3DQ and RIP absorption data for the DQ-TPA system (Figure 3, plot 2) have very similar slopes (SO = 8.3 m2/pmol). This correspondence indicates that the 3DQ static quenching by D results in RIP formation with 100% efficiency. On the other hand, equivalent slopes for the DQTMPD system are quite different from each other (Figure 3, plots 1 and 3, respectively). The SOvalue obtained from 3DQ absorption measurements is equal to 12 m2/pmol, which is 5 times larger than that extracted from RIP absorption data, indicating only 20% efficiency of RIP formation. The steady state diffuse-reflectance spectrum of the DQ-TPA system on the silica surface shows no evidence for significant ground state complexation. Spectra of DQ-TPA samples were close to the additive spectrum of DQ and TPA when measured separately. On the other hand, a new strong charge transfer absorption near 600nm appears in spectra of DQ-TMPD samples due to ground state complexation, as found in solutions of TMPD and quinones.26 The photoexcitation of charge transfer complexes of quinones and aromatic amines in solution results in formation of singlet RIPs which decay in a picosecond time ~ c a l e . ’The ~.~~ small yield of RIPs from the charge transfer system DQ-TMPD in silica samples may mean that the corresponding singlet excited charge transfer complexes are also short lived and do not result in formation of RIPs with lifetime 2 10 ns. RIP Decay Kinetics on the Silica Surface. The RIP decay kinetics on the silica surface (Figure 4) can be divided in two parts with very different lifetimes. More than 90% of RIPs disappear via a fast process, in a time scale shorter than 2 ps. The order of this time scale is similar to that observed for back ET within the same RIPS in solution^,'^ on the surface of porous glass,8 and on silica with 6 nm pore diameter.l0 Normalized RIP decay curves are independent of the wavelength of observation and of the initial concentration of the RIP. Imposition of a magnetic field leads to a noticeable retardation of the fast component up to 30%, as was repeatedly observed for the back ET within the same triplet RIPs in moderately polar solvents14 as well as on other Si02 surfaces, namely, on Vycor porous glass8 and on silica with 6 nm pore diameter.1° In view of these results as well as our earlier similar results and discussion,8*10it may be definitely concluded that the fast component corresponds to the back ET within triplet RIPs on silica surface. Only a minor part of RIP decays, which is usually less than 5%, corresponds to a slow process with a time scale greater than 100 ps. This component reflects the decay of RIPs which have been separated due to the escape of radical ions from the cage or due to some specificity for the surface charge separation processes, e.g. electron or hole hopping by some specific sites on the silica surface, as was observed for many transient radical pairs on solid ~ u r f a c e s . ~ ~ ~ ~ ~ ~ ~ - ~ I

J. Phys. Chem., Vol. 99, No. 4, 1995 1271

Triplet Radical Ion Pairs on Silica

0.251

A

0.20

.o

ko

0.15

u

0 O

0.10 0 Cn

0.05

I) Q

0.00 -0.05

' B

,

3.2

2 0

3 4

36

38

1ooo/1

Figure 5. Arrhenius temperature dependences for the time dependent rate constant k ( t ) at t = 20 ns (1, E, = 0) and 1 ps (3, E, = 2.0 kcaV mol) and average rate constant k, (2, E, = 1.3 kcaumol) of the decay of the transient absorption at 650 nm of 9,lO-anthraquinone in the presence of 0.5 pmoVm* triphenylamine on the silica surface. 0' -0.5

!

