Electrochemical and Spectroelectrochemical Study of a Bis

The ER signal involved responses due to both the redox reaction and the Stark effect ... Hidenori Murata , Martha Baskett , Hiroyuki Nishide , and Pau...
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Langmuir 1998, 14, 3682-3690

Electrochemical and Spectroelectrochemical Study of a Bis(arylgalvinol)-Substituted Alkyl Disulfide Monolayer and Mixed Monolayers on Polycrystalline Gold Takamasa Sagara* and Takahiro Midorikawa Department of Applied Chemistry, Nagasaki University, Bunkyo 1-14, Nagasaki 852-8521, Japan

David A. Shultz* and Qi Zhao Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204 Received February 4, 1998. In Final Form: April 22, 1998 The monolayer of bis(arylgalvinol)-substituted alkyl disulfide and mixed monolayers with decanethiol (DT) on a polycrystalline gold electrode were characterized in an aqueous solution by means of cyclic voltammetry and in situ electroreflectance (ER) spectroscopy. Both neat and mixed monolayers exhibited a quasi-reversible redox response of galvinol/galvinoxyl radical couple (GalH/Gal•) at the same formal potentials. The ER signal involved responses due to both the redox reaction and the Stark effect on Gal•. The ER spectrum of the redox reaction showed a difference absorption spectral feature, and it was found that the absorption bands of both immobilized GalH and Gal• are red shifted in comparison to their absorption bands measured in organic solvents. The ER spectral profile for the redox reaction was unaffected by mixing with DT, while the contribution of Stark effect on Gal• to the ER signal was significantly suppressed by increasing the amount of DT in the monolayer. The amount of immobilized GalH increased to larger than that of the neat monolayer as the mixing ratio of DT was increased up to 0.65, though further increases in the ratio resulted in a decrease in the amount of immobilized GalH. The increased mixing of DT narrowed the width of the CV peaks, consistent with an accelerated electron-transfer rate constant for the mixed monolayers. These results suggest that the neat monolayer may be disordered, while DT may mix well with the disulfide on the Au surface and force the monolayer to be more well-ordered.

Introduction Multilayers are well-suited to serve as molecular magnets since they provide a direct link between surfaces and magnetic properties, that is, magnetic information storage. An important design element of a multilayer scheme could be the incorporation of paramagnetic ligands. With this design one could utilize paramagnetic ligand to metal exchange couplings to create high-Tc ferrimagnetic films.1 Since the monolayer is the basis for the multilayer, an understanding of the structure and properties of paramagnetic monolayers is necessary. Recently, we described the synthesis of galvinolsubstituted alkanethiols and electrochemistry of their monolayers on polycrystalline gold.2,3 The half-wave potential for the galvinol/galvinoxyl couple (GalH/Gal•) was directly proportional to pH and moved 59 mV per unit change in pH, consistent with a one electron/one proton process:2 Gal• + H+ + e–

GalH

Herein, we describe spectroscopic characterization of monolayers of bis(arylgalvinol)-substituted alkyl disulfide 13 and mixed monolayers of 1 and decanethiol (DT) and use the results to describe the structures of the monolayers. The structure of 1 is shown below. * To whom correspondence should be addressed. E-mail: sagara@ net.nagasaki-u.ac.jp; [email protected]. Fax: (+81) 958-49-4999; (919) 515-8920. Voice: (+81) 958-47-1111 ext. 2747; (919) 515-6972. Web: http://www.eng.nagasaki-u.ac.jp/ch/gousei/ MS_home_E.html; http://www2.ncsu.edu/ncsu/chemistry/das.html.

Since the arylgalvinol tail group of the disulfide is sterically bulky, the orientation of the arylgalvinol will not be well-ordered in a neat monolayer of 1. The alltrans chain length of DT is slightly less than the length of undecyloxyl (-O-(CH2)11-) of 1 in the all-trans conformation; i.e., the DT chain should fill the space beneath the arylgalvinol tail group of upright oriented 1. Therefore, the orientation of 1 immobilized on the electrode surface is expected to be regulated by DT, provided that a well-mixed film can be formed. Additionally, DT may prevent unfavorable aggregation of arylgalvinol groups. In the present investigation, we used electroreflectance (ER) spectroscopy for in situ characterization of the monolayers. The ER signal represents the ac change of the reflectance at the monolayer-modified electrode surface in response to the ac potential modulation.4 We previously found that methylene blue and hypericin partitioned in an alkanethiol monolayer as well as viologen covalently tethered to the surface-confined alkanethiol on an Au electrode surface display an ER spectrum of difference absorption spectral features of oxidized and reduced

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Mixed Monolayers on Polycrystalline Gold

