Nitronyl Nitroxide and Imino Nitroxide Mono- and Biradicals in

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Langmuir 1998, 14, 7484-7492

Nitronyl Nitroxide and Imino Nitroxide Mono- and Biradicals in Langmuir and Langmuir-Blodgett Films J. Le Moigne,*,† J. L. Gallani,† P. Wautelet,† M. Moroni,† L. Oswald,† C. Cruz,† Y. Galerne,† J. C. Arnault,† R. Duran,‡ and M. Garrett‡ Institut de Physique et Chimie des Mate´ riaux de Strasbourg, UMR 7504, 23 rue du Loess BP 20, 67037 Strasbourg, France, and Butler Polymer Laboratory, University of Florida, Gainesville, Florida 32611 Received June 22, 1998. In Final Form: October 2, 1998 In an effort to prepare high-spin-ordered layers, several conjugated molecules bearing one nitronyl nitroxide radical (NN) or two imino nitroxide magnetically coupled radicals (IN) have been synthesized. The monoradical and the biradical molecules proved to be amphiphilic enough to form Langmuir films on a water surface. For the two monoradicals, the molecular area extrapolated at zero-surface pressure in the final part of the isotherms is nearly the same (A0 ∼ 0.55 nm2 molecule-1) and close to what may be expected for such molecular shapes. For the longest rigid-rod biradical, bis imino nitroxide pentamer, 5p(bisIN), the pressure-area isotherm shows a low compressibility and Brewster angle microscopy has revealed that the film is solidlike, the molecules having a tendency to aggregate, forming plates on the water surface. The bis imino nitroxide trimer, 3p(bisIN), has a larger final molecular area A0 ∼ 0.80 nm2 molecule-1, the molecules being, by then, oriented perpendicular to the surface as deduced from grazing incidence X-ray analysis (GIXA) and surface potential measurements. After deposition of monolayers on various hydrophilic substrates by the Langmuir-Blodgett (LB) technique, the biradical 3p(bisIN) happens to be the most interesting molecule since good transfer ratios (TR) are easily obtained and electron paramagnetic resonance (EPR) measurements indicate paramagnetic properties. In addition, atomic-force microscopy (AFM) imaging of the films shows defects in the form of elevated zones. The AFM reveals that these zones are partially made of molecules piling up, their heights being multiples of single-molecule lengths. These elevated domains occur in different shapes, either randomly distributed circular zones or aligned, coalesced domains lying along preferred directions. Y-type multilayers up to nine layers were also transferred on hydrophilic glass, with a somewhat poorer TR.

Institut de Physique et Chimie des Mate´riaux de Strasbourg. Butler Polymer Laboratory.

playing a triplet ground state and an intramolecular ferromagnetic spin coupling.7 In the present work, we focused on the preparation of a 2D organization of radicals using both approaches described above: the intermolecular interaction control through 2D spin alignment and the intramolecular approach by using molecules where the spin interactions are realized through a π-conjugated oligomeric backbone. Recently we have reported the existence of intramolecular long-range spin exchange in biradical molecules where the radicals galvinoxy, imino nitroxide or nitronyl nitroxide are separated by a conjugated phenylene ethynylene segment of variable length.8,9 A magnetic coupling has been shown by EPR in isolated molecular systems. It was demonstrated that the exchange coupling between the two radicals through the conjugated rigid segment decreases exponentially with distance, although it is still efficient at 3.6 nm.10 Four amphiphilic molecules have been synthesized, two of them being monoradicals, while the others were made of two radicals attached at both ends of a rigid rod. Since 2-D ferromagnetic interactions require molecular order and stability in two dimensions, the Langmuir-Blodgett technique11 has been utilized to create two-dimensional monolayers of each species. Monomolecular films of these

(1) Veciana, J.; Cirujeda, J.; Rovira, C. Adv. Mater. (Weinheim, Ger.) 1995, 2, 221. (2) Iwamura, H. Pure Appl. Chem. 1993, 1, 57. (3) Epstein, A. J.; Miller, J. S. Synth. Met. 1996, 80, 231. (4) Kinoshita, M.; Turek, P.; Tamura, M.; Nozawa, K.; Shiomi, D.; Nakazawa, Y.; Ishikawa, M.; Takahashi, M.; Awaga, K.; Inabe, T.; Maruyama, Y. Chem. Lett. 1991, 1225. (5) Chiarelli, R.; Rassat, A.; Rey, P. J. Chem. Soc., Chem. Commun. 1992, 1081. (6) Mataga, N. Theor. Chim. Acta 1968, 10, 372.

(7) Nishide, H.; Hozumi, Y.; Nii, T.; Tsuchida, E. Macromolecules 1997, 30, 3986. (8) Wautelet, P.; Le Moigne, J.; Turek, P. Mater. Res. Soc. Symp. Proc. 1994, 328, 313. (9) Turek, P.; Wautelet, P.; Le Moigne, J.; Stanger, J. L.; Andre´, J. J.; Bieber, A.; Rey, P.; De Cian, A.; Fisher, J. Mol. Cryst. Liq. Cryst. Sci. Technol., Sect. A 1995, 272, 99. (10) Wautelet, P.; Bieber, A.; Turek, P.; Le Moigne, J.; Andre´, J. J. Mol. Cryst. Liq. Cryst. Sci. Technol., Sect. A 1997, 305, 55.

