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Amphiphilic Phenylene-Ethynylene Oligomers in Langmuir-Blodgett Films. Self-Assembling Multilayers for Electroluminescent Devices E. Arias-Marin,† J. C. Arnault,† D. Guillon,† T. Maillou,† J. Le Moigne,*,† B. Geffroy,‡ and J. M. Nunzi†,‡ Institut de Physique et Chimie des Mate´ riaux de Strasbourg, UMR 7504, 23 Rue du Loess BP 20 CR, 67037 Strasbourg Cedex, France and LETI (CEA-Technologies Avance´ es), DEIN-SPE, Groupe Composants Organiques, CEA Saclay, 91191 Gif-sur-Yvette Cedex, France Received October 5, 1999. In Final Form: January 20, 2000 New rigid amphiphilic molecules based on a p-phenylene-ethynylene unit with hydrophilic side chains were synthesized by a step by step method up to the heptamer. The two most interesting materials, the pentamer and the heptamer, are amphiphilic enough in nature to produce stable Langmuir films on hydrophilic substrates such as hydrophilic glass, ITO, or hydrophilic silicon. A transfer ratio of 1, observed only by lifting, suggests a Z-type deposited film. The multilayer deposition can be carried out up to 36 layers. The films were analyzed by X-ray reflectivity and are revealed as well structured with a layering period of 3.7 nm. This suggests a rearrangement in a Y-type bilayer occurring after transfer deposition from the water surface. Using AFM, the surfaces of films deposited on glass or Si are shown to exhibit steps of 3.6-3.7 nm height or multiples, which are coherent with a self-rearrangement of the single deposited layer to a double layer during the drying process. The heptamer and pentamer show high photoluminescence and large Stokes shifts with emission peaks at 516 and 504 nm. LED properties are demonstrated using the ITO/oPEn/LiF/Al sandwich yielding photon emission at 516 nm for the heptamer. The luminescencevoltage characteristics of two diodes using 22 and 36 LB layers show threshold voltage at 4.5 and 6 V respectively and in those conditions the electroluminescence yield is close to 10-3%. It is concluded that the electroluminescence in a LB film of molecules aligned parallel to the substrate is interesting because it confirms the possibility of tailoring conduction and emission properties of devices using a layer by layer deposition technique.
Introduction In past years, the photophysical properties of highly fluorescent conjugated polymers and oligomers received considerable attention because of their potential application in producing light emission in large area panels and colored displays or else in the production of coherent light.1-3 Early polymeric electroluminescent devices were based on poly(p-phenylene vinylene) (PPV),4 poly(pphenylene) (PPP),5 or poly(thiophene)6 and efficient light electroluminescent diodes are now currently realized using vacuum deposited molecules7 or spin coated polymers.8 Some years ago the acetylene analogues of PPV, based on the phenyl-ethynyl unit, the poly(phenylynylenes) (PPE) were also shown to exhibit photoluminescence and electroluminescence properties.9 However due to their low solubility related to their aggregation properties10,11 and † ‡
Strasbourg. Gif-sur-Yvette.
(1) Kraft, A.; Grimsdale, A.; Holmes, A. Angew. Chem. Int. Ed. 1998, 37, 403. (2) Tessler, N. Adv. Mater. 1999, 11, 363. (3) Gelinck, G.; Warman, J.; Remmers, M.; Neher, D. Chem. Phys. Lett. 1997, 265, 320. (4) Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackay, K.; Friend, R. H.; Burns, P. L.; Holmes, A. B. Nature 1990, 347, 539. (5) Grem, G.; Leditzky, G.; Ullrich, B.; Leising, G. Adv. Mater. 1992, 4, 36. (6) Berggren, M.; Ingana¨, O.; Gustafsson, G.; Rasmusson, J.; Andersson, M. R.; Hjertberg, T.; Wennerstro¨m, O. Nature 1994, 372, 444. (7) Tang, C. W.; Van Slyke, S. A. Appl. Phys. Lett. 1987, 51, 913. (8) Friend, R. H.; Gymer, R. W.; Holmes, A. B.; Burroughes, J. H.; Marks, R. N.; Taliani, C.; Bradley, D. C. C.; Dos Santos, D. A.; Bredas, J. L.; Lo¨gdlund, M.; Salaneck, W. R. Nature 1999, 397, 121. (9) Swanson, L. S.; Shinar, J.; Ding, Y. W.; Barton, T. J. Synth. Met. 1993, 55-57, 1.
to their poor processability, these materials were not investigated for device applications. More recently PPE received an increasing interest, owing to their π-conjugation, their conformational rigid rodlike character and their attractive optical properties.12-14 As a matter of fact, the oligo(phenylene-ethynylenes) are presented as “molecular wires” for molecular conduction15 or magnetooptic responses in future nanoelectronic devices.16,17 In addition, these well-defined molecular systems, where the precise length and molecular shapes are strictly controlled, open a very large field of molecular applications in biology and in nanophotonics.18,19 As for the light emitting diodes (LED), a tradeoff exists in developing techniques to achieve a precise molecular organization of the active moieties in order to control transport and emission properties. In this respect, the Langmuir-Blodgett technique (LB) is a promising way for the achievement of improved self-organized systems of active electroluminescent molecules. Indeed this tech(10) Wautelet, P.; Moroni, M.; Oswald, L.; Le Moigne, J.; Pham, T. A.; Bigot, J.-Y.; Luzzati, S. Macromolecules 1996, 29, 446. (11) Halkyard, C. E.; Rampey, M. E.; Kloppenburg, L.; StuderMartinez, S. L.; Bunz, U. H. F. Macromolecules 1998, 31, 8655. (12) Weder, C.; Wrighton, M. S. Macromolecules 1996, 29, 5157. (13) Yang, J. S.; Swager, T. J. Am. Chem. Soc. 1998, 21, 5321. (14) Kukula, H.; Veit, S.; Godt, A. Eur. J. Org. Chem. 1999, 277. (15) Schumm, J. S.; Pearson, D.; Tour, J. M. Angew. Chem., Int. Ed. Engl. 1994, 33, 1360. (16) Le Moigne, J.; Gallani, J. L.; Wautelet, P.; Moroni, M.; Oswald, L.; Cruz, C.; Galerne, Y.; Arnault J. C.; Duran, R., Garrett, M. Langmuir 1998, 14, 7484. (17) Mu¨llen, K.; Wegner, G. Electronic Materials: The Oligomer Approach; Wiley-VCH: Weinheim, 1998. (18) Tour, J. Chem. Rev. 1996, 96, 537. (19) Pesak, D. J.; Moore, J. S.; Wheat, T. E. Macromolecules 1997, 30, 6467.
