Creutz, Sutin, et al.
/
Excited States of Polypyridine Complexes
Sci. 1967, 37, 7431 and Fourier program FFSYNT [ J . Appl. Crystallogr. 1973, 6. 2461. Johnson's WTEPthermal ellipsoid plotting program was used for drawing. (18) Muetterties, E. L.; Guggenberger, L. J. J. Am. Chem. SOC. 1974, 96, 1748. (19) Raymond, K. N.; Corfield, P. W. R.; Ibers, J. A. Inorg. Chem. 1968, 7, 1362. (20) Allen, G. C.; Hush, N. S. Inorg. Chem. 1967, 6, 4. (21) Ray, N.;Hulett, L.; Sheanan, R.; Hathaway, B. Inorg. Nucl. Chem. Lett. 1978, 14, 305. (22) Figgis, 8. N.;Lewis, J. Adv. Inorg. Chem. 1964, 6
I309
(23) Boudreaux, E. A. Trans. faraday SOC.1963, 59, 1055. (24) Ferraro, J. R. "Low-Frequency Vibrations of Inorganic and Coordination Compounds": Plenum Press: New York, 1971. (25) Nakamoto, K. "Infrared and Raman Spectra of Inorganic and Coordination Compounds"; Wiley: New York. 1978. (26) Bellamy, L. J. "The Infra-Red Spectra of Complex Molecules"; Chapman and Hall: London, 1975; pp 267-291. (27) Colthrum, N. B.; Daiy, L. H.; Wiberley, S. E. "Introduction to Infrared and Raman Spectroscopy"; Academic Press: New York, 1975; p 282. (28) Vallarino, L. M.; Goedken, V. L.; Ouagliano, J. V. Inorg. Chem. 1973, 72, 102.
Lifetimes, Spectra, and Quenching of the Excited States of Polypyridine Complexes of Iron( 11), Ruthenium( II), and Osmium( 11) Carol Creutz,* Mei Chou, Thomas L. Netzel, Mitchio Okumura,' and Norman Sutin* Contribution from the Chemistry Department, Brookhacen National Laboratory, Upton, New York 1 1973. Receiced April 23, I 9 7 9
Abstract: The lifetimes of the excited states formed by 530-nm excitation of polypyridine complexes of iron(I1) (FeL3*+) and osmium(1l) ( O S L ~ ~have + ) been determined by laser flash-photolysis techniques. The FeL32+ lifetimes, measured in water at room temperature using picosecond absorption spectrometrj, are as follows (L, 7 f u (ns)): 2,2',2"-terpyridine (2.54 f 0.1 3): 2,2'-bipyridine (0.81 f 0.07); 4,4'-dimethy1(2,2'-bipyridine) (0.76 f 0.04); I ,IO-phenanthroline (0.80 f 0.07); 4,7-(diphenyl sulfonate)-I ,IO-phenanthroline (0.43 f 0.03). Lifetimes for the analogous'complexes of osmium(l1) lie in the range IO- 100 430-460 nm. t -5 X I O 3 M-' ns under the same conditions. Unlike the excited states of Ru(bpy)j2+ and Os(bpy)3*+ (A,, cm-I), the excited state of Fe(bpy)32+ does not luminesce or absorb significantly in the visible ( t < I O 3 M-' cm-I at X 2350 nm) but does exhibit intense absorption below -325 nm. Rate constants for the quenching of the excited states of polypyridine complexes of osmium(l1) and ruthenium(l1) by ground-state polypyridine complexes of iron(lI), ruthenium(]]), and osmiuin(l1) are reported and are ascribed to either electron-transfer or energy-transfer processes. The excited states of tris(2,2'bipyridine)iron(ll) and of bis(2,2',2"-terpyridine)iron(ll) undergo reaction with Feaq3+(0.5 M H ~ S O J25 , "C) with a rate constant < 1 X IO' M-' SKI.Based on a comparison of its properties with those of the luminescent charge-transfer excited states of ruthenium(l1) and osmium(1l) polypqridine complexes the excited state of FeL3*+ is identified as a ligand-field state. The potential of the excited-state couple F e ( b ~ q ) 3 ~t+e *Fe(bpy)32+ is estimated to be t0.1 V.
Introduction The photophysics and photochemistry of polypyridine complexes of ruthenium, 'especially tris(2,2'-bipyridine)ruthenium(I1) (Ru(bpy)i2+), have been extensively investigated in recent years.2 Both kinds of study have implicated a major role for the luminescent metal-to-ligand charge-transfer excited state of complexes of this family. Similar conclusions have been drawn for the luminescent osmium( 11) polypyridine complexes, although fewer data are available for this By contrast, the analogous complexes of iron( 11) have received relatively little attention. Fink and Ohnesorges have found that FeL,2+ complexes (L = bpy, 1 , I 0-phenanthroline = phen, 2,2',2"-terpyridine = terpy, 2-methyl- I , 10-phenanthroline) d o not luminesce in absolute ethanol a t either room temperausing picosecond absorption specture or 8 0 K . Kirk et trometry, determined a 0.83-ns lifetime for a noneniitting excited state of F e ( b ~ y ) 3 ~ while +, Street et a1.I" have made analogous measurements on Fe(phen)3'+. Chum et al.' have obtained evidence for photoinduced oxidation of iron( 11) chelates, including Fe(bpy)32+, in aluminum chloride-ethylpyridinium bromide melts, while Phillips et a1.I2 have reported that F e ( b ~ y ) 3 ~undergoes + photooxidation in the presence of Feaq3+. I n this work we have used laser flash-photolysis techniques to measure the lifetimes and spectra of the excited states of polypyridine complexes of ruthenium(l1). osmium( I I ) , and iron( 11). In addition we have studied the quenching of the ex0002-7863/80/ 1502-I309SOl .00/0
cited states of the osmium(1l) and ruthenium(I1) complexes by ground-state polypyridine complexes of iron( I I ) , ruthenium(II), and osmium(I1) and the quenching of Fe(bpy)32+ and Fe(terpy)z2+ by Feaq3+ions. T h e mechanisms of these reactions and the properties of the excited states of the iron(l1) complexes are discussed below.
