J . Phys. Chem. 1989, 93, 5151-5164
5757
Photoinduced Electron-Transfer Reactions of Ruthenium( I I ) Complexes. 2. Oxidative Quenching of Excited Ru(bpy);+ by Neutral Organic Electron Acceptors Haeng-Boo Kim, Noboru Kitamura,* Yuji Kawanishi, and Shigeo Tazuke* Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 227, Japan (Received: October 24, 1988; I n Final Form: March 7, 1989)
Oxidative quenching of * R ~ ( b p y ) ~(bpy ~ + = 2,2’-bipyridine) by neutral organic electron acceptors such as quinones, nitroaromatics, and aromatic esters in acetonitrile was studied in detail. The quenching exhibited an anomalous negative temperature dependence at AG23 (free energy change of an electron-transfer step, k23) > -3 kcal/mol, which accompanied large and negative activation entropies (-18 to -44 eu). The abnormal behavior was explained satisfactorily by the large contribution of back electron transfer regenerating the excited-state reactants (k32). In a narrow AG23 region between -2 and 0 kcal/mol, a “bell-shaped” Eyring plot was observed for several quenchers. The activation parameters for the forward (k23) and backward (k32)electron-transfer processes were determined on the basis of the bell-shaped Eyring plot and the standard enthalpy and entropy changes of the process. Although the temperature dependence of the quenching is anomalous, the activation parameters for the elementary process, k23 or k32, are quite normal. The mechanism of the quenching was discussed on the basis of the rate constant as well as of the activation parameters relevant to k23, k32, and k30(deactivation from the product ion pairs).
1. Introduction
In the preceding paper in this issue, we reported temperature effects on reductive quenching of * R ~ ( b p y ) , ~(bpy + = 2,2’-bipyridine) by various aromatic amines (electron donor, D) in acetonitrile.’ Besides the reductive quenching, * R ~ ( b p y ) , ~ + undergoes oxidative quenching by various organic and inorganic electron acceptors.2 Among oxidative quenching, photoinduced electron-transfer reactions between * R ~ ( b p y ) , ~or+ its analogous * R u L ~ complexes ~+ ( L = diimine ligands) and viologen derivatives (N,N’-diaIkyL4,4’-bipyridinium salts) have been extensively studied in special reference to chemical conversion and storage of light In contrast to the quenching of *RUL,~+ by viologens2-’ or however, the number of studies on redox reactions of *Ru(bpy)32+with neutral electron acceptors (A) is still limited.4-8 Furthermore, these studies were conducted a t a fixed temperature. In 1982, we showed, for the first time, that the mechanism of oxidative quenching of *Ru(bpy)32+by nitroaromatics, quinones, or aromatic esters was essentially different from that of reductive quenching by D as demonstrated by the temperature dependence of the quenching rate constants ( k , ) . s Namely, oxidative D,’*29496,8-’2
(1) Part I : Kitamura, N.; Kim, H.-B.; Okano, S.; Tazuke, S. J . Phys. Chem., preceding paper in this issue. (2) (a) Kalyanasundaram, K. Coord. Chem. Rev. 1983,46, 159. (b) Juris, A.; Balzani, V.; Barigelletti, F.; Campagna, S.; Belser, P.; von Zelewsky, A. Coord. Chem. Rev. 1988, 84, 85. (3) Amouyal, E.; Zidler, B. Isr. J . Chem. 1982, 22, 117. (4) 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. (5) Kitamura, N.; Kawanishi, Y.; Tazuke, S. Chem. Lett. 1983, 1185. (6) Shioyama, H. Ph.D. Thesis, Osaka University, 1985. (7) (a) Rau, H.; Franck, R.; Greiner, G. J . Phys. Chem. 1986,90,2476. (b) Milosavlzevic, B. H.; Thomas, J. K. J . Am. Chem. SOC.1986, 108, 2513. (c) McGuire, M.; McLendon, G. J . Phys. Chem. 1986, 90,2549. (d) Prasad, D. R.; Hoffman, M. Z. J . Am. Chem. SOC.1986, 108, 2568. (e) Chiorboli, C.; Indelli, M. T.; Scandola, M. A. R.; Scandola, F. J. Phys. Chem. 1988, 92, 156. (f) Olmsted, J., 111; Meyer, T. J. J . Phys. Chem. 1987, 91, 1649. (9) Olmsted, J., 111; McClanahan, S. F.; Danielson, E.; Younathan, J. N.; Meyer, T. J. J . Am. Chem. SOC.1987, 109, 3297. (8) Kitamura, N.; Okano, S.; Tazuke, S. Chem. Phys. Lett. 1982, 90, 13. (9) (a) Anderson, C. P.; Salmon, D. J.; Meyer, T. J.; Young, R. C. J . Am. Chem. SOC.1977, 99, 1980. (b) Ballardini, R.; Varani, G.; Indelli, M. T.; Scandola, F.; Balzani, V. J . Am. Chem. SOC.1978, 100, 7219. (c) Sandrini, D.;Maestri, M.; Belser, P.; von Zelewsky, A,; Balzani, V. J . Phys. Chem. 1985, 89, 3675. ( I O ) Baggott, J. E. J . Phys. Chem. 1983.87, 5223. ( I I ) Garrera, H. A.; Gsponer, H. E.; Garcia, N. A,; Cosa, J. J.; Previtali, C. M. J . Photochem. 1986, 33, 257. (12) Shioyama, H.; Masuhara, H.; Mataga, N. Chem. Phys. Lett. 1982, 88. 161.
