5764
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, Japan (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~(bpy),~+-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),~+.-Q-pairs, 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 RuCN 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
Redox Quenching of Ruthenium( 11) Complexes
The Journal of Physical Chemistry, Vol. 93, No. 15, 1989 5765
TABLE 11: Rate Constants and Activation Parameters for Redox Quenching of *RuCN by Various Neutral Electron Acceptors and Donors in Acetonitrile (298 K. u = 0) ~
AG239
no. 1
2
3 4
5 6 7 8 9 10 11 12 13 14 15 16 17 18
quencher (El12(A/A-) or E1/2(D+/D))"
kcal/mol
pyromellitic dianhydride (-0.51)d p-xyloquinone (-0.67)e 1,4-naphthoquinone (-0.68)d tetrachlorophthalic anhydride (-0.83)e duroquinone (-0.84)d m-dinitrobenzene (-0.90)d methyl p-nitrobenzoate (-0.94)d methyl m-nitrobenzoate (-1 .04)d p-chloronitrobenzene (-1.06)' p-fluoronitrobenzene (-1.13)' m-nitroanisole (-1.14)d p-nitrotoluene (-1.21)d o-nitrotoluene (-1.26)e p-nitroanisole (-1.27)d 2,5-dimethylnitrobenzene(-1 .28)e
-13.3 -10.1 -9.4 -5.9 -5.7 -4.3 -3.4 -0.8 -0.5 +1.0 +1.3 +2.9 +4.1 +4.3 +4.5
N,N,N',N'-tetramethylphenylenediamine (0.12)g
-3.6 -0.2 +3.5
p-aminodiphenylamine (0.27)e
N,N,N',N'-tetramethylbenzidine (0.43)g
k , (k,)? M-1 s-l
AH',
AG*, kcal/mol
X lo7 X lo7 X lo7
1 .o -0.1 -0.4 -1.1 -1 .o -1.8 -1.4 -1.6 -1.6
AS*: eu -10.8 -10.6 -10.1 -11.1 -1 1.3 -9.9 -1 1.2 -16.6 -17.7 -22.1 -21.8 -27.4 -30.0 -29.9 -3 1.6
6.9 (5.2) X l o 9 1.1 (1.0) x 109 1.4 (1.4) X IO8
0.3 -0.2 -2.9
-14.4 -19.9 -32.8
4.6 5.7 6.9
1.6 (0.9) 2.3 ( 1 . 1 ) 2.4 (1.1) 8.7 (6.1) 1.4 (0.8) 1.3 (0.8) 9.9 (6.7) 4.5 (3.7) 4.5 (3.6) 1.4 (1.3) 1.5 (1.4) 3.4 (3.4) 4.8 (4.8) 6.8 (6.8) 2.7 (2.7)
X X X X X X
kcal/mol 1Olo
1O1O IO'O
IO9 10"
1Olo x 109 x 109 x 109 X IO9 X lo9
x 108
1.1 0.7 0.1 1.1
0.5 1.2
4.1 3.9 3.8 4.4 4.1 4.2 4.4 4.8 4.8
5.5 5.5 6.4 7.5 7.3 7.8
*
Electrode potential of quencher, volts vs SCE, in acetonitrile. Activation-controlled ( k , ) and experimentally observed bimolecular quenching rate constants ( k J . 'Calories per mole degree. dDetermined in this study in the presence of 0.1 M tetra-n-butylammonium perchlorate in acetonitrile. 'Meites. L.. Zuman. P.. Eds. Electrochemical Data: Wilev: New York, 1974. f M a k i , A. H.; Geske, D. H. J . Am. Chem. Sot. 1961, 83, 1852. ZReference 4.
