J. Phys. Chem. 1994,98, 1145-1150
1145
Quenching of the Excited States of Ruthenium(I1)-Diimine Complexes by Oxygen Quinto G. MU1azzani,*JaHai Sun,lb Morton Z. Hoffman,*JbWilliam E. Ford,lc and Michael A. J. Rodgers*Jc Istituto di Fotochimica e Radiazioni dAlta Energia del CNR, Via de’castagnoli 1 , 401 26 Bologna, Italy; Department of Chemistry, Boston University, Boston, Massachusetts 02215; and Center for Photochemical Sciences, Bowling Green State University, Bowling Green, Ohio 43403 Received: July 16, 1993; In Final Form: October 6, 1993’
The quenching by
0 2
of the M L C T excited states of 10 Ru(II)-diimine complexes (RuL2+) of the form
R~(bpy)3-~-~(bpm),(bpz),~+ (bpy = 2,2’-bipyridine, bpm = 2,2’-bipyrimidine, bpz = 2,2’-bipyrazine, m and z = 0, 1, 2, 3 and m + z I3) in aqueous solution has been investigated using the techniques of laser flash photolysis and time-resolved and steady-state near-infrared emission spectrophotometry. Values of kq have been determined in H20 and D2O and range between 5.1 X lo8 and 3.4 X lo9 M-I s-I. The quantum yields (@A) of singlet molecular oxygen, O2(IAg), originating from the energy-transfer quenching of *RuL2+ by 0 2 and determined in D2O by comparison with @A from the excited state of tetrakis(4-sulfonatopheny1)porphine (TPPS”), are 0.5 for R ~ ( b p y ) 3 ~and + 1 for all the other complexes. It has been established that for Ru( b ~ y ) 3 ~and, + presumably, all the other complexes the yield of electron-transfer products, RuL3+ and 0 2 * - , in bulk solution, is negligibly small. The lower value of @A for R ~ ( b p y ) 3 ~is+attributed to competitive chargetransfer quenching, followed by efficient back electron transfer within the solvent cage. The values of the rate constants of energy transfer for the six complexes that contain at least one bpz ligand decrease with increasing driving force.
-
Introduction Ru(I1) complexes have been shown to be efficient photosensitizers of the photoinduced cleavage of DNA2-*and the oxidation of phen01;~it has been established that singlet oxygen, 02(lAg), is an important reactant in these processes. In order to understand the detailed mechanisms of these photosensitization reactions, it is important that the yield of singlet oxygen generation, especially in aqueous solution, be known. Furthermore, the potential use of Ru(I1) complexes as photosensitizers for 02(lAg) needs to be evaluated. Among the Ru(I1) complexes that have been the focus of extensive research effort are those of the general form R~(bpy)3-~-~(bpm),,,(bpz),~+ (bpy = 2,2’-bipyridine, bpm = 2,2’bipyrimidine, bpz = 2,2’-bipyrazine, m and z = 0, 1,2, 3 and m z I3; RuL2+);details of their photophysics, photochemistry, and redox chemistry have been widely explored. l w 5 With regard to the generation of O2(lAg) from the quenching of *RuL2+by 0 2 , there are several reports in the literature of the quantum yield of O2(lAg) from *Ru(bpy)3,+ in methanol; values range from 0.81 to O.90.l6 Recently, it was reported” that O2(lAg) is formed withunitary efficiency from thequenching of *R~(bpy)3~+ in HzO, D20, methanol, and acetonitrile; there are no values available for the other members of this family of complexes. The quenching of *RuL2+by 0 2 can occur via two mechanisms: charge transfer (reaction 1) to form RuL3+and 0 2 . - and/ or energy transfer (reaction 2) to form ground-state sensitizer
+
*RuL2+ *RuL2+
-
+ 0,
+ 0,
RuL3+
RuL2+
+ 02*- k,,
+ 02(’Ag)
k,,
(1)
(2)
and O2(]A,). Inasmuch as 02(lAg)lies 0.98 eV above the triplet ground state, and the 0-0 transition energies (Em)of the complexes are about 2 eV (Table l), it is thermodynamically possible for all the complexes in their excited state to react with 0 2 and generate O2(lAg) via reaction 2. On the other hand, the value is -0.16 V (in aqueous solution at pH 1 7 and 0 2 of E0(O2/O2*-) Abstract published in Aduance ACS Abstracts, January 1, 1994.
0022-365419412098-1145$04.50/0
at unit activity) whereas thevalues of E0(RuL3+/*RuL2+)range between -0.88 and -0.20 V (Table 1); therefore, it is also thermodynamically possible for the excited states of these complexes to react via reaction 1. Leeand Wrighton18showedthat thequenchingof *Ru(bpy)j2+ and *Ru(bp~)3~+ by ferrocene derivatives in acetonitrile proceeds by both electron and energy transfer, with the latter path being strongly favored over the former at equal driving forces. It has also been concluded19that the quenching of *Ru(bpy)3,+, *Ru( b ~ m ) 3 ~and + , *Ru(bpz)32+by 0 2 is dominated by the energytransfer mechanism, based on the fact that electron transfer is energetically less favorable than is energy transfer. The establishment of the values of the singlet oxygen yield would allow the relative importance of processes 1 and 2 to be evaluated. Since the prediction, made by Marcus in 1960,*Oof the existence of the so-called “inverted region” for electron transfer, where the electron-transfer rate constant decreases with increasing driving force, many experimental confirmations of that phenomenon have been obtained.2143 Similarly, inverted behavior has been found for the energy-transfer quenching of the MLCT excited states of a series of 2,2’-bipyridine complexes of Os(I1) by anthracenes.44 Inasmuch as the photophysical properties of *RuL2+ are finetuned through the mixing of the orbital energies of the ligands, the variation of Eo0 offers an opportunity to evaluate k,, as a function of the energetics of the system. In this paper, the quantum yields of O2(]A8) formation from the quenching of *RuL2+by 0 2 in aqueous solution and k,, as a function of the energetics of the reaction are reported.