forms.5-7 Therefore, we expect that the ER spectrum of the 1(+DT)-modified Au electrode should also display the difference absorption spectrum between the phenolic and radical states. The ER signal may arise from not only the redox reaction but also nonfaradaic processes such as the dipole-field interaction, i.e., the Stark effect. Furthermore, the ER signal can be used to track the dynamics of the surface processes including the redox reaction and the dipole-field interaction.8,9 Electroreflectance measurements were carried out in neutral solutions with various mixing ratios of DT to explore the effect of mixing on the electrochemistry of the arylgalvinol tail group from a spectroelectrochemical viewpoint. Experimental Section Materials. The preparation of a bis(phenoxygalvinol)substituted alkyl disulfide 1 was described previously.3 Water was purified through an Ultrapure water system Milli-Q Plus (Millipore Co.) to a 18.3 MΩ cm resistivity. 1-Decyl mercaptan (decanethiol, DT) was purchased from Aldrich and used as received. Ethanol was of ultrapure analytical grade. All other chemicals were of reagent grade and were used without further purification. The gold electrode was a polycrystalline gold (Au) disk (geometric area 0.02 cm2) purchased from Bioanalytical Systems. Measurements. The Au electrode was polished with a 0.05 mm alumina slurry to a mirror finish and then sonicated in water. (1) For papers on radical-metal complexes and their potential as molecule-based magnetic materials, see the following: (a) Caneschi, A.; Gatteschi, D.; Sessoli, R.; Rey, P. Acc. Chem. Res. 1989, 22, 392. (b) Caneschi, A.; Gatteschi, D.; Rey, P.; Sessoli, R. Chem. Mater. 1992, 4, 204. (c) Gatteschi, D.; Sessoli, R. J. Magn. Magn. Mater. 1992, 104, 2092. (d) Stumpf, H. O.; Ouahab, L.; Pei, Y.; Grandjean, D.; Kahn, O. Science 1993, 261, 447. (e) Gatteschi, D. Adv. Mater. 1994, 6, 635. (f) Luneau, D.; Laugier, J.; Rey, P.; Ulrich, G.; Ziessel, R.; Legoll, P.; Drillon, M. J. Chem. Soc., Chem. Commun. 1994, 741. (g) Kitano, M.; Koga, N.; Iwamura, H. J. Chem. Soc., Chem. Commun. 1994, 447. (h) Kitano, M.; Ishimaru, Y.; Inoue, K.; Koga, N.; Iwamura, H. Inorg. Chem. 1994, 33, 6012. (i) de Panthou, F. L.; Belorizky, E.; Calemczuk, R.; Luneau, D.; Marcenat, C.; Ressouche, E.; Turek, P.; Rey, P. J. Am. Chem. Soc. 1995, 117, 11247. (j) Inoue, K.; Hayamizu, T.; Iwamura, H.; Hashizume, D.; Ohashi, Y. J. Am. Chem. Soc. 1996, 118, 1803. (k) Pei, Y.; Kahn, O.; Ouahab, L. Inorg. Chem. 1996, 35, 193. (l) Iwamura, H.; Inoue, K.; Hayamizu, T. Pure Appl. Chem. 1996, 68, 243. (m) Ottaviani, M. F.; Turro, C.; Turro, N. J.; Bossman, S. J.; Tomlalia, D. J. Phys. Chem. 1996, 100, 13667. (n) Sokolowsky, A.; Bothe, E.; Bill, E.; Weyhermueller, T.; Wieghardt, K. Chem. Commun. 1996, 6, 1671. (o) Nakatsuji, S.; Anzai, H. J. Mater. Chem. 1997, 7, 2161. (p) Pierpont, C. G.; Buchanan, R. M. Coord. Chem. Rev. 1981, 38, 45. (q) Gans, P.; Buisson, G.; Duee, E.; Marchon, J. C.; Erler, B. S.; Scholz, W. F.; Reed, C. A. J. Am. Chem. Soc. 1986, 108, 1223. (r) Miller, J. S.; Calabrese, J. C.; McLean, R. S.; Epstein, A. J. Adv. Mater. 1992, 4, 498. (s) Gatteschi, D.; Dei, A. Inorg. Chim. Acta 1992, 198-200, 813. (t) Pierpont, C. G.; Lange, C. W. Prog. Inorg. Chem. 1994, 41, 331. (u) Adams, D. M.; Li, B.; Simon, J. D.; Hendrickson, D. N. Angew. Chem., Int. Ed. Engl. 1995, 34, 1481. (v) Hintermaier, F.; Su¨nkel, K.; Volodarsky, L. B.; Beck, W. Inorg. Chem. 1996, 35, 5500. (w) Zhao, H.; Heintz, R. A.; Dunbar, K. R.; Rogers, R. D. J. Am. Chem. Soc. 1996, 118, 12844. For general texts on molecular magnetism, see the following: (x) Gatteschi Molecular Magnetic Materials; Kluwer Academic Publishers: Amsterdam, 1991. (y) Kahn, O. Molecular Magnetism; VCH: New York, 1993. (z) Molecular Magnetism: From Molecular Assemblies to the Devices; Coronado, E., Delhaes, P., Gatteschi, D., Miller, J. S., Eds.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1996; Vol. 321. (aa) MoleculeBased Magnetic Materials. Theory, Technique, and Applications; Turnbull, M. M., Sugimoto, T., Thompson, L. K., Eds.; ACS Symposium Series 644; American Chemical Society: Washington, DC, 1996. (2) Shultz, D. A.; Tew, G. J. Org. Chem. 1994, 59, 6159. (3) Shultz, D. A.; Zhao, Q. Tetrahedron Lett. 1996, 37, 8837. (4) For a review, see: Sagara, T. Electroreflectance Spectrosopy at Electrode Interfaces and Its New Application. In Recent Developments in Physical Chemistry; Transworld Research Network: Trivandrum, 1998; pp 159-173. (5) Sagara, T.; Kawamura, H.; Nakashima, N. Langmuir 1996, 12, 4253. (6) Sagara, T.; Kawamura, H.; Ezoe, K.; Nakashima, N. J. Electroanal. Chem., in press. (7) Sagara, T.; Kaba, N.; Komatsu, M.; Uchida, M.; Nakashima, N. Electrochim. Acta, in press. (8) Sagara, T.; Fukuda, M.; Nakashima, N. J. Phys. Chem. B 1998, 102, 521. (9) Feng, Z. Q.; Sagara, T.; Niki, K. Anal. Chem. 1995, 67, 3564.

