J. Phys. Chem. 1991,95, 10681-10688 10681

Figure 2. The experimental (triangles) and theoretical (circles) Do values for the second ligand plotted against the theoretical l / r : values. The solid and dashed lines are least-squares fits to the theoretical and ex- perimental data, respectively. The theoretical and experimental values for the same ion are connected by a dotted line. Note the Mn+ Do values are not included in the least-squares fit as they are derived from a dif- ferent bonding mechanism.

experiment for the water systems.'-4 The successive binding energies of the M(NH3)2+ systems are

compared with experiment in Table IV. Except for the fmt ligand binding energy to V+ and both ligand binding energies to Co+, agreement with experiment is very good. The difference for the second ligand binding energy to Mn+ is also relatively large, but in this case our value may be somewhat low. We analyze the difference between theory and experiment by making use of the fact that the bonding is primarily electrostatic, and therefore a correlation is expected between l/r(M - N)z and the binding energy. To illustrate that this is the case we have plotted our Do values and those of Marinelli and Squires for the first and second binding energies of M(NH3)z+ against l/r(M - N)z in Figures 1 and 2, respectively. Although there are exceptions, the binding energies generally display a linear relationship with 1 /$. For the single ligand case, there is a much larger variation from linear for both experiment and theory. This probably represents the greater diversity in the bonding mechanisms. However, the plot

suggests that the experimental values1 for VNH3+ and CoNH3+ are too high. The second ligand binding energies show a much more linear behavior, excluding Mn+, which is exceptional because the 5A,, ground state is derived from a highly excited asymptote. This results in a short bond distance, but a relatively small Do value because of the large promotion energy required to reach the excited atomic asymptote. From this plot it is clear that the experimental value for Co(NH3)2+ is much too large, and the value for V(NH3)2+ is probably a little large.

The binding energies of the ammonia systems are larger than the corresponding water systems in all cases, but the differences tend to increase from left to right in the transition row. For example, in the single ligand case, the difference increases from 5.3 kcal/mol for Sc+ to 14.5 kcallmol for Cu+. This reflects the larger electrostatic contribution to the bonding on the right side of the row as a result of the smaller getal-N bond lengths. This in turn results from the contraction of the 3d orbitals and the increasing stability of the 3d"I asymptote with respect to 3d"4s1. As discussed above the origins of the changes in bonding between the first and second ligand are similar to those for the transition metal-water systems. This results in a t least a qualitative sim- ilarity in the successive binding energies for the ammonia and water systems (see Table IV).

IV. Conclusions The successive binding energies in M(NH3)2+ have been com-

puted for the metals Sc through Cu at the MCPF level of cor- relation treatment. While the bonding is predominantly elec- trostatic, the ground state is dictated primarily by minimizing the Pauli repulsion. Using a point-charge model we show that for a given m e t a l 4 or metal-N distance, the dipole moment of ammonia is effectively much closer to the metal than is that of water. Thus the binding energies of the ammonia complexes are all larger than the corresponding water complexes even without allowing ligand polarization, which further increases the ammonia binding energy relative to water.

Acknowledgment. M. Sodupe gratefully acknowledges a Fulbright fellowship.

Registry NO. NH,, 7664-41-7.

Pulsed Laser Induced Photoelectrochemistry of Polypyridinic Ru( I I) Complexes in Water and in Acetonitrile

Katrin Karlsson and And& Kirsch-De Mesmaeker* Chimie Organique et Chimie Organique Physique, Facultd des Sciences, Universitd Libre de Bruxelles, CP 160, 50 Avenue F. D. Roosevelt, 1050 Brussels, Belgium (Received: February 28, 1991; In Final Form: June 26, 1991)

The photoelectrochemistry (PEC) of a series of complexes Ru(bpy),(tap),-,2+ (bpy, 2,2'-bipyridine; tap, 1,4,5,8-tetraaza- phenanthrene) is examined on a transparent SnO, electrode under pulsed laser illumination. The kinetics of the photoinduced open-circuit photopotentials in long time domains extending to a few milliseconds, are analyzed as a function of various parameters such as the nature of the solvent, the complex, and the oxygen concentration. Different SnOZ photosensitization processes are shown to occur, and some of them are explained on the basis of the bulk photochemistry. Biphotonic (or bimolecular) photoelectron transfers that generate long-lived reducing and oxidizing electroactive intermediates in the bulk solution are mainly responsible for the SnO, sensitization in long time domains; they give rise to electron injection into, or electron ejection from, the electrode, according to the experimental conditions and the complex. The measurement of the laser-induced open-circuit photopotentials as a function of time is shown to be a very useful method for studying these photosensitization mechanisms and may also be regarded as a technique complementary to the classical flash photolysis for the examination of photoinduced charge-transfer processes.

Introduc tion In the past 10 years considerable attention has focused on kinetic

studies of pulsed laser induced photopotentials or photocurrents

a t different "semiconductor/solution" interfaces in short time domains.

In a first approach, the semiconductor itself is irradiated and the separation of the photoinduced electron-hole pairs and the subsequent charge-transfer reactions at the interface are studied *To whom correspondence should be addressed.

0022-3654/91/2095-10681$02.50/0 0 1991 American Chemical Society

10682 The Journal of Physical Chemistry, Vol. 95, No. 26, 1991 Karlsson and Kirsch-De Mesmaeker

kinetically.] Willig et al. examined the rates of these processes in the picosecond time domain.,

In a second approach, a dye solution in contact with the sem- iconductor is excited by the pulsed laser beam. In such systems, the kinetics of the electron injection from the excited dye into a semiconductor (Figure 1) may also be examined in a picosecond time domain2-) as well as the subsequent processes of charge transfers a t the interface that occur in longer time scale^.^ We have examined these processes with different dye solutions such as Rhodamine B5 and Ru(I1) complexes, R ~ ( b p y ) , ~ + (bpy, 2,2’-bi~yridine)~ and Ru(tap),,+ (tap, 1,4,5&tetraazaphe- nar~threne)~ (I), in contact with a transparent SnO, electrode.