10.1021/la991313e CCC: $19.00 © 2000 American Chemical Society Published on Web 03/25/2000
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nique allows a good molecular order and a molecular alignment needed in the case of polarized light emission. Examples of polarized light emitting devices based on the deposition of oriented LB of poly(p-phenylene) or oxadiazole derivatives have been successfully achieved.20,21 Moreover the use of LB or absorbing layers improves the charge injection at the ITO electrode, i.e., the operating voltage could be drastically reduced, and as a result the lifetime of LEDs can be increased.22 In the present work we report on the synthesis of new oligo(phenylene-ethynylenes) (oPEn) by a step by step method. A single side chain (alkyl-hydroxyl ester group) per phenyl ring ensures the solubility and the amphiphilic character. The pentamer and the heptamer homologues can develop stable Langmuir films. It is then possible to produce multilayer films by successive transfers from the water surface to different hydrophilic substrates such as glass, ITO or silicon. The multilayer films are analyzed by X-ray reflectivity and the surface film morphologies are investigated by atomic force microscopy (AFM). It is shown that the film morphologies strongly depend on the transfer parameters and the substrate surface. Finally (20) Cimrova, V.; Remmers, M.; Neher, D.; Wegner, G. Adv. Mat. 1996, 8, 146. (21) Tokuhisa, H.; Era, M.; Tsutsui, T. Appl. Phys. Lett. 1998, 72, 2639. (22) Nuesch F.; Si-Ahmed, L.; Franc¸ ois, B.; Zuppiroli, L. Adv. Mater. 1997, 9, 222.
we demonstrate that the LB multilayer films of phenyl ethynyl heptamer can be used in a blue-green light emission device. Experimental Section Materials. The following chemical reactives, acetic acid, 2-amino-5-iodobenzoic acid, 2,5-dibromobenzoic acid, 11-bromo1-undecanol, boron trifluoride diethyl etherate (BF3‚OEt2), 1,5diazabicyclo[4.3.0]non-5-ene (DBN), copper(I) iodide, iodomethane, palladium chloride (PdCl2), trimethylsilylacetylene (TMSA), triphenylphosphine (TPP), tetrabutylamonium fluoride in THF (TBAF), trimethylchlorosilane, and tert-butyl nitrite, were obtained from Aldrich and Lancaster. Diethylamine and triethylamine (NEt3) from Acros, were previously dried from KOH. Benzene, CH2Cl2, CHCl3, diethyl ether, hexane, methanol and THF were purchased from Prolabo. Dry THF was obtained by vacuum transfer from sodium benzophenone. All the chemical materials were used without further purification. The heptamer (oPE7) and pentamer (oPE5) were synthesized following the protecting general route of oligophenylethynylenes.15,18 The main steps of the chemical route to oPE7 are given in Scheme 1. The intermediates have been synthesized according to the literature.23,10 Synthesis of (11-undecanol) 2,5-diethynylbenzoate (1). A round-bottom flask charged with (11-undecanol) [2,5-bis((trimethylsilyl) ethynyl)]-benzoate (7.2 mmol, 3.51 g) and wet (23) Moore, J. S.; Weinstein, E. J.; Wu, Z. Tetrahedron Lett. 1991, 32, 2465.
P-E Oligomer SAMs for Electroluminescent Devices THF (100 mL) was cooled to -20 °C. Then a 1 M solution of TBAF (2 mmol, 2 mL) was added and after 5 s of stirring, the reaction was stopped by passing through a plug of silica gel. After THF evaporation the crude product was chromatographed to obtain a red viscous liquid (1), SiO2 column, eluent CH2Cl2/ THF, (90/10 v/v). 1H NMR (200 MHz, CDCl3): δ(ppm) 8.05(s, 1H, -PhH), 7.57(s, 2H, -PhH), 4.34(t, 2H, -COOCH2-), 3.64(t, 2H, -CH2OH), 3.48(s, 1H, -CtCH), 3.22(s, 1H, -HCtC-), 1.76(q, 2H, -CH2-β-COO), 1.58(q, 2H, -CH2-β-OH), 1.30(m, 14H, -CH2-); Anal. Calcd (%) for C22H28O3: C, 77.64; H, 8.23; O, 14.13. Found: C, 77.41; H, 8.34; O, 14.54. Synthesis of [2-(3,3-diethyltriazene)-5-iodo]- (11-undecanol) benzoate (2). 