J . Phys. Chem. 1988,92, 3431-3440 greater than or equal to 8.9 X cm3 molecule-’ s-l. Feed into the a state from higher electronic states does not appear to be significant on the basis of spectral surveys and N2(C) state normalization. A substantial redistribution of the vibrational populations within the a state as a function of pressure is observed. Total quenching rate coefficients for vibrational levels 4-6 approach gas kinetic values. Intersystem energy transfer from the N2(a’12[) state to lower vibrational levels of the a state cannot be distinguished from intrasystem vibrational relaxation within the a-state manifold. Future state-specific laser-excitation ex-
Reductive Quenching of Ru( bpy):’
3437
periments are now under way to distinguish these competing pathways.
Acknowledgment. We acknowledge useful discussion with L. G. Piper and G. E. Caledonia of PSI and R. Huffman of the Air Force Geophysics Lab. This work was supported by the U S . Air Force Office of Scientific Research under Task 23 10G4 and the The Defense Nuclear Agency under Project SA, Task SA, Work Unit 115. Registry No. N2, 7727-37-9.
at High Pressures
Monty L. Fetterolf and Henry W. Offen* Department of Chemistry, University of California, Santa Barbara, California 931 06 (Received: October 2, 1987)
The reductive quenching of R~(bpy),~+ by several aromatic amines was studied at high pressures (0.1-300MPa) in acetonitrile (CH3CN) and n-butyl alcohol (n-BuOH). The luminescence lifetimes are lengthened with increasing pressures. This results in positive activation volumes for the quenching rate constants k, ranging in value from =1 mL/mol for dimethylaniline quencher in CH3CN to 13 mL/mol for benzidine in n-BuOH at 25 OC. The pressure dependence of k, for Ru(bpy),*+/DMA in CH3CN was insensitiveto temperature in the 15-45 OC range. The pressure results are discussed in terms of the commonly accepted mechanism for electron transfer in the activation- and diffusion-controlled limits.
Introduction Photoinduced electron transfer is an important mechanism for luminescence quenching.’v2 Both oxidative and reductive quenching have been studied with excited ruthenium metal comp l e ~ e s . ~In, ~this high-pressure study of luminescence quenching we focus on the tris(2,2’-bipyridine)ruthenium(II) complex paired with several electron donors Q chosen from related aromatic amines. The excited-state reaction in eq 1 has been shown to lead * R ~ ( b p y ) , ~++ Q
A Ru(bpy)3+ + Q+
to the typical plot of In k, against the quencher reduction potentials, in which a diffusion-limited plateau in the highly exergonic region precedes the predicted linear drop-off with increasingly positive activation barriers3-’ The reductive quenching of * R ~ ( b p y ) by ~ ~neutral + organic molecules leads to products of the same charge, so that back electron transfer to form the excited precursor complex can be ignored in the mechanistic description.8 The ordinary behavior of rate vs free-energy change for these reactants in both the diffusion- and activation-controlled regimes makes them instructive examples for pressure studies of bimolecular quenching reactions. A prior pressure study of photoinduced electron transfer of * R ~ ( b p y ) , ~with + a series of (charged) metal complexes revealed a complex pressure response of k,.9 These results could not be (1) Kavarnos, G. J.; Turro, N. J. Chem. Reu. 1986, 86, 401. (2) Marcus, R. A.; Sutin, N. Biochim. Biophys. Acta 1985, 811, 265. (3) Meyer, T.J. Prog. Inorg. Chem. 1983, 30, 389. (4) Whitten, D. G. Acc. Chem. Res. 1980, 23, 83. (5) Ballardini, R.; Varani, G.; Indelli, M. T.; Scandola, F.; Balzani, V. J. A m . Chem. SOC.1978, 100, 7219. (6) Bock, C. R.;Connor, J. A,; Gutierrez, A. R.; Meyer, T. J.; Whitten, D. G.; Sullivan, B. P.; Nagle, J. K. J. A m . Chem. SOC.1979, 101, 4815. (7) Sandrini, D.; Maestri, M; Belser, P.; von Zelewsky, A.; Balzani, V. J. Phys. Chem. 1985, 89, 3675. (8) Kitamura, N.;Obata, R.; Kim, H.-B.; Tazuke, S. J . Phys. Chem. 1987, 91, 2033. (9) Ueno, F. B.; Sasaki, Y.; Ito, T.; Saito, K. J . Chem. SOC.,Chem. Commun. 1982. 328.