Langmuir, Vol. 14, No. 13, 1998 3683 In the present study, we omitted the oxidation-reduction cycle treatment of the Au electrode5 before the adsorption procedure, since otherwise considerable desorption of the adsorbed 1 was observed during the measurement. After being polished and sonicated, the Au electrode surface was dried in an Ar gas (99.998% purity) stream and immediately immersed in a freshly prepared ethanolic solution of 1 + DT for adsorption. The solution contained (1 - x) mM 1 + x mM DT. The 17 h immersion period was carried out in the dark at 23 ( 2 °C. The electrode was then rinsed thoroughly with ethanol, dried with an Ar gas stream, and then placed in a quartz spectroelectrochemical cell filled with predeaerated 50 mM phosphate buffer solution (pH 7.0) prepared from potassium salts. An Ag/AgCl in saturated KCl solution (the equilibrium potential is 197 mV vs NHE) was used as the reference electrode. A coiled Au wire served as the counter electrode. It was pretreated by flame-annealing and subsequent quenching in purified water. All the measurements were carried out in Ar atmosphere at 24 ( 2 °C. The instruments for ER measurements were the same as reported previously.4,10 A sine-wave potential modulation at frequency f applied to the electrode is described as

E ) Edc + Eac ) Edc + ∆eac exp(-jωt)

(1)

where E is the electrode potential, the subscript under E denotes dc or ac, ∆Eac is the amplitude, j ) x-1, ω ) 2πf, which is the angular frequency, and t is the time. Under steady, unpolarized monochromatic incident light irradiation, the ac intensity of the reflected light (Iac) and the time-averaged dc intensity of the reflected light (Idc) were simultaneously detected. For the phasesensitive detection of Iac, a lock-in-amplifier (EG&G PARC model 5210) was employed. The ER signal is defined as Iac/Idc. The ER signal has both the real part (in-phase component) and the imaginary part (90° out-of-phase component) in response to the potential modulation. The incident angle was fixed at 45° in the present study. We made three types of ER measurements. The ER spectrum, the plot of ER signal as a function of the wavelength of the incident light (λ), was obtained by stepwise scanning of λ at a given set of Edc, Eac, and f. The ER voltammogram, the plot of ER signal as a function of Edc, was measured during the course of a linear scan of Edc at a given set of λ, Eac, and f. The frequency dependence of the ER signal was measured at a given set of λ, Edc, Eac.

Results and Discussion Voltammetric Measurements. The typical cyclic voltammograms (CVs) at a sweep rate (v) of 40 mV s-1 are shown in Figure 1. A redox wave is observed for any mole fraction of DT, x, less than unity. The formal potential (E0′) was obtained by extrapolating the v-dependence of midpoint potential between anodic and cathodic peaks to v ) 0 mV s-1. The value of E0′ for a neat monolayer (381 ( 4 mV as an average of five different experiments) is in accord with the value of E0′ of a related galvinol-substituted alkanethiol monolayer.2 The value of E0′ was independent of x. The peak current of the CV response is proportional to v in the range from 10 to 200 mV s-1, indicative of the redox reaction of the surface-confined species. The voltammetric response is presumably assigned as the redox reaction of immobilized GalH/Gal• couple as observed previously.2 This assignment will be confirmed by ER measurements (vide infra). Figure 2 shows the x-dependence of the peak separation (∆Ep), the full width of the anodic peak at the half-height (∆W1/2), and the amount of immobilized galvinol groups (Γ), all of which were obtained from the CVs at 40 mV s-1. The value of Γ was calculated from the anodic CV peak charge using a geometrical electrode area provided that the redox reaction is a one-electron-transfer process. Note that Γ represents the amount of GalH but not of 1. (10) Sagara, T.; Takeuchi, S.; Kumazaki, K.; Nakashima, N. J. Electroanal. Chem. 1995, 396, 525.

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Figure 2. Dependence on x of the peak separation (∆Ep, open circles), the full width of the anodic peak at the half-height (∆W1/2, closed circle) and the amount of immobilized galvinoxyl groups (Γ, squares) obtained from cyclic voltammograms at 40 mV s-1.

Both ∆Ep and ∆W1/2 decrease monotonically with increasing x. The ∆Ep decreased with slower sweep rates when x ) 0, indicating that the reaction is quasi-reversible. On the other hand, the v-dependence of ∆Ep at x ) 0.8 and 0.9 was negligible in the v-range of 10 to 200 mV s-1. The value of ∆Ep tends to zero when x is extrapolated to 1.0. When x is extrapolated to 1.0, the value of ∆W1/2 reaches nearly 90.6 mV, the theoretical value for the reversible one-electron-transfer reaction of noninteracting sites without distribution in the formal potential.11 The decrease of ∆Ep in harmony with ∆W1/2 indicates that the redox reaction shifts from quasi-reversible to reversible when increasing x. These results also indicate that either the interaction among the immobilized arylgalvinols is suppressed or the distribution in the formal potential is narrowed when increasing x. Both ∆Ep and ∆W1/2 are governed by the microscopic environments of the arylgalvinol but not directly by the total amount of the immobilized redox active centers (in fact, the x-dependence of Γ is in line with the dependence of ∆Ep and ∆W1/2 as described below). Therefore, 1 and DT are mixed on the

