J. Phys. Chem. 1988, 92, 1771-1774
1771
Pressure Effects on the Intervalence-Transfer Electronic Absorption Band of the Mixed-Valence Creutz-Taube Ion in Various Mediat William S. Hammack,'S2 Michael D. Lowery,' David N. Hendrickson,*J and Harry G. Drickamer*JJ School of Chemical Sciences, Department of Physics, and Materials Research Laboratory, University of Illinois, Urbana, Illinois 61801 (Received: August 7, 1987)
In this paper we present the results of a pressure tuning spectroscopy (PTS) study of the intervalence-transfer (IT) band of the (C15.5H20) salt of the Creutz-Taube ion. We studied this compound in three media: (1) as a crystalline solid, (2) doped in the polyelectrolyte poly(styrenesu1fonic acid) (PSS), and (3) in water (D20). In all cases the principal effect of pressure on the IT band was found to be a shift of the band to higher energy. For the microcrystalline sample this shift was -400 cm-' over a pressure range from 0.001 to 200 kbar (1 kbar = 986.9 atm). In this same pressure range the blue shift of the IT band for the PSS-doped sample was -600 cm-I. In water (D20) the shift was -35 cm-' in a pressure range from 0.001 to 7 kbar. In addition to the Gaussian fits described above we have fit the spectra using a vibronic coupling model developed by Piepho, Krausz, and Schatz (PKS). When analyzed in terms of this simple small-polaron model, this peak shift corresponds primarily to an increase in the electronic coupling between the metal centers of the ion. The degree of vibronic coupling changes either only slightly or not at all with pressure.
Introduction Intramolecular electron transfer (IET) is one of the most fundamental and important chemical processes.3 Creutz and Taube4 and Cowan and Kaufmad pioneered the use of binuclear transition-metal mixed-valence complexes6to study IET. Perhaps the paradigm for a mixed-valence complex is the so-called Creutz-Taube (C-T) ion, a pyrazine-bridged complex of ruthenium with the structure r
lecular effects, an excitation from a bonding to an antibonding orbital should increase in energy with an increase in pressure. On the other hand, as a result of attractive van der Waals intermolecular interactions, a bonding to antibonding excitation localized on one molecular unit will generally shift to lower energy upon increasing pressure. This shift to lower energy as a result of intermolecular effects reflects the greater polarizability of the excited-state molecule with an electron in the antibonding orbital compared to the ground-state molecule. The strength of the van der Waals attraction, of course, depends also on the polarizability of the medium. These concepts are discussed in detail elsewhere."
L
I
In a mixed-valence complex, the ground state may be either delocalized (valence averaged) or localized (valence trapped). Although several salts of cation I have been studied by a battery of experimental techniques and theoretical models,'J intense controversy rages about whether the ground state of the C-T ion is trapped or detrapped. A distinguishing characteristic of the electronic spectrum of a binuclear mixed-valence complex is a low-energy band that cannot be attributed to either metal center alone, since it arises when electron transfer is photoinduced from one metal center to the other. For the C-T ion this intervalence transfer (or IT) band maximum occurs at -6400 ~ m - ' . ' ~In this paper we present the results of a pressure tuning spectroscopy (PTS) study of the IT band of the (Cl5dH20)salt of cation I. We studied this compound in three media: (1) as a crystalline solid, (2) doped in the polyelectrolyte poly(styrenesu1fonic acid) (PSS), and (3) in water (D,O).In all cases the principal effect of pressure on the IT band was found to be a shift of the band to higher energy. When analyzed in terms of a simple small-polaron modelg, this peak shift corresponds primarily to an increase in the electronic coupling between the metal centers of the ion. The degree of vibronic coupling changes either only slightly or not at all with pressure. PTS has been shown to be effective in studying the energy perturbations on valence orbitals resulting from pressure-induced compression of condensed phases.1° Compression of crystals consisting of molecular units or large molecular ions leads to a reduction in interatomic and intermolecular distances which increases the overlap of valence orbitals. An electronic absorption band for a molecular moiety will shift either to higher or lower energy, dependent on the balance between intramolecular and intermolecular effects of compression. In the case of intramo'This work was supported in part by the Materials Science Division, Department of Energy, under Contract DE-AC02-76ER01198 and in part by NIH Grant HL13652.