I In those studies, however, the photopotentials induced by pulsed

illumination (pulse width of a few tens of nanoseconds) have a rise time too long for the measurements of the rate of the primary photoelectron injection (process 1, Figure 1). In contrast, the kinetics of the charge-transfer processes that take place after the pulse in a longer time domain may easily be ~ t u d i e d . ~ - ~ This emphasizes photoelectrochemistry (PEC) as an interesting and attractive method for the detection of photochemically generated transient species, which is also complementary to the well-known technique of flash photolysis.

The interest of this method is further illustrated in this paper by PEC studies of a series of complexes Ru(bpy),(tap),-?+ (i = 0, 1, 2, 3) under pulsed illumination at a transparent SnO, electrode. As the photophysics* and the photochemistry of this series vary not only with the number of bpy and tap ligands in the compound but also with the nature of the solvent, with the presence of oxygen, and with the complex concentration, the influence of these parameters on the PEC behavior is also exam- ined.

Experimental Section The experimental setup for the laser-induced open-circuit

photopotentials measurements has been described previ~usly.~ A Molectron UV 24 N2 laser is used to pump a tunable DL-I1

dye laser. The coumarine 440 is chosen for the excitation of the different Ru(I1) complexes at 448 nm. The pulse power is of the

(1) Richardson, J. H.; Perone, S. P.; Deutscher, S. B. J . Phys. Chem. 1981, 85, 341. Gottesfeld, S. Eer. Eunsenges. Phys. Chem. 1987, 91, 362. Cook, R. L.; Dempsey, P. F.; Sammels, A. F. J. Electrochem. Soc. 1986,133,2287. Gottesfeld, S.; Feldberg, S. W. J . Electroanal. Chem. 1983, 146, 47. Rich- ardson, J. H.; Perone, S. P.; Steinmetz, L. L.; Deutscher, S. B. Chem. Phys. Lett. 1981, 77, 93. Feldberg, S. W.; Evenor, M.; Huppert, D.; Gottesfeld, S. J . Electroanal. Chem. 1985, 185, 209. Hartig, K. J.; Grabner, G.; Getoff, N.; Popikirov, G.; Kanev, S. Eer. Eunsenges. Phys. Chem. 1985, 89, 831. Wilson, R. H.; Sakata, T.; Kawai, T.; Hashimoto, K. J. J. Electrochem. SOC. 1985, 132, 1082. Sakata, T.; Janata, E.; Jaegerman, W.; Tributsch, H. J . Electrochem. SOC. 1986, 133, 339.

(2) Willig, F. Eer. Eunrenges. Phys. Chem. 1988, 92, 1312. Bitterling, K.; Willig, F. J . Electroanal. Chem. 1986, 204, 211.

(3) Brasil, M. J.; Matisuke, P. Solid State Commun. 1990, 74, 935. Willig, F.; Bitterling, K.; CharlC, K. P.; Decker, F. Eer. Eunsenges. Phys. Chem. 1984, 88, 374. Sakata, T.; Hashimoto, K.; Hiramoto, M. J . Phys. Chem. 1990, 94, 3040. Prybyla, S.; Struve, W. S.; Parkinson, B. A. J . Electrochem. Soc. 1984, 131, 1587. Norton, A. P.; Bernasek, S. T.; Bocarsly, A. B. J . Phys. Chem. 1988, 92, 6009.

(4) Kamat, P. V.; Fox, M. A. J . Phys. Chem. 1983, 87, 59. (5) Frippiat, A,; Kirsch-De Mesmaeker, A. J. Phys. Chem. 1985,89, 1285.

Frippiat, A,; Kirsch-De Mesmaeker, A.; Nasielski, J. J . Electrochem. SOC. 1983, 130, 237. Frippiat, A,; Kirsch-De Mesmaeker, A. J . Electrochem. SOC. 1987, 134, 66.

(6) Kirsch-De Mesmaeker, A.; Rochus-Dewitt, M.; Nasielski, J . J . Phys. Chem. 1986, 90, 6657. Karlsson, K. Ph.D. Thesis, in preparation, UniversitC Libre de Bruxelles.

(7) Masschelein, A.; Kirsch-De Mesmaeker, A. Nouu. J . Chim. 1987, 11, 329.

(8) Masschelein, A.; Jacquet, L.; Kirsch-De Mesmaeker, A,; Nasielski, J. Inorg. Chem. 1990, 29, 855.

Sn02 electrolyte

Figure 1. Electron-transfer processes at the interface SnO,/solution induced from the excited complex. RU*+*,~: excited complex in the solution. RU~+*,~,: excited complex at the electrode. Ru? oxidized complex, at the electrode (elec) or in the solution (sol). +0.24 V = rest potential of the S n 0 2 electrode.

order of 0.5 mJ, of which 18% is collected by the PEC cell. Polycrystalline SnO, (ND = 7 X 1019 donors cm-), 320 nm

thick), deposited on a glass plate, is commercially prepared by Glaverbel.

The cell, purged with the corresponding gas or vacuum de- gassed, contains the working electrode (SnO,) and a large area platinum electrode playing the role of both the counter and ref- erence electrode in the time domains that are investigated. The working and counter electrodes are connected via the 1-Ma ex- ternal resistance of the oscilloscope. The electrical contacts on SnO, and the connections to the conducting wires are made with silver paint, and the SnO, electrode is mounted on the cell glass by a device composed of Teflon rings and screws.

The flat band potential of the Sn0, electrode in water a t pH 4.5 has been determined and is equal to -0.1 1 V/SCE.6 As the semiconductor is highly doped, we consider this value as ap- proximately the one for the conduction band edge (Figure 1). The ‘rest potential” in darkness, of the SnOz electrode in contact with the aqueous solution and connected via the 1-Ma resistance to the Pt counter electrode, is equal to +0.24 V/SCE. The bands thus present a downward bending extending over 0.35 V. With acetonitrile solutions the situation is not very different; the flat band potential is slighly more negative (some tens of millivolts), and the rest potential is the same as in water.

The transient potential difference AV photoinduced between the working and the reference electrodes is monitored on a 100- MHz dual-trace storage oscilloscope (Philips PM 3266, 1 -MR input).

