J. Phys. Chem. 1992, 96, 10880-10888
10880
Intramolecular Electron Transfer in Photoexcited Ru( II)-Rh( III)Binuclear Compounds Koichi Nozaki, Takeshi Ohno,* Chemistry Department, College of General Education, Osaka University, Osaka 560, Japan
and Masa-aki Haga Department of Chemistry, Faculty of Education, Mie University, Tsu, Mie 514, Japan (Received: July 30, 1992; In Final Form: October 5, 1992)
Lifetimes of metal-to-ligand charge-transfer excited states, (3CT)Ru, of ruthenium(I1) were measured for ruthenium(II)-rhodium(III) compounds, [R~(bpy)2(L-L)Rh(bpy),]~+, and its analogous compounds in a wide temperature range (170-300 K) in the fluid solvent of a mixture of propionitrile and butyronitrile. The interventing ligands (L-L) used were 2,6-bis(2’-pyridyl)benzdiimidazole (dpimbH,), 2,2’-bis(2’’-pyridyl)bibenzimidazole (bpbimH2), l,l’-dimethyl-2,2’-bis(2”pyridyl)-6,6’-bibenzimidazole(dmbpbim), and bis[2-(2’-pyridyl)benzimidazoyl]ethane (dpbime). The rapid quenching of (’CT)Ru can be explained in terms of an intramolecular electron transfer (ET), (’CT)Ru-Rh(III) Ru(II1)-Rh(II), which is followed by a fast backward ET to regenerate the ground state. The frequency factors obtained from the temperature dependence of the ET rates were found to be almost constant, (1.1-4.1) X 10” s-], irrespective of the intervening ligands, indicating that the ET processes are adiabatic. These factors were in agreement with the value (4-6) X 10” s-I calculated provided that the ET is influenced by the relaxation dynamics of the solvent. The activation energy (E,) ranged from 0.17 to 0.22 eV depending on the bridging ligands. Using the reorganization energy, A, determined from the metal-to-metal Ea was evaluated on the basis of the charge-transfer transition of a mixed-valence compound, [R~(bpy)~(L-L)Ru(bpy),]~+, classical ET theory. The E, values calculated by considering the temperature dependence of X and the solvent motion were in good agreement with the observed values except for bpbimH2.
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Introduction Much attention has been paid toward the understanding of the factors controlling the rates of electron transfers (ET).l-s It has been shown by the classical ET theory that a first-order ET rate can be represented by the product of an electronic transmission coefficient K ~ a~ nuclear , vibrational frequency Y,, and a nuclear factor K,:+II
ET
= KcIvnKn
When nuclear tunneling effects are negligible, K,
= exp[-AG*/RT]
AG* = (A
+ AG0)2/4X - IHrJ
(1) K,
is given by (2) (3)
The reorganization energy X is comprised of outer-sphere (Aoul) and inner-sphere (Xi,) reorganization energy. Hrpis the matrix element representing the extent of the electronic interaction between the reactant and the product states. The representation of K , ~ Y , depends on the adiabaticity of the ET process. When Hrpis sufficiently great, the ET is adiabatic and this term is reduced to Y,. On the contrary, when the ET is nonadiabatic ( K ~ + Rh(bpy),(bpbimH,)" Rh(phen)2(bpbimH2)3t Rh(bpy)2(bpbimH)2t Rh(phen),(bpbin~H)~' Ru( bpy),(dmbpbim)2t Ru( bpy),(dmbpbim)Rh(bpy),'+ Ru( b~y),(dpbime)~+ Ru(bpy),(dpbime)Rh(b~y)2Jt
R~(bpy),(dpbime)Rh(phen),~+ Rh(bpy),(dpbime)'+ Rh(phen),(dpbime)'+
wavenumber/103 cm" Figure 6. TA spectra for [R~(bpy),(dpimbH~)Rh(bpy)~]~+ (40 pM) at 168 K in 0.1 mM HC104 BN/PN (a) and for [Ru(bpy),(dp~mbH)Rh(bpy),I4+ at 183 K in PN/BN containing 2 mM pyridine (b).
