Spectroscopic and Electrochemical Characterizations of Dilithium

and Classical Molecular Dynamics Studies of the Dilithium Phthalocyanine/Pyrite Interfacial Structure. Yingchun Zhang , Yixuan Wang , Lawrence G. ...
0 downloads 0 Views 100KB Size
Download PDF
Langmuir 2002, 18, 2223-2228

2223

Spectroscopic and Electrochemical Characterizations of Dilithium Octacyanophthalocyanine Langmuir-Blodgett Films Hong-Qi Xiang,† Keiji Tanaka,† Atsushi Takahara,‡ and Tisato Kajiyama*,† Department of Applied Chemistry, Faculty of Engineering, and Institute for Fundamental Research of Organic Chemistry, Kyushu University, Fukuoka 812-8581, Japan Received September 7, 2001. In Final Form: December 31, 2001 Langmuir-Blodgett (LB) films of dilithium octacyanophthalocyanine [Li2Pc(CN)8] were first prepared, and their electrochemical reduction/reoxidation behaviors were studied in detail. An overwhelming majority of the Li2Pc(CN)8 molecules in the LB film were stacked in a face-to-face aggregated state and stood obliquely with an edge-on configuration on the substrate surface. Two overlapping redox waves were observed in the voltammogram of the LB film in 1 mol dm-3 HCl solution. Their relative intensity was found to be largely dependent on the molecular aggregation state in the film. On the basis of spectroscopic and electrochemical analyses, the first and second reduction waves upon cathodic potential scanning were principally assigned to the aggregated and monomeric Li2Pc(CN)8 molecules, respectively, in the solid film structure. In comparison with the corresponding solvent-cast film, charge transfer within the LB film and/or through the electrode interface was facilitated, and the LB film showed excellent dynamic character in the redox process.

Introduction The family of phthalocyanines (Pc’s) exhibits many intriguing properties, such as intense color, high thermal and chemical stabilities, redox activity, semiconductivity, photovoltaic effect, and so on, on account of its highly conjugated and disc-shaped molecular structure. This extraordinary versatility has established Pc’s as attractive candidates for photovoltaic cells, molecular metals, chemical sensors, nonlinear optics, and electrochromic display devices.1 To realize such highly functionalized materials, extensive research has been conducted on Pc-based thin solid films. Many experimental techniques, e.g., vacuum evaporation, solvent casting, spin coating, and LangmuirBlodgett (LB) methods, have been established to prepare organic thin films. Of these, the LB technique is most effective in controlling thickness and structure on a molecular level.2 Hence, the LB films of Pc derivatives substituted at the periphery of their structure with various bulky alkyl and/or alkoxy groups, including tert-butyl,3a,4-9b * To whom correspondence should be addressed. Tel.: +81-92642-3558. Fax: +81-92-651-5606. E-mail: [email protected]. † Department of Applied Chemistry, Faculty of Engineering. ‡ Institute for Fundamental Research of Organic Chemistry. (1) In PhthalocyaninessProperties and Applications; Lezonff, C. C., Lever, A. B. P., Eds.; VCH: New York, 1989, 1993, 1996; Vols. 1-4. (2) Ulman, A. An Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembly; Academic Press: New York, 1991; Chapter 2.7.B and references therein. (3) (a) Cook, M. J.; Dunn, A. J.; Daniel, M. F.; Hart, R. C. O.; Richardson, R. M.; Roser, S. J. Thin Solid Films 1988, 159, 395-404. (b) Velez, M.; Vieira, S.; Chambrier, I.; Cook, M. J. Langmuir 1998, 14, 4227-4231. (4) Gupta, S. K.; Hann, R. A.; Twigg, M. V. Thin Solid Films 1989, 179, 343-349. (5) (a) Gobernado-Mitre, M. I.; Aroca, R.; DeSaja, J. A. Langmuir 1993, 9, 2185-2189. (b) Gobernado-Mitre, M. I.; Aroca, R.; DeSaja, J. A. Langmuir 1995, 11, 547-550. (6) Vertsimakha, Y. Synth. Met. 2000, 109, 287-289. (7) Emelianov, I. L.; Khatko, V. V. Thin Solid Films 1999, 354, 237244. (8) Liu, Y.-Q.; Shigehara, K.; Hara, M.; Yamada, A. J. Am. Chem. Soc. 1991, 113, 440-443.

