J. Phys. Chem. 1989, 93, 717-723 appear that the electron transfer occurs when the two molecules are edge to edgez4and not in the *-complex form which would involve a distance of approximately 3.5 kZ5
Conclusions From the time dependence of the observed rate constant for the quenching of the first excited singlet state of trans-stilbene by fumaronitrile in acetonitrile, it is possible to separate the intrinsic rate of electron transfer from the rate of diffusion. With the time resolution afforded by the picosecond absorption specSiders, P.; Cave, R. J.; Marcus, R. A. J . Chem. Phys. 1984,81,5613. (25) Birks, J. B. The Exiciplex; Gordon, M., Ware, W. R., Eds.; Academic: New York, 1975. (24)
717
trometer, diffusional processes are no longer rate limiting. Thus, for a reaction that involves intermolecular electron transfer, it is now possible to examine the intrinsic rate of electron transfer as a function of energy change. We are currently examining the rate of electron transfer from the SI of trans-stilbene to a variety of electron-poor olefins in order to examine the Marcus theory for electron transfer. Acknowledgment. This work is supported by a grant from the National Science Foundation (CHE-8418611). We thank Professor Carl Koval, University of Colorado, for his assistance in the measurements of diffusion coefficients. Registry No. trans-Stilbene, 103-30-0; fumaronitrile, 764-42-1; trans-stilbenelfumaronitrilecomplex, 70152-65-7.
pH- Induced Intramolecular Quenching. Ligand-Bridged Complexes Containing Osmium and Ruthenium Barbara Loeb L.,t Gregory A. Neyhart, Laura A. Worl, Earl Danielson, B. Patrick Sullivan, and Thomas J. Meyer* Chemistry Department, The University of North Carolina, Chapel Hill, North Carolina 27599, and Faculty of Chemistry, Catholic University, Santiago, Chile (Received: April 14, 1988)
In the ligand-bridged complex [(tpy)(bpy)O~~~(4,4'-bpy)Ru~~(H~O)(bpy)~]~+ (tpy is 2,2':6',2''-terpyridine; bpy is 2,2'-bipyridine, 4,4'-bpy is 4,4'-bipyridine) and its mixed-valenceanalogue, pH-induced photophysical effects are observed which are triggered by proton loss from the bound aqua ligand. In the Os(I1)-Ru(II)(HzO) complex, Ru(I1) bpy metal to ligand charge-transfer (MLCT) excitation is followed by rapid, efficient energy transfer to the lower energy Os(III)(tpy'-) MLCT state. If the is the dominant form, the Os(1II)tpy'--based emission is pH is raised so that [(tpy)(bpy)Os11(4,4'-bpy)Ru11(OH)(bpy)z]3+ quenched by the Ru(II)(OH) site. The mixed-valence complex [(tpy)(bpy)Os111(4,4'-bpy)Ru11(H20)(bpy)z]s+ can be converted from its nonemitting, Os(II1)-Ru(II)(HzO), form, to its emitting form, [(tpy)(bpy)O~"(4,4'-bpy)Ru~~'(OH)(bpy)~]~+,by using pH changes to adjust the electronic distribution in the ground state.
-
Introduction Recently, it was shown that the electronic distribution within ligand-bridged, mixed-valence complexes can be changed by making changes in the external medium.' For example, in the mixed-metal complex, [(bpy)z(Cl)Os(pz)Ru(NH3)5]4+ (bpy is 2,2'-bipyridine; pz is pyrazine), different degrees of interaction
bpy
PZ
exist between the Os or Ru sites and the surrounding solvent dipoles. The difference is sufficient that changes in solvent can induce intramolecular electron transfer and a change in the oxidation-state distribution, reaction 1.
[(bpy)z(Cl)Os111(pz)Ru11(NH3)5]4+ * [(bPY)z(Cl)os"(Pz)RU"'("3)*14+ (1) In a second experiment the electronic distribution in a ligand-bridged, mixed-valence complex was changed by varying the pH (tpy is 2,2':6',2''-terpyridine and 4,4'-bpy is 4,4'-bip~ridine)~
4.4'- bpy
tPY
In the pH-dependent example advantage was taken of the increase in acidity of bound water in Ru(II1) and the resulting pH dependence of the Ru(III/II) couple to induce intramolecular electron transfer. The ability to manipulate oxidation-state distribution by changes in pH is possible because there is a change in pK, between oxidation states. For example, pK,, = 13.1 f 0.1 for [ R U ( N H ~ ) ~ ( H ~ Oand ) ] ~4.1 + f 0.1 for [ R U ( N H ~ ) ~ ( H ~ O ) ] ~ + . ~ The conversion of an aqua to an hydroxo ligand stabilizes the higher oxidation state and lowers the potential of the Ru(III/II) couple. For example, for the couples [(bpy)z(py)Ru(H~O)]3+/2+ and [(bpy)z(py)Ru(OH)]2+/+,Eo' = 0.78 (pH 11) vs SCE at 22 f 2 OC, p = 0.1 M? For the latter case, in the intermediate pH range between pKal = 0.8 for Ru(II1) and pKal = 10.8 for Ru(II), the Ru(III/II) couple is pH dependent.
