7632
J. Phys. Chem. 1996, 100, 7632-7645
Highly Photoactive Molecular Semiconductors: Determination of the Essential Parameters That Lead to an Improved Photoactivity for Modified Chloroaluminum Phthalocyanine Thin Films F. Santerre, R. Coˆ te´ , G. Veilleux, R. G. Saint-Jacques, and J. P. Dodelet* INRS-EÄ nergie et Mate´ riaux, C.P. 1020, Varennes, Que´ bec, Canada J3X 1S2 ReceiVed: December 6, 1995; In Final Form: February 14, 1996X
Thin films of chloroaluminum, chlorogallium, and chloroindium phthalocyanines (ClAlPc, ClGaPc, and ClInPc) have been sublimed on SnO2 substrates maintained during sublimation at temperatures ranging from -130 to 190 °C. Using this procedure, it is possible to obtain molecular semiconductor layers with a structure varying from amorphous to polycrystalline. These layers were immersed in KI3/KI or KCl solutions at pH ) 3.0. This treatment was found to improve drastically the photoelectrochemical activity of ClAlPc thin films. Shortcircuit photocurrents Jsc ) 0.75 ( 0.25 mA/cm2 were obtained, using polychromatic illumination (35 mW/ cm2), after immersion of ClAlPc into KCl solutions while lower Jsc values (0.3 ( 0.1 mA/cm2) were obtained for KI3/KI solutions. No change in the photoactivity was observed either for ClGaPc or for ClInPc when they were immersed in the same solutions. Both molecular semiconductors provided lower short-circuit photocurrents (Jsc e 0.15 ( 0.03 mA/cm2 for ClGaPc; Jsc e 0.20 ( 0.02 mA/cm2 for ClInPc). The characterization of the chloro-trivalent metal phthalocyanine films indicates that the hydrolysis of the metalCl bond is essential for the occurrence of the physicochemical transformation leading to improved photoactivity. The Al-Cl bond of ClAlPc hydrolyzes, but this reaction does not occur for ClGaPc or for ClInPc. In contact with KI3/KI or KCl solutions at pH ) 3.0, bulk hydrolysis occurs for ClAlPc, only if both H3O+ and an anion could diffuse from the solution into the material. The large I3- anion is prevented from doing so for polycrystalline ClAlPc films obtained by sublimation on SnO2 substrates maintained at 180 °C. However, it can diffuse easily in more disorganized films obtained at lower substrate temperatures. Powders of the chlorotrivalent metal phthalocyanines as well as bromoaluminum phthalocyanine (BrAlPc) were used to quantify anion incorporation in these materials. After complete hydrolysis of BrAlPc (powder) and ClAlPc (films) there are ca. 50-85% of the anions, generated in situ by the hydrolysis reaction or diffusing from the solution as a consequence of the hydrolysis reaction, that remain in the Pc material. Thus, ca. 50-85% of the protons released by the hydrolysis either protonate the macrocycles or react with Pc+O2- already present in the film. In both cases, anions are necessary to neutralize the excess of positive charges. H2O is also found in the modified films. The presence of protonated Pcs, of anions, and of H2O into what is now HOAlPc (after ClAlPc hydrolysis) modifies the structure of the material as well as its photoactivity.
Introduction Phthalocyanines (Pcs) provide the most important classical organic pigment in the blue and green shade areas, owing to their intense absorption in the visible, their excellent durability, and their relatively low cost.1,2 Besides this important conventional application, Pcs also have interesting properties for new technological applications.3 Among these, phthalocyanines are important in technologies that use the photoelectrical properties of materials. For instance, Pcs are, among other dyes, replacing Se as a charge carrier generating layer in the photocopying process.4,5 In recent years, there has been an increased interest in extending the absorption band of phthalocyanines into the nearinfrared (NIR) region of the spectrum. These Pcs are extensively used to generate charges in the laser light emitting diode printing process. The NIR absorbing phthalocyanines are metastable, but they are known to be more photoactive than the stable polymorphs. This is demonstrated, for instance, by the 10 times larger efficiency for the photogeneration of holes by x or τ H2Pc compared to R or β H2Pc.6 For these applications, NIR absorption of H2Pc or metal-based Pcs is * To whom correspondence should be sent. X Abstract published in AdVance ACS Abstracts, April 1, 1996.
