Thin Films of Polymerized Rodlike Phthalocyanine Aggregates

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Thin Films of Polymerized Rodlike Phthalocyanine Aggregates† Carrie L. Donley, Wei Xia, Britt A. Minch, Rebecca A. P. Zangmeister,§ Anthony S. Drager,| Kenneth Nebesny, David F. O’Brien,‡ and Neal R. Armstrong* Department of Chemistry, University of Arizona, Tucson, Arizona 85721 Received February 17, 2003. In Final Form: May 11, 2003 We report here the synthesis, characterization, and thin film formation of a polymerizable octa-substituted phthalocyanine (Pc) with styryl-terminated side chains, CuPc(OCH2CH2OCH2CHdCHPh)8, 2,3,9,10,16,17,23,24-octakis(2-cinnamyloxyethoxy) phthalocyaninato copper(II) (1). We compare this Pc with a previously discussed phthalocyanine, also possessing styryl groups at the termini of the side chains, but with one alkoxy group in the side chain removed, CuPc(OCH2CH2CHdCHPh)8 (2) (J. Am. Chem. Soc. 2001, 123, 3595). Both 1 and 2 are related to the octa-substituted phthalocyanine CuPc(OCH2CH2OBz)8, 2,3,9,10,16,17,23,24-octakis (2-benzyloxyethoxy) phthalocyaninato copper(II) (3), which has been shown to form highly coherent rodlike aggregates in Langmuir-Blodgett (LB) films, with excellent control of rod orientation (J. Am. Chem. Soc. 1999, 121, 8628; Langmuir 2001, 17, 7071). Irradiation of the styryl π-π* absorbance bands (254 nm) for horizontally transferred LB films of 1 and 2 results in stabilization of their rodlike aggregates, through formation of cyclobutane links between adjacent side chains. Compound 1 shows a maximum 75% conversion of styryl groups versus ca. 55% conversion in 2. Polymerized thin films of 1 are insoluble in common solvents, and “ribbonlike” features can be lithographically produced with widths of 8 microns and heights of ca. 20 nm, maintaining control over the orientation of the Pc rods in the patterned features. Long-range order in both the as-deposited, annealed, and polymerized thin films was confirmed by atomic force microscopy and X-ray reflectometry, and small differences in the orientation of individual Pc’s, between films of 1 and 3, were determined by transmission and reflectance Fourier transform infrared spectroscopy. Higher dark and photoconductivities and higher electrical anisotropies were observed in films of 1, after annealing and polymerization, versus those seen for films of 3.

Introduction Discotic mesophase materials which form rodlike aggregates exhibiting long-range order and large electrical anisotropies are of interest for use in organic electronic devices requiring high charge mobilities and dense integration (e.g., organic field effect transistors (OFETs), organic light-emitting diodes (OLEDs), and photovoltaic (PV) cells).1-29 It is generally held that hole mobilities in * To whom correspondence should be addressed. E-mail: nra@ u.arizona.edu. † Part of the Langmuir special issue dedicated to David O’Brien. ‡ Deceased. § Present address: National Institute of Standards and Technology, Gaithersburg, MD. | Present address: Dow Chemical Corp., Midland, MI. (1) van de Craats, A. M.; Warman, J. M. Adv. Mater. 2001, 13, 130 and references therein. (2) Tracz, A.; Jeszka, J. K.; Watson, M. D.; Pisula, W.; Mu¨llen, K.; Pakula, T. J. Am. Chem. Soc. 2003, 125, 1682. (3) van de Craats, A. M.; Stutzmann, N.; Bunk, O.; Nielsen, M. M.; Watson, M.; Mullen, K.; Chanzy, H. D.; Sirringhaus, H.; Friend, R. H. Adv. Mater. 2003, 15, 495. (4) Silerova, R.; Kalvoda, L.; Neher, D.; Ferencz, A.; Wu, J.; Wegner, G. Chem. Mater. 1998, 10, 2284. (5) Gattinger, P.; Rengel, H.; Neher, D.; Gurka, M.; Buck, M.; van de Craats, A. M.; Warman, J. M. J. Phys. Chem. B 1999, 103, 3179. (6) Fox, M. A.; Grant, J. V.; Melamed, D.; Torimoto, T.; Liu, C. Y.; Bard, A. J. Chem. Mater. 1998, 10, 1771. (7) Liu, C.-Y.; Bard, A. J. Acc. Chem. Res. 1999, 32, 235. (8) Liu, C.-Y.; Bard, A. J. Nature 2002, 418, 162. (9) Brand, J. D.; Ku¨bel, D.; Ito, S.; Mu¨llen, K. Chem. Mater. 2000, 12, 1638. (10) Ku¨bel, C.; Chen, S.-L.; Mu¨llen, K. Macromolecules 1980, 13, 6014. (11) Petritsch, K.; Dittmer, J. J.; Marseglia, E. A.; Friend, R. H.; Lux, A.; Rozenberg, G. G.; Moratti, S. C.; Holmes, A. B. Sol. Energy Mater. Sol. Cells 2000, 61, 63. (12) Petritsch, K.; Friend, R. H.; Lux, A.; Rozenberg, G.; Moratti, S. C.; Holmes, A. B. Synth. Met. 1999, 102, 1776.

