J. Phys. Chem. 1994, 98, 6490-6495
6490
Luminescence Dynamics of Halodicyanocuprate(I) Exciplexes Attila HorvBth’ and Craig E. Wood’ Department of General and Inorganic Chemistry, University of Veszpr&m,Veszprsm, H-8201, P.O.B.158, Hungary
Kenneth L. Stevenson Department of Chemistry, Indiana University-Purdue University Fort Wayne, Fort Wayne, Indiana 46805 Received: September 17, 1993; In Final Form: March 22, 1994’ Coordinatively unsaturated Cu(CN)2- forms mixed-ligand complexes, of the general formula Cu(CN)2X2-, in the presence of halide ions in aqueous solution. The stepwise formation constants have values of 0.70 f 0.12, 1.92 f 0.08, and 16 f 4 M-I for X = C1-, B r , or I-, respectively. Moreover, the excited state formed by UV irradiation of the dicyano complex, i.e., *Cu(CN)2-, also associates with halide ions to form a highly luminescent species identified as an exciplex, with about an order of magnitude higher stepwise formation constant (1 1 f 3,27 f 4, and 430 f 40 for C1-, B r , and I-, respectively) than the ground-state reaction. The trend in formation constants demonstrates the increasing stability of the exciplex with polarizability of the halo ligand. Luminescence quenching studies lead to the assignment of the exciplex as a triplet species. Activation parameters for exciplex formation and decay, obtained from temperature dependence studies of luminescence lifetimes measured at varying halide ion concentration, are consistent with the assignment of the precursor and the exciplex moieties, and contribute to the elucidation of the dynamics of the system.
Introduction For more than a decade there has been a growing interest in the light-induced reactions of copper(1) and copper(I1) complexes. These species can participate in a number of interesting photochemicalprocesses including hydrogenformationfrom water and synthesis of organic compounds or degradation of pollutants such as cyanide ions.’-15 It has already been demonstrated that excitation of the low-lying metal-to-ligand charge-transfer (MLCT) states of bis-N-heterocyclic (e.g. 2,2’-bipyridine, or 1,10-phenanthroline and their substituted derivatives) copper(1) complexes leads to a net electron transfer in the presence of oxidizing species1”I8 and the excited complexes have the ability to form exciplexes with electron donor m o l e c ~ l e s . ~ ~ - ~ ~ Luminescence studies of aqueous solutions of halocuprate(1) complexes indicate that the UV excitation of the dichloro and dibromo species does not result in emission,8J5whereas UV-excited emission peaking at about 475 nm is observed if the equilibrium in solutionsof such complexesis shifted toward the tricoordinated species by the addition of excess halide ions. For example, in the chlorocuprate(1) system, which consists of the two complexes CuC12- and CuCl32-, the luminescencequantum yield, &,increases when the concentration of chloride ion is increased, but the nonlinear relationship between @L and the fraction of light absorbed by CuC132- indicates that the tricoordinated complex itself is stimulated to become more luminescent by the addition of chloride ion.8 Furthermore, the luminescent lifetime depends on the chloride ion concentration and ionic strength: and this interesting behavior has recently been interpreted.z2 On the other hand, UV excitation of aqueous solutions of cyanocuprate(1) complexes results in emission peaking near 400 nm when the cyanide ion is low enough (i.e. [CN-] < 10-4) to ensure that dicyanocuprate(1)is thedominant species,l4but when the cyanide concentration is increased to form the triscoordinated species, the emission is quenched. This contrasting luminescence behavior between the chloro and cyano complexes of copper(1) stimulated us recently to study the photochemistry of dicyanocuprate(1) in aqueous solutions containing chloride ion, where 0 On leave from the Nottingham Trent University, Clifton Lane, Nottingham, NGll 8NS U.K. *Abstract published in Advance ACS Abstracts, June 1, 1994.
