Article pubs.acs.org/Macromolecules
Patterning Carbazole−Phenylazomethine Dendrimer Films Ken Albrecht,† Roderick Pernites,‡ Mary Jane Felipe,‡ Rigoberto C. Advincula,*,‡ and Kimihisa Yamamoto*,† †
Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsutacho, Midoriku, Yokohama 226-8503, Japan Department of Chemistry and Department of Chemical Engineering, University of Houston, Houston, Texas 77204-5003, United States
‡
S Supporting Information *
ABSTRACT: The synthesis of double-layer-type dendrimers with carbazole and phenylazomethine as the dendron with a symmetric tetraphenylmethane core is reported. Structural modeling studies showed that the G3 dendrimer has a rigid and spherical structure. These dendrimers were thermally stable (Td10% over 500 °C) with the TGA-MS study revealing a degradation mechanism occurring first at the inner-layer phenylazomethine group. The metal (Lewis acid) complexation property of these dendrimers was also studied. Electrochemical measurements showed that these dendrimers have the appropriate HOMO level as a hole-transporting material with electropolymerizability on the peripheral carbazole groups. A photo-cross-linking property of the dendrimer film was also observed. Finally, electro-nanopatterning with conducting AFM and photopatterning of the dendrimer film were demonstrated. Thus, the new dendrimer is a potential hole-transporting material that is patternable through oxidation of the peripheral carbazole units by either photochemical or electrochemical methods.
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INTRODUCTION Organic electronics, which started with the discovery of chargetransfer complexes,1 chemical doping of polymers,2 and charge injection into organic single crystals,3 has been of high interest for materials chemists for organic light-emitting diode (OLED) and organic photovoltaic (OPV) devices.4 The possibility for lightweight, flexible, and low-cost electronics devices has led to a significant research effort on low to high molecular5 weight materials for organic photoconductive devices,6 OLED,7 OPV,8 OFET,9 organic lasers,10 organic light-emitting transistors,11 and organic memory devices.12 The device structure and material composition depends on the particular application. Film formation from solution (wet process), laminatability, and patternability remain important properties for the development of truly simple and low cost organic/polymer devices. However, materials that satisfy all these requirements are still rare.13 Dendritic molecules14 including hyperbranched polymers15 have been extensively studied.16 Dendrimers have precise molecular weights and uniform structure which are suitable as model compounds to study optoelectronic properties in devices.17 Additionally, the high solubility, amorphousness, high glass transition temperature, and low viscosity enable dendritic materials to be attractive even for practical use. To date, several dendritic materials have been developed for this purpose,18 especially the rigid phenylene-19 and carbazoletype20 dendrons having potential as high-performance OLED materials.21,22 Despite these developments, dendritic materials are not easily accessible to lamination and patterning compared © 2012 American Chemical Society
to low molecular weight materials (vacuum deposition and shadow mask process). Polymerization or cross-linking after or during film formation23 can be a viable way to fabricate insoluble (laminable) and patternable polymer films. However, many of these cross-linkage units (especially photo-cross-linking units) are electrochemically inactive and require an initiator which can inhibit charge transfer or reduce the lifetime of the device. Adopting these units in a dendritic system is difficult. For crosslinking, the units have to be at the periphery, and this can block the charge-transfer property between dendrimers. One possible approach to solve this problem is to use peripheral units that have both cross-linking and charge-transfer abilities. As previously mentioned, carbazole units are efficient holetransporting materials and, at the same time, known to have electrochemical and photo-cross-linking properties. In this paper, we report on a new carbazole dendrimer and demonstrate the potential for electrocemical and photo-crosslinkable (laminable and patternable) dendritic electronics materials.
