J. Phys. Chem. 1994,98, 8108-81 13
8108
Photoinduced Electron Transfer in Porous Glass Martin Braun and Harry D. Gafney’ Departments of Mathematics and Chemistry, City University of New York,Queens College, Flushing, New York 1 1 367 Received: February 16, 1994; In Final Form: June 14, 1994’ Photoinduced disproportionation of Ru(bpy)3’+ cation exchanged onto porous Vycor glass occurs within a fixed array of immobilized adsorbates by means of a mobile, photodetached electron. As the moles of complex adsorbed increases, the mean separation between the adsorbed ions decreases and approaches the electron migration distance. As a result, the quantum efficiency of disproportionation initially increases with loading, reaches a maximum, and then declines as adsorbate spacing approaches the distances over which the thermal back-reaction occurs. Surprisingly, expressing this Gaussian-like dependence of the quantum efficiency on initial Ru(bpy)3’+ loading as a difference between the probability of another redox partner within the electron migration distance, less the probability that the redox partners are within the thermal back-reaction distance, where the probabilities are expressed as exponentials, fails to agree with the experimental data. Introducing a fractal dependence by scaling the distance parameters also fails to achieve agreement. Results presented here show that agreement with the experimental data requires considering the mechanism of electron conduction. In the absence of an accessible conduction band, electron migration on this glass is thought to occur via the population of surface acceptor sites. Treating these sites as shallow energy wells that reduce the energy and migration distance of the photodetached electron leads to excellent agreement with the observed dependence.
Introduction Adsorption onto a support changes the photophysics and photochemistry of organic and inorganic In spite of the obvious differences in adsorbate and adsorbent, a common underlying strategy is to exploit the microstructure and microenvironment of the support to impose some control on the reaction system. In the photoredox chemistry of R ~ ( b p y ) 3 ~and + its analogues, for example, heterogeneous media are being examined as a means to curtail thermal reversibility and promote a multielectron-transfer chemistry.2,6,7,1*-22 Adsorption onto a semiconductor and optical excitation of the adsorbed complex leads to electron injection into the conduction band of the s e m i c o n d u c t ~ r . ~Band ~ - ~ ~bending promotes charge separation, and an accumulation of electron density at a removed reaction site promotes a subsequent multielectron-transfer chemistry. Supports that are not usually thought of as electron conductors, i.e., where population of a conduction band is not energetically possible, also promote charge ~eparation.2.6,~,~*’1~’Z* In these cases, the increase in lifetime of the charge-separated state appears to arise from the local order imposed by the medium. Zeolite L particles, for example, spontaneously organize a molecular triad, resulting in a significant improvement in the lifetimeof thechargeseparated state.29930 A spontaneous partitioning of R ~ ( b p y ) 3 ~ + and MV2+ in Corning’s Code 7930 porous Vycor glass (PVG) leads to a pronounced increase in the lifetime of the photoredox product^.^ Increases in photoproduct lifetime also occur in the photoinduced disproportionation of R~(bpy)3~+cation exchanged onto the glass.6 In the latter case, the reaction mechanism is essentially equivalent to that in aqueous s0lution.3~ Biphotonic excitation of the adsorbed complex, designated R~(bpy)3~+(ads), leads to ionization, Ru(bpy),’+(ads)
+ 2hv
-
+
R ~ ( b p y ) ~ ~ + ( a d se-)
(1)
and the photodetached electron, e-, reduces another R ~ ( b p y ) 3 ~ + (ads)
e Abstract published in Aduance ACS Absrracrs, August 1, 1994.