Au surface rather than segmented into domains since the x-dependent changes in ∆Ep and ∆W1/2 are continuous. Interestingly, with increasing x, the value of Γ first increases up to approximately x ) 0.65 and then decreases steeply. Although the fractional amount of 1 in the solution for adsorption decreases, an increase in Γ occurs in the range of x e 0.65. This is reasonable if the average area occupied by one molecule of 1 on the Au electrode becomes smaller because of better ordering of 1 with increasing x. The area of the phenyl galvinoxyl tail group has been estimated to be a 1 nm2 area,2,12 which corresponds to the full coverage of 1.7 × 10-10 mol cm-2. The maximal value of Γ in Figure 2 is 1.4 × 10-10 mol cm-2 at x ) 0.65, which is ca. 85% of the estimated full coverage of the tail group. On the other hand, the value of Γ at x ) 0 corresponds to the half of the estimated full coverage. Prolonged adsorption time to longer than the 17 h period did not result in the increase in Γ. These facts may indicate that the neat monolayer (x ) 0) is disordered even when adsorption is saturated, while mixed adsorption with DT results in a better ordered, upright orientation of 1. The CV data described above were obtained within 1 h after the immersion of the electrode in the electrolyte solution. Within this period, the CV response was stable even though continuous multiscanning was conducted, regardless of x. Electroreflectance measurements followed these CV measurements. During the ER measurements for a ca. 5-8 h period, we conducted CV measurements several times and found that the response gradually decreases with time. For example, the anodic CV peak current (ipa) decreased by ca. 25% within the first 5 h for a neat monolayer of 1 and ipa decreased by ca. 20% for a monolayer of x ) 0.8. The change in ipa with time is accompanied by a slight increase of ∆Ep and ∆W1/2, though the increase is less than a few millivolts.

(11) Bard, A. J.; Faulkner, L. R. Electrochemical Methods; John Wiley & Sons: New York, 1980; Chapter 12.

(12) Bock, H.; John, A.; Havlas, Z.; Bats, J. W. Angew. Chem., Int. Ed. Engl. 1993, 32, 416.

Figure 1. Cyclic voltammograms of an Au electrode covered with monolayers of 1 + DT at various mixing ratios at 40 mV s-1: a, x ) 0 (neat monolayer of 1); b, x ) 0.5; c, x ) 0.8; d, x ) 0.9. Initial potential is -0.2 V.

Mixed Monolayers on Polycrystalline Gold

In order for the conditions inducing the decay of the redox response to be clarified, we examined the stability by the potential hold experiments. When the potential was held constant at more positive than E0′ for 1 h, decay was easily observed. On the other hand, the decay was smaller when the potential was held at less positive potential than E0′. These observations did not depend on x and were not affected by UV-vis light illumination. Therefore, the decay is ascribed to the instability of adsorbed, oxidized form of 1 (Gal•), probably due to the desorption of 1/Gal• or chemical reactivity of Gal•. Desorption is more likely, because the interfacial capacitance increased in harmony with the decay of the redox response. In fact, it has been known that the monolayer formed from 1•• (biradical form of 1) shows lower surface coverage than that formed from 1.3 We also found that mixing with DT slightly improved the stability. It is worth noting that the decay of the redox response never introduced the change of the ER spectral profiles unless the electrode was kept at positive potentials for several hours. Electroreflectance Measurements for a Neat Monolayer of 1. Figure 3 shows ER spectra of a Au electrode covered with a neat 1 monolayer. Parts A and B of Figure 3 were obtained at two different modulation frequencies at Edc ) E0′. Figure 3C was obtained at the lower frequency at Edc ) 600 mV, which is 215 mV more positive than E0′. In Figure 3A, the real part spectrum has two positivegoing peaks at 463 and 423 nm as well as three negativegoing peaks at 516, 441 (we presume this valley as a peak), and ca. 370 nm (weak). However, the peak wavelengths of the imaginary part of the spectrum are all different from those of the real part of the spectrum, though the spectral profile is similar as a whole. It is known that the spectral profiles of real and imaginary parts are identical if not the same intensity, when the ER signal arises from a single process.5,6 Therefore, this ER spectrum measured at the low modulation frequency, f ) 14 Hz, is the sum of at least two components. In Figure 3B, the real part of the spectrum has three positive-going peaks at 545, 462 (strong), and around 390 nm (very weak) as well as two negative-going peaks at 511 and 438 nm. The imaginary part spectrum appears as a diminished mirror image of the real part of the spectrum with respect to the zero-ER response line. This indicates that this ER signal measured at a higher frequency, f ) 1024 Hz, is of a single component, as opposed to the spectrum at 14 Hz (Figure 3A). Although there are a few peaks with the peak wavelengths roughly similar to Figure 3A, the spectral profile is not the same. In the potential region covered by the potential modulation to obtain Figure 3C, nearly all of the electroactive centers are in the oxidized form (Gal•). Therefore, the ER signal in Figure 3C is not due to the redox reaction of galvinol. The real part of the spectrum has three positivegoing peaks at 538, 459 (strong), and around 390 nm (very weak) as well as two negative-going peaks at 507 and 434 nm. Except for a 3-7 nm blue shift, the ER spectrum is nearly identical with that of Figure 3B. The ER spectra of parts B and C of Figure 3 can be ascribed to an identical origin that is nonfaradaic. The imaginary part is much smaller than the real part in Figure 3C, as opposed to Figure 3A. This indicates that the nonfaradaic process is a kinetically fast process. We will return to this kinetic aspect later. In order for the potential dependence of the ER signal to be clarified, ER voltammograms were measured at various λ and f. Typical results are shown in Figure 4, which were measured at λ ) 516 nm. The ER voltam-