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EXPERIMENTAL SECTION
Carbazole was purchased from the Kanto Kagaku Co. and recrystallized prior to use. All other reagents were purchased from Aldrich, Received: November 10, 2011 Revised: December 27, 2011 Published: January 26, 2012 1288
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Scheme 1. Synthesis of Tetraphenylmethane Core Carbazole−Phenylazomethine Double Layer-Type Dendrimers
TCI, and the Kanto Kagaku Co. and used without further purification. Solvents for the UV−vis titration and electrochemical measurements were of the dehydrated grade. The silica gel for column chromatography was of neutral grade. All the dendrimers were reprecipitated from hexane with chloroform and dried in vacuo (150 °C) before use. The NMR spectra were obtained using a JEOL JNMGX400 (400 MHz) and a JEOL JNM-EX270 (270 MHz) with TMS as the internal standard. The MALDI TOF-MS data were obtained using a Shimadzu/Kratos AXIMA CFR plus in the linear positive ion mode. Dithranol was used as the matrix. The elemental analysis was performed at the Center for Advanced Materials Analysis, Technical Department, TIT. A preparative scale gel permeation chromatograph, LC-908 (Japan Analytical Industry Co., Ltd.), was used to isolate each compound with chloroform or THF as the eluent. The thermal analysis was performed using a Rigaku Thermoplus TG8120 with flowing nitrogen, and the temperature rate increase was 40 K/min. The thermal desorption gas analysis was performed with a Rigaku TPD type R Photo with flowing helium. The temperature rate increase was 20 K/min, and the gas was analyzed by EI (electron ionization) and PI (photoionization) MS. The UV−vis spectra were recorded using a Shimadzu UV-3150 spectrometer with a quartz cell having a 1 cm optical length at 20 °C. The electrochemical measurements were
done using a conventional three-electrode configuration with an ALS Chi660 electrochemical analyzer. The working, counter, and reference electrodes were a glassy carbon (0.09 mm2), Pt wire, and Ag/Ag+, respectively. The solvent was 1,2-dichloroethane, the concentration of the sample was 0.5 mM, and the supporting electrolyte was 0.2 M tetra-n-butylammonium perchlorate (TBAP). The solution was purged with nitrogen before the measurements, and the voltage sweep rate was 0.1 V/s. The increased voltage was 0.004 V, the amplitude was 0.05 V, the pulse width was 0.06 s, the sample width was 0.02 s, and the pulse period was 0.2 s for the DPV measurements. The molecular models were calculated with the CAChe Worksystem ver. 5.04 (Fujitsu) using the MM/AM1MOZYME method. All the electronanopatterning experiments were done with a Picoscan AFM system and piezoscanner (Agilent Technologies). For the current-sensing AFM mode, conductive Pt/Ir-coated Si tips (Agilent Technologies) were used to pole the local nanoscale area on the dendrimer spincoated films. Picolith software for the AFM nanolithography was used, having various options with regard to force, voltage, and speed draw the desired patterns. The UV lamp for the photopolymerization of the film was a MORITEX MUV-202U (mercury−xenon lamp), and the light intensity at the irradiation point was ca. 1.7 W/cm2. Analytical size-exclusion chromatography (SEC) was performed using an HPLC 1289
