J. Phys. Chem. 1987, 91, 251-254
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mences when the cis content reaches the critical value which can trigger the transformation. Both spiropyran and azobenzene are well-known photochromic chromophores. Recovery of the original state either by thermal or by photochemical process is principally possible. Although the stability of these chromophores, in particular the spiropyran in the present form, is unsatisfactory, the transformation can be demonstrated repeatedly, but with fatigue. The present experiments are a clear demonstration that reading a photoimage by physical changes, such as a phase change brought about by photochemical reaction, is more sensitive than by detecting direct photochromic phenomena.
, 0.2 0.4 0.6 AABS at L50 nm
0.8
Figure 5. Change in light scattering intensity of photoirradiation of micelles of trans-11. (11) = 5.1 X lo4 M. Cis content is expressed by the increment of absorbance at 450 nm. Photoirradiation was performed at around 360 nm at 25 OC.
Although we have not yet confirmed the shape of aggregates by scanning electron microscopy, the results of differential scanning calorimetry clearly indicate the change in aggregation state. The trans micelle showed a main endothermic peak at 14.2 "C (AH = 1.0 kcal/mol), corresponding to a gel-liquid crystal phase transition whereas the transition temperature for the cis micelle appeared at 11.9 'C (AH = 0.8 kcal/mol), as shown in Figure 4. This is unequivocal manifestation that the aggregation of cis micelle is weaker than that of the trans micelle, in accordance with the smaller aggregation number of the cis micelle. From the RBvalue of the trans isomer, we estimated the apparent molecular weight to be 1.1 X lo6, corresponding to an aggregation number of 1600. The cis form is more unstable, and we could not measure the dn/dc (differential index of refraction) value accurately. However, there is no doubt that the decreased cis isomerization. size of aggregation is due to trans Photoirradiation of the trans form aggregate, therefore, brings about the transformation of the large disk form to a smaller size accompanied by decreased light scattering intensity. As shown in Figure 5 , the change in RBis nonlinearly related to the cis content expressed by the increase in absorbance at 450 nm, a characteristic absorption band of cis-azobenzene. This nonlinearity is attributed to the intrinsic nature of a phase transformation occurring at a critical condition. A sudden change in RBcom-
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Experimental Section Materials. The cationic surfactants (I) were prepared by the method used in the preparation of the C1 analogue.6 Analytical data are as follows. Elemental analysis [obsd (calcd for the dihydrate form, C32H4205N3Cll)], %: Ia (R = CsH17,mp 147-149 "C): C, 65.82 (65.80); H , 6.92 (7.25); N, 7.27 (7.19). Ib (R = Cl2HZ5,mp 121-125 "C): C, 67.68 (67.54); H, 7.65 (7.87); N , 6.64 (6.65). IC(R = Cl8H3,, mp 160-162 'C): C, 69.57 (69.64); H, 8.81 (8.63); N, 5.78 (5.80). The azobenzene containing I1 was prepared by the method reported by Okahata.3 Analytical data agreed. Photoreaction. The sample solution was irradiated by monochromatized light from a 500-W xenon lamp at 25 f 1 'C. The reaction was followed spectroscopically with a Hitachi UV-320 spectrometer and a Shimadzu UV-ZOOSspectrometer. Physical properties (surface tension, light scattering intensity, and refractive index) were measured by a surface tension meter (Kyowa CBVP A l ) , a low-angle laser light scattering photometer (Chromatix KMX-6), and a differential refractometer (Union Giken, RM102), respectively. Acknowledgment. This work was partly supported by a Grant-in-Aid on Special Project Research for "Highly Efficient Photochemical Process" from the Ministry of Education, Science and Culture. ( 6 ) (a) Taylor, L. D.; Davis, R. B. J . Org. Chem. 1963,28, 1713. (b) Taylor, L. D. US.Patent 3320067, 1967.