Experimental Section Materials. Commercial R ~ ( b p y ) 3 ~ +as, the C1- salt, was purified using a SP-Sephadexcation-exchange column; the cation, which was eluted with 0.2 M NaCl, was precipitated and collected as the Clod- salt. All the other complexes were available from previous studies.l2-I5 Water was purified by passage through a Millipore purification train or similar devices. D2O (>99.8 atom 9%D) was obtained from Aldrich or Fluka and was used as received. Tetrasodium tetrakis(4-su1fonatophenyl)porphine (TPPS”) was obtained from Porphyrin Products Inc. (Logan, UT) and was 0 1994 American Chemical Society
1146
3.0 1
1 05 .~ '1-
I
6
Mulazzani et al.
The Journal of Physical Chemistry, Vol. 98, No. 4, 1994
2.0
1
I n
II
-
TABLE 1: Photophysical and Electrochemical Properties of R d 2 + Complexes E O -
complex'
Amax,
nm
&I,
eV
TOW)? TO(D):
(3+/2+*),d
E1/2:
ws
ps
VvsNHE
VvsSCE
0.61 0.073 0.90 0.012 0.081 0.031 0.52 0.42 0.74 0.23
0.98 0.12 1.4 0.021 0.18 0.062 1.0 0.78 1.3 0.46
-0.88 -0.39 -0.20 -0.58 -0.45 -0.51 -0.27 -0.36 -0.31 -0.34
1.27 1.69 1.98 1.40 1.49 1.55 1.78 1.72 1.87 1.66
7
I 2
"0
yyy A, nm
1.0 0
c
\
W
0.0
300
350
400
450
-n
500
550
A, nm Figure 1. Spectrum of *Ru(bpy)(bpm)(bpz)2+obtained from the laser flash photolysis of Nz-purged solution containing -70 pM Ru(bpy)(bpm)(bp@+ in aqueous solution; the spectrum was obtained from that shown in the inset by correcting for the bleaching of the ground state. Conditions for the inset: optical path = 0.5 cm; time = 20 ns after the laser pulse.
mmm zzz yym yyz mmy mmz zzy zzm
-
-
2.15 2.08 2.18 1.98 1.94 2.06 2.05 2.08 2.18 2.00
ymz a Abbreviations: y = bpy, m = bpm, z = bpz. In HzO. In DzO. See footnote 56. e For oxidation in CH3CN (ref 11). 6.5 7
used as received. The solutions were purged with an inert gas (Ar or N2) or saturated with 0 2 or with O2-N2 mixtures of known composition, which allowed the concentration of 02 to be varied. The solubility of 02 at 1 atm partial pressure in H2O at 23 OC was taken to be 1.30 mM from a linear interpolation of the solubilities at 20 OC (1.38 mM) and 25 O C (1.26 mM);45it is assumed that the solubilities of 0 2 in H2O and D20 are the same. All measurements were made at ambient temperature (23 f 2 "C). No ionic strength adjustments were made. Apparatus and Procedures. Laser flash photolysis experiments using kinetic spectrophotometric (absorption or emission) detection were performed a t the Center for Fast Kinetics Research (CFKR), University of Texas at Austin, and at the Center for Photochemical Sciences, Bowling Green State University, using Q-switched Nd:YAG lasers that generated pulses of IO-ns duration. If not otherwise stated, the excitation wavelength was 532 nm. The solutions were contained in 1 X 1 or 1 X 0.5 cm cells; in the latter case, the solution was excited along the larger dimension and probed by the analyzing light along the smaller dimension. The solutions were continuously stirred with a stream of the purging gas. Kinetic data were obtained from computer averaging of at least 10 individual laser shots. The point-bypoint differential spectra of *RuL2+ were obtained from the averaging of at least three individual shots at each wavelength; the absorbances of the solutions at 532 nm were set a t 0.13. R ~ ( b p y ) 3 ~was + used as the actinometer, taking the difference in the molar absorptivities of the excited and ground states (Ae) as -1 .O X lo4 M-I cm-I a t 450 nm.46 The quenching of *RuL2+ by 0 2 was investigated by monitoring the temporal changes of the 450-nm absorbance; at this wavelength, all the complexes showed a bleaching upon excitation. In all cases in the absence and in the presence of 0 2 , the decay of the excited state resulted in the complete recovery of the absorbance that was evident prior to irradiation. The instruments for the time-resolved and steady-state nearinfrared (NIR) emission measurements have been described p r e v i o ~ s l y .The ~ ~ decay ~ ~ ~ kinetics of O2(lAg) were obtained by the computer averaging of 200 laser shots. The steady-state measurements were performed by using the 488-nm line of an Ar ion laser (Coherent, Innova 90) as the excitation source. Most of these experiments were performed with solutions that had absorbances of -0.3 at 488 nm, although some experimental runs were performed with solutions that had absorbances of 0.1 + used as a standard, and measurements or 1.O. R ~ ( b p y ) 3 ~was of 02('A,) generated from it were made before and after the other complexes or TPPS" were examined; this confirmed that the instrument response had remained constant. Each set of experiments was performed with solutions that had almost identical absorbances a t 488 nm; if necessary, small corrections
610 622 602 680 695 660 634 640 613 666
'm u3 0
I
I
6.0 -
5.5 .