the range of 1 cm2/V‚s are desirable in organic thin films, using metal or conductive metal oxide contacts, to optimize OLED, OFET, and PV applications of these materials.30-35 (13) Schouten, P. G.; Warman, J. M.; Dehaas, M. P.; Vanderpol, J. F.; Zwikker, J. W. J. Am. Chem. Soc. 1992, 114, 9028. (14) Schouten, P. G.; Warman, J. M.; Dehaas, M. P.; Vannostrum, C. F.; Gelinck, G. H.; Nolte, R. J. M.; Copyn, M. J.; Zwikker, J. W.; Engel, M. K.; Hanack, M.; Chang, Y. H.; Ford, W. T. J. Am. Chem. Soc. 1994, 116, 6880. (15) Schmidt-Mende, L.; Fechtenkotter, A.; Mu¨llen, K.; Moons, E.; Friend, R. H.; MacKenzie, J. D. Science 2001, 293, 1119. (16) van der Pol, J. F.; Neeleman, E.; van Miltenburg, J. C.; Zwikker, J. W.; Nolte, R. J. M.; Drenth, W. Macromolecules 1990, 23, 155. (17) Kimura, M.; Wada, K.; Ohta, K.; Hanabusa, K.; Shirai, H.; Kobayashi, N. Macromolecules 2001, 34, 4706. (18) Makhseed, S.; Cook, A.; McKeown, N. B. Chem. Commun. 1999, 5, 419. (19) Osburn, E.; Schmidt, A.; Chau, L.-K.; Chen, S.-Y.; Smolenyak, P.; Armstrong, N. R.; O’Brien, D. F. Adv. Mater. 1996, 8, 926. (20) Smolenyak, P. E.; Osburn, E. J.; Chen, S. Y.; Chau, L. K.; O’Brien, D. F.; Armstrong, N. R. Langmuir 1997, 13, 6568. (21) Smolenyak, P.; Peterson, R.; Nebesny, K.; Torker, M.; O’Brien, D. F.; Armstrong, N. R. J. Am. Chem. Soc. 1999, 121, 8628. (22) Zangmeister, R. A. P.; Drager, A. S.; O’Brien, D. F.; Armstrong, N. R. Langmuir 2001, 17, 7071. (23) Drager, A. S.; Zangmeister, R. A. P.; Armstrong, N. R.; O’Brien, D. F. J. Am. Chem. Soc. 2001, 123, 3595. (24) Zangmeister, R. A. P.; Donley, C. L.; Drager, A. S.; O’Brien, D. F.; Armstrong, N. R. Chem. Mater., submitted. (25) Sauer, T.; Arndt, T.; Batchelder, D. N.; Kalachev, A. A.; Wegner, G. Thin Solid Films 1990, 187, 357. (26) Wegner, G. Thin Solid Films 1992, 216, 105. (27) Wegner, G. Mol. Cryst. Liq. Cryst. Sci. Technol., Sect. A 1993, 234, 283. (28) Wegner, G.; Neher, D.; Remmers, M.; Cimrova, V.; Schulze, M. Mater. Res. Soc. Symp. Proc. 1996, 413, 23. (29) Adam, D.; Schumacher, P.; Simmerer, J.; Haussling, L.; Siemensmeyer, K.; Etzbach, K. H.; Ringsdorf, H.; Haarer, D. Nature 1994, 371, 141. (30) Klauk, H.; Schmid, G.; Radlik, W.; Weber, W.; Zhou, L.; Sheraw, C. D.; Nichols, J. A.; Jackson, T. N. Solid-State Electron. 2003, 47, 297.

10.1021/la034271+ CCC: $25.00 © 2003 American Chemical Society Published on Web 07/11/2003