0022-3654/94/2098-8 108%04.50/0
lying within the electron migration distance. Unlike aqueous solution, however, where diffusional randomization limits redox product lifetime to ca. 2 ms,31 the reaction products in the glass persist for hours in vacuo at room temperature.6 R ~ ( b p y ) 3 ~cation + exchanges onto PVG by displacing the slightly acidic silanol protons? and theemission polarization ratio for the adsorbed complex, 0.16 f 0.02 at 465 nm at temperatures as high as 90 OC, implies that R~(bpy)3~+(abs) is immobile during the excited-state lifetime, 740 f 20 ns.32 Macroscopic diffusion of both reactants and cationic products is severely curtailed, and in both cases where charge separation has been found in these glasses, the reactants and products remain cationic throughout the reaction sequence, suggesting that both remain bound to the sites onto which they originally cation exchanged.6.’ Consequently, electron transfer occurs within a fixed array of immobilized reactants by means of a mobile, photodetached electron, and charge separation occurs when the migration distance of the photodetached electron, which is ca. 50 A in this glass, exceeds the distance over which the thermal back-reaction occurs.6 The latter will reflect the specificredox partners, but the largest driving force that will be encountered following electron-transfer quenching of * R u ( b ~ y ) 3 ~is+ that between the disproportionation products. In PVG, the thermal back-reaction
requires a spacing of 1 1 3 A between the reactants. Since the electron migration distance exceeds that for reaction 3, charge separation occurs and the redox products are stable. Loss of adsorbate mobility converts the biomolecular photoredox chemistry of adsorbed Ru(I1) diimines from that dependent simply on loading to that dependent on adsorbate distribution, i.e., the mean separation between the immobilized redox ~artners.63~Certainly in the systems we have examined, the glass spontaneously organizes the reaction system and, in so doing, ultimately determines the efficiency of charge separation. The same condition occurs on any support where, in the absence of a specific low-energy pathway or an applied bias, both of which vector the electron in a specificdirection, electron transfer occurs 0 1994 American Chemical Society
Photoinduced Electron Transfer in Porous Glass between separated, immobilized sites. The support determines adsorbate distribution and, in so doing, charge separation efficiency. Adsorbate distribution is proportional to, but not necessarily a linear function of, the amount adsorbed. Porous supports, in particular, function as chromatographic substrates where adsorbate distributions are functions of adsorbate size, lability, and ionic p o t e r ~ t i a l . ~ Different J ~ > ~ ~ redox partners can partition within the support, and electron transfer occurs within the boundary region common to the different partner^.^ Attempts to model these systems are in fact attempts to model reactant distributions within the matrix. In this sense, the disproportionation reaction is unique since electron transfer occurs via a mobile photodeteched electron within a fixed array of identical redox partners that possess identical adsorption characteristics. As a result, the average spacing between the adsorbates and, in turn, the quantum yield of charge separation can be expressed as functions of initial loading. Experimentally, the quantum efficiency of disproportionation in this glass, @dr exhibits a Gaussian-like dependence on initial Ru(bpy)32+(ads) loading.6 As loading increases, the average spacing between the adsorbatesdeclines, and thequantum efficiency increases, reaching a maximum at an adsorbate spacing that corresponds to the mean electron migration distance in the glass.6 Further increases in loading reduce the spacing between the redox products, increasing the probability of the thermal backreaction and reducing @& It is certainly possible, within this interpretation, that @d is the probability that another redox partner lies within the electron migration distance, less the probability that the redox partners are within the thermal back-reaction distance, and these probabilities are exponential functions af adsorbates spacing which, since the redox partners are identical, are inversely proportional to initial loading. As described here, however, @d cannot be expressed as the difference of two exponential functions. In fact, no combination of exponentials even comes close to approximating the observed dependence of @d on initial loading. Differentiating electron-transfer distance from adsorbate spacing by introducing a fractal dependence36to scale the distance dependence also fails to achieve agreement with the experimental data. Agreement requires consideration of the mechanism of electron transport on the glass surface. In the absence of an energetically accessible conduction band, electron migration on this glass is thought to occur via surface conduction, where the photodetached electron populates surface acceptor sites.6-35 Treating these sites as shallow energy wells35 that reduce the energy and migration distance of the photodetached electron leads to excellent agreement with the observed dependence on initial loading.
Experimental Section Materials. [Ru(bpy)3]Cl2 was prepared by the method of Palmer and Piper and twice recrystallized from distilled water.37 Absorption, emission, and resonance Raman spectra of aqueous solutions of the complex agreed with published s p e ~ t r a . Code ~?~~ 7930 porous Vycor glass (Corning Inc.) in the form of 25 X 25 X 2 mm3 polished plates was extracted and calcined as previously described.6-12 The moles of complex adsorbed per gram of PVG was determined spectrophotometrically, and the water incorporated during impregnation was removed under vacuum at room temperature.6 The samples examined in these experiments contain from 3.1 X 10-7 to 1.22 X 10-4 mol of R ~ ( b p y ) ~ ~ + ( a d s of )/g PVG. Photolysis Procedures. Impregnated samples were mounted in rectangular quartz or Pyrex cells and irradiated under vacuum (p I104 Torr) with 458-nm light from a Spectra Physics Ar+ laser. Quantum yields of disproportionation, @d, were calculated from the initial rate of [Ru(bpy)2(bpy-)]+(ads) formation, measured during photolysis at 5 10 nm.6 The absorbed intensity was calculated from the incident intensity, measured with a Laser
The Journal of Physical Chemistry, Vol. 98, No. 33, 1994 8109 Precision Model Rj 7100 meter, using the average absorbance of R~(bpy)3~+(ads) during photolysis.