0022-3654/94/2098-6490$04.50/0
< [Cl-] < 5 M. At highchloride ionconcentration theabsorption and emission spectra indicated the formation of a new species, chlorodicyanocuprate(I), Cu(CN)2Cl-, the stepwise formation constant of which, determined from absorption and emission spectra, is 0.70 0.12 at 5 M ionic strength.23 This complex ejects hydrated electrons when excited by UV light, with a quantum yield which peaks at 266 nm and is smaller than that of the dicyanocuprate(I) ion24 as one would expect for an increase in coordination number of the coordinating copper(1) ion in the presence of halo or cyano ligand~.~-~J3-l5 Moreover, the luminescence quantum yield and lifetime of emission detected at 480 nm depend on the chloride ion concentration, similarly to that observed in chlorocuprate(1) solutions. Recently, we have suggested that the luminescence characteristics of the Cu(CN)2--X- system can be interpreted by an exciplex mechanism.22 Although exciplexes are well-known molecular entities in the photochemistry and photophysics of organic systems, so far there have been only a few reports of exciplex formation in the inorganic literat~re.1~2~925-2’ In this paper we wish to present more detailed evidence and the justification for the mechanism by which exciplexes are created by the coordination of chloro, bromo, and iodo ligands to the excited dicyanocuprate(1) complex.
*
Experimental Section All solutions were prepared to the desired concentrations from stock solutions of reagent grade NaCl, NaBr, NaI, NaC104, or KCN. The crystalline potassium salt of dicyanocuprate( I) was prepared by the method described earlier.13J5 Solutions to be irradiated were deaerated with a stream of deoxygenated argon or dinitrogen in a quartz cuvette of 4-mL volume and 1-cm path length. Absorption spectra were recorded with a Beckman ACTA MVI, Hewlett-Packard HP845 1, or Zeiss SPECORD M-40 spectrophotometers, all of which were interfaced with computers for facile spectral analysis. Luminescence excitation and emission spectra were obtained on a Shimadzu RF-540 or Perkin-Elmer LSSOB spectrofluorophotometer interfaced to a HP-86 or PC/386 computer, respectively. Luminescencequantum yields were determined by using quinine sulfate as a reference.28 0 1994 American Chemical Society
Luminescence Dynamics of Halodicyanocuprate(1) Exciplexes The Journal of Physical Chemistry, Vof.98, No. 26, 1994 6491
TABLE 1: Absorption Spectra and Formation Constants of Cu(CN)2XL Complexes (X = Cl, Br, I, CN) Cu(CN)2ClZ Cu(CN)2Br2Cu(CN)21ZCU(CN)32AI, nm 224" 226' 20614 q, M-I cm-l 16 70W 16 50W 20 70014 An, nm 235" 238' 243' 24314 ell, M-1 cm-1 11 90W 13 56W 11 70014 K. M-I 0.70 f 0.12a.23 1.92 f 0.08" 16 4a-22 5.75 X 104 (refs 13, 14) a Measurements were performed at 5 M ionic strength. 0.50 I
I
0.40
8
2 0.30
*
f
8 0.20 P 4
2 20
*
nm.
0.10 0.00
TABLE 2 Wavelengths of Absorption,. Excitation, and Emission Maxima in Aqueous Solutions of the Cu(CN)z--XSystem at 5 M Ionic Strength X hat. (nm) Lc(nm) horn (nm) c1 275 3 272-2756 475 f 5 Br 278 3 272-2 78' 482 5 I 282 f 3 272-2856 486 6 a From the differencespectra of Cu(CN)zX*-and Cu(CN)2- between 260 and 310 nm. * Increases with concentration of halide ion from 272
240
260
280
300
Wavelength, nm Figure 1. Absorption spectra of Cu(CN)z--Br systeminaqueous solution M, [ B r ] = 0, at 5 M ionic strength, I = 0.01 cm, [Cu(I)] = 3 X 0.2, 0.4, 0.6, 0.8, 1.5, 2.5 M.
Time-resolvedemission spectra and luminescenceand transient absorbance lifetimes were determined on two laser flash photolysis systems: (1) the Veszprem University system consisting of a Spectron SL-402 Nd:YAG laser with frequency quadrupling, Applied Photophysics KS-347 kinetic spectrometer with Hamamatsu R955 or 1P28 photomultipliers, and Philips PM3320/A digitizing storage oscilloscope; (2) the IndianaPurdue University system consisting of Continuum Surelite Nd: YAG laser with frequency quadrupling, a Photon Technology monochromator, and 75-W horizontal xenon lamp, Hamamatsu R936 photomultiplier, Hewlett-Packard 545 10A digitizing oscilloscope, and Stanford Research System DG535 pulse and delay generator for synchronizingthe laser, shutters, and data acquisition. Both systems were interfaced with IBM-compatible computers for data acquisition and analysis. Temperaturedependence measurements were obtained using a home-built constant-temperature cuvette holder.