S0022-3654(95)03635-5 CCC: $12.00
obtained by ball milling, heating, treating with organic solvent, or changing the sublimation rate of the molecular materials.7-14 In 1987, we reported that immersing thin films (4000 Å) of chloroaluminum phthalocyanine (ClAlPc) in an acidic solution of KI3/KI for several hours resulted in the occurrence of an absorption in the NIR as well as an increase in the film photoactivity.15,16 Indeed, short-circuit photocurrents increased from Jsc ) 0.25 to 0.65 mA/cm2. The cathodic photocurrent was measured under polychromatic illumination (35 mW/cm2) in a photoelectrochemical cell using KI3/KI (0.005 M/0.4 M) at pH ) 3 as an electrolyte. This increase in photoactivity was the result of a physicochemical modification of the film, described as transformation I. A study on film thicknesses revealed that the largest rise in Jsc upon transformation occurred for films that were 2000-6000 Å thick. Transformation I also occurred when the ClAlPc film was left several hours in contact with an acidic solution of KI. In that case, the cathodic photocurrent, measured in KI3/KI, reached Jsc ) 0.80-0.85 mA/cm2 in the previously described illumination conditions. The quantum yield (number of electrons per incident photon) measured at the maximum in the action spectrum was 12%.17 Another physicochemical modification of the film, described as transformation H, was also discovered. It occurred when © 1996 American Chemical Society
Highly Photoactive Molecular Semiconductors ClAlPc was left for several hours in contact with an acidic solution of KCl or KBr. After transformation H, cathodic photocurrents Jsc ) 0.80-1.0 mA/cm2 were measured under a polychromatic illumination of 35 mW/cm2 in KI3/KI electrolyte. A quantum yield of 16% was measured at the maximum of the action spectrum.17 A rather extensive physicochemical characterization of these films was performed in order to understand the origin of the improvement in photoactivity after transformations I or H.15-26 A summary of these characterizations is now presented for ClAlPc before and after transformation. ClAlPc before Transformation. ClAlPc (4000 Å thick films) sublimed at 2000 Å/min on SnO2 may be depicted as a rather tight collection of tiny rods being perpendicular to the substrate and having a diameter of 500-750 Å. The length of the rods is equivalent to the thickness of the film. The porosity of these films was demonstrated by the presence of an X-ray photoelectron spectroscopy (XPS) signal due to SnO2. Polarized X-ray absorption experiments at the carbon and nitrogen K-edges performed on thin (500 Å) ClAlPc films sublimed on float glass were used to determine the parallel orientation of the phthalocyanine molecules with respect to the substrate. Electron diffraction experiments performed on thin (104 cm-1, is not observed. Such a behavior is typical of porous films with pores extending right through the entire film thickness. When the photogeneration of charges involves a singlet exciton mechanism, like it is the case for ClAlPc, a similitude between the action spectrum and the absorption spectrum is expected. In that context, the similarities of all action spectra and their differences with the film absorbances are quite puzzling and worth some discussion. Let us start with Figure 5A,B. SEM observations on these films (Figure 3E-H) indicate that a layer of needlelike crystallites is lying on top of the transformed film. PcAlOAlPc is found in these needles by ToF SIMS.26 It is also known16 that a dimer film displays a Q-band absorbance with a maximum at 625 nm. It corresponds to one of the peaks in Figure 5A. PcAlOAlPc is also known16,17 to be much less photoactive (by a factor of 3-4) than ClAlPc transformed with KCl. The influence of PcAlOAlPc in the needle layer will therefore be minimal on the action spectrum of the transformed film. On the other hand, we performed some liftoff experiments (with Scotch tape) on the films whose absorbance is depicted in Figure 5A. By lifting off a majority of needles, both peaks in Figure 5A disappear, and the absorption spectrum of the remaining film after three lifting is similar to the action spectrum depicted in Figure 5B. We do not believe that the peak appearing between 815 and 835 nm in Figure 5A may be identified as another polymorphic form of PcAlOAlPc. Only one crystallographic structure has been reported for the latter material.45 Instead, the following explanation is proposed for the NIR peak in Figure 5A; we know that the mechanism to obtain the PcAlOAlPc dimers detected in the upper layer of ClAlPc transformed in KCl involves a slow dissolution of ClAlPc, followed by its hydrolysis in solution and its precipita-
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Figure 10. Film morphologies of ClGaPc sublimed on SnO2 substrates maintained at -132 °C (top row) and 178 °C (bottom row): left column, as-sublimed ClGaPc; right column, ClGaPc after attempted transformation in contact with KCl at pH ) 3.0.
