10246
J. Phys. Chem. 1995, 99, 10246-10250
Volume Changes Associated with Electron Transfer Quenching of Excited Ru(bpy)32+and Xanthene Dyes. Time-Resolved Optoacoustic Studies? Jean-Louis Habib Jiwan, Alexander K. Chibisov,' and Silvia E. BraSlavsky" Max-Planck-Institut j k r Strahlenchemie, Poslfach 10 13 65, 0-45413 Miilheim an der Ruhr, Germany Received: December 22, 1994; In Final Form: April 13, 1995@
Volume changes associated with intermolecular electron transfer reactions between R ~ ( b p y ) 3 ~and + Fe(H20)63+ and between xanthene dyes and metal cyanides in aqueous solutions were measured by temperature-dependent laser-induced optoacoustic spectroscopy with nanosecond pulsed excitation. For electron transfer between R u ( b p y ) F and Fe(H20)63+ a contraction of 11 mL per absorbed Einstein was observed. Smaller volume changes (5 ImLRJinstein) were determined for the electron transfer between xanthene dyes and metal cyanides. A contraction resulted for the photooxidations while an expansion was observed for the photoreductions. In each case the volume change is largely attributed to the medium reorganization around the system present after the time window of the experiment (ca. 600 ns) rather than to intrinsic molecular parameters, such as, e.g., bond length changes.
Introduction Experiments investigating pressure effects on the equilibrium or rate constants of reactions in order to obtain information about volume changes occurring during the reaction (Av) or activation volume changes (AV),' are not easy to perform for fast reactions, e.g., for photoinduced electron transfer reactions. In these cases, laser-induced optoacoustic spectroscopy (LIOAS) is the method of choice for the evaluation of the reaction volume changes in the nanosecond to microsecond time range. Callis et aL2 drew attention to the fact that there are two possible contributions to the volume change in a photoinduced reaction in solution: the difference in volume between products and reactants (we call these changes structural volume changes) and the expansion of the medium (in a few solvents a contraction might result) upon release of heat by radiationless processes. Only the second contribution depends on the thermoelastic parameters of the solution (cp, the heat capacity at constant pressure; /?= (bV/BT),,(llv), the volume expansion coefficient; and e, the solution density3). LIOAS measurements as a function of these parameters can separate both contributions, thus allowing the calculation of structural molecular volume changes. This method has already been applied to various photoinduced reactions4 but not yet to intermolecular photoinduced electron transfer reactions. We now present studies of volume change taking place during well-known photoinduced reactions in order to test the factors determining such changes. An electron transfer reaction in solution can induce two types of volume changes: an internal variation (bond length and/or angle differences between products and reactants) and/or a reorganization of the solutes solvation induced by the new charge distribution. In order to analyze these effects, we studied the photoinduced electron transfer reaction between the triplet MLCT state of R ~ ( b p y ) 3 ~and + F e ( H ~ 0 ) 6 ~and + between the triplet state of xanthene dyes and metal cyanides in aqueous solutions, taking advantage of the fact that /3 for water varies
* To whom correspondence should be addressed. 'Dedicated to Professor H.-D. Scharf on the occasion of his 65th birthday. On leave from the N. N. Semenov Institute of Chemical Physics of the Russian Academy of Sciences, Kossygin street 4,117334 Moscow, Russia. Abstract published in Advance ACS Abstracts, June 1, 1995. @
0022-365419512099-10246$09.00/0
x2- +
Fe(CN)3i
x2-
M(CN~;
M =
t
hU/pH 9 . 2
Fe2+/3+, W 4 t / 5 t 1
hu'pH
X
-
' 2 x3-
7
M04+/5t o r
+
Fe(CN)4i
+
M(cN):-
RuZt/3t
X2- = e o s i n o r e r y t h r o s i n Y = 6 o r 8
Figure 1. Photoinduced electron transfer between charged species.