+
[(bpy)z(py)Ru1r1(OH)]2+ H+ + e
[ (bPY)APY 1Ru"(OH2) 1z+
For the ligand-bridged complex in reaction 2, the Os(III/II) couple is pH independent and the Ru(III/II) couple is pH dependent. This fact allows the site of oxidation, whether at Ru or Os, to be controlled by pH changes. In a closely related experiment it was shown that in the twice oxidized ligand-bridged + can be generated complex, a chemically reactive R U ' ~ = O ~site (1) Hupp, J. T.; Neyhart, G. A.; Meyer, T. J. J . Am. Chem. SOC.1986,
108, 5349.
'Catholic University, Santiago, Chile.
0022-3654/89/2093-0717$01.50/0
-
(2) (a) Neyhart, G. A,; Meyer, T. J. Inorg. Chem. 1986, 25, 4807. (b) Neyhart, G. A.; Meyer, T. J., manuscript in preparation. (3) (a) Kuehn, M.; Taube, H. J. Am. Chem. Soc. 1976,98,689. (b) Ford, P.; Rudd, F. P.; Gaunder, R.; Taube, H. J . Am. Chem. SOC.1968,90, 1187. (4) (a) Moyer, B. A. Ph.D. Dissertation, University of North Carolina at Chapel Hill, 1979. (b) Moyer, B. A.; Meyer, T.J. Inorg. Chem. 1981, 20, 436. (c) Binstead, R. A,; Meyer, T. J. J . Am. Chem. SOC.1987, 109, 3287.
0 1989 American Chemical Society
718
The Journal of Physical Chemistry, Vol. 93, No. 2, 1989
Loeb L. et al.
by pH-induced electron transfer above pH 7, reaction 2. OH-
+ [ (t~~)(b~~)O~"'(4,4'-bpy)R~"'(OH)(bpy)2] '+* [(tPY)(bPY)Os11(4,4'-bPY)Ru'V(0)(bPY)214+ + H2O (3)
a)
Ao7r
-.pH=22 pH = - 5 t o 9
---- p H
06r 05r
11
9
In earlier work the photophysical and photochemical properties of a series of M(I1)-M(I1) and M(I1)-M(II1) ligand-bridged complexes of Ru and Os were inve~tigated.~-' Following metal to ligand charge-transfer (MLCT) excitation in M(I1) polypyridyl complexes, a variety of light-induced electron-transfer processes have been observed. They include ligand to ligand electron transfer, the photochemical preparation of transient mixed-valence complexes, and the formation and decay of high-energy mixedvalence isomers. With the presence of the aqua groups in the Os(I1)-Ru(I1) ligand-bridged complex, [(tpy)(bpy)Os"(4,4'b p y ) R ~ " ( H ~ O ) ( b p y ) ~and ] ~ + its , mixed-valence form, the possibility exists of exploring possible photochemical processes in ligand-bridged complexes which are induced by changes in proton content.
