Electrochemical Surface Science - American Chemical Society

Chapter 29

Electroactive

Bipyridiniums

in

Octadecylmercaptan Downloaded via NORTHWESTERN UNIV on July 11, 2018 at 15:16:20 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

on

Gold

Self-Assembled

Monolayers

Electrodes

H. O. Finklea, J. Fedyk, and J. Schwab Department of Chemistry, West Virginia University, Morgantown, WV 26506 N-Octadecyl-n'-methyl-4,4'-bipyridinium d i c h l o r i d e and the n'-ethyl analog are co-deposited with octadecylmercaptan onto a gold substrate during self-assembly of an organized monolayer from a chloroform/methanol s o l u t i o n . Surface redox waves are observed in aqueous e l e c t r o l y t e corresponding to the reduction of the bipyridiniums. The bipyridiniums also assemble from aqueous e l e c t r o l y t e to form an e l e c t r o a c t i v e monolayer on both bare gold and gold coated with a mercaptan monolayer. The electrochemical behavior suggests that i n both cases the bipyridiniums reside on but not i n the mercaptan monolayer. Evidence is given f o r penetration of the mercaptan monolayer by bipyridiniums. Long chain alkylmercaptans and d i s u l f i d e s r e a d i l y self-assemble on gold surfaces to form compact organized monolayers i n which the s u l f u r i s chemisorbed to the gold and the hydrocarbon t a i l i s extended away from the surface 0.-5). The mercaptan monolayers strongly i n h i b i t gold oxidation i n d i l u t e s u l f u r i c acid and also block d i f f u s i o n of aqueous ions (e.g. Fe , Fe(CN) ~ > R u ( N H ) ' ) t o the gold surface (4-5)• The unique anisotropy of the organized monolayer provides an opportunity to explore the e f f e c t of both o r i e n t a t i o n and distance on electron transfer between a molecule and a metal electrode (6-9). Our strategy i s to incorporate a prolate redox molecule i n t o the hydrocarbon phase of the organized monolayer. Steric r e s t r a i n t s imposed by a close-packed monolayer would presumably force the redox molecule to adopt an o r i e n t a t i o n p a r a l l e l to the hydrocarbon t a i l s . Spacing can then be controlled by a short hydrocarbon chain between the redox center and the metal. A class of molecules f i t t i n g these requirements are the assymetric 4,4'-bipyridiniums: 2

1 3

3

6

2 +

3

3 +

6

c

0097-6156/88/0378-0431$06.00/0 1988 American Chemical Society

Soriaga; Electrochemical Surface Science ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

432

ELECTROCHEMICAL SURFACE SCIENCE

R i s the spacer group which should also terminate i n an anchor e.g. a mercaptan. We report here intermediate r e s u l t s i n which R i s the methyl and e t h y l moiety; the case of R^H i s the subject of a separate paper (K. A. B. Lee, R. Mowry, G. McLennan and H. 0. F i n k l e a , i n press). Our primary concern i s whether the bipyridiniums are incorporated i n t o the mercaptan monolayer and whether they are located at a f i x e d distance r e l a t i v e to the gold surface. Experimental Synthesis. N-0ctadecyl-4-pyridinium-4•-pyridyl iodide (mp 125°C) was synthesized by s t i r r i n g stoichiometric amounts of 4,4'-bipyridine and 1-iodooctadecane i n acetone f o r several days. The p r e c i p i t a t e was c o l l e c t e d and r e c r y s t a l l i z e d from methanol/ether. N-Methyl-n'-octadecy1-4,4*-bipyridinium d i i o d i d e [(C bpyMe)I ] (mp 270°C with decomposition) and N-ethyl-n'-octadecy1-4,4'-bipyridinium d i i o d i d e [(C b p y E t ) I ] (mp 255°C with decomposition) were synthesized by r e f l u x i n g the mono-alkylated b i p y r i d i n e with methyl iodide i n methanol. The iodide was replaced with chloride by d i s s o l v i n g the biipyridiniums i n a minimum amount of b o i l i n g water and adding excess saturated KC1 s o l u t i o n . A f t e r c h i l l i n g the yellow s o l i d was c o l l e c t e d and subjected to a second metathesis step to remove the l a s t traces of iodide. t8