interpreted on the basis of electrostatic effects, which is not surprising when compared to the more numerous studies of ground-state electron-transfer reactions.IO An earlier report demonstrated that diffusion-controlled quenching between neutral organic molecules is diminished under pressure, as expected from the pressure-induced increase in solvent viscosity.” In contrast, the quenching of *Ru(bpy);+ by the neutral complex C ~ ( a c a c ) ~ , for which k, is less than the diffusional rate constant, resulted in an increase in k, with pressure.12 A recent study of the Mo6C114z-/IrC162- system suggests that the volume change associated with the precursor complex may be an important factor in pressure-induced changes of electron-transfer processes. l 3 We report on several *Ru(bpy),2+/amine systems in acetonitrile and n-butyl alcohol at elevated pressures and discuss the quenching results in terms of the classical theory of electron transfer. The effects of solvent polarity and pressure provide an interesting comparison for these bimolecular quenching reactions.
Experimental Section The solvents acetonitrile (CH,CN) from Burdick-Jackson (high-purity grade) and n-butyl alcohol (n-BuOH) from Aldrich Co. (Gold Label) were stored under nitrogen once opened. The source. of tris(2,2’-bipyridine)ruthenium dichloride was G. F. Smith Chem. Co. The five quencher molecules were used as received: N,N-dimethylaniline (DMA), N,N-diethylaniline (DEA), and N,N,N’,N’-tetramethylbenzidine (TMB) came from Aldrich Co., N,N-dimethyl-p-toluidine (DMpT) was obtained from Kodak Chemicals, and the free base benzidine (B) arrived as an Isopac from Sigma Chemical. The high-pressure equipment and operation have been described earlier.I4 The lifetime station15employs a nitrogen laser to excite (10) van Eldik, R. In High Pressure Chemistry and Biochemistry; van Eldick, R., Jonas, J., Eds.; Reidel: Dordrecht, 1987; p 333. (11) Turley, W.D.;Offen, H. W. J . Phys. Chem. 1984, 88, 3605. (12) Kirk, A. D.;Porter, G. B. J. Phys. Chem. 1980, 84, 2998. (13) Tanaka, H.D.; Sasaki, Y.;Saito, K. Sci. Pap. Imr. Phys. Chem. Res. (Jpn.) 1984, 78, 92.
0022-3654/88/2092-3431$0~.50/0 0 1988 American Chemical Society
3438 The Journal of Physical Chemistry, Vol. 92, No. 12, 1988 TABLE I: Pressure Dependence of Solvent Properties Near 25 P, MPa
CH,CN
n-BuOH
0.1 100 200 300 0.1 100 200 300
g/mL 0.786" 0.853 0.896 0.922 0.810d 0.864 0.898 0.924
n2
p,
36.0" 39.0 41.0 42.6 17.1e 18.4 19.3 19.9
1.81b 1.90 1.95 2.01 1.96b 2.04 2.10 2.14
Fetterolf and Offen 19.5 7
O C
7,c p 0.344' 0.481 0.664 0.927 2.63' 5.50 9.10 15.1
"Srinivasan, K. R.; Kay, R. L. J . Solution Chem. 1977, 6 , 357. Estimated from the Lorenz-Lorentz equation. Reference 19. dExtrapolated from Bridgman, P. W. Physics of High Pressures; Bell: London, 1958; p 130. eThe pressure response is assumed to be the average of that of methanol, ethanol, and isobutyl alcohol; Isaacs, N. S . Liquid Phase High Pressure Chemistry; Wiley: New York, 1981; p 99. IIbid., p 103.