strongly with temperature between 3.9 'C (/3 = 0) and room temperature (20 'C, /3 = 207 x K-' 5 ) . The choice of these two systems was dictated by the fact that (a) the thermodynamic parameters of these reactions are well-known and (b) after electron transfer both resulting species bear the same type of charge (Figure l), decreasing the rate of geminate recombination and keeping it within the time window of the LIOAS experiment (larger than ca. 20 ns). A second condition increases the lifetime of the photoproduced ion pair; i.e.,in both systems the photoactive state (for R~(bpy)3~+ and for the xanthene dyes) is a triplet state. After electron transfer, the individual spins of the products are triplet and singlet, and the back electron transfer to the ground state is therefore spin forbidden. Thus, the resulting species live longer than the time window of the experiment. Materials and Methods All compounds were obtained from Aldrich in the highest purity available and were used as received except bromocresol purple which was obtained from Fluka. K&fo(CN)g2H20 and & W ( C N ) & € 2 0 were prepared according to the methods of Furman and Miller6a and Heintz,6b respectively. For LIOAS experiments, the concentration of the samples was ca. 1.5 x M for R ~ ( b p y ) 3 in ~ +10 mM H2S04 (to avoid precipitation of Fe(OH)3), ca. 7.8 x M for eosin, and ca. 3.9 x low6M for erythrosin in 10 mh4 Na2B407 buffer (pH = 9.18 at 20 'C7). The quenchers' concentrations used were enough to ensure almost quantitative quenching (see Results and Discussion). 0 1995 American Chemical Society
Quenching of Excited R~(bpy)3~+ and Xanthene Dyes
J. Phys. Chem., Vol. 99, No. 25, 1995 10247
Absorbances of the samples (between 0.27 and 0.30) at and 532 nm for excitation wavelength (460 nm for R~(bpy)3~+ xanthene dyes) were matched to within f0.005units to those of the calorimetric reference solutions. Solutions of NazCr207 for 460 nm and bromocresol purple for 532 nm excitation were used as reference^.^ The samples were thermostated to f O . 1 'C employing a P T l O O thermoelement placed directly into the sample. Absorption spectra were recorded on a Shimadzu W2102PC spectrophotometer. Luminescence spectroscopy experiments were performed with a Spex Fluorolog spectrofluorimeterwith samples deoxygenated by N2 bubbling for 5 min. For the LIOAS experiments, the samples were deoxygenated by 10-15 min Ar bubbling. The setup has been previously described in detail.8.9 Essentially it consisted of a 15 ns 460 nm laser pulse produced by a FL2000 Lambda Physik-EMG 101 MSC Excimer laser (XeCl) which pumped a Coumarin 47 laser used to excite the R~(bpy)3~+ with a repetition rate of 3 Hz. For the xanthene dyes, an 8 ns pulse at 532 nm (second harmonic of a Nd:YAG laser) was employed with a 5 Hz repetition rate. The fluence of the laser pulses was varied using a neutral density filter, and the energy values were measured with a pyroelectric energy meter (Laser Precision Corp. RJ7620 and RJP-735), whose output was registered by the computer system. The laser beam diameter was set at 0.9 mm, so that the effective acoustic transit time r', (=d/ua; d = diameter of beam; u, = sound velocity3) was ca. 600 ns, which is the heat integration time. The pressure pulse was detected with a PbZr-Ti ceramic transducer pressed against the side wall of a quartz cuvette parallel to the laser beam direction.8 The signal was amplified 100 times (two Comlinear E103 amplifiers) and fed into a transient recorder (Tektronix 7912). Two hundred signals were transferred to a Vax station 3 100, where they were averaged and sent to a Vax mainframe system for signal analysis. Signal Handling. The heat delivered by the excited molecules after absorption of a laser pulse generates a thermally induced volume change AV,h. This volume change gives rise to a pressure pulse detectable with a piezoelectric transducerlo which is proportional to the thermoelastic properties of the sample p/cpe. AV,,
0~
-
aeE,,,(l
= a&EL
%e
CDQ
(1)
In eq 1, a is the fraction of absorbed energy E,,, (1dissipated into the medium as "prompt heat"; i.e., within the heat integration time, E,,, is the total energy of the laser pulse, A the sample absorbance, En the energy per Einstein of the laser pulse, and ns the number of Einsteins absorbed. The optoacoustic signal amplitude H is proportional to the total volume change during 2': through an instrumental constant k. Thus, eq 2 represents the thermally induced contribution to the LIOAS amplitude.
