10587
J. Phys. Chem. 1992,96,10587-10590
Transient Absorptions Due to Mixed Valence Species in the Exclted-State Absorption Spectra of Cyano-Bridged Trinuclear Polypyridyl Complexes of Ru( I I ) K.Matsui, Md. K. Nazeeruddin, R. Humphry-Baker, M. Criitzel, and K. Kalyanasuadaram* Institut de Chimie Physique, Ecole Polytechnique FZdZrale de Lausanne, CH- 1015 Luusanne, Switzerland (Received: August 18,1992;In Final Form: October 30, 1992)
Singly oxidized forms of two cyanebridged trinuclear complexes [(CN)(bpy),R~~~N-Ru~~(dcbpy)~-NC-Ru~~(bpy)~(CN)] (1)and [(H20)(bpy)zRu1LNC-Ru11(dcbpy)z~N-Ru11(bpy)2(HzO)] (2)show intervalence transition bands in the infrared region typical of mixed valence systems. The 532-nm laser photolysis studies of the parent/nonoxidized complexes show transient absorptions in the near-infrared region (600-1200 nm) with lifetime comparable to that of the luminescence from the lowest-energy CT excited state. The near-IR absorption is attributed to the mixed valence species present during the lifetime of the CT excited state.
Latroduction Charge-transfer (CT) excited states of transition-metal polypyridyl complexes have been extensively studied, and their properties are fairly well characterized.'-3 The lowest excited state of R~(bpy)~(cN),, for example, is CT character. The formation of this excited state involves transfer of an electron from Ru(I1) to one of the bipyridine (bpy) ligands as shown in the equation [Ru11(b~~)2(CN)21 [Ru1'(bpy)2(CN)21 * =
[Ru"'(bPY)(bPY*-)(CN)zl* ( 1) There is ample spectroscopic evidence for thistype of formulation: (i) The transient absorption spectra of the excited state show absorption bands corresponding to the presence of bipyridine anion (the LMCT band indicating the presence of Ru(II1) is often too weak to be detected, c I500 M-I ~ m - 9 ~ 3(ii). Resonance Raman spectra of the excited state also show Raman bands characteristic of the bpy*-. (iii) The excited state acts as both an oxidant and a reductant, and the redox potential of the excited state for both these processes can be tuned by suitable substitution of electrondonating/withdrawing group on the bpy framework. These studies are being extended to polynuclear complexes.6 In our studies on the photophysical and redox properties of ligand-bridged polynuclear we have been concerned with implications of such descriptions. In polynuclear polypyridyl complexes, the presence of such an oxidized metal center should lead to transient formation of mixed valence compounds, as shown in eqs 2 and 3 (BL represents the bridging ligand that links the two chromophores):
-
[(X)(bPY)2M,-BL-M2(bPY)2(X)I
hu
[(XI (bPY) 2M ,-BL-Mz+(bpy) (bPY'-) (X)1* (2)
[(X)(bPY)2MI-BL-M,+(bPY)(bPY*-)(X)I*
IT
[@PYI'M 1+-BL-Mz(bpy 1(bpy*-)(X)I (3) Cyano-bridged polypyridyl complexes in particular are well suited for examination of this question due to the intensive molar absorptivity of the intervalence transitions (IT) of their mixed valence compounds.6J0J1Some positive indications on this question can be found in a recent study.I2 There have been a lot of interest in the photochemistry of these systems9J1J3and their applications as sensitizers in photovoltaic cells.14 The IT absorptions in these polynuclear polypyridyl complexes occur in the IR region, and unambiguous assignments require monitoring of transients at least in the near-IR region. Using a Si-based photodiode in a Nd laser flash photolysis setup, we have been able to probe the transient absorptions up to 1200 nm. Herein we present laser flash photolysis data that indicate clearly transient formation of such mixed valence species during the lifetime of the CT excited state. Results of chemical oxidation studies are first described for the acidic and basic forms of two representative cyano-bridged trinuclear com[ (CN)plexes (dcbpy = 4,4'-dicarboxy2,2'-bipyridine):
(bpy),R~-CN-Ru(dcbpy)~-NC-Ru(bpy)~(CN)] (1) and [(H,O) (bPY)2Ru-NC-Ru(dcbPY)Z-CN-Ru(bPY)2(H2o)l (2).