tion as PcAlOAlPc. It is now postulated that, depending upon the rate of dissolution of ClAlPc, either dimerization of HOAlPc or its hydration46 is obtained in solution. Both reactions end up with the precipitation of needles as the end product. It is proposed that hydration is expected for small dissolution rates of ClAlPc and small concentrations of HOAlPc while dimerization is obtained when the dissolution rate increases. Small dissolution rates are expected for ClAlPc films grown on substrates maintained at high temperatures. This is indeed suggested by Figure 3E-H where larger needles are obtained for higher substrate temperatures. This behavior corresponds to a slow dissolution rate of the films, favoring the growth of needles from a small number of nucleation sites. It is also postulated that the maximum absorbance of hydrated HOAlPc
corresponds to the NIR peak in Figure 5 and that, like for PcAlOAlPc, the photoactivity of HOAlPc is low compared to that of the remaining transformed film. In the PcAlOAlPc/ hydrated HOAlPc hypothesis, using substrate temperature ranging from -120 to 180 °C, would change the chemical nature of the needles from PcAlOAlPc to hydrated HOAlPc. This behavior corresponds to the evolution of the spectra depicted in Figure 5A. According to this analysis, the action spectra of Figure 5B would mainly originate from the organic layer situated below the needle layer. This material is HOAlPc obtained by hydrolysis of ClAlPc, eventually protonated and containing anions located in the voids of the structure resulting from the stacking of the macrocycles (Table 1). The same kind of
7642 J. Phys. Chem., Vol. 100, No. 18, 1996 material is also generating the action spectra of Figure 6B at least for substrate temperatures (from -120 to 60 °C) that allow a bulk transformation of the film. The difference between transformation H with KCl and I with KI3/KI is in the nature of the anion incorporated in the bulk of the film: it is Cl- or I3- for transformations H or I, respectively. For the highest temperature (180 °C) it is shown in Figure 6A that the film does not transform in the bulk. It is, however, necessary to propose some surface transformation (including in the pores) if the similarities between all action spectra are to be justified. The measured photocurrent would find its origin in this transformed surface layer thin enough to be barely detectable in Figure 6A. (The dotted line in Figure 6A helps to compare the absorbance of as-sublimed ClAlPc grown on SnO2 maintained at 180 °C with that of the same film transformed after immersion in KI3/KI at pH ) 3.0. The difference in NIR absorbance is the result of surface modification of the film.) The quantum yields of both surface and bulk transformed films are alike. This can be understood in terms of an exciton diffusion length of about 100-200 Å (which is usual in such materials47-49) and with the hypothesis that only excited state dyes at the interface with the aqueous electrolyte undergo exciton dissociation to inject charges into I3- in solution. Therefore, even in bulk transformed film, only a thin layer corresponding to the exciton diffusion length is photoactive. Transformation of ClGaPc and ClInPc. At this point of our investigations, our next concern was to find out whether the transformation of ClAlPc described in the two previous sections could be generalized to other trivalent metal phthalocyanines like ClGaPc and ClInPc. ClGaPc and ClInPc thin films were then grown in the same experimental conditions as ClAlPc: 4000 Å thick films, 2000 Å/min on SnO2 maintained at various substrate temperatures. ClGaPc. Figure 9 presents the absorbance of films grown on SnO2 maintained at temperatures ranging from -132 to 178 °C. The morphology of the corresponding films is presented in