SCHEME I
*Ru(bpy)y t 0 e ‘12* R u ( b p y ) 32+..... 0 e ‘23 Ru(bpyI3 +/3+ .....a’/-
II
‘2 1
/I
I I_
‘32
,
1 ‘30 1
quenching exhibited an anomalous negative temperature dependence of k, (AH* < 0) while the activation enthalpies for reductive quenching were always positive.’,8 In order to explain the mechanistic differences between these two quenching systems, the origin of the negative temperature dependence should be elucidated. In particular, AH* < 0 a t AG23 > 0 cannot be explained unless the relevant standard enthalpy of the electron-transfer step (AH,,) is negative. In the present paper, we report oxidative quenching of * R ~ ( b p y ) ~by~ +16 electron acceptors such as quinones, nitroaromatics, and aromatic esters in acetonitrile a t variable temperatures to elucidate the kinetic mechanistic feature~.’~ 2. Experimental Section Materials. All the quenchers were supplied from Tokyo Kasei Co., Ltd. (reagent grade), and were purified before use (for the numbering, see Table I). 2, 4, 5, and 8 were purified by sublimation in vacuo. 3 was purified by column chromatography (silica gel-chloroform) followed by sublimation in vacuo. All other electron acceptors were recrystallized from an appropriate solvent prior to use. Ferrocene (Tokyo Kasei; guaranteed reagent) as the standard for electrochemical measurements was recrystallized from ethanol several times. Acetonitrile was purified according to the literatureI4 and distilled over CaH2 before use. Acetone and propylene carbonate were purified by the accepted procedure^.'^ Tetra-n-butylammonium perchlorate (TBAP) as a supporting electrolyte was purified by successive :ecrystallizations from acetone-diethyl ether. Quenching Experiments. The activation-controlled quenching rate constants, k,, a t various temperatures were determined by (13) Preliminary results have been already reported: Kim, H.-B.; Kitamura, N.; Kawanishi, Y.; Tazuke, S. J . Am. Chem. SOC.1987, 109, 2506. (14) Perrin, D. D.; Armargo, A. L. F.; Perrin, D. R. Purification of Laboratory Chemicals, 2nd ed.; Pergamon Press: New York, 1980.
0 1989 American Chemical Societv
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The Journal of Physical Chemistry, Vol. 93, No. 15, 1989
Kim et al.
TABLE I: Rate Constants and Activation Parameters for Oxidative Quenching of * R u ( b p y ) p in Acetonitrile at 298 K AG23,
no.