Results and Discussion Absorption Spectra and the Equilibrium in the Ground State. The effect of the halide ion concentrationon the absorptionspectra of the cyanocuprate(1) complex in the UV region was studied in aqueous solutionsat 5 M ionic strength by the systematicaddition of suitable amounts of sodium halide and sodium perchlorate, at constant concentration of added Cu(CN)2-. Difference spectra between solvent medium and solvent medium containing CU(CN)~were obtained, and these clearly indicate the formation of a new complex which possesses absorption bands rather different from those of either the pure dicyano or any possible halocuprate(1) (di- or tricoordinated) complexes. In the case of the Cu(CN)z-C1- system the band developed between 200 and 300 nm was identified as the absorptionof C U ( C N ) ~ C PThecharacteristics ,~~ of the photoinduced electron transfer reactions in these solutions, e.g., the quantum yield of solvated electron formation and the rate constant of the electron scavenging by copper(1) species in the ground state are consistant with this a ~ s i g n m e n t . ~ ~ Similar trends with a small red-shift were observed when bromide or iodide ions were added to the aqueous solutions of dicyanocuprate(I), as shown in Figure 1 and Table 1. However, the investigation of such spectral changes was restricted to
wavelengths longer than 220 nm for bromide and 240 nm for iodide ions because of the very large molar absorbances of these halo anions at the shorter wavelengths. The longest wavelength absorption occurs as a shoulder between 260 and 300 nm, a region where the trihalocuprates are known to undergo a CTTS t r a n ~ i t i o n . ~ .Hence ' ~ . ~ ~it is reasonable to assume that these bands originate from a spin-allowedbut symmetry-forbiddentransition. In determining the stepwise formation constant of the Cu(CN)2X2- complexes, the concentration of copper(1) was restricted to a range such that the coordinated halide ion was always less than 2% of the total added halideconcentration. Hence, the molar absorbances and equilibrium constants (see Table 1) can be calculated2.6V8 using the simple expression
where D is the optical density, c is the concentration of copper(1) in mole per liter, 1 is the optical pathlength in centimeters and and e3 are the molar absorbances of the dicyano- and halodicyanocuprate(1) ions, respectively, at a given wavelength, in M-1 cm-I. The increase in formation constant as X changes from C1 to Br to I reflects the expected tendency toward a stronger bond between the coordinatively unsaturated copper(1) central atom and a softer ligand. Luminescence Spectra and Quenching Studies. Steady-state luminescencestudies indicate that as the halide ion concentration is increased, the weak emission of the UV-excited (b,,, = 266 nm) dicyanocuprate(1) at X = 400 nm14 disappears while a new, wide emission band developsbetween 400 and 600 nm (see Figure 2, which shows the effect of iodide ion). The maximum intensity observed and the wavelength at the maximum intensity both increase as the atomic number of the halide ion is increased, suggesting that these properties are related to the polarizability of the halo ligand coordinated to the copper center, a property which becomes especially pronounced in the case of iodide ion. Although the position of the emission does not depend on the concentration of halide ion, the excitation bands do show a redshift from 272 f 3 nm to 275 f 3,278 3, or 285 f 5 nm for C1-, B r , or I-, respectively, when the concentration of halide ion is increased (see Table 2). This excitation red-shift corresponds rather well to the change in absorption spectrum observed in the shifting of the ground-state equilibrium as a result of changing ligand concentration. Thus, it appears that the luminescentspecies can be formed by excitation of either the dicyanocuprate(1) or halodicyanocuprate(1) species. Figure 3 shows that the luminescence quantum yields, determined by quininesulfate method>*first increase dramatically and then reach a maximum value followed by a leveling off in aqueous solution as halide ion concentration is increased. Such
*
6492 The Journal of Physical Chemistry, Vol. 98, No. 26, 1994
a :
-180.0
t
$
Horvlth et al.
-
80.0
-
60.0
'8'00
0 0.0
1.o
010
020
2.0
030
3.0
o!o
4.0
5.0
Concentration of Halide Ion, M Figure4. Dependence of luminescence lifetimeon concentration of halide ion in aqueous solutions of Cu(CN)2--X- at 5 M ionic strength, X = CI (O), X = Br (U), X = I (*).
. 40.0
SCHEME 1
*Cu(CN&
.
products
*CU(CN~ X '-
20.0 n
CU(CN& +X'-
.