v: cm-l 16 130 (2.000) 15820 (1.961) 15800 (1.959) 16640 (2.063) 16580 (2.056) 16490 (2.045) 16310 (2.022) 16290 (2.020) 20 830 (2.580) 20750 (2.570) 19010 (2.360) 19010 (2.360) 16420 (2.036) 16420 (2.036) 16 500 (2.046) 16420 (2.036) 16390 (2.032) 20750 (2.570) 20750 (2.570)
notes a a
b a a
b a a a, d
?e
b
" In the presence of 1 mM HC104. In the presence of 2 mM pyridine. 'Numbers in parentheses denote emission energy in electronvolts. dExcited at 26.3 X lo3 cm-I. 'Excited at 27.0 X 10' cm-l.
nonradiative decay of the excited state as has been seen for The emission spectra observed at 77 K are (dpimbHJ3+, and 23.5 X lo3 cm-' in [R~(bpy)~]~(dpimbH~)~+.~~3Rh(phen)s3+.35s36 characteristicof ligand 3(-*) states. As for Rh(bpy)2bpbimH2+ These bands have been assigned to a 7mr* transition of dpimbH2' and Rh(pher~),bpbimH,~+, a dependence of the emission spectrum coordinating to Ru(II1) ion.20 Two peaks at 21 X lo3 and 23.5 on the excitation energy was observed. The details will be deX lo3 cm-I in Figure 6a are presumably due to an overlap of a scribed el~ewhere.~' broad absorption band with the breaching of the MLCT band at 22 X lo3 cm-I. The considerable shift in energy (25 X lo3 For the Ru(I1)-Rh(II1) compounds, the emission from (3CT)Ru was observed on the excitation of either the Ru or the Rh chro22 X lo3 cm-I) probably results from the stabilization of the LUMO energy of dpimbH2coordinating to a Rh(II1) ion because mophore. These findings can be interpreted in terms of a fast intramolecular energy transfer, 3(?r?r*)Rh(III) -. (3CT)Ru. of its high charge density. A band in the (10-15) X lo3 cm-' The highest energy peaks of the emission spectra for the moregion predominantly consists of the 7mr* band of dpimbH,' and nonuclear and the binuclear compounds are summanzed * in Table (dpimbH2'-)-to-Ru(II1) CTn20 No time lags were observed between the decay of the excited 11. The energy difference between the Ru(1I)-Rh(II1) binuclear and the relevant Ru(I1) mononuclear compounds is relatively large state and the recovery of the ground state at any temperatures (39 meV) for dpimbH2, while it is negligible for the other infor all of the binuclear compounds examined. Emission of the Mononuclear and Blwclear Compounds. No tervening ligands. The energy shift for the former is probably detectable emission was observed for the Rh(II1) mononuclear due to the stabilization of the LUMO energy of dpimbH2 upon compounds at room temperature, probably due to remarkably fast the coordination of a Rh(II1) ion.
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The Journal of Physical Chemistry, Vol. 96, No. 26, 1992 10885
Intramolecular Electron Transfer TABLE IIk Kinetic P8r8IO&la for the Photoidwed IntrmokcuLr EketrolTrrPrfer in Ru(II)-Rh(1II) B i s w k u Compouads in the PN/BN Mixed Solveat complexes Ru(bpy),(dpimbH2)Rh(bPY)IJ+ R~(bpy)~(bpbimH,)Rh(bpyhJ+ 75% 25% Ru(dmbpy),(bpbimH,)Rh(phen)IJ+ Ru(bpy),(dmbpbim)Rh( ~ P)2'+Y Ru(bpy),(dpbime)Rh( ~ P)?+ Y Ru(bpy),(dpbime)Rh(PW?+ Ru(bpy),(dpimbH)Rh(bpy)24+ Ru(bpy),(bpbimH)Rh(bpy),4+
-AGO, eV -0.02
E,,
eV 0.166
0.200 -0.22 0.170 -0.21 -0.13
A,
eV 0.17-0.18
2.3 -2.6 1.1
0.15-0.16 0.15-0.16 0.11-0.12
0.190
3.2
0.1854.195
-0.04
0.190
3.2
0.205-0.215
-+0.17
0.172" 0.194 -0.23
+0.11 -0.1 1
0.6" 2.3
-10 -0.0025
0.17-0.18 0.205-0.215 0.25-0.26 0.27-0.28
"Values obtained in BzN. *Calculated values using eq 18.
