J. Am. Chem. SOC.1993,115, 6398-6405
6398
Ultrafast Electron Transfer and Coupled Vibrational Dynamics in Cyanide Bridged Mixed-Valence Transition-Metal Dimers Stephen K. Doom,+R. Brian Dyer,* Page 0. Stoutland,’& and William H. Woodrufft Contributionfrom the Divisions of Chemistry and Laser Sciences (Mail Stop J-567) and Isotope and Nuclear Chemistry (Mail Stop C-345), Los Alamos National Laboratory, Los Alamos, New Mexico 87545 Received December 18, 1992
Abstract: Picosecond infrared spectroscopy has been used to investigate electron transfer and vibrational excitation and relaxation in the mixed-valence transition-metal dimer [(NC)SMIICNMII~(NH~)~]( M = Ru, Os). Optical excitation into the metal-to-metal charge transfer band results in formation of the excited state redox isomer [(NC)5M*WNMII(NH3)~]-.Subsequent back electron transfer results in reformation of the ground state and occurs with T < 0.5 ps. Upon return to the ground electronic state, large amounts of energy (up to 7 quanta, 14 000 cm-1) are placed into the terminal M-N stretching mode. The rate of the subsequent energy relaxation was measured; decay times ranged from > k7‘) intramolecular modes coupled to the electron transfer are responsible for this rate acceleration.“ Thus, in addition to considering the dynamical effects of the solvent, it has become ’ Author to whom correspondance should be addressed. t Isotope and Nuclear Chemistry.
and Laser Sciences. (1) See,for example: (a) Photosynthesis;Stachlin, L. A., Amtzen, C. J., Ed.; Springer-Verlag: New York, 1986; Vol. 1, p 3. (b) Photosynthesis; Govindje, Ed.; Academic Press: New York, 1982; Vol. 1. (c) Danks, S. M.; t Chemistry
Evans, E. H.; Whittaker, P.A. Photosynthetic Systems; Wiley: New York, 1983. (d) Sauer, K. Annu. Reo. Phys. Chem. 1979, 30, 155. (2) Molecular Electronic Devices, Carter, F.L., Ed.; Marcel Dekker, Inc.: New York, 1982. (3) For recent reviews, see: (a) Simon, J. D. Acc. Chem. Res. 1988.21, 128. (b) Barbara, P.F.;Jarzeba, W. Ado. Photochem. 1990, 15, 1. See,also: (c) Walker, G. C.; Aakesson, E.; Johnson, A. E.; Levinger, N. E.; Barbara, P. F. J. Phys. Chem. 1992, 96, 3728. (d) Barbara, P. F.;Walker, G. C.; Smith, T. P. Science 1992, 256, 975 and references therein. (4) (a) Sumi, H.; Marcus, R. A. J. Chem.Phys. 1986,844894. (b) Rips, I.; Jortner, J. J. Chem. Phys. 1987, 87, 2090. (c) Jortner, J.; Bixon, M. J. Chem. Phys. 1987,88, 167. (d) Newton, M. D.; Sutin,N. Annu. Reu. Phys. Chem. 1984,35,437. (e) Bixon, M.; Jortner, J.J. Phys. Chem. 1991,95,1941. (e) Weaver, M. J.; McManis, G. E., 111 Acc. Chem. Res. 1990, 23, 294.
\\
i
MMCT State
Ground State Figure 1. Potential energy diagram for the electron transfer process showing the reaction along the solvent coordinate. Inclusion of a highfrequency quantal vibration (is. vibrations orthogonal to the solvent coordinate) lowers the potential energy barrier (points b) to the back electron transfer reaction in comparison to the classical barrier crossing at point a.
increasingly apparent that, in some cases, it is also necessary to consider nuclear motions associated with intramolecular vibrational modes. Recent models5 have sought to quantify the dynamical effects of high-frequency intramolecular vibrations that have been shown to be especially important for electron transfer reactions occurring in the “Marcus inverted region”. In the classical view of these cases, electron transfer can occur only a t curve crossings (Figure 1). Inclusion of high-frequency quantal modes, however, accelerates the electron transfer by providing additional, lower energy, curve crossings. Importantly, these models suggest that not only should high-frequency modes accelerate the rate of electron transfer, but that the products should have high degrees of vibrational excitation in the coupled intramolecular vibrational modes. There have, however, been no direct experimental tests of this prediction. We have recently been interested in a class of compounds known as mixed-valence transition-metal dimers which provide a well( 5 ) For a review of progress in this area, see: Newton, M. D.; Sutin, N. Annu. Rev.Phys. Chem. 1985,35,437 and ref 3d.