nature of the complex (i.e., Ru(bpy)32+or RuCN) will be thus attributed to the changes in electrostatic interactions within product ion pairs. In this paper, we report the rate constant and activation parameters for the quenching of the excited RuCN by both D and ~ the results are compared with the redox A in a ~ e t o n i t r i l e 'and quenching of * R ~ ( b p y ) , ~ + . l ~ ~
4
1
2. Experimental Section ,*' I I Materials. RuCN was the same sample reported p r e v i ~ u s l y . ~ ~ J ~ -15 -10 -5 0 5 Purification of quenchers (Tokyo Kasei Co., Ltd.; Nakarai AGZ3 kcal/mol Chemical Co., Ltd.; or Kanto Chemical Industries Co., Ltd.) used Figure 1. Free energy relationships of k, for redox quenching of *RuCN in this study is as follows (for the numbering of quenchers, see in acetonitrile at 298 K (ionic strength, 0): 0, oxidative quenching; A, Table 11). 1 was sublimed in vacuo. 4, 9-12, and 14 were reductive quenching (the numbering of Q are the same with that in Table recrystallized from appropriate solvents (4, acetone; 9 and 10, 11). The data for the oxidative quenching of *Ru(bpy),2+ (e) were taken ethanol; 11, 12, and 14, aqueous methanol). 13 was used as from ref 2. Solid curves represent the best fit simulation of the data by supplied. 15 was purified by vacuum distillation prior to use. eq 4. Purification of all other quenchers has been reported elsewhere.'*2 Acetonitrile was purified according to the literature procedures.I6 intensity of the complex agreed satisfactorily with those by Procedures. Emission and electrochemical measurements inemission lifetime, so that the participation of the static quenching cluding the temperature dependence of the electrode potential were can be eliminated. Determination of the activation parameters carried out by the system described earlier.'*Z The bimolecular for the quenching has been reported in the companion papers of quenching rate constants (k,) were determined by Stern-Volmer this issue.',2s'8 plots (emission intensity) a t various temperatures as reported 3. Results and Discussion previously.' The activation-controlled quenching rate constant ( k , ) was calculated by k;' = k;' - klF1.l The diffusion rate The rate constant ( k , or k,) and the activation parameters (AG*, constant ( k I 2 )in acetonitrile and the emission lifetime of RuCN AH*, and AS*) for the quenching of *RuCN by a variety of at each temperature necessary to calculate k , or k , have been organic electron donors and acceptors in acetonitrile are sumreported else~here.'~.''For emission quenching experiments, the marized in Table I1 (ionic strength p = o). For oxidative sample solutions were degassed by several freeze-pump-thaw quenching, the experiments were performed with 15 electron cycles. It has been reported that static quenching takes place in acceptors, which covered the AGz3 (free energy change of the the RuCN-Cu2+ ion system in aqueous s ~ l u t i o n . 'For ~ the present forward electron-transfer step) range from -1 3.3 to +4.5 kcal/ systems, however, the Stern-Volmer plots determined by emission mol.19 However, *RuCN is a very weak oxidant relative to I ) so that the reductive quenching studied * R ~ ( b p y ) , ~(Table + was confined to only three aromatic amines. (12) Nagle, J . K.; Dressick, W. J.; Meyer, T. J. J . Am. Chem. Sot. 1979, 3.1. Free Energy Relationship of the Quenching Rate Con101, 3993. (13) (a) Demas, J . N.; Addington, J. W. J . A m . Chem. Soc. 1974, 96, stants. Figure 1 shows the relationship between AG23 and k, for 3663. (b) Demas, J. N.; Addington, J. W.; Peterson, S. H.; Harris, E. W. reductive and oxidative quenching of *RuCN as well as for oxJ . Phys. Chem. 1977. 81, 1039. idative quenching of * R ~ ( b p y ) , ~ + .Although ' the number of the ( I 4) Preliminary results have already been reported: Kitamura, N.; Obata, data for reductive quenching of *RuCN is limited, the correlation R.; Kim, H.-B.; Tazuke, S. J . Phys. Chem. 1987, 91, 2033. ( 1 5 ) Kitamura, N.; Sato, M.; Kim, H.-B.; Obata, R.; Tazuke, S . Inorg. Chem. 1988, 27, 651. (16) Perrin, D. D.; Armargo, A. L. F.; Perrin, D. R. Purvication of Laborarory Chemicals, 2nd ed.; Pergamon Press: New York, 1980. (17) The radii of RuCN (6.2 A) and a quencher (the average for all quenchers used 3.8 A) were estimated by taking the average of the van der Waals dimensions along three molecular axe^.^,'^
(18) The contribution of K I 2to the activation entropy for the quenching (AS') was corrected. K , 2 was calculated to be 2.52 M-' according to the Fuoss-Eigen equation (eq 8 in ref I ) . (19) For the calculation of AGZ3(eq 3 in ref I ) , Eo,owas taken to be 1.88 V (43.4 kcal/mol).'2
5766
The Journal of Physical Chemistry, Vol. 93, No. 15, 1989
Kitamura et al.