Intramolecular Electron Hopping In Reduced Ru Complexes. ESR Hyperfine for a Mixed Ligand Complex J. N. Gex, M. K. DeArmond,* and K. W. Hanck Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204 (Received: September 2, 1986; In Final Form: December I , 1986)
The ESR spectra for the one- and two-electron reduction products of [R~(bpy)~(bpz)]~+ as well as the two-electron reduction product of [ R ~ ( b p z ) ~ ( b p y )exhibit ] ~ + hyperfine structure. The simulation of these spectra indicates that a small amount of spin density resides on the Ru nucleus. Moreover, spectra of the one-electron reduction product of [Ru(bp~)~(bpy)]~+ and the one- and two-electron reduction products of [ R u ( b p ~ ) ~verify ] ~ + the intramolecular electron hopping model previously proposed to rationalize temperature-dependent line broadening. The effects of temperature and structural perturbation for these mixed ligand complexes upon the resolution of the ESR spectra permit speculation about the absence of hfs in complexes as [Ru(L-)J for which a high barrier should prevent electron hopping.
Introduction This Letter reports a well-resolved hyperfine structure (hfs) for the oneelectron reduction product of the mixed ligand complex [Ru(b~y)~bpz]z+. The simulation of the ESR spectrum for the one-electron complex and the free ligand radical anion enables the perturbing effect of the metal ion to be &mated. The absence of a temperature-dependent line broadening for the mixed ligand 0022-3654/87/2091-0251$01.50/0
[R~(bpy)~,(bpz),](~-")+(where n is the number of redox electrons, = 1, 2, 3) in which the barrier to intramolecular hopping can be estimated to be large as well as the Presence of the line broadening for those complexes in which the barrier is estimated to be small verifies the model proposed to rationalize the temperature-dependent ESR line broadening found for reduced RUL3" and [oS(bPY)312+ X
0 1987 American Chemical Society
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The Journal of Physical Chemistry, Vol. 91, No. 2, 1987
Letters
The series of reduced products generated by the controlled reduction of [ R ~ ( b p y ) ~ ](and ~ + its analogues) have unique properties best explained by the concept of a "spatially isolated redox ~ r b i t a l " .Thus, ~ for example, the first electron resides in a redox orbital localized on one of the three near degenerate and noncommunicating orbitals available for [ R ~ ( b p y ) ~ (eq ] ~ +1) rather than in a delocalized orbital (eq 2). This "localized" orbital [ R u ( b ~ y ) ~+ ] ~e-+
-
-05
-10 -15 -20 E (Volt vs SCE)
-15
-10
SCE)
E(Volt vs
-20
1
[R~(bpy)~-l/~]+
(2) behavior is verified by the cyclic voltammetry,5,6optical spect r o s ~ o p y , ~resonance -'~ Raman and ESR spectra'-4 of the stable reduced species. Key to this model of the reduced species is the ESR results since these data detail not only the stationary state phenomenon but also give insight into the dynamic aspects of the problem. For example, the ESR of the one-electron reduction product, [Ruis that of an S = species with g 2 (indicating (bp~)~(bpy-)]+, minimum Ru contribution in the orbital of the unpaired electron). A temperature-dependent line broadening of the sort observed for most one- and two-electron reduction Ru(I1) products in a variety of solvents is never observed for the three-electron Ru complex. This line broadening has been ascribed to an electron hopping between the identical ligands involving the degenerate localized orbitals. Therefore, the absence of line broadening for the three-electron species results from the fact that all three orbitals contain an electron and hopping is slow since the activation barrier is high. Since hopping is slow, hyperfine structure is expected for these tris three-electron species, in particular since a wellresolved hfs is observed for the free ligand bpy-. However, to date only two such hfs have been reported for complexes, the first for the [Fe(bpy-),]- species2 and the second by Kaim for a Ru dimer complex.I2 The magnitude (10-20 G) of the intrinsic fluid solution line width (from the three-electron products) and that of the low-temperature one- and two-electron spectra is consistent with that expected for a ligand localized K radical where the McConnell equation for hydrogen and nitrogen nuclei predicts a peak-to-peak derivative line width in this range. Since the broadening in the fluid solution temperature region for some of these reduced species gives peak-to-peak line widths over 100 G with no narrowing as a function of temperature apparent, a T, type mechanism must be dominant, and the source of hfs loss. However, a T2 process involving solvent motion or other fluctuation could result in the loss of hfs when the TI process is absent, as for the three-electron species. The measurement of hfs for these reduced species is necessary to answer the questions: (1) Is electron hopping a proper description? (2) Does an additional dynamic process (T,) exist? The ability to construct an empirical wave function for these systems from the hfs would allow a more quantitative estimate of spin density for the complex ion vs. the ligand. Consequently, the role, if any, that the metal plays in the hopping process (and the localization) can be assessed.