2-(3,3-diethyltriazene)-5-iodobenzoic acid (22.76 mmol, 7.90 g) was treated with 11-bromo-1-undecanol (22.76 mmol, 5.72 g) and DBN (22.76 mmol, 2.81 mL) in benzene (70 mL) using the general esterification procedure. The product was chromatographed to afford a yellow viscous oil (2), SiO2 column, eluent CH2Cl2/CH3OH (95/5 v/v). 1H NMR (200 MHz, CDCl3): δ(ppm) 7.9(d, 1H, -PhH), 7.67(dd, 1H, -PhH), 7.18(d, 1H, -PhH), 4.34(t, 2H, -COOCH2-), 3.75(m, 4H, -NCH2-), 3.63(t, 2H, -CH2OH), 1.78(q, 2H, -CH2-β-COO), 1.59(q, 2H, -CH2-β-OH), 1.30(m, 14H, -CH2-), 0.89(t, 6H, -NCH3); Anal. Calcd (%) for C22H36IN3O3: C, 51.06; H, 7.012; N, 8.12; O, 9.27. Found: C, 50.85; H, 6.98; N, 8.41; O, 9.53. Synthesis of Trimer [4,4′′′-bis(3,3-diethyltriazene)-((11undecanol) 3,2′′,3′′′-tribenzoate)]-1,4′′diethynyl (3). The general procedure was followed to couple (2) (12.62 mmol, 6.53 g) with (1) (6.3 mmol, 2.15 g), PdCl2 (1.26 mmol, 0.223 g), CuI (0.631 mmol, 0.120 g), and TPP (5.0 mmol, 1.32 g) in NEt3/THF (100/30 mL). The crude product was purified by flash chromatography to obtain a deeply red oil (3), eluent CH2Cl2, then CH2Cl2/THF (80/20 v/v), (yield 70%). 1H NMR (200 MHz, CDCl3): δ(ppm) 8.15(d, 1H, -PhH), 7.81(m, 2H, -PhH), 7.62(s, 2H, -PhH), 7.56(m, 2H, -PhH), 7.47(d, 2H, -PhH), 4.35(dt, 6H, -COOCH2-), 3.79(qu, 8H, -NCH2-), 3.63(t, 6H, -CH2OH), 1.78(q, 6H, -CH2-β-COO), 1.59(q, 6H, -CH2-β-OH), 1.30(m, 54H, -CH2-, -NCH3); Anal. Calcd (%) for C66H98N6O9: C, 70.81; H, 8.82; N, 7.51; O, 12.86. Found: C, 70.28; H, 8.90; N, 7.76; O, 12.93. Synthesis of Trimer [4,4′′′-bis(3,3-diethyltriazene)-((11(trimethylsilyl ether) undecyl) 3,2′′,3′′′-tribenzoate)]1,4′′diethynyl (4). Triethylamine (7.51 mmol, 1.04 mL) and (3) (2.50 mmol, 2.56 g) in dry THF (50 mL) were taken up under argon in a round-bottomed flask and stirred at room temperature for 20 min. Then, trimethylchlorosilane (7.5 mmol, 0.96 mL) in dry THF was added dropwise and stirred overnight. The ammonium salt was filtered off and the solvent was vacuum evaporated. The red oil product was used in the next step without further purification to avoid silyl ether hydrolysis (4), (yield 96%). 1H NMR (200 MHz, CDCl ): δ(ppm) 8.15(d, 1H, -PhH), 7.81(m, 3 2H, -PhH), 7.62(s, 2H, -PhH), 7.56(m, 2H, -PhH), 7.47(d, 2H, -PhH), 4.35(dt, 6H, -COOCH2-), 3.8(qu, 8H, -NCH2-), 3.63(t, 6H, -CH2OH), 1.8(q, 6H, -CH2-β-COO), 1.59(q, 6H, -CH2β-OH), 1.30(m, 54H, -CH2-, -NCH3), 0.12(s, 27H, -SiCH3). Synthesis of Trimer [4,4′′′-bis(diiodo)-((11-(trimethylsilyl ether) undecyl) 3,2′′,3′′′-tribenzoate)]-1,4′′diethynyl (5). A heavy-walled glass flask joined to a Teflon screw valve was charged with the ditriazene trimer with protected OH (4) (1.86 mmol, 2.48 g) and iodomethane (10 mL). The solution was degassed, placed under vacuum, and the flask was sealed and heated under stirring at 115 °C overnight. Later, hexane (15 mL) was added and the precipitate was filtered off. After evaporation of the solvent, a yellow oil (5) was collected (yield 87%). 1H NMR (200 MHz, CDCl3): δ(ppm) 8.14(s, 1H, -PhH), 8.0(dd, 2H, -PhH), 7.95(t, 2H, -PhH), 7.65(s, 2H, -PhH), 7.34(m, 2H, -PhH), 4.35(dt, 6H, -COOCH2-), 3.62(t, 6H, -CH2OH), 1.8(q, 6H, -CH2-β-COO), 1.59(q, 6H, -CH2-β-OH), 1.30(m, 42H, -CH2-), 0.12(s, 27H, -SiCH3). Synthesis of [4,4′′-bis(diiodo)-((11-undecanol) 3,2′′,3′′′tribenzoate)]-1,4′′diethynyl Trimer (6). To a round-bottomed flask containing methanol (10 mL) was added the diiodo trimer OH protected (5) (1.61 mmol, 2.24 g) and stirred until total product dissolution. Then, a 1 M solution of acetic acid (5 mL) was added portionwise and stirred for 15 min. The solution was stored in a refrigerator overnight and the precipitate filtered off and airdried to obtain a yellow powder (6), (yield 95%). Mp 93-96 °C;
Langmuir, Vol. 16, No. 9, 2000 4311 1H NMR (200 MHz, CDCl ): δ(ppm) 8.15(s, 1H, -PhH), 8.0(dd, 3 2H, -PhH), 7.95(t, 2H, -PhH), 7.64(s, 2H, -PhH), 7.31(m, 2H, -PhH), 4.35(t, 6H, -COOCH2-), 3.62(t, 6H, -CH2OH), 1.8(q, 6H, -CH2-β-COO), 1.58(q, 6H, -CH2-β-OH), 1.30(m, 42H, -CH2-); Anal. Calcd (%) for C58H78I2O9: C, 59.39; H, 6.70; O, 12.28. Found: C, 59.56; H, 6.89; O,12.29.