0022-365418812092-1771$01.50/0
Experimental Section Solid-state electronic absorption spectra were recorded with the samples loaded into an Inconel-gasketed diamond anvil cell, (1) School of Chemical Sciences. (2) Department of Physics and Materials Research Laboratory. (3) (a) DeVault, D. C. Quantum-Mechanical Tunnelling in Biological Systems; Cambridge University Press: Cambridge, 1984. (b) Chance, B.; DeVault, D. C.; Frauenfelder, H.; Marcus, R. A.; Schrieffer, J. B.; Sutin, N., Eds. Tunneling in Biological Systems; Academic: New York, 1979. (c) Reynolds, W. K.; Lumry, R. W. Mechanisms of Electron Transfer; Ronald: New York, 1966. (d) Cannon, R. D. Electron Transfer Reactions; Butterworths: London, 1980. (e) Mikkelsen, K. V.; Ratner, M. A. Chem. Reu. 1987, 87, 113. (4) Creutz, C.; Taube, H. J. Am. Chem. SOC.1969, 91, 3988. (5) Cowan, D. 0.; Kaufman, F. J . Am. Chem. SOC.1970, 92, 219. (6) (a) Day, P. Inf.Rev. Phys. Chem. 1969, I , 149. (b) Brown, D. B., Ed. Mixed-Valence Compounds, Theory and Applications in Chemistry, Physics, Geology, and Biology; Reidel: Dordrecht, 1980. (c) Creutz, C. Prog. Inorg. Chem. 1983, 30, 1. (7) (a) Beattie, J. K.; Hush, N. S.; Taylor, P. R. Znorg. Chem. 1976, 15, 992. (b) Hush, N. S. In Mixed-Valence Compounds; Brown, D. B., Ed.; Reidel: Dordrecht, 1980; p 151. (c) Furholz, U.; Burgi, H.-B.; Wagner, F. E.; Stebler. A.; Ammeter, J. H.; Krausz, E.; Clark, R. J. H.; Steed, M. J.; Ludi, A. J. Am. Chem. Soc. 1984,106, 121. (d) Krausz, E.; Ludi, A. Znorg. Chem. 1985, 24, 939. (e) Furholz, U.; Joss, S.; Burgi, H.-B.; Ludi, A. Znorg. Chem. 1985, 24, 943. (8) (a) Ondrechen, M. J.; Ellis, D. E.; Ratner, M. A. Chem. Phys. Lett. 1984. 109. 50. Ib) KO. J.: Ondrechen. M. J. J . Am. Chem. SOC.1985. 107. 6161: (c)'Zhang,'L.-T.; KO,J.; Ondrechen, M. J. J. Am. Chem. SOC.1987; 109, 1666. (d) Ondrechen, M. J.; KO, J.; Zhang, L.-T. J. Am. Chem. SOC. 1987. 109. 1672. (9) (a)'Piepho, S. B.; Krausz, E. R.; Schatz, P. N. J. Am. Chem. Soc. 1978, 100,2996. (b) Wong, K. Y.; Schatz, P. N.; Piepho, S. B. J. Am. Chem. Soc. 1979, 101, 2793. (c) Schatz, P. N. In Mixed-Valence Compounds; Brown, D. S., Ed.; Reidel: Dordrecht, 1980; p 115. (10) (a) Drickamer, H. G. Acc. Chem. Res. 1985, 18, 355. (b) Drickamer, H. G. Annu. Rev. Phys. Chem. 1982.33.25. (c) Drickamer, H. G. Znt. Reu. Phys. Chem. 1982,2, 171. (d) Weber, G.; Drickamer, H. G. Q.Rev. Biophys. 1982, 16, 89. (1 1) Drickamer, H. G. In High Pressure Chemistry and Biochemistry; van Eldik, R.; Jonas, J., Eds.; Reidel: Dordrecht, 1987; p 263. ~
0 1988 American Chemical Society
Hammack et al.