Depmtomtionof the Imidazdes C h r d u ia w to a Rh(III) Ion. Two protons of the imidazole moiety in [Ru(bpy),(bpbimH,)Rh(bpy),15+ have different acidities, viz., pK, = 2.65 for the Rh(II1) site and 5.79 for the Ru(I1) site in the 1:l v/v AN/ buffered water.,* The monodeprotonated species, [Ru(La),(bpbimH)Rh(Lb),I4+ was then prepared by the addition of an appropriate amount of pyridine (1-2 mM) into a solution of the complex. The phosphorescence lifetime of [Ru(dmbpy),(bpbimH)Rh(phen),]" was 175 ns at room temperature, and the TA spectrum was similar to that of R~(dmbpy)~(bpbimH~)~+. The lifetime was ca. 20-30 times longer than that of the protonated form. For [Ru(bpy),(bpbimH)Rh(bpy),I4+, the decay of the excited state was single exponential, and the TA spectrum was similar to that of the long-lifetime species of the protonated form (Figure 5b). Two peaks at 27 X lo3and 24.4 X lo3cm-' are presumably ascribed to the T-T* bands of bpy'- and bpbimH'-, respectively, indicating that the difference in state energy between [3Ru111(bpy)(bpy*-) (bpbimH)Rh(bpy),] 4+ and ['Ru"'(bpy),(bpbimH'-)Rh(bpy),I4+ is less than kT (25 meV). In the case of [R~(bpy),(dpimbH,)Rh(bpy)~l~+ of which the pK, values are 2.70 for the Rh(II1) site and 6.74 for the Ru(I1) site,38two TA peaks at 20 X lo3 and 23 X lo3 cm-' for the monodeprotonatcd form at 238 K are due to the overlap a broad peak around 22 X 10 cm-'and the bleaching of the MLCT band at 22 X lo3cm-' (Figure 6b). The TA intensity in a red region was fairly large compared to the protonated species. Temperature Depe&aw of tbe ET Rates. The rapid quenching of (3CT)Ru in the Ru(I1)-Rh(II1) compounds is attributable to an intramolecular ET producing a Ru(II1)-Rh(I1) state (eq 5) since this process is exergonic as shown in Table I11 (vide infra). The resulting Ru(II1)-Rh(I1) state will decay rapidly to regenerate the ground state (q6). ('CT)Ru(L-L)Rh"' Ru"'(L-L)Rh" kET (5) +
Rdl'(L-L)Rhl' Ru"(L-L)Rh"' keET (6) The decay rate of ('CT)Ru was determined by monitoring both the phosphorescence at 15.6 X lo3cm-' and the highest band in +
1
a.
b.
20
(E.)CdIC?
10" s-I 4.1
-0.08
-0.04 -0.04
20
lob
'Os
T-'XlO'
T'XlO'/K''
/ K-'
Figure 7. Plots of In kETvs T'for the intramolecular ETs in the phoin BN/PN mixed solvent. (a) toexcited [Ru(bp~)~(L-L)Rh(bpy)~]~~ L-L = dmbpbim (a),bpbimH2 (fast 0,slow V), and bpbimH- (m). (b) L-L = dpimbH, (A), dpimbH- (A), and dpbime (0).