Solutions are prepared from MilliQ water and acetonitrile (Gold Label, for spectroscopy). L iN03 (Aldrich) and (But)4NPF6 (Fluka) (0.1 M) are used as supporting electrolytes. Complex concentrations are adjusted to get an equal absorbance at 448 nm (the excitation wavelength) for all of the complexes (concentrations of 7.92, 7.66, 7.81, and 7.10 X lo-) M for aqueous solutions of

respectively). The pH of each solution is adjusted to 4.5 by adding small amounts of concentrated HC1 or NaOH solutions. For the acetonitrile solutions the complex concentrations are 100 times lower.

The homoleptic complexes Ru(bpy),,+ and Ru(tap)32+ are prepared according to published procedures?J0 and the heteroleptic

Ru(bpy)?, Ru(bp~)Atap)~+, Ru(bpy)( ta~) ,~+, and Ru(taph2+,

Photoelectrochemistry of Ru(I1) Complexes The Journal of Physical Chemistry, Vol. 95, No. 26, 1991 10683

TABLE I: hoperties of Ru(bpy),(tap)*.,2+" 7(H20), ns 7(CH3CN), ns

degassed complex Em Ered EX* E d * air degassed air 1.28 -1.35 -0.76 0.7 1 388 63014 170 855'4 1.51 -0.88 -0.21 0.84 135 145 338 930

R U ( ~ P Y h2+ R ~ ( ~ P Y )(tapL2' 1.70 -0.83 -0.22 1.09 605 778 706 2134 Ru(tap)32+ 1.94 -0.75 -0.15 1.35 221 223 58 68

R u ( b p y M t a ~ ) ~ +

'E,,, oxidation potential in the ground state; Ed, reduction potential in the ground state; E,,,*, oxidation potential in the excited state; Ed*, reduction potential in the excited state; these redox potentials in CH3CN (V/SCE) are obtained by cyclic ~oltammetry.~ From these values, the excited-state redox potentials are estimated by using the energy of the emission maximum as the 0-0 transition energy. Experimental error for the luminescence lifetimes ( 7 ) I 3%;7 concentration of complex, M.

A V / mV

t / ns

Figure 2. Pulsed laser induced initial photopotential AVO for a N2 sat- urated aqueous solution of Ru(bpy),(tap)2+ 7.66 X lo-' M.

complexes Ru(bpy)2(tap)2+ and Ru(bpy)(tap)?+ are obtained8 by reacting 1 equiv of tap (or bpy) with 1 equiv of Ru(bpy),Cl," (or Ru(tap),Cl,) as described by Belser et a1.12 PFs- salts are used for the acetonitrile and C1- salts for the aqueous solutions.

The absorption spectra are carried out with a Cary 219 spec- trophotometer, and the steady-state illuminations are performed with an Ostram GY16 lamp, with a UV cutoff filter (NaN02).

Results and Discussion 1. Photophysical Properties of the Complexes Ru(bpy)i-

(tap)3-:+. The redox potentials in the ground and excited states of the different complexes of the series Ru(bpy)i(tap),-:+ ( i = 0, 1,2,3), as well as their luminescence lifetimes in degassed or air-saturated aqueous or acetonitrile solutions, are collected in Table I. Ru(bpy)?+ is the most reducing compound of the series. The reducing power of the complexes decreases with the increasing number of tap ligands; the trend is similar for the complexes in the excited 3MLCT (metal to ligand charge transfer) state. Ru(tap),,+* is the least reducing compound of the series and consequently the least efficient electron donor vs the SnO,.

Table I shows also that the luminescence lifetimes (complex concentration of the order of 10" M) in degassed solutions are longer in acetonitrile than in water, except for that of Ru(tap)?+ for which the particular photophysics has been discussed previ- ously.8 We have observed for all the complexes of the series that for concentrations of M the emission decaysI3 cannot fit with a single exponential or a biexponential. This deviation from a single decay is attributed to biphotonic processes (or bimolecular processes) that take place in highly concentrated solutions of excited complex (see below). The absence of a measurable lu- minescence from a quartz cell which has been preliminary in contact with a M complex solution indicates that adsorbed species do not contribute to this nonexponential decay.

(9) Anderson, S.; Seddon, K. R. J. Chem. Res., Synop. 1979, 74. (!O) Kirsch-De Mesmaeker, A,; Nasielski, R.; Maetens, D.; Pauwels, D.;

( 1 1) Sullivan, B.; Salmon, D. J.; Meyer, T. J. Inorg. Chem. 1978,17,3334. (12) Belser, P.; von Zelzwsky, A. Helu. Chim. Acra 1980, 63, 1675. (13) The emissions for such concentrations are not measured perpendicu-

larly to the excitation but from the irradiated side of the cell, as for mea- surements by diffuse reflectance.

Nasielski, J. Inorg. Chem. 1984, 23, 377.

2. Photocbemical Properties of the Complexes Ru(bpy),- (tap),-:+. As the photoreactions of the complexes of this series play an important role in PEC in the long time domains, they are briefly discussed in this section.

2.1. Photooxidation by Oxygen. The luminescence lifetimes of the complexes are 0, sensitive (Table I). The mechanism of this quenching process, debated in the literature, may take place either by energy or by electron transferl4-I6 in which the excited complex plays the role of electron donor:

Ru2+* + 0, - Ru3+ + 02'-

Ru3+ + 02'- + Ru" + 02 (1)

(2)

0,'- + H30+ a HOO' + H20 (3)

(For the sake of simplicity we will omit the nature of the ligands of the different complexes.)

The correlation observed between the photoredox potentials and the luminescence quenching constants by 0, in water and ace- tonitrile, for the tap complexes of Table 1,'' indicates that the quenching by electron transfer is also present for these compounds. Thus, in water, steps 1-3 should take place and the back electron transfer (reaction 2) is prevented by protonation of the oxygen radical anion (pK, = 4.5, reaction 3), making the detection of the Ru3+ complex easier in acidic solution^.'^ However, with the tap complexes of Table I, a t low pH, no Ru3+ species can be observed from a permanent bleaching of the starting solution under flash photolysis with oxygen. This is attributed to excited-state pro- tonations into acidic excited species that have too short lifetimes to be quenched by O2.I8

In acetonitrile, due to the absence of the 4'- protonation, the photooxidized complex Ru3+ cannot accumulate in the organic solvent because of its reduction by 02'- or derivatives.