hexyl,3b octyl,10 decyl,3b,10 dodecyl,10 methoxy,11 butoxy,12,13 octyloxy,11,14b benzyloxyethoxy,15 decyloxy,16 or cumylphenoxy,3b,14,17,18 have been prepared and characterized in detail because of their good amphiphilic character. In contrast, Pc derivatives with electron-withdrawing substituents would show different physical behaviors. In particular, the phthalocyanine substituted by eight cyano groups shows strong n-type semiconductor properties, rather than the p-type properties of general phthalocyanines, as well as easy reduction at more positive potentials.19 Strong electron-withdrawing substituents on the phthalocyanine ring increase its ionization potential and electron affinity. Such features give rise to a marked sensitivity toward electron-donating gases, which is of particular interest in chemical sensors.20a Nevertheless, little study has been done on LB films of Pc derivatives (9) (a) Baker, S.; Petty, M. C.; Roberts, G. G.; Twigg, M. V. Thin Solid Films 1983, 99, 53-59. (b) Hua, Y. L.; Roberts, G. G.; Ahmad, M. M.; Petty, M. C.; Hanack, M.; Rein, M. Philos. Mag. B 1986, 53, 105-113. (10) (a) Nakahara, H.; Fukuda, K.; Kitahara, K.; Nishi, H. Thin Solid Films 1989, 178, 361-366. (b) Ogawa, K.; Yonehara, H.; Pac, C. Langmuir 1994, 10, 2068-2070. (11) (a) Sauer, T.; Arndt, T.; Batchelder, D. N.; Kalachev, A. A.; Wegner, G. Thin Solid Films 1990, 187, 357-374. (b) Ferencz, A.; Armstrong, N. R.; Wegner, G. Macromolecules 1994, 27, 1517-1528. (12) Jones, R.; Hunter, R. A.; Davidson, K. Thin Solid Films 1994, 250, 249-257. (13) Jing, F.; Kareem, R. A.; Srinivasan, M. P. Mater. Sci. Eng. C 1999, 8-9, 103-106. (14) (a) Snow, A. W.; Jarvis, N. L. J. Am. Chem. Soc. 1984, 106, 4706-4711. (b) Barger, W. R.; Snow, A. W.; Wohltjen, H.; Jarvis, N. L. Thin Solid Films 1985, 133, 197-206. (c) Pace, M. D.; Barger, W. R.; Snow, A. W. Langmuir 1989, 5, 973-978. (15) (a) Osburn, E. J.; Chau, L. K.; Chen, S. Y.; Collins, N.; O’Brien, D. F.; Armstrong, N. R. Langmuir 1996, 12, 4784-4796. (b) Smolenyak, P.; Peterson, R.; Nebesny, K.; Torker, M.; O’Brien, D. F.; Armstrong, N. R. J. Am. Chem. Soc. 1999, 121, 8628-8636. (c) Smolenyak, P. E.; Osburn, E. J.; Chen, S.-Y.; Chau, L.-K.; O’Brien, D. F.; Armstrong, N. R. Langmuir 1997, 13, 6568-6576. (16) Schutte, W. J.; Sluyters-Rehbach, M.; Sluyters, J. H. J. Phys. Chem. 1993, 97, 6069-6073. (17) Burack, J. J.; LeGrange, J. D.; Markham, J. L.; Rockward, W. Langmuir 1992, 8, 613-618. (18) Baker, A. M.; Danzer, J.; Desaire, H.; Credo, G.; Flitton, R. Langmuir 1998, 14, 5267-5273. (19) Wohrle, D.; Schumann, B.; Schmidt, V. Makromol. Chem., Macromol. Symp. 1987, 8, 195-198.

10.1021/la011401a CCC: $22.00 © 2002 American Chemical Society Published on Web 02/21/2002

2224

Langmuir, Vol. 18, No. 6, 2002

Xiang et al.