quencher ( E I l 2 ( A / A - )vs SCE)"
kcal/mol
1
benzoquinone (-0.49)d pyromellitic dianhydride (-0.5 l ) d p-toluquinone (-0.57)d p-xyloquinone (-0.67)e 1.4-naphthoquinone (-0.68)d p-dinitrobenzene (-0.69)' o-dinitrobenzene (-0.8 l ) g duroquinone (-0.84)d p-nitrobenzaldehyde (-0.86)' m-dinitrobenzene (-0.90)d methyl p-nitrobenzoate (-0.94)d 4,4'-dinitrobiphenyl (-1 .OO)d m-nitrobenzaldehyde (-1.02)' methyl m-nitrobenzoate (-1 .04)d p-chloronitrobenzene (-1.06)' p-fluoronitrobenzene (-1 .l3)'
-10.6 -10.2
2 3 4 5 6 7
8 9 10 11 12 13 14 15 16
-8.8
-6.5 -6.3 -6.0 -3.3 -2.6 -2.1 -1.2 -0.3 +1.1 +1.6 +2.1 +2.5 +4. I
k, ( k A b M-I s-I 2.2 (1.1) x 1010 1 . 1 (0.7) X l o L o 1.7 (0.9) X IO'O 1.3 (0.8) X 1 O l o 1.3 (0.8) X 1O'O 1.2 (0.8) x 1010 4.2 ( 3 . 5 ) x 109 4.7 (3.8) x 109 2.8 (2.5) X lo9 1.8 (1.7) x 109 5.3 (5.2) X IO8 4.4 (4.4) x 107 4.1 (4.1) X lo7 1.5 (1.5) x 107 7.8 ( 7 . 8 ) X IO6 7.2 (7.2) X I O 5
AH', kcal/mol 1.3 1.7 1.1 1.1 1 .o 0.4 -0.4 -2.2 -2.6 -2.8 -3.4 -5.2 -4.0 -4.2 -3.4 -3.1
M*,C eu -9.2 -9.4 -11.2 -11.0 -11.3 -13.1 -18.1 -24. I -26.2 -28.1 -32.3 -43.5 -39.3 -42.3 -40.7 -44.3
AG', kcal/mol 4.0 4.5 4.4 4.4 4.4 4.3 5 .O
5 .o
5.2 5.6 6.2 7.8 7.7 8.4 8.7 10.1
*
a Reduction potential of an electron acceptor (volts vs SCE) in acetonitrile. k and k , are the activation-controlled and experimentally observed bimolecular quenching rate constants, respectively. 'Calories per mole degree. jDetermined in this study. e Meites, L., Zuman, P., Eds. Elecrrochemical Data; Wiley: New York, 1974. fMaki, A . H.; Geske. D. H. J . Am. Chem. SOC.1961, 83, 1852. ZMaki, A . H.; Geske, D. H. J . Chem. Phys. 1960, 33, 825. ~~
10 -
e Oar
O7
," 9 m
1.
e
0 876-
0
AGz3 Figure 1. Free energy relationships of k, in acetonitrile at 298 K: oxifor the numbering, see Table I ) and reductive ( 0 ;the data dative (0; taken from ref 1 ) quenching of *Ru(bpy)32+. t h e reported procedures.' For emission quenching experiments, t e m p e r a t u r e below or above 0 "C was controlled t o 10.1 "C by a liquid nitrogen cryostat (Oxford Instruments, Models DN 1704 a n d 3 120) or circulating water-ethylene glycol with a thermoregulator ( Y a m a t o , Models CTR-220 a n d CTE-220), respectively. The obserued activation enthalpy ( A H * ) and entropy (AS*) for t h e quenching of * R ~ ( b p y ) , ~by + A in acetonitrile were determined on t h e basis of t h e Eyring equation:
In ( k , / T ) = in
(
K,2~:) -
AH* RT
hS'
-+ -
R
kcai/mol
Figure 2. Free energy relationships of AH' in acetonitrile: oxidative (0; for the numbering, see Table I) and reductive ( 0 ;the data taken from ref 1) quenching of * R ~ ( b p y ) , ~ + .
3 W
it
v,
a
(1)
is t h e electronic transmission coefficient and was assumed t o be unity throughout this study (e.g., adiabatic). K 1 2is t h e formation constant of an association complex ( = k I 2 / k z lS;c h e m e I) and is 3.27 M-I as calculated from t h e Fuoss-Eigen equation.lJs Determination of the Standard Enthalpy and Entropy for an Electron- Transfer Step. T h e standard enthalpy (AH,,) a n d entropy (AS2,) for t h e forward electron-transfer step (k23) were determined by temperature effects on t h e free energy change of k23 ( e g , AG23 vs T plot). T h e calculation of ACZ3has been K
reported. ',I6 The t e m p e r a t u r e dependence of the electrode potentials of Ru(bpy),,+ and A necessary t o calculate AH,, a n d AS2, was determined by temperature-controlled cyclic voltammetry (Hokuto Denko, a Model HA-301 potentiostat a n d a Model HB-104 ( I 5) Fuoss, R. M. J . Am. Chem. SOC.1972, 94, 75. (16) EIp(Ru3+/Ru2+)= 1.25 V (volt vs SCE in acetonitrile at 298 K ) : Kawanishi, Y . ;Kitamura, N.; Tazuke, S. Inorg. Chem., in press. EO,O= 2.1 eV: Reference 2a. Temperature dependence of wp is reported in Table I of ref I .