Introduction In recent years, an increasing interest in preparing organic ferromagnetic materials has arisen in the field of materials science.1,2 Due to their organic nature, such materials would possess interesting properties such as solubility, transparence, flexibility, and lightness. Inherent properties of organic magnetism would make these materials especially useful for low-frequency shielding applications.3 Two major approaches are being used in the design and synthesis of purely organic ferromagnetic materials: the intermolecular and the intramolecular spin alignment. The intermolecular approach gives rise to materials with ferromagnetic behaviors,4,5 but the crystalline molecular structure is difficult to predict, and the interaction between unpaired spins in the crystal lattice is unpredictable. In the intramolecular approach, the spin coupling between multiple radicals through a π-conjugated polymer chain is theoretically expected to give high-spin ground state polymers.6 However, new polymers based on phenylene vinylenes bearing nitronyl nitroxide or galvinoxy radicals have recently been reported as dis† ‡

10.1021/la9807309 CCC: $15.00 © 1998 American Chemical Society Published on Web 12/04/1998

Mono- and Biradicals in Langmuir Films

radicals have been prepared on a water surface in order to study molecular organization and stability. Some of these films have been successfully transferred on solid substrates for AFM and EPR studies. Experimental Section Materials. The following chemical reactants were obtained from Aldrich: 1-octadecyne, 3-bromobenzaldehyde, 5-bromosalicylaldehyde, palladium(II) chloride (PdCl2), triphenylphosphine (TPP), copper(II) acetate monohydrate (Cu(OAc)2), manganese(IV) oxide (MnO2), and isopropylamine (i-Pr2NH). The triethylamine (Et3N) was obtained from Acros. Octadecyltrichlorosilane was obtained from ABCR GmbH. The methanol, hexane, CH2Cl2, and CHCl3 were obtained from Prolabo. All the chemicals were used without further purification. Monoradical Synthesis. [3-(1-octadecyne)]benzaldehyde (1). 3-Bromobenzaldehyde (0.94 g, 5 mmol) and 1-octadecyne (1.25 g, 5 mmol) were dissolved in 50 mL of distilled isopropylamine. To the degassed solution were added TPP (52 mg, 0.20 mmol), PdCl2 (18 mg, 0.10 mmol), and Cu(OAc)2 (20 mg, 0.10 mmol). The mixture was heated at 80 °C for 48 h under argon. After cooling, the precipitated isopropylamine hydrobromide was filtered off, and the solvent was evaporated. The crude material was purified on a silica-gel column using CH2Cl2/hexane (1/4) as an eluent to give 1.17 g of the aldehyde (1). Yield: 66%. Mp: 36-38 °C. 1H NMR (CDCl3): δ 10.0 (s, 1H, -CHO), 7.90 (s, 1H, -PhH), 7.80 (d, 1H, -PhH), 7.65 (d, 1H, -PhH), 7.47 (t, 1H, -PhH), 2.45 (t, 2H, -CtC-CH2), 1.62 (m, 2H, -CH2 β acetylenic), 1.31 (s, 26H, -CH2), 0.89 (t, 3H, -CH3). Anal. Calcd for C25H38O: C, 84.69; H, 10.80; O, 4.51. Found: C, 84.21; H, 10.74. 1,3-Dihydroxy-4,4,5,5-tetramethyl-2-[3-(1-octadecyne)phenyl]tetrahydroimidazoline (2). A mixture of [3-(1-octadecyne)]benzaldehyde (1.01 g, 2.84 mmol) and 2,3-bis(hydroxyamino)-2,3-dimethylbutane (0.459 g, 3.1 mmol) in 40 mL of dry MeOH was stirred at room temperature for 72 h. The white precipitate was filtered and washed to give 2. Yield: 0.55 g (40%). Mp: 131132 °C. 1H NMR (CDCl3): δ 7.55 (s, 1H, -PhH), 7.36 (m, 3H, -PhH), 5.05 (s, 2H, -OH), 4.71 (s, 1H, -CH), 2.40 (t, 2H, -Ct C-CH2), 1.61 (m, 2H, -CH2 β acetylenic), 1.26 (s, 26H, -CH2), 1.11 (s, 12H, -CH3), 0.89 (t, 3H, -CH3). Anal. Calcd for C31H52N2O2: C, 76.81; H, 10.81; N, 5.78; O, 6.60. Found: C, 77.25; H, 10.91. Oxidation. To 0.54 g (1.11 mmol) of (2) dissolved in 50 mL of CH2Cl2 was added 0.2 g (2 mmol) of MnO2. The solution, which becomes blue immediately, was stirred at room temperature for 9 h. After filtration and concentration, the crude material was purified by chromatography (eluent: CH2Cl2/hexane (1/ 3).Yield: 42 mg (79%) C18mNN blue crystals. Mp: 56-58 °C. Anal. Calcd for C31H49N2O2: C, 77.29; H, 10.25; N, 5.82; O, 6.64. Found: C, 77.24; H, 10.38; N, 5.79. [5-(1-Octadecyne)]salicylaldehyde (3). 5-Bromosalicylaldehyde (1.01 g, 5 mmol) and 1-octadecyne (1.25 g, 5 mmol) were dissolved in 60 mL of distilled triethylamine. To the degassed solution were added TPP (65 mg, 0.25 mmol), PdCl2 (9 mg, 0.6 mmol), and Cu(OAc)2 (10 mg, 0.5 mmol). The mixture was heated at 70 °C for 72 h under argon. After cooling, the precipitated triethylamine hydrobromide was filtered off, and the solvent was evaporated. The crude material was purified on a silica-gel column using hexane as an eluent to give 0.51 g of the intermediate 3. Yield: 27%. Mp: 48-49 °C. 1H NMR (CDCl3): δ 11.03 (s, 1H, -CHO), 9.85 (s, 1H, -OH), 7.60 (d, 1H, -PhH), 7.53 (dd, 1H, -PhH), 6.92 (d, 1H, -PhH), 2.39 (t, 2H, -CtC-CH2), 1.58 (m, 2H, -CH2 β acetylenic), 1.26 (s, 26H, -CH2), 0.88 (t, 3H, -CH3). Anal. Calcd for C25H38O2: C, 81.03; H, 10.34; O, 8.63. Found: C, 81.30; H, 10.51. 1,3-Dihydroxy-4,4,5,5-tetramethyl-2-[3-(1-octadecyne)6-hydroxyphenyl]tetrahydroimidazoline (4). The same procedure used to obtain 2 was used for 4. [5-(1-Octadecyne)]salicylaldehyde (3) (1.20 g, 3.2 mmol) and 2,3-bis(hydroxyamino)2,3-dimethylbutane (0.40 g, 2.7 mmol) in 150 mL of dry MeOH were stirred at room temperature for 72 h. A 50 mL portion of H2O was added, and the solution was stored in a refrigerator overnight. The solid was filtered, washed with water, dried, and (11) Bai, C.; Zhang, P.; Zhu, D.; Han, M.; Xu, Y.; Zhang, D.; Liu, Y. J. Phys. Chem. 1995, 99, 8202.