This is followed by transient absorption spectral studies of these complexes in the red near-IR region. Experimental Section
Materiala The two cyano-bridged trinuclear complexes were synthesized using the standard schemes described in the literature.+l' The complexes were purified by repeated reprecipitation from aqueous solutions at their isoelectric point (pH 2.8). The isolated neutral complexes can be dissolved in aqueous or nonaqueous solvents using few drop of sodium or tetrabutylammonium hydroxide (to ionize the carboxyl groups). Chemical oxidants used (Br2and ceric ammonium nitrate) were analytical grade chemicals from Fluka and were used as supplied. Metbods. Absorption spectra of the mixed valence compounds were recorded in the 200-2000-nm range on a Cary 5 spectrophotometer. Emission spectra were recorded on a Spex Fluorolog spectrofluorimeter equipped with photon counting detection and, a Hamamatsu R2658 photomultiplier whose sensitivity extends to the near-IR region. Emission maxima reported have been corrected for variations in the instrument response. Laser photolysis studies were made on a fast kinetic spectrophotometricunit consisting of a Qswitched Nd laser (delivering -15 ns, 532 nm, -5 mJ pulses) for excitation, an ELG photodiode (SHS-100) for detection, and a Tektronix 76 12 transient digitizer coupled to a HP 3000 series computer for data acquisition and analysis. Solutions were freshly prepared and were purged with ultrapure Ar for at least 15 min prior to measurements. The acidic and basic forms of two cyano-bridged complexes of Ru(I1) are examined in this work. The central Ru is linked to the peripheral Ru centers via isonitrile (N-bonded cyanide) in complex 1and via nitrile (C-bonded cyanide) in complex 2. Thus, complexes 1 and 2 can be considered as linlrage isomem. By having the dcbpy ligand in the protonated (carboxylic acid, dcbpyH2) or deprotonated (carboxylate, dcbpy2-) forms, it is possible to vary the overall charge of these complexes. The complexes are thus cationic in acidic solutions and anionic in neutral/basic solutions. The complexes were dissolved in acidic or basic acetonitrile and are identified in these media as lA,lB,2A,and 2B. The complexes in the CT excited state will be referred to as 1A*,1B*,
-
1A: [(CN) (bpy)2Ru1LCNRU"(~C~P~H~),-NC-RU"(~~~)~(CN)] 2+ 1B: [(CN)(bpy)2Ru'LCN-
Ru"(~c~~~)~-NC-RU"(%~~)~(CN)]~2A: [(H~O)(~PY)~RU'LCNR~"(dcbpyHz)rNC-Ru"(bpy)2(H20)]~+ 2B: [(H0)(bpy)2Ru1LCNR u " ( ~ c ~ P ~ ) ~ - N C -bpy)2( R U ~ OH)] ~(
0022-3654/92/2096-10587$03.00/00 1992 American Chemical Society
'-
105%8 The Journal of Physical Chemistry, Vol. 96, No.26, 1992 TABLE I: Ca38)uiroa of t k Pmpwtke of CT Akorptioll d E"with Tbore of Intm.lclceT d t i o n s (IT) Observed in the Excited sbite of Cyum-Bridgcd Trlouckar Complexes of Ru(II) in Acetonitrile at 293 K DroDcrtv 1B* lA* 3." 2B* 2A* 498 486 536 -520 X,(CT a b ) , nm 520
.. -
723 (720)' 122 980 109
Xm,(Em), nm dEmh ns ALx(if)nm T(IT),ns
793 30 1050 31
"Values shown are of Scandola et al.