Experimental Section Materials. The polypyridine ligands were used as received from the G. F. Smith Co. or from Fisher. The method of preparation of the
osmium polypyridine complexes followed that given by Dwyer and ~ o - n o r k e r s . The ' ~ procedure for Os(bpq)312 is typical: 1.0 g of K2OsC16 and 1.28 g of 2,2'-bipqridine were mixed with 50 mL of gllcerol and heated at 240 O C for I h . The hot glycerol mixture was poured into 150 mL of hot water and then acidified with -10 mL of 5 M HCI. The green solution uas filtered, and solid potassium iodide was added to the filtrate. The resulting black powder was separated by filtration and washed with cold water and ether. The perchlorate salt was prepared from the iodide as follows. A 10% solution of sodium perchlorate was added drophise to a solution of the iodide salt in hot water. The wall of the container uas scratched with a glass rod a n d the solution was cooled in an ice bath. The solid perchlorate was collected on a filter, washed with ether, then dried in vacuo overnight. The purity of the products was ascertained bq analysis for Os and halogen. When it proved difficult to separate OsL3*+ from unreacted ligand L (L. a phenanthroline derivative). the impure OsL3X2 v,as digested with benzene in order to dissolve the excess ligand. Stock (0.01 M ) solutions of the iron(l1) pollpyridine complexes were prepared by mixing I .0 nimol of Fe(NH1)2(SOJ)2,3.1 mmol of
8 1980 American Chemical Society
Journal of the Anierican Cheniical Society
1310
/
102:4
/ February
1 3 , 1980
E L VIDICON 'CONTROLLER
W
I
I
C
R
O PROCESSOR
I Y
100yoR'
TO DYE PUMP
Figure 1. Picosecond absorption spectrometer: R. reflector; GLASS OSC, Nd/glass laser oscillator; BS, beam splitter; PD, photodiode; PI and P2, crossed polarizers; PD TO OSC, fast photodiode signal to a SI9 oscilloscope; AMP, hd/glass amplifier: SHG, second harmonic generating crystal; CC14, 5-cm path length cell of carbon tetrachloride; ADJ DELAY, translation stage with 1 m of travel: I and l o . paths of the white light pulses through the sample and reference cells. respectively; SIT VID, silicon intensified-target vidicon; '/4 m SPEC, '14 m spectrograph; 3/4m MC, m monochromator, The dashed lines indicate the beam paths when the vidicon detection system is used. A I-mm aperture and ground-glass diffuser plate are used after the CC14 cell to homogenize the white light pulses. ligand (for FeL32+ complexes), and a spatula tipful of ascorbic acid and diluting to 100 mL. The solutions were characterized spectrallyi4 and stored in the dark under argon, then diluted to the desired concentration with water before use. Lifetimes of the O S L ~ Compounds. ~+ Solutions 10-5-10-4 M in O S L ~ (€530 ~ + typically 5 X IO3 M-' cm-I) were excited using a 5- or 8-ns (full width at half-height) pulse of 530-nm light from a frequency-doubled neodymium laser. The basic laser, frequency dou(The bling, and detection systems have been described previ~usly.'~-'~ Q-switched 22-ns pulse was, however, sliced to give a shorter pulse: the diameter of the oscillator pulse was reduced to 0.5 cm by the installation of a glass diaphragm within the oscillator cavity and a Korad Model KQS4 optical gate was inserted between the oscillator and the amplifier head. The spark gap uhich opened the gate was routinely operated at 20 k V and 60 psi of nitrogen. Approximately 100 mJ 530-nm pulses (after doubling) could be obtained under optimal conditions. Emission associated with the deca) of the OsL3'+ excited states was monitored at -71 5 nm using a Hamamatsu R928 photomultiplier. Measurement of the Ru(bpy)32+,Osibpy)3z+, and Fe(bpy)3*+ Excited-State Spectra. Deaerated solutions of Ru(bpy)j2+, Os(bpy)3*+, or Fe(bpy)12+contained in a 1 X I cm "microcell" (interrogation path length 0.37 cm)Ihhere excited with a pulseof 530-nm light.'5,'hFor Os(bpy)j2+ the absorbance changes observed immediatelq following the 8-11sexcitation pulse were recorded as a function of uavelength at high excitation intensity ( 2 2 0 einsteins cm-? s - ' ? see below) at 10or 15-nm intervals. For Ru(bpy)12+ the absorbance changes following a 22-ns excitation pulse "ere similarly determined. The spectrum of *Fe(bpy)32+was measured in an analogous fashion using the same apparatus as above. but in this work the absorbance changes taking place during a 22-11s excitation pulse were measured. I n calculating the absolute spectrum of *M(bpy)3>+,the directly determined absorbance changes ( A A , defined as postflash absorbance-or, i n the case of * F e ( b ~ y ) 3 ~maximum +, absorbance attained during flashminus preflash absorbance Ao) were corrected for the ground-state bleaching (c*Ao/co, where e* is the excited-state concentration and co is the preflash ground-state concentration) to give the excited-state absorbance A* according to the equation
The value of Ao uas determined using the flash-photolqsis monitoring optics (i.e.. same wavelength, slit width. sample cell. etc., as for A A ) . The evaluation of c*/co is described in the Results section. Typical excitation intensities in this experiment were 2 to 5 X 10' einsteins cni-2 s-1. Measurement of 530-nm Excitation Intensities. The relative laser pulse intensity was monitored routinely by diverting a small fraction of thc 530-nm beam into a photodiode ( E G & G ) uhose o u t p u t was displayed on a storage oscilloscope or monitored with a Laser Precision Corp. Encrgy Rationieter (Rk 3232) with digital display. Two approaches were used in estimating the absolute excitation intensities. I n the first. the pulse was collected in a Control Data thermopile (TRG 117) and the thermopile output w a s measured with a Keithley 150 B 'Clicrovoltnieter (41 p V J-l. 4.4 X einstein J-I at 530 nm). The area of thc focused laser beam was determined by burning a