with electron-withdrawing substituents owing to their poor solubility in common organic solvents. In this study, the LB film of a metallo phthalocyanine derivative with eight strong electron-withdrawing groups, dilithium octacyanophthalocyanine [Li2Pc(CN)8], is successfully prepared for the first time. Molecular aggregation states and electrochemical reduction/reoxidation behaviors of the LB film are studied by polarized/nonpolarized electronic absorption spectroscopy, cyclic voltammetry, and differential pulse voltammetry. It is apparent that the redox behaviors of the Li2Pc(CN)8 films are closely related to the molecular aggregation states in the films. In comparison with the corresponding solvent-cast film, the Li2Pc(CN)8 LB film exhibits excellent dynamic character in the electrochemical redox process. Experimental Section Preparation of Li2Pc(CN)8 LB Films. Li2Pc(CN)8 was synthesized through a lithium-propoxide-catalyzed cyclotetramerization of 1,2,4,5-tetracyanobenzene in refluxing n-propanol, following the procedure proposed by Wohrle et al.21 and Hieber.22 The product was purified by being passed through a TSK Toyopearl HW-40C gel column with dimethyl formamide (DMF) as the eluent. Complexes with other metal atoms, for example, CoPc(CN)8 and CuPc(CN)8, can be obtained by further treating Li2Pc(CN)8 with the corresponding metal salts;21 however, their synthesis and purification suffer possible side reactions such as oligomerization and saponification of the peripheral cyano groups.22 Hence, the current study focuses on the dilithium complex of octacyanophthalocyanine. LB films of Li2Pc(CN)8 were prepared on a computer-controlled Langmuir trough. The subphase used was distilled, deionized, and filtered (Millipore Milli-Q Plus unit) water, with a resistivity of greater than 18 MΩ. The subphase temperature was controlled to be 293 ( 0.5 K for all experiments. The solubility of the Li2Pc(CN)8 in common spreading solvents for monolayer preparation is quite poor on account of its polar nature. Hence, a mixed solvent composed of benzene (Bz) and DMF was used as the spreading solvent. Although DMF itself does not spread on the water surface, it is a good solvent for Li2Pc(CN)8. Thus, DMF was chosen as a component of the mixed solvent. In the case that a Li2Pc(CN)8 solution of 0.105 mmol dm-3 in Bz/DMF (2/1 v/v) was used, a monolayer was successfully formed at the air/water interface.23 A surface pressure (π) vs occupied molecular area (A) isotherm was obtained at a constant compression speed of 0.08 mm s-1 after allowing 50 min for complete evaporation of the solvent. The magnitude of π was measured by the Wilhelmy balance technique with a filter paper plate. Monolayer transfer onto a solid substrate, indium tin oxide (ITO) glass or highly oriented pyrolytic graphite (HOPG), was performed at π ) 11 mN m-1. The ITO substrate was washed successively with detergent, (20) (a) Fujiki, M.; Tabei, H. Langmuir 1988, 4, 320-326. (b) Fujiki, M.; Tabei, H.; Kurihara, T. Langmuir 1988, 4, 1123-1128. (21) (a) Wohrle, D.; Wahl, B. Tetrahedron Lett. 1979, 227-228. (b) Wohrle, D.; Meyer, G.; Wahl, B. Makromol. Chem. 1980, 181, 21272135. (22) Hieber, G. Study on the synthesis of 2,3,9,10,16,17,23,24Octacyanophthalocyanino Complexes. Master’s thesis, Eberhard-Karls University, Tubingen, Germany, 1987. (23) Yanagi et al. prepared thin films of octacyanophthalocyanines coordinated with three metal atoms, M2Pc(CN)8-M (M ) Na, K, Ru, Cs), by the chemical vapor deposition (CVD) method. They reported the hydrolysis of M2Pc(CN)8-M to H2Pc(CN)8-M after dipping their CVD films into water by high-resolution electron microscopy [See: (a) Ashida, M.; Ueda, Y.; Yanagi, H.; Uyeda, N.; Fujiyoshi, Y.; Fryer, J. R. Acta Crystallogr. 1988, B44, 146-151. (b) Ueda, Y.; Yanagi, H.; Hayashi, S.; Ashida, M. J. Electron Microsc. 1989, 38, 101-110]. However, in the case of our Li2Pc(CN)8 molecules, Hieber found, by the elemental analysis method using an atomic absorption spectrophotometer, that the Li2Pc(CN)8 molecules still hold a lithium content of 36.6% after being stirred for several days in a HCl solution of pH 1 at room temperature and that, even at higher reaction temperatures, 32.6% remained (see ref 22). The substitution reaction did not proceed as smoothly as expected. Hence, it is quite reasonable to consider that little substitution of Li atoms by hydrogen atoms, at most a small amount, occurred for Li2Pc(CN)8 on the time scale and under the experimental conditions of our study.