0.
300.
0.0 600.0
Figure 2. Uncorrected emission spectra of the Cu(CN)2--1- system in aqueous solution at 5 M ionic strength, sodium perchlorate medium. The eM,[I-] = 0.6.66 sample temperature was 303 K, [Cu(I)] = 7.83 X l X 10-3, 1.66 X 10-2, 3.31 X 10-2, and 6.58 X 10-2 M. 0.004 O
E 0.002
5
.
O
o
4
m
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1
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;1
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0.000 0.0
1.0
2.0
--
0.1
0.2
3.0
03
. - I II
0.4
4.0
0!5
5.0
Concentration of Halide Ion, M Figure 3. Dependence of luminescence quantum yield on concentration of halide ion in aqueous solutions of Cu(CN)2--X- at 5 M ionic strength, X = CI (a), X = Br (m), X = I (*). behavior can be rationalized, as will be shown later, by the concerted influenceof an equilibrium between the excited copper(I) complex and halide ion, and the ground-state equilibrium, Cu(CN)2- + X- = Cu(CN)2XZ-. The addition of cyanide to solutions of Cu(CN)2- containing halide ions at a concentration corresponding to the maximum luminescence quantum yield (Le., [Cl-] = 3 M, [ B r ] = 2 M, or [I-] = 0.15 M, see Figure 3) leads to a decrease in emission intensity, but the luminescencelifetime is unchanged. Such static quenching confirms that the excitation of the tricyanocuprate(1) ion results in a nonemitting species and is consistent with the idea that halo-assisted luminescence is favored when the lightabsorbing species is CU(CN)~-. Strong electron-donormolecules, such as CH3CNand DMSO, are efficient dynamic quenchers as indicated by the fact that
plots of the luminescencelifetime vs quencher concentration obey normal Stern-Volmer kinetics, resulting in quenching rate constants of 4.6 X lo7 and 3.5 X lo7 M-' s-l, for CH3CN and DMSO, respectively, in aqueous solution of Cu(CN)2- and C1at 5 M ionic strength. These values are rather similar to those obtained for quenching of the triplet *Cu(dmp)2+ (dmp = 2,9dimethyl- l,10-phenanthroline),20~21 and thus they suggest a weak electron-acceptor ability of the emitting species. The electron acceptor, methylviologen cation (MV+: l,ltdimethyl-4,4'-bipyridinium),was also found to be a very efficient dynamic quencher; e.g., in a glycerol/water solvent mixture (& = 0.1 5) a value, k = (6.1 f 0.2) X lo9M-' s-l, was obtained from lifetime measurements for MV2+at 1.5 M bromide ion concentration. Thisclearly indicates that the luminescent excitedspecies can add or transfer an electron to another molecule with a lowerenergy empty orbital or it can accept an electron from a species with an electron in a higher-energy orbital. It was also observed that there is a shortening in the luminescence lifetime of the exciplex when oxygen is dissolved in the solutions to be irradiated. The lifetime decreased, for example, from 485 to 324 ns when the C U ( C N ) ~ - - Bsystem ~ ( [ B r ] = 1.5 M) was excited in oxygen-free and air-saturated glycerol/water solvent (XdY= 0.13, indicating that the emitting species has triplet character. Luminescence Lifetime Measurements and the Excited-State Equilibrium. Time-resolved studies on the nanosecond time scale show characteristic halo ion concentration-dependent luminescence lifetimes. As can be seen in Figure 4, the lifetimeof emission first increases and then decreases with an increase in halide ion concentration, with the value of T,, dependent on the nature of the halide ion, occurring at nearly the same concentration for chloride and bromide ion (1.5-2.5 M) but at only 0.1 M iodide ion. The shape of these curves further suggests that the reaction of excited copper(1) complex with a halo ligand results in the luminescent species, or exciplex, while the coordination of the second halo ligand to the exciplex quenches the emission,a process that can occur only at rather high halide ion concentration. These experimental results can be interpreted by the general reaction Scheme 1, where P = precursor of the luminescent exciplex, E = exciplex, D = nonradiative decay, L = luminescent decay, and Products represents products formed by a bimolecular
Luminescence Dynamics of Halodicyanocuprate( I) Exciplexes The Journal of Physical Chemistry, Vol. 98. No. 26, 1994 6493
TABLE 3: Rate Constants and the Formation Constants of Exciplex Calculated from the T vs [X-]Data Measured at Ambient Temperature rate constant