the TA spectrum. The rate constant of the ET (kET) was calculated from the observed decay rate constant (kob)of ('CT)Ru in the Ru(I1)-Rh(II1) compound by subtracting that (ko)of the relevant Ru(I1) mononuclear compound. Figure 7 shows a plot of In kETagainst T'for [Ru(bpy),(LL)Rh(bpy),15+. Each plot shows a good linear relationship over a wide temperature range. Table I11 shows the values of activation energy (E,) and the frequency factors (A) calculated from the plots. For the monodeprotonated species, [Ru(bpy),(bpbimH)Rh(bpy),I4+ and [Ru(bpy),(dpimbH)Rh(bpy),14+,ko of the protonated species are used for the calculation of kET because it is difficult to prepare the relevant Ru(I1) compound in which the proton at the remote site from the Ru(I1) ion is released. The deprotonation of the imidazole moiety is expected to influence ko so that the E, values and the A factors may contain errors to some extent. MMCT in [R~(bpy)~(L-L)Ru(bpy)~P+. Table IV shows XoP (=hv,,) and Hw which were obtained from the peak energy and the intensity of the MMCT bands for the mixed-valence complexes, respectively.
Discussion Gibbs Energy Change for Intramolecular ET. The difference in the free energy (AGO) between the ('CT)Ru-Rh(III) and the Ru(II1)-Rh(1I) states can be represented by AGO = AG(Rul'LRhll)
- Eo0
(7)
Here Eo0 is the excitation energy of ('CT)Ru-Rh(III), which is estimated from the peak in the highest energy of the emission spectrum at 77 K. The first term representing the difference in the free energy between the Ru(II1)-Rh(I1) and the Ru(I1)Rh(II1) states is defined by AG(Ru"'-Rh")
= FE0'(Ru'''-Rh"/Ru"-Rh") FEo'(R~'LRh'''/Ru"-Rh")
(8)
where Eo' is a formal potential. Since E O ' ( R U ~ ~ L R ~ I ~ / R U ' L R ~ ~ ~ ) is difficut to be obtained, E O ' ( R U " ~ R U ~ ~ / R Uis~ substituted ~RU~~) for this potential provided that the electrostatic interaction between the Ru(I1) and the Rh(I1) ions is the same as that between the Ru(I1) ions in the Ru(I1)-Ru(I1) compounds. The AGO values calculated in this way are tabulated in Table 111. It is found that
TABLE I V P8rawters Obtained from MMtX BUNIS of [Ru(bpy)z(L-L)Ru(bpy)2p+ in AN L-L u,,.! 10) cm-I (hop). eV u t ,J 10) cm-' H,.. meV dpimbH2 6.1 0.76 4.34 58" bpbimH, 7.3 0.91 3.1 7.5-1W dmbpbim 7.7 0.96 3.35 20-24 dpbime 7.7 0.96 4.8 8.7
K,
r.C A 8 12-15 12-15 11
note
~~
236 5-76 15 -6
g
h
"Correction for comproportionation equilibrium was not made. bCalculated from the difference in the oxidation potentials of two Ru(I1) ions. 'The distance between Ru(I1) ions was estimated by using a molecular model. dThe energy at the peak of the MMCT band. /The bandwidth at the half-intensity of the peak. 8Referencc 20. hReference 21.
