Role of the Inner-Sphere Reorganization in the Photoinduced Electron

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J. Phys. Chem. 1995, 99, 6152-6162

6152

Role of the Inner-Sphere Reorganization in the Photoinduced Electron Transfer Reaction of Ru(I1) Complexes Containing Imine C=N or Azo N=N Double Bonds in the Ligands Mutsuhiro Maruyama and Youkoh Kaizu* Department of Chemistry, Tokyo institute of Technology, O-okayama, Meguroku, Tokyo, Japan Received: November 15, 1994; in Final Form: January 25, 1995@

Photoinduced oxidative and reductive electron transfer (ET) reactions of excited Ru(imin)32+ (imin = 2-(Nmethylfonnimidoyl)pyridine), Ru(imin)z(CN)z, and R u ( a ~ p y ) ~(azpy ~ + = 2-(phenylazo)pyridine), where imin and azpy contain imine C=N and azo N=N double bonds, respectively, with organic quenchers were investigated in acetonitrile solutions, and their AG dependencies of the quenching rate constants (k,) were compared with those of R ~ ( b p y ) 3 ~(bpy + = 2,2’-bipyridine) and Ru(L)z(CN)* complexes where L = 4,4’- or 5,5’-dmbpy (dmbpy = dimethyl-2,2’-bipyridine) and phen (phen = 1,lO-phenanthroline). The oxidative quenching rate constants of R ~ ( i m i n ) 3 ~and + Ru(imin)z(CN)z are smaller than those of the corresponding bpy, dmbpy, and phen complexes at the same AG value in the normal region. However, the AG dependencies of the reductive quenching rate constants of R~(imin)3~+ and R~(azpy)3~+ coincide with that of the corresponding bpy complex. The inner-sphere reorganization (Ai,,) caused by the deformation of the C=N bond of imin is considered to be the main reason for the disadvantage of ET in the normal region of the oxidative ET reactions of excited R ~ ( i m i n ) 3 ~and + Ru(imin)~(CN)2. On the other hand, the deformation of the C-N and N-N bonds of Ru(imin)j2+ and Ru(azpy)?+ is absent in the reductive ET reactions. The factors which govem these oxidative and reductive ET reactions are discussed and compared with other donor-acceptor systems.

Introduction There has been considerable interest in the photoinduced electron transfer (ET) reactions of transition metal complexes with a view to realizing efficient solar energy conversion and to elucidating the factors which govem the ET rates.’ The quenching ET reactions of the excited ruthenium(I1) polypyridine complexes have been the main subjects of these studies during the last decades.2 The ET rates are governed by several factor^.^,^ One of the most important factors which determine the ET rates is the reorganization energy (A), which consists of outer-sphere (solvent) (Aout) and inner-sphere (Ai,,) contributions. For metal complexes whose molecular geometry is significantly changed by the spin relaxation induced by ET, Ain plays a crucial role in determining the ET rates. For example, the oxidation process 4C0(II) ‘Co(II1) proceeds very s10wly.~~~ The transfer of the eg electron and the rearrangement of the remaining d electrons cause the substantial change of the metal-ligand (M-L) bond length, leading to the large A,, by which the ET rate is retarded. For such systems, Ain is often much larger than Aout. On the other hand, it has been accepted6,’ that Ai, of the oxidation and reduction processes of the excited ruthenium(I1) polypyridine complexes is negligible since the t2g, not the eg, orbital is involved in the ET. For such systems, A,, has an important effect on the ET rates. The inner-sphere reorganization could occur not only in M-L bonds but also in the ligand itself when ET occurs from the MLCT excited states of the complex. The substantial decrease of the electronic density on the ligand by ET will cause the deformation of the bonds in L. Although such a deformation is not likely to occur for the ligands bpy and phen which have a rigid framework of the aromatic ring, it is likely to occur for ligands with less rigid structures. Up to this point, however, the effect of the inner-sphere reorganization in the L on the ET rates has not been well studied.

-

* To whom correspondence should be addressed. @Abstractpublished in Advance ACS Abstracts, March 15, 1995.

In a previous paper,8 we reported that Ru(imin)z(CN)z, where imin = 2-(N-methylformimidoyl)pyridine,which has an imine C=N double bond in a less rigid structure, shows a highly redshifted emission in comparison with L = bpy and phen derivatives due to the inner-sphere distortion of the ligand in the MLCT excited state. Such distortion in the MLCT excited state is also reported for the Ru(I1) complexes which have an azpy ligand where azpy = 2-(pheny1azo)pyridine which has an azo N=N bond.9

imin

azPY -

In the lowest MLCT excited state of Ru(II) complexes which have C=N or N=N units in the ligand, the electron is relatively localized on these units in the antibonding phase, leading to bond extension in the excited state. These phenomena are analogous to the photochemistry of imine or azo aromatic compounds.lOJ1 The n-n* or n-n* transition of these aromatic compounds causes the electronic localization on the C=N and N=N double bonds, which leads to the cis-trans isomerization accompanying the bond extension. Although such an isomerization reaction has not been observed for imin and azpy ligands in the MLCT excited state of Ru(II) complexes, which is due to the chelate (bidentate) structure in the complex, the distortion of these bonds in the excited state causes the redshifted LMCT emission for these complexes, as mentioned above. An inner-sphere reorganization such as that observed in the transition between the ground and the MLCT excited states of Ru(I1) complexes which contain imin or azpy ligands is also considered to occur in the oxidative ET process of the excited state since the distortion of the ligand is considered to be released by the removal of the promoted electron. On the other hand, in the reductive quenching, the promoted electron in the

0022-3654/95/2099-6152$09.00/0 0 1995 American Chemical Society

J. Phys. Chem., Vol. 99, No. 16, 1995 6153

Inner-Sphere Reorganization in Electron Transfer ligand is considered to be left as it is since the dn orbital of Ru acts as the electron acceptor. It is interesting how such innersphere distortion in L in the MLCT excited state affects the oxidative and also the reductive ET rates of these complexes. We report here the AG dependencies of the photoinduced oxidative ET reactions from excited R~(imin)3~+ and Ru(imin)z(CN)2 to organic acceptors such as nitroaromatics and quinone derivatives and compare our results with those of other R u ( L ) ~ ~ + and Ru(L)2(CN)2 complexes where L = bpy, 4,4'-dmbpy, 53'dmbpy, and phen. We also investigate the reductive ET reactions of R~(imin)3~+ and R~(azpy)3~+ by aromatic amines and aromatic hydrocarbons and compare the AG dependencies of their quenching rates with that of the corresponding bpy complex. We discuss the role of the inner-sphere reorganization of these complexes on both the oxidative and reductive ET reactions. Materials The imin ligand was synthesized by a previously reported method.8 Ru(imin)3(PF& was synthesized by adding a saturated NHZF6 aqueous solution to a reaction filtrate after refluxing RuC13-xH20 and the ligand imin (3 times the molar quantity of RuClyxH20) in EtOH for 2 days. The product was purified by column chromatography (Merck, Aluminiumoxid 90) in acetonitrile, recrystallized from EtOH, and dried in vacuo.12 Ru(imin)z(CN)z,as well as the other Ru(L)z(CN)2 complexes where L = 4,4'-dmbpy and 5,5'-dmbpy, was synthesized according to the method of Demas et al.13 and purified by column chromatography (Wako, Wakogel) in methanol. The syntheses of Ru(azpy)z(CN)z and Ru(azpy)3(PF& followed the method of Krause and Krause.14 Most of the organic quenchers purchased from Wako Pure Chemical Industries, Ltd., Kanto Chemical Co., Inc., and Tokyo Kasei Kogyo Co., Ltd., were purified by recrystallization or sublimation in vacuo. Detailed procedures of the purification of the nitroaromatics and quinone derivatives used for oxidative quenching reactions have been previously d e ~ c r i b e d . ~ ~ The ,'~ aromatic amines used for the reductive quenching of R~(imin)3~+ were purified according to a literature method.7b The aromatic hydrocarbons used for the reductive quenching of R~(azpy)3~+ were used as received without further purification. The solvent acetonitrile was purified by distillation and used after the second distillation over CaH2. Apparatus Emission lifetime measurements of R~(imin)3~+, Ru(imin)2(CN)z, and Ru(azp~)3~+ were carried out by a single-photoncounting method on a PRA nanosecond fluorometer system. The samples were excited at around 300-380 nm from a PRA 510B nitrogen gas lamp through a Jobin-Yvon H-10 monochromator. Emission of the complex was detected through a Toshiba-Glass R-670 by a Hamamatsu R928 photomultiplier and counted on a Norland 5300 multichannel analyzer. The double wall cell compartment was thermostated at 298 K by a Haake thermostat. The emission lifetimes of Ru(L)z(CN)z where L = 4,4'-dmbpy and 5,s'-dmbpy were measured using the Nd3+:YAG laser system previously de~cribed.'~The sample solution was thermostated at 298 K by the Yamato NeoCool (Model CTE-21). Emission spectra of R u ( L ) ~ ~and + Ru(L)2(CN)2 where L = imin and azpy were determined by a single-photon-counting method, which is more sensitive in the red. The sample was excited at 488.0 or 514.5 nm from the Ar ion laser system, and the emission was detected through a monochromator (Nikon P-250) by a Hamamatsu photomultiplier R316 (S1 sensitivity) equipped with a photomultiplier cooler (HTV Model C659).