SCHEME I
RTln k q . V
I
*RUCN
+
k12 R-*R~cN..-.Q
11
k23 -R~CN+/-.
k21
0
'32
RuCN
+
.ope
1
1
I1 RuCN + Q
..d+
R ~ c N + / -+
Q
Q-/+
seems to be similar to that for the oxidative quenching. At AC23 < -5 kcal/mol, the quenching proceeds with the nearly diffusion-controlled rate (kit = 2.1 X 1O'O M-'8 ) in acetonitrile,l indifferent of the combination of a quencher and a complex. At AC23 > -5 kcal/mol, on the other hand, k, decreases with increasing AC23 for both *RuCN and * R ~ ( b p y ) ~ ~In+the . case of the quenching of *RuCN, however, the correlation between k, and AG23 shifts to the more endoergic region as compared with that of * R ~ ( b p y ) , ~ +The . quenching of *RuCN is faster than that of * R ~ ( b p y ) ~when ~ + k, of both complexes are compared a t the same AC23 (>-5 kcal/mol). According to Scheme I, k, is given as in eq 1.20 Applying the
k, =
kl2 1
(1)
+ kZl[k23-' + k32k3o-'k23-']
Marcus's equation (eq 1 1 in ref 1) to the forward electron-transfer step (k23), eq 1 can be rewritten as in eq 2.20
-10
10
0
-05
05
10
E(ox/red), V
Figure 2. Plot of RT In k, vs Elj2(ox/red) for oxidative (0)and reductive ( 0 )quenching of *RuCN in acetonitrile at 298 K (ionic strength, 0). For the numbering of Q, see Table I1
be detailed in the succeeding papers of this series, both k, (or k,) and the activation parameters for the quenching of * R ~ ( b p y ) , ~ + can be satisfactorily explained by the Marcus theory and Ao? Therefore, the simulation of the data in Figure 1 was performed by varying ~ 2 and 3 k30 with a fixed A,. The best fits of the RuCN and R ~ ( b p y ) ~data ~ + were attained with ~ 2 = 3 1.2 X lOI3 s-I, k30 = 3.5 X loio s-l, A, = 19.7 kcal/mol and ~ 2 = 3 2.9 X 10" s-l, k3, = 2.6 X IO8 s-I, A, = 19.3 kcal/mol, respectively (solid curves in Figure 1. It is noteworthy that k30 is almost constant at (3.5-3.6) X l o i o s-l and (2.5-2.6) X IO8 s-l for the quenching of RuCN and R ~ ( b p y ) , ~ respectively, +, irrespective of the initial estimate of u23 or k30 determined for RuCN is much larger than that observed for Ru(bpy)t+ by a factor of 100. Although the value estimated by the simulation of the data in Figure 1 does not necessarily represent the actual k30 of the quenching, it is obvious that k30 is strongly dependent on the quenching system and, thus, the charge type of product ion pairs (e&, RuCN+/--Q-/+ vs Ru(bpy)33+/+...Q-/+). As discussed in the following sections, the variation of k30with the quenching system is the primary reason for the differences in the AC23 dependence of k, as well as in the temperature dependence of k, (or k,). An alternative approach to elucidate the quenching mechanism based on the quenching rate constants is to examine the slope of a R T In k, (not k,) vs AC23 plot as has been reported by Bock et aL4 k, in eq 5 can be simplified to eq 6 or 7 depending on the
-
k30-I exp(AG23/RT) where we employed the relations in eq 3 and 4. K23
=
k23/k32
k23 =
u23
= exp(-AG23/RT)
exp(-AC23*/RT)
~ 2 is 3
I)
(2)
the fre(3) (4)
quency factor of k23 and A is the sum of the inner- (A,) and outer-sphere (A,) reorganization energies. In the present study, we assume X = A, and A, was calculated to be 19.7 or 19.3 ~+, kcal/mol for the quenching of *RuCN or * R ~ ( b p y ) ~respectively (eq 14 in ref k,, and kzl are determined by the viscosity of the medium] and the size of the reactants'' (see eq 2 and 8 in ref I ) , so that AC23 dependence of k, in Figure 1 can be in principle simulated by eq 2 with varying ~23,A, and k30. Sandrini et al. simulated the AC23 vs k, plot for the reductive quenching of * R ~ ( b p y ) ~by~ aromatic + amines by varying A (intrinsic barrier, AG23*(O) = AG23* at AC23 = 0) with fixed ~ 2 and 3 k30.25 They obtained larger AG23*(O) as compared with the values expected from the Marcus cross relations, which was explained by the nonadiabaticity of the k23 process. In the present case, however, the simulation of the data in Figure 1 failed with fixed ~ 2 and 3 k30.27 Furthermore, as will (20) Rehm, D.; Weller, A. Ber. Bunsen-Ges. Phys. Chem. 1969, 73, 834; Isr. J . Chem. 1970, 8, 259. (21) Do, = 1.807 and D,= 39.2 (in acetonitrile at 298 K), and the radii of RuCN and Q in ref 17 were used to calculate A,. X, was assumed to be negligibly smallia few kilocalories per mole at the maximum)22as compared with A,. I n fact, k, (or ko), AHij*, and AS,' can be explained by A, as will be described in part 4.28 (22) (a) Sutin, N. In Tunneling in Biological Systems; Chance, B., et al., Eds.; Academic Press: New York, 1979; p 201. (b) Brown, G. M.; Sutin, N . J . Am. Chem. SOC.1979, 101, 883. (23) Baggott, J. E. J . Phys. Chem. 1983, 87, 5223. (24) To fit the plateau region in Figure 1, the following values were assumed: k I 2= I X 1Olo M'' s" for both quenching systems and k2' was 4.0 X 109or 3.1 X IO9 s-' for RuCN or Ru(bpy)32+, respectively. (25) Sandrini, D.; Maestri, P.; Belser, P.; von Zelewsky. A,; Balzani, V . J . Phys. Chem. 1985, 89, 3675. (26) (a) Scandola, F.; Balzani, V.; Schuster, G. B. J . Am. Chem. SOC. 1981, 103, 2519. (b) Balzani, V.; Scandola, F.; Orlandi, G.; Sabbatini, N.; Indelli, M. T. J . Am. Chem. SOC.1981, 103, 3370. (c) Sandrini, D.; Gandolfi, M. T.; Maestri, M.; Bolletta, F.; Balzani, V . Inorg. Chem. 1984, 23, 3017.
kq
case I: case 11:
= K12k23k30/(k30 + k32)
>> k32 k30 -3 kcal/mol, both oxidative and reductive quenching of *RuCN exhibit a negative temperature dependence of k, (AH* < 0; typical examples of the temperature dependence ~ is quite contrasting to of k, have been already r e p ~ r t e d ) . 'This the quenching of * R ~ ( b p y ) ~ AH* ~ + ; is always positive for the reductive quenching' while the oxidative quenching shows a negative temperature dependence at AG23 > -3 kcal/moL2 The observation of the negative temperature dependence for both oxidative and reductive quenching of *RuCN is the key to elucidate the quenching mechanism and the origin of the temperature dependence of k,. (ii) AH* determined for *RuCN are more positive than those for the oxidative quenching of * R ~ ( b p y ) ~(Figure ~ + 3 ) . AH* of the cyanide complex ranges from +1 to -2 kcal/mol while that is from + I to -5 kcal/mol for R ~ ( b p y ) ~ ~ + . (iii) The quenching of *RuCN exhibits large and negative AS*, similar to the oxidative quenching of * R ~ ( b p y ) ~ ~However, +. values of AS* are more positive than those for *Ru(bpy):+ (Figure 4). As discussed in the previous section, the quenching of *RuCN proceeds via the case I1 mechanism (k3o 0). On the other hand, free product ions are likely to form ion pairs again in the case of the reactions in eq 11-13 (K34 = 0.094-0.0059 M-l), and consequently, the fate of product ions will be back electron transfer to the excited state (case 11; AH* < 0) and/or to the ground state. However, if back electron transfer to the ground state (kb) is the major deactivation path, AH* should be positive (case I). Since kb is a spin-forbidden process as demonstrated by Ohno et al.,39 kb will contribute to k, to a lesser extent as compared with k32.
4. Summary and Concluding Remarks In a series of publication^,'^^,^^^^ we revealed the mechanisms of the redox quenching of several excited-state Ru(I1) complexes by various organic electron donors and acceptors. Although the free energy relationships of k, are very similar to each other, the activation enthalpies and entropies are strongly dependent on the quenching system. Observation of (i) a "bell-shaped" Eyring plot for oxidative quenching of *Ru(bpy)2+,2(ii) large ionic strength effects on k, as well as on the activation parameters for redox quenching of * R ~ ( b p y ) ~ ~ + ,and ~ * ~(iii) * ' Othe negative temperature dependence of both reductive and oxidative quenching of *RuCN (this paper) enabled us to elucidate the overall quenching (38) k34 was calculated by eq 8 in ref 2. K3, is given as K34 = (3000/ 4rN~f)[I/exp(-w,/RT)], where d is the sum of the effective radii of Ru(I1) (rR = 6.8 A for RuCN and 7.1 A for Ru(bpy)?+) and Q (rQ = 3.8 A). See also: Reference 17. (39) Ohno, T.; Yoshimura, A,; Mataga, N. J. Phys. Chem. 1986, 90,3295, and the related papers cited therein.