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(1) Motten, A.; Hanck, K. W.; DeArmond, M. K. Chem. Phys. Left.1981, 79, 541. (2) Morris, D.; Hanck, K. W.; DeArmond, M. K. J. Am. Chem. Soc. 1983, 105, 3032. ( 3 ) Morris, D.: Hanck, K. W.; DeArmond, M. K. Inorg. Chem. 1985,24, 977. (4) DeArmond, M. K.; Hanck, K. W.; Wertz, D. W. Coord. Chem. Rev. 1985, 64, 65. (5) Ohsawa, Y.; DeArmond, M.K.; Hanck, K. W.; Morris, D. E.; Whitten, D. G.; Neveux, Jr., P. E. J. Am. Chem. SOC.1983, 105, 6522. (6) Ohsawa, Y.; Hanck, K. W.; DeArmond, M. K. J. Electroanal. Chem. 1984, 175, 229. (7) Heath, G . A,; Yellowlees, L. J.; Braterman, P. S.J. Chem. Soc., Chem. Commun. 1981, 287. ( 8 ) Elliot, C . M.; Hershenhart, E. J. Am. Chem. SOC.1982, 104, 7579. (9) Donohoe, R. J.; Tait, C. D.; DeArmond, M. K.; Wertz, D. W. Spectrochim. Acta, Part A 1986, 42A, 233. (10) Tait, C . D.; MacQueen, D. B.;Donohoe, R. J.; DeArmond, M. K.; Hanck, K. W.; Wertz, D. W. J. Phys. Chem. 1986, 90, 1766. (11) Donohoe, R. J.; Angel, S.M.; DeArmond, M. K.; Hanck, K. W.; Wertz, D. W. J. Am. Chem. SOC.1984, 106, 3688. (12) Kaim, W.; Ernst, S.; Kohlmann, S.;Welkerling, P. Chem. Phys. Lett. 1985, 118, 431.
MO
I
MO
J
Figure 1. Cyclic voltammograms of (a) [ R u ( b p ~ ) ( b p y ) ~ ] ~(b) ' , [Ru(bpz),12+ in 0.1 M TBAH/DMF, and (c) qualitative energy levels for an ML2L' and M L 3 molecule. TABLE I: ESR Parameters of the Bipyrazine Radical Ion (bpz-) and of [ R ~ ~ P Y ) & P Z ] + "
Q-(g b PZ
a bpz-
0.33 0.55 0.09 0.05
nX
2N 2H 2H 2N
U
1,l' 5,5'
3,3' or 6,6' 4,4'
g = 2.0045 Ru(bpy)2(bp~)+
0.47 0.47 0.55
2N 2H
1,l' $5'
1Rub
g = 1.9931 "Coupling constant are given in m T (temp = 298 K, solvent 0.1 M TBAH/DMF). b 9 9 R ~I, = 5/2, natural abundance 12.72%, p = -0.6430. 'O'Ru, I = 5/2, natural abundance 17.07%, p = -0.7207.
Experimental Section Materials. The 2,2'-bipyridine (bpy) and 2,2'-bipyrazine (bpz) were purchased from Aldrich. The tris bpz ruthenium complex was synthesized as described in the l i t e r a t ~ r e , 'while ~ the mixed ligand complexes were made by first preparing the bis ligand ruthenium dichloride intermediate.14 All complexes were precipitated as the PF6 salt, recrystallized from either methanol or acetone and dried under vacuum. Tetrabutylammonium hexafluorophosphate (TBAH) was prepared by the metathesis of the corresponding perchlorate and was purified by recrystallization from water and methanol. D M F (Fisher Scientific) was dried according to the procedure previously r e ~ 0 r t e d . l ~ Cyclic voltammetric measurements and bulk electrolysis were carried out under a nitrogen atmosphere. Electrochemical apparatus and procedure have been reported previously.I6 Electron spin resonance spectra were obtained with a Bruker spectrometer from the ER-200 series at the frequency of 9.5 GHz (X-band). Simulation was performed on a AT&T 6300 using SIMESR."