Langmuir, Vol. 14, No. 26, 1998 7485 purified on a silica-gel column using CH2Cl2/hexane (1/1) as an eluent. Yield of 4: 1 g (74%). Mp: 88 °C. 1H NMR (CDCl3): δ 7.25 (m, 2H, -PhH), 6.73 (d, 1H, -PhH), 4.82 (s, 1H, -CH), 2.36 (t, 2H, -CtCCH2), 1.54 (m, 2H, -CH2 β acetylenic), 1.26 (s, 26H, -CH2), 1.19 (s, 12H, -CH3), 0.89 (t, 3H, -CH3). Anal. Calcd for C31H52N2O2: C, 74.35; H, 10.47; N, 5.59; O, 9.58. Found: C, 74.70; H, 10.63. Oxidation. To 0.5 g (1 mmol) of 4 dissolved in 50 mL of CH2Cl2 was added 0.2 g (2 mmol) of MnO2. The solution, which becomes blue immediately, was stirred at room temperature overnight. After filtration and evaporation of the solvent, the crude material was purified on a silica-gel column using CH2Cl2/hexane (1/1) as an eluent. Yield: 21 mg (42%) C18OHmNN blue crystals. Anal. Calcd for C31H49N2O3: C, 74.81; H, 9.92; N, 5.63; O, 9.64. Found: C, 73.92; H, 9.82; N, 5.53. Biradical Synthesis. The trimer biradical 1,1′-bis-2-[4,4,5,5tetramethylimidazoline-1-oxyl]{4-4′-[phenyl-(1,4-diethynyl-2,5bis(dodecanoxy)benzene)]} and the corresponding bis-2-[4,4,5,5tetramethylimidazoline-1-oxyl] pentamer will be hereafter named 3p(bisIN) and 5p(bisIN), respectively. The 3p(bisIN) and the 5p(bisIN) syntheses were done following a protecting general route already used for the oligophenyl ethynyl derivatives.12 The detailed chemical routes have been published elsewhere.13 Trimer 3p(bisIN). Recrystallization in hexane/CH2Cl2 yielded 170 mg (85%) of red crystals. Mp: 102 °C. Anal. Calcd for C60H84N4O4: C, 77.92; H, 9.09; N, 6.06; O, 6.92. Found: C, 77.85; H, 9.23; N, 5.92; O, 6.87. Pentamer 5p(bisIN). Recrystallization in heptane/CH2Cl2 yielded 133 mg (89%) of red crystals. Mp: 169 °C. Anal. Calcd for C80H100N4O4: C, 81.35; H, 8.47; N, 4.74; O, 5.42. Found: C, 80.62; H, 8.43; N, 4.53; O, 5.46. Langmuir and Langmuir-Blodgett Films. Spreading solutions were prepared using CHCl3 or CH2Cl2 (Analysis Grade from Carlo Erba) at 0.2-1.0 mg/mL concentrations. Volumes of 50-200 µL were spread using a microsyringe. Films were left 15 to 20 min to obtain equilibrium before the compression started. Data were collected with a KSV LB5000 system (KSV Instruments, Helsinki, Finland) using a symmetrical compression Teflon trough and hydrophilic barriers in a clean, dust-free environment. The trough temperature was controlled to (0.1 °C, and the trough itself was in a Plexiglass enclosure. All isotherms were taken at 20 °C unless otherwise specified. The ultrapure water (F ) 18.2 MΩ cm) used for the subphase was obtained from a Milli-RO3 Plus system combined with a MilliQ185 Ultra Purification system from Millipore. The Wilhelmy plate method (platinum or paper) was used for surface-pressure measurements. Film images at the Brewster angle (BAM) were obtained on an apparatus from Nanofilm Technology GmBh Goettingen, Germany (BAM2plus). The surface potential measurements were performed using a KSV 5000 SP module. The monolayers were compressed with typical speeds of 0.1 to 0.3 nm2/(molecule‚min). Isotherms were reproducible from run to run and showed no noticeable hysteresis.14 LB films were obtained by transfer on glass slides, quartz plates, or hydrophilic silicon wafers (100) at surface pressures ranging from 8 to 10 mN/m for 3p(bisIN) to 20-22 mN/m for C18mNN and C18OHmNN. Transfers started from below the surface, with a typical emersion speed of 1 mm/min. Prior to transfer, quartz and glass substrates were cleaned using the following procedure: the plates were immersed in a hot detergent solution (Decon 90), rinsed several times with hot water, treated by a hot sulfochromic solution, and then rinsed at least 15 times with ultrapure water. The sulfochromic bath was replaced by an oxidizing mixture H2SO4-H2O2 (1:1) for the hydrophilic treatment of the silicon wafers (100). All the treatments were done in an ultrasonic bath. The hydrophobic plates were obtained using the standard procedure: the clean glass substrate is immersed in octadecyltrichlorosilane in solution in ethyl alcohol (3% w/w, with a few drops of acetic acid as a catalyst), dried in a nitrogen stream, and cured for 2 h at 150 °C. (12) Wautelet, P.; Moroni, M.; Oswald, L.; Le Moigne, J.; Pham, T. A.; Bigot, J. Y.; Luzzati, S. Macromolecules 1996, 29, 446. (13) Wautelet, P.; Turek, P.; Le Moigne, J.; Andre´, J. J.; Bieber, A. Submitted for publication. (14) Except for 5p(bisIN) for which aggregation prevented the isotherms from being reversible.