OOO2-7863/93/1515-6398$04.00/0 Q 1993 American Chemical Society
J. Am. Chem. Soc.. Vol. 115, No. 14, 1993 6399
Study of Ultrafast Electron Transfer defined environment in which to study electron transfer reactions.6 Specifically, we have used picosecond infrared spectroscopy to investigate a number of cyanide-bridged mixed-valence dimers of the form:
Mixed-valence dimers have historically been attractive model systems for studies of electron transfer.' This attraction arises from the presence of two well-defined metal sites of differing oxidation state in which there is the possibility of observing an inner-sphere electron transfer between the two metal centers. Optical excitation into the metal-to-metal charge transfer (MMCT) band leads to formation of the excited state redox isomer: hv
[(NC),Ru*'CNRu"'(NH,),]-~ ET
[ (NC),Ru"'CNRu"(NH,),]Subsequent thermal back electron transfer results in reformation of the original species. Simple models originally developed by Hush8 have linked the energy of the optical charge transfer to the barrier for the thermal back electron transfer. Despite the simplicity of these models, they have been shown to be consistent with the available experimental evidence, and thus have proven to be valuable tools in understanding electron transfer reactions. For example, simple static optical absorption experimentsghave yielded information on such aspects of electron transfer as the electronic coupling between donor and acceptor, solvation effects on band energies and coupling,and the magnitude of the activation barriers to the thermal electron transfer. While these studies have been instrumental in testing and modifying various aspects of electron transfer theories, direct tests of electron transfer barrier (and thus the rate) predictions have beendifficult, in part because MMCT bands are typically located in the near-infrared region of the spectrum and have very low oscillator strengths. The absorption characteristics of the cyanide-bridged dimers, however, make them more suitablefor kinetic studies from which the electron transfer barrier might be directly extracted. Because of the large differences in redox potentials between the two metal centers (about 0.9 eV for Ru-Ru and 1.2 eV for Os-0~10)the MMCT band is in thevisible region of the spectrum and is isolated from other optical transitions. This feature made possible the recent measurements of the electron transfer dynamics by Barbara et al.11 of the Ru-Fe and the Ru-Ru derivatives using femtosecond visible absorption spectroscopy. Furthermore, the accessibility of lasers in the visible region of the spectrum has also made it possible to obtain resonance Raman spectra of these molecules.12 (6) Preliminary reports of our work have been published: (a) Doorn, S. K.; Stoutland,P. 0.; Dyer, R.B.; Woodruff, W. H. J. Am. Chem.Soc. 1992, 114,3133. (b) Stoutland,P. 0.;Dyer, R.B.;Woodruff, W. H. Science 1992, 257, 1913. 17) See. for examde: [a) Mixed- ValenceComoounds: Brown. D. B.. Ed.: D. Reidel: 'Dordrecdt, TheNetherlands, 1980. (6) Creutz, C. Prog. I k r g ; Chem. 1983. 30. 1. - .