Experimental Section Materials. Solvents for preparative experiments were reagent grade. The water used as solvent for spectroscopic studies was triply distilled nanopure. The pH for absorption and emission studies was fixed with appropriate phosphate buffers prepared from 85% H3P04 or standard solutions containing Na2HP04, NaH2P04,and/or Na3P04. Ionic strength was maintained at 0.1 M. The pH values were measured by a glass electrodepH meter. Preparations. The salt [ (tpy)(bpy)Os"(4,4'-bpy)Ru"(H20)(bpy)2](BF4)4was prepared by a literature procedure? The corresponding mixed-valence compound was synthesized by electrolytic oxidation immediately prior to any spectroscopic work. In a typical preparation 3 mg of dimer was dissolved in approximately 3 mL of H3P040.1 M, (pH 1.6). The solution was subjected to oxidative electrolysis at 0.72 V vs SSCE, until the current had leveled off at 9. Figure 3a depicts spectra at pH 9 and pH 12, and Figure 3b the result of summing up the absorption spectra of the Ru(I1) and Os(I1) component complexes in Figure 1. The near coincidence of the spectra between parts a and b of Figure 3 in the region 400-600 nm where the spectra are dominated by d a a*(bpy) and d a a*(tpy) charge-transfer transitions demonstrate that the Ru and Os centers maintain their identities and must interact only weakly across the ligand bridge. The loss of intensity in the 350-400-nm region for the ligand-bridged complex is consistent with loss of the d?r a*(4,4'-bpy) charge-transfer bands in this region. Presumably, coordination shifts these transitions to lower energies where they are masked by the more intense dn a*(bpy) transitions. A value of pKa = 10.3 (22 f 2 OC; p = 0.1 M) was determined for the aqua-hydroxo equilibrium in the dimer
-
-
-
OH-
+
-
e
tbO~~~(4,4'-bpy)R~'*b2~+
I
OH2
+
t b O ~ ~ ' ( 4 ~ 4 ' - b p y ) R u ' ~ b 2 ~H+2 0 ( 5 )
I OH
Loeb L. et al.
720 The Journal of Physical Chemistry, Vol. 93, No. 2, 1989 TABLE I: Excited-State Properties at 22 i 2 OC complex (dominant form)
(/.t
= 0.1 M)' L a x , c m r b ~ cnm
798 [ ( ~ P1Y ( ~ P )Os1'(4,4'-bpy Y 112t [(~PY)(~PY)~~"(~,~'-~PY)R~"(H~O)(~PY)~I~~ 798 [(~PY)(~PY)O~"(~,~'-~PY)R~"(OH)(~PY)~I~~ 798 795 [(~PY)(~PY)O~"'(~,~'-~PY)R~"(H~~)(~PY)~I 795 mixed-valence mixtured 795 [(tpy)(bpy )Os1'(4,4'-bpy) Ru"'(OH) ( ~ P)21 Y4t
'+
PH 5.2 5.0 11.9 1.o 4.7 10.9
+,b ns
1034,~ 5.5 8.5 1.1 2.2 7.1 2.5
20 31 30 34 35' 24
'Uncertainties; pH i l % , A,, = f 2 % , ,pC = &lo%, 7 = f 5 % . "Excitation at 480 nm; lifetimes at Amx,a. 'Emission spectra were corrected for instrumental response. d A t this pH the Os(II1)-Ru(II)(H20) and Os(I1)-Ru(III)(OH) complexes are present in equal amounts. CNonexponential decay; the lifetime cited is for the dominant, longer lived component.
Uavel ength [r!rnj i.95E 04
\ \
0 50403-
J.
02-
7-
.-
01 -
n
I 351
Loo
I-
450
--
I
__
503
_L.2 550 600
h(nm)
Figure 3. (a) Absorption spectra of [(tpy)(bpy)Os"(4,4'-bpy)RuL1(H20)(bpy)z](BF4)4at room temperature in water at 3.38 X lo-' M and 1.1 = 0.1 M. (b) Sum of the absorption spectra of the component complexes in Figure 1 at the same concentration.
(t is tpy, b is bpy) by spectrophotometric titration. In Table I are collected quantum efficiencies, lifetimes, and emission wavelengths of maximum intensity for the Os(I1)-Ru(I1) complex in its aqua and hydroxy forms. The emission characteristics of the Os(I1) model are also included. In spectral distribution, emission from the Os(I1)-Ru(I1) complex (Figure 4a) exactly overlays the monomer emission at all pH values studied. However, the intensity of emission decreases sharply with pH as the pK, of the Os(I1)-Ru(II)(H20) complex is approached. In Figure Sa is shown a plot of the fractional loss of emitted light at 798 nm as a function of pH. The intensity is shown as an overlay on the pH titration curve derived from spectrophotometric measurements. In Figure 5b is shown the linear decrease in emission intensity that occurs as the concentration of the hydroxo form increases at the expense of the aqua form. The excitation spectrum of [(tpy)(bpy)0s1'(4,4'-bpy)Ru"(H20)(bpy)z]4+(Figure 4b) coincides qualitatively with the absorption spectrum of the Os(I1)-Ru(I1) complex (Figure 2b). At pH 11.9, compared to pH 5.9, the same excitation spectral profile is observed but the intensity of emission is sharply decreased. The Mixed-Valence Complex. As mentioned in the Introduction the electronic distribution in the mixed-valence complex [ (tpy)(bpy)O~~~~(4,4'-bpy)Ru'~(H~O)( bpy),] s+ is pH dependent, reaction 2, as shown by electrochemical and absorption spectral measurements.* Because of the pH dependence of the Ru(III/II) couple, at pH 1 the dominant form of the complex is [(tpy)(bpy)Os"'(4,4'b p y ) R ~ ' ~ ( H ~ O ) ( b p y ) , ]at~ +pH , 10.9 the dominant form is [ (tpy)(bpy)Os"( 4,4'- bpy)Ru"I(OH)( b p ~ ) ~ Jand ~ +at, pH 4.9 the
C
G Y
c
++
B.00E 00 350.00
475.0e
Wave) ei;th
EaE. Qfl
!IT:
Figure 4. (a) Emission spectra of [(tpy)(bpy)Os"(4,4'-bpy)Ru"(H20)(bpy)2]4t at pH 5.0 and 11.9; 22.0 OC;p = 0.1 M; excitation wavelength = 480 nm. (b) Excitation spectra for [(tpy)(bpy)Os"(4,4'b p y ) R ~ " ( H ~ O ) ( b p y )at~ ]pH ~ ~5.1 and pH 12.0; 22.0 OC;p = 0.1 M; emission wavelength = 783 nm.