photograph positioned at the front of the photolysis sample cell. In the second method eq 2 was applied: c * / c g = 1 - exp(-2.3 X 103c$*loAt) (2) (See Appendix for the derivation of eq 2.) Here c* is the concentration of excited state after the laser pulse, co is the ground-state concentration before the pulse, e is the molar absorptivity of the ground state at the excitation wavelength, $* is the quantum yield for production of the excited state, and the product loAt is the number of moles of photons per square centimeter per pulse. The quantity c*/q was evaluated for a solution I.O X M in Ru(bpy)3*+ and 5.2 X M in Feaq3+in I N H$S043,'5 as a function of laser intensity by monitoring the magnitude of the absorbance decrease at 490 nm.I6 These data were plotted as In (I - c*/co) vs. relative excitation intensity to give linear plots: the slopes of these plots provided an absolute calibration of the monitoring photodiode. The laser pulse shape and duration were determined by deflecting part of the doubled light into a photodiode (Korad Laser systems Model K D I ) whose output was displayed on a Tektronix 7834 storage oscilloscope. Quenching Rate Constants. Rate constants (kq) for quenching of * R u L ~ and ~ + *OsL3*+ were determined for solutions 2 to 5 X 10-5 M in R U L ~ or ~ OsL3?+ + and 0 to 5 X M in quencher. The lifetimes of *ML3*+ in the absence ( T O ) and presence ( 7 ) of quencher were determined as a function of quencher concentration [Q]. Rate constants were evaluated from the slopes of plots of ( T O / T ) - I vs. [Q]; slope = k,ro. The range of media used was dictated by the solubilities of the polqpyridinc complexes. Lifetimes of the FeLj2+ Complexes. Lifetimes for ground-state repopulation of the iron(l I ) polypyridine complexes were measured using picosecond absorption s p e c t r o s c ~ p y . Figure ~ ~ . ~ ~1 shows a schematic diagram of the experimental apparatus. A Nd-glass laser (I-ppm repetition rate) uas mode locked by flowing a dichloroethane solution of Kodak 9860 d i e through a cell in optical contact with the rear mirror of the cavity. A single pulse was selected with a Pockels cell energized by a Lasermetrics high-voltage pulse generator. The rejected pulses Mere directed to a high-speed photodiode coupled to a Tektronix 519 oscilloscope. Every laser shot was monitored and data nere accepted only i f the laser output consisted of a single train. The selected pulse of 1054-nm light was amplified to about 60 mJ of energy. Approximatelq 10% of this energy was converted to the second harmonic in a KDP (potassium hydrogen phosphate) crystal. The remaining 1 054-nm light was focused into a 5-cm cell of either DzO or carbon tetrachloride to produce uhite Iight.'9,20 The white light extended from about 420 nm to beyond 8.50 nm and its energy was about 0. I nJ/nm from 500 to 600 nm. To image the continuum light in the photolysis region of the sample, it was focused onto a ground glass diffuser plate located behind a I-mm diameter aperture. The diffuser smoothed out spatial inhomogeneities in the continuum beam. A lens positioned at twice its focal length from both the sample and the monochromator relayed the continuum to the entrance slit of the spectrograph and onto the detector. Recently a 0.25-111 double monochromator with subtractive dispersion has been inserted between the sample and the dispersing spectrograph and has significantly reduced the magnitude of scattered excitation light. The 400-500-nm measurements on Fe(phen)j2+ were made after these improvemen t s.
Creutz, Sutin, et al.
/ Excited States of Polypyridine Complexes
1311 A , nm
i
O t
a
a
-4-5
A
-0.3 I
I
0
I
L
I
2 TIME ( n s ) -S
-4
1
3
Figure 2. Kinetic data for the restoration of ground-state absorbance
at 557 nm for Fe(terpy)22+(crosses) and Fe(4,4’-(CH3)2bp~)3~+ (solid circles). The natural logarithm of the absolute value o f A , - At (absorbances at times infinity and t , respectively) is plotted vs. time. For Fe(bpy)32+the points at “negative” time indicate the excitation pulse width. A beam splitter was inserted in the continuum beam prior to the sample to create a reference b e a ~ n . ~The ’ - reference ~~ beam bypassed the excited sample and was directed into the spectrograph just above the continuum beam that traversed the sample. At the exit slit the two beams were again separated and directed either to different photodiodes (0.75-m Spex monochromator) or to different parts of a vidicon detector (0.25-m J-Y spectrograph). The presence or absence of a mirror after the sample (see dashed lines i n Figure I ) determined which detection system was employed. The vidicon detector (SIT tube, Princeton Applied Research OMA-11) could monitor a 320-nm spectral region in a single shot with 2.6-nm resolution (100-p slits). About 24 pairs of laser shots were averaged for each time point. (A pair consisted of one laser shot with the photolysis pulse blocked and one without blocking; a reference beam was used because the spectral shape of the continuum pulse varied from shot to shot). This procedure resulted in an average scatter in AA of about 0.02. The change in the ratio of the light in the two continuum beams following irradiation of the sample with 527-nm light was also measured with a pair of photodiodes. While only a single wavelength (1.7-nm resolution) could be monitored with the photodiodes, there was less error than with the vidicon detector: using an equivalent number of laser shots, an average scatter in AA of about 0.005 was obtained with the photodiodes. The energy of the 527-nm photolysis pulse was about 3-4 mJ at the sample cell (4.5-mm2cross-sectional area). After generation in a KDP crystal, the pulse was deflected along a I-m delay line. By sliding a pair of 45’ incident mirrors along a track, delays could be created between the arrival times of the photolysis and continuum pulses at the sample. This meant that absorbance measurements could be made coincident with the photolysis pulse ( t = 0) and up to 6 ns after it. The experimental uncertainty was about f 4 ps due to the -8-ps duration of the 1054 pulse. The FeL32+ samples were dissolved in water at concentrations of -6 X M. The monitoring path length was 2 mm. The samples were shaken after each photolysis shot.