c1
X Br
kpD X lo-', S-' kE X 10-8, M-'S-' kE X lo-', S-' (keL+keD)xlv,S-l kER x 10-6, S-l Km keJ k d
7.1 f 1.7 2.9 f 0.6 2.5 f 0.2 3.8f0.8 1.1 f 0.2 llf3
6.3 f 0.8 14 f 1 5.2 f 0.2 2.7f0.1 0.7 f 0.1 27 f 4
8
I 7.2 f 1.1 78 f 4 1.8 f 0.1 4.5f0.3 17 f 1 430 f 40
14.013.0-
s=
W
reaction of the exciplex with excess halide ion. If we assume that the exciplex precursor, *Cu(I), and the exciplex itself, *Cu(I).-X-, areobtained by excitation of thesystem with a Diracdelta function the decay rates of the two excited species are given by
The general solution of these equations is well-known30 and can be applied to this system by considering that under the general conditions, [*Cu(I)lt.0 = A I and [*Cu(I)...X-],,o = A,, the time dependencies of the concentration of the precursor and exciplex are given by
+ b,, exp(h,t)
(4)
where
a 12*0-
X
+
x t
x ym T
m
t
X
m
11.0-
lO.O{
\
0.003
0.0032
0.0034
0.0036
lnemperature, 1/K Figure 5. Eyring plots of rate constantas for Cu(CN)z--X- systems in aqueous solutions at 5 M ionic strength: k ~ X, = Br (M); kpD, X = CI (X), x = Br (+), x = I (*); keL + keD, x = Br (A); kER, x = Cl(0).
TABLE 4 [*Cu(I).-X-] = 621 exp(X,t)
I
reaction
Activation Parameters for Reactions in Scheme 1 AH* (kJ mol-l)o A S (J mol-' K-L)b AG* (kJ mol-')c C1 Br I C1 Br I C1 Br I
kpD 2 2 2 ke -7 -7 -1 +14 +16 +33 kEkEL+ kED +IS +16 +2O keR +21 +20 +3 0
-88 -a9 -54 -69 -58
-88 -94 -42 -68 -65
-88 -58 +3 -52 -97
+28 +24 +30 +35 +38
+28 +21 +29 +37 +39
+28 +i7 +30 +35 +32
Estimated error in AH* is 1 6 M mol-'. Estimated error in AS*is
1 1 5 J mol-' K-I. At 298 K.
X
= kp,
+ k,[X-]
J'
= k ~ +-
EL + k,, + k ~ ~ [ x -(9) ]
If x >> y and the formation of exciplex is nearly diffusioncontrolled, then the exciplex should be formed in several nanoseconds or subnanoseconds; hence the luminescence decay should be a simple exponential function with lifetime
Since the luminescencedecays we have observed are indeed firstorder, the assumption above is correct, and the rate constants involved in the mechanism can be estimated by fitting eq 10 to the lifetime-vs-halide concentration data by the Marquardt proced~re.3~ Rate constants calculated by the methods outlined above are compiled in Table 3. Since the decay constants of the precursor for threedifferent halide systemsare identical within experimental error, it is reasonable to conclude that the exciplex is formed from the same molecular entity in the presence of different halide ions. Moreover, rate constants for the exciplex decay and backreactions also are essentially independent of which halide ion is
coordinated to the complex, suggestingnearly identical structures for the different halocyanocuprate(1) exciplexes. On the other hand, the values of k~ and ~ E R which , are the rate constants for processes which result in an increase in coordination number of the copper center, vary significantly when changing the halo ligand. The increasingvalue of ke with polarizabilityof the ligand is especially pronounced in the case of iodide ions. Temperature Dependence of Emission Lifetime. The luminescence decay rates of these systems have been observed to be very sensitive to temperature. For example, the emission lifetime in the Cu(CN)*--Br system increases nearly 2 times when the temperature is changed from 50 to 25 OC. A similar lifetime doubling effect is observed in Cu(CN)2--Cl- solution when the temperature is decreased to 10 "C from room temperature. Such temperature sensitivitypermitted a reasonable estimation of activation parameters in the exciplex model (Scheme 1) by the systematic measurement of decay lifetimes at 5 M ionic strength at varying halide ion concentrations and temperatures between 10 and 55 OC. Using lifetime-vs-halide ion concentration isotherms, rate constants were calculated by the procedure described earlier. Figure 5 is an Eyring plot of some representative results of these calculations, and Table 4 summarizes all of the activation parameters obtained from such plots. The precursor decay constant, k p ~is, plotted in Figure 5 for all three halo ligand systems, and the fact that all three plots are coincident supports the proposal that the exciplex of chloride, bromide, and iodide originates from the same molecular entity, namely, Cu(CN)z-.