Emission spectra of R u ( L ) ~ ~and + Ru(L)z(CN)z where L = bpy, phen, and their derivatives were determined on the Hitachi 850 fluorescence spectrophotometer. All of the emission spectra were corrected with the standard solution of 4-(dimethylamino)4'-nitrostilbene. l6 Electrochemical and absorption measurements were carried out using the same instruments reported in previous public a t i o n ~ . * ~The ~ ' ~ electrochemical data in acetonitrile were determined against the Ag(s)/AgNOs (0.1 M) in CH3CN reference electrode with 0.1 M tetraethylammonium perchlorate as a supporting electrolyte. The redox potential of Ag(s)/AgNOs (0.1 M) in CH3CN vs SCE is +0.36 V. Measurements The quenching rate constants (Q in acetonitrile of R~(imin)3~+, Ru(imin)2(CN)2, and Ru(L)~(CN)Z(L = 4,4'-dmbpy and 5 3 ' dmbpy) were determined by a Stem-Volmer plot for the emission lifetime. For some of the donor-acceptor systems, that is, D = Ru(imin)~(CN)2and A = methyl m-nitrobenzoate and methyl p-nitrobenzoate, and D = R~(imin)3~+ and A = 1,Cnaphthoquinone and 2-methyl-1,Cnaphthoquinone, however, the emission lifetime measurements were not possible due to the strong absorption of the excitation light (300-380 nm) by the quenchers. For such systems, the kq values were determined by the emission intensity measurement using 514.5 nm light (which is out of the absorption range of the quenchers) from an Ar ion laser. We checked that the emission intensity measurements gave the same k, values as the lifetime measurements for some of other donor-acceptor systems. The concentrations of Ru(imin)32+,Ru(imin)2(CN)z, and Ru(L)z(CN)z were about 10-4-10-5 M, and that of the quencher was up to 10-1 M for the Ru(imin)z(CN)z system at its maximum. The 4 values of Ru(a~py)3~+ were determined by an emission intensity measurement using 514.5 nm light from an Ar ion laser, since a large amount of quencher (0.8 M at a maximum for the case of naphthalene), which is needed to quench the emission of this complex whose lifetime is very short (22 ns), strongly absorbed the excitation light (300-380 nm) of the N2 lamp used in the lifetime measurements. The absorption of 514.5 nm light by quenchers is negligible for intensity measurements. Sample solutions containing a donor and an acceptor gave a superposed absorption spectra in the visible region for all D-A systems in the concentration range used in this work. After the laser radiation of the intensity measurement of Ru(azpy)3*+ D-A systems, however, the absorption spectra were slightly changed.17 This is due to the reduction of R~(azpy)3~+ to Ru(azpy)3+ by q~enchers.'~ The amounts of the reduced species were estimated to be 0-2.8% depending on the quencher and its concentration. Such an extent of the reduction leads to some error (as large as 5% at a maximum) for the intensity measurement of this complex. Sample solutions were freed from oxygen by bubbling argon gas just before measurements. Results and Discussion Spectroscopic Character of the Ru(II) Complexes. Figure 1 shows the absorption and the emission spectra of Ru(L)3*+ (L = imin, azpy, and bpy) complexes in CH3CN at room temperature. The absorption band around 20-22 x lo3 cm-' could be assigned to the 'MLCT transition. Although the energy of the emission maximum decreases in the same order of L = bpy, imin (C=N), azpy (N-N) as the decrease of the energy of the 'MLCT absorption maximum, the emissions of R~(imin)3~+ and Ru(a~py)3~+ are highly red-shifted compared with that of

6154 J. Phys. Chem., Vol. 99, No. 16, I995

Maruyama and Kaizu

60x 1O3

40

20

0 20

25

30

35

40

Wavenumber ( 1 0 3 d ) Figure 1. Absorption and corrected emission (inset) spectra of R u ( L ) ~(L ~ += imin, dotted line; L = azpy, dashed line; L = bpy, solid line) in CH3CN solutions at room temperature ( E in M-' cm-'). TABLE 1: Lowest MLCT Absorption Maximum (Eabmax), Emission Maximum (E,,-), and Energy Difference between EabsmaX and E,,(A&) of Ru(L)3*+and Ru(L)z(CN)z in CH3CN at Room Temperature complex L (cm-I) E,,"'" (cm-') AEsf' (cm-I) Ru(L)?+ bpy 22.3 x IO3 16.0 x IO3 6.3 x IO3 imn

azpy Ru(L)2(CN)2 bpy imin azpy a

&ST

21.5 x IO3 20.3 x IO3

13.3 x io3 11.2 x IO3

8.2 x io3 9.1 x lo3

20.2 x IO3 19.9 x IO3 17.3 x IO3

14.0 x IO3 12.6 x IO3 '10.0 x lo3

6.2 x IO3 7.3 x IO3 '7.3 x lo3

= EabsmaX - E,"".