Results and Discussion If an electron hopping process does cause the temperaturedependent line broadening then a necessary condition for the (13) Crutchley, R. J.; Lever, A. B. P. Inorg. Chem. 1982, 21, 2276. (14) Belser, P.; Zelewsky, A. V. Helu. Chim. Acta 1980, 176, 1675. (15) Angel, S. M.; DeArmond, M. K.; Donohoe, R. J.; Wertz, D. W. J. Phys. Chem. 1985, 89, 282. (16) Morris, D. E.; Hanck, K. W.; DeArmond, M. K. J . Electroanol. Chem. 1983, 149, 115. (17) D a d , C . University of Fribourg, Switzerland.
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The Journal of Physical Chemistry, Vol. 91, No. 2, 1987 253
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observation of hfs must be that this hopping rate be decreased. However, this condition obviously was not sufficient for the observation of hfs for [ML31n+type species since only in one case had hyperfine been observed and here hyperfine could be observed only over a narrow temperature range., A spectral density and/or inhomogeneous line broadening could preclude observation of the reduction hfs for the [ML,-] species. So, the [Ru(bpy),bp~]~+ products were examined since the cyclic voltammetry of this complex (Figure 1) and the parent [ R ~ ( b p z ) ~predicts ] ~ + that the first electron is localized in the bpz unit. Moreover, since MI,*, the difference in peak potential for the first two reversible waves, is large, no electron hopping should occur. The presence of four nitrogens in the ligand was intended to diminish the spectral density by introducing (nearly) magnetically equivalent nuclei. However, the simulation (Figure 2b, Table I) indicates that this was not accomplished since the simulation indicates that the two nitrogens not bonded to the Ru have minimal spin density but that the hydrogens on positions 5 and 5' have nearly equivalent magnitude splitting to the coordinating nitrogens resulting in the relatively simple 9-line ("Ru I = 0) spectrum observed at room temperature. A surprising feature is that the spin '/, lolRu and 99Ru nuclei produce additional lines (1 3-line pattern) thus indicating a small spin density on the Ru nucleus. While a few percent Ru spin density would likely be consistent with the small g anisotropy measured, any larger electron density would not be readily rationalized. The hyperfine pattern for the free bpz ligand is complicated and so the N and 5 position hydrogen coupling cannot be precisely estimated but the shape of the spectrum does indicate that the magnitude of the N and H coupling constants must be reversed from the complex, a result that it not too surprising in view of the polarization of the spin density by the positively charged Ru ion. The fluid solution ESR spectra at -54 OC in DMF is not resolved, likely due to a small g factor or hfs anisotropy; only at higher temperatures can well-resolved spectra be obtained. However, the integrated (low-resolution) peak-to-peak line width is essentially invariant over the fluid solution temperature range indicating the absence of the TI line broadening effect attributed
I
V
I
Figure 2. Experimental (a) and simulated (b) spectra of [Ru(bpz-)( b ~ y ) ~ in ] +0.1 M TBAH/DMF at 298 K. Parameters used for the simulation are those of Table I.