A Gz3
k c a i /mol
Figure 3. Free energy relationships of aS' in acetonitrile at 298 K: oxidative (0;for the numbering, see Table I) and reductive ( 0 ;the data taken from ref 1) quenching of *Ru(bpy)32+. function generator) in acetonitrile with a conventional two-compartment glass cell (solution volume - 5 mL). The temperature of t h e working compartment (a platinum disk working electrode a n d a platinum wire counter electrode) was controlled [(5-40) f 0.1 "C] by circulating water while a reference electrode (saturated calomel electrode, SCE) was held a t a fixed ambient temperature (nonisothermal cell c ~ n f i g u r a t i o n ) . ' ~T h e electrode (17) (a) Weaver, M. J . J . Phys. Chem. 1979, 81, 1748. (b) Yee, E. L.; Cave, R. J.; Guyen, K. J.; Tyma, P. D.; Weaver, M . J. J . Am. Chem. SOC. 1979,101, 1131. (c) Hupp, J.T.; Weaver, M. J. Inorg. Chem. 1984, 23,3639. Two compartments of a CV cell were linked by a salt bridge. The potential determined by this method was slightly different from that obtained by a conventional CV cell (without a salt bridge). AG23 in Tables I (conventional) and I1 (nonisothermalcell configuration) are thus slightly different with each other.
Oxidative Quenching of Ruthenium(I1) Complexes
0
-1
a
" .
U
u
an electron-transfer process based on k, (Figure 1) alone is not warranted. In the following sections, we will describe the origin of the negative temperature dependence as well as the mechanism of oxidative quenching. 4. Discussion 4.1. Kinetic Analysis. k, in eq 2 can be simplified to eq 3 and 4 as reported previo~sly,'.~ where k30 is the sum of the rate constants of the deactivation processes from a product ion pair to free product ions (k34) and to the ground-state reactants (kb): k30 = ku kb (Scheme I). When the transition-state theory (eq 5)
CI
+"
The Journal of Physical Chemistry, Vol. 93, No. 15, 1989 5759
-1
!A >
+
-
c
0
>
kq
-1
= K12k23k30/(k30 + k32)
(2)
>> k32 k30 -3 kcal/mol. Moreover, the apparent negative AH* accompanies a large decrease in AS*.For 4,4'-dinitrobiphenyl (12), the observed AH* and AS* were -5.2 kcal/mol and -43.5 eu, respectively. Although the activation parameters for the quenching of * R ~ ( b p y ) ~by~ +aromatic amines have been reported by Baggott,lo and Garrera et al.," those for the oxidative quenching have not been reported except for our preliminary The mechanism of the emission quenching has not been discussed on the basis of the activation parameters. The large differences in the AG23 dependence of AH* or AS* between the oxidative and reductive quenching in Figure 2 or 3 unequivocally prove that the mechanism is strongly dependent on the nature of a quencher, D or A. Also, the present results indicate that the discussion of
-
K-
=
k, = K12K23k30
case I1 In (k,/T) = In
( xhs)
AH23*
( ):
~
KI~K- -
~
hs23*
-RT +R
KI~K-
+2 ~* ~ 3 +0 A S 2 3 * - A S 3 2 * + AS30* 2
-3 ~ * ~
3
=
AS* = =
AH23* AH23
-
AH32*
+ AH30*
AS23* AS23
1
RT
R
AH* =
(6a)
- AS32*
+ AS30*
(7a)
+ AH30* (7b)
+ AS30* (7c)
is applied to the k23, k32, and k3o processes, the temperature dependence of k, for cases I (eq 3) and I1 (eq 4) can be expressed as in eq 6 and 7, respectively. Equations 6 and 7 indicate that the activation parameters for k23 can be determined directly from an Eyring plot of k, for the reactions that proceeded via case I, while those obtained for case I1 (k32 >> k30) are the sum of the activation parameters of k23, k32, and k30. The relative magnitude of k32 to k30 is thus of primary importance to explain k, as well as the activation parameters for the quenching. 4.2. Origin of the Negative Temperature Dependence. Oxidative quenching of * R ~ ( b p y ) , ~by + neutral A has been reported to proceed via the case I1 m e c h a n i ~ m .It~ is obvious from eq 6 and 7 that the negative temperature dependence of k, can be explained by case I1 (eq 7) when AH23 is sufficiently large and negative as compared with A H 3 0 ' : AH23 + AH30* 0, rendering AS23 0 manifests t h a t t h e present negative t e m p e r a t u r e dependence of k, a t AG23 > -3 kcal/mol is reasonably explained by the large and negative AH,,. Similarly, t h e large and negative AS* is attributable to t h e large a n d negative AS23. A H z 3 increases with increasing AG23 ( T a b l e 11) so t h a t A H * should become positive a t AG23 >> 0. As clearly seen in T a b l e I a n d / o r Figure 2, A H * decreases with increasing ACX3 a n d reaches t h e minimum value AG23 = 1 k c a l / m o l (quenching by 4,4'-dinitrobiphenyl; AH* = -5.2 k c a l / m o l ) . F u r t h e r increase in AG23 leads to t h e increase in A H * (AG23 < 1 kcal/mol; T a b l e I ) a s expected from the above a r g u m e n t . The observed negative temperature dependence of k, as well as the AG23 dependence of A H * c a n b e explained satisfactorily by e q 7 . T h e a n o m a l o u s negative temperature dependence of k, is attributable to the large contribution to k, of the back electron transfer to the excited-state r e a c t a n t s (k32): k32 >> k30. 4.3. "Bell-Shaped" Eyring Plot. To obtain more detailed information on oxidative quenching of * R ~ ( b p y ) ~we ~ +extended , the emission quenching experiments to low t e m p e r a t u r e ( - - 3 5 "C) a n d found t h a t the s h a p e of the Eyring plot was strongly dependent on AC23. AG23 dependence of the Eyring plot can b e classified into three cases. ( a ) AC23 < -2 kcal/mol: k, showed a positive t e m p e r a t u r e dependence ( F i g u r e 5). ( b ) -2 < AC23 < 0 kcal/mol: k, has a maximum value a t a certain temperature, T, (Figure 6). (c) 0 < AC23: k , exhibits a negative temperature dependence alone in the temperature range examined (-35 to +50 O C ; Figure 7 ) .