0.0

0.5

1.0

Time,

1.5

2.0

2.5

,US

Figure 4. (A) Decay of the transient absorption of the radical ion pairs on silica obtained for duroquinone in the presence of 0.5 ,umoYm2 p-methoxy-NJV-dimethylaniline (lower curve) or triphenylamine (upper curve) at 470 or 650 nm, respectively, under the 0.05 T magnetic field: (solid lines) approximation by eq 2. The respective parameters are given in Table 1. (B) Plots of corresponding time dependent rate constants k(t) versus time. The decay kinetics of RIPs on silica is strongly nonexponential (Figure 4) and is dependent on the content of D on the surface. Figure 2A (plots 2 and 3) and Figure 2C show the typical dependences of ka and k(t) on D concentration for RIP decay on the silica surface. The decay becomes faster with the increase of D content with a tendency to saturation. At early times, after the laser pulse, the saturation can be achieved at very high D content only (Figure 3C, plot 1). However, at long times the time dependent rate constant is independent of D content in the range 20.1 pmol/m2 (Figure 2C, plot 3). Figure 5 shows the typical temperature dependence of k, and k(t) at different times. The slopes of Arrhenius plots for averaged rate constants give the activation energy E, = 1-2 kcal/mol for all the systems studied. For time dependent rate constants the values of E, are close to zero at early times. This is in agreement with the behavior in homogeneous solution, where the corresponding activation energies for back ET within the same RIPs are close to zero for most of the systems.14dThe time dependent values of Ea increase up to 2-3 kcal/mol for long times. The time dependence of E, observed results in the decrease in reaction heterogeneity constant, k, or in distribution width, y , with increase in temperature. The values of k,, y , and qaas well as kf, h, and qfobtained with the D content on the surface 0.5 pmol/m* are summarized in Table 1. The k, and kf values change 1 order of magnitude with the variation of A and D in a manner similar to that observed in liquid solution^.'^ There is also a tendency for an increase of values of h and y with an increase of ka or kp In other words, the acceleration of back ET within RIPs due to corresponding change in A and D electronic structure results in an increase of dispersive character of the RIP decay curves. It was found in liquid solutions that the rate constant of return ET within the same triplet RIPs depends on the free energy

TABLE 1: Rate Constants (in lo6 s-l) and Parameters Determined for the Decay of Triplet Radical Ion Pairs on the Silica Surface at a D Content of 0.5 pmol/m2 Using Albery (Eq 1) and Fractal (Eq 2) Function# - A G ~ , fractal function Albery function quinone eV kf h Vjr ka Y Pa Triphenylamine 9.10-anthraquinone H=O 2.02 1.4 0.14 0.04 1.3 0.70 0.03 H = 0.05 mT 1.2 0.10 0.06 1.2 0.56 0.04 duroquinone H=O 1.90 4.5 0.36 0.04 2.9 1.0 0.05 H = 0.05 mT 3.1 0.31 0.05 2.1 0.76 0.07 2,6-diphenylbenzoquinone H=O 1.53 6.2 0.42 0.03 4.3 1.2 0.03 H = 0.05 mT 5.0 0.42 0.03 3.4 1.3 0.03 p-Methoxy-NJV-dimethylaniline

9,lO-anthraquinone H=O H = 0.05 mT

1.51

16 0.53 0.02 9.2 1.6 0.03 14 0.55 0.03 7.5 1.7 0.03

duroquinone H=O 1.39 15 0.54 0.01 7.9 1.9 H = 0.05 mT 13 0.57 0.02 7.0 1.9 2,6-diphenylbenzoquinone H=O 1.02 13 0.58 0.01 6.9 2.0 H = 0.05 mT 10 0.57 0.01 5.7 2.0 N,N,W,W-Tetramethyl-p-phenylenediamine

0.01 0.02 0.01 0.01

9,lO-anthraquinone 1.08

8.0 0.48 0.02 5.6 1.5 0.01 7.0 0.49 0.02 4.9 1.6 0.02

H=O 0.96 H = 0.05 mT 2,6-diphenylbenzoquinone H=O 0.59 H = 0.05 mT

4.1 0.46 0.02 3.0 1.3 0.02 3.7 0.47 0.02 2.6 1.4 0.02

H=O H = 0.05 mT

duroquinone

a

2.4 0.37 0.02 1.8 1.0 0.03 2.2 0.36 0.03 1.6 1.1 0.03

Estimated errors 5 10%.

gap between the initial and final states (-AGT)in a bell-shaped manner. l 3 Moreover, these bell-shaped energy gap dependences for ET within triplet RIPs as well as for ET in some singlet systems in solution are generally consistent on a quantitative level with the current ET theories.13 In particular, a'single quantum mode approximation within the nonadiabatic multiphonon ET theory has been successfully sed.'^^^^ The rate of the ET process involving very weakly coupled reactants is expressed in terms of the Fermi golden rule due to the validity of the nonadiabatic approximation:

1272 J. Phys. Chem., Vol. 99, No. 4, 1995

Levin et al. r.3 r

kET= (2n/h)V2F (4) The electronic matrix coupling element V is due to the spin-

0

; 6.8}

orbit coupling (SOC) between radical ions within the RIP for the case of triplet RIPs under di~cussion.'~The element V allows for the dependence of ET on some structural features like spatial separation and orientation of the radicals in the RIP and the nature of the centers and orbitals providing SOC. The Franck-Condon-weighted density of states F depends on properties of both solvent and reactants and provides for the dependence on reaction exergonicity, solvent reorganization energy, and nuclear vibrational frequencies involved in the process. In terms of the single quantum mode approximation the system is divided into two parts: a classical one with lowenergy vibrations and reorganization energy A, and a quantum one with a single mode with frequency w and reorganization energy A". Then the following expression for F is frequently used:

0

y

'

\\

6.2 6.0

i' d

7.2 r

r T

1

__

n=O

where S = A,/hw. The dependences of k, and k(r) at several times on A&T are plotted (Figure 6), where the A&T values are those used before for the same systems in alcoholic solutions (see Table l).l4,l5 The A&T values in hydroxylic solvents seem reasonable for the silica surface which is covered by OH-groups. Bell-shaped energy gap dependences similar to those in s ~ l u t i o n ' are ~ observed fork, and k(t) at early times. These dependences may be well fitted in terms of eq 4 and 5 using reasonable parameters (Figure 6A,B, plots 1 and 2). On the other hand, the rate of intersystemET within adsorbed triplet RIPSon silica under study is independent of the energy gap at long times after the beginning of the reaction (Figure 6B, plot 3), as was already noticed for other silica surfaces.1° Some control experiments have been carried out with wet silica. The presence of physisorbed water on the silica surface results in the increase of ka values for A-TPA systems up to (5-8) x lo6 s-' and in the decrease of k, values for A-MDA systems to (7-5) x lo6 s-l, with the maximum ka value equal to 8 x lo6 s-l for the PQ-TPA system. The width of distribution does not alter significantly, while it remains approximately 2 times larger for the A-MDA system with respect to A-TPA systems. In general the bell-shaped energy gap dependence becomes smoothed with the maximum being shifted to larger lA&~l values.

Discussion The evacuated silica surface if not thermally pretreated is covered with silanol functionalities (typically 5 silanols/nm2) due to the chemisorption of water.29 The adsorption of aromatic and related species on the silica surface is caused by hydrogen bonds of different types, e.g. with hydrogen-bonded or isolated hydroxyl g r o ~ p s Spectroscopic . ~ ~ ~ ~ ~proof ~ ~ of ~ the participation of A in hydrogen bonding in a variety of different microenvironments on the silica surface are shifts and broadening of 3AQ and AQ- absorption The kinetic evidence for an,inhomogeneous silica surface has been derived from the numerous observations of multiexponential decays of phototransients, which were treated with different distribution models, in particular the Gaussian 0ne.2f*10*24,30 The translational diffusion of the molecules under discussion on the silica surface seems to be slow as compared with that in solution. The dynamic quenching rate constants obtained are

-2.2

-1.8 - 1.4 - 1.0 -0.6 F r e e Energy of E l e c t r o n Transfer, eV

Figure 6. Dependences of rate constants of radical ion pair decay on silica on the free energy of electron transfer. (A) Average rate constant k, without magnetic field (0)and under a 0.05 T magnetic field (0). (B) Time dependent rate constant k(t) at t = 20 ns (l), 100 ns (2), and 1 ps (3) under a 0.05 T magnetic field. Lines A and B1,2 were calculated using eqs 4 and 5 with the following parameters: S = 3, hw = 0.15 eV and (A) 1,= 0.95 eV, I l l = 3.0 x eV; (Bl) As = 0.90 eV, I l l = 4.1 x eV; (B2) As = 0.80 eV, I l l = 2.9 x

eV .

more than 3 orders of magnitude smaller than those in nonviscous solvents. Therefore, the time for translational diffusion of quinones and amines of distances of a molecular size is on the order of 1 ps, which coincides with the time scale of crossover from reaction-limited to diffusion-limited back ET within triplet RIPS (in other words, the time when the value of k(t) becomes independent of A&T; see discussion below). Moreover, the Pemn type static nature of the RIP formation within pairs of reactants randomly distributed within some selected areas on the silica surface implies a static interaction within the systems. Indeed, the kinetics of RIP decay depends on the D concentration that is on the structure of the precursor A-D pair. Activation energies for the average rate of back ET are significantly smaller than those expected for diffusioncontrolled processes on the silica ~ u r f a c e . ~All~ those . ~ ~ findings support the feasibility of a static model. A statistical approach with the Gaussian distribution of the activation free energy is able to describe well the kinetics of 3A quenching by D and of back ET within triplet RIPS on the silica surface. A reasonable bell-shaped energy gap dependence similar to those in solution is observed for average rate constants3' (see Figure 6A), indicating that intersystem ET (due to the intermolecular spin-orbit coupling in contact RIPS and the Franck-Condon term included in eq 5 ) is the ratedetermining step in average RIP decay on the surface, as well as in nonpolar or moderately polar solvents,14 rather than the diffusion. On the other hand, the width of the distribution increases with the increase of the ET rate (see values y in Table 1). This is evidence for the relatively fast relaxation, e.g. a diffusion of some kind participating in the process of back ET within RIPs on the silica surface. The presence of a relatively significant magnetic field effect in RIP decay on the surface is strong