18

2

2

Deposition of the mixed monolayer. Deposition solutions were prepared by d i s s o l v i n g octadecylmercaptan [C SH] and the respective bipyridinium i n a mixture of chloroform and methanol. The electrode was cleaned by heating i t i n a gas-air flame. A f t e r cooling, the electrode was immersed i n the deposition s o l u t i o n f o r 1 5 - 3 0 minutes, withdrawn, and rinsed i n clean methanol or chloroform. Q u a l i t a t i v e l y the most reproducible surface redox waves and lowest charging currents during c y c l i c voltammetry were obtained with a freshly-prepared deposition s o l u t i o n containing 50 mM C i e S H and 10 mM of the bipyridinium i n a 1:1 volume r a t i o of chloroform and methanol. 18

Electrochemistry. C y c l i c voltammetry was performed using gold f l a g electrodes (area 0.7 cm ) and a SCE reference electrode. The e l e c t r o l y t e was 0.1 M KC1 buffered to pH8 with 0.01 M Na HP0*. Data a c q u i s i t i o n , manipulation and p l o t t i n g were performed using a Zenith computer interfaced to a potentiostat v i a a Metrabyte DASH-16 board and running MacMillan ASYSTANT+ software. 2

2

Results and Discussion Mixed monolayers. An electrode coated with a mixed monolayer of C SH and C b p y M e e x h i b i t s a surface redox wave assignable to the reduction of the bipyridinium to the r a d i c a l cation (Figure 1) ( i n a l l 3 figures the c y c l i c voltammograms at 0.1 V/s ( s o l i d l i n e ) , 1 V/s (dashed l i n e ) and 10 V/s (dotted l i n e ) are plotted with the current axis scaled i n proportion to the scan r a t e ) . A second reduction wave at -0.9 V (not shown) i s obscured by increasing background currents. The electrochemical parameters f o r the surface 2+

18

18


29.

F I N K L E A ET AL.

433

Bipyridiniums in Octadecylmercaptan Monolayers

redox wave given i n Table I along with t y p i c a l values f o r the surface redox wave of a mixed monolayer containing d b p y E t . At scan rates less than 0.1 V/s the redox wave obeys the normal r e l a t i o n s h i p s f o r a surface bound species i . e . peak s p l i t t i n g (AE ) i s close to zero, peak currents are proportional to scan r a t e , and the peak f u l l width at h a l f maximum ( A E ^ ^ J i s close to the t h e o r e t i c a l 90 mV. The coverage represents ca. 10% of a close packed monolayer ( 3 x l 0 ~ " mol/cm assuming 50 A per bipyridinium). At higher scan rates the peaks broaden ( p a r t i c u l a r l y the anodic peak), the peak currents are not proportional to scan r a t e , and the apparent coverage decreases. 2 +

8

xo

2

Table I . Electrochemical Parameters f o r the F i r s t Redox Wave of Deposited or Adsorbed Bipyridiniums Al

Scan Rate E (V/s) (

Coating

0 1

AE mV

cat

P

an )

.1 r cat a n cat an (yA/cm ) ( x l O " mol/cm ) p

2

1 0

2

mixed monolayer

.10

-471

24

116

126

2.3

2.2

.30

.30

Ci bpyMe2+ + C SH

1.0 10.

-470 -475

61 63

147

181

-

-

15 56

16 63

.24 .08

.31 .09

mixed monolayer Ci bpyEt

.10

-480

30

110

120

3.1

2.9

.36

.31

adsorbed C bpyMe clean gold

.10

-459

4

124

116

15

14

1.8

1.2

1.0 10.