-
15,5t
30
34
3.8
4.2
1000/T IiK Figure 6. Eyring plot for the quenching of * R ~ ( b p y ) ~by~ p-nitro+ benzaldehyde in acetonitrile.
30
3.4 3.8 4.2 1000/T 1/K Figure 7. Eyring plot for the quenching of *Ru(bpy),,+ by methyl pnitrobenzoate in acetonitrile. T h e most striking feature is the appearance of a "bell-shaped" Eyring plot (Figure 6). Below and above T,, positive a n d negative t e m p e r a t u r e dependence w a s obtained, respectively. S u c h bellshaped temperature dependence is frequently found in e ~ c i m e r ' ~ or exciplex formation.20 By analogy to the t e m p e r a t u r e depen( 1 9) Birks, J. B. Photophysics of Aromatic Molecules; Wiley-Interscience: London, 1970. (20) Meeus, F.; Van der Auweraer, M.; De Schryver, F. C. J . Am. Chem. Sor. 1980, 102, 4017.
Oxidative Quenching of Ruthenium( 11) Complexes
Figure 8. Ionic strength effect on temperature dependence of the quenching by duroquinone: fi = 0 (0), fi = 0.1 (O), and p = 0.5 (A) in acetonitrile. Tetra-n-butylammonium hexafluorophosphate was used as an electrolyte.
1000/T 1 / K Figure 9. Solvent effect on temperature dependence of the quenching by duroquinone: acetonitrile (0),acetone (A),and propylene carbonate (W).
The Journal of Physical Chemistry, Vol. 93, No. 15, 1989 5761
1000/T 1 / K Figure 10. Temperature dependence of k23, k32, k30,and k,/KI2 for the quenching by p-nitrobenzaldehyde in acetonitrile.
0
3.5 40 45 1000/T 1 / K Figure 11. Temperature dependence of k23,k32rk30,and k,/K12 for the quenching by m-dinitrobenzene in acetonitrile.
dence of excimer/exciplex formation, the quenching below T, is assumed to proceed via case I since the negative slope of the plot indicates k32 9 8 > 10) coincides very well with the decreasing order of AG23. Besides AG23, T, is governed by the ionic strength of the medium (p)and solvent properties as well. In the case of the quenching by duroquinone, T, appears at higher temperature at higher p: -255 and -270 K a t p = 0.1 and 0.5, respectively, in acetonitrile (Figure 8). Also, T, decreases in the following order: propylene carbonate (PC; 308 K) > acetonitrile (ACN; 252 K) > acetone (Ac; 230 K) a t p = 0 (Figure 9). The shift of T, with A623 and p will be reasonably explained as the results of the variation of the relative contribution of k32 or k30 to k,. With decreasing AG23, k32 becomes smaller so that the relative contribution of k32 to k , becomes smaller (e.g., K23 = k23/k32 = exp(-AG,,/RT)) and, thus, k30 >> k32. The observation of the negative temperature dependence of k, at AG23 > -3 kcal/mol is in good support of this. Similarly, the increase in p brings about the situation k30(=k34+ kb) >> k32 since large p facilitates dissociation of the product ion pair R ~ ( b p y ) , ~ + . - A to the free product ions, k34. It is obvious that the shift of the quenching mechanism from case I1 (k32 >> k30)to case I (k32 -3 kcal/mol. 4.5. Charge-Separation Processes. The activation parameters for k30 are AG30* = 5.3-7.1 kcal/mol, A H 3 0 * = 1.7-9.4 kcal/mol, and AS3o*= -1.8 to -1 1.7 eu (Table 111). These values, however, include the contributions from both k3, and kb and should be separated into the relevant values for each process. The temperature dependence of k3, is originated essentially from that of the viscosity of the solvent (7) and wp (eq 8 ) . In eq 8, 2kBT
k3, =
-
rd3q 1
wp/ R T
(8)
- exP(-wp/RT)
d is taken to be the sum of the effective radii of R ~ ( b p y ) , ~(rR + = 7.1 A) and A (ra = 3.8 A); d = rR+ r4.1,4 Although the Eyring plot of k3, is not linear in the temperature range between 273 and 333 K, AH3,*(calcd) was estimated to be 1.2 kcal/mol from a In k3,/Tvs T I plot a t 273 < T < 300 K. Also, aS34*(calcd)was evaluated to be -15 eu by the transition-state theory (eq 5). AG,,*(calcd) was then calculated to be -5.6 kcal/mol. For photoredox reactions of R u L , ~ +in acetonitrile, on the other hand, kb has been reported to be (1-5) X 10" s-1.4330Equation 5 gives AGb*(cakd)= 2.9-3.8 kcal/mol. The upper and lower limits of AG,,*(calcd) are 8.5-9.4 kcal/mol (AG30*(calcd) = AC34*(calcd) + AG,*(cakd)) and 2.9-3.8 kcal/mol (AG,,'(calcd) AGb* (calcd)), respectively. The observed AG30* (5.3-7.1 kcal/mol;