H =ka
aEe,,(1
-
cP@
As mentioned in the Introduction, a structural volume change AV, originating in molecular processes other than heat release in the medium can take place during the integration time. The optoacoustic signal amplitude for a sample Hsresults from the sum of the contributions from the thermal and the structural volume change (eq 3)
H" = k[AV,,
+ AV,]
(3) The quantum yield @R of the photoinduced process must be taken into account for the evaluation of AVR (AV, = ns @R
AVR,with AVR= structural volume change per photoconverted mole). (4) In order to eliminate the instrumental constant k, it is necessary to use a calorimetricreference with a = 1 (or known) and AV, = 0. Therefore, the ratio of energy-normalized signal amplitude for sample, H",, and reference, M,,yields the following equation.
Assuming that AVR is temperature independent in the relatively small temperature range analyzed, it is convenient to use the strong variation of jf? for water (and dilute aqueous solutions) with temperature to evaluate AVR from eq 5. The ratios of the energy-normalizedsignals for sample and reference measured at different temperatures are then plotted against ( c g / p ) ~ The . intercept affords the a value, while the slope of the linear plot yields (@RAVR/EA)(eq 5). An altemative method is to use two temperatures for the measurements, one at which jf? = 0 (TO);therefore, there is no thermally produced signal (eq 2) and the signal from the sample is entirely due to the volume change (eq 3). The other temperature serves to determine the value of k in eqs 2 and 3 with a calorimetric reference. For neat water, jf? = 0 at 3.9 "C. For dilute aqueous solutions containing additives, the temperature (TO)giving a zero signal for the reference should be found. By measuring the sample signal at this temperature (TO)and the signal for sample and reference at another temperature (T), the values of a and AVR are derived using eqs 6 and 7.",12
(7) The ratio @lcpe)at different temperatures for the medium (10 mM H2SO4 and 10 mM Na2B407 in this work) is determined by measuring the energy-normalized signal for the reference in neat water (WW(T)) and in the solvent used (fFsolv(T)):
The solvent parameters determined by this method are shown in Table 1. In our case, the second factor on the right side of eq 8 could be ignored, since solutions were too dilute to significantly affect the density. The value of u,, the sound velocity, estimated by the signal arrival time to the detector was the same for the reference in the buffer as for it in neat water. Results and Discussion Reaction between R ~ ( b p y ) 3 ~and + Fe(Hz0k3+. For this reaction, temperature-dependent LIOAS measurements were made in 10 mM H2S04 in the range 7-20 "C. The concentration of Fe(H20)3+ was 16 mM in order to assure complete quenching of the excited states. By luminescence quenching experiments, at 20 and 5 'C, Stem-Volmer constants of 1051 M-' and 767 M-I were obtained, respectively. These constants
10248 J. Phys. Chem., Vol. 99, No. 25, 1995
Jiwan et al.