Films of Phthalocyanine Aggregates

Close cofacial contact between adjacent aromatic molecules can lead to low activation energies (low reorganization energies) for charge migration via hopping and therefore to high charge mobilities along a particular crystallographic axis,36,37 suggesting that sufficiently ordered systems should achieve mobilities as high as 1 cm2/V‚s. Properly modified disklike monomers self-organize into columnar structures, which have been shown recently to have sufficient coherence to approach these high charge mobilities. Warman and co-workers have suggested that hole mobilities in discotic mesophase materials may be related to the absolute size of the core of the discotic molecule.1 Time-of-flight methods were used by Adam et al. to find hole mobilities of up to 0.1 cm2/V‚s in modified triphenylenes, with nearly nondispersive transport.29 Hole mobilities of up to 1 cm2/V‚s have been observed in discotic materials derived from modified hexabenzocoronenes (HBCs), and hole mobilities as high as 0.67 cm2/V‚s have been observed for modified phthalocyanines, these last sets of observations arising from noncontact pulseradiolysis time-resolved microwave conductivity measurements.1,13,14 In geometries where contact must be made between an electrode and an organic semiconductor, lower mobilities are generally measured. For example, recent attempts to create organic field effect transistors from discotic mesophase HBCs and gold source/drain top contacts have shown mobilities limited to ca. 10-3 cm2/ V‚s.3 It is also clear that thin film processing conditions which lead to long, defect-free columnar aggregates, coupled with postdeposition stabilization of the assembly, and formation of low contact resistances, will be essential features of any successful technology based on these materials. There have been several previous attempts to force an optimum cofacial alignment of adjacent disklike monomers through polymerization. Wegner and co-workers produced a series of easily processed, high molecular weight silicon octa-alkoxy phthalocyanine (Pc) polymers, linked by central -O-Si-O- bonds between the Pc rings, with eight alkoxy side chains per Pc (C1-C18 alkoxy chains, PcPS).4,5,25-28,38 These PcPS polymers represented significant improvements to earlier, less easily processed, silicon phthalocyanine oligomers and polymers, introduced by Kenney and co-workers39 and Marks and co-workers.40 PcPS typically consists of ca. 100-200 Pc monomers per chain, with a high degree of polydispersity. LangmuirBlodgett thin films of the SiPc polymer contain highly aligned rods with lengths up to 100 nm and have excellent optical and electrical anisotropies. These materials are readily doped through chemical or electrochemical oxidation and show hole mobilities of 10-7-10-6 cm2/V‚s, measured in the dark on interdigitated microelectrodes.4 Other attempts to create Pc polymers from simple demetalated or divalent metal Pc monomers have intro(31) Klauk, H.; Jackson, T. N. Solid State Technol. 2000, 43, 63. (32) Seshadri, K.; Frisbie, C. D. Appl. Phys. Lett. 2001, 78, 993. (33) Chwang, A. B.; Frisbie, C. D. J. Appl. Phys. 2001, 90, 1342. (34) Chwang, A. B.; Frisbie, C. D. J. Phys. Chem. B 2000, 104, 12202. (35) Dimitrakopoulos, C. D.; Afzali-Ardakani, A.; Furman, B.; Kymissis, J.; Purushothaman, S. Synth. Met. 1997, 89, 193. (36) Cornil, J.; Beljonne, D.; Calbert, J. P.; Bredas, J. L. Adv. Mater. 2001, 13, 1053. (37) Kazmaier, P.; Hoffman, R. J. Am. Chem. Soc. 1994, 116, 9684. (38) Ferencz, A.; Armstrong, N. R.; Wegner, G. Macromolecules 1994, 27, 1517. (39) Mezza, T. M.; Armstrong, N. R.; Ritter, G. W.; Iafalice, J. P.; Kenney, M. E. J. Electroanal. Chem. 1982, 137, 227. (40) Gaudiello, J. G.; Kellog, G. E.; Tetrick, S. M.; Marks, T. J. J. Am. Chem. Soc. 1989, 111, 5259. Almeida, M.; Gaudiello, J. G.; Kellog, G. E.; Tetrick, S. M.; Marcy, H. O.; McCarthy, W. J.; Butler, J. C.; Kannewurf, C. R.; Marks, T. J. J. Am. Chem. Soc. 1989, 111, 5271.

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duced vinyl groups in the terminus of the side chains of the monomer Pc16,17 or have used vinyl or styrene groups in asymmetric Pc’s to incorporate the chromophore into another polymer.17,18 For the symmetric Pc’s, illumination of the absorbance band for the vinyl group or irradiation with electron beams leads to links between side chains in adjacent Pc rings. Similar strategies have been adopted by Mu¨llen and co-workers for the stabilization of HBC aggregates.9,10 This strategy introduces the risk of extensive cross-linking between adjacent Pc rods and some scrambling of the order in these materials. Our strategy to stabilize and enhance the ordering in rodlike Pc aggregates has been to add side chains with cross-linkable groups in the interior of the chain (e.g., CuPc(OCH2CH2OCH2CHdCHPh)8 (2,3,9,10,16,17,23,24octakis((2-cinnamyl)ethoxy) phthalocyaninato-copper(II) (1)), discussed here, and its predecessor CuPc(OCH2CH2CHdCHPh)8 (2), Figure 1).23 Both 1 and 2 are related to a nonpolymerizable phthalocyanine which shows exceptional aggregate formation with eight ethylene oxide side chains terminated with benzyl groups (CuPc(OCH2CH2OBz)8, 2,3,9,10,16,17,23,24-octakis (2-benzyloxyethoxy) phthalocyaninato-copper(II), 3 in Figure 1).19-24,41 Design of the side chains in 1 and 2 was based on the studies of Whitten and co-workers, where it was shown that styryl groups in the interior of an alkyl side chain could be photodimerized upon irradiation of the π-π* absorbance band of the styryl group (λmax ) ca. 250 nm) to form cyclobutane links.42 We hypothesized that forming ca. 2-3 links per Pc unit would be sufficient to stabilize the Pc aggregate. Pc 2 does not have the full ethylene oxide moiety in its side chains and is a more crystalline material than 1 or 3. It nevertheless forms rodlike aggregates in thin film formats which can be polymerized by irradiation at 254 nm.23,43,44 From polymerized firstgeneration thin films of 2, we recovered some of the Pc polymer in nonpolar solvents and characterized the molecular weights of the soluble fraction: aggregates with molecular weights up to 204,200 MW (117mer) were observed by matrix-assisted laser desorption ionization (MALDI) mass spectrometry.23 Atomic force microscopy (AFM) images of rodlike polymeric aggregates formed from recast thin films of this soluble fraction showed average rod lengths of 72 nm, with some rods in excess of 250 nm.23 Pc 1 represents an extension of the synthetic philosophy which led to 2 but contains the full ethylene oxide group in the side chains, as in 3. This Pc is more reactive toward polymerization at 254 nm, apparently forming up to six cyclobutane links per Pc in the polymerized material. The overall orientation of the Pc rods achieved in the asdeposited film is retained in the polymerized material. Changes in Pc tilt angles and column-column spacing are observed after annealing and after polymerization. Dark and photoconductivities and electrical anisotropies are also significantly enhanced after polymerization. The resultant Pc polymers are insoluble in common solvents, which provides for the lithographic patterning of these materials to create “ribbon-cable-like” features down to 2 microns in width. (41) Drager, A. S.; O’Brien, D. F. J. Org. Chem. 2000, 65, 2257. (42) Zhao, X.-M.; Perlstein, J.; Whitten, D. G. J. Am. Chem. Soc. 1994, 116, 10463. (43) Drager, A. S. Ph.D. Dissertation, University of Arizona, Tucson, AZ, 2001. (44) Zangmeister, R. A. P. Ph.D. Dissertation, University of Arizona, Tucson, AZ, 2001.