two forms are present in equal amounts.z Shown in Figure 6 are emission spectra for the ligand-bridged mixed-valence complex [ (tpy) (bpy)Os"'( 4,4'-bpy) Ru"( HzO)( b ~ y )s+~ as ] a function of pH. Although the emission spectral profiles are the same at all three pH values, there is a slight but reproducible decrease in lifetime at pH 10.9 and a maximum is reached in the emission quantum yield at pH 4.7 as shown in Table I. The emission-time decay profiles at pH 1 and 10.9 exhibited good first-order kinetics and were fit as a single-exponential decay. However, at pH 4.7 nonexponential behavior is apparent in the temporal data suggesting that more than one component may be contributing to the emission at this pH. Discussion The Ligand-Bridged Os(Il)-Ru(Il)(HzO) Complex. The coincidence in emission spectral profiles between the Os(I1) model and the Os(I1)-Ru(II)(H20) complex (Figures 2a and 4a), strongly suggests that the emitting state in both complexes is the same, probably an Os(I1)-tpy based MLCT state. The absence of a significant effect on the Os-based emission by the neighboring
CC t p y * - ) ( bpy 10s 11'(4,4
I-
bp y ) R u IICH 20)( bpy)*I
l/z(Os-RuH,O)
A495
'5 2 -IDH '5 2
A460
I
x
lo6
L
5.50E 03
11,
a' 11
'**
---right ax15 left axis
-
i\
0.00E 00 690.00
b)
I
790.00
890.00
Wave7 e n g t h Inml
Figure 6. Emission spectra as a function of pH for the mixed-valence complex [ (tpy) (bpy) Os"'( 4,4'-bp y) Ru"( H20)(bpy),] s+; excitation wavelength = 480 nm; 22.0 OC. At pH 10.9 the dominant form of the complex is [(tpy)(bpy)~"(4,4'-bpy)R~'~'(OH)(bpy)~]~~ and at pH 4.7 both forms are present in equal amounts.
1
12 I 1
0
1
211
3
4
[Os -Ru" (OH)]x105 Figure 5. Dependence of the intensity of emission from [(tpy)(bpy)0~~~(4,4'-bpy)Ru~~(H,O)(bpy)~]'+ on pH. (a) Overlay of the absorption vs pH data at 495 and 460 nm which were utilized to calculate the pK, value for the Os(I1)-Ru(II)(H,O) complex and the fractional loss of emitted light at 798 nm relative to the intensity emitted at pH 5.2 where there is no quenching. (b) The decrease in the absolute intensity of emitted light (0 as the concentration of the hydroxo complex, [(tpy)(bpy)0~~~(4,4'-bpy)Ru~~(OH)(bpy),]~+,increases. The concentrations were calculated by using pK, = 10.3. The total concentration of the M. ligand-bridged complex was 3.9 X
Ru site is consistent with the absorption spectra in Figure 3 which show relatively slight perturbations on the Os(I1)-bpy and Ru(11)-bpy MLCT spectra in the ligand-bridged complex. We have obtained no evidence for emission from the Ru(11)-aqua monomer, [(bpy)2(HzO)Ru(4,4'-bpy)]2+ or from the corresponding Ru(I1) site in the Os(I1)-Ru(I1) complex. However, in [(tpy)(bpy)O~~~(4,4'-bpy)Ru"(H~O)(bpy)~]~+ the close correspondence between the pH 5-9 absorption spectrum (Figure 3a) and the excitation spectrum (Figure 4b) shows that the light absorbed by d?r(Ru) n*(bpy) and by dn(Ru) ?r*(4,4'-bpy) transitions appears with high efficiency as an 0s"-tpy-based emission. The appearance of the Os(I1)-tpy-based emission following Ru(I1)-MLCT excitation is energetically reasonable. The energy of emission for the related complex [(bpy)2(C1)Ru(4,4'-bpy)]+ in CH3CN/CH2C12mixtures at low temperature is higher (A, = 698 nm, 14300 cm-1)6 than is the emission from the Os(I1)Ru(I1) complex. The lowest Os(I1) polypyridyl absorption band occurs at 725 nm (1 3 800 cm-*) and the emission energy a t 798 nm (12 SO0 cm-l) is lower still. In terms of the states involved, the situation is illustrated schematically in Scheme I. In Scheme I, l/T(Os-RuHzO) and 1 /T( Os-RuHzO) are the intrinsic excited-state decay lifetimes, including both radiative and nonradiative decay, at the Ru(111)-bpy*- and Os(II1)-tpy'- based sites, respectively. k,, is the
-
-
-
rate constant for energy transfer from Ru(II1)-bpy'- to Os(III)-tpy*-. For the Os model, the lifetime is determined by the rate constants for radiative (k,) and nonradiative (k",) decay 1/T = k, + k,, (6) and the quantum yield for emission is given by +em
=7 k ~
(7)
In eq 7, 7 is the quantum efficiency for the appearance of the emitting MLCT state. 7 has been shown to be -1 for [Ru(bpy)J2+ over a broad range of excitation wavelengths.I6 Since the Os(I1)-tpy site is the only emitter in the Os(I1)-Ru(II)(H20) complex, the lifetime expression in eq 6 is also appropriate for that complex as well. In both cases the complexes are weak emitters and 1 / =~ I/.& k,,. From the data in Table 1, for both the monomer and ligand-bridged complex (&,,/T) = vk, = 2.7 X los s-l assuming that 7 = 1 and k, = 2.7 lo5 s-l in both cases. However, in a kinetic sense, reaching the emitting Os(II1)-tpy'state in the ligand-bridged complex is more complicated than in the model. In the Os(I1)-Ru(II)(H20) complex there are both Ru(I1)- and Os(I1)-based MLCT light absorbers and a single Os(II1)-tpy'--based emitter. It follows from Scheme I that + , for the ligand-bridged complex, +em(Os-RuH20), is given by
-
+,,(Os-RuH20)
:[
= ~,T(OS-RUI&O) -&
21
+-
(8)
In eq 8 ARuand A , are the absorbances of the Ru- and Os-based chromophores, AT = A& + ARu,and det(=ket(ket+ 1 / ~ & ) is Os intramolecular energy transfer. the efficiency of Ru In deriving eq 8 it was assumed that (1) the light absorbed by all Ru-based transitions is transferred to Os-based states with the same efficiency, dct;(2) regardless of whether the Os-based emitting state(s) is reached by direct absorption or indirectly by intramolecular energy transfer, the emitting state is reached with the same efficiency, 7 = 1.
-
(16) Demas, J. N.; Taylor,
D.G . Inorg. Chem. 1979, 18, 3111.
722 The Journal of Physical Chemistry, Vol. 93, No. 2, 1989
Loeb L. et al.