Ru(bpy)?+. The energy differences between the 'MLCT absorption maximum and the emission maximum (A&) of R~(imin)3~+ and Ru(a~py)3~+ are larger than that of R~(bpy)3~+ by 1.3 x lo3 and 2.1 x lo3 cm-', respectively. In Table 1, A E ~ Tvalues of R U ( L ) ~ ~and + Ru(L)z(CN)z 'complexes are summarized, together with the energies of the absorption and emission maxima. A E ~ T of Ru(imin)z(CN)z is also larger than that of Ru(bpy)Z(CN)z by 1.1 x lo3 cm-'. In general, red-shifted emissions come from two sources, that is, large outer-sphere (solvent) and inner-sphere reorganizations. For Ru(I1) complexes containing imin or azpy, the large AEST mainly comes from the latter contribution. We have shown for the R u ( L ) ~ ( C Nseries )~ that the solvent reorganization in the MLCT excited state of the L = imin complex is smaller than that of L = bpy and phen complexes due to the smaller n-electron system of the ligand L = imin.8a Wolfgang et al. directly observed the distortion of the azo N=N bond of azpy in the MLCT excited state of azpy Ru(I1) complexes by resonance Raman spectros~opy.~ The distortion of the ligands imin and azpy which contain C=N and N-N double bonds, respectively, in the MLCT excited state is analogous to that of aromatic azo and imine

compounds in the n-n* or n-n* excited state. It is wellknown that, in these excited states, the electron is localized on the N=N or C-N bond in the antibonding phase, by which the bond is extended and a cis-trans isomerization reaction is induced.lOJ1For olefins which have C-C double bonds as well, the bond extension is caused by the z-z* transition. For the case of ethylene, it is reported that the bond extension in the excited state caused by the electronic localization on the double bond in the antibonding phase exceeds 0.3 8, (1.35 A 1.69

A) 18

-

*

In Figure 2, the electronic distributions on the ligand of the vacant orbitals of Ru(imin)z(CN)z and Ru(azpy)z(CN)z, which are calculated by DV-Xa MO calculations assuming Cz symmetry for the complex,lg are shown together with that of azobenzene. The HOMO (main character is Ru &)-LUMO (main character is L n*)transition corresponds to the MLCT transition. It is found that the electron is relatively localized on the C=N and N=N double bonds in the LUMO orbitals (49b and 52a for Ru(imin)z(CN)z and 65b and 68a for Ru(azpy)z(CN)z) in the antibonding phase, which is analogous to the case of the LUMO of azobenzene (24h). Although the cis--trans isomerization reaction of imin and azpy ligands is considered to be restricted by the chelate structure of these ligands in the complex, the bond extension is likely to occur for these less rigid ligands in the lowest MLCT excited state, leading to redshifted emissions due to the reorganization of these ligands in the excited state. Table 2 shows the electrochemical data and the lifetimes (70) of R u ( L ) ~ ~and + Ru(L)z(CN)z (L = imin, azpy, and bpy) complexes in CH3CN at room temperature. decreases in the order of L = bpy (860 ns), imin (67 ns), and azpy (22 ns) for R U ( L ) ~ ~which + , is the same as the decrease of the energy of the emission maximum of these complexes. This is also the case for Ru(L)2(CN)z. 70 decreases from L = bpy (210 ns) to

J. Phys. Chem., Vol. 99, No. 16, 1995 6155

Inner-Sphere Reorganization in Electron Transfer

azobenzene

-2

'

L

Ph

-4

z

v

A

2 a,

8

-6

-8

Llc

-10 Figure 2. The main character and the electronic population of the frontier orbitals of azobenzene and Ru(L)z(CN)~(L = imin and L = azpy) obtained by the DV-XaMO calculation. For Ru(L)z(CN)~,only the population on the ligand is shown. TABLE 2: Electrochemical Data and the Lifetimes of R U ( L ) ~and ~ + Ru(L)~(CN)~ in CHJCN at Room Temperature

Oxidative potential vs Ag/O.l M Ag+ in CH3CN. Reductive potential vs Ag/O.l M Ag+ in CH3CN. Not observed in the applied potential range in this work (x+1.9 V). Measured at 298 K. e f 2 ns. f Could not be determined due to the low-energy emission of this complex.

(IAEgapl). It may be pointed out, however, that the short lifetimes, or the large k,, of imin and azpy complexes originate not only from small IAEgapI values but also in part from the large displacement (AQ,) of the potential surfaces between the ground and the MLCT excited states of these complexes caused by the large distortion of the ligands in the excited state. Quenching Mechanism and the Estimation of AG. The Stern-Volmer plots for the emission lifetime and intensity gave straight lines for all donor-acceptor systems in this work. The kq values obtained are summarized in Tables 3-6. The quenching mechanism of R~(imin)3~+-quinonederivatives (Table 3) and Ru(L)~(CN)~ (L = imin or dmbpy)-nitroaromatics and Ru(L)2(CN)2-quinone derivatives (Table 4) is oxidative electron transfer (ET) (reactions l a and lb) since kq correlates with the reductive potentials of quenchers. On the

imin (40ns). zo of Ru(azpy)2(CN)2 could not be determined since the emission of Ru(azpy)2(CN)2 is highly red-shifted (>>850 nm) and is out of the sensitivity range of the photomultiplier used in the lifetime measurements. These lifetime data are qualitatively consistent with the energy gap law21,22which predicts that the nonradiative transition rate constant (k,) increases with a decrease of the energy gap

other hand, the quenching mechanism of R~(imin)3~+-aromatic amines (Table 5 ) and R~(azpy)3~+-aromatichydrocarbons (Table 6) is reductive ET (reaction 2) since kq correlates with

electrochemical data complex RU(L)32+

L

E(M+/M)" (V) E(M/M-)b (V) lifetimed (ns)

C

-1.70 -1.53 -0.40

860 67' 22'

+0.47

-1.97

270

+OS0

-2.00

C

-0.80

bpy imn azPY

+0.91

Ru(L)~(CN)Z bpy imin azPY

+0.99

4w

f

'

6156 J. Phys. Chem., Vol. 99, No. 16, 1995

Maruyama and Kaizu

TABLE 3: Rate Constants for Oxidative Quenching of Ru(imin)32f by Quinone Derivatives

2,3,5,6-tetracNoro-p-benzoquinone -0.34 2,6-dichloro-p-benzoquinone -0.45 chloro-p-benzoquinone p-benzoquinone pyromellitic dianhydride methyl-p-benzoquinone

2,5-dimethyl-p-benzoquinone 1,4-naphthoquinone 2-methyl-1,4-naphthoquinone

-0.58 -0.47 -0.25 -0.06 -0.05 -0.01 0.08 0.10 0.18

-0.67 -0.86 -0.87 -0.91 -1.00 -1.02 -1.10

9.2 x 6.8 x 4.6 x 1.4 x

lo9 lo9 lo9 lo9

b 4.1 6.1 6.1 7.8

x lo8

x lo7 x 107 x 106

"Reductive potential of quenchers vs Ag/O.l M Ag+ in CHsCN.

* Could not be determined due to the low solubility of the quencher. the oxidative potentials of quenchers.

+

M~+* Q

-M+ +

Q+

(2)

In the above reactions, M2+ and M represent R u ( L ) ~ ~and + Ru(L)2(CN)2, respectively. Mn+* and Q denote the excited state of these Ru(1I) complexes and the quencher, respectively. Energy transfer is not considered to take part in the quenching since it is highly endothermic because of the high triplet energies of the aromatic quenchers used in this work (e.g., NJVdiethylaniline (2.99 eV)23aand naphthalene (2.64 eV)23b). The oxidative quenching of azpy Ru(II) complexes (Ru(az~y)3~+ and Ru(azpy)z(CN)2) and also the reductive quenching of Ru(imin)2(CN)2 could not be studied here because of the absence of a series of appropriate organic quenchers; azpy Ru(I1) complexes and Ru(imin)z(CN)z are a very weak reductant and oxidant, respectively. The reductive potential E(*M/M-)(= E(M/h4-) EO-0) of Ru(imin)2(CN)2 is -0.31 V vs Ag/Ag+ which is about 0.6 V more negative than that of the excited R~(bpy)3~+ (E(*M2+/M+) = +0.33 V) and R~(imin)3~+ (+0.29 V). On the other hand, azpy Ru(I1) complexes are too stable for oxidation to be observable in cyclic voltammetry (CV) (Table 2), as was shown by Krause and co-worker~.'~They have reported9 that it is necessary to use a strong oxidant such as Ce4+ to quench the MLCT excited states of these complexes oxidatively. The free energy change (AG) of ET reaction could be written as the f o l l o ~ i n g : ~ ~ , ~ ~