-461 -467

16 86

119 123

112 134

164 163 1.9 1300 1270 1.5

1.6 1.4

adsorbed C bpyMe

.10

-445

9

153

153

17

17

2.0

2.0

SH1.0 coated gold 10.

-458

54

180 890

1.9 1.2

1.5

-

170 -

114

-

128 185

8

1 8

2

8

+ C SH 1 8

2+

18

2

18

C

1 8

-

-

The f i r s t two e n t r i e s refer to mixed monolayers deposited p r i o r to the electrochemical measurements; the l a s t two e n t r i e s refer to bipyridinium monolayers adsorbed from the e l e c t r o l y t e . J i s the peak current i n the cathodic and anodic d i r e c t i o n s f o r t h i f i r s t redox wave of the bipyridiniums; T i s the coverage found by i n t e g r a t i o n of the respective cathodic and anodic peaks. The other headings have been defined i n the preceding text. Data i s omitted where the surface wave i s not well-defined r e l a t i v e to the background current.



434

I f the bipyridiniums are located at a f i x e d distance from the electrode surface, the surface coverage would be independent of the scan rate. The loss of apparent coverage at fast scan rates indicates that the bipyridiniums are not r i g i d l y f i x e d near the electrode surface. Daifuku et. a l . (10) have also observed an apparent loss of coverage at fast scan rates f o r a Langmuir-Blodgett monolayer of surfactant osmium b i p y r i d i n e complexes. They postulated that parts of the monolayer were not i n contact with the electrode and that the remote 0s(bpy) complexes were oxidized or reduced v i a l a t e r a l e l e c t r o n exchange i n the monolayer. Further experiments with the coated electrode suggests that the surfactant bipyridiniums are mobile w i t h i n the mercaptan monolayer. I f the electrode i s transferred to a f r e s h e l e c t r o l y t e , the coverage (measured at 0.1 V/s) g r e a t l y diminishes. Likewise r i n s i n g the coated electrode with a stream of pure water between c y c l i c voltammograms removes most of the e l e c t r o a c t i v e bipyridiniums. These treatments do not remove the mercaptan monolayer (5). We therefore hypothesize that once the mixed monolayer i s i n contact with water, the surfactant bipyridiniums reside not w i t h i n the mercaptan monolayer but adsorbed to i t s e x t e r i o r . The adsorbed bipyridiniums communicate with the electrode by a small f r a c t i o n of bipyridiniums which penetrate the monolayer. A short-chain analog, methyl viologen (N,N -dimethy1-4,4 -bipyridinium d i c h l o r i d e ) , can penetrate the mercaptan monolayer at s i t e s not accessible to the f e r r i c ion or other inorganic ions (3,5). For example, i n a 1 mM methyl viologen s o l u t i o n the cathodic peak current on a C SH-coated electrode i s attenuated only by 30% r e l a t i v e to the peak current on a clean electrode (0.1 V/s). In a more d i l u t e s o l u t i o n (0.05 mM) the cathodic peak currents are i d e n t i c a l f o r clean and coated electrodes. Yet the same coated electrode i s strongly blocking to F e reduction i n d i l u t e s u l f u r i c a c i d ; the cathodic peak current i s reduced by a f a c t o r of 500 i n the presence of the monolayer. A d d i t i o n a l support f o r the preceding model i s found i n experiments i n which the surfactant bipyridinium i s dissolved i n the e l e c t r o l y t e rather than co-deposited with the mercaptan monolayer. 3