Figure 10A,B. Both absorption spectra and film morphologies for ClGaPc are similar to absorption spectra (Figure 4) and film morphologies (Figure 3A-D) presented for ClAlPc. The appearance of electron diffraction rings for substrates maintained at room temperature and above is also similar for both ClAlPc and ClGaPc. However, ClGaPc does not transform, even when mainly amorphous films, obtained at the lowest substrate temperatures, are immersed in either KI3/KI or KCl solutions at pH ) 3.0. Lowering the pH of the solutions to pH ) 1 has no effect either. The film morphology does not change (Figure 10C-D). The photocurrents under polychromatic illumination remain low. Jsc values are presented in Figure 11 as a function of the temperature of the substrate during ClGaPc sublimation and after attempts to transform ClGaPc in KI3/KI (1) or KCl (O) solutions at pH ) 3.0. They are 0.06 ( 0.02 mA/cm2 for amorphous ClGaPc and rise slightly to 0.15 ( 0.03 mA/cm2 when ClGaPc becomes polycrystalline. These values are much lower than the ones presented in Figure 7 for transformed ClAlPc. ClInPc. Figure 12 presents the absorbance of films sublimed on SnO2 maintained at temperatures ranging from -97 to 191 °C. The shoulder at about 800 nm becomes a peak in the Q-band absorbance as the substrate temperature rises. The absorption spectra will be discussed later on. The morphology of the corresponding films is presented in Figure 13A,B. The low-temperature SEM is similar to the one obtained for ClAlPc (Figure 3A,B) or ClGaPc (Figure 10A). At high substrate temperature, however, the morphology of ClInPc films is very similar to the one reported for polycrystalline TiOPc32 or
Santerre et al.
Figure 11. Evolution of the short-circuit cathodic photocurrent obtained under polychromatic illumination after transformation attempts of ClGaPc films by immersion in KI3/KI (1) or KCl (O) solutions at pH ) 3.0; films were sublimed on SnO2 maintained at temperatures varying from -132 to 178 °C.
Figure 12. Absorbance in the visible and NIR of ClInPc thin films sublimed on SnO2 substrates maintained at various temperatures from -97 to 191 °C.
VOPc,13 both pigments displaying also an absorption peak in the NIR.14,50 TEM results indicate the same behavior as the one observed for ClAlPc and ClGaPc: the appearance of electron diffraction rings for substrates maintained at room temperature and above. Similarly to ClGaPc, ClInPc does not transform, even when mainly amorphous films obtained at the lowest substrate temperature are immersed in either in KI3/KI or KCl solutions at pH ) 3.0. The lack of transformation is confirmed by the behavior of the film morphology. It remains identical to the one obtained for as-sublimed films (Figure 13C,D). The photocurrent values under polychromatic illumination remain low. Curiously, the maximum photocurrent is observed for the lowest substrate temperature (0.20 ( 0.02 mA/cm2) while it decreases to 0.10 ( 0.02 mA/cm2 when the substrate temperature is raised. This unexpected behavior is not understood. ClGaPc and ClInPc. The reasons for the lack of transformation of ClGaPc and ClInPc are found in the IR absorbance of the material. For both compounds, IR spectra were recorded for as-sublimed Pcs, after attempts to transform the powders in KBr solutions at pH ) 3.0 and after exposure of the Pcs to saturated water vapor at room temperature. After normalization for the peak occurring in the 700-750 cm-1 region, no difference was observed in the 375-700 cm-1 range between
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Figure 13. Film morphologies of ClInPc sublimed on SnO2 substrates maintained at -97 °C (top row) and 191 °C (bottom row): left column, as-sublimed ClInPc; right column, ClInPc after attempted transformation in contact with KCl at pH ) 3.0.