(8) (a)Hush, N. S . Prog. Inorg. Chem. 1967, 8, 391. (b) Hush, N. S . Electrochim. Acta 1968,13, 1005. (c) Hopfield, J. J. Proc. Natl. Acad. Sci. U.S.A. 1974, 21, 3640. (d) Hopfield, J. J. Biophys. J. 1977, 18, 311. (e) Potasek, M.J.; Hopfield, J. J. Proc. Narl. Acad. Sci. U.S.A.1977, 74,229, 3817. ( f ) Meyer, T. J. Prog. Inorg. Chem. 1983, 30, 389. (9) (a) Itskovitch, E. M.; Ulstrup,J.; Vorotyntscv,M.A. In The Chemical Physics of Solvation; Dogonadze, R. R.,Kalman,E., Komyshev,A. A,,Ulstrup, J., Us.; Elsevier: Amsterdam, 1986; Part B. (b) Blackbourn, R. L.; Doorn, S. K.; Roberts, J. A,; Hupp, J. T. Langmuir 1989, 5, 696 and references therein. (10) Eptimated from redox potentials for the appropriate monomer pairs, scc: Haim, A. Inorg. Chem. 1985,14, 113. (11) (a) Walker, G.C.; Barbara, P. F.; Doorn, S.K.; Dong, Y.;Hupp, J. T. J. Phys. Chem. 1991, 95, 5712. (b) Kliner, D. A. V.;Tominaga, K.; Walker, G. C.; Barbara, P. F. J. Am. Chem. Soc. 1992, 114, 8323. (12) (a) Doom, S.K.; Hupp, J. T. J. Am. Chem. Soc. 1989,111, 1142. (b) Doom, S.K.;Blackbourn, R. L.; Johnson, C. S.; Hupp, J. T. Electrochim. Acta 1991, 36, 1775. (c) Doom, S. K.Ph.D. Dissertation, Northwestern University, 1990.
Resonance Raman investigations in which the probe laser is resonant with the charge transfer band provide direct, detailed information on the vibrational modes coupled to the electron transfer reaction. Our measurements are complementary to these in that picosecond infrared spectroscopyprovides, within one experiment, the possibility of observing both the dynamics of the electron transfer and vibrational excitation/relaxation of the modes coupled to the transition. For example, the frequenciesof M-N stretching vibrations are very sensitive to the formal charge at the metal center and, because of anharmonicity, to the degree of excitation in a particular mode. Historically, there have been questions concerning the extent of localization of the excess electron in mixed-valence systems (Le. Is the system best described as M2.5+-M2.5+ or M3+-M2+?). In the cyanide-bridged dimer described here, strong asymmetry in the molecule leads to a localized ground state despite the strong coupling between the two metal centers, but it is conceivable that in the electronically excited state the excess electron might be delocalized. The experiments described here are thus able to address questions concerning the rate of electron transfer, the extent of charge transfer, and the role of coupled intramolecular vibrations.
Experimental Section Experiments were performed using visible pump pulses and infrared probe pulses where time resolution is obtained by spatially delaying the probe pulse relative to the pump pulse (Figure 2).13 Picosecond pulses originate in a dual-jet dye laser synchronously pumped at 76 MHz by the frequency-doubled output of a cw mode-locked Nd:YAG laser (Coherent Antares 764). Operating on Rhodamine 6G/DODCI, the dye laser (homebuilt) provides pulses of ca. 2 ps (fwhm = 8 cm-l, pulse energies 1 nJ/pulse) tunable using a two-plate birefringent filter throughout the gain curve of the dye (ca. 580-640 nm). Useful pulse energies are obtained by passing the dye laser output through a threestage dye amplifier (Kiton Red 620) pumped at 30 Hz by the frcquencydoubledoutput(ca. 15mJ/pulseat 532 nm) of a regeneratively amplified Nd:YAG laser (Continuum RGA-60-30). Amplification increases the pulse energy to -200 pJ/pulse with minimal pulse broadening. The pump (or excitation) pulse is either the amplifieddye laser pulse itself (typically attenuatedto 30pJlpulse) or is formedusingthe technique of continuum amplification. In this case, the amplified dye laser pulse is focused into a 2-cm cuvet of HzO to form a white light continuum.This isthencolliiatedandsentthrougha filter toselect tbedesiredwavelength. Subsquent amplification in two stages results in 2-ps pulses of 100 pJ/pulse. For the experiments described here, the 700-nm pump pulse wasobtainedbypassingthecontinuumthrougha 10nmfwhminterference filter centered at 700 nm and subsequently amplifying it in LDS 698 to 50-100 pJ/pulse (later attenuated to 30 pJ/pulse)." Infrared probe pulses in the 5-pmregion are generated in a 5 mm thick LiI03crystal by difference frequency mixing the visible dye