Results and Discussion PVG is obtained by cooling a borosilicate melt below its phase transition temperature.384 The boron oxide-alkali oxide phase separates, and acid leaching of this phase yields a material composed of Si02 nodules with an intervening, random, threedimensional array of interconnected pores.41 This is an amorphorous material, and the irregularity of the surface usually has a negative connotation with respect to organizing a reaction system or describing the chemistry occurring on it. Amorphorous, however, is a length-dependent term, whose significance depends on the dimensions of the events under consideration.20J6 Smallangle X-ray (SAXS) and neutron (SANS) scattering from PVG yield spectra characteristic of a spinodally decomposed material with a correlation length, i.e.,a length of uniform density, of 242 f 8 A.42,43Porous glasses do not possess the geometric regularity of a crystalline substrate, but with respect to the distances over which electron transfer occurs, ca. 50 %I in ,5302-based materials,6*u6 porous Vycor glass is treated as a uniform surface. The surface on which the chemistry occurs consists of B203 Lewis acid sites and Si-OH Bronsted acid groups,3*4 to which R ~ ( b p y ) 3 ~cation + exchanges by displacing the slightly acidic proton.6 The silanol groups are distributed throughout the glass with an average coverage of 4-7/100 A2.47Although the highest coverage is in the pores, the complex does not uniformily impregnate the entire sample. The narrower constrictions within the pore structure and the complex's size curtail diffusion into the interior of the glass, so that with a typical exposure time of 1 2 4 h, the complex impregnates to depths of 10.5 f 0.1 mm in a S m m - t h i c k ~ a m p l eNevertheless, .~ absorption spectra recorded at different locations on the individual samples, and as functions of relative thickness, establish a uniform distribution of complex within the impregnated volumes. Taking 7.4 A as the radius of R~(bpy)~2+(ads)and 183* lSmZ/gand 1,38g/mLasthesurface area and density of the glass, respectively, impregnations of to 1.22 X 10-4 mol/g correspond to surface coverages ranging from 5 1% to monolayer coverage in the impregnated volumes. Cation exchanged onto PVG, R ~ ( b p y ) ~ ~ + ( aexhibits ds) an emission polarization ratio, 0.16 f 0.02, that is independent of temperature up to 90 OC and equivalent to that in 77 K hydrocarbon g l a ~ s e s . ~This ~ - ~implies * that the adsorbed complex is immobilized during the excited lifetime, 740 f 20 ns, and absorption measurements indicate that macroscopic diffusion of both reactants and products is severely curtailed. In fact, in all cases where charge separation has been observed in this electron transfer appears to occur within a fixed array, where the reactants and products remain cationic and apparently fixed to the sites onto which they originally adsorbed. Half of the moles of adsorbed complex, n, are uniformly distributed within a volume defined by the geometric area of one side, A, and the penetration depth, d. Within this volume, the mean separation between the R ~ ( b p y ) ~ ~ + ( aions d s ) is given by (AdpS/Nn)ll2,where p and S are the density and surface area of the glass and N is Avogadro's number. Mean separations calculated from this expression as a function of initial loading and the measured values of the quantum yield of disproportionation, @d, at the different loadings show that as the mean separation between the R ~ ( b p y ) ~ z + ( a dions s ) declines, @d increases (Figure 1). The maximum value occurs at a loading corresponding to a mean separation between the ions of 50 f 10 A, which is in excellent agreement with previous determinations of the electron migration distance in PVG and Si02,6+- the principal constituent of PVG. Further increases in loading reduce the spacing between the redox products and increase the probability that the photoredox products are within the thermal back-reaction distance. As a result, the quantum efficiency of a net reaction
Braun and Gafney
8110 The Journal of Physical Chemistry, Vol. 98, No. 33, 1994
given by (AdpS/Nn)1/2.y reflects the distribution of the adsorbate on the surface, which in turn is a function of support surface area, number and distribution of binding sites, and the mobility of the electron on the support surface. In essence, y is a measure of how effective the support is in achieving charge separation and, therefore, potentially useful in comparing different supports. The probability that an electron travels a distance r without interacting with another R ~ ( b p y ) ~ ~ + ( aisd s )
0 0
150
15
5?