1772 The Journal of Physical Chemistry, Vol. 92, No. 7 , 1988 1
,
1
1
I
l
l
I
1
I
(Erystolline)
TABLE I: Data for the Near-IR IT Absorption Band of the (CIS-5H20)Salt of the Creutz-Taube Ion (Microcrystalline)
peak max, press, kbar
€*
x
-5.10 -5.85 -5.80 -5.80 -5.80 -5.90 -6.60 -6.60 -6.60
2.15 2.15 2.75 2.15 2.15 2.75 2.75 2.15 2.15
-5.70 -5.80 -5.85 -5.90 -5.90 -5.95 -6.00 -6.10
2.75 2.15 2.75 2.75 2.15 2.75 2.75 2.75
cm-I X
Run No. 1
PKS Fit
, 1c
12
14
Quanta (one auantum
16
1.4 23 48 13 90 114 138 163 190
6.21 6.32 6.38 6.39 6.44 6.49 6.55 6.61 6.61
4 19 40 70 90 100 123 141
6.33 6.44 6.40 6.42 6.48 6.51 6.52 6.66
500 cm-1)
Run No. 2
(Crystalline) 138 Kbar
Fit
Fit with a Gaussian line shape. Units of hij = 500 cm-l, tabulated to nearest 0.05 unit of hij.
TABLE 11: Data for the Near-IR IT Absorption Bend of the *‘\\k (Cl&H20) Salt of the Cruetz-Taube Ion Doped in e-10
14
16
18
-..--.e-
Quanta (one quantum = 500 cm-1)
Figure 1. (Top) Near-IR electronic absorption spectrum at 23 kbar of the (C1S.5H,0) salt of microcrystalline Creutz-Taube ion. (Bottom) Near-IR electronic absorption spectrum at 138 kbar of the (CIs.5H20) salt of microcrystalline Creutz-Taube ion.
using the ruby fluorescence method of pressure calibration.I2 A 100-W Oriel quartz tungsten halogen lamp with a Kratos quartermeter monochromator using a 2100 blaze grating at a slit width of 1.00 mm was used. Transmitted light was detected by an Opto-Electronics PbS photoconductor. Light pipes were used to connect the diamond anvil cell to the monochromator and the PbS cell. The PbS cell was operated with a 100-V dc bias. A light chopper was placed between the light source and the monochromator. The AC component of the PbS cell was measured with a PAR Model 8 lock-in amplifier. Solution absorption spectra were taken by placing the D20 solution (0.92 mM) of compound I in a stainless-steel inner cell with pistons containing sapphire windows.13 This cell was placed in a larger bomb which was filled with the pressurizing fluid, fluorotrichloromethane. The PbS photoconductor system described above was used to record the spectra. The (C15.5H20)salt of compound I was prepared as described by Creutz and Taube.I4 Compound I was studied dissolved in poly(styrenesu1fonic acid sodium salt) (PSS).The PSS thin film was formed by codissolving I and PSS in water and subsequently evaporating the solution with a spin coater. The concentration of compound I in the polymer was 2.5% by weight. Compound I was also studied as a crystalline solid with mineral oil as a pressure-transmitting medium. Electronic absorption spectra were least-squares fit to Gaussian peaks by using the computer program SKEW. Results and Discussion Origin of Pressure Shijt of the IT Band. The IT band of the (C15.5H20) salt of compound I occurs in the near-IR at -6400 cm-I. We have studied the shift of this IT band with pressure in three media: (1) microcrystalline, (2) doped in poly(styrene(12) Barnett, J. D.; Block, S.; Piermanini, C. J. Rev. Sci. Insfrum. 1973, 44, 1.
(13) Okamato, B. Y. Ph.D. Thesis, University of Illinois, Urbana, IL, 1974. (14) Creutz, C.; Taube, H. J . Am. Chem. SOC.1969, 91, 3988.