7486 Langmuir, Vol. 14, No. 26, 1998 Grazing Incidence X-ray Analysis (GIXA). The grazing incidence X-ray studies of LB films were performed on a X′PERTMPD device from Philips, equipped with a Nickel β filter, a programmable divergence slit (1/32°), a parallel plate collimator, a flat Ge monochromator, and a proportional Xe detector. The Cu KR line (wavelength ) 0.1542 nm) was used. All measurements were recorded immediately after the LB transfer. The intensity of the X-ray reflection pattern diminishes rapidly with time so that the fringes are not visible anymore after a few days. This is most probably due to molecular rearrangements within the film on the substrate. Some authors15 have also reported that LB films are very labile when not stored properly. AFM Studies and Magnetic Characterizations on LB Films. The AFM measurements were carried out with a Nanoscope III multimode microscope from Digital Instruments operating at ambient atmosphere. The images were recorded at room temperature using the tapping mode, where a silicon cantilever is vibrated near its resonance frequency (290-420 kHz). During the scan, the distance from tip to surface is of several nanometers so that the interaction with the surface is minimized. The sharpest tips were selected; their estimated nominal radius of curvature was between 5 and 10 nm. The EPR experiments have been carried out with a Bruker ESP 300E spectrometer. Average DC magnetic field was 3500 G, and RF frequency was around 9.7GHz. Spectra were recorded at room temperature. The X-ray, AFM, and EPR measurements were recorded immediately after the LB transfer. Nevertheless, AFM pictures show that defects are already present in the film immediately after the deposition.