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Figure 1. Schematic views of octa-substituted phthalocyanines: (1) the subject compound of this paper CuPc(OCH2CH2OCH2CHdCHPh)8; (2) a previous version of a polymerizable Pc with styryl-terminated side chains CuPc(OCH2CH2CHdCHPh)8 (ref 23); (3) the parent compound CuPc(OCH2CH2OBz)8 (refs 19-22). At lower right, we show the proposed formation of cyclobutane links between styryl groups in adjacent side chains, of 1 and 2, upon irradiation at the absorbance band maximum of the styryl π-π* transition.

Experimental Section A brief description of the synthesis of 1 is given here, and a more detailed description is given in the Supporting Information and in ref 41. Commercially available trans-cinnamylaldehyde was converted to the corresponding 1,3-dioxolane. The dioxolane was then reduced with a LiAlH4/AlCl3 mixture, and the resulting 2-cinnamlyoxy ethanol was indirectly converted to the bromide. The bromide was then coupled onto the dimethyl ester of 4,5bis(hydroxy) phthalic acid, and this was converted to the 4,5bis(2-cinnamyloxyethoxy) phthalonitrile utilizing previously published methods.41 The phthalonitrile was converted to the corresponding copper Pc in 24% overall yield. Pc’s 2 and 3 were synthesized as previously reported.23,41,43 1,1,1,3,3,3-Hexamethyldisilazane and 1,3-diphenyl-1,1,3,3tetramethyldisilazane were purchased from Aldrich and used without further purification. Benzyloxyethane thiol was synthesized in our lab as previously reported.20,22 HPLC grade CHCl3 (EM Science) was used without further purification. Silicon (100) and quartz surfaces were cleaned by immersing in Klean AR (93% sulfuric acid and 0.4% chromium trioxide, Mallinckrodt) for a few minutes and then rinsing with copious amounts of water. These surfaces were then hydrophobized by sonicating the clean surface in a 5% 1,1,1,3,3,3-hexamethyldisilazane, 5% 1,3diphenyl-1,1,3,3-tetramethyldisilazane solution in CHCl3 for 30 min with heating.22 Au surfaces were cleaned by immersing the surface into a piranha solution (4:1 H2SO4/H2O2) and then modified by soaking clean Au in a 1 mM solution of benzyloxy-

ethane thiol in ethanol for at least 24 h. Caution: piranha reacts violently with many organic compounds and should be handled with care. A Riegler & Kerstein RK3 Langmuir-Blodgett (LB) trough was used to form LB films by applying a solution of the molecules to the air-water interface, allowing the CHCl3 solvent to evaporate, and then compressing the barriers.20,21 A Wilhelmy balance was used to measure the pressure at a position centered between the barriers. Films of 1 were transferred at an area of ca. 45 Å2/molecule (Figure 2). After film compression, the film was lowered onto a metal baffle placed underneath the surface of the water by removing some of the water in the subphase. Contact of the rigid film to this baffle served to segment the film into 15 equal-sized pieces and allow for horizontal (Schaefer) film transfer from one section without disturbing adjacent sections of the film. Film transfer by the horizontal transfer technique has been shown to give more ordered Pc films than the vertical transfer technique and has been used in all of these studies.20,21 Multilayer films were deposited either one monolayer or one bilayer at a time, removing any residual water from the sample with a stream of N2 between each transfer step. Finished films were annealed at 120 °C for at least 4 h to remove any residual water and to impart more order in the film by raising their temperature above their liquid crystal transition temperature (KfDh ) ca. 70 °C, and decomposition occurs prior to the DhfI transition for both 1 and 3). All AFM images were taken with a Nanoscope III system (Digital Instruments, Santa Barbara, CA). Images of phthalo-