-
(28) Part 3: Kitamura, N.; Obata, R.; Kim, H.-B.; Tazuke, S. J . Phys. Chem., following paper in this issue. (29) Kitamura, N.; Obata, R.; Kim, H.-B.; Tazuke, S. J . Phys. Chem. 1987, 9 / , 2033. (30) Ohno, T.; Yoshimura, A.; Mataga, N.J . Phys. Chem. 1986, 90. 3295.
Kim et al. Table 111) agrees quite well with the above estimation; -3 < AG30* < 9.4 kcal/mol. For the redox quenching of the excited cis-Ru(phen),(CN), (phen = I,l0-phenanthroline) by D or A, the observed AG30* was again 3.4-8.2 kcal/mol, which satisfies the above estimation of the upper and lower limits of AG30*(calcd) as Figure 12 shows the reaction coordinate diagrams of four quenching systems, together with the rate constant and the activation free energy of each process (k23, k32, k,,, or kb) in acetonitrile. The energy of the free product ions is higher than that of the product ion pair by wp = 2.3 kcal/mol. In the present discussion, we took AC30*(calcd)to be the lower limit, 3 kcal/mol, for simplicity. (a) 1,4-Naphthoquinone (AG23 = -5.2 kcal/mol) a n d 0-Dinitrobenzene (AG23 = -2.1 kcal/mol). For the quenching of * R ~ ( b p y ) by ~ ~ 1,4-naphthoquinone + (Figure 12a), k23 is as fast as -4 X IO9 M-' s-l while k32 is slower than k23 or k3, by a factor of IO-, or re~pectively.~'No back electron transfer to the excited-state reactants occurs, and therefore, AH*is positive. The product ion pair should deactivate via k3, or kb; F = k3o/(k3,3 + k32) 1. However, the relative contribution of kj4 or kb to k30 could not be resolved by the present experiments. For the quenching by o-dinitrobenzene (Figure 12b), the situation is similar to that by 1,4-naphthoquinone; k32 is still a slow process as compared with k3, as well as with kb. However, AH* = -0.4 kcal/mol indicates a small but finite contribution of k32 to k Slightly smaller AG30' (=7.1 kcal/mol) relative to A6304'(calcd) = 9.4 kcal/mol will support this as well. The product ion pair mostly deactivates via both k3, and k,. (b) m-Dinitrobenzene (A623 = -0.3 kcallmol) and Methyl m-Nitrobenzoate (A623 = +3.1 kcal/mol). Further increase in A623 leads to the dramatic changes in the overall quenching paths (Figure 12c,d). For m-dinitrobenzene or methyl m-nitrobenzoate, k32 is faster than k3, by a factor of -5 or -85, r e ~ p e c t i v e l y , ~ ~ so that k32 is expected to play a dominant role for determining k, and, thus, the temperature dependence of k,. Indeed, AH* = -2.8 to -4.2 kcal/mol manifests that efficient back electron transfer from the quencher anion radical to Ru(bpy)?+ regenerates the excited-state reactants. Furthermore, smaller AC3o* (5.3-5.9 kcal/mol) relative to that for the quenching by o-dinitrobenzene or AG30*(calcd) (upper limit value 8.5-9.4 kcal/mol) suggests that the major deactivation path from the product ion pair will be kb with a relatively minor contribution of k3, tok30: k3, 0), whereas the electrostatic attraction between product ions ( R ~ ( b p y ) ~ ~ + - - Adisfavors -) dissociation to free ions. The product ions are likely to regenerate the excited-state reactants (case I1 and AH* < O).29,30 Recent results on redox quenching of excited c i ~ - R u ( p h e n ) ~ ( C Nas) ~ ~ ~ ~ ~ ~ well as on fluorescence quenching of pyrene by D or A in acetonitrile provide more explicit evidence.34 Since both redox ~~~~
~~
~~
~~~~~
(34) Fluorescence quenching of pyrene exhibits a negative temperature dependence by several electron donors or acceptors in acetonitrile; AH* (AS') are -1.9 (-32.4), -0.3 (-12.6). -1.0 (-l2.6), -0.5 ( - l S S ) , and -3.2 kcal/mol (-33.5 eu) for the quenching by dimethyl phthalate, p-fluorobenzonitrile, N-methylaniline, N,N-dimethylaniline, 1,2,4-trimethoxybenzene,and 1,4-dimethoxybenzene, respectively: Kitamura, N.; Obata, R.; Tazuke, S. Unpublished results.