1

1

18

3

Adsorbed bipyridiniums. Surfactant bipyridiniums adsorb onto bare gold from aqueous s o l u t i o n ; saturation i s reached at concentrations of 10 M or greater f o r Ciz,bpyMe (11,12). Figure 2 i l l u s t r a t e s the c y c l i c voltammogram of a 1x10 * M s o l u t i o n of CiebpyMe using a clean gold electrode; Table I contains the electrochemical parameters of the f i r s t redox wave near -0.46 V. The data i n Table I c l e a r l y indicates that the currents are due to an adsorbed monolayer of C i b p y M e . The peak currents due to d i f f u s i o n of the bipyridinium to the electrode are predicted (based on a d i f f u s i o n c o e f f i c i e n t of 7 x l 0 " cm /s (13) to be 23, 71 and 230 yA/cm at the three scan rates. As expected, the coverage i s constant with increasing scan rate (0.1 and 1.0 V/s). At 10 V/s the coverage decreases, possibly because the monolayer i s not f u l l y re-assembled a f t e r desorption at the p o s i t i v e p o t e n t i a l l i m i t (see below). Analysis of the cathodic peak at 10 V/s assuming i r r e v e r s i b l e reduction of an adsorbed reactant (14) y i e l d s values of 70/s f o r the standard rate constant and 0.6 f o r the t r a n s f e r c o e f f i c i e n t . u

2+

2

2+

8

6

2

2


29.

FINKLEA ET AL.


435

Reduction to the neutral r a d i c a l appears as an i r r e v e r s i b l e wave at -0.9 V. Neither anodic peak e x h i b i t s the shape c h a r a c t e r i s t i c of s t r i p p i n g a s o l i d coating from the electrode; hence p r e c i p i t a t i o n of the r a d i c a l cation or neutral r a d i c a l on the electrode i s not evident (11-13). The sharp peaks at +0.46 V are t e n t a t i v e l y assigned to desorption and adsorption of the CiebpyMe ; there are no anticipated redox reactions at that potential. The voltammetry of CiebpyMe ( l x l 0 ~ M) i n the presence of a CisSH monolayer i s s t r i k i n g (Figure 3 and Table I ) . The mercaptan monolayer displaces e l e c t r o l y t e from the e l e c t r o d e / e l e c t r o l y t e i n t e r f a c e ; consequently the i n t e r f a c i a l capacitance and charging current decrease dramatically. For comparison, the i n t e r f a c i a l capacitance at 0.0 V i s 120, 100, and 10 yf/cm f o r clean g o l d i n the KC1 e l e c t r o l y t e , gold with the adsorbed layer of C bpyMe and gold with the mercaptan monolayer. The adsorption/desorption wave at +0.46 _jf i s no longer v i s i b l e . Most notably, a monolayer CiebpyMe i s absorbed on the electrode (compare the f i r s t reduction peak of Figure 3 with the corresponding peak i n Figure 2; the current scales are i d e n t i c a l ) . At 0.1 V/s the coverage on the mercaptan-coated electrode i s i d e n t i c a l to that obtained on the clean electrode (Table I ) . I t i s l i k e l y that the bipyridinium monolayer assembles on the e x t e r i o r of the mercaptan monolayer to form a b i l a y e r structure (15). At 0.1 V/s the bipyridiniums penetrate the C SH monolayer at a s u f f i c i e n t rate that the e n t i r e adsorbed Ci bpyMe monolayer i s reduced. At faster scan rates transportation of charge to the remote CiebpyMe monolayer l i m i t s the current and the surface coverage apparently decreases. The fate of the bipyridinium r a d i c a l cation i n the mercaptan monolayer i s not c l e a r . The second reduction wave (Figure 3) possesses structure suggesting a p r e c i p i t a t e d phase; s i m i l a r sharp peaks are seen during the reductive p r e c i p i t a t i o n of C i 4 b p y M e (11). Yet at 0.1 V/s the anodic peak due to the oxidation of the r a d i c a l cation does not e x h i b i t the shape c h a r a c t e r i s t i c of s t r i p p i n g of a s o l i d phase. At f a s t e r scan rates the anodic peak broadens considerably and s p l i t s into two peaks; the same behavior i s noticeable i n Figure 1. We do not have an explanation f o r t h i s phenomenon. A recent t h e o r e t i c a l treatment of redox molecules attached to electrode surfaces predicts that under c e r t a i n conditions an anodic surface wave can broaden and s p l i t with increasing scan rate i n a manner shown i n Figure 3 (16). However the same theory predicts that the corresponding cathodic peak normalized to constant scan rate w i l l increase with increasing scan rate. The l a t t e r p r e d i c t i o n i s not observed i n our system. 2