mechanisms and the factors determining k, and the activation parameters. We showed that one of the most important factors determining k, and its temperature dependence was the electrostatic interaction within product ion pairs (e.g., repulsive or attractive), which influences AH2, (m23-)and A S 2 3 (AS,,-) through the solvation change before and after electron transfer. Dissociation of product ion pairs to free product ions as well as back electron transfer to the excited-state reactants is also governed by the electrostatic interactions, wp. Such effects of wp are not confined to the present photoredox reactions since analogous negative temperature dependence of k, can be observed in fluorescence quenching of pyrene by a series of organic electron donors or acceptors in acetonitrile as For efficient photoredox reactions to be constructed, it is obvious that the reactions producing an electrostatically attractive pair such as R ~ ( b p y ) ~ ~ + - are Q - disadvantageous since efficient back electron transfer to the excited-state reactants occurs. Quantitative formation of free product ions in the R ~ ( b p y ) ~ ~ + - a r o m aamine tic is indicative of the favorable k34caused by electrostatic repulsion within the ion pairs (wp > 0). Appropriate choices of reactant pairs avoiding electrostatic attraction in product ion pairs in combination with the ionic strength of the medium will be promising to improve photoredox reaction yields. The activation parameters for the overall quenching as well as for the elementary electron-transfer processes (k23, k32, and k30) will be discussed based on the Marcus theory in the forthcoming paper of this series.28
Acknowledgment. This work (parts 1-3) was partly supported by a Grant-in-Aid from the Special Project Research, Ministry of Education, Science and Culture: Research on Chemical Conversion and Storage of Solar Energy (61040046). Registry No. 1, 89-32-7; 2, 137-18-8; 3, 130-15-4; 4, 117-08-8; 5, 527-17-3; 6, 99-65-0; 7,619-50-1; 8, 618-95-1; 9, 100-00-5; 10, 350-46-9; 11, 555-03-3; 12, 99-99-0; 13, 88-72-2; 14, 100-17-4; 15, 89-58-7; 16, 27215-51-6; 17, 101-54-2; 18, 366-29-0; RuCN, 112087-85-1. (40) Kitamura, N.; Obata, R.; Kim, H.-B.; Tazuke, S., unpublished results. Analogous results have been also reported: Battott, J. E.; Pilling, M. J. J . Chem. SOC.,Faraday Trans. 2 1983, 79, 221. (41) Shioyama, H.; Masuhara, H.; Mataga, N. Chem. Phys. Lett. 1982, 88, 161.
Temperature Dependence of the Reaction of Nitrogen Atoms with Methyl Radicals George Marston,+ Fred L. Nesbitt,t David F. Nava, Walter A. Payne, and Louis J. Stief* Code 691, Astrochemistry Branch, Laboratory for Extraterrestrial Physics, NASAICoddard Space Flight Center, Greenbelt, Maryland 20771 (Received: November 9, 1988; In Final Form: February 15, 1989)
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The discharge-flow mass spectrometry technique has been used to measure the kinetics of the reaction N + CH3 products over the temperature range 200-423 K. The results are as follows (lo-" cm3 s-'): k1(200K) = (6.4 f 2.1), k1(298 K) = (8.5 f 2.0), kl(363 K) = (14 f 3.0), and k1(423 K) = (17 k 5.0). Quoted uncertainties include statistical (95% confidence) and systematic (15%) errors. Interpreting the temperature dependence is difficult, as there is a possibility that the reaction behaves in a non-Arrhenius manner. Possible causes of this behavior are discussed, and comparisons are made with reactions showing similar properties. The results of this study have implications regarding the formation of HCN in the atmosphere of Titan.
Introduction The reaction of ground-state N atoms with methyl radicals is a potentially important process in the atmosphere of Titan.' It
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NAS/NRC Postdoctoral Research Associate. *Research Associate, Chemistry Department, The Catholic University of America, Washington, DC 20064.
0022-3654/89/2093-5769$01.50/0
has also been invoked to explain some of the results obtained from active nitrOgen/hYdrocarbon Three thef"odYnamicallY accessible channels exist: ( I ) Yung, Y. L.; Allen, M.; Pinto, J. P. Astrophys. J . Suppl. Ser. 1984, 55, 465. (2) Safrany, D. R. Prog. React. Kine?. 1971, 6 , 1.
0 1989 American Chemical Society