735
50
749 15 940 I20
737 14 1100 I20
(refs 9 and 10).
etc. The singly oxidized form will be referred to simply as 1A+, lB+,etc.,or with the indication of the oxidation states of the metal centers [2,3,2]lA, [3,2,2]2A, etc.
Radb and Discussion 1. AbsorptioamdEm&dnnpropertiesoftkcompkxesinthe [2,2,2]Form. The absorption spectra of the cyano-bridged complexes consist of two bands in the visible region. Based on the electrochemical data (cf. section 2) and a comparkn of the spectra of model compounds, the lowest energy absorption band (located in the 500-540-nm region) is assigned to the MLCT transition Ru dcbpy of the central chromophoric unit in complexes 1A and 1B and to the Ru bpy of the peripheral units in complexes 2A and 2B
-
-
[2,2,211-
hu
[2,2*,211; [2,2,212 -!L [2*,2,212
(4)
Letters TABLE II: Properb of Iaterv8lence T r a " (IT) obrmred in the Mixed Valence Forms of the CymBridged Trlouckar Complexes of Ru(II) in Acetdtrile at 293 K property 1B+ lAt 3+4 2Bt 2A+ A,,
1280 0.781 0.348 4600
nm
vmpX, pm-'
Au. pm-' emax,
M-'
cm-' HAB,pm-' Eox(l), EoX(2),V AE1p V a2 b
1280 0.780 0.307 4660
1124 0.890 0.35 -9OOO
895 1.117 0.328 1900
1105 0.905 0.314 2830
0.145 0.137 -0.2 0.108 0.116 0.65, 1.20 0.68, 1.20 0.66, 1.19 -0.3, 1.0 0.69, 0.97 0.55 0.035
0.52 0.031
0.53 0.07
0.7 0.01
0.28 0.017
"Values are those of Scandola et al. (refs 9 and 10). bCalculated using a metal-metal distance (r) = 5 A.
cyanides.'IJ2 On this basis, the first oxidation in complexes 1A and 1B (observed at a0.65 V) is assigned to the central Ru, and this is followed by oxidation of one of the peripheral Ru centers at 1.20 V:
[2,2,2]1B
- 0.65 V
[2,3,2i IB
1.20 v
[3,3,2]1B
(6)
It can be noted that deprotonation of the carboxybipyridine ligands in complexes 1A and 1B causes only minor changes (130 mV) in the first and second oxidation potentials. In complexes 2A and 2B,the peripheral Ru centers carry the N-bonded cyanides. The first oxidation in 2A occurs at =0.7 V, but it occurs at less positive potentials of 0.3 V in 2B. Due to the ease of oxidation of the Ru centers that carry N-bonded cyanides, the first oxidation in complexes 2A and 2B is assigned to oxidation of the these terminal Ru centers:
The solvent dependence of the two absorption bands is consistent with such an assignment. In complexes 1A and lB,change of solvent from acetonitrile to water causes a blue shift of the higher energy absorption band (498 430 nm), but the shift is much d e r for the lowest energy band (522 520 nm). The MLCT transition of R u ( b p ~ ) ~ ( c N is )known1 ~ to be more strongly de0.70 V pendent as compared to that of Ru(bpy),2+, and the enhanced [2,2,2]2A [3,&2]2A (7) sensitivity of the former has been attributed to the interactions 0.30 V of the lone pair of electrons on the nitrogen of monodentate/ (8) [2,&2]2B [3,2,2]2B terminal cyanide ligand with solvent. The enhanced solvent The large difference in the oxidation potential of the dicationic sensitivity caused by terminal cyanide can be used to assign the and dianionic forms in complex 2 is due to the effect of deproMLCT transitions in the trinuclear complexes. Only the termha1 tonation of the aquo ligands of the peripheral Ru centers. The units of species 1 have such monodentate cyanides. effect of deprotonation of carboxyl and aquo ligands observed here All four complexes show weak emission in deaerated solution is similar to what has bten observed" in mononuclear complexes at room temperature. Data on measured absorption, emission Ru(dcbpyHz)s2+and R~(bpy)~(H20)2~+. maxima, and d o n lifetimes are listed in Table I. The emission Due to their low first oxidation potential, the complexes can spectra with a maxima in the 720-790-nm region are typical of be readily oxidized in solution using chemical oxidants such as polypyridyl