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(Shimadzu, LC-10AP) equipped with a TSK-GEL CMHXL (Tosoh) at 40 °C. Tetrahydrofuran (THF) was used as the eluent at the flow rate of 1 mL/min. The detection line was connected to a triple detector (refractive index, light scattering, and viscosity; Viscotek, TriSEC model 302). Dendrons. The carbazole-substituted benzophenone derivatives (dendrons) were obtained according to the literature method (G1 to G4).20a The model compounds G2A, G3A, and G3Ph were obtained according to the literature method.20a Tetrakis(4-aminophenyl)methane (TPMNH2) was prepared according to the literature methods24 from aniline and trityl chloride, and TPMG0 was also prepared according to the literature method.25 TPMG1 (General Procedure for Dehydration Reaction). TPMNH2 (84.0 mg, 0.221 mmol), G1B (1069 mg, 2.09 mmol), and DABCO (1959 mg, 17.5 mmol) were dissolved in chlorobenzene (30 mL) and heated to 75 °C. TiCl4 (253 mg, 1.33 mmol) dissolved in 5 mL of chlorobenzene was dropwise added, and the addition funnel was then rinsed with 5 mL of chlorobenzene. The mixture was heated to 125 °C and stirred for 15 h under a nitrogen atmosphere. The reaction mixture was cooled to room temperature, stirred overnight in air, and then filtered through Celite. The filtrate was concentrated, and the product was isolated by silica gel column chromatography (hexane:toluene = 1:50 with 2% Et3N) and purified by preparative GPC (eluent: chloroform). Yield: 61% (318 mg, 0.135 mmol). TPMG1: 1H NMR (400 MHz, CDCl3, TMS standard, 20.1 °C, ppm): δ 8.17 (8.0H, d, J = 7.3 Hz), 8.07 (8.0H, d, J = 7.3 Hz), 8.01 (8.0H, d, J = 8.3 Hz), 7.71 (8.0H, d, J = 8.8 Hz), 7.56 (8.0H, d, J = 8.3 Hz), 7.46 (8.0H, t, J = 7.8 Hz), 7.34 (16.0H, q, J = 7.6 Hz), 7.25−7.23 (16.0H, m), 7.18 (8.0H, d, J = 8.3 Hz), 7.05 (8.0H, d, J = 8.3 Hz), 6.96 (8.0H, d, J = 8.3 Hz), 6.55 (8.0H, d, J = 8.3 Hz). 13C NMR (100 MHz, CDCl3, TMS standard, 20.6 °C, ppm): δ 166.83, 149.15, 142.08, 140.36, 140.20, 140.11, 137.66, 137.58, 134.63, 131.13, 131.05, 130.80, 126.36, 126.08, 125.80, 123.62, 123.39, 120.30, 119.81, 109.76, 109.41, 63.03. MALDI TOF-MS (matrix: dithranol): calcd: 2358.9 ([M + H]+); found: 2357.5. Anal. Calcd for C173H112N12: C, 88.09; H, 4.79; N, 7.13. Found: C, 87.80; H, 4.65; N, 6.98. TPMG2. As per the general procedure for the dehydration reaction, TPMNH2 (41.3 mg, 0.109 mmol), G2B (1161 mg, 0.989 mmol), and DABCO (934 mg, 8.33 mmol) were dissolved in chlorobenzene (30 mL). TiCl4 (190 mg, 1.00 mmol) was added, and the mixture was stirred for 5 h at 125 °C. The product was isolated by silica gel column chromatography (hexane:toluene = 1:10 to 1:20 with 2% Et3N) and purified by preparative GPC (eluent: chloroform). Yield: 28% (153 mg, 0.03 mmol). TPMG2: 1H NMR (400 MHz, CDCl3, TMS standard, 22.0 °C, ppm): δ 8.26 (16.0H, dd, J = 7.6, 1.7 Hz), 8.16 (16.0H, d, J = 7.8 Hz), 8.03 (16.0H, d, J = 7.8 Hz), 7.94 (8.0H, d, J = 8.3 Hz), 7.59−7.54 (32.0H, m), 7.48 (8.0H, d, J = 8.8 Hz), 7.42−7.38 (48.0H, m), 7.30−7.28 (40.0H, m), 7.14 (16.0H, t, J = 7.3 Hz), 7.06 (8.0H, d, J = 8.3 Hz), 7.00 (8.0H, d, J = 8.3 Hz), 6.53 (8.0H, d, J = 8.3 Hz). 