Figure 1. Surface pressure vs molecular area isotherm of the Li2Pc(CN)8 monolayer at the air/water interface. The chemical structure of Li2Pc(CN)8 is also shown. distilled water, methanol, and acetone using ultrasonic waves for 30 min each. The cleaned substrate was dried and immediately used. In the case of HOPG, a very smooth surface was cleaved just prior to use. Three methods were applied to transfer the Li2Pc(CN)8 monolayer onto the solid substrate: (i) lowering the subphase level by slowly sucking out the subphase water through the area beyond a barrier (LSL), (ii) horizontal dipping (HD), and (iii) vertical dipping (VD). The latter two methods were performed with upstroke and downstroke speeds of approximately 5 mm min-1. Electronic Absorption Spectroscopic Characterization. Electronic absorption spectra were recorded using a Shimadzu UV-3100PC spectrometer with a slit width of 2.0 nm. A bare ITO substrate or a solvent-filled quartz cell was used as the reference for the measurements of the film or solution samples, respectively. In the case of the film samples, a sample stage that could be rotated about three normal axes was used for polarized electronic absorption spectroscopic analysis. To increase the absorption signal intensity for a quantitative dichromism evaluation, a stack of eight pieces of monolayer-coated ITO substrate was assembled. In the stack configuration, frame-shaped thin spacers were sandwiched between each pair of adjacent substrates to protect the monolayers from damage and also to allow the light pass. Eight pieces of monolayer-free ITO substrate were stacked in the same way and used as a reference for the polarized spectrum measurements. Electrochemical Measurements. Electrochemical measurements were performed on a BAS CV-50W bioanalytical system with a conventional three-electrode electrolysis cell. The electrolyte solution of 1 mol dm-3 HCl was deoxygenated by being bubbled with nitrogen gas prior to use. A nitrogen atmosphere was maintained in the cell throughout the measurements. The working electrode was constructed by attaching an aluminum string to the ITO glass or the HOPG substrate with silver paste. The ITO or HOPG substrate was vertically dipped into the electrolyte solution with its aluminum string over the liquid level. The HOPG electrode was wrapped with PTFE tape to expose the LB-film-coated surface solely to the electrolyte. A platinum coil was used as the counter electrode, and a saturated calomel electrode (SCE) as the reference electrode.

Results and Discussion Monolayer Formation and Transfer. Figure 1 shows the π-A isotherm for the Li2Pc(CN)8 monolayer at the air/water interface. Because π abruptly increased in the vicinity of the limiting area, which was specified by extrapolation to π ) 0 of the straight portion of the π-A curve, it seems most likely that a solid monolayer was formed at the air/water interface.24 The monolayer would be stable up to the collapse pressure of around 22 mN

Characterizations of Li2Pc(CN)8 LB Films

Figure 2. Electronic absorption spectra of Li2Pc(CN)8 in 0.105 mmol dm-3 Bz/DMF solution and in the monolayer LB film on ITO glass substrate.

m-1. The reproducibility of the π-A curve was excellent for all runs of the monolayer preparation. On the basis of the π-A curve, the limiting area was determined to be 0.52 nm2 molecule-1, as shown in Figure 1. The edge areas of unsubstituted and tetra-tert-butylsubstituted Pc have been reported to be about 0.40 and 0.47 nm2, respectively, assuming that the diagonal distance and thickness of the molecular ring were 1.95 and 0.34 nm,9a,9b,14a respectively. Because Li2Pc(CN)8 has eight small cyano groups as substituents, it is reasonable to infer that its edge area would be intermediate between those of unsubstituted and tetrabutyl-substituted Pc’s. This means that, if the Li2Pc(CN)8 molecules were densely packed in a face-to-face orientation with an edge-on configuration on the water surface,25 the surface area per molecule would be in the range of 0.40-0.47 nm2 molecule-1. However, the limiting area of Li2Pc(CN)8 was 0.52 nm2 molecule-1, as shown in Figure 1. Hence, it is plausible that the Li2Pc(CN)8 molecules are in a tilted edge-on arrangement at the air/water interface. This issue is discussed later in detail in relation to the results of polarized electron absorption spectroscopy. The Li2Pc(CN)8 monolayer on the subphase was transferred onto an ITO substrate at π ) 11 mN m-1 by the LSL method. When the HD method was applied, the transfer ratio was approximately 2, indicating that the bilayer Li2Pc(CN)8 film was formed on the ITO substrate. On the other hand, the VD method gave only poor transfer, probably because of the stiff character of the film. Molecular Aggregation States. In general, it has been widely accepted that planar Pc molecules are easily stacked in a face-to-face manner to form aggregates in the solution state. Therefore, in solution, the Pc molecules exist in an equilibrium partition between monomeric and aggregated states.14a The electronic absorption spectrum of the Li2Pc(CN)8 solution for monolayer spreading, curve 1 in Figure 2, shows such an equilibrium partition. The spectrum in the Q-band range consists of a strong narrow π-π* absorption at 698 nm and a second broad and partially resolved π-π* absorption at 652 nm. According to previous studies of Pc solutions,26-29a the narrow absorption band at longer wavelength can be attributed to monomeric Li2Pc(CN)8 molecules, and the broad par(24) (a) Kajiyama, T.; Kozuru, H.; Takashima, Y.; Oishi, Y.; Suehiro, K. Supramol. Sci. 1995, 2, 107-116. (b) Kajiyama, T.; Oishi, Y. In New Developments in Construction and Functions of Organic Thin films; Kajiyama, T., Aizawa, M., Eds.; Elsevier Science B.V.: Amsterdam, 1996; Chapter 1. (25) Palacin, S. Adv. Colloid Interface Sci. 2000, 87, 165-181.