10886 The Journal of Physical Chemistry, Vol. 96, No. 26, 1992 the ET process (eq 5) is exergonic except for the binuclear compounds with the deprotonated bpbimH, or dpimbHz. Then, we assume that the rapid quenching of (3CT)R~is attributed to the intramolecular ET except for the deprotonated species. Reorganization Energy. In evaluating h for kET, hi, is unable to be calculated because the nuclear configurations of either the reactant or the product states are unknown. However, Creutz et al. have reported that the collisional ET rates for both Rhself-exchange (L')33+-Rh(L')2+ and RU(L')~~+-(~CT)RU(L')~~+ (where L' is a diimine ligand such as bpy, phen) are as considerably great as -2 X lo9 M-I s-1.43 Such rates imply very low inner-sphere barriers in both processes (Xi, < 0.1 eV). Then we assume that Xi, contributes to h to a small extent for the intraUsing the dielectric molecular ET examined here, that is, X z L,. cortinuum model, A,, is given by6
A,", = .U/eop
- 1/%)
(9)
where t, and cop (=n2, n: a refractive index) are the static and optical dielectric constants, respectively. The coefficient a depends on the shape of the chromophores and the distance between them. This coefficient is then calculated from eq 10 using the reorg(Y
X0P/(l/tOp- 1/ts)AN,TE298K= 1.89X0P
(IO)
anization energy XoP for the optical ET in the Ru(I1)-Ru(II1) compound bridged by the same tetradentate as that of the Ru(11)-Rh(II1) compound. This is based on the assumption that a for the intramolecular ET in (jCT)Ru(L-L)Rh(III) is the same as that for the optical ET in Ru(II)(L-L)Ru(III). This assumption is valid because (1) ,,A IS primarily dominant in both X and XOP, and (2) the sizes of the chromophores and the distance for (3CT)Ru(L-L)Rh(III) are almost the same as those for Ru(II)(L-L)Ru(III). Consequently, X is expressed by using the approximation that l/ts >
IAGOl. AG* = a/4n2
+ AGo/2 - IHrpl
(12)
Since ASo is presumed to be very small in such a simple charge shift ET, we assume that AGO = AHo. The activation energy, E, = -R(d In kET/dT1),and the frequency factor for the intramolecular ET studies here are represented by eqs 13 and 14, respectively, by neglecting the temperature dependence of Hrp, K , ~ , v,, and a.
+ a/4n2(1 + 2T/n dn/dT) - lHrpl (13) In A = In ~,p,+ a/2Rn3 dn/dT (14)
E, = AH0/2
When ET processes are nonadiabatic, the rates depend on the electronic interaction. In our systems, the electronic interaction between the chromophores in the dpimbH, compounds is anticipated to be the strongest from the findings as follows: (1) the energy shift of the emission with the coordination of the Ru(bpy)z(L-L)2+to a Rh(II1) ion is the largest (39 meV) when L-L is dpimbH2, (2) Hrpfor the optical ET in the Ru(I1)-Ru(II1) compound bridged by dpimbHz is more than twice greater than that by the other tetradentates. The frequency factors listed in Table I11 are, however, independent of the bridging ligands, which indicates that K~~ is close to unity, that is, the ET process is adiabatic since the second term in eq 14 is almost constant (1.40-1.76) for a series of the tetradentates. It has been found that the reduction of Rh(L&(b~bimH,)~+ occurs at the metal center from the spectroscopic study of Rh(L@)z(bpbimHz)2+ 37 which is generated in the reductive quenching of the 3(?rr*) state of Rh(L)2(bpbimH2)3+by 1,2,4-trimethoxybenzene. Therefore, the intramolecular ET in the photoexcited Ru(I1)-Rh(II1) compound takes place from the K* oibital of the pyridylimidazole moiety to the du* orbital of the Rh(II1) ion. Since the electronic exchange interaction between the donor and the acceptor is unfavorable in the sense of orbital symmetry, the