+

AG = E(D/D+) - E(A-/A) -

+ (wP- wr)

(3)

Here, E(D/D+) is the oxidation potential of the donor, E(A-/ A) is the reduction potential of the acceptor, and wp and wr are the work terms to bring the products and reactants together to the close-contact distance, respectively. wr is zero in this work since the organic quenchers have no charge. wp could be written as (4) where e is the charge of the electron, E is the static dielectric constant of the solvent, r is the distance between the donor and acceptor, and ZI and Z2 are the charge numbers of the counterpairs after the ET reaction, respectively. Using the values of 37Sz6for E and 10.9 and 10.0 AZ7for r of M2+ and M redox systems, respectively, wp is estimated as -0.09 eV for reaction la, -0.03 eV for reaction lb, and t0.03 eV for reaction 2. EO-0 is the zero-zero excitation energy of the 3MLCT state of Ru(II) complexes. Figure 3 is a schematic representation of

the potential surfaces of the ground and the excited states ('MLCT and 3MLCT states). 'EabS,3Eabs, and Eem in Figure 3 represent the energy of the 'MLCT absorption, 3MLCT absorption, and emission in fluid solution, respectively. In this figure, it is assumed that the gap of the potential minimum (AQeq) between the 'MLCT and 3MLCT states is zero. If we knew the exact position of the absorption maximum of the 3MLCT state (3EabJ, EO-0could be directly estimated as EO-0= (3E,b, Eem)/2. However, the 3MLCTstates of R u ( L ) ~ ~and + Ru(L)2(CN)2 are too weak to be resolved in the absorption spectrum, and thus the exact value of 3Eabs is uncertain. Under these conditions, several efforts have been made to estimate EO-0 for these complexes. One of the approaches for EO-0 uses the energy of the emission maximum at 77 K.28 However, it has been pointed out for Ru(L)~(CN)~ that the EO-o value estimated from the emission at 77 K does not necessarily represent that at room temperature since a great change of the solvation occurs around the complex at the glass transition temperature of the medium.29 Another approach for EO-ois to estimate the appropriate AE for the energy of the emission (Ee,) in fluid solutions. Then, EO-o is the sum of E,, and AI3 as follows (see Figure 3).

+

Eo-0

= E,,

+ AE

(5)

AE in eq 5 corresponds to the sum of the solvent and intramolecular reorganization energies between the ground and the excited states. Bock et aL30 applied the ET theory to the quenching experiments of the excited R~(bpy)3~+ to estimate EO-oand found that EO-ois larger than Eemby a factor of around 0.05 eV, which is considered to correspond to AE. Using the value of 0.05 eV for Ru(L)z(CN)~ (L = bpy or phen), EO-0values for these complexes were often estimated as EO-0 = Eem 0.05 eV7a,31as well. Following Bock et al., 0.05 eV was used as AE for R~(bpy)3~+ and Ru(L)z(CN)2 (L = 4,4'-dmbpy, 5,5'-dmbpy, or phen) complexes in this work. For R~(imin)3~+, Ru(imin)2(CN)2, and R~(azpy)3~+ whose AESTis much larger than that of bpy and phen derivatives, however, this AE value is not suitable since AE is dependent on AEST as shown in eq 8. From Figure 3, the following relation is found to hold neglecting an entropic difference between the states.

+

'Eabs= E,,

+ AE + 2AE

(6)

Here, A6 is the energy separation between the 'MLCT and 3MLCT states. Using the following relationship, (7) where AESTis the energy difference between defined previously, AE is rewritten as

AE=

'Eabs

and Eem as

AEST- Ac 2

Assuming that the values of the Ac are the same32for a series of R u ( L ) ~ ~and + Ru(L)2(CN)2 complexes (L = bpy, imin, or azpy), the energy difference between AE of R~(imin)3~+ (=AE(imin)), for example, and that of Ru(bp~)3~+ (=AE(bpy)) could be estimated as follows:

where hEsT(imin) and AEsT(bpy) are & ? S Tvalues for the

J. Phys. Chem., Vol. 99, No. 16, 1995 6157

Inner-Sphere Reorganization in Electron Transfer

TABLE 4: Rate Constants for Oxidative Quenching of Ru(L)z(CN)z (L = imin, 5,5'-dmbpy, 4,4'-dmbpy) by Nitroaromatics and Quinone Derivatives quenchers &MA-)"

imin 10-9kq (M-* s-1 )/AG (eV

55'-dmbpy 10-9kdAG

4,4'-dmbpy 10-9kq/AG

15.3/- 1.10 12.6/-1.04 12.6/-0.88 8.54/-0.77 9.99/-0.36 b 4.32/-0.22 5.13/-0.18 5.331-0.17 1.46/-0.03 0.71/+0.08 0.02/+0.18 b b b

17.3/- 1.40 16.6/- 1.34 15.0/- 1.18 15.0/- 1.07 13.7/-0.66 10.2/-0.65 11.11-0.52 11.3/-0.48 10.4/-0.47 8.04/-0.33 8.06/-0.22 5.16/-0.12 2.02/+0.05 0.959/+0.10 0.555/+0.13

17.9/-1.33 15.7/- 1.27

tetracyanoethylene (-0.12) TCNQd (-0.18) 2,3,5,6-tetrachloro-p-benzoquinone 2,6-dichloro-p-benzoquinone p-benzoquinone pyromellitic dianhydride

2,5-dimethyl-p-benzoquinone 1,4-naphthoquinone

1,2,4,5-tetracyanobenzene(- 1.05) tetrachlorophthalic anhydride (- 1.19) methyl p-nitrobenzoate (- 1.30) methyl m-nitrobenzoate (- 1.40) p-nitrotoluene (- 1.57) p-nitroanisole (- 1.62) 2,5-dimethylnitrobenzene(- 1.65)

C

C

11.3/-0.59 8.63/-0.58 8.95/-0.45 9.36/-0.41 9.271-0.40 C

6.30/-0.15 4.69/-0.05 C

0.35/+0.17 C

Reductive potential of quenchers vs Ag/O.l M Ag+ in CH3CN. That of the quencher, which has been listed in Table 3, is not shown here. Could not be measured because of the strong inner-filter effect of quenchers or low solubility of them. Not measured. d7,7,8,8tetracyanoquinodimethane.

TABLE 5: Rate Constants for Reductive Quenching of R~(imin)3~+ by Aromatic Amines

N,A',N"-tetramethylbenzidine phenothiazine 2,4-dimethylaniline p-anisidine

N,N-dimethyl-p-toluidine o-anisidine NJV-diethy lanilhe

E(Df/D)" (VI 0.07 0.17 0.34 0.35 0.35 0.40 0.40

I

AG (eV)

(M-Is-l)

-0.19 -0.09 0.08 0.09 0.09 0.14 0.14

7.7 x 8.5 x 3.1 x 4.0 x 6.8 x 2.2 x 1.1 x

kg

\

I

109 10' lo6 lo7 lo7 lo6 107

Oxidative potential of quenchers vs Ag/O.l M Ag+ in CH3CN.