Results and Discussion Chemical Synthesis and Characterization. The chemical structures of mono- and biradicals used in this work are schematized in Figure 1a. The two monoradicals C18mNN and C18OHmNN were synthesized according to the chemical routes given in Figures 1b,c. The detailed chemical schemes for 3p(bisIN) and 5p(bisIN) have been published elsewhere.13 The chemical purities of the intermediates and the final products in the synthetic routes were investigated by elemental analysis and 1H NMR. The chemical structures of the final products are confirmed by infrared, UV-vis, and EPR spectroscopy. The spin concentrations of the mono- and biradicals in the solid state were determined at room temperature by integration of the EPR signal and were calibrated against a standard (VARIAN pitch). The results are given in Table 1 together with ns, the number of spins 1/2 per molecule. It is concluded from these estimations, that these molecules may be considered as monoradicals or biradicals. The slight spin deficits observed in some cases may originate from uncertainties ((15% on the spin number) inherent to the experimental procedure.16 EPR integration demonstrates that 0.89 spins/molecule and 0.63 spins/ molecule are obtained, respectively, for C18mNN and C18OHmNN and 1.8 and 1.9 spins/molecule for the biradicals 3p(bisIN) and 5p(bisIN), respectively. The monoradical C18mNN and the two biradicals may be considered as a pure monoradical and pure biradicals within the error limit, while the monoradical C18OHmNN should be considered as 15-20% deficient in radical character. The EPR spectra in solution of the different radical derivatives have been recorded at room temperature in toluene for the monoradicals C18mNN and C18OHmNN (15) Mikrut, J. M.; Dutta, P.; Ketterson, J. B.; MacDonald, R. C. Phys. Rev. B: Condens. Matter 1993, 48, 14479. (16) The sample being in powder form, the filling of the EPR tube is difficult because of electrostatic effects which cause the product to stick to the walls. Hence the exact amount of sample in the cavity is not precisely known.

Le Moigne et al.

(Figure 2a,b). The typical concentrations for the roomtemperature studies are in the range 10-5-10-4 mol‚l-1. The spectra of C18mNN and C18OHmNN exhibit the typical five lines hyperfine pattern of an nitronyl-nitroxide radical with relative intensities 1:2:3:2:1,17 the g factor is 2.0087. The hyperfine coupling constant a ) 7.42 G is typical for freely tumbling molecules in which the electron is coupled with two equivalent N atoms.17 The roomtemperature measurements of the biradicals 3p(bisIN) and 5p(bisIN) show a much more complex spectra due to the asymmetric imino radical and to the intramolecular exchange between the two imino nitroxide radicals interacting within the strong exchange limit for the 3p(bisIN) and within the intermediate exchange limit for the 5p(bisIN). The complete discussion of the magnetic exchange coupling in linear biradicals has been developed elsewhere.10 The optical-absorption spectra in solution of the monoradicals and biradicals are given in Table 2 and Figure 3a,b. In Figure 3 a the spectrum of C18mNN in CH2Cl2 exhibits two main peaks, one in UV with a maximum centered at λ ) 247 nm ( ) 3.7 × 104 L‚mol-1‚cm-1) and one in the near-UV at λ ) 365 nm ( ) 1.8 × 104 L‚mol-1‚cm-1). A very weak and broad absorption in the visible at λ ∼ 600 nm ( ) 6.5 × 102 L‚mol-1‚cm-1) is characteristic of the blue NN radical. In Figure 3b, the biradicals 3p(bisIN) and 5p(bisIN), which have an ether as a lateral donor side group, show similar spectral features with two main bands in the near-UV and a weak broad absorption in the visible. The first one is centered at λ ) 320 nm ( ) 5.2 × 104 L‚mol-1‚cm-1) and λ ) 337 nm ( ) 8.2 × 104 L‚mol-1‚cm-1) respectively for 3p(bisIN) and 5p(bisIN). The second broad absorption band appears at λ ) 383 nm ( ) 4.8 × 104 L‚mol-1‚cm-1) and λ ) 390 nm ( ) 8.8 × 104 L‚mol-1‚cm-1), respectively, for the 3p(bisIN) and 5p(bisIN). Langmuir Films. The pressure-area isotherms are given in Figure 4 for the two monoradicals. The surfacepressure and surface-potential isotherms for the biradicals are given in Figure 5a,b. All compounds proved to be amphiphilic enough to form Langmuir films at the water surface. The isotherms of C18mNN and C18OHmNN at 20 °C have the same shape: the surface pressure π starts to rise slowly for a molecular area of 0.70-0.75 nm2 and film collapse occurs at pressures in the range of 28-32 mN/m. The specific molecular areas A0 (extrapolated at zero pressure, see dotted lines in the figures) are, respectively, A0 ) 0.57 ( 0.04 nm2 molecule-1 and A0 ) 0.53 ( 0.04 nm2 molecule-1 for C18mNN and C18OHmNN. Brewster angle microscopy and the surface potential measurements (not shown) indicate that these films are coherent without any holes or defects when the molecular area goes below 0.65 nm2. These observations indicate that the films are in a liquid expanded phase,18 where molecules interact with each other more weakly than in a solid film. The fact that both isotherms have similar shapes indicates that the OH group on the phenyl ring does not influence the packing behavior or change the amphiphilic character of these molecules. The heads of the molecules are probably perpendicular to or slightly tilted with respect to the water surface, with the aliphatic tails extending away. This hypothesis is sustained by the fact that GIXA analysis of monolayers transferred on silicon (see below) at a pressure of 20mN/m revealed that the organic layer has a thickness of 2.3 ( 0.3 nm, close (17) Ulman, E. F.; Osiecki, H.; Boocock, D. G. R.; Darcy, R. J. Am. Chem. Soc. 1972, 94, 7049. (18) Adamson, A. W. Physical Chemistry of Surfaces, 5th ed.; Wiley & Sons Inc.: New York, 1990; p 132.