TI 0)
n 0
Y
4.0 0.0
1 .o
0.5
1.5
[021> mM Figure 2. Values of kom for the decay of *Ru(bpy)(bpm)(bpz)*+ monitored at 450 nm as a function of [OZ]in aqueous solution.
were made for differences in absorbance a t the excitation wavelength. It was proven that the ratio of the quantum yield of 0 2 ( l A g ) formation (@A) from *Ru(bpy)32+ and from *Ru( b p ~ ) 3 (vide ~ + infra) was independent of the value of absorbance at 488 nm and of the intensity of the laser beam in the ranges over which they were changed. Ground-state spectra were measured with a Perkin-Elmer Lambda Array 3840 spectrophotometer operated in the high-resolution mode (0.25 nm) (Center for Photochemical Sciences) or with a Hewlett-Packard 8450A spectrophotometer (CFKR).
Results The excitation (& = 532 nm) of aqueous solutions of RuL2+ resulted in the transient bleaching of the ground state with the concomitant generation of the excited state, according to reaction 3; as a result, an increase of absorbance in the 350-380-nm region and an increase of transmission (bleaching) in the 450-nm region were observed for all the complexes. The differential spectrum obtainedwithRu(bpy)(bpm)(bpz)2+isshownin theinset toFigure 1.
-
R U L ~+ + hv *RUL~+ (3) In the absence of 0 2 , the bleaching a t 450 nm recovered to the original base line in an exponential manner, yielding the values of 70 given in Table 1; the bleaching kinetics always paralleled the decay of the emission in the 600-700-nm region. The values of TO in D20 are also reported in Table 1. In the presence of 0 2 , the decay of *RuL2+was enhanced, with k o b d increasing linearly with increased [Oz];Figure 2 shows, for example, the variation of kobsd with Io,] for Ru(bpy)(bpm)(bpz)2+. From the slopes of plots of kobsd vs [ 0 2 ] , the values of k, in H 2 0 reported in Table 2 were obtained; values of k, for Ru(bpy)2(bp~)~+ and Ru(bpm)~(bpy)~+ in H2O and all thevalues in DzO were evaluated by determining kobsd for 02-saturated solutions only. The value of k , for Ru(bpy)z(bpm)2+ could not
The Journal of Physical Chemistry. Vol. 98, No. 4, 1994 1147
Quenching of Ru(I1)-Diimine by 02
TABLE 2 Thermodynamic and Kinetic Parameters and Relative Singlet Oxygen Yields for RuL2+Complexes AGetO,
complex* YYY
mmm zzz
YYm YYZ
Y"
mmz ZZY
zzm Ymz
AGct",
kq(H)?
kq(D):
eV
eV
109 M-1 s-I
109 M-1 s-1
$@Ic
-1.17 -1.10 -1.20 -1.00 -0.92 -1.08 -1.07 -1.10 -1.20 -1.02
-0.72 -0.23 -0.04 -0.42 -0.29 -0.35 -0.11 -0.20 -0.15 -0.18
3.4 1.5 0.51 1.6c 1.8 1.5 0.74 0.91 0.68 1.3
3.2 1.3 0.58 1.6c 1.3 1.6 0.8 1 1.1 0.71 1.2
0.80 0.17 0.51 0.04c 0.23 0.11 0.51 0.53 0.55 0.42
(h'/h')tr
(hX/hr)ssd (@A'/@A%
1.o 0.43 1.3 0.11 0.69 0.30 1.2 1.5 1.2 1.1
(@A'/@A%
1.o 2.0 2.0
1.o 0.52 1.2
1.o 2.4 1.8
2.4 2.2 1.8 2.3 1.8 2.2 2.1 f 0.21
1.1 1.5 1.4
1.7 2.3 2.0
'
2.0 f 0.3'
Abbreviations: y = bpy, m = bpm, z = bpz. In H20. In D20. Evaluated by dividing the area under the IO2 luminescence spectral peak (corrected for background) by that for Ru(bpy)s2+.e Evaluated as described in text and in footnote 51. f Mean value. 81
6oo
2 1
c .-
: I
Y
.k
-200
1
0
40
80
120
160
I
200
w
I
A
I \
4
I
/
01 1200
I
\
1300
1250
1350
h , nm Figure 4. Example of steady-state NIR emission experiment showing the spectrum obtained in 02-saturated D20 solution containing Ru(bPY)32+.