45 D
I
Y
c -
185
175
-I
,
!
I
0
100
CH, CN
1
200
PressJre
401i
OC
~
300
,MPoj
Figure 2. Plot of In k , vs P for quenching of Ru(bpy),C12 by DMA in CH3CN at 15 and 45 OC ([DMA] = 8.0 X M) TABLE 11: Temperature Dependence of DMA Quenching in CH,CN"
T , OC kq(O.l) kq(3PO)
vs
15
25
35
45
E,, kcal/mol
7.5 5.8 2.0
9.1 7.8 1.3
15 11 2.3
27 19 2.9
7.9 7.0
"Quenching rate constants k, in units of lo7 M-' SKI, , k q(O.l) and k,(300) refer to 0.1 and 300 MPa pressure, respectively; [DMA] = 8.0 X M; AV: in mL/mol.
Figure 1. Stern-Volmer quenching plots of the Ru(bpy),Cl*/DMA system at 25 OC and at pressures of 0.1 (0),100 (O),200 (A), and 300 MPa ( 0 )in CH$N and n-BuOH.
the sample, a 0.5-m Ebert monochromator to disperse the luminescence, an RCA 8852 PMT to detect the signal and a PAR Mod 4420 Boxcar Averager with Mod4402 Signal Processor to analyze the intensity-time data for typically five sweeps at each pressure, temperature point. Analysis of the data was done by an exponential fitting routine, use of which was judged appropriate by the x2 test and led to *2% errors in the reported luminescence lifetimes. Lifetime measurements are obtained at 0.1, 100, 200, and 300 MPa (1 MPa = IO bar = 10.1 atm). The temperature is regulated to f 0 . 2 OC at 15, 25, 35, and 45 "C. The acceptor/donor solutions are used within 24 h of preparation. After nitrogen purging of the solutions, the quartz capsule is loaded with the sample, closed with a movable stainless steel piston, and placed into the highpressure optical cell. Solvent placed outside the capsule serves to transmit the hydrostatic pressure. If the lifetime at 1 atm after a pressure run agrees with its prior value within 2%,the experiment is judged successful. Estimated errors are 15% in the quenching constants and 1 mL/mol in activation volumes. The pressure dependence of solvent properties are listed in Table I, where p is the density, E is the dielectric constant, n2 is the square of the refractive index, and 7 is the solvent viscosity. Results In all experiments the concentration of Ru(bpy),Cl, was 5 X M. In the absence of quencher, the pressure dependence of (14) Dawson, D. R.; Offen, H. W. Reo. Sci. Instrum. 1980, 51, 1349. (15) Watts, R. J ; Harrington, J. S.; Van Houten, J. J . Am. Chem. Sor. 1977, 99. 2179
the luminescence lifetime T~ in CH3CN has been reported previously.'6 The observed increase in T~ with pressure in n-BuOH parallels that for CH3CN. The donor DMA was chosen to define the appropriate quenching regime, with concentrations below, above, and near 50%quenching, the latter corresponding to 1.2 M DMA in CH3CN and n-BuOH, reX and 1.6 X spectively. When the lifetimes in the presence of quencher T are plotted in Figure 1 according to eq 2, a linear behavior in this
concentration and pressure range, except in CH3CN at high concentration and at high pressure, is demonstrated. The concentrations chosen for this pressure study are well below the nonlinear region, where pressure-induced deviations from linear Stern-Volmer plots may be due to such complications as complex formation in the ground state. The pressure-induced decrease in the bimolecular quenching constant k , is evident from Figure 1. The pressure dependence of k , is expressed in terms of the activation volume AVqt AV,' = -aRT
(3)
where u is the slope (assumed linear) of In k , vs P plots. As seen from Figure 2, such a plot is linear and the modest decrease in k, with P translates into small, positive activation volumes for quenching. On the basis of the results shown in Figures 1 and 2, the consequent pressure experiments at different temperatures and with different quenchers were limited to one concentration near 50% quenching. The temperature influence on k, at 1 atm and 300 MPa is shown in Table 11. As expected, the quenching rate constant increases with temperature. The activation energy E, measured over this small interval is somewhat larger than the 6.6 kcal/mol reported previously." Figure 3 and Table I1 reveal that E , is approximately constant in the 0.1-300-MPa interval and that AV; values are roughly constant in the 15-45 OC interval. The observed pressure insensitivity of the bimolecular quenching process at different temperatures is in sharp contrast to unimolecular (16) Fetterolf, M. L.; Offen, H. W. J . Phys. Chem. 1985, 89, 3320 (17) Kitamura, N.; Okano, S.; Chem. Phys. Lett. 1982, 90, 13.