F(r) = exp[-ar]
x
Q
(5)
Equation 5 arises from the fact that for small Ar F(r+Ar) = F(r)[ 1 - u A ~ ]
(6)
where 1 - a b is the probability that the electron moves a distance Ar without interacting. Rearranging equation 6 yields
[F(r+Ar) - F ( r ) ] / k = -aF(r) and taking the limit of (7) as Ar -7
-
(7)
0 yields
-5
F'(r) = -aF(r)
Log(mol adsorbedg)
Figure 1. Quantum yields of disproportionation ( 0 ) and calculated adsorbate spacings (A) (see text) as functions of Ru(bpy)j2+loading.
Quantum yields ( 0 ) calculated from the simple difference of the probabilities and by the equation @iP(rl) (m) (see text).
(8)
Equation 8 implies that
-
F(r) = c exp[-ar]
(9)
for some constant c. Since the probability of not interacting goes to one as r 0, c = 1 and
F(r) = exp[-ar]
Figure 2. Schematic representation of electron transfer, where Ru(bpy)32+(ads)undergoing photoionization is at the center and re and ?th
represent the distances over which electron migration and the thermal back-reaction occur, respectively. initially increases, reaches a maximum value, and then declines and approaches zero as loading approaches monolayer coveragee6 Electron transfer within a fixed array of immobilized adsorbates by means of a mobile, photodetached electron is represented schematically in Figure 2. R~(bpy)~2+(ads) undergoing photoionization (reaction 1) is at the center, and the radii re and r t h represent the maximum migration distance of the photodetached electron and the distance over which the thermally activated backreaction occurs, respectively. If the photodetached electron encounters another adsorbed ion within rth, reduction occurs, but the redox products, R~(bpy)3~+(ads) and [Ru(bpy)~(bpy-)]+,are transient since the redox products are within the thermal backreaction distance. Similarily, no net reaction occurs when the average spacing between the adsorbates exceeds the electron migration distance, re. Consequently, the radii define an annulus about a specific adsorbate (Figure 2) in which charge separation occurs, and photolysis leads to a net reaction. The probability that the photodetached electron interacts with another Ru(bpy)32+(ads) within a distance Ar is uAr, where a = exphr,I/r,
(4)
and rl is the average distance between the R ~ ( b p y ) ~ z + ( a dions, s)
(10)
where a is given by eq 4. F(r) is the probability that the electron travels a distance r without interacting, whereas the probability that it does interact is 1 - exp[-ar]. Since the correlation length of PVG, 242 f 8 A,42-43 exceeds the electron migration distance,6-the glass is considered a nondescript, flat surface. Consider the simplest situation first, where the glass defines the array but does not itself participate in the electron-transfer process or the thermal back-reaction. Thequantumefficiency of net electron transfer, a d , is = @iP(rl), where @i is the quantum efficiency of photoionization of the Ru( b p ~ ) ~ ~ + ( aatd thecenter s) (Figure 2) andP(rl) is the probability of the photodetached electron encountering another Ru(bpy)32+(ads) within the annulus defined by re and rth. The probability of an interaction within the annulus, P(rl), is the probability of an interaction within re, 1 - exp[-ar,], minus the probability of an interaction within rth, 1 - exp[-arth], or
P ( r l ) = exp[-ar,,]
- exp[-are]
(1 1)
were a = exp[-yrl]/rl. The function P(rl) exhibits the general Gaussian-like dependence shown in Figure 1, but surprisingly, @d = @iP(rl)fails to reproduce the experimental data with the appropriate spacing, rl, as given by (AdpS/Nn)1/2.In fact, regardless of the values of ai, y, re,or rth, as shown in Figure 1, the function @$(?I) fails to reproduce the maximum and the steep narrow dependence of @d on Ru(bpy)s2+ loading. In the derivation of eq 11,it was implicitly assumed that electron transfer occurs on a nondescript, flat surface where the electrontransfer distance is equivalent to the mean separation between the reactants. In fact, the surface is highly irregular,41 and since the electron is a relatively small particle, its actual migration distance may reflect the irregularity and topology of the surface over which it migrates.36 Whether porous Vycor glass is fractal remains to be established, since Drake and co-workers have shown that the variation in the Hausdorff dimension describing porous glass surfaces reflects the mode of interaction between the adsorbate probe and the substrate.49 Nevertheless, if one considers
Photoinduced Electron Transfer in Porous Glass
The Journal of Physical Chemistry, Vol. 98, No. 33, 1994 8111