-200 1
0
40
80
120
160
I 200
Time, pi Figure 3. Examples of time-resolved experiments showing the decay of
the NIR emission of singlet oxygen obtained from the laser flash photolysis of 02-saturated D20 solutions containing TPPS' (A) or Ru(bpy)?+
(B). be obtained because of the low value of TO (21 ns); a value of k, -< 1 X IO9 M-'s-l for this compound has been estimated by Akasheh et al.49 Relative Yields of 02(1Ag). Time-Resolved Experiments. Relative quantum yields of 0 2 ( l A g ) in D20 with R ~ ( b p y ) 3 ~as+ the reference were obtained as follows. The exponentially decaying slow component of the NIR luminescence (Figure 3) was extrapolated to zero time. Referring to Figure 3A,B, this is the point defined by the intersection between the extrapolated part of the curve and the vertical rise of the luminescence signal. The signal intensity a t this point (&) is given as & k,nA, where k, is the radiative rate constant for the {Ag 32,transition in 0 2 and nA is the number of Oz(lAg) species generated in the sample. Within the same solvent, k, is invariant, and when the excitation conditions are held constant, &x/Lo' = @ A ~ ~ ~ / @ A ' V ' , where 7' and vx are the fractions of the excited states that are quenched by oxygen in the reference and unknown samples, respectively. Values of $'and qxwere calculated from the relevant values of TO, k,, and [Oz] for each experiment. As Figure 3 shows, the rapid decay of the 'prompt" luminescence component50 exhibits a tail that is merged into the O2(lAg) component. To obviate errors from this, we delayed the start of our fitting routine until -30 ps after the pulse. Consistent values for T A (60 f 3
--
ps) were derived from the fits, which, together with the excellent consistency with the data from the steady-state experiments (see below), provide convincing evidence that our &values are accurate representations of @A. For R ~ ( b p y ) 3 ~R+~, ( b p m ) 3 ~R+~, f b p z ) 3 ~ + , Ru(bpm)2(bp~)~+, Ru(bp~)z(bpy)~+, and Ru(bpz)~(bpm)~+, this procedure proved reliable and resulted in values of (W/L&, the ratio between & for the various complexes and & for Ru(bpy)j2+ (Table 2). After correction for vq, these quantities resulted in values of ( @ ~ ~ / @ A ' ) t r (Table 2). For Ru(bpy)2(bp~)~+, R~(bpm)2(bpy)~+, and Ru(bpy)(bpm)(bpz)2+, the 02(lAg)component could not be separated from the prompt emission. Using the same method, we obtained @~(RU(bpy)3~+)/@~(TPPS') = 0.80 f 0.07. Steady-State Experiments. Upon 488-nm excitation of 0 2 saturated DzO solutions containing RuL2+,composite luminescence spectra of the tail of the emission of *RuL2+and the emission of O2(lAg) wereobtaind, the spectrumobtained withR~(bpy)3~+ is shown in Figure 4. From these spectra, the component originating from 02(lAg) was extracted by interpolating a base line and measuring the area above it. Using solutions having the same absorption a t 488 nm and with constant laser beam intensity, the signals associated with 02(lAg) formed by the 10 complexes were normalized as above, using the values of vr and vx (=kq[02]/(kq[02] 1/70)), given in Table 2. The ratio between the signals from the various complexes and that from R ~ ( b p y ) 3 ~ + are also reported in was calculated; these values ((JV/LO')~~) Table 2,51as are the (@A'/@A')~~ values. As with the time-resolved experiments, we also compared Ruusing the steady-state method. From this ( b p ~ ) and ~ ~ TPPS" + we obtained @.a(RU(bpy),2+)/@A(TPPS") = 0.80 f 0.10.
+
Discussion The excitation of RuL2+generates the luminescent metal-toligand charge-transfer (MLCT) excited state, which can be described as a one-electron-reduced ligand coordinated to a Ru-
Mulazzani et al.