Results Typical kinetic data for the repopulation of the ground state of FeL32+ after excitation with a -6-ps pulse of 527-nm light are shown in Figure 2 for L = terpy and 4,4’-dimethyl-2,2’bipyridine (4,4’-(CH3)2bpy). Bleaching of ground-state absorption resulted from the excitation pulse; then the groundstate absorption was restored exponentially. The same behavior was observed for all of the iron(I1) complexes investigated, including F e ( ~ h e n ) 3 ~ which +, was studied in the wavelength range 400-500 nm. See Figure 3. Thus we find no evidence for the 70-ps state of Fe(phen)3*+ reported by Street et al.’O I n fact, we estimate that for all of these iron(I1) complexes the nanosecond state is formed quantitatively in > terpy, but for iron it is terpy > bpy phen > 4,4’-(CH3)2bpy. The relatively short, ligand-insensitive lifetimes of the FeL32+ excited states suggest that the radiationless decay of the FeL32+ complexes differs in at least one important respect from the decay of the R u L ~ ~ O+ S, L , ~ + ,
265 134 270
78 84
0.80 f 0.07 0.43 f 0.03
Lifetimes in 1 M HCI, from ref 28. Lifetimes in water. See ref 11. This work. This work, 22-25 “C. e Young, R. C.; Nagle, J. K.; Meyer, T. J.; Whitten, D. G.; J . Am. Chem. SOC.1978, 100, 4773. Brunschwig, B. S., unpublished observations.
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1314
Journal of the American Chemical Society I
E -
I
4s
. I W
P
2.c
0
L
20
-5 I
*
b I .o
bI
0 3.0
+Q *ML32+ + Q *ML32+ + Q
2.0
102:4
/ February
13, I980
transitions. T h e excited-state spectra of these complexes a r e presented in Figure 7. As was noted previously,2a the spectral features of *Ru(bpy)32+ and *Os(bpy)32+ are similar and reminiscent of the spectrum of the 2,2'-bipyridine radical anion reported by Mahon and Reynolds.30 For * R ~ ( b p y ) 3 ~and + * O ~ ( b p y ) 3 ~ two + , maxima are seen. These occur a t -360 and a t 430 or 460 nm, respectively, and may correspond to the radical anion peaks a t 386 and 420 nm. T h e spectral features of these excited states are thus consistent with the ligand-localized transitions of a ( t 2 g ) 5 ( ~ * )metal-to-ligand 1 chargetransfer excited state. On the other hand, the spectrum of *Fe(bpy)3*+ is featureless above -300 nm. Intense absorption is seen below 300 nm, but no absorption maxima are found between 270 and 510 nm. This striking contrast between the spectra of * F e ( b ~ y ) 3 ~on+ the one hand and of *Os(bpy)3*+, * R ~ ( b p y ) 3 ~ and + , bpy- on the other suggests that * F e ( b ~ y ) 3 ~ + does not possess the bipyridine anion chromophore, i.e., that * F e ( b ~ y ) 3 ~is+not a metal-to-ligand charge-transfer excited state. This conclusion is consistent with the assignment made above, namely, that * F e ( b ~ y ) 3 ~is+a ligand-field state, either 3Tl or 5T2.The spectral features of either state would include ligand localized transitions ( F T * around 290 nm as observed for Fe(bpy)32+, etc.), weak ligand-field transitions of the metal center, and weak metal-to-ligand charge-transfer transitions in the visible. I n particular, neither state is likely to exhibit intense low-energy metal-to-ligand charge transfer as is seen for Fe(bpy)3*+ in the v i ~ i b l e . ~ I - ~ ~ Quenching Rate Constants. Three processes must be considered as possible quenching pathways for the polypyridine excited states: *ML32+
-
/
-
+ QML3+ + Q+ MLj2+ + Q* ML33+
4
(6) (7)
(8)
T h e first two, eq 6 and 7 , involve respectively oxidation and reduction of the *ML32+ excited state, while in eq 8 electronic excitation is transferred by a n energy-transfer process to Q. These three quenching processes may proceed in parallel so that the observed quenching rate constant may contain contributions from all three:
E
-
I W
t
10
kq,obad
=
k6
+ k7
kg
Despite this complication it is possible to deduce the major quenching mechanism in a number of the systems studied. As the electron-transfer quenching reactions are expected to be Figure 7. The spectra of *Fe(bpy)3*+,*R~(bpy)3~+, and *O~(bpy)3~+ a function of the oxidation-reduction potentials of the reacbased on the data presented in Figure 5 . tants, while energy-transfer processes should depend on spectral overlap considerations, the reduction potentials and spectral data of the polypyridine complexes used in this study and CrL33+ complexes. This difference may have its origin in a r e summarized in Table IV and will be used as a basis for the geometries of the excited states of the FeL3*+ complexes, discussion in the following paragraphs. unlike those of the other complexes considered, being subElectron-Transfer Quenching. Models and information stantially distorted from the ground-state geometries. This is amassed for ground-state reactions provide a useful guide for expected because of the population of