40 00 1O.C
20 00
---B
7%
000
-0.51
"
v
5
& . -2ow -40 00
12% 10%
-6000 000
C
100
200
300
400
500
600
700
800 -2.5-
Laser Fluence (PI)
Figure 2. LIOAS signal amplitude H , as a function of the absorbed energy for the quenching of Ru(bpy)32+ by Fe(H~0)6~+ in 10 mM H,so4 at different temperatures. Positive slopes correspond to the reference Na2Cr04, and negative slopes to the sample. TABLE 1: Thermoelastic Parameters of the Solvents As Determined from LIOAS Measurements with Bromocresol Purple in the Various Media (mL kJ-9 ~
~~
ca/cFQ)
10 mM
(PIcFQ)~
T("C)
water
7
0.011
10
0.021 0.027 0.033 0.042 0.049
12 14 17 20
+
colc~~)
GRIcg)
Na2B407
10mM
10 mM
10 mM
Na2B407
&Ru(CN)6
0.0012 0.0023 0.029 0.037 0.047 0.057
It is worthwhile to make a comparison with the work of Marcus and SutinI6 devoted to the study of the electron transfer between Fe(H20),j2+ and R~(bpy)3~+. The authors found the following thermodynamic parameters, AGO = -55.1 kJ/mol, LWD = -112.0 kJ/mol, and AS" = -179.7 J/(mol "C).I6 In our case, the stored energy [( 1 - a)El] = 114.5 kJ/mol, which corresponds to the enthalpic content of the final state for our experimental window (600 ns), Fe(H20)6*+ R~(bpy)3~+, is in very good agreement with the value determined by Marcus and Sutin. We note that this electron transfer reaction, which is considered as an exception with its large entropic variation (-179.7 J/(mol "C)), is associated with a relatively large expansion (4-11 mL/mol, i e . , the opposite sign of that determined for the opposite reaction). The unexpected result of an expansion for a reaction with a negative AS" should be related to the fact that with our method we measure a global volume change including the ions and the surrounding solvent molecules, making it difficult to define system and surroundings. This result confirms that our measurements provide information on the structural changes around the dipoles in solution and including the medium. Since it is well-known that ASo for redox reactions between metal complexes strongly depends on the ligands,I7 more data on AV values should be obtained in order to test a possible correlation with AS" values. Reaction between Xanthene Dyes and Metal Cyanides. The method of using the temperature (TO)giving zero signal for the reference together with measurements for reference and sample solutions at another temperature was used for these systems in 10 mM NazB407. The signal for the reference bromocresol purple was zero at 1.8 "C in 10 mM Na2B407 (Figure 4) and at 3.4 "C in 10 mM NazB407 10 mM &Ru(CN)6. The second temperature was 20 "C. The concentration of metal cyanides was around 2 mM (the exact concentration depended on the particular solution) except for the less efficient quencher &Ru(CN)6, for which a concentration of 10 mM was used. These concentrations were calculated to yield more than 95% quenching efficiency (the precise number depends on each system).I8 Again in these bimolecular photoinduced electron transfer reactions the products live several milliseconds before collapsing to the original system.I8 Thus, again in this case, they are the final product for our 600 ns experimental window. In this series of experiments, the laser energy was also kept low, since at laser energies 26-7 pJ the plots of H against E,,, showed deviation from linearity (Figure 5). For each pair of dye and quencher, two or three experiments with freshly
+
0.052
0.056
and a F ~ ( H z O ) ~ concentration + of 16 mM afford 94% and 92% quenching at 20 and 5 'C, respectively. Under these conditions, the products (R~(bpy)3~+ and Fe( H z O ) ~ ~have + ) a lifetime exceeding 14 ms before collapsing to the original states.I3 This means that they are the final products for our experimental window of ca. 600 ns. In order to avoid multiphotonic processes, the LIOAS measurements were performed at low energy densities of the laser pulse (max x 7 pJ; corresponding to a ratio of photons/ molecules 5 1). The LIOAS signal after excitation of the Ru(bpy)32f/Fe(H*0)63+at all temperatures had an opposite sign to those of the calorimetric reference solution (Figure 2), indicating that a contraction takes place in the system. The slopes of the linear plots of H versus Ea = Eexc(l-lO-A) afforded the energy-normalized signals. The ratio of the normalized signals for sample and reference was plotted against cpe/p(eq 5, Figure 3). The values obtained after a linear least mean square analysis of the data were a = 0.56 f 0.15 and the structural volume change per absorbed Einstein AVe = @)RAVR = -10.8 f 1 mLEinstein, Le., slope x El (EA= 460 nm 259.96 kJ/mol). The relatively large contraction cannot be explained by simple bond changes after absorption of light, since the electron transfer reaction leads to a reduction of Fe(H20)63+ into Fe(H20)6*+ corresponding to an expansion and to an oxidation of R~(bpy)3~+ into Ru(bpy)s3+,probably corresponding to a small contraction of the ligands around the metal. The negative volume change should thus be attributed to some solvent reorganization. In general, electrostriction is considered a result of solvent re~rganization'~ and is an important phenomenon in cases where there is a net change in the number of charges. In our case, however, this phenomenon is neither due to charge creation nor destruction, the reaction corresponding to a charge shift. In addition, the value expected for the electrostriction effect in water is extremely small (1-2 mL/mol for the development of a single ionic chargeI5).