complexes of Ru(I1). Only in complex 1B is the Br, or Ce4+. Figure 1 shows the absorption spectra in acetonitrile emission fairly long-lived ( T 122 ns in acetonitrile at 293 K). of the singly oxidized complexes 1A+ and 1B+ (top panel) and The spectral shifts in the absorption and emission maxima of the of complexes 2A+ and 2B+ (bottom panel). Isosbestic points are lowest-energy CT band upon protonation of the four complexes obtained at 620 nm (for 1) and 4 4 0 nm (for 2)up to the addition are consistent with the assignments shown in eq 4. Going from of 1 equiv of the oxidant. As indicated above, the singly oxidized 1B to lA,the lowest-energy abrption and emission bands show compounds are of the mixed valence form [2,3,2] for lA+,1B+ a large red shift. In these complexes, the CT transition is assoand [3,2,2] for 2A+,2B+. The absorption band of the mixed ciated with Ru dcbpy of the central chromophore, and the valence compound in the IR region is assigned to intervalence (IT) protonation involves this ligand directly involved in the CT transitions involving the central and peripheral Ru centers. S i transition. The behavior is similar to that observed in complexes assignment has been made earlierg-" on the analogous complex such a9 [R~(bpy)~(dcbpy)].l~ Protonation of the carboxyl group lowers the w* level of the dcbpy ligand and a decrease in the energy [(CN)( ~ P)~RU-CN-RU(~PY Y )2-NC-Ru ( ~ P Y2(CN) ) 1 (3). All the singly oxidized complexes show another broad maximum of the MLCT transition. A small blue shift is o b s e ~ e dupon in the 420-480-nm region. This is a composite band containing protonation of complex 2B. The protonation effect is indirect for the Ru(I1) L CT transition of the nonoxidized chromophores the excited state involves Ru bpy excitation of the peripheral Ru(1II) CT transition of (L = bpy in 1, dcbpy in 2) and L' unit(s). The behavior is similar to that observed in complexes the oxidized unit (L' = dcbpy in 1, bpy in 2). The latter (LMCT Protonation of the hysuch as [R~(bpy),(4,7-(OH)~-phen)].~~ transition) is very weak (c I 500)4*sin mononuclear complexes, droxyl group decreases the effective charge density of the Ru and hence it is likely that the absorption band in the 420-480-nm center involved in the MLCT. A lowering the Ru(tD) level causes region is mainly due to MLCT of the nonoxidized chromophores. an increase in the energy of the MLCT transition. Z ~ V ~ ~ O b b i n o d d . a I e 5 k a lIn ~all four cases examined, the IT band is fairly intense,as has been observed earlier with analogous cyano-bridged Ru comCyclic voltammetric studies in acetonitrile show three distinct plexes.1°J2 The IT band in all cases can be fitted to a Gaussian oxidation waves, and the measured redox potentials are collected with a bandwidth in the range 3000-3500 cm-'. Data on the in Table 11. Earlier studies on cyano-bridged polypyridyl comspectral analysis of the IT band (fitted Gaussian maximum, plexes have been shown that the Ru centers carrying N-bonded bandwidth, and molar extinction coefficient) are listed in Table cyanides are more readily oxidized than those with C-bonded
- -
-
-
-
-
-
The Journal of Physical Chemistry, Vol. 96, No. 26, 1992 10589
Letters
40
30 1110
20 /
/o 10 0.0 400
800
800
1000 1200 Wavelength (nm)
1400
1000
1800
05
-
oo
v
400
800
800
1000
1200
1400
1600
1800
Wwelength (nm)
Figure 1. (top) Absorption spectra of the mixed valence form [2,3,2] of the trinuclear complexes 1A (dashed line) and 1B (solid line) obtained by chemical oxidation in acetonitrile. (bottom) Absorption spectra of the mixed valence form [3,2,2] of the trinuclear complexes 2A (dashed line) and 2B (solid line) obtained by chemical oxidation in acetonitrile. Spikes seen at X 2 1350 nm are artifacts from problems of base line/solvent correction.