the eg orbitals in the the excited-state electron-transfer reactions, eq 6 and 7 . (ligand-field) excited states of the FeL32+ complexes. As a Ground-state outer-sphere electron-transfer reactions of consequence the ground- and excited-state potential energy FeL32+/3+, R u L3 2+/3+,and O S L ~ ~ +a/r ~e very + rapid. This surfaces of the FeL32+ complexes may cross (strong coupling limit) rather than nestle as they d o for * R u L ~ and ~ + *OsL3*+ is attributed to the low intrinsic barriers to electron transfer that obtain in this series and which a r e most evident in the (and the M L C T state of FeL32+) and their respective ground self-exchange process: states. This could account for the insensitivity of the *FeL32+ lifetimes to the nature of L: in the strong-coupling limit, exML32+ ML33+ + ML33+ ML32+ k,, (9) cited-state deactivation is determined by the magnitude of inner shell and solvent distortion and by spin-orbit coupling R a t e constants of 5 X lo7 to 1 X lo9 M-I s - I in water a t 25 factors which are expected to be similar for the various ligands O C have been determined for reaction 9.34,35In addition there studied. is compelling evidence that the couples involving the chargetransfer excited states of R u L ~ ~and + O S L ~ also ~ + possess very Excited-State Spectra. T h e visible absorption spectra of ground-state Fe(bpy)32+, Ru(bpy)32+, and O ~ ( b p y ) 3 ~a+r e high self-exchange rates (eq 10 and 11); rate constants of b 1 Os dominated by intense t~ a* metal-to-ligand charge-transfer and lo9 M-I s-I have been estimated for the excited-state 0
I
I
I
400 A.nm
500
+
-
+
Creutz, S u t i n , et ai. / Excited States of Polypyridine Cowiplexes
1315
Table IV. Reduction Potentials and Absorption and Emission Maxima for Polypyridine Complexes of Iron(II), Ruthenium(II), and Osmium(I1) at 25 'CU
ground staten complex
E03,2r
R~(bpy)3*+ R~(S-Clphen)3~+ Ru(4,7-(CH3)~phen)3~+ Ru(terpy)?*+ R~(S-NO*phen)3~+ Ru(TPTZ)>*+ Os(bpy)3'+ Os( 5 - C l ~ h e n ) 3 ~ + Fe(phen)32+ Fe(bpy h2+ Fe(terpy)Z2+
v
excited stateu
absorptionb X m m , nm
emissionb 6 13,627 605, 625 613, 627 628 606 605 750d 700e
-1.26'
452,560 422, 447, (560)f 425,445, (560)f 473 (560) 449 (560) 501 (560) 480,590,650 477, 560, (650)f 5109 522,g 870',
-1.17'
522g
E02,1. V
+ 1.26
*E03,23
-1.28
+1.36 +1.09 +1.25 +1.46b 1.49 f0.82 +0.93 1 .066 +1.05b
v
*E02,1.V
-0.84 -0.77 -0.94
-1.15
- 1.47
+0.84 + I .OO
+0.67
- 1.36
-0.67'
+
-0.77 -1.22 - 1.09
+
+1.05c
-0.96 -0.9
$0.59 +0.7
Amax,
nm
a Data taken from Tables I1 and I I I of ref 2a unless otherwise stated. See ref 2a for original literature references. From ref 15 unless otherwise noted. See ref 1 5 for original literature reference. Unless otherwise noted, corrected emission maxima are reported. Dwyer, F. P.; Gyarfas, E. C. J . Am. Chem. Soc. 1954, 76, 6320. Reference 3. e Uncorrected emission maximum. /Assumed by analogy with the spectrum of M(bpy)j2+. See text. g Reference 14. Reference 27. Saji, T.; Aoyagui, S. J . EIectroanaL Chem. Interfacial Electrochem. 1975, 58, 401. Table V. Quenching Mechanisms for Polypyridine Complexes
reactants
-AE',l,~, -AE0,1,7,
probable reaction
V
V
AE*, eV
k,, M-'s-'
1.
+0.38
-0.02
-0.33
1.5
2.
t0.45
-0.18
-0.33
2.5 x 109
3.
f0.28
+O. I 5
-0.33
2.5 x 109
4.
+0.44 +0.52
t0.41 +0.35 +0.62
-0 -0 -0
< I x 108 1.5 x 109
none
2.0 x 109
(6)
donor
5.
auencher
6.
x
109
-0.07
+0.65
1.2 x 109
-0
+
8
6 8
8.
to.5
$0.6
9.
-0. I
+0.8
IO.
+0.42
II.
12. +0.4
13.
+ ML33+ * ML33+ + *ML3'+
+ *ML3'+ * *ML32+ + ML3+ ML3+ + ML32+ * ML3'+ + ML3'
ML3+
< I x 108
2.6 x
109
f0.22
1 . 1 x 109
+0.21
+0.4
1.0x 109 1 . 4 x 109
+0.4
2.3 x
exchange processes (10) and ( 1 I ) , respectively.2 Furthermore, i~~ a rate constant of the work of Saji and A ~ y a g u implicates B 1 O8 M-l s-l for eq 12. Finally, both the ground-state and *ML32+
t0.33 $0.33
(IO) (1 1 )
(12)
excited-state reactions respond to reaction driving force ( K I 2 ) in a way which a t moderate driving force is given by the Marcusequation, log kl2 = 0.5 log kllk22K12.~'Herek l l and k22 are the self-exchange rates of the reactant couples (eq 9-12) and kl2 is the rate constant for the cross reaction (eq 6 or 7). It becomes evident that reactions 7 and 8 should be very rapid indeed when Q is a polypyridine complex of iron, ruthenium, or osmium provided that K B 1. From these considerations, it is concluded that for oxidative quenching kg should be -3 X IO8 M-' s-l if K = 1 and, in general, k6 should ) s-l where K may be calculated from be -3 X IO8 ( K 1 / 2 M-I AE0,1,6 = Eo2,1;, - *E03,2;d. Similarly, for reductive quenching, k7 should be -3 X 1OX(K'/')M-' s - I where K is obtained from AE0,1,7 = *E"*.I:. - E 0 3 , 2 , q . Both electrontransfer processes should approach diffusion-controlled rates (-3 X IO9 M-' s-l) when K 2 10' or AE" B 0.12 V.