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Figure 3. ER spectra of an Au electrode covered with a neat monolayer of 1: solid line, real part; broken line, imaginary part. A: Edc ) 382 mV, ∆Eac ) 99 mV, f ) 14 Hz. B: Edc ) 382 mV, ∆Eac ) 99 mV, f ) 1024 Hz. C: Edc ) 600 mV, ∆Eac ) 99 mV, f ) 14 Hz.

mograms shown in parts A and B of Figure 4 were measured, respectively, with f ) 14 and 1024 Hz. The imaginary part at f ) 14 Hz displays an almost symmetrical bell-shaped curve with the peak potential ca. 15 mV more positive than E0′. The curve shape is an expected one for the ER response to the quasi-reversible redox reaction.13,14 On the other hand, both real and imaginary parts of the ER voltammogram at 1024 Hz (Figure 4B) exhibit a sigmoidal curve. The magnitude of the 1024 Hz (13) Sagara, T.; Igarashi, S.; Sato, H.; Niki, K. Langmuir 1991, 7, 1005. (14) Feng, Z. Q.; Imabayashi, S.; Kakiuchi, T., Niki, K. J. Electroanal. Chem. 1996, 408, 15.

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Figure 5. Modulation-frequency dependence of ER signal at an Au electrode covered with a neat monolayer of 1: open circles and dotted line, real part; closed circles and solid line, imaginary part. λ ) 516 nm, Edc ) 385 mV, ∆Eac ) 42 mV.

Figure 4. ER voltammograms of an Au electrode covered with a neat monolayer of 1 measured at λ ) 516 nm, ∆Eac ) 42 mV, and the potential sweep rate of -1 mV s-1: open circles and dotted line, real part; closed circles and solid line, imaginary part. A, f ) 14 Hz; B, f ) 1024 Hz.

ER signal, which is attributable to a nonfaradaic signal, is almost independent of Edc when the redox active center is in fully oxidized form, while it is nearly zero when in fully reduced form. Therefore, the nonfaradaic signal arises from the oxidized form. However, the magnitude is not directly proportional to the fractional amount of the oxidized form, since the half-wave potential is ca. 325 mV less positive than E0′. The real part of the ER voltammetric curve at 14 Hz (Figure 4A) resembles the factored sum of the redox reaction-based curve (viz., the sign-inverted curve of the imaginary part at 14 Hz) and the nonfaradaic process-based curve (viz., the real part at 1024 Hz). The ER voltammetric measurements were also carried out at various f and λ. All the obtained curves could be categorized as one of three types: redox reaction-based curve, nonfaradaic process-based curve, or the sum of the two. The ER signal in the potential region more negative than 180 mV, where the redox active center is in fully reduced form, was zero in any combination of f and λ (other than 516 nm, wavelengths of 511, 459, 438, 434, and 423

nm were used). This indicates that the reduced form (GalH) does not produce the nonfaradaic ER signal. Figure 5 shows a plot of ER signal at 516 nm as a function of f. In the measured range of f, two kinetically wellseparated components of the ER signal are clearly observed. The kinetically faster component is observed as a peak of imaginary part and a sigmoidal rising of the real part. Both the peak frequency of imaginary part and the half-wave frequency of the real part rising are ca. 3.5 kHz. The kinetically slower component exhibits an onset from ca. 100 Hz and increases continuously with decreasing f down to 1 Hz. When the complex plane plot of the ER signal (i.e., the plot of imaginary part versus real part; not shown here) was made for the identical data, the kinetically faster component showed a semicircle trajectory. The peak frequency of the semicircle was also 3.5 kHz. We also made an ac admittance measurement and found that the peak frequency of the semicircle trajectory in the high-frequency region on the complex plane plot of the admittance is 3.51 kHz; that is, the cell time constant (τcell) is ca. 45 µs. Therefore, the peak frequency of the ER response is identical with the peak frequency of the ac admittance, which is equal to (2πτcell)-1. This fact indicates that the kinetically faster component synchronizes without delay with the ac change of the interfacial electric field at the position of the chromophore producing the signal.9 The ER spectrum of Figure 3B attributable to the nonfaradaic process was measured at 1024 Hz, at a frequency where the ER signal involves only the kinetically faster component as shown in Figure 5. Therefore, the kinetically faster component corresponds to the nonfaradaic ER signal. On the other hand, the kinetically slower component corresponds to the redox reaction. Note that these results support that the redox reaction of galvinol is quasi-reversible. Figure 5 also tells us that the contribution of the nonfaradaic process to the imaginary part of the ER signal at f lower than 100 Hz is negligibly small, though a constant contribution to the real part of the ER signal is present in the low-frequency region. To explore the difference electronic spectrum between oxidized and reduced forms of the redox couple of interest, it is necessary to figure out the ER spectrum due solely