2.2. Photooxidation by Persulfate. S20B2- is a powerful irre- versible oxidant involving two electrons. Photooxidation of the complexes in aqueous solution by this oxidant has been performed under continuous illumination to detect and characterize the Ru3+ species that play an important role in pulsed laser induced PEC in long time domains.

When R ~ ( b p y ) , ~ + is illuminated in the presence of S2082-, Ru(bpy),,+ is evidenced by the disappearance of the MLCT absorption of the starting complex and by the appearance of a weaker absorption centered a t 415 and 680 nm, typical of the oxidized c0mp1ex.I~ When left in darkness, the produced Ru- ( b ~ y ) ~ , + restores very slowly the starting complex, but not with

(14) Mulazzani, Q. C.; Ciano, M.; D'Angelantonio, M.; Venturi, M.; Rodgers, M. A. J. J. Am. Chem. SOC. 1988,110,2451. Demas, J. N.; Harris, E. W.; McBride, R. P. J . Am. Chem. SOC. 1977, 99, 3547. Lin, C. T.; BBttcher, W.; Chou, M.; Creutz, C.; Sutin, N. J . Am. Chem. SOC. 1976.98, 6536. Baqawi, K. R.; Akasheh, T. S.; Beaumont, P. C.; Parsons, B. J.; Phillips, G. 0. J . Phys. Chem. 1988,92,291. Albery, J. W.; Foulds, A. W.; Darwent, J. R. J . Photochem. 1982, 19, 37.

(15) Winterle, J. S.; Klinger, D. S.; Hammond, G. S. J . Am. Chem. SOC. 1976, 98, 3719.

(16) Navarro, J. A.; Roncel, M.; De la Rosa, F. F.; De la Rosa, M. A. J. Photochem. Photobiol. A: Chem. 1987, 40, 279.

(17) Jacquet, L. Ph.D. Thesis, 1990, Free University of Brussels. (18) Kirsch-De Mesmaeker, A.; Jacquet, L.; Nasielski, J. Inorg. Chem.

(19) Neumann-Spallaert, M.; Kalyanasundaram, K.; Gratzel, C.; Griltzcl, 1988, 27, 4451.

M. Helu. Chim. Acta 1980, 63, 116.

10684 The Journal of Physical Chemistry, Vol. 95, No. 26, 1991 Karlsson and Kirsch.De Mesmaeker

100% recovery. The slight permanent loss of absorption after long illumination times corresponds to some decomposition.20

If the same illumination experiments are carried out with the tap complexes, no transient decreases of absorption are detected, only slight permanent decreases of the starting absorbance being observed after long illumination times. The photooxidation of these complexes by S20s2- should, however, be possible thermodynam- ically, as is indicated by the luminescence quenching by this oxidant. We thus conclude that the photooxidized tap complexes would be too unstable in water to be detected with the spectro- photometer.

The flash photolysis of the tap complexes in the presence of S20s2- does not allow a better detection of the Ru3+ species, except with R~(bpy) , ( tap)~+ for which a solution photobleaching is observed; its recovery is, however, too slow to be followed in the time scale of the flash photolysis equipment. Consequently, R ~ ( b p y ) ~ ( t a p ) ~ + has a lifetime in water too long for the time scales in flash photolysis but too short for recording its absorption spectrum with a Cary spectrophotometer.

2.3. Biphotonic (or Bimolecular) Process. The biphotonic process of electron transfer between two excited states (reaction 4) in conditions of high-intensity irradiation has been reported in the literature.16.21 As stated before, this bimolecular process might explain the deviation of the luminescence decays from a single exponential with concentrations of complexes of the order of M. In the absence of irreversible reaction of the mo- noreduced (Ru'+) or oxidized complex (Ru3+), these two species react by back electron transfer (reaction 5) and restore the starting complex:

Ru2+* + Ruz+* - Ru" + Ru3+ (4)

Rul+ + Ru3+ - 2Ru2+ ( 5 )

The presence of process 4 has been tested by laser flash pho- tolysis of the complexes of Table I, highly concentrated in water and acetonitrile, and in either the absence or the presence of oxygen. This latter reoxidizes the Ru'+ and should allow the easier detection of the remaining Ru3+ species.

Although the detection of these species under intense laser- pulsed illuminations has been reported in the literature,'6-2' they cannot be observed in our experimental conditions.22

3. Principles of the Photoelectrochemical Mewurements. The complex solution is illuminated by the laser pulse through the Sn02 electrode. If the excited complexes are sufficiently reducing (donor level of the excited state well above the conduction band edge; Figure l), the adsorbed excited molecules inject electrons into the Sn02 during the laser pulse. This process gives rise to an open-circuit photopotential measured on an 0scilloscope.2~ With our experimental setup the appearance of a positive photopotential corresponds to an oxidation process (or electron injection).

In addition to the adsorbed excited complex, solution excited species can also inject electrons into the semiconductor after diffusion toward the electrode (Figure 1) (section 7). Thus, if the excited states have a lifetime longer than the laser pulse, an injection process may take place after the light flash.

Electron ejections from the semiconductor are also possible (appearance of a negative photopotential). At the Sn02 rest potential the electron ejection corresponds either to tunneling processes through the thin space charge layer of the semiconductor

~

(20) Nord, G.; Pedersen, P.; Bjergbakke, E. J . Am. Chem. SOC. 1983, 105, 1913. Ghosh, P. K.; Brunschwig, B. S.; Chou, M.; Creutz, C.; Sutin, N. J . Am. Chem. Soe. 1984, 106,4172.

(21) Milosavijevic, B. H.; Thomas, J. K. J . Phys. Chem. 1983, 87, 616. Meisel, D.; Matheson, M. S.; Mulac, W. A,; Rabani, J. J . Phys. Chem. 1977, 81, 144. Memming, R.; Schroppel, F.; Bringman, U. J. Electroonu/. Chem. 1979, 100, 307.

(22) A high complex concentration is of course a drawback for the de- tection, perpendicularly to the excitation, of transient absorption by laser flash photolysis, especially with the low power of the Molectron laser used in this work.