Triplet Radical Ion Pairs on Silica evidence for the involvement of the translational diffusion, since the magnetosensitiveS-T evolution mechanisms are active only in the distance-separated RIPs, where the exchange interaction between radicals is negligible.32 The application of the fractal model with the time dependent rate constant gives an opportunity to separate in time processes of different nature that are involved in RIP decay. The simulation of k(t) versus time for RIP decay (Figure 4B) shows the contribution of a fast energy gap dependent ET followed by a slow process, which is practically independent of A and D electronic structure. Energy gap dependences similar to that for the average rate constant are only observed for k(t) at early times (Figure 6B, plots 1 and 2 ) . The rate of intersystem ET within adsorbed triplet RIPs on silica is independent of the energy gap at times 2 0.5 p s (at times larger than the time of diffusion of distances of molecular size; see discussion above) and seems to be controlled by the diffusion on the surface (Figure 6B, plot 3).15 There is a noticeable amount of RIPS for which the rate-determining step is the diffusion and not the back ET. The increase of the role of diffusion in time is demonstrated also by the corresponding increase of activation energy for RIP decay (Figure 5, plots 1 and 3). At the same time it is worthwhile pointing out that even at long times the activation energy for back ET within RIPs is still much less than that for diffusion-controlled quenching or electron-ion recombination on the silica surface ( 2 4 k ~ a l / m o l ) . ~ ~ . ~ ~ ~ On the other hand, the magnetic field effect on k(t) is observed at early times as well as at long times, indicating the participation of molecular dynamics with the time scale shorter than that for intersystem ET within RIPs due to intermolecular SOC. The nature of these dynamics is not very clear at the present stage of our knowledge on the molecular movements on the surface.33 However, the vibrations and rotations of reactants bound to the given group of silanol functionalities may be a reasonable source of fast transitions between contact and distance-separated states of RIPs on the surface. The contribution of distance-separated states of RIPs seems to be small, considering that the magnetic field effects observed are relatively and do not exceed 30% (Table 1). One other reason for the small contribution of the magnetosensitive pathway may be due to modulation and dephasing effects in the interplay of spin evolution and encounters within RIPs. The frequent forced encounters between radical ions within RIPs on the surface, being of frequency comparable to or even higher than that of hyperfine interaction (hfi, x5 x lo7 s-l, time scale of T-S interactions due to typical hfi in a pair of aromatic radicals35),may result in retardation of RIP recombination via the pathway with the hfiinduced T-S transition^.^^ Nevertheless, the most important route of the RIP recombination on the silica surface still remains the intersystem backward ET in the contact state due to the spin-orbit coupling, as was concluded for the same triplet RIPs in other environments.8-10%14 The above model is consistent with the finding that the magnetic field effect is practically independent of time and of the lifetime of RIPs (see parameters in Table l).37 The relatively small value of the heterogeneity constant, h, for the bimolecular dynamic quenching of 3A by D (between theoretical limits for a simple two-dimensional lattice and a critical percolation cluster38)implies that the reaction takes place in a heterogeneous environment which, however, does not present distinct domains. However, the values of h for ET within short-lived RIPs are even larger than 0.5, which is the theoretical asymptotic value for diffusion-limited bimolecular reaction on a one-dimensional lattice.38 The computer simula-

J. Phys. Chem., Vol. 99, No. 4, 1995 1273

.o/

0

I.