the three spectra recorded for ClGaPc or ClInPc. This energy range encompasses the νGa-Cl vibration energy reported to be in the 430-440 cm-1 region.13 Since it is similar to the energy of the νAl-Cl vibration, we will assume that νIn-Cl will also occur at about the same energy (430-440 cm-1). From the IR spectra, we conclude that neither ClGaPc nor ClInPc is hydrolyzed. Since at least partial hydrolysis of ClAlPc powders was observed when transformation of that material occurred, a hydrolysis of the metal-halogen bond is therefore a requisite in order to further transform the film. Returning to Figure 12, Armstrong et al. studied in detail the behavior of ClInPc (and also ClGaPc and ClAlPc) ultrathin films obtained by slow epitaxy (100 °C. In these films, the transitions at 660670 nm and the one at 800-820 nm are attributed to exciton splitting resulting from a dimeric slipped structure equivalent to the one proposed for phase II of VOPc.14 The latter material displays transitions at 670 and 815 nm. (The slipped dimers of VOPc and those in ClInPc multilayer assemblies are not characterized by the same parameters.) In Figure 12, there is a shoulder at 800 nm when ClInPc is sublimed on SnO2 maintained at -97 and 23 °C. This shoulder indicates that a fraction of ClInPc molecules in the film have already the high-temperature configuration despite the fact that TEM diffractions show essentially amorphous films. The remaining features of these films are a peak at about 720 nm and a shoulder at about 660 nm. Similar features are also seen on the absorbances of the most amorphous films of ClGaPc (Figure 9) and ClAlPc (Figure 4) and on the absorbance of phase I VOPc which is also proposed to be dimeric in nature.14 In the latter case, the lower excitonic splitting (compared to phase II VOPc) was attributed to parallel plane rotated cofacial dimers. The amorphous character of trivalent metal halogen phthalocyanines observed in this work for low-temperature substrates indicates that these films lack long-range order between the constituting dimeric units. Conclusion The hydrolysis of the halogeno-trivalent metal bond is a necessary condition for the occurrence of the physicochemical modification of halogeno-trivalent metal phthalocyanines in contact either with water vapor or with acidic solution. This hydrolysis does not occur for ClGaPc or ClInPc. When ClAlPc (or BrAlPc) is hydrolyzed by the diffusion of water vapor in the material, H3O+ and Cl- released by the reaction remain in the phthalocyanine, and the physicochemical modification reaches completion. When ClAlPc is hydrolyzed in acidic solution, the reaction is only possible in the bulk of the material if H3O+ (not only H2O) as well as an anion of the solution are able to diffuse
Santerre et al. together in the material. This diffusion is facilitated when the Pc structure is disorganized, but the diffusion of large anions in the material may become difficult and even impossible when the Pc is polycrystalline. It has indeed been demonstrated that, in contrast to Cl-, I3- is not able to diffuse in the bulk of a ClAlPc film sublimed on a substrate maintained at high temperature (180 °C). Only the surface of the film in contact with the electrolyte is modified in that case on a depth equivalent at least to the exciton diffusion length (e200 Å). Hydrolysis of BrAlPc in solution is complete. It is also complete for thin films of ClAlPc while hydrolysis is only partial (between