Experimentally, we find that, relative to [(tpy)(bpy)Os(4,4'bpy)12+*, &,,(Os-RuH20) is independent of whether the excitation wavelength is 480 or 560 nm. From Figure 1, the Ru(I1)and Os(I1)-based chromophores are roughly comparable light absorbers at 480 nm but absorbance at 560 nm is dominated by Os(I1). It follows from the wavelength independence of &,(Os-RuHzO) and eq 8 that for intramolecular energy transfer, 4Jet
-
1'
The -40% increase in &,, for the Os(I1) site in the ligandbridged complex compared to the model (Table I) has its origin in the 40% decrease in k,, for Os(I1)-Ru(II)(H,O). The nearly superimposable emission spectral profiles for the two point to equivalent Franck-Condon factors for nonradiative decay.8c The relative enhancement in nonradiative lifetime for the ligandbridged complex may have its origin in the influence of electronic effects through the ligand bridge upon the vibrationally induced electronic coupling term that mixes the excited and ground states .8cJ 'J The Ru-bpy-based MLCT excited states in Os(11)-Ru(11)(HzO) are, by inference, very short-lived, as they are in related monomers. The bound aqua ligand apparently plays an important role in the nonradiative decay of these comple~es.'~Given our measurement capabilities, the failure to observe a detectable emission suggests that l / ~ ( o s - R u H ~ O< ) 1 ns (Scheme I). Given the conclusion that de, 1, it follows that the rate constant for intramolecular energy transfer, k,, in Scheme I, must be exceedingly rapid with ket > 1Olo s-l. In an energetic sense there may be a special pathway that assures rapid energy transfer. The special pathway must be based on the utilization of intermediate energy levels on the 4,4'-bpy ligand. The absorption spectra in Figure 3 show that, upon complexation and formation of the ligand-bridged complex, the d a a*(4,4'-bpy) transitions move to lower energy. Although the relative energies of the Ru(I1)-bpy and Ru(II)-4,4'-bpy MLCT states are not known, they are probably close. There may be an "energy funnelling" pathway for energy transfer through the ligand bridge involving sequential electron and energy transfers
1
- R U O H ) t b0-Os"(4,
4 !-b p y ) R U " ' ( o ~ ) b 2 3 *
-
-
-
hv
- -
tbO~~~(4,4-bpy)Ru~~'(H~O)b'-~+* tbO~"(4,4'-bpy'-)R~"'(H~O)b~~+* tb'-Os"'( 4,4'-bpy)Ru"( H 2 0 )b:+*
Intramolecular Quenching in [(tpy)(bpy)Os11(4,4'-bpy)Ru"(OH)(bpy)2]3+. As the p H of solutions containing the Os(I1)Ru(II)(H,O) complex is increased, the intensity of emission falls (Figure 5). The overlay between the pK, curve and the fractional loss of emitted light in Figure 5a shows that the loss of emission is triggered by the conversion of the ligand-bridged aqua ion to its hydroxo form, [(tpy)(bpy)Os"(4,4'-bpy)Ru"(OH) (bpy)2] which is nonemitting. This conclusion is reinforced by two additional observations. The first i s shown by the data in Figure 5b. The intensity of emitted light at 798 nm decreases linearly as the concentration of Os(I1)-Ru(II)(OH) increases. The second reinforcing observation is that through the pH-induced transition region where the emission intensity is falling, the experimental emission-time profile remains simply exponential with the lifetime ( T = 30 ns) being that of the Os(I1)-Ru(II)(H20) complex. From Figures 1 and 3, excitation at 480 nm results in roughly equivalent light absorption by the Os(I1) and Ru(I1)-MLCT chromophores in Os(I1)-Ru(II)(OH). In contrast to [(tpy)(bpy)Os"( 4,4'-bpy) Ru"( H 2 0 )(bpy)2]4+, Ru( 11)-bpy-based excitation does not lead to energy transfer and emission from the
'+,
(17) (a) Henry, B. R.; Siebrand, W. Organic Molecular Photophysics; Birks, J. B., Ed.; Wiley: London, 1973; Vol. 1, Chapter 4. (b) Fong, F. K. Top. Appl. Phys. 1976, IS. (c) Avouris, P.; Gelbart, W. M.; El-Sayed, M. A. Chem. Rev. 1977, 77,793. (d) Freed, K. F. Acc. Chem. Res. 1978, 11, 74. (e) Lim, E. C. Excited States; Acddemic: New York, 1979. (f) Lin, S . H. Radiationless Transitions; Academic: New York,1980. (9) Heller, E. J.; Brown, R. C. J . Chem. Phys. 1983, 79, 3336. (18) (a) Robinson, G. W.; Frosch, R. P . J. Chem. Phys. 1962, 37, 1962. (b) Byrne, J. P.; McCoy, E. F.; Ross, I. G. Aust. J . Chem. 1965, 18, 1589. (c) Siebrand, W.; Williams, D. F. J . Chem. Phys. 1968, 49, 1860. (19) Lumpkin, R., unpublished results.
(20) Em was estimated from the emission spectral profile as the energy where the emitted intensity reaches 25% of the maximum intensity on the high-energy side. The basis for the estimate comes from previous determinations of the relative position of ,500 by emission spectral-fitting of the emission profiles for related polypyridine complexes of osmium(II).8'*c
Ligand-Bridged Complexes Containing Os and Ru SCHEME IVa 21 b O s 1 ' ( 4 . 4 ' - b p y ) Ru"'(OH) bz4+ (T
= 2 4 ns)
12H+ +n+,
21 bOs ' 1 1 ( 4 , 4 ' - b p y ) R ~ 1 1 ( H 2 0 ) b 2 5 *
--H*
t bO~"'(4,4'-bpy)Ru"'(OH)b~'
+
tbOs"(4.4'- bpy)RuII( H20)b:' (T
"5
O C in
= 31 n s )
H20;p = 0.1 M.