Polr(stvrenesu1fonic acid)
peak max, cm-’ x Run No. 1
Dress. kbar 5.5 19 32 50 73 91 118 134
6.46 6.41 6.54 6.59 6.67 6.16 6.18 6.82
I 26 54 84 91
6.45 6.51 6.63 6.68 6.15
15 53 120 150 163 195
6.49 6.65 6.81 6.83 6.86 6.96
€b
x
-5.85 -5.90 -5.95 -6.05 -6.10 -6.20 -6.20 -6.25
2.75 2.75 2.75 2.15 2.75 2.15 2.15 2.15
-5.90 -5.95 -6.05 -6.15 -6.20
2.15 2.15 2.15 2.15 2.15
-5.90 -6.15 -6.20 -6.25 -6.30 -6.30
2.75 2.15 2.15 2.15 2.80 2.85
Run No. 2
Run No. 3
“Fit with a Gaussian line shape. bunits of to nearest 0.05 unit of he.
hij
= 500 cm-I, tabulated
TABLE I11 Data for the Near-IR IT Absorption Band of the (CI,-5HzO) Salt of the Creutz-Taube Ion Dissolved in D,O
peak max, press, kbar
press, kbar
cm-’ X
peak max, cm-’ X lo-)”
Run No. 1 0.0 1 .o 2.0 3.0 4.0
6.42 6.42 6.43 6.43 6.43
1.o 2.0 3.0 4.0
6.43 6.43 6.44 6.44
5.0 6.0 6.5 1.0
6.44 6.44 6.44 6.45
5.0 6.0 7.0
6.45 6.46 6.41
Run No. 2
Fit with a Gaussian line shape.
The Journal of Physical Chemistry, Vol. 92, No. 7, 1988 1773
Pressure Effects on IT Band of Creutz-Taube Ion
0
IN PSS, ~ 0 . 6 4 0 0cm-1
200 - 0 Crystalline,
v, = 6 3 0 0 c m - l
0.
0 0
150 -
e
0
-
l
'
l
'
l
'
l
'
0
0 U
0
-
--
0
0
0
100-
ui -
Quanta (one quantum = 500cm-1)
0
0
.-I
6 -
0
0
0
0-
50 -
uo
0
0.
l
a
(PSS)
0
150 Kbar
o&0
.Gaussian
I
5o
I
100
1
1
150
I
200
Pressure (Kbar) Figure 4. Pressure dependence of the PKS electronic coupling parameter
Fit
c
at some applied pressure compared to the electronic coupling parameter
at ambient pressure co for microcrystallineand PSS-doped Creutz-Taube ion (C15.SH20).
Figure 2. (Top) Near-IR electronic absorption spectrum at 19 kbar of the (CIS.5H20) salt of the Creutz-Taube ion doped in poly(styrenesulfonic acid). (Bottom) Near-IR electronic absorption spectrum at 150 kbar of the (C15.5H20)salt of the Creutz-Taube ion doped in poly(styrenesulfonicacid).
,
6001
I
I
1
I
I
I
IN PSS, v0=6400 cm-1 0 Crystalline, vo =6300cm-1
1
0
5001
B400t
0 0 0
0
0
4- 300
0
0
0
L .-
r
I
l0OL
0
0
-O
I
U
U I
0
Pressure
I
I
I
(Kbar)
Figure 3. Pressure dependence of the intervalence band maximum of microcrystalline and PSS-doped Creutz-Taube ion.