__ 227K
Figure 3. ESR spectra (a) of [Ru(bpz-)(bpz)(bpy)]*at 227, 260, and 300 K in 0.1 M TBAH/DMF and (b) ESR spectrum of [Ru(bpz-),(bpy)]" in 0.1 M TBAH/DMF at 300 K. TABLE 11: Activation Energies for Electron Hopping (AI1 Measurements in DMF)
compd [RU(~P~-)(~PZ)Z~+ [W~PZ-)~(~PZ)~I [Wbpz-)(bpz) ( ~ P Y1' )
m. cm-' 7 20 510 520
to electron hopping. The peak-to-peak contour of the low-resolution line is comparable to that of the complex species [Ru(bpy-)J for which no hfs is observed. However, the total spectral width is, due to the two 5 / 2 spin Ru isotopes, almost double the width of the typical light atom heterocycle radical (-30 G). Nevertheless, no temperature-dependent broadening of the hyperfine components can be measured so no T, process corresponding to inter-ring bending or solvent-solute interaction can be identified. A variety of solvent and concentration effects upon the ESR of other reduced Ru complexes have been identified and will be reported later.I8 The ESR of the two-electron species is very similar to that of the one-electron spectrum with well-resolved hfs and no temperature-dependent broadening. Since the simple model and the voltammetry predicts that the second electron is localized in a bpy ring, this spectrum might be expected to be a composite of the well-resolved one-electron and a broad-line spectrum for the appearance rather Ru-bpy- species. Nevertheless, the S = than S = 1 or S = 0 is consistent with the spatially isolated model. The three-electron reduction product gives a very weak unresolved signal, a result not readily understood. The ESR spectra of the reduced [Ru(bpz),bpy12+ (Figure 3, a and b) further corroborate the electron hopping model since now the voltammetry pattern predicts that the first two electrons enter redox orbitals localized on each of the bpz rings. Moreover, the one-electron species, [Ru(bpz-)(bpz)bpy]+ (Figure 3a), should have near isoenergetic orbitals on the bpz ring; consequently (!8) Gex, J. N.; Hanck, K. W.; DeArmond, M. K., to be submitted for
publication.
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J. Phys. Chem. 1987, 91, 254-257
hopping between these rings should be fast as seen for the reduced RuL3"+ type complex. An Arrhenius plot (In line width vs. 1/T) does give a straight line with an activation energy of 520 cm-' (Table 11). The two-electron product, [ R ~ ( b p z - ) ~ b p (Figure y]~ 3b), gives an ESR spectrum with hfs (and no line broadening). The spectrum of this species is very similar to that of the reduced mono bpz complex, thus the coupling of the spins is small as expected. The three-electron reduction product, [Ru(bpz-),(bpy-)]-, gives an ESR spectrum with hyperfine structure (no line broadening) but the structure is less resolved than that of the two-electron species as expected if now a Ru-bpy- spectrum is superimposed upon the Ru-bpz- spectrum. The spectra for the mono and bis bpz complexes are in sharp contrast to those for the reduced [ R ~ ( b p z ) ~species. ]~+ N o hyperfine structure is observed for the one- and two-electron reduced species but a temperature-dependent line broadening is measured (Table 11). The three-electron reduced product, for which hopping should not occur, unfortunately gives a very weak signal; consequently, no hfs can be observed. Comparison of the ESR spectra of the two-electron reduction product, [R~(bpz-)~bpy]~, and the [Ru(bp~-)(bpy)~]+ does provide a clue to the absence of hfs for the three-electron reduction product of [ R ~ ( b p y ) ~and ] ~ +other RuL3"+ species. In both of these bpz complexes the barrier to electron hopping is high so line broadening should not occur; but the hfs for the two-electron bis bpz- species is much less well resolved than that of the mono bpz-, due likely to the inexact superposition of the two bpz- spectra in the bis complex. Therefore resolution of hfs for the three-electron reduction products of R U L , ~ would + be critically dependent upon the exact superposition of the spectra for three nearly identical
L- species. Thus residual g factor or hfs anisotropy or a small g shift (0.5 G ) would result in the loss of the hfs. That the -54 OC spectrum of the [Ru(bpz-)(bpy),]+ does not give hfs (because of such an anisotropy effect) while the room temperature spectrum of this species is well resolved (Figure 2) is verification of this hypothesis for the loss of hyperfine structure. The identification of the conditions necessary to obtain hyperfine splitting will enable systematic structural perturbations. Ultimately, the desire is to adjust AElj2to be sufficiently large to observe hfs at low temperature but sufficiently small to facilitate electron hopping at high temperature. The one-electron [Ru(pq-)(bpy),]+ complex (pq = 2,2'-pyridylquinoline) does show both a region of temperature-independent line width as well as a higher temperature line broadening region.19 Unfortunately, the asymmetry of the ligand precludes any observation of hfs in the no line broadening region. The use of ENDOR techniques may enable more rapid determination of the spin density and the related dynamics processes. N M R techniques20 can also be useful but the necessity for determination of proton, nitrogen, and ruthenium signals makes N M R an expensive and slow approach to spin density determination for these complexes.