Mixed Monolayers on Polycrystalline Gold

Figure 6. Comparison of ER spectrum due to the redox reaction with the solution absorption spectra of 1: upper part, absorption spectrum of 1 in phenol form (GalH) in tetrahydrofuran + ethanol (1:1); middle part, sign-converted imaginary part ER spectrum of Figure 3A (Edc ) E0′, f ) 14 Hz); lower part, absorption spectrum of 1•• (i.e., biradical form) in tetrahydrofuran (shown in an overturned manner for the sake of comparison).

to the redox reaction. Recall that analyses of Figures 4 and 5 have demonstrated that the imaginary part of the ER response at 14 Hz involves no contribution from the nonfaradaic ER signal. In other words, the imaginary part of the ER spectrum of Figure 3A is due solely to the redox reaction of galvinol. The spectrum is shown again in Figure 6 after inverting the sign of the signal. Additionally, solution absorption spectra of 1 in the reduced and oxidized biradical forms are presented above and below the ER spectrum, respectively. The ER spectrum can be understood apparently to be corresponding to the difference absorption spectrum between absorption spectra of GalH and Gal• species. When the absorption is more intense at more positive potentials than that at less-positive potentials at a given λ, the sign of the imaginary part of the ER response is positive.13 Therefore, positive-going peaks and negative-going peaks of the ER spectrum of the redox reaction shown in Figure 6 correspond apparently to the absorption band of reduced and oxidized forms, respectively. There are three negativegoing ER bands in Figure 6: 514, 453, and 385 nm. Correspondingly, the absorption spectrum of the biradical form (1••) possesses three bands with maxima at 508, 442, and 364 nm. All three ER band “maxima” are red-shifted compared to the absorption maximum of 1•• in solution. The reduced form (1) in solution has an absorption band with a maximum at 422 nm, which is broader than the absorption bands of 1••. The ER spectrum has two positivegoing bands between 400 and 500 nm. However, it is also

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possible that these are not two distinct bands but, alternatively, one broad positive-going band superposed by a sharp negative-going band at 453 nm, and as a result it appears as two bands. We prefer the latter assignment of the positive-going band corresponding to an absorption band adsorbed 1 with a maximum between 429 and 477 nm, red-shifted compared to the solution absorption band. At present, we cannot completely rule out the possibility that the absorption band of GalH immobilized on the Au electrode is split into two bands. However, no matter which assignment of the positive-going band is correct, the ER spectral profile exhibits a difference absorption feature. This is the case of the chromophores immobilized closely to the Au electrode surface but not in direct contact with the Au surface, as exemplified by previous ER spectral measurements for methylene blue or hypericin at alkanethiol-modified Au electrodes,4,6 alkanethiol-substituted viologen immobilized on an Au electrode,7 and heme proteins at modified Au electrodes.15,17 Note that it cannot be quantitatively determined at present whether the relative ER peak heights are in line with absorption coefficients in the solution absorption spectrum, since the ER peak intensity also depends on the chromophore orientation.7 We are currently analyzing the ER spectrum with plane-polarized incident light to explore chromophore orientation. What is the origin of the nonfaradaic ER signal? The nonfaradaic ER signal has the following features as described above: (1) The signal synchronizes with the modulation of the interfacial electric field without delay. (2) The spectrum is blue-shifted with the positive shift of Edc (recall the comparison between Figures 3B and 3C as an example). These features are presumably consistent with the Stark effect. The Stark effect originates from the interaction between the electric dipole and the static electric field.8,17-19 The Stark effect brings about a shift in the absorption bands that can be expressed as

∆a ) hc/∆λ ) k∆µF cos R

(2)

where ∆a is the band energy shift, h is Planck’s constant, c is the velocity of light in a vacuum, ∆λ is the wavelength shift, k is a constant, ∆µ is the change in the static dipole moments responsible for absorption between the ground and excited states of the chromophore (∆µ b)b µexcited b µground), F is the static electric filed at the position of the chromophore, and R is the angle between the static dipole and F. The ER spectrum due to the Stark effect should appear as the first derivative of the absorption spectrum according to the following consideration: An ER spectrum represents the absorption spectrum at a less positive potential from which that at a more positive potential is subtracted. When an absorption band is bell-shaped and it is forced to be blue-shifted by the change of Edc to the positive direction, the resulting ER band should be a bipolar-shaped curve, which appears as the first derivative of the absorption spectrum when ∆Eac is infinitesimal. That is, on the energy scale of the ER spectrum, the blueshifted peak is positive-going, and the red-shifted peak is negative-going, and the energy of the zero-ER response corresponds to the isosbestic point of the unshifted and shifted absorption spectra. According to the interpretation (15) Hinnen, C.; Niki, K. J. Electroanal. Chem. 1989, 264, 157. (16) Sagara, T.; Murakami, H.; Igarashi, S.; Sato, H.; Niki, K. Langmuir 1991, 7, 3190. (17) Plieth, W. J.; Gruschinske, P.; Hensel, H.-J. Ber. Bunsen-Ges. Phys. Chem. 1978, 82, 615. (18) Platt, J. R. J. Chem. Phys. 1961, 34, 862. (19) Leow, L. W.; Simpson. L. L. Biophys. J. 1981, 34, 353.