+
Quenching of Excited R ~ ( b p y ) 3 ~and + Xanthene Dyes
J. Phys. Chem., Vol. 99, No. 25, 1995 10249 TABLE 2: Fraction of Energy Dissipated Promptly as Heat, a, Volume Change per Absorbed Einstein, AVe, Quantum Yield of Ion Pairs Escape Calculated with eqs 9 and 10, ~ESC and , Eficiency of Escape, Calculated (eq ll),~ E S C , and from the Literature, vESC(lit),for the Photoinduced Electron Transfer between Xanthene Dyes and Metal Cyanides ab AVec,d Qscd 7 ~ s7ESC(litS8 ~ ~
T : 1.8OC
0.04i
I
l
l
4
b
b
6
0.66 0.34 0.30 0.21 0.28 0.67 0.38 0.34 0.25 0.19
f
Tim (miaosec)
Figure 4. LIOAS signal for reference (bromocresol purple) and sample [ErZ- f Fe(CN)&] in 10 mM NazB407 at 1.8 "C. The difference between the laser trigger and the first maximum used for the determination of the amplitude (H) is due to the sound velocity in the medium.
0.00
5.00
10.00
15.00
20.00
25.00
30.00
Laser Fluence (pl)
Figure 5. Laser energy dependence of the amplitude, H, of the LIOAS signal for the quenching of Er2- by F ~ ( C N ) Gand ~ - for bromocresol purple in 10 mM Na~B407: (A) sample at 1.8 "C; (0)sample at 20 "C; and (0)reference at 20 "C. .Q" jx2-
+ Q "
hv
between xanthene dyes (X2-) and metal cyanides (Q"-). The energies of the ion pair and free ions are considered similar.
prepared solutions were performed. The a and AVe values obtained using the data and eqs 6 and 7 are shown in Table 2. The stored energy [(l - a)&]was used to calculate the efficiency of charge separation, i.e., @ESC using eq 9, derived from simple energy balance considerations, together with eq 10 for the calculation of the energy stored. (9)
In eq 9, El = 225 kJ/mol ( ~ 5 3 2nm) is the energy of an Einstein of photons. At pH 9, @f = 0.19 and 0.02, Ef = Esl =
0.7 1.2 1.2 1.2 1.0 1.0 1.1 1.1 1.0 1.0
0.38 0.46 0.89 0.79 0.11 0.35 0.48 0.99 0.99 n.d.
TABLE 3: Free Energies for the Quenching Reaction, AGO, and Energy Level of the Ion Pair, Em -AGO 109.7 58.0 48.4 23.3 5.9 110.0 54.5 44.8 18.8 3.1
EIP' 67.3 118.9 128.6 154.7 171.1 74.0 129.5 139.2 165.2 180.9
M; [Er-] = 3.9 x M; for [quencher], see a [Eo-] = 7.8 x text. From ref 18, kJ/mol. In kJ/mol; see eq 10.
209 and 212 kJ/mol, and ET, = 177 and 184 kJ/mol for eosin and erythrosin, respectively.20.2' In order to calculate the energy stored by the free ions (FIS), it was assumed that (a) it is identical to that for the solventseparated ion pair (IP) and (b) the entropic term has a negligible contribution to the magnitude of AGO; i.e., the energy of the IP was calculated with the Rehm-Weller equation (19) (column 3 in Table 3). The values calculated for @ESC are listed in Table 2 together with the escape efficiencies, VESC, calculated according to eq 11 with = 0.7 1 and 1 for eosin and erythrosin, respectively, and VET = 1 for both triplets. Also listed are the values of these efficiencies determined by Chibisov and Zakharova, VESC(1it). QESC
Figure 6. Mechanism of the photoinduced electron transfer reaction
0.5 0.89 0.89 0.87 0.70 1.00 1.07 1.06 1.02 1.00
a [Eo--] = 7.8 x l o 6 M; [Err] = 3.9 x M; for [quencher], see text. Error &20%. mLEinstein. Error +lo%.