sulfonic acid) (PSS), and (3) in a water solution. In all three media the effect of pressure was to shift the IT band to higher energy. A tabulation of the peak positions at each pressure and for each medium appears in Tables 1-111. Typical spectra for the microcrystalline sample and the PSS-doped sample are presented in Figures 1 and 2. In Figure 3 is presented the shift of the IT band maximum over a pressure range of 0.001-200 kbar (1 bar = 0.9869 atm) for the PSS-doped sample and the microcrystalline sample. Note that the shift of the polymer-doped material is roughly twice that of the microcrystalline sample. The IT band shifted to higher energy by 35 cm-' in 10 kbar when the
C-T ion was dissolved in D20. In all media the IT band intensity did not change significantly. The peak shifts are probably best described in terms of the vibronic coupling model discussed below. Nevertheless it is useful to discuss them briefly in terms of the balance between intra- and intermolecular forces mentioned earlier. Ondrechen et al. suggested that an IT transition for a complex with a delocalized ground state is best described as a bonding to a less bonding transition rather than as a metal-to-metal charge transfer.* Ondrechen et al. pointed out that in a qualitative way the three electronic states of the C-T ion may be pictured as bonding, nonbonding, and antibonding orbitals resembling the three x molecular orbitals of the Hiickel allyl radical. In the C-T ion the bonding orbital is fully occupied and the nonbonding orbital is half-occupied. What is called an IT transition could also be viewed as a bonding-to-nonbonding transition in which charge density is transferred from the bridging ligand to the metal ions. In any case the excited state is probably more polarizable than the ground state, but the difference may not be large. The blue shift observed indicates the predominance of intramolecular interaction. The smaller blue shift for the crystal may result from a larger polarizability and therefore larger van der Waals interaction in this medium. It could also be that the polymer is more compressible than the crystal so that a given pressure corresponds to a larger perturbing effect of compression on the bond lengths for the polymeric medium. Analysis of the IT Bands in Terms of a Vibronic Coupling Model. In addition to the Gaussian fits described above we have fit the spectra using a vibronic coupling model developed by Piepho, Krausz, and Schatz (PKS). Details of this PKS vibronic coupling model are given el~ewhere.~ The PKS model is a two-site, one-dimensional model that uses two parameters to describe the IT band of mixed-valence dimers. The two parameters are the electronic-exchange coupling between the two metal centers (c) and the coupling between the electronic and nuclear motion on each site, X. X is directly proportional to the difference in the equilibrium value of the alg monomer normal coordinate in the two different oxidation states. Solving the PKS model yields a complete set of vibronic energies and eigenfunctions which can be used to calculate the IT band absorption profile. By use of a high-speed computer with interactive graphics, c and X can be varied until the calculated spectrum matches the observed spectrum. The method we have used is precisely as described by Schatzgc All energies used in this model are scaled by the energy (ho) of the totally symmetric metal-ligand stretching of the subunits. For the C-T ion Schatz et al. used hij = 500 cm-I; we
J . Phys. Chem. 1988, 92, 1774-1777
1774
have used the same value in our c a l c ~ l a t i o n s . ~ Typical fits for the microcrystalline and the PSS-doped sample are shown in Figures 1 and 2. All fits are very similar to the ones presented by Schatz in Figure 4 of ref 9c. Listed in Tables I and I1 are the values for t and X at each pressure (0.OOl-200 kbar) and for each medium. Presented in Figure 4 is the change of t with pressure for the PSS-doped sample and the microcrystalline sample. In solution there was little change (-0.05) of t in 10 kbar, while h did not change at all. Perhaps the most interesting feature of Figure 4 is how the change of t parallels very closely the shift in the IT band maximum with pressure. This behavior is similar to what we have seen for other mixed-valence compounds. The tendency for h to depend little on pressure is reasonable since X is directly proportional to the difference in the metal-ligand bond lengths in the two different oxidation states. Since bond compressibilities are in general an order of magnitude less than intermolecular (bulk) compressibilities, the difference in compressibility of the Ru(I1)-ligand and Ru(1II)-ligand bonds is likely to be quite small. Within the context of the PKS model we can conclude that the primary effect of pressure on the C-T ion is to increase the amount of electronic coupling between the metal moieties and to change the vibronic coupling only slightly. Comments and Conclusions
The main conclusions from this PTS study of the IT band of
the (C1,.5H20) salt of the Creutz-Taube ion are fourfold. (1) In all media [microcrystalline, doped in the polyelectrolyte poly(styrenesu1fonic acid), and in water] the principal effect of pressure is to shift the IT band to higher energies with increasing pressure. (2) Within the context of the PKS vibronic coupling model this blue shift corresponds primarily to an increase in the amount of electronic coupling between the transition-metal moieties of the C-T ion. (3) The degree of vibronic coupling changes either only slightly or not all all with pressure. (4) IEI/X' increases with pressure but is never greater than -0.85. It appears that the Cruetz-Taube ion is borderline between the trapped and detrapped states at the highest pressures. Although we fit our data to the simple PKS theory, it should be remarked that recent electronic absorption results indicate that the IT band contour for salts of the C-T ion is comprised of several electronic transitions. Furholz et al.7cstudied the electronic absorption spectrum of an X-polarized single crystal of the (CI,. 5 H 2 0 ) salt at 10 K and resolved three or more bands. In fact, close examination of our IT band contours for the (C1,.5HZO) salt of the C-T ion in either a crystalline (Figure 1) or doped in PSS polymer (Figure 2) environment shows that there is a sensitivity of the IT band contour to environment. The line width and position of the "shoulder" on the IT band change from one environment to another. It seems likely that a theory more complicated than the PKS or Ondrechen models, particularly a theory incorporating the effects of spin-orbit interactions, is needed.