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J. Phys. Chem. 1989, 93, 5764-5769
quenching systems produce electrostatically attractive pairs, a temperature dependence of the quenching becomes negative as expected from the present arguments. Further discussion on the electrostatic effects within product ion pairs on the quenching of *Ru(bpy)32+and cis-*Ru(phen),(CN), will be developed in detail
in the following paper in this issue.28 Registry No. I , 106-51-4;2, 89-32-7;3, 553-97-9;4, 137-18-8;5, 103-15-1;6 , 100-25-4; 7 , 528-29-0; 8,527-17-3; 9, 555-16-8; IO, 99-65-0; 1 1 , 619-50-1;12, 1528-74-1; 13, 99-61-6; 14,618-95-1;15,100-00-5;16, 350-46-9;Ru(bpy),2+, 151 58-62-0.
Photoinduced Electron-Transfer Reactions of Ruthenium( I I ) Complexes. 3. Redox Quenching of Excited cis-Dicyanobis( 1,I0-phenanthroline)ruthenium(I I ) Noboru Kitamura,* Ritsuko Obata, Haeng-Boo Kim, and Shigeo Tazuke* Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 227, J a p a n (Receiaed: October 24, 1988; In Final Form: March 7 , 1989)
An anomalous temperature dependence of the rates was observed for both reductive and oxidative quenching of excited cis-dicyanobis( I , IO-phenanthroline)ruthenium(II), *RuCN, by aromatic amines and nitroaromatics, respectively, in acetonitrile. The observed activation enthalpies and entropies were more positive than those observed for oxidative quenching of the excited R ~ ( b p y ) , ~(bpy ' = 2,2'-bipyridine). The results were satisfactorily explained by the standard and/or electrostatic enthalpy and entropy changes of the electron-transfer step. Efficient back electron transfer from product ion pairs to the excited-state reactants in the redox quenching of *RuCN is ascribable to a favorable entropy change of the process as well as to electrostatic attraction within the ion pairs. The electrostatic interactions within the product ion pairs (repulsive or attractive) determine the overall quenching paths and, thus, the temperature dependence of the quenching.