the surface fractal, at least on the dimensions of the photodetached electron, Avnir has shown that distance-dependent events can be treated by scaling the distance dependence as a power, Le., rln.36 Introducing a fractal dependence in eq 11 by expressing r as a power, however, fails to produce agreement with the experimental data. In scaling the distance dependence, it should be noted that both reactions were scaled identically. This is an approximation since reactions 1 and 2 are photochemically driven, whereas reaction 3 is thermally driven. Nevertheless, the assumption appears justified. Since the leW&i& of Figure 1 (low loading) principally reflects photoinduc& electron transfer, whereas the right side (higher loading) reflects the thermal back-reaction, significant differences in the distance dependencies would skew the observed dependence. Judging from the symmetry of Figure 1, scaling the distance dependencies in the same manner appears justified. To obtain agreement with the data, it is necessary to consider the mechanism of electron transport on the glass surface. In the absence of an energetically accessible conduction band, electron migration on these glasses is thought to occur via surface conduction, where the photodetached electron populates intermediate surface acceptor sites.35 The availability and Lewis acid character of the B2O3 sites present in PVG suggests that these sites could act as an electron acceptor, but in forming a radical species would remain sufficiently reactive to liberate the electron on thermal activation. Selective NH3 adsorption experiments, however, establish that the surface acceptor sites are nor the B2O3 Lewis acid sites.50 The specific nature of the acceptor site is not known, but the temperature dependence of +d indicates that these are shallow energy wells, 16.87f 0.1 1 kcal/mol, from which the photodetached electron can be thermally a c t i ~ a t e d . ~ ~ The probability that an electron populates a surface acceptor site within a small distance Ar is bAr, where
Figure 3. Schematic representation of an electron interacting with Ru(bpy)32+(ads)in the annulus rth + z and rth + z + PI without populating a surface acceptor site.
Figure 4. Reduced electron migration distance, r:, as a result of the photodetached electron falling into a surface acceptor site.
and s1 is the average spacing between the acceptor sites. In the absence of information on the number and spacing of these sites in the support, this approach is useless. However, the probability of an electron traveling a distance r without falling into a surface acceptor site is exp[-br]. The probability that the electron interacts with a R ~ ( b p y ) ~ ~ + ( aind sthe ) annulus between rth + z and r t h I + Az without populating a surface acceptor site is
+
P = exp[-a(rth
+ z)] exp[-b(rth + z)]aAz = a exp[-(a
+ b)rth] exp[-(a + b)z]Az
(13)
where z is the distance traveled in the annulus (Figure 3). Summing 13 for all values of z between 0 and re - r t h gives
P = a exp[-(a
+ b ) r t h ] ~ F ‘ e ~ p [ - ( u+ b)z] d z
(14)
Integrating (14) and combining terms yields
P = a/(a
+ b)(exp[-(a + b)rth] - exp[-(a + b)r,])
(15)
Figure 5. Schematicrepresentation of the angular dependenceof random electron migration after leaving a trap when ro < rlh.
leaving the surface acceptor site is random, we must integrate over all values of 8 (Figure 5 ) . Furthermore, in the case where ro > rth, the possibility that the electron interacts in the short side of the annulus or the long side of the annulus (Figure 6) must also be considered. Approximating this integral by the average of its value where 8 = 0 and 8 = A and simplifying yields
which reduces to eq 11 when b = 0. To compute the probability that an electron interacts with a R~(bpy)~Z+(ads) between r t h and re after populating a surface acceptor site, two simplifying assumptions are made. First, the electron populates only one surface acceptor site, and second, since these sites are energy wells with a depth of ca. 7 k c a l / m ~ l , ~ ~ activation from the site reduces the electron’s energy and in turn reduces its maximum travel distance tor: (Figure 4). Even with these simplifying assumptions, however, the situation remains complex since all cases where the electron falls into a site after Equation 16 represents the probability that the photodetached traveling a distance ro < r t h and ro > r t h have to be treated electron will interact with another Ru(bpy)32+(ads) within the separately. Also, since the direction of electron migration after
Braun and Gafney
8112 The Journal of Physical Chemistry, Vol. 98, No. 33, 1994
a maximum, and then declines with increasing loading. Although the reaction occurs within a fixed array of adsorbates by means of a mobile, photodetached electron, the quantum efficiency of the reaction cannot be expressed as the difference between exponential functions describing the probability of the photodetached electron encountering another adsorbate and the probability that the redox partners are within the thermal backreaction distance. Consistent with the experimental data, which suggest electron migration via population of a surface acceptor site, agreement requires the population of a single surface acceptor site, and these sites can be treated as a shallow energy well that reduces the energy and migration distance of the photodetached electron. Figure 6. Schematic representation of the angular dependence when ro > rth: sas denotes the surface acceptor site, ss the short side of the annulus, and Is the long side of the annulus.