(23) It is considered an open-circuit photopotential because in the time scale of the measurements the injected electrons do not cross the 1-MR external load, as explained in previous works.56

TABLE I 1 Dependence of Initial Photopotentials (A V,) on Solvent and Complex"

complex AVo(H20) AVo(CH3CN) R u ( ~ P Y ) 3'' 28.5 27 Ru(bPy)dtaP)2+ 9 18.5

R ~ ( t a p ) , ~ + 5.4 5.6 Ru(bpy)(taph2+ 6 12

"AVO (mV) for aqueous (7.5 X M) and acetonitrile (7.5 X M) solutions, a same laser pulse power, and a same percentage of ab- sorption by the complex (see Experimental Section). (For R ~ ( b p y ) , ~ + 7.5 X M in an aqueous solution, AVO is approximately 2.5 mV.)

(process 2, Figure 1) or to processes of electron detrapping from band gap energy states, which may take place in long time do- mains. Therefore, only long-lived oxidizing species formed during the laser light are able to scavenge these "slow" trapped electrons. Complexes in the MLCT excited states are too short-lived (maximum 1 ps). In contrast, the oxidized complexes Ru3+ formed at the electrode after the electron photoinjections (Figure 1) are possible scavengers. Moreover, the Ru3+ species originating from the bulk photochemical reactions (section 2) may also play the role of such electron acceptors but will influence the AVbehavior in a longer time domain because they have first to diffuse toward the electrode.

As different photoinduced charge-transfer processes may occur in different time scales, it is important to be aware of the time domain accessible for the study of these processes with our ex- perimental setup. This time domain is limited for the short and long times by two time constants. The shortest one is determined by the pulse width of the laser (10 ns) and the rise time of the oscilloscope (10 ns a t 2 mV/cm); therefore, the open-circuit photopotential is reached in a few tens of nanoseconds. Thus, processes taking place in a time domain shorter than this rise time cannot be examined with our equipment. The limit for the long time scale is given by the R C constant controlling the discharge of the SnO, space charge capacitance into the external I-MQ resistance (of the order of 0.8 s as R C = 1 X lo6 Q X 0.8 pF). Consequently, the various processes of charge transfer may be observed up to 0.8 s.

For the clarity of the paper, we divide the description and discussion of the PEC results according to the time scale examined after the laser pulse: a time domain shorter than 5 ps and a time domain longer than 5 ps.

4. Laser-Induced Open-circuit Photopotentials for Times Shorter than 5 ps. With the aqueous solutions of complexes in contact with the Sn02 electrode, the laser-induced photopotentials are established after approximately 50 ns (Figure 2); they are called "initial photopotentials" (AVO), are oxygen insensitive, and correspond to the electron injections into the semiconductor conduction band from the excited complexes, either adsorbed or close to the electrode. An injection rate faster than the quenching by O2 for the adsorbed excited molecules is probably responsible for this 0, insensitivity of AVO.

The AVO values for the aqueous solutions are collected in Table I1 for the same rest potential of the Sn02 and for the same absorbed light intensity. When one bpy ligand in the Ru(bpy)t+ is replaced by one tap ligand, AVO drops to one-third of its value and decreases further by replacing a second and a third bpy. As expected, the photopotentials AVO increase with the reducing power of the excited states (Eox*, Table I) or, in other words, depend on the donor level of the excited complex in relation to the level of the SnO, conduction band edge. These photopotentials do not remain constant but relax during a few microseconds; these re- laxations are independent of the deoxygenation conditions and are attributed to the back electron transfer process from the semiconductor to the photoproduced oxidized complex (process 2, Figure 1, and reaction 6 ) . For other dyes, this AVrelaxation

Ru3+,'= + le-ce - Ru2+ was analyzed according to an exponential decay$ for these com- plexes, however, such simple decays are not observed probably because other processes (see below) disturb the relaxations.

Photoelectrochemistry of Ru(I1) Complexes The Journal of Physical Chemistry, Vol. 95, No. 26, 1991 10685

on C I M

Om

I I ms I I mV

(C)

2 -

3 -

4.' , I . I ,

AVIAV, t / ms

& / G

t / mr 0.0

0 1 0 0 2 0 0

Figure 5. (a) Slow AVrise for a degassed aqueous solution of Ru(tap)?' 7.10 X lo-' M. The asterisk indicates the value of the starting point for the analysis of the slow rise in (c). (b) Slow AYrise for an 02-, air-, and Nrsaturated aqueous solution of Ru(tap)32t. (c) Error function (-) of (k , t )1/2 for k, = 7' = 57-1 (ms-') and experimental data (0) AV/AV, vs time for the signal in (a). h

longer than a few microseconds, the AVbehavior for the aqueous solutions (complex concentration of M) depends on the nature of the complex and on the deoxygenation conditions. Therefore, each complex will be examined successively.

5.1.1. R ~ ( b p y ) , ~ + and R~(bpy),(tap)~+, For air- and oxy- gen-saturated aqueous solutions of these two complexes, AV continues relaxing in longer time domains, with a crossing of the zero potential line and an increase of AV toward negative values (Figure 4a,b). As shown in Figure 4c, this AVvaries according to the square root of the time. With deoxygenated solutions, however, the photopotential, after its relaxation in the micro- second-millisecond time scale, reaches a constant positive value (Figure 4a) until the discharge of the Sn02 space charge layer across the external load.

5.1.2. Ru(bpy)(tap),2+. With this complex, the behavior of the photopotential is similar except that less negative AVvalues are reached in the presence of oxygen.

5.1.3. R ~ ( t a p ) , ~ + . In this case, instead of a slow relaxation of the initial photopotential, a slow increase of AV is recorded (Figure 5a,b), the amplitude and rise time of which depend on the O2 content of the solution; they are enhanced when the oxygen concentration decreases (from oxygen-saturated, to air-saturated, to nitrogen-saturated, and to degassed solutions). In these slow rises, AVdoes not vary according to the square root of the time.

5.2. Acetonitrile Solutions. For low complex concentrations ( M) in acetonitrile, no influence of oxygen is observed on the AVrelaxations; they are similar to the one shown in Figure 4a (curve for the degassed solution).