11. For comparison, Table I1 also includes literature data on the IT of the trinuclear complex (3). As per the Hush modelY1* for mixed valence s y s t e m with similar inner- and outer-sphere reorganization energies, the energy of the IT (Eop)should increase with increasing M l l z(difference in the oxidation potential of the two metal centers). As per the data presented in Table I, M l l z is -0.55 V for complexes 1A+ and 1B+. The mixed valence compounds derived from these show IT "aat 1280 nm, in agreement with the magnitude of the AEllz values. MIlz is 4 . 2 8 V for complex 2A+ and 0.7 V for complex 2B+. The properti= of the IT band (energy, bandwidth, and molar absorptivity) can be used to calculate, according to the Hush model,I4 the electronic interaction matrix element HA, and the degree of delocalization a2between the adjacent metal centers of the mixed valence complexes. The calculated values (HAB = 0.15 pm-l and a*= 0.01-0.035) indicate that these compounds are of the class I1 mixed valence type. Notable differences do exist in the mixed valence forms derived from complexes 1 and 2. The direction of the IT is from terminal unit@) to the central unit in 1A+ and 1B+ but in the opposite direction in complexes 2A+ and 2B+. The molar extinction coefficients are in the range 4600 M-' cm-l for 1A+ and 1B+, but they are r e d u d to e2000 M-' cm-l in complexes 2A+ and 2B+. Most of this difference possibly comes from the fact that, in complexes 1A+ and 1B+, there are two donor units (terminal chromophores) involved in the IT while it is only one donor chromophore interacting with an acceptor unit. The difference in the ,Y of the mixed valence species 1A+ and 1B+ is negligibly small but is very significant between 2A+ and 2B+. This again values. The degree reflects the magnitude of the relative MIlz of electron delocalization as given by the azvalues is larger in 1+ than 2+. 3. Transient Absorption Spectra of the C T Excited State of the Parent Trinuclear Complexes. Transient formation of mixed
800
700
1000
900
1100
Wivelength(nm)
Figure 2. (top) Transient absorption spectra of the trinuclear complexes 1A (open circles) and 1B (filled circles) measured following 532-nm excitation in acetonitrile ( t = 0). (bottom) Transient abgorption spectra of the trinuclear complexes 2A (open circles) and 2B (filled circles) measured following 532-nm excitation in acetonitrile ( f = 0).
--
SCHEME I: Representation of Excited-State Electron Transfer (2,2',23 [2,3(-), 21' [2',2,2) 3 I?(-), 22)'
1 1J-J
mixed valence
Lxat
mixed valence
,i
5550nm LXdl
5 550 nm
[2,2,23
*-( [2,3,2]+0)
lA',
18'
[2,2,2]
i
+--( [3,2,2] + Q )
lA',
18.