compound*
P
-4.7 f0.6 +1.1 +2.7 -0.3 -4.2 +1.3 +1.2 +1.9 +OS
= @'ncVET VESC
(11)
For the quenching of triplets of eosin, as well as of erythrosin, we find values of V E ~ C= 1 for every quencher (with the exception of the quenching of Eo- by Fe(CN)63-), at variance with the findings of Chibisov and Zakharova. The disagreement might be related to the assumptions used for the calculation of the energy level of the IP, in particular the assumption of a negligible entropy change, which in a way is in contradiction with the fact that a structural volume change is measured (vide infra). In any case, the values of V E ~ C, with the exception of 3Eo- quenched by Fe(CN)63-, are similar for the quenching of both triplets by the same quencher. For the quenching of eosin and erythrosin triplets by R u ( C N ) ~ ~, for - which the exothennicity of the electron transfer is very small ( 5 6 kJ/mol), other quenching mechanisms might compete with electron transfer. One possible explanation might be the occurrence of quenching by an extemal heavy atom effect inducing a faster decay of the eosin and erythrosin triplets. Such
Jiwan et al.
10250 J. Phys. Chem., Vol. 99, No. 25, 1995
a quenching mechanism has already been proposed for the quenching of the triplet state of eosin and rose bengal by &RU(CN)~.~~ The absolute values of AV, are relatively small, from ca. 0 to 5 rnLlEinstein, corresponding to a contraction for the electron transfer from the excited dye to the metal cyanide (photooxidation process) and to an expansion for the electron,transfer from the metal cyanide to the excited dye (photoreduction process). Notwithstanding the uncertainty in the value of @ESC needed to calculate the value of the structural volume change, AVR,the trend remains the same. For this series of experiments, no large volume change is expected from bond variations, since it is known that the structures of the metal cyanides in their two oxidation states are similar.23 This is probably also the case for the xanthene dyes, since they are large aromatic compounds with full charge delocalization. The fact that the trend in AV, is similar for both series of photoinduced electron transfer reactions (Le., quenching of eosin and of erythrosin) constitutes an important proof for internal consistency of the measurements. In an ion pair, the Coulombic interaction (attraction or repulsion) is proportional to the product of the charges. In our series we can distinguish two situations. For the photooxidation, the product is (-2 x -3) = 6 before and (-1 x -4) = 4 after the electron transfer, indicating a less repulsive character after the transfer and thus the possibility of a tighter ion pair, in agreement with the contraction observed. For the photoreduction, the product of the charges is +8 before and f 9 after the electron transfer, corresponding to an increased repulsion after the electron transfer, in agreement with the positive volume change observed. However, for the photoreductions the change in charges A(qlq2) is always 1. Should the coulombic term be the only reason for the observed structural volume change, the same AVRwould be observed in all cases. This is not the case. Therefore, some other factor should add to the simple effect of Coulombic interaction. One of the possible factors is again variation in the solvation of the final versus initial species. The large contraction observed for the quenching of the R ~ ( b p y ) 3 ~ + by Fe(H20)3+ with A(qlq2) = 0 indicates that the variation of the solvation corresponding to a real change of density around the solutes may be the predominant factor, at least in this case. Conclusions We have shown that the volume change associated with a bimolecular photoinduced electron transfer is an important factor which cannot be a priori neglected in photothermal studies. In every case there is indication that the main contribution to the volume change is a reorganization of the medium around ion pairs with a charge distribution different from that of the parent excited separated complex.