13C NMR (100 MHz, CDCl3, TMS standard, 23.1 °C, ppm): δ 166.71, 149.67, 142.10, 141.68, 141.59, 140.15, 140.00, 139.70, 137.46, 137.14, 135.65, 131.27, 131.18, 130.91, 130.80, 126.38, 126.20, 125.93, 124.33, 124.21, 123.20, 120.34, 119.92, 119.78, 111.22, 110.98, 109.63, 109.53, 63.11. MALDI-TOF-MS (matrix: dithranol): calcd: 5002.9 ([M + H]+); found: 5002.5. Anal. Calcd for C365H224N28: C, 87.65; H, 4.51; N, 7.84. Found: C, 87.47; H, 4.23; N, 7.84. TPMG3. As per the general procedure for the dehydration reaction, TPMNH2 (11.1 mg, 0.029 mmol), G3B (414 mg, 0.166 mmol), and DABCO (598 mg, 5.33 mmol) were dissolved in chlorobenzene (20 mL). TiCl4 (102 mg, 0.538 mmol) was added, and the mixture was stirred for 5 h at 125 °C. The product was isolated by silica gel column chromatography (toluene with 2% Et3N) and purified by preparative GPC (eluent: chloroform). Yield: 14% (43 mg, 0.004 mmol). TPMG3: 1H NMR (400 MHz, CDCl3, TMS standard, 19.0 °C, ppm): δ 8.47 (16.0H, d, J = 14.2 Hz), 8.28 (32.0H, d, J = 13.7 Hz), 8.15 (32.0H, d, J = 7.8 Hz), 8.00−7.94 (40.0H, m), 7.71−7.52 (80.0H, br m), 7.36−7.35 (80.0H, br m), 7.30−7.14 (128.0H, br m), 7.06− 7.05 (32.0H, br m), 6.60 (8.0H, br s). 13C NMR (100 MHz, CDCl3, TMS standard, 19.3 °C, ppm): δ 167.53, 150.26, 142.12, 141.63, 141.49, 141.35, 141.26, 140.83, 140.03, 139.76, 136.89, 136.65, 136.46,
131.41, 130.89, 130.40, 130.24, 126.75, 126.51, 126.19, 125.81, 124.38, 124.19, 123.73, 123.06, 120.36, 120.25, 119.78, 119.64, 111.23, 110.95, 109.56, 109.32, 63.20. MALDI-TOF-MS (matrix: dithranol): calcd: 10285.7 ([M + H]+); found: 10285.4. Anal. Calcd for C749H448N60: C, 87.44; H, 4.39; N, 8.17. Found: C, 87.18; H, 4.19; N, 7.95.
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RESULTS AND DISCUSSION The carbazole dendrimer20,22 with metal coordination sites (phenylazomethine26) on the inner layer of the dendron is expected to be an efficient hole-transporting material enhanced by metal coordination.22a−d,27 This dendrimer was synthesized by the dehydration reaction between the carbazole dendron substituted benzophenones (GnB) and tetrakis(4aminophenyl)methane (TPMNH2) (Scheme 1). Because of the high steric hindrance (high generation dendrimers had 3substituted products as a byproduct), the yield of the dendrimerization reaction decreased from G1 to G3 (61, 28, and 14%). All products were finally purified by preparative scale GPC and identified by 1H NMR, 13C NMR, MALDI-TOF MS (Figure S1), and elemental analysis, and the purity of the double layer-type dendrimers was verified by HPLC (SEC) analysis (Figure S2). Interestingly, all the dendrimers were soluble in common organic solvents, such as benzene, toluene, chloroform, and THF (at least more than 10 mg/mL), without bulky end-capping groups at the 3,6-position of the peripheral carbazole.20b The intrinsic viscosity of the dendrimers and dendrons were measured and plotted versus the logarithm of the molecular weight (Mark−Houwink−Sakurada plot,14b,28 Figure S3). The viscosity of the dendrons linearly increased with generation, but the viscosity of the dendrimer did not increase from G2 to G3. This means that the carbazole dendrons have rigid and linear molecular structures (no back-folding). On the other hand, the four-substituted tetraphenylmethane core dendrimer (G3) has a spherical structure due to the symmetric and compact core structure. This can also be confirmed by the molecular modeling (Figure S4) which verified that the peripheral carbazoles exist on the surface of the molecule. The thermal stability of the dendrimer was evaluated by thermogravimetric analysis (TGA) measurements (Figure 1).