TABLE 6: Rate Constants for Reductive Quenching of Ru(a~py)3~+ by Aromatic Hydrocarbons 1-naphthylamine 9,lO-dimethylanthracene anthracene pyrene benz[a]anthracene 2,6-dimethylnaphthalene 2,3-dimethylnaphthalene 2-methylnaphthalene 1-methylnaphthalene phenanthrene naphthalene

X(D+/D)' (V) 0.32 0.65 0.84 0.97 1.01 1.08 1.08 1.22 1.24 1.30b 1.34

AG (eV) kqc (M-l s-l 1 -0.85 (V) 2.8 x 10'' -0.52 3.0 x 10" -0.33 1.8 x 10" -0.20 8.9 x 109 7.9 x 109 -0.16 -0.09 8.1 x lo8 -0.09 7.1 x lo8 0.05 4.8 x lo8 0.07 4.3 x 108 0.13 3.7 x 107 0.17 5.8 x lo6

Oxidative potential of quenchers vs Ag/O. 1 M Ag' in CH3CN cited from the following reference except for phenanthrene: Neikam, W. C.; Desmond, M. M. J. Am. Chem. SOC.1964, 86, 4811. Estimated by subtracting 0.04 V from the value for naphthalene. Naphthalene data are taken from Pysh, E. S . ; Yang, N. C. J. Am. Chem. SOC.1963, 85,2124. Determined by the Stem-Volmer plot for emission intensity at 800 nm.

R~(imin)3~+ and R ~ ( b p y ) 3 ~complexes, + respectively. Using values of 1.9 x IO3 cm-' (0.24 eV) for h E ~ ~ ( i m i-n msT(bpy) ) and 0.05 eV for AE(bpy), AE(imin) is estimated to be 0.17 eV. By the same procedure, AE of Ru(a~py)3~+, whose AESTis 2.8 x lo3 cm-' (0.35 eV) larger than that of R~(bpy)3~+, is estimated to be 0.22 eV. Since the values of AESTfor Ru(L)z(CN)z (L = bpy, 4,4'dmbpy, 5,5'-dmbpy, or phen) are nearly identical to each other, the AE values of these complexes are considered to be nearly the same (0.05 eV). AEST for Ru(imin)2(CN), in CH3CN solution is 1.1 x 103-1.4 x lo3 cm-' larger than that of the 4,4'-dmbpy, 5,5'-dmbpy, and phen derivatives, leading to 0.13 eV for the AE value for Ru(imin)z(CN)z.

Figure 3. Schematic representations of the transitions between the ground and the 'MLCT and 3MLCT excited states for Ru(I1) polypyridine complexes.

Using these AE values and the energy of the emission maximum (Eem), EO-0 is estimated as follows by using eq 5: R u ( L ) ~ ~(L+ = bpy, 2.03 eV; imin, 1.82 eV; or azpy, 1.60 eV) and Ru(L)z(CN)z (L = 4,4'-dmbpy, 1.77 eV; 53'-dmbpy, 1.87 eV; phen, 1.86 eV; or imin, 1.69 eV). The estimated AG values are summarized in Tables 3-6. Eemand E(D/D+) values for Ru(L)z(CN)z (L = 4,4'-dmbpy, 5,5'-dmbpy, or phen) used to estimate AG have been reported in previous publication^.^^^'^ AG Dependence of Log k, of Quenching Reactions. In Figure 4, log kq is plotted against AG for the oxidative quenching of R~(imin)3~+ by quinone derivatives. The rate constants decrease as the driving force (-AG) decreases in the region of -AG < 0.4 eV (the normal region). At 0.4 eV < -AG < 0.6 eV, the rate constants remain at almost a constant value of 1 x 1OloM-I s-l (the plateau region). In the same figure, the data for the oxidative quenching of Ru(bpy)32+by quinone derivatives and nitroaromatics, which were reported by Kim et al.,7c are also plotted.34 It is found that, in the normal region, the quenching rate constants of R~(imin)3~+ complexes are smaller than those of R~(bpy)3~+ complexes when compared at the same AG value. In Figure 5, log k, is plotted against AG for the oxidative quenchings of Ru(L)z(CN)z (L = 4,4'-dmbpy, 5,5'-dmbpy, or imin) by quinone and nitroaromatics, together with the data of Ru(phen)2(CN>2?@ The quenching rate constants of Ru(imin)~(CN)2 are smaller than those of Ru(L)z(CN)2 (L = 4,4'-dmbpy,

Maruyama and Kaizu

6158 J. Phys. Chem., Vol. 99, No. 16, 1995

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1

1

1

1

1

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I

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-

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-

-

-

-

-

7.5

-

-

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-

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J. Phys. Chem., Vol. 99, No. 16, 1995 6159

Inner-Sphere Reorganization in Electron Transfer 11

I

o

I

0

10

0 0

9

0

sM 0

CI

8

7 M

c

M ;'

mi:

6 I

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1

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-1.0

-0.8

-0.6

-0.4

I

-0.2 AG(eV)

1

I

I

0.0

0.2

0.4

Figure 6. Plots of log k, vs AG for the reductive quenching of R u ( L ) ~ complexes ~+ (bow tie, L = imin; circle, L = azpy; square, L = bpy). The dotted curve represents the best-fit correlation using the parameter set 1, = 0 eV, KV = 2 x 10".

classical treatment, using a Marcus f u n ~ t i o n , ~ the , ~ overall quenching rate constant kq is written as a function of AG in eq 10 under the assumption that kp is much larger than Le, kd

kq =

1

+ -exp{ -(I+ AG*(O) kd KVKd

-r}

(10)

4AG* AG(0)

where K is the transmission coefficient which represents the probability that the reactants convert into the products in the transition state, v is the frequency factor, Kd is the equilibrium constant of the encounter complex (& = kd/k-d), and AG*(O) is the activation free energy when AG equals 0 eV (intrinsic barrier). Here, AG*(O) is considered to be the sum of the innersphere (AG*(0)in) and the solvent (AG*(O),,,) activation energies. AG*(O) = AG*(OIi, iAG*(O),,,

(1 1)

AG*(O),,, is estimated by the f ~ l l o w i n g , ~ ~ ~ e 2 1 1 I 1 AG*(O),,, = - -+ -- - - - 4(2rD 2rA a)(n2

I)

(12)

where r - ~and rA are the radii of the donor and the acceptor, a is the distance between the donor and the acceptor, and n is the refractive index of the solvent. The value of n for acetonitrile at 298 K is 1.34.26 By using the radii of the reactants,27 AG*(0)outis estimated to be 0.215 eV for the D-A systems of Ru(L)3*+ (reactions l a and 2) and 0.219 eV for those of Ru(L)2(CN)2 (reaction lb). The inner-sphere (Ai,,) and outersphere (Aout) reorganization energies are written as

The diffusion rate constant (kd) is approximated by the experimental plateau value, leading to a value of 1 x 1Olo M-' SKI for the oxidative quenchings of R u ( L ) ~ ~and + Ru(L)~(CN)~ and to a value of 1.5 x 1OloM-' s-l for the reductive quenching of R u ( L ) ~ ~ +The . equilibrium constant of the encounter complex (&) is represented as kd 4000 Kd = - = - , U N A k-d

