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Thermal Characteristics of the Langmuir-Blodgett Film Fabricated with Bipyridinium Bis-(Tetracyanoquinodimethane Anion Radical) Salts Dong-Myung Shin* Department of Chemical Engineering, Hong-Ik University, 72-1 Sangsoo-dong, Mapo-ku, Seoul, 121-791, Korea Received April 2, 1999. In Final Form: July 29, 1999 The phase transition of the headgroup in a Langmuir-Blodgett film fabricated with bipyridinum bis(tetracyanoquinodimethane anion radical) salts attached with long chain alkyl groups has been studied using electron spin resonance spectroscopy. The C18 double chain attached more efficiently to the dipyridinium stabilized the alkyl chain and headgroup structure than the C22 single chain. The stability of the film may be due to the size matching between headgroup and hydrocarbon. The headgroup melting temperature of the double chain compound was 367 K and that of the single chain compound was 347 K, which was identified by ESR spectroscopy. A pretransition between two gel states was found. The viscosity estimated with the rotational correlation time of electron spin resonance spectroscopy was 0.8 poise for the LangmuirBlodgett films.
Introduction The thermal stability of organic thin films has drawn a lot of attention for the past 20 years. Most of the investigations have focused on the alkyl chain stability. The alkyl chain stability is important for molecular scale devices as well as for biomembranes. Alkyl chain stability was mostly studied with vibration frequencies obtained from FTIR spectroscopy. The alkyl chain frequencies were correlated with the phase transition of the films. A two step melting process in a LB monolayer of cadmium arachidate on silver was reported. A premelting stage showed no substantial change in headgroup area and considerable change in intensities of CH2 stretching at 2918 and 2850 cm-1.1-3 At the second stage above melting, the monolayer is liquidlike and the alkyl chains are completely randomized. Low-temperature studies showed that the alkyl terminuses are disorganized at around 100 K. The van der Waals interaction among alkyl groups determines the thermal behavior of membranes. It is wellknown that the melting temperatures of the amphiphilic molecules are dependent on the chain length of the alkyl group. Between room temperature and 80 °C, the alkyl chain of arachidic acid (AA) changes slowly. Increased competition between gauche bond concentration and chain untilting was found. It is rather difficult to generalize the melting behavior of the headgroup. Randomization of the carboxylic headgroup in LB films of cadmium arachidate on silver and aluminum oxide was observed around 125 °C.1-3 Electron spin resonance has been a very sensitive technique that enables us to detect a very small amount of unpaired spins in an organic film. ESR is used mainly to determine the angular distribution of the molecules in * Tel.: 82-2-320-1652. Fax: 82-2-334-5842. E-mail: shindm@ wow.hongik.ac.kr. (1) Nasselli, C.; Rabolt, J. F.; Swalen, J. D. J. Chem. Phys. 1985, 82, 2136. (2) Nasselli, C.; Rabe, J. P.; Rabolt, J. F.; Swalen, J. D. Thin Solid Films 1985. 134, 173. (3) Cohen, S. R.; Naaman, R.; Sagiv, J. J. Chem. Phys. 1985, 82, 2136.
the films. Most of the ESR studies of LB films were centered on organometallic compounds. Barraud et al. studied electronically conducting LB films which contained the TCNQ anion radical or the TTF cation radical.4,5 A series of nitroxide containing stearic acid derivatives, where the position of the five membered paramagnetic unit in the chain was changed systematically, were studied.6 The bulky paramagnetic unit introduced free volume and disorder into the monolayer, which was indicated by ESR spectra. ESR studies of merocyanine dyes, which form a LB film, reveal the orientation and thermal characteristics of the film.7,8 A series of TCNQ anion radicals have been studied.1-3,9,10 these molecules can be seen in Figure 1. The cationic partner of the anion radical determines the structure of the crystals and the electronic nature of the anion radical, which eventually influences the conductivity of the material.11 The arrangement and orientation of the TCNQ anion radical in LB films has been studied.5,12 The studies indicated that the TCNQ anion radicals have preferential orientation as these form LB films. The angle of the TCNQ plane is about 50° from the surface normal for the N-eicosyl-N′-methylbipyridinium bis (tetracyano-quinodimethane anion radical), 2. We studied the thermochemical behavior of the headgroup using ESR spectroscopy in connection with the structural characteristics of 2 in LB films. (4) Vandevyver, M.; Richard, J.; Ruaudel-Teixier, A.; Lequan, M.; Lequan, R. M. J. Chem. Phys. 1987, 87, 111. (5) Barraud, A.; Lequan, M.; Lequan, R. M.; Lesieur, P.; Richard, J.; Ruaudel-Teixier, A.; Vandevyver, M. J. Chem. Soc., Chem. Commun. 1987, 797. (6) Taupin C.; Dvolaitzky, M. In Surfactant Solutions; Zana, R., Ed.; Dekker: New York, 1987; Vol. 22, p 359. (7) Kuroda, S.-I.; Sugi, M.; Iizima S. Thin Solid Films 1985, 133, 189. (8) Kuroda, S.-I.; Ikegami, K.; Saito, K.; Saito, M.; Sugi, M. Thin Solid Films 1988, 159, 285. (9) Lapinski, A.; Swietlik, R.; Graja, A.; Strzelecka, H.; Veber, M. Synth. Met. 1995, 71, 1923. (10) Kudo, K.; Nagaoka, M.; Kuniyoshi, S.; Tanaka, K. Synth. met. 1995, 71, 2059. (11) Malatesta, V.; Millini, R.; Montanari, L. J. Am. Chem. Soc. 1996, 117, 7. (12) Shin, D. M.; Choi, K. H.; Kang, D. Y.; Park, K. Y.; Kwon, Y. S. Mol. Cryst. Liq. Cryst. 1993, 227, 159.