00

/

55

60

65

71;

LOS (ko)

Figure 7. Plot of heterogeneity ( h ) versus log k, for the intersystem electron transfer within triplet radical ion pairs on silica measured without magnetic field (0)and under a 0.05 mT magnetic field (0).

tions for both diffusion-limitedand reaction-limitedbimolecular reactions in low-dimensional or fractal media predict a sharp change of h values with local reaction probability (when two reactants collide). The observed dependence of h on k, (k, value is taken as a measure of reaction probability) has a sigmoid form (Figure 7), reflecting the temporal behavior of the back ET, and it is very similar to that predicted the~retically.~~, The crossover from reaction-limited to diffusion-limited kinetics in small-dimensional media is not smooth. The lower the dimensionality of the reaction media, the more likely it is to reach a diffusion-limited reaction kinetics within the time scale of the experiment.38a The back ET within triplet RIPs is limited by diffusion at times greater than 0.5 p s . This time of crossover seems to decrease with an increase of water content on the surface due to the acceleration of diffusion on the wet silica surface since the contribution of the diffusion-controlledprocess increases. The description of the bell-shaped energy gap dependence in terms of expressions 4 and 5 provides the possibility to obtain the corresponding ET parameters (see caption to Figure 6) and to compare them with those for ET in other systems, although this discussion is qualitative and restricted due to the importance of many processes involved. The calculations were performed using constant values of hw and A,, identical to those of the J ~ values of SOC same RIPs in moderately polar s ~ l v e n t s . ' ~The coupling, V, are smaller than those in homogeneous solvents presumably due to the lack of conformations favorable for SOC within RIPs adsorbed on the surface.39 The values of A, obtained with an error of -10% for back intersystem ET on the surface are considerably larger than those in moderately polar liquid alcohols (0.7 eV)13J4and seem to be close to that in strongly polar solvents like acetonitrile and water, where the maximum of bell-shaped dependence is near 1.5 eV.28b,ds40This result agrees with numerous observations that the silica gel surface is equivalent to a strongly polar local e n v i r ~ n m e n t . ~The ~,~~~ uncertainty introduced in As by using the solution data for A&T may be quantitative but does not change the qualitative conclusions. The value of A, seems to increase with the increase of water content on the surface. The polar environment on the silica surface is due to the presence of adsorbed water (for example, see ref 30b). The bell-shaped dependence of k, on wet silica is smoothed due to the increase in the contribution of diffusion.

1274 J. Phys. Chem., Vol. 99, No. 4, 1995

Conclusions In general, it should be noted that the use of diffusereflectance laser flash technique applied to the direct kinetic study of electron transfer kinetics within radical ion pairs on the surface provides important information on the properties of the surface and its capabilities as regards controlling electron transfer processes. We have observed the bell-shaped energy gap dependence for charge recombination on the silica surface, which can supply the primary information about the most important parameters of electron transfer, e.g. reorganization energy of the environment. Two a priori different kinetic models have been used, namely, a static one with Gaussian distribution of first-order rate constants and the stretched exponential one with a time dependent rate constant, and were also shown to be related. This kinetic treatment provided an opportunity to quantify both the average behavior of the system and the specific surface kinetic effects, e.g. the crossover from reaction-limited to diffusion-limited reaction. A reasonable agreement of theory and experiment has been found. Further investigations of the effects concerning the nature and pretreatment of the surfaces, as well as the nature of the free radicals, on the kinetics of the electron transfer within adsorbed radical ion pairs seem to be very informative.