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Figure 1. (a) Conjugated mono- and biradical molecules based on nitronyl nitroxide or imino nitroxide groups used in this work. (b) Synthetic chemical scheme of the monoradical C18mNN. (c) Synthetic chemical scheme of C18OHmNN.

to the full length of an extended molecule (lo ≈ 2.1 nm). The possibility of the molecules changing their orientation during the transfer can be ruled out since the transfer ratio was close to one; hence, each molecule occupies the same area on water as it does on silicon.

The surface-pressure and surface-potential isotherms of the two biradical molecules (Figure 5a,b) show two strongly different behaviors. The 5p(bisIN) surfacepressure isotherm shows a low compressibility and a stiff, nearly linear, increase of the pressure up to the collapse

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Table 1. Spin Number Determination ns from EPR and SQUID for the Mono- and Biradical Molecules molecule

ns from EPR (spin 1/2 molecule-1)

C18mNN C18OHmNN 3p(bisIN) 5p(bisIN)

0.89 0.63 1.8a 1.9a

a

ns from SQUID (spin 1/2 molecule-1)

1.99a

Data from ref 8 for biradical.

Table 2. UV-vis Spectroscopy of Mono- and Biradical Solutions and Films molecule

absorption peaks of solutions λmax (nm)

λmax films (nm)

C18mNN 3p(bisIN) 5p(bisIN)

247, 274a, 348a, 365, 600 320, 383, 458, 494a, 534a 291, 337, 390, 459, 494a, 534a

327, 393

a

The shoulder on absorption peaks.

Figure 3. (a) UV-vis spectrum in solution of C18mNN. (b) Spectra of 3p(bisIN) and 5p(bisIN) in CH2Cl2.

Figure 2. (a) EPR spectra of C18mNN. (b) C18OHmNN in CHCl3 solution.

which occurs at πc ) 52-54 mN/m, the final molecular area being A0 ) 0.55 ( 0.04 nm2 molecule-1 (Figure 5b). Brewster angle microscopy has revealed that the film is never perfect, the molecules having a tendency to aggregate, forming plates on the water surface (Figure 5c). The increase of the surface pressure starts when these plates begin to interact with each other, around a molecular area of 0.80 nm2. If the barriers of the trough are moved back, these plates break into smaller ones, and the pressure drops abruptly. These observations indicate that the film is solid. Moreover, the surface potential starts to rise for A ≈ 1.0 nm2, which indicates the formation of a more or less continuous film on the water surface. As a result the solidlike behavior of the aforementioned plates, defects are present, and the curve becomes noisy. The molecules initially lay flat on the water, but around A ≈ 0.75 nm2 the change in the slope indicates that they begin to stand up. The process is complete when the collapse pressure is reached.

Figure 4. Langmuir isotherms at 25 °C of the 2 monoradicals C18mNN and C18OHmNN.

On the contrary, the 3p(bisIN) film is highly compressible at surface areas between 1.60 and 0.80 nm2. Brewster angle microscopy reveals that the film is in a liquid expanded phase and is very homogeneous as soon as the pressure starts to rise. The leveling of the surface pressure at the rather moderate value of πc ) 12 mN/m is the signature of the collapse of the film, which coincides with the formation of a second layer with undefined structure. The final area is A0 ≈ 0.80 nm2 molecule-1, extrapolated at zero pressure (Figure 5a). The molecules begin to interact with each other when the surface pressure starts to rise at 1.60 nm2 molecule-1, and the surface potential shows a steep increase. The further increase of the surface

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a

Figure 6. UV-vis spectroscopy of nine layers of 3p(bisIN) LB film on hydrophilic glass (T ) 25 °C).

b

c

Figure 5. Surface-pressure and surface-potential isotherms of the conjugated rigid-rod biradicals at T ) 20 °C for (a) 3p(bisIN) and (b) 5p(bisIN), respectively. (c) BAM picture of 5p(bisIN) Langmuir film upon decompression at the area of 0.64 nm2 molecule-1.

potential indicates that the molecules initially lay flat on the water and then begin to stand up. The value of 1.60 nm2 molecule-1 is consistent with the film being in a liquid expanded phase. An estimate value of the minimum molecular area, deduced from crystalline structure19 (with the two dodecyloxy side chains out of the phenyl-ethynyl plane), would be ≈1.25 nm2 molecule-1. The final molecular