Synthesis of [(11-undecanol) benzoate]-Containing Heptamer (8). The general procedure was followed to couple (6) (0.78 mmol, 0.92 g) with (7) (1.56 mmol, 0.99 g), PdCl2 (0.078 mmol, 0.014 g), CuI (0.02 mmol, 0.004 g), and TPP (0.24 mmol, 0.062 g) in NEt3/THF (40/10 mL). The crude product was chromatographed, SiO2 column, eluent CH2Cl2, then CH2Cl2/ THF (80/20 v/v). Recrystallization in methanol, yellow fluorescent powder (8), (yield 87%). Mp 104-107 °C; 1H NMR (200 MHz, CDCl3): δ(ppm) 8.24(s, 1H, -PhH), 8.18(s, 6H, -PhH), 8.04(dt, 2H, -PhH), 7.76(dt, 2H, -PhH), 7.68(s, 10H, -PhH), 7.46(t, 2H, -PhH), 4.38(m, 14H, -COOCH2-), 3.63(m, 14H, -CH2OH), 1.80(m, 14H, -CH2-β-COO), 1.54(m, 14H, -CH2-β-OH), 1.31(m, 98H, -CH2-); 13 C NMR(50 MHz, CDCl3) δ(ppm) 165.94, 165.51, 165.40, 135.78, 134.28, 133.73, 132.69, 132.44, 130.84, 129.66, 128.51, 123.50, 123.12, 122.87, 95.17, 94.85, 90.89, 90.75, 88.97, 65.79, 65.44, 62.96, 32.75, 29.52, 29.45, 29.25, 28.70, 26.03, 25.70; FAB-MS (3-nitrobenzyl alcohol matrix, positive ion mode) m/z (rel intensity %): 2177.9 (30), 1863.7 (23), 1549.7 (59), 1121.1 (22), 987 (66), 969 (48), 699 (59). Anal. Calcd (%) for C138H184O21: C, 76.06; H, 8.51; O, 15.41. Found: C, 75.73; H, 8.58; O, 15.18. Synthesis of [(11-undecanol) benzoate]-Containing Pentamer (9). The general procedure was followed to couple (11undecanol) 2,5-dibromobenzoate, (2.22 mmol, 1 g) with (7) (4.44 mmol, 2.80 g), PdCl2 (0.22 mmol, 0.04 g), CuI (0.11 mmol, 0.022 g), and TPP (0.66 mmol, 0.175 g) in NEt3/THF (50/15 mL). The crude product was chromatographed, SiO2 column, eluent CH2Cl2, then CH2Cl2/THF (80/20 v/v). Green fluorescent powder, crystallization in methanol, (9), (yield 87%). Mp 91-94 °C; 1H NMR (200 MHz, CDCl3): δ(ppm) 8.24(s, 1H, -PhH), 8.18(s, 4H, -PhH), 8.04(dt, 2H, -PhH), 7.76(dt, 2H, -PhH), 7.67(s, 6H, -PhH), 7.45(t, 2H, -PhH), 4.38(m, 10H, -COOCH2-), 3.63(m, 10H, -CH2OH), 1.80(m, 10H, -CH2-β-COO), 1.54(m, 10H, -CH2-β-OH), 1.31(m, 70H, -CH2-);13 C NMR (50 MHz, CDCl3) δ(ppm) 165.94, 165.51, 165.40, 135.78, 134.28, 133.73, 132.69, 132.44, 130.84, 129.66, 128.51, 123.50, 123.12, 122.87, 95.17,94.85, 90.89, 90.75, 88.97, 65.79, 65.44, 62.96, 32.75, 29.52, 29.45, 29.25, 28.70, 26.03, 25.70; FAB-MS (3-nitrobenzyl alcohol matrix, positive ion mode) m/z (rel intensity %): 1549.8 (56), 931.2 (25), 699.1 (76), 681.1 (71), 655.1 (31), 605.1 (24), 291 (76); Anal. Calcd (%) for C98H132O15: C, 75.93; H, 8.58; O, 15.48. Found: C, 75.31; H, 8.55; O, 15.12. Langmuir and Langmuir-Blodgett Films. Spreading solutions were prepared using CHCl3 (Analysis Grade, Carlo Erba) at 2-2.5 mg/mL concentrations. Volumes of 10 to 60 µl were spread using a microsyringe. Spreading solutions were left 15 to 20 min to equilibrium before the compression started. Data were collected with a KSV LB5000 system (KSV Instruments) using a symmetrical compression Teflon barrier in a clean dust free environment. The trough temperature was controlled to ( 0.1 °C and the trough itself was in a Plexiglas enclosure. The ultrapure water (F ) 18.2 MΩ cm) used for the subphase was obtained from a Milli-RO3-plus and Milli-Q185 ultra-purification system from Millipore. The Wilhelmy plate method (platinum) was used for surface pressure measurements. The monolayers were compressed with a typical speed of 2.5 mm/min. Isotherms were reproducible from run to run and showed no noticeable hysteresis. LB films were obtained by transfer on glass slides, glass plates, or hydrophilic silicon wafers (100) at surface pressures ranging from 15 mNm-1. Transfers on substrates started from below the surface, with a typical lifting speed of 5 mm/min. Prior to transfer, glass substrates were cleaned using the following procedure: the plates were immersed in a hot detergent solution (Decon 90), rinsed 10 times with hot water, or treated by a hot sulfochromic solution, rinsed with ultrapure water in an ultrasonic bath for
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Table 1. Spectroscopic Data of Oligomers (oPEn for n 2 to 7) and Corresponding Polymer (PPE) in Solution and in Solid Films solutions, absorption peak (nm) and molecular absorption (L g-1cm-1)
molecules oPE2 oPE3 oPE5 oPE7 pPEn
320 345 364 374 380
24.6 45.2 50.3 56.5 46.9
20 min, and then dried in nitrogen flux and heated to 50 °C. Si substrates were treated according to a modified RCA procedure.24 Grazing Incidence X-ray Analysis (GIXA). The grazing incidence X-ray studies of LB films were performed on an X’PERTMPD apparatus from Philips, (with nickel beta filter, programmable divergence slit (1/32°), parallel plate collimator, flat Ge monochromator, and Xe detector). A Cu KR beam at wavelength 0.1542 nm was used. All measurements were recorded immediately after the LB transfer. Data were analyzed by using GIXA (V2.1) software from Philips Electronics Instruments. AFM Studies. The AFM measurements were carried out with a Dimension 3100 from Digital Instruments operating at ambient atmosphere. The images were recorded at room temperature using the tapping mode, at resonance frequency (290-420 kHz). Low vibration amplitudes were chosen in order to improve the AFM lateral resolution; the distance tip-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 being between 5 and 10 nm. From the AFM recorded images, the roughness of the surface topography was characterized. The standard deviation of the height values, noted Rq, was obtained as follows:
∑ (Z - Z
Rq ) [1/N
i
i
m)
2 1/2
] , for i from 1 to N
where Zm is the average of the Z values within the given image, Zi is the current Z value and N is the number of points of the image. Electroluminescent Devices. Electroluminescent diodes were prepared as described previously25 on a patterned ITO (15 Ω/0) coated glass. LB films were directly deposited on top of the ITO coated glass. Two samples were investigated by transfer of oPE7 monolayers. 22 and 36 layers were transferred, and the layer thicknesses, 50 and 81 nm, were determined by DEKTAK 3 profilemeter. Films were subsequently dried 12 h in a 10-7 mB vacuum. After drying the LB multilayer were subsequently covered by a Li/Al cathode, as an efficient electron injection electrode.26,27 The thickness of these evaporated layers, controlled by the quartz balance monitor, was respectively 1 and 49 nm. The active area of the EL device was 0.33 cm2. Diodes were studied in air, electroluminescence and photoluminescence spectra being recorded with a broad-band JobinYvon spectrometer coupled to an amplified Hamamatsu-CCD multichannel detector and with an F 4500 Hitachi fluorescence spectrophotometer. Light intensities were measured with a calibrated 1 cm2 area Hamamatsu silicon photodiode.
Results and Discussion Chemical Synthesis and Characterization. A series of rodlike 1,4-benzoate ethynyl oligomers (oPEn with n from 2 to 7) has been synthesized according to the step by step method described previously.28 To ensure, from (24) Kern, W.; Puotinen, D. RCA Review 1970, 31, 187. Oxidative treatment: the substrates were soaked in a hot mixture of H2O/H2O2/ NH4OH (40:30:30)% for 20 min, then rinsed in an ultrasonic bath with ultrapure water. (25) Gautier, E.; Nunzi, J. M.; Sentein, C.; Lorin, A.; Raimond, P. Synth. Met. 1996, 81, 197. (26) Hung, L. S.; Tang, C. W.; Mason, M. G. Appl. Phys. Lett. 1997, 70, 152. (27) Jabbour, G. E.; Kawabe, Y.; Shaheen, S. E.; Wang, J. F.; Morrell, M. M.; Kippelen, B.; Peyghambarian, N. Appl. Phys. Lett. 1997, 71, 1762. (28) Schumm, J. S.; Pearson, D. L.; Tour, J. M. Angew. Chem., Int. Ed. Engl. 1994, 33, 1360.
solid films, absorption peak and shoulder* (nm)
photoluminescence, emission peak (nm)
339, 365* 367, 396* 374, 403* 382, 415* 404, 430*
504 516 539
Figure 1. Optical absorption spectra of oligomers and related polymer in CHCl3. The spectral absorbances are in L g-1 for the series of oPEn, for n ) 1 to 7. The insert shows the wavelength at the absorption peak vs 1/n, n ) number of phenyl groups.
the dimer to the heptamer, both an amphiphilic character and a good solubility in the reaction solvent, the first compounds were synthesized with a phenyl-ethynyl unit bearing ester undecanol as a side chain. The pentamer and heptamer show a good solubility in common solvents such as toluene, CHCl3, or MeOH. The chemical purities of the all intermediates in the synthetic routes were checked by elemental analysis and 1H NMR; the final products were characterized by elemental analysis, 1H, 13 C NMR, and mass spectroscopy. The optical absorption spectra in solution of the oligomer series oPEn are given in Table 1 and Figure 1. The spectra in CHCl3 exhibit a broad peak in UV-Vis with maxima centered at λ ) 320, 345, 364, 374 nm, where the electronic absorbances are respectively ) 24.6, 45.2, 50.3, 56.5 L g-1 cm-1 for n ) 2, 3, 5, 7, (see insert λ vs 1/n). In addition the absorption spectrum of the corresponding polymer shows a peak at λ ) 380 nm and ) 46.9 L g-1 cm-1. It demonstrates that the red shift due to the electron delocalization increases with the oligomer length and that the limit value (λ ) 380 nm) is obtained in the polymer or in oligomers including more than 7 phenyl units. Thermogravimetric Analysis and Differential Scanning Calorimetry. TGA of oPE7 and oPE5 were performed under air and nitrogen. Under air, the weight loss is less than 2% up to 200 °C for the two materials. The loss increases after 200 °C and the material decomposed completely at 700 °C. The remaining weight at 700 °C under nitrogen was less than 1.0%, which shows that the phenyl-ethynyl backbone was completely decomposed. The oPEn oligomers show a better thermal stability than the corresponding polymer (weight loss of fractionated polymer < 2% only at 161 °C). The DSC of oPE7 and oPE5 are given in Figure 2a and 2b. On heating, oPE7 shows a broad endothermic transition between 42 and 108 °C with two peaks at 90 and 99 °C. On cooling, a reversible exothermic transition is observed at 80 °C. These results were corroborated by optical observations under a microscope that showed a fluid and birefringent state of the materials at temperatures higher than 60 °C. Similar properties for oPE5 are shown in Figure 2b: a broad
P-E Oligomer SAMs for Electroluminescent Devices
Figure 2. DSC of oPE7 (a); second heating and cooling at the scanning rates 5 °C/min. (b) DSC of oPE5, second heating and cooling in the same conditions.