109
Ru(bpy)3+ + O ~ ( b p y ) 3 ~ + Ru(bpy)3'+ *Os(bpy)3*+ Ru(S-CIphen)3+ t O ~ ( b p y ) 3 ~ + R~(S-Clphen)3~+*Os(bpy)3*+ Ru(4,7(CH3)2phen)3'+ *Os(bpy)3*+ none Ru(bpy)32+ + *Ru(terpy)3?+ R ~ ( b p y ) 3 ~++Ru(S-X02)phen)3+
+
8
7.
postulated products
none 6 (6) 8 8
(6) 8 8
+
Ru(bpy)3'+ t * R ~ ( 5 - N 0 2 ) -
phen)3*+ R ~ ( b p y ) 3 ~ +Ru(TPTZ)2+ Ru(bpy)3*+ + *Ru(TPTZ)>*+ none O~(S-Clphen)3~+Ru(TPTZ)2+ R ~ ( b p y ) 3 ~t+Fe(phen)3+ Ru(bpy)32+ + *Fe(phen)32+ Ru(bpy)32+ *Fe(bpy)3l+ O~(S-Clphen)3~+ t Fe(phen)3+ O~(S-Clphen)3~+*Fe(phen)3*+ Os(5-Clphen)32++ *Fe(bpy)3*+
+
+
+
+
In Table V the data for quenching by polypyridine complexes have been organized so that these predictions can be systematically examined. The third and fourth columns contain values of AEO for quenching according to eq 6 and 7, respectively. Values observed for k , a r e given in the penultimate column. For reactants I , 2, 6, 7, 9, 10, and 12 in Table V AEO is BO V for one or the other of the electron-transfer quenching reactions and, notably, all these reactions proceed with quenching rate constants > I O 9 M-l s-I. Furthermore, for systems 4 and 8 for which Kl2 for quenching by either eq 6 or 7 is less than IOp5, the quenching rate constants a r e less than IO8 M-l s-l. Energy-Transfer Quenching. T h e reactivity patterns examined so far generally support the rate criteria for electrontransfer quenching developed above. W e consider now the rate patterns expected for energy-transfer processes. Within the Dexter model for energy transfer, the efficiency of the transfer increases as spectral overlap between donor and acceptor inc r e a s e ~ . ~For * the donor, the maximum. and shape of the emission band returning the donor to the ground state are considered; for the acceptor, the absorption spectrum is of relevance. With * R u L ~ ~and + *OsL3'+ as donors, the emission spectra maximize a t -600-630 and a t -750 nm, respectively, and the excited-state energies are estimated to be 16.9 and 14.4
Journal of the American Chemical Society
1316
X IO3 cm-I, respectively. T h e properties of the potential acceptors R u L ~ ~and + OSL3*+ will be considered in turn. T h e principal low-energy absorption features for the Ru( 11) complexes (Table IV) a r e the intense, charge-transfer transitions in the range 420-470 nm, but a weak, spin-forbidden transition has been recognized for R ~ ( b p y ) 3 ~a +t -590 nm. T h e latter is the lowest energy absorption band detected and is the counterpart of the emission band a t 600-630 nm. T h e osmium( 11) complexes also undergo intense charge-transfer absorption a t -480 nm with a secondary fairly intense band a t -560 nm. In addition, weaker absorption is seen a t -650 nm,6 a band which probably corresponds to the inverse of the 750-nm emission. W e now consider donor/quencher pairs to determine for which the Dexter criteria for energy transfer a r e met. ( 1 ) * R u L ~ ~ + / O S LThe ~ ~ +Ru(I1) . emission spectra ,A,(, 600-630 nm) overlap the absorption spectra of the Os(1I) complexes ,A,(, 590, 650 nm) so that energy transfer from * R u L ~ ~to+O S L ~ is~ generally + to be expected. Quenching is thus likely to proceed by energy transfer in systems 1-3 in Table V. I n systems 1 and 2 electron-transfer quenching (eq 7) is also expected to be rapid, so that the high values of the quenching rate constants a r e not readily interpreted. For system 3, however, redox quenching is thermodynamically unfavorable as it is in system 4 for which k , d IO8 M-I s-l. Therefore the high value of k , = 2.5 X IO9 M-I s - I with *R~(4,7-(CH3)2phen)3’+/Os(bpy)~~+ is ascribed to rapid energy transfer. This is a n important conclusion: “thermodynamically favorable” energy transfer from * R U L ~ to ~ +oSL32+ occurs with a rate constant approaching the diffusion-controlled value. (2) * R u L ~ ~ + / R u LBy ~ ~contrast, +. energy transfer from the excited state of one R u L ~ ~complex + to the ground state of another is not likely to be very favorable thermodynamically, but rather “thermoneutral”. In the R u L ~ ~series + the absorption and emission spectra (and thus the excited-state energies) a r e remarkably insensitive to the nature of L.I5 Consequently energy transfer from one R u L ~ ~complex + to another is rather analogous to the self-exchange process for electron transfer, except that it formally involves two simultaneous electron transfers. The data for entries 4-7 bear on this question. For systems 6 and 7 electron-transfer quenching may occur in parallel with energy transfer. It is, however, evident from system 5 ( * R ~ ( b p y ) 3 ~ + / R u ( t e r p y ) 2 ~for + ) ,which electron-transfer quenching is highly unfavorable, but k , = 1.5 X IO9 M-’s-l, that energy transfer may also be very rapid between various ruthenium( 11) complexes. Entry 4, R ~ ( b p y ) 3 ~ + “self-quenching”, is a n interesting case in point: according to our arguments, “exchange” energy-transfer quenching could also be facile here. However, since each such quenching act must regenerate an excited molecule (eq 13), no net diminution in donor concentration and lifetime can result.