The situation is different for higher complex concentrations (5 X IOw3 to M). With Ru(bpy)>+ the AV relaxation is de- pendent on the O2 concentration. For instance, with an air-sat-

10686 The Journal of Physical Chemistry, Vol. 95, No. 26, 1991 Karlsson and Kirsch-De Mesmaeker

S V / mV

1 / ms A V / mV

I / ms A V I mV

I " l;i 10 .A

v7 1 6 2 4 6 8 I O 1 2

Figure 6. (a) AV(t) for an air-saturated acetonitrile solution of Ru- ( t a ~ ) ~ ~ + 7.3 X M. (b) Same solution as (a) but saturated with Nz. (c) A V V S & ' / ~ for the signal in (b).

urated solution AVrelaxes slowly in the millisecond time domain, crmes the zero potential line, and reaches a negative value. This latter increases with the O2 content and the complex concentration and varies with the square root of the time.

With Ru(tup):+ solutions saturated with air or deoxygenated solutions (Figure 6a,b), a slow rise of AV in a few tens of mil- liseconds is observed, this is more clearly seen with a deoxygenated medium. In this latter case AVrises with the square route of the time (Figure 6c).

6. Correlations of the AVBehaviors (5 ps < t < 0.8 s) with the Nature of the Electroactive Species. The origin of the elec- troactive species responsible for the observed AVis discussed in this section on the basis of the photochemical processes of each complex in the different experimental conditions.

For high concentrations M) of these two complexes in aqueous solution, the slow AVrelaxations in the presence of oxygen toward negative AV values (Figure 4a,b) indicate the presence of an excess of reducible species produced during the laser flash, such as oxidized complex Ru3+. The dependency of AV on the square route of the time suggests that the reduction at the electrode of these species is diffusion controlled and that the oxidized complexes may be considered stable species in water, a t least in the time scale of the PEC studies. This agrees with the conclusions drawn from the photooxidation experiments under continuous and flash illumination of these two complexes (section 2).

These Ru3+ species are produced either by reaction 1 or, as we are dealing with highly concentrated and oxygenated solutions, by process 4 in which the monoreduced complex is reoxidized by oxygen and does not recombine with Ru3+. The deviation of the luminescence decays of such solutions from a single exponential indicates that a biphotonic process might indeed be present, in

6.1. AqueouS solutiola 6.1.1. Ru(bpy)?+ and R~(bpy)z(tap)~+.

accordance with the fact that the amplitude of the negative AV depends on the laser focalization on the electrode surface.24

In the absence of 02, the constant positive AVreached after the AVrelaxation in the microsecond time domain indicates that (i) some Ru3+ produced from the photoelectron injection escapes the back electron transfer (reaction 6) either by diffusing from the electrode surface to the bulk solution or by reaction and (ii) the species Rul+ and Ru3+ from process 4 either recombine in the bulk solution according to reaction 5 or contribute equally to electron injections and ejections so that no resulting AV is observed in a long time domain.

6.1.2. Ru(bpy)(tap)$+ and R ~ ( t a p ) , ~ + . With these two com- plexes in the presence of 02, the less important negative AV, or even its absence with R ~ ( t a p ) ~ ~ + , indicates of course a lesser contribution or the absence of long-lived Ru3+ species. However, there are no reasons a priori to exclude their production according to process 1 or 4; instead, their instability in water could be invoked, as this has already been concluded from the photo- oxidation experiments by S2OS2- under continuous illumination. If such is the case, the Ru3+ species stemming from process 4 decompose very rapidly and do not recombine with Ru'+. De- pending on the O2 concentration, the reoxidation of Rul+ by O2 enters in competition with its diffusion toward the electrode, where Rul+ injects an electron into the Sn02 conduction band; this generates the slow AV rises observed with Ru(tap),2+. With Ru(bpy)(tap)22+, whose lifetime in the oxidized state in water is probably intermediate between the long-lived nonreactive Ru- (bpy),,+ and the short-lived reactive R ~ ( t a p ) ~ , + , a slight AV relaxation is observed with 02-saturated solutions, as was the case with the Ru(bpy),2+, but more pronounced.

In an earlier work we showed that the lifetime of the transient electroactive Ru(tap),'+ species, produced by a photoreduction of Ru(tap),2+ with hydroquinone under a pulsed illumination at the Sn02 electrode, can easily be determined from the rise time of the induced ph~topotential.~ In those experiments, the lifetime of the bulk Ru'+ species was limited by a pseudo-first-order process (k,) which corresponded to the reoxidation of Ru" by benzo- quinone added to the solution. The diffusion-controlled oxidation of the transient Rul+ at the electrode leads, in those conditions, to a photopotential that increases as a function of time according to

where erf is the error function, n the number of transferred electrons, F the Faraday constant, S the surface of the electrode, C its capacitance, D the diffusion coefficient of Rul+, and Co the initial concentration of Rul+ produced after the laser pulse. The normalization of AV(t) by AVm for a time t = gives

AV(t)/AV, = erf (k,t)I/2 (11)

Equation I1 shows quite clearly that the lifetime of Rul+ (thus l/k,) can be determined when AV(t)/AV, is equal to the error function of I , Le., when AV(t)/AV, = 0.8427; thus, the time corresponding to 84.27% of the AVplateau is equal to the lifetime of Rul+.

If we apply the same analyses based on eq I1 to the AV(t) signals recorded with R ~ ( t a p ) , ~ + in the presence of O2 (Figure 5b) and originating from R ~ ( t a p ) ~ I + produced by process 4, the curves correspond also to an error function. Consequently, the R ~ ( t a p ) ~ ' + species are transient as they are reoxidized by O2 according to a pseudo-first-order process25 which is in competition with the

(24) In a previous paper6 this biphotonic process was excluded because we failed to observe such deviations of the luminescence dacays from single exponentials. This is attributed to the fact that the luminescence measure- ments had not been performed with an optical arrangement such as the one used in diffuse reflectance and which allows the analysis of the solution volume where the steady concentration of the excited species is at its highest.

( 2 5 ) A disappearance according to a bimolecular reaction does not lead to an erf.