6494
The Journal of Physical Chemistry, Vol. 98, No. 26, 1994
1
primary excited species
I
luminescent species
Horvath et al. which can be separated into two terms:
where products
I
,
j
/excitation
jemission
Ground Stale
b = kE&43X/(XY
(14)
-
At high [X-] the absorbance fraction, f2, approaches zero, and hence 4~ b. It can also be shown that under this condition (see Appendix)
Figure 6. Simplified energy diagram of the system.
The slightly positive slope in the plot of the exciplex formation rate constant, corresponding to a very small, negative enthalpy, can be regarded as further evidence of exciplex formation.20.21 The negative entropies for this reaction are consistent with an increase in the coordination number of the central copper(1) atom from two to three. The positive enthalpy and moderate negative or small positive entropy of activation for the exciplex back-reaction and firstorder decay of the exciplex are indicative of the importance of solvent reorganization effects in these processes. The relatively large negative entropy of activation for the second-order decay of the exciplex, which in the cases of the chloro and bromo species must be considered only very qualitatively because of the nonlinearity of activity versus concentration at high halide concentration,92canbe attributed to thecoordination of thesecond halide ion to the central copper atom of the excited species. These results and conclusions are in accordance with the proposed model of Scheme 1,and their essencecan be summarized in an energy diagram, shown in Figure 6. This scheme shows that the energy level of the exciplex is lower than that of its precursor. The reaction of the exciplex to the products in the ground state or back to the precursor is governed by the size of the energy barriers which depend on the nature of the halo ligand coordinated to the excited dicyanocuprate(1) or to the exciplex. The formation of the exciplex seems to be most favored, both kinetically and thermodynamically, in the case of the iodide ion. The back-reactionof the exciplex to the precursor is more probable than the first- or second-orderdecays of the exciplex for bromide and chloride exciplexes, whereas the reaction of iodide exciplex is determined by nearly equal potential energy barriers for all reaction routes at room temperature. Quantum Yield of Exciplex Emission: Effect of the Groundand Excited-State Equilibria. According to the proposed model (Scheme l), the luminescent species can be formed by excitation of either of the two equilibrated ground-state complexes, Cu(CN)2- and Cu(CN)2X2-, since both absorb UV light. For Cu(CN)2-, excitation results in the formation of light-emitting exciplexvia reaction between the excited complex and halide ion, whereas excitation of Cu(CN)zXZ- leads to the exciplex with no chemical reaction. Under conditions of continuous photolysis, the rates of change of [*Cu(I)] and [*Cu(I)-.X-] are expressed by eqs 1 and 2,above, each with an added term, 1$242and ZJ343, respectively, to account for their continuous formation, where la is the absorbed light intensityJ2 and f3 are the fractions of light absorbed by each of the two ground-state species, and 42 and 43 are the quantum yields for formation of the bis- and triscoordinated excited species, respectively. Since the system is in a stationary state, these two rates are zero, and one can determine the net quantum yield of luminescence:
- kEkE-[X-l)
4 L = = kELd3/b - k 4kE-(kED + kER[X-] - kpD) when [X-] 2 1 M; hence 7
2/[(x
+ Y) - ( x - y + 2kE-)] = 1/0- kE-)
Acknowledgment. This work was supported by grants from the Hungarian Academy of Sciences, the Petroleum Research Fund of the American Chemical Society, and the National Science Foundation. References and Notes (1) Davis, D. D.; King, G. K.; Stevenson, K. L.; Birnbaum, E. R.; Hageman, J. H. 3. Solid Stute Chem. 1977,22,63. (2) Davis, D. D.; Stevenson, K. L.; Davis, C. R. J. Am. Chem.Soc. 1978,
100.5344. ~.~ (3) Stevenson, K. L.; Kaehr, D. M.;Davis, D. D.; Davis, C. R. Inorg. Chem. 1980,19, 781. (4) HorvBth, A.; Papp, S.;D k y , Z . J. Photochem. 1984, 23, 331.