area A0 ≈ 0.80 nm2 molecule-1 is also consistent with the molecular shape and size calculated from crystallographic measurements, if one considers that the molecules are perpendicular to the surface or slightly tilted. Repeated compression-decompression cycles for the monoradical and the 3p(bisIN) molecules, measured at various barrier speeds, show that the isotherms remain unchanged with negligible hysteresis for pressures lower than the collapse. This indicates that the stress-relaxation processes are faster than the movement of the barrier. If the collapse pressure is exceeded, the isotherms remain reversible as long as the decompression of the film is sufficiently slow. It is also not clear why the 5p(bisIN) molecules aggregate while the 3p(bisIN) molecules show no such tendency. Langmuir-Blodgett Films. LB films of the two monoradicals C18mNN and C18OHmNN and of the biradical 3p(bisIN) were obtained by transfer on glass or silicon substrates. A very good transfer ratio (TR ) 1.0 ( 0.05) is observed for C18OHmNN on glass, while for C18mNN transfers are poorer (TR about 1.2). A single layer of 3p(bisIN) can easily be transferred (TR ) 1.0 ( 0.1) on a hydrophilic Si wafer, but this layer is unstable, thus preventing the deposition of multilayers. The deposition of 3p(bisIN) in multilayers is easier on a hydrophilic glass slide, with a good transfer ratio and up to 9 layers being deposited. No deposition or incomplete transfer resulted from hydrophobic glass substrates for these monoand biradicals. LB Films Optical and Magnetic Properties. Figure 6 shows the UV-vis absorption spectrum of a multilayer of 3p(bisIN) on hydrophilic glass, where nine layers have been deposited on each of the two faces of the glass slide. It shows two main bands, one in the near-UV and one in the visible, quite similar to the spectrum in solution. The first peak is centered at λ ) 325 nm; the second peak at λ ) 394 nm, is broadened and contains a long tail at long wavelength which corresponds to the radical-absorption band observed in solution in the range of 450-580 nm. The magnetic properties of mono-molecular layers20 of the monoradical C18mNN and of the biradical 3p(bisIN) have been studied by EPR. Films21 had been deposed on hydrophilic quartz plates. EPR spectra taken at room (19) The crystallographic structure was partially given in ref 9. The crystallographic system of 3p(bisIN) is monoclinic, the space group P21/ c, the lattice parameters a ) 0.9337 nm, b ) 1.1956 nm, c ) 2.4788 nm, β ) 99.912°. Other data are available on request. (20) Unfortunately the nine-layer LB film on glass could not fit into the EPR tube and has not been investigated. (21) The 2D magnetic structure of the LB films has been ascertained through EPR-anisotropy measurements. These results, together with the temperature dependence of the susceptibility, will be discussed in a coming paper on the magnetic properties of LB films.

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Le Moigne et al.

Table 3. EPR Data of C18mNN and 3p(bisIN) in LB and Cast Films on Hydrophilic Quartz Plates EPR signal C18mNN LB film C18mNN cast film 3p(bisIN) LB film 3p(bisIN) cast film

bandwidth (G) 11.2 6.6 11.5 6.7

frequency (GHz) 9.73861 9.74277 9.69228 9.74838

g factor 2.0066 2.0068 2.0065 2.0064

temperature are shown in Figure 7a,b. In contrast to the well-resolved EPR spectra of radical solutions, the LB EPR spectra consist of a broad single line, without hyperfine splittings. The line shape fits well with a Lorentzian, and the resulting parameters are given in Table 3. The values for the g factor and the bandwith for cast films are also provided for comparison. The main difference lies in the broadening of the line which can be attributed to dipolar interactions and reduced dimensionality (2D) of the LB film. The relative discrepancy in the spin number between the estimated number and the measured value may arise from the fact that within the resonant cavity all parts of the substrate do not experience the same electromagnetic field intensity. LB Films Grazing Incidence X-ray Scattering. To ascertain the structure of LB films, grazing incidence X-ray scattering studies have been performed on single layers of C18mNN and 3p(bisIN) deposited on Si wafers. Figure 8a,b shows the X-ray intensity versus the scattering angle for a single layer of C18mNN and a single layer of 3p(bisIN), respectively. Only three reflection fringes are visible, which indicate that the films are not very well structured. The electronic density profile within the organic layer has been calculated in accordance with the structure of each molecule. The resulting simulated diffraction pattern is represented as a continuous line. The calculated layer thickness from the fit is 2.3 ( 0.3 nm and 2.4 ( 0.3 nm for the C18mNN and 3p(bisIN) monolayers, respectively. These values, close to the length of the extended molecules (respectively, ≈2.3 nm and ≈2.9 nm9), indicate that for each film the molecules are more or less perpendicular to the substrate. In both cases, the roughness is important around 0.6 nm, which indicates that defects are present either in the form of free space, flat-lying molecules, or stacks of molecules (see below). This was, indeed, to be expected, given the large molecular area at which the transfers were processed. LB Films Morphology, AFM Characterization. To get a more precise idea of the surface organization, we investigated a transferred LB film of the biradical 3p(bisIN) by AFM. Figure 9a shows an AFM picture of a monolayer of the rigid biradical deposited on a silicon wafer. The monolayer was imaged about 1 h after LB film deposition. In this image, the darker gray levels correspond to the higher positions. A flat background is observed, with circular domains higher than the background randomly arranged on the surface. Their diameters ranged from 0.05 to 0.17 µm in size. The statistical analysis shows that high domains only occupy 39% of the total surface for a z-scale threshold at 1.5 nm with respect to the background. For the highest domains (more than 15.0 nm), the surface ratio decreases to 4.8%. This confirms the previous hypothesis that parts of the film are either collapsed or made of stacks of molecules. Such topographic images were also found on LB multilayers of an alkoxyphenyl-NN transferred on glass slide,11 where a large number of grains with different sizes and heights were randomly distributed on the whole surface. These authors analyzed the grains in terms of defects of the surface or contaminants. On another part of the Si surface, the AFM picture (Figure 9b) shows oriented domains along parallel direc-

number of spins, deposited 1.0 ×

number of spins, measured

1015

1.2 × 1015

9.2 × 1014

6.8 × 1014

Figure 7. EPR signals of the LB monolayers of (a) C18mNN and (b) 3p(bisIN), respectively, observed on hydrophilic quartz plate.