Langmuir, Vol. 18, No. 6, 2002 2225

tially resolved absorption to aggregated Li2Pc(CN)8 molecules. Extensive exciton coupling between adjacent faceto-face π-electron systems in aggregated Pc species was considered to account for the blue shift of the second absorption.30,31 Also, curve 2 in Figure 2 illustrates the electronic absorption spectrum of the Li2Pc(CN)8 LB monolayer. In contrast to the spectrum for the solution system, the absorption band at 652 nm became the most intense one, and only a weak shoulder peak was detected at 698 nm for the monomeric Pc species, indicating that the aggregated species dominated in the Li2Pc(CN)8 LB film. Further information on the molecular organization in the Pc LB film can be obtained by using polarized light. Parts a and b of Figure 3 show the experimental configuration and the polarized electronic absorption spectra, respectively, of the monolayer Li2Pc(CN)8 LB film on an ITO substrate. An obvious dichroism at both incident angles of 0 and 30° was observed, as shown in Figure 3b. The dichroic ratio D is defined as A|/A⊥, where A| and A⊥ are the absorbances of the film obtained with s- and p-polarized light, respectively. Assuming that the π-π* transition dipoles of the Q-band are uniformly distributed in the phthalocyanine ring plane, the orientation angles can be calculated by the method of Yoneyama et al.32 Table 1 reports the absorbance at 652 nm and the mean orientation angles of a Li2Pc(CN)8 molecule in the LB film. The azimuthal angle θ was calculated to be 29.8°. Postulating that the Li2Pc(CN)8 monolayer on the water surface was transferred onto the ITO glass without any structural changes, this result is in good accordance with the conclusion made on the basis of the π-A isotherm that the Li2Pc(CN)8 molecules were in a tilted edge-on arrangement at the air/water interface. Hence, it can be claimed that the planar Li2Pc(CN)8 molecules formed an inclined stack with the stacking axis parallel to the substrate. Molecular aggregation states in a film are strongly dependent on the method of preparation. Hence, such an effect was examined next. Figure 4 illustrates the electronic absorption spectra for Li2Pc(CN)8 films prepared by the different methods discussed in the Experimental Section. For LB films deposited on the ITO substrate by both the lowering-subphase-level (LSL) and horizontaldipping (HD) methods, a strong absorption corresponding to the aggregated species and a weak shoulder arising from the monomeric ones were observed at 652 and 698 nm, respectively. In contrast to these LB films, the control solvent-cast film exhibited a shoulder peak with increased intensity in the monomeric absorption region, indicating that a relatively large portion of monomeric Li2Pc(CN)8 molecules remained in the film. (26) Mack, J.; Stillman, M. J. J. Phys. Chem. 1995, 99, 7935-7945. (27) (a) Pasimeni, L.; Meneghetti, M.; Rella, R.; Valli, L.; Granito, C.; Troisi, L. Thin Solid Films 1995, 265, 58-65. (b) Capone, S.; Mongelli, S.; Rella, R.; Siciliano, P.; Valli, L. Langmuir 1999, 15, 1748-1753. (28) Yang, Y. C.; Ward, J. R.; Seiders, R. P. Inorg. Chem. 1985, 24, 1765-1769. (29) (a) Kobayashi, N.; Higashi, R.; Ishii, K.; Hatsusaka, K.; Ohta, K. Bull. Chem. Soc. Jpn. 1999, 72, 1263-1271. (b) Isago, H.; Leznoff, C. C.; Ryan, M. F.; Metcalfe, R. A.; Davids, R.; Lever, A. B. P. Bull. Chem. Soc. Jpn. 1998, 71, 1039-1047. (c) Kobayashi, N.; Lam, H.; Nevin, W. A.; Janda, P.; Leznoff, C. C.; Koyama, T.; Monden, A.; Shirai, H. J. Am. Chem. Soc. 1994, 116, 879-890. (30) Mataga, N.; Kubota, T. Molecular Interactions and Electronic Spectra; Marcel Dekker: New York, 1970; Chapters 2-16 and 2-17. (31) Cook, M. J. Optical and Infrared Spectroscopy of Phthalocyanine Molecular Assemblies. In Spectroscopy of New Materials; Clark, R. J. H., Hester, R. E., Eds.; John Wiley & Sons: New York, 1993. (32) Yoneyama, M.; Sugi, M.; Saito, M.; Ikegami, K.; Kuroda, S.; Iizima, S. Jpn. J. Appl. Phys. 1986, 25, 961-965.