1148 The Journal of Physical Chemistry, Vol. 98, No. 4, 1994
(111) center with the electron localized on the most easily reduced ligand (bpz > bpm > bpy);1° theefficiency of formation of *RuL2+ is 1 for all the complexes. The differential absorption spectra of *RuL2+relative to those of the ground states reflect the change in absorbance of the solution upon excitation; AA = Aelc, where AEis the difference in the molar absorptivities of the excited- and ground-state complexes (e* - egs) at the wavelength of observation, I is the optical path length, and c is the concentration of excited state formed and ground state depleted. By using * R ~ ( b p y ) 3 ~ + (Ae = -1.0 X lo4 M-1 cm-I at 450 nm)46 as the actinometric standard, the corrected spectra of *RuL2+were calculated from the differential and ground-state spectra; as an example, the corrected spectrum of *Ru(bpy)(bpm)(bpz)2+is given in Figure 1. The corrected spectrum of * R ~ ( b p m ) 3 ~we + reported prev i o ~ s l ywas ~ ~ based on a lower value of Ae450(-7.6 X lo3 M-l cm-I) for * R ~ ( b p y ) 3 ~ + . Yields of Singlet Oxygen. Table 2 shows that for eight complexes the ( @ A T / @ ~ r ) s s values exhibit a mean of 2.1 f 0.2 relative to that of R ~ ( b p y ) 3 ~ + In. addition, the (@AX/@Ar)trvalues, where determined, have a mean of 2.0 f 0.3 based on the same reference. In order to put these determinations onto an absolute ~ + measured relative scale, the yield of 0 2 ( IAg)from * R ~ ( b p y ) 3was to that from TPPS4-. As mentioned above, it was found that the relative yield of O2(lAg) from * R ~ ( b p y ) 3 ~was + -80% of that from the TI state of TPPS". The absolute value of @A for TPPS" from several laboratories has been reported as being between 0.33 and 0.76.16 By discarding the clearly suspect values of 0.33 and 0.42, the remaining seven display a mean of 0.63 f 0.1, which corresponds to that of 0.64 f 0.1 derived from the careful study of Davila and H a r r i m a r ~ .By ~ ~using @A = 0.63 for TPPS" in D20,54@A = 1.1 f 0.1 for all RuL2+except R ~ ( b p y ) 3 ~and +, @A = 0.5 for R ~ ( b p y ) 3 ~ + . That we should find @A for R ~ ( b p y ) 3 ~to+ be so low was surprising in view of the much higher values found in other solventsI6and the recent claim" that the value is unity in H2O. Nevertheless, our data, obtained in both steady-state and timeresolved experiments, all point unequivocally to this conclusion and are not inconsistent with the value of 0.41 obtained by Tanielian using oxygen consumption methods.55 Reaction Mechanism. It is apparent that, with the exception of R ~ ( b p y ) 3 ~all + , the complexes are quenched by 0 2 through energy transfer to give O2(lAg) with unitary efficiency; for *Ru( b ~ y ) at ~ least ~ + one other path must be competitive with energy transfer with an efficiency of 0.5. Thevalues ofE0(3+/*2+) and AGo(ct) for thecharge-transfer quenching of *RuL2+ (reaction 1) can be obtained from relationships 4 and 5; the values of these parameters are given in Table 2.56 Thus, although one-electron-transfer quenching is energetically feasible for all the complexes, with AGO (ct) ranging between -0.04 and -0.72 eV (Table 2), energy transfer is always more exoergonic with AGo(et) ranging between -0.92 and -1.2 eV (Table 2).
-
E0(3+/*2+) = E0(3+/2+) -Eoo
(4)
AGo(ct) = E0(3+/*2+) - E o ( 0 , / O 2 * - )
(5)
For R ~ ( b p y ) 3 ~ +E0(3+/*2+) , is at least several hundred millivolts more negative than those of the other complexes; it is expected that electron transfer from the excited state would be more favorable. With 50%of the collisional interactions between *Ru(bpy)32+and 0 2 leading to products other than O2(lAg), it is tempting to think that electron transfer accounts for the remainder. However, this is not immediately obvious. As stated earlier, the bleaching at 450 nm upon excitation of the ground state into the MLCT state recovers to the level of that before the flash, following a clean, single-exponential process; there is no residual component of the bleaching signal that could correspond
to the conversion of some of the Ru(I1) species into Ru(II1). Such a component would recover to the base line over a much longer time scale.58 Nevertheless, we are forced to conclude that the process competing with energy transfer is, indeed, charge separation. There are no other possible routes that lead from the collision complex.59 We infer that the geminate redox pair undergoes back electron transfer within the solvent cage much more rapidly than it diffuses into bulk solution; k b t >> kce. Reactions 6-8 summarize this mechanism.
[RuL3+-.02'-]
-
RuL2+
+ 0,
kbt
(7)
Support for this conjecture comes from the values of the bimolecular rate constant of reaction 9, where the species are present in homogeneous solution rather than within the solvent cage as in reaction 8. Literature values of k9 in H2O range from 8.0 X lo9 to 3.5 X 1Olo M-I s-l, depending upon the ionic strength.6u2 These values are very close to the diffusion limit at ambient temperature and indicate that the RuL3+ and 0 2 ' species undergo mutual annihilation at virtually every collision. In the case of reaction 8, where the electron-transfer reaction is proposed to occur within the confines of the solvent case, it becomes very unlikely that cage escape will occur to any significant extent.
(9) Thus, for Ru(bpy)j2+,although the energy transfer is more exoergonic than is electron transfer, both processes proceed with approximately thesamerateconstant. Thisseems to beincontrast with theconclusions of Lee and Wrighton18 that "the extra solvent and internal reorganization energy needed to accommodate the electron transfer strongly favors quenching by energy transfer at equal driving force." However, the energy gap dependence of the rate constant for these processes may be quite complicated, and direct comparison must be made cautiously. For example, if the energy-transfer rate constant follows a Marcus relationship as observed and has a "bell-shaped" dependence on driving for the quenching of some Os(I1) complexes by anthracenes,44 then the maximum of the curve, which corresponds to the reorganization energy, will move along the AGO axis according to the absolute value of 1,. Indeed, the maximum of the bellshaped curve observed by Murtaza et al.44 for energy transfer occurs at a driving force of approximately -0.5 eV, while the maximum for the back electron transfer within the solvent cage upon thequenching of metal complexes by aromatic amines occurs in the vicinity of -1.7 eV.3841 Clearly, if compared at the same driving force, the electron transfer could be slower than energy transfer at certain driving forces due to a higher-energy barrier, while the opposite could be true at yet other driving forces, depending on the shapes of the bell-shaped curves, the values of the reorganization energies, and the driving force at which the comparison is made. Energy Transfer. Because of the competition between energy and electron transfer in the reaction of * R ~ ( b p y ) 3 with ~ + 0 2 , the yieldof 02(lAg)and the quenching rate constant can be expressed by eqs 10 and 11, respectively. Combining the two equations, a value of 1.7 X lo9 M-l s-l for k,, is obtained. For the other 1, and k,, = k,. complexes, @
-
+
@ = ket/(ket kct)= 0.5
k, = k,, + k,, = 3.4
X
lo9 M-' s-'
(1 1)
The Journal of Physical Chemistry, Vol. 98, No. 4, 1994 1149
Quenching of Ru(I1)-Diimine by 0 2 9.5 1
8.7.