(tpy'-)Os" couple can be estimated to be -0.42 V. From the electrochemical results reported in ref 2 and pK, = 10.3 for the Os(I1)-Ru(II)(H20) complex, = 0.27 V for the Ru"'(OH)/Ru"(OH) couple. From the two potentials, intramolecular electron-transfer quenching, Scheme 111, is spontaneous by -0.15 eV. A closely related intramolecular electron-transfer quenching step has been proposed to occur in [(bpy)(bpy'-)(CO)Os"'(L)0~~~Cl(phen)(cis-Ph~PCH=CHPPh~)]~+ (L is 4,4'-bpy or 1,2bis(pyridy1ethane): phen is 1,lO-phenanthroline) following Os(I1) bpy excitation.' In the "remote" MLCT state produced in the quenching step in Scheme 111, the ligand-based acceptor and metal based donor sites are separated by >13 A. The same state could also be reached by bpy tpy electron transfer following Ru(I1) bpy excitation, via the electron-transfer sequence bpy 4,4'-bpy tPY,
-
-
-
hv
- --
- -
tbO~"(4,4'-bpy)R~"'(OH)b*-~+* tbO~~~(4,4'-bpy'-)Ru"'(OH)b$+* tb'-Os"'(4,4'-bpy)R~~~~(OH) b23+*
At this point we have no experimental basis for choosing between quenching by intramolecular energy or electron transfer. In either case, when excited, the capability exists within the ligand-bridged hydroxo complex to transfer energy (Scheme 11) or electrons (Scheme 111) across the bridge when the pH is increased. The Mixed- Valence Complex. From the pH dependence of the redox potentials for the ligand-bridged Os-Ru complex, the oxidation-state distribution in the mixed-valence Os(II1)-Ru(I1) complex is pH dependent. In strongly acidic solution a t pH 1, the dominant form of the complex is [(tpy)(bpy)0s1I1(4,4'b p y ) R ~ ~ ~ ( H ~ O ) ( b5+.p yGiven ) ~ ] the absence of emission from the Ru(I1)-based aqua and hydroxo sites, it is surprising that emission is observed under these conditions (Table I). The emission lifetime and energy in acidic solution are in the same range as for the Os(II1)-tpy'--based MLCT excited state in [(tpy)(bpy)O~"(4,4'-bpy)Ru~~(H,O)(bpy)~]~+. At pH 4.7 the lifetime is nonexponential and the emission efficiency increases by -3. At pH 4.7 the Os(II1)-Ru(II)(H20) and Os(I1)-Ru(III)(OH) forms of the complex are present in solution in equal amounts. By p H 10.9, where [(tpy)(bpy)Os"(4,4'-bpy)Ru"'( o H ) ( b p ~ ) ~is] ~the + dominant form in solution, both the lifetime and efficiency of emission decrease noticeably. The most sensible explanation of the results is based on the fact that in solutions containing the mixed-valence complex there are also equilibrium amounts of the Os(I1)-Ru(I1) and Os(II1)Ru(III), dimers, Scheme IV. All three species are present because of the close proximity of the potentials for the III,III/III,II and III,II/II,II couples.2 From the pH dependences of the couples, even at pH 1 there is a small, finite concentration of [(tpy)(bpy)OsI1(4,4'-bpy) Ru"( H,O) (bpy),] 4+. In agreement with the coincidence of T values in Table I, we conclude that emission at pH 1 is dominated by the Os(II1)-tpy'- MLCT excited state in the Os(I1)-Ru(I1) complex, although the emission quantum yield is higher than we estimate from the pK, data in ref 2. By pH 10.9, where [(tpy)(bpy)O~~~(4,4'-bpy)Ru"~(OH)( b p ~ ) ~ ]is~the ' dominant form, the lowered emission yield and lifetime under conditions where the equilibrium concentration of
The Journal of Physical Chemistry, Vol. 93, No. 2, 1989 723