3

3

following Fuoss and E i g e ~where ~ , ~ ~NA is Avogadro's number, leading to values of 3.27 M-' for the D-A systems of Ru(L)32+ (reactions l a and 2) and 2.53 M-' for those of Ru(L)2(CN)2 (reaction lb). By using these & and kd values, the dissociation rate constant (k-d) are estimated as 3.1 x lo9 and 4.0 x lo9 s-l for the R u ( L ) ~ ~and + Ru(L)2(CN)2 D-A systems, respectively. The two parameters Ah and KY were adjusted to fit the experimental results. However, several parameters sets reproduced the results and both KV and Ai, varied in a wide range. For example, the parameter sets ( K V , Ai,,) = (2 x 1013, 0 eV) and (5 x 1013,0.09 eV) reproduced the results of the oxidative quenching of Ru(L)z(CN)~(L = 4,4'-dmbpy, 53'-dmbpy, or phen). We could not obtain the optimum parameter set by this procedure. Thus, we fixed Ai, for the quenching reactions of R~(bpy)3~+ and Ru(L)2(CN)2 (L = 4,4'-dmbpy, 5S'-dmbpy, or phen) to be zero in this work. This treatment is generally accepted in many p u b l i ~ a t i o n s ~since ~ , ~ ,(i) ~ ,the ~ ~metal-ligand distortion is small because the Ru dn,not do, orbital is involved in the ET and (ii) the distortion of the ligands bpy and phen and their derivatives is small due to the rigid ring structure of these ligands. In fact, this treatment gives good KV values for our quenching systems as discussed later. The correlation in Figure 4 for the R~(bpy)3~+ oxidative quenching system was obtained by using the parameter set Ai, = 0 eV and KV = 3 x 10I2. Using the same KV value 3 x loL2

Maruyama and Kaizu

6160 J. Phys. Chem., Vol. 99, No. 16, 1995 for the R~(imin)3~+ system, a value of Ai, = 0.201 eV is needed to fit the results. For the Ru(L)z(CN)2 (L = 4,4‘-dmbpy, 53’dmbpy, or phen) oxidative quenching system, the parameter set Ai, = 0 eV and KV = 2 x 1013reproduced the results.37 Using the same KV value 2 x 1013 for the Ru(imin)z(CN)z system, a value of Ai, = 0.186 eV is needed to fit the quenching results (Figure 5). For the reductive quenching of R u ( L ) ~ ~(L+= bpy, imin, or azpy), a value of Ai, = 0 eV and KV = 2 x 10” reproduced the results (Figure 6). The important points revealed by these fitting calculations could be summarized as the following: (1) For the oxidative quenchings of excited R~(imin)3~+ and Ru(imin)2(CN)z,a value of Ai, as large as 0.2 V is needed to fit the results. (2) On the other hand, Ai, is nearly zero for the reductive quenchings of excited R~(imin)3~+ and R~(azpy)3~+ as is the case for Ru( b p ~ ) 3 ~ +(3) . KV for reductive quenching (2 x 10”) is smaller than that for oxidative quenching (3 x 10l2 and 2 x 1013). Points 1 and 2 could be explained as follows. In the lowest MLCT excited states of R~(imin)3~+, R~(azpy)3~+, and Ru(imin)2(CN)z, the C=N and N=N bonds of the imin and azpy ligands are distorted due to the relatively localized electronic character on these bonds. In the oxidative ET process, by which the promoted electron in the ligand is removed (reaction 15a), the distortion of the ligand is released. This inner-sphere deformation in the ET process is considered to act as Ai, for ET. In *Ru(III)-(L-) *Ru(III)-(L-)

+ Q -Ru(II1)-(L) + Q+ Q -.Ru(II)-(L-) + Q+

(15a) (15b)

reactions 15a and 15b, *Ru(III)-(L-) represents the MLCT excited state of Ru(I1) in which a d, electron of Ru(II) is transferred to L. In the reductive ET process, however, the electron in the ligand is not removed; an electron from a quencher enters into the dx orbital of *Ru(III) (reaction 15b). Thus, the distortion of the ligand in the MLCT excited state is not released in the reductive ET process, which implies that Ai, is negligible for this case. Point 3 could be discussed in terms of the transmission coefficient (K). The frequency factor (Y)depends on the fastest vibration which destroys the activated encounter complex. It ranges from 10I2 to 1014 depending on the vibrations involved (solvent or intramolecular) in ET. It is known that the fastest vibrational mode of the aromatic ligands is 4.5 x 1013s-1.39 Taking the order of 1012-1013for V: K is estimated to be around unity for the oxidative quenchings of R~(imin)3~+ and Ru(imin)z(CN)z, as well as for those of bpy and phen complexes. This means that these ET reactions proceed via the adiabatic mechanism. This seems reasonable since the orbital overlap between the aromatic ligand of a Ru(II) complex and a quencher is adequate because of the absence of bulky substituents, which often makes K smaller than unity due to the weak orbital overlap with a quencher,6 in the aromatic ligands studied in this work. On the other hand, K of the reductive quenchings of R~(imin)3~+ and R~(azpy)3~+, as well as that of Ru(bpy)?+, is estimated as 10-1-10-2 from KV = 2 x loll. This means that the ET proceeds nonadiabatically. The degree of the transmission coefficient (K) depends on the orbital overlap between the donor and acceptor as mentioned above. The small K value for the reductive quenchings of Ru(imin)3*+ and Ru(a~py)3~+ is considered to come from the weak orbital overlap between the dn orbital of *Ru(III) and the quencher since the dn orbital of metal complexes is shielded by ligands. The disadvantage of the orbital overlap of the dn orbital of metal complexes for ET

reactions is also reported by Sandrini et al.6 and Maruyama et al.15 for other D-A systems. It is concluded that the disadvantage of the oxidative ET reactions of R~(imin)3~+ and Ru(imin)z(CN)z comes from the large inner-sphere reorganization in the ligand imin. Such deformation of a ligand is not expected to occur for bpy and phen since they have rigid ring structures. In the reductive quenchings of R~(imin)3~+ and Ru(a~py)3~+, Ai,, is negligible such as in the case of Ru(bpy)P. In the reductive ET studied in this work, k, is affected by K, not by Ai,. Comparison with Other Systems. The role of the innersphere reorganization of the metal complexes in ET reactions has been extensively studied both theoretically and experimentally, most of the studies have been directed to the electronexchange reactions!,@ The metal-ligand (M-L) bond distance is changed (i) by the spin relaxation and (ii) by the change of the electrostatic interaction between M-L during the redox reaction of M. Crystallographic and EXAFS measurements have been used to determine the differences in the M-L distance ( A r ) between the oxidized and the reduced states of the It has been reported that Ar values of C O ( L ) ~ ~ +(L ’~+ = en, phen, or bpy) and C O ( L ) ~ ~ +(L / ~= + H2O or NH3) are 0.190.22 A, leading to values of 2.12-2.84 eV for Ai,, for these CO~+’~ exchange + reactions! Such large Ai,, values could be attributed to the spin relaxation (4T lA), that is, the rearrangement of d electrons accompanied by the transfer of the eg electron. The slow ET rates of the exchange reaction of C O ~ + ’ ~mainly + come from large Ah, In these cases, A,, is estimated to be 1.18-0.57 eV, which is ‘/2-l/3 of Ai,,. For Rh(III)/Rh(II)36 and Cr(III)/Cr(II) redox systems as well, substantial inner-sphere reorganization occurs in M-L bonds by the transfer of the eg electron. For the redox systems that involve tzg redox orbitals, Ru(II)/ Ru(III), Os(II)/Os(Il’I),and Re(I)/Re(II), for example, the change of the M-L distance by ET is small. The difference of the M-L bond length is not caused from the spin relaxation but from the change of the electrostatic interaction between M and L; the increase and the decrease of the charge of the metal ion make the M-L distance shorter and longer, respectively. Smaller Ar values are reported for Ru2+13+ compared with C O ( L ) ~ ~ + ’systems; ~+ 0.09 for Ru(H20)62+’3+,0.04 for R u ( N H ~ ) ~ ~ +and / ~ +0.02 , A4 for R~(en)3~+”+. AI, values for these Ru2+13+exchange reactions were calculated as 0.36-0.64 eV (L = H20), 0.10-0.12 eV (L = NH3), and 0.04 eV (L = en), all of which are much smaller than A,, (1.20 eV (L = H20), 1.11 eV (L = N H 3 ) , and 0.919 eV (L = en)). The solvent reorganization plays the important role in determining the ET rates for these R u ~ + / exchange ~+ reactions. The Ar value for the R~(bpy)3~+”+ system is reported as nearly zero4 and thus the main reorganization barrier of this redox system comes from solvent rearrangement. It has been accepted that Ai, values for the oxidative and the reductive ET reactions of the excited polypyridine (bpy and phen and their derivatives) Ru(I1) complex is negligible,4b~6~7~36 because (1) the ET process is identical to the Ru2+13+redox system from the viewpoint of the change of the electronic configuration (the eg orbital is not involved in ET) and (2) the distortion of these polypyridine ligands is negligible due to the rigid ring structure. The solvent reorganization is considered to be the main contribution to 1 for the quenching reactions of excited Ru(I1) complexes. On the other hand, Ai, values as large as 0.2 eV put the oxidative ET reactions of Ru(imin)32+and Ru(imin)z(CN)z at a disadvantage in the normal region compared with those of