Figure 3. Temperature dependence X-ray diffraction pattern of oPE7 powder in the range 30-100 °C on the second heating. The relative X-ray intensities are normalized at the large Bragg angles (at maximum of the diffuse band 2θ ≈ 20°).
transition is observed on heating between 47 and 96 °C and two peaks at 83 and 94 °C. On cooling a reversible exothermic transition is observed at 47 °C. Powder X-ray Diffraction. X-ray diffraction experiments were performed on the oPE7 and oPE5 powders with similar results. The X-ray diffractograms of oPE7, recorded at 30, 70, 95, and 100 °C are shown in Figure 3. At low temperature in the small angle region, the diffractogram shows two sharp reflections in the ratio of 1:2, corresponding to a lamellar system with a layer spacing of 3.87 nm. In the wide angle region, two diffuse bands are observed at 0.44 and 0.35 nm. These two bands correspond to the liquidlike order of the side-chains and to weak π-π interactions between phenyl rings, respectively. All these features are the signature of a disordered smectic phase of type A or C. Since the layer spacing
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corresponds to exactly twice the width of the oPE7 molecule with the side chains in an extended conformation, it can be stated that oPE7 exhibits a smectic A mesophase from room temperature up to 110˚C. It has to be noted that the optical textures observed under the microscope do not correspond to the classical textures usually reported in the literature for such a mesophase. Nevertheless, their strong birefringence clearly confirms the existence of a mesomorphic order in the temperature range of 25-110 °C. Langmuir and Langmuir-Blodgett Films. Langmuir Isotherm. The pressure-area isotherms of oPE7 at various temperatures are given in Figure 4. Successive compression-decompression cycles for the Langmuir isotherm show hysteresis at 10 and 16 °C, while the Brewster angle microscopic (BAM) pictures show a heterogeneous surface. The compression and decompression isotherms become reversible at 20, 25, 30, and 40 °C, where no hysteresis is observed when the surface pressure is lower than 15 mNm-1. In Figure 5 recorded at 25 °C the pressure-area isotherm shows a high compressibility at low pressure, beginning indeed near 8.5 nm2molecule-1. Around 2.00 nm2molecules-1 the pressure levels rise and the compressibility decreases. It demonstrates that the monolayer is in a liquid expanded phase. At 1.25 nm2molecules-1 and surface pressure of 15 mN/m1, the specific molecular area extrapolated at zero pressure is A0 ) 1.46 ( 0.04 nm2molecule-1. The BAM pictures a, b, c, and d, corresponding to the molecular area 0.08, 1.40, 1.50, and 1.95 nm2molecules-1 on the pressure-area isotherm, show the heterogeneous character of the film at low pressure. An improvement of the film homogeneity at higher pressure was observed around 15 mN/m, where no holes or defects can be observed by BAM. At higher pressure than 15 mNm-1, when the film compressibility increases, the BAM observation shows an increase of defects, the molecular layer becoming more and more heterogeneous. Then the collapse is reached at 32 mNm-1. The oPE5 shows the same behavior. The specific molecular area extrapolated at zero pressure is A0 ) 1.43 ( 0.04 nm2molecule-1 and the homogeneous Langmuir film is also observed in the same pressure range around 15 mNm-1 at 30 °C. The two specific molecular areas calculated from molecular modeling in a comblike conformation are respectively 1.75 and 1.25 nm2molecule-1 for oPE7 and oPE5. Langmuir-Blodgett Films. A very good transfer ratio (TR ) 0.95 ( 0.05) is observed for oPE7 and oPE5 on hydrophilic glass. For oPE7, at the surface pressure of 15mNm-1, the transfer of a monolayer can be achieved by lifting only. The deposited layer is then dried in air for 2 or 3 h and for a multilayer deposition, after every upstroke. Under these conditions, a multilayer film with more than 36 layers can be transferred. The transfer ratio was unity in the upward direction but very low and irregular in the downward direction, approaching Z-type-like deposition. At lower or higher surface pressures, i.e., 14.0 and 19.5 mNm-1, the transfer ratio becomes weak or irregular. No transfer is observed on hydrophobic glass but good transfer ratios are obtained on ITO glasses and oxidized Si. LB Films Properties. Figure 6 shows the UV-Vis absorption spectrum of oPE7 for t transfers (t from 1 to 16) on the 2 sides of the glass substrate. The absorption spectra show a broad band in the near-UV and in the visible at 382 nm with a shoulder around 415 nm, quite similar to the spectrum in solution. The insert shows that the peak absorption at 382 nm is linear with the layer number up to 12 and then deviates slowly from linearity. This indicates that the transfers could be irregular for a
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Figure 4. Pressure-area isotherms of the heptamer oPE7 at various temperatures (10, 16, 20, 25, 30, and 40 °C).
Figure 5. Pressure-area isotherms of oPE7 at 25 °C and the relative BAM pictures for the successive surface pressures (a) ∼ 0 mN/m (at the molecular area 6.00 nm2), (b) 11 mN/m, (c) 15 mN/m, and (d) 32 mN/m (collapse pressure).