3688 Langmuir, Vol. 14, No. 13, 1998

Figure 7. ER spectra of an Au electrode covered with a 1 + DT monolayer produced by adsorption in 1 + DT (x ) 0.8) ethanolic solution: solid line, real part at f ) 14 Hz; dotted line, imaginary part at f ) 14 Hz; dashed line, real part at f ) 1024 Hz. Edc ) 382 mV, ∆Eac ) 99 mV. •

of Figure 6, the oxidized form (Gal ) has two successive absorption bands in the wavelength range longer than 400 nm. Therefore, the ER spectrum due to the Stark effect on Gal• should appear as a doublet bipolar curve. Additionally, close comparison of the ER spectral profiles of the redox response (Figure 6) and the nonfaradaic response (Figure 3C) leads to the conclusion that the nonfaradaic ER spectrum has in fact doublet bipolar feature and the center wavelengths of the two bipolarshaped responses correspond to the negative-going peak wavelengths of the ER spectrum of the redox process shown in Figure 6. Both bipolar curves in Figure 3C have a positive-going peak at longer wavelength than the center wavelength and a negative-going peak at a shorter wavelength than the center wavelength. That is, the bipolar response at shorter wavelengths is centered at 450 nm (provided that the bipolar curve is inversionsymmetric) and corresponds to the negative-going peak at 453 nm in the ER spectrum of Figure 6. Another bipolar response at longer wavelengths is centered at 525 nm, which corresponds to the negative-going peak at 523 nm in Figure 6. Both negative-going peaks at 453 and 523 nm are the absorption bands of the oxidized form (Gal•). Therefore, the nonfaradaic ER response is the first derivative of the absorption spectrum of Gal• immobilized on the electrode surface. This indicates that (1) the electric dipole of Gal• responsible for the transitions at 453 and 523 nm is not oriented parallel to the electrode surface, since a parallel orientation would not be expected to give rise to a Stark effect, and (2) the static electric field at the Gal• tail group is perturbed by potential modulation. It is worth noting that the Stark effect on the reduced form (GalH) was not observed as described above. It may be that k(GalH) is very small or R is nearly 90° for GalH; however, the reason is not clear at present. Electroreflectance Measurements for 1 + DT Monolayers. We will first show the results of ER measurements when x ) 0.8 as representative, and then discuss the dependence of the ER spectral features on x. Figure 7 shows the ER spectra for a 1 + DT (x ) 0.8) monolayer at Edc ) E0′ using two different f. Solid and dotted lines are, respectively, the real and imaginary parts measured at f ) 14 Hz. The differences in the peak wavelengths between real and imaginary parts are much smaller than those in Figure 3A. This indicates a smaller contribution of the nonfaradaic response than that obtained at a neat monolayer-modified electrode. The

Sagara et al.

Figure 8. ER voltammogram of an Au electrode covered with a 1 + DT monolayer produced by adsorption in 1 + DT (x ) 0.8) ethanolic solution: open circles, real part; closed circles, imaginary part. λ ) 516 nm, ∆Eac ) 42 mV, f ) 14 Hz, and a potential sweep rate of -2 mV s-1.

dashed line is the real part of the spectrum measured at f ) 1024 Hz. The nonfaradaic ER spectrum is observed as nearly the same profile as Figure 3B, but the intensity relative to the spectra at f ) 14 Hz is smaller than that shown in Figure 3. The spectral characteristics are similar to a neat monolayer. Figure 8 shows the ER voltammogram for a 1 + DT (x ) 0.8) monolayer at λ ) 516 nm and f ) 14 Hz. The imaginary part is due totally to the redox reaction. The real part involves the signals of both redox reaction and the nonfaradaic process. Once again, the smaller contribution of the nonfaradaic response than that at the neat monolayer-modified electrode is obvious. On the basis of the results in Figures 7 and 8, the nonfaradaic ER signal is attributable to the Stark effect of Gal•. Figure 9 shows the frequency dependence of the ER response at λ ) 516 nm. The curves are similar to those in Figure 5. However, two significant differences are pointed out: (1) The intensity at higher frequencies relative to the intensity at lower frequencies in Figure 9 is much smaller than that in Figure 5. (2) The imaginary part of the redox ER signals is seen to exhibit a peak around 1 Hz in Figure 9, while the peak seems to occur far beyond 1 Hz in Figure 5. It is known then that as the electron-transfer rate constant becomes greater, the peak frequency shifts to higher frequency.9,12 Result (2) above implies, therefore, that the redox reaction for a 1 + DT (x ) 0.8) monolayer is kinetically faster than that for a monolayer of neat 1. This is consistent with the CV results (vide supra). Assuming that the peak frequency is 1 Hz for the 1 + DT (x ) 0.8) monolayer, the heterogeneous electron-transfer rate constant ks can be obtained using the equation13

ks ) -πfp[Re]p/[Im]p

(3)

where fp is the peak frequency and [Re]p and [Im]p are, respectively, the ER intensity of real and imaginary parts at fp. Since fp is far smaller than the reciprocal of τcell, neither the solution resistance nor double-layer capacitance appears in eq 3. The calculated value of ks is 7.3 s-1. For the other mixed monolayers of x < 0.8, the frequency exhibiting the maximal imaginary part of the

Mixed Monolayers on Polycrystalline Gold

Langmuir, Vol. 14, No. 13, 1998 3689 Table 1. ER Spectral Band Maxima Due to the Absorption by Gal• for Neat and Mixed Monolayers of 1 λmax/nm (imaginary part at 14 Hz at Edc ) E0′) x

N1a

N2a

N3a

0 0.20 0.35 0.50 0.65 0.80 0.90

379 ( 7 370 376 364 374 376 372

453 ( 1 451 450 452 451 452 449

522 ( 2 519 517 513 515 516 ( 5 514

a N1 through N3 are the positive going peaks due to the absorption by Gal• in order from shorter to longer wavelengths.