valence species was examined following 532-nm laser pulse excitation of the complexes in the [2,2,2] form in degassed acetonitrile at ambient temperature. It may be recalled that at this wavelength there is selective excitation of the central Ru chromophore in complex 1 and peripheral units in 2. Using a EG&G photodiode (Si-based), we could measure the transient absorption changes up to 1150 nm. Figure 2 shows the transient absorption spectrum in the 550-1 150-nm range obtained in this manner. The transient absorptions in the infrared region decay exponentially with a lifetime identical to the emission decay of the luminescent Ru chromophore (e.g., =30 ns for complex 1A*). The absorption in the red near-IR region (2600 nm) is assigned to intervalence transitions (IT) of transient mixed valence species as shown in Scheme I. The assignment of the transient absorption to an IT is based on the following grounds: (i) It is well-known that excitation of polypyridyl complexes of Ru(I1) to the CT excited state in effect creates an oxidized metal center (Ru"') and a reduced ligand (LV-).Thus, the trinuclear complexes with the central Ru in the CT excited state can be visualized as transient mixed valence species as shown in the equation
10590 The Journal of Physical Chemistry, Vol. 96, No. 26, 1992
[RU'LRIJ"(L)~-RU"]
hv
[Ru'~Ru"(L)~*-Ru"]= [Ru"-Ru"'(L) (L'-)-Ru"]
(9)
The mixed valence species can be detected via their IT absorption only during the lifetime of the CT excited state (lifetime of Ru"'). Matching decay kinetia of the transient absorption of the IR band (IT) with the embion decay of CT excited state is consistent with such a description. (ii) No such absorption in the near-IR region can be detected in the transient spectra of the parent mononuclear complexes Ru(bpy),(CN), or Ru(dcbpy),(CN),. Observance of intervalence (IT) transitions requires along with Ru(II1) additional metal centers linked via the cyanide ligand. An analogous situation is observed during chemical oxidation. (iii) Experiments in the presence of quenchers allow differentiation of the transient absorption due to products from that are purely T-T absorption of the excited state. In the presence of suitable electron acceptors, the transient absorption decay in the IR is biphasica kinetic behavior quite different from that of the emission (cf. Scheme I). Exponential decay of the emission decay is accelerated and lifetime ( 7 ) reduced to nanoseconds. The fast component of the IT band is similar to emission decay and the slow component decays over several microseconds. The interpretation of the IT decay behavior is as follows. In the presence of quencher Q, the transient absorption of 2A* (for example) is converted to that of 2A+ very rapidly. This is followed by back-reaction between 2A+ and Q-. An analogous situation is observed in homogeneous photoredox processes. The oxidized sensitizer taking part in back electron transfer in this case is a mixed valence compound. In principle, one would expect the absorption maximum of the IT band of the transient mixed valence species to be blue-shifted with respect to the absorption of the chemically oxidized species. This is because the IT process involving transfer of an electron from Ru" to Ru"I(dcbpy(-)) (or Ru"'(bpy-)) in the transient excited state is more difficult than that involving Ru" and Ru"'dcbpy (or RuIIIbpy) in the chemically oxidized species. The blue-shifted nature of the IT in the excited state can also be considerations. The oxidation potential of predicted from A,?%,,, the CT excited state is less than that of the ground-state species. Comparison of the IT spectra presented in Figures 1 and 2 shows that this prediction is clearly borne out in species 1. The situation is not at all clear in 2. Due to the broad nature of the IT bands, we would consider the IT maxima to be, at best, very similar to the transient excited state and chemically oxidized forms derived from species 2. There are differences in the mixed valence species derived from 1 and 2 (directionality of IT and the number of donor units involved). The data are too limited, and any interpretations invoking consequences of the directionality of IT can only be speculative at this time. Currently we are in the process of -e several other related complexes to identify some trends before advancing any explanations.
Letters Detection of the transient absorptions due to photoinduced mixed valence species in the IR region provides a useful additional tool in the studies of photoredox reactions of polynuclear complexes. A possible, important application of polynuclear complexes of the present type is as light-harvesting devices. Monitoring of the IT absorption can provide data on the kinetics and efficiencies of excited-stateintramolecular electron/energy-transferprocesses. Detailed studies on the application of the near-IR absorption to study mechanism of sensitization of Ti02 electrodes are in progress, and results will be reported at a later date.