Acknowledgment. We are indebted to Professor K. Schaffner for his constant support and interest. Dr. M. S . Churio is thanked for her help at the beginning of this work, and Dr. D. Meissner (Hannover) and Dr. C. Viappiani are thanked for interesting discussions. The able technical assistance by S. Porting, A. Keil, D. Lenk, and T. Huestege is gratefully acknowledged. A.K.C. was the recipient of a DFG research grant during his stay in Germany. References and Notes (1) Van Eldik, R.; Asano, T.; Le Noble, W. J. Chem. Rev. 1989, 89, 549. (2) Callis, J. B.; Parson, W. W.; Gouterman, M. Biochim. Biophys. Acra 1972, 267, 348. (3) Braslavsky, S. E.; Heibel, G. E. Chem. Rev. 1992, 92, 1381. (4) (a) Peters, K. S.; Snyder, G. J. Science 1988,241, 1053. (b) Churio, M. S.; Angermund, K. P.; Braslavsky, S. E. J. Phys. Chem. 1994,98, 1776. (c) Hung, R. R.; Grabowski, J. J. J. Am. Chem. SOC. 1992, 114, 351. (d) Goodman, J. L.; Herman, M. S. Chem. Phys. Lett. 1989, 163, 417. (e) Herman, M. S.; Goodman, J. L. J. Am. Chem. SOC. 1989, 111, 1849. (5) Weast,R. C., Ed. CRC Handbook of Chemistry and Physics, 67th ed.; CRC Press: Boca Raton, FL, 1986-87; pp F-4, F-5. (6) (a) Furman, N. H.; Miller, C. 0. Inorgorganic Synthesis; Mc GrawHill: New York, 1950; Vol. 111, p 160. (b) Heintz, E. A. Inorganic Synthesis; Mc Graw-Hill: New York, 1963; Vol. VII, p 142. (7) Robinson, R. A.; Stokes, R. H. Electrolyre Solutions, 2nd ed.; Buttenvorths: London, 1959; p 545. (8) Rohr, M.; Ghtner, W.; Schweitzer,G.; Holzwarth, A. R.; Braslavsky, S. E. J. Phys. Chem. 1992, 96, 6055. (9) Braslavsky, S. E.; Heihoff, K. In CRC-Handbook of Organic Photochemisrry; Scaiano, J. C., Ed., CRC Press: Boca Raton, FL, 1989; Vol. 1, Chapter 14. (10) Patel, C. K. N.; Tam, A. C. Rev. Mod. Phys. 1981, 53, 517. (11) Malkin, S.; Churio, M. S.; Shochat, S.; Braslavsky, S. E. J. Photochem. Photobiol. B: Biol. 1994, 23, 79. (12) Yruela, I.; Churio, M. S.; Gensch, T.; Braslavsky, S. E.; Holzwarth, A. R. J. Phys. Chem. 1994, 98, 12789. (13) Ferreira, M. I. C.; Harriman, A. J. Chem. SOC., Faraday Trans. 2 1979, 75, 874. (14) Hamann, S. D. Rev. Phys. Chem. Jpn. 1980, 50, 147. (15) Le Noble, W. J.; Kelm, H. Angew. Chem. Int. Ed. Engl. 1980, 19, 841. (16) Marcus, R. A.; Sutin, N. Inorg. Chem. 1975, 14, 213. (17) (a) Yee, E. L.; Cave, R. J.; Guyer, K. L.; Tyma, P. D.; Weaver, M. J. J. Am. Chem. Soc. 1979,101, 1131. (b) Hupp, J. T.: Weaver, M. J. Inorg. Chem. 1984, 23, 256. (18) Chibisov, A. K.; Zakharova, G. V. Usp. Nauchn. Fotogr., 1989, 25, 114. (19) Rehm, D.; Weller, A. Isr. J. Chem. 1970, 8, 259. (20) Wilkinson, F. In Organic Molecular Photophysics; Birks, J. B., Ed.; Wiley: New York, 1975; Vol. 2, Chapter 3. (21) Murov, S. L.; Carmichael, I.; Hug, G. L. Handbook of Photochemistry, 2nd ed.; M. C. Dekker: New York, 1993. (22) Zakharova, G. V.; Knoll, H.; Hennig, H.; Chibisov, A. K. Bull. Acad. Sci. USSR, Div. Chem. Sci. 1987, 36, 490. (23) Sharpe, A. G. The Chemistry of Cyano Complexes of Transition Metals; Academic Press: London, 1976.
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