Figure 9. Modulation-frequency dependence of ER signal at an Au electrode covered with a 1 + DT monolayer produced by adsorption in 1 + DT (x ) 0.8) ethanolic solution: open circles and dotted line, real part; closed circles and solid line, imaginary part. λ ) 516 nm, Edc ) 382 mV, and ∆Eac ) 42 mV.

Figure 10 clearly shows that the relative contribution of the Stark effect becomes smaller with increase in x. The coadsorbed DT plays a role as the Stark effect inhibitor. According to the discussions in the previous section, the Stark effect becomes smaller either when the electric dipole of Gal• comes further apart from upright orientation and turns into flat orientation or when the relative amplitude of the modulated static electric field at the position of Gal• becomes smaller. In the former case, the change of the orientation with x should be able to be detected by measuring the redox ER signal by the use of polarized incident light.7,8,20 Therefore, we are planning experiments to determine whether the orientation of the electric dipole depends on x by carrying out the ER measurement with polarized incident light. In the latter case, if the majority of the interfacial potential drop takes place in the interior of the film, and the distance of Gal• from the Au surface becomes greater with increasing x, the electric field change felt by Gal• becomes smaller. As a result, the Stark effect is suppressed by an increase in x. At any rate, further studies are required to understand the mixed film structure and x-dependence of the Stark effect. Table 1 summarizes the wavelengths of peak maxima of the ER spectral band for Gal•. The peak wavelengths were read from the imaginary part of the ER spectrum at f ) 14 Hz, and thus they correspond to the absorption maxima of Gal• immobilized on the electrode surface. Although the peak wavelengths vary from film-to-film by (1.2 nm for N2 and (4.0 nm for N3, no correlation with x was found. That is, the absorption spectrum (i.e., the electronic structure) of Gal• immobilized on the electrode surface is unaffected by mixing with DT. Conclusions

Figure 10. Plot of G value as a function of x. G is defined as ER (1 kHz)/Γ. For details see text.

ER signal in low-frequency range seems to be lower than 1 Hz, and thus ks may be smaller than 7 s-1. The ER spectral features of mixed monolayers were similar to the monolayer of x ) 0.8, while the magnitude of the Stark effect ER signal with the other mixing ratios depends on x significantly. Figure 10 shows the plot of G ) ER(1 kHz)/Γ as a function of x, where ER(1 kHz) is the ER signal intensity at f ) 1 kHz and defined as

ER(1 kHz) ) {[Re]2 + [Im]2}1/2

(4)

where [Re] and [Im] are, respectively, the ER intensities of real and imaginary parts at f ) 1 kHz measured with ∆Eac ) 42 mV at the peak wavelength around 516 nm. The value of G is an approximate measure of the magnitude of the Stark effect signal relative to the amount of Gal•, since ER(1 kHz) is due to the Stark effect signal.

The neat monolayer of 1 immobilized on a polycrystalline Au electrode exhibits a quasi-reversible redox response of a GalH/Gal• couple in a neutral aqueous solution. The ER response due to the redox reaction shows difference absorption spectral features, and the absorption bands of both immobilized GalH and Gal• are found to be red-shifted in comparison to the absorption spectra measured in organic solvents. A strong nonfaradaic ER response due to the Stark effect on Gal• is also observed. The mixed monolayer of 1 + DT also exhibits a quasi-reversible redox response of the GalH/Gal• couple at the same formal potential as the neat monolayer. The ER spectral profile for the redox reaction is unaffected by mixing with DT. However, mixing with DT affects the CV and ER responses in the following manner: (1) mixing with DT accelerates the electron-transfer rate constant and narrows the halfwidth of the CV peak as x is increased; (2) the amount of (20) Sagara, T.; Nomaguchi, N.; Nakashima, N. J. Phys. Chem. 1996, 100, 6393.

3690 Langmuir, Vol. 14, No. 13, 1998

Sagara et al.

increased mixing with DT may increase order within the monolayer. Since further discussion of the orientation of the GalH tail group is warranted in light of the present results, we are planning ER measurements using polarized incident light to explore chromophore orientation. Present results and additional studies of the film structure and details of spin-spin interaction may promise to elucidate the structural requirements best-suited for our multilayer approach toward the construction of molecular magnets. For instance, the finding that surface coverage of largetailgroup alkanethiols can be increased by addition of a straight-chain alkanethiol could be an important design element for maximizing surface coverage of paramagnetic building blocks of a magnetic multilayer. Figure 11. Schematic picture of the neat and mixed monolayers.

immobilized GalH increased to larger than that of the neat monolayer as x is increased up to x ) 0.65, though a further increase in x results in a decrease in adsorbed 1; (3) the ER response due to the Stark effect on Gal• is significantly suppressed with increasing x. The x-dependent monolayer structure is schematically summarized in Figure 11. The neat monolayer may be disordered and

Acknowledgment. This work is supported financially in part by the Grant-in-Aid from the Ministry of Education, Science, Sports and Culture of Japan to T.S. D.A.S. thanks the Research Corporation (CS0127) for a Cottrell Scholars Award and the Kenan Foundation and Hoechst-Celanese for a seed grant. T.S. thanks Drs. N. Nakashima and M. Tominaga (Nagasaki University) for helpful discussions. LA980136Z