Reductive Quenching of R ~ ( b p y ) ~at~High + Pressures
The Journal of Physical Chemistry, Vol. 92, No. 12, 1988 3439
19.5
0
19.0
1
\
cr Y
c -
18.5
i
18.0
4
175
I
31
\ 32
33
34
1 /Tx 1 3'
(K"
300 MPa
35
1
1
Figure 3. Arrhenius plots of the quenching rate constant of the Ru(bpy),CI,/DMA system in CH,CN at 0.1 (0) and 300 MPa ( 0 )
([DMA] = 8.0 X
M).
TABLE 111: Pressure Dependence of Electron-Transfer Quenching at 25 O C "
DMA
DEA
quencher DMpT
E,l2?v concn,mM
0.78' 8
0.76b d
kq(O.1) k,(3p) AVq
9.1 7.8 1.3
35 31 1.1
kq(O.1) kq(300) A V:
11 9.0 1.8
'
TMB
0.65b 5
B 0.46c 0.3
130 120 1 .o
300 100 8.9
760 300 7.7
n-BuOH 78 14 9.1 36 4.0 6.5
140 29 13
220 62 11
8
0 0
b
0 1
0 2
0 3
0 6
07
08
Figure 4. Plot of In kqvs E l l z for quenching of R~(bpy)~Cl, by several electron donors (see Table 111) in CH3CN and n-BuOH at 0.1 and 300 MPa. The lines signify only trends. Shown at left are the In kd values for the two solvents at the two pressures. 20
--
0.32c 0.5
-0
0
CH,CN
n-BuGH 0
16
CH9CN 0 0
0 0 0
41
kq is expressed in units of 10' M-I s-l and given at pressures of 0.1 and 300 MPa; AV? in mL/mol. 'Hino, T.; Akazawa, H.; Masuhara, H.; Mataga, N. J . Phys. Chem. 1976, 80, 33. CMann,C. K.;Barnes, K. K. In Electrochemical Reactions in Nonaqueous Systems; Marcel Dekker: New York, 1970;pp 272-278. [DEA] = 4 mM in CH3CN and 27 mM in n-BuOH. deactivation of the ruthenium metal complexes.l6+l8 Finally, the pressure dependence of quenching is compared for several aromatic amines with variable redox potentials. A decrease in the oxidation potential E , in this homologous series, given in CH3CN solvent in Table I h , increases the observed k,. Table I11 and Figure 4 show that pressure reduces quenching in all cases; Le., k,(300) < k,(O.l). These plots of In k, against the thermodynamic driving force AGO, proportional to have the expected shape and approach but do not reach, in the case of CH3CN,5the diffusion limit defined by kd(O.l)= 1.9 X 10" and 2.5 X lo9 M-' s-l for acetonitrile and butyl alcohol, respectively. The pressure influence on kd is smaller in CH3CN19 than nBuOH?O in parallel with the k,(P) response. The plateau governed by kd is only reached in n-BuOH for the free-energy interval defined by these five quenchers. Figure 5 illustrates the pressure-induced decrease in k, in another way by plotting activation volume against the oxidation potential. Here, the activation volume for quenching approaches the activation volume for viscous flow in the diffusion limit only in the case of CH3CN.