ca. 50 and 80%) for ClAlPc powder. After complete hydrolysis of BrAlPc or ClAlPc, there are ca. 50-85% of the anions generated in situ by the reaction or diffusing from the solution that remain into the Pc material. It means that ca. 5085% of the protons released by hydrolysis either protonate the macrocycles or react with Pc+O2- in the film. In both cases, anions are necessary to neutralize the excess of positive charges. The presence of protonated Pcs, of anions, and of H2O in the structure of the phthalocyanine results from the hydrolysis of ClAlPc or BrAlPc. It modifies the nature and the structure of the as-sublimed material. The new material is more photoactive. The increase of the photoactivity varies with the type of anion found into the film, being lower for I3- than for Br- or Cl-. After this study, it seems that the mechanism governing the modification of ClAlPc thin films is now elucidated. It remains, however, to confirm whether the rise in photoactivity for transformed films is the result of an increased charge density upon modification as it was concluded from Figure 8 or whether it is related to a larger hole mobility in the new molecular semiconductor. Experiments are in progress to answer that question. Acknowledgment. Support for this work has been provided by NSERC. The authors want to thank CANMET-EMR for the access to their SEM. References and Notes (1) Smith, H. M. Phthalocyanines. In Pigment Handbook, 2nd ed.; Lewis, P. A., Ed.; John Wiley: New York, 1988; Vol. 1, p 663. (2) Moser, F. H.; Thomas, A. L. In The Phthalocyanines; CRC Press: Boca Raton, FL, 1983; Vols. I and II. (3) Leznoff, C. C.; Lever, A. B. P. In Phthalocyanines; Properties and Applications; VCH: Weinheim, 1989, Vol. I; 1993, Vol. II; 1994, Vol. III. (4) Gregory, P. In High-Technology Application of Organic Colorants; Plenum Press: New York, 1991; p 59. (5) Loutfy, R. O.; Hor, A. M.; Hsiao, C. K.; Baranyi, G.; Kazmaier, P. Pure Appl. Chem. 1988, 60, 1047. (6) Kanemitsu, Y.; Yamamoto, A.; Funuda, H.; Masumoto, Y. J. Appl. Phys. 1991, 69, 7333. (7) Sharp, J. H.; Lardon, M. J. Phys. Chem. 1968, 72, 3230. (8) Enokida, T.; Hirohashi, R.; Mizukami, S. J. Imaging Sci. 1991, 35, 235. (9) Lee, S. N.; Hoshino, K.; Kokado, H. Denshi Shashin Gakkaishi 1992, 31, 2. (10) Loutfy, R. O.; Hor, A. M.; Rucklidge, A. J. Imaging Sci. 1987, 31, 31. (11) Loutfy, R. O.; Hor, A. M.; DiPaola-Baranyi, G.; Hsiao, C. K. J. Imaging Sci. 1985, 29, 116. (12) Arishima, K.; Hiratsuka, H.; Tate, A.; Okada, T. Appl. Phys. Lett. 1982, 40, 279. (13) Sims, T. D.; Pemberton, J. E.; Lee, P.; Armstrong, N. R. Chem. Mater. 1989, 1, 26. (14) Griffiths, C. H.; Walker, M. S.; Goldstein, P. Mol. Cryst. Liq. Cryst. 1976, 33 149. (15) Guay, D.; Coˆte´, R.; Marque`s, R.; Dodelet, J. P.; Lawrence, M.; Gravel, D.; Langford, C. H. J. Electrochem. Soc. 1987, 134, 2942. (16) Guay, D.; Dodelet, J. P.; Coˆte´, R.; Langford, C. H.; Gravel, D. J. Electrochem. Soc. 1989, 136, 2272. (17) Gastonguay, L.; Veilleux, G.; Coˆte´, R.; Saint-Jacques, R. G.; Dodelet, J. P. J. Electrochem. Soc. 1992, 139, 337.