Photoelectrochemistry of Ru(I1) Complexes The Journal of Physical Chemistry, Vol. 95, No. 26, 1991 10687

TABLE III: Lifetimes ( 7 ) of the Transient Electroactive R ~ ( t a p ) ~ ' + Species and the Corresponding A VPlateaus (A V,)"

7, ms AV-, mV under O2 5 2 under air 12.5 4.4 under N2 45 1.6 degassed 57 8.2

Aqueous solutions, under different conditions of gas saturation; the lifetime is determined from AV(r) /AV, according to an error function (Figure 7) as explained in the text. AV, represents the corresponding values of the AV plateau.

diffusion and oxidation of Ru(tap),'+ a t the electrode. The Ru(tap),I+ lifetimes can thus be determined from the AVrise times in different conditions of oxygenation of the complex solutions (Table 111). As expected, the lifetime increases for lower O2 concentrations, and given that the AVplateau (AV,) is inversely proportional to the square root of the constant k, (eq I), AV- decreases when R ~ ( t a p ) ~ I + disappears faster (i.e., an 02-saturated solution). The fact that for N,-saturated and degassed solutions AV(t) varies also according to an error function (Figure 5c) indicates that, even in the absence of any oxidant, Ru(tap),'+ disappears by a pseudo-first-order process (7 = 57 ms) which probably corresponds to its degradation in water.26

6.2. Acetonitrile Solutions. In acetonitrile for low complex concentrations ( lo4 M) the biphotonic process 4 does not take place and the oxidative luminescence quenching by oxygen (re- action 1) is immediately followed by the back electron transfer (reaction 2) so that long-lived electroactive species (Ru" or Ru3+) are not expected in this solvent. Indeed, AValways reaches, after a certain relaxation, a constant positive value and is unaffected by the presence of oxygen, for all the complexes of the series.

M) in acetonitrile, the biphotonic process 4 is the only possible reaction producing oxidized species when the recombination re- action 5 is prevented by the reoxidation of Rul+ by oxygen. Thus, the photoproduction of Ru(bpy),3+ in the presence of 02, according to this mechanism, explains once more the observation of slow AV relaxations toward negative values. The dependence of AV(t) on the square root of the time is compatible with a stable oxidized complex diffusing from the bulk solution toward the electrode.26

With R ~ ( t u p ) ~ ~ + in high concentration in acetonitrile, AV(t) is once more different from the other complexes and a slow AV rise depending on the oxygen concentration is observed. Conse- quently, in acetonitrile too, R ~ ( t a p ) ~ , + produced by process 4 would be unstable26 (maybe due to some water traces in the organic solvent), so that this species has no chance to reoxidize R ~ ( t a p ) ~ l + . This reduced entity generates a positive AV which rises, in the absence of 02, differently from the aqueous solution, i.e., with the square root of the time (Figure 6c); this indicates that the Ru(tap),l+ may be considered a stable species in ace- tonitrile, diffusing from the bulk solution to the electrode.

7. Photoelectrochemical Detection of ,MLCT' Excited States Produced in tbe Bulk of the Solution. As the excited 3MLCT states have long lifetimes, especially in acetonitrile (Table I), their oxidation at the electrode after their diffusion from the bulk solution should produce a slow AVrise (reaction 7) in the time domain of the excited-state lifetime.

In contrast, for high complex concentrations (5 X to

Ru'+*,,I - R u ~ + * , ~ ~ + RU,',~,~ + e-CB

R u ~ + ~ ~ ~ ~ + TEA - RU'+,~,~ + TEA'+

(7)

(8) However, the detection of this signal will be possible only if the following experimental conditions are fulfilled: (i) Use deoxy- genated acetonitrile solutions at low complex concentration with a view to prevent reactions 1 and 4. (ii) Add triethylamine (TEA) to scavenge the Ru3+ species (reaction 8)27 generated at the

(26) These observations are in accordance with the cyclic voltammetric data (For Ru(tap),2c: a first reduction wave, irreversible in water and re- versible in acetonitrile, and an oxidation wave, not totally reversible in ace- tonitrile. For Ru(bpy)?+: a reversible oxidation wave in acetonitrile).

A V / mV

(0)

t / nr

AVlAVm

I

0 400 800 1200 I600

Figure 7. (a) AV rise for an N,-saturated acetonitrile solution lo4 M in Ru(bpy),*+ and 1.25 M in TEA (triethylamine). The asterisk indi- cates the value of the starting point for the analysis of the slow rise in (b). (b) Error function (-) of ( k p t ) i / 2 for k, = 7-l = 390-' (ns-I) and experimental data (a) A V / A V , vs time from (a).

electrode by process 7. The oxidized species may indeed quench oxidatively the bulk excited states diffusing toward the electrode or give rise to the AV relaxation in the microsecond time scale, which masks the observation of a AV rise in this time domain. (iii) Choose R ~ ( b p y ) ~ ~ + for testing the presence of process 7 as its luminescence is not easily quenched reductively by TEA.

In those conditions, with lo4 M R ~ ( b p y ) , ~ + in deoxygenated acetonitrile, in the presence of 1.25 M TEA, a pulse illumination through the S n 0 2 electrode generates a slow AVrise (Figure 7a) of a few hundreds of nanoseconds, which corresponds to an error function (Figure 7b). The lifetime of the electroactive species determined at 84% of the AV plateau is 390 ns, whereas the luminmnce lifetime of Ru(bpy):' in 1.25 M TEA in acetonitrile is 652 m2* The origin of this discrepancy is not clear; some adsorption of TEA on the Sn02 could explain this difference. TEA in high local concentration on the electrode would enhance the luminescence quenching and thus would shorten the lifetime of the Ru(bpy),*+* in the electrode vicinity, as compared to its lifetime in the bulk solution.

Conclusions The results of this kinetic study of laser-induced photopotentials

for the series of complexes R~(bpy),(tap)~-?+ in water and in acetonitrile show clearly the different advantages of the photoe- lectrochemical (PEC) method for studying photoinduced charge-transfer processes at the interface and in the bulk solution.

If the absorption spectroscopy and the photoelectrochemistry are compared as detection methods of transients produced by flash photolysis, PEC is a very good technique when the transient species are electroactive. PEC is especially attractive when the transients have low absorption coefficients (in the case of Ru3+ species). Moreover, PEC is superior to the conventional flash photolysis when the electroactive species are formed by biphotonic processes

(27) Kalyanasundaram, K.; Kiwi, J.; Gratzel, M. Helu. Chim. Ada 1978, 61,2720. Chan, S.-F.; Chou, M.; Creutz, C.; Matsubara, T.; Sutin, N. J . Am. Chem. Soc. 1981, 303, 369.