Discussion The quenching mechanism for the R~(bpy),~+/amine systems (eq 4) gives the result that k, = kd in the diffusion-controlled limit *Ru(bpy)32++ Q
KA
-
*Ru(bpy)3Z+--Q
k,
products (4)
and k, = k,KA in the activation-controlled limit, where KA = (18)Fetterolf, M.L.;Offen, H.W. J . Phys. Chem. 1986, 90, 1828. (19) Salmon, 0.A. Diss. Abstr. Int., B 1983, 43, 2974. (20)Stearn, A. E.;Eyring, H. Chem. Rev. 1941, 29, 509.
'1
01 00
I
01
,
0 2
,
I
03 El / 2O 4
(by
o , & 0 7 0 8 1
0 6
Figure 5. Activation volume for quenching plotted against the oxidation potential for several quenchers (see Table 111) in CH3CN and n-BuOH. Shown at left are the respective activation volumes for diffusion.
kd/kd is the association constant for the precursor complex? Their pressure dependence (a In k,/aP = -AKt/RT) gives AhV:
= -AV,'
= AVdt
(5)
+ AV,
(6)
or AV:
= AVA
for the respective limits of diffusion and activation. The effect of pressure on electron-transfer quenching within this mechanistic framework can be estimated from a knowledge of the solvent physical properties as a function of pressure. First, the diffusion limit is applicable to quenchers TMB and B (Table 111). In this limit the q data in Table I and the Debye equation (kd = 8RT/30007) yield AVdt values of 7.5 and 18 mL/mol for CH3CN19and TZ-BUOH,~' respectively. The predictions of this model generally fit the experimental results for electron donors B and TMB in acetonitrile but not butyl alcohol (Figure 5 ) . One possible reason for this discrepancy is that the hydroxylic environment around the reactant molecules, particularly the amines, is in a state of compression even at 1 atm; consequently, the volume change associated with diffusion would be smaller. For example, it is known that AV,' is reduced from 18 to 10 mL/mol at a pressure of 600 MPa?O In the other limit (eq 6) two terms contribute to the observed AV.: The volume change for the association of an ion and neutral molecule in polar solvent may be estimated from the density data in Table I and the Eigen-Fuoss equation for ion-neutral reactions
3440
The Journal of Physical Chemistry, Vol. 92, No. 12, 1988
18.6
'8.5
18.1
7
1 j
I
0.40
,
0.42
I
,
,
0.44
,
, 0.48 n-=I
0.46
,
,
0.50
I
,
0.52
I
,
0.54
Figure 6. Plot of In k, vs (n-2 - 6-I) for Ru(bpy)$12/DMA system in CH,CN (0) and n-BuOH (0)at 0.1, 100,200, and 300 MPa (pressure increases toward the left). The lines signify only trends.
(KA= 4rLd3/3000) to give AVA.equal to 1.3 and 1.1 mL/mol for CH3CN and n-BuOH, respectively. Charge redistribution in the precursor complex may add to this value, but this is not expected to be an appreciable correction. For the second term in eq 6, the Marcus equation for the solvent reorganization energy X1-4,21 (eq 7) can be used to estimate the pressure effect on
X = (e2/2)(r+ - t-l)(rQ-I
+ rRu-]- 2 h I )
(7)
outer-sphere electron transfer through the external influence on the solvent continuum properties. Since AGO N 0 for the quenchers DMA and DEA (E,j2[*RuZ+/Ru+]= 0.78 V)*, the Marcus model applied to these quenchers yields AV:
= (1 /4)(dX/dP)
(8)
Assuming that only n and t vary with pressure (Table I) and that rQ = 4, r, = 7, and d = 11 A, eq 8 yields -6.1 and -4.4 mL/mol for CH3CN and n-BuOH, respectively. If the reactant distance in eq 7 is also allowed to decrease as intermolecular distances according to p(P) of the solvent, the respective numbers become -5.8 and -4.1 mL/mol for the electron-transfer step. This contribution to eq 6 would overpower the AVAcontribution estimated above, resulting in negative activation volumes and predicting increased quenching under pressure-in contradiction with the experimental results. The influence of solvent polarity and pressure on the observed quenching rate constant is shown in Figure 6. The plot of In k against the solvent parameter n-2 - t-l has the predicted trend for the two solvents a t 1 atm with respect to solvent polarity;2-22 Le., the solvent parameter is smaller and hence k, is predicted to be larger for n-BuOH than for CH3CN. Even though pressure decreases the solvent parameter, predicting an increase in k, and kq, the opposite trend is observed with respect to pressure-induced increases in solvent polarity. However, if we assume t(P) according to Table I and the refractive index to be pressure-independent, (21) Marcus, R. A. J. Chem. Phys. 196543, 619. (22) Li, T. T.; Brubaker, C. H., Jr. J. Organomel. Chem. 1981,216,223, Chan, M.-S.;Wahl, A. C. J. Phys. Chem. 1982,86, 126.