higher number of layers. No dichroism on UV-Vis spectrum is observed on LB transfer with oligomers. On the polymer LB films (pPEn), a monolayer transferred at high compression speed (10 mm/min), a slight dichroism is observed in polarized light at 435.5 nm; the dichroic ratio being A|/A⊥ ) 1.3, A| and A⊥ are respectively the optical absorbances parallel and perpendicular to the lifting direction. Photoluminescence. The absorption and emission properties of the solid-state film of oPE7 are shown in Figure 7. The oPE7 film exhibits an absorption and emission maximum at 382 and 516 nm respectively. As expected, due to the reduced conjugation length in oPE5, the observed absorption and emission maxima are slightly blue shifted to 374 nm and 504 nm respectively. LB Film Grazing Incidence X-ray Scattering. To ascertain the structure of LB films, grazing incidence X-ray scattering studies have been performed on deposited layers of oPE7 on glass plates. Figure 8 a shows the X-ray intensity versus the scattering angle for 4 and 11 deposited layers, respectively. Kiessig as well as Bragg peaks are clearly visible on both curves, which indicate that the
transferred films are well structured. The Bragg peak at 2θ ) 2.44° indicates a well-defined internal periodic electron density corresponding to a layered organization. The calculated periodic spacing of 3.7 nm matches well with the periodic structure (3.87 nm) observed by X-ray diffraction on the oPE7 powder after thermal rearrangement. Other peaks on the reflectivity curves can be referred as Kiessig fringes, which arise from the X-ray interference at the two major interfaces, the air/film and the film/glass substrate. The total film thickness was evaluated from the spacing between the Kiessig fringes. A simulated diffraction pattern is calculated using an electron density profile within the organic layer in accordance with the electron structure of each molecule. The GIXA simulated reflection patterns are represented on Figure 8a as continuous lines. The calculated curves fit well with the experimental data. The structural parameters are given in Table 2 for oPE7 LB films. The Bragg period at 3.7 nm suggests a structural arrangement in double layer in a Y-type assembly for the oPE7 molecules. The thicknesses of the LB films are estimated to be 11.1 ( 0.1 nm and 33.3 ( 0.1 nm respectively, thicker than the expected values
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Figure 6. Optical absorption spectra of LB films for 1 to 16 deposited layers. The insert shows the linear dependence of the absorbances vs the layer number (t upstrokes).
Figure 7. Normalized absorption, photoluminescence, and electroluminescence spectra of LB multilayers on glass.
corresponding to a single layer transfer. These results are coherent with a self-organization of the oPE7 molecules in a Y-type double layer 3.7 nm thick. Starting from a Z-type single layer transferred on the substrate (TR ) 1), a rearrangement to a Y-type double layer could be expected by self-organization during drying (see Figure 8b). This explains why the successive layers can be deposited in upstroke only after drying. As a consequence of this selfrearrangement in bilayer, we infer that it could induce defects in the multilayer. LB Film Morphology, AFM Characterization. To investigate possible defects on the LB films, their morphology was studied by AFM. Several areas of the sample were observed in order to check the homogeneity of the transfer. First a freshly deposited film (Figure 9a) corresponding to 4 transferred layers of oPE7 was deposited on a glass plate (the same as in Figure 8a). The AFM picture shows an irregular granular surface randomly distributed, where no large flat domain can be detected. The typical size of the grains is included between 50 and 100 nm. By AFM we do not identify clearly other possible types of defects such as voids or pores. In Figure 9a the lighter gray levels correspond to the higher position and the Z-height amplitude is between 0 and 10 nm. The smallest imaged grain has a diameter of 25 nm. The Rq value is close to 1.8 nm for a surface of 9 µm2. A line scan of this image shows two main levels of height values of 3.9 ( 0.2 nm and 7.6 ( 0.2 nm. The first level height is close to the
double layer period 3.7 nm already found by X-ray diffraction and reflectometry, while the second level is twice this period. The sample roughness depends also on the substrate itself. Direct AFM observations of the free glass surface shows irregularities as pits and deep holes. The Rq value on 400 µm2 of free glass is 6.6 ( 0.5 nm, while a roughness calculated on a reduced surface of 9 µm2 between the holes gives an Rq value of 1.1 ( 0.1 nm only. These values are significantly lower than those of the sample after the LB transfer of 4 layers and these measurements suggest a roughening of the LB surface by a molecular rearrangement. From the high periodic substructure 3.7 nm observed on the same sample by X-ray reflectometry we expected defects or at least a multistep surface. It is clear that the irregular film morphology observed by AFM and the measured thickness of the film from X-ray reflectometry do not match with a Z-type single layer deposition. The structure is more consistent with a self-assembly of Y-type layers. First, the layer period is 3.7 nm, about twice the molecular size of the side chains in an extended conformation (1.8 nm). Unfortunately the film substructure cannot be explained by a bilayer transfer, because the transfer ratio is only 1 ( 0.05 for each transfer. Moreover, the calculated thickness from X-ray reflectivity is deficient relative to the Y-type deposition. For the sample of 4 liftings the calculated thickness of 1.1 nm corresponds to a mean value of 3 double layers only (Y-type). For the second sample of 11 liftings the value of 33.3 nm corresponds to a mean thickness of 9 double layers. Consequently, as expected before, a single layer is transferred according to TR ) 1 on the glass substrate by lifting. The measured X-ray Bragg reflection suggests a reorganization in Y-type double layers and this rearrangement can occur only by self-organization during the drying phase. The multistep surface observed on the glass substrate by AFM and the well-defined internal periodicity of 3.7 nm are consistent with such a lamellar organization. To control the in-plane morphology we studied by AFM the surface of a transferred monolayer and of a preassembled bilayer on a Si oxidized wafer. For this substrate, the Rq roughness is strongly reduced to 0.33 ( 0.03 nm for a surface of 400 µm2. The LB transfers were carried out at the same surface pressure than on glass substrate. oPE7 monolayer and bilayer were transferred
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Figure 8. X-ray reflectivity plots for 4 and 11 deposited layers on glass plate, (a). The experimental data are represented by the scattered points and the simulated reflectivity pattern, using 3 and 9 bilayer models, by the continuous lines. In (b), the Z-type and Y-type models of bilayer. In (c) the X-ray reflectivity for 1 single layer and 1 bilayer on Si, respectively. The scattered points show the experimental data and the continuous line the simulated reflectivity pattern. Table 2. Structural Parameters of the oPE7 and oPE5 Oligomers in Multilayers Determined by X-ray Reflectometrya
a
sample
LB transfers at TR ) 1
Bragg peak spacing (nm)
total thickness from Kiessig fringes (nm)
calculated thickness for the transfer layer (nm)
oPE7-4 SL-Fig 8a oPE7-11 SL-Fig 8a oPE7-1 SL-Fig 8c oPE7-1 DL-Fig 8c
4 11 1 1
3.7 3.7 s s
11.1 33.3 3.3 3.7
7.4 20.3 1.8 3.7
oPE7 multilayers are related to the transfers of figure 8a.