I
0
Acknowledgment. This research was supported in part by Consiglio Nazionale delle Ricerche, Progetto Finalizzato Chimica Fine I1 (Q.G.M.), in part by the Office of Basic Energy Sciences, Division of Chemical Sciences, U.S. Department of Energy (M.Z.H.), and in part by N I H Grant GM24235 and the Center for Photochemical Sciences at Bowling Green State University (M.A.J.R.). CFKR is supported jointly by the Biotechnology Resources Program of the N I H (Grant RR00886) and by The University of Texas at Austin. References and Notes
Figure 5. Values of log ket vs Aceto for bpz-acceptor complexes (0),
(1) (a) Istituto FRAE-CNR. (b) Boston University. (c) Bowling Green State University. (2) Kelly, j.M.; McConnell, D. J.; OhUigin, C.; Tossi, A. B.; Kirsch-De Mesmaeker,A.; Masschelein,A,; Nazielski, J. J . Chem.Soc.,Chem. Commun. 1987. 1821-1823. ~ - - ~ (3) Kelly, J. M.; Feeney, M. M.; Tossi, A. B.; Lecomte, J.-P.; Kirsch-De Mesmaeker, A. Anti-Cancer Drug Design 1990, 5, 69-75. (4) Orellana, G.; Kirsch-De Mesmaeker, A.; Barton, J. K.; Turro, N. J. Photochem. Phorobiol. 1991, 54, 499-509. (5) Lecomte, J.-P.; Kirsch-De Mesmaeker, A.; Kelly, J. M.; Tossi, A. B.; GBrner, H. Photochem. Phorobiol. 1992, 55, 681-689. (6) Fleisher, M. B.; Waterman, K. C.; Turro, N. J.; Barton, J. K. Inorg. Chem. 1986, 25, 3549-3551. (7) Mei, H.-Y.; Barton, J. K. Proc. Natl. Acad. Sci. U.S.A. 1988, 85, 1339-1 343. ( 8 ) Tossi, A. B.; Kelly, .I. M. Photochem. Photobiol. 1989,49,545-556. (9) (a) Okamoto, K.; Honda, F.;Itaya, A.; Kusabayashi, S. J . Chem. Eng. (Jpn.) 1982,15,368-375. (b) Tratnyek, P. G.; Hoignb, J. Enuiron. Sci. Technol. 1991,25,1596-1604. (c) Pizzocaro,C.; Bolte, M.;Sun, H.; Hoffman, M. Z. New J . Chem., in press. (10) Allen, G. H.; White, R. P.; Rillema, D. P.; Meyer,T. J.J. Am. Chem. SOC.1984, 106, 2613-2620. (11) Ross, H. B.; Boldaji, M.; Rillema, D. P.; Blanton, C. B.; White, R. P. Inorg. Chem. 1989, 28, 1013-1021. (12) Sun, H.; Neshvad, G.; Hoffman, M. Z. Mol. Cryst. Liq. Cryst. 1991, 194, 141-150. (13) Venturi, M.; Mulazzani, Q. G.; DAngelantonio, M.; Ciano, M.; Hoffman. M. Z . Radiat. Phvs. Chem. 1991. 37. 449-456. (14) DAngelantonio, MI; Mulazzani, Q. G:; Venturi, M.; Ciano, M.; Hoffman, M. Z . J. Phys. Chem. 1991, 95, 5121-5129. (15) Sun, H.; Hoffman, M. Z. J . Phys. Chem. 1993, 97, 11956-11959. (16) Wilkinson, F.; Helman, W. P.; Ross, A. B. J. Phys. Chem. Ref. Data 1993, 22, 113-262 and references therein. (17) Zahir, K. 0.;Haim, A. J. Photochem. Phorobiol. A : Chem. 1992, 63, 167-172. (18) Lee, E. J.; Wrighton, M. S. J . Am. Chem. SOC.1991, 113, 85628564. (19) Timpson, C. J.; Carter, C. C.; Olmsted, J., 111 J . Phys. Chem. 1989, 93,41164120. (20) Marcus, R. A. Discuss. Faraday SOC.1960, 29, 21-31. (21) Miller, J. R.; Calcaterra, L. T.;Closs, G. L. J . Am. Chem. SOC.1984, 106. 3047-3049. (zi) Miller, J. R.; Beitz, J. V.; Huddleston, R. K. J . Am. Chem.Soc. 1984, 106, 5057-5068. (23) McLendon, G.; Miller, J. R. J. Am. Chem. SOC.1985, 107, 78117816. (24) Closs, G. L.; Calcaterra, L. T.; Green, N. J.; Penfield, K. W.; Miller, J. R. J. Phys. Chem. 1986, 90, 3673-3683. (25) Wasielewski, M. P.; Niemczyk, M. P.; Svec, W. A,; Pewitt, E. B. J . Am. Chem. SOC.1985,107, 1080-1082. (26) Irvine, M. P.; Harrison, R. J.; Beddard, G. S.; Leighton, P.; Sanders, J. K. M. Chem. Phys. 1986, 104, 315-324. (27) Harrison, R. J.; Pearce, B.; Beddard, G. S.; Cowan, J. A.; Sanders, J. K. M. Chem. Phys. 1987, 116, 429-448. (28) Gould, I. R.; Ege, D.; Mattes, S. L.; Farid, S. J. J . Am. Chem. SOC. 1987, 109, 3794-3796. (29) Gould, I. R.; Moody, R.; Farid, S. J. Am. Chem. SOC.1988, 110, 7242-7244. (30) Gould, I. R.; Moser, J. E.; Armitage, B.; Farid, S.;Goodman, J. L.; Herman, M. S. J. Am. Chem. SOC.1989,111, 1917-1919. (31) Gould, I. R.; Ege, D.; Moser, J. E.; Farid, S. J . Am. Chem.Soc. 1990, 112,4290-4301. (32) Gould, I. R.; Young, R. H.; Moody, R. E.; Farid, S. J . Phys. Chem. 1991, 95, 2068-2080. (33) Chen, P.; Deusing, R.; Tapolsky, G.; Meyer, T. J. J . Am. Chem. SOC. 1989,111, 8305-8306. (34) Moser, C. C.; Keske, J. M.; Warncke, K.; Farid, R. S.;Dutton, P. L. Nature 1992, 355, 796-802. (35) Fox, L. S.;Kozik, M.; Winkler, J. R.; Gray, H. B. Science 1990,247, 1069-1071. (36) McCleskey, T.M.; Winkler, J. R.; Gray, H. B. J . Am. Chem. SOC. 1992,114, 6935-6937. (37) Asahi, T.; Mataga, N. J. Phys. Chem. 1989, 93, 6575-6578. (38) Ohno, T.; Yoshimura, A,; Mataga, N. J. Phys. Chem. 1986, 90, 3295-3297. ~
bpm-acceptor complexes (O), and Ru(bpy)3*+ (A).
A plot of log ket vs Aceto is given in Figure 5 . It is clear that linear relationships exist for bpz-acceptor and bpm-acceptor complexes; the rate constants for R ~ ( b p y ) 3 ~and + the bpmacceptor complexes are higher than those for bpz-acceptor complexes but do not lie on the same line. This indicates that the properties of the unique reduced ligand play an important role in controlling the rate of energy transfer. Indeed, it has been observed beforelo that the nonradiative decay rate constant (knr) for bpm-acceptor complexes is higher than that for bpz-acceptor complexes and that they lie on distinctly different lines when correlated with the emission energy of the excited state; this behavior has been attributed to a higher vibrationally-induced electronic coupling term for bpm-acceptor complexes. Similarly, the electronic coupling term for the energy transfer could be greater for R ~ ( b p y ) 3 and ~ + for bpm-acceptor complexes than for bpz-acceptor complexes. The more interesting feature in Figure 5 is the fact that for the bpz-acceptor complexes the rate constant decreases as the reaction becomes more exoergonic, which clearly indicates the existenceof the inverted behavior for the energy-transfer reaction; an inverted region was observed for energy transfer when -Aceto > 0.5 eV.44 If the reorganization energy for those reactions44is similar to that associated with the reactions we have examined, it is reasonable for the energy-transfer quenching of * R ~ ( b p y ) , ~ + by 0 2 to be in the inverted region since the energy gap is about 1 eV. Although a linear plot is also observed for bpm-acceptor complexes,the correlation cannot be reliably evaluated inasmuch as the variation in the values of Em and ket are very small. Su"ary The excited states of the 10 Ru(II)-diimine complexes, which possess an excitation energy of about 2 eV, react with 0 2 via bimolecular energy-transfer reactions to generate 0 2 ( *Ag). Only Ru(bpy)32+in this group of complexes is believed to react with 02 via both electron- and energy-transfer pathways; no redox products can be detected, presumably due to fast back electron transfer between the geminate redox pair within the solvent cage. For all the complexes, the only product generated from the quenching reaction is 02('Ag). The quantum yield of generation + approximately unity for all of O2(lAg) is 0.5 for R ~ ( b p y ) 3 ~and the other complexes. The energy-transfer quenching rate constants for Ru(bpy)32+and bpm-acceptor complexes are distinctly higher than those of bpz-acceptor complexes at equal driving force, suggesting that the properties of the unique reduced ligand in the excited state are important in controllingthe energy-transfer process. Inverted behavior has beenobserved for the bpz-acceptor complexes, in which the rate constants decrease as the driving forces for the reaction increase. Because of their clean generation of singlet oxygen in high yields, these complexes may find utility as efficient visible light photosensitizers for singlet oxygen in aqueous solution.
.