Films of Phthalocyanine Aggregates

Figure 2. Pressure-area isotherms for 1 and 3 (Figure 1). Both materials show a sharp transition to the compact monolayer at Π1. The transition to a compact bilayer for 3 is easily seen, and horizontal transfer of coherent bilayers occurs typically at Π2 for this material. The transition to a stable bilayer is less clear for 1, but transfer of coherent films typically could be carried out at Π2. The collapse of these thin films to stable fiber structures (ref 19) is indicated by (/). Structures of the side chains are also shown. cyanine molecular columns were recorded in contact mode displaying the friction channel with the sample immersed in 1 mM potassium chloride in a standard solution cell, and images of patterned materials were recorded in tapping mode, with the sample immersed in 18 MΩ Millipore water. Oxide-sharpened silicon nitride probes, with a nominal force constant of 0.38 N/m, were used for all AFM imaging and were ozone cleaned for ca. 1 h prior to use. X-ray reflectivity measurements of 7 bilayer films of 3 and 14 monolayer films of 1, on silane-modified Si(100), were obtained with a Philips X’Pert Pro instrument with copper (KR) radiation (λ ) 0.154 nm) at 45 kV (40 mA target current). The measurements were conducted with a continuous scan along the Omega2θ axis. Assignment of Bragg peaks and analysis of Kiessig fringes were assisted by the use of the modeling program WinGixa, supplied with the Philips instrument. UV/visible spectra were obtained with a Hitachi U-2000 doublebeam spectrometer on Pc films that were 4 monolayers thick. A polarizing filter was used for the dichroic ratio studies, and spectra were obtained with the polarizer oriented parallel and perpendicular to the Pc column axis to obtain the two absorbances A| and A⊥, respectively. Solution spectra were taken in CHCl3 solutions. Fourier transform infrared (FTIR) transmission and reflection-absorption infrared spectra (RAIRS) were obtained with a dry-air-purged Nicolet 550 spectrometer with a tungsten source and a liquid-nitrogen-cooled MCT detector. A gold wire grid polarizer (Cambridge Physical Sciences) was used in transmission experiments, where spectra were taken with the incident beam normal to the substrate and the polarizer oriented parallel and perpendicular to the Pc column axis. RAIRS spectra were obtained with an FT-80 fixed 80° grazing angle accessory (Spectra-Tech). The polymerization of LB films of 1 was carried out in either a nitrogen- or argon-purged environment to minimize the production of ozone in the polymerization environment. Two Hgvapor pen lamps (Spectroline, model 11SC-1OP, and Pen-Ray, model 11SC-1) were placed adjacent to each other and were used to irradiate the samples through a 254 nm band-pass filter (Oriel Corp.; 254.91 nm filter, full width at half-maximum (fwhm) ) 10.66 nm). Polymerizations took place for 5 h unless otherwise

Langmuir, Vol. 19, No. 16, 2003 6515 noted. For the photopatterning experiments, a photomask was placed into intimate contact with the surface of a 7 monolayer film of 1 (ca. 20 nm) deposited on a Si(100) substrate, and the film was irradiated through the mask for periods up to 5 h. Following irradiation, the film was vigorously rinsed with CHCl3 to remove the unpolymerized material. Measurements of the dark and photoconductivity in films of 1 were carried out on interdigitated microelectrodes (IMEs) with 10 micron finger spacing (Abtech) before and after polymerization. Twenty-eight layer Pc films were deposited from the LB trough with the molecular columns oriented either parallel or perpendicular to the electrodes. A N2-gas-purged cell was used to minimize interference from adsorbed gases (especially O2) during measurements of both dark and photoconductivities.21,24,44 Temperature variations were monitored using a proximal thermocouple, which is especially important during characterization of photoconductivity, to ensure that increases in current are not simply the result of local heating of the microcircuit. A Keithley Source Meter, controlled using LabVIEW software, was used for the application of linear bias potential scans and simultaneous current monitoring. Conductivity measurements were made in both the ohmic regime, from 0 to 10 V applied bias, and in the space charge limited regime with applied voltages of up to 50 V. Photoconductivity experiments were carried out at a 5 V bias with either HeNe laser (15 mW, 633 nm) or Xe arc lamp (430 mW, 400-700 nm, IR filtered) irradiation. A simple solubility test was used to determine whether films of 1 remained unpolymerized following photocurrent measurements.

Results and Discussion Pressure-Area Isotherms. Figure 2 shows pressurearea isotherms for both Pc’s 1 and 3. Compression of freshly deposited 3 on an LB trough results in the arrangement of the Pc molecules into rodlike aggregates with the column axis parallel to the compression barriers.19-22 Two sharp transitions are observed for formation of films of 3, corresponding to the formation of a compact monolayer (Π1) at ca. 105 Å2/molecule and a compact bilayer (Π2) at ca. 50 Å2/molecule. Further compression of these films past Π2 results in the formation of robust fibers as the films collapse (point marked * in the upper plot of Figure 2); fibers up to several centimeters in length can be removed from the trough with tweezers.19 Films of 1 also show two transitions during compression, one at ca. 90 Å2/molecule and a second broader transition at ca. 40 Å2/molecule. Π1 on this pressure-area isotherm corresponds to the formation of a compact monolayer of Pc, but in contrast to CuPc(OCH2CH2OBz)8 films, the transition to a stable bilayer is not sharp (i.e., the pressure continues to increase gradually after Π1) and the film collapses (* in the bottom plot of Figure 2) prior to the formation of a stable bilayer. Point Π2 on the isotherm for 1 therefore does not correspond to completion of a compact bilayer but is a pressure that was used for horizontal transfer of coherent films, ca. 1 monolayer thick. We will, therefore, refer to the deposition of monolayers of 1 but bilayers of 3; however, in all cases multilayers of both were used for structural and electrical property characterization. UV-Visible Spectroscopy. Solution and thin film absorbance spectra of 1 are shown in Figure 3. The dilute solutions of both 1 and 3 show Q-band absorbances at ca. 677 nm, corresponding to the monomer Pc. With either concentrated solutions or thin films, the Q-band broadens and blue-shifts to yield a peak at ca. 625-632 nm, which arises from cofacial aggregation of the Pc.45,46 The peak (45) Kasha, M. Spectroscopy of the Excited State; Plenum Press: New York, 1976. (46) Chau, L. K.; England, C. D.; Chen, S. Y.; Armstrong, N. R. J. Phys. Chem. 1993, 97, 2699.