annulus defined by rth and re either directly or after falling into one (1) surface acceptor site. The function OiP(rl), with P(rJ given by eq 16 now yields excellent agreement with the observed values of @d (Figure l ) , and fitting the function to the observed data by means of the method of steepest descent,s' the function yields values of the individual parameters that agree with prior estimates. Fitting thedatawitheq 16,forexample,yields16 X 1Wforthequantum efficiency of Ru(bpy)32+(ads) photoionization, @ i . Meisel and co-workers report a quantum yield of [Ru(bpy)2(bpy-)]+ formation, a lower limit of the photoionization efficiency, of (1.5 f 0.2) x 10-3 in aqueous solution, while analyses of flash photolysis experiments carried out in this laboratory yield L 1.2 X 10-3 for ai. Unfortunately, unique estimates for a, b, re, r:, and rth are unaccessible since P (eq 16) exhibits a degeneracy, where the function is unchanged when a Aa, b Ab, re re/A, r i ?\/A, and rth rth/X. Ratios of the parameters, however, are unaffected by the degeneracy, and fitting theobserveddependence yields -2.5 for re/rth. re is the migration distance of the photodetached electron on PVG, which Wong and Allen originally reported to be 30 A. Subsequent estimates yield 49 A, while Berglund and Powell report an electron migration distance in Si02, the principal constituent of PVG, of 34 A. Previous estimates in this laboratory indicate that, for the thermal backreaction to occur, the distance between the Ru(bpy)33+(ads)and [Ru(bpy)2(bpy-)]+(ads) coordination shells, i.e,rth, must be 1 1 3 A. Using thelatter value, and the averageof thereported electron migration distances, re = 38 A, yields 2.9 for re/rth,which agrees with the calculated ratio of 2.5. The parameter a is proportional to the probability of encountering another ion, whereas b reflects the probability of the photodetached electron falling into a trap. As such, a reflects the distribution of the redox partners in the matrix and their spacing on the glass surface. In turn, these are related to the mechanism by which the complex binds onto the glass surface, the number and distribution of the binding sites, and the substrate morphology. The parameter b reflects the probability of the photodetached electron falling into a surface acceptor site. Unlike a, which reflects the binding and distribution of the photoredox reagent on the substrate, b is a function of the substrate itself and the mechanism by which the substrate conducts the photodetached electron on the substrate surface. Currently, we are working on eliminating the previously described degeneracy. Once this is accomplished, the algorithm will be implemented to compute a definitive b for a given surface. This will then enable us toevaluate different porous glasses, such as PVG and acid- or base-catalyzed xerogels, with respect to promoting charge separation.
-
- - -.
+
Conclusion The quantum yield of the photoinduced disproportionation of Ru(bpy)32+cation exchanged onto PVG initially increases, reaches
Acknowledgment. Support of the research by the BHE-PSC Research Award Program of CUNY and the National Science Foundation (CHE-89 13496) isgratefully acknowledged. H.D.G. also thanks Corning Inc. for samples of porous Vycor glass. References and Notes (1) Turro, N. J. Pure Appl. Chem. 1986, 58, 1219. Milosavijevic, B. H.; Thomas, J. K. J. Phys. Chem. 1983, 87, 616. Jackson, R. L.; Trusheim, M. R. J. Am. Chem. Soc. 1982,104,6590. Gafney, H. D. Coord. Chem. Reus. 1990, 104, 113. Gafney, H. D. In Photochemistry of Solid Surfaces; Anpo, M., Matsuura, T., Eds.; Elsevier: New York, 1989; p 272. (6) Kennelly, T.; Gafney, H. D.; Braun, M. J. Am. Chem. SOC.1985, (2) (3) (4) (5)
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