10.1021/la990388f CCC: $19.00 © 2000 American Chemical Society Published on Web 01/28/2000
Thermal Properties of TCNQ Derivative LB Films
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Figure 1. Molecular structure of TCNQ anion radical derivatives, 1 N,N′-dioctadecyl viologen-bis(tetracyanoquinodimethane anion radical), and 2 N-docosyl-N′-methyl viologen-bis(tetracyanoquinodimethane anion radical).
Figure 3. UV/visible absorption spectra of 2 fabricated as a seven-layer LB film on a quartz plate.
Figure 2. UV/visible absorption spectra of 1 solubilized in a 1:1 mixture of methylene chloride and acetonitrile and fabricated as a seven-layer LB film on a quartz plate.
Experimental Section Compounds 1 and 2 were synthesized and recrystallized twice from ethanol.12 The water used as a subphase was purified by distillation followed by deionization with an Elgastat Spectrum C. The resistivity of the purified water was higher than 18 MΩcm. The LB films were deposited using a moving wall type deposition apparatus (Nippon Laser, Model: NL-LB-1405-MWC). The compounds were dissolved in a mixture of acetonitrile and methylenechloride (1:1, v/v, 10-3 mol/L) and the solutions were spread over the water subphase. The surface pressure for the LB deposition was 30 mN/m with the dipping speed of 15 mm/min for the down stroke and 10 mm/min for the up stroke. For the ESR measurement, the LB film has to be more than 30 layers to get enough spin signal. The temperature of the TCNQ salts (the weight of the sample was 1-2 mg) loaded in DSC (Dupont thermal analysis system-9900) was increased at a rate of 10 °C/min. The ESR spectra were obtained from the Brucker ESR spectrometer (EST300S). The powder samples were prepared in a cell, and LB film samples were cut to insert the probe. The angle was set by a goniometer, which can be rotated to an angle from the external magnetic field of the spectrometer.
Results The UV-visible spectra of 1 and 2 layered as LB films and those solubilized in solution are shown in Figures 2-3. The absorption spectra of 1 and 2 in solution show the absorption band of the TCNQ anion radical at 400 nm and the localized excitation of the dimer at around 700900 nm.13,14 These bands are shifted to 390 nm and to around 650 nm respectively for LB films. The hypsochromic shift observed in the spectrum is due to H-aggregation.15 The structured spectrum becomes broad and structureless. The broad absorption of the dimer can be (13) Shin, D. M.; Choi, K. H.; Park, J. S.; Choi, J. S.; Kang, D. Y. Thin Solid Films 1996, 284-285, 523. (14) Tanner, D. B. Optical Properties of One-Dimensional Systems. In Extended Linear Chain Compounds; Miller, J. S., Ed.; Plenum: New York, 1982; Vol. 2, p 221. (15) Kasha, M.; Rawls, H. R.; El-Bayoumi, M. A. In Molecular Spectroscopy (IUPAC Symposium, Volume VIII), Butterworths: London, 1965, p 371. Pure and Appl. Chem. 1965, 11, 371.