-

A41942

A41942

J. Phys. Chem., Vol. 99, No. 16, 1995 6161

Inner-Sphere Reorganization in Electron Transfer bpy or phen complexes. This Ah value is considered to originate from the reorganization in the ligand L = imin not from the change of the M-L distance. The value of Ain = 0.2 eV is larger than that which comes from the change of electrostatic interaction between M and L (for Ru(I1) Ru(1II) processes of N-coordinated Ru(I1)-L complexes, 0.05-0.06 eV (L = NH3), 0.02 eV (L = en)), but it is much smaller than the 1 , value which comes from the spin relaxation (for Co(I1) Co(I11) processes, 1.42 eV (L = NH3), 1.3 eV (L = en), 1.3 eV (L = HzO), 1.06 eV (L = bpy)). Note here that the values of Ah for the M(II) M(II1) processes listed above are one-half of those for the exchange reaction in which two reactants are involved in ET. It should be pointed out that the major contribution to A comes from the solvent reorganization even for R~(imin)3~+ and Ru(imin)2(CN)2 oxidative quenching systems; the value of Ah around 0.2 eV is still smaller than the solvent reorganization energy A,, (= 0.86-0.87 eV). Such inner-sphere reorganization in the ligands of metal complexes in the photoinduced ET reactions as studied in this work is also reported by Chen et a1.& for the photoinduced intramolecular ET reaction of [(bpy)Re(C0)3(MQ+)l2+ (MQ+ = N-methyl-4,4'-bipyridinium cation). MLCT (Re dn bpy n*)excitation of the complex induces the bpy n* MQ+n* electron transfer, that is, [(bpy-)Re(C0)3(MQ+)l2+* [@PY)Re(C0)3(MQ')l2+*. In this process, the nonplaner conformation of MQ+ (its dihedral angle is 47" in the ground state) changes to the planar one due to the strong n-n interaction between the rings in the one-electron reduced state. Such a conformational change is analogous to the case of biphenyl which favors a planar conformation in the n-n* excited state.& Chen et al. estimated Aln for the conformational change of MQ+ in the ET process to be 0.57 eV at a maximum. The degree of the distortion of a ligand in ET reactions is related to the coordinating structure of the ligand. The rotational distortion is allowed for MQ+ due to the monodentate structure of the ligand in the complex. However, the rotational distortion around C=N and N=N, which is observed for the aromatic imine and azo compounds in the n-n* or n-n* excited state as the cis-trans isomerization reaction, is not allowed for imin and azpy ligands since they chelate to the Ru(I1) ion in the complex. The distortion of imin in the oxidative ET reaction is considered to be caused by the stretching of the C=N bond since it has a less rigid structure in the ligand. Such deformation is not likely to occur in bpy, phen, and their derivatives since they consist of rigid aromatic rings.

-

-.

-

-

-+

-+

Summary Inner-sphere .reorganization occurs not only in M-L bonds but also in the ligand itself. We investigated the photoinduced oxidative ET reactions from excited Ru(imin)?+ and Ru(imin)2(CN)2 to organic quenchers and found that kq values of these donor systems are smaller than those of the corresponding bpy, dmbpy, and phen complexes in the normal region. The bond deformation of the C=N bond of the ligand L = imin in the ET reaction, which could be estimated to be around 0.2 eV, is considered t o be the main reason for the disadvantage of the quenching reaction of these complexes. In the reductive ET reactions of R~(imin)3~+ and R~(azpy)3~+, however, the AG dependencies of log kq of these complexes are nearly identical with that of R~(bpy)3~+. &, is zero for these cases; the distortion of C=N and N=N in the MLCT excited state is not released in the reductive ET process. Since the accepting dn orbital of the excited Ru(I1) complexes is sterically shielded by ligands, the reaction proceeds nonadiabatically ( K 1).