laser pulse (- 10 pJ/pulse) with the 532-nm pulses (75 ps, 100 pJ/pulse) from the regeneratively amplified Nd:YAG laser. The resultantIR pulses arc -2 ps fwhm, 10-100 nJ/pulse, with a frquency bandwidth ofca. 8 cm-l. Tuningof the infrared probe pulse is accomplishedby changing the output wavelength of the dye laser. Using the present dye/crystal combination, the infrared probe can be tuned between 2900 and 1800 cm-1 which corresponds to changing the dye laser wavelength from 629 to 588 nm. The infrared pulses are split into reference and samplebeams and are detectedby matchedindium:antimonide(InSb) detectors. Subsequently, the detector signals are integrated (Stanford SR250), ratioed (Stanford SR235), and transferred to a Macintosh IIx computer for further data analysis. The visible pump beam is blocked at 15 Hz with a shutter to yield light on/light off data, allowing the determinationof absolute A A ' s
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(13) For a rcccnt review of the technological aspects of ultrafast infrared spectroscopy, see ref 6b. (14) When performing experiments using a pump pulse formed by amplifying part of a white light continuum, larger 'coherence artifacts" were observed. We believethisoccurs because the pulse is actuallyhighly structured (temporally) due to the process of continuum formation. Thus while the werage pulse shape is well-described by a sech*function, a single pulse. would be more accurately described as a highly structured pulse composed of many subpicosecondfeatures. Such structure would tend to accentuate "coherence artifacts".
Doom et al.
6400 J. Am. Chem. SOC.,Vol. 115, No. 14, 1993
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Nd: YAG Laser
Infrared Detectors
Figure 2. Schematic of the picosecond time resolved infrared apparatus.
and improving the S/N ratio by 3-4 times.15 Typical transients are averages of 4 to 10 scans, with 50 samples being collected at each delay time for each individual scan. Our apparatus as described is capable of detecting absorbance changes of -0.1%. The instrument response function and the zero of time are determined by substituting a wafer of silicon in place of the sample.16 Excitation by the pump pulse results in a subpicosecond decrease in transmission throughout the infrared, thus decreasing the observed probe intensity as the two pulses are overlapped in time. Typical pump-probe crosscorrelations are 3-4 ps fwhm and are well described by a sech2function. Using deconvolutiontechniqueson transients with typical signal to noise, we can resolve events with rise/decay times of -0.5 ps. The mixed-valence dimers were prepared according to literature methods." Typical samples are 0.04 M solutions of dimer in D20 or twice the concentration in H20. Solutions are loaded into 100 or 50 pm (for D2O and H20, respectively) pathlength IR cells fitted with CaF2 windows. The ground state visible spectra of the Ru-Ru and Os-Os dimers are shown in Figure 3. The MMCT bands are centered at 683 (t = 2800 M-' cm-l) and 560 nm (e = 2100 M-l cm-l), respectively. The structure observed in the Os-Os spectrum is due to partial resolution of splitting of the charge transfer transition due to spin-orbit coupling. The ground state infrared spectrum of the Ru-Ru dimer is shown in Figure 4. We observe one strong band at 2053 cm-l (fwhm = 23 cm-l, t = 2500 M-l cm-l) corresponding to the terminal R u G N stretching mode.'* We do observe a band attributable to the bridging cyanide at 21 18 cm-l, but it is ca. 50 times weaker than the terminal stretching mode. The spectrum for the Os-Os dimer is similar, with the terminal stretching mode occurring at 2040 cm-l. (15) While we are unsure as to why this improves our S/N ratio, we assume it is because it diminishes the effect of low-frequency noise. (16) Yen, R.; Shank, C. V.; Hirlimann,C. Muter. Res. SOC.Symp. Proc. 1983, 13, 13. (17) (a) Vogler, A.; Kisslinger, J. J. Am. Chem. SOC.1982,204,231 1. (b) Vogler, A.; Osman, A. H.; Kunkely, H. Znorg. Chem. 1987, 26, 2337. (1 8) In C4,symmetry there should be more than one band in this region
(excluding the bridging cyanide stretch). Compounds of this type, however, often show only one IR active stretching vibration, perhaps because of their closeness to oh symmetry.