Figure 1. Temperature−weight loss plot of the dendrimers.
The 10% weight loss temperatures (Td10%) of the dendrimers increased when the generation increased. However, the derivative TG (DTG) plots (Figure S5) showed that all the dendrimers started to degrade at ca. 500 °C, which can be attributed to the degradation of the phenylazomethine units.25 For further study, the EI- and PI-MS analyses of the thermal desorption gas of the model compounds20a (phenylazomethine substituted carbazole dendrons = one-fourth of the dendrimer; 1290
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the carbazole-substituted phenylazomethines.20a Therefore, it can be concluded that the electronic density of the phenylazomethine and the binding constant decreases when the generation of the dendrimer increases due to the generation dependent withdrawal of the carbazole dendrons. This kind of Lewis acid coordination to the phenylazomethine group is reported to enhance hole-transporting property,22a−d,27 which will be an advantage as a hole-transporting material. The electrochemistry of the dendrimer was studied by cyclic voltammetry (CV) and differential pulse voltammetry (DPV) techniques. Cyclic voltammograms of the dendrimer (Figure 3) show a typical electropolymerization behavior due to the oxidative coupling of the peripheral carbazole units. The oxidation of the carbazole unit starts at about 0.8 V for TPMG3 and TPMG2 and at 1.0 V for TPMG1 because of the difference in the oxidation potential due to the higher conjugation and substituent effect of the carbazole units20a,d (Figure S12). As is known,31 the carbazole dimer will have two redox sites, and the first redox will occur at a voltage lower than the monomer. The CV of TPMG1 from 0 to 1.5 V clearly shows this behavior; i.e., a new oxidation peak at about 0.9 V appeared during the second cycle. Additionally, the first redox peak current in the CV from −0.2 to 1.0 V of TPMG1 was very low compared to the other generations (Figure S13) and to the CV from 0 to 1.5 V of TPMG1. This also suggests that the oxidative coupling occurs around 1.0 V. When the cycle is stopped at 1.0 V, the number of the formed dimers is small compared to the other generation dendrimers that have lower oxidation potentials. Other generations also show similar CV behavior, but due to the π-conjugation of several carbazole sites in the dendrimer, the redox behavior is more complicated and difficult to analyze. The peak current of the oxidation peak of the first cycle oxidation has increased although the diffusion constant of the dendrimer decreases when the generation increases (Figure S12). (Theoretically, when the diffusion constant decreases, the peak current decreases.32) This indicates that larger numbers of carbazole units are oxidized at once in the higher generation and reflect the higher number of peripheral carbazole units. Next, the HOMO levels of the dendrimers were estimated from the onset voltage of the oxidation in each dendrimer using DPV.33 The HOMO levels were converted to the vacuum level using the redox potential of ferrocene,34 and the results were TPMG1 (5.59 eV), TPMG2 (5.43 eV), and TPMG3 (5.40 eV). When the generation increased, the HOMO level also increased. This can be similarly explained by the redox potential, i.e., the higher conjugation and substituent effect of the carbazole units.20a,d These values show that the dendrimers are suitable as hole-transporting semiconducting materials, injecting holes from ITO (4.5−5.1 eV35) or Au (4.6−5.2 eV35,36) electrodes, and also indicate that higher generation dendrimers have a better matching. The electropolymerization and deposition that were mentioned in the last section can be certainly used to obtain patterned semiconductive films by using patterned electrodes. Moreover, this electropolymerization can be expanded to direct patterning with current-sensing AFM (CS-AFM).37 A spincoated film of TPMG3 (115−130 nm thickness) on a goldcoated glass substrate was prepared, and the patterning was done by tracing a “UH” pattern at a 10 V bias and 0.5 μm/s scan rate (19 °C and 37% relative humidity). The scanning of the film was then done at a 5 V bias and 1.5 μm/s scan rate (Figure 4). The image clearly shows that the pattern is written in the dendrimer film. The negative current that was observed