References rad Notes (1) Kalyanasundaram, K. Photochemistry of Polypyridine and Porphyrin Complexes; Academic Press: London, 1992. (2) (a) Meyer, T. J. Pure Appl. Chem. 1986, 58, 1193. (b) DeArmond, K. A. Acc. Chem. Res. 1989, 22, 364. (3) Juris, A.; Barigelletti, F.;Balzani. V.; Campagna, S.; Belser, P.; von Zelewsky, A. Coord. Chem. Reo. 1988, 84, 85. (4) Lever, A. B. P. Inorganic Electronic Spectroscopy, 2nd ed.; Elsevier: Amsterdam, 1984; Sections 5.10 and 5.12. ( 5 ) (a) Liu, D. K.; Brunschwig, B. S.; Creutz, C.; Sutin, N. J . Am. Chem. Soc. 1986,108,1749. (b) 0hno.T.; Nozaki, K.; Haga, M. Inorg. Chem.1992, 31, 548. (6) (a) Scandola, F.; Indelli, M. T.; Chiorboli, C.; Bignozzi, C. A. Top. Curr. Chem. 1991, 73, 158. (b) Scandola, F.; Balzani, V. Supramoleculur Photochemistry; Harwood: London, 1991. (7) (a) Kalyanasundaram, K.; Nazeeruddin, Md. K. Inorg. Chem. 1990, 29, 1888. (b) Kalyanasundaram, K.; Nazeeruddin, Md. K. J . Chem. Soc., Dalron Trans. 1990, 1657. (c) Kalyanasundaram, K.; Gritzel, M.; Nazeeruddin, Md. K. J . Chem. Soc., Dalton Trans. 1991, 343. (d) Kalyanasundaram. K.: Nazeeruddin. Md. K. Chem. Phvs. Lett. 1989.158.45. (8) 'Kalyanasundaram, K.; Grltzel, M.; Nazeeruddin, 'Md. 'K.J . Phys. Chem. 1992, 96, 5865. (9) Kalyanasundaram, K.; Grltzel, M.; Nazeeruddin, Md. K. Inorg. Chem., in press. (10) (a) Bignozzi, C. A.; Paradisi, C.; Roflia, S.;Scandola, F. Inorg. Chem. 1988,27,408. (b) Bignozzi, C. A.; Roffia, S.;Scandola, F. J . Am. Chem. SOC.1985,107, 1644. (1 1) Bignozzi, C. A.; Roffia, S.;Chiorboli, C.; Davila, J.; Indelli, M. T.; Scandola, F. Inorg. Chem. 1989. 28, 4350. (12) Bignozzi, C. A.; Argazzi, R.; Chiorboli, C.; Roffia, S.; Scandola, F. Coord. Chem. Rev. 1991, 1 1 1 , 261. (13) (a) Bignozzi, C. A.; Indelli, M. T.; Scandola, F. J . Am. Chem. Soc. 1989,111, 5192. (b) Lei, Y.;Buranda, T.; Endicott, J. F. J . Am. Chem.Soc. 1990, 112, 8820. (14) (a) Nazeeruddin, Md. K.; Liska, P.;Moser, J.; Vlachopoulos, N.; Grltzel, M. Helu. Chim. Acta 1990,73, 1788. (b) ORegan, B.; Gritzel, M. Nature 1991, 353, 737. (c) Amadelli, R.; Argazzi, R.; Bignozzi, C. A.; Scandola, F. J. Am. Chem. Soc. 1990, 112, 7099. (15) (a) Giordano, P. J.; Bock, C. R.; Wrighton, M. S.;Interrante, L. V.; Williams, R. F. X . J . Am. Chem. Soc. 1977,99,3187. (b) Nazeeruddin, Md. K.; Kalyanasundaram, K. Inorg. Chem. 1989, 28, 4251. (16) Giordano, P. J.; Bock, C. R.; Wrighton, M. S.J . Am. Chem. Soc. 1978, 100, 6960. (17) (a) Takeuchi, K.; Samuels, G. J.; Gertsen, S. W.; Gilbert, J. A.; Meyer, T. J. Inorg. Chem. 1983,22, 1407 and references therein. (b) Liska, P.; Vlachopoulos, N.; Nazeeruddin, Md. K.; Comte, P.; Griteel, M. J. Am. Chem. SOC.1988, 110, 3686. (18) (a) Hush, N. S. Prog. Inorg. Chem. 1%7,8,391; Electrochim. Acta 1968, 13, 1005. (b) Creutz, C. Prog. Inorg. Chem. 1983, 30, 1.