(15) Sinha, U.; Lowery, M. D.; Hammack, W. S.; Hendrickson, D. N.; Drickamer, H. G. J . Am. Chem. SOC.1987, 109, 7340.
Registry No. I.5C1, 94780-98-0.
Methine and Methyl Stretchlng Vibrations in Substituted Phenylethanes Prasad L. Polavarapu* and Howard E. Smith* Department of Chemistry, Vanderbilt University, Nashville, Tennessee 37235 (Received: August 10, 1987)
Raman and infrared spectra of several phenylethanes were measured so as to identify the stretching vibrations of methine and methyl groups. These identifications are facilitated by the deuteriation of the methine group in all of the molecules considered and of the methyl group and phenyl group in 1-phenylethanol. The methine stretching vibrational frequency and its intensity are found to be significantly affected by the substitution at the carbon atom to which the methine hydrogen atom and methyl group are directly attached.
Introduction Vibrational bands of methine and methyl groups are generally believed to be assignable in a straightforward manner. Empirical vibrational assignments for a large number of benzene derivatives are available in literature,' but most of those assignments need to be verified through isotopic substitution studies. Such verification is becoming increasingly important as new types of spectroscopic analysis emerge. Vibrational circular dichroism (VCD) is one such area where the interpretation of the observed CD bands need2 correct vibrational assignments. In the last few years we have investigated the VCD in 1-substituted phenylethanes, with particular attention to the bending vibrations involving the methine group.3 To facilitate the identification of the bands assignable to such bending vibrations, the corresponding compounds with the methine hydrogen atoms replaced by deuterium atoms have been synthesized in our laboratories. In this present paper we report the Raman and infrared spectra in the C-H and C-D stretching regions of a series of 1-substituted (1) Varasanyi, C. Assignments for Vibrational Spectra of Seven Hundred Benzene Derivatives; Wiley: New York, 1974. (2) Freedman, T. B.; Balukjian, G. A.; Nafie, L. A. J . Am. Chem SOC. 1985, 107, 6213. (3) Polavarapu, P. L.; Fontana, L. P.; Smith, H. E. J. Am. Chem SOC 1986, 108, 94.
0022-3654/88/2092-1774$01.50/0
phenylethanes: C6HjCH(CH3)R,C6H5CD(CH3)R;R = OH, NH2, SH, and CI. Also included are other deuteriated 1phenylethanols and benzylamine: C6H5CH(CD3)0H,C6DjCH(CH3)OH, and C6HjCHDNH2. The results indicate confirmation for some and disagreement for other literature112assignments. The revised assignments offered here are useful for the analysis of the C-H stretching VCD of 1-substituted phenylethanes2 and also for the analysis of the C-H stretching overtone bands4 in related molecules. Experimental Details
The infrared spectra were obtained on a Fourier transform infrared spectrometer (Nicolet 6000C) as neat liquids, and Raman spectra were obtained on a multichannel Raman spectrometer, also as neat liquids. The Raman band positions were calibrated with respect to indene bands in the C-H stretching region and with respect to acetone-d6 bands in the C-D stretching region. Two deuteriated compounds, C6DjCH(CH3)OHand C ~ H S C H (CD,)OH, were obtained from Cambridge Isotope laboratories and Merck, Sharp and Dome Isotopes Ltd. and had 99% and 98% deuterium incorporation, respectively. 1-Phenylethanol-I -d was (4) Khalique Ahmed, A,; Swanton, D. J.; Henry, B. R. J . Phys. Chem. 1981, 91, 293.
0 1988 American Chemical Society