+
-
*R~(bpy)3~+R~(bpy)3~+ R ~ ( b p y ) 3 ~ + * R ~ ( b p y ) 3 ~ +(13)
+
(3) * O S L ~ ~ + / R U LT h~e~absorption +, spectrum of R U L , ~ + does not detectably overlap O S L ~ emission. ~+ Often such a n observation is of no great value in predicting energy-transfer efficiencies because energy transfer may occur via acceptor states which a r e spectroscopically unobserved-for example, via spin-forbidden transitions. In the present case, however, the fact that R u L ~ emission ~+ from the lowest excited state occurs a t -600-630 nm, while the O S L , ~ +emission lies a t -750 nm, indicates that the relevant spectral overlap must be poor, largely because the Os( I I) complexes have lower excited state energies than the Ru(l1) excited states. In other words, energy transfer from * O S L ~ to ~ +R u L ~ is ~ energetically + uphill. Rates for this process are thus expected to be slower than
/
102.4
/ February 13, 1980
for those discussed above. The validity of this conclusion is supported by the data for system 8, * O ~ ( S - C l p h e n ) 3 ~ + / Ru(terpy)z2+, for which both electron-transfer and energytransfer quenching are unfavorable processes; here k , is d 1 O8
M-1 s-l Quenching by Polypyridine Iron(I1) Complexes. Analysis of
the quenching rate patterns for R u L ~ ~and + O S L ~ has ~ + led to two conclusions: when electron-transfer quenching is thermodynamically favorable, very rapid quenching rate constants ( IO9 M-’ s - I or greater) are seen. Similarly if energy-transfer quenching is “thermodynamically favorable” ( * E , < *Ed), energy-transfer quenching is very rapid. Conversely, when either process is uphill, k , is diminished ( < I O s M-’ s - ] ) . For entries 10-1 3 in Table V, Fe(bpy)3*+ and F e ( ~ h e n ) 3 ~were + used as quenchers for *Ru(bpy)3*+ and *Os( 5-Clphen)3*+. In entries 1 1 and 13 electron-transfer quenching is unfavorable by a t least 0.2 V. The rapid quenching observed ( k , 3 1 X 1 O9 M-l s-I) is thus very likely due to energy transfer. The lowest absorption feature observed for Fe(bpy)32+ is a weak ( 6 -I M-l cm-I), broad band a t -870 nm. As discussed by Palmer and Piper,2i this band is probably the ligand-field transition 3TI;the three related transitions ’ A ’ T I , ’ A 3T2, ‘A and ‘ A IT2 must then lie a t higher energy, that is, a t wavelengths shorter than 870 nm. (The highly forbidden (and therefore very weak) transition ‘ A I 5T2may occur at higher or lower energy than ‘ A I 3 T I ,but should, in any case, occur 3T2or IT!.) T h e principal visible a t lower energy than ’ A absorption feature for F e ( b ~ y ) ~is~ the + metal-to-ligand charge-transfer absorption a t 522 nm. Presumably the spinforbidden counterpart of this band lies a t longer wavelength. For energy-transfer from * R ~ ( b p y ) 3 ~to+ F e ( b ~ y ) 3 ~overlap + of Fe(bpy)3*+ absorption with the 630-nm emission of * R ~ ( b p y ) , ~is+ required; with *Os(5-Clphen)32+ as donor F e ( b ~ y ) 3 ~absorption + a t -750 nm is needed. Although no such absorptions a r e observed in the F e ( b ~ y ) 3 ~spectrum, + it has been inferred that several unobserved spin- or symmetryforbidden transitions a r e present between 522 and 870 nm. These may provide acceptor states through which energy transfer from the ruthenium( 11) and osmium( 11) donors takes place. Properties of *FeL32+. T h e spectrum and lifetime of * F e ( b ~ y ) 3 ~ the + , nanosecond state achieved by 530-nm excitation, support its assignment a s a ligand-field state. From spectroscopic considerations the lowest ligand-field state of F e ( b ~ y ) 3 ~lies + a t or below 1 1.5 X IO3 cm-l ( I .43 eV) and is either 3T1 or 5T2.2i W e postulate that * F e ( b ~ y ) 3 ~is+3 T or ~ 5T2, if the latter is the lowest energy state, and that the excited-state energy is consequently < 1.4 eV. From the picosecond experiments *FeL32+ is formed within - I O ps after excitation. Thus internal conversion and intersystem crossing to reach the nanosecond state must occur with an overall rate constant of a t least -lo1’ s-l. Assuming the initially populated M L C T state to be a singlet, formation of 3T1or 5T2states involves a net spin change of 1 or 2, respectively. Rate constants > lo7 s-I have been reported for intersystem crossing between iron(l1) ‘A and 5T ligand-field states of comparable energy.39 In the present case the intersystem crossings postulated are to states of much lower energy. Since the potential energy surfaces for the M L C T and 3TI or 5T2 states cross4o (strong coupling), the rate of intersystem crossing should be enhanced by the fact that the ligahd-field states a r e a t lower energy than the M L C T state. Consequently, the intersystem crossing rate should exceed loi s - I and could be a s great as 10” s-I. T h u s it is not unreasonable that 3Ti or 5T2 be the nanosecond state of Fe(bpy)32+ The ligand-field states 3TIand 5T2have the configurations (t2#(eg)’ and (t2g)4(eg)2, respectively. Because the antibonding e&orbitals a r e populated, the metal-ligand bonds in these states are likely to be -0.14 8, longer than in the (t2g)6
--
-
-- -
-
Creutz, Sutin, et al.