Highly Photoactive Molecular Semiconductors (18) Guay, D.; Veilleux, G.; Saint-Jacques, R. G.; Coˆte´, R.; Dodelet, J. P. J. Mater. Res. 1989, 4, 651. (19) Tourillon, G.; Coˆte´, R.; Guay, D.; Dodelet, J. P. J. Electrochem. Soc. 1989, 136, 2931. (20) Guay, D.; Tourillon, G.; Gastonguay, L.; Dodelet, J. P.; Nebesny, K. W.; Armstrong, N. R.; Garrett, R. J. Phys. Chem. 1991, 95, 251. (21) Dodelet, J. P.; Gastonguay, L.; Veilleux, G.; Saint-Jacques, R. G.; Coˆte´, R.; Guay, D.; Tourillon, G. Proc. SPIE 1991, 1436, 38. (22) Dodelet, J. P.; Tourillon, G.; Gastonguay, L.; Coˆte´, R.; Guay, D.; Ladouceur, M.; Flanck, A. M.; Lagarde, P. J. Phys. Chem. 1992, 96, 7202. (23) Gastonguay, L.; Veilleux, G.; Coˆte´, R.; Saint-Jacques, R. G.; Dodelet, J. P. Proc. SPIE 1992, 1729, 13. (24) Coˆte´, R.; De´ne`s, G.; Gastonguay, L.; Dodelet, J. P. Proc. SPIE 1992, 1729, 222. (25) Gastonguay, L.; Veilleux, G.; Coˆte´, R.; Saint-Jacques, R. G.; Dodelet, J. P. Chem. Mater. 1993, 5, 381. (26) Santerre, F.; Coˆte´, R.; Lalande, G.; Gastonguay, L.; Guay, D.; Dodelet, J. P. J. Phys. Chem. 1995, 99, 17198. (27) Wynne, K. J. Inorg. Chem. 1984, 23, 4658. (28) Unpublished results. (29) Emsley, J. In The Elements, 2nd ed.; Clarendon Press: Oxford, 1991. (30) Schramm, C. S.; Scaringe, R. P.; Stojakovic, D. R.; Hoffman, B. M.; Ibers, J. A.; Marks, T. J. J. Am. Chem. Soc. 1980, 102, 6702. (31) Owens, J. E.; Kenney, M. E. Inorg. Chem. 1962, 1, 331. (32) Ghosez, Ph.; Coˆte´, R.; Gastonguay, L.; Veilleux, G.; De´ne`s, G.; Dodelet, J. P. Chem. Mater. 1993, 5, 1581. (33) Panayatatos, P.; Whitlock, J. B.; Bird, G. R. Proc. SPIE 1992, 1729, 230. (34) Whitlock, J. R.; Bird, G. R.; Cox, M. D.; Panayotatos, P. Thin Solid Films 1992, 215, 84. (35) Simon, J. J.; Andre, J. J. In Molecular Semiconductors; SpringerVerlag: Berlin, 1985. (36) Dahlberg, S. C.; Musser, M. E. J. Chem. Phys. 1980, 72, 6706. (37) Musser, M. E.; Dahlberg, S. C. Surf. Sci. 1980, 100, 605. (38) Day, P.; Price, M. G. J. Chem. Soc. (A) 1969, 236.
J. Phys. Chem., Vol. 100, No. 18, 1996 7645 (39) Wo¨hrle, D.; Schlettwein, D.; Kirschenmann, M. J. Macromol. Sci. Chem. A 1990, 27, 1239. (40) Zhang, X. F.; Xu, H. J.; Chen, D. W. J. Photochem. Photobiol. B: Biol. 1994, 22, 235. (41) Sawyer, D. T.; Valentine, J. S. Acc. Chem. Res. 1981, 14, 393. (42) Cafre´, P.; Sawyer, D. T. Anal. Chem. 1986, 58, 1057. (43) Kobayashi, T.; Uyeda, N. Philos. Mag. B 1988, 57, 493. (44) Be´langer, D.; Dodelet, J. P.; Lombos, B. J. Electrochem. Soc. 1986, 133, 1113. (45) Wynne, K. J. Inorg. Chem. 1985, 24, 1339. (46) Owens, J. E.; Kenney, M. E. Inorg. Chem. 1962, 1, 334. (47) Loutfy, R. O.; Sharp, J. H.; Hsiao, C. K.; Ho, R. J. Appl. Phys. 1981, 52, 5218. (48) Blanzat, B.; Barthon, C.; Terrier, N. J. Am. Chem. Soc. 1987, 109, 6193. (49) Chau, L. K.; Chen, S. Y.; Armstrong, N. R.; Collins, G. E.; England, C. D.; Williams, V. S.; Anderson, M. L.; Schuerlein, T. J.; Lee, P. A.; Nebesny, K. W. Mol. Cryst. Liq. Cryst. 1994, 252, 67. (50) Klofta, T. J.; Danziger, J.; Lee, P.; Pankow, J.; Nebesny, K. W.; Armstrong, N. R. J. Phys. Chem. 1987, 91, 5646. (51) Nebesny, K. W.; Collins, G. E.; Lee, P. A.; Chau, L. K.; Danziger, J.; Osburn, E.; Armstrong, N. R. Chem. Mater. 1991, 3, 829. (52) Collins, G. E.; Nebesny, K. W.; England, C. D.; Chau, L. K.; Lee, P. A.; Parkinson, B. A.; Armstrong, N. R. J. Vac. Sci. Technol. A 1992, 10, 2902. (53) Chau, L. K.; Arbour, C.; Collins, G. E.; Nebesny, K. W.; Lee, P. A.; England, C. D.; Armstrong, N. R.; Parkinson, B. A. J. Phys. Chem. 1993, 97, 2690. (54) Davidov, A. S. In Theory of Molecular Excitons; McGraw-Hill: New York, 1962. (55) Kasha, M. In Spectroscopy of the Excited State; Bartolo, B. D., Ed.; Plenum: New York, 1976; p 337. (56) Chau, L. K.; England, C. D.; Chen, S.; Armstrong, N. R. J. Phys. Chem. 1993, 97, 2699.
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