Figure 4. Dependence of the ESR spectra of 1 in LB films on temperature.
explained by the less defined structure of aggregates in the LB film. The oxidation of the TCNQ anion radical produces TCNQ, which shows a 400 nm absorption peak and an aggregate absorption peak at 445 nm. The CN stretching mode band of 1 and 2 is located at 2182 cm-1 which is 20 cm-1 shifted from that of TCNQ. The shift was due to the double bond character of the CN bond due to the anion radical. The sharp peak at 1507 cm-1 is attributed to the b1uν20 mode of the TCNQ anion radical species in the dimeric state.16 The 1362 cm-1 peak is complex (agν4 + b2u)2. The peaks at 1587 and 1179 cm-1 are assigned respectively to the agν3 and agν5 IR-activated transitions with dipole moments both parallel to the direction of the charge-transfer interaction.16 These two spectroscopic data clearly show that the films contain TCNQ anion radical aggregates. The ESR spectra show a symmetrical triplet signal arising from TCNQ anion radical dimers. Figure 4 is the first derivative of the ESR absorption signals of 1 that was deposited as an LB film on a fused quartz plate. The ESR signals clearly exhibit a narrow line at g ) 2, and the dimer signals are strongly dependent on the angle between the normal to the substrate and the permanent magnetic (16) Richard, J.; Vandervyer, M.; Lesieur, P.; Ruaudel-Teixier, A.; Barraud, A.; Bozio, R.; Pecile, C. J. Chem. Phys. 1987, 86, 2428.
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Figure 5. Dependence of the ESR spectra of 2 in LB films on temperature.
field. The half bandwidth of the signals is 6.3 G at a normal angle, which decreases to the minimum value, 4.7 G, at 32° from surface normal.12 The angle at which the minimum bandwidth was generated was 54° for 2.17 Since the orientation of the radical is perpendicular to the TCNQ plane, the angles, discussed above for the TCNQ anion radical, represent an angle between TCNQ plane and the substrate. The TCNQ anion radical of 1 lies rather flat on the substrate and that of 2 has a tilt angle of 36° from the surface normal.17 The ESR bandwidth and intensity showed temperaturedependent variation. The ESR bandwidth of 1 and 2 became narrow at temperatures above 365 K and 348 K, respectively (Figures 4, 5). Above these temperatures, the ESR bandwidth became smaller than 2 G. The narrow signal was maintained up to 403 K, above which the signal intensity reduced significantly and broadened. Figure 5 shows the temperature-dependent half bandwidth variation of 2. We also investigated the low-temperature behavior of these samples with ESR spectroscopy. A gradual increase in ESR signal intensity of 1 and 2, for both the powdered sample and the LB film, was observed as the temperature fell from 297 K to 100 K (Figure 6). However, the half bandwidth of 4 G was maintained. The ESR signal intensity of 1 in the LB film remained strong upon reducing the temperature of the substrate from 400 K to 297 K, and the half bandwidth was 2 G. The signal intensity gradually decreased as low as that of the sample without heat treatment; however the half bandwidth became 3 G, which was an overlap of the 4 G and 2 G bandwidth signals. Figure 6 shows that the ESR signal intensity of the powder sample of 1 increases linearly between 370 K and 100 K as the temperature was reduced. A clear inversion of the intensity of the slope was observed at 370 K. The 390 nm band of 1 indicates a characteristic absorption of TCNQ anion radicals. Above 393 K, this oxidized to TCNQ, which had a 400 nm absorption band. The spectra obtained up to 340 K were the same as those obtained at 298 K. This suggests that the TCNQ anion radicals are intact below 340 K. The absorption peak of the TCNQ anion radicals shifts from 650 nm to 612 nm, which overlaps with the absorption band of the viologen cation radical. (17) Lee, Y. S.; Shin, D. M.; Kim, T. W.; Kang, D. Y. J. Kor. Inst. Elec. Eng. 1998, 47, 2133.
Shin
Figure 6. Signal intensity of 1 in powdered film as a function of reciprocal temperature. The signal intensity was linear with the reciprocal temperature between 330 K and 100 K.
The thermal characteristics of 1 and 2 were studied with differential scanning calorimetry (DSC). Two endothermic peaks were observed for 1 at the temperatures of 340 K and 350 K. Endothermic transitions were observed at 329 K and 340 K from 2. Since 2 has a longer alkyl chain than 1, it generally melts at a higher temperature. It is interesting to observe the lower temperature melting of 2 than of 1. Discussions Viscosity. The melting behavior of biomembranes has been studied extensively to understand the thermal stability and transporting process of biomaterials.18 The thermal stability of the conducting films is also important for the application of high-temperature conducting films. The arrangement of the headgroups is critical for the conducting properties. Melting and crystallization characteristics for the annealing process can be utilized to improve electrical properties of the films. For example, the side chain and headgroup areas melt at different temperatures. Ordered structures, such as biomembranes and block copolymers, are constituted with multiple ordered segments which have their own melting temperature. The local viscosity of the melting zone is reduced as melting progresses. The ESR bandwidth is dependent on the local viscosity. The rotational correlation time τ is in effect equal to τ ) 4πηa3/3kT, following Stokes’ relation, with a the particle radius and η the viscosity of the solvent.19 Above the melting temperature, the viscosity of the environment becomes drastically reduced, which eventually reduced the rotational correlation time. The ESR spectrum undergoes a strong modification as the memory of the anisotropic effect is completely lost. Above the critical frequency which is at 1/τC ≈ γδH, the broader line exhibits motion narrowing, where γ ) gβ/h. It is then no longer possible to distinguish between the two states because of the uncertainty principle. Knowing that the γ for ESR is about 3 × 106 Hz/G, and the size of the TCNQ anion radical is about 8 Å, we can deduce the viscosity of the headgroup. The viscosity of the TCNQ anion radical at its melting temperature is about 0.8 poise, which is more viscous than (18) Gennis, R. B. Biomembranes; Springer-Verlag: New York, 1989. (19) Taupin, C.; Dvolaitzky, M. In Surfactant Solutions; Zana, R., Ed.; Dekker: New York, 1987; Vol. 22, Part 7, pp 379-380.