see Chart S1 for the structure) have been performed. These data (Figures S7−S10) indicated that the model compounds (G2A and G3A) degrade at 500 °C to m/z 93 (aniline), at 540 °C to m/z 103 (benzonitrile), and at 630 °C to m/z 78 (benzene) and m/z 167 (carbazole). Additionally, the TGA curves of these compounds and the benzene-substituted carbazole dendron (G3Ph) showed that the degradation of the carbazole dendron occurs at ca. 600 °C (Figure S6). These results clearly show that the degradation of the dendrimer starts from the phenylazomethine units at around 500 °C followed by the degradation of the carbazole dendrons around 600 °C. When the generation of the dendrimer increases, the proportion of the phenylazomethine to total weight of the dendrimer decreases. At the same time, the molecular weight of the remaining carbazole dendrons increases and the sublimation will be suppressed. Therefore, the apparent Td10% increases when the generation of the dendrimer increases. However, the degradation starting at 500 °C of all the dendrimers is still a high value which indicates the high thermal stability of these dendrimers. The coordination chemistry of the dendrimer was confirmed by UV−vis titration with SnCl2.26a Upon the addition of SnCl2 to the dendrimer solution (benzene:acetonitrile = 4:1), the spectra gradually changed with one isosbestic point (Figure 2
Figure 2. UV−vis spectra of TPMG3 during the addition of SnCl2 (solvent was benzene:acetonitrile = 4:1 and the concentration was 10 μM). Inset is the titration curve of three dendrimers (10 μM benzene:acetonitrile = 4:1 solution) with SnCl2. Aobsd is the observed absorption, Af is the final absorption, and Ac is the initial absorption.
and Figure S11). The pattern was similar to other phenylazomethine compounds26 or the carbazole-substituted phenylazomethine compounds20a,22a−c which indicate the complexation of SnCl2 to the inner phenylazomethine sites of the dendrimers. The equivalent amount of SnCl2 that was needed until the spectral change became saturated increased when the generation increased (note that the number of binding sites and the concentration were the same for all dendrimers). The reason for this decrease in the binding constant can be twofold: the higher steric hindrance of the higher generation dendrimer and the substituent effect of the carbazole dendrons. As was noted in the Mark−Houwink−Sakurada plot, these dendrimers have an ideal “dense shell”19b,29 structure without back-folding. It can be assumed that the increase in the steric effect is not very high. The substituent effect of the carbazole (especially when it is tilted) is reported to have a withdrawal character,30 and the same decrease in the binding constant is reported for 1291
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Figure 3. Cyclic voltammogram of the dendrimers during two sweeping ranges (0.5 mM with 0.2 M TBAP).
Figure 4. Electronanopatterned “UH” feature with 10 V bias and 0.5 μm/s scan rate (19 °C and 37% relative humidity) using the conducting-AFM mode.
TPMG3 is shown in Figure 5. After UV irradiation, a decrease in the absorption of carbazole (around 300 nm) and increase in the longer wavelength region (350−550 nm) was observed. This corresponds to the oxidative coupling of the peripheral carbazole units at the 3,6-position.22d,38 After this UV irradiation, the film was rinsed twice with toluene (it was soluble before irradiation), and the UV−vis spectra were measured again. Indeed, no change in the UV−vis spectra was observed, which indicates that the dendrimer film has become insoluble in toluene. This experiment was performed for all three dendrimers for several irradiation times. The minimum time for insolubility to be reached (Figure 6) increased when
in the pattern indicates the lower resistance of the film due to polymerization and electrodoping. On the other hand, the topographic image of the same area did not show a big change in the height (maximum 0.25 nm change). This morphological stability shows the capability of periphery unsubstituted and rigid dendrimers as patternable materials for electropolymerization. The oxidative polymerization reaction can be possibly applied to photopolymerization reactions and for patterning with photolithography.22d To verify this assumption, a film of the dendrimer was formed in a quartz cell and irradiated by UV light. The spectra before and after UV irradiation in air of 1292
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to the photochemical conversion of diphenylamine to carbazole.41 However, the results indicate that the reaction without oxygen is slow and the presence of oxygen accelerates the dimerization reaction. Finally, to demonstrate photopatterning using this photoreaction, the UV irradiation through a photomask (300 mesh copper TEM grid was used) was performed. After the UV irradiation, the film was washed with toluene to remove the un-cross-linked dendrimer. A clear pattern of the 45 μm squares was observed in the optical microscope image (Figure 7), which means that the photo-
Figure 5. UV−vis spectra of the TPMG3 film before, after a 3 min UV irradiation, and after washing with toluene.