(51 Braish. T. F.; Duncan. R.E.; Harber, J. J.; Steffen. R.L.; Stevenson. K. L: Inorg. Chem. 23,4072. (6) HorvBth, 0.; Papp, S. J. Photochem. 1985, 30,47. (7) HorvPth, 0.;Papp, S . J. Photochem. 1985, 31, 21 1. (8) Stevenson, K.L.; Braun, J. L.; Davis, D. D.; Kurtz, K. S.;Sparks, R. I. Inorg. Chem. 1988,27, 3412. (9) HorvBth, 0.; Stevenson, K. L. Inorg. Chem. 1989, 28, 2548. (10) Sykora, J.; Jacubcova, M.; Cvengrosova, Z . Coll. Czech. Chem. Commun. 1982,47,2061. (1 1) HorvBth, 0.; Papp, S . J. Chem. Educ. 1988.65, 1102. (12) Ferraudi, G. Inorg. Chem. 1978, 17, 1370. (13) ZsilBk, Z.; HorvBth, A.; Papp, S. mag^ Kem. Foly. 1984,90, 556. (14) Horvith, A.; ZsilBk, A.; Papp, S.J . Photochem.Photobiol. A: Chem. 1989, 50, 129. (15) Stevenson, K. L.; Berger, R. M.; Grush, M. M.; Stayanoff, J. C.; Horvith, A.; HorvBth, 0.J. Photochem. Phofobiol.A: Chem. 1991,60,215. (16) McMillin, D. R.;Buckner, M. T.; Ahn, B. T. Inorg. Chem. 1977,16, 943. (17) Blaskie, M. W.; McMillin, D. R. Inorg. Chem. 1980, 19, 3519. (18) Gamache, Jr., R.E.; Rader, R. A,; McMillin, D. R. J. Am. Chem. Soc. 1985,107, 1141. (19) McMillin, D. R.; Kirchoff, J. R.;Goodwin. K. V. Coord. Chem. Rea. 1985. _ _ 64. 83.
(20) Palmer, C. E. A.; McMillin, D. R.;Kirmaier, C.; Holten, D. Inorg. Chem. 1987,26, 3167. (21) Stacy, E. M.; McMillin, D. R. Inorg. Chem. 1990, 29, 395. (22) HorvBth. A.; Stevenson, K.L. Inorg. Chem. 1993, 32. 2225. (23) HorvBth, A.; Stevenson, K. L. InoG. Chim. Acfa 1991, 186, 61. (24) Horvath, A.; HorvBth, 0.;Stevenson, K. L. J. Photochem.Phofobiol. A: Chem., accepted. (25) Ayala, N. P.; Demas, J. N.; DeGraff, B. A. J . Am. Chem. Soc. 1988, 110, 61. (26) Lever, A. B. P.; Seymour, P.; Auburn, P. R.Inorg. Chim. Acfa 1988. 145, 43. (27) Vogler, A.; Kunkely, H. Inorg. Chim. Acta 1980, 45, L265. (28) Eaton, D. F. Pure Appl. Chem. 1988,60, 1107. (29) Horvith, 0.; Fendler, J. H.; Stevenson, K. L. Inorg. Chem. 1993,32, 327. (30) Demas, J. N. Excired Slate Lifetime Meusuremenrs, Academic Press: New York, 1983; pp 59,24&242. (31) Marquardt, D. W. J. Soc. Ind. Appl. Math. 1%3,11,431. (32) As a result of comments by a referee, we checked the activities of the chloride, bromide, and iodide ions in 5 M ionic strength solutions (NaClO, medium) using specific ion electrodes. The results indicated that activities vary linearly with molarity up to molar halide concentrations of 1.5-2 M. This suggests that our estimates are reliable for all rate constants except the values of keR for the chloride and bromide systems, where molarities higher than 2 M were necessary to obtain km. (33) Wagner, P.J.; Siebert, E. J. J . Am. Chem. Soc. 1981,103,431. (34) Wagner, P. J.; Lindstrom, M. J. J. Am. Chem.Soc. 1987,109,3057. (35) HorvBth,A.;Hodth,O.;Stevenson,K.L. 203rdAmerican Chemical Society National Meeting, San Francisco, CA, Apr 5-10, 1992, Division of Inorganic Chemistry, 630.