Nozaki et al. ET is anticipated to be slow. However, it is proved that the interaction is great enough for adiabatic ET even for the dpbime compounds in which the two chromophores are linked by several u bonds. Such adiabatic ET processes across long distance have where N-N is been reported for Fe11(CN)5(N-N)Co1*1(NH3)5, 1,3-bis(4'-pyridyl)propane or analogous ligands.40 Next, the frequency factor is calculated using eq 14. Weaver et al. have pointed out that solvent orientating motion is a dominant factor in determining Y, when the thermal barrier of adiabatic ET is primarily composed of solvent reorganizati0n.4'~~~ The frequency factor observed for [R~(bpy)~(dpbime)Rh(bpy)~]~+in benzonitrile (BzN) is rather smaller than that in the PN/BN mixed solvent (Table 111). This solvent effect on the frequency factor can be explained in terms of the slow dielectric relaxation of BzN compared to that of PN or BN and hence supports the idea that Y, is determined by the solvent orientating motion also in these ET processes. There are several theoretical studies on the ET dynamics influenced by a solvent inertia and a dielectric Calef and Wolynes have derived an equation for Y, in a weakly adiabatic case (Hrp< RT) as43 Y,
= (~TCTL)-~[O.~ + 0.5(1 + 2T,,~/C2e,yT~2)1'2]-1(15)
here y = (4r/3)(ppz/RT)/(s
- 1) and c = (RT/rE,)If2. Where
p is the molar density of solvent and p is its effective dipole moment and T~~ is the rotational relaxation time (rrot = (Z/RT)I12) as-
sociated with the moment of inertia (0of solvent. The T L is the longitudinal relaxation time which is calculated from the Debye relaxation time TD ( T L 3 (eOp/c,)~D). BN is a relatively low friction medium ( T L = 0.53 PS)~'and T,,, of 0.9 ps is estimated using a molecular model. We obtained Y, of 3.9 x 10" s-' at 298 K using c,y = 4.5 and E, = 0.2 eV. The value Y, of 8.5 X 1O1O s-l is obtained for BzN using the parameters T L = 5 ps, T~ = 1.0 ps, and sy = 3.2. Assuming that the temperature dependence of Y, is primarily determined by that of 7L-I and can be represented by the Arrhenius equation as Y, = yo, exp(-E,/RT) where E, = -R(d In TD-'/dT'), eq 14 is rewritten as
In A = In
YO,
+ a/2Rn3 dn/dT
(16) (17)
While few E, values for 7D-l are available, E, values are 0.04-0.05eV for aprotic polar solvents.& Using these values also for BN and BzN, the frequency factors are calculated to be (4-6) X 10" and (1.1-1.6) X 10" s-I for BN and BzN, respectively, with the parameters n = 1.382, dn/dT = -0.00043 for BN, n = 1.526, dn/dT = -0.00048 for BzN, and a = 1.63.49 These frequency factors are in accord with the observed ones, (1.1-4.1) X 10" s-l, in the PN/BN mixed solvent and 6 X 1O'O s-I in BzN. It is concluded that the intramolecular ET in the photoexcited Ru(I1)-Rh(II1) compound is adiabatic (probably weakly adiabatic) and Y, is determined by the solvent relaxation dynamics. Activation Eoegy. Taking the additional activation energy due to the solvent motion into account, E, is represented finally as E, = AH0/2 + a/4n2(1 + 2T/n dn/dT) - lHrpl+ E, (18) The value (10 meV) of Hrpis used for the calculation of E, although it is difficult to evaluate HIPin the Ru(I1)-Rh(II1) compounds examined here.5' The E, values calculated using dn/dT = -0.000436, n = 1.410 (weighted average for PN BN (3:7)) and E, = 0.04-0.05eV are shown as (E,)cakin Table 111. The (Ea)calc values are in fairly good agreement with the E, values except for the compound of bpbimH2. As for the compounds of bpbimH2,E, are considerably greater than (Ea)calc. This can be partly attributed to the presence of the isomers of the binuclear compounds. The intensity of the MMCT band in the Ru(I1)-Ru(II1) compound may be dominated by one of the isomers for which HVmay be stronger so that the use of XoP for the calculation of E, might be questionable for these compounds. To explain the observed E, values, the XoP value should be 1.15-1.20 eV ((9.3-9.7) X lo3 cm-I). It is difficult to recognize
+
The Journal of Physical Chemistry, Vol. 96, No. 26, 1992 10887
Intramolecular Electron Transfer an additional peak in this region in the spectrum of [Ru(bpy),(bpbimH2)Ru(bpy)2]s+due to an overlap with the strong ligand-to-metal CT band. The isomers corresponding to hop of 0.91 eV are predicted to have much greater kEP Attempts to observe such a fast decay component were unsuccessful even at a low temperature (