0.8-
400
500
600 700 Wavelength (nm)
aoo
Figure 3. Visible absorption spectra of Ru-Ru (solid line) and Os-Os (dashed line) dimers in H20.
Results
In our experiments we excite into the MMCT band of the dimer and probe the subsequent dynamics by monitoring spectral changes in the cyanide stretching region (1900-2200 cm-l) with the infrared probe pulse. Following excitation of the MMCT transition at 600 nm for Ru-Ru in DzO,we observe an instrumentlimited decrease in absorbance of the ground state infrared band followed by a recovery that is best fit by convolving the instrument response with a 6 f 1 ps exponential (Figure sa). Probing at higher frequencies reveals a new positive absorbance feature centered at 2110 cm-', which appears and decays within our instrumental resolution ( T C 0.5 ps, Figure 6). We also observe a new broad absorption at lower energies from the ground state band, appearing between 1950 and 2040 cm-l. Shown in Figure 7 are three kinetic traces taken throughout this region. The rise times of the transients at the low-energy side of this band are
J. Am. Chem.SOC.,Vol. 115, No. 14, 1993 6401
Study of Ultrafast Electron Transfer
I
I
(b) 1994 cm”
0.01
1
1850
I
1900
I
I
1950
2000
I
2050
I
2100
2150
Wavelength (cm”) Figure 4. Ground state FTIR spectrum of Ru-Ru in D2O.
(c) 2034 cm”
20
10
0
-10
0
10
20
Picoseconds Figure7. Transient decays obtained at (a) 1954, (b) 1994, and (c) 2034 cm-1 with 0.04 M Ru-Ru in D20 following 600-nm excitation.
Table I. Rise and Decay Times in Ru-Ru for the Best Fit to Data from 1954 to 2060 cm-1 (600-nm Excitation)CI wavelength (cm-I) rise time (ps) fall time (ps) 1954 0.1 0.7 1974 0.3 1.9 1994 0.6 3.6 2014 0.7 4.6 2034 1.4 5.9 0.0 6.9 20606 Picoseconds
Figure 5. Kinetic tracts of the ground state absorbances acquired at (a) 2060 cm-1 in Ru-Ru and (b) 2050 cm-1 in 0 4 s following excitation at 600 nm. Experimental traces are overlayed with calculated best fits to the data.
0 Values given correspond to the best fit obtained when convolvingthe instrument response (typically a 3 4 ps fwhm sech2 function) with a biexponential function; estimated relative error is AO.5 ps. Ground state bleach.
*
Picoseconds Figure 6. Transient decay obtained at 21 10 cm-l from 0.04 M Ru-Ru in D2O following 600-nm excitation. Offsets remaining at long times (see Figure 7 also) are due to thermal transients and do not affect the fits obtained.
3+ oxidation state. Such a change in oxidation state is expected to shift the ground state frequency to -2120 cm-l.I9 Thus, we assign the band centered at 2110 cm-1 in the transient spectrum to the initially populated MMCT excited state. The features between 1950 and 2040 cm-I are consistent with vibrationally hot ground state molecules formed upon return to the ground electronic state. Metal cyanide stretching vibrations exhibit anharmonicities of ca. 14 cm-l, leading to vibrationally excited states that are shifted to lower energy from the ground state band.20 The frequencies observed correspond to population of the u = 1-7 excited vibrational states. When the MMCT excitation wavelength is changed from 600 to 700 nm, we again observe an instrument limited decrease in absorbance at the ground state IR frequency with a similar recovery time as found for 600-nmexcitation. Due to interferences from coherence artifacts we were unable to conclusively observe the transient due to the MMCT excited state. We do observe production of vibrationally hot molecules of the ground electronic state. In the 700-nm case, however, we observe states u p to only u = 5. Decay times for these transients were found to be similar
instrumentally limited, but they become progressively slower toward the high energy side of the band. Additionally, the decay times of the transients increase from