Fetterolf and Offen then the predicted and observed trends match in Figure 6 and give AV; equal to + O S and +O. 1 mL/mol for CH3CN and n-BuOH, respectively. When this result is added to AV,, the comparison with experiment is excellent. What rationale can be offered for this approach? The distinction between n and t is that the former depends on short-range dispersion forces, while the latter responds to long-range electrostatic forces. At the microscopic level near the reactant pair, it is possible that the polarizability is largely unaltered, while the dipole forces are modified by the removal of free volume upon compression. The present knowledge of solvent and pressure effects on electron transfer are inadequate to offer a strong justification for this line of reasoning. There are of course many other features characterizing the nuclear and electronic barriers to electron transfer that might alter under pressure. For example, it is unknown how significantly the redox potentials and work terms are modified by pressure. Specific solvent effects23are always difficult to identify or exclude from consideration. Conformational packing considerations in the precursor complex and transition state may significantly change the probability of electron transfer.24 The observed decrease in AV: in both solvents when quencher B is replaced with TMB (Figure 5 ) , although barely outside experimental error, suggests that steric factors could be important in solvent compression. As important as the energetics might be the microscopic influence of rotational diffusion on the electron-transfer The observed pressure effect for these photosensitized reactions is altogether too small to verify compensating factors from several potential sources. It should be noted that these difficulties also pervade electron-exchange reactions in the ground state.I0 For example, Nielson et al.24find that electrostatic models are inadequate in explaining ligand effects on k,(P) for self-exchange reactions between Mn(I)/Mn(II) complexes and that the local packing of flexible ligands best track the observed trends in rates. This first pressure study of photoinduced electron transfer between inorganic complexes and neutral organic donors shows that modification in solvent viscosity dominates the diffusioncontrolled limit and that changes in dielectric constant are the dominant feature in activation-controlled electron transfer. However, in the latter case the pressure dependence of k, is small and difficult to assign. Acknowledgment. We are grateful for the financial support of the donors of the Petroleum Research Fund, administered by the American Chemical Society, and the Marine Science Institute. Also, we are indebted to Prof. R. J. Watts for access to his lifetime instrumentation. Registry No. DMA, 121-69-7; DEA, 91-66-7; TMB, 366-29-0; DMpT, 99-97-8; B, 92-87-5; Ru(bpy);2+, 15 158-62-0; [Ru(bpy);]*2Cl, 14323-06-9. (23) Hupp, J. T.; Weaver, M. J. J . Phys. Chem. 1985, 89, 1601. (24) Nielson, R. M.; Hunt, J. P.;Dodgen, H. W.; Wherland, S. Znorg. Chem. 1986, 25, 1964. (25) Rips, I.; Jortner, J. Chem. Phys. Lett. 1987, 133, 411. (26) McManis, G. E.; Golovin, M. N.; Weaver, M. J. J. Phys. Chem. 1986, 90, 6563. (27) Kakitani, T.; Mataga, N . J. Phys. Chem. 1986, 90, 993. (28) Harrer, W.; Grampp, G.; Jaenicke, W. Chem. Phys. Lett. 1984, 112, 263.