Os(I1)-Ru(II)(H20) is negligible show that emission occurs from the Os(I1)-Ru(III)(OH) mixed-valence complex. At p H 4.7, significant amounts of both of the emitting complexes, Os(I1)Ru(II)(H,O) and Os(I1)-Ru(III)(OH), exist in solution. Because of the higher emission efficiency for the Os(I1)-Ru(II)(H20) complex (Table I), it is the dominant emitter. The contribution to emission from the mixed-valence complex Os(11)-Ru(III)(OH) is sufficient to account for the observed nonexponential decay at this pH. The mixed-valence complex provides a second, somewhat subtle example of a pH-induced excited-state phenomenon based on the 4,4'-bpy bridging ligand. For the mixed-valence ion itself, variations in pH lead to a change in electronic distribution between the nonemitting, [(tpy) (bpy)Os"'(4,4'-bpy)Ru"(H2O)(bpy)2] 5+ and emitting, [(tpy)(bpy)Os"(4,4'-bpy)Run1(OH)( b p ~ )'+, ~ ]forms. The existence of pH-induced electron transfer in the ground state does provide an additional basis for pH-induced intramolecular quenching. However, a total accounting of the photophysical properties of the system must include the contribution to the observed excited-state properties from the other emitter in the system, [(t~y)(bpy)Os~~(4,4'-bp~)Ru"(H2O)(bp~)21~+. Intramolecular Quenching. We have been able to demonstrate here the existence of two different mechanisms for pH-induced quenching in ligand-bridged complexes. In the first, loss of a proton in the Os(I1)-Ru(II)(H20) complex leads to the hydroxo form, Os(I1) Ru(II)(OH), and intramolecular quenching of the Os(I1)-tpy-based MLCT state by energy or electron transfer. In the second, a pH-induced switch in ground-state electronic distribution from Os(11)-Ru( III)(OH) to Os(111)-Ru(I1) (H 2 0 ) results in an interconversion between emitting and nonemitting forms. Some of the observations made here are of relevance to those made earlier on related ligand-bridged complexes of Ru and Os. The intramolecular electron-transfer quenching step in Scheme I11 has also be suggested to occur in the complexes [ ( b ~ y ) ~ (C0)0~(L)0sCl(phen)(cis-Ph~PCH=CHPPH~)]~+. It has been suggested that Os(I1) bpy excitation of these complexes in a 4:l (V:V) EtOH MeOH glass is followed by intramolecular electron transfer,
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hu
c
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[(bpy) (bpy'-) (C0)0s111(L)Os11Cl(phen)(P2)] 3+* [(bpy) (bpy'-) (CO)Osl'( L)Os"'Cl(phen) (P2)]3+*
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As in the ligand-bridged complex [(bpy),ClR~"(4,4'-bpy)Ru~~(NH3)5]3+,Ru bpy excitation of [(tpy)(bpy)0s1I(4,4'-bpy)R~"(H~0)(bpy)~]'+, in fluid solution is followed by rapid, efficient intramolecular energy transfer. In the mixed-valence complexes [ (bpy),( CO)Os"( 4,4'-bpy)Os"'Cl( phen) (P2)I4+ and [(bpy)2C1Ru11(4,4'-bpy)Ru111(NH3)5]4+,6 neither Ru bpy nor Os bpy excitation leads to appreciable quenching by intramolecular electron transfer to the intramolecular Ru(II1) or Os(II1) sites. The same is true for [(tpy)(bpy)Os(4,4'-bpy)R ~ ~ ~ ' ( O H ) ( b p y ) , where l ~ + , there is no evidence for significant intramolecular quenching. As suggested earlier, the key in all three cases may be the large energy release that occurs upon electron transfer. The large energy release creates a dynamic limitation arising from the energy gap law, and the large separation distance between the electron transfer donor and acceptor sites.'
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Acknowledgment. B.L. acknowledges UNESCO through its project PNUD-UNESCO CHI-84/006 for a study tour to Chapel Hill. Additional support from the DIUC (P. Catholic University of Chile) is also acknowledged and to ARO-D under Grant DAAG29-85-K-0121 and N S F under Grant CHE-8503092 for support of this research. Registry No. [(bpy)2(H20)Ru11(4,4'-bpy)](PF6)2, 117183-36-5; [(tpy)(bpy)0s1'(4,4'-bpy)](PF6)2,
117183-37-6; [(tpy)(bpy)Os"(4,4'-
~PY)R~"(H~O)(~PY)~I(BF~)~, 105638-52-6;[(tpy)(b~y)os"(4,4'-b~~)-
Ru"(0H) (bpy)Z]'+, 1 17183-38-7; [ (tpy)(bpy)O~"'(4,4'-bpy)Ru"(HZO)(bpy)2] '+, 105693-40-1 ; [ (tpy)(bpy)Os"(4,4'-b~y)R~~~~(OH)(bpy)2]'+, 105638-54-8.