(28) The drop from 855 to 652 ns for the luminescence lifetime of Ru- (bpy),*+ is due to some quenching by TEA.

10688 J . Phys. Chem. 1991, 95, 10688-10693

such as those discussed in this paper for the following reasons. A biphotonic or bimolecular process such as process 4 is of course more probable if a high local concentration of excited states is produced, which is usually the case within the first millimeter of a highly concentrated complex solution absorbing the light. This is exactly the situation encountered in PEC since the illumination through the SnO, electrode generates the highest concentration of transient electroactive species a t the electrode where the de- tection is carried out. It is also the case when the luminescence, for concentrated solutions, is measured as in the method of diffuse reflectance, i.e., from the same side as the excitation. In contrast, the spectroscopic detection of absorbing transients perpendicularly to the exciting beam is of course much less appropriate for such highly concentrated solutions.

Interestingly this PEC study has also shown that the stability of the oxidized complexes (Ru3+) in water decreases with the number of tap ligands: Ru(bpy),,+ and R ~ ( b p y ) ~ ( t a p ) ~ + behave

as stable species in the investigated time domain of PEC, whereas Ku(tap)?+ and, to a lesser extent, R~(tap),(bpy)~+ are too unstable to be detected by PEC.

A last positive aspect of this PEC study is evidenced by the analysis of AVas a function of time, which may lead to the lifetime of the transient electroactive species in the presence and in the absence of oxygen. Thus, the Ru(tap),l+ species in deoxygenated water has a lifetime limited to 57 ms, whereas in deoxygenated acetonitrile it is quite stable.

Acknowledgment. K. K . thanks the IRSIA (Institut pour la Recherche Scientifique dans 1'Industrie et 1'Agriculture) for a fellowship.

Regisby No. Sn02, 18282-10-5; LiNO,, 7790-69-4; (Bu-&NPF6, 3109-63-5; CH,CN, 75-05-8; Ru(bpy),2t, 15158-62-0; R~(bpy)2(tap)~+, 117183-30-9; Ru(bpY)(tap)?', 117183-28-5; Ru(tap)Y, 88181-60-6; 0 2 , 7782-44-7; S2Og2-, 15092-81-6.

Kinetics of the Reactions of Partially Halogenated Methyl Radicals (CH,CI, CH,Br, CH,I, and CHCI,) with Molecular Chlorine

J. A. Seetula, D. Gutman,* Department of Chemistry, Catholic University of America, Washington D. C. 20064

P. D. Lightfoot,* Laboratoire de Photophysique et Photochimie Moltculaire LIRA 348- CNRS, Universitd de Bordeaux I , 33405 Talence Cedex, France

M. T. Rayes, Laboratoire de Physicochimie Thtorique URA 503- CNRS, Universitt de Bordeaux I , 33405 Talence Cedex, France

and S. M. Senkan Department of Chemical Engineering, University of California. Los Angeles, California 90024 (Received: April 24, 1991)

The gas-phase kinetics of the reactions of four partially halogenated methyl radicals (CH,Cl, CH2Br, CH21, and CHC12) with Cl, have been studied as a function of temperature using a tubular reactor coupled to a photoionization mass spectrometer. Radicals were homogeneously generated by pulsed 193- and/or 248-nm laser photolysis. Decays of the radical concentrations were monitored in time-resolved experiments as a function of [Cl,] to obtain bimolecular rate constants for the R + C1, - RCI + C1 reactions studied. The following Arrhenius expressions (k = A exp(-E/RT)) were obtained (the numbers in brackets are log (A/(" molecule-' s-')), E/(kJ mol-I); the temperature ranges are also indicated): R = CH2Cl [-11.82 f 0.12, 4.1 f 1.3, 295-719 K]; R = CH2Br [-11.91 * 0.14,2.4 f 1.4, 295-524 K]; R = CH21 [-11.94 * 0.19, 0.8 & 2.2, 295-524 K]; R = CHC12 [-12.07 * 0.15, 10.3 f 2.0, 357-719 K]. Errors are lu, including both random and an estimated 20% systematic error in the individual bimolecular rate constants. The Arrhenius parameters of these and two other R + C12 reactions are compared with theoretical determinations based on semiempirical AM1 calculations of transition-state energies, structures, and vibration frequencies. The calculations qualitatively reproduce the obsehed trends in both the Arrhenius A factors and in the activation energies. The use of molecular properties to account for reactivity differences among all the R + C12 reactions which have been studied to date are also explored using free-energy correlations with these properties.

Introduction Recent interest in the combustion,' atmospheric oxidation,2 and

oxidative pyrolysis3 of halogenated hydrocarbons has given rise to a need for kinetic data on the elementary reactions of halo- genated carbon-based radicals in order to improve our under- standing of these complex processes. The kinetic data base for such radicals is much less extensive than that for nonsubstituted hydrocarbon radicals, particularly a t the elevated temperatures

(1) Chang, W. D.; Senkan, S. M. Environ. Sci. Technol. 1989, 23, 442. (2) World Meteorological Organization, Global Ozone Research and

(3) Senkan, S. M. Chem. Eng. Prog. 1987, 12, 5 8 . Monitoring Project, Report No. 16, Geneva, 1986.

pertaining to combustion processes. As part of our program of study of the reactions of polyatomic

carbon-based free radicals with a number of halogen-containing diatomic molec~les,4-~ we have previously studied the reactions

(4) Timonen, R. S.; Gutman, D. J. Phys. Chem. 1986, 90, 2987. (5) Timonen, R. S.; Russell, J. J.; Sarzynski, D.; Gutman, D. J . Phys.

(6) Timonen, R. S.; Russell, J. J.: Gutman, D. Int. J. Chem. Kinet. 1986, Chem. 1987, 91, 1873.

18. 1193. --. (7) Timonen, R. S.; Seetula, J. A,; Niiranen, J.; Gutman, D. J . Phys.

( 8 ) Seetula, J. A.; Russell, J. J.; Gutman, D. J. Am. Chem. Soc. 1990, 112, Chem. 1991, 95,4009.

1 ?A7 (9) Seetula, J. A.; Gutman, D. J . Phys. Chem. 1991, 95, 3626.

0022-3654/91/2095-10688%02.50/0 0 1991 American Chemical Society