Acknowledgment. This work was supported by CQE-4 and JNICT (STRDA C/CEN/439/92) and in part by the Russian fund of Fundamental research (93-03-4217). P.P.L. thanks NATO for the award of a Senior Research Fellowship. The authors thank Mr. P. Coutinho and Dr. I. Khmelinski for helpful assistance in the data analysis programs. References and Notes (1) See review papers and references therein: (a) Kinetics and Catalysis in Microheterogeneous Systems; Gratzel, M., Kalyanasundraram, K., Eds.; Marcel Dekker: New York, 1991. (b) Photochemistry on Solid Surfaces; Anpo, M., Matsuura, T., Eds.; Elsevier: Amsterdam, 1989. (c) Photochemistry in Constrained and Organized Media; Ramamurthy, V., Ed.; VCH: New York, 1991. (d) Gratzel, M. Heterogeneous Photochemical Electron Transfer; CRC: Boca Raton, FL, 1989. (e) Yoon, K. B. Chem. Rev. 1993, 93, 321. (2) (a) Beck, G.; Thomas, J. K. Chem. Phys. Lett. 1983, 94,553. (b) Liu, X.; Iu, K. K.; Thomas, J. K. J . Phys. Chem. 1989, 93, 4120. (c) Iu, K. K.; Thomas, J. K. J . Phys. Chem. 1991, 95, 506. (d) Pankasem, S . ; Thomas, J. K. J . Phys. Chem. 1991, 95, 6990. (e) Pankasem, S.;Thomas, J. K. J. Phys. Chem. 1991, 95,7385. (f) Krasnansky, R.; Thomas, J. K. J . Photochem. Photobiol. A: Chem. 1991, 57, 81. (g) Liu, X.; Iu, K. K.; Thomas, J. K. Chem. Phys. Lett. 1993,204, 163. (h) Thomas, J. K. Chem. Rev. 1993, 93, 301. (3) (a) Wilkinson, F.; Willsher, C. J. Chem. Phys. Lett. 1984,104,272. (b) Wilkinson, F. J . Chem. Soc., Faraday Trans. 2 1986, 82, 2073. (c) Oelkrug, D.; Krabichler, G.; Honnen, W.; Wilkinson, F.; Willsher, C. J . Phys. Chem. 1988, 92, 3589. (d) Oelkrug, D.; Reich, S.; Wilkinson, F.; Leicester, P. A. J . Phys. Chem. 1991, 95, 269. (4) (a) Slama-Schwok, A.; Avnir, D.; Ottolenghi, M. J. Phys. Chem. 1989, 93, 7544. (b) Samuel, J.; Ottolenghi, M.; Avnir, D. J . Phys. Chem. 1991, 95, 1890. ( 5 ) (a) Basie, A.; Gafney, H. D.; Perettle, D. J.; Clark, J. B. J. Phys. Chem. 1983, 87, 4532. (b) Wolfgang, S.;Gafney, H. D. J. Phys. Chem. 1983, 87, 5395. (c) Shi, W.; Gafney, H. D. J. Phys. Chem. 1988,92,2329. (6) (a) Kuo, P. L.; Okamoto, M.; Turro, N. J. J . Phys. Chem. 1987, 91,2934. (b) Ryan, M. A,; Fitzgerald, E. C.;Spitler, M. T. J. Phys. Chem. 1989, 93, 6150. (c) Kamat, P. V.; Ford, W. E. J . Phys. Chem. 1989, 93, 1405. (d) Gopidas, K. R.; Kamat, P. V. J. Phys. Chem. 1989, 93, 6428. (e) Draper, R. B.; Fox, M. A. J . Phys. Chem. 1990, 94,4628. (f) Kim, Y., 11; Mallouk, T. E. J . Phys. Chem. 1992, 96, 2879. (g) Dutta, P. K.; Turbeville, W. J . Phys. Chem. 1992, 96, 9410. (h) Colon, J. L.; Yang, C.; Clearfield, A.; Martin, C. R. J . Phys. Chem. 1990, 94, 874. (i) Kamat, P. V. Chem. Rev. 1993, 93, 267. (7) (a) Sankararaman, S.;Yoon, K. B.; Yabe, T.; Kochi, J. K. J. Am. Chem. SOC. 1991, 113, 1419. (b) Yoon, K. B.; Huh, T. J.; Corbin, D. R.; Kochi, J. K. J. Phys. Chem. 1993, 97, 6492. (8) (a) Levin, P. P.; Katalnikov, I. V.; Kuzmin, V. A. Bull. Akad. Sci. USSR, Diu. Chem. Sci. 1989,38, 1095. (b) Levin, P. P.; Katalnikov, I. V.; Kuzmin, V. A. Khim. Fiz. 1989, 8, 1604. (c) Levin, P. P.; Katalnikov, I.