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Figure 3. UV/visible solution (CHCl3) and thin film spectra of 1 and 3. The peak at 677 nm corresponds to the Q-band absorbance for the monomer, while that at 625 nm corresponds to the cofacial aggregate. An MLCT band is seen at ca. 407 nm, while the π-π* transition for the styryl band (unique to 1 and 2) is seen at ca. 250 nm.

at 407 nm corresponds to a metal-to-ligand charge transfer (MLCT) transition and is unaffected by aggregation;47 Soret bands appear at 293 and 339 nm. The large absorbance at 250 nm in the spectra of solutions or thin films of 1 is assigned to the πfπ* transition of the styryl moiety (also seen in spectra of 2, but missing in spectra of 3), which is not significantly affected by aggregation.23 The molar absorptivity of the styryl absorbance band at 250 nm (eight styryl groups per Pc) is estimated to be ca. 1.8 × 105 M-1‚cm-1 per molecule (2.2 × 104 M-1‚cm-1 per styryl group), compared to the Q-band absorbance of 9 × 104 M-1‚cm-1. The Q-band absorbance spectra of LB-deposited films of 1 show a majority of species in the aggregated state; however, as seen in Figure 3, the intensity of the shoulder at ca. 677 nm indicates the presence of disordered material in the as-deposited films. Some narrowing and loss of the long-wavelength shoulder was observed in these Q-band spectra upon annealing of these films. Using light sources polarized perpendicular and parallel to the Pc column axis, we observe dichroic ratios, R ) A⊥/A| (A⊥ ) absorbance with excitation polarization perpendicular to the column axis, and A| ) absorbance with excitation polarization parallel to the column axis), of R ) 1.5 for as-deposited films and R ) 3 for annealed films of 1. No changes in dichroic ratios were seen following polymerization. These dichroic ratios are only slightly lower than what has been observed for annealed films of 3 (R ) 3.5).19-22 Photopolymerization of the Pc Films. The 250 nm absorbance peak for films of 1 was monitored as a function of time during 254 nm irradiation (Hg lamp) and is illustrated in Figure 4. The intensity of the 250 nm peak decreases monotonically with irradiation time, indicating (47) Simon, J. Phthalocyanines: Properties and Applications; VCH: New York, 1993; Vol. 2.

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the disappearance of the styryl functionality from the molecule, but with no appreciable change in either the Q-band absorbance or the MLCT absorbance. We estimated a baseline for this spectrum based on the absorbance at 250 nm from a comparable thickness film of 3 and calculated a conversion percentage of styryl groups, as shown in the inset of Figure 4 based on the disappearance of the styryl absorbance band. Most polymerizations were carried out for 5 h, at which time there was about 50% loss of the styryl absorbance band in films of 1. From longer exposure times, it appears that the maximum conversion approaches 75% of all styryl groups, which implies that up to 6 cyclobutane links are formed per Pc.23,43,44 Such high conversion efficiencies in the annealed films indicate excellent registry between the side chains in adjacent molecules, given the Pc-Pc spacing of ca. 3.4 Å and the 3.5-4.2 Å spacing required for cyclobutane formation.42 Previous studies of thin films of 2 showed a maximum loss of styryl absorbance band intensity of ca. 30%,23 which upon further purification of this Pc increased to ca. 55%,44 suggesting that side chain registry in this material was not as complete as in thin films of 1. Illumination of firstgeneration films of 2 for periods of a few minutes, followed by MALDI mass spectrometry of the polymerization products, indicated the presence of monomers, dimers, trimers, tetramers, pentamers, and so forth in the partially polymerized thin film, decreasing monotonically in population with increasing molecular weight, with intensities of the larger oligomers just above the detection limit in the experiment.23,43,44 The highest molecular weight fragments determined from these experiments were ca. 204,200 corresponding to ca. 117 monomer units per aggregate rod. Repeated attempts at MALDI characterization of the polymerized versions of LB films of 1, regardless of the time of polymerization and in a variety of matrixes, did not produce similar oligomer products. We attribute our inability to produce oligomer ions of 1 to the highly efficient polymerization of the Pc side chains in well-ordered films during the MALDI experiment. The pulsed nitrogen laser used for ionization overlaps the red side of the absorbance band of the styryl groups so that we expect each laser shot to polymerize some percentage of the illuminated region of the Pc film. The efficiency of polymerization of 1 in an organized (LB) film is apparently sufficient to create oligomers and polymers too large to ionize and propel into the mass spectrometer. (Note: There is not sufficient ordering in cast films of 1 to allow for extensive polymerization. The molecular ion peak has been observed from these cast films with MALDI and is reported in the Supporting Information.) AFM and X-ray Characterization of Film Structure. AFM images of two monolayer films of 1 and 3 transferred to hydrophobized silicon surfaces are shown in Figure 5. We attribute the striations seen most clearly in images a and b of Figure 5 to the tops of molecular columns of the rodlike aggregates of 3 and 1, respectively.21,22,44 Pc columns in films of 3 show an average diameter of ca. 2.8 nm with an average length of ca. 100150 nm, as opposed to lengths of 20-50 nm in films of 1.22 The images in Figure 5c,d suggest a restructuring of films of 1 upon annealing and polymerization. Fourier transforms of each 100 nm × 100 nm image were required to estimate the periodicity (column-column spacing). The average column spacing appears to increase to ca. 5.1 nm for the polymerized films, the images become less well defined, and there is some “texturing” of these films on the 100-300 nm distance scale. This increase in columncolumn spacing may be due to a change in the lattice unit