Figure 7. Optical microscope image of photopatterned TPMG3 film. The object on the right-hand side is a 500 μm diameter wire that was inserted as a standard.
patterning of the dendrimer film has been successfully done and shows that this dendrimer can be potentially used as a photopatternable hole-transporting material.
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CONCLUSION In conclusion, periphery unsubstituted carbazole−phenylazomethine double-layer-type dendrimers with a tetraphenylmethane core were synthesized and characterized. The structure, thermal stability, and metal coordination behavior were determined. The electrochemical polymerization and application to patterning with CS-AFM and patterning with photopolymerization of the dendrimer were demonstrated. Carbazole dendrimers are well-known as an efficient holetransporting material, and these results are promising such that this dendrimer is an efficient and patternable semiconducting to conductive (doped) material for several organic devices.
Figure 6. Rate of insolubilization of the double-layer-type dendrimers for different irradiation times.
the generation decreased. This can be explained with the next three reasons: (1) higher generation dendrimers have higher molecular weights, (2) higher generation dendrimers have more reactive units and higher density of them at the periphery, and (3) higher generation dendrimers have higher HOMO levels; i.e., higher generation dendrimers reach the molecular weight that insolubilizes the film faster than the lower generations by photopolymerization. In addition, it is reported that the dimerization reaction of 9-phenylcarbazole cation radical (intermediate of the photo-cross-linking reaction) in solution is accelerated in the presence of neutral 9-phenylacarbazole.39 Higher generation dendrimers have higher concentration of carbazole unit at the periphery, and presumably this accelerates the photochemical reaction. Interestingly, under a nitrogen atmosphere (nitrogen was flowed through the cell) this reaction was very slow. After 10 min irradiation to TPMG3 film the insolubilization was less than 10%. This suggests that the existence of air (oxygen) accelerates this photo-cross-linking reaction. The photooxidation (dimerization) reaction of carbazole and the participation of oxygen to this reaction are pointed out in some reports.40 Electron transfer from excited carbazole group generates carbazole cation radicals that dimerize. At the same time, highly reactive oxygen anion radical is generated which can eliminate as H2O or H2O2 or react with carbazole (or carbazole dimer) to produce quinones or oxygen-bridged dimer structures. In the absence of oxygen, the dimerization reaction may take place by elimination of H2 or disproportion, similarly
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ASSOCIATED CONTENT
S Supporting Information *
Some chemical structures, identification data, calculations, thermal degradation analysis, UV−vis titration, and cyclic voltammograms. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (K.Y.);
[email protected] (R.C.A.).
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ACKNOWLEDGMENTS This work was supported in part by the Core Research for Evolutional Science and Technology (CREST) program of the Japan Science and Technology (JST) Agency and by an Innovative Area (Coordination Programming, Area 2107, no. 21108009) from the Ministry of Education, Culture, Sports, 1293
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Science and Technology, Japan. The authors thank the Rigaku Corp. for performing the thermal desorption gas analysis. R.C.A. thanks partial funding support of this work from the Robert A. Welch Foundation E-1551.
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