2226

Langmuir, Vol. 18, No. 6, 2002

Xiang et al.

Figure 4. Electronic absorption spectra of the Li2Pc(CN)8 solvent-cast film, and Li2Pc(CN)8 LB films prepared by the HD and LSL methods. Each spectrum is normalized by its maximum value.

Figure 5. Polarized electronic absorption spectra of the Li2Pc(CN)8 solvent-cast film on an ITO glass substrate. Figure 3. Polarized electronic absorption analysis for a stack of eight pieces of the monolayer Li2Pc(CN)8 LB film on an ITO substrate. (a) Schematic representation of the experimental geometry. The Y and Z axes in Cartesian coordinates (XYZ) are chosen to be perpendicular to the barrier of the trough and normal to the substrate, respectively. Also, the Z′ axis is the center axis of the Pc molecule represented by the circular plate. The polarized light is directed onto the film at an incident angle of β′ in the X-Z plane. (b) Resulting spectra obtained with sand p- polarized light at β′ ) 0 and 30°. Table 1. Experimental Data and Calculated Results for the Polarized Electron Absorption Spectroscopy of a Stack of Eight Pieces of Monolayer Li2Pc(CN)8 LB Film on ITO Substratea β′ ) 0° β′ ) 30°

A|

A⊥

D

〈cos2 θ〉b

〈cos2 φ〉b

〈θ〉b

〈φ〉b

0.175 0.169

0.151 0.127

1.158 1.334

0.753

0.760

29.8°

29.4°

a No refractive index (n) data for Li Pc(CN) LB film is available. 2 8 However, the most preferable refraction angle for 45° incidence was found to be 38° for several Pc LB monolayers (see ref 32), meaning a refractive index value of 1.148. Hence, n ) 1.148 was taken in this calculation. b Angle brackets denote a statistical average.

Solvent evaporation breaks the equilibrium partition between the aggregated and monomeric Pc species and is the sole driving force for the formation of more aggregated species in the solvent-cast film. It is plausible that some portion of the Li2Pc(CN)8 molecules are unable to adjust their orientations before the completion of solvent evaporation and are present as monomeric species in the

resulting film. In addition, the stacking axes of the aggregates might be randomly distributed. Figure 5 shows the polarized electronic absorption spectra of the solventcast film on an ITO substrate. In comparison with the obvious dichroism shown in Figure 3, a weak dichroism was observed for the solvent-cast film, indicating its much less ordered molecular alignment. In contrast, for the LB method, the Li2Pc(CN)8 molecules would be allowed to adjust their orientation better. The ordering process of the Pc molecules spread at the air/water interface in this case is driven not only by solvent evaporation but also by compression with the aid of the moving barrier of the LB trough. The ordering of the formed Pc aggregates would especially be improved by the latter factor. Electrochemical Reduction/Reoxidation Behaviors. Because a bare ITO substrate is electrochemically inactive in the potential range from 600 to -400 mV vs SCE, as depicted in Figure 6a, it can be used as a conductive substrate for electrochemical measurements. Figure 6b shows the cyclic voltammetric (CV) curve of the second cycle for the bilayer Li2Pc(CN)8 LB film coated on the ITO electrode. The LB film was electrochemically reduced and then reoxidized. Two overlapping cathodic current waves can be distinguished at 30 and -38 mV, as marked by I and II, and also two anodic waves appeared at 80 and -9 mV, as denoted by I′ and II′. The shape of each redox pair was symmetric. In addition, the separation of each cathodic and corresponding anodic current wave was less than 50 mV. These results apparently indicate the good reversibility of the solid-state redox process. The

Characterizations of Li2Pc(CN)8 LB Films

Langmuir, Vol. 18, No. 6, 2002 2227

Figure 7. Dependence of the cathodic peak current on the scan rate and its square root for the monolayer LB film and the solvent-cast film. The straight lines represent linear fits to the data.