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Figure 4. Absorbance spectra of a 28 monolayer film of 1 (on quartz) as a function of irradiation time, during illumination at 254 nm (Hg pen lamp). There is a clear loss of absorbance intensity for the styryl band and no appreciable change in the Q-band region. The inset shows the percentage loss of styryl group absorbance versus time, based on the loss of absorbance intensity at 250 nm, calculated after correction of the baseline absorbance, which was estimated from the absorbance spectrum of a film of 3 with comparable thickness.

Figure 5. AFM images of (a) 1 bilayer of an annealed film of 3 and comparable thickness films of 1 which are (b) as-deposited, (c) annealed, and (d) polymerized, with arrows indicating the presumed alignment directions of the Pc columns. Pc films were deposited on hydrophobized Si(100) substrates and were taken in contact mode, displaying the friction channel, in 1 mM KCl solution, to improve image contrast. All images are 100 nm × 100 nm.

cell (movement away from hexagonal close-packed (hcp) packing toward a distorted rectangular lattice) and/or termination of the hcp lattice with displaced columns (as shown in Scheme 1), as has been proposed recently for close-packed arrays of other rodlike molecular objects.48,49 Each of these distortions creates a larger separation

between the columns which would be imaged by AFM and would also affect the lattice periodicity revealed by X-ray reflectivity measurements. (48) Stocker, W.; Karakaya, B.; Schurmann, B. L.; Rabe, J. P.; Schulter, A. D. J. Am. Chem. Soc. 1998, 120, 7691. (49) Schulter, A. D.; Rabe, J. Angew. Chem., Int. Ed. 2000, 39, 863.

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Table 1. X-ray Reflectivity Data for Films of CuPc(OCH2CH2OBz)8 and CuPc(OCH2CH2OCH2CHdCHPh)8 sample

first Bragg peak (deg in 2θ)

lattice periodicity (nm)

column-column spacing (nm)a

CuPc(OCH2CH2OBz)8 annealed CuPc(OCH2CH2OCH2CHdCHPh)8 as-deposited CuPc(OCH2CH2OCH2CHdCHPh)8 annealed CuPc(OCH2CH2OCH2CHdCHPh)8 polymerized

3.72 3.38 3.54 3.69

2.38 2.61 2.50 2.40

2.75 3.02 2.89 2.77

a

Assuming hexagonal close packing of adjacent Pc rods; therefore (column-column spacing) ) (lattice periodicity)/cos 30°.

Figure 6. X-ray reflectivity data for 14 bilayer films of 3 and 14 monolayer films of 1 on hydrophobized Si(100) substrates. The film of 3 was annealed, while the film of 1 is shown before and after annealing, and after polymerization. The spectra have been offset for clarity. Scheme 1. Hexagonal Close-Packed and Distorted Rectangular Lattice Terminations

Figure 6 shows X-ray reflectivity data for 14 bilayer films of 3 (only the annealed thin film is shown)21,44 and for 14 monolayer films of 1 in the as-deposited, annealed, and polymerized states. Table 1 includes a summary of such data for these types of thin films. Both first- and second-order Bragg peaks are seen for all films characterized. The Bragg peak at ca. 3.71° for annealed thin films of 3 corresponds to a column-column spacing of ca. 2.7 nm, assuming hexagonal close packing of adjacent rods in the thin film (cos 30° ) (lattice periodicity)/(columncolumn spacing)).21 The primary Bragg peak for asdeposited films of 1 occurs at ca. 3.38°, suggesting a larger Pc column-column separation versus that seen for films of 3. Annealing and polymerization of thin films of 1 shifts the Bragg peak back toward higher angles, suggesting a decrease in the separation distance between Pc columns to ca. 2.77 nm, which is closer to that seen for annealed films of 3. Kiessig fringes are seen in all of the X-ray reflectivity data and are an indication of films with a sharp