Thermal Properties of TCNQ Derivative LB Films
moderately viscous organic solvents, such as cyclohexanol (0.4 poise) or diethylene glycol (0.3 poise).20 However, it is low enough for the rotation of TCNQ anion radicals in a microsecond time scale. The dynamic effects of the nitroxide group have been studied to understand the mobility of proteins in micelles and vesicles. The spin labeling methods has some limitations, such as that the spin probe acts as an impurity of the membrane and changes the melting temperature. In this paper, the melting viscosity of TCNQ radical anion, which constitutes the headgroup structure of the LB film, was estimated; this would be difficult with any other technique. Phase Transition Temperature. It is interesting to correlate the alkyl chain length and number of alkyl chain with the physical characteristics of a film. Two alkyl groups are attached to the dipyridinium unit of 1 and 2, and they influence the headgroup arrangement and packing density. It is known that the melting temperature of phosphatidylcholine derivatives increases as the number of carbon increases, such that the melting temperatures of dipalmitoyl phosphatidylcholine (DPPC, C16), distearoyl phosphatidylcholine (DSPC, C18), and diarachidoyl phosphatidylcholine (DAPC, C20) are 315 K, 328 K, and 337 K, respectively.18 IR and Raman spectroscopy have also been used to study the melting behavior of cadmium arachidate LB films on silver, which had a strong increase in rotation about the chain axis at around 353 K and 363 K.1,2 We observed two endothermic peaks of 1 and 2 from DSC. Low-temperature peaks are at the temperature of 340 K for 1 and 329 K for 2, and high-temperature peaks are at 350 K for 1 and at 340 K for 2. These are in the range of the melting temperature of the long chain dipolar molecules. The high temperature also compares with the melting temperature, 366 K, of the N-eicosylquinolinium TCNQ anion radical complex, which has a smaller headgroup than 1 and 2. The discrepancy in melting temperature may result from the crystallinity of the alkyl (20) Gordon, A. J.; Ford, R. A. The Chemist’s Companion; Wiley: New York, 1972; p 2.
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chain, which can be determined by the balance between headgroup and alkyl chains. Since 1 has a double chain, the alkyl chains can be packed better in the solid state and it melts at higher temperature compared to 2, which has a much bigger headgroup than the alkyl chain. The charge-charge interactions between headgroup molecules are critical for influencing the melting temperature. The neutral TCNQ headgroup with the long alkyl chain generally melts at a higher temperature, around 400 K.21,22 There is a similarity between the melting processes of phosphatidylcholines and TCNQ complexes, which exhibited a pretransition between two gel states. Lipids with a large area requirement for the polar headgroup, such as phosphatidylcholines, show a pretransition.23 The two TCNQ anion radicals and dipyridinium require a large area in the film, which presumably shows a pretransition. The melting temperatures of 1 and 2 are 18 K lower than the temperature at which ESR bandwidth narrowing occurs, 368 K and 358 K, respectively. The rotational motion of the TCNQ headgroup is somewhat related to the melting of the alkyl chains. However, the TCNQ stack itself has to be melted and becomes free in rotational motion. It needs additional thermal energy which is 18 K in temperature. We discussed the structure and melting behavior of TCNQ radical anion complexes. The mismatch in headgroup and alkyl chain lowers the melting temperature, and double alkyl chains helps the thermal stability. The headgroup area melts about 18 K higher than the alkyl chain in our case. Acknowledgment. This work was supported by the Korea Research Foundation. LA990388F (21) Wang, Y.; Nichogi, S. T.; Iriyama, K.; Ozaki, Y. J. Phys. Chem. 1996, 100, 368. (22) Nakagoshi, A.; Wang, Y.; Ozaki, Y. Langmuir 1995, 11, 3610. (23) Lichtenberg, D.; Menasche M.; Donaldson S.; Biltonen, R. L. Lipids 1984, 19, 395.