/
Excited S t a t e s of Polypyridine Complexes
1317
process is not involved (or, possibly, both). I n fact, even if the reaction occurs by "static quenching", the ligand-field state cannot be the active state unless its energy and reactivity are drastically altered by association with Etpy+ (exciplex). Thus it is tempting to propose that associated Etpy+ intercepts a higher excited state of F e ( b ~ y ) 3 ~(perhaps + indeed the M L C T state as implied by Chum et al.). This process would, however, have to be exceedingly rapid-as much as 10' I s-'-since the rate of formation of F e ( b ~ y ) 3 ~from + the M L C T state (in water) is of this order. Clearly, it is not possible to draw any F e ( b ~ y ) 3 ~ +e *Fe(b~y)3~+ (14) firm conclusions without further studies of this system. We have concluded that * F e ( b ~ y ) 3 ~is+a ligand-field rather Because of the distortion of * F e ( b ~ y ) 3 ~relative + to the ground than a charge-transfer excited state. This assignment seems state, the self-exchange rate of t h e excited s t a t e reasonable in view of the fact that the ligand-field splitting for * F e ( b ~ y ) 3 ~ + - F e ( b p y ) 3couple ~+ is expected to be relatively ' to slow. The exchange rate of the ground state F e ( b ~ y ) 3 ~ + - iron( 11) is relatively small ( 10- 14 X I O 3 ~ m - l , ~compared 20-30 X I O 3 cm-' for R U ( I I ) ~ Iand >20 X lo3 cm-I for F e ( b ~ y ) 3 ~couple + is -2 X I O 9 M-' s-I 3s and is consistent Os(I1)). No photoaquation has been repcrted for F e ( b ~ y ) 3 ~ + with essentially zero inner-sphere difference between or related complexes. I n general oxidation-reductions of F e ( b ~ y ) 3 ~and + F e ( b ~ y ) 3 ~ The + . ~ -0.5-eV ~ distortion4I of *FeL32+ are expected to be rather slow. Consequently, because * F e ( b ~ y ) 3 ~relative + to F e ( b ~ y ) 3 ~will + contribute -0.36 eV47 the lifetimes of these excited states are short, photoinduced to the activation energy for the * F e ( b ~ y ) 3 ~ + - F e ( b p y ) 3ex~+ electron transfer in FeL32+ systems may be expected only when change reaction. On this basis the rate constant for the the electron acceptor is highly reactive and present in high * F e ( b ~ y ) j ~ + - F e ( b p y ) 3exchange ~+ reaction is calculated to concentration. In summary, the excited states of FeL32+ be < I O 3 M-' S K IThis . exchange rate together with thevalue complexes are much less reactive than *RuL3'+ and *OsL3?+ of the excited-state potential may be used to calculate a rate both because of their lower excitation energy (-0.9 vs. 2.1 and constant for the reaction of * F e ( b ~ y ) 3 ~(or + *Fe(terpy)z2+) 1.8 eV for R u L ~ and ~ + O S L ~ ~respectively) +, and because their with Feaq3+.From substitution in the Marcus equations37 we electronic configuration creates high kinetic barriers to electron calculate a rate constant for the * F e ( b ~ y ) 3 ~ + - F e , , ~reaction + transfer. of < I O 6 M-' s-' which is consistent with the observed value
ground ~ t a t e . ~As ~ . a~ consequence ' of this distortion, the thermally equilibrated 3TIand 5T2states should lie -0.5 eV42 below the Franck-Condon states. On this basis the thermally equilibrated 3Tl (or sT2) state is not expected to lie more than -0.9 eV above the ground state. In summary, a n excitation energy of 1 0 . 9 eV is postulated for *Fe(bpy)3*+. Combining this estimate with the reduction potential of F e ( b ~ y ) 3 ~(1.05 + V, 0.5 M H2S0443,"), *E03.2, the reduction potential of the excited-state couple (eq 14), is 1 +0.1 V.
+
I 107 M-I
s-1.4*
I n this study we have presented evidence that, in water a t
Acknowledgment. Helpful discussions with Drs. Bruce S. Brunschwig, Marshall D. Newton, and Gil Navon are acknowledged. This work was performed under the auspices of the U S . Department of Energy and supported by its Office of Basic Energy Sciences.
25 "C, the excited state of F e ( b ~ y ) 3 ~arising + from 530-nm light absorption undergoes oxidation only rather slowly. There are, however, two reports which appear to conflict with these conclusions. Phillips, Koningstein, Langford, and Sasseville describe electrochemical evidence for photooxidation of Appendix F e ( b ~ y ) 3 ~by + Feaq3+.12Using exciting light in the range 459-5 14 nm they determined photocurrents a t a n SnO2 elecBy analogy with other scattering processes, a cross section trode and deduced a 33-11s lifetime for the F e ( b ~ y ) 3 ~excited + for the interaction of a molecule with an impinging photon may state and a rate constant of 2.4 X lo9 M-' s-I for its oxidation be defined by by Feaq3+.T h e present studies and those of Kirk et aL9 (in d l = aNAcllo/ 1 O3 ('41) which ground-state bleaching produced by a subnanosecond pulse of 530-nm light was monitored) provide no evidence for where u is the molecular cross section (cm2 molecule-'), N A the population of such a long-lived state. Although the exciis Avogadro's number, IO is the incident photon intensity tation wavelength ranges for the electrochemical experiments (einstein cm-2 s-l), d I is the decrease in the photon intensity, and the laser excitation experiments do not overlap, it would c is the concentration of absorbing molecules (mol L-I) and be quite remarkable if the state produced by the shorter 1 is the path length of the solution (cm) along the excitation wavelength light did not rapidly convert to that produced by beam. The relation between and the usual molar absorptivity 530-nm excitation. It is more likely that the photocurrents ( L mol-' cm-I) is given by reported arose from species adsorbed on the SnOz or from some interfacial artifact. The operation of such effects on Sn02 is a N ~ / 1 0= ~2.3036 (A2) also suggested by the failure of Ohsawa, Saji, and A o y a g ~ i ~ ~ Substitution of eq A 2 into eq AI gives to observe photocurrents a t a platinum electrode when F e ( b ~ y ) 3 ~was + irradiated i n the presence of Feaq3+. d l = 2.303~cllo (A3) The photooxidation experiments of Chum, Koran, and which is Beer's law for a dilute solution ( d l