air/film interface on a smooth substrate.50a Annealing films of 1 and 3 clearly creates a more uniform film, while polymerization of 3 increases the surface roughness and reduces the relative intensity of the fringes. The width of the Bragg peak is inversely related to the coherence of these assemblies in the z-direction (Scherrer equation).50b Increases in this width, especially for the polymerized thin films, suggest more disordering within the films of 1, versus the annealed films of 3. The decreasing columncolumn spacing in the z-direction observed in the X-ray data and the increasing column-column spacing measured by AFM in the x-y plane are not necessarily in conflict; that is, the packing structures shown in Scheme 1 would lead to similar or slightly smaller lattice spacings as measured by X-ray reflectivity, but quite different AFM column spacing. X-ray reflectivity and electron diffraction experiments are underway for 1-2 layer thickness films of 1, as a function of annealing and polymerization, to fully characterize the exact packing of these layers where both the AFM data and the X-ray data are obtained on the same coverage of Pc. FTIR Characterization of Film Structure. Further characterization of the microstructure in these thin films was conducted using reflection-absorption (RAIRS) and transmission FTIR on modified Au and Si(100) substrates, respectively. These techniques are used in combination to determine the orientation of the individual Pc disks within the columns,21,22,25,51,52 which we think will ultimately correlate with differences in electrical conductivities and mobilities of these materials. Previous vibrational spectroscopic studies of aggregated Pc films have used a modification of a procedure developed by Debe,51 assuming a single linear dipole to model the in-plane vibrational transitions. We have extended this characterization protocol to include two orthogonal linear dipoles in the molecular plane, which provides a better model of the dipoles, and thus a more accurate picture of the orientation of the individual Pc’s within the columnar aggregate. A detailed description of the treatment of these data will be published elsewhere;52 a general overview of the method is presented here and in the Supporting Information. The intensities of in-plane (υ(Pc-O-C,ip) at 1204 and 1283 cm-1) and out-of-plane (δ(ringC-H,op) at 745 cm-1) molecular vibrations are monitored along the three laboratory axes by using polarized incident IR radiation. Three Euler angles (φ, θ, and ψ) describe three sequential rotations, which are used to define the position of the molecular plane.51-53 φ defines a rotation about the z-axis, θ defines a second rotation about the x-axis, and ψ defines a third rotation, again about the z-axis. Examples of IR spectra for the isotropic sample in a KBr pellet and thin films on Au and on Si are shown in Figure 7, and they clearly show (50) (a) Basu, J. K.; Sanyal, M. K. Phys. Rep. 2002, 363, 1. (b) Guinier, A. X-ray Diffraction; W. H. Freeman: San Francisco, 1963; pp 121125. (51) Debe, M. K. Appl. Surf. Sci. 1982, 14, 1. (52) Donley, C. L.; Nebesny, K.; Mendes, S.; Armstrong, N. R. Manuscript in preparation. (53) Goldstein, H. Classical Mechanics; Addison-Wesley: Reading, MA, 1950; pp 107-109.

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Figure 7. (A) Normalized KBr (transmission) and RAIRS FTIR spectra for annealed films of 1 showing changes in relative intensity in the in-plane and out-of-plane peaks indicating an ordered film on the gold surface. The transmission FTIR spectra in (B) also show changes in the relative peak intensities as the polarization of the incoming beam is changed. In-plane transitions are intense when the polarizer is oriented perpendicular to the column axis, and out-of-plane transitions are more intense when the polarizer is parallel to the column axis.

evidence of film ordering as the relative intensities of the peaks in the ordered sample on Au are different than those in the randomly oriented KBr sample. This is most clearly seen in the out-of-plane peak at 745 cm-1 that almost completely disappears in the RAIRS spectrum, while the in-plane transitions remain intense (Figure 7a). The differences in the transmission spectra as a function of polarization also indicate an ordered film; when the incident IR radiation is perpendicular to the columns, the in-plane transitions are intense and the out-of-plane transitions are weak (Figure 7b, top). The opposite is observed when the incident IR radiation is parallel to the columns (Figure 7b, bottom). From the treatments of the RAIRS and transmission IR data described in the Supporting Information, Euler angles for thin films of 1 and 3 were calculated, and the orientation of Pc molecules within a small section of a rodlike aggregate of 1 is illustrated in Figure 8 for asdeposited, annealed, and polymerized films. The symmetry of the Pc molecule leads to a number of possible solutions (orientations), most of which are equivalent (i.e., positive and negative solutions, or solutions that differ by 90° or 180°). After eliminating all of the degenerate solutions, there are two valid but slightly different orientations. For either set of orientations, the same trends are observed as a function of annealing and polymerization. In general, the effect of annealing and polymerization on these Pc assemblies is to bring the individual Pc’s to a more “upright” configuration, with more complete cofacial overlap between adjacent Pc rings. Following polymerization, the molecules display an orientation almost perpendicular to the surface, similar to the PcPS system, although there is still some twisting of the molecules about the surface normal within the column. The distribution of orientations of the molecules about the column axis within a single column must be small due to the high polymerization efficiencies (75%) reported earlier in this paper and the strict geometrical requirements for a [2+2] cycloaddition reaction to occur.42 Figure 8 shows one set of orientations and illustrates the trends described above. Three different views of the molecules are given for each treatment of the film: a view down the column axis (x-axis, front view), a view down the y-axis (side view of the column), and a view down the z-axis (top view of the column). The inset in the upper right corner

of each part of Figure 8a shows the orientation of the molecule if all of the Euler angles were 0°. Annealed films of 3 exhibit Euler angles of φ ) 4°, θ ) -87°, and ψ ) 24°, which results in an orientation similar to that of annealed films of 1. Previous reports on the orientation of annealed films of 3 utilized a less sophisticated model to determine orientation and reported a tilt angle away from the surface normal of ca. 30°.21 The treatment described here results in a tilt angle of