Figure 6. Cyclic voltammograms of (a) the bare ITO substrate, (b) the bialyer Li2Pc(CN)8 LB film on ITO, and (c) the monolayer Li2Pc(CN)8 LB film on ITO (the first 160 cycles) at a scan rate of 40 mV s-1. The arrows in the part c indicate the directions of change with cyclic potential scanning.

charge passed under the curve was calculated to be 3.1 F mol-1 for both the cathodic and anodic branches of the voltammogram, which is larger than the value calculated for the corresponding solvent-cast film, 2.9 F mol-1 (not shown). The Li2Pc(CN)8 molecules in the LB film configuration were more deeply reduced in comparison with the molecules in the solvent-cast film. This result makes it clear that the ordered molecular orientation in the LB film facilitated the charge transfer through the electrode interface and/or the charge transport within the LB film. Figure 6c illustrates the CV curves of the monolayer Li2Pc(CN)8 LB film for the first 160 cycles. The intensities of the first pair of redox waves gradually decreased with increasing number of repeated cycles, whereas the intensities of the shoulder-shaped second pair of waves gradually increased, as shown by the arrows. Because the change in wave intensity was quite small even after 160 cycles, however, it can be claimed that the Li2Pc(CN)8 LB film exhibited high electrochemical stability. The dynamic character of the electrode process was further examined. Figure 7 shows the scan rate dependence of the cathodic peak current for the monolayer LB film and the solvent-cast film of Li2Pc(CN)8. In the case of the monolayer LB film, the linear relation between peak current and scan rate was maintained up to 4000 mV s-1. This limiting rate was approximately 3 times faster than that of the solvent-cast film, suggesting the possibility of

Figure 8. Cyclic voltammetry and differential pulse voltammetry of the solvent-cast film, the LB films prepared by the LSL method on HOPG and ITO, and the LB film prepared by the HD method on ITO. Each curve is normalized by its maximum value and vertically shifted for clarity.

a more rapid and sensitive response in chemical sensor applications.1 Nernstian behavior of the electrode over such a wide scan rate range should be related to the molecular-level ordering in the LB film configuration. Beyond 6000 mV s-1, the cathodic peak currents of both films increased linearly with the square root of the scan rate. This result makes it clear that the electrode process is diffusion-controlled in this scan rate region. Figure 8 shows the CV and differential pulse voltammetric (DPV) curves of Li2Pc(CN)8 films prepared by different methods. The DPV analysis was carried out because it provides better resolution and sensitivity than CV. It was found that, in the case of the LB films transferred onto the ITO substrate by the LSL and HD methods, the first cathodic reaction was dominant, and

2228

Langmuir, Vol. 18, No. 6, 2002

the shoulder-shaped second wave was small. On the other hand, the second cathodic wave had an intensity close to that of the first wave for the solvent-cast film. For the LB film transferred on the HOPG substrate by the LSL method, an intermediate case was observed. Hence, it is clear that the relative intensities of the redox waves were strongly dependent on the film preparation method and the substrate character. Because different film preparation methods correspond to different molecular aggregation states, it is conceivable that the discrepancy in the voltammograms of the different films arose from the different aggregation states in the films. A careful comparison between Figures 4 and 8 reveals a resemblance in outline between the electronic absorption and the corresponding voltammetric results for the Li2Pc(CN)8 film. That is, both the Q-band absorption and the voltammetric current response were composed of two parts. The aggregated Q-band absorption and the first cathodic wave were dominant in the spectrum and in the voltammogram, respectively. In addition, the relative intensities of the two absorption bands and of the two redox waves varied in the same manner when a different technique was applied to prepare the Li2Pc(CN)8 film. This phenomenon implies that the first and second voltammetric waves upon cathodic potential scanning can be assigned to the redox reaction of the aggregated and monomeric Li2Pc(CN)8 molecules, respectively. To strengthen this assignment, two points should be addressed. The first is the reduction sequence of the aggregated and monomeric Li2Pc(CN)8 species. Because the aggregates were formed through a face-to-face stacking of the Li2Pc(CN)8 molecules, a coupling effect between adjacent phthalocyanine π-electron systems is present. This coupling effect favors a wide distribution of charges throughout the system and confers extra stability to the reduced Li2Pc(CN)8 molecules. Thus, it is quite reasonable that the reduction of the aggregated Li2Pc(CN)8 molecules first occurred during the course of the cathodic potential scanning, as has already been reported for the solutionstate reduction of other Pc molecules.29b,33 The second point relates to the stability of the aggregated Li2Pc(CN)8 species after reduction. In the electrochemical reduction of MPc(CN)8 (where M ) Zn, Cu, H2) in DMF solution, only monoanions of the aggregated MPc(CN)8 were detected, and further reduction resulted in their disassociation.33 In analogy, the molecular aggregation state should affect the first reduction wave of the Li2Pc(CN)8 films greatly and the latter step minimally. Nevertheless, the experimental result turned out to be opposite, as shown in Figure 8. To overcome this difficulty, the idea that dianions and even trianions of the aggregated Li2Pc(CN)8 molecules were also stable in the solid-state films was taken into account. Therefore, on one hand, the potentials of approximately three single-electron-transfer steps merged to give broad waves in the solid-state reduction.34,35 On the other hand, because of the extensive (33) (a) Giraudeau, A.; Louati, A.; Gross, M.; Andre, J. J.; Simon, J.; Su, C. H.; Kadish, K. M. J. Am. Chem. Soc. 1983, 105, 2917-2919. (b) Louati, A.; Elmeray, M.; Andre, J. J.; Simon, J.; Kadish, K. M.; Gross, M.; Giraudeau, A. Inorg. Chem. 1985, 24, 1175-1179. (34) Schumann, B.; Wohrle, D.; Jaeger, N. I. J. Electrochem. Soc. 1985, 132, 2144-2149. (35) Takeshita, K.; Aoyama, Y.; Ashida, M. Bull. Chem. Soc. Jpn. 1991, 64, 1167-1172.