+

2

4

2

+

2

18

2

18

2+

8

2

2+

Conclusion. Our major conclusion i s that surfactant bipyridinium dications do not remain at a f i x e d l o c a t i o n w i t h i n a mercaptan monolayer. In order to c o n t r o l the spacing and o r i e n t a t i o n of the bipyridiniums i n the monolayer, i t w i l l be necessary to anchor them w i t h i n the monolayer or to the electrode. We are currently pursuing that goal by synthesizing a bipyridinium with a terminal mercaptan.


436


0

E

4.00

2.00 .000-

-2.00-4.00.700

.500

.300

.100 E 0

-.100

Figure 1. C y c l i c voltammograms of a mixed monolayer of C SH + (C bpyMe)Cl . 18

18

2

-1.00

-.600

Figure 2 . C y c l i c voltammograms of C bpyMe s o l u t i o n on a clean gold electrode. i8

E

adsorbed from

0 2.00 1.00 .000

-1.00 -2.004 -1.00

-.600

.200

.200

.600 E 0

Figure 3 . C y c l i c voltammograms of CiebpyMe adsorbed from s o l u t i o n on a gold electrode coated with a C I B S H monolayer. 2


29.

FINKLEA ET AL.


Acknowledgments Acknowledgement i s made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, f o r support of t h i s research. L i t e r a t u r e Cited

1.

Nuzzo, R. G.; Allara, D. L. J . Am. Chem. Soc. 1983, 105,

2.

Nuzzo, R. G.; Fusco, F. A . ; Allara, D. L. J . Am. Chem. Soc.

3.

Porter, M. D.; Bright, T. B . ; Allara, D.; Chidsey, C. E. D.

4481. 1987, 1 0 9 , 2358.

J. Am. Chem. Soc. 1987, 109, 3559.

4.

Sabitini, E . ; Rubinstein, I . ; Maoz, R.; Sagiv, J . J. Electroanal. Chem. 1987, 219, 365. 5. Finklea, H. O.; Avery, S.; Lynch, M . ; Furtsch, T. Langmuir 1987, 3, 409. 6. Lane, R. F . ; Hubbard, A. T. J . Phys. Chem. 1973, 77, 1401. 7. Bravo, B. G.; Mebrahtu, T.; Soriaga, M . ; Zapien, D. C.; Hubbard, A. T.; Stickney, J . L. Langmuir 1987, 3, 595. 8. Li, T. T.-T.; L i u , H. Y . ; Weaver, M. J . Am. Chem. Soc. 1984, 106,

1233.

9. Li, T. T.-T.; Weaver, M. J . Am. Chem. Soc. 1984, 106, 6107. 10. Daifuku, H . ; Aoki, K . ; Tokuda, K . ; Matsuda, H. J . Electroanal. Chem. 1985, 183, 1. 11. Enea, O . Electrochim. Acta 1986,31,789. 12. Crouigneau, P.; Enea, O.; Beden, B. J . Electroanal. Chem. 1987, 218,

13.

307.

Bruinink, J.; van Zanten, P.

J . Electrochem. Soc. 1977, 124,

1232.

14. 15. 16.

Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications, J. Wiley & Sons: New York, 1980: p. 525. M i l l e r , C. J.; Majda, M. J . Am. Chem. Soc. 1986, 108, 3118. Matsuda, H . ; Aoki, K . ; Tokuda, K. J . Electroanal. Chem. 1987, 217, 1 and 15.

RECEIVED May 1 7 , 1988


437

Electrochemical Surface Science - American Chemical Society

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