tions. While Figure 9a shows a random distribution of circular domains, Figure 9b shows aligned domains along a privileged direction. The coalescence of some of these domains gives rise to long parallel ridges. A cross section of the film along a line perpendicular to these ridges is represented in Figure 9c. It shows a great roughness of the LB film, but closer examination reveals that some levels are around 3.0 and 6.0 nm corresponding to 1 or 2 times the molecule length. Other authors22 report that upon aging, monolayers of arachidic acid spontaneously reorganize to multilayers. They first observe granulelike defects such as the ones we see, which evolve into platelike islands with time, the height of these defects being commensurate with the length of the molecules. Still, statistical analysis along several line scans shows a rather wide distribution of heights. Clearly, other defects are present, such as tilted or flat-lying molecules. The two AFM images presented here correspond to two different zones of the same sample. Both have been extracted from the same image, and therefore have the same referential. The privileged directions, observed on the pictures, make an angle of 50°; they cannot be related to any crystallographic angle of the substrate (Si 100) nor of the molecular crystal. At the present time, the formation process of the oriented domains is unclear. We can discard the eventuality of dipping-induced orientation since it (22) Kondrashkina, E. A.; Hagedorn, K.; Vollhardy, D.; Schmidbauer, M.; Ko¨hler, R. Langmuir 1996, 12, 5148, (See also ref 15 for the building up of multilayers).

Mono- and Biradicals in Langmuir Films

a

Langmuir, Vol. 14, No. 26, 1998 7491

a

b

b

Figure 8. Plots of X-ray intensity versus the scattering angle plot for a single layer of (a) C18mNN and (b) 3p(bisIN), respectively, on an Si surface. C18mNN and 3p(bisIN) were transferred at a surface pressure of 22 mN/m and 10 mN/m, respectively; the corresponding molecular areas are 0.43 and 0.66 nm2 molecule-1.

should not give rise to two different orientations on the same substrate. We have checked the topography of the silicon substrates before the transfer, and no such defects have been observed even at much smaller scales. Conclusion To prepare high-spin-ordered layers, several conjugated molecules bearing one or two nitronyl nitroxide radicals or two imino nitroxide radicals have been synthesized. The amphiphilic properties, where the nitronyl nitroxide and imino nitroxide radicals are used as polar groups, have been demonstrated through the processing of Langmuir films. The isothermal compression of the film results in strongly different behaviors for a single radical and biradicals on a rigid core. While the general behavior and the final molecular areas for the two monoradicals are nearly the same, the biradicals differ: the 3p(bisIN) is in a liquid expanded phase before the film collapses, whereas the 5p(bisIN) aggregates and form clusters. After deposition of mono- or multilayers on silicon, glass, or quartz by the Langmuir-Blodgett technique, it was found that the most interesting molecule was the biradical 3p(bisIN). Good transfer ratios were obtained for a monolayer on hydrophilic Si and Y-type multilayers of up to nine layers on hydrophilic glass. The monolayer exhibited paramagnetic properties. The morphology of the monolayer surface, obtained by AFM imaging, shows that even though the transfer was done at pressures lower than the collapse pressure, defects are present in the film. The AFM revealed that these defects are either depletion zones or bilayered zones. Different shapes are observed for these defects: randomly distributed circular zones or aligned coalescent domains lying along privileged directions of the substrate.

c

Figure 9. (a and b) Tapping-mode AFM images of two places of the surface monolayer of 3p(bisIN) on hydrophilic Si wafer. The scan rate was 0.39 Hz. Figure (c) Cross-section profile along the line (b).

Although several organic magnetic materials have already been obtained in bulk crystals, we demonstrate in this work that their 2D engineering using the LB technique gives rise to weak but detectable paramagnetic properties. Further data on monolayers or multilayers

7492 Langmuir, Vol. 14, No. 26, 1998

obtained by near-field microscopy, including magnetic force imaging, will be necessary to understand the 2D magnetic interactions. Acknowledgment. We wish to acknowledge the CNRS and the NSF-CNRS, USA-France exchange program (Action Incitative No. 4378-97). We also acknowledge support from NSF grant No. CHE9424058, the NSF REU

Le Moigne et al.

program through which Matt Garret received support, NSF Grant No. DMR9357462, and DOE No. DE-FG0296ER.45589.A001. We would also like to thank Mr. M. Keyser, the Analytical Services Group of the Institut Charles Sadron in Strasbourg for the elemental analyses, and M. Bernard for EPR spectra. LA9807309