Levin et al. V.; Kuzmin, V. A. Chem. Phys. Lett. 1990, 167, 73, (d) Levin, P. P.; Katalnikov, I. V.; Kuzmin, V. A. Chem. Phys. 1991, 154, 449. (9) (a) Levin, P. P.; Vieira Ferreira, L. F.; Costa, S . M. B. Chem. Phys. Lett. 1990, 173, 277. (b) Levin, P. P.; Vieira Ferreira, L. F.; Costa, S . M. B. hngmuir 1993, 9, 1001. (10) Levin, P. P.; Vieira Ferreira, L. F.; Costa, S. M. B.; Katalnikov, I. V. Chem. Phys. Lett. 1992, 193, 461. (11) (a) 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. (b) Kazanis, S.; Azarani, A,; Johnston, L. I. J . Phys. Chem. 1991, 95, 4430. (c) Johnston, L. J.; Scaiano, J. C.; Shi, J. L.; Siebrand, W.; Zerbetto, F. J . Phys. Chem. 1991, 95, 10018. (12) The Fractal Approach to Heterogeneous Chemistry: Polymers, Colloids, Surfaces; Avnir, D., Ed.; Wiley: Chichester, 1989. (13) (a) Marcus, R. A. J. Chem. Phys. 1984, 81,4494. (b) Ulstrup, J.; Jortner, J. J . Chem. Phys. 1975, 63, 4358. (14) (a) Levin, P. P.; Pluzhnikov, P. F.; Kuzmin, V. Chem. Phys. Lett. 1988, 147, 283. (b) Levin, P. P.; Pluzhnikov, P. F.; Kuzmin, V. A. Chem. Phys. Lett. 1988, 152, 409. (c) Levin, P. P.; Pluzhnikov, P. F.; Kuzmin, V. A. Chem. Phys. 1989, 137, 331. (d) Levin, P. P.; Pluzhnikov, P. F.; Kuzmin, V. A. Khim. Fiz. 1989,8, 752. (e) Levin, P. P.; Raghavan, P. K. N.; Kuzmin, V. A. Chem. Phys. Lett. 1990, 167, 67. (f) Levin, P. P.; Raghavan, P. K. N. Chem. Phys. Lett. 1991, 182, 663. (g) Levin, P. P.; Kuzmin, V. A. Russ. Chem. Rev. 1987, 56, 307. (15) Levin, P. P.; Costa, S. M. B.; Vieira Ferreira, L. F. J. Photochem. Photobiol. A: Chem. 1994, 82, 137. (16) Vieira Ferreira, L. F.; Freixo, M. R.; Garcia, A. R.; Wilkinson, F. J . Chem. Soc., Faraday Trans. 1992, 88, 15. (17) (a) Oelkrug, D.; Honnen, W.; Wilkinson, F.; Willsher, C. J. J . Chem. Soc., Faraday Trans. 2 1987,83,2081. (b) Drake, J. M.; Levitz, P.; Turro, N. J.; Nitsche, K. S.; Cassidy, K. F. J . Phys. Chem. 1988, 92, 4680. (18) Marquardt, D. W. J. Soc. Indust. Appl. Math. 1963, 11, 431. (19) Albery, W. J.; Bartlet, P. N.; Wilde, C. P.; Danvent, J. R. J. Am. Chem. Soc. 1985, 107, 1854. (20) (a) Klafter, J.; Drake, J.; Blumen, A. In Chapter 14 of ref la. (b) Prasad, J.; Kopelman, R. J. Phys. Chem. 1987, 91,265. (c) Siebrand, W.; Wildman, T. Acc. Chem. Res. 1986,19,238. (d) Klymko, P. W.; Kopelman, R. J . Phys. Chem. 1983, 87, 4565. (21) (a) Hulme, B. E.; Land, E. J.; Phillips, G. 0.J . Chem. SOC., Faraday Trans. 1 1972, 68, 2003. (b) Hamanoue, K.; Nakayama, T.; Yamamoto, Y.; Sawada, K.; Yuhara, Y.; Terenishi, H. Bull. Chem. Soc. Jpn. 1988, 61, 1121. (c) Land, E. J. Trans. Faraday Soc. 1969, 65, 2815. (d) Kemu, D. R.; Porter, G. Proc. R. SOC.A 1971,326, 117. (e) Amouyal, E.; Bensakon, R. J. Chem. Soc., Faraday Trans. 11977, 73, 1561. (22) Oelkmg, D.; Uhe;S.; Wilkinson, F.; Willsher, C. J. J . Phys. Chem. 1989, 93, 4551. (23) (a) Carlson, S. A.; Hercules, D. M. Photochem. Photobiol. 1973, 17, 123. (b) Burrows, H. D.; Greatorex, D.; Kemp, T. J. J . Phys. Chem. 1972,76,20. (c) Das, P. K.; Bobrowski, K. J . Chem. Soc., Faraday Trans. 2 1981, 77, 1009. (d) Albrecth, A. C.; Simpson, W. T. J . Am. Chem. Soc. 1955, 77, 4454. (24) Marro, M. A. T.; Thomas, J. K. J . Photochem. Photobiol. A: Chem. 1993, 72, 251. (25) (a) Pemn, J. C. R. Acad. Sci. 1929, 178, 1978. (b) Frank, J. M.; Vavilov, S. I. Z. Phys. 1931, 69, 100. (c) Inokuti, M.; Hirayama, F. J . Chem. Phys. 1965, 43, 1978. (26) Peover, M. E. Trans. Faraday Soc. 1962, 58, 1656. (27) (a) Kobashi, H.; Funabashi, M.; Kondo, T.; Morita, T.; Okada, T.; Mataga, N. Bull. Chem. Soc. Jpn. 1984, 57, 3557. (b) Kobashi, H.; Funabashi, M.; Shizuka, H.; Okada, T.; Mataga, N. Chem. Phys. Lett. 1989, 160, 261. (28) (a) Closs, G. L.; Miller, J. R. Science 1988,240,440. (b) Mataga, N.; Asahi, T.; Kanda, Yu.; Okada, T.; Kakitani, T. Chem. Phys. 1988,127, 249. (c) Asahi, T.; Ohkohchi, M.; Matsusaka, R.; Mataga, N.; Zhang, R. P.; Osuka, A.; Maruyama, K. J. Am. Chem. Soc. 1993, 115, 5665. (d) Gould, I. R.; Young, R. H.; Moody, R. E.; Farid, S. J . Chem. Phys. 1991, 95, 2068. (e) Harrison, R. J.; Peace, B.; Beddard, G. C.; Cowan, J. A.; Sanders, J. K. M. Chem. Phys. 1987, 116, 429. (f) Harriman, A,; Heitz, V.; Sauvage, J.-P. J. Chem. Phys. 1993, 97, 5940. (29) (a) Hair, M. L. Infrared Spectroscopy in Surface Chemistry; Marcel Dekker: New York, 1967. (b) Kiselev, A. V.; Lygin, V. I. Infared Spectra of Surface Compounds; Wiley: New York, 1975. (c) Iler, R. K. The Chemistry of Silica, Solubility, Polymerization, Colloid and Surface Properties, and Biochemistry; John Wiley: New York, 1979. (30) (a) de Mayo, P.; Natarajan, L. V.; Ware, W. R. In Organic phototransformations in nonhomogeneous media; Fox, M. A,, Ed.; Symposium Series No. 278; American Chemical Society: Washington, DC, 1985; p 1. (b) Liu, Y. S.;de Mayo, P.; Ware, W. R. J . Chem. Phys. 1993, 97, 5987, 5995. (31) The energy gap dependence fork, was not observed on silica with 6 nm pore size for relatively small D content on the surface. However, it was obeyed for k(t) at early times.I0 (32) Salikhov, K. M.; Molin, Yu. N.; Sagdeev, R. Z.; Buchachenko, A. L. Spin Polarization and Magnetic Effects in Radical Reactions; Elsevier: Amsterdam, 1984.