in a single step on the Si wafer and then dried under high vacuum. For the monolayer, AFM images of several areas show slightly different morphologies, some of them exhibit smooth curved surfaces and folding zones (well represented in Figure 9b) and others irregular surfaces with steps. In all smooth areas the roughness Rq is low, close to 0.36 ( 0.03 nm on a 25 µm2 surface. The AFM observations of the oPE7 bilayer reveal a more homogeneous and regular topography than in monolayer; the film surface is characterized by convolutions and folding zones (Figure 9c). The defects in black are characterized by a depth of 1.8 ( 0.5 nm, which correspond to a single step
of molecules. The roughness including black defects Rq is 0.48 ( 0.03 nm. From X-ray reflectometry (Figure 8a) on the two samples, the mean film thicknesses of 3.3 and 3.7 ( 0.1 nm are calculated, respectively. The corresponding surface roughnesses,29 from Kiessig fringes, are also reduced to 0.4 nm for the two samples (the surface of the X-ray exposure ≈ 19.2 mm2 at 2θ ) 1°). Nevertheless, as we expected film thicknesses of 1.8 and 3.7 nm for a single layer and a bilayer, the reflectivity simulations fit better with 3.3 and 3.7 nm respectively. The AFM observations (29) Ne´vot, N.; Croce, P. Revue Phys. Appl. 1980, 15, 761.
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Figure 10. (a) Electroluminescence intensity-voltage characteristics of the ITO/LB/LiF/Al LEDs for 22 and 36 layers. (b) Luminance-voltage and current density-voltage plot of a 22 layer display. The insert shows the luminance-current dependence in logarithmic scale.
Figure 9. AFM image of LB deposited multilayer (4 upstrokes) on hydrophilic glass (a). The scan was in tapping mode with scanning rate 0.39 Hz. (b) and (c) show AFM images of a monolayer and bilayer, transferred in a single upstroke on a hydrophilic Si wafer.
on the film morphology and the X-ray measurements confirm that the deposited layers are more regular on the
Si wafer than on glass.30 It points also that the preorganized bilayer is more regular and stable, in terms of molecular organization, than the monolayer. For the monolayer, the unexpected thickness in the smooth areas and the morphology could be explained by a partial selfarrangement in the bilayer, which takes place during the transfer or during drying.31 Electroluminescent Devices. The EL spectrum is represented on Figure 7. The two cells with 22 and 36 layer films have similar EL spectra. The maximum of the emission spectra is centered on 527 nm, slightly red-shifted with respect to the maximum of the photoluminescence spectra at 516 nm. This shift of 15 nm may be attributed to a localized recombination of the charges close to one of the electrodes. The luminescence-voltage characteristics of the diodes are given in Figure 10a. The threshold voltage for electroluminescence is initially 4.5 and 6 V for 22 and 36 layers, respectively. It increases to a more stable 8 and 12 V value after some minutes for the 22 and 36 layer diodes. The EL intensity saturates at the highest values. The same phenomenon appears in the I-V curve (Figure 10b) indicating that the current follows the luminescence quite well. This effect may be attributed to the orientation of internal dipoles, as suggested by Zou et al.32 In these conditions, the external quantum efficiency is close to 10-3% photon per injected electron. The devices presently studied were not optimized with regard to the film morphology, the injection process, and lifetime. However, the material itself is not highly photoluminescent in the (30) Kim, Y. K.; Kim, K. Y.; Kang, W. H.; Yang, S. S.; Sohn, B. C. Thin Solid Films 1998, 312, 291. (31) Reiche, J. Thin Solid Films 1996, 284-285, 453. (32) Zou, D.; Yahiro, M; Tsutsui, T. Jpn. J. Appl Phys. 1998, 37, L1406.
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solid state. Presently, its main interest is as concerns the molecular order. Conclusion New rigid-rod molecules based on oligo(p-phenyleneethynylenes) with amphiphilic side chains were synthesized by a step by step method up to the heptamer. The pentamer and the heptamer are amphiphilic enough to produce stable Langmuir films and the two oligomers can be deposited as LB films on hydrophilic substrates. The multilayer deposition can be carried out by lifting only, up to 36 layers. The transfer ratio of 1 for the upstroke deposition suggests a Z-type film. Nevertheless a welldefined periodic substructure at 3.7 nm, observed by X-ray reflectivity, suggests an arrangement in a Y-type bilayer occurring during the transfer deposition or during the drying process. By AFM, the observed surface shows steps of the same periodicity (3.6 nm) which is consistent with a self-arrangement of the single deposited layer to a double layer. The heptamer and pentamer in LB multilayers show a high photoluminescence with blue-green emission. Organic light emitting diodes have been elaborated using these LB multilayers in a sandwich of ITO/oPE/LiF/Al. The device with the heptamer gives rise to a photon emission at 527 nm and a quantum efficiency yield in the
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range of 10-3 %. In this work we did not try to optimize the device parameters such as carrier injection or lifetime, but the demonstration of electroluminescence in diodes using Langmuir-Blodgett films of molecules aligned parallel to the substrate is interesting. Indeed, it confirms the possibility of being able to tailor conduction and emission properties of the devices using a layer by layer deposition technique. The LB deposition technique applied to amphiphilic oligo(p-phenylene-ethynylenes) will be successfully developed for the preparation of molecularly ordered thin films for light emitting devices. Acknowledgment. We wish to acknowledge the CNRS for the financial support, the GDR “Mate´ riaux pour l’Optique nonline´ aire”(GDR 1181) for the financial support to LED characterization, the Mexican National Council for Science and Technology through the program CONACyT/SFERE for the scholarship 112097 of E. Arias-Marin. We thank also B. Heinrich (IPCMS) for X-ray diffraction measurements, and A. Lorin and A. Rosilio (LETI) for their valuable assistance in LED elaboration and measurements. LA991313E