References and Notes (1) For reviews, see: (a) Balzani, V.; Barigelletti, F.; Cola, D. L. Top. Curr. Chem. 1990, 1.58, 31. (b) Willner, I.; Willner, B. Top. Curr. Chem. 1991, 159, 153. (2) For reviews, see: (a) Juris, A.; Balzani, V.; Barigelletti, F.; Campagna, S.; Belser, P.; Von Zelewsky, A. Coord. Chem. Rev. 1988,84, 85. (b) Kalyanasundaram, K. Coord. Chem. Rev. 1982, 46, 159. (3) Marcus, R. A. J . Chem. SOC.1956, 24, 966. (4) (a) Sutin, N. Prog. Inorg. Chem. 1983,30,441.(b)Sutin, N. Acc. Chem. Res. 1982, 15, 275. (5) Yoshimura, A,; Nozaki, K.; Ikeda, N.; Ohno, T. J . Am. Chem. SOC. 1993, 115, 7521. (6) Sandrini, D.; Maestri, M.; Belser, P.; von Zelewsky, A,; Balzani, V. J . Phys. Chem. 1985, 89, 3675. (7) (a) Kitamura, N.; Obata, R.; Kim, H.-B.; Tazuke, S. J. Phys. Chem. 1989, 93, 5764. (b) Kitamura, N.; Kim, H.-B.; Okano, S.; Tazuke, S. J . Phys. Chem. 1989, 93, 5750. (c) Kim, H.-B.; Kitamura, N.; Kawanishi, Y.; Tazuke, S. 1989, 93, 5757. (8) (a) Maruyama, M.; Matsuzawa, H.; Kaizu, Y. Inorg. Chem., submitted for publication. (b) To a cooled solution of 2-pyridinecarboxaldehyde in benzene, a methylamine (1.5 times the molar quantity of 2-pyridinecarboxaldehyde)solution of benzene was added. After standing for 1 h, the solvent was caused to reflux with a Dean-Stark water separator until no more water separated. After removal of the solvent, the product was distilled at 78-79 "C118 " H g . (9) Wolfgang, S.; Strekas, T. C.; Gafney, H. D.; Krause, R. A,; Krause, K. Inorg. Chem. 1984, 23, 2650. (10) For example, Fischer, E. J . Am. Chem. SOC.1960, 82, 3249. (11) (a) Fischer, E.; Frei, Y. J. Chem. Phys. 1957, 27, 808. (b) Wettermark, G.; Dogliotti, L. J . Chem. Phys. 1964, 40, 1486. (12) Elemental analysis. Calcd for Ru(C~N~H&(PF&:c, 33.55; H, 3.20; N, 11.18. Found: C, 33.39; H, 3.51; N, 11.02. (13) Demas, J. N.; Tumer, T. F.; Crosby, G. A. Inorg. Chem. 1969,8, 674. (14) Krause, R. A.; Krause, K. Inorg. Chem. 1982, 21, 1714. (15) Maruyama, M.; Sonoyama, N.; Kaizu, Y. J. Phys. Chem. 1994, 98, 5332. (16) Lippert, E.; Nagele, W.; Blankenstein, I. S.; Staiger, U.; Voss, W. Z.Anal. Chem. 1959, 170, 1. (17) The intensity of the long wavelength side of the lowest MLCT absorption band of R~(azpy)3~+ (500-600 nm) increases after the laser radiation. This is due to the production of the reduced species Ru(azpy)3+. The absorption spectrum of Ru(azpy)3+ could be obtained by the electrochemical reduction of Ru(azpy)sZ+. The MLCT absorption maximum of Ru(azpy)s+ (518 nm) is red-shifted compared with that of Ru(az~y)3~+ (490 nm), and their molecular extinction coefficients ( E ) are nearly identical. The increase of the intensity of absorption by laser radiation around 600 nm is due to fact that the 6 value of Ru(azpy)3+ is larger than that of ( E = 800)). Using R~(azpy)?~+ (Ru(azpy)3+ ( E = 7000) and R~(azpy)3~+ the E values of these complexes, the amount of the reduction could be estimated as described in the text. (18) Willkinson, P. G.; Mulliken, R. S. J . Chem. Phys. 1955,23, 1895. (19) The metal atom and two cyanides were placed at the coordinate origin and on the X and Y axes, respectively. Two imin and azpy ligands were placed in the X-Z and Y-Z planes in such a way that the nitrogen atom of the pyridine ring of these ligands is on the Z and -2 axes. The structure of the azpy ligand is followed by the X-ray crystallographic dahzo Other details including the structure of the ligand imin were described in a previous paper.8a (20) Seal, A,; Ray, S. Acta Crystallogr. 1984, C40, 929. (21) (a) Englman, R.; Jortner, J. Mol. Phys. 1970, 18, 145. (b) Freed, K. J.; Jortner, J. J. Chem. Phys. 1970, 52, 6272. (22) Meyer, T. J. Prog. Inorg. Chem. 1983, 30, 389. (23) (a) Dubroca, C.; Lozano, P. Chem. Phys. Lett. 1974, 24, 49. (b) Fry, A. J.; Liu, R. S. H.; Hammond, G. S. J . Am. Chem. SOC. 1966, 88, 478 1. (24) Rehm, D.; Weller, A. Isr. J. Chem. 1970, 8, 259. (25) Chen, J.-M.; Ho, T.-I.; Mou, C.-Y. J . Phys. Chem. 1990,94,2889. (26) Organic Solvenrs, 3rd ed.; Techniques of Chemistry; WileyLnterscience: New York, 1979. (27) The radii of Ru(L)3*+, Ru(L)z(CN)2, and the organic quenchers studied in this work are estimated as 7.1, 6.2, and 3.8 A, respectively.' (28) Demas, J. N.; Addington, J. W.; Peterson, S. H.; Harris, E. W. J . Phys. Chem. 1977, 81, 1039. (29) Kitamura, N.; Kim, H.-B.; Kawanishi, Y.; Obata, R.; Tazuke, S. J . Phys. Chem. 1986, 90, 1488.

Maruyama and Kaizu

6162 J. Phys. Chem., Vol. 99, No. 16,1995 (30) Bock, C. R.; Connor, J. A.; Gutierrez, A. R.; Meyer, T. J.; Whitten, D. G.; Sullivan, B. P.; Nagle, J. K. J . Am. Chem. SOC. 1979, 101, 4815. (31) Nagle, I. K.; Dressick, W. J.; Meyer, T. J. J. Am. Chem. SOC.1979, 101, 3993. (32) The singlet-triplet splitting (A€)of the MLCT state of Ru(II) polypyridine complexes is expressed as 2K where K is the exchange integral between the JC*orbital of L and the d~ orbital of the ruthenium atom. Since the electron of imin and azpy is localized relatively close to the metal orbitals in the LUMO, K of imin and azpy complexes may be larger than that of bpy and phen complexes to some extent. Although we could not estimate K values of these complexes, it is expected that K is small for these complexes and the difference of K between imin, azpy complexes and bpy, phen complexes is not so large since the degree of the Ru-L mixing is small. K becomes larger as the degree of the metal-ligand mixing becomes greater.33aIn fact, the large K values of the MLCT transition from Pd@), Pt(II), Ir(I), and Rh(1) to the n* orbital of CN-, CNR (R = alkyl) or to a d orbital of PR3 (200-5000 cm-' 33a) is due to the large metal-ligand mixing. On the other hand, K of M(bp~)3~+ complexes where M = Fe, Ru, and Os is reported as 800-850 ~ m - The ~ . small ~ ~K ~value of M(b~y)3~+ comes from the relatively small metal-ligand mixing and also from the diffuse JC electron systems of the ligand. It is considered that the K value of Ru(Q polypyridine complexes, including imin and azpy, is small and thus the difference between them is also small. (33) (a) Lever, A. B. P. Inorganic Electronic Spectroscopy, 2nd ed.; Elsevier: Amsterdam, 1984; Chapter 5. (b) Kober, E. M.; Meyer, T. J. Inorg. Chem. 1982, 21, 3967. (34) A G of the reference was reestimated following our way described in the text.

(35) (a) Fuoss, R. M. J . Am. Chem. SOC.1958,80,5059. (b) Eigen, M. Z. Phys. Chem. 1954, 1, 176.

(36) Creutz, C.; Keller, A. D.; Sutin, N.; Zipp, A. P. J. Am. Chem. SOC. 1982, 104, 3618. (37) The parameter set obtained here (A, = 0 eV, KY = 2 x 1013) is slightly different from our previously reported values (A,, = 0 eV, KY = 3 x 1013).15This comes from the difference of estimation of A G and also from the parameters used in the calculation. Previously, we used the energy of the emission maximum at room temperature as A G without the correction for AE. We estimated r~ = 7 %,forRu(L)z(CN)z (L = 4,4'- or 5,5'-dmbpy, 4.7-diphenyl-phen) as the mean radius of these complexes. In this work, we used Q, = 6.2 8, for Ru(L)z(CN)z (L = 4,4'- or 5.5'-dmbpy, phen). In addition, in this work, we used 1.0 x 1Olo M-I s-l for kj instead of 1.5 x 1OloM-I s-l since the correlation in the normal region is much improved. (38) Legros, B.; Vandereecken, P.; Soumillion, J. Ph. J . Phys. Chem. 1991, 95, 4752. (39) Scandola, F.; Indelli, M. T.; Chiorboli, C.; Bignozzi, C. A. Top. Curr. Chem. 1990, 158,73. (40) Buhks, E.; Bixon, M.;Jortner, J.; Navon, G. Inorg. Chem. 1979, 18, 2014. (41) Brunschwig, B. S.; Creutz, C.; Macartney, D. H.; Sham, T.-K.; Sutin, N. Faraday Discuss. Chem. SOC. 1982, 74, 113. (42) Stynes, H. C.; Ibers, J. A. Inorg. Chem. 1971, 10, 2304. (43) Bemhard, P.; Biirgi, H.-B.; Hauser, J.; Lehmann, H.; Ludi, A. Inorg. Chem. 1982,21, 3936. (44)Chen, P.; Curry, M.; Meyer, T. J. Inorg. Chem. 1989, 28, 2271.

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