1. Introduction
Among photoredox reactions of R ~ ( b p y ) , ' + , ' - ~oxidative quenching of *Ru(bpy):+ or its analogous complexes by viologen derivatives (V2'; 4,4'-bipyridinium salts) have been extensively ~ t u d i e d . ~ -In~ the absence of a sacrificial electron donor, photoreduction of V2' by Ru(I1) complexes proceeds with a quantum yield of -0.4 as determined by transient absorption spectroscopy.' Contrarily, Bock et al. reported that no transient ions could be observed in the oxidative quenching of * R ~ ( b p y ) , ~by+ neutral electron acceptors (A) such as nitroaromatics in a ~ e t o n i t r i l e .In ~ the Ru(bpy),2+-V2+ system, products are an electrostatically repulsive pair ( R ~ ( b p y ) ~ ~ + - . Vwhile + ) the ions produced in the R~(bpy),~+-nitroaromatic (A) systems attract with each other ( R ~ ( b p y ) , ~ + - A - ) . The difference in electrostatic interactions within the product ion pair between two modes of quenching by V2' (repulsive) and A (attractive) must be the primary reason for the variation of the product ion yields with the nature of a quencher . As reported in the previous paper^,',^*^ we showed that electrostatic interactions within the product ion pair determined the quenching pathways and, thus, the temperature dependence of the quenching. Ionic strength effects on the rate constant as well ( I ) Part 1: Kitamura, N.; Kim, H.-B.; Okano, S . ; Tazuke, S. J . Phys. Chem., companion paper in this issue. (2) Part 2: Kim, H.-B.; Kitamura, N.;Kawanishi, Y.; Tazuke, S. J . Phys. Chem., companion paper in this issue; J . Am. Chem. SOC.1987, 109, 2506. ( 3 ) (a) Kalyanasundaram, K. Coord. Chem. Reu. 1983, 46, 159. (b) Juris, A.; Balzani, V . ; Barigelletti, F.; Campagna, S.; Belser, P.; von Zelewsky, A. Coord. Chem. Reo. 1988,84, 85. (4) 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. ( 5 ) Amouyal, E.;Zidler, B. Isr. J . Chem. 1982, 22, 117. (6)Kitamura, N.;Kawanishi, Y.; Tazuke, S . Chem. Lett. 1983, 1185. (7)Shioyama, H. Ph.D. Thesis, Osaka University, 1985. (8) (a) Rau, H.; Franck, R.; Greiner, G. J . Phys. Chem. 1986, 90, 2476. (b) Milosavlzevic, B. H.; Thomas, J. K . J . Am. Chem. SOC.1986, 108, 2513. (c) McGuire, M.; McLendon, G. J . Phys. Chem. 1986, 90, 2549. (d) Prasad, D. R.; Hoffman, M. Z. J . Am. Chem. SOC.1986, 108, 2568. ( e ) Chiorboli, C.; Indelli, M . T.; Scandola, M. A. R.; Scandola, F. J . Phys. Chem. 1988, 92, 156. (f) Olmsted, J., III; Meyer, T. J. J . Phys. Chem. 1987, 91, 1649. (g) Olmsted, J., 111; McClanahan, S . F.; Danielson, E.; Younathan, J. N.; Meyer, T. J . J . Am. Chem. SOC.1987. 109, 3297. (9)Tazuke, S.;Kitamura, N.; Kim, H.-B. In Supramolecular Photochemisrry; Balzani, V . , Ed.; Reidel: Dordrecht. The Netherlands, 1987;p 87.
0022-3654/89/2093-5764$01.50/0
TABLE I: Spectroscopic and Redox Properties of RuCN and Ru(bov)22+in Acetonitrile at 298 K RuCN" R~(bpy),~'~ Xab(MLCT), nm (log e, M-' cm-') 490 (4.06) 449 (4.17) 620 (580) Xem(MLCT),nm (at 77 K)C 686 (586) $em d 0.054 0.062 rem,ns 1210e 840 +0.83 + 1.25 EIj2(M"'/M") V -1.64 -1.35 Elj2(M"/M')/V
"Reference 15. bData taken from: Kawanishi, Y.; Kitamura, N.; Tazuke, S. Inorg. Chem., in press. 'In ethanol-methanol, 4:l v/v. Emission quantum yield. e Reference 14. fEIj2(M"'/M1') and (M"/M') are the oxidation and reduction potentials of Ru(I1) complexes (volts vs SCE), respectively. as on the activation parameters for quenching of *Ru(bpy)?+ also prove the importance of the electrostatic interaction^.^.^^^^ Since Ru(bpy),,+ possesses dipositive charge, reductive and oxidative quenching of the excited complex by a neutral quencher (Q) produces Ru(bpy),+--Q+ and R~(bpy),~+.-Qpairs, respectively, which brings about large differences in the quenching pathways. If the excited complex is a neutral molecule, the quenching by a neutral electron donor (D) or acceptor (A) leads to the formation of an electrostatically attractive pair of Ru--.D+ or Ru+-.A-, respectively. In this case, a temperature dependence of the quenching will be similar for both reductive and oxidative quenching. As an example of such complexes, we chose cis-dicyanobis( 1,lO-phenanthroline)ruthenium(II) complex, R u C N . The lowest excited state of R u C N has been assigned to the metal to ligand charge-transfer state similar to the excited state of Ru(bpy),*'," so that both absorption and emission spectra of + for their R u C N are comparable to those of R ~ ( b p y ) ~ 'except peaking wavelengths (Table I). Although the electrode potentials of R u C N are slightly difference from those of Ru(bpy),,+, the excited state of R u C N has been known to undergo electrontransfer reactions with various organic and inorganic compounds as Variation of the quenching mechanisms with the ( I O ) Kitamura, N.; Kawanishi, Y.; Kim, H.-B.; Tazuke, S . Manuscript in preparation ( 1 I ) Klassen, D. M.; Crosby, G . A. J . Chem. Phys. 1968, 48, 1853.
0 1989 American Chemical Society