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Triplet Radical Ion Pairs on Silica (33) (a) Preliminary results33bshow that the kinetics of RIP decay is only slightly dependent on the pore size of silica in the range of pore diameters from 6 up to 100 nm. Therefore, the nature of the silica surface on the molecular size scale is important for the geminate RIP behavior rather than the surface geometry in larger scales. (b) Levin, P. P.; Costa, S. M. B.; Vieira Ferreira, L. F. Unpublished results. (34) The magnetic field effects may exceed 300% in recombination of triplet radical pairs in micelles or in recombination of some triplet biradicals, where distance-separated states of radical pairs are very important.34 (35) (a) Levin, P. P.; Shafirovich, V. Ya.; Kuzmin, V. A. J. Phys. Chem. 1992, 96, 10044. (b) Shafirovich, V. Ya.; Batova, E. E.; Levin, P. P. J . Phys. Chem. 1993, 97, 4877. (36) (a) Tarasov, V. F.; Ghatlia, N. D.; Avdievich, N. I.; Shkrob, I. A,; Buchachenko, A. L.; Turro, N. J. J . Am. Chem. SOC. 1994, 116, 2281. (b) Tarasov, V. F. Unpublished results.

(37) The other common model which is capable of grounding the small magnetic field effects is one in tenns of slow transitions between contact and distance-separated RIPS. However, this model implies the dependence of magnetic field effects on time and on the lifetime of RIPS. (38) (a) Shi, Z.-Y.; Kopelman, R. J . Phys. Chem. 1992, 96, 6858. (b) Taitelbaum, H.; Kopelman, R.; Weiss, G. H.; Havlin, S.Phys. Rev. A 1990, 41, 3116. (c) Kopelman, R. Science 1988, 241, 1620. (39) The SOC within RIPS should be very sensitive to the mutual orientation of radicals within RIP (see ref 13c and references therein). (40) (a) Ohno,T.; Yoshimura, A.; Mataga, N.; Tazuke, S.; Kawanishi, Yu.; Kitamura, N. J. Phys. Chem. 1989,93,3546. (b) Ohno, T.; Yoshimura, A.; Mataga, N. J . Phys. Chem. 1990, 94, 4871.

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