Xiang et al.

coupling effect, the merged reduction potentials of the Li2Pc(CN)8 molecules in the aggregation state were shifted to values of higher voltage than in the monomeric state. Consequently, in the final voltammogram, the aggregated Li2Pc(CN)8 molecules contributed mainly to the current response at the higher-potential side, and the monomeric species contributed mainly to the current response of the lower potential side. The reduction of the Li2Pc(CN)8 films was dependent on their aggregation state and, in turn, reacted to it.36 In this study, although aggregated dianions and even trianions were formed in the reduction process, strong electrostatic repulsion might lead in part to disassociation of the reduced aggregated species because of their intrinsic instability as reported for their reduction in the solution state.33 On the time scale of the subsequent anodic potential sweeping process, the disassociation might be unable to be completely recovered. Therefore, the second redox waves, denoted by II and II′ and relating mainly to monomeric Pc species, gradually increase in intensity with cyclic potential scanning. Such a result was already observed in the voltammetric curves of the first 160 cycles for the monolayer Li2Pc(CN)8 LB film, as shown in Figure 6c. This fact strongly supports the proposed assignment model for the voltammetric waves of the Li2Pc(CN)8 solid films. Conclusions LB films of dilithium octacyanophthalocyanine have been successfully prepared. Electronic absorption spectroscopic analysis revealed that the Li2Pc(CN)8 LB film was composed of an overwhelming proportion of aggregated Pc molecules. The aggregated Li2Pc(CN)8 molecules were stacked in a face-to-face manner and obliquely stood with an edge-on configuration on the ITO substrate. The electrochemical reduction/reoxidation behavior of the Li2Pc(CN)8 films was found to be strongly dependent on the molecular aggregation state, which can be altered by using different film preparation techniques and different types of solid substrates. In comparison with the corresponding solvent-cast film, the Li2Pc(CN)8 LB film showed excellent dynamic character in the redox process, and also the charge transfer within the LB film and/or through the electrode interface was facilitated. In particular, in contrast to the solution-state reduction, not only monoanions but also dianions and even trianions of the aggregated Li2Pc(CN)8 molecules could be formed in the solid-state reduction. The first and second reduction waves observed upon cathodic potential sweeping of the Li2Pc(CN)8 LB film were principally assigned to the aggregated and monomeric species in the film, respectively. Acknowledgment. The authors are grateful to Prof. D. Wohrle of Brem University, Prof. K. Shigehara of Tokyo University of Agriculture and Technology, and Prof. N. Kimizuka of Kyushu University for their helpful discussions. This work was supported, in part, by a Grant-inAid for Center of Excellence Research (08CE2005) from the Ministry of Education, Science, Sports and Culture, Japan. LA011401A (36) Kouzeki, T.; Tatezono, S.; Yanagi, H. J. Phys. Chem. 1996, 100, 20097-20102.