1150 The Journal of Physical Chemistry, Vol. 98, No. 4, I994 (39) Ohno, T.; Yoshimura, A.; Shioyama, H.; Mataga, N. J. Phys. Chem. 1987, 91, 43654370. (40) Ohno, T.; Yoshimura, A.; Mataga, N.; Tazuke, S.; Kawanishi, Y.; Kitamura, N. J . Phys. Chem. 1989, 93, 3546-3551. (41) Ohno, T.; Yoshimura, A.; Mataga, N. J . Phys. Chem. 1990, 94, 4871-4876. (42) Zou, C.; Miers, J. B.; Ballew, R. M.; Dlott, D. D.; Schuster, G. B. J. Am. Chem. SOC.1991, 133, 7823-7825. (43) Yonemoto, E. H.; Riley, R. L.; Kim, Y. I.; Atherton, S. J.; Schmehl, R. H.; Mallouk, T. E. J. Am. Chem. SOC.1992, 114, 8081-8087. (44) Murtaza, 2.; Zipp, A. P.; Worl, L. A,; Graff, D.; Jones, W. E., Jr.; Bates, W. B.; Meyer, T. J. J. Am. Chem. SOC.1991, 113, 5113-5114. (45) Battino, R.; Rettich, T. R.; Tominaga, T. J . Phys. Chem. Ref. Data 1983, 12, 163-178. (46) Yoshimura, A,; Hoffman, M. 2.; Sun,H. J . Photochem. Photobiol. A: Chem. 1993, 70, 29-33. (47) Rodgers, M. A. J. J. Am. Chem. SOC.1983, 105,6201-6205. (48) Firey, P. A.; Ford, W. E.; Sounik, J. R.; Kenney, M. E.; Rodgers, M. A. J. J. Am. Chem. SOC.1988, 110, 7626-7630. (49) Akasheh, T. S.; Beaumont, P. C.; Parsons, B. J.; Phillips, G. 0. J. Phys. Chem. 1986, 90, 5651-5654. (50) The prompt componentof theNIR luminescencearisesfrom radiation in the spectral region 1100-1600 nm, Le., that to the red of the silicon filter cutoff and prior to the band edge of the Ge detector. As the steady-state spectrum shows (Figure 4), there is a significant contribution from the MLCT state luminescence in this region. The time profile of this signal corresponds to the decay time of this component convoluted with the time constant of the detector-amplifier combination (-600 ns). (51) The value of ( @ A ~ / @ A ~ for ) ~ Ru(bpy)z(bpm)2+, based on the use of k, 5 1 X lo9 M-I s-l$9 was 23.4, Le., much higher than that obtained from all the other complexes (Table 2). Using the mean value of ( @ A ’ / @ A ~ ) ~ , 9 = 0.04 i 0.003 was calculated for Ru(bpy)z(bpm)z+; by taking TO = 20 ns (Table l), a value of kq = 1.6 X lo9 M-1 s-1 was calculated for this complex.
Mulazzani et al. (52) Neshvad, G.; Hoffman, M. 2.;Mulazzani, Q.G.; Venturi, M.; Ciano, M.; D’Angelantonio, M. J . Phys. Chem. 1989, 93, 6080-6088. (53) Davila, J.; Harriman, A. Photochem. Photobiol. 1990, 51, 9-19. (54) Our use of @A for TPPS’ of 0.63 is supported by the fact that this reference value leads to @A = 1.1 i 0.1 for the nine other RuLz+complexes. If nine compounds are to exhibit a common quantum yield for singlet oxygen formation, the most likely common value is surely unity. ( 5 5 ) (a) Tanielian, C.; Esch, M. Proceedings of the 6th International Conference on Photochemical Conversionandstorage of Solar Energy, Paris, July21-25,1986; F-7.(b) Esch, M.; DoctoralThesis,UniversittLouisPasteur, Strasbourg, France, 1986. (56) We note here that, except for Ru(bpy)12+,the oxidation potentials for the complexes are available only in acetonitrile.” However, it has been demonstrateds’ that for Ru(bpy)12+and Ru(phen)12+the values of Elp(3+/ 2+) in aqueoussolution with respect toNHEdiffered from thevaluesobtained in acetonitrile with respect to SCE by 30 mV. On the basis of this, we have assumed that for all the complexes examined the values of E1/2(3+/2+) vs SCE could be used as the values of E0(3+/2+) in aqueous solution. (57) Lin, C.-T.; Bdttcher, W.; Chou, M.; Creutz, C.; Sutin, N. J. Am. Chem. SOC.1976, 98,6536-6544. (58) The decay of uncaged Ru(II1) and Oz’entities follow a second-order rate law and will extend over severaltens of microsecondsunder our conditions. (59) Thedirect conversion toground-statespecies is conceivable,but such a process suffers from severe Franck-Condon restrictions. In any case, there appears to be no reason why such a process would not compete with the energytransfer reaction for the other RuL2+ species. (60) Sassoon, R. E.; Aizenshtat, 2.;Rabani, J. J . Phys. Chem. 1985,89, 1182-1190. (61) Mulazzani, Q.G.; Ciano, M.; D’Angelantonio, M.; Venturi, M.; Rodgers, M. A. J. J. Am. Chem. SOC.1988, 110, 2451-2457. (62) Miller, S. S.;Zahir, K.; Haim, A. Inorg. Chem. 1985,24,3978-3980. (63) Scandola, F.; Balzani, V. J . Chem. Educ. 1983, 60, 816823.