Acknowledgment. Support by the National Science Foundation (Grant No. CHE-8507901) and the Swiss National Science Foundation (support of J.N.G.) is gratefully acknowledged. (19) Vess, T. M.; Tait, C. D.; DeArmond, M. K.; Hanck, K. W.; Wertz, D. W., unpublished results. (20) Ohsawa, Y.; DeArmond, M. K.; Hanck, K. W.; Moreland, C. G.J . Am. Chem. Soc. 1985, 107, 5383.
Infrared Spectroscopic Investigation of the Ethylene Chemisorption Reaction on Supported Metallic Catalyst Surfaces: Ethylldyne Formation on Pt, Rh, Pd, and Ru Supported on Alumina Thomas P. Beebe, Jr., and John T. Yates, Jr.* Surface Science Center, Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 (Received: September 12, 1986)
The reaction of ethylene with Pt, Rh, Pd, and Ru supported on alumina has been studied at 300 K by transmission infrared spectroscopy. Small metal particles of all four elements form the ethylidyne species, =CCH3,and the vibrational frequencies of these species are in close agreement with those reported for ethylidyne adsorbed on the close-packed faces of single crystals of these same metals. Our Pt and Rh surfaces produce ethylidyne IR bands of higher intensity than Pd and Ru. This is consistent with the presence of smaller Pt and Rh average particle sizes, where quantitative CO adsorption measurements indicate an approximately 2- to 3-fold greater chemisorption capacity of Pt and Rh surfaces than Pd and Ru surfaces. In addition, Pt and Rh surfaces exhibit relatively strong ethylidyne asymmetric modes [v,,(CH,) and 6,,(CH3)], indicating that the surface-dipole selection rule is not strictly in operation on these surfaces. This weakening of the selection rule for Pt and Rh surfaces is consistent with the small average particle sizes for Pt and Rh (19 and 28 A, respectively) compared to larger average particle sizes for Pd and Ru (84 and 66 A, respectively).
Introduction There has been much interest recently in the characterization of reactions between small hydrocarbon molecules and the surfaces of the platinum group metals (Pt, Pd, Rh, Ru, etc.). In the past 4 years, it has been shown that ethylene adsorption on these surfaces often yields the ethylidyne species, =CCH3. Ethylidyne has been shown to form upon reaction of C2H4with P t ( l 1 l),] Rh( 11l),, Pd( 11l),, Ru(OOO~),~ and, most recently, Pd/A1203,5 (1) Steininger, H.; Ibach, H.; Lehwald, S. Surf. Sci. 1982, 227, 685. (2) Koel, B. E.; Bent, B. E.; Somorjai, G. A. Surf. Sei. 1984, 246, 211. (3) Kesmodel, L. L.; Gates, J. A. Surf. Sci. 1981, 211, L747. (4) Barteau, M. A.; Broughton, J. Q.; Menzel, D. Appl. Surf. Sci. 1984, 19, 92.
0022-3654/87/2091-0254%01.50/0
Pt/A&O3,6 and Pt/Si02 and Pd/Si02.7,8 In addition, the surface capacity for ethylidyne has been postulated to reflect the abundance of Pd( 111) sites on Pd/Al,O, catalyst^.^ It has been suggested that on P t ( l l 1 ) the related species, ethylidene, = CHCH,, is a crucial hydrogen-transfer agent or "co-catalyst" in ( 5 ) Beebe, Jr., T. P.; Albert, M. R.; Yates, Jr., J. T. J . Catal. 1985, 96,
1.
(6) Wang, P-K.; Slichter, C. P.; Sinfelt, J. H. J . Phys. Chem. 1985, 89, 3606. (7) Bandy, B.J.; Chesters, M. A,; James, D. I.; McDougall, G. S.; Pemble, M. E.; Sheppard, N. Philos. Trans R . SOC.London, A 1986, 318, 141. (8) Sheppard, N.; James, D. I.; Lesiunas, A.; Prentice, J. D. Commun. Dep. Chem. (Bulg. Acad. Sci.) 1984, 17, 95. (9) Beebe, Jr., T. P.; Yates, Jr., J. T. Surf. Sci. 1986, 173, L606.
0 1987 American Chemical Society
